Use of Zero-Valent Iron Nanoparticles (nZVIs) from Environmentally Friendly Synthesis for the Removal of Dyes from Water—A Review

Abstract

This review article addresses the increasing environmental concerns posed by synthetic dyes in water, exploring innovative approaches for their removal with a focus on zero-valent iron nanoparticles (nZVIs) synthesized through environmentally friendly methods. The article begins by highlighting the persistent nature of synthetic dyes and the limitations of conventional degradation processes. The role of nanoparticles in environmental applications is then discussed, covering diverse methods for metallic nanoparticle production aligned with green chemistry principles. Various methods, including the incorporation of secondary metals, surface coating, emulsification, fixed support, encapsulation, and electrostatic stabilization, are detailed in relation to the stabilization of nZVIs. A novel aspect is introduced in the use of plant extract or biomimetic approaches for chemical reduction during nZVI synthesis. The review investigates the specific challenges posed by dye pollution in wastewater from industrial sources, particularly in the context of garment coloring. Current approaches for dye removal in aqueous environments are discussed, with an emphasis on the effectiveness of green-synthesized nZVIs. The article concludes by offering insights into future perspectives and challenges in the field. The intricate landscape of environmentally friendly nZVI synthesis has been presented, showcasing its potential as a sustainable solution for addressing dye pollution in water.

1. Introduction

Water pollution stems from various sources, including industrial, agricultural, and domestic effluents, as well as geologic weathering, mining effluent, and atmospheric sources [1]. Water pollution, primarily from industrial and agricultural activities, poses a significant threat to water quality, rendering it unfit for consumption and harming aquatic life [2]. This widespread pollution affects drinking water from rivers, streams, lakes, and oceans globally, contributing to a high number of fatalities, particularly in developing countries [3]. Hazardous water pollutants pose severe health risks to humans and the environment. For instance, drinking polluted water can result in gastrointestinal, hepatobiliary, and neurological complications, even leading to mortality [4]. Inorganic contaminants such as arsenic, lead, and cadmium in high concentrations can cause cancer, diabetes mellitus, and cardiovascular diseases, further exacerbating the health impact [5]. The economic consequences of water pollution are significant, affecting sectors such as health, agriculture, tourism, real estate, and aquaculture/fisheries. Regions downstream of heavily polluted rivers experience reduced economic growth, reflecting underestimated costs of environmental degradation and inefficient pollution management [6]. Economic burdens associated with poor water quality include health-related expenses, high water treatment costs, and adverse impacts on economic activities like agriculture and manufacturing [7]. Regulatory measures to tackle water pollution include efforts like the European Union’s Water Framework Directive 2000/60/EC, aimed at addressing chemical pollution of surface waters. Such regulations play a crucial role in mitigating the adverse effects of water pollution and safeguarding human health and the environment.

Emerging contaminants, also known as pollutants of emerging concern, encompass a wide range of compounds found in surface water and groundwater. These pollutants originate from industrial, pharmaceutical, and municipal sources, as well as various human activities [8,9]. Examples of emerging contaminants include pharmaceuticals, personal care products, illicit drugs, nanomaterials, and endocrine-disrupting chemicals [10,11]. Due to their widespread distribution, ecological impact, and potential risks to human health and aquatic life, dyes can be considered as emerging pollutants [12]. These contaminants are synthetic or naturally occurring chemicals not commonly monitored or regulated in the environment but are suspected to have adverse health effects [13]. They pose serious threats to ecosystems, as even low concentrations can be mobile or persistent in air, water, soil, and sediments, directly or indirectly exposing aquatic species and populations to potential harm [14]. One of the significant challenges in addressing emerging contaminants is the lack of legislation to regulate their discharge or monitor these compounds, presenting obstacles for water sustainability and environmental protection [15].

Emerging pollutants, including dyes, are dumped in natural environments, posing serious health risks, and affecting life in general [16]. The application of multiple dyes and pigments in sectors derived from the textile, paper, pharmaceutical, and cosmetics industries have caused them to be present in the environment, being considered as persistent pollutants in ecosystems and, therefore, of great concern [17].

Studies indicate that during the dyeing process, a percentage of the dye used remains as a substance fixed to the fabric, but there is another quite high percentage (15–50%) that remains in the effluents of the textile industry [18]. The presence of dyes in the effluents of the industry has been corroborated. In aquatic environments, dyes represent a great threat to aquatic organisms, as they can participate in the reduction of dissolved oxygen, generating anoxic and anaerobic conditions, and impede the passage of sunlight and the diffusion of oxygen, thus interfering with the development of different forms of aquatic life [19]. Studies have revealed that dyes are the cause of illnesses and can even cause cancer [20]. In this sense, water pollution by dyes has become a great threat due to their high toxicity and their chemical stability, which makes them resistant to many degradation processes and difficult to eliminate in conventional treatment plants. This is why, over the years, studies have been carried out to evaluate the efficiency of non-conventional treatment processes for the elimination of this type of emerging pollutant [21].

Within the alternative treatments and, more specifically, considering the processes of dye removal with non-conventional materials, given their associated benefits, the implementation of metallic nanoparticles, specifically nZVI nanoparticles synthesized through green chemistry, is positioned as an alternative of great interest.

In the pursuit of well-being, modern society frequently requires resources and carries out practices for their transformation into useful products. Over time, an increase in environmental pollution has occurred due to these human activities, introducing external chemical or biological agents. This pollution has negative impacts not only on the environment but also on the organisms living within it. Consequently, the primary sources of water pollution predominantly come from human activities, including urban discharges of organic pollutants, as well as contributions from agriculture and livestock, like pesticides and fertilizers, and various industrial processes [22,23,24,25].

Urban pollution arises from residential, commercial, and service sectors, generating wastewater containing refuse and chemicals such as bleach, detergents, pharmaceuticals, and cosmetics [26,27,28,29]. These chemicals cannot be released indiscriminately into lakes or traditional waterways without proper treatment. Additionally, contaminants like metals and hydrocarbons originate from vehicle emissions. Agricultural pollution primarily results from the runoff of fertilizers [30] and pesticides [31] into water streams, often containing nitrogen, phosphorus, or sulfur, which subsequently leads to soil contamination and groundwater pollution [32]. However, the most significant and dangerous source of pollution comes from industrial activities, given the multitude of materials and substances present in industrial wastewater. These pollutants stem from various industrial processes, including manufacturing, transformation, utilization, cleaning, maintenance, and consumption, each associated with specific products and industry types. Notably, industries such as petrochemicals, textiles, energy, and paper manufacturing contribute significantly to the production of the most environmentally harmful substances. For instance, the textile industry [33], known for its extensive use of dyes and pigments [34], often releases these chemicals into wastewater without adequate treatment, resulting in substantial environmental waste.

Considering the rapid pace of industrialization, coupled with population growth and urbanization, the World Health Organization (WHO) reports that more than 2.1 billion individuals lack access to safe drinking water [35] (as per the Sustainable Development Goals by the World Health Organization). Additionally, the European Environment Agency (EEA) estimates that roughly three million locations within its member countries have been impacted by contaminants originating from local sources (as stated in the European Environment Agency’s report on Contamination from Local Sources).

In response to the mentioned pollution problem, the scientific community has directed significant attention towards exploring the potential of nanotechnology in creating nanomaterials endowed with exceptional characteristics and properties, ideally suited for remedying water pollution issues. It is worth noting that the term “nanotechnology” refers to a collection of procedures and techniques that enable the manipulation and operation of molecular structures and their constituent atoms on a nanoscale, typically spanning lengths from 1 to 100 nanometers [36]. In recent years, the studies and advancements in nanotechnology and nanoscience have rapidly translated into a wide array of products, including nanoparticles, nanowires, nanotubes, and nanolayers, that have been synthesized for application in diverse sectors of modern society [37]. Nanotechnology is not merely an area of promising research [38,39]; it has already begun to demonstrate its initial commercial applications across various industries, including electronics, automotive, sports equipment, cosmetics, and more. This expands research areas, both in advanced and emerging economies [40].

Simultaneously, nanotechnology has emerged as a captivating research subject, reflecting our fascination with the ability to manipulate the shape and composition of matter at the atomic and molecular scale through various techniques and approaches. The potential of nanotechnology is the outcome of scientific advancements throughout the 20th century, which have progressively deepened our understanding of matter’s properties at increasingly smaller scales, evolving from what can be termed “microtechnologies” to “nanotechnologies”. The remarkable development of characterization and detection techniques such as scanning and transmission electron microscopes (SEM, TEM) and atomic force microscopy (AFM) has contributed to our deliberate and active comprehension and control of matter’s properties at the atomic and molecular levels, allowing for the feasible modification of structures within this size range [41]. In addition, they can provide various types of physicochemical information on nanoparticles, but their usefulness will depend on the technology and the properties of the material, such as composition and scale.

The properties exhibited by nanomaterials [42] and their behavior differ from those of their counterpart bulks and expand when compared to the same materials outside the nanoscale. Furthermore, within this size range, all properties—physical, chemical, and biological—undergo alterations, both at the individual atom and molecule levels and as composite materials [37]. Many of these alterations depend on the behavior of electrons within the nanomaterials or how the atoms are arranged within the matter. It is important to note that in nanometer-sized materials, the movement of electrons is significantly constrained by the material’s dimensions. Additionally, the ratio of surface atoms to interior atoms is much higher than in larger materials, resulting in a heightened density of exposed sites per unit mass, and thus, might increase chemical reactivity. Notably, metallic materials like iron, silver, and gold, semiconductors such as cadmium chalcogenides, gallium arsenide, and indium phosphide, and insulators like iron oxide and titanium are among the most extensively researched materials [43].

By precisely controlling synthesis conditions and appropriate functionalization, it is possible to obtain a narrow range of sizes and shapes. Thus, the properties of nanomaterials can be tailored to some extent [44]. Consequently, nowadays there exists methodologies and technology to tune the dimension and shape of the material, and therefore, its properties, enabling the design of materials with specific characteristics [37].

In this review, the use of zero-valent iron nanoparticles (nZVIs) as an eco-friendly solution for dye removal from water is analyzed. The manuscript unfolds progressively, commencing with an examination of the role of nanoparticles in environmental applications, followed by an in-depth analysis of various synthesis strategies employed for nanoparticle production. Next, it delves into conventional and environmentally friendly methods for synthesizing zero-valent iron nanoparticles (nZVIs), highlighting their advantages and limitations. A comprehensive discussion ensues on the influence of synthetic methods on nZVI preparation, alongside strategies for stabilizing nZVIs to enhance their effectiveness in dye removal. The review also describes emerging green alternatives for nZVI synthesis, emphasizing sustainability and environmental compatibility. Furthermore, the prevalent issue of dye pollution in wastewater and current removal strategies is explored, underscoring the importance of efficient and environmentally benign approaches. The manuscript concludes with a forward-looking discussion on future perspectives and challenges in the field, aiming to provide insights for advancing sustainable water remediation practices.

This review also aims to fill a critical knowledge gap in the existing literature by consolidating and synthesizing dispersed information on the synthesis, stabilization, and application of zero-valent iron nanoparticles (nZVIs) for dye removal in water. While previous studies have explored various aspects of nZVI synthesis and its potential in water treatment, there has been a lack of comprehensive and integrated analysis that encompasses the diverse synthesis methods, stabilization techniques, and practical applications of nZVIs specifically for dye removal. By systematically reviewing and synthesizing findings from a wide range of research studies, reviews, and technical reports, this review provides a comprehensive resource that tries to bridge this gap.

Our approach involved meticulous analysis and synthesis of the existing literature, ensuring a thorough examination of each aspect of nZVI-based dye removal. By consolidating scattered knowledge and offering insights into the efficacy, limitations, and future directions of environmentally friendly nZVIs for sustainable water remediation, this review aims to inform and guide researchers, policymakers, and practitioners in the field toward more effective and environmentally benign water treatment solutions.

2. Nanoparticles in Environmental Applications

Nanotechnology continues to gain momentum as a powerful tool in addressing environmental challenges, establishing itself as one of the most significant applications in different fields, such as water depuration [44], gas cleaning [45], green synthesis [46], catalysis [43], etc. This can be attributed to the profound understanding of nanoparticle properties, enabling the development of novel synthesis methods, and enhancing environmental applications [47]. Nanoparticles find their place in environmental applications due to their remarkable capacity for removing pollutants, thanks to their high surface area-to-volume ratio. As particle size decreases, the proportion of atoms on the surface increases, leading to enhanced adsorption capacity, ease of interaction with other atoms and molecules, and improved reactivity on chemical or biological surfaces compared to bulk materials [48]. However, this heightened chemical reactivity can result in nanoparticle agglomeration, reducing their surface energy.

Zero-valent metals, exemplified by nanoscale iron (nZVI or nanozero-valent iron), possess highly reactive surfaces, and are commonly employed in the treatment of water, sediments, and soil. Noble metals, such as gold, platinum, and palladium, have been used for degradation of gas pollutants like volatile organic compounds (VOCs). Silver has a special application in the biomedical field due to its great antibacterial activity. Semiconductors like titanium, zinc, and cerium have been widely used as catalysts for water cleaning using light as a source of energy. On many occasions they are combined with other metals like copper or noble metals to increase their activity.

As a counterpoint, the nanoparticles used can reach the environment and act as a contaminant. Westerhoff et al. showed the removal of over 96% of titanium in influent sewage, with effluents typically containing <25 μg/L [49]. Batley et al. discussed the future of the bioaccumulation of nanoparticles in the environment, concluding that it is rising and potentially it will happen [50]. In recent years, some authors have reported the bioaccumulation of nanoparticles in some foods that we can find in supermarkets today. For example, Gallocchio reported how mussels exposed to TiO₂ nanoparticles can adsorb them at concentrations of 209–1119 µg/kg [51]. Thus, great attention must be paid to releasing nanoparticles to the environment and, additionally, low toxicity must be a characteristic.

3. Nanoparticle Synthesis Strategies

Various techniques have been described for synthesizing nanoparticles, and the synthesis method significantly influences the size and shape of the resulting nanoparticles. Special attention is given to controlling factors such as size, shape, composition, crystallinity, and structure, as these parameters determine the intrinsic properties of metal nanostructures [52]. To achieve specific characteristics, process conditions must be meticulously controlled. These characteristics include uniform size across all particles (referred to as monodisperse samples), consistent shape or morphology, the chemical composition of both the core and the modified surface, crystalline structure, and the prevention of aggregation. To address the latter, nanoparticle syntheses typically employ stabilizing agents that attach to the surface, providing a stabilizing charge to keep the nanoparticles suspended and prevent aggregation [47].

Nanoparticle synthesis processes may seem conceptually straightforward but are challenging to execute according to the literature. Typically, authors describe two distinct approaches for the controlled synthesis of nanostructures, each with its own set of advantages and disadvantages [52]. The first approach, termed “top-down”, begins with larger materials and uses powerful techniques to break them down into particles of the desired size and is the most conventional method for nanoparticle synthesis. In contrast, the second and more recently developed approach, known as “bottom-up”, starts with individual atoms and molecules, assembling them into nanoparticles. Nowadays, some methods combine both approaches.

Top-down methods involve the subdivision of solid metals (bulk materials). This subdivision can be made by mechanical forces (ball milling, grinding, ultrasound, etc.) or chemically (degradation, consumption, etc.). A key drawback is the imperfect surface structure, along with a broad particle size distribution, typically exceeding 10 nm, and limited reproducibility. Within the “top-down” approach, various methods are utilized, with the most representative ones being thermal evaporation, high-energy milling, laser ablation, etching, etc.

Bottom-up methods focus on constructing nanomaterials atom by atom, molecule by molecule, or cluster by cluster. A common synthetic approach involves reducing metal ions to their corresponding atoms and subsequently controlling the aggregation of these atoms. Key methods within the “bottom-up” approach include the colloidal method, photochemical and radiochemical reduction, microwave irradiation, dendrimer utilization, solvothermal synthesis, and the sol–gel method [53]. Also, a bulky precursor can be fragmented into ions or molecules that are further combined to form nanoparticles. This could be called a mixed method (top–bottom method, Figure 1), and some examples are chemical vapor deposition, solvolysis, thermal evaporation, etc. Each method has inherent advantages and disadvantages in the synthesis process.

Figure 1 summarizes these processes. The main methods of preparation of nanoparticles are described below, together with wide variety of processes involving physical, chemical, or a combination of both methods in the synthesis of metal nanoparticles to develop environmentally friendly solutions for water treatment, detection of persistent pollutants, and remediation of soil and water.

Initially, any synthesis approach can be categorized as either environmentally harmful or environmentally friendly. In scientific discourse, techniques employing chemical reagents are typically deemed polluting, referred to as conventional or gray methods. Conversely, methods emphasizing the utilization of renewable resources like biomass or waste from other processes are considered environmentally friendly or green. To accurately classify them, various parameters need to be assessed (Figure 1), including CO₂ levels, energy efficiency, waste generation, and overall performance. However, essential data for these evaluations, such as the yield of the synthetic process, are often lacking in the literature. Consequently, there is a pressing need for studies that rigorously assess the potential minor contaminations associated with purportedly green methods.

In the following paragraphs, the most reported methods are briefly described, giving some examples of each one.

3.1. Sol–Gel

The sol–gel process is a bottom-up nanoparticle production method. The process involves two steps, first the obtaining of a sol, which consists of dissolving the precursors, normally in an acidic solvent, obtaining a colloidal solution. This solution evolves into a network, or gel, of the aforesaid monomer. Once the gel is formed, the excess of solvent must be removed, and finally, a drying or calcination step is required [55,56,57]. The efficiency of the process is usually very high, above 70%, and with moderate energy consumption, most of which is consumed in the drying/calcination process.

3.2. Hydrothemal

The hydrothermal synthesis method is one of the most used (Figure 1). In summary, it consists of introducing the precursor salts into a suitable solvent and subjecting them to a certain temperature and pressure. The solubility and stability of the salts in the solvent help make the growth of the nanoparticles controllable. Furthermore, this technique allows the incorporation of stabilizing, emulsifying, or reducing agents. As a rule, it has a high performance, and the energy consumption is not excessive [58,59], although in many cases it must be completed with a calcination or thermal decomposition process.

3.3. Chemical Vapor Condensation of a Metal

In the chemical vapor condensation approach for nanoparticle production, the metal is evaporated on a non-wetting surface in the presence of a solvent. This results in the deposition of metal in the form of islands on the surface [60]. Subsequent condensation of the solvent covers these particles, preventing the formation of a continuous metal film. Typically, high temperatures are required. However, a modified method has been developed recently, involving the thermal decomposition of pentacarbonyl iron (Fe(CO)₅) within a surfactant/solvent system [61]. This compound has a lower enthalpy of formation (−185 kcal/mol), making its decomposition easier to achieve. This approach can yield small nZVI particles (10–20 nm) at lower temperatures (140–160 °C).

3.4. Chemical Reduction Methods

This method is commonly used for reduction in solutions at atmospheric pressure. The solution of metal precursors is usually in water, and therefore, the most common precursors are inorganic salt, e.g., sulfates, nitrates, and chlorides. The chemical reduction agents also are soluble chemical reagents, such as NaHB₄ or similar. The specific case of iron nanoparticles will be discussed in more detail in Section 4. However, some other reduction methods are described below.

In some cases, the reducing agent can be a gas flow, so that some authors called this method the gas-phase reduction technique. For nZVI production, the process involves reducing iron compounds like goethite (FeO(OH)) or hematite (Fe₂O₃) at elevated temperatures using H₂ as the reducing agent [62]. Typically, nanoparticles generated through this method are larger, around 100 nanometers, and exhibit more irregular shapes with a crystalline core structure. The composition of the oxide shell is generally magnetite (Fe₃O₄). This synthesis process is easily scalable and has found application in the production of commercial nZVIs.

3.5. Co-Precipitation

The co-precipitation method is probably the simplest and most efficient chemical method for the synthesis of magnetic nanoparticles of the M-Fe_XO_Y-type, where M can be Co, Ni, Cu, etc. It is based on the precipitation of salt precursors of the metals forming the nanoparticles in a basic medium. The main advantage of this method is the large amount of material that can be synthesized, but the main disadvantage is the lack of control over the particle size generated, since the only control on the reaction is the kinetics. The process can be described in two stages: in the first one, rapid nucleation occurs until the concentration of the species reaches supersaturation; in the second one, the nuclei formed grow uniformly by diffusion of the species towards the surface of the particle until the final size is reached. To obtain monodisperse nanoparticles, these two stages must be separated, and nucleation must be avoided during the growth process. In general, it is difficult to maintain a homogeneous level of supersaturation throughout the reaction volume. The shape and size of the nanoparticles can be modified relatively easily by adjusting the pH, the ionic strength, the synthesis temperature, the nature of the reagents, or the proportion of Fe and metal (Co, Cu, Ni) salts [63,64].

3.6. Electrochemical Methods

The method of electrolysis synthesis involves the creation of metal nanoparticles from a solution containing Mⁿ⁺/M^m+ salts by applying an electric current between the anode and cathode. The metal particles produced are deposited on the cathode, usually within a size range of between 20 and 30 nm, and are spherical and uniform shape nanoparticles [65]. This method is straightforward, cost-effective, and requires minimal time. However, a drawback is that the nanoparticles tend to aggregate and form clusters on the cathode, often necessitating the use of surfactants to disperse them.

3.7. Precision Milling

In precision milling synthesis, micro-sized metal is milled in a high-speed chamber using stainless steel balls [66]. This method has several advantages, including the absence of toxic reagents, short production times, and scalability [67]. The resulting nZVIs are characterized by small size (~20 nm) and a large surface area (~40 m²/g) [68]. However, the nanoparticles exhibit irregular shapes due to deformation during milling and have a propensity to agglomerate.

Methods ranging from hydrothermal and solvothermal synthesis, pyrolysis, ultrasonication, sol–gel methods, precipitation and co-precipitation, chemical reduction methods using chemical agents such as NaBH₄, citrates, ascorbic acid, sodium hydroxide, hydrogen, among others (see Table 1), are described in the literature. These strategies aim to explore new material properties at both the atomic and molecular level, but they often involve complex processes, form precipitates that are mostly amorphous, have high energy requirements with difficulties controlling the growth rate of metal nanoparticles, use hazardous solvents and expensive reagents, and pose a potential biohazard. Due to these drawbacks, alternative techniques and sources for the synthesis of nanoparticles are being promoted more decisively such as the use of plant extracts as reactive agents for the synthesis of nanoparticles from green chemistry.

Table 1. Use of various metals in nanoparticle and nanocomposite synthesis and their subsequent use in environmental pollutant removal.

Nanoparticles	Support	Synthesis Techniques	Environmental Applications	Reference
			Removal of Dyes
ZnO	-	Sol–gel co-precipitation in KOH	Methylene blue	[55]
Zr-doped Fe2O3	CoOx	Hydrothermal	Orange II	[59]
Ca peroxide	Starch	Precipitation	Methylene blue	[63]
TiO2	g-C3N4(g-CN) nanolayers	Co-precipitation; thermal polymerization	Methylene blue	[64]
Au	Au/VO2/CeO2	Photodeposition	Methylene blue	[69]
Magnetite (MN)	Fe3O4@C	Microwave-induced	Methylene blue	[70]
Ag	Na0.5Bi0.5TiO3 nanospheres	Chemical solution, hydrothermal	Rhodamine B	[71]
HRC-Ag/Agar	Agarose hydrogel	Reduction with KBH4	Methylene blue, rhodamine B	[72]
Cationic polyethyleneimine (PBVPR218)	-	-	Dyes	[73]
EC-Ag	Elettaria cardamomum	Reduction with NaBH4	Methylene blue, rhodamine B	[74]
nZVI	Janus particles	Reduction with NaBH4	Methyl orange	[75]
CdAl2O4	Fe	Co-precipitation	Brilliant blue, brilliant green	[76]
CuNiFe2O4	g-C3N4	Gel auto combustion	Methylene blue	[77]
Black phosphorus (BPQDs)-IO TiO2	-	Sonication assisted; liquid exfoliation	Rhodamine B, methylene orange	[78]
g-C3N4-Fe3O4@KF	Kapok fiber	One step	Rhodamine B	[79]
CeO2	-	Co-precipitation, calcination	Methylene blue	[80]
PLA/CMC/GO _f-COOH@Ag	Polymeric matrix	Reduction with ascorbic acid	Methylene blue	[81]
(B-GNP)	Graphene	Polyvinyl alcohol film	Atrazine	[82]
Magnetite nanoparticles (MNs)	β-cyclodextrin (β-CD) and quaternary ammonium salts	Co-precipitation	Methylene blue, Orange G	[83]
C3N5-LDH-Ag	C3N5-LDH	Precipitation in NaOH	Tartrazine	[84]
			Removal of Organic Compounds
Ferrihydrite (Fh)	Non-ionic surfactant Brij L4	Co-precipitation	Cooking oils	[44]
TiO2	-	Sol–gel	Phenol	[56]
Zr-doped Fe2O3	CoOx	Hydrothermal	Bisphenol A	[59]
AuNPs	Au/VO2/CeO2	Photodeposition	Aromatic alcohols, p-nitrophenol	[69]
nZVI	Janus particles	Reduction with NaBH4	Trichloroethylene	[75]
Fe3O4/FeS	Biochar	Pollen pyrolysis	phenol	[85]
AA@Fe°	Amino acids	Reduction with KBH4	Tributyl phosphate, n-dodecane	[86]
Biochar	-	Pyrolysis	Bisphenol A	[87]
Fe3O4	rGO	Co-precipitation	4-Aminophenol	[88]
Aptamer-MrGO@Au and ssDNA-AuNP@MBs	BPA aptamer	Reduction with trisodium citrate	Bisphenol A	[89]
ZnO@SCF SppCobalt Ferrite	Spinel cobalt ferrite	Precipitation	Phenantrene	[90]
Fe⁰@C/surfactant	Carbon	Hydrothermal; carbothermic; reduction with NaBH4	Nitrobenzene	[91]
Zn/FeO	Surfactant foams	Reduction with NaBH4	Diesel	[92]
			Removal of Pharmaceutical Compounds
CuO and nZVI	-	Sol–gel; precipitation in NaOH; reduction with NaBH4	Levofloxacin	[57]
CuO	-	Co-precipitation; calcination	Metronidazol	[93]
Ag-Cu-Li	Multimetal nanorods	Precipitation in NaOH	Antibacterial activity	[94]
WS2	-	Hydrothermal thiourea	Tetracycline	[95]
ZnO	Alginate nanofibers	Hydrothermal	Tetracycline	[96]
BiFeO3	-	Combustion	Ofloxacin	[97]
TGC/NiCr2O4	Tubular g-C3N4 (TGC)	Calcination	Tetracycline	[98]
ZnO	Carbon cloth (CC)	Hydrothermal	Hydroxychloroquine	[99]
Ag/Ag3PO4-VAg	Ag nanocluster with vacancies of Ag (Ag/Ag3PO4-Vag)	Reduction with NaBH4	Sulfamethoxazole	[100]
Au	-	Reduction with NaBH4/320 °C	Penicillin G, sulfamethazine, tetracycline, enrofloxacin, skewered fish	[101]
Bi-Ag nanoalloys	-	Solvothermal	Bacterial infections	[102]
TiO2NW-CuO- cellulose	Cellulose	Solvothermal	Escherichia coli	[103]
			Removal of Pesticides
Ag (SNP)	-	Reduction with NaBH4	Miticidal activity	[104]
Au	Ovalbumin	Reduction with trisodium citrate	Carbaryl	[105]
Au@ZIF-67	ZIF-67	Hydrothermal	Thiram, Carbendazim	[106]
Chitosan (CS)	-	Ionic gelation	Plant diseases	[107]
Cu (ChNC@Cu)	Chitin nanocrystals (ChNC)	TEMPO oxidation	Pesticides	[108]
SiO2	Biopolymers	Sol–gel	Plastic films	[109]
ZnO and TiO2	-	One-pot	Polycyclic aromatic hydrocarbons	[110]
Pd-nZVI and S-nZVI	-	Reduction with NaBH4	Trichloroethylene	[111]
			Toxicity abatement
PbO2	Humic acid	Chlorination	Toxicity in medaka fish	[112]
Metal, metal oxide, carbon, plastic	-		Phytotoxicity	[113]
nFe@Fe3O4, nFe3O4 nFe2O3		-	Toxic effects on zebra fish	[114]
ZnO and Ag	-	Sol–gel+ reduction with NaBH4	Acute toxicity in zebra fish	[115]
			Chemical processes
Pt@S-1	β zeolite	-	Synthesis of naphtha	[116]
ZnO	Molasses and urea	Precipitation	Synthesis of fertilizer	[117]
OMGR Oximagnesite/green rust	-	Hydrothermal	Phosphate recovery	[118]
CrMnFeCoNi-HEO; MgCoNiCuZn-HEO	Spinel-structure sHEO	Sol–gel	Desalination techniques of seawater	[119]
RuNi-rGO@ MNCs	N-doped C nanosheets	Reduction with H2	Hydrogen evolution reaction (HER)	[120]
			Others
TiO2	Recycled rubber tiles	Sol–gel	Airborne pollution	[121]

Table 1. Use of various metals in nanoparticle and nanocomposite synthesis and their subsequent use in environmental pollutant removal.

Nanoparticles	Support	Synthesis Techniques	Environmental Applications	Reference
			Removal of Dyes
ZnO	-	Sol–gel co-precipitation in KOH	Methylene blue	[55]
Zr-doped Fe2O3	CoOx	Hydrothermal	Orange II	[59]
Ca peroxide	Starch	Precipitation	Methylene blue	[63]
TiO2	g-C3N4(g-CN) nanolayers	Co-precipitation; thermal polymerization	Methylene blue	[64]
Au	Au/VO2/CeO2	Photodeposition	Methylene blue	[69]
Magnetite (MN)	Fe3O4@C	Microwave-induced	Methylene blue	[70]
Ag	Na0.5Bi0.5TiO3 nanospheres	Chemical solution, hydrothermal	Rhodamine B	[71]
HRC-Ag/Agar	Agarose hydrogel	Reduction with KBH4	Methylene blue, rhodamine B	[72]
Cationic polyethyleneimine (PBVPR218)	-	-	Dyes	[73]
EC-Ag	Elettaria cardamomum	Reduction with NaBH4	Methylene blue, rhodamine B	[74]
nZVI	Janus particles	Reduction with NaBH4	Methyl orange	[75]
CdAl2O4	Fe	Co-precipitation	Brilliant blue, brilliant green	[76]
CuNiFe2O4	g-C3N4	Gel auto combustion	Methylene blue	[77]
Black phosphorus (BPQDs)-IO TiO2	-	Sonication assisted; liquid exfoliation	Rhodamine B, methylene orange	[78]
g-C3N4-Fe3O4@KF	Kapok fiber	One step	Rhodamine B	[79]
CeO2	-	Co-precipitation, calcination	Methylene blue	[80]
PLA/CMC/GO _f-COOH@Ag	Polymeric matrix	Reduction with ascorbic acid	Methylene blue	[81]
(B-GNP)	Graphene	Polyvinyl alcohol film	Atrazine	[82]
Magnetite nanoparticles (MNs)	β-cyclodextrin (β-CD) and quaternary ammonium salts	Co-precipitation	Methylene blue, Orange G	[83]
C3N5-LDH-Ag	C3N5-LDH	Precipitation in NaOH	Tartrazine	[84]
			Removal of Organic Compounds
Ferrihydrite (Fh)	Non-ionic surfactant Brij L4	Co-precipitation	Cooking oils	[44]
TiO2	-	Sol–gel	Phenol	[56]
Zr-doped Fe2O3	CoOx	Hydrothermal	Bisphenol A	[59]
AuNPs	Au/VO2/CeO2	Photodeposition	Aromatic alcohols, p-nitrophenol	[69]
nZVI	Janus particles	Reduction with NaBH4	Trichloroethylene	[75]
Fe3O4/FeS	Biochar	Pollen pyrolysis	phenol	[85]
AA@Fe°	Amino acids	Reduction with KBH4	Tributyl phosphate, n-dodecane	[86]
Biochar	-	Pyrolysis	Bisphenol A	[87]
Fe3O4	rGO	Co-precipitation	4-Aminophenol	[88]
Aptamer-MrGO@Au and ssDNA-AuNP@MBs	BPA aptamer	Reduction with trisodium citrate	Bisphenol A	[89]
ZnO@SCF SppCobalt Ferrite	Spinel cobalt ferrite	Precipitation	Phenantrene	[90]
Fe⁰@C/surfactant	Carbon	Hydrothermal; carbothermic; reduction with NaBH4	Nitrobenzene	[91]
Zn/FeO	Surfactant foams	Reduction with NaBH4	Diesel	[92]
			Removal of Pharmaceutical Compounds
CuO and nZVI	-	Sol–gel; precipitation in NaOH; reduction with NaBH4	Levofloxacin	[57]
CuO	-	Co-precipitation; calcination	Metronidazol	[93]
Ag-Cu-Li	Multimetal nanorods	Precipitation in NaOH	Antibacterial activity	[94]
WS2	-	Hydrothermal thiourea	Tetracycline	[95]
ZnO	Alginate nanofibers	Hydrothermal	Tetracycline	[96]
BiFeO3	-	Combustion	Ofloxacin	[97]
TGC/NiCr2O4	Tubular g-C3N4 (TGC)	Calcination	Tetracycline	[98]
ZnO	Carbon cloth (CC)	Hydrothermal	Hydroxychloroquine	[99]
Ag/Ag3PO4-VAg	Ag nanocluster with vacancies of Ag (Ag/Ag3PO4-Vag)	Reduction with NaBH4	Sulfamethoxazole	[100]
Au	-	Reduction with NaBH4/320 °C	Penicillin G, sulfamethazine, tetracycline, enrofloxacin, skewered fish	[101]
Bi-Ag nanoalloys	-	Solvothermal	Bacterial infections	[102]
TiO2NW-CuO- cellulose	Cellulose	Solvothermal	Escherichia coli	[103]
			Removal of Pesticides
Ag (SNP)	-	Reduction with NaBH4	Miticidal activity	[104]
Au	Ovalbumin	Reduction with trisodium citrate	Carbaryl	[105]
Au@ZIF-67	ZIF-67	Hydrothermal	Thiram, Carbendazim	[106]
Chitosan (CS)	-	Ionic gelation	Plant diseases	[107]
Cu (ChNC@Cu)	Chitin nanocrystals (ChNC)	TEMPO oxidation	Pesticides	[108]
SiO2	Biopolymers	Sol–gel	Plastic films	[109]
ZnO and TiO2	-	One-pot	Polycyclic aromatic hydrocarbons	[110]
Pd-nZVI and S-nZVI	-	Reduction with NaBH4	Trichloroethylene	[111]
			Toxicity abatement
PbO2	Humic acid	Chlorination	Toxicity in medaka fish	[112]
Metal, metal oxide, carbon, plastic	-		Phytotoxicity	[113]
nFe@Fe3O4, nFe3O4 nFe2O3		-	Toxic effects on zebra fish	[114]
ZnO and Ag	-	Sol–gel+ reduction with NaBH4	Acute toxicity in zebra fish	[115]
			Chemical processes
Pt@S-1	β zeolite	-	Synthesis of naphtha	[116]
ZnO	Molasses and urea	Precipitation	Synthesis of fertilizer	[117]
OMGR Oximagnesite/green rust	-	Hydrothermal	Phosphate recovery	[118]
CrMnFeCoNi-HEO; MgCoNiCuZn-HEO	Spinel-structure sHEO	Sol–gel	Desalination techniques of seawater	[119]
RuNi-rGO@ MNCs	N-doped C nanosheets	Reduction with H2	Hydrogen evolution reaction (HER)	[120]
			Others
TiO2	Recycled rubber tiles	Sol–gel	Airborne pollution	[121]

3.8. Commercial Nanoparticles

Commercial nanoparticles (CNPs) are also widely used in various fields such as medicine, pharmaceuticals, and consumer products. The main advantage of using CNPs lies in the fact that they offer well-established synthesis methods and scalability for mass production, making them readily available for industrial applications. CNPs are extensively characterized using various spectroscopic techniques like TEM, SEM, FTIR, and UV–visible absorption spectroscopy, ensuring consistent quality and properties. A wide variety of uses of CNPs have been detailed in the literature and are illustrated in Table 2.

Table 2. Commercial nanoparticles for environmental applications.

Nanoparticles	Support	Environmental Applications	Reference
SiO2@CA MSs	Microspheres	Removal of dyes	[122]
UHMWPE/TiO2	Polyethylene	Removal of methyl orange, methylene blue, Congo red, and tetracycline	[123]
nZVI, Fe2O3, Fe3O4	-	Florfenicol containing cow manure	[124]
ZnO	-	Escherichia coli	[125]
nZVI	-	Tetracyclines	[126]
ZnO and TiO2	-	Tetranychus urticae and Neoseiulus californicus	[127]
NiO	-	Neurotoxicity in zebra fish	[128]
ZnO	Film of chitosan	Loaf bread shelf life	[129]
nZVI	-	Polycyclic aromatic hydrocarbons	[130]
ZnO	-	Gut microbiome alterations in rats	[131]
Ag	-	Improved quality of Capsicum annum crops	[132]
SiO2, Al2O3, and O-CNT	-	Clofibric acid, acetaminophen, sulfamerazine	[133]
nZVI	-	Hydrophobic organic compounds, sediments	[134]
ZnO	-	Biofortification of Dracocephalum moldavica	[135]
ZnO	Pannonibacter- phragmitetus	Inhibition of microbial Cr(VI) reduction	[136]
Se	-	As (III), soybean roots	[137]

Table 2. Commercial nanoparticles for environmental applications.

Nanoparticles	Support	Environmental Applications	Reference
SiO2@CA MSs	Microspheres	Removal of dyes	[122]
UHMWPE/TiO2	Polyethylene	Removal of methyl orange, methylene blue, Congo red, and tetracycline	[123]
nZVI, Fe2O3, Fe3O4	-	Florfenicol containing cow manure	[124]
ZnO	-	Escherichia coli	[125]
nZVI	-	Tetracyclines	[126]
ZnO and TiO2	-	Tetranychus urticae and Neoseiulus californicus	[127]
NiO	-	Neurotoxicity in zebra fish	[128]
ZnO	Film of chitosan	Loaf bread shelf life	[129]
nZVI	-	Polycyclic aromatic hydrocarbons	[130]
ZnO	-	Gut microbiome alterations in rats	[131]
Ag	-	Improved quality of Capsicum annum crops	[132]
SiO2, Al2O3, and O-CNT	-	Clofibric acid, acetaminophen, sulfamerazine	[133]
nZVI	-	Hydrophobic organic compounds, sediments	[134]
ZnO	-	Biofortification of Dracocephalum moldavica	[135]
ZnO	Pannonibacter- phragmitetus	Inhibition of microbial Cr(VI) reduction	[136]
Se	-	As (III), soybean roots	[137]

CNPs exhibit consistent sizes and morphologies, which surpass those synthesized in the laboratory, resulting in less variability in their chemical properties. Furthermore, they exhibit superior adherence to various supports like microspheres and films, as outlined also in the table, facilitating the attainment of more comparable results.

CNPs are produced using traditional chemical and physical methods, which may involve the use of hazardous compounds and harsh reaction conditions. In contrast, green-synthesized nanoparticles are characterized by being less toxic, cost-effective, and eco-friendly, as they utilize biogenic materials such as plants, algae, waste biomass, and microorganisms as reducing agents. Nonetheless, despite the benefits of green synthesis, the commercial production of green-synthesized nanoparticles has not scaled up, highlighting limitations in the scaling of green synthesis for commercial production. In other words, commercial nanoparticles offer established synthesis methods, scalability, and extensive characterization. The differences in production processes highlight the challenges in scaling-up green synthesis for commercial production.

3.9. Emerging Green Alternatives

The use of plant extracts in the green synthesis of nanoparticles has demonstrated significant advantages over conventional synthesis methods. The synthesis of nanoparticles by chemical reduction from plant extracts represents a sustainable and environmentally friendly approach with numerous environmental applications, including water treatment, for the removal of a wide variety of pollutants, such as dyes, pharmaceutical compounds, pesticides, inorganic compounds, and heavy metals. The synthesis of green nanoparticles takes advantage of the reducing and stabilizing properties of phytochemicals present in various plant extracts to convert metal ions into nanoparticles. As shown in Table 3, a comprehensive review of this method and its environmental applications has been performed. Plant extracts rich in bioactive compounds such as flavonoids, phenolics, terpenoids, and alkaloids serve as reducing and stabilizing agents for metal salts, usually salts of silver, gold, copper, iron, magnesium, titanium, or zinc, dissolved in solution. The plant extract is added to the metal salt solution, initiating the reduction process. The phytochemicals in the extract reduce the metal ions to form nanoparticles. The nanoparticles are stabilized by the phytochemicals in the extract, which prevents their aggregation and ensures their colloidal stability.

Table 3. Synthesis of nanoparticles by chemical reduction using plant extracts (green chemistry) for environmental applications.

Nanoparticles	Support	Reactive Agent	Environmental Applications	Reference
			Removal of Dyes
Ag/Ti	-	Aloe vera L.e.	Rhodamine B	[46]
MoO3 and WO3 nanorods	-	Leidenfrost (H2O) with NaOH	Methylene blue	[138]
Ag	-	Antidesma acidum L.e.	Congo red, methylene blue	[139]
B-doped g-C3N4/TiO2	g-C3N4	Spinacia oleracea	Methylene blue	[140]
Au	-	Wedelia urticifolia	Rhodamine B	[141]
ZnS/Fe3O4	Carboxymethyl cellulose	Co-precipitation of Fe(II) and Fe(III)	Methylene blue, methyl orange Congo red, and rhodamine B	[142]
ZnO	-	Citrus x lemon	Reactive green 19	[143]
IRCFA-PDA@Ag	Iron-rich coal fly ash (IRFA)-Polydopamine (PDA)	Floral	Methylene blue	[144]
ZnO, CuO, MnO2 and MgO	-	Leucaena leucocephala	Golden yellow-145, Direct red-31	[145]
nZVI	-	Cow and goat milk	Methyl orange	[146]
Au	-	Cell-free filtrate of Penicillium rubens	Methylene blue, phenol red, bromothymol blue, methyl orange	[147]
ZnO	Mesoporous carbon	Firecracker waste	Methyl orange	[148]
ZnO	-	Aloe vera	Malachite green, Basic violet 3	[149]
ZnO and SiO2	-	Cyperus alternifolius	Methylene blue	[150]
			Organic Compounds
Pt-SnO2	rGO-CH	Amaranthus spinosus	Methanol	[151]
nZVI	-	Tung, aspen, and holly L.e.	Tetrabromobisphenol A	[152]
CeO2	-	Dillenia indica	2,2-difphenyl-1- -picrylhydrazyl	[153]
GM-Ag	-	Gnetum montanum	3-nitrophenols and 4-nitrophenols	[154]
MgO	-	Jatropha oil	Compounds in air, HC, CO, CO2	[155]
Fe-C, Co-C, and Ni-C	-	Tea residue	Cystine	[156]
			Removal of Microorganisms
ZnO		Brassica oleracea	Gram-negative Bc Escherichia coli	[157]
Ag		Pseudomonas canadensis bacterial isolate	Pseudomonas tolaasii Pt18	[158]
CuO		Alpinia officinarum	Colletotrichum gloeosporioides	[159]
nZVI		Common mantle	Pathogenic fungi, wheat plants	[160]
			Removal of Pharmaceutical Compounds
ZnO	-	Aloe vera	Amoxicillin	[149]
Ag	-	Passiflora foetida	Antibacterial	[161]
Ag	-	Calotropis procera	Fungal and bacterial pathogens	[162]
Ag, Cu, and Fe		Catharanthus roseus L.e.	Several anti-inflammatory drugs	[163]
Lignin	Nanocellulose cryogels	Kraft lignin	Diclofenac, metropolol, tramadol, carbamazepine	[164]
			Removal of Inorganic Compounds
MgO	-	Jatropha oil	CO, CO2 in air	[155]
Fe-C, Co-C, and Ni-C	-	Tea residue	CrO42¯	[156]
Chitosan	-	Salix subserrata bark extract	As in rats	[165]
Magnetite	-	Amla (tree bark)	U(VI)	[166]
ZnO	-	Acacia catechu L.e.	As	[167]

Note: L.e.: leaf extract.

Table 3. Synthesis of nanoparticles by chemical reduction using plant extracts (green chemistry) for environmental applications.

Nanoparticles	Support	Reactive Agent	Environmental Applications	Reference
			Removal of Dyes
Ag/Ti	-	Aloe vera L.e.	Rhodamine B	[46]
MoO3 and WO3 nanorods	-	Leidenfrost (H2O) with NaOH	Methylene blue	[138]
Ag	-	Antidesma acidum L.e.	Congo red, methylene blue	[139]
B-doped g-C3N4/TiO2	g-C3N4	Spinacia oleracea	Methylene blue	[140]
Au	-	Wedelia urticifolia	Rhodamine B	[141]
ZnS/Fe3O4	Carboxymethyl cellulose	Co-precipitation of Fe(II) and Fe(III)	Methylene blue, methyl orange Congo red, and rhodamine B	[142]
ZnO	-	Citrus x lemon	Reactive green 19	[143]
IRCFA-PDA@Ag	Iron-rich coal fly ash (IRFA)-Polydopamine (PDA)	Floral	Methylene blue	[144]
ZnO, CuO, MnO2 and MgO	-	Leucaena leucocephala	Golden yellow-145, Direct red-31	[145]
nZVI	-	Cow and goat milk	Methyl orange	[146]
Au	-	Cell-free filtrate of Penicillium rubens	Methylene blue, phenol red, bromothymol blue, methyl orange	[147]
ZnO	Mesoporous carbon	Firecracker waste	Methyl orange	[148]
ZnO	-	Aloe vera	Malachite green, Basic violet 3	[149]
ZnO and SiO2	-	Cyperus alternifolius	Methylene blue	[150]
			Organic Compounds
Pt-SnO2	rGO-CH	Amaranthus spinosus	Methanol	[151]
nZVI	-	Tung, aspen, and holly L.e.	Tetrabromobisphenol A	[152]
CeO2	-	Dillenia indica	2,2-difphenyl-1- -picrylhydrazyl	[153]
GM-Ag	-	Gnetum montanum	3-nitrophenols and 4-nitrophenols	[154]
MgO	-	Jatropha oil	Compounds in air, HC, CO, CO2	[155]
Fe-C, Co-C, and Ni-C	-	Tea residue	Cystine	[156]
			Removal of Microorganisms
ZnO		Brassica oleracea	Gram-negative Bc Escherichia coli	[157]
Ag		Pseudomonas canadensis bacterial isolate	Pseudomonas tolaasii Pt18	[158]
CuO		Alpinia officinarum	Colletotrichum gloeosporioides	[159]
nZVI		Common mantle	Pathogenic fungi, wheat plants	[160]
			Removal of Pharmaceutical Compounds
ZnO	-	Aloe vera	Amoxicillin	[149]
Ag	-	Passiflora foetida	Antibacterial	[161]
Ag	-	Calotropis procera	Fungal and bacterial pathogens	[162]
Ag, Cu, and Fe		Catharanthus roseus L.e.	Several anti-inflammatory drugs	[163]
Lignin	Nanocellulose cryogels	Kraft lignin	Diclofenac, metropolol, tramadol, carbamazepine	[164]
			Removal of Inorganic Compounds
MgO	-	Jatropha oil	CO, CO2 in air	[155]
Fe-C, Co-C, and Ni-C	-	Tea residue	CrO42¯	[156]
Chitosan	-	Salix subserrata bark extract	As in rats	[165]
Magnetite	-	Amla (tree bark)	U(VI)	[166]
ZnO	-	Acacia catechu L.e.	As	[167]

Note: L.e.: leaf extract.

There are many environmental applications, such as water treatment. Silver nanoparticles are used for the removal of dyes, pharmaceutical compounds, and pesticides and can also be used to disinfect water because of their strong antimicrobial properties. Zinc and magnesium oxide nanoparticles synthesized using green chemistry methods can be used to remove heavy metals and organic pollutants from contaminated soil and water. Gold nanoparticles can catalyze the degradation of organic pollutants such as dyes (rhodamine B, methylene blue, phenol red, bromothymol blue, methyl orange) in wastewater through different processes, contributing to the remediation of contaminated water bodies. Furthermore, nanoparticles obtained by green chemistry methods can be used for the elimination of conventional pesticides, reducing the environmental impact of agricultural activities. They can also be incorporated into air filtration systems to capture and remove pollutants and particulates, helping to improve indoor and outdoor air quality.

The advantages of green synthesis using plant extracts are that it reduces reliance on hazardous chemicals and minimizes the generation of toxic by-products, making it environmentally sustainable. Green synthesis methods are often cost-effective compared to traditional chemical synthesis routes, as they use readily available plant materials and simple reaction conditions.

In conclusion, the literature review underscores the viability of applying nanotechnology to combat emerging contaminants, notwithstanding inherent challenges. Through the synthesis of nanoparticles utilizing diverse metals, a spectrum of methodologies, ranging from traditional techniques to eco-friendly approaches and readily available commercial routes, has emerged as a potent strategy in environmental remediation efforts. Traditional synthetic procedures have long been employed, taking advantage of the inherent properties of metals to engineer nanoparticles for specific applications. Green chemistry principles have further enriched this landscape, advocating for environmentally benign methodologies that minimize waste and energy consumption. Concurrently, the accessibility of commercial nanoparticles has streamlined research endeavors, offering standardized materials with consistent properties that enhance reproducibility and comparability of results.

Beyond these established methods, recent innovations continue to expand the arsenal of nanotechnology in environmental stewardship. Emerging approaches such as functionalized nanoparticles and nanocomposite materials exhibit promising capabilities for targeted contaminant removal, paving the way for more efficient and selective remediation strategies. Furthermore, the interdisciplinary nature of nanotechnology facilitates holistic solutions to complex environmental challenges by integrating principles from materials science, chemistry, and environmental engineering.

4. Zero-Valent Iron Nanoparticles: Conventional and Eco-Friendly Synthesis

4.1. Zero-Valent Iron Nanoparticles

Given the need to develop sustainable methods and solve environmental problems, iron is used as a biocompatible material with a relatively non-toxic nature and high catalytic efficiency among the transition elements. It has been widely used in the reduction of environmental pollutants in an environmentally friendly way [168,169,170], in biomedical applications [171,172,173], in batteries [174,175,176], as a catalyst in a large variety of processes [177,178,179], etc. Iron has the advantage of being suitable for use in sustainable technologies as it avoids the toxic load generated by other materials. It normally substitutes Ni or Co in a reduction catalyst [180,181], Pd in hydrogenation reactions [182,183], and metal oxides like TiO₂ or CuO in pollutant degradation [184,185,186,187]. Iron has two oxidation states, Fe²⁺ and Fe³⁺, which give rise to oxide minerals such as hematite (Fe₂O₃), magnetite (Fe₃O₄), goethite (FeO(OH)), and siderite (FeCO₃); in water, the predominant species are Fe²⁺ and Fe³⁺ and ferrous and ferric organic complexes. In the absence of air and at pH ≈ 2, the inorganic Fe³⁺ salts are the most stable species. Under normal conditions the most abundant species is Fe²⁺, while at basic pH the insoluble hydroxide predominates [188]. Reduction of Fe ions into Fe(0) obviously requires the presence of Fe(II) or Fe(III) in solution. Fe(III) starts to precipitate at pH above 7.5, and Fe(III) at pH above 3–3.5, thus giving rise to the presence of solid iron hydroxides and oxohydroxides. Hence, when attempting to prepare Fe(0) nanoparticles from dissolved iron species, it is of the utmost importance to operate within the pH interval at which Fe(II) and/or Fe(III) exist in solution. This can be easily understood from the Pourbaix diagrams of iron that can be found in Ref. [189]. The Pourbaix diagram indicates that the stability of iron in solution is related to both the electrode potential and the pH of the aqueous solution. It predicts the direction of spontaneous iron reduction, analyses the trend in metal oxidation from thermodynamics and estimates the composition of the reduction products [190]. The strong reducing character of the metal core of nZVI nanoparticles acts synergistically in the removal of pollutants from groundwater, soil, and wastewater [189,191].

Because of their exceptional characteristics, effectiveness in removing contaminants, low toxicity, and ability to move effectively through porous media, nZVIs can serve as cost-effective alternatives to in situ contaminant removal methods. These nZVIs contain donor electrons, which are responsible for their high reactivity in aqueous environments. This reactivity is a result of both iron (Fe⁰) corrosion reactions and non-selective reactions with dissolved oxygen. In the particular case of dyes, understanding factors influencing the oxidation process, such as nZVI modifications, concentrations, pH, temperature, and soil constituents, is crucial for optimizing treatment efficiency [192]. Dyes are primarily removed by destroying their chromogenic groups through reduction or Fenton-like reactions with zero-valent iron nanoparticles [193]. Nonetheless, the presence of active oxygen species in solution also plays a key role in the degradation process. For instance, persulfate activation, facilitated by nZVIs, yields sulfate radicals with potent oxidizing power, offering high selectivity in pollutant degradation [194,195,196]. Additionally, aerobic degradation pathways, enabled by nZVIs, involve the reductive activation of molecular oxygen, generating reactive oxygen species for the oxidation or mineralization of organic pollutants [197]. The core–shell structure-dependent pathways (see below) and the molecular oxygen activation mechanism provide valuable insights into aerobic degradation processes mediated by nZVIs.

It is commonly accepted that nZVI nanoparticles exhibit a core–shell structure, where a metallic iron core is enveloped by a thin layer of iron oxides or oxyhydroxides [198,199]. The thickness of the passivation layer, which varies depending on synthesis conditions, significantly impacts the electron transfer process; thicker passivation layers hinder effective electron transfer from the metallic core to the outer surface [200]. In environmental applications, the core–shell nZVI model satisfactorily explains the efficacy in wastewater treatment and catalytic processes, as the outer shell prevents iron nanoparticle aggregation while offering protective coverage [201]. Characterization techniques such as spherical-aberration-corrected scanning transmission electron microscopy (Cs-STEM) and energy-dispersive X-ray spectroscopy (EDS) provide confirmation of the core–shell structure and enable analysis of the passivation layer composition [199]. The core’s structural attributes hold significant importance for the chemistry of nZVIs. While the core serves as the source of electrons for reactions involving nZVIs, the shell is the focal point for intricate chemical reactions and electrostatic interactions [202,203,204,205]. The shell oxide is flawed and its disordered nature renders it more reactive when compared to a straightforward passive oxide.

The typical size of Fe nanoparticles falls in the range of ten to several hundred nanometers, depending on the technique used. The surface area of the nanoparticle depends on the particle size, being theoretically between 150 and 15 m²/g for the usual range (10 to 100 nm; see Figure 2a). Most of the articles that have reported their surface area obtained a range of 30 to 50 m²/g for nanoparticles of 20–40 nm.

In cases where iron nanoparticles aggregate, they feature a continuous oxide shell, with the metal cores separated by a thinner interfacial oxide layer. Nemati et al. synthesized Fe nanoparticles by thermal decomposition of organometallic compounds [206]. A transmission electron microscopy (TEM) image reveals a typical cluster of nanoscale zero-valent iron (nZVI) particles. The core shell ranging from 3 nm to 7 nm is visible. Montferand et al. synthesized iron nanoparticles by reducing a salt precursor in liquid media at temperatures above 200 °C using stabilizing and surfactant agents [207], resulting in several shapes depending on the stabilizing agent and concentration.

Ansari et al. [208] prepared Fe nanoparticles using ferric iron and sodium borohydride as the reducing agents under ambient conditions. These nanoparticles visibly clustered together, forming interconnected chain-like structures, primarily due to magnetic and electrostatic interactions. Each particle consisted of a dense core surrounded by a thin shell, which exhibited noticeably lower contrast than the interior core. When examining the core region through selected area electron diffraction (SAED), diffused ring patterns are observed, characteristic of nanocrystalline body-centered cubic (bcc) iron metal. Notably, the chains of interconnected nanoparticles exhibit a continuous oxide shell, but each metallic core is separated from its neighbors by a thinner interfacial oxide layer, approximately 1 nanometer in thickness. The observed shell thickness under TEM aligns reasonably well with measurements from previous X-ray photoelectron spectroscopy (XPS) studies. These earlier investigations estimated an average oxide thickness of approximately 2–3 nanometers based on the relative intensities of oxidized and metallic iron signals. A phase-contrast TEM image shell further illustrated that the oxide displays a speckled contrast and lacks periodic lattice fringes, indicating an amorphous character in the oxide layer’s structure. This disordered oxide layer can be partly attributed to the exceedingly small radio of the nanoparticles and the curvature of the oxide shell, both of which introduce significant strain, hindering the formation of crystalline structures. Additionally, the presence of a small amount of boron in the form of iron borides [209] in the oxide film, stemming from the borohydride precursor used during the synthesis step, may contribute to defective sites and alter the oxide structure [203].

4.2. Influence of Synthetic Methods for the Preparation of nZVIs

The selected synthetic method might allow rational control of the size and shape of the nanoparticles to tailor as much as possible the properties of the formed nanomaterials to a specific application [193]. The most common are illustrated in Figure 3, with the typical particle size range and the yield obtained.

The most used methods for the synthesis of iron nanoparticles are electrochemical, chemical reduction using chemical agents or plant extracts (sometimes call biomimetic), hydrothermal, and sol–gel. The range of particle sizes is quite wide within each technique and varies depending on the preparation conditions. However, to finely control the range some techniques are better, especially the ones that allow the use of additives, such as chemical reduction, hydrothermal, sol–gel, or microemulsion. Techniques such as chemical vapor deposition lead to smaller particle sizes, but the size range is very narrow, which can be an advantage or disadvantage depending on the application. The yield is not commonly reported, and it is one of the most influential factors in being able to declare the technique as “green”. The reported yields in Figure 3 are extracted from several publications and are very approximate. Reporting the yield of nZVIs obtained should be encouraged. The methods with the largest yield are chemical reduction, hydrothermal, sol–gel, microemulsion, and ball milling. Regarding morphology, the most common shape is spheres; however, it varies with the size and the technique. Normally, small nanoparticles tend to be spherical, while large nanoparticles can be rods, spheres, or cubes. The introduction of additives or other metals during the synthesis helps in tuning the morphology.

Chemical reduction methods require a metal precursor, a reducing agent, and sometimes the use of a stabilizing agent. Regarding the metal precursors, the most common are inorganic salts; nitrates, chlorides, or sulfates. The reducing agent can be divided into two large groups: common chemical reagents and plant extracts. In the former case, dissolved iron salts are reduced, using NaBH₄ or KBH₄ as a reducing agent, effectively converting Fe (II) or Fe (III) salts into Fe nanoparticles [210], typically at room temperature and with a reaction time of one hour or less. Next, iron nanoparticles must be separated and washed thoroughly to remove all impurities of the reducing agent. Normally, this methodology leads to a narrow range of particle size distribution and the method is quite reproducible. However, it is worth noting that NaBH₄ is highly toxic, corrosive, and flammable, posing environmental risks. Moreover, nanoparticles synthesized with NaBH₄ tend to agglomerate easily, leading to reduced reactivity and stability. During reduction with NaBH₄, it is oxidized to various species, such as B₂O₃, B(OH)₃, and some intermediates. The reuse of these species could go a long way towards the sustainability of the method, giving a very reproducible and sustainable method. There are currently projects that consider boron as a possible agent for the storage of H₂ and boron oxides are already being used to obtain NaBH₄. The Kotai Hydrogen Project is a collaborative project with experts from Curtin University in Perth. The research project is investigating the feasibility of using sodium borohydride (NaBH₄) as a safe hydrogen ‘carrier’ that can be used on demand where needed.

In chemical reduction using plant extracts, dissolved iron salts are reduced using plant extracts as natural reducing agents, which usually is claimed as green synthesis by the scientific community. However, this is not supported by evidence such as life cycle assessment studies, calculation of CO₂ emissions, etc. This point will be discussed in detail in Section 5.

4.3. Stabilization of nZVIs

Fe(0) nanoparticles tend to become unstable under atmospheric conditions and readily form oxides and hydroxides, including Fe₂O₃, Fe₃O₄, and FeO(OH), under specific conditions. In addition, like other nanomaterials, they have a naturally strong inclination to agglomerate into larger particles due to van der Waals interactions and subsequently settle, as graphically shown in Figure 4. In efforts to prevent the aggregation of nZVIs and enhance their environmental remediation capabilities, various modifications to its structure and synthesis methods have been developed. These modifications aim to boost the stability, reactivity, and surface area of nZVIs [211]. Figure 4 also schematizes such procedures. Techniques for modifying nZVIs include (among others):

• Incorporating a second metal into nZVIs.
• Coating the surface of nZVIs with organic substances.
• Synthesizing emulsified nZVIs.
• Fixed support.
• Encapsulation.
• Electrostatic stabilization.
• Steric stabilization.

Figure 4. Scheme of different stabilization methods.

Figure 4. Scheme of different stabilization methods.

4.3.1. Incorporating a Second Metal into nZVIs

To enhance the reaction rate of nZVIs in pollutant degradation, small amounts of transition metals like Pt, Ag, Cu, and Ni can be added to the nanoparticle surface, serving as catalysts [43]. These metals are believed to accelerate degradation through mechanisms such as facilitating electron transfer via galvanic coupling and generating reactive atomic hydrogen. Additionally, bimetallic particles reduce the accumulation of corrosion products on the particle surface and protect against passivation. Consequently, bimetallic nanoparticles find applications in the decolorization of aromatic compounds and polychlorinated biphenyls, which typically degrade slowly. Bimetallic nanoparticles can be synthesized through two methods [212]. The first involves mixing the as-synthesized nZVIs with a solution containing the second metal salt to be added. As the reduction potential of transition metals (Pt, Ag, Cu, or Ni) is more positive than that of Fe [213], a metal replacement reaction occurs, and the zero-valent metal is deposited on the nZVI surface. This is known as the solution deposition method. The second method, called the co-reduction method, entails the simultaneous reduction of iron and the second metal salt using sodium borohydride [214]. Regardless of the method used, the transition metal forms a thin, discontinuous layer on the nZVI surface.

Among the studied transition metals, Pd has proven to be the most effective due to its optimal structure and chemical properties for generating active hydrogen species and breaking carbon–halogen bonds in dehalogenation reactions [215]. For instance, Fe/Pd nanoparticles have demonstrated reaction rates over one order of magnitude higher than common nZVIs for trichloroethylene degradation. However, a key drawback of bimetallic systems is their acceleration of iron corrosion via galvanic effects, which can rapidly decrease system reactivity. Further research is required to enhance the properties of bimetallic systems and extend their reactivity over prolonged periods.

4.3.2. Coating of nZVI Surfaces

Various compounds, including synthetic and natural polymers, surfactants, carboxylic acids, and polysaccharides [216], have been employed to coat the surfaces of nZVIs. Generally, molecules with high molecular weights and a high density of functional groups are more effective stabilizers [217]. The size of the resulting nanoparticles can be controlled by adjusting the concentration and chain length of the surfactant. Notably, biopolymers have garnered significant attention due to their cost-effectiveness and environmentally friendly nature. Examples include food-grade polysaccharides like chitosan [218,219], carboxymethyl cellulose (CMC) [218], starch [63,220], and guar gum [221], which have demonstrated their effectiveness as coatings for nZVIs in multiple studies. Among these, CMC and polyacrylic acid (PAA) have emerged as particularly promising stabilizers. CMC has proven to enhance nZVI reactivity and in situ derivability for various applications, while PAA/nZVI has shown improved mobility compared to CMC/nZVI under specific conditions, such as carbonate porous media and concentrated Ca²⁺ groundwater [222].

4.3.3. Emulsified nZVIs

Nanoparticle emulsification represents a cutting-edge technique that combines the properties of nanoparticles with the versatility of emulsions, opening a range of possibilities in diverse fields such as pharmaceuticals, cosmetics, food, and materials science, as well as contaminant removal. This process involves the dispersion of nanoscale particles within a continuous phase to form a stable emulsion. Several preparation methods are used to emulsify nanoparticles, including high-energy methods such as ultrasonication, high-pressure homogenization, and microfluidics. These techniques help break down larger nanoparticle aggregates into smaller, uniform particles dispersed throughout the emulsion. Stabilizing agents such as surfactants, polymers, and proteins are often used to prevent agglomeration or settling of the nanoparticles within the emulsion. These agents form a protective layer around the nanoparticles, preventing them from coalescing and maintaining the stability of the emulsion.

Recent studies indicate that the type of emulsion used in nZVIs is a water/oil emulsion, which offers the advantage of protecting the surface, preventing aggregation, and improving the degradation efficiency of non-polar substances. Two types of emulsions are presented: (i) oil-in-water, where nZVIs are placed in a non-polar substance that is dispersed in an aqueous solution [223]; and (ii) water-in-oil, nZVIs contained in water droplets of 10–20 µm in size are surrounded by an oil film [224]. Different surfactants are used to ensure the stability of the emulsions.

4.3.4. Fixed Support

The literature has also reported the utilization of Fe(0) nanoparticles supported on diverse materials such as clays, activated carbon, sawdust, and natural and synthetic zeolites to improve their effectiveness in removing contaminants from aqueous environments and prevent nanoparticle aggregation. Hydrophobic supports also extend the reactivity of nanoparticles and enable the adsorption of hydrophobic contaminants like PCBs onto the nZVI surface. Materials such as phyllosilicate minerals [225], granitic residual [226], silica [227], activated carbon [228], zeolites [229], biochar [230] or polymer membranes have been utilized as supports for nZVIs. Immobilization of nZVIs on these supports can be achieved through carboxyl, hydroxyl, or amine groups serving as chelating sites. Depending on the carrier and its characteristics, the adsorption properties of nZVIs can be improved. Numerous studies have demonstrated that immobilizing nZVI particles on solid supports simplifies operation while preserving the excellent reducing capacity of nZVIs [231]. However, a notable drawback of this modification method is that nanoparticles are exposed on the support surface, making them susceptible to rapid oxidation by air. Encapsulation of nZVIs is thus preferred, to enhance stability without compromising their contaminant remediation efficiency.

4.3.5. Encapsulation of nZVIs

Another method for modifying nZVIs involves their capture or encapsulation within a matrix, such as chitosan [232], arabic gum, calcium carbonate [233], calcium alginate, or carbon materials [91]. Among these, encapsulation within carbon nanospheres or microspheres has shown significant promise. Such encapsulation results in unique core–shell nanomaterials that exhibit both the adsorption properties of carbon and the reducing agent properties of nZVIs. Methods for synthesizing these hybrid materials include the confined plasma arc method and hydrothermal carbonization.

4.3.6. Electrostatic Stabilization

Electrostatic stabilization consists of repulsion between nanoparticles by forces related to charges or surface charge densities. These forces appear in nanoparticles, especially in those that have an interface between two crystallinity states, have doping agents, impurities of the precursor salts, etc. This effect can also be enhanced if components are adsorbed on the surface of the charged nanoparticles [234].

4.3.7. Steric Stabilization

Steric stabilization of nanoparticles is a technique used to prevent their aggregation and favor their dispersion in a solvent or medium. This technique involves the use of long-chain polymers or surfactants adsorbed on the surface of nanoparticles to create a steric barrier that hinders interactions between nanoparticles, thus improving their stability in dispersion. The advantage is that they result in stable dispersions that can be stored for extended periods without significant changes in the size or properties of the nanoparticles. Polyethylene glycol (PEG) or polyvinyl alcohol (PVA) are long-chain polymers that have a flexible structure that extends away from the nanoparticle surface, creating a physical barrier between the nanoparticles [235]. Surfactants, such as the Tween or Triton series, can also provide steric stabilization. The surfactant molecules adsorb on the surface of the nanoparticles, forming a layer that prevents the particles from coming into close contact with each other. Steric stabilization of the nanoparticles is usually achieved during their synthesis or post-synthesis.

Table 4 summarizes the most widely used conventional methods for nZVI synthesis and stabilization.

Table 4. Conventional synthesis of zero-valent iron nanoparticles.

FeNPs	Precursor	Support	Size/nm	Morphology	Characterization	Reference
MCMZVI	FeCl3·6H2O+ glucose+CO(NH2)2	Mesoporous carbon	20–100	Uneven size	XRD, SEM, XPS, FTIR, BET	[194]
CS-nZVI (core–shell)	FeCl3·6H2O+NaBH4	-	15.42–97.57	Spherical	UV–vis, FTIR, TEM, SEM EDX, XRF	[198]
nZVI	FeCl3·6H2O+NaBH4	-	34–110	Spherical	UV, XRD, SEM, EDX, TEM, DSL	[208]
nZVI@FSG	FeSO4·7H2O+NaBH4	Flaxseed gum	73–87	Spherical	DLS, FESEM, EDX, FTIR, DR5000	[216]
CMC-S-nZVI	FeSO4·7H2O+NaBH4	Carboxymethyl cellulose (CMC)	90	Spherical	TEM-EDS, UV–vis, PSD, ζ-potential	[218]
nZVI/SS/BC	FeSO4·7H2O+NaBH4	Biochar with stable starch	45.7–37	-	SEM, EDS, BET, FTIR, XRD	[220]
nZVI	FeSO4·7H2O+KBH4	Guar gum	87.4	Cubic	TEM, XRD, SEM, XPS	[221]
nZVI	FeSO4·7H2O+KBH4	Attapulgite	-	-	FTIR, SEM-EDS, XPS	[225]
Fe@SiO2	FeSO4·7H2O+KBH4	Silica	40–50	Nanospheres	XRS, FTIR, TEM, EDS	[227]
nZVI/AC	FeSO4·7H2O+NaBH4	Activated carbon	40–100	Rough	SEM, XRD, BET XPS	[228]
CS@nZVI	FeSO4·7H2O+NaBH4	Chitosan	13.12	-	XRD, FESEM, EDS, FTIR	[232]
CaCO3-nZVI	FeSO4·7H2O+NaBH4	CaCO3	75–89	Spherical	SEM-EDX, XRD, TEM, FTIR, XPS, BET	[233]
nZVI-A and nZVI-S	FeCl3·6H2O+NaBH4	Rhamnolipids	60 and 42	-	SEM, XRD, FTIR, TG, DLS, ζ-potential	[236]
FG-nZVI	FeSO4·7H2O+NaBH4	Flaxseed gum extract	<100	Spherical	DLS, FESEM, RDX, FTIR	[237]
nZVI	FeCl3·6H2O+NaBH4	-	20–60	Uniform morphology	XRD, SEM, TEM	[238]
nZVI	FeSO4·7H2O+NaBH4	-	14.3	-	XRD, SEM, TEM, FTIR	[239]
nZVI/GO	Fe (NO3)3·9H2O+NaBH4	Graphene oxide	10–20	Dispersed	SEM, TEM-EDS, FTIR	[240]
nZVI-kaol/PES	FeCl3·6H2O+NaBH4	Kaolin and poly-ethersulfone (PES)	42	Ridge and valley	XRD, FESEM, FTIR	[241]
NC-nZVI	FeCl3·6H2O+NaBH4	Nanocelluloses (NC)	116–200	Spherical	SEM, TEM-EDX, FTIR, XRD, XPS	[242]
Fe@BC	Black liquor lignin and Fenton sludge in one step	Biochar	20–50	Notable aggregation	XRD, FTIR, BET, BJH, FESEM, TEM	[243]
MFO@nZVI	FeCl3·6H2O+NaBH4	MnFe2O4 (MFO) hydrogel	-	Mass of spheroidal particles	XRD, SEM, FTIR, UV–vis DRS	[244]
CDLA@nZVI and CDCA@nZVI	FeCl3·6H2O+NaBH4	β-cyclodextrin (CD): CDLA and CDCA	25 and 30	Amorphous	NMR, FTIR, HRTEM, DLS, ζ-potential, FESEM, EDAX, VSM, XRD, XPS, TGA	[245]
Fe3O4@nZVI-PEI	Fe3O4+FeSO4·7H2O+NaBH4	Poly ethylenimine	-	-	SEM, TEM, FTIR, XRD, XPS	[246]
S-nZVI	FeSO4·7H2O+KBH4				-	[247]
nZVI-LBC	FeCl3·6H2O+NaBH4	Biochar	-	-	FTIR, XRD, TEM, XPS, VSM, BET, ζ-potential	[248]
nZVI/GAC	FeSO4·7H2O+NaBH4	Granular activated carbon (GAC)	40	Spherical	SEM, BET, XRD	[249]
nZVI/GO	FeCl3·6H2O+NaBH4	Graphene oxide (GO)	4.97	-	SEM-EDS, XRD, FTIR	[250]
nZVI/Sch-AP and nZVI/ Sch-CO	FeSO4·7H2O+NaBH4	Schwermannite (Sch)	50	Spherical	SEM, XRD, BET, XPS	[251]
A400-nZVI	FeSO4·7H2O+NaBH4	Polystyrenic gel (Purolite A400)	75–150	-	FTIR, SEM, EDAX, XRD, TGA	[252]
nZVI@ Zr(OH)4	FeCl3·6H2O+NaBH4	Zirconium hydroxide	-	-	TEM-EDS, XRD, BET, FTIR, XPS	[253]
pyGA-nZVI	FeCl3·6H2O+NaBH4	Pyrogallic acid (pyGA)	40–90	Spherical	SEM, TEM-EDS, BET, ζ-potential, XRD, FTIR, XPS	[254]
Ox-nZVI	FeCl3·6H2O+NaBH4	Oxalate	30–40	Spherical	BET, SEM-EDS, FTIR, XRD, XPS	[255]
nZVI 1 and nZVI 2	FeSO4·7H2O+NaBH4 FeCl3·6H2O+NaBH4	-	72 and 38	Spherical	SEM, TEM-EDS, XRD, BET, XPS, Raman spectroscopy	[256]
nZVI/ copper slag	FeCl3·6H2O+NaBH4	Copper slag	30	-	FE-SEM, EDX, XRD, FTIR, BET, VSM, ζ-potential	[257]
WPANF/ nZVI	FeCl3·6H2O+NaBH4	Polyacrylonitrile fiber (WPANF)	15–50	-	XRD, FTIR, BET, XPS, SEM-EDS, TEM	[258]
nZVI, ds-coated nZVI, and ds-FeS	FeSO4·7H2O+NaBH4	-	60.12 and 110	Spherical	SEM, XRD, FTIR, ζ-potential, DLS	[259]
nZVI	Self-combustion of Fe2O3 and NaBH4	-	2–8	Amorphous	XRD, HRTEM, EDS	[260]
nZVI	Laser fragmentation in liquids (LFL) ethylene glycol and polyethylene glycol 400	-	10.5 and below 3	-	DLS, LDE, TEM, XPS	[261]
CS@BC/ S-nZVI	FeSO4·7H2O+NaBH4	Chitosan and Biochar	-	-	SEM, BET, FTIR, XRD, XPS	[262]
3D-RGO@nZVI/ Al2O3	FeSO4·7H2O+NaBH4	Reduced graphene oxide	<100	Spherical	SEM, BET, Raman spectroscopy, XRD, XPS	[263]
nZVI/n-lignin	FeCl3·6H2O+NaBH4	Lignin	-	-	TEM, XPS, XRD	[264]
BP-S-nZVI	FeCl3+NaBH4+sulfate- reducing bacteria (SRB)	-	-	-	FESEM, TEM, XRD, BET	[265]
nZVI-DE	FeCl2·4H2O+NaBH4	Diatomaceous earth (DE)	20–40	Spherical	XRD, SEM, EDX, TEM, BET	[266]
CnZVI	FeSO4·7H2O+NaBH4	-	80–99	Spherical	UV, FTIR, XRD, TEM	[267]
G-nZVI-BC and C-nZVI-BC	FeSO4·7H2O+NaBH4	Biochar	-	-	SEM, XRD, FTIR, XRF	[268]
Iron oxide	FeCl3·6H2O+FeCl2·4H2O+ NaOH	-	10 ± 4	Regular crystalline	TEM	[269]
CS/nZVI	FeCl3+NaBH4	Chitosan	25	Spherical	SEM, FTIR, XRD, EDX, VSM, BET, TGA, DSC	[270]
BC@nFe-CA	FeSO4·7H2O+NaBH4	Biochar	-	-	SEM, EDS, FTIR, Raman spectroscopy, XRD, XPS	[271]
PDA@Fe/rGO	FeSO4·7H2O+NaBH4	Reduced graphene oxide (rGO) and polydopamine (PDA)	51	-	TEM, XRD, FTIR, XPS, VSM	[272]
PDCA@nZVI	FeCl3+NaBH4	2,6-pyridinedicarboxylic acid, (PDCA)	115	Nanospheres	SEM, EDS, EDX, XRD, FTIR	[273]
nZVI	FeSO4·7H2O+NaBH4	-	-	Spherical	SEM, TEM, BET	[274]
nZVI/BC	Pyrolysis (Fe2O3+biochar)	-	200	-	XRD, SEM	[275]
CMC-nZVI, bare-nZVI, PAA-nZVI, PSM-nZVI, PVP-nZVI	FeCl3+NaBH4	CMC, PAA, PSM, PVP	9.53, 65.4, 106.4, 106.6, and 109	Spherical	SEM-EDX, XRD, FTIR, TEM	[276]
nZVI-HPB	FeCl3·6H2O+NaBH4	Hydrophilic biochar	52–243	-	SEM, TEM, XRD, FTIR, XPS	[277]
nZVI	FeSO4·7H2O+NaBH4	-	20–60	Spherical	BET, SEM EDX, BET, XRPD, XPS	[278]
nZVI/SBA-15	FeCl3·6H2O+NaBH4	Mesoporous silica (Santa Bárbara-15)	50–80	Spherical	SEM EDS, TEM, BET	[279]
nZVI	FeCl3·6H2O+NaBH4	-	36	Regular and irregular	XRD, SEM, EDX, UV–vis	[280]
nZVI-chitosan	FeCl3·6H2O+NaBH4	Chitosan	15–20	-	SEM, XRD, TEM, FTIR	[281]
nZVI	Bulk iron disks+ solvents+ laser	-	9.4 and 3.5	Spherical	TEM EDS, XPS	[282]
nZVI/RS	FeSO4·7H2O+NaBH4	Biochar	-	-	FTIR, XRD, Raman spectra, BET	[283]
AHG@nZVI	FeSO4·7H2O+KBH4	Aluminum hydroxide gel	-	Irregular and rough	SEM, FTIR, XPS, XRD	[284]
S-nZVI/GA	FeSO4·7H2O+NaBH4	Graphene aerogel (GA)	-	Flake-like shell	TEM, SEM, BET, XRD, XPS, FTIR, Raman spectrum	[285]
nZVI and SnZVI	FeCl2·4H2O+ FeSO4·7H2O+NaBH4	-	-	Cross-linked spherical	TEM EDX, XPS, XRD, XAS	[286]
Fe0@p-SiO2	FeCl3·6H2O+KBH4	SiO2	30–40	-	TEM, XRD, XPS, ζ-potential	[287]
Fe/TRGO	Carbothermal GO+ Fe(NO3)3·9H2O	Graphene oxide	-	-	HRTEM, XRD, XPS, Fe Mössbauer spectroscopy	[288]
nZVI	FeCl2·4H2O+NaBH4	-	-	Chain-like	FTIR, SEM, TEM, XRD, XPS	[289]
Fe@CQDs MNCs	FeCl3+NaBH4	-	-	Smooth	FTIR, XRD, SEM, TEM	[290]
nZVI	FeCl3·6H2O+NaBH4	-	-	Irregular and noncircular	XRD, TEM EDS, FESEM	[291]
Nanostructured nZVI	FeSO4·7H2O+NaBH4	-	-	Rough	XRD, SEM, TEM, DLS	[292]

Table 4. Conventional synthesis of zero-valent iron nanoparticles.

FeNPs	Precursor	Support	Size/nm	Morphology	Characterization	Reference
MCMZVI	FeCl3·6H2O+ glucose+CO(NH2)2	Mesoporous carbon	20–100	Uneven size	XRD, SEM, XPS, FTIR, BET	[194]
CS-nZVI (core–shell)	FeCl3·6H2O+NaBH4	-	15.42–97.57	Spherical	UV–vis, FTIR, TEM, SEM EDX, XRF	[198]
nZVI	FeCl3·6H2O+NaBH4	-	34–110	Spherical	UV, XRD, SEM, EDX, TEM, DSL	[208]
nZVI@FSG	FeSO4·7H2O+NaBH4	Flaxseed gum	73–87	Spherical	DLS, FESEM, EDX, FTIR, DR5000	[216]
CMC-S-nZVI	FeSO4·7H2O+NaBH4	Carboxymethyl cellulose (CMC)	90	Spherical	TEM-EDS, UV–vis, PSD, ζ-potential	[218]
nZVI/SS/BC	FeSO4·7H2O+NaBH4	Biochar with stable starch	45.7–37	-	SEM, EDS, BET, FTIR, XRD	[220]
nZVI	FeSO4·7H2O+KBH4	Guar gum	87.4	Cubic	TEM, XRD, SEM, XPS	[221]
nZVI	FeSO4·7H2O+KBH4	Attapulgite	-	-	FTIR, SEM-EDS, XPS	[225]
Fe@SiO2	FeSO4·7H2O+KBH4	Silica	40–50	Nanospheres	XRS, FTIR, TEM, EDS	[227]
nZVI/AC	FeSO4·7H2O+NaBH4	Activated carbon	40–100	Rough	SEM, XRD, BET XPS	[228]
CS@nZVI	FeSO4·7H2O+NaBH4	Chitosan	13.12	-	XRD, FESEM, EDS, FTIR	[232]
CaCO3-nZVI	FeSO4·7H2O+NaBH4	CaCO3	75–89	Spherical	SEM-EDX, XRD, TEM, FTIR, XPS, BET	[233]
nZVI-A and nZVI-S	FeCl3·6H2O+NaBH4	Rhamnolipids	60 and 42	-	SEM, XRD, FTIR, TG, DLS, ζ-potential	[236]
FG-nZVI	FeSO4·7H2O+NaBH4	Flaxseed gum extract	<100	Spherical	DLS, FESEM, RDX, FTIR	[237]
nZVI	FeCl3·6H2O+NaBH4	-	20–60	Uniform morphology	XRD, SEM, TEM	[238]
nZVI	FeSO4·7H2O+NaBH4	-	14.3	-	XRD, SEM, TEM, FTIR	[239]
nZVI/GO	Fe (NO3)3·9H2O+NaBH4	Graphene oxide	10–20	Dispersed	SEM, TEM-EDS, FTIR	[240]
nZVI-kaol/PES	FeCl3·6H2O+NaBH4	Kaolin and poly-ethersulfone (PES)	42	Ridge and valley	XRD, FESEM, FTIR	[241]
NC-nZVI	FeCl3·6H2O+NaBH4	Nanocelluloses (NC)	116–200	Spherical	SEM, TEM-EDX, FTIR, XRD, XPS	[242]
Fe@BC	Black liquor lignin and Fenton sludge in one step	Biochar	20–50	Notable aggregation	XRD, FTIR, BET, BJH, FESEM, TEM	[243]
MFO@nZVI	FeCl3·6H2O+NaBH4	MnFe2O4 (MFO) hydrogel	-	Mass of spheroidal particles	XRD, SEM, FTIR, UV–vis DRS	[244]
CDLA@nZVI and CDCA@nZVI	FeCl3·6H2O+NaBH4	β-cyclodextrin (CD): CDLA and CDCA	25 and 30	Amorphous	NMR, FTIR, HRTEM, DLS, ζ-potential, FESEM, EDAX, VSM, XRD, XPS, TGA	[245]
Fe3O4@nZVI-PEI	Fe3O4+FeSO4·7H2O+NaBH4	Poly ethylenimine	-	-	SEM, TEM, FTIR, XRD, XPS	[246]
S-nZVI	FeSO4·7H2O+KBH4				-	[247]
nZVI-LBC	FeCl3·6H2O+NaBH4	Biochar	-	-	FTIR, XRD, TEM, XPS, VSM, BET, ζ-potential	[248]
nZVI/GAC	FeSO4·7H2O+NaBH4	Granular activated carbon (GAC)	40	Spherical	SEM, BET, XRD	[249]
nZVI/GO	FeCl3·6H2O+NaBH4	Graphene oxide (GO)	4.97	-	SEM-EDS, XRD, FTIR	[250]
nZVI/Sch-AP and nZVI/ Sch-CO	FeSO4·7H2O+NaBH4	Schwermannite (Sch)	50	Spherical	SEM, XRD, BET, XPS	[251]
A400-nZVI	FeSO4·7H2O+NaBH4	Polystyrenic gel (Purolite A400)	75–150	-	FTIR, SEM, EDAX, XRD, TGA	[252]
nZVI@ Zr(OH)4	FeCl3·6H2O+NaBH4	Zirconium hydroxide	-	-	TEM-EDS, XRD, BET, FTIR, XPS	[253]
pyGA-nZVI	FeCl3·6H2O+NaBH4	Pyrogallic acid (pyGA)	40–90	Spherical	SEM, TEM-EDS, BET, ζ-potential, XRD, FTIR, XPS	[254]
Ox-nZVI	FeCl3·6H2O+NaBH4	Oxalate	30–40	Spherical	BET, SEM-EDS, FTIR, XRD, XPS	[255]
nZVI 1 and nZVI 2	FeSO4·7H2O+NaBH4 FeCl3·6H2O+NaBH4	-	72 and 38	Spherical	SEM, TEM-EDS, XRD, BET, XPS, Raman spectroscopy	[256]
nZVI/ copper slag	FeCl3·6H2O+NaBH4	Copper slag	30	-	FE-SEM, EDX, XRD, FTIR, BET, VSM, ζ-potential	[257]
WPANF/ nZVI	FeCl3·6H2O+NaBH4	Polyacrylonitrile fiber (WPANF)	15–50	-	XRD, FTIR, BET, XPS, SEM-EDS, TEM	[258]
nZVI, ds-coated nZVI, and ds-FeS	FeSO4·7H2O+NaBH4	-	60.12 and 110	Spherical	SEM, XRD, FTIR, ζ-potential, DLS	[259]
nZVI	Self-combustion of Fe2O3 and NaBH4	-	2–8	Amorphous	XRD, HRTEM, EDS	[260]
nZVI	Laser fragmentation in liquids (LFL) ethylene glycol and polyethylene glycol 400	-	10.5 and below 3	-	DLS, LDE, TEM, XPS	[261]
CS@BC/ S-nZVI	FeSO4·7H2O+NaBH4	Chitosan and Biochar	-	-	SEM, BET, FTIR, XRD, XPS	[262]
3D-RGO@nZVI/ Al2O3	FeSO4·7H2O+NaBH4	Reduced graphene oxide	<100	Spherical	SEM, BET, Raman spectroscopy, XRD, XPS	[263]
nZVI/n-lignin	FeCl3·6H2O+NaBH4	Lignin	-	-	TEM, XPS, XRD	[264]
BP-S-nZVI	FeCl3+NaBH4+sulfate- reducing bacteria (SRB)	-	-	-	FESEM, TEM, XRD, BET	[265]
nZVI-DE	FeCl2·4H2O+NaBH4	Diatomaceous earth (DE)	20–40	Spherical	XRD, SEM, EDX, TEM, BET	[266]
CnZVI	FeSO4·7H2O+NaBH4	-	80–99	Spherical	UV, FTIR, XRD, TEM	[267]
G-nZVI-BC and C-nZVI-BC	FeSO4·7H2O+NaBH4	Biochar	-	-	SEM, XRD, FTIR, XRF	[268]
Iron oxide	FeCl3·6H2O+FeCl2·4H2O+ NaOH	-	10 ± 4	Regular crystalline	TEM	[269]
CS/nZVI	FeCl3+NaBH4	Chitosan	25	Spherical	SEM, FTIR, XRD, EDX, VSM, BET, TGA, DSC	[270]
BC@nFe-CA	FeSO4·7H2O+NaBH4	Biochar	-	-	SEM, EDS, FTIR, Raman spectroscopy, XRD, XPS	[271]
PDA@Fe/rGO	FeSO4·7H2O+NaBH4	Reduced graphene oxide (rGO) and polydopamine (PDA)	51	-	TEM, XRD, FTIR, XPS, VSM	[272]
PDCA@nZVI	FeCl3+NaBH4	2,6-pyridinedicarboxylic acid, (PDCA)	115	Nanospheres	SEM, EDS, EDX, XRD, FTIR	[273]
nZVI	FeSO4·7H2O+NaBH4	-	-	Spherical	SEM, TEM, BET	[274]
nZVI/BC	Pyrolysis (Fe2O3+biochar)	-	200	-	XRD, SEM	[275]
CMC-nZVI, bare-nZVI, PAA-nZVI, PSM-nZVI, PVP-nZVI	FeCl3+NaBH4	CMC, PAA, PSM, PVP	9.53, 65.4, 106.4, 106.6, and 109	Spherical	SEM-EDX, XRD, FTIR, TEM	[276]
nZVI-HPB	FeCl3·6H2O+NaBH4	Hydrophilic biochar	52–243	-	SEM, TEM, XRD, FTIR, XPS	[277]
nZVI	FeSO4·7H2O+NaBH4	-	20–60	Spherical	BET, SEM EDX, BET, XRPD, XPS	[278]
nZVI/SBA-15	FeCl3·6H2O+NaBH4	Mesoporous silica (Santa Bárbara-15)	50–80	Spherical	SEM EDS, TEM, BET	[279]
nZVI	FeCl3·6H2O+NaBH4	-	36	Regular and irregular	XRD, SEM, EDX, UV–vis	[280]
nZVI-chitosan	FeCl3·6H2O+NaBH4	Chitosan	15–20	-	SEM, XRD, TEM, FTIR	[281]
nZVI	Bulk iron disks+ solvents+ laser	-	9.4 and 3.5	Spherical	TEM EDS, XPS	[282]
nZVI/RS	FeSO4·7H2O+NaBH4	Biochar	-	-	FTIR, XRD, Raman spectra, BET	[283]
AHG@nZVI	FeSO4·7H2O+KBH4	Aluminum hydroxide gel	-	Irregular and rough	SEM, FTIR, XPS, XRD	[284]
S-nZVI/GA	FeSO4·7H2O+NaBH4	Graphene aerogel (GA)	-	Flake-like shell	TEM, SEM, BET, XRD, XPS, FTIR, Raman spectrum	[285]
nZVI and SnZVI	FeCl2·4H2O+ FeSO4·7H2O+NaBH4	-	-	Cross-linked spherical	TEM EDX, XPS, XRD, XAS	[286]
Fe0@p-SiO2	FeCl3·6H2O+KBH4	SiO2	30–40	-	TEM, XRD, XPS, ζ-potential	[287]
Fe/TRGO	Carbothermal GO+ Fe(NO3)3·9H2O	Graphene oxide	-	-	HRTEM, XRD, XPS, Fe Mössbauer spectroscopy	[288]
nZVI	FeCl2·4H2O+NaBH4	-	-	Chain-like	FTIR, SEM, TEM, XRD, XPS	[289]
Fe@CQDs MNCs	FeCl3+NaBH4	-	-	Smooth	FTIR, XRD, SEM, TEM	[290]
nZVI	FeCl3·6H2O+NaBH4	-	-	Irregular and noncircular	XRD, TEM EDS, FESEM	[291]
Nanostructured nZVI	FeSO4·7H2O+NaBH4	-	-	Rough	XRD, SEM, TEM, DLS	[292]

5. Environmentally Friendly Methods for the Synthesis of nZVI

As an eco-friendly alternative to the use of chemical reductants in nanoparticle synthesis, the utilization of natural materials, such as plants or plant extracts, has emerged as one of the most effective strategies [293]. The so-called green synthesis method, or biomimetics, makes use of natural extracts from environmentally friendly substances, such as agricultural or food industry waste materials (fruit residues, vines, or eucalyptus leaves) as well as natural plant extracts typically used for human consumption, like coffee, tea, or grape must, water hyacinth, barley, lemon balm (Melissa officinalis), parsley (Crispum spp.), and sorghum bran (Sorghum spp.), known for their rich polyphenolic content. Very rarely is the concentration of polyphenols reported, and therefore, it is not possible to establish a relationship between their concentrations and the particle size, yield, or morphology of nanoparticles. It should be noted that there are some limitations with certain plant sources; for example, tea can be relatively expensive, while barley, although chemically stable, may not always be available for harvesting. Even so, these extracts, which are rich in phenolic compounds (polyphenols), reducing sugars, ascorbic acid, flavonoids, and amino acids, play a crucial role in the generation of iron nanoparticles. On the one hand, like borohydride, they make the reduction of iron ions possible; on the other hand, and this is a noticeable advantage compared to NaBH₄, they contribute to the formation of stable iron complexes, rather than borohydrides.

The advantages of this green chemical reduction method commonly reported are:

• Enhanced removal capacity and longevity: The presence of polyphenols in these extracts enhances their removal capabilities because bioactive compounds such as polyphenols have several benzene groups substituted by hydroxyl functional groups in their structure, so they can reduce metal ions to their elemental state, obtaining nanoparticles. They are also low cost or economically viable, reducing the consumption of organic compounds and the generation of toxic products.
• Environmentally friendly: This method is hailed as a potential environmentally friendly process, characterized by lower toxicity levels, the absence of aggressive reagents, and the generation of non-harmful by-products.
• Cost-effective: Many authors claim it might provide a low-cost alternative for nanoparticle synthesis.
• Inherent stabilization: The extract matrix often acts as a stabilizer, reducing nanoparticle agglomeration without requiring the addition of dispersants.
• Biomass valorization: The method offers potential for biomass valorization, further contributing to its sustainability.

Despite these advantages, widespread acceptance of green synthesis remains limited due to a lack of comprehensive understanding regarding the reactivity, physicochemical properties, and agglomeration tendencies of the resulting nanoparticles. Studies suggest that the choice of plant extract used leads to variations in nanoparticle size and specific surface area (refer to Table 5). Additionally, the reduction of iron to nZVIs by plant extracts can sometimes result in the formation of different iron compounds, including iron oxides or hydroxides, throughout the process.

The process of green synthesis typically involves the extraction of biomolecules from plants through heating in water, usually near the boiling point. Subsequently, the extract is separated from the residual plant material and combined with a solution containing Fe²⁺ or Fe³⁺. However, a key limitation of this method lies in the inability of wet plant extracts to efficiently reduce Fe²⁺/Fe³⁺ to nZVIs, potentially leading to the formation of iron (hydr)oxides rather than metallic iron [294,295]. The typical pH for extraction is around 5, which may affect the reduction potential. Additionally, the characteristics of the resulting nZVIs, including size and surface area, can vary significantly depending on the specific plant extract utilized.

Considering that polyphenols are potent antioxidants capable of trapping free radicals, their reaction with iron salts, such as FeCl₃, results in the formation of ortho-diphenol compounds. These compounds exhibit a green coloration, while those with three adjacent hydroxyl groups appear dark gray. The presence of isolated phenolic hydroxyls is indicated by a yellow color. This color change occurs due to chloride ions attacking the hydrogen in the hydroxyl group, leading to bond breakage and the formation of a complex with iron (complex formation). A transition from the initial yellow to dark yellow signifies the presence of phenol and the reduction of Fe³⁺ to Fe⁰. Phenols interact with the empty orbitals of FeCl₃, while the unreacted polyphenols continue to reduce the iron complex, forming an aromatic organic complex and further reducing Fe, resulting in the release of Fe nanoparticles in the nZVI state [296].

The nanoparticles are synthesized by the formation of phenoxy groups, dimerized by hydrogen bonds which bind to the Fe⁰. If the pH is increased (OH⁻ concentration is increased), more phenoxy groups will be generated, which can act as dye reductants and adsorbents. If the pH of the zero-charge point is acidic, this means that most of its surface is negatively charged, so better cation bonds will be formed.

It has been reported that the existence of hydroxyl and carbonyl groups in the structure of biomolecules makes them powerful reductants, giving rise to unstable zero-valent iron nanoparticles. However, if they have double bonds in their structure, these biomolecules act as a coating agent, which translates into an increase in the chemical stability of the iron nanoparticles [297].

Table 5. Synthesis of iron nanoparticles through chemical reduction using plant extract.

FeNPs	Precursor	Support	Size/nm	Morphology	Characterization	Reference
BC-nZVI-BC	Oak+FeCl3	Biochar	68–521	Twister and serpentine	SEM, EDS, XRD, DLS	[230]
nZVI, ds-coated-nZVI+ds-FeS	Phoenix dactylifera +FeSO4·7H2O	-		Spherical	SEM, XRD, FTIR, ζ-potential, DLS	[259]
GnZVI	Amaranthus dubius leaf extract+ FeSO4·7H2O	-	1–3	Spherical	UV, FTIR, XRD, TEM	[267]
G-nZVI-BC and C-nZVI-BC	Green tea residues+FeSO4·7H2O	-	-	-	SEM, XRD, FTIR, XRF	[268]
Iron oxide	Cymbopogon citratus extract+ FeCl3·6H2O+Na2CO3	-	9 ± 4	Regular crystalline	TEM	[269]
nZVI	Azadirachta indica (neem) Mentha longifolia (mint) L.e.+FeCl3·6H2O	-		Spherical	SEM, TEM, BET	[274]
GT-nZVI	Black tea+FeCl3·6H2O	-	80	Regular and irregular	XRD, SEM, EDX, UV–vis	[280]
Fe-NP-GV	Mansoa alliacea +FeSO4·7H2O	-	18.22	Spherical	XRD, UV–vis, AAS FTIR, TGA	[295]
nZVI	Shirazi thyme L.e.+FeSO4·7H2O/ Pistachio green hulls pomegranate/banana/ mango+FeCl3 black tea+ FeCl3·6H2O	-	40–70 114, 76, 95	-	-	[298]
nZVI	Green tea+ Fe (NO3)3·9H2O	-	5–45	Amorphous spherical	TEM, SEM/EDS, XRD, BET	[299]
AC/nZVI	Pomegranate peel extract+FeCl2	Activated carbon		Crystalline	FTIR, XRD, BET, FESEM	[300]
nZVI	Cleistocalyx operculatus L.s+FeCl3	-	100	Spherical	SEM, XRD, FTIR	[301]
nZVI @gBC	Carbothermal (sawdust+FeCl3·6H2O)	Graphene	-	-	-	[302]
nZVI	Cleistocalyx operculatus L.s+FeCl3	-	100	Spherical	SEM, XRD, FTIR	[303]
TP-nZVI/PE	Tea polyphenols	Polyethylene	-	Rough	SEM, TEM, ζ-potential, XRD, FTIR, XPS	[304]
Fe@C	Rice powder+ Fe (NO3)3·9H2O	Biomass- derived carbon	30–150	Irregular	XRD, FTIR, SEM, TEM, EDS	[305]
nZVI@GNPs	Cleistocalyx operculatus Leaf extract+ graphene NPs	Graphene nanoplatelets (GNPs)	30–100	Spherical	SEM, XRD, EDS, FTIR	[306]
G-nZVI	Ripe mango peels+FeCl3·6H2O	-	-	-	XRD, FTIR, TEM, BET, SEM-EDX	[307]
EGnZVI	Eucalyptus grandis+FeSO4·7H2O	-	50–500	Spherical	XRD, FTIR, Raman, SEM, TEM/EDS	[308]
G-nZVI/B	Green tea+ FeSO4·7H2O	Calcined bentonite	8 30	Irregular	BET, SEM, TEM, FTIR, XRD, XPS	[309]
nZVI	Corn+ FeCl3·6H2O	-	150–300	Spherical	SEM, TEM-EDS, XRD, XPS	[310]
EL-nZVI	Eucalyptus L.e.+FeSO4·7H2O	-	87	Spherical	SEM, FTIR	[311]
SnZVI@HPAC	Green tea+FeSO4·7H2O	-	-	-	SEM-EDS, TGA, FTIR, XRD	[312]
SC-nZVI	Green tea waste +FeCl3·6H2O	Silty clay	-	-	FTIR, SEM, XRD, BET, ζ-potential	[313]
nZVI/CF	Carbothermal (cotton fiber +Fe (NO3)3·9H2O)	Cotton carbon fiber	-	-	XRD, SEM, BET	[314]
nZVI @TP-Mont	Green tea+FeSO4·7H2O	Montmorillonite	15–30	Spherical	TEM, XRD, FTIR, SBET, XPS, ζ-potential	[315]
RCL-nZVI	Ricinus communis L.e.+FeCl3·6H2O	-	4.84 25.6	Irregular	SEM, TEM, FTIR, EDS, XRD, XPS, ζ-potential	[316]
B-BT-nZVI	Black tea+ FeCl3	Bentonite	<50	-	AFM, SEM, ζ-potential, BET	[317]
GT-nZVI@VC	Green tea+ Vit C+ FeSO4·7H2O	-	100	-	XRD, TEM, SEM, FTIR	[318]
nZVI	Pomegranate peel extract+ FeCl3·6H2O	-	40–60	Spherical	UV–visible, FTIR, SEM	[319]
k-nZVI	Ruellia tuberosa+FeCl3·6H2O	Kaolin	20–40	Spherical	XRD, SEM, TEM, EDS	[320]
R-FeNPs	Green tea+FeCl3·6H2O	Resin	20–40	Spherical	SEM, TEM, EDS	[321]
nZVI @Fe3O4 @HMIMPF6	Camellia sinensis+ FeCl3·6H2O+ FeCl2·4H2O	Magnetite+ 1-hexyl-3-methylimidazolium hexafluorophosphate	30	Spherical	FTIR, XRD, VSM, BET, SEM, TEM	[322]
FeNPsJF	Jackfruit peel (JFP) extract+FeCl2	-	33	Irregular spherical	FTIR, TEM, XRD, SEM, EDX	[323]
G-nZVI	Pomegranate fruit peel+FeCl3·6H2O	-	60–75	Spherical to cubical	UV–vis, XRD, TEM, SEM, DLS, ζ-potential	[324]
nZVI coupling with MR-1	Green tea+ FeSO4·7H2O	Shewanella oneidensis MR-1	-	-	SEM-EDS, XPS, FTIR, Raman, EEM	[325]
RC-nZVI	Ricinus communis seed extract+Fe3+	-	20	Spherical	SEM, TEM, FTIR, XRS, EDS, XRD, XPS, ζ-potential	[326]
FeNPs	Denitrifying bacteria+ +FeCl3	-	-	-	UV–vis, XPS, FTIR, TEM	[327]
AC/nZVI	Pomegranate peel extract+FeSO4·7H2O	Activated carbon	-	-	XRD, FTIR, FESEM	[328]
Zeolite/ nZVI	Pomegranate peel extract+FeCl2·4H2O	Zeolite	30	-	FTIR, FESEM, BET, XRF	[329]
BGT-nZVI	Green tea+FeCl3·6H2O	Bentonite	-	-		[330]
PPAC-nZVI	Pomegranate peel extracts+FeCl2·4H2O	Activated carbon	19–24	-	FESEM, BET, FTIR	[331]
nZVI-RBC	Rice husk+FeSO4·7H2O Rice husk-derived biochar (RBC)	Rice husk-derived biochar (RBC)	100	-	SEM EDS, XRD, FTIR, BET, XPS	[332]
S-nZVI/AC	Ulva. prolifera+ FeCl2·4H2O	Algal carbon	-	Flower-like	SEM EDS, TEM, XRD, BET, XPS	[333]
Fe/N-OB	Carbothermal (hematite oak wood biochar)	-	-		XRD, XPS, SEM EDS, FTIR, BET	[334]
GT-nZVI	Black tea+FeCl3·6H2O	-	83	Irregular	XRD, SEM, EDAX	[335]
n-ZVI-NPs	Mentha piperita+FeCl3	-	5–10	Spherical	UV–vis, SEM-EDX, DLS	[336]
Fe0+ Fe1.91C0.09+ Fe3O4	Neurospora crassa +urea+FeSO4	-	50	-	SEM EDX, XRD, XPS, BET	[337]
TP-ZVI-OB	Green tea+FeCl3·6H2O	Oak wood biochar	-	-	BET, FTIR, SEM, EDS, XPS, XRD	[338]
F–Fe0 ads	Ficus sycomorus dry L.e.+FeCl3·6H2O	Wheat bran (B), rice bran (RB), activated charcoal (Ach), and bentonite (Bent)	2.46–11.49	Circular	UV–vis, HRTEM	[339]
NA-FeNPs	Nephrolepis auriculata+ FeCl3	-	40–70	Spheroidal	TEM, XRD, EDS, XPS, FTIR	[340]
GMP-nZVI	Mango peel extract+FeCl3·6H2O	-	1–10	-	UV–vis, EDX, XRD, XPS, FTIR	[341]
DOX@GTCs-FeNPs	Green tea catechin powder+FeCl3·6H2O		159		TEM, UV–vis, AAS,	[342]
Micro/FeNPs	Mango, rose, Neem L.e. carom seeds, and clove buds+FeCl2·4H2O	Polyvinyl alcohol (PVP)	75–6500	Spherical or irregular	SEM, XRD, EDX, FTIR, UV–vis	[343]
Iron oxide	Eucalyptus L.e. +FeCl3·6H2O	Cetyltrimethylammonium bromide (CTA)	80–90	Spherical	XRD, EDS, FTIR, TGA	[344]
FeNPs	Eichhornia crassipes L.s +FeSO4·7H2O	-	-	Rod	UV–vis, SEM, TEMXRD, FTIR	[345]
Iron oxide	Cocos nucifera+FeCl3	-	10–100	Clustered	UV–vis, TEM, XRD, XPS	[346]
FeNPs	Eucalyptus globulus, Mangifera indica, Syzygium cumini, Psidium guajava+FeCl3·6H2O	-	38–47	Irregular	UV–vis, FTIR, FESEM EDS, XRD	[347]
VI	Oak L.e.+FeCl3·6H2O	-	20–100	Irregular	TEM, EDS, XRD	[348]
Fe3O4NPs	Coriandrum sativum L.e. +FeCl3	-	20–90	Spherical	UV–vis, FTIR, XRD SEM EDX	[349]
Iron oxide	Lantana camara fruit+FeSO4·7H2O+ FeCl3·6H2O	-	28	Spherical	FTIR, TGA, PSA, SEM EDAX, ζ-potential	[350]
Iron oxide	Lantana camara L.e.+FeSO4·7H2O	-	10–20	Nanorods	XRD, FTIR, SEMEDX, UV–vis	[351]
LGFeNPs	Eucalyptus L.e.+ laterite	-	20–70	Spherical	FESEM EDX, XRD, FTIR, BET	[352]
FeNPs	Eucalyptus L.e.+FeSO4·7H2O	-	70 ± 20	Spherical	SEM EDS, FTIRE, XRD, TEM, XPS, XRD, BET	[353]
Fe2O3@SiO2	Zanthoxylum rhetsa +FeCl3·6H2O	SiO2	12.2 ± 0.8	Cluster-like	FTIR, XRD, SEM EDX, HRTEM	[354]
SJA-FeNPs	Syzgium jambos +FeCl3	-	13.7 ± 5	Spherical	UV–vis, TEM, XRD, XPS	[355]
nZVI	Vaccinium corymbosum +FeCl3·6H2O	-	52.4	Irregular	TEM, SEM, BET, XRD	[356]
Fe3O4@ZnO	Azadirachta indica(neem)+ FeSO4·7H2O+ Fe (NO3)3·9H2O	ZnO	38	Brick-like	XRD, FTIR, SEM EDX, TEM, TGA	[357]
Ec-Fe-NPs	Eichhornia crassipes+FeCl3	-	20–80	Amorphous	SEM, EDS, TEM, XPS, FTIR, DLS, ζ-potential	[358]
FeNPS	Moringa oleifera+FeCl3	-	2.6–6.2 and 3.4–7.4	Spherical	UV–vis, XRD, FTIR, TEM	[359]
Iron oxide	Black tea+FeSO4·7H2O	-	5–50	Amorphous	XRD, FTIR, SEM, TEM, EDS	[360]
nZVI	Green tea L.e.+FeCl3	-	116	-	FTIR, SEM	[361]
FeNPs	Rosa damascene (RD), Thymus vulgaris (TV), and Urtica dioica (UD) +FeCl3·4H2O	-	100	Nonuniform	FTIR, SEM, TEM, XRD	[362]
FeNPs	Euphorbia cochinchensis +FeCl3	-	100	Spherical	GMS, TEM, XPS, XRD, BET	[363]
FeNPs	Eucalyptus L.e. +FeSO4·7H2O	-	20–80	Polydisperse	SEM, XRD, XPS, FTIR	[364]
Iron oxide	Sapindus mukorossi +Fe(NO3)3·9H2O)+FeCl3	-	<50	Nanorods	XRD, FESEM, TEM	[365]
Iron oxide nanorods (IONRs)	Mangifera indica L.e. +FeSO4·7H2O		3.0 ± 0.2	Nanorods	FESEM, EDX, XRD, TEM	[366]
FeNPS	Mangifera indica, Murraya koenigii, Azadiracta indica, Magnolia champaca +FeSO4·7H2O		AI (96–110), MC (99–129), MIAND MK (100–150)	Spherical	UV–vis, SEM-EDS-FTIR	[367]
GT-Fe NPs and EL-Fe NPs	Eucalyptus L.e.+green tea L.e.+FeSO4·7H2O		20–80	Quasi-spherical	SEM, XRD, FTIR	[368]
nZVIs	Waste from citrus juice (orange, lime, lemon and mandarin) +FeSO4·7H2O		3–300	Spherical, cylindrical, irregular	TEM, XRD, Mössbauer spectroscopy	[369]
nZVI	Black tea, grape mark vine L.e. +FeCl3·6H2O		15–45	-	TEM	[370]
(ZVI) NPs	Terminalia chebula+FeSO4·7H2O		<80	Amorphous	TEM, XRD, UV–vis, FTIR	[371]
nZVI	26 tree leaf extracts+FeCl3		10–20	Spherical	TEM	[372]

Note: L.e.: Leaf extract.

Table 5. Synthesis of iron nanoparticles through chemical reduction using plant extract.

FeNPs	Precursor	Support	Size/nm	Morphology	Characterization	Reference
BC-nZVI-BC	Oak+FeCl3	Biochar	68–521	Twister and serpentine	SEM, EDS, XRD, DLS	[230]
nZVI, ds-coated-nZVI+ds-FeS	Phoenix dactylifera +FeSO4·7H2O	-		Spherical	SEM, XRD, FTIR, ζ-potential, DLS	[259]
GnZVI	Amaranthus dubius leaf extract+ FeSO4·7H2O	-	1–3	Spherical	UV, FTIR, XRD, TEM	[267]
G-nZVI-BC and C-nZVI-BC	Green tea residues+FeSO4·7H2O	-	-	-	SEM, XRD, FTIR, XRF	[268]
Iron oxide	Cymbopogon citratus extract+ FeCl3·6H2O+Na2CO3	-	9 ± 4	Regular crystalline	TEM	[269]
nZVI	Azadirachta indica (neem) Mentha longifolia (mint) L.e.+FeCl3·6H2O	-		Spherical	SEM, TEM, BET	[274]
GT-nZVI	Black tea+FeCl3·6H2O	-	80	Regular and irregular	XRD, SEM, EDX, UV–vis	[280]
Fe-NP-GV	Mansoa alliacea +FeSO4·7H2O	-	18.22	Spherical	XRD, UV–vis, AAS FTIR, TGA	[295]
nZVI	Shirazi thyme L.e.+FeSO4·7H2O/ Pistachio green hulls pomegranate/banana/ mango+FeCl3 black tea+ FeCl3·6H2O	-	40–70 114, 76, 95	-	-	[298]
nZVI	Green tea+ Fe (NO3)3·9H2O	-	5–45	Amorphous spherical	TEM, SEM/EDS, XRD, BET	[299]
AC/nZVI	Pomegranate peel extract+FeCl2	Activated carbon		Crystalline	FTIR, XRD, BET, FESEM	[300]
nZVI	Cleistocalyx operculatus L.s+FeCl3	-	100	Spherical	SEM, XRD, FTIR	[301]
nZVI @gBC	Carbothermal (sawdust+FeCl3·6H2O)	Graphene	-	-	-	[302]
nZVI	Cleistocalyx operculatus L.s+FeCl3	-	100	Spherical	SEM, XRD, FTIR	[303]
TP-nZVI/PE	Tea polyphenols	Polyethylene	-	Rough	SEM, TEM, ζ-potential, XRD, FTIR, XPS	[304]
Fe@C	Rice powder+ Fe (NO3)3·9H2O	Biomass- derived carbon	30–150	Irregular	XRD, FTIR, SEM, TEM, EDS	[305]
nZVI@GNPs	Cleistocalyx operculatus Leaf extract+ graphene NPs	Graphene nanoplatelets (GNPs)	30–100	Spherical	SEM, XRD, EDS, FTIR	[306]
G-nZVI	Ripe mango peels+FeCl3·6H2O	-	-	-	XRD, FTIR, TEM, BET, SEM-EDX	[307]
EGnZVI	Eucalyptus grandis+FeSO4·7H2O	-	50–500	Spherical	XRD, FTIR, Raman, SEM, TEM/EDS	[308]
G-nZVI/B	Green tea+ FeSO4·7H2O	Calcined bentonite	8 30	Irregular	BET, SEM, TEM, FTIR, XRD, XPS	[309]
nZVI	Corn+ FeCl3·6H2O	-	150–300	Spherical	SEM, TEM-EDS, XRD, XPS	[310]
EL-nZVI	Eucalyptus L.e.+FeSO4·7H2O	-	87	Spherical	SEM, FTIR	[311]
SnZVI@HPAC	Green tea+FeSO4·7H2O	-	-	-	SEM-EDS, TGA, FTIR, XRD	[312]
SC-nZVI	Green tea waste +FeCl3·6H2O	Silty clay	-	-	FTIR, SEM, XRD, BET, ζ-potential	[313]
nZVI/CF	Carbothermal (cotton fiber +Fe (NO3)3·9H2O)	Cotton carbon fiber	-	-	XRD, SEM, BET	[314]
nZVI @TP-Mont	Green tea+FeSO4·7H2O	Montmorillonite	15–30	Spherical	TEM, XRD, FTIR, SBET, XPS, ζ-potential	[315]
RCL-nZVI	Ricinus communis L.e.+FeCl3·6H2O	-	4.84 25.6	Irregular	SEM, TEM, FTIR, EDS, XRD, XPS, ζ-potential	[316]
B-BT-nZVI	Black tea+ FeCl3	Bentonite	<50	-	AFM, SEM, ζ-potential, BET	[317]
GT-nZVI@VC	Green tea+ Vit C+ FeSO4·7H2O	-	100	-	XRD, TEM, SEM, FTIR	[318]
nZVI	Pomegranate peel extract+ FeCl3·6H2O	-	40–60	Spherical	UV–visible, FTIR, SEM	[319]
k-nZVI	Ruellia tuberosa+FeCl3·6H2O	Kaolin	20–40	Spherical	XRD, SEM, TEM, EDS	[320]
R-FeNPs	Green tea+FeCl3·6H2O	Resin	20–40	Spherical	SEM, TEM, EDS	[321]
nZVI @Fe3O4 @HMIMPF6	Camellia sinensis+ FeCl3·6H2O+ FeCl2·4H2O	Magnetite+ 1-hexyl-3-methylimidazolium hexafluorophosphate	30	Spherical	FTIR, XRD, VSM, BET, SEM, TEM	[322]
FeNPsJF	Jackfruit peel (JFP) extract+FeCl2	-	33	Irregular spherical	FTIR, TEM, XRD, SEM, EDX	[323]
G-nZVI	Pomegranate fruit peel+FeCl3·6H2O	-	60–75	Spherical to cubical	UV–vis, XRD, TEM, SEM, DLS, ζ-potential	[324]
nZVI coupling with MR-1	Green tea+ FeSO4·7H2O	Shewanella oneidensis MR-1	-	-	SEM-EDS, XPS, FTIR, Raman, EEM	[325]
RC-nZVI	Ricinus communis seed extract+Fe3+	-	20	Spherical	SEM, TEM, FTIR, XRS, EDS, XRD, XPS, ζ-potential	[326]
FeNPs	Denitrifying bacteria+ +FeCl3	-	-	-	UV–vis, XPS, FTIR, TEM	[327]
AC/nZVI	Pomegranate peel extract+FeSO4·7H2O	Activated carbon	-	-	XRD, FTIR, FESEM	[328]
Zeolite/ nZVI	Pomegranate peel extract+FeCl2·4H2O	Zeolite	30	-	FTIR, FESEM, BET, XRF	[329]
BGT-nZVI	Green tea+FeCl3·6H2O	Bentonite	-	-		[330]
PPAC-nZVI	Pomegranate peel extracts+FeCl2·4H2O	Activated carbon	19–24	-	FESEM, BET, FTIR	[331]
nZVI-RBC	Rice husk+FeSO4·7H2O Rice husk-derived biochar (RBC)	Rice husk-derived biochar (RBC)	100	-	SEM EDS, XRD, FTIR, BET, XPS	[332]
S-nZVI/AC	Ulva. prolifera+ FeCl2·4H2O	Algal carbon	-	Flower-like	SEM EDS, TEM, XRD, BET, XPS	[333]
Fe/N-OB	Carbothermal (hematite oak wood biochar)	-	-		XRD, XPS, SEM EDS, FTIR, BET	[334]
GT-nZVI	Black tea+FeCl3·6H2O	-	83	Irregular	XRD, SEM, EDAX	[335]
n-ZVI-NPs	Mentha piperita+FeCl3	-	5–10	Spherical	UV–vis, SEM-EDX, DLS	[336]
Fe0+ Fe1.91C0.09+ Fe3O4	Neurospora crassa +urea+FeSO4	-	50	-	SEM EDX, XRD, XPS, BET	[337]
TP-ZVI-OB	Green tea+FeCl3·6H2O	Oak wood biochar	-	-	BET, FTIR, SEM, EDS, XPS, XRD	[338]
F–Fe0 ads	Ficus sycomorus dry L.e.+FeCl3·6H2O	Wheat bran (B), rice bran (RB), activated charcoal (Ach), and bentonite (Bent)	2.46–11.49	Circular	UV–vis, HRTEM	[339]
NA-FeNPs	Nephrolepis auriculata+ FeCl3	-	40–70	Spheroidal	TEM, XRD, EDS, XPS, FTIR	[340]
GMP-nZVI	Mango peel extract+FeCl3·6H2O	-	1–10	-	UV–vis, EDX, XRD, XPS, FTIR	[341]
DOX@GTCs-FeNPs	Green tea catechin powder+FeCl3·6H2O		159		TEM, UV–vis, AAS,	[342]
Micro/FeNPs	Mango, rose, Neem L.e. carom seeds, and clove buds+FeCl2·4H2O	Polyvinyl alcohol (PVP)	75–6500	Spherical or irregular	SEM, XRD, EDX, FTIR, UV–vis	[343]
Iron oxide	Eucalyptus L.e. +FeCl3·6H2O	Cetyltrimethylammonium bromide (CTA)	80–90	Spherical	XRD, EDS, FTIR, TGA	[344]
FeNPs	Eichhornia crassipes L.s +FeSO4·7H2O	-	-	Rod	UV–vis, SEM, TEMXRD, FTIR	[345]
Iron oxide	Cocos nucifera+FeCl3	-	10–100	Clustered	UV–vis, TEM, XRD, XPS	[346]
FeNPs	Eucalyptus globulus, Mangifera indica, Syzygium cumini, Psidium guajava+FeCl3·6H2O	-	38–47	Irregular	UV–vis, FTIR, FESEM EDS, XRD	[347]
VI	Oak L.e.+FeCl3·6H2O	-	20–100	Irregular	TEM, EDS, XRD	[348]
Fe3O4NPs	Coriandrum sativum L.e. +FeCl3	-	20–90	Spherical	UV–vis, FTIR, XRD SEM EDX	[349]
Iron oxide	Lantana camara fruit+FeSO4·7H2O+ FeCl3·6H2O	-	28	Spherical	FTIR, TGA, PSA, SEM EDAX, ζ-potential	[350]
Iron oxide	Lantana camara L.e.+FeSO4·7H2O	-	10–20	Nanorods	XRD, FTIR, SEMEDX, UV–vis	[351]
LGFeNPs	Eucalyptus L.e.+ laterite	-	20–70	Spherical	FESEM EDX, XRD, FTIR, BET	[352]
FeNPs	Eucalyptus L.e.+FeSO4·7H2O	-	70 ± 20	Spherical	SEM EDS, FTIRE, XRD, TEM, XPS, XRD, BET	[353]
Fe2O3@SiO2	Zanthoxylum rhetsa +FeCl3·6H2O	SiO2	12.2 ± 0.8	Cluster-like	FTIR, XRD, SEM EDX, HRTEM	[354]
SJA-FeNPs	Syzgium jambos +FeCl3	-	13.7 ± 5	Spherical	UV–vis, TEM, XRD, XPS	[355]
nZVI	Vaccinium corymbosum +FeCl3·6H2O	-	52.4	Irregular	TEM, SEM, BET, XRD	[356]
Fe3O4@ZnO	Azadirachta indica(neem)+ FeSO4·7H2O+ Fe (NO3)3·9H2O	ZnO	38	Brick-like	XRD, FTIR, SEM EDX, TEM, TGA	[357]
Ec-Fe-NPs	Eichhornia crassipes+FeCl3	-	20–80	Amorphous	SEM, EDS, TEM, XPS, FTIR, DLS, ζ-potential	[358]
FeNPS	Moringa oleifera+FeCl3	-	2.6–6.2 and 3.4–7.4	Spherical	UV–vis, XRD, FTIR, TEM	[359]
Iron oxide	Black tea+FeSO4·7H2O	-	5–50	Amorphous	XRD, FTIR, SEM, TEM, EDS	[360]
nZVI	Green tea L.e.+FeCl3	-	116	-	FTIR, SEM	[361]
FeNPs	Rosa damascene (RD), Thymus vulgaris (TV), and Urtica dioica (UD) +FeCl3·4H2O	-	100	Nonuniform	FTIR, SEM, TEM, XRD	[362]
FeNPs	Euphorbia cochinchensis +FeCl3	-	100	Spherical	GMS, TEM, XPS, XRD, BET	[363]
FeNPs	Eucalyptus L.e. +FeSO4·7H2O	-	20–80	Polydisperse	SEM, XRD, XPS, FTIR	[364]
Iron oxide	Sapindus mukorossi +Fe(NO3)3·9H2O)+FeCl3	-	<50	Nanorods	XRD, FESEM, TEM	[365]
Iron oxide nanorods (IONRs)	Mangifera indica L.e. +FeSO4·7H2O		3.0 ± 0.2	Nanorods	FESEM, EDX, XRD, TEM	[366]
FeNPS	Mangifera indica, Murraya koenigii, Azadiracta indica, Magnolia champaca +FeSO4·7H2O		AI (96–110), MC (99–129), MIAND MK (100–150)	Spherical	UV–vis, SEM-EDS-FTIR	[367]
GT-Fe NPs and EL-Fe NPs	Eucalyptus L.e.+green tea L.e.+FeSO4·7H2O		20–80	Quasi-spherical	SEM, XRD, FTIR	[368]
nZVIs	Waste from citrus juice (orange, lime, lemon and mandarin) +FeSO4·7H2O		3–300	Spherical, cylindrical, irregular	TEM, XRD, Mössbauer spectroscopy	[369]
nZVI	Black tea, grape mark vine L.e. +FeCl3·6H2O		15–45	-	TEM	[370]
(ZVI) NPs	Terminalia chebula+FeSO4·7H2O		<80	Amorphous	TEM, XRD, UV–vis, FTIR	[371]
nZVI	26 tree leaf extracts+FeCl3		10–20	Spherical	TEM	[372]

Note: L.e.: Leaf extract.

6. Dye Pollution in Wastewater and Current Removal Strategies

Numerous organic compounds are introduced into the environment as waste pro-ducts stemming from diverse industries, including textiles, paper, leather, cosmetics, and more recently, photovoltaics, batteries, and light-emitting diodes. Of these industries, textiles take the lead as one of the most significant contributors to global water pollution [373]. The textile sector consumes millions of tons of water daily in its production processes, with a substantial portion of this water being discharged into rivers and lakes, frequently without prior treatment to eliminate pollutants that detrimentally impact aquatic ecosystems. Synthetic dyes constitute a substantial portion of the toxic compounds present in textile industry wastewater. This predicament is exacerbated by the fact that approximately 2% of the dyes manufactured within this industry are directly released into natural effluents [374].

Generally, dyes are recognized as potential sources of carcinogenic, mutagenic, and teratogenic substances, capable of infiltrating the food chain. Hence, it becomes imperative to exert stringent control over the industrial utilization of dyes to prevent their discharge in concentrations that exceed the environment’s assimilation capacity.

Dyes exhibit structural commonalities across different types, categorized based on their chemical structures as well as industrial applications [375]. Polymethine dyes, for instance, feature a conjugated polymethine chain with terminal functional groups and an odd number of π-centers and π-electrons [376,377]. Various functional groups, such as anthraquinone, azo, phthalocyanine, sulfur, indigo, nitro, and nitroso, also determine their structural classification [378]. The chemical structures of all the dyes referred to in this work, together with their common and IUPAC names can be found as Supplementary Material.

Chemical stability is a notable trait of dyes, rendering them resistant to light, temperature, and environmental degradation. This persistence poses ecological concerns, particularly for freshwater aquatic ecosystems [379]. Synthetic dyes, in particular, exhibit high resistivity towards washing, heat, light, and biological agents, exacerbating their environmental persistence [380]. Dyes’ adverse effects on aquatic life are well documented, including inhibition of photosynthesis, reduction of dissolved oxygen levels, and disruption of behavior and reproductive patterns in aquatic organisms [381]. Toxicity testing, measuring parameters like LC50 and EC50, confirms their detrimental impact on aquatic ecosystems, directly affecting photoautotrophic organisms and causing ecological imbalances [381,382]. In many instances, however, the pollution caused by dyes employed in sectors like the textile industry goes largely unnoticed. This lack of awareness among both workers and the exposed population makes it a silent and unaddressed issue. Consequently, the release of effluents containing these dyes into the environment is deemed unacceptable without prior treatment to bring their concentrations within permissible levels. The paramount challenge revolves around reducing the dye concentration in industrial effluents cost-effectively.

However, these types of wastewater-containing dyes are very difficult to treat, because the organic molecules that compose them have complex aromatic groups in their structure, which makes them resistant to light and temperature; these characteristics make them bio-accumulative and difficult to degrade in living beings, which causes illness or can even be fatal. In addition, the colloidal matter together with these colloids can cause diseases or even death. Also, the greasy foam present in the water increases the turbidity and gives the water a bad appearance. Evidently, the human being is not free of the polluting action of the colorants spilled in the environment. If the spillage occurs in crop fields, it can affect soil productivity and, through filtration or runoff, it can reach aquifers or surface waters (rivers, lakes, reservoirs) with the consequent problems in the water supply for human consumption.

Today, the manufacturing and utilization of synthetic dyes for fabric dyeing have evolved into a colossal industry. The allure of vibrant colors on textiles has a history dating back to at least 3500 B.C. However, it was in 1856 when Perkins unveiled the realm of synthetic dyes, ushering in a wide spectrum of vivid hues. Yet, the environmentally detrimental aspects of these dyes have raised profound concerns.

Dyes are water-soluble compounds that possess the ability to impart color to fibers while remaining unaffected by factors such as light, temperature, and soap. There are several ways to classify the colorants, according to their chemical structure, their characteristics, and their applications [383].

Treatment technologies for dyes differ only slightly from those generally used for other pollutants. Such differences are mainly due to some specific characteristics of dyes, such as, for instance, low biodegradability, which makes them persistent and refractory to many conventional treatments [384,385]. The typical methods for dye wastewater treatment include physical, chemical, and biological approaches, as well as advanced oxidation processes and bio-adsorption techniques [386,387,388,389,390]. Physicochemical methods and adsorption using activated carbon are commonly employed for dye abatement [391,392]. Bio-adsorption, particularly using bio-adsorbents derived from local sources, is favored for its long-term viability [393]. Bioremediation, including microbial treatment, enzyme-mediated dye removal, and phytoremediation, has emerged as a sustainable approach for dye degradation [389]. Furthermore, nanotechnology and advanced oxidation processes (AOPs) hold promise for effective dye removal [394], as will be detailed in the following paragraphs.

In some cases, conventional treatment methods for the removal of dyes in wastewater are not effective due to the recalcitrant nature of the pollutants. Such methods include adsorption, degradation, and mineralization.

Regarding adsorption and degradation methods, in many cases the treatment involves both. Adsorption using biomass as the source for creating the adsorbent has received special attention. Degradation methods focus on oxidation process of dyes. Some advanced oxidation processes involving the use of iron cations in solution have been used for the degradation of dyes. Specifically, these are the processes known as Fenton (if the cation used is Fe²⁺) or Fenton-like (if Fe³⁺ is used), in which the dissolved iron catalyzes the decomposition of hydrogen peroxide with the consequent formation of two highly oxidizing hydroxyl radicals (·OH), which result in the decomposition of the dye, the latter being reduced (ideally) to innocuous substances such as CO₂ and water. Its major drawback is the large amount of peroxide that must be added if such a method is used alone [395]. nZVIs are currently being used in conjunction with advanced oxidation processes such as Fenton or Fenton-like processes for dye removal. The aim is to achieve a synergistic effect by increasing the efficiency of the nanoparticles and decreasing the peroxide concentrations in the oxidative processes used. The resulting combination shows promising effects, and its development is in full growth, especially focusing on the development of green chemistry methods for the synthesis of nZVIs.

Metallic iron nanoparticles have emerged as an exceedingly efficient alternative for mitigating this problem within the textile industry. Fe nanoparticles may act as adsorbent, degradation catalyst, and flocculant or additive for flocculation. As mentioned before, this work focuses mainly on degradation. Figure 5 shows no clear trend in dye removal efficiency as a function of either particle size, precursor, or dye. This huge variability probably is due to the multiple factors that condition the removal activity, such as experimental conditions, nanoparticle stabilization and accessibility, oxidant agent, etc.

Table 6 summarizes studies involving the synthesis of iron nanoparticles using plant extracts for use in dye removal. Parameters influencing the conditions include the nanoparticles synthesized, the different precursors, the plant extracts used, the concentration of the dyes, the yield, and the reaction time for dye removal to take place.

The plant extract, whose natural composition includes polyphenols, sugars, alkaloids, phenolic acids, proteins, and coenzymes, plays a triple role, acting as a reducing, coating, and stabilizing agent. The result is the rapid initiation of the reduction process and the generation of large quantities of stable nanoparticles. This multifunctionality expedites the onset of the reduction process and yields substantial quantities of enduring nanoparticles. These nanoparticles possess an extensive surface area and demonstrate thermal and electrical conductivity. Moreover, they have the capacity to acquire magnetic properties, facilitating convenient extraction from the environment and potential reuse [396]. When the concentration of polyphenols in the plant is high, the size of the nanoparticles decreases and they are highly reactive, thus the degradation rate increases and there are no iron oxides present in the nanoparticles.

According to the literature, the synthesis of nanoparticles is influenced not only by the plant extract, but also by various plant parts such as stems, leaves, bark, and fruits. The composition and percentage of biomolecules in these plant parts significantly influence the morphology of the nanoparticles and the properties of their metal precursors as they can act as stabilizing agents [397]. Biomolecules containing -OH and C=C groups serve as reductants for iron salts, transforming them into elemental iron while undergoing oxidation to COOH. It has been shown that if these biomolecules also have conjugated double bonds, they act as capping agents. This capping process can lead to steric hindrance around the nanoparticles, preventing their aggregation. Other large biomolecules such as cellulose, hemicellulose, proteins, etc., seem to play the role of stabilization, preventing agglomeration and favoring flocculation.

Grape leaf extracts have been analyzed, where biomolecules such as phytols, terpenoids (sitosterols), and antioxidants (vitamin E) play the dual role of reducing agent and coating [398,399]. The nanoparticles generated are quasi-spherical with a diameter of 20–60 nm. Other biomolecules such as hydroquinone, benzenediol, 2-methoxy-4-vinylphenol, and some polyphenolic compounds are also responsible for the reduction and coating of iron salts. In eucalyptus leaf extract, benothein B also forms complexes with iron and acts as a reducing and stabilizing agent [296,308].

As seen above, a promising strategy to achieve better adsorption/reduction performance together with increased stabilization of the nanoparticle is the use of zero-valent iron nanoparticles immobilized on porous supports. The use of ecological supports such as biochar or activated carbon, among others, is being sought to a greater extent. The advantages of using these types of carriers are the easy availability of biomass, as indicated in Table 6 (cellulose, corn stover, dried distillers’ grain, red oak, bamboo charcoal, and pine sawdust), and the synergistic effect produced in the adsorption mechanism on the surface of the activated carbon or biochar and the nanoparticle. The most advantageous carriers used are those derived from biomass, biochars, or pyrolyzed material. Another advantage is the high surface area and easy availability of biomass. It should be noted that if nanoparticles are supported on resins, polymers, and heavy metals they may contribute to toxic effects or give rise to other harmful pollutants.

When nanoparticles are synthesized in situ for use in decontamination in water or soil, the use of biopolymers (guar gum, chitosan, starch, xanthan gum, riboflavin, cellulose, and agar-agar) that have hydroxy groups on their surface prevents the nanoparticles from agglomerating due to possible repulsion effects, or to the increased nucleation speed of the iron atoms that gives rise to a thin negatively charged layer, so that electrostatic stabilization occurs.

Chitosan, which is a glucosamine biopolymer derived from crustaceans, modifies the surface of the nanoparticles significantly as it has -NH₂ and -OH groups that provide bin-ding groups, resulting in greater stabilization of the nanoparticle. The addition of amino acids has also been used as a step after the synthesis of the nanoparticles. Groups in their chemical structure such as -NH₂, -SH, and -COOH can interact with the nanoparticle surface of the zero-valent iron by ionic interaction or by formation of -OH bonds on their oxidized surface. It is very important to choose the correct amino acid to mediate the generation of the zero-valent iron nanoparticles. L-glutamic acid proved to be the most effective while L-cysteine gave rise to the transformation of zero-valent iron into oxides and oxyhydroxides [400].

Organic dyes used in the textile and leather industry are degraded by using nZVI nanoparticles formed from extracts of green tea, oolong tea, black tea, eucalyptus plant leaves, Ferula persica root, pomegranate leaf, and grape, following pseudo-first- and pseudo-second-order kinetic models (such as bromothymol blue, orange II, malachite green, methylene blue, methyl orange, crystal violet, and black acid 194). Activation energies less than 15 kJ/mol indicate that diffusion is the predominant phenomenon during adsorption. The degradation of orange II in the LC-MS analysis shows the presence of intermediates such as 1,2-dihydroxynaphthalene, 1-diazo-2-naphthol, 2-naphthol 4-hydrobenzenesulfonate, 4-sulfophenylhydroperoxide, and benzene sulfonate, suggesting that orange II is adsorbed on the surface of the nanoparticle [401]. Possibly, by the interaction of FeO(OH) groups and iron oxides followed by the excision of the azo bond by the zero-valent iron electrons present in the core.

The bleaching of methylene blue or methyl orange can be attributed to a process involving the cleavage of C=C and C=N bonds of the dyes. It can also be explained by an enhanced mass transfer rate at the surface of the nanoparticles followed by their adsorption on the reactive sites of the nanoparticles. Although ultrasound generates some ·OH radicals, the amount remains too low to drive the reaction to a significant extent. The decrease in total organic carbon (TOC) could be attributed to fixation of the dye molecule by long-chain capping agents, followed by co-precipitation and adsorption. The superior removal performance obtained with ultrasound was explained by Wang et al. through a better mass transfer rate of dyes on the nZVI surface, followed by adsorption on reactive sites of the nZVI particles [402]. The electrons generated by Fe⁰ can directly migrate to dyes. Subsequent trapping by H₂O/H⁺ yields highly reactive hydrogen species (H*) or dissociation of dissolved oxygen, generating strong oxidative species such as CHO₂, CO, and COH, followed by cleavage of the chromophore groups and conjugated systems. Furthermore, corrosion products deposited on functional nZVIs (TP-Fe) [402] could be expelled by ultrasound and replenish the surface of nZVIs. Coagulation of the corrosion products (i.e., iron hydroxides/oxides) leads to sedimentation and co-precipitation, which might be the mechanisms responsible for the TOC removal process.

In recent studies, green synthesis of iron nanoparticles by tea (Camellia sinensis) po-lyphenols has been used for the removal of bromothymol blue. The nanoparticles created were spherical with a size range of 5–15 nm. The degradation of bromothymol blue was performed with H₂O₂ and synthesized nZVIs, the kinetics of the reactions ran through a first-order equation depending on the concentration of bromothymol blue. The results suggested a decrease in the rate of bromothymol degradation with increasing amounts of iron up to a maximum of 0.55 mM. In addition, EDTA was used to increase the stability of hydrogen peroxide and slow down its decomposition and production of hydroxyl radicals [403].

According to Shahwan [404], the removal efficiency of dyes is higher with iron nanoparticles obtained from green tea extract and used as a Fenton-type catalyst than if iron nanoparticles are obtained with sodium borohydride. These nanoparticles act as a substrate for the generation of ferrous ions with a consequent decrease in pH (to ~3.5) and increase in the concentration of ·OH radicals. As for the kinetics of the removal process, it is faster when green-synthesized nanoparticles are used. It should be noted that the kinetics of the removal process runs through a second-order or first-order equation.

Huang et al. [405] used oolong tea extract for the reductive degradation of malachite green in aqueous solution. The removal rate decreased with aggregation and oxidation of the nanoparticles, so the size and reactivity of the nanoparticles played an important role in their degradation. It is very likely that the final structures and size of the nZVIs were related to the polyphenol/caffeine concentrations present in the tea extracts. Spherical nanoparticles within the range of size of 40–50 nm were synthesized. XRD showed characteristic peaks corresponding to iron structures such as maghemite (Fe₂O₃), magnetite (Fe₃O₄), and iron oxyhydroxide (FeO(OH)), and the Fe⁰ peak was small as it was not very crystalline in nature. When iron nanoparticles from oolong tea were used, the removal efficiency improved as the contact time increased, reaching equilibrium in 60 min and a removal rate of 75.5%. This is attributable to the fact that the polyphenol and caffeine content in the oolong tea extract not only act as coating agents, preventing the aggregation of the nanoparticles, but are also reducing agents in the synthesis of nanoparticles. The kinetics conformed to a pseudo-first-order kinetic equation with high correlation coefficients (r² > 0.98) and whose activation energy was of the order of 23.86 kJ/mol, suggesting that the relationship was controlled by the chemical diffusion factor. According to the EDS data, the compositions of the oolong tea extract were 20.34% for O, 49.51% for C, 5.05% for S, and 15.10% for K.

Luo et al. [401] address the synthesis of iron nanoparticles using vine leaf extract and apply them to the elimination of orange acid II. The biomolecule extract in methanol and in aqueous medium is compared. The methanol extract has a high concentration of biomolecules capable of increasing the reduction of iron II to zero-valent iron, and thus, increasing the concentration of iron nanoparticles, which results in a high reactivity in the removal of the dye (removal is 80% for the methanolic extract and only 4% when using the aqueous extract). This is attributed to the high concentration of polyphenols and other biomolecules that not only act as reducing agents but also as coating agents, preventing the nanoparticles from aggregating. As a result, the nanoparticles are more stable and reactive. The authors propose a degradation mechanism based on asymmetric cleavage of azo bonds for the removal of orange II when using iron nanoparticles prepared from vine leaf extract.

From eucalyptus leaves, iron nanoparticles are obtained for the elimination of the acid black dye 194, forming an iron–polyphenol complex which is used as a flocculant [296], containing a large amount of hydroxyl groups and acting as chelating agents to form the iron–polyphenol complex. The formed iron–polyphenol nanoparticles (nZVIs) are amorphous, with sizes in the range of 60 nm and with an iron(III) ion located in the chelated nanoparticle. The nanoflocculant can remove acid black 194 more efficiently than FeCl₃ for the same dose (1 mL) 80.5% and 63%, respectively.

Removal of crystal violet in aqueous media was carried out by Nasiri et al. [406] using zero-valent iron nanoparticles functionalized with β-cyclodextrin from Ferula persica root extracts. When a surfactant (β-cyclodextrin) was added to the plant extract, the BET surface area of the nanoparticle increased (from 46.68 to 47.10 m²/g), its size decreased (from 34.20 to 24.20 nm), and its morphology was spherical. Biomolecules extracted also served as a coating agent, stabilizing the zero-iron nanoparticle and decreasing the concentration of iron oxides.

Table 6. Synthesis of Fe nanoparticles by green chemistry for the removal of dyes.

NPs	Leaf Extract of Plant	Fe Source	Dye	Concentration (mgL−1)	Removal (%)	React. Time (min)	Reference
nZVI	Green tea	Fe (NO3)3∙9H2O	Malachite green	50	93	60	[294]
nZVI	Eucalyptus	FeCl3·6H2O	Acid black 194	100	80.5	200	[296]
nZVI	Eucalyptus L.e.	FeSO4·7H2O	Crystal violet	30	97.6	1800	[311]
nZVI	Black tea	FeCl3·6H2O	Reactive blue 238	49.6	90.5	60	[317]
nZVI	Ruellia tuberosa	FeCl3·6H2O	Reactive black 5	25–400	95.3–99.8	30	[320]
nZVI	Artocarpus heterophyllus	FeCl3·6H2O	Fuchsin basic	4	87.5	20	[323]
nZVI	Ricinus communis	FeCl3·6H2O	Methylene blue	25–200	96.8	160	[326]
nZVI	Green tea	FeCl3·6H2O	Reactive blue 238	49.5	96.2	40	[330]
nZVI	Mango peel	FeCl3·6H2O	Methyl orange	100	94.23	60	[341]
nZVI	Pomegranate L.e.	FeCl3·6H2O	Malachite green	156	95	36	[396]
nZVI	Vine L.e.	FeCl2·4H2O	Orange II	100	80	120	[398]
nZVI	Vine L.e.	FeCl2·4H2O	Orange II	100	92	200	[401]
nZVI	Tea	FeSO4·7H2O	Malachite green, methylene blue	200	90.7, 90.75	30	[402]
nZVI	Camellia sinensis tea	FeCl3·6H2O	Bromothymol blue	500	-	60	[403]
nZVI	Green tea	FeCl2·4H2O	Methylene blue, methyl orange	50	360	80	[404]
nZVI	Oolong tea	FeSO4·7H2O	Malachite green	50	75.5	-	[405]
nZVI	Ferula persica	FeSO4·7H2O	Crystal violet	200	99.8	210	[406]
nZVI	Catharanthus roseus	FeSO4·7H2O	Methyl orange	50	50	360	[407]
α-Fe2O3	Phyllanthus niruri, Moringa stenopetala	FeCl3·6H2O	Methylene blue	156	92–96	36	[408]
nZVI	Chlorophytum comosum	FeCl3·6H2O	Methyl orange	4	77	25	[409]
nZVI	Parthenium	FeSO4·7H2O	Crystal violet	120	95	30	[410]
Fe3O4	Thunbergia grandiflora	FeSO4·7H2O	Acid blue 113	25	94.38	210	[411]
Fe2O3	Raphanus sativus L.e.	FeCl3·6H2O	Methylene blue, methyl red	100	100	60	[412]
Fe2O3	Fan palm, Dombeya wallichii, Pyrus communis	FeSO4·7H2O	Reactive blue	200	94, 81, 88	210	[413]
Fe3O4	Jatropha curcas, Cinnamomum tamala	FeCl2·H2O+ FeCl3	Methylene blue	200	46.66	120	[414]
Iron oxide	Artemisia vulgaris L.e.	FeCl3·6H2O	Methyl orange	25	98.6	360	[415]
Iron oxide	Daphne mezereum	FeCl3·6H2O	Methyl orange	25	81	360	[416]
Fe3O4	Fraxinus sinensis Roxb	FeCl3·6H2O+ FeSO4·7H2O	Crystal violet	4	98.57	25	[417]
nCluster	Cupressus sempervirens	FeCl3·6H2O	Methyl orange	25	95.8	360	[418]
nZVI	Psidium guajava L.e.	FeCl3·6H2O	Methylene blue	50	94	5	[419]
nZVI	Hibiscus sabdariffa, roselle flower	FeCl3·6H2O	Rhodamine B	17	100	5	[420]
nZVI	Trigonella foenum-graecum	FeCl3·6H2O	Methyl orange	25	95	90	[421]
ZVI	Camellia sinensis and pomegranate L.e.	FeSO4·7H2O	Textile wastewater	2.330 -	>95 pH 8.5 (Pt-Co)	120	[422]
Fe3O4	Azolla filiculoides and fig. L.e.	FeCl3·6H2O+ FeSO4·4H2O	Crystal violet Methylene blue	500	100	210	[423]
Fe3O4	Pissum sativum peel	FeCl3·6H2O	Methyl orange	100	96.2	360	[424]
nZVI	Eucalyptus	FeSO4·7H2O	Methyl orange	10	99.6	180	[425]
Iron oxide	Pomegranate L.e.	FeCl3·6H2O	Congo red	100	93	60	[426]
Ni-iron oxide	Moringa oleifera	FeCl3·6H2O + NiCl2·6H2O	Malachite green	20	~91.6	25	[427]
Fe3O4	Green tea L.e.	FeCl3·6H2O+ FeSO4∙ 4H2O	Methylene blue	3.5	95	16	[428]
Pd-iron oxide	Pepper	Fe (NO3)3∙9H2O+ FeCl2·4H2O	Acid black, acid brown	20.2	97.85	120	[429]
Iron oxide	Cynometra ramiflora fruit	FeCl2·H2O+FeCl3	Methylene blue	20	100	110	[430]
Iron oxide	Cynometra ramiflora	FeSO4·7H2O	Rhodamine B	134	100	15	[431]
Iron oxide	S. cumini	Ferrous oxalate	Reactive blue 235	10	98.75	240	[432]
nZVI	Tea	FeSO4·7H2O	Methylene blue	50	85.7, 34.4	5	[433]
Fe3O4	Ridge gourd peels	FeCl3·6H2O	Methylene blue	120	96	30	[434]
nZVI	Tieguanyin tea	FeCl3·6H2O	Bromothymol blue	100	>90	30	[435]
nZVI	Green tea	FeCl3·6H2O	RBB-R, DR80 mixture	250	90	20	[436]
Fe3O4	Maize cob	FeCl2·4H2O+ 2FeCl3·6H2O	Methylene blue	156	99.63	36	[437]
nZVI	Eucalyptus tereticornis, Melaleuca nesophila, Rosemarinus officinalis	FeSO4·7H2O	Acid black 194	100	100	200	[438]
nZVI	Green tea, oolong tea, black tea	FeSO4·7H2O	Malachite green, methylene blue	50	81.2, 75.6, 67.1	60	[439]
nZVI	Aspalathus linearis	Acid mine drainage	Orange II	50	94	30	[440]
nZVI	Calotropis gigantea	Fe (NO3)3·9H2O	Methylene blue	50–400	83.9	30	[441]
nZVI	Amaranthus dubius L.e.	FeCl3·6H2O	Methyl orange	20	81	360	[442]

Note: L.e.: Leaf extract.

Table 6. Synthesis of Fe nanoparticles by green chemistry for the removal of dyes.

NPs	Leaf Extract of Plant	Fe Source	Dye	Concentration (mgL−1)	Removal (%)	React. Time (min)	Reference
nZVI	Green tea	Fe (NO3)3∙9H2O	Malachite green	50	93	60	[294]
nZVI	Eucalyptus	FeCl3·6H2O	Acid black 194	100	80.5	200	[296]
nZVI	Eucalyptus L.e.	FeSO4·7H2O	Crystal violet	30	97.6	1800	[311]
nZVI	Black tea	FeCl3·6H2O	Reactive blue 238	49.6	90.5	60	[317]
nZVI	Ruellia tuberosa	FeCl3·6H2O	Reactive black 5	25–400	95.3–99.8	30	[320]
nZVI	Artocarpus heterophyllus	FeCl3·6H2O	Fuchsin basic	4	87.5	20	[323]
nZVI	Ricinus communis	FeCl3·6H2O	Methylene blue	25–200	96.8	160	[326]
nZVI	Green tea	FeCl3·6H2O	Reactive blue 238	49.5	96.2	40	[330]
nZVI	Mango peel	FeCl3·6H2O	Methyl orange	100	94.23	60	[341]
nZVI	Pomegranate L.e.	FeCl3·6H2O	Malachite green	156	95	36	[396]
nZVI	Vine L.e.	FeCl2·4H2O	Orange II	100	80	120	[398]
nZVI	Vine L.e.	FeCl2·4H2O	Orange II	100	92	200	[401]
nZVI	Tea	FeSO4·7H2O	Malachite green, methylene blue	200	90.7, 90.75	30	[402]
nZVI	Camellia sinensis tea	FeCl3·6H2O	Bromothymol blue	500	-	60	[403]
nZVI	Green tea	FeCl2·4H2O	Methylene blue, methyl orange	50	360	80	[404]
nZVI	Oolong tea	FeSO4·7H2O	Malachite green	50	75.5	-	[405]
nZVI	Ferula persica	FeSO4·7H2O	Crystal violet	200	99.8	210	[406]
nZVI	Catharanthus roseus	FeSO4·7H2O	Methyl orange	50	50	360	[407]
α-Fe2O3	Phyllanthus niruri, Moringa stenopetala	FeCl3·6H2O	Methylene blue	156	92–96	36	[408]
nZVI	Chlorophytum comosum	FeCl3·6H2O	Methyl orange	4	77	25	[409]
nZVI	Parthenium	FeSO4·7H2O	Crystal violet	120	95	30	[410]
Fe3O4	Thunbergia grandiflora	FeSO4·7H2O	Acid blue 113	25	94.38	210	[411]
Fe2O3	Raphanus sativus L.e.	FeCl3·6H2O	Methylene blue, methyl red	100	100	60	[412]
Fe2O3	Fan palm, Dombeya wallichii, Pyrus communis	FeSO4·7H2O	Reactive blue	200	94, 81, 88	210	[413]
Fe3O4	Jatropha curcas, Cinnamomum tamala	FeCl2·H2O+ FeCl3	Methylene blue	200	46.66	120	[414]
Iron oxide	Artemisia vulgaris L.e.	FeCl3·6H2O	Methyl orange	25	98.6	360	[415]
Iron oxide	Daphne mezereum	FeCl3·6H2O	Methyl orange	25	81	360	[416]
Fe3O4	Fraxinus sinensis Roxb	FeCl3·6H2O+ FeSO4·7H2O	Crystal violet	4	98.57	25	[417]
nCluster	Cupressus sempervirens	FeCl3·6H2O	Methyl orange	25	95.8	360	[418]
nZVI	Psidium guajava L.e.	FeCl3·6H2O	Methylene blue	50	94	5	[419]
nZVI	Hibiscus sabdariffa, roselle flower	FeCl3·6H2O	Rhodamine B	17	100	5	[420]
nZVI	Trigonella foenum-graecum	FeCl3·6H2O	Methyl orange	25	95	90	[421]
ZVI	Camellia sinensis and pomegranate L.e.	FeSO4·7H2O	Textile wastewater	2.330 -	>95 pH 8.5 (Pt-Co)	120	[422]
Fe3O4	Azolla filiculoides and fig. L.e.	FeCl3·6H2O+ FeSO4·4H2O	Crystal violet Methylene blue	500	100	210	[423]
Fe3O4	Pissum sativum peel	FeCl3·6H2O	Methyl orange	100	96.2	360	[424]
nZVI	Eucalyptus	FeSO4·7H2O	Methyl orange	10	99.6	180	[425]
Iron oxide	Pomegranate L.e.	FeCl3·6H2O	Congo red	100	93	60	[426]
Ni-iron oxide	Moringa oleifera	FeCl3·6H2O + NiCl2·6H2O	Malachite green	20	~91.6	25	[427]
Fe3O4	Green tea L.e.	FeCl3·6H2O+ FeSO4∙ 4H2O	Methylene blue	3.5	95	16	[428]
Pd-iron oxide	Pepper	Fe (NO3)3∙9H2O+ FeCl2·4H2O	Acid black, acid brown	20.2	97.85	120	[429]
Iron oxide	Cynometra ramiflora fruit	FeCl2·H2O+FeCl3	Methylene blue	20	100	110	[430]
Iron oxide	Cynometra ramiflora	FeSO4·7H2O	Rhodamine B	134	100	15	[431]
Iron oxide	S. cumini	Ferrous oxalate	Reactive blue 235	10	98.75	240	[432]
nZVI	Tea	FeSO4·7H2O	Methylene blue	50	85.7, 34.4	5	[433]
Fe3O4	Ridge gourd peels	FeCl3·6H2O	Methylene blue	120	96	30	[434]
nZVI	Tieguanyin tea	FeCl3·6H2O	Bromothymol blue	100	>90	30	[435]
nZVI	Green tea	FeCl3·6H2O	RBB-R, DR80 mixture	250	90	20	[436]
Fe3O4	Maize cob	FeCl2·4H2O+ 2FeCl3·6H2O	Methylene blue	156	99.63	36	[437]
nZVI	Eucalyptus tereticornis, Melaleuca nesophila, Rosemarinus officinalis	FeSO4·7H2O	Acid black 194	100	100	200	[438]
nZVI	Green tea, oolong tea, black tea	FeSO4·7H2O	Malachite green, methylene blue	50	81.2, 75.6, 67.1	60	[439]
nZVI	Aspalathus linearis	Acid mine drainage	Orange II	50	94	30	[440]
nZVI	Calotropis gigantea	Fe (NO3)3·9H2O	Methylene blue	50–400	83.9	30	[441]
nZVI	Amaranthus dubius L.e.	FeCl3·6H2O	Methyl orange	20	81	360	[442]

Note: L.e.: Leaf extract.

7. Future Perspectives and Challenges

Environmental issues are not inherent to chemistry per se; rather, they frequently originate from anthropogenic processes or the imprudent utilization of chemical knowledge, resulting in adverse environmental ramifications. Consequently, there exists a pressing need to adopt clean and sustainable methodologies, as well as to utilize raw materials derived from nature.

This review undertakes a comprehensive analysis of methodologies employed for the removal of dyes, with a specific focus on nanoparticles synthesized through both conventional and green chemistry techniques. Notably, this entails the utilization of plant extracts as reductants for the formation of nZVIs. Such an approach prioritizes environmental safety and long-term sustainability over immediate gains. This necessitates exploration into more environmentally benign alternatives, refinement of Fe nanoparticle synthesis processes for heightened efficacy, and advocacy for responsible chemical practices.

Some key aspects that need to be addressed in the near future are summarized below.

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Critical to note is the considerable variability in extracted biomolecules from plant extracts, constituting an unpredictable factor in green chemistry nanoparticle synthesis. The diverse nature of plants, encompassing distinct plant parts and geographical locales, complicates the prognostication of extracted biomolecules and their quantities. Moreover, not all biomolecules involved in nanoparticle synthesis are comprehensively understood. Controllable factors such as solvent, temperature, pH, precursor iron salts, and solution agitation profoundly influence nanoparticle growth, morphology, size, aggregation, coating, and stability within green chemistry procedures.

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Furthermore, it is imperative to recognize the industrial implications of nanoparticle synthesis, necessitating large-scale production. Consequently, material supply assumes critical importance, with the agrifood industry wielding significant influence due to the pronounced variability influenced by geographical location and climate.

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A standardized protocol is imperative for achieving greater reproducibility in nanoparticle synthesis concerning size and shape, thereby optimizing dye decontamination processes in the environment. Furthermore, the development of methodologies aimed at minimizing waste generation or enabling waste reuse is critical for fostering more sustainable processes. The extraction processes from agrifood industries entail significant solvent consumption, resulting in solid residues, necessitating a reduction in energy consumption. Analogous to numerous other fields, the reutilization and recycling of these by-product streams are pivotal for practical industrial implementation.

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Regarding applications, substantial challenges persist. Ongoing efforts are directed towards exploring novel applications in diverse sectors such as food, textiles, healthcare, and construction, extending beyond environmental objectives.

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In the realm of water purification, wastewater treatment plants are transitioning towards pollutant mineralization and biorefinery models, aiming to derive commercially viable products. Accordingly, pollutant removal processes must align with this philosophy and be seamlessly integrated into wastewater treatment plants. This mandates that new nanoparticles, acting as catalysts, should adsorb and convert pollutants into products that can be easily desorbed at low energy costs.

Regarding the main challenges that must be faced in connection with the use of nZVIs in decontamination processes of waters polluted with dyes, some crucial aspects have to be taken into account.

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The authors underscore the potential for the wastewater industry to operate as a biorefinery, extracting commercial products from wastewater. However, these treatment methodologies are still nascent, as depicted in Figure 6.

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It is noteworthy that a mere 242 articles, less than 0.001%, focus on dye recovery, with the majority concentrating on repurposing waste for dye treatment using alternative methods. Only a fraction of these articles address the recovery of solid waste from the textile industry [443,444]. Marazzi et al. [445] propose the use of algae for the treatment of dyes and their subsequent transformation into energy in biogas plants through anaerobic digestion.

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Since clean, high-quality water is a valuable and essential commodity, and given that one of the main and most obvious parameters indicating water quality is its color, the application of iron nanoparticles as an available technology for dye removal is easy to use, cost-effective, and very efficient. The two main removal processes are adsorption and decolorization. This implies that studies should be directed to investigate the optimal conditions of these processes, such as the influence of the effects of initial dye concentration, pH, temperature, as well as nanoparticle size, morphology, and dosage, and to generate new general trends based on these studies.

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A plethora of literature exists on environmentally friendly methods, often associated with biomass or bio-resources as feedstock sources. While these methodologies hold significant promise, presuming their environmental friendliness without thorough environmental impact assessments would be erroneous [446]. Thus, it is imperative to evaluate the environmental footprint of the chosen method to ensure its sustainability and assess its economic viability for industrialization. Furthermore, methods employing industrial chemical reagents can be rendered environmentally friendly provided energy consumption is minimized, and by-products or residues are recycled or integrated into a circular economy framework.

Figure 6. Number of published articles for each treatment method (a), and schematic mechanism of each treatment method (b).

Figure 6. Number of published articles for each treatment method (a), and schematic mechanism of each treatment method (b).