Small particles, big environmental impact: the sustainable chemistry of iron sulfide nanomaterials

1 Introduction

The significance of iron-sulfide (FeS) has frequently been extended to discussions of early Earth chemistry and the origin of metabolism. It has been proposed that FeS minerals in hydrothermal vent systems acted as primitive catalysts for carbon fixation, driving the reduction of carbon dioxide by hydrogen to form simple organic molecules (Huber and Wachtershauser, 1997; Wächtershäuser, 1988). The exergonic conversion of FeS and hydrogen sulfide (H₂S) to pyrite has been reported to occur as shown in Equation 1. The reaction proceeds through oxidation of sulfide species to form disulfide (S₂ ^2-) units characteristic of pyrite. In this process, sulfur derived from H₂S undergoes partial oxidation, while Fe remains predominantly in the Fe²⁺ state within the pyrite lattice. The electrons released during sulfide oxidation reduce protons to molecular hydrogen, making the overall transformation thermodynamically favorable under anoxic conditions.

FeS + H 2 S → FeS 2 + H 2 (1)

Iron sulfide minerals have been proposed as early geochemical energy sources capable of fueling protometabolic pathways (Rickard and Luther, 2007). These concepts have been reinforced by the structural resemblance between mineral FeS clusters and iron–sulfur cofactors in enzymes such as ferredoxins and hydrogenases, which are ubiquitous in extant microbial metabolism (Russell and Martin, 2004). Representative structures of the active centers of [Fe-Fe]-hydrogenases is shown in Figure 1. These biological iron–sulfur cofactors mirror structural motifs that are also observed in naturally occurring iron sulfide minerals. In both systems, iron atoms are coordinated by bridging sulfide ligands to form metal-sulfur cluster which is capable of mediating electron transfer reactions.

1.1 Distinction from other iron sulfides

Iron sulfides occur in several mineral forms that differ markedly in crystal structure, stability, and electronic properties. The most common examples include pyrite (FeS₂), greigite (Fe₃S₄), pyrrhotite (Fe_1-xS), chalcopyrite (CuFeS₂), bornite (Cu₅FeS₄), and mackinawite (FeS). Each phase displays distinctive physical and chemical properties. Crystal structures and mineral forms of common iron-sulfides, including FeS₂, Fe₃S₄, Fe_1-xS, CuFeS₂, Cu₅FeS₄, and FeS, are shown in Figures 2A–F. These minerals exhibit diverse crystallographic arrangements of Fe and S atoms. This arrangement leads to variations in electronic structure, redox behavior, and stability. The structural diversity of naturally occurring Fe-S minerals provides an important framework for understanding the physicochemical properties and reactivity of iron sulfide materials.

FeS₂ has been investigated primarily as a semiconductor for photovoltaic and catalytic applications (Ennaoui and Tributsch, 1984). Fe₃S₄, with its ferrimagnetic properties (Chang et al., 2009a; Chang et al., 2009b), has been studied in magnetic recording and biomedical contexts (Feng et al., 2013). Similarly, Fe_1-xS is a non-stoichiometric iron sulfide phase. It exhibits metallic conductivity and magnetic ordering arising from iron vacancies within its lattice. Both CuFeS₂ and Cu₅FeS₄ are copper-iron sulfide minerals commonly associated with hydrothermal ore systems. They possess mixed-metal sulfide frameworks that enable semiconducting behavior and catalytic activity in electrochemical and photocatalytic processes. Among the various iron sulfide minerals, the FeS phase mackinawite is particularly important because of its high surface reactivity and environmental relevance. Mackinawite is a metastable tetragonal iron sulfide phase and is frequently formed in aqueous environments. It is characterized by high surface reactivity and rapid electron transfer. These properties have made it an important model phase for studying reductive transformations of environmental contaminants and the formation of nanoscale FeS materials.

Iron-sulfur frameworks are not limited to mineral phases. Similar Fe-S motifs also occur widely in biological systems, where they function as catalytic and electron-transfer cofactors in enzymes. These clusters typically occur as discrete assemblies such as [2Fe-2S] [3Fe-4S], and [4Fe-4S] units coordinated by cysteine residues. The structural similarity between mineral Fe-S frameworks and biological clusters highlights a shared motif based on Fe-S connectivity. Representative biological iron-sulfur clusters are shown in Figure 3.

When iron sulfide forms at the nanoscale, these structural and electronic features become amplified, producing materials with exceptionally high surface reactivity and rapid electron transfer kinetics. Nanoparticulate FeS remains metastable and poorly crystalline relative to bulk iron sulfide minerals. Nanoparticulate FeS typically refers to iron monosulfide phases with particle sizes in the nanometer scale, commonly <100 nm and often ranging between 5–50 nm depending on synthesis conditions. These particles are frequently poorly crystalline or exhibit short-range ordered structures such as mackinawite, which represent metastable precursors to more ordered iron sulfide minerals including greigite and pyrite (Morse and Luther, 1999; Rickard and Luther, 2007). Structural disorder in FeS nanoparticles produces highly reactive surface Fe–S sites that facilitate rapid electron transfer at the mineral–solution interface. The high surface area and abundance of coordinatively unsaturated Fe(II) centers enhance reductive reactivity in aqueous systems. Natural FeS nanoparticles typically form through microbial sulfate reduction and abiotic sulfide precipitation in sediments and anoxic waters, whereas synthetic FeS nanoparticles are commonly produced via controlled aqueous precipitation methods that allow greater control over particle size, surface chemistry, and aggregation behavior (Han et al., 2020; Rickard, 2012b). It exhibits enhanced electron transfer kinetics. This characteristic has enabled its use in redox reactions with contaminants, such as the reduction of hexavalent chromium (Gong et al., 2017; Li et al., 2017; Wang et al., 2019) as presented in Equation 2. In this reaction, Fe(II) within the FeS lattice acts as the primary electron donor and transfers electrons to Cr(VI), reducing it to the less soluble Cr(III) form. Concurrently, sulfide species may participate in secondary redox pathways and facilitate electron shuttling at the particle surface. The high density of reactive Fe(II) sites and the conductive Fe–S framework together enable rapid interfacial electron transfer during contaminant reduction.

3 FeS + 2 CrO 4 2 − + 8 H + → 3 Fe 2 + + 3 S 0 + 2 Cr 3 + + 4 H 2 O (2)

Fe(II) of the FeS nanoparticles served as the primary electron donor in this reaction. Whereas sulfide species may participate in secondary redox pathways. These properties highlight the central role of nanoparticulate FeS as a highly reactive iron sulfide phase. It has motivated the growing interest in controlled synthesis strategies for producing FeS nanoparticles and its applications across fields.

1.2 Emergence of nanoscale FeS

Inspired by the structural diversity and reactivity of naturally occurring Fe-S systems, significant effort has been devoted to developing synthetic approaches for iron sulfide nanoparticles with controlled composition, morphology, and surface chemistry. The synthesis of FeS nanoparticles has been reported through multiple strategies, including aqueous co-precipitation (Agnihotri et al., 2020), solvothermal (Dutta et al., 2012) and hydrothermal methods (Dutta et al., 2012; Khabbaz and Entezari, 2016), and more recently, green synthesis using biogenic reducing agents (Saeed et al., 2023). Control over particle size, morphology, and crystallinity has been demonstrated by varying solvent composition, reaction temperature, and stabilizing ligands. However, a major challenge remains the inherent instability of FeS nanoparticles. Fe(II) is rapidly oxidized upon exposure and leads to the formation of oxyhydroxide layers and elemental sulfur as presented in Equation 3. This oxidative transformation proceeds through oxygen adsorption at FeS surfaces followed by electron transfer from Fe(II) to dissolved oxygen. The resulting oxidation produces Fe(III) oxyhydroxide phases while sulfide species are converted to elemental sulfur or sulfate intermediates. Formation of these surface layers progressively passivates reactive FeS sites and reduces long-term redox activity.

4 FeS + 4 O 2 → 2 Fe 2 O 3 + 4 S (3)

This transformation reduces the reactivity of FeS, thereby complicating its practical deployment. Encapsulation within carbon matrices or polymer coatings has been explored to enhance stability (Sheng et al., 2021).

The potential applications of FeS nanoparticles span multiple sectors. In environmental engineering, they have been deployed for the reductive immobilization of oxyanions, chlorinated solvents, and organic dyes (Ali et al., 2021; Rafeeq et al., 2022). In catalysis, FeS has been reported as an efficient, low-cost electrocatalyst for hydrogen evolution and oxygen reduction reactions, particularly when combined with heterometal dopants such as Co or Ni (Dutta et al., 2012; Tigwere et al., 2023; Yang et al., 2025). Applications in energy storage have included its use as a conversion anode for lithium-ion and sodium-ion batteries, where carbon encapsulation strategies have been shown to mitigate volume expansion and polysulfide dissolution (Xu et al., 2026; Yao et al., 2019). Although significant progress has been reported in the synthesis and application of FeS nanoparticles, important challenges remain, particularly regarding metastability, aggregation, and the variability of reactivity under environmental conditions. In addition, current studies are often dispersed across different disciplines, and a clear connection between FeS nanoparticle structure, surface chemistry, and environmental reactivity remains insufficiently discussed. This review addresses these gaps by critically examining the synthesis, stabilization, and structure-reactivity relationships of nanoparticulate FeS with emphasis on their behavior in environmental systems. Particular attention is given to the mechanisms governing contaminant transformation and the factors controlling nanoparticle stability in aqueous environments. By integrating these aspects, this review provides a focused perspective on the role of FeS nanomaterials in environmental chemistry and sustainable remediation strategies.

2 Synthesis and stabilization of FeS nanoparticles

The synthesis of FeS nanoparticles has been pursued via diverse chemical and physical routes to tailor phase purity, particle size, surface chemistry, and dispersibility. A major focus has been directed toward controlling the metastable nature of FeS, as rapid transformation to more stable sulfide phases such as FeS₂ or Fe₃S₄ has been frequently observed under both laboratory and environmental conditions (Rickard, 2012a). In nanoparticulate form, stabilization of FeS has been achieved through optimized synthetic environments and protective surface modifications (Kim et al., 2011). The major synthesis routes for nanomaterials are schematically summarized in Figure 4.

2.1 Aqueous precipitation methods

One of the simplest and most widely used techniques involves aqueous precipitation. In this method, ferrous salts react with sulfide donors under anoxic conditions and the reaction takes place as shown in Equation 4. The formation of FeS in aqueous systems proceeds through rapid nucleation following supersaturation of Fe²⁺ and sulfide species (HS⁻/S^2-). This is followed by particle growth via aggregation and ripening, with pH, ionic strength, and precursor concentration controlling particle size, crystallinity, and phase evolution.

Fe a q 2 + + HS a q − → FeS s + H a q + (4)

This approach has been implemented in batch reactors, where solutions of FeCl₂ or FeSO₄ were combined with Na₂S or NaHS solutions under inert atmosphere (Rickard, 2006). The rapid precipitation kinetics have typically produced nanoparticles with sizes in the 5–50 nm range. However, particle aggregation has been frequently reported, necessitating the use of stabilizing agents. Stabilizers such as carboxymethyl cellulose, gelatin, and related biomaterials have been introduced to improve colloidal stability and reactivity (Shao et al., 2016) (Figure 5).

The advantages of precipitation methods have included simplicity, scalability, and low cost. Nevertheless, disadvantages include poor control over crystallinity, heterogeneous particle size distributions, and susceptibility to rapid oxidation upon exposure to air. The reproducibility of such methods has therefore depended strongly on the exclusion of oxygen and the careful regulation of pH and ionic strength.

2.2 Hydrothermal and solvothermal synthesis

Under hydrothermal conditions, FeS formation is governed by dissolution-recrystallization processes, where elevated temperature and pressure promote controlled crystal growth and phase transformation. These conditions facilitate the development of more ordered structures compared to aqueous precipitation. Hydrothermal and solvothermal methods have been developed to achieve better control over the crystallinity and morphology of FeS nanoparticles (Sines et al., 2012). In these approaches, iron salts and sulfur precursors have been reacted in sealed autoclaves at elevated temperatures (120 °C–220 °C) under autogenous pressure. Reaction conditions such as precursor composition, temperature and duration strongly influence crystal growth and phase formation. For example, solvothermal treatment of an amorphous precursor in ethylene glycol at 200 °C for 4 h yielded highly crystalline FeS nanosheets, demonstrating the ability of elevated-temperature sealed-reactor systems to improve crystallinity and morphological control. It has also been demonstrated that the use of organic solvents can influence nucleation kinetics and suppress aggregation (Sines et al., 2012; Yuan et al., 2020).

Hydrothermal synthesis has also allowed the formation of hierarchical morphologies such as nanosheets, nanorods, and flower-like aggregates. These architectures have been shown to provide increased surface area and improved electrochemical performance, particularly in applications involving lithium-ion storage (Xing et al., 2015).

2.3 Microemulsion and template-assisted routes

Microemulsion systems have been used to confine nucleation within nanoscale droplets (Khatun et al., 2025). In microemulsion and template-assisted systems, nanoparticle formation occurs within confined nanoscale domains, where spatial restriction limits particle growth and aggregation. This confinement enables controlled nucleation and leads to narrower size distributions and more uniform morphologies. These routes has enabled size control with narrow distributions (Dadyburjor et al., 2000). Variety of surfactants have been employed to stabilize oil–water interfaces, at which the reactions between Fe²⁺ and S^2- were initiated (Shao et al., 2016; Sun M. et al., 2018; Sun Y. et al., 2018; Wu et al., 2017).

Template-assisted synthesis has further been attempted using mesoporous silica or carbon scaffolds, in which FeS nucleation occurred within ordered pores (Yu et al., 2025). Such confinement has yielded nanoparticles with well-defined geometries and enhanced resistance to aggregation.

2.4 Biogenic and green synthesis approaches

Biogenic routes have gained interest due to their alignment with green chemistry principles. Microorganisms including sulfate-reducing bacteria have been shown to produce FeS nanoparticles enzymatically during metabolic sulfate reduction (Donald and Southam, 1999; Moreau et al., 2013). Plant extracts containing polyphenols and reducing sugars have also been explored as reductants and stabilizers in the synthesis of FeS nanoparticles. It has yielded environmentally benign products with surface functionalization derived from natural biomolecules (Iravani, 2011). Such approaches offer environmentally benign routes for nanomaterial synthesis and may enhance surface functionalization and biocompatibility. Despite these advantages, biogenic and green synthesis approaches face some limitations. These include limited control over particle size and crystallinity, variability in precursor composition, and challenges in reproducibility across different biological systems. In addition, scale-up remains difficult due to slower reaction kinetics and sensitivity to environmental conditions.

The synthesis routes discussed above differ substantially in their degree of control over particle formation, susceptibility to oxidation, scalability, and practical relevance. A direct comparison is therefore useful to identify which approaches are most suitable for producing FeS nanoparticles for environmental applications. Table 1 summarizes the main synthesis and stabilization strategies discussed in this section, with emphasis on their typical strengths, limitations, and current level of use in the FeS literature.

TABLE 1. Comparative summary of synthesis strategies for nanoparticulate FeS.

Overall, aqueous precipitation remains the most practical route for environmentally relevant FeS nanoparticles, whereas more controlled methods such as solvothermal, microemulsion, and core-shell approaches are mainly useful when improved structural definition or stability is required.

2.5 Stabilization strategies

The stabilization of FeS nanoparticles has remained a critical issue. Several strategies have been employed in the past. But, despite these advances, challenges persist. Some of the reported stabilization strategies that are discussed below include coating, capping, doping and so on.

2.5.1 Carbon coating

Encapsulation of FeS nanoparticles in carbon shells has been reported in the literature. It was found that this coating potentially improve conductivity and protect against oxidation (Miao et al., 2020; Phogat et al., 2026; Wang et al., 2002; Xu et al., 2026). In these systems, nanocrystals are embedded within conductive carbon matrices or coated with thin carbon shells during synthesis. Such architectures are typically prepared by pyrolysis of organic precursors, hydrothermal carbonization, or solvothermal synthesis followed by carbonization. The major advantage is that it suppresses aggregation and slows down oxidation under ambient conditions (Zhou et al., 2022). Carbon-coated FeS nanostructures have been widely explored in lithium-ion batteries, sodium-ion batteries, and electrocatalysis.

2.5.2 Polymer capping

Surface modification with polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), or chitosan has been demonstrated to enhance colloidal stability and dispersion in aqueous media. It is one of the most effective strategies. Polymers such as carboxymethyl cellulose (CMC), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), or gelatin adsorb onto nanoparticle surfaces during synthesis (Gong et al., 2014). These macromolecules provide steric stabilization by forming a protective organic layer around each particle. This reduces particle-particle interactions. In addition to steric stabilization, certain polymers introduce electrostatic repulsion due to negatively charged functional groups. This further enhances colloidal stability. Nanoparticles stabilized by polymer capping have been widely studied in environmental remediation. They have been shown to be particularly useful during groundwater treatment and heavy metal immobilization (Shao et al., 2016; Sharma et al., 2024). The improved dispersion of stabilized FeS nanoparticles increases their effective surface area and enhances reactivity toward contaminants such as mercury, uranium, and arsenic.

2.5.3 Heterometal doping

Incorporation of cobalt, nickel, or molybdenum into FeS structures has been shown to stabilize it (Xue et al., 2024; Zheng et al., 2025). This incorporation prevents structural collapse and enhances catalytic activity. Doping is typically achieved during synthesis by introducing secondary metal precursors into the reaction. These heterometals may substitute for Fe atoms in the crystal lattice or form mixed-metal sulfide phases. Such modifications can improve electrical conductivity, introduce additional catalytically active sites, and enhance stability under reaction conditions (Gaikwad et al., 2024; Voiry et al., 2018). In electrocatalysis, Ni- and Co-modified iron sulfides have demonstrated improved activity for hydrogen evolution and oxygen evolution reactions. The enhanced performance has been attributed to synergistic interactions between Fe and the dopant metal, which modify the adsorption energies of reaction intermediates and facilitate electron transfer processes.

2.5.4 Core–shell structures

FeS@SiO₂ and FeS@Al₂O₃ systems have been reported to retain FeS reactivity while suppressing surface oxidation. In these systems, FeS forms the core material while a secondary shell material surrounds the nanoparticle surface. Carbon, silica, or metal oxides are often reported as secondary shell materials (Lou et al., 2008). Sequential deposition processes or in situ growth methods are the strategies most commonly used during solvothermal synthesis. The shell material serves multiple functions. It protects FeS from oxidation, prevents nanoparticle aggregation, and can introduce additional catalytic or electronic properties (Zhao et al., 2019). Silica shells have been reported to provide chemical stability and prevent particle sintering. Carbon shells have been reported to enhance electrical conductivity and improve electrochemical performance. Core–shell FeS materials have therefore been widely explored for energy storage devices and catalytic systems (Huang et al., 2023; Zhao et al., 2019).

While significant progress has been made and several strategies have been established, particle aggregation has remained problematic in aqueous suspensions, limiting surface area and reactivity. It is reported that the oxidation during handling and storage has frequently altered phase composition, producing mixed Fe(III) oxyhydroxides that confound performance evaluation. Reproducibility has often been limited by insufficient reporting of synthesis parameters, including atmosphere control, mixing rates, and precursor purity. It has therefore been recommended that standardized characterization, including X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Mössbauer spectroscopy, and Fe(II)/Fe (total) ratios should be consistently applied to confirm the identity and stability of FeS nanoparticles (Rickard and Luther, 2007). The metastable nature of FeS demands careful synthesis and stabilization strategies, and continued refinement of these methods will be necessary to unlock the full potential of FeS nanoparticles in technological and environmental applications.

3 Structure-property tools for FeS nanoparticles

Understanding the function of FeS nanoparticles has been critically dependent on systematic characterization of their structure, morphology, electronic configuration, and surface chemistry. Because FeS is metastable and prone to transformations during synthesis, storage, and experimental use, identifying its structural attributes has been essential for interpreting catalytic activity, redox behavior, and environmental stability. A comprehensive toolkit of spectroscopic, microscopic, and electrochemical techniques has therefore been employed and reported to link structural features with observed performance (Rickard and Luther, 2007). A summary of commonly used characterization techniques and their relevance to FeS nanoparticle properties is provided in Table 2.

TABLE 2. Summary of key characterization techniques used for FeS nanoparticles.

The accurate characterization of FeS nanoparticles still remains challenging due to their metastability, rapid oxidation under ambient conditions, and susceptibility to structural alteration during analysis. As a result, each characterization technique presents inherent limitations that must be carefully considered when interpreting FeS properties.

3.1 Crystallographic characterization

XRD has been used as the primary tool for distinguishing FeS phases. The tetragonal mackinawite structure (FeS) is typically identified by reflections near 2θ ≈ 17°, 33°, and 44° (Cu Kα radiation), corresponding to the (001), (110), and (200) lattice planes, respectively (Lennie et al., 1995; Rickard, 2012a; 2012b). In nanoparticulate systems, these peaks are often broadened due to reduced crystallite size and structural disorder characteristic of nanoscale materials. Such peak broadening is commonly interpreted using the Scherrer relation to estimate crystallite dimensions. TEM coupled with selected-area electron diffraction (SAED) has been further applied to confirm lattice spacing and to detect secondary phases or impurities. So, these techniques not only confirm the formation but also provide in-depth characteristics. For example, high-resolution TEM images have revealed lattice fringes corresponding to interplanar spacings of 0.27–0.29 nm (Lennie et al., 1995; Rickard and Luther, 2006; 2007; Wolthers et al., 2005). It has been assigned to the (200) plane of mackinawite and have provided direct evidence of nanoscale crystallographic order. In addition, TEM imaging has shown that FeS also exhibits a layered, platelet-like morphology (Bolney et al., 2021). Intensity profile analysis along the stacking direction indicates a periodic spacing of ∼0.5 nm, corresponding to the interlayer distance of FeS (Figure 6).

However, XRD analysis of FeS nanoparticles is often complicated by peak broadening, low crystallinity, and overlap with other iron sulfide phases, which can hinder precise phase identification. Additionally, amorphous or poorly ordered FeS phases may remain undetected by XRD.

3.2 Oxidation state and electronic environment

The redox flexibility of iron has necessitated the use of spectroscopic methods to determine the Fe(II)/Fe(III) ratio in nanoparticles. XPS has been widely employed, with Fe 2p₃/₂ binding energies near 707 eV indicating Fe(II) and satellite features near 711–713 eV indicating Fe(III). Sulfur species have been detected in the S 2p region, with signals corresponding to S^2- (161–162 eV), polysulfides (163–164 eV), and oxidized forms such as SO₄ ^2- (168–170 eV) (Kim et al., 2011). Representative high-resolution Fe 2p and S 2p XPS spectra of FeS nanomaterial are shown in Figure 7. The spectra reveal the coexistence of Fe(II)-S and Fe(III)-S species together with sulfur present in multiple chemical environments, including S^2-, S₂ ^2- and S_n ^2- species (Matamoros-Veloza et al., 2018). The persistence of these Fe and S species after Ar-ion etching further demonstrates the use of XPS for investigating near-surface chemical states and sulfur speciation in Fe–S nanomaterials.

Mössbauer spectroscopy has also been regarded as an indispensable method for quantifying Fe valence states and distinguishing between tetrahedral and octahedral coordination environments (Lefelhocz et al., 1967). Despite its utility, XPS is inherently surface-sensitive and may not accurately represent the bulk composition of FeS nanoparticles, particularly in systems where surface oxidation has occurred. Similarly, Mössbauer spectroscopy requires specialized instrumentation and complex spectral interpretation, which can limit its routine application. In addition, the presence of mixed-valence states and overlapping spectral features in FeS systems can complicate quantitative analysis.

3.3 Surface chemistry and reactive sites

The high surface reactivity of FeS nanoparticles has been attributed to exposed Fe(II) sites and sulfur anions. In particular, Fe(II) acts as a key electro donor in redox reactions, enabling the reduction of contaminants through interfacial electron transfer. Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy have been applied to detect Fe-S stretching vibrations and to probe the presence of polysulfide intermediates (Agoro and Meyer, 2022; Chen et al., 2019). Surface charge has been routinely evaluated using zeta potential measurements, with values typically near neutral to slightly negative at circumneutral pH. Such low absolute zeta potential values indicate weak electrostatic repulsion between particles, resulting in limited colloidal stability and a strong tendency toward aggregation (Chen et al., 2019; Marguš et al., 2016). In aqueous systems, this aggregation is further influenced by ionic strength and the presence of dissolved ions, which can compress the electrical double layer and promote particle–particle interactions. As a result, FeS nanoparticles frequently form larger aggregates or colloidal clusters, leading to a reduction in effective surface area and blocking of reactive Fe(II) sites. This aggregation-driven loss of accessible surface sites has been identified as a major factor limiting reactivity, particularly in environmental applications where dispersion and transport are critical. Electrochemical methods have also been employed (Theberge & Luther III, 1997). Cyclic voltammetry has revealed reversible Fe(II)/Fe(III) redox peaks, while impedance spectroscopy has been used to evaluate conductivity improvements imparted by carbon encapsulation or heterometal doping (Bura-Nakić et al., 2011; Helz et al., 2011; Kam and Johnson, 1980). But it should be noted that the FTIR and Raman signals for Fe–S systems are often weak and can overlap with signals from surface-bound species or stabilizing agents, complicating interpretation. Zeta potential measurements are highly sensitive to solution chemistry, including ionic strength and pH, which can lead to variability in reported values. Electrochemical measurements, while useful for probing redox behavior, may be influenced by electrode preparation, particle aggregation, and surface passivation effects, making direct comparison between studies difficult.

3.4 Morphology and size distribution

Literature reveals that the particle size and morphology have been characterized primarily using TEM and scanning electron microscopy (SEM) (Ahuja et al., 2019; Csákberényi-Malasics et al., 2012; Kim et al., 2011; Malek et al., 2018; Yu et al., 2020). Nanoparticles produced by precipitation methods have typically exhibited sizes of 5–20 nm, whereas hydrothermal synthesis has yielded nanorods and nanosheets exceeding 100 nm in length or width (Mansor et al., 2020). Brunauer–Emmett–Teller (BET) surface area measurements have been conducted to quantify available reactive surface sites, with values ranging from 30 to 120 m² g^-1 depending on synthesis conditions (Brumovský et al., 2020). Beyond simple surface area estimation, BET analysis provides an indirect measure of the density of accessible reactive sites, which play a critical role in adsorption and redox-driven transformations. Higher surface areas generally correlate with increased availability of exposed Fe(II) active sites and sulfur species, thereby enhancing adsorption capacity and catalytic reactivity toward contaminants. However, it is important to note that BET measurements do not distinguish between truly reactive surface sites and inaccessible or blocked pores, and therefore may overestimate the effective reactive surface area in aggregated or partially oxidized FeS systems.

Dynamic light scattering (DLS) has been employed to determine hydrodynamic diameters in aqueous suspension (Marguš et al., 2016). Discrepancies between TEM and DLS measurements have been interpreted as evidence of aggregation and the formation of colloidal clusters. Such aggregation has been shown to diminish reactivity by reducing the number of accessible surface sites. Nevertheless, accurate determination of particle size and morphology remains challenging. TEM provides high-resolution images but typically represents a limited sample area, which may not capture overall sample heterogeneity. In contrast, DLS measures hydrodynamic diameter and is highly sensitive to aggregation, often overestimating particle size in colloidal suspensions. BET surface area measurements provide indirect estimates of reactive surface area and do not distinguish between accessible and inactive surface sites, which may lead to overestimation of catalytic potential.

3.5 Phase transformations and aging

The metastable nature of FeS nanoparticles has necessitated careful evaluation of phase transformations over time. Studies conducted under oxic conditions have shown rapid conversion of FeS to ferric oxyhydroxides with concomitant sulfur oxidation (Chiriţă and Schlegel, 2015). Under anoxic aqueous conditions, FeS has been observed to transform to greigite (Fe₃S₄) within days to weeks, and further to pyrite (FeS₂) over longer timescales (Jeong et al., 2010; Rickard, 2012a; 2012b).

Operando techniques have increasingly been utilized to monitor these changes (Khatun et al., 2025; Zhao et al., 2025). X-ray absorption spectroscopy (XAS) has been applied to follow real-time alterations in Fe valence states and local bonding environments (Kruse, 1992). Time-resolved TEM has been used to document nucleation and growth of secondary phases (Chen et al., 2016). Such approaches have been essential in clarifying the mechanisms by which FeS evolves under realistic operating conditions. Despite these advances, the kinetics of phase transformation and their direct impact on reactivity remain insufficiently quantified. Most studies report phase evolution qualitatively, without linking transformation rates to performance loss. A more quantitative understanding of these processes is required for predicting material behavior under realistic conditions.

Thus, the characterization of FeS nanoparticles has relied on multidisciplinary methods. The identification of oxidation state, surface charge, morphology, and phase stability has been essential for understanding the observed reactivity. Strong correlations between structure and function have been consistently demonstrated. It therefore underscores the need for rigorous, standardized characterization protocols to advance FeS nanoparticle research.

Overall, while a wide range of characterization techniques has been applied to FeS nanomaterials, the field remains largely descriptive, with limited integration of structural, electronic, and functional insights. In particular, inconsistencies in experimental conditions and characterization methodologies hinder direct comparison across studies. Future efforts should focus on establishing quantitative structure–reactivity relationships supported by standardized protocols and advanced operando techniques, enabling more predictive understanding of FeS behavior in complex environmental systems.

4 Functional applications of FeS nanoparticles

Reactivity of the FeS nanoparticles arises from a combination of structural disorder, high surface area, redox states, and the presence of reactive sulfur species that participate in rapid electron transfer reactions (Gong et al., 2014; Patterson et al., 1997). These characteristics enable FeS nanoparticles to mediate wide range of processes including reductive transformation, catalytic transformation, and adsorption–precipitation pathways (Butler and Hayes, 2000; Rickard and Luther, 2007). The kinetics and extent of these transformations are strongly influenced by environmental variables such as pH, redox conditions, dissolved oxygen, ionic strength, and the presence of natural organic matter. For instance, acidic conditions generally enhance Fe(II)-driven reduction reactions, whereas oxic environments promote surface oxidation and passivation, thereby reducing reactivity. Similarly, background ions and organic matter can either inhibit or facilitate reactions by affecting surface charge, aggregation behavior, and accessibility of reactive sites.

In addition to environmental chemistry, FeS nanomaterials have attracted interest in areas such as electrocatalysis, energy conversion, and emerging biomedical systems due to their versatile redox behavior and structural adaptability. (Matamoros-Veloza et al., 2018). However, the extent of their practical application varies significantly across these domains, with environmental remediation remaining the most extensively explored and immediately relevant field. Figure 8 provides an overview of the diverse functional applications of nanoparticulate FeS across domains.

Compared to widely studied remediation materials such as nanoscale zero-valent iron (nZVI), TiO₂, and biochar-based systems, FeS nanoparticles exhibit competitive removal efficiencies, typically in the range of 80%–99% for metal ions and organic contaminants depending on reaction conditions (Gong et al., 2016; Rickard and Luther, 2007). However, their reaction kinetics are often slower than nZVI in purely reductive systems, where rapid electron release enables higher rate constants (Verma et al., 2025). In contrast, FeS offers advantages over TiO₂ by operating effectively under dark or reducing conditions without requiring external irradiation, making it more suitable for subsurface and anoxic environments (Fujishima et al., 2008). Compared to biochar composites, which rely primarily on adsorption mechanisms, FeS provides combined redox and catalytic functionality, enabling transformation rather than simple sequestration of contaminants (Ahmad et al., 2014). Despite these advantages, the practical performance of FeS remains constrained by instability, phase transformation, and aggregation, which can reduce long-term efficiency relative to more stable materials. This section focuses specifically on the application of FeS nanoparticles, where their redox behavior plays a central role.

4.1 Environmental remediation applications of FeS nanoparticles

The use of FeS nanoparticles in environmental remediation has been intensively investigated because of their strong reductive properties, low cost, and ability to immobilize or transform a wide spectrum of contaminants (Liu et al., 2022; Liu et al., 2016; Nawaz et al., 2025; You et al., 2025). Their deployment has been motivated by the need for alternative technologies to replace or complement conventional methods such as pump and treat, activated carbon adsorption, or advanced oxidation processes. Reported methods often suffer from inefficiency, secondary waste generation, or high operating costs (Dai et al., 2024). The high surface reactivity of FeS nanoparticles has been exploited for the remediation of heavy metals, oxyanions, chlorinated organic compounds, dyes, antibiotics, and other emerging pollutants (Nawaz et al., 2025; Sun et al., 2023). Contaminants addressed by FeS nanomaterials can be broadly categorized into (i) heavy metals (e.g., Cr(VI), Pb(II), Hg(II)), (ii) organic pollutants (e.g., dyes, pharmaceuticals, pesticides), and (iii) emerging contaminants (e.g., PFAS, microplastics-associated compounds). This highlights the versatility of FeS systems across diverse environmental remediation applications. Mechanisms of contaminant removal have been described as complex and multifaceted, involving adsorption, surface precipitation, electron transfer, and radical as well as non-radical oxidation pathways. In natural environmental systems, the performance of FeS nanoparticles is further governed by transport behavior, phase stability, and interactions with background constituents. Their mobility is strongly influenced by aggregation and surface charge, which are in turn affected by pH and ionic strength, often limiting transport in porous media (Phenrat et al., 2007). In addition, natural organic matter (NOM) can adsorb onto FeS surfaces, modifying surface reactivity by either enhancing dispersion or blocking reactive sites depending on its composition (Aiken et al., 2011). Redox conditions and dissolved oxygen also play a critical role in determining the environmental fate of FeS, as oxidation can lead to phase transformation and reduced reactivity over time (Rickard and Luther, 2007). These factors highlight that FeS behavior in natural systems may differ significantly from laboratory observations and must be considered for practical application. Recent studies have also explored FeS-based hybrid nanostructures that integrate catalytic and photo-assisted functionalities within a single platform (Nawaz et al., 2025). For example, as Figure 9 shows, FeS-based hybrid nanoreactors demonstrate synergistic effects arising from coupled catalytic and photothermal processes, which can enhance reaction kinetics and overall efficiency. Such multifunctional architectures highlight the potential of FeS nanomaterials for integrated catalytic systems, particularly in processes involving reactive species generation and pollutant transformation.

4.1.1 Immobilization and reduction of toxic metals and oxyanions

The capacity of FeS nanoparticles to immobilize toxic metals such as chromium, arsenic, cadmium, and uranium has been widely reported (Xu et al., 2024; Zou et al., 2021). The transformation of contaminants by FeS nanoparticles proceeds through coupled reductive and oxidative pathways at the particle–solution interface. In reductive pathways, surface-bound Fe(II) donates electrons directly to contaminants such as metal ions or chlorinated organics, resulting in their reduction while Fe(II) is oxidized to Fe(III). Concurrently, sulfur species (e.g., S^2- or polysulfides) can participate in electron transfer or nucleophilic reactions, further facilitating transformation. Under oxidizing conditions, Fe(II) can also activate dissolved oxygen or hydrogen peroxide to generate reactive oxygen species (e.g., •OH), enabling indirect oxidation of organic pollutants. These combined pathways govern the overall reactivity of FeS systems and determine contaminant fate under varying environmental conditions. For instance, Cr(VI) is reduced to Cr(III) by FeS through electron transfer from Fe(II) and sulfide species. A simplified version of the reduction of hexavalent chromium can be demonstrated by the reaction shown in Equation 5 below.

3 FeS + 2 CrO 4 2 − + 8 H + → 3 Fe 2 + + 3 S 0 + 2 Cr 3 + + 4 H 2 O (5)

This transformation has been considered particularly relevant to groundwater remediation, where Cr(VI) contamination persists as a widespread problem. Arsenic immobilization has also been reported, with As(V) reduced to As(III) and subsequently sequestered through the formation of insoluble Fe–As–S precipitates (Chen et al., 2025). Studies have demonstrated that the reactivity is highly dependent on pH, with acidic conditions favoring reductive removal, whereas near-neutral pH conditions have favored adsorption on sulfur-rich sites (Liu et al., 2016). It is interesting to note that when As(V) reacted with the FeS at pH 5, XRD peaks becomes less intense in the component peaks, an effect that was enhanced at lower Fe/As ratios (Zhou et al., 2018).

Similar mechanisms have been observed for U(VI), which is reduced to U(IV) and immobilized as uraninite-like precipitates in the presence of FeS nanoparticles (Duan et al., 2020; Hua and Deng, 2008). Other than redox-driven mechanisms has also been reported in the literature (Sun et al., 2018; Xiang et al., 2025; Xu et al., 2022). Co-precipitation has also played a role in the immobilization of metals such as Pb(II) and Cd(II), which form highly insoluble sulfide phases (PbS, CdS) when exposed to FeS Such pathways have been exploited in wastewater treatment where simultaneous removal of multiple metal ions has been required.

4.1.2 Degradation of chlorinated organic contaminants

Nanoparticulate FeS has been applied to the degradation of chlorinated solvents (Habib et al., 2025; Habib et al., 2024; He et al., 2015; Kim et al., 2013). Researchers have focused on solvents like trichloroethylene (TCE), perchloroethylene (PCE), and polychlorinated biphenyls (PCBs). The reductive capacity of FeS surfaces has been shown to promote dechlorination reactions, resulting in the stepwise removal of chlorine substituents and the eventual production of less harmful compounds. TCE could undergo reductive dechlorination in the presence of iron-based reductants, producing ethene as the final product according to the reaction presented below in Equation 6 (Matheson and Tratnyek, 1994). In this process, electrons are supplied by Fe⁰, Fe²⁺, or FeS under reducing conditions as shown in Equation 6.

C 2 HCl 3 + 3 H + + 3 e − → C 2 H 4 + 3 Cl − (6)

The performance of FeS nanoparticles in these systems has been enhanced by bimetallic modification, particularly with palladium or nickel dopants, which catalyze hydrogenation reactions and accelerate dechlorination rates (Nascimento et al., 2016; Wang et al., 2009). The enhanced performance in Pd- or Ni-modified FeS systems arises from their catalytic role in facilitating electron transfer and hydrogen activation. Pd and Ni sites promote the dissociation of molecular hydrogen (H₂) or the generation of reactive hydrogen species, which are then utilized for hydrogenation and reductive transformation of contaminants. In addition, these metals serve as electron conduits, accelerating interfacial electron transfer from Fe(II) sites to target compounds, thereby improving reaction kinetics and overall catalytic efficiency. The integration of FeS nanoparticles into permeable reactive barriers has also been demonstrated as a promising in situ remediation approach, although long-term stability and the potential for passivation by carbonate and sulfate ions have been recognized as limitations (Nawaz et al., 2025).

4.1.3 Treatment of organic dyes and antibiotics

The persistence of organic dyes and antibiotics in industrial and municipal effluents has led to their investigation as model contaminants for FeS-based treatment. Methylene blue, rhodamine B, and Congo red have been degraded by FeS nanoparticles (Esmaili et al., 2018; Kumar et al., 2024; Lianmawii et al., 2025; Zhang et al., 2016). This remediation is achieved either through direct electron transfer or by activating oxidants such as persulfate (PS) and peroxymonosulfate (PMS). Reductive pathways involve direct electron transfer from surface Fe(II) sites to contaminants such as metal ions and chlorinated organics, resulting in their reduction while Fe(II) is oxidized to Fe(III). Sulfur species may also contribute to electron donation or nucleophilic transformation processes. Oxidative pathways, in contrast, arise under oxic or peroxide-rich conditions, where Fe(II) activates dissolved oxygen or hydrogen peroxide to generate reactive oxygen species, which subsequently oxidize organic contaminants. A general scheme shown in Equation 7 (Lianmawii et al., 2025; Liu et al., 2024; Wang et al., 2020; Zhang et al., 2022).

Fe 2 + + H 2 O 2 → Fe 3 + + · O H + O H − (7)

Antibiotics such as tetracycline and ciprofloxacin have also been effectively degraded by FeS nanoparticles under both dark and photo-assisted conditions (Bhattacharjee et al., 2021; Lan et al., 2024; Lan et al., 2025; Rahmani-Aliabadi and Nezamzadeh-Ejhieh, 2018; Wang et al., 2019). In several cases, the non-radical pathway involving singlet oxygen (¹O₂) has been identified as dominant, particularly when PMS was activated by FeS in the presence of electron shuttles such as quinones (Zheng et al., 2023). Such pathways have been regarded as beneficial because of their selectivity and reduced scavenging by natural organic matter.

4.1.4 Emerging contaminants and complex mixtures

Beyond classical pollutants, FeS nanoparticles have been tested for the removal of endocrine-disrupting chemicals, perfluorinated compounds, and pharmaceutical residues. The reductive transformation of nitroaromatic compounds has been demonstrated, with FeS nanoparticles facilitating the conversion of nitrobenzene to aniline (Cheng et al., 2023; Hofstetter et al., 1999; Menezes et al., 2021; Ou et al., 2024). For perfluorooctanoic acid (PFOA), however, only partial defluorination has been achieved, indicating that additional catalysts or combined processes may be required. This limitation is primarily attributed to surface oxidation, where Fe(II) is converted to less reactive Fe(III) phases, leading to passivation of active sites and diminished electron transfer capacity. In addition, nanoparticle aggregation reduces the accessible surface area and limits the availability of reactive Fe(II) sites. For some contaminants, particularly highly stable organic compounds, reaction kinetics may also be slow, further restricting overall treatment efficiency.

The multifunctional reactivity of FeS nanoparticles has been attributed largely to the combined effects of surface Fe(II) sites, sulfur anions, and defect structures. Despite their success under lab conditions, FeS nanoparticles have shown reduced efficiency in realistic wastewater matrices. This is probably due to the competitive adsorption and radical scavenging by background constituents. To address this limitation, composite materials such as FeS-biochar and FeS-graphene have been fabricated, yielding enhanced stability and selectivity in complex environments. Oxidation and aggregation of FeS nanoparticles during application also reduce available surface sites and compromise long-term efficiency. It is noteworthy that the potential release of dissolved iron and sulfur species has also raised concerns about secondary contamination. Other factors that have complicated real-world applications include heterogeneities in groundwater chemistry and flow dynamics, as well as the presence of natural organic matter. Future work should prioritize the rational design of FeS nanomaterials with enhanced resistance to oxidation and aggregation through heterometal incorporation, surface functionalization, and structural tuning. Mechanistic investigations under environmentally relevant conditions are essential to bridge the gap between laboratory-scale observations and field performance. Furthermore, systematic evaluation of long-term stability, reusability, and scalability will be critical for advancing FeS-based systems toward practical remediation technologies.

4.2 Catalysis and energy applications of FeS nanoparticles

The catalytic properties of FeS nanoparticles have been increasingly recognized in recent years, owing to their ability to facilitate electron-transfer reactions, mediate redox processes, and serve as low-cost alternatives to noble-metal catalysts. Their applications have extended across electrocatalysis, photocatalysis, and advanced oxidation processes, with implications for renewable energy production, environmental sustainability, and industrial catalysis. Structural tunability, abundance of active Fe(II) and S^2- sites, and compatibility with heteroatom doping have contributed to their versatility in these domains.

4.2.1 Electrocatalysis for hydrogen evolution reaction (HER)

The HER has been widely studied as a key step in electrochemical water splitting, which has been considered central to hydrogen-based renewable energy systems (Kadam et al., 2024). FeS nanoparticles have been employed as electrocatalysts because of their reactivity (Yang et al., 2022). Their catalytic activity has been attributed to Fe-S motifs that mimic the active centers of natural hydrogenases.

The intrinsic catalytic activity has been demonstrated to depend strongly on crystallinity, particle size, and exposure of edge sites. Hydrothermally synthesized FeS nanosheets have displayed lower overpotentials and smaller Tafel slopes than bulk FeS, highlighting the role of morphology in facilitating proton adsorption and electron transfer. Heteroatom doping has further enhanced HER performance. Incorporation of cobalt or nickel into FeS lattices has been reported to increase the density of active sites and improve conductivity, resulting in higher current densities at lower overpotentials (Baste et al., 2025). Porous and hybrid structures, such as FeS@C and FeS–graphene composites, have demonstrated synergistic effects, with carbon matrices improving electron mobility while FeS providing catalytic sites (Xu et al., 2021).

4.2.2 Oxygen evolution and oxygen reduction reactions (OER/ORR)

In addition to HER, FeS nanoparticles have been studied for their role in oxygen electrocatalysis. Although FeS is less active than transition-metal oxides or platinum-based catalysts, improvements in performance have been achieved through compositional modification (Sari et al., 2024; Wei et al., 2026).

Nickel- and cobalt-doped FeS nanoparticles have shown enhanced bifunctional activity for OER and ORR, attributed to modified electronic structures that facilitated oxygen intermediate adsorption and desorption (Sari et al., 2024). Conductive substrates such as carbon nanotubes, reduced graphene oxide, and porous carbon have been used as supports to address the inherently low conductivity of FeS, with remarkable improvements in stability under alkaline electrolytes (Heift, 2019).

4.2.3 Electrocatalysis for carbon dioxide reduction (CO₂RR)

FeS nanoparticles have also been investigated for the electrochemical reduction of CO₂ (Nan et al., 2024; Parsons and Alotaibi, 2023; Yamaguchi et al., 2014). This approach is viewed as a potential pathway toward carbon-neutral fuel cycles. Surface Fe(II) sites have been shown to promote the conversion of CO₂ to formate and carbon monoxide (Roldan et al., 2015). However, the S^2- sites have been shown to influence product selectivity by stabilizing reaction intermediates (Nan et al., 2024).

When integrated with carbonaceous supports, FeS catalysts have shown higher current densities and faradaic efficiencies than unsupported FeS nanoparticles. The introduction of defects and vacancies has been suggested as a means of tuning binding energies of intermediates, thereby modulating product selectivity. Such findings have highlighted the importance of defect engineering in catalytic applications of FeS nanomaterials.

4.2.4 Photocatalysis and photo-assisted processes

The band gap of FeS has been reported to range between 0.8 and 1.0 eV, rendering it a narrow-band-gap semiconductor capable of absorbing in the visible to near-infrared region (Lennie et al., 1995; Rickard and Luther, 2007). Although this band structure has limited photocatalytic performance when compared to materials such as TiO₂, coupling with wide-band-gap semiconductors or sensitization with photosensitizers has enhanced light harvesting.

Composite materials such as FeS-TiO₂ and FeS-g-C₃N₄ have been reported to exhibit superior photocatalytic activity for hydrogen production and pollutant degradation under visible light (Li et al., 2016). In such systems, FeS serves as a co-catalyst by participating in interfacial charge transfer processes. Upon light irradiation, photoexcited electrons are generated in the semiconductor (e.g., g-C₃N₄) and are transferred to FeS, where Fe(II) sites act as electron acceptors and mediators, facilitating subsequent transfer to adsorbed reactants. Simultaneously, photogenerated holes remain in the semiconductor phase, enabling spatial separation of charge carriers and reducing recombination. This directional electron flow prolongs electron lifetimes and enhances overall photocatalytic efficiency. Photo-assisted activation of persulfates has also been achieved using FeS nanoparticles, where light irradiation enhances Fe(II)/Fe(III) redox cycling and promotes the generation of reactive sulfate radicals (SO₄•^-) (Sühnholz et al., 2021). This synergistic interaction between photogenerated charge carriers and Fe(II)-mediated electron transfer accelerates oxidant activation and improves degradation efficiency. As a result, efficient removal of antibiotics, pesticides, and dyes has been demonstrated, highlighting the potential of FeS in solar-driven advanced oxidation processes.

4.2.5 Heterogeneous catalysis in chemical transformations

Beyond electrocatalysis and photocatalysis, FeS nanoparticles have been studied as heterogeneous catalysts in organic transformations (Cheng et al., 2023; Ou et al., 2024). Hydrogenation of nitroaromatics, hydrodesulfurization of petroleum fractions, and coupling reactions have been facilitated by FeS-based catalysts under laboratory conditions. Although not yet fully established, such studies have demonstrated that FeS nanoparticles can serve as active and recyclable alternatives to more expensive catalysts. The main challenges have included surface passivation, sulfur leaching, and loss of catalytic activity after repeated cycles.

4.2.6 Energy storage and conversion

FeS undergoes a conversion reaction in which FeS reacts with lithium ions to form metallic Fe. Although this reaction provides high theoretical capacities, the main limitations have been volume expansion during cycling and dissolution of lithium polysulfides into the electrolyte. Encapsulation of FeS within carbon frameworks, coating with conductive polymers, and hybridization with graphene have been adopted to mitigate these effects. Significant improvements in cycling stability and rate capability have been reported when such strategies were applied.

In supercapacitors, FeS nanoparticles have provided pseudocapacitive contributions, with specific capacitances under optimized conditions (Balamurugan et al., 2018). These results have indicated that FeS-based materials can function as multifunctional components in energy storage and conversion systems. Challenges remain in catalytic and energy applications of FeS nanoparticles. Stability under electrochemical conditions has been identified as a major limitation, as surface passivation, sulfur leaching, and structural collapse have reduced catalytic lifetimes. The conductivity of pristine FeS has been inherently low, necessitating hybridization with conductive support.

4.3 Advanced oxidation and activation chemistry of FeS nanoparticles

The ability of FeS nanoparticles to activate oxidants and drive advanced oxidation processes (AOPs) has been studied extensively because of their dual role as sources of Fe(II) and S^2- and their ability to mediate electron transfer reactions (Wei et al., 2022). Advanced oxidation processes have been regarded as a central strategy for the degradation of recalcitrant organic contaminants, with reactive oxygen species (ROS) such as hydroxyl radicals (·OH), sulfate radicals (SO₄ ^·-), superoxide (O₂ ^·-), and singlet oxygen (¹O₂) serving as primary oxidants. FeS nanoparticles have demonstrated unique reactivity by activating hydrogen peroxide (H₂O₂), persulfate (S₂O₈ ^2-), and peroxymonosulfate (HSO₅ ⁻), as well as by mediating non-radical oxidation pathways (He et al., 2020; Wang et al., 2023). Their function has often been compared to that of Fe²⁺ in homogeneous Fenton systems, but the solid-phase character of FeS has provided additional advantages, including sustained Fe²⁺ release, surface catalysis, and the potential to limit secondary sludge formation (Li et al., 2016).

4.3.1 Activation of hydrogen peroxide: Fenton-like pathways

FeS nanoparticles have been applied in Fenton-like systems, where Fe(II) from the FeS lattice reacts with hydrogen peroxide to generate hydroxyl radicals (Cheng et al., 2016). The release of Fe(II) from the FeS matrix has been slower and more sustained compared to soluble Fe²⁺ salts, which has conferred the advantage of continuous radical production over extended timescales. In addition, the sulfur component has been shown to participate in redox cycling, where polysulfides and intermediate sulfur species reduce Fe(III) back to Fe(II), thereby regenerating active sites (Luther and Rickard, 2005; Wolthers et al., 2005).

Experiments conducted on dye degradation have demonstrated rapid color removal and mineralization efficiencies exceeding 80% when FeS nanoparticles were combined with H₂O₂ under acidic to neutral conditions. Compared to conventional Fenton catalysts, FeS nanoparticles offer greater stability against precipitation and broader pH applicability, though Fe²⁺ leaching remains a challenge for long-term operation.

4.3.2 Activation of persulfate (S₂O₈ ^2-)

Persulfate activation has emerged as a promising approach for in situ chemical oxidation (ISCO). The cleavage of the O–O bond in persulfate generates highly reactive sulfate radicals as shown in Equation 8 (Habib et al., 2025).FeS nanoparticles have acted as electron donors to initiate this reaction, with Fe(II) serving as the primary reducing agent. The additional presence of S^2- and polysulfides has been reported to accelerate electron transfer and maintain Fe(II) regeneration.

S 2 O 8 2 − + F e 2 + → F e 3 + + S O 4 · − + S O 4 2 − (8)

Experiments have shown that chlorinated solvents, such as TCE and carbon tetrachloride (CCl₄), can be effectively degraded using FeS-activated persulfate systems. The degradation of phenolic compounds and polycyclic aromatic hydrocarbons (PAHs) has also been demonstrated, with pseudo-first-order rate constants enhanced by more than twofold compared to conventional Fe²⁺/persulfate systems.

4.3.3 Stability, reusability, and environmental factors

One of the major challenges in applying FeS nanoparticles for advanced oxidation has been stability under repeated use. Surface passivation by Fe(III) species and sulfur oxidation products has been frequently observed, leading to reduced activity over successive cycles.

Environmental factors such as pH, ionic strength, and natural organic matter have exerted strong influence on degradation efficiency. At acidic pH, radical pathways are favored, whereas at neutral pH, non-radical pathways have shown to be dominated. The presence of humic acids has been found to both inhibit and promote degradation depending on the pathway involved. FeS nanoparticles have been shown to serve as effective catalysts for the activation of H₂O₂, persulfate, and peroxymonosulfate, enabling the generation of a diverse set of oxidizing species, But the study of FeS nanoparticle-based AOPs has revealed both opportunities and challenges. Sustainability assessments addressing long-term stability, leaching of iron and sulfur, and secondary contaminant formation should be focused in the future research.

4.4 Energy storage and conversion with FeS nanoparticles

The development of efficient, low-cost, and sustainable energy storage systems has been regarded as critical for the global transition toward renewable energy. Conversion and storage technologies, including rechargeable batteries and supercapacitors, have required electrode materials that exhibit high capacity, good conductivity, and long-term cycling stability. Iron sulfide nanoparticles have been widely investigated in this context, with attention focused on their high theoretical capacity, abundance, and structural tunability. Despite limitations in conductivity and structural stability, numerous strategies have been employed to exploit the potential of FeS in lithium-ion, sodium-ion, potassium-ion, and hybrid energy storage devices.

4.4.1 Lithium and sodium ion batteries

FeS has been studied extensively as an anode material for lithium-ion batteries (Xu et al., 2026). The theoretical capacity has been reported as ∼609 mAh g^-1, which exceeds that of conventional graphite anodes (372 mAh g^-1). However, FeS anodes have faced several intrinsic challenges, including significant volume expansion during lithiation and delithiation, dissolution of intermediate polysulfides into the electrolyte, and sluggish electron transport due to FeS’s semiconducting nature. Nanoscale engineering has been employed to shorten Li diffusion pathways and advance this field.

FeS has also been explored as an anode material for sodium-ion batteries (Sheng et al., 2021). Because the ionic radius of Na⁺ is larger than that of Li⁺, greater structural strain has been induced during cycling, leading to faster capacity fading. To overcome this problem, FeS nanoparticles with porous or hollow structures have been synthesized, which have accommodated mechanical stress. Carbon coating and graphene hybridization have further enhanced conductivity and stabilized the electrode–electrolyte interface. More recently, graphene-wrapped, nitrogen-rich carbon-coated iron sulfide nanofibers (FeS@NCG) have been developed and used as an anode for sodium-ion batteries (Yang et al., 2022). In this research, they achieved outstanding capacity retention of 535 mAh g^–1 at 5 A g^–1 over 1010 cycles. Such performance has indicated that FeS-based electrodes are viable candidates for next-generation SIBs.

4.4.2 Supercapacitors

FeS nanoparticles have been incorporated into supercapacitors as pseudocapacitive materials (Riaz and Bibi, 2026). The capacitive behavior has been associated with fast surface redox reactions of Fe(II)/Fe(III) and sulfur species. Carbon hybridization has been crucial in improving cycling stability and power density. FeS–graphene and FeS–carbon nanotube composites have retained more than 90% of their capacitance after 5000 cycles, highlighting the potential for high-performance energy storage applications (Yang et al., 2022). Such systems have bridged the gap between batteries and capacitors by offering both high capacity and fast charge–discharge capability.

The low cost, abundance, and high theoretical capacity of FeS nanoparticles have positioned them as attractive alternatives to conventional materials. However, their commercial applications still remain understudied. Future research in the field of defect and heterostructure engineering to tailor ion diffusion and charge transfer kinetics would be of great interest. But with continued progress in these areas, FeS nanoparticles can certainly emerge as practical components in next-generation rechargeable batteries, supercapacitors, and hybrid energy systems.

4.5 Industrial processing and sulfur management with FeS nanoparticles

The industrial relevance of iron sulfide nanoparticles has been increasingly recognized because of their multifunctional reactivity, low cost, and structural similarity to naturally occurring iron sulfide minerals. While their environmental and electrochemical applications have received greater attention in academic studies, their potential roles in industrial processing and sulfur management are equally significant. In the chemical industry, sulfur species must be controlled across a wide range of operations, including petroleum refining, natural gas processing, coal utilization, pulp and paper production, and wastewater treatment. The ability of FeS nanoparticles to react with hydrogen sulfide, catalyze desulfurization, and participate in sulfur cycling has rendered them attractive for integration into industrial technologies.

Hydrogen sulfide (H₂S) has long been regarded as a hazardous by-product of industrial processes, including oil and gas extraction, wastewater treatment, and anaerobic digestion. Because of its toxicity, corrosivity, and foul odor, H₂S removal has been considered essential for occupational safety and regulatory compliance. FeS nanoparticles have been widely studied as H₂S scavengers due to their strong affinity for sulfide and ability to form stable iron sulfide phases (Wei et al., 2024).

The petroleum refining industry has faced stringent sulfur regulations because sulfur-containing compounds in fuels lead to SO₂ emissions and catalyst poisoning. Hydrodesulfurization (HDS) has been the dominant industrial method for sulfur removal, relying primarily on MoS₂ and CoMoS-based catalysts (Sun et al., 2025). Although FeS has not traditionally been used as a primary HDS catalyst, FeS nanoparticles have been explored as promoters. Studies have shown that FeS nanoparticles can catalyze the hydrogenolysis of organosulfur compounds such as thiophenes, benzothiophene, and dibenzothiophene under elevated temperature and pressure (Dzade and de Leeuw, 2018).

In aqueous environments, FeS nanoparticles have been employed to treat industrial effluents containing sulfide, sulfite, and thiosulfate species (Yu et al., 2020). The redox reactivity of FeS enables both oxidation and reduction pathways, depending on system conditions. For example, sulfide has been oxidized to elemental sulfur or sulfate by FeS in the presence of dissolved oxygen as shown in Equation 9.

2 F e S + O 2 → 2 F e 2 + + 2 S 0 (9)

In strongly reducing environments, FeS catalyzes the reduction of sulfite to sulfide, facilitating sulfur recovery. Despite encouraging results, the metastable nature of FeS remains a major limitation for industrial utilization, as it is prone to oxidation and phase transformation under ambient or oxic conditions, leading to loss of active Fe(II) sites and diminished catalytic performance. To address these challenges, several mitigation strategies have been proposed, including surface modification or encapsulation to limit oxygen exposure, heterometal doping to enhance structural stability and redox resilience, and operation under controlled redox conditions to preserve Fe(II)-rich phases. In addition, integrating FeS into composite systems or supports can reduce aggregation and improve long-term stability. These approaches collectively offer pathways to improve durability and enable more reliable application of FeS nanoparticles in industrial processing and sulfur management.

4.6 Biomedicine and antimicrobial uses of FeS nanoparticles

The biomedical applications of FeS nanoparticles have been increasingly investigated (Shan et al., 2022). This research has mostly been motivated by their structural similarity to biologically relevant iron-sulfur clusters, their capacity to generate reactive oxygen species, and their low raw material cost. Quantitative evaluation of FeS-based systems has demonstrated promising biological performance, with reported cell viability reductions exceeding 70%–90% under treatment conditions, depending on dosage and exposure time (Cheng et al., 2017; Nawaz et al., 2025). While research in this area remains at an early stage compared to environmental and electrochemical domains, promising results have been reported in antimicrobial therapies, biofilm inhibition, drug delivery, and diagnostic applications. Considerable attention has also been directed toward the toxicological implications of FeS nanoparticles, as the release of iron and sulfur species within biological systems may lead to both beneficial and adverse outcomes. Recent studies have demonstrated that FeS-based nanoreactors can combine catalytic therapy, photothermal treatment, and glucose depletion pathways to suppress tumor growth and metastasis, as illustrated in Figure 10 (Ren et al., 2021). In photothermal applications, FeS-based nanostructures have shown high photothermal conversion efficiencies, typically in the range of 30%–45%, enabling effective tumor ablation under near-infrared irradiation (Cheng et al., 2017; He et al., 2020). Correspondingly, IC₅₀ values for cancer cell inhibition have been reported in the low micromolar range, indicating strong therapeutic efficacy at relatively low concentrations.

4.6.1 Antimicrobial properties

FeS nanoparticles have been reported to exhibit broad-spectrum antimicrobial activity (Cheng et al., 2026; He et al., 2025). Their mechanism of action has been attributed to multiple factors, including the release of Fe²⁺ ions, the production of reactive oxygen species (ROS), and the disruption of bacterial membranes. In antimicrobial applications, iron-sulfides have demonstrated high bactericidal efficiency, with reported inactivation rates exceeding 95% against common pathogens such as E. coli and S. aureus under optimized conditions (Cao et al., 2023).

The Fenton-like reactivity of FeS nanoparticles has been considered central to their antibacterial activity (Agnihotri et al., 2020; Jiang et al., 2022). In the presence of hydrogen peroxide, Fe(II) ions released from the FeS surface catalyze the formation of hydroxyl radicals as presented in Equation 10.

Fe 2 + + H 2 O 2 → Fe 3 + + · O H + O H − (10)

These radicals cause oxidative damage to bacterial membranes, proteins, and DNA. In studies against E. coli and S. aureus, FeS nanoparticles demonstrated significant antibacterial activity, with log reductions in viable cells exceeding 4–5 within hours of exposure (Xu et al., 2023). The sulfur component of FeS nanoparticles has also been implicated in antimicrobial activity (Kezia and Devi, 2025). Sulfide release has been shown to interact with thiol-containing proteins in bacterial cells, disrupting enzyme activity and metabolic processes. Furthermore, FeS nanoparticles have been observed to attach directly to bacterial membranes, causing mechanical damage and increased permeability. The therapeutic performance is closely linked to ROS generation, with FeS-based systems demonstrating significant enhancement in intracellular ROS levels, often exceeding two-fold increases compared to control conditions (He H. et al., 2020). This elevated ROS production contributes directly to oxidative stress-induced cell damage.

4.6.2 Biosensing applications

The catalytic properties of FeS nanoparticles have been utilized in biosensing platforms. FeS-modified electrodes have been developed for the detection of glucose, hydrogen peroxide, and biomarkers of oxidative stress. In these systems, FeS nanoparticles facilitated electron transfer and improved sensitivity compared to unmodified electrodes. In a recent study, the FeS nanoparticles showed highly sensitive and specific sensing toward neuroprotector when compared to unmodified 10.1 µA electrodes with a sensing value of 12.51 μA (Mathew et al., 2024). In another study, FeS-flower-decorated rGO nanosheets were tested positively for enhanced catechol biosensing (Sun et al., 2026). Additionally, FeS nanoparticles have been employed in colorimetric assays based on their peroxidase-like activity (Song et al., 2022).

4.6.3 Toxicological considerations

The biomedical promise of FeS nanoparticles has been accompanied by concerns regarding toxicity. Studies have shown that the cytotoxicity of iron sulfide nanoparticles depends strongly on concentration, surface functionalization, and exposure conditions (Yuan et al., 2020). Surface-modified particles often exhibit improved biocompatibility, whereas higher concentrations can induce oxidative stress and cellular damage by catalyzing the generation of reactive oxygen species. In vivo investigations have further demonstrated that iron sulfide nanomaterials may accumulate in organs associated with the reticuloendothelial system, particularly the liver and spleen, before gradual clearance (Nyström and Fadeel, 2012). These observations highlight the importance of systematic toxicity evaluation and careful material design for biomedical applications.

FeS nanoparticles have demonstrated significant potential in biomedicine. Their capacity to generate reactive oxygen species and interact with biological systems has conferred unique therapeutic advantages, while also raising safety concerns. Although clinical applications remain distant, continued progress in stabilization, surface functionalization, and toxicological evaluation is expected to advance FeS nanoparticles toward practical biomedical use. Expansion of FeS-based nanozyme applications for rapid, low-cost detection of disease biomarkers holds potential for future studies.

In addition to physicochemical stability, the environmental and health implications of FeS nanoparticles warrant more rigorous evaluation. Although iron sulfides are naturally occurring phases, nanoscale FeS can exhibit enhanced reactivity and altered dissolution behavior that influence exposure and biological response. Existing nanotoxicology studies indicate that particle aggregation, surface oxidation, and ion release are key determinants of nanoparticle fate and toxicity, yet these aspects remain poorly characterized for FeS systems under environmentally relevant conditions (Keller et al., 2013; Nel et al., 2006). Moreover, data on chronic exposure, ecotoxicological effects, and transport across environmental compartments are limited. A lifecycle-oriented framework that considers synthesis, environmental transformation, and end-of-life fate is therefore necessary to more accurately assess the risks associated with FeS nanoparticle deployment. Some recent studies have reported improved FeS nanoparticle performance through heterometal doping and composite design, highlighting enhanced redox activity and stability under environmentally relevant conditions (Miller et al., 2020; Moehring et al., 2020; Park et al., 2022; Roberts et al., 2018; Sano et al., 2020). In addition, emerging work employing in situ and operando techniques has provided new insights into phase transformation and electron transfer mechanisms, strengthening the understanding of FeS behavior in complex.

Overall, while nanoparticulate FeS have demonstrated versatility across multiple application domains, the current body of work is largely fragmented, with limited integration between mechanistic understanding and material design. Many reported systems prioritize performance metrics without adequately addressing stability, reproducibility, and environmental relevance. Advancing the field will require a shift toward more systematic studies that couple controlled synthesis, quantitative mechanistic analysis, and realistic application conditions to enable reliable and scalable technologies.

5 Challenges, risks, lifecycle of nanoparticulate FeS and future directions

Despite the promising applications of FeS nanoparticles in environmental remediation, catalysis, energy storage, and biomedicine, significant challenges have remained regarding their stability, reproducibility, safety, and environmental impact. Because FeS is a metastable material prone to phase transformations, controlling its structural and surface chemistry during synthesis, application, and storage has been recognized as a major challenge. These transformations are governed by both kinetic and thermodynamic factors. Under kinetic control, FeS (e.g., mackinawite) can form rapidly and persist as a metastable phase with high reactivity; however, over time or under favorable conditions, it undergoes thermodynamically driven conversion to more stable phases such as Fe₃S₄ or FeS₂. This transition typically results in reduced surface reactivity due to decreased availability of Fe(II) active sites and increased structural ordering.

Furthermore, potential risks related to toxicity, secondary contamination, and long-term environmental fate have raised questions about sustainability. A complete understanding of the lifecycle of FeS nanoparticles, from synthesis to deployment, reuse, and disposal, has therefore been considered essential for responsible development. By addressing these challenges, the potential of FeS nanoparticles may be realized in a manner consistent with both scientific rigor and societal responsibility.

Even though research on FeS nanoparticles has progressed significantly over the last 2 decades, the translation of laboratory findings into practical technologies remains limited due to challenges related to material stability, reproducibility, and performance under complex environmental conditions. The unique chemical versatility of FeS, which is simultaneously an electron donor, reductant, adsorbent, and catalyst, has suggested that it may become a multifunctional material of the future. To realize this, a coordinated research agenda that integrates fundamental mechanistic studies, advanced material design and standardized reporting is required.

Numerous studies have demonstrated FeS nanoparticles’ ability to activate oxidants, reduce contaminants, and mediate electrochemical reactions, but the specific electron transfer pathways remain incompletely characterized. Besides integration, the experimental findings with density functional theory (DFT) modeling have the potential to provide molecular-level insights into adsorption energies, defect roles, and charge transfer processes. This calls for more computational studies in the field.

Composite engineering, such as FeS encapsulated in carbon, immobilized on porous supports, or hybridized with conductive polymers, has already improved stability and conductivity. The next-generation of synthesis should focus on green, biogenic approaches, using plant extracts or microbial systems to produce biocompatible, environmentally sustainable FeS nanoparticles.

Furthermore, pilot-scale studies in contaminated aquifers, wastewater treatment plants, and industrial effluent streams must be conducted to evaluate real-world performance. Also regeneration and reuse strategies should be developed to minimize resource consumption.

6 Conclusion

The trajectory of FeS nanoparticle research suggests that FeS nanoparticles are poised to play an increasingly important role in sustainable technologies. Their multifunctional reactivity, low cost, and structural versatility have provided a foundation for applications spanning environmental remediation, energy storage, industrial processing, and biomedicine. The realization of this potential will however depend on overcoming fundamental challenges of instability, reproducibility, scalability, and safety. Despite sevaral advances, important gaps remain in linking structure–reactivity relationships with performance under environmentally relevant conditions, which limits direct translation to practical applications. Future research should prioritize mechanistic understanding, innovative synthesis and stabilization and environmental friendly strategies. By addressing these priorities, it can be ensured that advances move beyond laboratory toward practical, scalable, and sustainable technologies that contribute meaningfully to global energy, environmental, and health challenges.

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Footnotes

Edited by: Massoud Kaykhaii, Comenius University, Slovakia

Reviewed by: Worradorn Phairuang, Chiang Mai University, Thailand

Camil Rex M., VIT University, India

Jyoti Yadav, Inter-University Accelerator Centre (IUAC), India