Molecular Structure and Sulfur Content Affect Reductive Dechlorination of Chlorinated Ethenes by Sulfidized Nanoscale Zerovalent Iron

Single-Atom Iron Can Steer Atomic Hydrogen toward Selective Reductive Dechlorination: Implications for Remediation of Chlorinated Solvents-Impacted Groundwater

Atomic hydrogen (H*) is a powerful and versatile reductant and has tremendous potential in the degradation of oxidized pollutants (e.g., chlorinated solvents). However, its application for groundwater remediation is hindered by the scavenging side reaction of H2 evolution. Herein, we report that a composite material (Fe0@Fe-N4-C), consisting of zerovalent iron (Fe0) nanoparticles and nitrogen-coordinated single-atom Fe (Fe-N4), can effectively steer H* toward reductive dechlorination of trichloroethylene (TCE), a common groundwater contaminant and primary risk driver at many hazardous waste sites. The Fe-N4 structure strengthens the bond between surface Fe atoms and H*, inhibiting H2 evolution. Nonetheless, H* is available for dechlorination, as the adsorption of TCE weakens this bond. Interestingly, H* also enhances electron delocalization and transfer between adsorbed TCE and surface Fe atoms, increasing the reactivity of adsorbed TCE with H*. Consequently, Fe0@Fe-N4-C exhibits high electron selectivity (up to 86%) toward dechlorination, as well as a high TCE degradation kinetic constant. This material is resilient against water matrix interferences, achieving long-lasting performance for effective TCE removal. These findings shed light on the utilization of H* for the in situ remediation of groundwater contaminated with chlorinated solvents, by rational design of earth-abundant metal-based single-atom catalysts.

Fast degradation of vinyl chloride by green rust and nitrogen-doped graphene

Carbonaceous materials catalyze reductive dechlorination of chlorinated ethylenes (CEs) by iron(II) materials providing a new approach for the remediation of CE polluted groundwater. While most CEs are reduced via β-elimination, vinyl chloride (VC), the most toxic and recalcitrant CE, degrades by hydrogenolysis. The significance of carbon catalysts for reduction of VC is well documented for iron(0) systems, but hardly investigated with iron(II) materials as reductants. In this study, a layered iron(II)‑iron(III) hydroxide sulfate (green rust) was used as reductant for VC, with an N-doped graphene (NG), prepared by co-pyrolysis of graphene and urea, as catalyst. VC (80 μM) was completely reduced to ethylene within 336 h in the presence of 5 g Fe/L GR and 5 g/L NG pyrolyzed at 950 °C, following pseudo-first-order kinetics with a rate constant of 0.017 h-1. Dosing experiments demonstrated that dechlorination of VC takes place on the NG phase. Monitoring of hydrogen formation, cyclic voltammetry, and quenching experiments demonstrated that atomic hydrogen contributes significantly to the dehalogenation reaction, where NG is critical for formation of atomic hydrogen. CE competition experiments demonstrated the presence of specific VC reduction sites with hydrogenolysis being unaffected by concurrent β-elimination reactions. The system exhibited excellent performance in natural groundwaters and in comparison with iron(0) systems. This study demonstrates that GR + NG is a promising system for remediation of VC contaminated groundwater, and the mechanistic part of the study can be used as a reference for subsequent studies.

Key role of hydrogen atoms in the preparation of sulfidated zero valent iron

Sulfidated zero valent iron (ZVI) is a popular material for the reductive degradation of halogenated organic pollutants. Simple and economic synthesis of this material is highly demanded. In this study, sulfidated micro/nanostructured ZVI (MNZVI) particles were prepared by simply heating MNZVI particles and sulfur elements (S0) in pure water (50℃). The iron oxides on the surface of MNZVI particles were conducive to sulfidation reaction, indicating the formation of iron-sulphide minerals (FeSx) on the surface of MNZVI particles might not be from the direct reaction of Fe0 with S0 (Fe0 and S0 acted as reductant and oxidant, respectively). As an important reductant, hydrogen atom (H•) can be generated from the reduction of H+ by MNZVI particles and participate in the formation of FeSx. Quenching experiment and cyclic voltammetry analysis proved the existence of H• on the surface of MNZVI particles. DFT calculation found that the potential barrier of H•/S0 and Fe0/S0 were 1.91 and 7.24 eV, respectively, indicating that S0 would preferentially react with H• instead of Fe0. The formed H• can quickly react with S0 to generate hydrogen sulfide (H2S), which can further react with iron oxides such as α-Fe2O3 on the surface of MNZVI particles to form FeSx. In addition, the H2 partial pressure in water significantly affected the amount of H• generated, thereby affecting the sulfidation efficiency. For TCE degradation, as the sulfur loading of sulfidated MNZVI particles increased, the contribution of H• significantly decreased while the contribution of direct electron transfer increased. This study provided new insights into the synthesis mechanism of sulfidated ZVI in water.

Insights into the contrasting effects of sulfidation on dechlorination of chlorinated aliphatic hydrocarbons by zero-valent iron

Contrasting effects of sulfidation on contaminants reduction by zero-valent iron (ZVI) has been reported in literature but the underlying mechanisms remain unclear. Here, under well-controlled conditions, we compared the performance of ZVI and sulfidated ZVI (S-ZVI) toward a series of chlorinated compounds. Results revealed that, although S-ZVI was more reactive than ZVI toward hexachloroethane, pentachloroethane, tetrachloroethylene, and trichloroethene, sulfidation hindered the dechlorination of the other ten tested chlorinated aliphatics by a factor of 1.5-125. Moreover, S-ZVI may lead to an accumulation of toxic partially-dechlorinated products. Analogous to its effects on ZVI reactivity, sulfidation also exerted positive, negligible, or negative effects on the electron efficiency of ZVI. Solvent kinetic isotope effect analysis suggested that direct electron transfer rather than reaction with atomic hydrogen was the dominant reduction mechanism in S-ZVI system. Hence, the sulfidation enhancing effects could be expected only when direct electron transfer is the preferred reduction route for target contaminants. Furthermore, linear free energy relationships analysis indicated one-electron reduction potential could be used to predict the transformation of chlorinated ethanes by S-ZVI, whereas for chlorinated ethenes, their adsorption properties on S-ZVI determined the dechlorination process. All these findings may offer guidance for the decision-making regarding the application of S-ZVI.

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

Unravelling the removal mechanisms of trivalent arsenic by sulfidated nanoscale zero-valent iron: The crucial role of reactive oxygen species and the multiple effects of citric acid

The remediation of arsenic-contaminated groundwater by sulfidated nanoscale zero-valent iron (S-nZVI) has raised considerable attention. However, the role of trivalent arsenic (As(III)) oxidation by S-nZVI in oxic conditions (S-nZVI/O2) remains controversial, and the comprehensive effect of citric acid (CA) prevalent in groundwater on As(III) removal by S-nZVI remains unclear. Herein, the mechanisms of reactive oxygen species (ROS) generation and multiple effects of CA on As(III) removal by S-nZVI/O2 were systematically explored. Results indicated that the removal efficiency of As(III) by S-nZVI/O2 (97.81 %) was prominently higher than that by S-nZVI (66.71 %), resulting from the significant production of ROS (mainly H2O2 and OH) under oxic conditions, which played a crucial role in promoting the As(III) oxidation. Additionally, CA had multiple effects on As(III) removal by S-nZVI/O2 system: (i) CA impeded the diffusion of As(III) towards S-nZVI and increased the secondary risk of immobilized As(III) re-releasing into the environment due to the Fe dissolution from S-nZVI; (ii) CA could significantly enhance the yields of OH from 25.29 to 133.00 μM via accelerating the redox cycle of Fe(II)/Fe(III) and increasing the oriented conversion rate of H2O2 to OH; (iii) CA could also enrich the types of ROS (such as O2- and 1O2) in favor of further As(III) oxidation. This study contributed novel findings regarding the control of As(III) contaminated groundwater using S-nZVI technologies.

Reductive Dechlorination of Chlorinated Ethenes at the Sulfidated Zero-Valent Iron Surface: A Mechanistic DFT Study

Sulfidated nano- and microscale zero-valent iron (S-(n)ZVI) has shown enhanced selectivity and reactive lifetime in the degradation of chlorinated ethenes (CEs) compared to pristine (n)ZVI. However, varying effects of sulfidation on the dechlorination rates of structurally similar CEs have been reported, with the underlying mechanisms remaining poorly understood. In this study, we investigated the β-dichloroelimination reactions of tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), and trans-1,2-dichloroethene (trans-DCE) at the S and Fe sites of several S-(n)ZVI surface models by using density functional theory. Dechlorination reactions were both kinetically and thermodynamically more favorable at Fe sites compared to S sites, indicating that maintaining the accessibility of reactive Fe sites is crucial for achieving high S-(n)ZVI reactivity with contaminants. At Fe sites adjacent to S atoms, the reactivity for CE dechlorination followed the order trans-DCE ≈ TCE > cis-DCE > PCE. PCE degradation was hindered at these sites due to the steric effects of S atoms. At the S sites, the energy barriers correlated with the CEs' energy of the lowest unoccupied molecular orbital in the order PCE < TCE < DCE isomers. Our findings reveal that the experimentally observed selectivity of S-(n)ZVI materials for individual CEs can be explained by an interplay of the varying reactivities of Fe and S sites in CE dechlorination reactions.

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.

Intrinsic Effects of Sulfidation on the Reactivity of Zero-Valent Iron With Trichloroethene: A DFT Study

Sulfidation represents a promising approach to enhance the selectivity and longevity of zero-valent iron (ZVI) in water treatment, particularly for nanoscale ZVI (nZVI). While previous mechanistic studies have primarily concentrated on the impact of sulfidation on the (n)ZVI hydrophobicity, the fundamental effects of sulfidation on the (n)ZVI reactivity with target contaminants remain poorly understood. Herein, we employed density functional theory to elucidate reaction mechanisms of trichloroethene (TCE) dechlorination at various (n)ZVI surface models, ranging from pristine Fe0 to regularly sulfidated Fe surfaces. Our findings indicate that sulfidation intrinsically hinders the TCE dechlorination by (n)ZVI, which aligns with prior observations of sulfur poisoning in transition metal catalysts. We further demonstrate that the positive effects of sulfidation emerge when the surface of (n)ZVI undergoes corrosion. Notably, S sites exhibit higher reactivity compared to the sites typically present on the surface of (n)ZVI oxidized in water. Additionally, S sites protect nearby Fe sites against oxidation and make them more selective for direct electron transfer. Overall, our results reveal that the reactivity of sulfidated (n)ZVI is governed by an interplay of intrinsic inhibitory effects and corrosion protection. A deeper understanding of these phenomena may provide new insights into the selectivity of sulfidated (n)ZVI for specific contaminants.

Zero-valent iron-peroxydisulfate as synergistic co-milling agents for enhanced mechanochemical destruction of 2,4-dichlorophenol: Coupling reduction with oxidation

Mechanochemical (MC) remediation with zero-valent iron (ZVI) as co-milling agent enables the non-combustion and solvent-free disposal of solid halogenated organic pollutants (HOPs) via solid-phase reaction, but suffers from incomplete dechlorination (especially for less chlorinated chemicals). Herein, a reduction-oxidation coupling strategy using ZVI and peroxydisulfate as synergistic (ZVI-PDS) co-milling agents was investigated, with 2,4-dichlorophenol (2,4-DCP) as probe contaminant. By revisiting the MC destruction process of 2,4-DCP by ZVI, the contribution of both reductive and oxidative routes is confirmed, and the inefficient •OH generation is addressed. With ball-to-material and reagent-to-pollutant mass ratios of 30:1 and 13:1, respectively, ZVI-PDS achieves higher dechlorination ratio (86.8%) for 2,4-DCP within 5 h, outcompeting sole ZVI (40.3%) or PDS (33.9%), due to the accumulation of numerous SO4•-. As suggested by a two-compartment kinetic model, the optimal ZVI/PDS molar ratio of 4:1 is determined, which balances the relative contribution of reductive/oxidative routes and leads to a maximum mineralization efficiency of 77.4%. The analysis on product distribution verifies the generation of dechlorinated, ring-opening and minor coupling products (with low acute toxicity). This work validates the necessity to couple reduction with oxidation in MC destruction for solid HOPs, and may provide information on reagent formulation.

Enhanced removal of cis-1,2-dichloroethene and vinyl chloride in groundwater using ball-milled sulfur- and biochar-modified zero-valent iron: From the laboratory to the field

Sulfidated zero-valent iron (ZVI) and biochar-supported ZVI have received increasing attention for their potential to dechlorinate trichloroethylene. However, minimal data are available regarding the combined effect of sulfur and biochar ZVI on trichloroethylene byproducts. The primary aim of the current study is to determine whether sulfur- and biochar-modified ZVI (ZVI-BC-S) enhances the removal of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) from groundwater. Results show that biochar and sulfur facilitated the milling of ZVI-BC-S into micro- and nanoscale particles and increased FeS formation. Moreover, the rates of cDCE and VC removal by ZVI-S increased by 30.1% and 30.2%, respectively, compared to those obtained with ZVI, owing to enhanced dechlorination via β-elimination by sulfur. Meanwhile, treatment with ZVI-BC-S harnessed the benefits of biochar and sulfur to enhance the cDCE and VC removal rates by 62.0% and 67.7%, respectively. Mechanistically, biochar enhanced the corrosion of ZVI-S to increase FeS production and enhance the electron transfer, β-elimination, and hydrogenolysis involved in cDCE and VC dechlorination. The effectiveness of ZVI-BC-S was confirmed in a field demonstration, during which cDCE and VC concentrations significantly decreased within 10 days following injection. The findings of this study can help inform the rational design of ZVI for in-situ remediation of chlorinated hydrocarbons in groundwater.

Preparation of Fe/Cu bimetals by ball milling iron powder and copper sulfate for trichloroethylene degradation: Combined effect of FeSx and Fe/Cu alloy

The addition of a secondary metal (such as Cu, Co, Ni and Pd) to form iron-based bimetallic particles could enhance the reactivity of zero valent iron (ZVI). This study proposed a new synthesis method for preparing Cu-Fe bimetals (Cu-Febm (CuSO4)) by ball milling mZVI and CuSO4. During ball-milling process, 40% of Cu2+ can be reduced to Cu0, which formed galvanic couple with Fe0 in a way of Fe/Cu alloy structure. Part Cu2+ was only reduced to Cu+ (corresponding to Cu2O), while 29% of SO42- was reduced to Sx2- (corresponding to FeSx). The appearance of Cu2O was not conducive to the activity of Cu-Febm (CuSO4) particles, the formation of Fe0/FeSx structure compensated for the partial loss of Fe/Cu alloy. H•abs was identified as the main active species for TCE degradation by Cu-Febm (CuSO4) bimetals. The Cu-Febm (CuSO4) bimetals has great potential for the removal of chlorinated hydrocarbons in water.

Ferrous ion enhanced Fenton-like degradation of emerging contaminants by sulfidated nanosized zero-valent iron with pH insensitivity

In this study, the performance and mechanism of the integrated sulfidated nanosized zero-valent iron and ferrous ions (S-nZVI/Fe2+) system for oxygen activation to remove emerging contaminants (ECs) were comprehensively explored. The S-nZVI/Fe2+ system exhibited a 2.4-8.2 times of increase in the pseudo-first order kinetic rate constant for the oxidative degradation of various ECs compared to the S-nZVI system under aerobic conditions, whereas negligible removal was observed in both nZVI and nZVI/Fe2+ systems. Moreover, remarkable EC mineralization efficiency and benign detoxification capacity were also demonstrated in the S-nZVI/Fe2+ system. We revealed that dosing Fe2+ promoted the corrosion of S-nZVI by maintaining an acidic solution pH, which was conducive to O2 activation by dissolved Fe2+ and surface-absorbed Fe(II) to produce •OH. Furthermore, the generation of H* was enhanced for the further reduction of Fe(III) and H2O2 to Fe(II) and •O2-, resulting in the improvement of consecutive single-electron O2 activation for •OH production. Additionally, bisphenol A (BPA) degradation by S-nZVI/Fe2+ was positively correlated with the S-nZVI dosage, with an optimum S/Fe molar ratio of 0.15. The Fenton-like degradation process by S-nZVI/Fe2+ was pH-insensitive, indicating its robust performance over a wide pH range. This study provides valuable insights for the practical implementation of nZVI-based technology in achieving high-efficiency removal of ECs from water.

Nitrogen amended graphene catalyses fast reduction of vinyl chloride by nano zerovalent iron

Vinyl chloride (VC) is a dominant carcinogenic residual in many aged chlorinated solvent plumes, and it remains a huge challenge to clean it up. Zerovalent iron (ZVI) is an effective reductant for many chlorinated compounds but shows low VC removal efficiency at field scale. Amendment of ZVI with a carbonaceous material may be used to both preconcentrate VC and facilitate redox reactions. In this study, nitrogen-doped graphene (NG) produced by a simple co-pyrolysis method using urea as nitrogen (N) source, was tested as a catalyst for VC reduction by nanoscale ZVI (nZVI). The extent of VC reduction to ethylene in the presence of 2 g/L of nZVI was less than 1% after 3 days, and barely improved with the addition of 4 g/L of graphene. In contrast, with amendment of nZVI with NG produced at pyrolysis temperature (PT) of 950 °C, the VC reduction extent increased more than 10-fold to 69%. The reactivity increased with NG PT increasing from 400 °C to an optimum at 950 °C, and it increased linearly with NG loadings. Interestingly, N dosage had little effect on reactivity if NG was produced at PT of 950 °C, while a positive correlation was observed for NG produced at PT of 600 °C. XPS and Raman analyses revealed that for NG produced at lower PT (<800 °C) mainly the content of pyridine-N-oxide (PNO) groups correlates with reactivity, while for NG produced at higher PT up to 950 °C, reactivity correlates mainly with N induced structural defects in graphene. The results of quenching and hydrogen yield experiments indicated that NG promote reduction of VC by storage of atomic hydrogen, thus increasing its availability for VC reduction, while likely also enabling electron transfer from nZVI to VC. Overall, these findings demonstrate effective chemical reduction of VC by a nZVI-NG composite, and they give insights into the effects of N doping on redox reactivity and hydrogen storage potential of carbonaceous materials.

Tetrabromobisphenol A transformation by biochar supported post-sulfidated nanoscale zero-valent iron: Mechanistic insights from shell control and solvent kinetic isotope effects

Post-sulfidated nanoscale zero-valent iron with a controlled FeS_X shell thickness deposited on biochar (S-nZVI/BC) was synthesized to degrade tetrabromobisphenol A (TBBPA). Detailed characterizations revealed that the increasing sulfidation degree altered shell thickness/morphology, S content/speciation/distribution, hydrophobicity, and electron transfer capacity. Meanwhile, the BC improved electron transfer capacity and hydrophobicity and inhibited the surface oxidation of S-nZVI. These properties endowed S-nZVI/BC with highly reactive (∼8.9-13.2 times) and selective (∼58.4-228.9 times) over nZVI/BC in TBBPA transformation. BC modification improved the reactivity and selectivity of S-nZVI by 1.77 and 1.96 times, respectively. The difference of S-nZVI/BC in reactivity was related to hydrophobicity and electron transfer, particularly FeS_X shell thickness and morphology. Optimal shell thickness of ∼32 nm allowed the maximum association between Fe⁰ core and exterior FeS_X, resulting in superior reactivity. A thicker shell with abundant networks increased the roughness but decreased the surface area and electron transfer. The higher [S/Fe]_surface and [S/Fe]_particle were conducive to the selectivity, and [S/Fe]_particle was more influential than [S/Fe]_surface on selectivity upon similar hydrophobicity. The solvent kinetic isotope effects (SKIEs) exhibited that increasing [S/Fe]_dose tuned the relative contributions of atomic H and electron in TBBPA debromination but failed to alter the dominant debromination pathway (i.e., direct electron transfer) in (S)-nZVI/BC systems. Mechanism of electron transfer rather than atomic H contributed to higher selectivity. This work demonstrated that S-nZVI/BC was a prospective material for the remediation of TBBPA-contaminated groundwater.

Cations facilitate sulfidation of zero-valent iron by elemental sulfur: Mechanism and dechlorination application

The solid-solid reaction of microscale zero-valent iron (mZVI) with elemental sulfur (S0) in water can form sulfidated mZVI (S-mZVI) with high reactivity and selectivity. However, the inherent passivation layer of mZVI hinders the sulfidation. In this study, we demonstrate that ionic solutions of Me-chloride (Me: Mg2+, Ca2+, K+, Na+ and Fe2+) can accelerate the sulfidation of mZVI by S0. The S0 with S/Fe molar ratio of 0.1 was fully reacted with mZVI in all solutions to form unevenly distributed FeS species on S-mZVIs as confirmed by SEM-EDX and XANES characterization. The cations depassivated the mZVI surface by driving the proton release from the surface site (FeOH) and resulting in localized acidification. The probe reaction test (tetrachloride dechlorination) and open circuit potential (EOCP) measurement demonstrated that Mg2+ was most efficient in depassivating the mZVI and therefore promoting sulfidation. The decrease of surface proton for hydrogenolysis on the S-mZVI synthesized in MgCl2 solution also inhibited the formation of cis-1,2-dichloroethylene by 14-79% compared to other S-mZVIs during trichloroethylene dechlorination. In addition, the synthesized S-mZVIs exhibited the highest reduction capacity reported so far. These findings provide a theoretical basis for the facile on-site sulfidation of mZVI by S0 with cation-rich natural waters for sustainable remediation of contaminated sites.

The influence of various microplastics on PBDEs contaminated soil remediation by nZVI and sulfide-nZVI: Impedance, electron-accepting/-donating capacity and aging

The microplastics (MPs) existed in the environment widely has resulted in novel thinking about in-situ remediation techniques, such as nano-zero-valent iron (nZVI) and sulfided nZVI (S-nZVI), which were often compromised by various environmental factors. In this study, three common MPs such as polyvinyl chloride (PVC), polystyrene (PS), and polypropylene (PP) in soil were found to inhibit the degradation rate of decabromodiphenyl ether (BDE209) by nZVI and S-nZVI to different degrees due to MPs inhibiting of electron transfer which is the main way to degrade BDE209. The inhibition strength was related to its impedance (Z) and electron-accepting (EAC)/-donating capacity (EDC). Based on the explanation of the inhibition mechanism, the reason for different aging degrees of nZVI and S-nZVI in different MPs was illustrated, especially in PVC systems. Furthermore, the aging of reacted MPs, functionalization and fragmentation in particular, indicated that they were involved in the degradation process. Moreover, this work provided new insights into the field application of nZVI-based materials for removing persistent organic pollutants (POPs).

Functional materials contributing to the removal of chlorinated hydrocarbons from soil and groundwater: Classification and intrinsic chemical-biological removal mechanisms

Chlorinated hydrocarbons (CHs) are the main contaminants in soil and groundwater and have posed great challenge on the remediation of soil and ground water. Different remediation materials have been developed to deal with the environmental problems caused by CHs. Remediation materials can be classified into three main categories according to the corresponding technologies: adsorption materials, chemical reduction materials and bioaugmentation materials. In this paper, the classification and preparation of the three materials are briefly described in terms of synthesis and properties according to the different types. Then, a detailed review of the remediation mechanisms and applications of the different materials in soil and groundwater remediation is presented in relation to the various properties of the materials and the different challenges encountered in laboratory research or in the environmental application. The removal trends in different environments were found to be largely similar, which means that composite materials tend to be more effective in removing CHs in actual remediation. For instance, adsorbents were found to be effective when combined with other materials, due to the ability to take advantage of the respective strengths of both materials. The rapid removal of CHs while minimizing the impact of CHs on another material and the material itself on the environment. Finally, suggestions for the next research directions are given in conjunction with this paper.

Reductive dechlorination of chlorinated ethenes by ball milled and mechanochemically sulfidated microscale zero valent iron: A comparative study

Ball milling is an effective technique to not only activate and reduce the size of commercial microscale zero valent iron (mZVI) but also to mechanochemically sulfidate mZVI. Yet, little is known about the difference between how chlorinated ethenes (CEs) interact with ball milled mZVI (mZVI^bm) and mechanochemically sulfidated mZVI (S-mZVI^bm). We show that simple ball milling exposed the active Fe⁰ sites, while mechanochemical sulfidation diminished Fe⁰ sites and meanwhile increased S^2- sites. Mechanochemical sulfidation with [S/Fe]_dosed increased from 0 to 0.20 promoted the particle reactivity most for TCE dechlorination (∼14-fold), followed by PCE and 1,1-DCE while it diminished the reactivity for trans-DCE (∼0.4-fold), cis-DCE (∼0.02-fold) and VC (∼0.002-fold) compared to simple ball milling. Sulfidation also improved the electron efficiency of CE dechlorination, except for cis-DCE and VC. The k_SA of cis-DCE, VC and trans-DCE dechlorination positively correlated with surface Fe⁰ content, suggesting their dechlorination was mainly mediated by Fe⁰ site or reactive atomic hydrogen. The k_SA of TCE dechlorination positively correlated with surface S^2- content and the dechlorination mainly occurred on S^2- sites via direct electron transfer. Increased sulfidation favored direct electron transfer mechanism. The k_SA of PCE and 1,1-DCE was not dependent on either parameter and their dechlorination was equally achieved through either mechanism.

Adsorption-reduction coupling mechanism and reductive species during efficient florfenicol removal by modified biochar supported sulfidized nanoscale zerovalent iron

Sulfidized nanoscale zerovalent iron (S-nZVI) was a promising material for degrading halogenated contaminants, but the easy aggregation limits its application for in-situ groundwater remediation. Hence, S-nZVI was decorated onto modified biochar (mBC) to obtain better dispersity and reactivity with florfenicol (FF), a widely used antibiotic. Uniform dispersion of S-nZVI particles were achieved on the mBC with plentiful oxygen-containing functional groups and negative surface charge. Thus, the removal rate of FF by S-nZVI@mBC was 2.5 and 3.1 times higher than that by S-nZVI and S-nZVI@BC, respectively. Adsorption and dechlorination of FF showed synergistic effect under appropriate mBC addition (e.g., C/Fe mass ratio = 1:3, 1:1), probably due to the enrichment of FF facilitates its reduction. In contrast, the contact between FF and S-nZVI could be hindered under more mBC addition, significantly decrease the reduction rate of FF and the reduction capacity of per unit Fe⁰. In addition, sulfur dose altered the surface species of surface Fe and S, and removal rates of FF correlated well with surface reductive species, i.e., FeS (r = 0.90, p < 0.05) and Fe⁰ (r = 0.98, p < 0.01). These mechanistic insights indicate the importance of rational design for biochar supported S-nZVI, which can lead to more efficient FF degradation.