FeS particles are usually synthesized by physicochemical and biochemical methods. Physicochemical method is the co−precipitation of different types of iron salts and sulfide salts in the aqueous solution under the condition of hypoxia (Eq. 4). Common source of Fe²⁺ include ferrous chloride (FeCl₂), ferric sulfate heptahydrate (FeSO₄·7H₂O), and ammonium ferrous sulfate (Fe (NH₄)₂·(SO₄)₂·6H₂O). Sodium sulfide (Na₂S) is usually used as the source of S²⁻ (Chen, et al., 2019). F e 2 + + S 2 − → F e S (4)

Biosynthetic FeS has much attracted attention due to its environmental friendliness. In general, ferroreductive bacteria (FRBS) and sulfate−reducing bacteria (SRBS) reduce Fe³⁺ and sulfur substances (including sulfate, thiosulfate and elemental sulfur) to S²⁻ and Fe²⁺ respectively, and subsequently to produce FeS nanoparticles (Xie, et al., 2013; Zhou, et al., 2017).

However, FeS particles prepared by traditional methods, especially co−precipitation, tend to rapidly aggregate, which greatly reduces their specific surface area and leads to significant reduction in pollutant removal efficiency (Gong, et al., 2012). Therefore, the preparation of dispersible FeS nanoparticles with small particle size, large specific surface area and high reactivity has attracted extensive attention. Previous studies have explored various techniques to prepare FeS nanoparticles with controllable particle morphology and size distribution, such as reverse micelles (Ajay, et al., 1996), SRB−assisted production (Watson, et al., 2001), high−energy mechanical grinding (Soori, et al., 2016), wet chemical synthesis (Paknikar, et al., 2005), etc.

Recent studies have shown that the presence of polymer stabilizers and surfactants during the synthesis of FeS particles can effectively control the nucleation and growth of nanoparticles through simultaneous electrostatic repulsion and steric hindrance, thus effectively promoting the size control of FeS in aqueous solution. This can not only reduce the aggregation of nanostructured materials, but provide a large number of functional groups to degrade pollutants (Shao, et al., 2016). For example, polyelectrolyte stabilizer carboxymethyl cellulose (CMC) is widely used to modify nanoparticles to enhance the stability of nano FeS due to a large number of carboxylic and hydroxyl groups in its macromolecular chains (Xiong, et al., 2009; Gong, et al., 2012; Gong, et al., 2014; Van Koetsem, 2016). Additionally, other macromolecular biomaterials with similar physical and chemical properties, such as starch, cyclodextrin (CD), chitosan, polysaccharide sodium alginate (SA), etc., have also been used as stabilizers to synthesize nano FeS particles (Shao, et al., 2016; Wu, et al., 2017; Sun, et al., 2018a; Sun, et al., 2018b). The mechanism and processes of synthesis of iron sulfide are exhibited in Figures 1A, B.

In aqueous solution, the reducibility of ferrous ions will decrease with the pH value decreasing, which means acidic environment can inhibit the oxidation of ferrous ions to ferric ions (Liu, et al., 2022b). Therefore, the elemental valence state of the synthesized iron sulfide catalysts can also be adjusted. Liu et al. (Liu, et al., 2022a) obtained three kinds of iron sulfides by controlling the pH of the co−precipitation system of iron and sulfide salt to synthesize FeS₂, Fe₇S₈, Fe₃S₄ at p H = 3 , 5, 7, respectively. Hydrothermal method is also a common method for the synthesis of FeS_x. To mix dissolved ferrous salts (FeSO₄·7H₂O, FeCl₂·4H₂O, etc.) with sulfide (thiourea, Na₂S₂O₃, etc.) in a reaction vessel, and keep the vessel at 200°C for 24 h to obtain FeS₂ (Wang, et al., 2020a; Mohamed, et al., 2023). Magnetic Fe₃S₄ was obtained by dissolving FeCl₃·6H₂O and thiourea in ethylene glycol, and then heating in a hydrothermal kettle at 180°C for 12 h (Shi, et al., 2020; Li, et al., 2022b). The mechanism and processes of synthesis of FeSx are exhibited in Figure 1C

Because of the synergistic effect between zero−valent iron (ZVI) and Fe_xS_y (FeS and FeS₂), S−ZVI has high reactivity and selectivity for PS activation to degrade refractory contaminants (Li, et al., 2017a; Wei, et al., 2022). S−ZVI can be synthesized by the “one−pot” process, where ferrous ions react with borohydrides and sulfides. While in the two−step process, sulfur compounds (sodium sulfide and thiosulfate) are added to the synthesized nZVI to form S−ZVI (Li, et al., 2017b). For S−ZVI prepared by one−pot process, FeS_x are distributed both inside and on the surface of ZVI particles, while prepared by two−step method, FeS_x are only formed on the surface of ZVI to form a core−shell structure (Su, et al., 2018; Xu, et al., 2019). In addition, vulcanized micro−scale S−ZVI can be prepared by ball−milling S and ZVI powders under dry conditions (Gu, et al., 2017; He, et al., 2022). The structure, physicochemical properties and performance in PS based Fenton−like reactions of S−ZVI produced by various synthesis methods are mainly affected by the molar ratio of S/Fe (Fan, et al., 2017; Zhang, et al., 2020). Kim et al. (Kim, et al., 2011) reported that the degradation rate of TCE by S−nZVI increased linearly with increasing molar ratio of S/Fe, but decreased with increasing molar ratio of S/Fe when the concentration of Na₂S₂O₄ increased to 2.0 g/L (S/Fe molar ratio was 0.33). However, Han et al. (Han and Yan, 2016) reported that when S/Fe ratio is smaller than 0.025, the degradation rate of TCE becomes faster with the increase of S/Fe molar ratio, and then becomes stable when the molar ratio of S/Fe molar ratio exceeds 0.025. Rajajayavel et al. (Rajajayavel and Ghoshal, 2015) showed that the reducing ability of S−nZVI on TCE was strongly dependent on the S/Fe molar ratio, which provided the highest TCE dechlorination rate in the range of 0.04–0.083. Since the reaction conditions (including synthesis methods, reaction conditions and the properties of the target stain, etc.) used by different researchers to obtain the optimal S/Fe molar ratio are very different, the optimal S/Fe molar ratio values in different studies are not comparable. Based on the above examples, higher S/Fe molar ratio results in more production of FeSx and a larger surface area of the synthesized S−ZVI, thus facilitates the pollutants degradation (Gu, et al., 2017; Huang, et al., 2017). However, excess S content blocks the active Fe site on the surface, which will decrease their activity for PS activation. In addition, the conversion of Fe⁰ to ferric/ferrous (hydrogen) oxides may be intensified at high S/Fe ratio, leading to waste of ZVI with strong reducing capacity (Li, et al., 2017a). Therefore, it is essential to find the optimal S/Fe molar ratio in different reaction systems. The mechanism and processes of synthesis of S−ZVI are exhibited in Figure 2A.

The large surface area of carbon material can greatly reduce the agglomeration of iron−based nanoparticles, thus improving the activity of catalyst. Additionally, strong electron transfer capacity of carbon material can greatly enhance the electron transfer rate of iron−based catalyst (Li, et al., 2018; Ma, et al., 2021). Due to environmentally friendly, good surface physical and chemical properties, and rich oxygen−containing functional groups. Biochar (BC), graphene/graphene oxide (GO)/reductive graphene oxide (rGO), carbon nanotubes (CNTs), graphite carbon nitride (g−C₃N₄), etc., are commonly selected as supports to load FeS. They have been proven to significantly reduce the aggregation of FeS, resulting in better catalytic activity (Ma, et al., 2015; Sun, et al., 2020; Lyu, et al., 2018a; Zhang, et al., 2019a; Bin, et al., 2020; Zhuang, et al., 2020; Han, et al., 2022; Xu, et al., 2022). Generally, the composite of FeS and carbon materials can be obtained by loading iron salt on carbon materials firstly followed with sulfidation process, and freeze−drying, ultrasound or mechanical stirring are widely used loading methods (Sun, et al., 2020; Han, et al., 2022; Xu, et al., 2022). Specifically, these hybrids can be synthesized by mixing carbon materials with ferrous salt and sulfide and then reacted at high temperature under hypoxia condition (Ma, et al., 2015; Lyu, et al., 2018b; Hong, et al., 2021), or carbon materials mixed with pre−synthesized iron sulfide and then treated by hydrothermal method (Li, et al., 2022a). In addition, ball−milling can also achieve the combination of FeS with some carbon materials (Lyu, et al., 2018b; He, et al., 2021; Xia, et al., 2022). The mechanism and processes of synthesis of FeS@Carbon materials are exhibited in Figure 2B.

Multicomponent iron sulfides exhibit better electrochemical and catalytic properties due to the synergistic effect between metal ions and redox reactions. It can be synthesized by improved hydrothermal methods. Li et al. (Li, et al., 2020) mixed FeCl₂, Co (NO₃)₂, NH₄F and g−C₃N₄ powders at 25°C for 30 min, then heated the mixture in hydrothermal reactor at 140°C for 9 h. Put the Fe−Co precursor and thionoacetamide solution in hydrothermal reactor at 160°C for 6 h, FeCo₂S₄−CN composite can then be obtained. Nie et al. (Nie, et al., 2019) mixed CuCl, FeCl₃·6H₂O and (NH₄)₂S for 30 min, and heated at 200°C for 10 h in hydrothermal reactor to obtain CuFeS₂ NPs. Yan et al. (Yan, et al., 2020) loaded ferric nitrate, cobalt nitrate hexahydrate, ammonium fluoride, urea solution, and reduced graphene oxide film (RGOF) in a hydrothermal kettle at 120°C for 8 h, and then placed the obtained Fe−Co precursor and sodium sulfide solution in the hydrothermal reactor at 160°C for 8 h to obtain FeCo₂S₄/RGOF composite material. The mechanism and processes of synthesis of multicomponent iron sulfide are exhibited in Figure 1C.

Among various iron sulfides, FeS and FeS₂ are the most commonly used materials for persulfate activation. Since FeS is widely distributed in anoxic environment, and has strong reducibility and high reactivity to organic pollutants, it has been widely used as PS activators for water remediation. As shown in Figure 3, Xu et al. (Xu and Sheng, 2021) used FeS/PMS system to degrade chloramphenicol (CAP), sulfoxamycin (TAP), ciprofloxacin (CIP) and norfloxacin (NOR), and 100% NOR, 100% CIP, 93.5% CAP and 98.5% TAP was degraded within 2 h. Fan et al. (Fan, et al., 2018b) used FeS/PDS system to degrade PCA, and PCA degradation reached nearly 100% in acidic conditions within 240 min. And Sarah et al. used FeS as PDS activator and reached nearly complete removal of TCE within 20 min. These results showed that the FeS/PS system can effectively remediate the organic pollutants containing wastewater. In addition, the catalytic performance of iron−based catalysts in PS based Fenton−like reactions is summarized in Table 1.

So far, the mechanism of PS activation through FeS has been considered to be homogeneous and heterogeneous activation. Homogeneous activation refers to the continuous release of dissolved Fe²⁺ by FeS for persulfate activation, and the slow release of Fe²⁺ by FeS can effectively inhibit the self−quenching effect on SO₄ ^•− to promote the degradation of pollutants (Eqs. 5, 6). Heterogeneous activation refers to the surface of FeS combines Fe (II) or structural ≡Fe (II) for persulfate activation (Yuan, et al., 2015; Chen, et al., 2017; Fan, et al., 2018a; Sühnholz, et al., 2022). Fan et al. (Fan, et al., 2018b) found the presence of binding free radicals (≡SO₄ ^•−) on the surface of the catalyst through radical quenching experiments, indicating the activation of persulfate by structural Fe (Ⅱ). In addition, it was found that S_aq ²⁻ ion itself could not activate PS to produce oxidation radicals (Eq. 7) (Oh, et al., 2011), but SO₄ ²⁻, S⁰, polysulfide (S_n ²⁻) and S⁻ were detected on the surface of FeS after PS activation by X−ray photoelectron spectroscopy (XPS) and FT−IR analysis (Eq. 3 and Eqs. 8, 9), indicating that S (−Ⅱ) in FeS can indirectly provide electrons to PS by facilitating the reduction of Fe (III) to Fe (II). Once S²⁻ is exhausted, Fe (II) regeneration via PS reduction will dominate, since S²⁻ is non−renewable in FeS/PS systems (Eq. 10). In addition, Xu et al. (Xu and Sheng, 2021) also demonstrated that Fe (Ⅳ) was generated in the FeS/PS system, but its contribution to pollutant degradation as reactive species is not significant. S O 4 • − + H S O 5 − → S O 5 • − + S O 4 2 − + H + (5) S O 4 • − + S O 4 • − → S 2 O 8 2 − (6) S 2 − + S 2 O 8 2 − → 9 S O 4 2 − (7) n − 1 S 0 + H S − → S n 2 − + H + n = 2 − 6 (8) S n 2 − + F e S → S n − 1 2 − + F e S 2 (9) F e 3 + + H S O 5 − → F e 2 + + S O 5 • − + H + (10)

Therefore, the mechanism of FeS for persulfate activation can be proposed in Figures 4A, B, which is also explained as follows:

Homogeneous activation process: Firstly, FeS release Fe²⁺ ions (Eqs. 11, 12), which can activate PS to form SO₄ ^•− (Eqs. 1, 2). The Fe³⁺ can be reduced to Fe²⁺ by reacting with FeS or S²⁻ (Eq. 3, 13). Finally, the regenerated Fe²⁺ continue to maintain PS activation for pollutant degradation.

Heterogeneous activation process: ≡Fe (II) in FeS activates PS as electron donor to produce SO₄ ^•−(Eqs. 1, 2), then ≡S²⁻ and HS⁻ adsorbed on the surface of FeS can also give electrons to ≡Fe (III) and reduce it to ≡Fe (II), ensuring that the heterogeneous activation process can be continued (Eqs. 14, 15) (Yang, et al., 2022). F e S + H 2 O → F e 2 + + H S − + O H − (11) F e S + H + → F e 2 + + H S − (12) F e S + 8 F e 3 + + 4 H 2 O → 9 F e 2 + + S O 4 2 − + 8 H + (13) 2 ≡ F e 3 + + H S − → 2 ≡ F e 2 + + S 0 + H + (14) ≡ F e 3 + + S 2 − → ≡ F e 2 + + S 0 (15)

In addition, it is worth mentioning that solution pH is one of the most significant influencing factors in the remediation of contaminants by nano−sized FeS, which not only plays an important role in the decomposition of oxidants, but affects the surface charge of FeS−based catalysts and the speciation of substrates to be transformed (Chen, et al., 2019; Li, et al., 2021b). In general, when the solution pH is greater than the pH value at point of zero charge (pH_pzc), the surface of the catalyst is negatively charged, otherwise positively charged (Li, et al., 2020). Under strongly alkaline or strongly acidic conditions, the adsorption capacity is significantly reduced, which not only destroys the active sites on surface and accelerates the corrosion of FeS nanoparticles, but promotes the hydrolysis of bio−modifiers, thus decrease the stability and dispersion of modified nano−FeS. In addition, under strongly alkaline conditions, Fe (II) species will be reduced due to the precipitation of ferric hydroxide in reaction system and produce passivation layer on the surface of FeS. Additionally, negatively charged hydroxide ions compete for absorption with other negatively charged contaminants, which will influence the degradation of contaminants by FeS. Therefore, neutral conditions can provide optimal degradation of pollutants with FeS materials as catalysts for PS.

FeS₂ can also effectively activate persulfate to degrade pollutants, and the good persulfate activation performance is attributed to the low−valent Fe and S (Fe²⁺ and S⁻¹) (Hou, et al., 2021). By being fully oxidized to SO₄ ²⁻ and Fe³⁺, FeS₂ can provide 15 electrons. Therefore, FeS₂ can slowly and sustainably releases dissolved Fe²⁺, which activates persulfates to produce reactive free radicals to degrade pollutants (Eqs. 1, 2). Fe²⁺ comes from the water corrosion process of FeS₂ (Oh, et al., 2011), and Fe²⁺ will activate persulfate to be consumed, which will accelerate the water corrosion reaction of FeS₂ (Eq. 10 and Eqs. 16–18). ≡S⁻¹ could also give electrons to persulfate or Fe (III), which would cause Fe²⁺ to continue to form (Liang, et al., 2010). In addition, surface−bound ≡Fe (Ⅱ) can activate molecular oxygen to produce O₂ ^•− through single electron transfer pathway (Liu, et al., 2015), while surface−bound ≡Fe (Ⅱ) is also reduced, allowing the degradation to continue. The mechanism of FeS₂ for persulfate activation can be proposed in Figures 4C, D. F e S 2 + H 2 O → S 0 + F e 2 + + H S − + O H − (16) F e S 2 + 2 S 2 O 8 2 − → F e 2 + + 2 S O 4 • − + 2 S O 4 2 − + 2 S 0 (17) F e S 2 + 14 F e 3 + + 8 H 2 O → 15 F e 2 + + 2 S O 4 2 − + 16 H + (18)

S−ZVI has been widely used in activating persulfate to degrade refractory organic pollutants, such as tetracycline (Dong, et al., 2019a), bisphenol S (Cai and Zhang, 2022), tetrabromobisphenol A (Quoc, et al., 2021), trichloroethylene (Zhou, et al., 2021a), sulfadiazine (SDZ) (Guo, et al., 2020), etc., indicating that S−ZVI/PS system can effectively treat organic wastewater.

S−ZVI has high electron utilization efficiency, 10–50 times larger than that of unsulfide ZVI. Its sulfide layer can significantly enhance the activity of ZVI and promote to release ferrous ions into the environment (Fan, et al., 2016; Fan, et al., 2018b). This is because there are delocalized electrons in the FeS layer, which has good electrical conductivity and facilitates the transfer of electrons from Fe⁰, thus accelerating the ferrous ions formation (Kim, et al., 2011; Song, et al., 2017). According to electrochemical test, the results of Tafel curve and electrochemical impedance spectroscopy (EIS) also confirm that S−ZVI supports better electron transfer (Turcio−Ortega, et al., 2012; Wang, et al., 2019). Hence, the sulfide layer mainly acts as conductor of electrons to promote the release of Fe²⁺ during the reaction process, rather than source of Fe²⁺ production, which can be used to activate persulfate.

In addition to promoting the release of ferrous ions, the sulfur compounds in the sulfide layer have strong reducing capacity, which can reduce Fe³⁺ to Fe²⁺ (Eq. 3), and these ferrous species will be subsequently used to activate persulfate based on the electron transfer capacity of the sulfide layer (Fan, et al., 2018a; Li, et al., 2019a). Fe⁰ in S−ZVI can also reduce Fe (III) to Fe (Ⅱ) (Eq. 19), and a small amount of FeS₂ in S−ZVI can react with water to produce Fe²⁺ to further enhancing the iron cycle (Eq. 20). The cycle of iron species enables the degradation reaction to be carried out contumely and efficiently (Liu, et al., 2015). The mechanism of S−ZVI for persulfate activation can be proposed in Figure 5. F e 0 + 2 ≡ F e − S − F e 3 + → F e 2 + + 2 ≡ F e − S − F e 2 + (19) 2 F e S 2 + 7 O 2 + H 2 O → 2 F e S O 4 + 2 H 2 S O 4 (20)

FeS@Caobon materials has been widely used to activate persulfate for degradation of organic pollutants, such as petroleum hydrocarbons (Xia, et al., 2022), 2, 4−dichlorophenoxyacetic acid (Hong, et al., 2021), tetracycline (He, et al., 2021), sulfamethazine (Jin, et al., 2022), etc., indicating the potential of FeS@Caobon/PMS system in wastewater remediation.

It has been reported that in heterogeneous activation systems, radicals are first produced near the surface of the activator and then diffused into the solution to degrade pollutants (Liu, et al., 2014), but it has also been reported that both activation and degradation processes may occur near the surface of the carbon−based activator (He, et al., 2019). By measuring the levels of dissolved ion (dissolved Fe²⁺ and total Fe) in the reaction system, and using hydrophobic phenol and 1, 10−phenanthroline to chelate with surface−bound Fe (II) and dissolved Fe²⁺ as quenchers, confirming that the active radicals were mainly generated on the Fe@Carbon surface (He, et al., 2021; Han, et al., 2022). According to the XPS results of catalysts before and after the reaction, the ratio of Fe²⁺ to S²⁻ decreased significantly after the reaction, while that of other sulfur species such as S_n ²⁻ and SO₄ ²⁻ increased, indicating that Fe (II) and S (−II) species were involved in the reaction process, and S (−II) contribute to the conversion of Fe (III) and Fe (II), which will further enhance the activation of PS (Xia, et al., 2022; Yu, et al., 2022). All of these results suggest that surface−bound Fe (II) plays an important role in the PS activation process and generates ROSs on the FeS@Carbon surface. In addition, a small amount of Fe²⁺ dissolved in solution can also directly activate persulfate to produce free radicals (He, et al., 2021).

During the process of contaminants degradation in FeS@Carbon/PS system, carbon materials can prevent the agglomeration of FeS particles to make FeS particles evenly dispersed, which increase the chance of catalyst contact with solution and further increase the concentration of sustainably released Fe²⁺ (Wang, et al., 2017). Besides, the adsorption capacity of carbon materials can make contaminants adsorbed on the surface or inside of composite materials, which make the free radicals produced easier to contact with contaminants (Qu, et al., 2022). Additionally, carbon material itself can act as intermediary of electron transport to accelerate the electron transfer process and improve the degradation efficiency (Qiu, et al., 2021), CNTs and BC has strong electron donor groups on surface, such as hydrogen peroxide (−OOH) and hydroxyl (−OH), which can also activate persulfate to produce more radicals (Eqs. 21 − 2 4) (Zhou, et al., 2020; Qiu, et al., 2022). In the composite of FeS and graphene, the bond length between FeS and graphene layer is relatively long, which indicates the weaker bonding, resulting in the higher activity of S to react with SO₅ ²⁻ during PMS activation. With long bond length, greater electron localization could be facilitated on the S sites to reduce the barrier of pollutant bonding and accelerate the regeneration of metal species (Zhuang, et al., 2020).

Therefore, there are three possible mechanisms of FeS@Carbon to activate persulfate: 1) the adsorption of contaminants by FeS@Carbon; 2) The active sites on catalyst surface, such as oxygen−containing functional groups, Fe²⁺, S²⁻, etc. act as electron donors in reaction process to activate PS and then produce ROSs for pollutants degradation (Eqs. 21−30). In addition, S²⁻ also participates in the reduction of Fe³⁺, enabling the continuous generation of Fe²⁺, which can be further used for the activation of PS (Eqs. 1 − 3); 3) Carbon materials can accelerate the electron transfer process, which promote the electron transfer from pollutants to PS and further improve the generation rate of radicals on catalyst surface. The mechanism of FeS@Carbon for persulfate activation can be proposed in Figure 6. − O O H s u r f + S 2 O 8 2 − → − O O • s u r f + S O 4 • − + H S O 4 − (21) − O O H s u r f + H S O 5 − → − O O • s u r f + S O 4 • − + H 2 O (22) − O H s u r f + S 2 O 8 2 − → − O • s u r f + S O 4 • − + H S O 4 − (23) − O H s u r f + H S O 5 − → − O • s u r f + S O 4 • − + H 2 O (24) S O 4 • − + H 2 O → S O 4 2 − + • O H + H + (25) S O 4 • − + O H − → S O 4 2 − + • O H (26) S 2 O 8 2 − + 2 H 2 O → H O 2 − + 2 S O 4 2 − + 3 H + (27) S 2 O 8 2 − + H O 2 − → S O 4 • − + S O 4 2 − + H + + O 2 • − (28) O 2 • − + 2 H 2 O → H 2 O 2 + 2 O H − + O 2 1 (29) • O H / S O 4 • − / O 2 1 + c o n t a m i n a n t s → i n t e r m e d i a t e s → C O 2 + H 2 O (30)

Compared with single−component sulfide, multi−component metal sulfide exhibits better catalytic performance due to its richer redox reactions and synergistic effects between metals (Li, et al., 2019c). Moreover, as an electron donor, the low electronegativity of S²⁻ can promote the redox cycle of metal ions, making multi−component metal sulfide an effective catalyst for the activation of PMS. CuFeS₂ (Nie, et al., 2019), NiFe₂S₄ (Fan, et al., 2022), Cu₂FeSnS₄ (CFTS) (Li, et al., 2022a), CoFe₂S₄ (Li, et al., 2022b) etc., have been widely used in the field of persulfate activation to degrade organic pollutants, showing good pollutant removal effect.

During the process of persulfate activation by polymetallic sulfide, Cu⁺, Co²⁺, Fe²⁺, Ni²⁺ act as active sites to accelerate the generation of radicals such as SO₄ ^•−, •OH, O₂ ^•− by destroying the O−O bond of PS, and they are themselves oxidized into Ni³⁺, Fe²⁺, Ni³⁺, Co³⁺. (Eqs. 31, 32). Due to the strong reducibility of sulfur species such as S²⁻ and S₂ ²⁻, the high valent metal ions formed can be reduced to low valent states (Eqs. 33−35). The reduction of Fe³⁺ by Cu⁺ and Ni²⁺ is easy to achieve, which is thermodynamically advantageous (Eqs. 36, 37) (Nie, et al., 2019; Fan, et al., 2022). Therefore, the synergistic interaction between these metals on the catalyst surface facilitates interfacial electron transfer. In addition, ROSs produced by hydrolysis of persulfate can also oxidize high−valent metals to low−valent metals (Eq. 10), and the regenerated active sites such as Cu⁺, Ni²⁺ and Fe²⁺ on the surface can again participate in the continuous generation of ROSs induced by persulfate activation. In the CFTS/PMS system, there is S→M σ bond ((M = Cu, Fe, and Sn) which is favorable for electron transfer and contribute to form ≡Fe (II)*. It has been reported that Sn(Ⅱ) can also activate persulfate to produce active free radicals due to the synergy between ≡Fe (II)* and Sn (Eqs. 38−40) (Kong, et al., 2019). The mechanism of multicomponent iron sulfide for persulfate activation can be proposed in Figures 7A−C. F e 2 + / C u + / N i 2 + / C o 2 + + H S O 5 − → F e 3 + / C u 2 + / N i 3 + / C o 3 + + S O 4 • − + O H − (31) F e 2 + / C u + / N i 2 + / C o 2 + + S 2 O 8 2 − → F e 3 + / C u 2 + / N i 3 + / C o 3 + + S O 4 • − + S O 4 2 − (32) 2 S 2 − + F e 3 + / C u 2 + / N i 3 + / C o 3 + → F e 2 + / C u + / N i 2 + / C o 2 + + S n 2 − (33) S n 2 − + F e 3 + / C u 2 + / N i 3 + / C o 3 + → F e 2 + / C u + / N i 2 + / C o 2 + + S 0 (34) S 0 + F e 3 + / C u 2 + / N i 3 + / C o 3 + → F e 2 + / C u + / N i 2 + / C o 2 + + S O 4 2 − (35) ≡ C u Ⅰ / C u + + ≡ F e Ⅲ / F e 3 + → ≡ F e Ⅲ / F e 2 + + ≡ C u Ⅱ / C u 2 + (36) ≡ N i Ⅱ / N i 2 + + ≡ F e Ⅲ / F e 3 + → ≡ F e Ⅲ / F e 2 + + ≡ N i Ⅲ / N i 3 + (37) F e Ⅱ + ≡ → S → M σ ≡ F e Ⅱ * (38) 2 ≡ F e Ⅱ * + ≡ S n Ⅳ → 2 ≡ F e Ⅲ + ≡ S n Ⅱ (39) ≡ S n Ⅱ + 2 S 2 O 8 2 − → ≡ S n Ⅳ + 2 S O 4 • − + 2 S O 4 2 − (40)

Considering the poor dispersion of metal sulfides and large amount of metal ions leaching, the researchers further modified these catalysts. For example, Li. et al. (Li, et al., 2020) used FeCo₂S₄ modified g−C₃N₄ (FeCo₂S₄−CN) composite for PMS activation to degrade sulfamethoxazole (SMX). As shown in Figure 7D, in this system, there is a synergistic effect between FeCo₂S₄ and g−C₃N₄, thus the removal rate of SMX is higher than that of FeCo₂S₄/PMS, g−C₃N₄/PMS and PMS alone. Moreover, due to the synergistic effect between metal ions and g−C₃N₄, iron and cobalt ions, excessive leaching of metal ions is avoided. Li. et al. (Li, et al., 2022a) synthesized CoFe₂S₄/BC catalyst by a two−step hydrothermal method and combined it with PMS for the degradation of sodium sulfadimethacil (SMT). As shown in Figure 7E, in this system, in addition to the free radical pathway induced by the metal active site, there is also a non−free radical pathway. On the one hand, electrons can be transferred directly from the contaminants to the PMS through the active center of BC, leading to the direct decomposition of the pollutant. On the other hand, BC can activate the O−O bond in the PMS and directly oxidize the target compound.

FeS and its derivatives are considered promising candidates for photocatalytic water treatment due to their ability to absorb visible and/or ultraviolet light, narrow optical band gap, and charge transport properties (Ayodhya and Veerabhadram, 2018; Li, et al., 2021a). Bibhutibhushan et al. (Show, et al., 2017) synthesized FeS nanospheres by a simple electrochemical route, which act as photocatalysts to successfully degrade alizarin red S (ARS), methylene blue (MB), rose red (RB) and phenol, and no degradation of these dyes was observed in the dark and very slow degradation was observed in the absence of FeS but the presence of light, which indicates that FeS has a strong synergistic effect when working together with photoassisted technologies.

During the reaction process, when FeS are exposed to visible light, illumination causes the excitation of valence band electrons in the conduction band, and this charge separation leads to the formation of electron−hole pairs (Suroshe, et al., 2018). Photogenerated electron (e⁻)−hole pairs (h⁺) can react with adsorbed surface species such as O₂, H₂O, and OH⁻ to form ROSs such as O²⁻ and •OH, for pollutants degradation (Eqs. 41−45) (Nair, et al., 2011; Chabri, et al., 2016). In addition, the Fe²⁺ and S²⁻ in FeS can react as persulfate activator for pollutants degradation. Additionally, visible light will accelerate the cyclic conversion between Fe³⁺ and Fe²⁺ (Eq. 46) (Chen, et al., 2021), further improving the continuous degradation. Hence, possible light−induced reactions can be proposed as Eqs. 1−3 and Eqs. 41–46: F e S + h ν → F e S h + + e − (41) h + + H 2 O a d s → • O H + H + (42) h + + O H − / O H a d s → • O H (43) e − + O 2 a d s → O 2 − (44) • O H / O 2 − + c o n t a m i n a n t s → i n t e r m e d i a t e s → F e S h + C O 2 + H 2 O (45) F e Ⅲ O H 2 + + h ν → F e 2 + + • O H (46)

Electrochemical advanced oxidation processes (EAOPs) have attracted increasing attention due to their environmental compatibility, ease of scaling up and high efficiency in degrading refractory contaminants compared to conventional advanced oxidation processes (Luo, et al., 2020). Iron sulfide shows good electrocatalytic performance due to its excellent electrical conductivity, hybrid d orbital, and general redox properties (Li, et al., 2021b), which has great potential in the field of wastewater treatment when combined with EAOPs.

Ammar. et al. (Ammar, et al., 2015) used pyrite as a heterogeneous source of Fe²⁺ catalyst to degrade tyrosol (TY) in an electro−assisted process, which possesses superior performance due to the self−regulation of Fe²⁺ content in the medium. As shown in Figure 8A, (Labiadh, et al., 2015),used pyrite/EF system to generate H₂O₂ in situ and regenerate Fe²⁺ to completely remove azo dye (4−amino−3−hydroxy−2−p−toluene −naphthalene−1−sulfonic acid) (AHPS) from water, the mineralization rate of pyrite/EF system was superior to that of EF system alone under the same conditions, specifically, more than 90% TOC was removed in pyrite/EF within 300 min, whereas in the same reaction time only 70% TOC was removed with the conventional EF process. This is due to the self−regulation effect of pyrite on pH and soluble Fe²⁺ without additional acidification. As shown in Figure 8C, Ye et al. (Ye, et al., 2020) used FeS₂/C nanocomposites as highly active, stable and recyclable catalysts to treat fluoxetine in polyphase EF system, which achieved an impressive 90% TOC removal, while conventional EF processes produce a maximum TOC removal of 60%. The good performance of pyrite/EF system was attributed to the large amount of •OH produced by the pyrite induced Fenton reaction, which mainly attributed to the following aspects: (1) The mass transfer restriction of FeS₂ is very small, and it can act as the transfer intermediate of Fe²⁺ to participate in the homogeneous reaction to produce •OH. Meanwhile, pyrite can provide rich active sites, and ≡Fe (II) can activate H₂O₂ to produce •OH by Fe−S bond. (2) the ≡Fe (II) on the surface of FeS₂ can promote the activation of molecule O₂, thus accelerating the formation of O₂ ^•− (Liu, et al., 2015). In addition, the boron−doped diamond (BDD) anode used in the electro−assisted system also contributes to the generation of physical adsorption •OH in pyrite/EF system. The heterogeneous mechanism of the system is dominant because the concentration of dissolved iron is relatively low. The mechanism of electro−assisted pyrite/persulfate system for contaminants degradation can be proposed in Figures 8B, D.

Increasing attention has been paid to the application of ultrasonic (US) combined with advanced oxidation technology in water treatment. On the one hand, US has strong mechanical effect, which can enhance the mass transfer between interfaces, remove the passivation film on the metal surface and make the surface regenerate continuously. These properties can overcome the heterogeneous mass transfer barrier in the process of degrading organic matter, and achieve good contaminants removal effect (Xiang, et al., 2022). On the other hand, US lead to form cavitation effect, and its local high temperature and high pressure can produce •OH, O₂ ^•− and other ROSs (Chi, et al., 2022; Savun−Hekimoğlu, 2020; Wei, et al., 2017).

Chi et al. (Chi, et al., 2022) used US to enhance ferrous sulfide (FeS) to activate persulfate (PDS) for 2’−deoxycoformycin (DCF) degradation. US/FeS/PDS system with excellent activity presented an optimal DCF degradation efficiency (98.9%), which was 56.7, 5.81, 1.48 times than that of US, US/PDS and FeS/PDS systems (Figure 9B). Wei. et al. (Xiang, et al., 2022) demonstrated good degradation effect of carbamazepine (CBZ) by using ultrasonic−enhanced FeS/PDS system, which can remove 94.2% CBZ in 60 min, while FeS/PDS system can only remove 71.2% CBZ under same conditions (Figure 9A), confirming the synergistic enhancement effect of US and FeS contributed to the degradation of contaminants.

In ultrasonic enhanced FeS/persulfate system, US can promote the interfacial sulfur−iron electron transfer and the disintegration of passivation layer, which avoid the passivation and deactivation of FeS, and provide redox energy between S²⁻/S_x ²⁻ and Fe²⁺/Fe³⁺ to promote continuous production of Fe²⁺. Additionally, US can directly activate PDS, H₂O and dissolved oxygen to form SO₄ ^•− and •OH. In a word, US greatly promotes both heterogeneous and homogeneous iron cycles in the system. Hence, possible US−assisted reactions can be summarized as Eqs.1−3, Eqs. 47, 48, and the mechanism of US−assisted FeS/persulfate system for contaminants degradation can be proposed in Figure 9C H 2 o + ) ) ) U S → • O H + H • (47) S 2 O 8 2 − + ) ) ) U S → S O 4 2 − (48)

Microwave (MW) activation of persulfate has been widely studied. It has been proved to be superior to conventional thermal activation in terms of accelerating reaction rate, increasing yield and selectively activating or inhibiting reaction pathways, thus leading to higher degradation rate and significant savings in energy consumption and treatment time (Qi, et al., 2014). Wang et al. (Wang, et al., 2020a) used microwave radiation combined with FeS to activate persulfate to treat dinitrodiazophenol in explosive production wastewater. The Chemical Oxygen Demand (COD) removal efficiency of the MW–FeS/PS process reached 76.16%, which was 2.62, 2.57, and 1.42 times than that of MW/FeS, FeS/PS and MW/PS systems, indicating the strong synergistic effect between FeS and MW.

In MW–FeS/PS system, in addition to the persulfate activation performance of FeS, MW radiation can not only activate PS to produce SO₄ ^•− by its thermal effect, but make some functional groups and structures on organic pollutants vulnerable. Furthermore, the MW and FeS have strong synergic effect. Therefore, microwave−assisted FeS activation of persulfate can show higher treatment efficiency and higher PS utilization efficiency (Wang, et al., 2020b).