Energetic Valorization of Leather Solid Waste Through Thermochemical and Biochemical Methods

Abstract

The leather industry generates large amounts of solid waste, creating environmental concerns for the presence of hazardous compounds such as chromium. In fact, conventional disposal practices, including landfill and incineration, promote the formation of hexavalent chromium (Cr⁶⁺) and polluting emissions. This work reviews biochemical and thermochemical processes for the energetic valorization of different leather solid wastes, namely untanned, tanned with chromium or vegetable tanning agents, and post-consumer leather. Thermochemical routes, i.e., pyrolysis, gasification, and hydrothermal treatment (HT), can convert leather waste into energy carriers including bio-oil, syngas, and char, while anaerobic digestion (AD) is a biochemical method used to produce biogas. Particularly, pyrolysis is promising for fuel precursors and chromium stabilization, HT suits wet, raw waste, while gasification enables syngas recovery. In AD, microbial chromium inhibition is mitigated through the co-digestion of degradable substrates. This review takes a waste-type-driven rather than process-driven approach to provide new insights into the conversion of leather solid waste into value-added products, showing that the optimal recycling route depends on the waste characteristics. Moreover, these methods have not yet been directly compared in terms of their energy production performance with regard to leather waste. Future work should improve process conditions, evaluate chromium and finishing additive impacts, and assess scalability.

1. Introduction

The world’s oldest known leather shoe, over 5500 years old, demonstrates that leather manufacturing has long been integral to human history for millennia [1]. Leather is a versatile, insulating, and waterproof material widely used across various industries for its technical properties [2]. The global leather goods market is valued at approximately 394.12 billion USD [3], while also generating significant amounts of polluting waste [4]. According to the literature, it is estimated that processing one metric ton of raw hide results in 200 kg of tanned leather, 190–350 kg of non-tanned waste, and 200–250 kg of tanned leather waste; moreover, approximately 15–50 m³ of wastewater is generated, which collects the remaining ~350 kg of raw material [5].

The key steps in the leather manufacturing are as follows: (i) pre-tanning, where non-collagenous materials are removed and the hide is prepared for subsequent tanning (i.e., soaking, dehairing, bating, and degreasing); (ii) tanning, performed with mineral or vegetable tanning agents cross-linking the collagen fibers, making the skin imperishable against heat and microbial attacks; and (iii) post-tanning, which imparts the required functional and esthetic properties through re-tanning, dyeing, and fat liquoring [3,6,7,8,9,10].

Leather manufacturing is a complex process producing several types of waste at each operational unit. Based on the different steps, leather solid waste (LSW) is generally classified as follows [11,12]:

1.

Untanned leather solid waste (ULSW): This includes leftovers mainly from beamhouse processing, i.e., trimmed raw hide or liming skin. Fleshing waste contains about 87% of water, 4–6% dry wt. of proteins (e.g., collagen, keratin, elastin), and 1–2% dry wt. fat, so due to biological degradation, its handling and discharge is complicated. Islam et al. found that fleshing contained 82.57% of volatile matter, a low C/N ratio of 2.64, and a high pH of 10.99 [13].

2.

Tanned solid leather waste (TLSW): TLSW is leather tanned via (i) mineral tanning agents, i.e., chromium (III) salts, producing the so-called wet blue leather, or alternative minerals, like aluminum and silicates, which produce wet white leather, or via (ii) vegetable tanning agents that employ vegetable tannins such as plant polyphenols [12,14]. In this work, we will focus on the main types of TLSW, specifically:

• Chromium leather solid wastes (CLSW), i.e., wet blue, chrome shavings, buffing dust, and splitting and trimming leftovers, which contain about 1–3% of chromium, mainly Cr₂O₃, and almost 60–75% of it remains in the collagen structure [11,12].
• Vegetable-tanned leather solid waste (VLSW), i.e., leather tanned using tannins that are complex and heterogeneous polyphenolic secondary metabolites produced by plants, ranging from 500 to 20,000 Da, which are applied as tanning agents in leather manufacturing [15]. Tannin types are divided into hydrolysable, from pyrogallol (e.g., chestnut, valonea, and tara extracts), condensed, containing polyhydroxyflavan-3-ol oligomers (e.g., mimosa and quebracho), and complex tannins. In VLSW, the chemical nature of the chosen tannin influences the aging of the material, as well as the deterioration of collagen.

3.

Finished or post-consumer leather waste (FCLW): This includes industrial waste, i.e., leather trimmings, crust leather buffing dust, and finished leather scraps, as well as used leather goods, such as shoes, bags, and the end products of leather industry [12]. Due to the difficulties related to disposal, the majority of these wastes are directly discarded without proper treatment, causing environmental pollution [12] not only because of chromium but also because of the surface protective coating containing polymers, organic or inorganic pigments, lacquers, etc. [7].

Leather manufacturing produces not only toxic compounds with hazardous effects on the environment and human health, but also solid and liquid wastes that are difficult to recycle and detoxify [4,8]. The main pollutant is represented by chromium from chromium sulfate salts, which are applied as tanning agent by 90% of tanneries [16]. Its danger is related to the fact that non-toxic chromium (III) can spontaneously oxidize into its carcinogenic form, i.e., chromium (VI), that is part of IARC group 1 and can possibly spread to soil or groundwater when leather waste is landfilled [10,12,17]. Higher amounts of chromium are mainly found in leather industry wastewater, where it can reach the concentration of 3000–6000 mg L⁻¹ [18], but also are found in the CLSW and FLSW [19,20]. Along with this, the leather industry produces greenhouse gases, e.g., carbon dioxide (CO₂) and ammonia (NH₃), as well as organic compounds [21], e.g., aldehydes, hydrocarbons, amines [22], dyes, or finishing products [23], mainly present in wastewaters [24].

LSW management strongly depends on the composition of the waste stream. Particularly, primary wastes are generated during beamhouse operations (i.e., ULSW and TLSW) whereas secondary wastes originate from wet-end and finishing processes (i.e., FLSW). Indeed, while numerous studies have addressed the valorization of primary LSW, significantly less research has focused on secondary LSW [19], which is still mainly disposed of by direct landfilling without appropriate pre-treatment [11]. Incineration is often applied to generate heat from different types of LSW and to recover chromium from the resulting ash, which can be used as a substitute for chromite ore in metallurgical or chemical industries [25]. For instance, Famielec et al. described the incineration of CLSW in a 7 m tunnel incinerator, where the waste underwent an initial drying and volatilization for 25 min, followed by a combustion phase of 18 min at 850 °C, and a final cooling phase of 17 min. Each test processed approximately 15 kg of waste per hour and recovered 130–135 MJ of heat. The resulting ash residue contained a high concentration of Cr (III) while a minimal presence (<0.5 mg Cr g⁻¹) or no Cr (VI) was detected.

Kluska et al. reported similar findings when testing the co-combustion of CLSW with hardwood pellets in a laboratory-scale combustion reactor operating at 1050–1180 °C, where air was supplied at 50 L min⁻¹ [26]. They found that the high ash content in CLSW can negatively impact the combustion by reducing the air/fuel mixture and gas flow, potentially causing slagging. XRF analysis of the ash samples confirmed a concentration of around 67.1% of Cr (III) in pure CLSW. Similarly, Velusamy et al. used CLSW and FLSW in a comparative study of incineration vs. pyrolysis [27]. Incineration was carried out in an industrial-scale system with a capacity of 150 kg h⁻¹, where the primary combustion zone operated at 800 °C and the secondary at 1200 °C, followed by flue gas treatment with a wet scrubber. They observed that, unlike pyrolysis performed below 600 °C, high-temperature incineration can promote the conversion of Cr (III) into Cr (VI) more significantly.

Although incineration has a high technology readiness level (TRL 9) compared to other technologies [17] and the ash is generally safe due to the absence of carcinogenic Cr (VI), the emission of gaseous pollutants such as NO_X and SO_X remains a major challenge [25,26,27]. Despite technological advancements aimed at reducing emissions and improving combustion, such as flue gas cleaning systems and bubbling fluidized-bed technology [20], the potential presence of chromium in flue gases or fine particulates still requires thorough investigation to ensure the environmental sustainability of industrial-scale processes [27]. Additionally, landfilling and incineration neglect the potential application of the bio-based collagen fibers contained in leather [10]. As such, the management and recycling of LSW remain critical environmental issues, particularly in relation to Sustainable Development Goals No. 14, i.e., Life Below Water, and No. 15, i.e., Life on Land [6,11,16,28].

Alternative recycling and valorization processes towards LSW are classified as follows: direct use to make composites or absorbents (mechanical recycling), the application of collagen extracted by hydrolysis (chemical recycling), and energy recovery processes (biochemical and thermochemical recycling) [11,29]. Mechanical recycling, a direct utilization method for tanned and finished leather [29], is aimed at electrostatic flocking [11], fiber leather regeneration, adsorbents [30], conductive materials [11], or composites production [31,32,33]. Chemical treatments that are indirect recycling methods for wet blue, wet white, and raw hide, remove chromium, which can be recycled as a pigment, tanning agent, or in metallurgy, as well as recover collagen, which is employed in the medical and cosmetic industry, in agriculture for fertilizers production, or in polymer and composite design [16,34,35,36,37,38]. Energy recovery technologies (indirect utilization recycling) involve the decomposition or depolymerization of leather to recover the monomeric constituents or other valuable chemicals [28,39].

In this review, we focus on the energy recovery recycling methods applied to ULSW, CLSW, VLSW, and FLSW. Energy recovery can be achieved through two main processing routes: (i) the thermochemical pathway, which includes gasification, pyrolysis, and hydrothermal treatment (HT), and (ii) the biochemical pathway, mainly represented by anaerobic digestion (AD) and based on enzymatic and microbial degradation [40]. The overarching goal is to unify the available knowledge of these methods, with a specific emphasis on thermochemical and biochemical strategies capable of converting leather residues into valuable energy carriers and bioproducts, thereby bridging the gap between waste management, resource recovery, and sustainable industrial practices.

In this review, the main databases used for bibliographic research were Scopus and Google Scholar, using the keywords “leather AND pyrolysis OR gasification OR HT ORAD”, as well as “ULSW OR CLSW OR TLSW OR FLSW AND pyrolysis OR gasification OR HT or AD”. For this review, papers published between 2010 and 2024 were taken into consideration. The most studied LSW are ULSW and CLSW, followed by TLSW and FLSW, whose literature is fairly limited. The most studied method is pyrolysis, followed by gasification and AD, while HT is a more recent method.

2. Thermochemical Methods

Thermochemical treatments are high-temperature processes that transform materials by breaking chemical bonds under controlled reaction conditions, in terms of temperature, pressure, and oxidizing conditions. Thermochemical processes play a crucial role in waste and biomass management, as well as energy recovery and resource sustainability [12,39]. These processes can be classified as thermal methods, such as pyrolysis and gasification, or as hydrothermal treatments, which employ sub- or supercritical water both as a medium and as a reactant.

Thermochemical processes transform organic matter into valuable products and energy carriers, such as biogas, bio-oil, and char. Bio-oils generally exhibit high energy content (15–40 MJ kg⁻¹) and are suitable for chemical upgrading or combustion because of their storability and low oxygen content [41]. On the other hand, char shows a calorific value of 15–30 MJ kg⁻¹ and lower moisture and volatiles content, which make it applicable as a solid fuel after proper upgrading, or as activated carbon for environmental or agricultural uses [42,43]. The gas fraction is typically composed of CO, H₂, CH₄, and CO₂, with a calorific value of 4–15 MJ Nm⁻³ [44]. It is characterized by low energy density and good flammability, allowing its application in gas turbines, internal combustion engines, or fuel cells.

Additionally, recent studies on the thermochemical conversion of polymeric materials emphasized that reaction efficiency is governed by vapor accessibility to internal acid sites in microporous catalysts [45]. Since complex feedstocks generate bulky intermediates that limit diffusion and promote coke formation, hierarchical catalysts with combined meso- and microporosity have been developed to enhance transport and selective aromatization. Core–shell systems, such as ZSM-5/SBA-15, enable effective pre-cracking, improved intermediate enrichment, and reduced formation of heavy byproducts. These insights highlight how feedstock composition, reaction environment, and catalytic structure strongly influence product selectivity and process efficiency, which are relevant factors in the application of thermochemical routes to complex organic wastes.

Among thermal processes, pyrolysis produces (i) biochar, which is primarily used as soil amendment due to its carbon sequestration capacity and, secondarily, is employed for energy generation, (ii) bio-oil, which serves as a renewable energy source for fuel and chemical precursors, and (iii) gases, which can be directly utilized for energy recovery [46,47,48]. Gasification converts biomass into syngas, i.e., a mixture of CO and H₂ that is applied in power generation or as feedstock in chemical synthesis, reducing waste volume and pollutant emissions [46,47,48,49]. On the other hand, hydrothermal treatments are particularly suitable for wet biomasses, as they can directly process high-moisture feedstocks without energy-intensive drying, that is required in pyrolysis and gasification, thus producing hydrochar and biofuels’ precursors more efficiently [48,49,50,51,52,53]. Heating water to 250 °C under subcritical conditions requires about 1 MJ kg⁻¹, corresponding to 6–8% of dry biomass energy content, whereas latent heat vaporization needs 2.26 MJ kg⁻¹, and loss is avoided since water remains in a liquid or supercritical state [50]. This efficiency reduces energy losses, enhances overall energy yield, and improves the economic feasibility of biomass conversion while using water that is a non-toxic and inexpensive reaction medium. In contrast, pyrolysis and gasification require biomass drying and higher temperatures, leading to greater energy demand [48,54]. Nonetheless, these thermochemical routes can be higher energy-density fuels’ precursors when dry biomass is available, highlighting a trade-off between feedstock moisture content and conversion efficiency.

While these thermochemical methods represent a promising route for sustainable waste management, their large-scale implementation still faces challenges due to their optimization process and economic viability; reasonably, further research and development are needed. The following section will investigate the application of thermal conversion processes in the recycling of LSW.

2.1. Pyrolysis Processes

Pyrolysis offers a promising route for waste valorization, making it possible not only to recover energy but also to obtain valuable chemicals [8,40,55]. Through this process, biomass can be transformed into fractions with a high energy density and calorific value, such as char or fuel precursors, as well as into a variety of value-added compounds. When compared to landfilling, pyrolysis also produces less greenhouse gas emissions [56]. Pyrolysis occurs in inert atmospheres and promotes the formation of three main products: (i) a liquid (bio-oil) with a high density, (ii) a carbonaceous solid fraction (bio- or pyrochar), and (iii) a gas phase [56,57]. The yield and the composition of these fractions depend not only on the operational parameters, i.e., temperature, heating rate, and residence time [58,59,60] but also on the feedstock nature and features, pre-treatment methods, and reactor configurations [56].

Pyrolysis can be divided into three stages: (i) dehydration (30–144 °C), (ii) devolatilization (144–500 °C), where compounds such as CO₂, NH₃, polypeptides, carboxylic acids, and aromatic hydrocarbons are released, and (iii) carbonization (>500 °C), which leads to char formation [58]. Low temperatures (300–450 °C) and high residence times favor biochar production, reaching yield values between 35 and 50% [56,61,62]. Mild temperatures (400–550 °C) and short residence times maximize bio-oil yield, which peaks at up to 60–75% [56,61,62,63,64,65]. In contrast, temperatures >600 °C and higher residence times promote the secondary cracking reactions of volatile compounds, which mainly enhance gases’ production, up to 25–40%, at the expense of bio-oil formation [56,61,62].

The heating rate is a key parameter that mainly affects both the product yields and their nature. Fast pyrolysis, being characterized by rapid heating rates (10–200 °C s⁻¹) and shorter residence times, favors the formation of bio-oil, while limiting secondary reactions and char production [61,63]. Conversely, slow pyrolysis, which is determined by lower heating rates (0.1–1 °C s⁻¹), promotes char formation basically due to the polymerization reactions of primary pyrolysis products [64,65]. In the case of flash pyrolysis, where heating rates are up to 1000 °C s⁻¹, the formation of volatile products is maximized, while pyrochar is minimized. In addition, the size of feedstock particles plays a crucial role; in fact, smaller particles promote heat transfer and bio-oil synthesis, while larger particles enhance char, gas, and phenolic compounds’ formation [66].

The chemical composition of the pyrolysis fractions influences their applicability. For instance, bio-oil is a complex mixture of aliphatic and aromatic compounds that can be employed as renewable fuels’ precursors or as feedstock for chemicals’ extraction if the formation of a specific class of compounds is maximized [64]. On the other hand, the gas phase, which is rich in CO, H₂, CH₄, light hydrocarbons, and CO₂, is suitable for direct energy generation or to energetically sustain the pyrolysis process itself [67]. Pyrochar, the solid fraction, is a porous carbon-rich material that might be applied in different fields, including soil amendment [68], pollutant and dyes adsorption [69,70,71], conductive material in energy storage devices [72,73,74,75], and combustion [76].

The main advantages of pyrolysis are its flexibility, the possibility to easily control products’ distribution, and their wide applicability as fuels’ precursors, chemicals, or functional materials. On the other hand, the main drawbacks are the need for precision in process control, the instability of bio-oil, and the relatively high operating costs compared to traditional recycling methods.

2.2. Gasification Processes

The main objective of gasification is to convert carbonaceous materials into gaseous products for direct combustion, particularly syngas, i.e., CO, H₂, CO₂, CH₄, and light hydrocarbons [66]. Unlike pyrolysis, which can yield char, chemicals, or liquid fuels, gasification is primarily aimed at energy production. This thermochemical process promotes the feedstock’s partial oxidation by supplying less oxygen than that stoichiometrically required to complete combustion [66,77,78,79]. For this reason, it can be considered a process between combustion, which is a thermal degradation with excess of oxygen, and pyrolysis, which is a thermal degradation in the absence of oxygen; additionally, gasification normally proceeds this at temperatures ranging from 750 °C and 1500 °C [80]. During gasification, carbonaceous material is converted into gaseous fuel in the presence of a gasifying medium, i.e., air, steam, oxygen, or a mixture of these three, that is pivotal to the final high heating value (HHV) of obtained gases [12,77,81,82]. Air is the most common gasifying medium because of its easy availability and low cost, but gaseous products have a lower calorific value (4–6 MJ m⁻³) because of dilution and the presence of nitrogen [83]. On the one hand, oxygen in gasification produces cleaner, high-energy syngas with less tar and no nitrogen dilution, but it requires costly oxygen separation. On the other hand, steam produces H₂-rich gases with a higher H/C ratio and a HHV around 10–18 MJ m⁻³ [82,83] because steam promotes reactions like the water–gas and water–gas shift reactions, generating additional H₂ [83]. Unlike air, steam does not dilute, resulting in a higher energy content, making steam a more effective gasifying agent for producing high-quality syngas. In both cases, where oxygen and steam are gasifying agents, because of the high HHV, the obtained gases are then normally used as fuel to produce electricity or heat [78].

Important operational parameters include temperature and pressure, which determine gas quality, tar formation, reactor requirement, and capital costs, as well as bed material and retention time, which influence the overall efficiency [83]. The size and type of the gasifier affect the process and depend on the products, moisture content, and fuel availability; gasifiers are classified as fixed-bed and moving-bed gasifiers (up-draft, down-draft, and cross-draft gasifiers), fluidized-bed gasifiers (bubbling and circulating bed gasifier), entrained flow gasifiers, dual fluidized-bed gasifiers, and supercritical water gasifiers.

Gasification and pyrolysis are both thermochemical processes that convert biomasses into valuable products, but they differ significantly in their ability to handle heterogeneous feedstocks [83,84,85]. Gasification demonstrates greater flexibility, as it can process a wide range of materials [84,85], as well as producing lower amounts of tar, facilitating downstream processing [84]. In contrast, pyrolysis often encounters difficulties with variations in feedstock composition, that can lead to non-uniform thermal decomposition, complicating heating and conversion processes; despite that, pyrolysis remains attractive when liquid products are preferred [86].

In conclusion, gasification offers several advantages, including the ability to process a wide variety of heterogeneous feedstocks, the production of syngas with relatively low tar content, and the possibility of tailoring gas quality through the gasifying medium. However, it also presents some disadvantages, such as the lower calorific value of air-blown syngas, the high costs of O₂ separation, and the need for high levels of control in reactor design and operational conditions.

2.3. Hydrothermal Processes

Hydrothermal treatment (HT) stands out as a promising thermochemical technology as it is able to convert biomasses in water medium into three fractions, i.e., hydrochar (solid), biocrude (liquid phase), and gas, without any preliminary drying pre-treatment, in contrast with other thermochemical processes, such as combustion, gasification, and pyrolysis [79,87]. Consequently, HT is used for wet and high-moisture feedstocks as the water present in the starting materials acts as reactant [27]. In fact, HT exploits the changes in water properties near and above its critical point (T_C = 374 °C and P_C = 22 MPa), where the dielectric constant decreases by 80% (from ~80 at 25 °C to ~2 at 450 °C), causing water to behave as a non-polar solvent and to take part in the reaction; simultaneously, the ionic product is still high enough (above 10–14) to favor ionic reactions, which promote biocrude formation, rather than the solid phase that is favored by radical reactions [88,89]. Moreover, HT decreases the oxygen content of biocrude and increases its energy density, showing great feedstock flexibility, particularly towards complex, mixed, or contaminated waste, which would be otherwise incinerated [39,89]. The nature and quantity of the final products depend strictly on the substrate’s features.

In general, the yield distribution, hydrochar functionalization, and reaction mechanisms (e.g., hydrolysis, dehydration, decarboxylation, condensation, etc.) change according to the substrate features and operating conditions; in particular, temperature strongly controls the final products and energy efficiency [39,90,91]. In fact, depending on temperature, three methods can be distinguished: (i) hydrothermal carbonization (HTC), from 180 to 250 °C, whose main product is hydrochar; (ii) hydrothermal liquefaction (HTL), from 250 to 375 °C, producing water insoluble organics, i.e., biocrude, aqueous phase, and light gases (CO₂, CH₄, CO, and H₂); and (iii) hydrothermal gasification (HTG) or supercritical water gasification (SCWG), which produces syngas rich in renewable H₂ or CH₄ [91,92]. Hydrochar (solid phase) can be used for multiple applications, e.g., as adsorption medium or soil amendment, but the most convenient and profitable employment is as solid fuel coal [91]. In fact, when compared to the initial biomass feedstock, hydrochar presents improved properties in terms of fuel ratio and aromatic structure, which make its combustion performance aligned with characteristics of conventional solid fuels like coal. On the other hand, the obtained liquid and gas phases are suitable for subsequent valorization, upgrading, or combustion [39]. In addition, the nitrogen and sulfur initially contained in the biomass are converted into gases, primarily CO₂ and trace gases, and biocrude, i.e., the liquid phase mainly composed of organic acids, phenolic compounds, fatty acids, and nitrogenous structures [91]. In the case of LSW, the use of water in subcritical conditions, i.e., below its critical point, has not yet been extensively explored as a thermochemical recycling method and literature remains limited. Nevertheless, this approach could represent a promising thermochemical treatment, especially for substrates such as tannery fleshing or sludge, which typically contain up to 70% moisture [79].

To sum up, HT effectively converts wet biomasses into hydrochar, biocrude, and gas without prior drying. Its main advantages include high feedstock flexibility, the improved fuel properties of hydrochar, and the possibility to valorize both liquid and gaseous products. However, HT is strongly dependent on feedstock type and operating conditions. Additionally, high temperatures and pressures, as well as purification steps, due to the presence of nitrogen- and sulfur-containing compounds, might increase the overall process costs.

3. Biochemical Processes: Anaerobic Digestion

Biochemical methods use microorganisms to break down biomasses and produce valuable biomolecules, such as biomethane [66]. Anaerobic digestion (AD) is the most representative biochemical method for the valorization of LSW.

AD is a process that exploits the bioenergy potency of organic wastes (e.g., swine manure, municipal solid, or lignocellulosic waste) since its output is biogas, which is a biofuel consisting of 52–95% CH₄, 10–50% CO₂, 0.1–4% N₂, 0.01–2% H₂, 0.001–2% H₂S, 0.02–6.5% O₂, and some other gases [78,79]. Besides biogas production, a resultant sludge or digestate is obtained and, due to its high concentrations of nutrients, it is generally employed as fertilizer [22,79]. Anaerobic bacteria promote the decomposition of organic wastes in the absence of oxygen (anaerobic conditions) through a series of steps, i.e., hydrolysis, acidogenesis, acetogenesis, and methanogenesis [79]. Hydrolysis is the initial step where complex molecules, such as proteins, lipids, and carbohydrates, are reduced into smaller compounds, respectively, amino acids, fatty acids, and sugars, that are easier for microbial enzymes to access [27]. During the second step, i.e., acidogenesis, acidogenic bacteria oxidize these smaller compounds into volatile fatty acids (e.g., acetate, propionate, ethanol, methanol, butyrate, etc.), but also into CO₂, alcohols, and H₂ [79,93]. Consequently, acetogenesis is the third step where fatty acids are converted into CO₂, H₂, and acetic acid; those molecules are then employed by methanogens to produce CH₄ during the last step, methanogenesis [79]. The H₂ that is produced in this stage is crucial for digester maintenance, and different factors, such as feedstock features, temperature, organic loading rate, pH, hydraulic retention time, C/N ratio, moisture, and heat content, are critical to biogas production [66,79]. Digestion temperature is particularly pivotal: most studies use mesophilic bacteria that grow in the range of 30–42 °C, but thermophilic bacteria can also be applied at higher temperatures, ~55 °C [66,78,94]. In general, it can be said that microorganisms, particularly mesophilic bacteria, show stability and adaptability to shifting environmental conditions [66,94].

The application of bio-based energy, such as biogas, has proven its suitability in terms of power generation along with its significantly reduced environmental pollution [95]. This biogas can be used to generate electricity, reducing the need for external energy, dependence on fossil fuels, and greenhouse emissions [94].

In conclusion, AD represents a promising strategy for the valorization of organic wastes, as it allows energy recovery, with biogas production, and nutrient recycling, through the digestate. Its main advantage lies in the use of renewable feedstocks, reducing environmental impacts. However, the process is sensitive to fluctuations in feedstock composition and operating parameters, which can compromise stability and efficiency, requiring careful monitoring and control. Furthermore, the presence of chromium can exert various effects on AD microbial activity and performance; these aspects are examined in detail in the following sections.

4. Treatments Based on Waste Types

4.1. Untanned Leather

Untanned solid leather waste (USLW) primarily consists of tissues, flesh, and fat, with typical contents of proteins (5–7%), sulfides (2–4%), lime (2–6%), and fats (4–18%) [12]. USLW is the richest LSW in proteins and fats (10.5%) and contains approximately 60% of moisture. The higher HHV of untreated leather is around 24.4 MJ kg⁻¹. Thermogravimetric (TG/DTG) analysis reveals a combustion profile consisting of three main stages; (i) below 130 °C, the evaporation of moisture happens, (ii) between 130 and 480 °C, the volatile matter (i.e., proteins, lipids, and carbohydrates) goes under devolatilization and combustion, and (iii) above 480 °C, there is char combustion [90]. The presence of lime from the liming step causes an overlap of stages, resulting in a distinctive peak at 700–780 °C.

Pyrolysis is best suited for low-moisture wastes (<15%) with highly volatile content because it enables the conversion of the organic fractions into liquid and gaseous fuels’ precursors [12]. Although USLW is typically valorized via hydrolysis or fat recovery, its pyrolysis potential is notable due to its protein and fat content. Pyrolysis products include compounds like (i) phenols (7.42%), e.g., 4-methyl-phenol, formed by the decomposition of aromatic amino acids, and (ii) aromatic or aliphatic nitrogen- and oxygen-rich heterocyclic compounds (21.26%), generated via amino acid cleavage, dimerization, and cyclization reactions [96].

Experimental studies confirm these findings. Almeida et al. investigated the pyrolysis of USLW scraps in a vertical semi-batch reactor (9.7 cm diameter) under N₂ between 490 and 800 °C [97]. Increasing temperature reduced char yield (30.1% to 21.4%) and increased gas yield (52.0% to 66.4%), confirming typical pyrolysis trends. Liquid yields remained stable (~18%) up to 610 °C, then declined due to secondary cracking (14.8% at 700 °C and 12.2% at 800 °C). The heating rate (15 °C min⁻¹) had a minimal influence on product distribution, confirming temperature as the dominant variable. Char showed decreasing volatile matter and fixed carbon, but increased ash and Cr₂O₃ content (from 2.1% to 7.8%), with a final HHV around 18 MJ kg⁻¹. Chromium stabilization above 600 °C reduced leaching risks, making char suitable for combustion. The biphasic liquid contained phenols, alcohols, alkanes, ketones, acids, and aromatics. FTIR analysis (Fourier Transform Infrared spectroscopy) indicated a decline in O-H band intensity along with temperature, suggesting the increased degradation of polar compounds. In the gas phase, CO₂ dominated at 490 °C, while H₂ became prevalent at 610–800 °C; CO increased with temperature, and CH₄ remained the least abundant. Gas HHV rose from 0.34 to 1.08 MJ m⁻³, reflecting the energy value of cracked vapors. Complementary findings were reported by Amdouni et al., who studied the pyrolysis of pre-dried fleshing LSW in a fixed-bed reactor at 500–700 °C (15 °C min⁻¹, 1 h) [98]. At 600 °C, they achieved a 60% bio-oil yield (39.36 MJ kg⁻¹), due to the decomposition of fats, proteins, and secondary char breakdown. It was found that inorganic elements, e.g., Ca and Na, can act as inert catalysts, enhancing oil formation. Higher temperatures and heating rates favored gas formation via secondary cracking. Gases accounted for 35.9% of products, including CH₄ (5.25%) and C_nH_m hydrocarbons (8.26%), with a HHV of 10 MJ m⁻³, which would make them suitable as combustion gases for thermal processes like leather drying.

Gasification performance is significantly affected by the temperature, gasifying agent, and moisture content. Ongen et al. demonstrated improved gas yield and syngas quality at >700 °C, with optimal values between 850 and 900 °C [99]. At temperatures ≥500 °C, ANOVA analysis registered a significant increase in CO, H₂, CH₄, and CO₂ content, and it was seen that the highest syngas HHV (12.55 MJ m⁻³) and CO content was reached under pure oxygen (99%) at 0.04 L min⁻¹, while dried air (0.1–0.2 L min⁻¹) produced a syngas with a lower HHV (8.37 MJ m⁻³). High oxidant flow reduced H₂, CO, and CH₄, lowering HHV. Moderate flow rates (0.1–0.2 L min⁻¹) improved syngas composition. Moisture, despite its energy demand, enhanced CO and H₂ through steam–char and water–gas shift reactions. Biomass loading had a limited effect, though further study is needed. Overall, gasification with high temperatures and a controlled oxygen flow generated higher quality syngas than pyrolysis. Mohamadi-Baghmolaei et al. investigated H₂ production via the SCWG of raw leather waste using a thermodynamic model to predict gas yields and char formation [100]. By introducing a catalytic correction through the adjustment of the Gibbs free energy of char, the model accounted for catalytic effects and predicted a hydrogen yield of up to 89.3%. Temperature (300–750 °C) had a major impact on gas composition and conversion: higher values promoted H₂ through steam reforming and water–gas shift reactions, increasing CO₂ while reducing CH₄ and CO. At the same time, char formation decreased, improving carbon gasification efficiency and minimizing clogging. Increasing biomass concentration lowered the gas yield and enhanced char production due to limited water availability, negatively affecting H₂ and CO₂, while CO remained low because of secondary consumption. Pressure had little influence, whereas supercritical water proved essential as both solvent and reactant. Overall, high temperatures, combined with diluted feeds, optimized H₂-rich syngas generation, supporting SCWG as a promising valorization pathway.

Although hydrothermal carbonization (HTC) of tannery waste is less explored, Lee et al. successfully applied it to fleshing waste under mild conditions (180–200 °C for 30 min), yielding >82.9 wt% hydrochar with improved energy properties [90]. The hydrochar’s HHV (24.3–27.3 MJ kg⁻¹) exceeded that of lignite and sub-bituminous coal. Combustion tests showed enhanced fuel ratios, thermal stability, and suitability for co-firing. Chemically, the hydrochar underwent dehydration, decarboxylation, and de-methanation, increasing its carbon content while reducing its oxygen and hydrogen levels (Van Krevelen analysis). The nitrogen content dropped from 2.2% to 1.1–1.4%, and sulfur remained low (~0.1–0.3%), minimizing NO_X and SO_X formation during combustion. At temperatures above 220 °C, the carbon loss increased due to gas formation and lipid solubilization. Thus, moderate HTC conditions support energy recovery and offer a cleaner, sustainable, solid fuel pathway.

Taken together, these studies highlight that the thermal valorization of USLW is highly temperature dependent. Lower pyrolysis temperatures (490–600 °C) yield more char with lower HHV, while higher temperatures (>700 °C) reduce char yield and enhance gas production [97,98]. At 600–700 °C, CH₄ and C_nH_m formation is promoted [98], while temperatures above 700 °C favor H₂ and CO [100]. Pyrolysis between 600 and 700 °C maximizes bio-oil [97,98], while at 180–200 °C, HTC mainly produces hydrochar [90]. Under controlled oxygen conditions and high temperatures, gasification enhances syngas quality [77], whereas SCWG at high temperatures and diluted feeds yields hydrogen-rich syngas [100]. These findings confirm the potential of USLW as a valuable feedstock for sustainable energy production. In

Table 1. Optimal conditions and main products from thermochemical treatment of USLW.

References	Technology	Operational Conditions	Products	HHV	Advantages
Almeida et al., 2017 [97]	Pyrolysis	490–800 °C, 15 °C min−1, N2, semi-batch reactor	21.4–30.1% char 12.2–18.0% liquid 52.0–66.4% gas	Char: 18 MJ kg−1 Gas: 1.08 MJ m−3	Cr stabilized >600 °C
Amdouni et al., 2021 [98]	Pyrolysis	500–700 °C, 15 °C min−1, 1 h, fixed-bed reactor	60.0% bio-oil 35.9% gas	Gas: 10 MJ m−3	Ca and Na as inert catalysts
Ongen & Arayıcı, 2014 [99]	Gasification	850–900 °C, O2, air, moisture	CO, H2, CH4, CO2	Gas: 12.6 MJ m−3	High syngas quality
Mohamadi-Baghmolaei et al., 2022 [100]	SWG	300–750 °C	89.3% H2-rich syngas	-	Catalyst reduces char
Lee et al., 2019 [90]	HTC	180–200 °C, 30 min	82.9% char	Char: 27.3 MJ kg−1	Low N and S

a resume of the above-mentioned experiments on USLW is reported.

4.2. Tanned Leather

Pyrolysis is a promising thermochemical method for tanned leather (TLSW), particularly because it occurs in absence of oxygen and generates no CO₂ emissions, being limited to the CO₂ inherently derived from the feedstock itself (i.e., intrinsic CO₂) [57], but also because it prevents the oxidation of Cr (III) into its carcinogenic hexavalent form (Cr⁶⁺) [101,102]. Additionally, pyrolysis promotes the formation of chromium carbides rather than oxides [103].

The distribution of pyrolysis products is highly dependent on temperature [104]. Several studies concluded that the highest yield of gas is reached between 650 and 750 °C, due to the thermal decomposition of organic compounds. During pyrolysis, chromium might act as a catalyst by accelerating the mass loss between 300 °C and 400 °C, causing the formation of volatile compounds at higher temperatures. To maximize char production, lower temperatures (450–600 °C) should be used, as they minimize volatile release.

Tanning agent and waste type significantly impact pyrolysis product distribution and their chemical composition. Tanning agents, particularly chromium, account for 4.63% of the ash content, which represents 6.03% of the tanned leather [67]; for these reasons, the operating conditions need to be optimized depending on the waste type [12,57]. Early studies by Yilmaz et al. demonstrated that different TLSW, e.g., buffing dust, chromium-tanned, and vegetable-tanned shavings, exhibited distinct pyrolysis behaviors and yields when processed in a fixed-bed reactor at 450–600 °C at 5 °C min⁻¹ for 2 h [60]. While chromium-tanned (CLSW) and buffing dust leather yielded more gas at higher temperatures, vegetable-tanned leather (VLSW) maintained a stable gas fraction. The solid fraction, promoted at lower temperatures, with a high calorific value (18–25 MJ kg⁻¹), was suitable as a fuel, while bio-oil, rich in asphaltene and unspecified polar compounds, showed potential for chemical applications or upgrading via steam cracking, hydrogenation, or Fischer–Tropsch synthesis. CLSW and buffing dust leather registered a higher content of (NH₄)₂CO₃, whose formation occurred between NH₃, from polypeptide degradation, and CO₂, from decarboxylation reaction. In chromium-tanned wastes, decarboxylation is facilitated because of chromium, allowing NH₃ and CO₂ to react simultaneously. At 600 °C, (NH₄)₂CO₃ formation increases in CLSW but decreases in buffing dust due to particle size, which affects the reactivity and pyrolytic product formation.

Regarding the thermal behavior of different tanning agents, Marcilla et al. compared the decomposition temperatures and volatile compounds of inorganic and organic tanned leathers [105], while Sebastyén et al. examined the differences between hydrolysable and condensed tannins in vegetable tanning [15]. Rosu et al. observed that wet blue (i.e., chromium-tanned) leather exhibited thermal behavior similar to wet white (i.e., tanned with titanium and aluminum), both undergoing three main stages: (i) moisture loss, (ii) pyrolytic collagen degradation, and (iii) residue oxidation [8]. Nevertheless, wet blue leather showed slightly higher thermal stability, with collagen decomposition peaks at 322 °C and 425 °C, compared to 313 °C and 421 °C for wet white leather. These shifts are attributed to chromium-induced cross-linking, which enhances stability and increases the activation energy of wet white leather.

More recently, regarding the influence of tanning agents on the final pyrolysis products, González et al. compared vegetable- and chromium-tanned cowhides using microwave-assisted pyrolysis at 600–1000 W for 15–30 min [59]. Microwave (MW) heating is considered one of the most promising technologies for pyrolysis, as irradiation at 2.45 GHz enables the rapid and direct heating of MW-absorbing materials [106,107]. In fact, unlike conventional systems based on thermal conductivity and convection, MW heating penetrates the material, enhancing internal heat generation, which improves heat transfer efficiency, accelerates chemical reactions, and reduces both residence times and energy consumption [107]. Under these conditions, González et al. observed that the liquid fraction from VLSW primarily contained pyridine, pyrrole, aniline, phenol, and cresol, while CLSW yielded pyridine, pyrrole, 3-methylpyrrol, and succinimide [59]. The solid phase of both types had heating values comparable to solid fuels (20–28 MJ kg⁻¹) and after proper upgrading, the liquid products could be used either as chemicals’ precursors, due to their high nitrogen content, or as fuels’. More recently, analyzing the thermal properties of vegetable (mimosa), metal salt (chromium), and mixed (chromium–mimosa and aluminum–myrobalan) tanned bovine leathers, Czirok et al. demonstrated how pyrolyzates were dependent on the tanning agent and how tannins decreased the protein content, which were 73.62% and 86.11% for mimosa and CLSW, respectively [108].

Chromium is a tanning agent whose influence is particularly strong, due to its potential oxidation to Cr⁶⁺ when exposed to high pH values or temperatures, UV light, or unsuitable storage conditions [15]. However, pyrolysis retains the majority of heavy metals in the solid fraction, allowing their eventual recovery [27,104]. Oliveira et al. confirmed that in pyrochar, chromium is present in its trivalent, non-carcinogenic form and does not leach when in water [69,70]. Multiple studies have confirmed the absence of Cr⁶⁺ in both solid and liquid pyrolysis fractions: Marcilla et al. applied XPS (X-ray Photoelectron Spectroscopy) to Cr⁶⁺ detection in both the char and bio-oil [67], as well as Sethuraman et al., who showed that Cr⁶⁺ was below detection limits in the residual ashes in biochar [55,109]; Wells et al. found that Cr was stabilized as chromium carbide via XANES analysis (X-ray Absorption Near Edge Structure) via ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy) [103]; Guan et al. detected no Cr⁶⁺ on the leachates obtained by liquid and solid phases [101]; González et al. found that Cr⁶⁺ is generally present in negligible amounts (0–68 ppm) in the liquid and solid fraction [59]; Grycová et al. confirmed the absence of Cr⁶⁺ from carbonized products via XRD (X-ray Diffraction) [75].

Alternative thermochemical processes, such as gasification and hydrothermal treatment, also impact chromium retention. Gasification of trimmings, shavings, and footwear leather concentrates chromium in ash as Cr₂O₃, which can be used in ferrochromium alloy production [27]. Poletto et al. demonstrated that the resulting ash contained 50–60% Cr₂O₃, making it a viable precursor for sodium chromate (Na₂CrO₄) synthesis [110]. Similarly, hydrothermal treatment modifies chromium behavior: Yanik et al., while studying HTG at 500 °C on VLSW and CLSW, found that chromium decreased the gasification of protein, without altering the gas composition, as well as increasing the water-soluble organics in the liquid phase [111]. Compared to chromium-tanned waste, VLSW produced more gas (mainly H₂, CO₂, and CH₄) and coke, while chromium-tanned residues retained a significant amount of chromium. Additionally, they found that the reaction network of amino acids took two main paths, namely decarboxylation to produce carbonic acid and amines, and deamination to produce ammonia and organic acids, which are responsible for CO, CO₂, H₂, and water production. Catalysts, such as K₂CO₃, influence the balance between deamination and decarboxylation reactions by altering the reaction conditions, primarily through pH modification, since alkaline salts raise the pH; they also lead to increased yields of H₂ and CO₂ and a reduction in CO, thereby influencing the final composition of gaseous products derived from the degradation of organic acids.

Regarding AD, the experimental results show that chromium can have different effects on the final yields, both beneficial and adverse. For example, Agustini et al. studied the influence of chromium and vegetable tanning agents in the co-digestion of leather shavings mixed with sludge and concluded that sludge and shavings (i.e., hide, CLSW, and VLSW) provided a methane potential of 4.1–11.3 mL_biogas g_VS⁻¹ (Volatile Solids) [112]. In fact, chromium is beneficial to AD and produced about three times more biogas (around 27.9–16.4 mL_biogas g_VS⁻¹), with 55% of CH₄ on average, while vegetable tannins (590 mg L⁻¹) had an inhibitory effect on waste mineralization as well as methane production. Zupančič et al. investigated the employment of thermophilic 100-day AD with inoculum from wastewater sludge on different types of tannery waste and saw that the potential methane production at 55 °C was 0.377 L_biogas kg_VS⁻¹ for tannery trimmings and 0.649 L_biogas kg_VS⁻¹ for tannery fleshings [113]. The chromium content and salinity showed no adverse effects on the process, while a temperature fluctuation of 4.4 °C led to a drop in biogas production of 25%, indicating the need to keep temperature constant at 55 °C. On the other hand, chromium could be isolated via alkaline hydrolysis with lime and KOH and resulting collagen hydrolysates could be then digestated [114]. To avoid Cr³⁺ oxidation, some bacteria or filamentous fungi, e.g., Bacillus cereus or Pseudomonas putida, can help in the remediation of Cr⁶⁺ via chromium (VI)-reductase [115], and some other microorganisms can adapt to substrates containing chromium and tolerate increasing concentrations [22].

For the above-mentioned reasons, pyrolysis studies will be reported differentiating between VLSW and CLSW.

4.2.1. Chromium-Tanned Leather

As previously mentioned in paragraph 3.2, thermochemical treatments such as pyrolysis and gasification are promising strategies for managing chromium leather solid waste (CLSW), as they allow both energy recovery and partial chromium confinement in the solid fraction. As shown in various studies (e.g., 27, 59), the chemical composition and distribution of pyrolysis fractions is important for assessing the suitability of the process in relation to the expected products.

Concerning pyrolysis products, bio-oil is the fraction, whose composition reflects the nature of the processed LSW the most. In the case of CLSW, bio-oil is typically composed of aromatic compounds, such as phenols, and aliphatic compounds, e.g., aldehydes [8,64,116]. Due to the proteinaceous nature of LSW, mainly composed of collagen, but also because of the presence of tanning chemicals (e.g., amines, finishing agents, etc.), nitrogenated and oxygenated molecules are present in significant amounts [108]. In particular, diketopiperazines, which are nitrogen-rich cyclic dimers that derive from peptide bond degradation, are the most abundant compounds in bio-oil and are key indicators of protein decomposition, affecting the potential applications of the liquid.

To design efficient thermochemical processes, it is essential to understand the thermal degradation of CLSW. Thermogravimetric analysis of CLSW shows that the major weight loss occurs between 80 and 500 °C, with a maximum decomposition rate around 325 °C (0.47%/°C), which is associated with the breakdown of organic matter and collagen [26]. This thermal behavior falls in the operative range generally adopted in pyrolysis studies, as seen in various experimental setups, e.g., Marcilla et al. [67] and Velusamy et al., 2020 [27]. Furthermore, chromium salts improve the thermal stability of the substrate by forming strong bonds with collagen fibers, thus influencing both decomposition kinetics and product distribution [10].

The transformation and stabilization of chromium during pyrolysis of CLSW are strongly governed by specific process parameters, particularly temperature, atmosphere, and heating rate. Pyrolysis temperatures below 600 °C prevent the oxidation of Cr³⁺ into Cr⁶⁺ [55,109], whereas higher temperatures favor carbothermal reduction and the formation of stable chromium carbides (e.g., Cr₃C₂) [103]. XPS analyses reported in the literature confirm that even after pyrolysis and controlled post-treatment up to 800 °C, Cr⁶⁺ remains undetected when oxygen-free conditions are maintained, and chromium persists in non-leachable, stabilized forms [55,103,109]. Chromium itself can catalyze pyrolytic decomposition, accelerating mass loss between 300 and 400 °C and facilitating the complete conversion of organic matter near 630 °C [59,69,70,104]. Atmosphere is therefore a critical factor: inert gases such as N₂ or Ar, as well as CO₂ streams used for activation, maintain a reducing environment that prevents Cr³⁺ oxidation [55,69,70,75,108,109]. Heating rates typically adopted in CLSW pyrolysis, ranging from 10 °C min⁻¹ for conventional slow pyrolysis to 50 °C min⁻¹ for enhanced bio-oil formation, also influence chromium behavior by affecting volatile evolution, secondary reactions, and the structure of CLSW [27,67,117]. Overall, maintaining oxygen-free conditions and sufficiently high temperatures allows pyrolysis to act as a chemical “vault”, promoting the stabilization of Cr³⁺ while suppressing Cr⁶⁺ formation [59,69,70,103].

Based on these thermochemical principles, Velusamy et al. performed pyrolysis of CLSW, specifically chrome shavings and finished leather trimmings [27]. Their study revealed that at 500 °C, pyrolysis produced three main fractions: bio-oil reached a yield of 49–52%, char 28–31%, and non-condensable gases accounted for 18–20%. The bio-oil presented a HHV of 28 MJ kg⁻¹ and had phenols, hydrocarbons, and nitrogenated compounds. The biochar retained most of the chromium content, minimizing its environmental dispersion and allowing its possible valorization.

In contrast, González et al. applied MW-assisted pyrolysis to CLSW and observed a different phase distribution, producing around 40–55% of char, 35–45% of bio-oil, and 6–15% of gas depending on the experimental conditions [59]. The yield of solid residue, with a HHV of 12–14 MJ kg⁻¹, was limited by the high ash and moisture content of CLSW, while the bio-oil showed notable potential for chemicals recovery because it is rich in aromatic and nitrogenated compounds, including benzonitrile, phenol derivatives, and long-chain hydrocarbons. In particular, the main chemical species were pyridine, 3-methylpyrrole, succinimide, and pyrrole, which reached up to 15–25%. MW-assisted pyrolysis produces nitrogen-rich bio-oils with high HHV (>20 MJ kg⁻¹), comparable to those from conventional pyrolysis, making them suitable as fuels’ precursors or nitrogen fertilizers, although their specific chemical compositions differ.

Further exploring the influence of chromium on pyrolysis outcomes, Czirok et al. reported that pyrochar yield increased with chromium content; in fact, samples with 5% Cr produced up to 8% more char than chromium-free ones [108]. This was attributed to residual chromium and the catalytic effect of chromium sulfate in promoting pyrochar formation. On the other hand, a higher chromium content led to an increase in the inorganic fraction within the char; the article does not explicitly address whether CLSW produced lower quality pyrochar, but the presence of chromium as part of the ashes suggests that the fixed carbon percentage may have been lower than in pyrochar with reduced inorganic content. Thermogravimetric and mass spectrometric analysis (TG/MS) revealed that higher chromium levels shift decomposition to higher temperatures, with a DTG shoulder around 450 °C indicating alternative degradation pathways. Experiments conducted under argon up to 900 °C at 20 °C min⁻¹ confirmed that chromium sulfate catalyzes the formation of nitrogen-containing volatiles such as hydrogen cyanide, acetonitrile, and propionitrile, while suppressing ammonia evolution. This behavior reflects a stabilizing effect on the collagen backbone and enhanced oxidative dehydrogenation reactions. Pyrolysis-GC/MS (Gas Chromatography-Mass Spectrometry) at 400 and 600 °C further confirmed the increased production of nitriles and hydrocarbons. While markers of protein degradation like pyrrole and diketopiperazines were present, their abundance declined at higher temperatures and chromium concentrations due to secondary fragmentation.

In light of the complex role of chromium compounds, Marcilla et al. investigated the effect of different heating conditions by comparing flash (450–550 °C for 30 min) and slow pyrolysis (up to 750 °C at 10 °C min⁻¹) [67]. They observed that flash pyrolysis promoted higher liquid (41.0–44.5%) and solid yields (31.0–38.5%), while slow pyrolysis resulted in a higher gas fraction (41.8%). The liquid phase contained substantial nitrogen (7.1–15.1%) and oxygen (1.4–3.7%) and included aromatic amines, nitriles, and phenols, indicating its potential for fuel or chemical production after upgrading. The solid fraction was enriched with fixed carbon and chromium, while the gas phase contained CO, CO₂, CH₄, and light hydrocarbons.

Several other studies corroborate the versatility of CLSW pyrolysis under varying reactor types and conditions. Sethuraman et al. pyrolyzed CLSW at 700 °C in a batch gasification reactor, obtaining 33.03% of renewable fuel gases, of which 6.20% were short-chain hydrocarbons (e.g., CH₄, C₂H₆, C₃H₈, C₄H₁₀, and C₅H₁₂) [55]. Simioni et al., using a fluidized-bed reactor (450–550 °C, 15–24 °C s⁻¹), reported that higher oil yields were achieved above 550 °C, with liquids rich in oxygenated (e.g., alcohols, phenols, and ketones) and non-oxygenated compounds (e.g., alkanes, alkenes, and aromatics) [117]. Similarly, Poletto et al. performed pyrolysis at 450 °C in a semi-continuous pilot screw reactor and obtained 26.8% liquid, mainly composed of nitrogenated compounds (75%), such as nitriles and piperazines, and phenols (10%), and a gaseous phase (34.7%) rich in H₂, CO, CH₄, and CO₂, which are suitable for combustion [110]. Regarding the testing of different conditions, Alagöz et al. tested the effect of CaO from waste marble, applied as catalyst in CLSW pyrolysis to enhance the concentration of H₂, CO₂, and acetylene, while improving the liquid product [118]. Upon adding CaO, the bio-oil yield reached up to 49% and a HHV of 31.2 MJ kg⁻¹, an increase compared to the previous 35.5% and 27.6 MJ kg⁻¹ without the catalyst.

Beyond liquid and gas valorization, biochar from CLSW has shown potential for metallurgical applications. Fihlo et al. applied prolonged pyrolysis (8.5 h at 380–440 °C), producing a char suitable as a mineral coal substitute in iron ore pelletization, with 76.47% chromium recovery and enhanced pellet strength [104]. Similarly, Hemati et al. confirmed that such chars effectively reduce iron oxides, making them viable as alternative reductants in ironmaking [119]. Ferreira et al. also produced char (38.5%) via pyrolysis at 450 °C, yielding hydrogen-rich syngas during steam gasification (22.05 MJ kg⁻¹) and chromium-rich ashes [82]. Their bio-oil (26.8%) mirrored previous findings, with nitrogenated, non-oxygenated, and oxygenated compounds.

The gasification of CLSW, especially for energy generation, is effective. Midilli et al. demonstrated the feasibility of buffing dust leather gasification in a down-draft reactor at 966–1050 °C, producing 29–33% combustible gases (e.g., H₂, CO, CO₂, C₂H₂, and C₂H₆) [120]. They identified the optimal gasification range as 486.39–584.36 Nm³ m⁻² h⁻¹, balancing conversion efficiency and gas output. More recently, Dudyński et al. tested CLSW gasification both in a lab-scale up-draft gasifier and an industrial 2.5 MW system processing 1000 kg h⁻¹ of CLSW at 850 °C (2 s residence time) [121]. The resulting syngas (4.1–6.5 MJ m⁻³) was used for steam generation. Importantly, the process produced low-Cr (VI) ashes containing up to 55% Cr₂O₃, which could be recovered for reuse in tanning. However, due to the high moisture content, co-feeding with biomass or refuse-derived fuels was necessary. The study demonstrated that gasification offers advantages over incineration, including safer residues and valuable gas production. In

Table 2. Optimal conditions and main products from thermochemical treatment of CLSW; arrows (↑) indicate the increasing yield or amount at the reported operating conditions.

References	Technology	Operational Conditions	Products	HHV	Advantages
Velusamy et al., 2020 [27]	Pyrolysis	500 °C	49–52% bio-oil 28–31% char 18–20% gas	Bio-oil: 28 MJ kg−1	Cr in char
González et al., 2022 [59]	Microwaveassisted pyrolysis	–	35–45% bio-oil 40–55% solids 6–15% gas	Char: 12–14 MJ kg−1	Benzonitriles and phenols in bio-oil
Czirok et al., 2023 [108]	TG, TG/MS, Py-GC/MS	Up to 900 °C, 20 °C min−1, under Ar	↑ char with ↑ Cr, nitrile formation above 400 °C	–	Cr catalyzes nitriles
Marcilla et al., 2012 [67]	Flash and slow pyrolysis	Flash: 450–550 °C, 30 min Slow: 750 °C at 10 °C min−1	Flash: ↑ liquid and solid Slow: ↑ gas	–	Phenols, nitriles, and aromatics in bio-oil
Sethuraman et al., 2014 [55]	Batch gasification	700 °C, batch reactor	33.03% gas 6.20% light hydrocarbons	–	Renewable gas stream
Simioni et al., 2014 [117]	Fluidized-bed pyrolysis	450–550 °C, 15–24 °C s−1	↑ bio-oil above 550 °C	–	Alcohols, ketones, and aromatics
Poletto et al., 2016 [110]	Semi-continuous pyrolysis	450 °C, screw reactor	26.8% bio-oil 34.7% gas	–	Bio-oil: 75% N-compounds and phenols
Alagöz et al., 2024 [118]	Catalyzed pyrolysis	500 °C, 50 °C L min−1, 20 min, CaO	↑ bio-oil with CaO (49%)	Bio-oil: 31.2 MJ kg−1	↑ H2, CO2, and acetylene
Filho et al., 2016 [104]	Pyrolysis	8.5 h at 380–440 °C	Biochar for iron ore pellets	–	76.47% Cr recovery
Ferreira et al., 2023 [82]	Pyrolysis + gasification	450 °C pyrolysis; steam gasification	26.8% bio-oil 38.5% char H2-rich syngas	Syngas: 22.05 MJ kg−1	Cr-rich ash and N/O bio-oil
Midilli, 2004 [120]	Down-draft gasification	966–1050 °C, 486–584 Nm3 m−2 h−1	29–33% combustible gases	–	Cr(VI)-free gas
Dudyński et al., 2021 [121]	Up-draft and industrial gasification	Lab-scale and 2.5 MW pilot, 850 °C, 2 s residence time	Syngas	4.1–6.5 MJ m−3	Cr2O3-rich ash

a resume of above-mentioned experiments on CSLW is reported.

4.2.2. Vegetable-Tanned Leather

VLSW decomposed in the range of 150–600 °C, with the maximum rate at 290–320 °C, because of the weak bonding between the tannins and the collagen protein, i.e., hydrogen bridge bonds and dipolar bonds [28,120]. Additionally, VLSW is richer in carbon, hydrogen, nitrogen, and sulfur and presents lower ash and water content as well as higher calorific value when compared to chromium-tanned ones [59]. VLSW has a calorific value around 16–17 MJ kg⁻¹.

Considering studies about pyrolysis, Sebastyén et al. investigated the thermal decomposition of leather tanned with hydrolysable and condensed tannins by pyrolysis-GC/MS (Py-GC/MS) at 400 °C and found out the main decomposition products were mono-, di-, and trihydroxybenzenes, while 1,3-dihydroxybenzene and 3,5-dihydroxytoluene were specific markers for condensed tannins [15]. Gil et al. pyrolyzed bovine skin hides vegetable-tanned with chestnut, mimosa, and quebracho at 750 °C for 1 h and 5 °C min⁻¹ to obtain biochar feasible as a solid fuel due to its high calorific value and relatively low ash content, while the oil was suitable as a liquid fuel or a chemical feedstock, because it was formed by phenols, which are typical of VLSW, nitrogen compounds (e.g., nitriles, 2,5-diketopiperazines, etc.), alkanes, alkenes, acids, and esters, derived from tannins and collagen decomposition [59,122]. The gaseous phase, rich in CO and CO₂, also contained CH₄ and H₂, with syngas yield increasing with temperature. While the heating rate generally affects pyrolysis kinetics, the thermogravimetric analysis of collagenic LSW over a range of 2–20 °C min^-1 showed only minor shifts in decomposition peaks toward higher temperatures, likely due to heat transfer limitations. In Gil et al.’s study, the heating rate had a limited impact on the apparent kinetic parameters and total weight loss, and a rate of 5 °C min^-1 was chosen for conventional furnace experiments. In 2022, Hu et al. investigated the products of leather tanned with mimosa (hydrolysable tannin) and tannic acid (condensed tannin) via Py-GC/MS at 500 °C and found that the main gaseous products were CO₂, H₂O, and NH₃, followed by smaller amounts of HNCO and pyrrole, while oil presented nitrogen-containing compounds (i.e., nitriles and diketopiperazines), hydrocarbons, and phenols; char could be applied in the bioenergy field [123]. Leather tanned with tannic acid had poorer thermal stability and produced more CO₂ and phenols; on the other hand, nitro-ketone compounds accounted for 65% of the mimosa-tanned liquid phase.

Concerning HTC, the only study currently available is by Debina et al., who investigated the conversion of VLSW into hydrochar, not for energy purposes but for environmental remediation, specifically the removal of organic and inorganic pollutants from aqueous media [124]. The HTC process was optimized using response surface methodology (RSM), analyzing the influence of carbonization temperature, residence time, and initial moisture content on the hydrochar yield and adsorptive performance. Among several experimental conditions, the highest hydrochar yield (85%) was achieved at 190 °C after 75 min. The authors observed that increasing temperature led to a decrease in yield due to the enhanced release of volatiles and dehydration reactions. Characterization of the activated hydrochar revealed a specific surface area of 849 m² g⁻¹ and the presence of oxygenated functional groups (e.g., –OH, –COOH, –C=O), as confirmed by FTIR analysis. SEM-EDX analyses (Scanning Electron Microscopy coupled with Energy Dispersive X-ray analysis) showed a heterogeneous surface morphology with developed porosity and a composition rich in carbon and calcium. These findings highlight the potential of HTC as a sustainable valorization route for tannery waste, yielding a highly porous carbonaceous adsorbent suitable for water purification applications. In

Table 3. Optimal conditions and main products from thermochemical treatment of VLSW.

References	Technology	Operational Conditions	Products	HHV	Advantages
Gil et al., 2012 [122]	Pyrolysis	750 °C, 5 °C min−1, 1 h	Bio-oil, biochar, and gas	Bio-oil and char with high HHV (16–17 MJ kg−1)	Oil: phenols, nitriles, 2,5-diketopiperazines, and alkanes Gas: CO, CO2, CH4, and H2
Sebestyén et al., 2018 [15]	Py-GC/MS	400 °C	Volatile compounds	–	Condensed tannins
Hu et al., 2022 [123]	Py-GC/MS	500 °C	Gas, oil, and char	–	Gas: CO2, H2O, NH3 Oil: nitriles, phenols, hydrocarbons
González et al., 2022 [59]	Microwave-assisted pyrolysis	–	Bio-oil and char	16–17 MJ kg−1	Lower ash and moisture
Debina et al., 2023 [124]	HTC	190 °C, 75 min	Hydrochar (85%)	–	Oxygenated groups, porous C/Ca structure in char

a resume of above-mentioned experiments on VLSW is reported.

4.3. Finished and Post-Consumer Leather

Finished and post-consumer leather wastes (FLSW) are probably the most complicated LSW to recycle because they have been subjected to all the necessary treatments (e.g., dying, fatting, softening, etc.) that use a variety of additives to impart the desired esthetic features. FLSW is, in fact, mostly impregnated with chromium, synthetic fat, oil, tanning agents, dye chemicals, etc., and it contains 80–85% organic substances, whose energy value is nominally 20 MJ kg⁻¹ (dry materials) [109].

During the pyrolysis of FLSW, the main weight loss (58.13%) occurs at 185 to 620 °C, which corresponds to protein decomposition and is particularly high (<50%) due to cross-link bonds between adjacent peptide chains, enhancing the chemical stability and shrinkage temperature [58]. Sethuraman et al. applied on FLSW (i) normal pyrolysis up to 600 °C and (ii) controlled oxygen supply gasification from 600 to 800 °C; they obtained fuel gas, suitable for direct thermal cooking/heating or electrical energy generation, and liquid fuel oil, that was miscible with 2% (v/v) naphtha, 66% (v/v) kerosene, 24% (v/v) diesel, or 9% (v/v) vacuum gas oil [109]. The study confirmed that the oxygen supply of 1.44 L g⁻¹ for 1 h increased the gases content (e.g., 30% of CO₂ and 10% of H₂) but slightly decreased the hydrocarbon gases (C_xH_y) from 6.43% to 5.74%, because of their partial oxidation with oxygen contained in leather.

In 2021, Van Rensburg et al. pyrolyzed at 450–650 °C for 10 min a post-consumer leather shoe waste and saw that lower temperatures increased the solid yield (42.1% at 409 °C), while higher temperatures promoted the liquid (52.6% at 550 °C) and gas phases (29.1% at 650 °C) [125]. Liquid characterization confirmed not only the presence of nitrogenated compounds, as previously described in other studies, but also compounds that could be applied as adhesives and lubricants after proper impurities’ removal, e.g., octadecanamide et oleamides. Additionally, the HHVs of bio-oil (33.6 MJ kg⁻¹) and char (25.6 MJ kg⁻¹) suggested their potential application for energy purposes.

Silva et al. studied the effect of the operating temperature, ranging from 444 to 875 °C, on fixed-bed reactor pyrolysis of leather trimmings from the footwear industry [126]. As the temperature rose, char (HHV 23.1 MJ kg⁻¹) decreased (from 36.2 to 28.1%), gases’ (i.e., CO, CO₂, CH₄, and H₂) yield increased (from 33.2 to 47.6%), and the liquid phase (HHV 15.1 MJ kg⁻¹) was at its highest at 444 °C (30.6%) and decreased at 875 °C (24.3%). Higher yields were registered as the particle size increased (10 mm side length) and the liquid phase contained water (up to 43%), alcohols, phenols, carboxylic acids, alkanes, alkenes, and aromatics. In

Table 4. Optimal conditions and main products from thermochemical treatment of FLSW.

References	Technology	Operational Conditions	Products	HHV	Advantages
Sethuraman et al., 2013 [109]	Pyrolysis and Gasification	Pyrolysis: 600 °C Gasification: 600–800 °C, O2 (1.44 g h−1)	Fuel gases, and fuel oil precursors	–	Gas: 30% CO2, 10% H2, hydrocarbons Liquid: miscible with naphtha, kerosene, and diesel
Van Rensburg et al., 2021 [125]	Pyrolysis	450–650 °C, 10 min	Char, gas, and liquid	Bio-oil: 33.6 MJ kg−1 Char: 25.6 MJ kg−1	Liquid: nitrogenated compounds and finishing additives
Silva et al., 2024 [126]	Pyrolysis (fixed-bed)	444–875 °C, vary particle size (up to 10 mm)	Char, gas, and liquid	Char: 23.1 MJ kg−1 Liquid: 15.1 MJ kg−1	Gas: CO, CO2, CH4, H2 Liquid: alcohols, phenols, alkenes, and aromatics

, a summary of the above-mentioned experiments on FSLW is reported.

4.4. Mixed Waste

The co-pyrolysis of different materials is usually capable of releasing free radical macromolecules, increasing the content of volatile matter, and promoting secondary reactions, thus improving the quality of pyrolysis products [127]. Zhang et al. performed the co-pyrolysis of tanning sludge, CLSW, and buffing dust, mixed in the ratio 2:3:5, and this increased the oil and gas yields by 15% while reducing the pyrolysis activation energy [127,128]. In Liu et al.’s work, where the co-pyrolysis of wheat straw with CLSW was performed, the proximate analysis of CLSW revealed a low content of fixed carbon and a high percentage of volatile matter, which enhanced the ignition efficiency and feedstock reactivity during pyrolysis, supporting the generation of condensable/non-condensable gases [129]. Additionally, CLSW exhibited a high ash content, potentially attributed to the presence of inorganic salts in its manufacturing. Hu et al. examined the co-pyrolysis of waste textiles and leather with the presence of Ca/Fe-rich sludge ash, that accelerated weight loss rates of 12.8% min⁻¹ and improved the product quality by reducing N- and high-O-content compounds, e.g., acids, by 25.37% [130].

Anaerobic co-digestion refers to the simultaneous AD of multiple organic wastes in one digester, where co-substrates present a synergistic effect that depends on their microbial communities; this approach should help overcome the challenges associated with feedstock limitations, such as digestibility, stability, or nutrient value [93,131]. Generally, the leather waste C/N ratio is generally low and alone it wouldn’t be a proper feedstock for biodegradation, so it is normally mixed with other substrates that could act as bacterial inoculum and promote the biogas formation [12,132]. For instance, Sri Bala Kameswari et al. carried out co-digestion using fleshings and a mixture of primary and secondary tannery sludge with the addition of steapsin, a lipase that would enhance the AD hydrolysis; the experimental results revealed the optimum lipase dosage of about 0.75 g for a total VS input of 7.5 g of dry substrate [133]. In fact, lipases increased the biogas yield by about 15% and cut the retention time by about 30%, i.e., from 42 to 29 days. Further studies without lipase identified the feedstock ratio 1.00:2.70:0.30, which guarantees a biogas production of 385 mL g_VS⁻¹ in 45 days retention time; additionally, secondary metabolites, i.e., alcohols, amino acids, methane, and ammonia, were detected and the C/N ratio of digestate was around 8.0 [134]. The following experiments in semi-continuous mode increased the biogas yield up to 470 mL_biogas g_VS⁻¹ with a VS load of 68 g while decreasing the volatile fatty acids. The same feedstock was used by Islam et al., who performed AD on fleshing waste and domestic sewage (ratio 1:1) at 75% and cow dung at 25%; they reached 52% of VS destruction, 476 L g_VS⁻¹ of gas production, and 73% of methane yield [13]. They stated that fleshing waste could be used as a complementary substrate in small-scale biogas plants. Polizzi et al. used the IWA-ADM1-based mathematical model to study the feasibility of the AD of tannery primary sludge and fleshing, and their biodegradability and batch tests, at different inoculum/substrate VS–mass ratios, and confirmed that the substrates were suitable for AD, exhibiting methane productions of 0.26 and 0.47 m³ kg _VS⁻¹, respectively [135]. Bayrakdar et al. tested the single-phase and two-phase (hydrolysis + methane reactor) mesophilic anaerobic co-digestion of leather fleshings and sludge in the 120-day semi-continuous feeding mode [136]. The single-phase digestion produced 15% more CH₄ (0.46 m³ kg⁻¹) than the two-phase digestor (0.40 m³ kg⁻¹). However, the H₂S (<1%) caused the inhibition of CH₄ production; on the other hand, sulfides, normally contained in fleshings, and causing H₂S formation, were eliminated by rinsing the leather before the digestion, but produced foams affecting the performance of hydrolysis and methane reactor.

More recently, Rajamani et al. also explored the potential of AD applied to fleshing and bio-sludge from an effluent treatment plan to produce energy, and the process after 30 days at 20–35 °C produced 0.5 m³ kg⁻¹ of biogas, which could be further converted into electricity via a gas engine, while the digested sludge was suitable as fertilizer [137]. On the other hand, Priebe et al.’s work is interesting because they studied the inter-relation between microorganisms and substrates, and the biodegradation of collagenous feedstocks containing different concentrations of chromium via AD at the bench scale; four substrates, i.e., soybean meal, hydrolysed collagen, hide powder, and wet blue leather shaving, were inoculated with three different biological sludges from wastewater treatment plants, i.e., sewage anaerobic sludge, slaughterhouse anaerobic sludge, and tannery aerobic sludge [132]. CLSW shavings reached the highest biogas yield (162.2 mL g⁻¹) with a methane fraction of 73.7% after 36-day AD and it was found that, because chromium reduced biogas formation, leather substrate should be mixed with degradable materials that destabilize chrome–collagen complexes and increase water dispersion. In

Table 5. Optimal conditions and main products from thermal and biochemical treatment of LSW with mixed waste; arrows (↑) indicate the increasing yield or amount at the reported operating conditions.

References	Technology	Feedstocks	Products	Advantages
Zhang et al., 2021/2022 [127,128]	Co-pyrolysis	Tanning sludge, CLSW, and buffing dust (2:3:5 ratio)	Oil and gas yields ↑ by 15%	Reduced energy activation; improved product quality
Liu et al., 2024 [129]	Co-pyrolysis	Wheat straw and CLSW	Condensable and non-condensable gases	Low fixed carbon, highly volatile matter, and high ash content
Hu et al., 2024 [130]	Co-pyrolysis	Waste textiles, LSW, and Ca/Fe-rich sludge ash	Gases with reduced acids and nitrogen compounds	Ash accelerated weight loss rate (12.8% min−1)
Kameswari et al., 2011/2013 [133,134]	AD	ULSW, tannery sludge, and lipase	Biogas yield ↑ 15% 385 mLbiogas gVS−1	Retention time ↓ (45 days) Improved hydrolysis
Islam et al., 2014 [13]	AD	ULSW, sewage (1:1), cow and dung (25%)	476 Lgas gVS−1 73% CH4	52% VS destruction
Polizzi et al., 2017 [135]	AD	Tannery sludge, and ULSW	Methane: 0.26 m3 kgVS−1 (sludge), 0.47 m3 kgVS−1 (ULSW)	Biodegradability model
Bayrakdar, 2020 [136]	Single/Two-phase AD	Tannery sludge and ULSW	Single-phase: 0.46 m3 kg−1 Two-phase: 0.40 m3 kg CH4−1	Rinsing reduces H2S but causes foam
Rajamani et al., 2021 [137]	AD	Tannery sludge and ULSW	Biogas: 0.5 m3 biogas kg−1 Digestate	Biogas for electricity Digestate as fertilizer
Priebe et al., 2016 [132]	AD	Soybean, CLSW, and ULSW sludges	Biogas (CLSW): 162.2 mL g−1, 73.7% CH4	Degradable substrates and improved inhibition by Cr

a summary of the above-mentioned experiments on mixed wastes is reported.

5. Discussion

Pyrolysis offers several advantages, including the elimination of toxic pollutants, chromium recovery, and fuels’ precursors and chemicals’ production [17,55,117]. However, its feasibility depends on the ability to overcome certain technological limitations such as high operational costs, energy consumption, and complex separation steps [5,17,109,117]. Compared to combustion, pyrolysis recovers energy and chromium from LSW more efficiently, although its performance is highly sensitive to operating conditions [22].

The pyrolysis temperature strongly influences the distribution of products: temperatures above 450–500 °C favor the formation of gas and oil, especially in CLSW [55,67,110,117], while temperatures below 450 °C enhance biochar production [66,82,104]. In VLSW, gas production is maximized between 450 and 600 °C due to collagen and tannin decomposition [122,123]. Additionally, fixed-bed reactors are more advantageous for oil production within this temperature range, whereas fluidized-bed reactors improve heat transfer and homogeneity, but they may limit oil yields [66,138]. Particle size is a parameter affecting oil recovery [66], as well as chemical pre-treatments, such as NaOH or sulfuric acid soaking, because they reduce the decomposition temperature and modify the product composition [67,103].

Insights from the pyrolysis of other polymeric materials also support the thermochemical valorization of leather waste [45]. As in synthetic polymers, the decomposition of collagen and lipids produces aliphatic and aromatic compounds whose formation depends on diffusion limitations and inorganic species that may act as catalysts. Studies show that the microporosity of conventional zeolites cannot easily accommodate bulky cracking intermediates and cause the limitation of light aromatics while promoting coke or heavy products. Similar behavior is observed in leather pyrolysis, where mineral salts (Ca and Na), metallic oxides (e.g., Cr₂O₃), and finishing agents influence reaction pathways and may act as unintended catalysts. The use of hierarchical catalysts, already effective in polymer conversion, could therefore enhance product quality and selectivity in the thermochemical processing of animal-based wastes.

The amino acid composition of leather, which varies depending on the tanning method, influences both product yields and NO_X formation [20,22,139,140]. For instance, alanine is easily converted into gas products, yielding up to 95%, while valine reaches 88% of gaseous fraction [20]. Polyphenolic and chromium tanning agents modify the interaction with collagen, altering the resulting pyrolyzates [139,140]. Understanding the amino acid composition of the final products is essential for selecting the optimal conditions and minimizing emissions.

The calorific value of the LSW bio-oil fraction can exceed 20 MJ kg⁻¹, surpassing conventional biomass fuels such as wood or agricultural residues [57,67], and when properly upgraded, may reach up to 29 MJ kg⁻¹, aligning with diesel-range fuels [122]. Nevertheless, its high nitrogen and oxygen content limits direct fuel applications, often requiring catalytic cracking for improvement [60,104,118].

Despite its energy density, a deeper investigation of bio-oil composition reveals fundamental limits that can affect upgrading processes. In fact, bio-oils derived from collagen-rich feedstocks contain high amounts of oxygenated compounds, i.e., carboxylic acids, phenolics, carbonyls, and carbohydrate-derived species, that reduce stability and promote coke formation during processing [67,87,121,122,141]. For instance, carboxylic acids contribute to acidity and corrosion, whereas phenolic and sugar-derived structures readily undergo condensation and polymerization, accelerating catalyst deactivation [67,122,123,141]. These functionalities determine the dominant deoxygenation pathways (decarboxylation, decarbonylation, and hydrogenolysis) and influence process severity, hydrogen demand, and catalyst selection [57,87,141].

On the other hand, nitrogen-containing compounds introduce an additional level of complexity. The thermal decomposition of collagen generates heterocyclic aromatics, amide-type structures, nitriles, and volatile species such as NH₃ and HCN. Aromatic nitrogen heterocycles are particularly resistant to hydrotreatment and contribute to coke formation and catalyst fouling, whereas light nitrogenous species may form fuel-bound NO_X if not effectively removed. Consequently, the distribution of oxygen- and nitrogen-containing compounds governs both the stability of crude bio-oil and its behavior during catalytic upgrading [20,58,67,101,122,141]. As a result, tailored upgrading strategies are needed, since these molecular features determine catalyst compatibility, reaction selectivity, hydrogen consumption, and the potential for NO_X formation in downstream use [20,58,67,101,122,141,142].

During pyrolysis, nitrogenous compounds such as NH₃, HCN, and HNCO are released; although not directly responsible for NO_X and N₂O emissions, they act as important precursors that can be further oxidized into these species during subsequent combustion processes [143]. Even if temperatures above 400 °C promote oxidation which reduces these intermediates, NO_X emissions may still pose environmental challenges [143,144]. The gaseous fraction, rich in CO, CH₄, and H₂, can contribute to energy self-sufficiency or be used for heat and power generation, with yields increasing at higher pyrolysis temperatures and varying by reactor configuration. The biochar fraction, although often limited by high H/C and low O/C ratios, particularly in CLSW [116] can still achieve HHVs of 16.5–19.0 MJ kg⁻¹, comparable to lignite [59]. In specific cases, especially with VLSW, biochar exhibits up to 60% fixed carbon and low volatile matter, enhancing its use as a carbon-rich fuel or additive [97].

In contrast to pyrolysis, gasification primarily generates syngas, with its energy content strongly influenced by the gasifying agent. Air-blown gasification typically produces syngas with a lower HHV (4–6 m⁻³), whereas oxygen or steam increases this value (12–18 MJ m⁻³) [99]. Among the available reactor configurations, down-draft gasifiers are particularly suitable for LSW as they produce clean syngas with lower Cr (III) emissions [66,79].

Prior to gasification, LSW must undergo essential pre-treatments, e.g., maceration, flash drying, and densification, to ensure stable operation, maximize energy recovery, and to improve the cold gas efficiency and syngas quality [114,145]. Considering that LSW retains more than half the energy value of coal, gasification could meet internal thermal needs in tannery operations [114]. In fact, it is estimated that up to 70% of the intrinsic energy content of leather waste can be recovered as syngas, mostly usable in on-site recovery systems [78,114]. This potential was demonstrated by Bowden et al., who successfully coupled drying and gasification to simultaneously manage solid waste and generate renewable energy for manufacturing [114,146].

A practical application of this technology is the prototype plant at the BLC Leather Technology Centre in the UK, which processes various leather residues, e.g., ULSW, CLSW, and VLSW, at a feed rate of 50 kg h⁻¹ [145]. The plant achieves cold gas efficiencies of up to 70% on a two-ton-per-day scale, producing syngas with a HHV of 4.5–5 MJ m⁻³, suitable for use in boilers or combined heat and power (CHP) units, provided the appropriate pre-treatment is performed.

From an environmental standpoint, gasification also offers the advantage of stabilizing heavy metals in the solid residue, thereby reducing contamination risks [66,147]. Moreover, the reduction in volatile matter limits tar formation and might reduce the need for expensive catalysts [78,114]. Nonetheless, the presence of tars and aromatic compounds in the gas phase may require dedicated cleaning systems [20]. Although the economic viability of leather waste gasification remains under evaluation [23], the quality and calorific value of the syngas produced support its use in dual-fuel systems and decentralized CHP applications [114,145].

HT processes emerge as promising alternatives for the energetic valorization of wet leather solid waste. Despite being less explored compared to pyrolysis or gasification, these technologies offer distinct advantages for processing moist feedstocks without requiring pre-drying [79,87]. Among them, HTC of ULSW has shown significant potential in producing hydrochar with high heating values (24–27 MJ kg⁻¹), and low ash, nitrogen, and sulfur content, making it suitable for clean combustion in coal-based systems [90]. The produced biocrude had HHVs up to 32 MJ kg⁻¹, making it promising for renewable liquid fuel generation, provided that upgrading processes can be optimized. Nevertheless, the overall application of HT technologies to leather solid waste remains limited, both in literature, where only a few studies are available, and in practice, as the restricted number of experiments has kept this technology at the laboratory scale [90,148]. As a result, further research is needed to optimize operational parameters, evaluate technical feasibility at scale, and compare HT’s performance to more established thermochemical routes.

Although the few studies on leather HT primarily focus on hydrochar yield, fuel properties, and heteroatom transformation [90,124], existing evidence in the scientific literature indicates that the NO_X and SO_X emissions of HTC-derived hydrochar can be reasonably inferred [90,124,149,150]. Experimental investigations on ULSW treated at 180–200 °C demonstrated a reduction in nitrogen content and a stabilization into less volatile aromatic and heterocyclic structures, alongside the persistence of intrinsically low sulfur levels [90]. These compositional and structural changes are directly relevant to emission formation pathways as the conversion of proteins into refractory nitrogen functionalities limits the release of volatile precursors, such as NH₃ and HCN, that are known to be key intermediates in fuel-NO_X formation [90,149,150,151]. Similarly, the depletion of volatile sulfur species during HTC decreases the availability of sulfur-containing gases that would otherwise oxidize to SO_X during combustion. While direct flue gas measurements on hydrochar from leather waste remain limited in the literature, studies that combine HTC with subsequent combustion or thermogravimetric oxidation consistently report improved combustion stability, reduced volatile matter, and mitigated heteroatom release [89]. These experimentally verified transformations provide a mechanistic basis supporting the expectation of lowered NO_X and SO_X emissions compared to raw leather waste and highlight HTC as a promising pre-treatment strategy for producing cleaner solid fuels from the leather industry [90,152]. Nonetheless, dedicated combustion tests with continuous flue gas monitoring remain essential to fully validate the emission profile of leather-derived hydrochar, and future research should address this gap to strengthen its industrial applicability [89,149].

In addition to thermochemical methods, AD represents a viable biochemical approach for the energetic valorization of tannery waste, particularly thanks to the production of biogas that is primarily composed of CH₄ (55–70%) and CO₂ (30–45%) [66,112]. This biogas can be directly used for heat and electricity generation, and it is very attractive due to its potential to reduce energy consumption in tanneries [94,132]. For example, under semi-pilot conditions, the biogas produced on-site could replace part of the electricity and thermal energy that a midsize tannery would normally buy from the grid, reducing this demand by 6.8% and 1.6%, respectively, and also lowering overall waste management costs [94,112].

AD represents a useful approach to produce bioenergy from LSW because the leather industry produces many wastes rich in animal fats and proteins containing high thermal values, which creates an opportunity for their exploitation [79]. Among LSW, fleshing waste, rich in biodegradable carbohydrates and lipids, is particularly suitable for AD, as it degrades easily and reduces the costs associated with its high viscosity and pollutant load [93,97]. In addition to pH, bacterial growth and enzymatic activity during digestion are influenced by the hydrolysis rate, which is particularly slow in fat- and lipid-rich wastes, potentially limiting degradation and requiring enzyme addition, such as lipase [133]. Process stability, commonly assessed through the VFA/alkalinity ratio, is also critical, with optimal environmental conditions, i.e., redox potential, nutrient balance, and dilution of toxic compounds. However, tannery byproducts are protein-rich biomasses with high nitrogen content, leading to ammonia formation during degradation [134], which can contribute to the inhibition of enzymatic activity by increasing the intracellular pH and compromising process stability; such an effect can also be exacerbated by other factors, e.g., VFA, pH, and free ammonia [134,153]. Consequently, mono-digestion is often inefficient, and co-digestion with other organic substrates is recommended to optimize the C:N ratio (ideally 20–30:1 [112] or 16–25:1 [132]) and reduce inhibitory compounds [22,79]. LSWs are highly nitrogen-rich and generally fall far below the optimal C/N range; for instance, fleshing residues exhibit a C/N of 3.5, while shavings reach only 2.8 [112,132,154]. Therefore, the C/N ratio is a critical parameter to consider, as low values may also lead to ammonia inhibition.

Co-digestion has been shown to mitigate the inhibition of ammonia, sulfides, and long-chain fatty acids, enhance nutrient balance, improve particle size distribution, and increase overall reactor performance through synergistic effects and nutrient supplementation [79,116]. The benefits of AD co-digestion strongly depend on the characteristics of the initial substrates, whose selection is crucial for optimizing final yields and ensuring an adequate distribution of nutrients and trace elements (e.g., Ni, Co, Mo, and Se) [112,132,154]. Additionally, viable co-digestion performance at low C/N ratios can be achieved through microbial acclimation and the selection of microbial communities adapted to protein-rich wastes, the enrichment of specific functional groups (e.g., Bacteroidales, Methanosaetaceae), inoculation with activated sludge, and appropriate pH control [94,112,132,154].

The presence and high content of heavy metals represent the main challenges in this process [66]. Particularly, the presence of chromium has shown different effects depending on its concentration and the applied operational conditions. At low concentrations, Cr³⁺ may act as a micronutrient, stimulating microbial growth and enhancing biogas and methane production, as observed by Agustini et al., where its presence was beneficial to AD [112] because of the partial removal of the organic load, increasing the final biogas yields [79,112,113]. However, higher concentrations tend to reduce biogas production due to the stabilization of the collagen matrix, which becomes resistant to enzymatic hydrolysis and limits microbial degradation [109]. The inhibitory effect is primarily linked to substrate recalcitrance rather than to direct toxicity, although excessive chromium can further impair microbial activity, resulting in incomplete stabilization and lower methane yields. Consequently, AD is generally more efficient with chromium-free LSW, but co-digestion with more degradable substrates has been proposed to destabilize chromium–collagen complexes and improve microbial accessibility [132].

Residence time is also a critical factor: under mesophilic conditions, AD typically requires about five weeks, which increases contamination risk and limits throughput [93]. To improve efficiency, various pretreatments, i.e., mechanical, thermal, or biological, are often applied to accelerate hydrolysis, the rate-limiting step of the process, thereby improving mass transfer and methane generation [79]. Reported methane yields vary widely, ranging from 70 to 300 mL_CH4 g_VS⁻¹ depending on the substrate characteristics and inoculum used, e.g., in the studies of Mozhiarasi et al. [79] and Priebe et al. [132]. Despite its potential for bioenergy recovery, AD does not ensure complete solid waste elimination, and high initial investment costs remain a major limit [109].

6. Conclusions

The energetic valorization of LSW can be pursued through two main technological routes: biochemical and thermochemical processes.

AD represents a biochemical approach to producing biogas for heat and power generation, but it still faces limitations such as slow conversion rates, high moisture content, and the presence of residual organic matter in the digestate. The TRL of AD depends heavily on the feedstock: while food-waste-to-biofuel conversion has reached TRL 9 (fully commercial), many biochemical waste valorization processes remain at TRL 4–5, particularly for complex feedstocks such as leather residues [155,156]. AD has been mainly applied to ULSW, including fleshing residues and sludge, while applications to TLSW and FLSW remain limited due to the inhibitory effects of chromium and finishing products on microbial activity. Nevertheless, bench-scale experiments demonstrated that CLSW shavings can still produce relevant amounts of biogas, with yields up to 162.2 mL g⁻¹ and a methane fraction of 73.7% after 36 days of AD, although bio-gas formation is significantly reduced compared to chromium-free substrates [132]. A solution is represented by mixing CLSW with more degradable co-substrates, that help to destabilize chrome–collagen complexes and enhance microbial accessibility.

Thermochemical processes, in contrast, are gaining increasing attention for their ability to handle a broader spectrum of LSW and generate multiple energy carriers, although their TRL is still generally low and requires further development before industrial deployment. Among them, pyrolysis is the most extensively studied method showing strong potential for fuel precursors’ diversification and chromium stabilization; in addition, pyrolysis is expected to reach TRL 9 (i.e., system proven in operational environment) in the next two decades, indicating its high potential for commercial application [157]. Gasification represents a viable pathway as well for syngas production and energy recovery, but its application to TLSW and FLSW remains underexplored. On the other hand, like pyrolysis, gasification is anticipated to achieve TRL 9, reflecting its readiness for commercial application.

HT is particularly suited for wet, raw ULSW residues, producing solid or liquid fuels’ precursors with high HHV, though research on its employment with leather is still scarce. For instance, HTC of ULSW under mild conditions (180–200 °C, 30 min) yielded >82.9 % wt. hydrochar with a HHV of 24.3–27.3 MJ kg⁻¹, exceeding that of lignite and sub-bituminous coal, and showing improved combustion properties particularly in lower water and volatiles contents, compared to initial feedstock [90]. Concerning TRL, HT has a low value due to challenges in operating at high temperatures and pressures, and limited understandings of reaction kinetics and reactor complexity, which can reduce heat and mass transfer efficiency [157,158,159]. Its endothermic nature demands substantial energy input, while high initial investment, specialized materials, and increased maintenance under continuous operation further challenge economic feasibility and large-scale implementation.

Advances in the thermochemical conversion of polymeric feedstocks suggest that hierarchical catalysts, combining mesoporous pre-cracking with selective reactions in acidic micropores, also offer promising opportunities for leather recycling [45]. These catalysts improve access to internal acid sites, reduce unwanted reactions and coke formation, and address challenges similar to those which are observed in the conversion of collagen and tanning residues. Their application could enhance both the energy yield and the chemical quality of products derived from leather waste.

In conclusion, the valorization of LSW cannot rely on a single universal route, as each waste stream presents distinct chemical features that shape process feasibility and product quality. Advancing sustainable solutions therefore requires the transition from traditional waste disposal to resource-oriented strategies framed within the principles of the circular economy and aligned with broader sustainability objectives. Future research should therefore prioritize the optimization of thermochemical and biochemical processes, focus on clarifying the influence of chromium and finishing agents, improve the upgrading of derived fuels and chemicals, and assess the environmental implications of large-scale deployment. Equally important is the assessment of environmental and economic performance at a large scale, including the role of pre-treatment steps that can enhance processes’ robustness and efficiency, and the need to address challenges associated with chromium stabilization and nutrient recovery. A comparative and integrative approach across biochemical and thermochemical pathways appears essential to identify synergies, minimize limitations, and promote closed-loop pathways capable of maximizing resource recovery. By combining these complementary strategies, LSW valorization can progress toward more sustainable, efficient, and commercially viable solutions that support the transition to a circular and environmentally responsible leather industry.