Mechanochemistry in Waste Valorization: Advances in the Synthesis of Catalysts, Polymers, and Functional Materials

Abstract

The growing accumulation of waste from diverse human activities has intensified the search for sustainable strategies. Mechanochemistry offers a promising pathway, transforming residues into high-value products with reduced energy demand, shorter reaction times, and minimal use of solvents and reagents. Various wastes—including biomass, food residues, fly ash, used batteries, and halogenated polymers—can be converted into environmental adsorbents, industrial biopolymers, biocompatible compounds, electrodes, and catalysts. Unlike previous reviews that addressed specific waste streams, this study provides the first systematic and comparative analysis of mechanochemical valorization across multiple residues, following PRISMA guidelines (2000–2025). A total of 656 studies indexed in Scopus and Web of Science were evaluated. This integrative approach highlights recent advances, current challenges, and future prospects, offering a rigorous and transparent guide for scaling mechanochemistry toward circular and sustainable solutions.

1. Introduction

Mechanochemistry, the science of reactions induced by mechanical force, dates back to ancient times with the grinding of pigments [1]. However, it gained notoriety in the 19th century with M. Carey Lea’s research on photomechanical and mechanochemical reactions, connecting early manual processes with modern mechanochemistry [2]. Recently, it has experienced a resurgence as a modern and sustainable approach to chemical synthesis. Today, mechanochemistry has enabled new chemical processes to create valuable compounds for pharmaceuticals [3,4,5], polymers [6], biopolymers [7,8], and others functional products in many industrial sectors. In this context, mechanochemistry emerges as a solution to the challenge of reducing the dependence on organic solvents in chemical processes, which can be harmful and represent the majority of a process’s composition. Thus, its importance lies in: (i) promoting green chemistry by minimizing or eliminating solvents; (ii) using recyclable raw materials (such as waste), aligning with fundamental sustainability principles [9,10].

Chemical methods used to generate desired products include thermochemical, electrochemical, photochemical, and mechanochemical approaches. All these methods require an energy source to overcome the natural energy barriers involved in breaking chemical bonds, thereby enabling new molecular arrangements and bond formation. Mechanochemical methods, in particular, have attracted increasing interest due to their simplicity, operating under varied conditions such as low temperatures, minimal solvent use, or entirely solid-state systems in inert or reactive environments. The objective of mechanochemical methods is to convert reactants into products through mechanical energy, applied via physical force with tools like crucibles and mortars [9,10]. The friction between components allows the direct absorption of energy, creating favorable conditions to overcome activation energy barriers. This process depends on the contact interfaces of the reactants, as reactions occur at the surface, promoting wear, fracture, and microstructural refinement [11].

Although this technique has employed specific tools since ancient times, its modern evolution is remarkable, as the milling process can now be carried out using ball mills equipped with jars made of resistant materials such as polytetrafluoroethylene (Teflon©), polymethylmethacrylate (PMMA), zirconia, and tungsten carbide. Therefore, the process can be conducted in both open and enclosed systems, using mills that protect the reaction from environmental interference. It can also be monitored in real time when transparent jars are used as reaction vessels. Analytical techniques such as synchrotron XRD (X-ray diffractometers) and Raman spectroscopy facilitate real-time monitoring of reaction kinetics [12,13]. Moreover, mechanochemical methods can incorporate small amounts of liquid additives in the form of “Liquid-Assisted Grinding” (LAG), thereby expanding their applications in pharmaceutical and biological research.

Thanks to its ease, versatility, and solvent-free conditions, mechanochemistry competes effectively with standard synthetic methods, offering efficient, single-step reactions without the need for traditional heating or toxic reagents. Mechanically assisted methods are increasingly recognized for their ability to synthesize a wide array of products. Despite its long history, mechanochemistry’s most significant developments in synthesis have taken place primarily over the past two decades [9,10,14,15]. As shown in Figure 1, studies related to the application of mechanically assisted protocols in organic chemistry and the preparation of new materials have increased considerably, especially over the last 14 years.

The applications of mechanochemical methods encompass catalytic processes such as Suzuki–Miyaura coupling [17,18], olefin metathesis, and C-H activation [12,19]. Other applications of mechanochemical reactions are in the synthesis of metal–organic materials such as MOFs and metal nanoparticles where the extensive use of solvents traditionally required to control particle size is avoided [20,21]. In general, mechanochemistry has found productive applications in the synthesis of porous materials [6]. These protocols have been used to prepare a wide range of products, as represented schematically in Figure 2.

Significantly reducing the solvent makes mechanochemical methods both cost-effective and broadly applicable. This aligns with the principles of green chemistry and broader environmental protection goals. In this context, waste recovery has emerged as a prominent application. Waste often presents challenges for industrial production, as it can constitute up to 50% of the total output and requires conversion into functional products. At the same time, many synthesis processes require precursors with specific chemical compositions to obtain functional materials, derived from both organic and inorganic sources, such as zeolites, mesoporous silica, biopolymers, nanoflowers, and biochar. These functional products typically undergo physicochemical processes, including hydrothermal reactions, sol–gel synthesis, or thermal treatments. Such precursors can be derived from various waste sources (e.g., minerals, biomass, ashes, and rice husk) [22]. Recent studies have highlighted the central role of mechanochemistry in waste transformation. Indeed, mechanochemistry-assisted techniques can be used for the preparation of new materials or value-added products from various industrial, agro-industrial, and urban wastes [23,24,25,26].

Fly ash is an abundant byproduct generated by various industries, including coal combustion and energy generation from agricultural byproducts such as sugarcane bagasse. In many cases, the incomplete carbonization of these materials results in fly ashes with a high content of unburned carbon [27]. Mechanochemical methods, such as high-energy milling and chemical activation, can be used to upcycle this type of waste. This approach has been applied in several studies, including enhancing adsorption characteristics for the effective removal of pollutants, such as dyes, from aqueous solutions. Furthermore, mechanochemically modified fly ash has shown potential in various applications, ranging from the stabilization of hazardous waste and contaminated soil to the synthesis of sustainable geopolymers and hydraulic cement. These innovations represent significant opportunities for the environmentally friendly utilization of fly ash and the valorization of this commonly overlooked industrial residue through mechanochemical methods [28,29].

The valorization of lignin residues has become an area of great interest, as lignin is one of the primary components of biomass and holds significant potential for the production of high-value chemicals and materials. Various mechanochemical methods have been explored to break lignin bonds and transform it into useful products. Some of these methods include the use of high-energy mills and readily available reagents, avoiding the need for expensive transition metals or solvents. Furthermore, the utilization of mechanochemical approaches has proven effective in producing a variety of products such as monomers, energy storage materials, and bioplastics. Research in this field continues to advance, and it is expected that mechanochemical strategies will play a crucial role in the sustainable valorization of lignin residues, thus contributing to the transition to a greener and more circular economy [30,31,32,33].

Plastic disposal, on both micro- and macro-scales, is a long-term environmental challenge. Sustainable practices are needed to reduce plastic waste, as global plastic production, reaching megatons annually, is heavily used in packaging (40%), construction (20%), and textiles (13%). In 2015, it was estimated that polyolefins, PVC (Polyvinyl Chloride), and PET (Polyethylene Terephthalate) comprised 79% of global plastic production, generating 69% of plastic waste. Besides the use in bottles, PET is included in the production of polyester fibers which has a production of 57 million tons by year that are discarded in landfills or incinerated [34,35]. Mechanochemical depolymerization offers a promising route for reducing plastic waste [36]. Applications include recycling automotive shredder residues through solid-state mechanochemical techniques [37], using mechanoenzymatic methods for polymerization and depolymerization [38], and filling PVC structures with eggshell additives, as described by Skórczewska [39]. These approaches could transform polymer production and waste management, reducing environmental pollution.

Animal-derived feedstocks provide an eco-friendly source for producing functional products using mechanochemistry. Chitin, a polysaccharide of D-glucosamine derived from crustacean shells, is commonly transformed into chitosan, which has wide applications in papermaking, food, bioengineering, and agriculture [40]. Chitosan, synthesized by the deacetylation of chitin, is a biopolymer with high water solubility, biodegradability, and biocompatibility which makes it useful for the production of sustainable derivatives. In textiles, chitosan offers antimicrobial, antiviral, and anti-odor properties [41]. Mechanochemical milling of chitin from European green crabs yields eco-friendly biomaterials, replacing harmful inorganic acids with organic solid acids [33]. Mechanochemical and mechanoenzymatic methods are also applied to produce oligomers of N-acetyl-D-glucosamine and in fine chemistry for the production of 5-hydroxymethylfurfural (5-HMF) [42,43,44]. Thus, it is clear that mechanochemistry has wide applications and has become globally impactful, with research groups advancing its applications in various fields, such as catalyst preparation, depolymerization, geopolymerization, and the synthesis of high-value molecules from agro-industrial and urban biomass, fly ash, and other wastes [4,45,46,47,48,49,50,51,52,53].

Therefore, this systematic review aims to present the current landscape of waste valorization through ecologically sustainable mechanochemical processes, producing high-value functional products with diverse applications. The next section will describe mechanochemically assisted protocols for transforming low-cost, waste-based raw materials into value-added products.

2. Methods

This section describes the research and study selection process. It can be divided into subheadings to detail the search strategy, inclusion and exclusion criteria, and the data extraction and synthesis method.

2.1. Search Strategy

This systematic review was prepared in accordance with the recommendations described in the Preferred Reporting Items for Systematic Review 2020 (PRISMA 2020) methodology [54]. The literature was retrieved from Scopus and Web of Science to search for citations between 15 March 2025 and 30 April 2025 in the Scopus and Web of Science databases, using the keywords “mechanochemistry AND waste OR valorization OR reject OR processing OR waste recovery”, for studies published in the period from 2000 to 2025, limiting only research articles to the areas of chemistry, chemical engineering, materials engineering, materials science, and environmental science. In addition, the inclusion of additional literature was necessary, added from the Capes Journals platform.

2.2. Inclusion and Exclusion Criteria

All mechanochemistry articles that were related to the valorization or beneficiation of waste or rejects were included. The exclusion criteria were articles that did not address mechanochemistry, or mechanochemistry articles unrelated to waste or rejects, duplicate articles, review articles, and studies with insufficient data or low quality.

2.3. Study Selection

The selection of studies was carried out independently by two authors, using the Rayyan selection platform [55]. Initially, the articles were selected after reading the title and abstract, and those that met the eligibility criteria and that had consensus between the two authors were read in full for inclusion or exclusion in the review. Disagreements in the full reading phase were resolved by consensus by a third author [56].

3. Results

Initially, searching with keywords identified 656 potential articles: 336 from the Scopus database and 320 from the Web of Science database (Figure 3). Of the initial 656 articles, 25 were excluded for being duplicates files. The remaining 631 files were evaluated by title, abstract, and keywords, with 203 articles excluded for not being suitable for the scope of the study. There remaining 428 articles were selected for full-text evaluation, 31 of which were not retrieved, 62 were excluded because they were not related to mechanochemistry, 82 articles were not related to mechanochemistry applied to waste or rejects, and another 103 were reviewed articles. Therefore, 150 articles were selected from the Scopus and Web of Science databases and included in the review.

3.1. Mechanochemistry: A Tool to Enhance Waste Valorization into Valuable Products

Mechanochemistry has proven itself as an established method across various research fields and has seen growing interest in recent years. This method contributes significantly to advancements in the utilization of waste in materials science, with a particular focus on catalysis, pretreatment of biomass, and preparation of active materials. Additionally, waste treatment has emerged as a compelling area for the mechanochemical community, with numerous studies employing mechanochemical techniques for waste processing [4,9,10,51,52,57]. In the following subsections, updates will be provided on publications that explore the reuse of various waste materials using a mechanochemical approach for the synthesis of catalytic materials, ceramics, polymers, valuable chemicals, and the degradation of halogenated organic pollutants.

These studies focus on both the synthesis and characterization of materials or products, aiming not only for the economic benefits derived from waste reuse but also for the significant environmental advantages it offers.

3.2. Fly Ashes Waste

Fly ash (FA) is a significant byproduct resulting from the combustion of coal, biomass, or municipal solid waste in power plants and industrial boilers, including those fueled by agricultural residues such as sugarcane bagasse [27]. It is typically defined as a fine, powdery residue composed of inorganic particles expelled from combustion chambers during high-temperature burning (see Figure 4). Based on the source material, fly ash can be classified into three main types: coal fly ash (CFA), biomass fly ash (BFA), and municipal solid waste fly ash (MSWFA) [48,49]. These categories represent the principal residues generated from both fossil fuel and renewable energy combustion processes. From an environmental perspective, fly ash presents considerable concern due to the presence of hazardous contaminants, including polycyclic aromatic hydrocarbons (PAHs), heavy metals, and dioxins [57]. Despite this, its unique physical and chemical properties render it a valuable raw material for environmental applications. Coal fly ash, in particular, has a complex composition, comprising a mixture of aluminosilicates, iron oxides, titanium dioxide, unburned carbon, and various inorganic oxides. Its morphological features, such as particle size distribution, surface area, and pore structure, vary significantly depending on its origin and combustion conditions. Additionally, it contains transition metals (often as oxides) and a range of functional groups that contribute to its reactivity. These properties make fly ash an attractive candidate for the synthesis of adsorbent materials, offering a low-cost and sustainable solution for pollutant removal from aqueous environments. Mechanochemical treatments have been shown to enhance these properties, improving its adsorption capacity and expanding its potential for use in environmental remediation strategies [58].

Mechanochemical techniques have been explored as an effective approach to improve the properties of fly ash for pollutant removal. According to Geng et al. [58], mechanochemical activation—achieved by grinding solid reagents together, with or without solvents—induces surface defects and increases reactivity through frictional and shear forces. In their study, the authors utilized residual coal fly ash from power plants to produce low-cost adsorbents for mercury removal from wastewater. Mechanochemical bromination, performed using a planetary omnidirectional ball mill, was employed to modify the surface of the fly ash. This technique proved to be a cost-effective and efficient alternative to conventional solution-based functionalization, owing to the effective incorporation of bromine onto the adsorbent surface. The improved adsorption capacity was attributed to the formation of carbon–bromine (C–Br) bonds in carbon-rich domains and iron–bromine (Fe–Br) bonds in Fe₂O₃-containing phases. Based on Fourier-transform infrared spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) analyses, the proposed mechanism involved the generation of unsaturated carbon structures within the activated carbon matrix, which subsequently reacted with bromine radicals to form stable C–Br bonds. Overall, the study provides valuable insight into the mechanisms underlying the enhanced mercury adsorption performance of fly ash modified via mechanochemical bromination, reinforcing its potential for cost-effective wastewater treatment applications.

Chumpiboon et al. [27] reported that the use of mechanochemical methods can increase the adsorption capacity of fly ash, such as modifying fly ash through high-energy milling and chemical treatments with acidic and alkaline solutions. In this study, the authors investigated the treatment of sugarcane bagasse fly ash (BFA) with various acids and simultaneous carbonization under a nitrogen atmosphere to remove a cationic dye, methylene blue (MB), from aqueous solutions. This treatment significantly improved both the surface area and adsorption performance of the material. The resulting modified material—referred to as Treated BFA—achieved an MB adsorption capacity of 39.0 mg/g in 90 min, closely approaching that of commercial activated carbon (CAC), which showed a capacity of 42.1 mg/g but with a much larger surface area (1130 m²/g). Kinetic studies revealed that the adsorption process followed pseudo-second-order and intra-particle diffusion models, while equilibrium data were best fitted by the Langmuir isotherm model, with maximum MB adsorption capacities of 27.2 mg/g for untreated BFA, 39.0 mg/g for Treated BFA, and 42.1 mg/g for CAC. A comparative analysis demonstrated the high performance of the modified material relative to the commercial standard. These results highlight that, beyond surface area, surface functional groups and electrostatic interactions also play a critical role in dye adsorption efficiency.

Any material containing silica and alumina phases—whether primary (e.g., kaolinite, illite) [59] or secondary (e.g., fly ash, steel slag, red mud) [60]—is suitable for the production of geopolymers. Among these, fly ash stands out as a promising precursor for geopolymer synthesis through mechanochemical activation. Geopolymers are synthetic aluminosilicate binders formed by the reaction of silicon (Si) and aluminum (Al) oxides under alkaline conditions, resulting in an amorphous framework composed of interconnected SiO₄ and AlO₄ tetrahedra. Fly ash typically contains these oxides in both crystalline (e.g., mullite, quartz) and reactive amorphous forms. The effectiveness of geopolymerization depends largely on the reactivity of the aluminosilicate phases, which is influenced by factors such as the reactive content of SiO₂ and Al₂O₃, and the particle fineness of the raw material. Insufficient reactivity leads to slower setting and poor mechanical strength in the final geopolymer [28,61]. Mechanochemical activation via high-energy milling plays a key role in enhancing this reactivity by increasing the specific surface area, generating structural defects, and inducing partial amorphization, which facilitates the dissolution of reactive species during alkali activation [29]. These effects have been shown to improve the mechanical performance of the resulting materials. In particular, the incorporation of mechanochemically activated fly ash into cementitious or geopolymeric matrices has led to significant increases in compressive strength, positioning this strategy as a viable route for both waste valorization and the development of sustainable construction materials [28].

Oyun-Erdene and Temuujin [29], investigated the effect of mechanochemical activation of fluidized bed coal fly ash (CFA) on the mechanical performance of geopolymers. In this study, geopolymer samples were prepared from both raw and mechanically activated CFA using alkaline solutions composed of 10 mol/L sodium hydroxide (NaOH) and mixtures of sodium silicate (Na₂SiO₃) and sodium hydroxide at a 2:1 weight ratio. After 30 min of vibratory milling, the Brunauer–Emmett–Teller (BET) method surface area of CFA increased from 4.8 m²/g to 9.2 m²/g, attributed to particle size reduction and the amorphization of crystalline phases, as confirmed by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and FTIR analyses. The incorporation of sodium silicate into the alkaline solution increased the compressive strength of raw CFA-based geopolymers from 6.03 MPa to 16.23 MPa, while the mechanically activated CFA-based geopolymers exhibited an increase from 27.6 MPa to 32.4 MPa. In addition, geopolymers derived from mechanically treated CFA showed a denser and more homogeneous microstructure. These results demonstrate that mechanochemical activation is an effective strategy to enhance the reactivity of fluidized bed fly ash, thereby improving the geopolymerization process and the resulting mechanical properties of the final material.

In another study, Matsuoka et al. [62] studied the effect of mechanochemical activation on the surface of coal fly ash particles. The article reports that mechanochemical activation modifies the surface morphology and the crystalline phase of the fly ash particles using a friction-type mill (Figure 5A,B). Consequently, an increased dissolution of Si (IV) and Al (III) species in 10 mol/L NaOH for 3 h solution is observed (Figure 5C,D). This increase results in an increased reactivity of the mechanochemical-activated CFA allowing the use of milder geopolymerization conditions (28 days at 70 °C, 7 days at 70 °C, and 28 days at room temperature). Mechanochemical activation of CFA results in improved compressive strength (45.2 MPa for 30 min of treatment after 28 days at 70 °C) of the corresponding geopolymer, along with an increased acid resistance by densification of the hardened body (Figure 5E). The compressive strength of geopolymers after an acid resistance test is shown in Figure 5F. Thus, with these results, it is clearly demonstrated that mechanochemical activation is effective for producing geopolymers with beneficial mechanical properties under milder curing conditions.

In another subsequent study, Kalinkin and collaborators [15] examined the synthesis of geopolymers at room temperature using mechanically activated fly ash (FA) milled for 30 s, 180 s, and 400 s, combined with varying contents of natural calcite (0–10 wt%) (Calcium carbonate, CaCO₃) and activated with an 8.3 mol/L sodium hydroxide solution. The incorporation of 10 wt% calcite into 90 wt% FA (denoted as GFA10C) resulted in compressive strength values that were 8.0, 3.5, and 2.9 times higher, respectively, than those of the calcite-free geopolymer (GFA0C) for the same milling times, particularly after 7 days of curing (Figure 6A,B). According to specific surface area data (Figure 6C), milling for 180 s led to a substantial strength improvement compared to 30 s, whereas the additional milling from 180 to 400 s yielded only marginal or even negative effects. Furthermore, the main calcite diffraction peak, observed at 2θ ≈ 29.4° in the XRD pattern of GFA10C, showed a slight decrease in intensity relative to the unreacted FA10C blend (Figure 6D), suggesting partial interaction between calcite and the geopolymeric matrix.

Research in this field has focused on synthesizing geopolymers from waste and by-products, with fly ash standing out due to its aluminosilicate composition, flow ability, and large waste volume. Geopolymer samples synthesized at a temperature of 27 °C using fly ash mechanically activated for different periods (60–90 min) in an eccentric vibrating mill (EVM) were reported by Kumar et al. [28]. The study also analyzed the effect of MA on the geopolymerization reaction using an isothermal conduction calorimeter. FTIR analysis justified the formation of the new band at 1080–1096 cm⁻¹ in MA samples which is related to the highly polymerized sheet structure corresponding to SiQⁿ (n = 3–4) which is related to the restructuring of quartz, also corroborated by the results from QXRD (Quantitative X-ray Diffraction).

Nikolić et al. [63] investigated the effectiveness of geopolymers derived from FA (initial FA) or mechanically activated FA in lead immobilization. To simulate the ability to use these FAs to trap and immobilize toxic Pb²⁺ ions during the geopolymerization process, they were mixed, at room temperature, with the geopolymerizing agent (alkali solution) in the presence of Pb(NO₃)₂. The results of the studies indicated that geopolymers based on mechanically activated FA were more effective in immobilizing lead compared to geopolymers based on initial FA, showing a reduction in the leaching of Pb²⁺ present in the geopolymers. The greater effectiveness of Pb²⁺ immobilization was attributed to the decrease in the pore sizes (from 18.8 to 6.9 nm) of the geopolymers based on mechanically activated FA compared to those based on the initial FA. In addition, the mechanically activated FA led to a geopolymer with a significantly increased compressive strength (58.46 N/mm², with 1% Pb after 28 days of aging). The study reports that mechanochemical activation of FA can replace thermal activation which is of great importance for practical applications of geopolymers.

Regarding mechanical treatment and the presence of heavy metals in FA, studies have focused on the mechanochemical stabilization of Pb. In this sense, Li et al. [64] associated the Pb stabilization mechanism in ground fly ash with the entrapment of Pb in a process of particle re-agglomeration. Interestingly the authors reported a counterintuitive trend of the particle size vs. the mechanochemical process time. Indeed, the size decreases at low treating times while increasing again due to a re-agglomeration process occurring at higher times, to arrive at the size of the starting non-treated particles. In terms of leaching, the results show that 93.11% of Pb was partitioned into the ground ash, and the leaching of Pb was inhibited after short-term grinding (from 5.2 to 1.2 mg/L after 1 h of grinding) and further reduced by around 96% after 96 h of ball milling. This can be attributed to the formation of water-insoluble species, upon interaction with the higher surface energy the freshly ground particles exhibited. Nomura et al. [65] also reported that through mechanochemical treatment of FA, the Pb leaching was reduced by 92.8% due to the formation of water-insoluble species. Among these, is the formation of Pb₃O₄. Chen et al. [25] also noted that mechanochemical treatment of FA decreases the concentration of leached Pb, Cr, and Cu, while water prewashing effectively increases stabilization efficiency through the removal of soluble salts.

In another study reported by Li et al. [66] red mud (RM) was used as an additive for the stabilization of heavy metals in mechanochemical-treated FA. The results of this study demonstrated that the addition of RM under the optimized conditions of 30% by weight of red mud, 0.1 g/L solid–liquid ratio, and 24 h ball milling time significantly decreased the leaching of Pb, Cr and Zn from FA, in 99%, 81.18% and 100%, respectively. Even in this case, a re-agglomeration phenomenon was reported: FA was broken into small particles and then agglomerated into larger particles after mechanochemical treatment. It was evident that the main reasons responsible for the entrapment of heavy metals were attributed to precipitation and sealing by fine particles, captured by new phases which RM contributed to form.

On the other hand, Li et al. [67] investigate the adsorption properties of methylene blue (MB) on mechanochemically modified FA. The results showed that the modification of the grinding process increased the MB adsorption capacity of FA, with the adsorption capacity value increasing from 5.06 to 7.97 mg/g for raw FA and ground FA, respectively. Kinetic modeling revealed that the adsorption follows a pseudo-second-order reaction rate. Furthermore, the authors mentioned that some assessment of environmental pollution caused by FA should be carried out before application.

Modification of fly ash was carried out by dry grinding in a planetary ball mill was reported by Sundum et al. [68] and studied its efficiency in preparing thermoplastic starch composites (TPS) containing different amounts of unmodified (UFA) or modified (MFA) fly ash powder. It was found that the UFA particle size decreased from 59.60 μm to 13.17 μm while the surface area increased from 3.68 m²/g (UFA) to 4.44 m²/g (MFA), after grinding for 1 h at 400 rpm. Both UFA and MFA powders were incorporated into thermoplastic starch. The high maximum tensile strength (7.78 MPa) was obtained in a composite with 2.50% MFA, which is about 9 times higher than TPS (0.86 MPa) and 2 times higher than the TPS/UFA composite. According to this study, composites with MFA showed better water resistance and delayed degradation compared to TPS and composites with UFA. While the presence of UFA or MFA had a slight effect on the thermal stability of the samples, it promoted the final phase of the thermal decomposition. This property can be exploited for their thermal degradation as waste.

Resuming, the use of mechanochemically treated FA in geopolymers presents several advantages, including transforming waste into a value-added material, improving the mechanical properties of geopolymers, and reducing environmental impact. These and the other results presented in this section are schematically resumed in

Table 1. General summary of studies on mechanochemical activation of fly ash and its applications.

Raw Material	Milling Pretreatment Conditions	Final Product Properties	Applications	Results Without Pretreatment	Results with Pretreatment	Ref.
Fly ash	Ball mill—filling rate of 1.1, material filling rate of 30, speed (e) 80%,	Increased specific surface area (>3000 cm2/g), decrease in average particle size (10.6 μm), FA amorphization	Geopolymer (curing at 150 °C for 4 h)	2.01 MPa (Compressive strength)	15 MPa (Compressive strength)	[60]
Fly ash	Vibratory mill—filling rate of 1.1, material filling rate of 70, speed (e) 80%,	Increased specific surface area (>3000 cm2/g), decrease in average particle size (9.7 μm), FA amorphization	Geopolymer (curing at 150 °C for 4 h)	2.01 MPa (Compressive strength)	16 MPa (Compressive strength)	[60]
Fly ash	Stirred ball mill—filling rate of 1.1, material filling rate of 70, speed of 5 m/s.	Increased specific surface area (>10,000 cm2/g), decrease in average particle size (5.2 μm), FA amorphization	Geopolymer (curing at 150 °C for 4 h)	2.01 MPa (Compressive strength)	22 MPa (Compressive strength)	[60]
Fly ash	Sample of 75 g, 300 rpm. 60 min, dry	Decrease in average particle size, increased structural disorder, FA amorphization	Geopolymer (cured at 20 °C for 90 days)	>8.1 MPa (Uniaxial compressive strength)	>10.2 MPa (Compressive strength)	[69]
Fly ash	FA to ball mass ratio was 1:35, milling time 60; dry	Increased specific surface area, decrease in average particle size (5–7 μm), FA amorphization	Geopolymer (cured at 27 °C for 28 days)	<10 MPa (Compressive strength)	40 MPa (Compressive strength)	[70]
Fly ash	FA to ball mass ratio was 1:35, milling time 45; dry	Increased specific surface area, decrease in average particle size (5–7 μm), FA amorphization	Geopolymer (cured at 27 °C for 24 h followed by curing at 60 °C for 4 h)	>10 MPa (Compressive strength)	>60 MPa (Compressive strength)	[70]
Fly ash	Sample of 40 g, 180 s, dry	Increased specific surface area, decrease in average particle size, FA amorphization	Geopolymer (curing at 22 °C for 7 days)	-	<20 MPa (Compressive strength)	[15]
Fly ash	Sample of 40 g, 180 s, dry	Increased specific surface area, decrease in average particle size, FA amorphization	Geopolymer (curing at 22 °C for 28 days)	-	>20 MPa (Compressive strength)	[15]
Fly ash	FA to ball mass ratio was 1:20, 380 rpm; milling time 15; dry	Increased specific surface area, decrease in average particle size, FA amorphization	Geopolymer (curing at 95 °C for 4 h)	<5.24 N/mm2 (Compressive strength)	>50 N/mm2 (Compressive strength)	[24]
Fly ash	FA to ball mass ratio was 1:20, 380 rpm; milling time 15 min; dry	Increased specific surface area (67.7 m2/g), decrease in average particle size (6.9 nm), FA amorphization	Geopolymer/reduced lead leaching (curing at 90 °C for 28 days)	10.66 N/mm2 (Compressive strength)	60.69 N/mm2 (Compressive strength)	[63]
Fly ash	Ball mill—until the particles retained on a sieve no. 325 were <2% of the original weight.; dry	Decrease in average particle size (10.5 μm), FA amorphization	Geopolymer (cured in a controlled chamber at 25–28 °C for 28 days)	14 MPa (Compressive strength)	25.5 MPa (Compressive strength)	[71]
Fly ash	FA to ball mass ratio was 1:10, milling time 60 min; dry	Decrease in average particle size (6.8 μm), FA amorphization	Geopolymer (cured at ambient temperature for 28 days)	25 MPa (Compressive strength)	45 MPa (Compressive strength)	[72]
Fly ash	Sample of 10 g, 400 rpm, 1 h, dry	Increased specific surface area (4.44 m2/g), decrease in average particle size (13.17 μm); FA amorphization	Preparation of thermoplastic starch composites	3.27 MPa (Tensile strength)	7.78 MPa (Tensile strength)	[68]
High calcium fly ash	Attrition mill—steel balls and steel container; milling time 1 h, dry	Increased specific surface area (20.47 m2/g), decrease in average particle size (6.25 μm); FA amorphization	Geopolymer (curing at 70 °C for 24 h)	21.3 MPa (Compressive strength)	61 MPa (Compressive strength)	[50]
High calcium fly ash	Vibration mill—steel balls and steel container; 1750 rpm; milling time 1 h, dry	Increased specific surface area (17.8 m2/g), decrease in average particle size (7.25 μm); FA amorphization	Geopolymer (curing at 70 °C for 24 h)	21.3 MPa (Compressive strength)	49.53 MPa (Compressive strength)	[50]
Fly ash	Attrition-type mill—4000 rpm, 20 min, dry	Increased specific surface area (2.2 m2/g), decrease in average particle size (4.7 μm); FA amorphization	Geopolymer (curing at 70 °C for 28 days)	26.9 MPa (Compressive strength)	45.2 MPa (Compressive strength)	[62]
Municipal solid waste incinerator (MSWI) fly ash	Sample of 10 g, 93 rpm, 96 h, wet	Decrease in average particle size (2 μm); FA amorphization;	Lead stabilization	5.2 mg/L	0.20 mg/L (>96% Inhibition of Pb leaching)	[64]
Fly ash	Sample of 10 g, 700 rpm, 8 h, dry	Decreased average particle size (39 μm); FA amorphization	Lead stabilization	540 μg/g-fly ashes	39 μg/g-fly ashes (92.8% Inhibition of Pb leaching)	[65]

.

3.3. Biomass

In recent years, concerns about the severity of environmental problems and the growing energy shortage have led to the exploration of renewable resources as a viable alternative. Biomass is the most abundant renewable source globally, with approximately 130 billion tons produced annually. Lignocellulosic biomass, derived from agricultural residues, bioenergy crops, and biomass waste, is composed of three biopolymers: cellulose (38–50%), hemicellulose (20–35%), and lignin (15–25%), varying by species and biomass sources [31,33]. While cellulose and hemicellulose are primarily exploited for conversion into various products such as pentoses, xyloligosaccharides, and liquid fuels, lignin can be employed in the production of aromatic compounds. However, the recalcitrance of biomass and the complexity of its structures pose significant challenges in the efficient separation and utilization of its components [30,31,33,73,74,75,76].

The pre-treatment to isolate lignin from other components is a crucial step in biorefinery operations, presenting a challenge to achieve maximum production efficiency. Various technologies of mechanochemical and biological pre-treatment have been developed for the effective separation of this biomass. Among them, mechanochemical activation is a sustainable approach compared to techniques involving toxic solvents during pre-treatment. This technique disrupts the biomass structure, reducing cellulose crystallinity and enhancing reactive activity [77].

Mechanochemical methods applied in lignin depolymerization, enable the breaking of bonds and the production of economically valuable aromatic compounds. This is because mechanochemistry can fasten reactions and create high-energy microenvironments through localized pressures and friction-induced heating [30,33]. According to Scimmi et al. [31], lignin depolymerization to recover fine chemicals involves hydrogenolysis and oxidation methods, with the latter being more commonly used due to milder conditions and cleaner final product production. Traditional oxidation processes using chlorine and nitrate are considered environmentally unfriendly, posing environmental and health risks. In pursuit of more sustainable alternatives, new oxidative methods have been optimized, replacing oxidants with hydrogen peroxide and molecular oxygen. Additionally, the application of heterogeneous catalysts, following green chemistry principles, aims to improve recoverability, reusability, and waste reduction.

The work of Kleine et al. [74] was a pioneer in reporting the use of mechanochemistry for biomass degradation, demonstrating that with this methodology the use of solvents and metal catalysts can be avoided. This study developed a base-assisted ball milling process, demonstrating its potential as a mechanochemical technique for wood degradation. NMR spectroscopy indicated that cellulose and hemicellulose in beech wood were degraded into low molecular weight compounds, mainly breaking the β-O-4 linkage and hydrolyzing oligosaccharides (Scheme 1). Overall, these observations supported the authors’ initial hypothesis that base-assisted milling can also be applied to the degradation of untreated natural wood.

On the other hand, Dabral et al. [78] reported a mechanochemically activated oxidation of beech lignin using HO–TEMPO, KBr, and Oxone^® in the ratio 0.2:0.2:1.5 and using tungsten carbide (WC) as grinding medium. In this study, mechanochemical oxidation of model compounds, methoxy-substituted monolignol derivatives, 1a and 1b proved to be successful, as it provided the corresponding ketones 2a and 2b in 93% and 87% yields, respectively, after purification by column chromatography (Scheme 2).

The same methodology was applied to a beechwood lignin. The Gas Chromatography coupled to Mass Spectrometry (GC-MS) analysis highlighted the formation of 3,5-dimethoxyquinone as the major monomer (2.5 wt%), along with 2-methoxybenzoquinone (0.5 wt%) (Figure 7). Furthermore, when the reaction was carried out on 10 g of beechwood lignin, gel permeation chromatography showed a strong reduction in molecular weight, demonstrating that the lignin was successfully depolymerized. Overall, this study demonstrates that this oxidative protocol can be applied on a large scale to biomass.

Another successful solvent-free and environmentally friendly oxidative depolymerization protocol of model lignin (lignin^ox) was developed by Sun et al. [79], and the results showed successful depolymerization with some selectivity in syringate production (72.7% in weight). Zakaria et al. [80] used lignocellulosic oil palm residues, namely empty fruit bunches (OPEFB) and palm leaf fiber (OPFF), to produce glucose and xylose through enzymatic hydrolysis after ball milling pretreatment. The pretreatment of OPFF for 60 min of ball milling followed by OPEFB for 120 min demonstrated high glucose recovery with yields of 80.3 and 67.5%, respectively. Meanwhile, the highest xylose yield was obtained from OPEFB (80.1%), followed by OPFF (78.6%). The total sugar conversion yield of OPEFB and OPFF was 71.9 and 79.8%, respectively. The BM-treated OPEFB and OPFF produced 4 times more glucose and 4 to 16 times more xylose, respectively, compared to untreated oil palm biomass. The study further showed that higher recoveries of xylose (80.35 and 84.23%), glucose (62.4 and 78.69%) and total conversion (68.1 to 81.5%) were obtained from OPEFB and OPFF samples, respectively, when they were hydrolyzed at different loadings (30 and 40 FPU/g) of acremonium cellulase substrate. This suggested that BM effectively increased the surface area to enhance enzymatic hydrolysis.

Ball milling pretreatment was considered by Pang et al. [81] an efficient method to promote the catalytic conversion of lignocellulosic biomass into ethylene glycol. According to the report, Miscanthus subjected to dry grinding for 6 h at 450 rpm using a planetary ball mill, was converted by a binary catalyst of tungstic acid and Ru/AC into water. The maximum ethylene glycol yield from Miscanthus reached 52.4%, much higher when compared to untreated Miscanthus (22.5%). The study by Schneider et al. [82] describes a procedure for converting lignocellulosic barley straw into total reducing sugars (ATR) by mechanocatalytic pretreatment. Under optimized conditions, it resulted in high ATR release (42%) using oxalic acid dihydrate as catalyst. This study revealed that acid strength plays an important role in the depolymerization of barley straw and furthermore, it showed that the reaction catalyzed by oxalic acid generates low levels of the degradation product 5-hydroxymethylfurfural (HMF).

The ball mill has provided many relevant advantages compared to other lignocellulosic biomass pretreatment techniques, such as easy operation and avoidance of solvents and post-treatments. Zhang et al. [83] compared the feasibility and effectiveness of different biomass pretreatment methods, including microwave-assisted alkali (MAP), ultrasound-assisted alkali (UAP), and ball milling (BMP) to pretreat digested waste (2.5%-DR and 10%-DR) of rice straw, with 2.5% and 10.0% solids, for ethanol production. The result of this study revealed that BMP was found to be the best method to increase ethanol production from 116.65 and 147.42 mg/g to 2.5%-DR and 10%-DR, respectively. Digestion of ruminal fluid from rice straw with 2.5% solids content, combined with ethanol fermentation, produced a total energy production of 7.010 kJ/g, higher than the 5.464 kJ/g with 10% solids content. Overall, the integrated system was an effective biorefinement process with high energy output and reaction time savings.

A combined pretreatment of ball milling and ultrasound (MU) was investigated by Li et al. [77], corn cob with solid acid catalyst (${\mathrm{S}\mathrm{O}}_4^{2-}$/SiO₂⁻Al₂O₃/La³⁺) for the conversion selectively catalytic hydrothermal processing of corncob into furfural. It was evident from this study that combined combination of ball milling and MU pretreatment was an efficient mechanocatalytic approach to destroy the complex structure of corn cob. So much so that the highest furfural yield of 197.76 mg/g corresponding to 82.90% of theoretical productivity was obtained at 190 °C for 30 min.

The work reported by Jiang et al. [76] compared the effects of ionic liquid and ball milling pretreatment on glucose yield from cellulose. According to the report, the glucose yield from untreated cellulose was only 20.9%, while both ball mill and ionic liquid pretreated cellulose exhibited remarkably higher cellulose digestibility for efficient enzymatic hydrolysis. The results proved that the yield of cellulose pretreated by ball milling (84.5%) was higher than that of cellulose pretreated with ionic liquid (78.0%).

Studies prove that ball mill pretreatment is a simple operation with high efficiency and is environmentally friendly. Powders, ball milling is a physical process that eliminates the use of potentially dangerous chemical reagents. The pretreated sample could be used directly without washing and filtration steps. For example, Kim et al. [84] reported that conventional pretreatment processes with sodium hydroxide and aqueous ammonia showed a rice straw loss of 34.2% and 14.8%, respectively. Comparatively, no biomass loss was observed with ball milling. Furthermore, milling produced significantly lower concentrations of soluble phenolics than alkaline treatments.

A solid cellulase acid catalyst (SA-SO₃H) prepared without the use of sulfuric acid (H₂SO₄) was prepared by Shen et al. [85] and applied it to the conversion of microcrystalline cellulose. According to the report, cellulose can be effectively converted to levulinic acid in pure water at 180 °C in 12 h without additives with a maximum yield of 51.5% double SA-SO₃H. However, cellulose pretreatment by ball milling improved levulinic acid yields by only a few percent (52.2%), showing that the cellulose binding sites (-Cl) and catalytic sites (-SO₃H) of the catalyst are fundamental to the activity of the catalyst. A greater recycling efficiency of 95% is reported in the 5th round, and that the spent catalyst was regenerated with hydrogen peroxide (H₂O₂) solution. The study investigated the use of glucose as a starting material under the same reaction conditions and with the catalyst it promoted a yield of 61.5% of levulinic acid. The route of converting carbohydrates to levulinic acid in pure water with the biomimetic catalyst prepared with an H₂SO₄ free method provides an environmentally friendly method for producing bio-based platform chemicals from renewable resources.

Su et al. [86] successfully obtained a solid acid catalyst, notable for its absence of strong sulfonic acid (-SO₃H) functional groups. Instead, the material, prepared from cow dung, exhibits the presence of weak acidic groups, namely carboxylic and phenolic. The synthesis process involved the initial carbonization of the dung at 400 °C for 2 h, followed by an activation step at 600 °C under a nitrogen (N₂) atmosphere for 2 h, using potassium hydroxide (KOH) in a 1:2 weight ratio (substrate/activator).

A solid carbonaceous acid catalyst functionalized with -SO₃H group by co-carbonization of sucralose was synthesized by Qiu et al. [87] and studied its efficiency in converting cellulose to glucose in pure water (Figure 8A). It was established by the study that the mixture of cellulose with the catalyst prepared by ball milling, used in the cellulose hydrolysis process provided a glucose yield of 52.8% in pure water at 200 °C in 1 h, and the glucose yields increased to 71.9% and 88.0% when it was diluted in 0.02% by weight of H₂SO₄ and hydrochloric acid (HCl) (with the same proton content) aqueous solutions, respectively, (Figure 8B). Furthermore, the prepared solid carbonaceous acid catalyst exhibited relatively stable catalytic activity over five cycles (Figure 8C). According to this study, the excellent catalytic activity for the production of glucose from cellulose was attributed to the reduced degree of crystallinity of cellulose and the improved contact between the active sites and the β-1,4-glycosidic bonds in cellulose during the cellulose and solid acid milling process. The work provided a promising strategy for high-yield glucose production from cellulose hydrolysis in aqueous solutions and offers broad application for transforming lignocellulosic biomass into valuable chemicals.

Research conducted by Qi et al. [88], demonstrated a promising method for the synthesis of weakly acidic carbon catalysts from biomass byproducts. The team used black liquor, a substance generated from rice straw pretreated with an aqueous potassium hydroxide (KOH) solution, as the main raw material. These innovative catalysts demonstrated excellent catalytic performance in hydrolysis processes, efficiently converting cellulose and alkali-pretreated rice straw into valuable sugars. The study highlighted remarkable results: cellulose hydrolysis achieved an impressive 76.3% glucose yield. For rice straw, the process resulted in high yields of 52.1% glucose and 66.5% xylose when the reaction was conducted in a 0.015% (by weight) HCl solution at 200 °C for 60 min. In addition to high activity, the carbon catalysts exhibited remarkable stability and recyclability in the aqueous reaction system. One specific catalyst, BC-600, provided a glucose yield of 84.6% and maintained this yield after three recycling cycles, highlighting its robustness and potential for industrial use. In summary, this work not only offers an innovative strategy for the efficient hydrolysis of lignocellulose using catalysts derived from the biomass itself but also represents a significant advance in the comprehensive utilization of lignocellulosic biomass.

The valorization of compounds derived from biomass can benefit from mechanochemical techniques as well. The research group of Oh et al. [89], developed a simple and sustainable mechanochemical technique for the production of polyurethanes (PUs). The method, which does not require the use of solvents and operates at room temperature, uses 2,5-bis(hydroxymethyl)furan (BHMF), a biomass-derived compound, in combination with a vibratory milling process (Figure 9). This approach, in addition to being fast and straightforward, produced PUs with a maximum molecular weight of 163 kJ. The resulting polymers are flexible, with a glass transition temperature (T_g) of 96 °C, and thermally stable, with a decomposition temperature (T_d) of 197 °C. The ball milling technique also proved versatile, enabling the synthesis of PU copolymers with different diols and diamines, which allows varying the final properties of the polymer, such as its T_g.

In summary, mechanochemistry has proven to be a promising tool in transforming biomass, including lignin, into sustainable chemicals and materials. This approach contributes to the search for environmentally friendly solutions for the efficient use of renewable resources and addresses the challenges in biomass separation. As research advances in this field, it is expected that new developments will lead to more effective and sustainable processes for biomass valorization.

Table 2. Mechanical milling pretreatment on the product yields from different types of biomass.

Substrate	Ball Milling Pretreatment	Catalyst	Product	Yield Without Pretreatment	Yield with BM Pretreatment	Ref.
Sugarcane bagasse	Sample of 1.0 g, 400 rpm, 90 min, room temperature.	Sulfuric acid	Glucose	22% (80 mg/g)	78.7% (338.6 mg/g)	[90]
Rice straw	Sample of 50 g, 1700 rpm, 60 min, room temperature	Sulfuric acid	Glucose	23.4%	89.4%	[91]
Cellulose	Sample of 0.50 g, 2 h	Sulfuric acid	Hexitols	44.7%	87.6%	[92]
Cellulose	Sample of 10 g, 60 rpm, 24 h	Activated carbon	Glucose	36%	57%	[75]
Oil palm empty fruit bunch	Sample of 20 g, 250 rpm, 120 min, room temperature	Enzyme	Xylose	5.4%	80.1%	[80]
Oil palm empty fruit bunch	Sample of 20 g, 250 rpm, 120 min, room temperature	Enzyme	Glucose	15.9%	67.5%	[80]
Oil palm frond fiber	Sample of 20 g, 250 rpm, 60 min, room temperature	Enzyme	Xylose	17.7%	78.6%	[80]
Oil palm frond fiber	Sample of 20 g, 250 rpm, 60 min, room temperature	Enzyme	Glucose	23.2%	80.3%	[80]
Cellulose	Sample of 1.5 g, 20 Hz, 4 h, room temperature	Ru/AC	Sorbitol	49.4%	68.0%	[93]
Jatropha hulls	24 h	C-SO3H/Fe3O4	Total reducing sugar (TRS)	29.5%	35.4%	[94]
Bagasse	24 h	C-SO3H/Fe3O4	TRS	57%	68.4%	[94]
Plukenetia hulls	24 h	C-SO3H/Fe3O4	TRS	34%	40.8	[94]
Eucalyptus wood chips	Sample of 1.0 g, 400 rpm, 2 h, room temperature	Enzyme	Glucose	-	89.7%	[95]
Cellulose	Sample of 1.0 g, 400 rpm, 4 h, room temperature	Biochar	Glucose	28.9%	52.8%	[85]
Douglas-fir forest residuals	Sample of 250.0 g, 270 rpm, 30 min, room temperature	Enzyme	Glucose	14%	60%	[96]
Cotton textile	Sample of 1.5 g, 20 Hz, 4 h, room temperature	Ru/CNT	Sorbitol	64 mg/g	212 mg/g	[97]
Cotton wool	Sample of 1.5 g, 20 Hz, 4 h, room temperature	Ru/CNT	Sorbitol	66 mg/g	183 mg/g	[97]
Tissue paper	Sample of 1.5 g, 20 Hz, 4 h, room temperature	Ru/CNT	Sorbitol	83 mg/g	338 mg/g	[97]
Cellulose	4 h	SA-SO3H	Levulinic acid	46%	52.2%	[84]
Hydroxymethylfurfural	60 Hz, 5 min	NaOH	2,5-dihydroxy- methylfuran	67% (24 h)	64% (5 min)	[98]
Miscanthu	Sample of 6.0 g, 450 rpm, 4 h, 40 °C	5%Ru/AC t	Ethylene glycol	22.5%	52.4%	[81]
Corn stover	120 min, <30 °C	Sulfuric acid	Ethyl levulinate	15.59%	20.46%	[99]
Digested residue of rice straw	Sample of 24 g, 750 rpm, 2 h	Enzyme	Ethanol	15.08% (30.8 mg/g)	57.84% (116.65 mg/g)	[82]
Cellulose	Sample of 1.5 g, 500 rpm, 4 h	Sulfonated carbon	Glucose	35.6%	52%	[87]
Rice straw	Sample of 1.0 g, 500 rpm, 4 h	Biochar	Xylose	41.1%	61.5%	[88]
Rice straw	Sample of 1.0 g, 500 rpm, 4 h	Biochar	Glucose	3.7%	19.4%	[88]
Cellulose	Sample of 2.0 g, 300 rpm, 3 h, room temperature	Enzyme	Glucose	20.9%	84.5%	[76]
Cellulose	Sample of 1.3 g, 350 rpm, 4 h,	Al2(SO4)	Hydroxymethylfurfural	39.8%	44.6%	[100]

summarizes the effect of ball milling pretreatment on the conversion of lignocellulosic biomass into different chemicals.

3.4. Polymer Waste

Polymers are one of the main materials developed in the XX century with the growth of petroleum exploitation, to supply the demand of an extensive range of industrial activities making them ubiquitous. It is estimated that the production of plastics in the world was almost 368 million tons in 2019, with over 80% being waste. In 2020 the production of fibers included polymers such as polyester and polyamide, representing 109 million tons together with natural fibers. In this scenario, polyester is responsible for 52% of fiber production with only polyethylene terephthalate contributing with 57 million tons [34,101].

Plastic waste treatment includes incineration, landfill, mechanical recycling, and chemical upcycling. Among these, chemical upcycling offers a path to obtain highly valuable monomers and oligomers. An example is the reported selective depolymerization of PET and other polymers to originate useful compounds [36,101,102]. The process of chemical upcycling demands the application of heating and the use of chemicals besides catalysts. In addition, classical methods are more time-consuming and less cost-effective than mechanochemical methods which also avoid the use of solvents and additives and offer fewer risks in downgrading the properties of the recycled polymer [102].

The mechanochemical treatment of polymers and their wastes makes the use of mills for the generation of new structures, such as the method called Solid State Shear Milling (SSSM) that allows the formation of ultrafine powder polymer with the application of strong force. Polypropylene (PP) waste and automotive shredder residue (ASR) have been used with this technique to produce a composite with good toughness [37]. The authors fabricated a PP/ASR composite that exhibited robust mechanical performance due to the excellent dispersion of ASR particles and in situ compatibility between the ASR matrix and PP (Figure 10A). The results of this study showed that the 50/50 wt% PP/ASR-10 composite presented excellent mechanical performance combined with good processability and high thermal stability (Figure 10B–E).

Access to new polymer structures based on mechanochemistry methods for recycling polymer waste demands some considerations about experimental parameters such as crushing speed, crushing time, and the knowledge of feed amount. In polyurethane processing for recycling, the action of mechanochemical forces decomposes chemical groups such as methyl and aldehyde groups, generating a material with strong reactivity and plasticity. Improved thermomechanical features are reached in this process [101].

In this scenario, polyurethane thermoset waste recycling is a challenge. A process of milling and grinding in the presence of an organocatalyst, triazabicyclodecene, followed by molding, has been applied to promote a vitrimerization process. This results in a vitrimerized polymer that can relax stress quickly, mainly by a carbamate exchange reaction, allowing the material to be reprocessed by simple techniques such as extrusion and injection molding [103]. Plastic wastes of polyolefins from domestic sources have the potential to be transformed into materials with improved properties by high-energy mechanochemical processes using planetary ball milling [104].

The prospects for eco-friendly methods involving mechanochemical treatments highlight the urgent need to address the disposal of massive amounts of plastic waste. These methods aim to avoid traditional recycling procedures such as incineration and landfill storage, which, while reducing polymer waste, contribute significantly to global pollution. One significant source of waste is car tires, composed of thermosetting polymer materials that can persist in the environment for hundreds of years. The cross-linking in polymer structures has great occurrence [105,106,107,108] and increases their stability. Mechanochemical processing breaks the cross-linking, increasing the surface area and roughness of rubber from tires [109].

The production of polymer blends involves the bonding of a polymer structure to another one. In this process, the mechanochemical technique is applied to split the polymers, with disproportionation or recombination reactions, forming a copolymer block in conditions of high shear. The formation of these structures allows the improvement of mechanical properties in the final material [110]. Mechanochemical ball milling plays a key role in the development of these materials. For example, the formation of polylactic acid (PLA) with improved mechanical properties demonstrates the high efficiency of this method. The high-energy collisions between reactants enable the synthesis of new compounds without the need for solvents. Additionally, mechanochemical processes offer advantages such as shorter polymer degradation times and the ability to avoid C-C bond cleavage. This makes them a versatile route for the synthesis of polymers like polyphenylene/vinylene, polyphenylene, polyazomethine, polylactides, and polyurethane [111].

Polyurethane copolymers can be synthesized via ball milling through the polyaddition of diisocyanates in the presence of a catalyst. To obtain a high molecular weight product, it is crucial to control factors such as reaction time and energy input [89]. In this context, few studies have investigated the kinetics of copolymer formation, such as the melting process of two polymers to produce a copolymer. The mechanical force applied during the process yields a product with an architecture similar to the primary structures involved, often with the addition of a multifunctional reagent that interacts with both polymers. Radical-radical coupling frequently occurs in these reactions. Ball milling, used to pulverize shredded recycled plastic, affects polymer size and has been applied in the treatment of polyolefins, such as in the formation of the polypropylene–low-density polyethylene (PP-LDPE) copolymer. This process acts as a compatibilizer, reducing interfacial tension and improving interfacial bonding [51].

The development of functional materials through the mechanochemical treatment of polymer waste is driving the growth of this research area, offering new concepts for waste management across various industries. In this context, the production of mesoporous carbon materials is particularly attractive for polymer waste-based synthesis, as these materials serve as effective catalyst supports due to their inert nature. They are widely used in reactions such as hydrodeoxygenation, nitrogen reduction, hydrogen evolution, and oxygen evolution. Carbon-based structures can be synthesized from PET waste, as reported by Xu et al. [112]. In this study, functional porous carbons with variable shapes formed in situ and with high surface areas (up to 1001 m²/g) were obtained after carbonization and chemical attack (Figure 11). Furthermore, metallic Mo species were impregnated in a well-dispersed form into the porous carbons through solvent-free extrusion to support solid catalyst design particles (Mo₂C@MC). The results indicated that Mo₂C@MC not only has good catalytic performance in the reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), but also the linearity of ln(C/C₀) over time was consistent with pseudo-first-order kinetics with an apparent rate constant (K_app) of 0.5812 min⁻¹ (Figure 12).

Another polymer with potential application in the synthesis of functional materials is polyurethane. Wastes generated from this material are estimated to accumulate at about 19 million tons annually. This is a precursor for N-doped porous carbon materials, used in supercapacitor applications, provided with high surface area and electrical conductivity. The modification with heteroatoms improves features of the carbonous structures in terms of energy storage, being nitrogen incorporated in the carbon skeleton by immersion of it in a liquid medium such as urea or by post-treatment in the gas phase using ammonia (NH₃) or nitrogen (N₂) at high temperatures [113]. Schneidermann and his collaborators [113] developed a polyurethane upcycling process obtaining porous carbon materials already doped with nitrogen that were applied to supercapacitor electrodes (Figure 13). In this study, polyurethane residues served as a source of carbon (and nitrogen) that were converted via mechanochemistry with K₂CO₃ and, optionally, urea. Finally, optimized carbon materials with excellent properties were obtained, such as a high specific surface area of 2.150 m²/g and a total pore volume of 0.9 cm³/g (PUUPC-800-1). Furthermore, such N-doped carbon materials performed similarly to commercial carbon material supercapacitors such as YP-50F, showing a specific capacitance of up to 99 F/g in Li₂SO₄, as well as stable performance in TEA-BF₄ with 83 F/g. By mechanochemical upcycling with additional urea, the rate capacity of the supercapacitor was increased and the obtained device exhibits 80% of its capacitance at a high specific current of 10 A/g in aqueous electrolyte. The authors report that such materials have the possibility of being applied to other energy storage systems, such as lithium-ion batteries or wastewater purification, whenever materials with a high surface area and better wettability are required. This study presents a broader perspective, aiming to extend the process described here to other challenging polymeric waste streams, thereby further reducing waste generation.

The presence of halogenated organics in plastic waste complicates chemical recycling. Therefore, recycling methods for these polymeric wastes focus on developing mechanochemical processes with dehalogenation, such as the removal of bromine, chlorine, and fluorine from polypropylene after ball milling for a certain period [114].

Mechanochemistry has proven to be an effective method for mineralizing halogenated organic pollutants. Balema et al. [115] reported the mechanochemical depolymerization of polystyrene to styrene monomer via ball milling. After the milling process, the molecular weight of polystyrene dropped from 88 kDa to 7–8 kDa, and 7 wt% styrene monomer was detected, as characterized by ¹H NMR (hydrogen-1 proton nuclear magnetic resonance) and GC-MS (Gas Chromatography with Mass Spectrometry). Although the depolymerization yield is low, this work showed promise for depolymerizing polystyrene in an economical and scalable manner.

The recycling process for this material involves the use of a solvent (benzyl alcohol) under high-pressure conditions. It was observed that extended soaking periods resulted in less undissolved resin in the final product, improving the quality of the recovered fibers. However, the deposition of the catalyst on the surface may compromise this quality [116]. Some additives such as flame retardants, that use halogens in composition, are present in plastic from vehicle components, that in the end-of-life cycle turn to waste. Hexabromocycledodecane (HBCD) is one of the additives. Plastic components made of ABS (acryl nitrile-butadiene-styrene) and PP/PE (polypropylene/polyethylene) have undergone a mechanochemical dehalogenation process using ball milling with the addition of Al- and Si-based additives. This process is particularly relevant since HBCDs were listed as potential pollutants under the Stockholm Convention on Persistent Organic Pollutants in 2013 [117].

In a previous study, Grause et al. [118] performed the almost complete debromination (98%) of decabromodiphenyl ethane (DBDPE) in high impact polystyrene (HIPS) through 24 h wet grinding with an agitated ball mill. In this study, NaOH in ethylene glycol was used as dehalogenation reagent at a relatively high temperature (90 °C). The diffusion-controlled activation energy was about 205 kJ/mol. The strong reagent, high temperature, and long treatment highlight the weak reactivity of brominated compounds in polymer matrices. However, the ball mill had the advantage of deforming and flattening the plastic particles. Therefore, the thermal stability of treated HIPS was also improved after mechanochemical treatment, according to Thermogravimetric analysis (TGA).

Cagnetta et al. [114] studied the mechanochemical treatment of commercial polypropylene (PP) pellets, which contain 15.4 wt% DBDPE, through high-energy planetary grinding for 8 h. The highest debromination efficiency of 90% was achieved with Fe-SiO₂ additives, due to the redox activity of Fe, which reduces DBDPE and facilitates the debromination/degradation. The report estimated the energy consumption, assessing that the polymer matrix absorbed about 85% of the milling energy, while only about 15% was used for effective debromination, approximately proportional to the mass proportion of DBDPE. Furthermore, the proposed mechanochemical process carried out in dry conditions at room temperature with cheap reagents, is significantly faster than previously studied [118].

On the other hand, Wang and collaborators [119] investigated the mechanochemical degradation of polybrominated diphenyl ethers (PBDEs) contained in Waste Printed Circuit Boards (WPCBs). The results showed that a degradation conversion of 65.2% for PBDEs was achieved, with PBDEs with higher bromination being reduced with increasing grinding time, and PBDEs with lower bromination being generated.

Considering the strong need for disposal of waste containing HBCD, mechanochemical pretreatment technology as a dehalogenation method was investigated by Lu et al. [117] to destroy brominated flame retardants (BFRs). In this study, the results showed that HBCD was effectively degraded during mechanochemical treatment with the aid of Si-Al-based additives in the ratio SiO₂/Al = 7:2 with a reagent ratio of 15:1. With the optimized additive system, 4 h of ball milling treatment was sufficient to degrade HBCD. According to the experimental and characteristic results, the possible degradation pathway of HBCD was proposed, as shown in Scheme 3. There are two reactions: (a) debromination and (b) fragmentation. Both reactions occur at the same time, but debromination is the dominant reaction. As it is only a qualitative and not a quantitative result, more in-depth research must be carried out on the mechanism.

The application of recycling techniques enables the exploration of methods beyond ball milling, such as solid-state shear milling, which has been reported as a useful approach for recycling epoxy resin waste with potential applications in coatings and functional materials [120]. To conclude this section, it is important to highlight that mechanochemical methods are as environmentally friendly as other methods of recycling polymeric waste. Giving new life to these materials offers alternatives to obtain composites, blends, catalyst supports, electrodes, and electric components, all products which are essential for many human activities. The mechanochemical processes offer many positive features, e.g., the problem of contaminant presence that can be ignored because contaminants may act as milling assistants. This process can be combined with chemical processing for the recycling of a variety of wastes and for the synthesis of new materials [121], which have already been and will be mentioned throughout this review. It is proven that the mechanochemical method has a certain degradation effect on brominated organic matter. The charge ratio (i.e., ball-to-powder ratio) and the rotation speed are considered fundamental parameters in dehalogenation reactions. These control the specific energy density within the matrix. Essential information on the most effective operating parameters of mechanochemical treatment of the chlorinated, brominated, and fluorinated compounds described in the reviewed papers is reported in

Table 3. Summary of experimental conditions utilized in reviewed papers that achieved best dehalogenation/degradation results.

Contaminant	Ball Milling Conditions	Chemical Precursor	Reagent Ratio a	Charge Ratio b	Destruction Percentage (%)	Ref.
Chlorinated pollutants
2,4-dichlorophenol (2,4-DCP)	Vibrational milling, sample of 14 g, 20.5 Hz,	zero-valent iron (ZVI)	ZVI-PDS = 13:1	30	86.8%	[122]
2,4-dichlorophenol (2,4-DCP)	Vibrational milling, sample of 14 g, 20.5 Hz,	CaO, peroxydisulfate (PDS)	CaO-PDS:2,4-DCP = 13:1	30	99.8	[123]
Hexachloroethane (HCE)	Planetary ball mill, sample of 7.25 g, 600 rpm, 2 h	Fe/Fe3O4	Fe/Fe3O4:HCE = 13.5:1	20	98.7	[124]
Hexachlorobenzene (HCB)	Planetary ball mill, sample of 7.25 g, 600 rpm, 4 h	Fe/Fe3O4	Fe/Fe3O4:HCB = 13.5:1	20	98.3	[124]
1,1,1-trichoro-2,2-bis(p-chlorophenyl)-ethane, (D DT)	Planetary ball mill, sample of 10.2 g, 250 rpm, 4 h	Fe-Zn	9 (wt)	35	98	[125]
Polychlorinated biphenyls (PCBs)	Planetary ball mill, sample of 20 g, 400 rpm, 4 h	CaO	CaO:Soil = 1:1 (wt)	10	>99.9	[126]
Polychlorinated dibenzo-p-dioxins (PCDD)/Fs (fly ash) PCBs	Planetary ball mill, sample of 9.0 g, 275 rpm, 7 h	CaO + SiO2	CaO:SiO2:fly ash = 4:1:5 (wt)	90	84.8	[127]
Pentachlorophenol	Planetary ball mill, sample of 5.0 g, 300 rpm, 5 h	CaO + SiO2	PCP:CaO:SiO2 (mol) = 1:60:60	40 c	58.4	[128]
Hexachlorobenzene	Planetary ball mill, sample of 9.0 g, 275 rpm, 8 h	Fe + SiO2	15 (wt)	36	99.9	[129]
Dechlorane plus	Planetary ball mill, sample of 9.0 g, 275 rpm, 2 h	Al + SiO2	11 (wt)	30	99.9	[130]
Mirex	Planetary ball mill, sample of 9.0 g, 550 rpm, 2 h	Fe + SiO2	24 (wt)	36	100	[131]
Polychlorinated naphthalene	Planetary ball mill, sample of 38 g, 700 rpm, 1 h	CaO	57 c	25 c	99.9	[132]
Pentachlorophenol	Planetary ball mill, sample of 38 g, 700 rpm, 1 h	MnO2 (birnessite)	20 (wt)	6.6 c	>99.0	[133]
Pentachloronitrobenzene	Planetary ball mill, sample of 5.0 g, 275 rpm, 4 h	Fe	24 (wt)	36	94	[134]
Pentachloronitrobenzene	Planetary ball mill, sample of 5.0 g, 275 rpm, 4 h	Fe + SiO2	24 (wt)	36	99.9	[134]
Pentachloronitrobenzene	Planetary ball mill, sample of 5.0 g, 275 rpm, 4 h	Fe + Ni	24 (wt)	36	99.9	[134]
Pentachloronitrobenzene	Planetary ball mill, sample of 5.0 g, 275 rpm, 3 h	Fe + Ni + SiO2	24 (wt)	36	99.9	[134]
1,3,4,5,6-pentachlorocyclohexene (γ-PCCH)	Planetary ball mill, sample of 38.0 g, 700 rpm, 2 h	CaO	60 (mol)	98	100	[135]
2,4,6-trichlorophenol	Planetary ball mill, sample of (not reported), 400 rpm, 6 h	CaO + SiO2	5.329 (wt)	22.24 c	99.0	[136]
Pentachlorophenol	Planetary ball mill, sample of 38 g, 700 rpm, 1 h	MnO2 (birnessite)	20 (wt)	22 c	>99.0	[137]
Pentachlorophenol (soil)	Planetary ball mill, sample of 38 g, 700 rpm, 1 h	MnO2 (birnessite)	40 (wt)	7 c	75.0	[137]
Pentachloronitrobenzene	Planetary ball mill, sample of 5.0 g, 275 rpm, 4 h	Nano-Fe	15 (wt)	36	100	[138]
PCDD/Fs	Planetary ball mill, sample of 10 g, 400 rpm, 20 h	CaO	100 (wt)	4 c	44.2	[139]
PCDD/Fs	Planetary ball mill, sample of 10 g, 400 rpm, 20 h	Ca + CaO	100 (wt)	4 c	82.7	[139]
PCDD/Fs	Planetary ball mill, sample of 10 g, 400 rpm, 20 h	Ca + CaO	100 (wt)	4 c	100	[139]
Dechlorane plus	Planetary ball mill, sample of 5.0 g, 550 rpm, 4 h	CaO	25 (wt)	36	100	[140]
PCDD/Fs (fly ash)	Planetary ball mill, sample of 13 g, 400 rpm, 8 h	Eggshells	-	60 c	>50	[141]
PCDD/Fs (fly ash)	Planetary ball mill, sample of 13 g, 400 rpm, 2 h	CaO	Ca:PCDD/F = 1.49 x 107	-	76.8 (PCDD)	[142]
PCDD/Fs (fly ash)	Planetary ball mill, sample of 13 g, 400 rpm, 2 h	CaO	Ca:PCDD/F = 1.49 x 107	-	56.8 (PCDF)	[142]
Pentachlorophenol	Planetary ball mill, sample of 13 g, 400 rpm, 1 h	CaO + SiO2	Ca:Cl (mol) = 4	9 c	98.4	[143]
PCBs	Planetary ball mill, sample of (not reported), 500 rpm, 3 h	Zn + KOH + PEG2000	Glycol:KOH = 1.33 Glycol:PCBs oil = 0.6 Zn = 1.53 mol/kg oil	-	100	[144]
PCBs (sediment)	Planetary ball mill, sample of 16 g, 480 rpm, 30 min	NaBH4	H:Cl = 14 (mol)	10	<98	[145]
PCDD/Fs (fly ash)	Planetary ball mill, sample of 10 g, 350 rpm, 2 h	CaO	0.6 (wt)	-	>60	[146]
3-chlorobiphenyl	Planetary ball mill, sample of 38 g, 700 rpm, 6 h	La2O3	0.05 (wt)	14 c	100	[147]
4-chlorobenzene	Planetary ball mill, sample of 38 g, 700 rpm, 2 h	CaO	20 (wt)	-	100	[148]
Octachlorodibenzo-p-dioxin	Planetary ball mill, sample of 38 g, 700 rpm, 2 h	CaO	200 (wt)	-	100	[148]
Octachlorodibenzofuran	Planetary ball mill, sample of 38 g, 700 rpm, 2 h	CaO	200 (wt)	-	100	[148]
PCBs (sand), dichlorobenzene, pentachlorophenol (sand)	Planetary ball mill, sample of 22 g, 530 rpm, 5 h	Mg	0.04 (wt)	9 c	100	[149]
Atrazine (soil)	Attritor (ring mill), sample (not reported), 750–1000 rpm, 2 h	LiAlH4	0.05 (wt)	36	100	[150]
PCBs (soil)	Attritor (ring mill), sample (not reported), 750–1000 rpm, 2 h	LiAlH4	0.05 (wt)	36	99.9	[150]
PCBs (soil)	Attritor (ring mill), sample (not reported), 750–1000 rpm, 18 h	LiAlH4	0.05 (wt)	36	99.9	[151]
1,2,3-trichlorobenzene	Planetary ball mill, sample of 38 g, 700 rpm, 6 h	CaO	3.71 (wt)	35 c	>99.9	[152]
3-chlorobiphenyl	Planetary ball mill, sample of 38 g, 700 rpm, 6 h	CaO	2.33 (wt)	-	>99.0	[153]
3-chlorobiphenyl	Planetary ball mill, sample of 38 g, 700 rpm, 6 h	CaO + SiO2	0.05 (wt)	42 c’	99.5	[154]
Brominated pollutants
Decabromodiphenyl ether	Planetary ball mill, sample of 38.0 g, 700 rpm, 1 h	CaO	20 (wt)	23 c	99.0	[155]
Hexabromobenzene	Planetary ball mill, sample of 38.0 g, 700 rpm, 3 h	CaO	Ca: Br (mol) = 2	35 c	100	[156]
Hexabromocyclododecane	Planetary ball mill, sample of 9.0 g, 275 rpm, 1.5 h	CaO	11 (wt)	27	98.0	[157]
Hexabromocyclododecane	Planetary ball mill, sample of 9.0 g, 275 rpm, 1.5 h	Fe + Quartz	11 (wt)	27	<98.0	[157]
Hexabromocyclododecane	Planetary ball mill, sample of 9.0 g, 275 rpm, 3 h	CaO	11 (wt)	27	98.0	[157]
Hexabromocyclododecane	Planetary ball mill, sample of 9.0 g, 275 rpm, 2 h	Fe + Quartz	11 (wt)	27	100	[157]
Decabromodiphenyl ether	Planetary ball mill, sample of 38.0 g, 700 rpm, 1 h	Bi2O3	2.43 (wt)	27	100	[158]
Decabromodiphenyl ether	Planetary ball mill, sample of 38.0 g, 700 rpm, 2 h	CaO	2.43 (wt)	27	85	[158]
Tetrabromobisphenol A	Planetary ball mill, sample of 6.0 g, 550 rpm, 4 h	Fe + SiO2	11 (wt)	30	99.6	[159]
Tetrabromobisphenol A	Planetary ball mill, sample of 6.0 g, 550 rpm, 2 h	CaO+ Na2S2O8	Na2S2O8:CaO:TBBPA = 13:52:1 (mol)	30	100	[160]
Fluorinated pollutants
Perfluorooctane sulfonate, Perfluorooctanoic acid	Planetary ball mill, sample of 380 g, 700 rpm, 6 h	CaO	4 (wt)	20 c	99.0	[161]
Perfluorooctane sulfonate, Perfluorooctanoic acid	Planetary ball mill, sample of 380 g, 700 rpm, 18 h	CaO	4 (wt)	20 c	98.4	[161]
Perfluorooctane sulfonate, Perfluorooctanoic acid	Planetary ball mill, sample of 9.0 g, 275 rpm, 6 h	KOH	23 (wt) c	19 c	99.8	[162]
Chlorinated polyfuorinated ether sulfonate (F-53B)	Planetary ball mill, sample of 9.0 g, 275 rpm, 8 h	Na2S2O8 +NaOH	Na2S2O8:NaOH:F-53B = 83:40:1 (wt) c	10 c	88.0	[163]

^a Weight (wt) or molar (mol) ratio expressed as reagent (or total reagents mixture) amount over contaminant amount. ^b Milling bodies (balls) to reactant mixture (powder) ratio. ^c Value inferred from data available on the paper.

.

3.5. Crustaceans

As large amounts of residual shells still remain underexploited, the concept of shell biorefinery is developing rapidly and has attracted increasing attention in recent years. Fractionating crustacean shells into their main components and processing each component into value-added chemicals and materials can bring ecological and economic benefits [164].

Chitosan (CS), a biopolysaccharide composed of glucosamine units linked together by β-1,4 linkages, is generally produced by deacetylation of chitin, which is mainly extracted from crustacean shells, insect cuticles, or other microorganisms such as bacteria, fungi, and others. In particular, low molecular weight chitosan (LMWC) between 2 and 10 kDa exhibits enhanced solubility that can be dissolved in neutral water for direct use under physiological conditions. Furthermore, LMWC is specifically suited for DNA delivery, showing more enhancement in biological, antibacterial, antitumor, and immunostimulatory activities than chitin and high molecular weight chitosan (HMWC), making it attractive in biomedical and pharmaceutical applications. LMWC is also superior compared to HMWC and chitin in materials synthesis due to remarkably improved reactivity in grafting, crosslinking, and other types of modifications [165,166]. Both chitin and chitosan are biocompatible, biodegradable, non-toxic, and quite valuable due to their lower mineral content. However, the deacetylation of chitin into chitosan is still an expensive, difficult process and is usually accompanied by depolymerization, providing low molecular weight chitosan [167].

The first report of a chitinolytic system using combinations of mechanochemical grinding and direct enzymatic degradation of crustacean shells was by Nakagawa and colleagues [168]. This study developed an intensive “convergence” ball mill for rapid mechanochemical conversion of chitin and crustacean shells (crab or shrimp) into amorphous microparticles sensitive to chitinase (Figure 14). The mechanochemical pre-treatment allowed a complete degradation of crab shells into N-acetylglucosamine (GlcNAc).

Chen et al. [169] transformed shrimp shells into LMWC in the presence of NaOH by single-step mechanochemistry. The results showed that raw shrimp shells were successfully transformed into LMWC (7.9 kDa) with a high purity of 90% under NaOH-catalyzed grinding conditions (8 cycles, 700 rpm). The FTIR spectra of the product demonstrate the formation of chitosan-like compounds, as well as the weak bands at 1536 cm⁻¹ and 1395 cm⁻¹ which suggest the presence of residual CaCO₃ and proteins. The content of protein residues in the final product was analyzed using the Bradford method, while analysis by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) revealed a CaCO₃ content of 6.7% in weight. This new simple, solvent-free approach avoids the generation of wastewater and the use of oxidants, acids, or other environmentally harmful reagents. Furthermore, the work not only emphasizes that base usage has been drastically reduced to 1/10 compared to traditional methods but also inhibits side reactions under ball milling conditions.

Di Nardo and colleagues [170] reported a new method, to produce high molecular weight chitosan with minimal energy and solvent consumption, based on the combination of mechanochemistry and aging. The results showed that the method was versatile and applicable to a range of chitin sources, including crude crustaceans and insect shells, allowing a deacetylation degree of up to 98% and remarkably high molecular weights. According to reports, this process provides chitosan in a safer way and with lower consumption of materials and energy compared to the classic hydrothermal process. It minimizes material dependency (achieving excellent results with NaOH: chitin ratios of 4 or 5), eliminates the need for solvents during the reaction, and significantly lowers energy consumption, particularly in pure aging experiments. This article describes an innovative methodology for producing chitosan-based materials resistant to dissolution in water. This opens up the possibility of using them in new areas, such as environmental remediation or food packaging production. Because this methodology is remarkably simple to implement, requiring minimal technical equipment, it is highly adaptable to the challenge of bark biorefinery. Overall, this approach offers solutions for the local production of high-value-added products, especially in locations close to fishing areas and insect farms.

Su et al. [171] also developed a contemporary method for reusing shrimp shell waste, transforming it into new value-added materials. This process, called mechanochemistry, is considered gentle and waste-free. One of the main advantages of this method is that it eliminates the use of harsh chemicals such as strong acids and bases, as well as oxidants. It also does not require extreme temperature or pressure conditions and consumes less energy than traditional methods (

Table 4. The types, quantities, and energy consumption of chemical reagents required to treat each ton of shrimp shells [171].

Process	Agent	Deproteinization	Demineralization	Decoloration
Traditional solution method	Chemicals	NaOH: 500 kg	HCl: 540 kg	H2O2: 296 kg
	Electricity consumption	1.02 × 106 kW h	1.53 × 106 kW h	1.02 × 106 kW h
	Water	19.5 × 103 L;	14.4 × 103 L	12.7 × 103 L
	Component loss	Loss of protein	Loss of minerals	Loss of astaxanthin
	Products	_	–	Chitin: 184 kg
Mechanochemical solution method	Electricity consumption	1.02 × 106 kW h	1.53 × 106 kW h	1.02 × 106 kW h
	Water	19.5 × 103 L;	14.4 × 103 L	12.7 × 103 L
	Component loss	Loss of protein	Loss of minerals	Loss of astaxanthin
	Products	_	–	Chitin: 184 kg
	Electricity consumption	1.02 × 106 kW h	1.53 × 106 kW h	1.02 × 106 kW h

). According to the authors, the aim of the study is to contribute to the circular economy by promoting the sustainable use of crustacean waste. With this approach, they aim to achieve United Nations (UN) Sustainable Development Goals (SDG) 3, 6, 12, and 14 [172] proposing a method of economical and sustainable reuse of crustacean waste.

Hajiali et al. [33] described a sustainable approach based on mechanochemistry and aging for extracting chitin directly from crab shells using a commercial blender (Figure 15). According to the results, different solid acids (i.e., citric, ascorbic, malic, salicylic, and succinic acids) promoted chitin extraction with yields reaching 27% and low ash contents (0.9%). Using citric acid, chitin from 200 g of crab shells could be extracted using a commercial blender, achieving low values of 190 and 18 kJ/g for process mass intensity (PMI) and energy consumption, respectively. Although 30 min of grinding can provide high-quality materials, yields could be increased further by decreasing the grinding time to 10 min. These results showed viability when compared with other traditional chitin extraction methods and alternative strategies, which is summarized in Figure 16.

Recently, Fatika et al. [173] successfully synthesized chitosan Schiff bases (ChSB) from shrimp shells through a consecutive process involving demineralization, deproteinization, and deacetylation at room temperature, followed by solvent-free mechanochemical grafting using an environmentally friendly method. The resulting ChSB was found to be more amorphous and exhibited increased thermostability compared to chitosan. Notably, solvent-free mechanochemical grafting enabled ChSB synthesis in just 10 min, significantly reducing the processing time compared to conventional methods (>3 h). The authors highlighted the need for further studies to explore this approach for generating ChSB from other raw materials and to investigate its biological activity.

3.6. Synthesis of Materials

Eggs, a widely consumed natural food source, provide essential amino acids, vitamins, and minerals. As a result, chicken eggshells are a common byproduct of food waste. These residues are fully biodegradable, recyclable, and biocompatible [174]. The primary chemical component of eggshells is calcium carbonate (CaCO₃) in the form of calcite, comprising approximately 94–97% of their composition. Other constituents include calcium phosphate (Ca₃(PO₄)₂) at 1%, magnesium carbonate (MgCO₃) at 1%, and organic matter at 4% [4]. Studies highlight the significance of CaCO₃ as the main component of natural waste materials such as oyster and eggshells, both of which have been successfully employed in mechanochemical dechlorination processes [175,176,177]. This review provides an updated overview of recent publications utilizing a mechanochemical approach for eggshell waste treatment, summarizing key applications and findings.

In line with efforts to develop new green chemistry methods, Mosaddegh and Hassankhani [174] explored a simple and rapid ball mill-assisted preparation of nano-CaO (calcium oxide) based on eggshell waste. Additionally, they developed a green, fast, and highly efficient protocol for the one-pot synthesis (Scheme 4) of 2-amino-7-methyl-5-oxo-4-phenyl-4,5-dihydropyran[4,3-b]pyran-3-carbonitrile derivatives (6). The reaction involved a carefully stirred mixture of 4-hydroxy-6-methyl-2H-pyran-2-one (3), substituted benzaldehydes (4), and malononitrile (5) at 120 °C under solvent-free conditions, using a nano-CaO catalyst. This method achieved excellent yields (93–98%) in a remarkably short reaction time. The nano-CaO was still reused several times without loss of catalytic activity. According to the authors, this study represents the first report on the use of eggshell waste as a natural precursor of nanometer-sized CaO catalyst in organic synthesis, offering a sustainable approach to reducing environmental impact.

Focusing on the production of bioceramics, the report Ferro and Guedes [178] is of particular interest, as they managed to prepare hydroxyapatite (HA) using ball milling, without the need for subsequent sintering. Eggshells or cuttlefish bones have been used as sources of calcium (containing Ca in the form of calcite or aragonite, respectively), maintaining a constant Ca/P ratio of 1.67, in the presence of 6.4% by weight of water. It was found that the reaction progress for the synthesis of HA was a function of energy input and was different for each precursor, for the eggshell it was 52.5 kWh/g, while only 6.2 kWh/g was required from the cuttlefish bone. However, the final product was formed more easily when using an aragonite-containing precursor. In all cases, crystalline HA nanoparticles were produced. From a general point of view, the results obtained in this study demonstrate the potential of chicken eggshell and cuttlebone as natural precursors and open a window of opportunity for the development of a reliable, scalable, fast, and economical HA production method. The use of cuttlefish bone as a source of calcium for the mechanochemical production of HA has been previously reported [179], where however, the cuttlefish bone was calcined in CaO before grinding.

On the other hand, Cestari et al. [180] studied the use of three different biogenic materials, such as eggshell (calcite), cuttlefish bone (aragonite), and mussel shell (aragonite/calcite) as sources of calcium to produce HA through grinding in an aqueous medium ((NH₄)₂HPO₄ or H₃PO₄) at relatively low temperature (120–150 °C). Drying temperature was shown to have a fundamental influence on process efficiency, while grinding time appeared to have a negligible impact on HA formation. Furthermore, they confirmed that aragonite from cuttlefish bones was transformed into HA more easily compared to eggshell calcite. They also found that the synthesis was favored by acidic aqueous media compared to basic media.

Another strategy was developed by Shen et al. [181] to prepare hierarchically porous carbons derived from biomass using mechanical grinding with the aid of CaCO₃ or eggshell. This work used dairy manure residues and eggshells as carbon precursors (Figure 17). They were pretreated using a ball mill at 350 rpm for 30 min. Subsequently, the mixtures were pyrolyzed at a high temperature (>800 °C) and part of the CaCO₃ was decomposed into CO₂ (carbon dioxide), which acted as an activating agent to produce micropores in the carbon. The subsequent removal of residual CaCO₃ and newly formed CaO by the HCl solution led to the formation of mesopores. The findings were corroborated by nitrogen physisorption analysis. The produced materials applied as supercapacitor electrodes showed a relatively high specific capacitance of 226.6 F/g in KOH aqueous electrolyte (6 mol/L). Furthermore, it exhibited excellent cycling stability with nearly 100% specific capacitance retention after 2500 galvanostatic charge–discharge (GCD) cycles.

Seeharaj et al. [182] have manufactured a superhydrophobic coating from eggshell biowaste. The work reports that the coating was prepared by grinding chicken eggshells, followed by surface hydrophobization with stearic acid. The modified eggshell particles were then dispersed in a polystyrene (PS) binder and dip-coated onto a substrate. The resulting material exhibited water contact angle (WCA) values above 150° on cotton fabric substrates when a 4:1 weight ratio of modified eggshell to PS was used, classifying it as superhydrophobic. The material showed good environmental stability, self-cleaning, and oil/water separation properties (Figure 18). These results suggest that biological waste from eggshells can be used for superhydrophobic applications.

Previously, Tongamp and collaborators [175] had already reported a new process for dechlorinating polyvinyl chloride (PVC) using oyster shell waste. The study reports that with a 1:4 molar ratio of PVC and oyster shell mixture, the complete chlorine extraction rate can be achieved in 2 h of grinding at 700 rpm, while only 65% and 25% of chlorine are extracted to 600 rpm and 500 rpm, respectively. The present process offers a potential route for the handling and disposal of oyster shells and PVC waste.

A semi-industrial approach for simultaneous treatment of eggshell and industrial polyvinyl chloride waste using ball milling tools is reported by Baláž et al. [183]. In this study, the laboratory-scale experiment proved to be more efficient, as complete dechlorination was achieved after 4 h of grinding, while only 56% was achieved after 12 h of treatment in a semi-industrial vibrating ball mill. The Ca:Cl molar ratio was a crucial factor in the reaction’s progression. Increasing it from 2.34 to 5.06 significantly improved the outcome, with the yields rising from 4% to 30% after 8 h of grinding. The rate constant obtained followed the zero-order kinetics with the rate constant 1.23 × 10⁻⁵ s⁻¹. The results of XRD and FTIR measurements showed a high probability of calcium chloride formation. This study also highlights the green, environmental and sustainable nature of mechanochemistry and shows the possibility of increasing the process to a semi-industrial scale using the eccentric vibrating mill.

Interesting studies showing the acid resistance properties of a new nanosized eggshell-titanium dioxide material (TiO₂-EB), which can potentially be applied in dentistry, were reported by Onwubu et al. [184,185] (see Figure 19). In one study, Onwubu et al. [185] collected bovine enamel and evaluated the resistance properties of TiO₂-EB in situ when exposed to HCl in the presence of various toothpaste solutions, as well as pure eggshell and an eggshell-TiO₂ composite. The acid-resistant properties could be clearly traced from the SEM images of the enamels obtained after the action of the corresponding solutions (Figure 20). The image observed in Figure 20G visibly confirmed that the titanium dioxide coating on the surface of TiO₂-EB effectively improves its resistance in an acidic environment.

Calcium titanate (CaTiO₃) was successfully synthesized by combustion synthesis using duck eggshell as a source of calcium, mixed with anatase titanium dioxide (A-TiO₂) and using magnesium (Mg) as fuel [186]. The eggshell and A-TiO₂ were ground for 30 min in a high-energy planetary mill for 30 min at 400 rpm. These powders were then mixed separately with Mg in a ball mill for 120 min. According to XRD patterns, the calcium titanate ceramic sintered at 1350 °C for 180 min presented a single CaTiO₃ perovskite phase. Furthermore, the ceramic product exhibited a maximum density of 3.65 g/cm³ and a minimum porosity of 0.54%. The same manufactured product also exhibited the highest dielectric constant (~78) with the lowest dielectric loss (~0.02), which resulted from the simplified charge polarization process.

A biochar composite (CE) was successfully prepared by Liu et al. [187] using eggshell waste as a source of Ca, and rice straw as a source of C through ball milling and pyrolysis steps. Eggshell was co-milled with rice straw and subsequently heated at 800 °C in order to obtain effective adsorbent of phosphate ions. The result of adsorption studies says that CaO-biochar (EC) composite can efficiently remove (231 mg/g) phosphate from aqueous solution (evidenced for eggshell: rice straw 1:1 ratio). The adsorption was very well described by the pseudo-second order (R² > 0.975) and Langmuir (R² > 0.979) models. Thermodynamic analysis revealed that the adsorption process was spontaneous (ΔG 0 < 0) and endothermic (ΔH 0 > 0). The FTIR, XRD, SEM and EDS analyzes justified the adsorption of phosphate on the adsorbent (Figure 21A,B and Figure 22A–C). The elevated temperature improved the outcome, which, together with other results in the paper confirmed the chemical character of adsorption. The article also highlights that the prepared CaO-biochar composite is an efficient adsorbent for removing/recovering phosphate from aqueous solution and has potential application as a multifunctional fertilizer after phosphate adsorption.

In another study by Sari et al. [188], calcined eggshells were used to produce biogas from palm oil factory effluents and cow dung mixture. The study investigated the effect of particle size reduction achieved by ball milling. According to the results, the increasing amount of nano-Ca led to improved performance; however, the excessive amount of calcium had a detrimental effect on biogas production, as when the Ca concentration is 10 g/L, the result is even worse than in the case of the control experiment. The authors report that further investigation is needed to improve the methane produced during anaerobic digestion.

Egg waste is valuable not only for its calcium-rich inorganic shell but also for its organic inner components, which have been utilized in the mechanochemical preparation of catalytic materials and electrodes. Below are two examples reported in the literature that demonstrate these applications. In a promising contribution on mechanochemistry, Rodríguez-Padrón et al. [189] addressed a facile and environmentally friendly synthesis of a titanium-based nanocomposite (PT-TiO₂) using expired egg whites through a model methodology for catalytic and energy storage applications (Figure 23A). The article reports that the catalytic performance of PT-TiO₂ sample was evaluated in the microwave-assisted oxidation of diphenyl sulfide (7) to the corresponding sulfoxide (8a) and sulfone (8b) (Scheme 5). The study also showed that the catalytic system remained stable, showing a conversion of 86% after the fifth use (Figure 23D). A comparative study between the prepared material and a commercial one (titania P25), showed that both materials led to comparable results in terms of conversion; however, a critical change in selectivity was observed (Figure 23B,C). On the other hand, the PT-TiO₂ nanocomposite was successfully proven as an anode material for lithium-ion batteries, providing a reversible capacity of 107 mA h/g at 0.1C with an excellent Coulombic efficiency of 100% from the second cycle onwards (Figure 24A–D). Furthermore, the synthesized material showed significant capacity retention values of 76% between the 2nd cycle and the 100th cycle. PT-TiO₂ turned out to be a versatile material with potential catalytic and energy storage applications.

Karuppiah et al. [190], prepared a carbon composite lithium iron orthosilicate (LFS@ESM) using a polyol-assisted ball milling method to enhance its poor electronic conductivity, with carbon derived from eggshell membrane. In this study, complete carbonization of the eggshell membrane was achieved through post-grinding calcination in an Ar atmosphere, and the successful iron coating was confirmed by TEM (Transmission Electron Microscopy), which revealed the porous nature of the carbon and the atomic arrangement. The LFS/C composite was characterized as a monolayer, with an average surface area of 24 m²/g, and exhibited excellent electrical properties, including a high Coulombic efficiency of 98.5%, which remained stable for up to 50 cycles. The key findings of the study are summarized in Figure 25.

Biomass, with an estimated carbon content of 45–50% by weight, is considered a promising and sustainable precursor to prepare advanced carbonaceous materials. Such carbon materials have been widely used in many fields, such as catalysts, adsorbents, and energy storage [181,191]. In general, ordered mesoporous carbons (OMCs) and hierarchically porous carbons (HPCs) are synthesized by hard or soft template methods using toxic regents such as HF (hydrofluoric acid), concentrated NaOH, or formaldehyde among others. These wet processing methods require a large amount of solvent and restrict the widespread application of these materials. The primary self-assembly process for the synthesis of these materials by gentle templating is also induced by the acidic or basic catalyst in the solvent [192]. To overcome these disadvantages, microwave grinding and ball milling are also employed during the synthesis process to enhance the functionality of carbonaceous materials and improve preparation efficiency. Some ball milling methods for preparing biomass-derived carbon materials will be discussed in detail in this section.

In a single-step, solvent-free mechanochemical approach, Zhang and co-workers [193] synthesized ordered mesoporous carbon catalysts from a renewable tannin precursor. The process allowed incorporating cobalt nanoparticles (Co NPs) at high concentrations (up to 21.5 wt%), confining them in an ordered pore structure with tunable diameters (4.3–9.4 nm) and high surface area (up to 700 m²/g) even at high temperatures (Figure 26A). Among the catalysts designated as Co@OMC, the nanocatalyst Co@P123_0.8, with 20.9 wt% Co NPs (4.9 nm) uniformly dispersed in 5.1 nm mesoporous channels, stood out with high catalytic efficiency in the deoxygenation of ketones to alkylbenzenes. This catalyst efficiently converted different functional groups (ketones, aldehydes, alcohols) into target molecules in high yields (Scheme 6). The stability of the catalyst was confirmed by the low cobalt leaching (less than 0.1% of total Co, as detected by ICP-AES) in the post-reaction solution. The ultrastability of Co@P123_0.8 was evidenced in reuse tests, in which the catalytic performance and textural properties, including the mesoporous structure and dispersion of cobalt nanoparticles, were consistently maintained after multiple reaction cycles (Figure 26B,C).

According to the study by Shan et al. [194] magnetic hybrid materials of biochar/Fe₃O₄ and AC/Fe₃O₄ were synthesized and evaluated for the removal of pharmaceuticals, specifically carbamazepine (CBZ) and tetracycline (TC), through adsorption and mechanochemical degradation. Both hybrid materials demonstrated fast adsorption kinetics, with most of the removal occurring within the first hour. Biochar/Fe₃O₄ exhibited a maximum adsorption capacity of 62.7 mg/g for CBZ and 94.2 mg/g for TC, while AC/Fe₃O₄ presented values of 135.1 mg/g for CBZ and 45.3 mg/g for TC. Degradation by ball milling was more effective for pharmaceuticals adsorbed on biochar/Fe₃O₄. After 3 h, TC was 99% degraded on biochar/Fe₃O₄ and 97% on AC/Fe₃O₄. For CBZ, degradation was less efficient on both materials. However, the addition of 300 mg SiO₂/g of adsorbent significantly improved degradation, reaching 98.4% for CBZ on biochar/Fe₃O₄ and 88.2% on AC/Fe₃O₄.

Pure and ball-milled wheat stalk biochars pyrolyzed at 300 °C, 450 °C, 600 °C were studied by Xiang et al. [195] for adsorption of TC hydrochloride from aqueous solution. The result of this study revealed that compared to original biochar, 600 °C pyrolyzed and ball-milled biochar (BM-biochar) adsorbs more TC in the order of 84.54 mg/g. The study also assessed that the adsorption performance of TC was significantly improved largely due to the strong positive correlation between adsorption and specific surface area (257.50 m²/g), total pore volume (0.2678 cm³/g) or mesoporous volume (0.1966 cm³/g). The adsorption results showed a better fit with pseudo second order kinetics. The Langmuir isotherm values show maximum fit for the adsorbent-adsorbate interaction. The paper adequately explains the influence of pH on the maximum adsorption of TC; however, less attention is given to aspects such as the surface charge of the adsorbent, thermodynamics, and the underlying adsorption mechanism, which remain unknown.

Sugarcane bagasse biomass ground into balls and pyrolyzed at 450 °C (BMBG450) was prepared by Lyu et al. [196] to be applied as an adsorbent of methylene blue (MB). Compared to unground bagasse biochar (BG450), BMBG450 showed greater specific surface area (increased from 51 to 331 m²/g), larger pore volume (increased from 0.008 to 0.099 cm³/g), smaller hydrodynamic radius (reduced from 0.5–1 mm to 170 nm), stronger negative zeta potential (about 1.6-fold increase) and more oxygen-containing functional groups (increase 1.05 mmol/g). The adsorption isotherm results indicated that the adsorption fit well with the Langmuir model and the BMBG450 composite showed a maximum removal capacity of 354 mg/g vs. 17.2 mg/g of original BG450. The work pointed out that π-π and electrostatic interactions were dominant mechanisms for MB sorption. The article satisfactorily explained the adsorption mechanism, the influence of pH on adsorption, and the pH_pzc of the prepared material and, therefore, justified the adsorption process (Figure 27).

Dual-functional MgO-biochar nanocomposites were successfully synthesized by Zheng et al. [197], by a novel and facile method that ground walnut biochar with MgO particles under a solvent-free condition. The study reported that MgO-biochar nanocomposites exhibited sorption performance of 62.9% removal of phosphate, and 87.5% removal of methylene blue at low adsorbent dosages of 1.0 g/L and 0.2 g/L, respectively.

Being the byproduct of biodiesel synthesis, glycerol valorization as reagent [198,199,200] and/or solvent [201,202] is a hot topic in scientific community. Searching for an alternative process for the production of calcium diglyceroxide (CaDG) was successfully synthesized by Lukić et al. [26], using mechanochemical treatment between glycerol (18) and CaO in a ratio of 1:5. This was used as a catalyst of the transesterification of sunflower oil to afford biodiesel (F.A.M.E in Scheme 7) under different working conditions: amount of catalyst, stirring speed, temperature and methanol/oil molar ratio. The kinetic model involved the physical effect of CaDG on the reduction in mass transfer resistance, which enhanced the transesterification rate. According to the report, it was found that the maximum (<100%) conversion of triglycerides (15) can be achieved with a molar ratio of methanol (16): sunflower oil of 10:1, 0.5 wt% catalyst, at 60 °C, after 2 h of reaction. However, the reusability of the catalyst and the possible mechanism were not evaluated in this study.

In subsequent studies, the synthesis of CaDG using a continuous mechanochemical reactor (MCR) was reported, using the same equipment used for biodiesel production [203,204]. Specifically, Glycerol and CaO were mixed in two different molar ratios, 3:1 and 5:1, stirred in a mixing tank before being pumped into the MCR (0.5 L volume) at a flow rate of 4 and 150 L/h, respectively. In this study, XRD data was used for both the refinement and quantification of CaDG crystal formation, both of which were carried out using the Rietveld method. According to the results of the Rietveld method, with glycerol: CaO molar ratio of 3:1, the temperature needed to be increased from 25 °C to 50 °C for optimal yield (91 and >99%, in that order). To achieve yields greater than 99% at room temperature (25 °C), it was necessary to increase the molar ratio of glycerol to CaO to 5. The researchers also evaluated the influence of the presence of water due to the hydrophilic nature of glycerol. In the presence of 10 wt% water and glycerol: CaO molar ratio of 5:1, CaDG was obtained in excellent quantitative yield (>99%) with a residence time of 30 min. The same research group investigated the production of biodiesel from both sunflower oil and cooking oil via transesterification using 1.5 wt% CaDG as the catalyst and employing methanol to oil molar ratio close to stoichiometry (4:1). With a continuous inlet flow between 4 and 45 L/h, the same semi-continuous mechanochemical reactor (volume of 0.5 L) containing zirconia spheres doped with yttrium (diameter of 0.3 to 2.0 mm) was used, occupying a volume of 55% to 70% (Figure 28). The subsequent stirring reaction mixture for 4 h at 50 °C or 24 h at room temperature completed the reaction and allowed the production of biodiesel with a yield greater than 90%. According to the authors, this new biodiesel production process proved to be efficient and scalable and can be applied to the conversion of used cooking oils without any significant loss of yield. Furthermore, this new approach proved to be more economical than the conventional batch stirring process. The results showed that the methanol-oil molar ratio can be reduced from 12:1 to 4:1, and the catalyst weight can be reduced from 4% to 1.5% in a standard laboratory-scale reactor, using a more economical process compared to the conventional batch stirring method.

To develop a clean production process, Li et al. [205] explored the reaction of CaC₂ (calcium carbide) and glycerol (18) under varied conditions. The result of this study revealed that CaC₂ can react with glycerol at room temperature in a stirred mill with a rotation speed of 450 rpm, with 96% conversion of glycerol after 1 h of grinding. As a result, the desired acetylene (21) was obtained in good yield, with 96% glycerol conversion (Scheme 8). Importantly, when compared to conventional acetylene production methods, the authors did not detect any PH₃, AsH₃, and H₂S formed as side reactions involving Ca₃P₂, Ca₃As₂, and CaS, respectively. It was evident from this study that the mechanochemical reaction performance of CaC₂ with glycerol is more efficient than the corresponding thermochemical reaction at high temperatures, and CaC₂ shows much greater reactivity than CaO with glycerol.

In summary, the mechanochemical-assisted method is an efficient way to improve product yield and reduce biomass conversion reaction time compared to the solution-based catalytic method. Literature data on some typical examples of improving the yield of biomass-based products through mechanochemical-assisted methods are listed in

Table 5. Composites synthesized by mechanical grinding from different wastes as precursors.

Precursor	Chemical Precursor	Ball Milling Conditions	Composites	Application	Activity	Reuse Cycles	Activity	Ref.
Humins	FeCl2 or Fe (NO3)3	350 rpm, 45 min	Catalyst, H-TD-W	Oxidation of isoeugenol to vanillin	Conversion >87%	Four	Conversions >87%	[206]
Mimosa tannin	Co(OAc)2	30 min	Catalyst, Co@P1230.8	Hydrogenation of acetophenone to alkylbenzene	Conversion (>99%) and Selectivity (94%)	Ten	No significant loss in catalytic performance	[193]
Avian eggshell	NCM811	600 rpm for approximately 6 h	Electrode, CaO-NCM811	Energy storage	Discharge capacity (177 mA h/g)	Fifty	Maintained a retention capacity of 92.6%	[207]
Chicken eggshells	-	Ground in a planetary ball mill for 2 h and then calcined at 900 °C for 1 h	Catalyst, nano-CaO	Suzuki reaction pyrano[4,3-b]pyrans	yields (93–98%)	Seven	no loss of catalytic activity	[174]
Polysaccharide	Ammonium niobate (V) oxalate	350 rpm, 30 min	Catalyst, S13-Nb2O5	Oxidation of isoeugenol to vanillin	Conversion (70%) and Selectivity (65%)	Not reported	Not reported	[23]
Rice straw and Eggshell	-	30 min	Adsorbent, CaO-biochar (E-C 1:1)	Adsorption of phosphate	231 mg/g	Not reported	Not reported	[187]
Lignin	HAuCl4, PdCl2RuCl3, Re(CO)5Br	29.5 Hz, 90 min	MNPs where M = Au, Pd, Ru, Re.	Not reported	Not reported	Not reported	Not reported	[73]
Glycerol	CaO	550 rpm for 5 h	Catalyst, CaDG	Methanolysis of sunflower oil	Conversion ~100%	Not reported	Not reported	[26]
Coconut, pinenut and walnut shells	Fe3O4	550 rpm for 6 h	Adsorbent, biochar/Fe3O4	Carbamazepine (CBZ) removal	62.7 mg/g	Not reported	Not reported	[194]
Coconut, pinenut and walnut shells	Fe3O4	550 rpm for 6 h	Adsorbent, biochar/Fe3O4	Tetracycline (TC) removal	94.2 mg/g	Not reported	Not reported	[194]
Wheat stalk	-	300 rpm for 12 h	Adsorbent, BM-biochar	TC removal	84.54 mg/g	Not reported	Not reported	[195]
Glycerol	CaC2	450 rpm, 1 h	Catalyst, Ca-DG	Synthesis of acetylene	96% (Conversion)	Not reported	Not reported	[205]
Glycerol	CaO	Mixture was pumped into the MCR (V = 0.5 L) at a flow rate of 4 and 150 L/h	Catalyst, CaDG	Transesterification of vegetable oil	>90% (Yield)	Not reported	Not reported	[204]
Sugarcane bagasse	-	300 rpm for 12 h	Adsorbent, BMBG450	MB removal	354 mg/g	Not reported	Not reported	[196]
Pine wood	-	575 rpm for 16 h	Adsorbent, nanobiochar	CBZ of removal	95%	Not reported	Not reported	[208]
Bagasse	NH4OH	300 rpm for 12 h	Adsorbent, BMBG600-N	Sorption of CO2	48.2 mg/g	Not reported	Not reported	[209]
Bagasse	NH4OH	300 rpm for 12 h	Adsorbent, BMBG600-N	Sorption of reactive red	27.4 mg/g	Not reported	Not reported	[209]
Hickory	MgO	500 rpm for 12 h	Adsorbent, BMMg50	Removal of phosphate	62.9%	Not reported	Not reported	[197]
Hickory	MgO	500 rpm for 12 h	Adsorbent, BMMg50	Removal of MB	87.5%	Not reported	Not reported	[197]
Hickory chips	NH4OH	300 rpm for 12 h	Adsorbent, BMHC600-N	Sorption of CO2	52.5 mg/g	Not reported	Not reported	[209]
Hickory chips	NH4OH	300 rpm for 12 h	Adsorbent, BMHC600-N	Sorption of reactive red	37.4 mg/g	Not reported	Not reported	[209]
Lignin	AgNO3	30 Hz for 90 min	Antibacterial silver nanomaterials, AgNP@TM-PLig/PAM.	Antimicrobial filters	Killing 99.99% of bacteria	Five	Bacterial activity completely eradicated	[210]
Urea and h-BN	Cu(NO3)2·3H2O	800 rpm for 8 h	Catalyst, CuO/BCN	Depolymerization of lignins	10 wt% (Yield of monomers) 70 wt% (Yields of bio-oils)	Three	high yields of bio-oil were maintained	[211]
Glycerol	K2CO3	350 rpm for 2 h	K2CO3	Synthesis of benzoxazine	35% (Yield)	Not reported	Not reported	[212]

.

3.7. Recovery of Used Battery Metals

The lithium-ion battery (LIB) industry has developed rapidly in recent years, accounting for approximately 85% of the rechargeable battery market. And the outlook is that by 2030, the world is expected to have more than 200 million electric vehicles and a battery demand of more than 3 TWh by 2030. The production of LIBs consumes a significant amount of metal resources, especially lithium and cobalt, which will potentially trigger a strategic metals crisis. In addition, compared with other batteries, LIBs do not contain toxic heavy metals such as lead and mercury, but they may cause certain harm to the environment and human health in their production, consumption and disposal. And the development of a recycling process for used lithium-ion batteries is necessary and urgent from the perspective of environmental protection and circular economy of resources [213,214].

Lithium carbonate is an inorganic salt that is used in a variety of applications such as catalysts, ceramics and pharmaceuticals. Recovery of lithium as lithium carbonate salt by precipitation is a common recovery method performed after leaching and LIB recovery processes. The predominant LIB recovery technologies are mainly based on hydrometallurgical approaches and pyrometallurgical methods [215]. Initially, several pretreatment steps are required for both methods, which include classification, discharge, disassembly, crushing, screening and heat treatment. Both techniques require the synergistic use of solvents and heat to drive reactions along the path of selective leaching of high-value metals and enrichment of impurity elements, which can bring environmental problems due to the emission of toxic gases and effluents [216]. As both methods have many disadvantages, an alternative needs to be found to make LIB recycling more environmentally friendly and at the same time profitable for the industry [217]. To promote an even more efficient leaching process, enhanced leaching technologies such as ultrasonication [218], microwave [219] and mechanochemical activation (MCA) were explored.

Saeki et al. [220] and Zhang et al. [221] developed a process for recovering Co and Li metals from waste lithium-ion secondary batteries (LIS) and LiCoO₂ powder by co-grinding with polyvinyl chloride (PVC) using the mechanochemical activation method. Meanwhile, more than 90% of Co and nearly 100% of Li were extracted from the used LIS by a mechanochemical method after 30 h of grinding between LiCoO₂ and polyvinyl chloride (PVC) to form chlorides that are soluble in water. Consequently, about 90% of the chlorine in the PVC sample was transformed into inorganic chlorides at the time. Wang et al. [222] also achieved conversion rates of 100% to 91.9% to Co, respectively, with 12 h of grinding. During the mechanochemical reaction, PVC played two important roles: one as a chloride source, and the other as a carbon source that acted as a reducing agent of Co(III) in LiCoO₂ to Co(II) in cobalt chloride (CoCl₂). These studies revealed that the mechanochemical processes not only obviously simplified the leaching of metals but also avoided the generation of liquor residues at the solid–solid reaction point.

On the other hand, Guan and his colleagues [223] examined the feasibility of mechanochemical processes to enhance the acid leaching of Co and Li at room temperature from spent LIBs. In this study, metallic iron powder was used as a reducing reagent and served as grinding aids with good performance for the extraction of valuable metals. A mechanochemical reduction time of 250 min operating at a rotation speed of 650 rpm and a mass ratio of Fe/LiCoO₂ of 1:1 enabled a positive leaching of 80% of Co compared to the extraction of Li (63%). The mechanochemical reduction mechanism with Fe powder altered the valence state of Co and played a dominant role in the high-efficiency extraction of Co, supported by the characterization analyses conducted by XRD, SEM, and XPS. In a comparative study with pure materials, the extractions of Li (77%), Co (91.25%), Mn (100%), and Ni (99.9%) were obviously enhanced after 2 h of leaching in HNO₃ (nitric acid) solution (1 M) at room temperature. Furthermore, more than 99% of Fe could be leached by this mechanochemical reduction process in acidic solutions.

Meanwhile, Liu et al. [224], in their study, proposed a selective (>90%) and acid-free Li extraction process to successfully achieve isomorphic substitution of LiCl in LiFePO₄ crystals with sodium (Na). The method uses LiFePO₄ powder co-ground with NaCl in the ratio of 1:2 at a rotation speed of 500 rpm for 6 h. Thus, NaFePO₄ and LiCl were obtained due to the substitution of Li and Na. Li was recovered as Li₂CO₃ and NaCl was regenerated simultaneously using Na₂CO₃ as the sole reagent, without acid and no detection of leached Fe.

Mechanochemical recovery of Li and Co from pure LiCoO₂ and commercial LIB cathodes using Al as a reducing agent was successfully demonstrated by Dolotko et al. [225]. The recovery rates achieved during experiments with pure LiCoO₂ are 90% for Co and 76% for Li. In the mechanochemical reduction reaction experiments involving commercial LIB cathodes for 3 h, polyvinylidene fluoride (PVDF) and Al were used as reducing agents. After dispersion in water and magnetic separation, the magnetic material was sonicated in water for 5 min to remove traces of PVDF. Recycling 4 g of the cathode material yields 0.6 g of metallic Co and 0.3 g of Li₂CO₃ (Figure 29). In a recent publication, Dolotko et al. [226] reported a systematic study of Li recovery up to 70% from various cathodes, such as LiCoO₂, LiMn₂O₄, Li(CoNiMn)O₂, LiFePO₄ and their mixtures, using Al as a reducing agent in the mechanochemical reaction, without applying corrosive leachates or using high temperatures in the recycling process. Both studies report that the developed processes are simple, fast, energy efficient and do not involve or produce highly corrosive liquid waste or toxic gases, thus offering clear advantages over other known recycling techniques for any type of LIB.

Liang et al. [213] demonstrated that advanced oxidation processes enhanced by mechanochemical activation can also enhance the effective leaching of valuable metals from spent LIBs. In this study, spent cathode material (LNCM) was subjected to co-milling with oxidant (Ammonium persulfate (NH₄)₂S₂O₈ or NHS) and reductant (sucrose, Suc) simultaneously in a ball milling process followed by acid leaching. The study reported that leaching rates of 99.1%, 96.2%, 99.9%, and 99.2% were achieved for Li, Co, Ni, and Mn, respectively, under the following conditions: rotational speed of 800 rpm, grinding time of 3 h, mass ratio of NHS/LNCM and Suc/LNCM of 2:1, leaching temperature of 25 °C, for 15 min at pH 2. The study demonstrated the synergistic effect of ammonium persulfate, iron (0.03 g/g LNCM, activating persulfate to form strong oxidative species), and sucrose in significantly improving the leaching efficiency. The successful leaching of the desired metals within a short period after the mechanochemical activation event could be well tracked by FT-IR, XRD, and XPS analyses that served to support the reaction mechanism.

Zhang et al. [227] reported a one-step mechanochemical leaching process using grape skin powder (GS) as a reducing agent to recover metals from cathode residues of LIBs. It was reported that maximum leaching rates of Li and Co metals reached 99.33% and 98.87%, respectively, using mechanochemical activation at a speed of 400 rpm for 60 min, 0.3 gGS/gLCO of GS added, 0.15 M H₃Cit (citric acid), and a solid/liquid ratio of 30 g/L GS. The mechanochemical leaching mechanism was analyzed by BET, PSD, XRD, and XPS. The study proved the superiority of using one-step direct mechanochemical activation as the leaching method for this system over the two-step process because a significantly lower amount of H₃Cit can be used in the absence of heating.

Cobalt leaching from spent LIBs using a mechanochemically assisted method in sulfuric acid and hydrogen peroxide system was studied by Qiao et al. [228]. This process was also optimized for better results by response surface methodology (RSM). The optimum conditions determined were H₂SO₄ concentration of 0.5 M, H₂O₂ concentration of 2 M, ball-to-powder mass ratio of 35:1, solid-to-liquid ratio of 20 g/L, reaction time of 120 min, revolution speed of 100 rpm, rotation speed of 468 rpm. Under these conditions, the cobalt leaching efficiency reached 98.96%. The work also demonstrated that the Co leaching rate was improved compared to the results in the relevant literature.

Liu and his collaborators [229] developed an advanced and rapid green recovery process for recycling used LiFePO₄ (LFP) batteries. In this study, a water leaching process using an ultrafast mechanochemical reaction (UMR) flash metallurgy technology was shown to be highly efficient, acid-free and at room temperature, for selective Li recovery. Recovery rates for Li and Fe of 99.17% and 2.39%, respectively, were achieved under the optimized conditions: a mass ratio of C₁₀H₁₄N₂Na₂O₈ to LFP of 3:1, the addition of 0.5 mL of H₂O₂, a rotation speed of 30 Hz, 4 min of UMR time, and a filling coefficient of 0.79. After filtration and precipitation, Fe and Li are finally recovered as FePO₄ and Li₂CO₃ precursors, respectively. Furthermore, based on XRD, XPS and TEM characterizations, along with simulations, the study reveals the chemical and mechanical mechanisms of how UMR extracts Li from the orthorhombic structure (Figure 30). Overall, this research highlighted the significant potential that advanced mechanochemical oxidation technology creating an acid-free environment can have to improve industrial efficiency for a greener future that aligns with the era of green chemistry in the entire process that is recycling of used LIBs.

Mechanochemistry has been increasingly applied in recent years to address some of the difficulties in recycling metals from different wastes. As reviewed previously, mechanochemical methods can successfully recover a variety of metals. Table 6 summarizes studies that have applied mechanochemical methods for recycling metals from different wastes, the processing parameters used, and the recycling efficiencies achieved.

Table 6. Summary of applications of assisted mechanochemical methods in recycling of waste metals.

Materies	Type of Mill	Milling Parameters	Leaching Parameters	Leaching Efficiency (%)	Ref.
LCO	Planetary	Ratio of the rotational-to-revolution speed is fixed at 1, air atmosphere Time: 36 h, Co-grinding reagent: polyvinyl chloride (PVC)-LiCoO2: = 1:1	H2O, L/S = 250 mL/g, 1 h, agitation	90% Co and >90% Li	[221]
LCO	Planetary	Ratio of the rotational-to-revolution speed is fixed at 1, Time: 30 h, air atmosphere Co-grinding reagent: polyvinyl chloride (PVC)/LiCoO2 = 1:1	H2O, L/S = 250 mL/g, 1 h, agitation	90% Co and 100% Li	[220]
CRT	Planetary	Speed: 500 rpm, Time: 240 min	H2O, L/S = 20 mL/g, agitation, 95 °C, 2 h 3 M HNO3, L/S = 150 mL/g, agitation, 95 °C, 2 h	92.5% Pb	[230]
LCO	Planetary	Speed: 600 rpm Time: 240 min Co-grinding reagent: LiCoO2/EDTA = 1:4	H2O: 100 mL, agitated for 30 min	98% Co and 99% Li	[231]
LCO	Planetary	Speed: 600 rpm Time: 12 h Co-grinding reagent: LCO/PVC/Fe = 1:1:2	H2O: 100 mL	81.1% Co, 91.9% Co was rearranged to CoFexOy and 100% Li	[222]
LCO	Planetary	Speed: 650 rpm, Time: 250 min Co-grinding reagent: Fe/LiCoO2 = 1:1	1 M HNO3; 2 h, 25 °C, magnetic stirring	99.9% Ni, 91.25% Co, 100% Mn and 77.15% Li	[223]
LFP	Planetary	Speed: 550 rpm, Time: 120 min, Co-grinding reagent: LiFePO4/EDTA-2Na = 3:1	0.6 M H3PO4, S/L = 50 g/L, 20 min, 25 °C	97.67% Fe and 94.29% Li	[232]
LCO	Planetary	Speed: 500 rpm Time:60 min	1.0 M L-ascorbic acid; 20 min; 298 K; 10 g/L	99% Co and 100% Li	[233]
LCO	Planetary	Speed: 500 rpm Time:30 min Co-grinding reagent: C	20 vol% acetic acid; 5 vol% H2O2; 15 min; 298 K	99.7% Co and 99.8% Li	[234]
LFP	Planetary	Speed: 500 rpm Time:120 min Co-grinding reagent: oxalic acid ball milling medium:1 mL H2O	H2O	94% Fe and 99% Li	[235]
LFP	Planetary	Speed: 500 rpm Time: 360 min Co-grinding reagent: NaCl	H2O	>90% Li	[224]
WPCB	Planetary	Speed: 400 rpm Time: 240 min Co-grinding reagent: K2S2O8/WPCB = 3:2	H2O: 50 mL; magnetic stirring; 300 rpm. temperature: 25, 35, 45, and 55 °C	>98% Cu	[236]
LCO	SPEX 8000 shaker mil	Speed = 1725 rpm Time = 180 min, Co-grinding reagent: Al/LiCoO2 = 1:1 under air atmosphere and LiCoO2/Li = 1:3 and LiCoO2/Ca = 1:1.5 under argon atmosphere	H2O, stirred for a few minutes in air, 1 M Na2CO3, stirred for 1 h, room temperature	90% Co and 70% Li	[225]
LCO	Planetary	Speed = 700 rpm Time = 90 min, and LiCoO2/Li and LiCoO2/Ca atmospheric pressure Co-grinding reagent: dry ice; mass ratio = 1:20, NaCl, SiO2; LCO/NaCl mass ratio = 1:6, LCO/SiO2 mass ratio = 1:2, room temperature	H2O, magnetic stirring, room temperature, 5 min	95% Li	[237]
LCO	Planetary	Speed: 500 rpm, Time: 720 min, Co-grinding reagent: LiCoO2/NaCl = 1:6 and LiCoO2/SiO2 = 1:2, room temperature and atmospheric pressure	H2O, magnetic stirring, room temperature, 5 min	92.89% Li	[238]
NCM	Planetary	Speed: 550 rpm Time: 120 min Co-grinding reagent: Zn powder	1.5 M H2SO4; 15 min; 323 K; 20 g/L	96.2% Ni, 94.3% Co, 91.0% Mn and 99.9% Li	[239]
LNCM	Planetary	Speed: 800 rpm Time: 180 min Co-grinding reagent: (NH4)2SO4, sucrose and powder Fe	1 M H2SO4; 298 K, 15 min; pH 2.0; 25 g/L.	99.1% Li, 96.2% Co, 99.9% Ni and 99.2% Mn,	[213]
LCO	Planetary	Speed: 500 rpm Time: 120 min	1 M H2SO4; 0.03 M NH4Cl; 60 min; 353 K; 20 g/L	99.22% Co and 100% Li	[240]
LCO	Planetary	Speed: 500 rpm, Time: 240 min, Co-grinding reagent: LCO/alginic acid = 1:10 and 2 mL of H2O2	H2O, room temperature, and agitated for 5 min	97.58% Li and 98.59% Co	[241]
LCO	Planetary	Speed:400 rpm Time:60 min Co-grinding reagent: Grape skin powder	0.15 M citric acid; 323 K; 30 min; 30 g/L	98.87% Co and 99.33% Li	[227]
LCO, NMC, LMO and LFP	SPEX 8000 shaker mill	Speed = 1725 rpm Time = 180 min, Co-grinding reagent: Al/LiCoO2 = 1:1, Al/NMC = 1:2.33, LiMn2O4/Al = 1:2.33, LiFePO4/Al = 1:3 and LCO:NMC:LMO:LFP:7.33Al, all experiments were performed in air atmosphere	H2O, stirred for a few minutes in air, room temperature	29.8–39.6% Li	[226]
LCO, NMC, LMO and LFP	SPEX 8000 shaker mill	Speed = 1725 rpm. Time = 180 min, Co-grinding reagent: Al/LiCoO2 = 1:1, Al/NMC = 1:2.33, LiMn2O4/Al = 1:2.33, LiFePO4/Al = 1:3 and LCO:NMC:LMO:LFP:7.33Al, all experiments were performed in air atmosphere	H2O, stirred for a few minutes in air, room temperature, carbonatization and recrystallization at 70 °C	55.6–75.9% Li	[226]
LCO	Planetary	Speed = 650 rpm, time = 90 min, ball-to-powder mass ratio = 60:1	EDTA 0.052 M, H2O2 0.3 vol.% at room temperature and leaching time = 60 min	100% Co and 98.2% Li	[242]
LCO	Planetary	Speed: 600 rpm, Time: 60 min, Co-grinding reagent: citric acid/LCO = 1 and ascorbic acid/LCO = 0.5	H2O, S/L = 775 g/L, 15 min, room temperature	97.2% Co and 99.7% Li	[243]
LCO	Planetary	revolution speed of 100 r/min, rotation speed of 468 r/min, Time: 120 min, Co-grinding reagent: H2SO4 0.5 M, H2O2 2 M and LCO	Saturated H2C2O4 solution, S/L = 20	98.96% Co	[228]
LFP	_	Speed of 30 Hz Time: 4 min Co-grinding reagent: C10H14N2Na2O8 and H2O2	H2O; 298 K; 30 min	99.17% Li and 2.39% Fe	[229]

Table 6. Summary of applications of assisted mechanochemical methods in recycling of waste metals.

Materies	Type of Mill	Milling Parameters	Leaching Parameters	Leaching Efficiency (%)	Ref.
LCO	Planetary	Ratio of the rotational-to-revolution speed is fixed at 1, air atmosphere Time: 36 h, Co-grinding reagent: polyvinyl chloride (PVC)-LiCoO2: = 1:1	H2O, L/S = 250 mL/g, 1 h, agitation	90% Co and >90% Li	[221]
LCO	Planetary	Ratio of the rotational-to-revolution speed is fixed at 1, Time: 30 h, air atmosphere Co-grinding reagent: polyvinyl chloride (PVC)/LiCoO2 = 1:1	H2O, L/S = 250 mL/g, 1 h, agitation	90% Co and 100% Li	[220]
CRT	Planetary	Speed: 500 rpm, Time: 240 min	H2O, L/S = 20 mL/g, agitation, 95 °C, 2 h 3 M HNO3, L/S = 150 mL/g, agitation, 95 °C, 2 h	92.5% Pb	[230]
LCO	Planetary	Speed: 600 rpm Time: 240 min Co-grinding reagent: LiCoO2/EDTA = 1:4	H2O: 100 mL, agitated for 30 min	98% Co and 99% Li	[231]
LCO	Planetary	Speed: 600 rpm Time: 12 h Co-grinding reagent: LCO/PVC/Fe = 1:1:2	H2O: 100 mL	81.1% Co, 91.9% Co was rearranged to CoFexOy and 100% Li	[222]
LCO	Planetary	Speed: 650 rpm, Time: 250 min Co-grinding reagent: Fe/LiCoO2 = 1:1	1 M HNO3; 2 h, 25 °C, magnetic stirring	99.9% Ni, 91.25% Co, 100% Mn and 77.15% Li	[223]
LFP	Planetary	Speed: 550 rpm, Time: 120 min, Co-grinding reagent: LiFePO4/EDTA-2Na = 3:1	0.6 M H3PO4, S/L = 50 g/L, 20 min, 25 °C	97.67% Fe and 94.29% Li	[232]
LCO	Planetary	Speed: 500 rpm Time:60 min	1.0 M L-ascorbic acid; 20 min; 298 K; 10 g/L	99% Co and 100% Li	[233]
LCO	Planetary	Speed: 500 rpm Time:30 min Co-grinding reagent: C	20 vol% acetic acid; 5 vol% H2O2; 15 min; 298 K	99.7% Co and 99.8% Li	[234]
LFP	Planetary	Speed: 500 rpm Time:120 min Co-grinding reagent: oxalic acid ball milling medium:1 mL H2O	H2O	94% Fe and 99% Li	[235]
LFP	Planetary	Speed: 500 rpm Time: 360 min Co-grinding reagent: NaCl	H2O	>90% Li	[224]
WPCB	Planetary	Speed: 400 rpm Time: 240 min Co-grinding reagent: K2S2O8/WPCB = 3:2	H2O: 50 mL; magnetic stirring; 300 rpm. temperature: 25, 35, 45, and 55 °C	>98% Cu	[236]
LCO	SPEX 8000 shaker mil	Speed = 1725 rpm Time = 180 min, Co-grinding reagent: Al/LiCoO2 = 1:1 under air atmosphere and LiCoO2/Li = 1:3 and LiCoO2/Ca = 1:1.5 under argon atmosphere	H2O, stirred for a few minutes in air, 1 M Na2CO3, stirred for 1 h, room temperature	90% Co and 70% Li	[225]
LCO	Planetary	Speed = 700 rpm Time = 90 min, and LiCoO2/Li and LiCoO2/Ca atmospheric pressure Co-grinding reagent: dry ice; mass ratio = 1:20, NaCl, SiO2; LCO/NaCl mass ratio = 1:6, LCO/SiO2 mass ratio = 1:2, room temperature	H2O, magnetic stirring, room temperature, 5 min	95% Li	[237]
LCO	Planetary	Speed: 500 rpm, Time: 720 min, Co-grinding reagent: LiCoO2/NaCl = 1:6 and LiCoO2/SiO2 = 1:2, room temperature and atmospheric pressure	H2O, magnetic stirring, room temperature, 5 min	92.89% Li	[238]
NCM	Planetary	Speed: 550 rpm Time: 120 min Co-grinding reagent: Zn powder	1.5 M H2SO4; 15 min; 323 K; 20 g/L	96.2% Ni, 94.3% Co, 91.0% Mn and 99.9% Li	[239]
LNCM	Planetary	Speed: 800 rpm Time: 180 min Co-grinding reagent: (NH4)2SO4, sucrose and powder Fe	1 M H2SO4; 298 K, 15 min; pH 2.0; 25 g/L.	99.1% Li, 96.2% Co, 99.9% Ni and 99.2% Mn,	[213]
LCO	Planetary	Speed: 500 rpm Time: 120 min	1 M H2SO4; 0.03 M NH4Cl; 60 min; 353 K; 20 g/L	99.22% Co and 100% Li	[240]
LCO	Planetary	Speed: 500 rpm, Time: 240 min, Co-grinding reagent: LCO/alginic acid = 1:10 and 2 mL of H2O2	H2O, room temperature, and agitated for 5 min	97.58% Li and 98.59% Co	[241]
LCO	Planetary	Speed:400 rpm Time:60 min Co-grinding reagent: Grape skin powder	0.15 M citric acid; 323 K; 30 min; 30 g/L	98.87% Co and 99.33% Li	[227]
LCO, NMC, LMO and LFP	SPEX 8000 shaker mill	Speed = 1725 rpm Time = 180 min, Co-grinding reagent: Al/LiCoO2 = 1:1, Al/NMC = 1:2.33, LiMn2O4/Al = 1:2.33, LiFePO4/Al = 1:3 and LCO:NMC:LMO:LFP:7.33Al, all experiments were performed in air atmosphere	H2O, stirred for a few minutes in air, room temperature	29.8–39.6% Li	[226]
LCO, NMC, LMO and LFP	SPEX 8000 shaker mill	Speed = 1725 rpm. Time = 180 min, Co-grinding reagent: Al/LiCoO2 = 1:1, Al/NMC = 1:2.33, LiMn2O4/Al = 1:2.33, LiFePO4/Al = 1:3 and LCO:NMC:LMO:LFP:7.33Al, all experiments were performed in air atmosphere	H2O, stirred for a few minutes in air, room temperature, carbonatization and recrystallization at 70 °C	55.6–75.9% Li	[226]
LCO	Planetary	Speed = 650 rpm, time = 90 min, ball-to-powder mass ratio = 60:1	EDTA 0.052 M, H2O2 0.3 vol.% at room temperature and leaching time = 60 min	100% Co and 98.2% Li	[242]
LCO	Planetary	Speed: 600 rpm, Time: 60 min, Co-grinding reagent: citric acid/LCO = 1 and ascorbic acid/LCO = 0.5	H2O, S/L = 775 g/L, 15 min, room temperature	97.2% Co and 99.7% Li	[243]
LCO	Planetary	revolution speed of 100 r/min, rotation speed of 468 r/min, Time: 120 min, Co-grinding reagent: H2SO4 0.5 M, H2O2 2 M and LCO	Saturated H2C2O4 solution, S/L = 20	98.96% Co	[228]
LFP	_	Speed of 30 Hz Time: 4 min Co-grinding reagent: C10H14N2Na2O8 and H2O2	H2O; 298 K; 30 min	99.17% Li and 2.39% Fe	[229]

3.8. Synthesis of Battery-Based Nanocomposites

An efficient strategy for recycling used LIBs is the direct regeneration of cathode materials from the used cathodes themselves. Successful results have been reported in the literature for the regeneration of cathode materials. Wang et al. [244] synthesized a material in 30 min, Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ nanometric, from Li₂CO₃ cathode materials used by a microwave technique assisted by mechanical and chemical activation. The material showed optimal electrochemical performance with an initial discharge capacity of 239 mA h/g and initial coulombic efficiency of 73% at 25 mA/g and 228, 215, 193 mA h/g at 50, 150 and 300 mA/g, respectively, with almost no capacity loss after 100 cycles at 0.1 C.

Meng et al. [245] regenerated LiMn₂O₄ used involving a solid-state technique assisted by mechanochemical activation to synthesize a high-performance nano-LiMnPO₄/C composite. The obtained material exhibited promising electrochemical performance with discharge capacities of 148.5 mA h/g at a rate of 0.05 C, respectively, in the voltage range of 2.5 to 4.5 V. The result also demonstrated that the LiMnPO₄ material has high structural stability and stable cycling performance with a capacity retention of 98% after 100 cycles at a rate of 1 C.

Meng et al. [246] also reported an integrated method of mechanochemical activation and high-temperature solid-state sintering (800 °C for 10 h). In this study, the ball milling process was introduced to facilitate the diffusion of lithium and nickel ions, thereby decreasing the cationic disorder and rebuilding the layered crystal structure. Combined with sintering, the recycled MA-1.20-800 provided a specific discharge capacity of 165 mA h/g at 0.2 C in the first cycle and a capacity retention maintained above 80% after 100 cycles.

The preparation of a highly efficient and easily scalable solid-phase recycling procedure for the cyclic utilization of waste LFP was proposed by Song et al. [247]. In this study, the waste LFP material (W-LFP) was regenerated by mechanical separation (ball milling), removing impurities. Then, 5% carbon nanotubes, 15 wt% glucose, and 5 wt% Li₂CO₃ were added to the W-LFP (1:1 aqueous ethanol solution as dispersant), followed by ball milling, and the subsequent heat treatment at 650 °C for 12 h. Finally, the obtained regenerated LFP (R-LFP) achieves an excellent specific capacity of 155.47 mA h/g at 0.05 C, which is about 99% of the capacity of a fresh LFP, and long-term cyclic stability (more than 800 cycles at 1 C). The work emphasizes that the main consumption costs in the regeneration process account for only 33.7% of the price of a fresh LFP. Likewise, Liu and his collaborators [248], prepared a new cathode material doped with 3 mol% V, after calcination, to prepare a new material R3, with a discharge capacity of 134.3 mA h/g after 200 cycles at 1 C maintained 99.1% discharge capacity retention. At 2, 5, 10 C, the discharge capacities are 134, 124.1, 111.5 mA h/g, respectively.

Gou et al. [249] described the sample preparation process, first using ethanol or aqueous solution as the dispersant, supplementing the lithium source with waste LiFePO₄ cathode powder, and adding melamine as the carbon and nitrogen source. The slurry was mixed by ball milling, dried, and sintered at high temperature under inert gas to obtain a repaired nitrogen-doped carbon-coated LiFePO₄ sample (Figure 31). The regenerated R-(C + N)-LiFePO₄ exhibits a large discharge capacity of 168 mA h/g. In addition, it exhibited an excellent capacity preservation rate of 99.03% after 200 cycles and an exceptional charge capacity at 5 C, with a discharge capacity of 116 mA h/g.

Representative publications on direct regeneration of cathode materials from spent cathodes themselves are listed in

Table 7. Electrochemical characteristics of various materials as cathodes.

		Regeneration Process			Electrochemical Performance
Materials	Type of Mill	Ball Milling Parameters	Sintering Temperature (°C)	Regenerated Cathode	Specific Charging Capacity (mA h/g)	Cycle	Retention Rate	Ref.
NCM	Planetary	Speed = 400 rpm, Time = 120 min	900 for 30 min	Li[Li0.2Mn0.54Ni0.13Co0.13]O2	239	100	98%	[244]
NCM	Planetary	Speed = 400 rpm; Time: 240 min, Co-grinding reagent: Li/TM (Ni, Co, Mn) = 1.20/1	800 for 10 h	MA-1.20-800	165	100	>80%	[246]
NCM	Without mechanochemical activation	Li/TM (Ni, Co, Mn) = 1.20/1	800 for 10 h	non-MA-1.20-800	<165	-	-	[246]
LiMn2O4	Planetary	Speed: 800 rpm; Time: 240 min; Co-grinding reagent: NH3H2PO4, polyvinyl alcohol (PVA) and ethyl alcohol	650 for 6 h	LiMnPO4/C	148.5	100	98%	[245]
LFP	Planetary	Speed: 400 rpm; Time: 240 min Co-grinding reagent: LFP of Li:V: = 1.05:0.03)	700 for 9 h	R3	134.3	200	99.10	[248]
W-LFP	Planetary	Speed: 500 rpm; Time: 120 min; Co-grinding reagent: 5 wt% CNTs, 15 wt% glucose, 5 wt% Li2CO3, W-LFP (1:1 ethanol aqueous solution as dispersant)	650 for 12 h	R-LFP	155.47	800	70.84	[247]
LFP	Planetary	Speed: 500 rpm; Time: 120 min; nitrogen-doped carbon-coated LiFePO4 composites, lithium carbonate and melamine; LFP (1:1 ethanol aqueous solution as dispersant)	650 for 12 h	R-(C + N)-LiFePO4	168	200	99.03	[249]
LFP	Planetary	Ball milling; Time: 2 h Co-grinding reagent: PVDF/LFP = 5 wt%	650 for 2 h	R3-LFP	141.5	100	99.60	[250]
LFP	Planetary	Ball milling; Time: 2 h Co-grinding reagent: sucrose, g-C3N4 powder and ethanol	700 for 8 h	RLFP/g10–700/8	135.50	300	-	[251]

. So far, heat treatment is almost an inevitable process for repairing cathodes and effectively improves electrochemical performance.

3.9. Technology Assessment

According to the studies analyzed in this review, there are many advantages of the mechanochemical process over the solution-based wet process. Mechanochemical processing bypasses the requirement for reagent solubility and large solvent consumption, as well as avoiding complex post-treatment steps, such as solvent removal and product purification [48,49,52,191]. Another advantage of mechanochemistry is its operation at room temperature. Therefore, from an environmental and economic point of view, mechanochemistry is an environmentally friendly and labor- and time-saving approach for the synthesis of chemicals and materials from various types of substrates. Throughout this review, several advantages of mechanochemical reactions were highlighted and most of these benefits were related to efficiency and sustainability [252].

Mechanochemistry has become an innovative, advantageous, and very attractive method for producing various materials, polymers, and small chemical products with great added value. Furthermore, most of these advantages are in line with the twelve principles of green chemistry [52]. In general, mechanochemical treatment is well known as a key technology for surface modification and reactivity improvement of various types of substrates (

Table 8. The most important benefits and drawbacks of mechanochemical activation [191].

Benefits

Drawbacks

Alternative to conventional methods Easy operation Low cost Consumption of very low amounts of solvents, including water No or minimal amounts of chemical waste Ability to control intermediate products Reproducible milling parameters Generating new surface structure (revealing fresh new surfaces that are easily susceptible to chemical reactions) Fast and reproducible fragmentation of materials Usually mechanochemical reaction time is shorter than in traditional methods Increasing the surface area and pozzolanic activity of FA, as well as decreasing particle size Partial or complete amorphization of crystalline materials during milling

Energy consumption High cost of professional milling devices Necessity to purchase additional accessories (i.e., cups and milling balls, often made of non-reactive and durable materials, in the case of reactions aimed at obtaining, MOF and COF for example) Long milling time for specific and demanding reactions Abrasion of the milling balls and walls of the milling cup Impossibility of obtaining monocrystals in the case of chemical syntheses Difficult to control temperature

).

Another problem is the long grinding time usually required, especially to achieve complete degradation of different residues, such as halogenated pollutants, discarded batteries, residual biomass, among others. This represents an obstacle, since energy consumption is the main weakness of mechanochemical technology. In general, high load rates and rotation speeds reduce grinding times, making the process more efficient, but require more powerful mills that, in most cases, consume more energy. Consequently, energy-intensive grinding for very long periods makes the full-scale mechanochemical process economically impractical [53]. Therefore, the selection of suitable grinding devices and a suitable mixture of reagents to improve the reaction kinetics would allow reducing energy costs, in terms of energy consumption and grinding duration, which continue to be the focus of research.

3.10. Limitations of Technology

As discussed in the previous sections, mechanochemistry emerges as a sustainable alternative to traditional synthesis, reducing the use of solvents and expanding reactive possibilities. However, some obstacles and limitations to the technique can be identified below.

3.10.1. Product Purification

As mentioned in the previous sections, even though mechanochemistry is sometimes a solvent-free technique, it may still require the use of solvents to purify the final product. Removing non-volatile byproducts or traces of reagents may require extraction or recrystallization processes that use solvents. Therefore, mechanochemistry is unlikely to make all chemical syntheses completely solvent-free, but it is important to identify situations in which it offers a clear advantage over conventional methods [9,253].

3.10.2. Difficulty of Scalability

In the previous sections, it was demonstrated that all mechanochemical processes were performed at laboratory scales, ranging from a few hundred milligrams to a few grams. Although grinding equipment for large-scale material processing is available, the issue of scalability in mechanochemistry has not yet been widely explored. One of the main barriers is the transition from laboratory to industrial scale. While mechanochemistry is ideal for small production runs, its large-scale application is complex. Many mechanochemical processes are performed in batch reactors, which are less efficient than the continuous reactors used in conventional industry. This makes it difficult to produce large volumes of materials economically and reproducibly [9,52,253,254]. The lack of efficient continuous mechanochemical reactors for the chemical industry is a significant challenge [9,253]. Therefore, the perception persists that scaling up this technique presents difficulties.

3.10.3. Parameter Control and Reproducibility

Controlling variables such as temperature, pressure, impact frequency, and degree of grinding is more complex in mechanochemistry than in solution reactions. Small variations in grinding frequency, for example, can significantly influence reaction yield. Mechanical energy and heat dissipation during milling are difficult to control and monitor in real time, which can lead to undesirable variations in the structure and purity of the final product, compromising reproducibility [251,253,254,255]. Knowledge about mechanosynthesis parameters, such as milling conditions and the use of activating agents and precursors, is still developing. Despite notable progress in specific examples presented in this review, the technique has not yet reached the same level of sophistication and versatility as solution-phase synthesis strategies.

3.10.4. Equipment Wear and Contamination

The constant collision of grinding balls and friction can cause wear on the reactors and the balls themselves, leading to product contamination. Although modern mills are designed to minimize this wear, contamination by metal or ceramic particles can be a problem, especially in the production of high-purity materials for the pharmaceutical and electronics industries [52,252,253,254].

In general, mechanochemistry focuses on making known solution-based synthetic procedures more environmentally friendly by avoiding the use of solvents, which is also one of the main disadvantages. Therefore, the development of innovative bond-forming reactions under mechanical milling should be highly valued, as they are inaccessible through solution-phase chemistry.

4. Conclusions

The growing global demand for the sustainable use of natural resources has driven the development of green processes, including the recycling and valorization of waste generated by various anthropogenic activities. These processes enable the production of functional materials with applications across multiple sectors such as medicine, engineering, pharmaceuticals, and chemistry.

Although nature offers a vast diversity of raw materials—many of which are still underexplored—waste materials, often improperly discarded in landfills or natural environments, have demonstrated significant potential for reuse. Over the past decades, a substantial body of literature has emerged highlighting the economic and environmental benefits of converting waste into useful products, reinforcing the growing interest of research groups worldwide in this field.

Among the innovative strategies explored, mechanochemistry has shown particular promise in addressing the challenges associated with waste management, offering a solvent-free, energy-efficient route for the modification and transformation of diverse waste streams. This review presented current advances in the application of mechanochemical techniques to the modification of fly ash, geopolymer synthesis, dehalogenation of electronic waste, metal ion extraction, and the generation of value-added materials.

These studies demonstrate that mechanochemistry is a versatile and environmentally friendly platform with applications in catalysis, electrochemistry, and advanced material production. While this approach offers clear advantages—such as reduced processing time, lower costs, and elimination of harmful reagents—it still remains at a pre-commercial stage, requiring further research, optimization, and scale-up.

The widespread implementation of mechanochemical strategies would represent a significant step forward in waste valorization, contributing to more sustainable production systems, the mitigation of environmental liabilities, and the generation of economically valuable materials from solid waste.