Performances and Interactions of Contaminants Removal from Water by Sulfidated Zerovalent Iron

Kaili Gu; Haozhen Li; Jinhua Zhang; Jinxiang Li

doi:10.7536/PC200920

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (10) : 1812-1822 DOI: 10.7536/PC200920

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Performances and Interactions of Contaminants Removal from Water by Sulfidated Zerovalent Iron

Kaili Gu ¹^,²
, Haozhen Li ¹^,²
, Jinhua Zhang ¹^,²
, Jinxiang Li ¹^,²^,^*

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Abstract

In recent years, synchronously mediating the reactivity and electron selectivity(ES) of zerovalent iron(ZVI) toward target contaminant has been of great interest but challenging to researchers. Sulfidation can suppress the side reaction of ZVI with water under anaerobic conditions and thus improve the ES toward target contaminants. As such, this review systematically summarizes the physicochemical characteristics of sulfidated ZVI(S-ZVI) as function of the sulfidation approaches, reagents and extents. Then, this work analyzes the interactions of contaminants removal by S-ZVI. Typically, the sulfidation can tune the electrical conductivity and/or hydrophobicity of ZVI, thereby enhancing the reactivity and ES toward target contaminants under aerobic or anaerobic conditions, which is strongly dependent on S/Fe molar ratio but without regard to the approaches and reagents of sulfidation. Finally, the application potentials of S-ZVI for groundwater remediation and wastewater treatment are also outlooked.

Contents

1 Introduction

2 Preparations and morphology of S-ZVI

2.1 One-step synthesis

2.2 Two-step synthesis

2.3 Ball-milling synthesis

3 Surface and bulk characteristics of S-ZVI

3.1 Surface information

3.2 Valence state and distribution of S/Fe

3.3 Bulk contents of S and Fe

4 Physical and chemical properties of S-ZVI

4.1 Conductivity

4.2 Hydrophobicity

4.3 H₂evolution rate

5 Decontamination performances of S-ZVI

5.1 Reactivity

5.2 Electron selectivity

6 Engineering applications

7 Conclusion and outlook

Graphical abstract

Keywords

sulfidation / zerovalent iron / reactivity / electron selectivity / physicochemical characteristic

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Kaili Gu, Haozhen Li, Jinhua Zhang, Jinxiang Li. Performances and Interactions of Contaminants Removal from Water by Sulfidated Zerovalent Iron. Progress in Chemistry, 2021, 33(10): 1812-1822 DOI:10.7536/PC200920

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1 Introduction

Zero-valent iron (ZVI), as a cheap, environmentally friendly and highly reducible water treatment agent, can remove various pollutants such as heavy metals, halogenated organic compounds, nitro compounds and azo dyes through oxidation, reduction and adsorption co-precipitation^[1,2]^[3,4]^[5,6]^[7,8]. On this basis, zero-valent iron has played an important role in the fields of in-situ remediation of groundwater, treatment of industrial wastewater and purification of drinking water^[9,10]. However, in practical engineering, ZVI is easy to be oxidized and passivated, which leads to poor reaction activity and low electron selectivity (ES) with target pollutants, which limits the wide application of ZVI, especially nano-ZVI (nZVI)^[11,12]. Therefore, how to simultaneously improve the reaction activity and electron selectivity of zero-valent iron for pollutant removal has become a research hotspot in recent years.

The improvement measures for ZVI focus on two aspects: one is to enhance the pollutant removal efficiency of ZVI by adding ultrasound, weak magnetic field or strong oxidant in the reaction system^[4]^[13]^[14]; On the other hand, from the point of view of ZVI itself, the improvement is to reduce the size of particles, modify the surface, modify or load them^{[15⇓⇓⇓~19]}. Acid pickling can dissolve the oxide film on the surface of ZVI to expose the core Fe⁰ and increase the number of active sites, but the effect of improving the reactivity is not good^[6]. The construction of bimetallic system can significantly improve the reaction performance of ZVI, but it still can not avoid the passivation of ZVI and faces the risk of high cost and secondary pollution of the environment^[20]. Most of these improvements are limited to the improvement of reaction rate and removal ability of pollutants, ignoring the electronic selectivity of ZVI reduction. The electronic selectivity here refers to the orientation of the reaction between ZVI and pollutants, and its quantitative method is basically consistent with the electronic efficiency (EE). The electron selectivity of the ZVI is better described by the electron efficiency, which is defined as the proportion of the electrons (N_e) used to reduce the target pollutant to the total electrons (N_t) supplied by the ZVI in the process of pollutant removal^[21,22]. The specific calculation formula is as follows:

(1)

EE = N e N t

When ZVI is used to remove pollutants, as an electron donor, it will react with other electron acceptors such as H₂O and O₂, which will lead to corrosion failure. How to shield the side reactions in the process of removing pollutants based on ZVI is a key problem that must be faced and solved in the practical application of water pollution control technology based on ZVI^[23]. Recently, sulfidation can inhibit the hydrogen evolution reaction of nano-ZVI (nZVI) in the absence of oxygen, which provides a new way to improve the efficiency of ZVI reduction and decontamination^[24⇓~26]. The improvement of electron selectivity of ZVI reduction by sulfidation is essentially attributed to the weakening of the ability of non-target pollutant H₂O to compete for S-ZVI electrons, thus increasing the "targeted" reduction equivalent of system electrons to target pollutants. At first, the research of sulfidation mainly focused on the effects of sulfidation methods, sulfiding agents, S/Fe molar ratio, sulfiding time and other factors on the activity of nZVI for removing pollutants and the related mechanisms. Until Rajajayavel and Ghoshal confirmed that sulfidation under anaerobic conditions could inhibit the hydrogen production rate of nZVI and H₂O, the researchers gradually paid attention to sulfidation^{[27⇓⇓⇓⇓~32]}^[33]. The reaction activity and electron selectivity of sulfurized zero-valent iron are higher than that of ZVI. The main mechanisms are as follows: (1) sulfurization changes the surface morphology of ZVI and inhibits its agglomeration^[34]; (2) Sulfidation improves the electrical conductivity of ZVI and promotes the transfer of electrons from the core Fe⁰ to the target pollutants; (3) Sulfidation increases the hydrophobicity of ZVI, which makes it easy to react with organic pollutants with strong hydrophobic properties; (4) H adsorption sites on the surface of ZVI are blocked by sulfidation; (5) Iron sulfide (FeS_x) produced in the synthesis process can directly participate in the reduction and removal of various pollutants.

It can be seen that the incorporation of S is very important in the synthesis process of S-ZVI, and the morphology, structure and inherent properties of the particles will change accordingly, which ultimately affects the pollutant removal efficiency of ZVI. At present, the research on the relationship between the structural characteristics of S-ZVI and its pollutant removal efficiency has made some progress, but there is no systematic understanding^{[35⇓⇓⇓~39]}. Based on the preparation method of S-ZVI, this paper summarizes the changes of particle structure caused by sulfurization, combs the content, valence and distribution of sulfur, and clarifies the influence law and interaction mechanism of sulfur on particle intrinsic properties, pollutant removal reactivity and electronic selectivity, in order to provide a solid theoretical basis for the design and application of S-ZVI.

2 Synthesis and Morphology of Zero-Valent Iron Sulfide

The preparation and synthesis methods of S-ZVI are mainly divided into chemical synthesis and physical synthesis^[40].

As shown in Figure 1, the chemical synthesis method is the most commonly used method to prepare S-nZVI, which is divided into one-step synthesis method and two-step synthesis method, while the mechanical ball milling method in the physical synthesis method is usually used to prepare S-mZVI. The unvulcanized nZVI is a spherical particle with a core-shell structure, has a smooth surface, is easy to agglomerate due to a small size, and has a chain-like structure^{[45⇓⇓~48]}. The morphology and structure of ZVI were changed by different vulcanization methods. The physical and chemical properties of zero-valent iron are also affected by the experimental drugs, operating conditions, instruments and equipment involved in different vulcanization methods.

2.1 One-step synthesis

The one-step synthesis method is also known as the pre-treatment method, and the preparation steps are relatively simple, as shown in Figure 1 (a), after the strong reducing agent and the vulcanizing agent are dissolved together, the solution containing the Fe²⁺/Fe³⁺ is added, and after being fully mixed by mechanical stirring, the S-nZVI_one-step can be obtained by taking out and drying. Kim et al. First proposed this vulcanization method, using sodium borohydride (NaBH₄) reduction method and sodium dithionite (Na₂S₂O₄) to prepare Fe/FeS particles with excellent performance, which laid the foundation for the research of S-nZVI^[27]. The Na₂S₂O₄ is unstable and easy to hydrolyze into thiosulfate (S₂

O 3 2 -

) and sulfite (

SO 3 2 -

) in the reaction process,Hydrogen sulfide (H₂S) is further produced and combined with Fe²⁺ to produce insoluble ferrous sulfide (FeS), which is commonly used as a vulcanizing agent in one-step process because of its moderate price, safety and easy availability.

The shape of S-nZVI_one-step particles is mainly spherical, and the surface will appear needle-like and flaky structures, and the agglomeration of particles is reduced, showing a dispersed state, as shown in Figure 1 (d). With the increase of vulcanization dosage, the acicular and flaky substances on the surface of the particles increased significantly, and the surface roughness also increased^[42,49]. The surface morphology of such particles is different from the typical nZVI structure, the original chain-like structure is destroyed, and the particle spacing is enlarged due to the continuous formation of iron sulfides on the surface, resulting in a "shear effect".

2.2 Two-step synthesis method

The two-step synthesis method, also known as the post-synthesis method, operates differently from the one-step method, as shown in Figure 1 (B). Firstly, nZVI is prepared by a NaBH₄ reduction method, and then the nZVI is placed in a solution containing a vulcanizing agent and is uniformly mixed in an ultrasonic or oscillating mode to obtain a S-nZVI_two-step. During the synthesis process, nZVI reacts with water to release Fe²⁺, and sodium sulfide (Na₂S) decomposes into HS^-, which combine to form FeS and cover the surface of nZVI, forming an iron sulfide layer^[25]. As a vulcanizing agent, Na₂S is used in the preparation of S-nZVI_two-step, which can retain the Fe⁰ and maintain its reactivity, so it is widely used.

As shown in fig. 1 (e), the shape of the S-nZVI_two-step particles is also mainly spherical, maintaining the chain-like structure of nZVI. As far as the particle structure is concerned, the S-nZVI_two-step is composed of an outer shell dominated by FeS_x and an inner core of Fe⁰^[36]. In the case of high sulfur dosage and long vulcanization time, the hollow structure shown in the inset of Figure 1 (e) will appear in the particles, and the formation process is similar to the Kirkendall effect, which may be caused by the outward diffusion of Fe⁰ faster than the inward diffusion of S^[28]. In the two-step method, the sulfiding agent is combined with the nZVI after the nZVI is synthesized, so that the deposition and stacking of S are mainly concentrated on the outer surface of the particle, and only a small amount of S enters the inner part of the particle, and the chain structure of nZVI is still intact.

2.3 Mechanical Ball Milling

Mechanical ball milling is a physical synthesis method, which does not need to add additional chemical reagents in the synthesis process, and has been used to synthesize nZVI, bimetallic systems and iron-carbon composites^[50]^[51]^[52].

Sulfur is abundant and easy to react with ZVI to form iron sulfide, and micron zero-valent iron (mZVI) has a high application prospect. Therefore, the ball milling method is a simple method. The specific process is shown in Figure 1 (C). Zero-valent iron and elemental sulfur are put into the planetary ball mill at the same time, and the S-mZVI_bm is obtained after a period of mutual extrusion, collision and grinding. The oxide film on the original ZVI surface is destroyed by ball milling, so that the Fe⁰ and the S⁰ can directly contact to form FeS^[53].

After ball milling, the surface of ZVI is smooth, the shape of ZVI changes from spherical to irregular, and the flake structure is the majority^[44]. As shown in Figure 1 (f), the particle size distribution of S-mZVI particles synthesized by ball milling method is wide, and there are not only lamellar structures, but also approximately spherical particles. The surface roughness of the S-mZVI_bm changes with the amount of sulfur, and is generally coarser than that of the ZVI_bm, and the particle size decreases more, which is mainly due to the fact that more FeS_x are generated on the surface and penetrate into the interior of the ZVI due to the incorporation of sulfur^[53,54].

3 Surface and bulk characteristics of zero-valent iron sulfides

Different sulfurization methods lead to different morphologies and structures of S-ZVI, which are also indirectly reflected by the specific surface area and other surface information. The influence of sulfurization mode on the morphology and structure of S-ZVI is also due to the change of sulfur and iron valence distribution and content when S is doped, so understanding the surface and internal characteristics of S-ZVI is helpful to understand the inherent properties of particles and the performance of pollutant removal.

3.1 Surface Information

The surface morphology and structure of S-ZVI particles have a certain impact on other surface information such as specific surface area and dispersion performance, as shown in Figure 2.

The effect of the vulcanization method on the specific surface area of S-ZVI at different S/Fe molar ratios is shown in fig. 2 (a). The specific surface area of S-ZVI prepared by any vulcanization method is higher than that of unvulcanized ZVI, and the formation of FeS_x on the surface of particles and the change of morphology and structure play an important role. The specific surface area of S-nZVI prepared by one-step method is better than that of the original nZVI, while the specific surface area of S-nZVI prepared by two-step method is not much different from that of the original nZVIs^[34]. Li et al. Prepared S-nZVI with S/Fe molar ratio of 0.2 by one-step method, and its specific surface area increased from 8.6 m²·g^-1 to 19.9 m²/g without sulfidation^[56]. The specific surface area of the S-nZVI_one-step prepared by Lv et al. Did not increase continuously. With the gradual increase of S/Fe molar ratio, the specific surface area of the particles decreased from 26 m²/g to 10.7 m²/g and then increased to 50.3 m²/g^[42]. When the S/Fe molar ratio is low, the particle size of the S-nZVI_one-step increases and the specific surface area decreases correspondingly, and then the increase of the needle-like structure on the particle surface and the enhancement of the "shear effect" further promote the increase of the specific surface area. Xu et al. prepared S-nZVI by one-step method and two-step method respectively under low sulfur dosage, and found that the specific surface area of S-nZVI_two-step did not change significantly, while the specific surface area of S-nZVI_one-step decreased by 50%, except for the increase of particle size, which may also be caused by incomplete incorporation of S^[57]. The reason why the specific surface area of S-nZVI_two-step is similar to that of nZVI is related to their similar structures. As described in Section 2.2, S-nZVI_two-step can well retain the core-shell and chain-like structure of the original nZVI, while the stacking of iron sulfides layer by layer in the vulcanization process will increase the roughness of the particle surface and the specific surface area. At high sulfur dosage, the specific surface area of S-nZVI prepared by both methods decreased slightly, which may be due to the formation of excessive FeS_x blocking the surface pores^[27].

The particle size of mZVI is larger than that of nZVI, and the specific surface area is generally smaller. Vulcanization can make its surface rougher and the specific surface area can also be improved. The specific surface area of S-mZVI_bm prepared by Li et al increased rapidly from 0.97 m²/g to 33.28 m²/g at low sulfur dosage, but decreased significantly at high sulfur dosage, which may be due to the fact that a small amount of S can be fully fused on the surface of ZVI, and the roughness of the inherent oxide film on the surface of ZVI increased significantly after ball milling, resulting in a significant increase in the specific surface area^[54]. Zou et al. Used S-mZVI_bm to remove hexavalent chromium (Cr (Ⅵ)), and found that when the S/Fe molar ratio increased, the specific surface area increased from 0.21 m²/g to 2.08 m²/g, and the removal ability of S-mZVI_bm to Cr (Ⅵ) also showed a linear increase, which also meant that the effect of sulfidation on the specific surface area of ZVI would positively affect its pollutant removal efficiency^[58].

In addition to the specific surface area, the aggregate size and dispersity of particles can also be reflected by the average pore diameter (D_ads), pore volume (V_ads), hydrodynamic diameter (DLS) and polydispersity index (PDI), as shown in Figure 2 (B). Although the specific surface area of FeS @ Fe particles prepared by Du et al. Under low sulfur dose is smaller than that of nZVI, it can also fit the type IV isotherm well, indicating that there are developed mesopores in the material^[55]. During the preparation of S-ZVI, insoluble FeS_x will deposit on the surface of the particles and occupy the surface adsorption sites, and the average pore size and pore volume may decrease, which will also have a certain impact on the adsorption performance of S-ZVI for heavy metal pollutants. Bhattacharjee and Ghoshal measured the hydrodynamic diameter and polydispersity index of nZVI, S-nZVI_one-step and S-nZVI_two-step by dynamic light scattering. It was found that sulfidation decreased the aggregation of nZVI. For example, when the S/Fe molar ratio was 0.4, the hydrodynamic diameter of S-nZVI_one-step was 1.3 ± 0.1 μm, and the PDI was 0.38, while the hydrodynamic diameters of S-nZVI_two-step and nZVI were 4.4 ± 0.4 μm and 7.2 ± 0.8 μm, respectively, and the PDI was 0^[36]. The difference in the degree of particle dispersion corresponds to the structure, the "shear effect" of the S-nZVI_one-step is obvious, and the distance between particles increases because of its independence, while the chain-like structure of nZVI will deepen the degree of compactness between particles, and the deposition of FeS_x on the surface of S-nZVI_two-step will increase this distance, but the restriction of chain-like structure limits the increase of the distance.

3.2 S/Fe valence distribution

The valence distribution of S and Fe on the surface of S-ZVI under different sulfidation modes is shown in Fig. 3 (a). For S-nZVI prepared by chemical synthesis, S^2-,

S 2 2 -

and

S n 2 -

are the main forms of S on the surface.The

SO 3 2 -

is a product of the hydrolysis of the Na₂S₂O₄ during the synthesis process, and may also be present on the particle surface as a result of oxidation, as is the case with

SO 4 2 -

^{[37,57,59,60]}. The valence state and quantity of sulfur species on the surface of S-nZVI are different from those in the interior of S-nZVI. The deeper the particles go into the core, the

SO 3 2 -

and

SO 4 2 -

will disappear, and the S^2- and

S 2 2 -

will account for more^[49,57]. The surface and internal sulfur oxide species of S-nZVI_one-step are less than those of S-nZVI_two-step, which means that S-nZVI_one-step is more resistant to oxidation than S-nZVI_two-step, and the oxygen incorporation in the one-step synthesis of the former is less than that in the two-step synthesis of the latter. The absence of

S n 2 -

in the core of the S-nZVI_two-step also reflects that the incorporation of S^2- into the particles is limited, and the chain-like structure and the close connection between particles limit the further penetration of S. There is S⁰ in the preparation of S-mZVI_bm, and there will be a certain amount of S⁰ on its surface, and the oxidation problem can not be avoided. The oxide film on the surface of ZVI can hinder the incorporation of sulfur, but the sulfur can be uniformly distributed in the particles, which is related to the uniform stress caused by mechanical milling during synthesis^[53,54].

On the surface of S-ZVI, the content of Fe⁰ is less, while the content of Fe²⁺ and Fe³⁺ is dominant, which can also explain that with the addition of vulcanizing agent in the synthesis process.Most of the Fe⁰ on the surface of the particles will react to form iron sulfides or be oxidized to form iron oxides, and the Fe²⁺ of reduced iron species and the increase in the number of Fe⁰ will be found in the core of S-ZVI^[49,62]^[57]. The increase of these sulfur species and the decrease of iron oxides indicate that sulfidation can inhibit the oxidative passivation of ZVI and prolong its service life.

3.3 Sulfur and iron content

With the increase of sulfur dosage, the content of S on the surface of particles under different vulcanization methods is undoubtedly increased, and the content of Fe⁰ is correspondingly decreased, as shown in Figure 3 (B). The content of Fe⁰ on the surface of S-mZVI_bm prepared by Gu et al. By ball milling decreased from 92.7% to 85.5% due to the incorporation of S and the reaction between Fe⁰ and S⁰^[53]. Bhattacharjee and Ghoshal used one-step and two-step methods to prepare S-nZVI, and the theoretical S/Fe molar ratio increased from 0 to 0.4.The content of Fe⁰ on the surface of the S-nZVI_one-step particles is reduced from 0.82 mol Fe⁰/mol Fe to 0.4 mol Fe⁰/mol Fe,It is possible that part of the Fe⁰ reacts with the HS^- to form FeS, which causes the content of FeS to decrease^[36]. Due to the "shear effect" of the S-ZVI_one-step particles, the contact between the surface Fe⁰ and the curing agent will increase with the increase of the particle spacing, and the reaction between them will be enhanced. At the same time, S will go deep into the particles, thus limiting the consumption of a large amount of surface Fe⁰. However, the content of Fe⁰ on the surface of the S-nZVI_two-step did not change much, and the contact between the vulcanizing agent and the surface Fe⁰ was reduced due to the smaller spacing under the chain structure of the particles and the FeS_x stacking. Wu et al. prepared S-nZVI_two-step with different amounts of sulfur, and found that even when the S/Fe molar ratio varied from 0.1 to 0.5, the Fe⁰ content on the surface of the particles remained similar at 83 wt%^[59]. Xu et al. Increased the amount of vulcanization and extended the vulcanization time for the preparation of S-nZVI_two-step, and the content of Fe⁰ on the surface of particles was not significantly affected, but only decreased slightly^[28]. The small change of Fe⁰ content may be due to the limited incorporation of S. During the synthesis process, FeS_x deposited on the surface of nZVI, which hindered its further combination with S^2- in the solution^[36,57]. Under the difference of particle morphology and structure, the incomplete incorporation of S also causes the actual S/Fe molar ratio on the particle surface to be less than the theoretical S/Fe molar ratio, and to some extent affects the formation and distribution of iron sulfide species on the particle surface and inside.

4 Physical and Chemical Properties of Sulfide Zero-valent Iron

4.1 Conductivity

The iron oxides formed during the sulfidation process are considered to be semiconductors due to the presence of free electrons, which can promote the electron transfer from the core of the Fe⁰ to the surface of the particles, thus improving the electron transfer ability of ZVI^[27,33,55].

The corrosion tendency and electron transfer ability of the material can be characterized by electrochemical experiments, and the effects of vulcanization mode and vulcanization dose on the electron transfer ability of S-ZVI are shown in Figure 4 (a). The charge transfer resistance of the S-nZVI_two-step decreases continuously with the increase of the amount of sulfide, while the charge transfer resistance of the S-nZVI_one-step and the S-mZVI_bm fluctuates, but it is still lower than that of nZVI. The smaller the charge transfer resistance is, the stronger the electron transfer ability of S-ZVI is, and the conductivity is gradually enhanced. The inset in Figure 4 (a) shows the effect of S content on the open circuit potential of S-ZVI under different vulcanization modes. The more negative the corrosion potential value, the greater the tendency of the material to corrode in water. The three types of S-ZVI show different corrosion tendency when the sulfur content changes, which is related to the difference of experimental conditions and the amount of sulfur doped.

4.2 Hydrophobic property

Iron sulfides are more hydrophobic than iron oxides, and it is generally believed that the FeS_x layer of S-nZVI enhances the ability of the material to bind to hydrophobic organics^[27,33,34]. By measuring the contact angle of S-ZVI, the hydrophobic property of S-ZVI was explained. When the contact angle is greater than 90 °, the hydrophobicity of the material is better, otherwise the hydrophilicity is stronger.

Fig. 4 (B) show that influence of the vulcanization mode and the vulcanization amount on the hydrophilicity and hydrophobicity of S-ZVI. The contact angle of nZVI is small and its hydrophilicity is strong, while the contact angle of S-nZVI_two-step with similar structure increases slightly, but it is still in a hydrophilic state. The contact angle of the S-nZVI_one-step increases to more than 100 ° with the increase of the vulcanization amount, and then decreases. The hydrophobicity of S-nZVI_one-step will be enhanced at low sulfur content, but not the higher the sulfur content, the stronger the hydrophobicity, and the reason for the decline is not clear^[39]. Both nZVI and S-nZVI_two-step are chain-like structures with small particle spacing and poor hydrophobicity. The "shear effect" of S-nZVI_one-step is obvious, and the distance between particles is large, showing good hydrophobic properties. This is also the performance of the change of particle structure caused by vulcanization mode and the difference of hydrophilic and hydrophobic properties. Therefore, the change of contact angle reflects that the vulcanization mode, particle structure and S content can affect the hydrophilicity and hydrophobicity of S-ZVI surface.

4.3 H₂ release rate

While ZVI can remove the target pollutants, it is easy to react with water to produce hydrogen evolution, which will reduce the efficiency of pollutant removal, while vulcanization can effectively inhibit this side reaction^{[24,28,34,63]}.

The degree of inhibition of the reaction of ZVI with water by vulcanization can be evaluated by the surface normalization rate of the H₂. Fig. 4 (C) shows the effect of vulcanization mode and vulcanization amount on the release rate of the H₂. It can be seen from the figure that vulcanization can always significantly reduce the release rate of H₂, but it can also accelerate the release of H₂. The S-nZVI_one-step with low S/Fe molar ratio promoted the release of H₂, which may be due to the fact that the resistance of the material decreased more, that is, the conductivity increased, but at the same time the contact angle was smaller, that is, the hydrophilicity was still good, and only a small number of H adsorption sites were blocked when the sulfur content was low. With the increase of sulfur dosage, S blocked more H adsorption sites on the surface of S-ZVI and accelerated the release of H₂^[39]. The H₂ release rate of S-ZVI decreased due to the low affinity of iron sulfide for water, while the faster H₂ release rate of S-nZVI_one-step at high sulfide content may be due to the increase of electron conduction sites on the surface of S-ZVI due to the increase of FeS content.The release rate of H₂ in S-nZVI_two-step is lower than that in S-nZVI_one-step, which may be due to the small amount and uneven distribution of FeS^[36]. The inhibition of H₂ release by vulcanization can not only prolong the service life of ZVI, but also play a positive role in improving its pollutant removal efficiency.

5 Effect of sulfuration on pollutant removal efficiency

The effect of the sulfurization method and the amount of sulfur on the morphology and intrinsic properties of ZVI will be reflected in the change of the reaction activity and electronic efficiency of pollutant removal, as shown in Figure 5.

5.1 Reactivity

The research of S-ZVI mainly focuses on the degradation of organic pollutants in anaerobic system and the removal of heavy metal ions in aerobic system. As shown in Figure 5 (a), sulfurization can generally improve the removal rate of pollutants by ZVI, but the greater the amount of sulfurization, the higher the reaction activity, which may be related to the type of pollutants removed and also affected by the reaction system. Trichloroethylene (TCE) is one of the most studied organic pollutants in the world.Sulfurization can greatly improve the removal rate of TCE, which is related to the conductivity and hydrophobicity of particles in addition to the sulfidation method and dosage^[27,33,39]. Bhattacharjee and Ghoshal compared the TCE removal performance of S-nZVI_one-step and S-nZVI_two-step, and found that the TCE degradation ability of both increased with the increase of sulfur dosage.The maximum degradation ability of S-nZVI_one-step can reach 0.46±0.02 mol TCE/mol Fe⁰, but the maximum degradation ability of S-nZVI_two-step is only 0.28±0.02 mol TCE/mol Fe⁰ in the same dose range, and the removal rate of the former is also greater than that of the latter^[36]. Gu et al. Found that under the limited conditions of ZVI and TCE, the specific surface area normalized rate constant of the reduction of TCE by S-mZVI_bm with S/Fe molar ratio of 0.1 increased by 2 and 5 times, respectively^[53]. Wu et al. Used S-nZVI_two-step to degrade tetrabromobisphenol-a (TBBPA), and the k_SA of TBBPA increased from (6.63±0.25)×10^-4L·m^-2·h^-2However, when the S/Fe molar ratio was further increased to 0.5, the k_SA decreased, and Li et al found that when the S/Fe molar ratio was 0.5, the maximum removal rate constant of TBBPA by S-nZVI_one-step was 0.315 h^-1, which was 6.6 times higher than the minimum^[59]^[31]. Cao et al. Also found that the removal efficiency of S-nZVI for florfenicol (FF) did not increase continuously with the increase of S/Fe molar ratio. When the S/Fe molar ratio was 0.14, the maximum k_SA was 3.1×10^-4L·m^-2·min^-1, and the removal efficiency reached 90%. When the S/Fe mole ratio increased to 0.28, the k_SA decreased^[65]. The difference of S/Fe molar ratio when the removal ability is the strongest will have a certain impact on the nature of pollutants, and the vulcanization mode, particle morphology and structure, and vulcanization dosage are also part of the reason.

The removal of heavy metal ions by S-ZVI is relatively rare, and the reaction system is different from the degradation of organic pollutants, which is mainly carried out in aerobic system. Zou et al considered that the removal ability of Cr (Ⅵ) by S-mZVI_bm was closely related to its specific surface area, which had good correspondence^[58]. Lv et al. used the pseudo-second-order kinetic model to fit the adsorption process of Cr (Ⅵ) by S-nZVI. The q_e of the equilibrium adsorption capacity of nZVI was 20.2 mg/G, while when the S/Fe molar ratio was 0.1, the q_e of S-nZVI decreased to 16.8 mg/G, and then increased to 53.2 mg/g,q_e with the increase of sulfide dosage. The decrease corresponds to the change of specific surface area, as shown in Figure 2 (a)^[42]. Lv et al. investigated the ability of S-nZVI to remove cadmium (Cd) at different dosages. When the S/Fe molar ratio increased from 0.1 to 0.3, the removal rate of Cd increased significantly, which was largely promoted by the increase of specific surface area^[66]. Wu et al. Observed that the ability of S-nZVI to remove trivalent arsenic (As (Ⅲ)) was stronger with the increase of sulfide dosage, which may be due to the increase of surface roughness. In addition, the morphology of particles with S/Fe molar ratio of 0.07 was similar to that of nZVI, which was also approximately consistent with the ability to remove pollutants^[49].

5.2 Lectron selectivity

In practical application, ZVI not only reacts with target pollutants, but also easily combines with other coexisting oxidizing substances, which leads to low utilization rate of ZVI and weak targeted binding ability with target pollutants. Sulfidation can significantly inhibit the side reaction of ZVI with water, as described in Section 4.3, which is also an important manifestation of the improvement of electronic selectivity of ZVI by sulfidation.

As shown in Figure 5 (B), the effect of vulcanization on the improvement of ZVI electron efficiency is obvious, and it varies with different vulcanization methods and vulcanization doses. The electronic efficiency of S-ZVI can be improved by different vulcanization methods at low vulcanization dose, but the electronic efficiency may decrease with the increase of vulcanization dose. Li et al. Sulfurized ZVI by mechanical ball milling and reduced Cr (Ⅵ) in the presence of oxygen. The electron efficiency of ball milled sulfurized ZVI was 14.6%, which was 10.7 times of that of unsulfurized ball milled ZVI^[54]. Gu et al. Found that when the pH value was 9, the electron efficiency of ZVI for removing TCE was improved more significantly by ball milling vulcanization, which could be increased from 3.2% to 22%^[30]. One-step and two-step vulcanization have a stronger effect on improving the electronic efficiency of ZVI than ball milling vulcanization. The electron efficiency of S-nZVI prepared by He et al. Through one-step method for TCE increased from 2% of nZVI to 72%, and its dechlorination ability was enhanced, which may be due to the FeS_x acting as an electron mediator^[67]. The electron efficiency is related to the particle morphology, iron sulfide content, conductivity, hydrophobicity and other properties. With the increase of sulfur dosage, the amount of FeS_x on the particle surface increased, the surface roughness and specific surface area increased, and the hydrophobicity improved significantly, while the charge transfer resistance decreased, which promoted the electron transfer from the core Fe⁰ to the particle surface, and the electron efficiency also improved. When the sulfur dosage is high, although the conductivity of the particles is still good, the hydrophobicity of the S-nZVI_one-step decreases, and excessive FeS_x will block the pores on the surface of the particles, resulting in a decrease in H adsorption sites and a decrease in electronic efficiency^[39]. Xu et al. pointed out that the ability of S-nZVI_two-step to inhibit the release of H₂ was enhanced at higher S/Fe molar ratio, while the electronic efficiency of TCE was reduced, which may be due to the decrease of dechlorination ability of particles^[61]. Zou et al. found that the electronic efficiency of S-ZVI for removing Cr (Ⅵ) alone can reach 100%, but when it coexists with TCE, the electronic efficiency of S-ZVI for removing TCE is lower due to its passivation^[58]. The research on the electronic efficiency of S-ZVI for coexisting pollutants is worthy of further study, which is of great help to its practical application. To sum up, the removal efficiency of ZVI by sulfidation is closely related to the particle morphology and structure, and the amount of sulfidation also plays a key role.

6 Engineering Application

Zero-valent iron (ZVI) permeable reactive barrier (PRB) has been applied to the in-situ remediation of groundwater contaminated by chlorinated organic compounds since the 1990s, and the removal effect is good, thus the engineering application based on ZVI technology began^[69].

However, the water environment is more complex in practical application, ZVI is easy to deactivate and has low reactivity with target pollutants, while sulfurized ZVI can effectively improve its reactivity, electron selectivity and long-term stability^[23]. Service life is an important manifestation of the long-term stability of ZVI. The service life of S-ZVI can be estimated according to the initial Fe⁰ content and H₂ release rate, which is an idealized result without considering other coexisting oxidants and ions, but it can also reflect the influence of vulcanization mode and vulcanization dosage on the long-term stability of the material^[39,63]. S-nZVI_one-step can effectively improve the service life, which is related to its strong hydrophobicity. However, when the sulfur content is high, the hydrophobicity decreases, and the affinity between particles and water increases again, which is the possible reason for the decrease of service life. The difference of service life between S-nZVI_two-step and S-nZVI_one-step is related to their hydrophilicity and hydrophobicity, and the decrease of service life of S-nZVI_two-step under high sulfur content is probably related to the hollow structure formed. Service life can also be reflected by reactivity to contaminant removal. The positive effect of vulcanization on prolonging the service life of the ZVI can also be shown if the vulcanized ZVI is still able to maintain some reactivity when used for contaminant removal after aging for different times. In the actual water environment, the removal efficiency of ZVI is affected by many aspects, including the electronic competition between water and oxygen, the interference of coexisting ions and the simultaneous pollution of multiple substances, and the actual service life may be shorter than expected. Therefore, the effect of vulcanization on the long-term stability of ZVI in practical engineering applications needs to be further studied.

The practical application fields of ZVI mainly include aerobic industrial wastewater treatment and anaerobic groundwater remediation, and the improvement of its pollutant removal efficiency by vulcanization can also seek breakthroughs in these two aspects. Li et al. Evaluated the removal performance of S-mZVI_bm for hexavalent chromium in continuous operation. The results showed that the rate constant of S-mZVI_bm for removing Cr (Ⅵ) decreased gradually from the first operation to the seventh operation, but its reaction activity was still higher than that of ZVI in the first operation^[54]. The cumulative removal capacity of the S-mZVI_bm was 30. 8 mg/G in seven cycles, which was much higher than that of the ZVI_bm (0. 26 mg/G), and the corresponding electron selectivity was increased by 10. 7 times. Therefore, the high reactivity, high removal capacity and high electron selectivity of S-ZVI make it have broad application potential in aerobic industrial wastewater treatment.

Recently, Nunez Garcia et al. Used S-nZVI for in situ remediation of groundwater and surrounding soil contaminated by chlorinated solvents (see Figure 6)^[70,71]. They prepared carboxymethyl cellulose (CMC) stabilized S-nZVI in situ, which showed excellent dispersibility and colloidal stability, and had a sheet structure related to FeS on the outside. CMC-S-nZVI is injected into the sandy aquifer by gravity through injection wells and is well distributed horizontally and vertically in the study area with strong mobility. When CMC-S-nZVI was first injected into groundwater, the concentration of chlorinated volatile organic compounds (cVOCs) decreased rapidly. The dechlorination capacity of the particles remained stable during the long-term degradation process, and low chlorine cVOCs such as vinyl chloride did not appear, and the concentration of cVOCs in the surrounding soil area also showed a significant decrease after treatment. This study is the first pilot test from the laboratory to large-scale field application, and combined with the results of these studies, it shows that the water pollution control technology based on zero-valent iron sulfide is a suitable and promising in-situ groundwater remediation technology.

7 Conclusion and prospect

The improvement of sulfuration on the performance of zero-valent iron is mainly reflected in the enhancement of reaction activity and electron selectivity, which is closely related to the incorporation of sulfur and the change of particle structure. S-ZVI shows distinct structural characteristics due to the different ways of sulfur incorporation, while the sulfur content and morphology distribution will also cause the difference of the inherent properties of particles, which will lead to the change of pollutant removal efficiency. From the improvement of synthesis method to the improvement of reaction activity and mechanism of pollutant removal, to the exploration of the enhancement of electronic selectivity of target pollutants, the related research of S-ZVI is gradually deepening, but there are still some problems and challenges that need to be paid attention to:

(1) The incorporation of sulfur into zero-valent iron is incomplete, which leads to the decrease of the actual S/Fe molar ratio of the particles and a certain deviation from the theoretical value. Therefore, it is necessary to consider the actual amount of sulfur, find out the reasons and propose improvement measures to achieve the optimization of S-ZVI performance.

(2) S-ZVI can greatly improve the removal effect of specific pollutants, such as TCE with strong hydrophobicity, but it may also hinder the removal of some pollutants, and the range of pollutants to be removed needs to be further confirmed.

(3) In industrial wastewater and groundwater environment, there are often a variety of pollutants coexisting, and most of the studies on S-ZVI only focus on the removal of a certain pollutant, so the performance of S-ZVI in the coexistence of multiple pollutants should be strengthened.

(4) Although S-ZVI has been used in the remediation of contaminated groundwater, its mobility and long-term stability still need to be further explored, and the large-scale production and economic benefits of S-ZVI should also be taken into account. In addition, whether S-ZVI has toxic effects on organisms and ecological environment needs to be further clarified.

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