Sulfidation Of Zerovalent Iron Research Articles

Facilitating Se(VI) adsorption and electron transfer by introducing αFeOOH to sulfidated zero-valent iron

Sulfidation of zerovalent iron (SZVI) enhances electron transfer ability due to the formation of FeSx, which facilitates pollutant removal. However, for pollutants that are difficult to adsorb onto SZVI, the longer electron transfer distance results in inefficient removal. The presence of iron (hydr)oxides on ZVI and SZVI is inevitable, and it can adsorb highly mobile pollutants (such as Se(VI)). Here, we regulated the composition of iron (hydr)oxides on ZVI and SZVI surfaces by aging treatment, which improves their Se(VI) removal performance. Based on this, αFeOOH which has a strong affinity for Se(VI), is introduced onto SZVI to synthesize the high-quality composite SZVI-αFeOOH. The characterization results showed that Fe-OH from αFeOOH and FeSx successfully combined and uniformly distributed on SZVI-αFeOOH, creating active sites, and the Se(VI) removal rate (kobs = 0.2211 min−1) increased ∼ 7.4 times over SZVI (kobs = 0.0297 min−1). The adsorption and reduction of Se(VI) were optimized by regulating the mass ratio of αFeOOH to SZVI. In addition, different initial Se(VI) concentrations and solution pH were designed to explore the effects on Se(VI) removal and evaluate the best removal reaction condition. The mechanism of Se(VI) removal by SZVI-αFeOOH is as follows: Se(VI) is firstly adsorbed on αFeOOH, which shortens the electron transfer distance, and then the absorbed Se(VI) is reduced to readily immobilized states by SZVI. This work proposes a targeted strategy to modify SZVI to efficiently remove pollutants with strong mobile and difficult adsorption.

Intervention of HCHO on the FeSx structure transition during sulfidation of zero-valent iron

Iron sulfides (FeSx) enhancing the properties of sulfidated zero-valent iron (SZVI) to contaminants removal has been extensively studied, however, it is difficult to further regulate the FeSx species to control the SZVI properties. Based on the transformation of formaldehyde (HCHO) on FeSx, this work introduced HCHO into SZVI preparation process to regulate the FeSx structure. Results show that the HCHO-intervened SZVI (SZVIHCHO) has better Cr(VI) removal performance, which is due to the batter electrochemical properties. The FeSx structure of SZVIHCHO is evidently changed by HCHO, that is flaky FeSx transforms into granular FeSx and locally aggregated on SZVIHCHO surface. The composition of FeSx species shows that HCHO can weaken the conversion of FeS to FeS2, but directionally induce the conversion of FeS to Fe3S4. The electrostatic attraction of Fe atom in FeSx to the –CHO and the electron-withdrawing property of –CHO are the causes of electron loss from Fe atom and thus increase the Fe valence state. This study provides the inspirations for the future regulation of SZVI by inserting organics with specific structure and functions during SZVI preparation.

Decoding the divalent cation effect on sulfidation of zero-valent iron: Phase evolution and FeSx assembly

The decontamination ability of sulfidated zero-valent iron (S-ZVI) can be enhanced by the effective assembly of iron sulfides (FeSx) on neglected heterogeneous surfaces by liquid-phase precipitation. However, S-ZVI preparation with the usual pickling is detrimental to orderly interfacial assembly and leads to an imbalance between electron transfer optimization and electron storage. In this work, S-ZVI was prepared in solutions containing trace divalent cation, and it removed Cr(VI) up to 323.25 times higher than ZVI. This result is achieved by surface sites protonation of divalent cations regulating the phase evolution on the ZVI surface and inducing FeSx chemical assembly. Regulation of divalent cation and S(-II) content further promotes FeSx targeted assembly and reduces electron storage consumption as much as possible. The barrier for FeSx assembly is found to lie at the ZVI interface rather than in the deposition between FeSx. Chemical assembly at heterogeneous interfaces is a prerequisite for the ordered assembly of FeSx. In addition, S-ZVI prepared in simulated groundwater showed extensive preparation pH and universality for remediation scenarios. These findings provide new insights into the development of in-situ sulfidation mechanisms with particular implications for S-ZVI applied to soil and groundwater remediation by the regulation of heterogeneous interfacial assembly.

Dominant pathways of FeSx formation and evolution during sulfidation of zero-valent iron by different sulfur agents

Sulfidation of zero-valent iron (SZVI) is an effective method to improve the properties of ZVI for decontamination. Among various sulfidation methods, sulfur agent and sulfidation parameters play decisive roles in the sulfidation reactions. A correct understanding of the reactions to form FeSx is helpful to regulate the sulfidation. In this study, Na2S, Na2S2O3 and Na2S2O4 were used as sulfur agents to reveal the dominant pathways of FeSx formation. Results demonstrate that FeSx tends to form via the reaction of S2- and Fe2+ (S2- pathway) when Na2S and Na2S2O3 are used as sulfur agents, whereas it tends to occur via the reaction of S0 and Fe0 (S0 pathway) when Na2S2O4 is used. During the sulfidation, FeSx gradually grows and covers the surface, and then the FeSx layer flaks off locally. The sulfur agents can also affect the FeSx composition, that is, Na2S and Na2S2O3 tend to form FeS, while Na2S2O4 tends to form FeS2. Such FeSx composition can further affect the performance of SZVI to Cr(VI) removal due to their different conductivity. The study of the influence on SZVI properties with different sulfur agents is helpful to select sulfur agents for targeting design of SZVI.

Alcohothermal synthesis of sulfidated zero-valent iron for enhanced Cr(VI) removal

Sulfidation of zero-valent iron (ZVI) has attracted broad attention in recent years for improving the sequestration of contaminants from water. However, sulfidated ZVI (S-ZVI) is mostly synthesized in the aqueous phase, which usually causes the formation of a thick iron oxide layer on the ZVI surface and hinders the efficient electron transfer to the contaminants. In this study, an alcohothermal strategy was employed for S-ZVI synthesis by the one-step reaction of iron powder with elemental sulfur. It is found that ferrous sulfide (FeS) with high purity and fine crystallization was formed on the ZVI surface, which is extremely favorable for electron transfer. Cr(VI) removal experiments confirm that the rate constant of S-ZVI synthesized by the alcohothermal method was 267.1- and 5.4-fold higher than those of un-sulfidated ZVI and aqueous-phase synthesized S-ZVI, respectively. Systematic characterizations proved that Cr(VI) was reduced and co-precipitated on S-ZVI in the form of a Fe(III)/Cr(III)/Cr(VI) composite, suggesting its environmental benignancy.

Interaction of pyrite with zerovalent iron with superior reductive abilityviaFe(ii) regeneration

Sulfidation of zero-valent iron (SZVI) emerged as a promising approach to enhance both the reactivity and selectivity of nano zero-valent iron (ZVI).

Phase evolution of the surface iron (hydr)oxides to the iron sulfide through anion exchange during sulfidation of zero valent iron

The naturally-formed iron (hydr)oxides on the surface of zero valent iron (ZVI) have long been considered as passivation layer and inert phases which significantly reduce the reaction activities when they are employed in environmental remediation. Although it seems there are no direct benefits to keep these passivation layers, here, we show that such phases are necessary intermediates for the transformation to iron sulfides through an anion exchange pathway during sulfidation of ZVI. The pre-formed (hydr)oxides undergo a phase evolution upon aging and specific phases can be effectively trapped, which can be confirmed by a combination of different characterization techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRPD), and X-ray absorption near edge structure (XANES) spectroscopy. Interestingly, after sulfidation, the resultant samples originated from different (hydr)oxides demonstrate different activities in the Cr(VI) sequestration. The XANES investigation of Fe K edge and Fe L2,3 edge indicates Fe remains the same after sulfidation, suggesting a non-redox, anion exchange reaction pathway for the production of iron sulfides, where O2- anions are directly replaced with S2-. Consequently, the structural characteristics of the parent (hydr)oxides are inherited by the as-formed iron sulfides, which make them behave differently because of their different structural natures.

Reaching beyond the sulfided surface: Doping of copper cation to expand the pH tolerance range for SZVI

Sulfidation of zero-valent iron (SZVI) is an important means to greatly inhibit the formation of inert hydr/oxide and improve the overall reactivity in pollutant removal reactions, which is mainly benefiting from the formation of FeSx layer on surface of the zero-valent iron (ZVI). However, FeSx phase can act as reducing reagent in the treatment of pollutants, resulting in inevitable consumption. Considering that, copper cation was introduced to the FeSx phases to prepare copper-doped sulfides with improved durability. In this study, we successfully prepared copper-doped SZVI (Cu-SZVI). The composition and structure have been clearly confirmed by a combination of several techniques, including Raman spectroscopy and X-ray powder diffraction. Cu-SZVI exhibits a superior sequestration reaction to remove Cr(VI) with higher removal capacity of 82.93 mg/g, with the comparison of SZVI (57.54 mg/g) and ZVI (8.23 mg/g). Moreover, Cu-SZVI possesses better operational pH tolerance and longer lifetime of usage, which makes it particularly attractive to large-scale environmental applications. This work proposes a new way to modify SZVI by doping a secondary cation, which can be used to further explore new SZVI materials based on specific needs.

Characteristics, kinetics, thermodynamics and long-term effects of zerovalent iron/pyrite in remediation of Cr(VI)-contaminated soil

Development of efficient, green and low-cost natural mineral-based reductive materials is promising to remediation of hexavalent chromium(Cr(VI))-contaminated soil. Considering the synergetic effect between pyrite and zerovalent iron (ZVI), an activated pyrite supported ZVI(ZVI/FeS2) with high reducing activity was developed by ball milling activation of natural pyrite and sulfidation of ZVI. The remediation property of ZVI/FeS2 for Cr(VI)-contaminated soil was evaluated with different ZVI/FeS2 dosage, soil-water ratio, initial pH, time and temperature, as well as the stability of Cr. The results showed that ZVI/FeS2 possessed high reduction activity with soil Cr(VI) removal rate up to 99 % even under alkaline condition, and soil with different pH values eventually converged to neutral after 90 days, indicating that ZVI/FeS2 has a good self-regulating alkaline ability. The reduction process conformed to Langmuir-Hinshelwood first-order kinetics and was a spontaneous and endothermic process. The lower activation energy of 17.97 kJ mol−1 (usually 60–250 kJ mol−1) indicated that the reduction reaction of Cr(VI) was particularly easy to occur. The speciation change of Cr in soil within 30 days demonstrated that the Cr in the soil was converted from a readily migratable state to a more stable state, where the Fe–Mn oxide bound fraction reached 85.03 % due to the generation of Cr(III)/Fe(III) co-precipitation. The results of long-term stability experiments showed that the leaching concentrations of Cr(VI) and total Cr decreased significantly after the ZVI/FeS2 treatment and remained stable at very low levels for 180 days. This study provided a sustainable way to fully utilize natural pyrite minerals to obtain iron-bearing reductive materials for feasible, effective and long-term stable immobilization of Cr(VI) in soil.

Looking deeper: Decoding the core structure of a micron-sized S-ZVI particle

• The diffusion of sulfur to the core of the micron ZVI was confirmed for the first time. • The inner core composition of the micron ZVI was confirmed and quantified. • XRPD and XANES analysis pointed out that about 12.5% iron was transformed to FeS x . Although it has been widely accepted that the sulfidation of zero-valent iron (ZVI) represents a typical diffusionless process, which mainly happens on the surface of the ZVI particle and results in a FeS x @Fe core-shell structure, such an assumption has never been experimentally proved due to the difficulties in probing the interior domains of the large particles, especially those in micro size. In this work, we demonstrate that the sulfidation process not only takes place on the surface but also synchronizes throughout the entire body of the micron-sized ZVI, which was confirmed by the close inspection of the cross-section of a finely sliced ZVI particle. Coupled with the X-ray powder diffraction and X-ray absorption near-edge structure analysis, it is revealed that the whole S-ZVI particle consists of a mixed lattice of the dominant metallic iron (87.5%) and minor iron sulfides (12.5%), according to the established averaged oxidation state for iron. This work is the first to unambiguously reveal the compositional information of the micron-sized S-ZVI particles, especially the inner components that are typically difficult to reach. It clarified a long-time misunderstanding that the sulfidation process on ZVI is initiated from the surface and continuously undergoes a diffusion pathway, reaching to the center of the particle.

Sulfidation of Zero-Valent Iron by Direct Reaction with Elemental Sulfur in Water: Efficiencies, Mechanism, and Dechlorination of Trichloroethylene.

Sulfidation can enhance both the reactivity and selectivity (i.e., electron efficiency, εe) of zero-valent iron (ZVI) in contaminant removal, which may make this technology cost-effective for a wider range of water treatment applications. However, current sulfidation methods involve either hazardous or unstable sulfidation agents (e.g., Na2S, Na2S2O3, and Na2S2O4) or energy-intensive preparations (e.g., mechanochemical sulfidation with elemental sulfur). In this study, we demonstrate that very efficient sulfidation of microscale ZVI (mZVI) can be achieved at all S/Fe molar ratios (∼100% sulfidation efficiency, εs) simply by direct reaction between elemental sulfur (S0) and ZVI in an aqueous suspension at ambient temperature. In comparison, the εs values obtained using Na2S, Na2S2O3, or Na2S2O4 as the sulfidation agents were only ∼23, ∼75, and ∼38%, respectively. The sulfidated mZVI produced using the new method reacts with trichloroethylene (TCE) with very high rates and electron efficiencies: rate constants and electron efficiencies were 800- and 79-fold higher than those of the unsulfidated mZVI. The enhanced performance of this material, together with the operational advantages of S0 for sulfidation (including safety, stability, and cost), may make it a desirable product for full-scale engineering applications.

Activated peroxydisulfate by sulfidated zero-valent iron for enhanced organic micropollutants removal from water

Recently, sulfidation of zerovalent iron (S-ZVI) has gained increasing attention due to its merits to mediate the electron transfer from Fe0 core to reactants at the water-particle interface. However, few studies have attempted so far to use the unique properties of S-ZVI to activate peroxydisulfate (PDS) under both oxic and anoxic conditions. Hence, in this study, the performance of S-ZVI in activating PDS was systematically evaluated at various conditions. As expected, under oxic conditions, S-ZVI could enhance the acetaminophen (ACT) removal rate by 2.4–5.1 folds compared to unmodified ZVI, and this enhancing effect works well over the tested initial pH range of 3.0–8.5. Likewise, under anoxic conditions, S-ZVI still performed better than unmodified ZVI. But interestingly, both ZVI and S-ZVI achieved better PDS activation with the absence of O2, despite in which comparable or much less Fe0 were consumed. This may be ascribed to the multiple roles of O2 which could not only enhance the Fe0 to generate Fe2+ but also compete with PDS significantly for the generated Fe2+. Through quenching experiments and electron spin resonance spectra analysis, SO4•− and HO• were identified to be involved in Fe0/PDS process. Moreover, taking PMSO as a probe, Fe(IV) was also confirmed to be generated in both ZVI/PDS and S-ZVI/PDS systems. Taking four other microcontaminants and a real groundwater sample, the broad applicability of S-ZVI/PDS was further evaluated. All these results would favor the fundamental understanding of Fe0/PDS process and thus help to facilitate its application.

Influence of coexisting ions on the electron efficiency of sulfidated zerovalent iron toward Se(VI) removal

Recently, sulfidation of zerovalent iron (ZVI) has gained increasing attention due to its merits to enhance both the reactivity and electron efficiency (EE) of ZVI. While few studies have been conducted to elucidate the influence of coexisting ions on EE of sulfidated ZVI, which is important for up-scaling such a decontamination strategy. In this study, taking Se(VI) as a probe, the influence of coexisting ions, including Cl−, SO42−, PO43−, Ca2+, and Mg2+, on the removal capacity and EE of ZVI under aerobic conditions was investigated. Results revealed that, compared to unamended ZVI, sulfidated ZVI synthesized by ball-milling with elemental sulfur (S-ZVIbm) could enhance EE of ZVI toward Se(VI). However, this enhancing effect is not very effective and the maximum of EE value obtained in this work is only 8.8%. Chloride with concentration range of 1–20 mM was found to exert little influence on EE of ZVI. While for SO42− and PO43−, relative to 10 mM NaCl, their presence could depress EE of S-ZVIbm, and this inhibiting effect became more pronounced as their concentration elevated. In contrast, Ca2+ or Mg2+ hardness ions (0.05–2 mM) increased the EE values of S-ZVIbm by 19.2–83.3%. X-ray absorption fine structure analysis revealed that all of Fe0 in S-ZVIbm was exhausted within 24 h with lepidocrocite and magnetite being the primary corrosion products. This suggested that it was the amounts of electrons accepted by Se(VI) rather than that donated by S-ZVIbm accounted for the variation of EE. In general, these co-solutes may impose their influence on EE through affecting Se(VI) adsorption and the subsequent electron transfer from Fe0 core to Se(VI) at the water-particle interface.

Advances in Sulfidation of Zerovalent Iron for Water Decontamination.

Sulfidation has gained increasing interest in recent years for improving the sequestration of contaminants by zerovalent iron (ZVI). In view of the bright prospects of the sulfidated ZVI (S-ZVI), this review comprehensively summarized the latest developments in sulfidation of ZVI, particularly that of nanoscale ZVI (S-nZVI). The milestones in development of S-ZVI technology including its background, enlightenment, synthesis, characterization, water remediation and treatment, etc., are summarized. Under most circumstances, sulfidation can enhance the sequestration of various organic compounds and metal(loid)s by ZVI to various extents. In particular, the reactivity of S-ZVI toward contaminants is strongly dependent on S/Fe molar ratio, sulfidation method, and solution chemistry. Additionally, sulfidation can improve the selectivity of ZVI toward targeted contaminant over water under anaerobic conditions. The mechanisms of sulfidation-induced improvement in contaminants sequestration by ZVI are also summarized. Finally, this review identifies the current knowledge gaps and future research needs of S-ZVI for environmental application.