Recent Advances in Sulfidated Zerovalent Iron for Contaminant Transformation

Uranium immobilization via sulfur-modified Fe0 nanoparticles: U(VI) trapping kinetics and long-term stability evaluation

The modification of nanoscale zero-valent iron (nZVI) by loading or incorporating sulfur into the iron crystal lattice can augment their efficacy in the removal of hydrophobic contaminants from wastewater. Nevertheless, the reactivity of sulfur-embedded nZVI (SnZVI) in immobilizing hydrophilic uranyl ions and the long-term stability of the sequestered uranium has received little attention. This study employed Na2S2O4 to modify the nZVI with different S/Fe molar ratios (0.1 and 0.3), following one-step and two-step approaches to create SnZVI-1 and SnZVI-2, respectively. Both experimental and theoretical calculation results revealed that the U(VI) ions exhibited low affinity for the surface of SnZVI. Additionally, the hindered electron transfer between the electron donors of SnZVI and U(VI) led to a diminished U(VI) reduction efficiency for SnZVI-1 (50.71 %∼67.74 %) and SnZVI-2 (68.03 %∼86.89 %), inferior to that of nZVI (78.63 %∼90.78 %). Consequently, the uranium detachment ratios of SnZVI (0.04 %∼0.85 %) during the 210-day stability assessment were higher compared to those of nZVI (0.04 %∼0.34 %). Hence, this study offered novel insights into how sulfur affected the adsorptive and redox properties of nZVI for U(VI) immobilization through solid and aqueous samples analyses, complemented by theoretical calculations. The findings are instrumental in designing SnZVI for effective and environmentally sound treatment of uranium-contaminated radioactive wastewater.

Sulfidated zero-valent iron bimetals for passive remediation of chlorinated vapors in the subsurface

This study explores a novel application of sulfidated zero-valent iron (S-ZVI) bimetals for the treatment of chlorinated solvents in the vapor phase. The potential of these reactive materials was investigated through batch, column, and modeling tests. The materials were produced by disc milling of ZVI, sulfur (S), copper (Cu), and nickel (Ni) with molar ratios of 0.05 and 0.2. The reactivity of the materials was assessed through vapor degradation batch tests conducted under partially saturated conditions using trichloroethylene (TCE) as a model compound. Sulfidated materials with a 0.05 S/ZVI molar ratio were the most reactive, achieving up to 99 % degradation of TCE vapors within 18 h and first-order degradation constants of 5-5.7 d-1. Compared to the non-sulfidated materials, sulfidated ones remained reactive even after aging by exposure to air for 30 days. In all tests, C3-C6 hydrocarbons were detected as main byproducts, indicating β-elimination as the dominant TCE degradation pathway, with minor dichloroethylene and vinyl chloride amounts from the hydrogenolysis pathway. To evaluate the use of sulfidated bimetals as Horizontal Permeable Reactive Barriers (HPRBs) for treating chlorinated vapors in the subsurface, TCE diffusion column tests were performed using a 5 cm thick reactive layer of S-ZVI-Ni. These tests demonstrated up to 70 % degradation over 25 days. By integrating the column test results into an analytical model, it was estimated that an 18 cm HPRB could ensure up to 99 % degradation of TCE vapors. These findings highlight the potential of S-ZVI bimetals as an effective passive mitigation system for reducing chlorinated solvent vapor emissions from the subsurface.

Integrated application of nanoscale zero-valent iron for sulfide and methane control in sewers and improved wastewater treatment

Sewer systems are critical water infrastructures for sewage collection and transportation services but are frequently challenged by odour nuisance, corrosion and greenhouse gas emissions, primarily driven by sulfide and methane production. This study investigated the effectiveness of multifunctional nanoscale zero-valent iron (nZVI) in controlling sulfide and methane, along with its downstream impacts on wastewater treatment. Two continuous flow laboratory-scale reactor systems were used: sewer reactors and sequencing batch reactors (SBRs). Intermittent doses of 50 mg Fe/L of nZVI were introduced daily for a 6-h cycle in the experimental sewer reactors. Results indicated reduced sulfide (by 8.5±0.5 mg S/L during dosing; 4.2±0.6 mg S/L off-dosing) and methane (by 16.6±1.9 mg COD/L during dosing; 12.6±1.3 mg COD/L off-dosing) concentrations compared to the control. This reduction involved sulfide removal (0.12±0.01 g S/g Fe or 0.20±0.02 mol S/mol Fe) and the inhibition of microbial sulfate-reducing and methanogenic activities. Sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) activities exhibited dynamic inhibition with long-term nZVI addition (SRB: 58 % after the first dose, 21 % after 3 months; MA: 27 % to 39 %). Additionally, the sewer-dosed nZVI improved downstream phosphorus removal (0.42±0.04 mg P/mg Fe or 0.76±0.07 mol P/mol Fe) and enhanced sludge settleability and dewaterability. These findings highlight the potential of intermittent nZVI dosing for effective sulfide and methane control in sewers while delivering downstream benefits for integrated urban wastewater management.

Mechanochemical Molten-Salt-Assisted Surface Nitridation Promotes Electron Transfer Dechlorination of Zerovalent Iron

The widespread application of zerovalent iron (ZVI) in water treatment and remediation has motivated research into modifications of the material that result in improved performance. The newest and most promising involves nitridation, but available methods for nitridation are not environmentally sustainable processes. Here, we demonstrate that mechanochemical molten-salt-assisted synthesis can be an effective method to produce surface-nitridated ZVI (sN-ZVI). This modification promoted the dechlorination rate of ZVI by up to 82-fold toward chlorinated ethenes and chloroform (CF). Enhanced conductivity of the nitridated shell boosts outward electron transfer from the iron core, while the exposed iron nitrides are efficient in activating the C-Cl bond of both trichloroethene and chloroform and facilitate electron-transfer-mediated C-Cl cleavage with an energy barrier of 0.20 eV, lower than that of iron oxides (0.78 eV). More importantly, sN-ZVI minimizes side hydrogen evolution and promotes deep dechlorination, thus preventing the formation of any toxic chlorinated intermediates, even in the case of highly chlorinated carbons such as tetrachloroethene and chloroform. sN-ZVI maintains its dechlorination performance even after 100 days of water aging, highlighting its great potential for field remediation applications.

Co-incorporation of lattice S and P into nano zero-valent iron induces multiple Kirkendall effects for enhanced trichloroethylene reduction efficiently

Conventional zero-valent iron (ZVI) materials are limited by the constraints of reactivity-selectivity-stability trade-offs, so designing multi-heteroatom co-modified ZVI with synergistic effects is gradually gaining popularity. Herein, we developed a novel co-modified nZVI by simultaneously doping sulfur (S) and phosphorus (P) heteroatoms into nZVI using the one-step liquid-phase reduction method. In this case, the faster diffusion rate of core iron atoms compared to shell components triggeres multiple Kirkendall effects, causing the inward diffusion of vacancies with further coalescing into radial nanocracks. Regarding the reactivity and selectivity, sulfidation and phosphorylation co-modified nZVI exhibited the best performance, with a trichloroethylene (TCE) dechlorination rate (kobs,TCE) of 0.65 h-1 and an electron efficiency (εe) of 14.5 %, which are 20.9 and 13.8 times higher than those of unmodified nZVI. A series of characterizations and electrochemical analyses indicated that S and P doping significantly altered the physicochemical properties of the core and shell layers, generating distinctive "lemon slice" nanocracks that could be used as electron transport channels, and FeSX significantly reduced the availability of hydrogen evolution reaction (HER) active surface sites and attenuates the passivation of nZVI. In addition, the co-modified S/P-nZVI exhibited excellent stability in different groundwater conditions, indicating its strong potential for application.

Integrative Lattice and Surface Engineering of Nanoscale Fe0 for Superior Dechlorination of Trichloroethene in Groundwater: Coordination in Reactivity, Selectivity, and Stability

Nanoscale zero-valent iron (nFe0) materials hold great promise in environmental remediation, yet achieving high reactivity, selectivity, and stability in reduction remains a long-standing challenge. Here we address this challenge by employing Ni lattice and FeS surface engineering to fabricate novel nFe0-based nanomaterials (dubbed as FeNix@FeSy), featuring FeNi as the core and FeS as the shell. The FeNi5@FeS10 delivered approximately 242.7- and 81.2-times higher reactivity and selectivity, respectively, over unmodified nFe° for the remediation of trichloroethene (TCE; a notorious environmental pollutant), while maintaining high stability in groundwater remediation. We found that the core composition (i.e., Ni/Fe ratio) of FeNix@FeSy primarily determined reactivity, governed by a tradeoff between the galvanic effect and lattice strain, while shell properties mainly controlled selectivity, despite some interactions between them. Density functional theory (DFT) calculations revealed that the FeS surface served as a favorable adsorption site for TCE, and the low energy barriers (TS2, 0.19 eV) of FeNi5@FeS10 facilitated the cleavage of the first chlorine from TCE. Moreover, the core-shell structure promoted electron transfer from the core to the shell and TCE. This integrative lattice and surface engineering strategy provides a new avenue for designing advanced functional materials for environmental remediation and beyond.

A green sulfidated micro zero-valent iron based-hydrogel for the synergistic removal of heavy metal cations and anions in groundwater

Heavy metal cations and anions contaminated groundwater was a big challenge to water resource safety. Herein, a green sulfidated micro zero-valent iron-based hydrogel (SA-S-mZVI) was synthesized using sodium alginate biomass for the simultaneous removal of heavy metal cations (Cu(II), Pb(II), Cd(II)) and anions (Cr(VI)). The sulfur modification and incorporation of sodium alginate hydrogel facilitated the efficient and sustainable removal of both single and multi-heavy metals. The co-existing heavy metal cations benefited the removal of Cr(VI), and heavy metals were mostly transformed into stable precipitates. The presence of organic substance and ions slightly affected the removal of heavy metals. Long-term column experiments (240 days) showed that SA-S-mZVI maintained over 99.9 % removal efficiency for heavy metal cations and anions, without adverse impacts on the groundwater environment. This study provided new insights into the development of eco-friendly, long-lasting zero-valent iron-based hydrogels for in-situ remediation of heavy metals-contaminated groundwater.

Sulfophilic metal ions in groundwater induce particle structure and dechlorination efficiency change of sulfidated zero-valent iron

Sulfidated microscale zero-valent iron (S-mZVI) is a promising ZVI material for remediation of chlorinated hydrocarbons (CHCs). However, the structure and dechlorination behavior change of S-mZVI induced by sulfophilic metal (Me) ions in groundwater are barely studied. Here we show that Me ion-amended S-mZVI (S-mZVIMe) have a rate sequence of S-mZVICo>S-mZVINi>S-mZVI>S-mZVICu>S-mZVICd≈S-mZVIZn and S-mZVINi>S-mZVICd>S-mZVI≈S-mZVIZn≈S-mZVICu≈S-mZVICo for trichlorethylene (TCE) dechlorination and hydrogen evolution reaction (HER), respectively. This results in the highest ever reported electron efficiency (98.6 %) for TCE dechlorination by S-mZVICo. Cross-section SEM-EDS, XRD, and XPS analyses confirm the formation of MeSx on the surface of all S-mZVIMe. Additionally, Ni0, Cu0, and possibly Cd° formed on the S-mZVINi, S-mZVICu, and S-mZVICd, respectively. Theoretical calculations indicate that the nascent metal sulfides are more hydrophobic than FeS, indicating the faster HER with Ni and Cd amendment is likely due to formation of bimetallic structures. Correlation analyses suggest that both low band gap and high work function of the semi-conductive Co sulfide contribute to the high reactivity of S-mZVICo. Column studies further show that implementing Co2+ enables the dechlorination of TCE from 2000 µg/L to <70 µg/L up to 1000 pore volumes by S-mZVI, compared to >1.2 mg/L without Co2+. These findings have important implications for remediation of CHC-contaminated sites using S-mZVI.

An azide-based sampler for monitoring abiotic reduction of chlorinated solvent contaminants in groundwater

There is significant interest in monitoring abiotic decomposition of chlorinated solvents at contaminated sites due to large uncertainties regarding the rates of abiotic attenuation of trichloroethylene (TCE) and perchloroethylene (PCE) under field conditions. In this study, an innovative passive sampling tool was developed to quantify acetylene, a characteristic product of abiotic reduction of TCE or PCE, in groundwater. The sampling mechanism is based on the highly specific and facile click reaction between acetylene and an azide compound to form a biologically and chemically stable triazole product. In preliminary investigations using static solutions of acetylene, a strong linear relationship was observed between the quantity of azide formed and the concentration of aqueous acetylene. The product yield was stable within a solution pH range of 5-9 and in the presence of common groundwater salts or gaseous hydrocarbon molecules. Based on these results, a prototypical acetylene passive sampler was constructed, which featured a piece of PDMS membrane embedded with an azide compound and Cu(0) catalyst to form a reactive phase for acetylene capture. The sampler was evaluated in microcosm reactors containing TCE and an abiotic reductant (viz., sulfur-treated iron powder). Acetylene produced during abiotic dechlorination of TCE was effectively captured by the sampler, and the quantity of the product recovered from the sampler correlates with the amount of TCE converted. The specificity and sensitivity of the proposed acetylene sampler proffers a new diagnostic tool for monitoring abiotic degradation of chlorinated ethenes at complex contamination sites.

Sulfidation of Magnetite for Superior Dechlorination of Trichloroethene

The reported contributions of magnetite to the abiotic natural attenuation of chlorinated ethenes have generated interest in its potential for soil and groundwater remediation. In this study, we investigated the impact of the two-step sulfidation method on the physicochemical properties and reactivity of magnetite with trichloroethene (TCE). We systematically evaluated the effect of different sulfur precursors (dithionite, thiosulfate, and sulfide) and sulfur-to-iron ([S/Fe]dosed) molar ratios on the reactivity. Results were compared to those of sulfidated nZVI (S-nZVI) as a benchmark for assessing the efficacy of sulfidated magnetite (S-Fe3O4). The findings indicated limited reactivity of magnetite when sulfidated with dithionite and thiosulfate. However, sulfidation with sulfide yielded reaction rates comparable to those of S-nZVI, particularly at lower [S/Fe]dosed ratios. At higher [S/Fe]dosed ratios (>0.1), sulfide-sulfidated magnetite (S-Fe3O4_S) exhibited reaction rates surpassing those of S-nZVI, with the major dechlorination product being acetylene. Nonetheless, reusability experiments demonstrated that the performance of S-Fe3O4 diminished with aging. These results show that S-Fe3O4_S achieved complete transformation of TCE to acetylene, with reaction rates comparable to S-nZVI. Given its lower cost of production, engineered S-Fe3O4_S remediation systems could serve as a more affordable alternative for in situ chemical reduction of TCE with further research and development.

Unveiling trends in migration of iron-based nanoparticles in saturated porous media

Nanoscale zero-valent iron (nZVI) particles are routinely used for environmental remediation, but their transport dynamics in different settings remain unclear, hindering optimization. This study introduces a novel approach to predicting nZVI transport in saturated porous model environment. The method employs advanced long column devices for real-time monitoring via controlled magnetic susceptibility measurements. Numerical modeling with a modified version of the MNMs 2023 software was then used to predict nZVI and its derivatives mobility in field-like conditions, offering insights into the radius of influence (ROI) and shape factor (SF) of their distribution. A standard nZVI precursor was compared with its four major commercial derivatives: nitrided, polyacrylic acid-coated, oxide-passivated, and sulfidated nZVI. All these iron-based nanoparticles exhibited identical particle sizes, morphologies, surface areas, and phase compositions, isolating surface properties, dominated by charge, as the sole variable affecting their mobility. The study revealed optimal transport when the surface charge of nZVI and its derivatives was strongly negative, while rapid aggregation of nZVI derivatives due magnetic attraction reduced their mobility. Modeling predictions based on column scale-up, indicated that detectable concentrations of 20 g L⁻1 were found at distances ranging from 0.4 to 1.1 m from the injection well. Slightly sulfidated nZVI traveled farther than the nZVI precursor and ensured more homogenous particle distribution around the well. Organically modified nZVI migrated the longest distances but showed particle accumulation close to the injection point. The findings suggest that minimal sulfidation combined with organic modification of nZVI surfaces may effectively enhance radial and vertical nZVI distribution in aquifers. Such improvements increase the commercial viability of modified nZVI, reduce their adverse impacts, and boosts their practical applications in real-world scenarios.

Recent Progress in Molecular Oxygen Activation by Iron-Based Materials: Prospects for Nano-Enabled In Situ Remediation of Organic-Contaminated Sites

In situ chemical oxidation (ISCO) is commonly used for the remediation of contaminated sites, and molecular oxygen (O2) after activation by aquifer constituents and artificial remediation agents has displayed potential for efficient and selective removal of soil and groundwater contaminants via ISCO. In particular, Fe-based materials are actively investigated for O2 activation due to their prominent catalytic performance, wide availability, and environmental compatibility. This review provides a timely overview on O2 activation by Fe-based materials (including zero-valent iron-based materials, iron sulfides, iron (oxyhydr)oxides, and Fe-containing clay minerals) for degradation of organic pollutants. The mechanisms of O2 activation are systematically summarized, including the electron transfer pathways, reactive oxygen species formation, and the transformation of the materials during O2 activation, highlighting the effects of the coordination state of Fe atoms on the capability of the materials to activate O2. In addition, the key factors influencing the O2 activation process are analyzed, particularly the effects of organic ligands. This review deepens our understanding of the mechanisms of O2 activation by Fe-based materials and provides further insights into the application of this process for in situ remediation of organic-contaminated sites.

Enhanced breakage of the aggregates of nanoscale zero-valent iron via ball milling

Aggregates of nanoscale zero-valent iron (nZVI) are commonly encountered for nZVI in aqueous solution, particularly during large-scale nZVI applications where nZVI is often in a highly concentrated slurry, and such aggregates lower nZVI mobility during its in-situ remediation applications. Herein, we report that the ball milling is an effective tool to break the nZVI aggregates and thereby improve the nZVI mobility. Results show that the milling (in just five minutes) can break the aggregates of a few tens of microns to less than one micron, which is one-tenth of the size that is acquired via the breakage using the mechanical mixing and ultrasonication. The milling breakage can also improve the efficacy of the chemical conditioning method that is commonly used for the nanoparticle stabilization and dispersion. The milling breakage is further optimized via a study of the milling operational factors including milling time, bead velocity, bead diameter, and chamber porosity, and an empirical equation is proposed combining the bead collision number during the milling. Mechanistic study shows that the high efficacy of the milling to break the aggregates can be explained by the small eddy created by the high shear rate produced by the close contact of the milling beads and may also relate to the direct mechanical pulverization effect. This study provides a high efficacy physical method to break the nanoparticle aggregates. The method can be used to improve the nZVI mobility performance by milling the nZVI slurry before its injection for in-situ remediation, and the milling may also replace the mechanical mixing during the nZVI stabilization via surface modification.

Quantifying remediation of chlorinated volatile compounds by sulfidated nano zerovalent iron treatment using numerical modeling and CSIA

Sulfidated nanoscale zerovalent iron (S-nZVI) has demonstrated promising reactivity and longevity for remediating chlorinated volatile compounds (cVOC) contaminants in laboratory tests. However, its effectiveness in field applications remains inadequately evaluated. This study provides the first quantitative evaluation of the long-term effectiveness of carboxymethyl cellulose-stabilized S-nZVI (CMC-S-nZVI) at a cVOC-contaminated field site. A reactive transport model-based numerical approach delineates the change in cVOC concentrations and carbon isotope values (i.e., δ13C from compound-specific stable isotope analysis (CSIA)) caused by dissolution of dense non-aqueous phase liquid, sorption, and pathway-specific degradation and production, respectively. This delineation reveals quantitative insights into remediation effectiveness typically difficult to obtain, including extent of degradation, contributions of different degradation pathways, and degradation rate coefficients. Significantly, even a year after CMC-S-nZVI application, degradation remains an important process effectively removing various cVOC contaminants (i.e., chlorinated ethenes, 1,2-dichloroethanes, and chlorinated methanes) at an extent varying from 5 %-62 %. Although the impacts of CMC-S-nZVI abundance on degradation vary for different cVOC and for different sampling locations at the site, for the primary site contaminants of tetrachloroethene and trichloroethene, their predominance of dichloroelimination pathway (≥ 88 %), high degradation rate coefficient (0.4-1.7 d-1), and occurrence at locations with relatively high CMC-S-nZVI abundance strongly indicate the effectiveness of abiotic remediation. These quantitative assessments support that CMC-S-nZVI supports sustainable ZVI-based remediation. Further, the novel numerical approach presented in this study provides a powerful tool for quantitative cVOC remediation assessments at complex field sites where multiple processes co-occur to control both concentration and CSIA data.

Trade-off between sulfidated zero-valent iron reactivity and air stability: Regulation of iron sulfides by ammonium dihydrogen phosphate

The reactivity and stability of zero-valent iron (ZVI) and sulfidated zero-valent iron (S-ZVI) are inherently contradictory. Iron sulfides (FeSX) on the S-ZVI surface play multiple roles, including electrostatic adsorption and catalyzing reduction. We proposed to balance the reactivity and air stability of S-ZVI by regulating FeSX. Benefiting from the superior coordination and accelerate electron transport capabilities of phosphate, herein, eco-friendly ammonium dihydrogen phosphate (ADP) was employed to synthesize N, P, and S-incorporated ZVI (NPS-ZVI) and regulate the FeSX. Raman, FTIR, XPS, and density functional theory (DFT) calculations were combined to reveal that HPO42- acts as the main P species on the Fe surface. The superior reactivity of NPS-ZVI was quantified by kobs, kSA, and kM of Cr(VI), which were 210.77, 27.44, and 211.17-fold than ZVI, respectively. NPS-ZVI demonstrated excellent reusability, with no risk of secondary pollution. Critically, NPS-ZVI could effectively maintain FeSX stability under the combination of diffusion limitation and surface protection mechanisms of ADP. The superior reactivity of NPS-ZVI was attributed to the fact that ADP maintains FeSX stability and accelerates electron transport. This study provides a novel strategy in balancing the reactivity and air stability of S-ZVI and offers theoretical support for material modification.

Sulfidation of Nanoscale Zero-Valent Iron by Sulfide: The Dynamic Process, Mechanism, and Role of Ferrous Iron

Sulfidation of nanoscale zerovalent iron (nZVI) can enhance particle performance. However, the underlying mechanisms of nZVI sulfidation are poorly known. We studied the effects of Fe2+ on 24-h dynamics of nZVI sulfidation by HS- using a dosed S to Fe molar ratio of 0.2. This shows that in the absence of Fe2+, HS- rapidly adsorbed onto nZVI particles and reacted with surface iron oxide to form mackinawite and greigite (<0.5 h). As nZVI corrosion progressed, amorphous FeSx in solution deposited on nZVI, forming S-nZVI (0.5-24 h). However, in the initial presence of Fe2+, the rapid reaction between HS- and Fe2+ produced amorphous FeSx, which deposited on the nZVI and corroded the surface iron oxide layer (<0.25 h). This was followed by redeposition of colloidal iron (hydr)oxide on the particle surface (0.25-8 h) and deposition of residual FeSx (8-24 h) on S-nZVI. S loading on S-nZVI was 1 order of magnitude higher when Fe2+ was present. Surface characterization of the sulfidated particles by TEM-SAED, XPS, and XAFS verified the solution dynamics and demonstrated that S2- and S22-/Sn2- were the principal reduced S species on S-nZVI. This study provides a methodology to tune sulfur loading and S speciation on S-nZVI to suit remediation needs.

Photo-induced protonation assisted nano primary battery for highly efficient immobilization of diverse heavy metal ions

Highly-stable heavy metal ions (HMIs) appear long-term damage, while the existing remediation strategies struggle to effectively remove a variety of oppositely charged HMIs without releasing toxic substances. Here we construct an iron-copper primary battery-based nanocomposite, with photo-induced protonation effect, for effectively consolidating broad-spectrum HMIs. In FCPBN, Fe/Cu cell acts as the reaction impetus, and functional graphene oxide modified by carboxyl and UV-induced protonated 2-nitrobenzaldehyde serves as an auxiliary platform. Due to the groups and built-in electric fields under UV stimuli, FCPBN exhibits excellent affinity for ions, with a maximum adsorption rate constant of 974.26 g∙mg-1∙min-1 and facilitated electrons transfer, assisting to reduce 9 HMIs including Cr2O72-, AsO2-, Cd2+ in water from 0.03 to 3.89 ppb. The cost-efficiency, stability and collectability of the FCPBN during remediation, and the beneficial effects on polluted soil and the beings further demonstrate the splendid remediation performance without secondary pollution. This work is expected to remove multi-HMIs thoroughly and sustainably, which tackles an environmental application challenge.

Unraveling the interplay of common groundwater ions in arsenic removal by sulfide-modified nanoscale zerovalent iron

Sulphidation of nZVI (S-nZVI) has shown to significantly improve the arsenic removal capacity of nZVI, concurrently modifying the sequestration mechanism. However, to better apply S-nZVI for groundwater arsenic remediation, the impact of groundwater coexisting ions on the efficacy of arsenic uptake by S-nZVI needs to be investigated. This present study evaluates the potential of S-nZVI to remove arsenic in the presence of typical groundwater coexisting ions such as Cl-, HA, HCO3-, PO43- and SO42- through batch adsorption experiments. Individually, PO43- and HA had a dominant inhibition effect, while SO42- promoted As(III) removal by S-nZVI. Conversely, for As(V) removal, HCO3- and SO42- impeded the removal process. X-ray spectroscopic investigation suggests that the coexisting ions can either compete with arsenic for the adsorption sites, influence the S-nZVI corrosion rates and/or generate distinct corrosion products, thereby interfering with arsenic removal by S-nZVI. To investigate the cumulative effects of these ions, a 25-1 Fractional Factorial Design of experiments was employed, wherein the concentration of all the ions were varied simultaneously in an optimized manner, and their impact on arsenic removal by S-nZVI was observed. Our results shows that when these ions are present concurrently, PO43-, SO42- and HA still exerted a dominant influence on As(III) removal, whereas HCO3- was the main ions affecting As(V) removal, although the combined influence of the ions was not merely a summation of their individual effects. Overall, the finding of our study might provide valuable insight for predicting the actual performance of S-nZVI in field-scale applications for the remediation of arsenic-contaminated groundwater.

Unveiling the Roles of Alloyed Boron in Hexavalent Chromium Removal Using Borohydride-Synthesized Nanoscale Zerovalent Iron: Electron Donor and Antipassivator

Nanoscale zerovalent iron synthesized using borohydride (B-NZVI) has been widely applied in environmental remediation in recent decades. However, the contribution of boron in enhancing the inherent reactivity of B-NZVI and its effectiveness in removing hexavalent chromium [Cr(VI)] have not been well recognized and quantified. To the best of our knowledge, herein, a core-shell structure of B-NZVI featuring an Fe-B alloy shell beneath the iron oxide shell is demonstrated for the first time. Alloyed boron can reduce H+, contributing to more than 35.6% of H2 generation during acid digestion of B-NZVIs. In addition, alloyed B provides electrons for Fe3+ reduction during Cr(VI) removal, preventing in situ passivation of the reactive particle surface. Meanwhile, the amorphous oxide shell of B-NZVI exhibits an increased defect density, promoting the release of Fe2+ outside the shell to reduce Cr(VI), forming layer-structured precipitates and intense Fe-O bonds. Consequently, the surface-area-normalized capacity and surface reaction rate of B-NZVI are 6.5 and 6.9 times higher than those of crystalline NZVI, respectively. This study reveals the importance of alloyed B in Cr(VI) removal using B-NZVI and presents a comprehensive approach for investigating electron pathways and mechanisms involved in B-NZVIs for contaminant removal.

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.

Soluble metal porphyrins - Zero-valent zinc system for effective reductive defluorination of branched per and polyfluoroalkyl substances (PFASs)

Nano zero-valent metals (nZVMs) have been extensively utilized for decades in the reductive remediation of groundwater contaminated with chlorinated organic compounds, owing to their robust reducing capabilities, simple application, and cost-effectiveness. Nevertheless, there remains a dearth of information regarding the efficient reductive defluorination of linear or branched per- and polyfluoroalkyl substances (PFASs) using nZVMs as reductants, largely due to the absence of appropriate catalysts. In this work, various soluble porphyrin ligands [[meso‑tetra(4-carboxyphenyl)porphyrinato]cobalt(III)]Cl·7H2O (CoTCPP), [[meso‑tetra(4-sulfonatophenyl) porphyrinato]cobalt(III)]·9H2O (CoTPPS), and [[meso‑tetra(4-N-methylpyridyl) porphyrinato]cobalt(II)](I)4·4H2O (CoTMpyP) have been explored for defluorination of PFASs in the presence of the nZn0 as reductant. Among these, the cationic CoTMpyP showed best defluorination efficiencies for br-perfluorooctane sulfonate (PFOS) (94%), br-perfluorooctanoic acid (PFOA) (89%), and 3,7-Perfluorodecanoic acid (PFDA) (60%) after 1 day at 70 °C. The defluorination rate constant of this system (CoTMpyP-nZn0) is 88-164 times higher than the VB12-nZn0 system for the investigated br-PFASs. The CoTMpyP-nZn0 also performed effectively at room temperature (55% for br-PFOS, 55% for br-PFOA and 25% for 3,7-PFDA after 1day), demonstrating the great potential of in-situ application. The effect of various solubilizing substituents, electron transfer flow and corresponding PFASs defluorination pathways in the CoTMpyP-nZn0 system were investigated by both experiments and density functional theory (DFT) calculations. SYNOPSIS: Due to the unavailability of active catalysts, available information on reductive remediation of PFAS by zero-valent metals (ZVMs) is still inadequate. This study explores the effective defluorination of various branched PFASs using soluble porphyrin-ZVM systems and offers a systematic approach for designing the next generation of catalysts for PFAS remediation.

Impacts of Perfluoroalkyl Substances on Aqueous and Nonaqueous Phase Liquid Dechlorination by Sulfidized Nanoscale Zerovalent Iron

Per- and poly fluoroalkyl substances (PFASs) are often encountered with nonaqueous phase liquid (NAPL) in the groundwater at fire-fighting and military training sites. However, it is unclear how PFASs affect the dechlorination performance of sulfidized nanoscale zerovalent iron (S-nFe0), which is an emerging promising NAPL remediation agent. Here, S-nFe0 synthesized with controllable S speciation (FeS or FeS2) were characterized to assess their interactions with PFASs and their dechlorination performance for trichloroethylene NAPL (TCE-NAPL). Surface-adsorbed PFASs blocked materials' reactive sites and inhibited aqueous TCE dechlorination. In contrast, PFASs-adsorbed particles with improved hydrophobicity tended to enrich at the NAPL-water interface, and the reactive sites were re-exposed after the PFASs accumulation into the NAPL phase to accelerate dechlorination. This PFASs-induced phenomenon allowed the materials to present a higher reactivity (up to 1.8-fold) with a high electron efficiency (up to 99%) for TCE-NAPL dechlorination. Moreover, nFe0-FeS2 with a higher hydrophobicity was more readily enriched at the NAPL-water interface and more reactive and selective than nFe0-FeS, regardless of coexisting PFASs. These results unveil that a small amount of yet previously overlooked coexisting PFASs can favor selective reductions of TCE-NAPL by S-nFe0, highlighting the importance of materials hydrophobicity and transportation induced by S and PFASs for NAPL remediation.

Surface wettability control and electron transport regulation in zerovalent iron for enhanced removal of emerging polystyrene microplastics-heavy metal contaminants

Emerging microplastics-heavy metal (MPs-HM) contaminants in wastewaters pose an emerging health and environmental risk due to their persistence and increasing ecological risks (e.g., "Trojan horse" effect). Hence, removing MPs in solution and preventing secondary releases of HM has become a key challenge when tackling with MPs pollution. Leveraging the hydrophobic nature of MPs and the electron transfer efficiency from Fe0 to HM, we demonstrate an alkylated and sulfidated nanoscale zerovalent iron (AS-nZVI) featuring a delicate "core-shell-hydrophobic film" nanostructure. Exemplified by polystyrene (PS) MPs-Pb(II) removal, the three nanocomponents offer synergistic functions for the rapid separation of MPs, as well as the reduction and stabilization of Pb(II). The outmost hydrophobic film of AS-nZVI greatly improves the anticorrosion performance of nanoscale zerovalent iron and the enrichment abilities of hydrophobic MPs, achieving a maximum removal capacity of MPs to 2725.87 mgMPs·gFe-1. This MPs enrichment promotes the subsequent reductive removal of Pb(II) through the electron transfer from the iron core of AS-nZVI to Pb(II), a process further strengthened by the introduced sulfur. When considering the inevitable aging of MPs in wastewaters due to mechanical wear or microbial degradation, our study concurrently examines the efficiencies and behaviors of AS-nZVI in removing virgin-MPs-Pb(II) and aged-MPs-Pb(II). The batch results reveal that AS-nZVI has an exceptional ability to remove above 99.6 % Pb(II) for all reaction systems. Overall, this work marks a pioneering effort in highlighting the importance of MPs-toxin combinations in dealing with MPs contamination and in demonstrating the utility of nZVI techniques for MPs-contaminated water purification.

Doped Cu0 and sulfidation induced transition from R-O• to •OH in peracetic acid activation by sulfidated nano zero-valent iron-copper

Peracetic acid (PAA) has emerged as a new effective oxidant for various contaminants degradation through advanced oxidation process (AOP). In this study, sulfidated nano zero-valent iron-copper (S-nZVIC) with low Cu doping and sulfidation was synthesized for PAA activation, resulting in more efficient degradation of sulfamethoxazole (SMX, 20 μM) and other contaminants using a low dose of catalyst (0.05 g/L) and oxidant (100 μM). The characterization results suggested that S-nZVIC presented a more uniform size and distribution with fewer metal oxides, as the agglomeration and oxidation were inhibited. More significantly, doped Cu0 and sulfidation significantly enhanced the generation and contribution of •OH but decreased that of R-O• in S-nZVIC/PAA/SMX system compared with that of nZVIC and S-nZVI, accounting for the relatively high degradation efficiency of 97.7% in S-nZVIC/PAA/SMX system compared with 85.7% and 78.9% in nZVIC/PAA/SMX and S-nZVI/PAA/SMX system, respectively. The mechanisms underlying these changes were that (i) doped Cu° could promote the regeneration of Fe(Ⅱ) for strengthened PAA activation through mediating Fe(Ⅱ)/Fe(Ⅲ) cycle by Cu(Ⅰ)/Cu(Ⅱ) cycle; (ii) S species might consume part of R-O•, resulting in a decreased contribution of R-O• in SMX degradation; (iii) sulfidation increased the electrical conductivity, thus facilitating the electron transfer from S-nZVIC to PAA. Consequently, the dominant reactive oxygen species transited from R-O• to •OH to degrade SMX more efficiently. The degradation pathways, intermediate products and toxicity were further analyzed through density functional theory (DFT) calculations, liquid chromatography-mass spectrometry (LC-MS) and T.E.S.T software analysis, which proved the environmental friendliness of this process. In addition, S-nZVIC exhibited high stability, recyclability and degradation efficiency over a wide pH range (3.0∼9.0). This work provides a new insight into the rational design and modification of nano zero-valent metals for efficient wastewater treatment through adjusting the dominant reactive oxygen species (ROS) into the more active free radicals.

Combined exposure to decabromodiphenyl ether and nano zero-valent iron aggravated oxidative stress and interfered with metabolism in earthworms

Decabromodiphenyl ether (BDE-209) is a common brominated flame retardant in electronic waste, and nano zero-valent iron (nZVI) is a new material in the field of environmental remediation. Little is known about how BDE-209 and nZVI combined exposure influences soil organisms. During the 28 days study, we determined the effects of single and combined exposures to BDE-209 and nZVI on the oxidative stress and metabolic response of earthworms (Eisenia fetida). On day 7, compared to CK, malondialdehyde (MDA) content increased in most combined exposure groups. To remove MDA and reactive oxygen species (ROS), superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities were induced in most combined exposure groups. On day 28, compared to CK, the activities of SOD and CAT were inhibited, while POD activity was significantly induced, indicating that POD plays an important role in scavenging ROS. Combined exposure to BDE-209 and nZVI significantly affected amino acid biosynthesis and metabolism, purine metabolism, and aminoacyl-tRNA biosynthesis pathways, interfered with energy metabolism, and aggravated oxidative stress in earthworms. These findings provide a basis for assessing the ecological impacts of using nZVI to remediate soils contaminated with BDE-209 from electronic waste.

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.

Processes and mechanisms in remediation of aqueous chromium contamination by sulfidated nano-scale zerovalent iron (S-nZVI): Experimental and computational investigations

Sulfidated nano-scale zerovalent iron (S-nZVI) has emerged as an advanced functional nanomaterial for efficiently remediating Cr(VI) contamination in aqueous environments. However, there is an insufficient understanding of its coherent process, removal pathway, and hydrochemical reactive mechanisms, presenting potential challenges for its future environmental applications. To address this gap, this study successfully synthesized S-nZVI through a chemical precipitation method and effectively applied it for the removal of Cr(VI). Additional characterization revealed that the removal of Cr(VI) followed a sequence of rapid chemisorption and intraparticle diffusion processes, concomitant with an increase in pH and a decrease in oxidation-reduction potential. The remediation mechanism encompassed a synergistic reduction of Cr(VI) to Cr(III) and simultaneous immobilization via Cr2FeO4 coprecipitation. The highest Cr(VI) removal capacity of 75 mg/g was attained during dynamic removal experiments in the sand column packed with S-nZVI. Further computational analysis, employing density functional theory calculations based on the experimental data, revealed the involvement of multiple molecular orbitals of Cr(VI) in the removal process. It also elucidated a step-by-step reduction pathway for Cr(VI) characterized by decreasing free energy. These findings provide evidence-based insights into Cr(VI) remediation using S-nZVI and can serve as valuable technical support for future environmental management of heavy metals.

Robust Activity and Stability of P-Doped Fe-Carbon Composites Derived from MOF for Bromate Reduction

Iron-based materials are effective for the reductive removal of the disinfection byproduct bromate in water, while the construction of highly stable and active Fe-based materials with wide pH adaptability remains greatly challenging. In this study, highly dispersed iron phosphide-decorated porous carbon (Fe2P(x)@P(z)NC-y) was prepared via the thermal hydrolysis of Fe@ZIF-8, followed by phosphorus doping (P-doping) and pyrolysis. The reduction performances of Fe2P(x)@P(z)NC-y for bromate reduction were evaluated. Characterization results showed that the Fe, P, and N elements were homogeneously distributed in the carbonaceous matrix. P-doping regulated the coordination environment of Fe atoms and enhanced the conductivity, porosity, and wettability of the carbonaceous matrix. As a result, Fe2P(x)@P(1.0)NC-950 exhibited enhanced reactivity and stability with an intrinsic reduction kinetic constant (kint) 1.53-1.85 times higher than Fe(x)@NC-950 without P-doping. Furthermore, Fe2P(0.125)@P(1.0)NC-950 displayed superior reduction efficiency and prominent stability with very low Fe leaching (4.53-22.98 μg L-1) in a wide pH range of 4.0-10.0. The used Fe2P(0.125)@P(1.0)NC-950 could be regenerated by phosphating, and the regenerated Fe2P(0.125)@P(1.0)NC-950 maintained 85% of its primary reduction activity after five reuse cycles. The study clearly demonstrates that Fe2P-decorated porous carbon can be applied as a robust and stable Fe-based material in aqueous bromate reduction.

Enhanced hydrodechlorination of 4-chlorophenol through carboxymethylcellulose-modified Pd/Fe nanosuspension synthesized by one-step methods

Palladized iron (Pd/Fe) represents one of the most common modification strategies for nanoscale zero-valent iron (nZVI). Most studies prepared Pd/Fe by reducing iron salts and depositing Pd species on the surface of pre-synthesized nZVI, which can be called the two-step method. In this study, we proposed a one-step method to obtain Pd/Fe by the concurrent formation of Fe0 and Pd0 and investigated the effects of these two methods on 4-chlorophenol (4-CP) removal, with carboxymethylcellulose (CMC) coated as a surface modifier. Results indicated that the one-step method, not only streamlined the synthesis process, but also Pd/Fe-CMCone-step, synthesized by it, exhibited a higher 4-CP removal rate (97.9%) compared to the two-step method material Pd/Fe-CMCtwo-step (82.4%). Electrochemical analyses revealed that the enhanced activity of Pd/Fe-CMCone-step was attributed to its higher electron transfer efficiency and more available reactive species, active adsorbed hydrogen species (Hads*). Detection of intermediate products demonstrated that, under the influence of Pd/Fe-CMCone-step, the main route of 4-CP was through hydrodechlorination (HDC) to form phenol and H* was the main active specie, supported by EPR tests, quenching experiments and product analysis. Additionally, the effects of initial 4-CP concentration, initial pH, O2 concentration, anions such as Cl-, SO42-, HCO3-, and humic acid (HA) were also investigated. In conclusion, the results of this study suggest that Pd/Fe-CMCone-step, synthesized through the one-step method, is a convenient and efficient nZVI-modifying material suitable for the HDC of chlorinated organic compounds.

Carboxymethyl cellulose stabilization induced changes in particle characteristics and dechlorination efficiency of sulfidated nanoscale zero-valent iron

Polymer stabilization, exemplified by carboxymethyl cellulose (CMC), has demonstrated effectiveness in enhancing the transport of nanoscale zero-valent iron (nZVI). And, sulfidation is recognized for enhancing the reactivity and selectivity of nZVI in dechlorination processes. The influence of polymer stabilization on sulfidated nZVI (S-nZVI) with various sulfur precursors remains unclear. In this study, CMC-stabilized S-nZVI (CMC-S-nZVI) was synthesized using three distinct sulfur precursors (S2-, S2O42-, and S2O32-) through one-step approach. The antioxidant properties of CMC significantly elevated the concentration of reduced sulfur species (S2-) on CMC-S-nZVIs, marking a 3.1-7.0-fold increase compared to S-nZVIs. The rate of trichloroethylene degradation (km) by CMC-S-nZVIs was observed to be 2.2-9.0 times higher than that achieved by their non-stabilized counterparts. Among the three CMC-S-nZVIs, CMC-S-nZVINa2S exhibited the highest km. Interesting, while the electron efficiency of CMC-S-nZVIs surged by 7.9-12 times relative to nZVI, it experienced a reduction of 7.0-34% when compared with S-nZVIs. This phenomenon is attributed to the increased hydrophilicity of S-nZVI particles due to CMC stabilization, which inadvertently promotes the hydrogen evolution reaction (HER). In conclusion, the findings of this study underscores the impact of CMC stabilization on the properties and dechlorination performance of S-nZVI sulfidated using different sulfur precursors, offering guidance for engineering CMC-S-nZVIs with desirable properties for contaminated groundwater remediation.

Enhanced thermal desorption of chlorinated hydrocarbons by nanoscale zero-valent iron: the effect of in situ dechlorination

Thermal desorption provides an efficient solution to remediate soil contaminated with chlorinated organic pollutants. However, enhanced desorption efficiency is desired to facilitate easier and less costly remediation. Hence, nanoscale zero-valent iron (nZVI) was combined with thermal desorption to remove trichloroethene (TCE) and trichlorobenzene (TCB) from soil in a laboratory-scale study. The addition of nZVI greatly improved the desorption efficiency, especially at low temperature with 99.6% of TCE and 98.8% of TCB removed at 300 °C for 2 h. Characterization results revealed that the addition of nZVI loosened the structure of soil, preventing the soil from agglomerating during the thermal treatment. Besides, the analyses of dechlorination intermediates and the variation of Fe species proved the in situ dechlorination effect of nZVI and the redox cycle of Fe was revealed. Moreover, the influences of nZVI dosage and treatment time on thermal treatment were assessed. This study not only offers new perspectives for contaminated soil remediation, but also provides mechanistic insights into the dechlorination effect of nZVI in the thermal desorption.

In-situ reactivation and reuse of micronsized sulfidated zero-valent iron using SRB-enriched culture: A sustainable PRB technology

Sulfidated zero-valent iron (S-ZVI) is an attractive material of permeable reactive barriers (PRBs) for the remediation of contaminated groundwater. However, S-ZVI is prone to be passivated due to the oxidation of reactive and conductive iron sulfide (FeSx) shell and the formation of inactive and non-conductive ferric (hydr)oxides, which serve as electron transfer barriers to hinder the electron flow from Fe° core to contaminants. This study thus proposed a novel approach for in-situ reactivation and reuse of micronsized S-ZVI (S-mZVI) in PRB using sulfate-reducing bacteria (SRB) enriched culture to realize long-lasting remediation of Cr(VI)-contaminated groundwater. S-mZVI were passivated after reactions with Cr(VI) due to the formation of electron transfer barriers (mainly inactive and non-conductive Fe(III) (hyd)oxides, which increased the polarization resistance from 16.38 to 27.38 kΩ cm2 and hindered the electron transfer from the Fe° core. Interestingly, the passivated S-mZVI was efficiently reactivated by providing the SRB-enriched culture and organic carbon within 12 h, and the Cr(VI) removal capacity of S-mZVI in the three use cycles increased to 37.4 mg Cr/g, which was 2.1 times higher than that of the virgin S-mZVI. After biological reactivation, the Rp of reactivated S-mZVI decreased to 12.30 kΩ cm2. SRB-mediated reactivation removed the electron transfer barriers via biotic and abiotic reduction of Fe(III) (hyd)oxides. Especially, the microbial Fe(III) reduction mediated by FmnA-dmkA-fmnB-pplA-ndh2-eetAB-dmkB protein family enhanced the Fe2+ release from the surface and the subsequent re-formation of reactive and conductive FeSx shell. A long-term PRB column test further demonstrated the feasibility of in-situ biological reactivation and reuse of S-mZVI for enhanced Cr(VI)-contaminated groundwater remediation. Within 64 days, the Cr(VI) removal capacity of S-mZVI in the four use cycles increased by 3.2 times, compared to the virgin one. The bio-reactivation using the SRB-enriched culture and sulfate locally-available in groundwater will reduce the chemical and maintenance costs associated with the frequent replacement of reactive ZVI-based materials. The PRB technology based on the bio-renewable S-mZVI can be a sustainable alternative to the conventional PRBs for the long-lasting and low-cost remediation of groundwater contaminated by oxidative pollutants.

Recycling of waste aluminum scraps to fabricate sulfidated zero-valent iron-aluminum particles for enhanced chromate removal

Massive waste aluminum scraps produced from the spent aluminum products have high electron capacity and can be recycled as an attractive alternative to materials based on zero-valent iron (Fe0) for the removal of oxidative contaminants from wastewater. This study thus proposed an approach to fabricate micron-sized sulfidated zero-valent iron-aluminum particles (S-Al0@Fe0) with high reactivity, electron selectivity and capacity using recycled waste aluminum scraps. S-Al0@Fe0 with a three-layer structure contained zero-valent aluminum (Al0) core, Fe0 middle layer and iron sulfide (FeS) shell. The rates of chromate (Cr(VI)) removal by S-Al0@Fe0 at pH 5.0‒9.0 were 1.6‒5.9 times greater than that by sulfidated zero-valent iron (S-Fe0). The Cr(VI) removal capacity of S-Al0@Fe0 was 8.2-, 11.3- and 46.9-fold greater than those of S-Fe0, zero-valent iron-aluminum (Al0-Fe0) and Fe0, respectively. The chemical cost of S-Al0@Fe0 for the equivalent Cr(VI) removal was 78.5% lower than that of S-Fe0. Negligible release of soluble aluminum during the Cr(VI) removal was observed. The significant enhancement in the reactivity and capacity of S-Al0@Fe0 was partially ascribed to the higher reactivity and electron density of the Al0 core than Fe0. More importantly, S-Al0@Fe0 served as an electric cell to harness the persistent and selective electron transfer from the Al0-Fe0 core to Cr(VI) at the surface via coupling Fe0-Fe2+-Fe3+ redox cycles, resulting in a higher electron utilization efficiency. Therefore, S-Al0@Fe0 fabricated using recycled waste aluminum scraps can be a cost-effective and environmentally-friendly alternative to S-Fe0 for the enhanced removal of oxidative contaminants in industrial wastewater.

Photochemical-promoted ZVI reduction for highly efficient removal of 4-chlorophenol and Cr(VI): Catalytic activity, performance and electron transfer mechanisms

Zero-valent iron (ZVI) reduction represents a promising methodology for water remediation, but its broad application is limited by two critical challenges (i.e., aggregation and passivation). Here, we report a hybrid strategy of photochemical-promoted ZVI reduction with high efficiency and reduction capacity for removing coexisting refractory pollutants in water. A composite material with Pd/Fe bimetallic nanoparticles supported onto semiconducting metal oxide (Pd/Fe@WO3-GO) was prepared and subsequently used as the model catalyst. By using the developed strategy with visible light as light source, this catalyst showed a remarkable catalytic performance for simultaneously eliminating 4-chlorophenol (4-CP) and Cr(VI), with dehalogenation rate as high as 0.43 min-1, outperforming the reported ZVI-based catalysts. A synergistic interaction of photocatalysis and ZVI reduction occurred in this strategy, where the interfacial electron transfer on particles surface were greatly strengthened with light irradiation. The activation was attributed to the dual functions of semiconducting material as support to disperse Pd/Fe nanoparticles and as (photoexcited) electron donor to directly trigger reduction reactions and/or indirectly inhibit the formation of oxides passivation layer. Both direct electron transfer and H*-mediated indirect electron transfer mechanisms were confirmed to participate in the reduction of pollutants, while the later was quantitatively demonstrated as the predominant reaction route. Importantly, this strategy showed a wide pH applicability, long-term durability and excellent catalytic performance in different real-water systems. This work provides new insights into ZVI reduction and advances its applications for the removal of combined organic and inorganic pollutants. The developed photochemical-promoted ZVI reduction strategy holds a great potential for practical applications.

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.

Characteristics of iron (hydr)oxides and Cr(VI) retention mechanisms in soils from tropical and subtropical areas of China

Though both iron (hydr)oxides and soil organic matter (SOM) significantly influence heavy metal behaviors in soils, studies on the characteristics of natural minerals and the synergic effects of the two on Cr(VI) transformation are limited. This study investigated Cr(VI) retention mechanisms in four soils from tropical and subtropical regions of China based on a comprehensive characterization of Fe (hydr)oxides. These soils exhibited varying quantities of hematite, ferrihydrite and goethite, with distinct Al substitution levels and varied exposed crystallographic facets. Adsorption experiments revealed a positive correlation between Fe (hydr)oxide content and Cr(VI) fixation amount on colloid, which was influenced by the mineral types, Al substitution levels and facet exposures. Further, Cr(VI) was sequestered on soil by adsorption and reduction. In soils enriched with crystalline Fe (hydr)oxides, Cr(VI) reduction was primarily governed by SOM, while in soils enriched with poorly crystalline Fe (hydr)oxides, mineral-associated Fe(II) also contributed to Cr(VI) reduction. Aging experiments demonstrated that SOM and mineral-associated Fe(II) expedited Cr (VI) passivation and diminished the Cr leaching. These results improve our understanding of natural Fe (hydr)oxide structures and their impact on Cr(VI) behavior in soils, and shed light on complex soil-contaminant interactions and remediation of Cr(VI) polluted soils.

Rapid removal of decabromodiphenyl ether by mechanochemically prepared submicron zero-valent iron with FeC2O4·2 H2O layers: Kinetics, mechanisms and pathways

The utilization of nano zero-valent iron (nZVI) in polybrominated diphenyl ethers remediation has been studied extensively. However, challenges in balancing cost and reactivity have been encountered. A submicron zero-valent iron coated with FeC2O4·2 H2O layers (OX-smZVI) was synthesized via a mechanochemical method, aiming to resolve this contradiction. Characterization via SEM, TEM, and XPS confirmed the structure as FeC2O4·2 H2O coated iron lamellate with a surface area 24-fold higher than ball-milled zero-valent iron (smZVI). XRD highlighted an Fe/C eutectic in OX-smZVI, boosting its electron transfer capacity. Decabromodiphenyl ether degradation by OX-smZVI follows a two-stage process, with initial degradation by FeC2O4·2 H2O and a subsequent phase dominated by electron transfer. OX-smZVI exhibits a 4.52-34.40 times faster BDE209 removal rate than nZVI and scaled-up OX-smZVI displayed superior reactivity with preparation costs only 1/680 of nZVI. Given its enhanced reactivity and cost-efficiency, OX-smZVI emerges as a promising replacement for nZVI.

New insights into long-lasting Cr(VI) removal from groundwater using in situ biosulfidated zero-valent iron with sulfate-reducing bacteria

Sulfidation enhances the reactivity of zero-valent iron (ZVI) for Cr(VI) removal from groundwater. Current sulfidation methods mainly focus on chemical and mechanical sulfidation, and there has been little research on biosulfidation using sulfate-reducing bacteria (SRB) and its performance in Cr(VI) removal. Herein, the ability of the SRB-biosulfidated ZVI (SRB-ZVI) system was evaluated and compared with that of the Na2S-sulfidated ZVI system. The SRB-ZVI system forms a thicker and more porous FeSx layer than the Na2S-sulfidated ZVI system, resulting in more sufficient sulfidation of ZVI and a 2.5-times higher Cr(VI) removal rate than that of the Na2S-sulfidated ZVI system. The biosulfidated-ZVI granules and FeSx suspension are the major components of the SRB-ZVI system. The SRB-ZVI system exhibits a long-lasting (11 cycles) Cr(VI) removal performance owing to the regeneration of FeSx. However, the Na2S-sulfidated ZVI system can perform only two Cr(VI) removal cycles. SRB attached to biosulfidated-ZVI can survive in the presence of Cr(VI) because of the protection of the biogenic porous structure, whereas SRB in the suspension is inhibited. After Cr(VI) removal, SRB repopulates in the suspension from biosulfidated-ZVI and produce FeSx, thus providing conditions for subsequent Cr(VI) removal cycles. Overall, the synergistic effect of SRB and ZVI provides a more powerful and environmentally friendly sulfidation method, which has more advantageous for Cr(VI) removal than those of chemical sulfidation. This study provides a visionary in situ remediation strategy for groundwater contamination using ZVI-based technologies.

Sulfidated nanoscale zero-valent iron derived from iron sludge for tetracycline removal: Role of sulfur and iron in reactivity and mechanisms

Iron sludge, produced during the drinking water treatment process, can be recycled as potential iron resource to create environmental functional material. In this study, sulfur-iron composites derived from iron sludge (S-Fe composites) was synthesized through sulfidation and carbonization, and used for the tetracycline (TC) removal under aerobic and anoxic conditions. The reactivities of these as-prepared products were strongly depended on pyrolysis temperatures. In particular, sulfidated nanoscale zero-valent iron loaded on carbon (S-nFe0@CIS) carbonized at 800 °C exhibited the highest TC removal efficiency with 86.6% within 30 min at circumneutral pH compared with other S-Fe composites. The crystalline structure of α-Fe0, FeSx and S0 as main active sites in S-nFe0@CIS promoted the degradation of TC. Moreover, the Fe/S molar ratios significantly affected the TC removal rates, which reached the best value as the optimal S/Fe of 0.27. The results illustrated that the optimized extent of sulfidation could facilitate electron transfer from nFe0 towards contaminants and accelerate Fe(III)/Fe(II) cycle in reaction system compared to bared nFe0@CIS. We revealed that removal of TC by S-nFe0@CIS in the presence of dissolved oxygen (DO) is mainly attributed to oxidation, adsorption and reduction pathways. Their contribution to TC removal were 31.6%, 25.2% and 28.8%, respectively. Furthermore, this adsorption-oxygenation with the formation of S-nFe0@CIS-TC* complexes was a surface-mediated process, in which DO was transformed by the structural FeSx on complex surface to •OH with the generation of H2O2 intermediate. The intermediates of TC and toxicity analysis indicate that less toxicity products generated through degradation process. This study provides a new reclamation of iron sludge and offers a new insight into the TC removal by S-nFe0@CIS under aerobic conditions.

Insights into the long-term immobilization performances and mechanisms of CMC-Fe0/FeS with different sulfur sources for uranium under anoxic and oxic aging

Nanoscale zerovalent iron (Fe0)-based materials have been demonstrated to be a effective method for the U(VI) removal. However, limited research has been conducted on the long-term immobilization efficiency and mechanism of Fe0-based materials for U(VI), which are essential for achieving safe handling and disposal of U(VI) on a large scale. In this study, the prepared carboxymethyl cellulose (CMC) and sulfurization dual stabilized Fe0 (CMC-Fe0/FeS) exhibited excellent long-term immobilization performances for U(VI) under both anoxic and oxic conditions, with the immobilization efficiencies were respectively reached over 98.0 % and 94.8 % after 180 days of aging. Most importantly, different from the immobilization mechanisms of the fresh CMC-Fe0/FeS for U(VI) (the adsorption effect of -COOH and -OH groups, coordination effect with sulfur species, as well as reduction effect of Fe0), the re-mobilized U(VI) were finally re-immobilized by the formed FeOOH and Fe3O4 on the aged CMC-Fe0/FeS. Under anoxic conditions, more Fe3O4 was produced, which may be the main reason for the long-term immobilization U(VI). Under oxic conditions, the production of Fe3O4 and FeOOH were relatively high, which both played significant roles in re-immobilizing U(VI) through surface complexation, reduction and incorporation effects.

Reductive sequestration of Cr (VI) by phosphorylated nanoscale zerovalent iron

The cracked phosphorylated nanoscale zerovalent iron (p-nZVI) has a strong electron selectivity towards the reductive removal of many heavy metal ions in waters. However, the unintended environmental risk after interactions with impurities or wastewater are not involved. Therefore, in this study, the phosphate group was successfully adsorbed into p-nZVI, and the cracked p-nZVI was successfully prepared with an optimal P/Fe ratio of 0.5%. The dosages of p-nZVI and temperatures were positively correlated with the removal rates. The removal process of Cr(VI) was more suitable by the Langmuir isothermal model（R2 > 0.99). The process of Cr (VI) (10, 20 and 40 mg/L) removal more fitted the pseudo first-order reaction model, while the process of Cr (VI) (60, 80 mg/L) removal more fitted the pseudo second-order reaction model. The Cr (VI) removal rates gradually decreased when the pH was increased. Dissolved oxygen slowed nanoiron reaction rates. The order of inhibition on the reactivity towards Cr(VI) was SiO32- > SO42- > PO43- > NO3- > HCO3-.The facilitation followed the order of Cd2+>Cu2+>Mg2+>Mn2+>Ca2+. Ca2+ showed an inhibitory effect, but all other cations showed different degrees of facilitation. The promotion effect is relatively similar in presence of Mn2+ or Mg2+. HA had a significant inhibitory effect. Environmental friendly p-nZVI had a good effect in simulated groundwater, seawater, river water and secondary effluent of the urban sewage treatment plant. The main pathway to remove Cr (VI) was in situ reduction by p-nZVI. The improved adsorption and reduction effect of p-nZVI on heavy metal ions in water was due to the structural change and the phosphate group.

Dechlorination Helps Defluorination: Insights into the Defluorination Mechanism of Florfenicol by S-nZVI and DFT Calculations on the Reaction Pathways

Defluorination is essential to eliminate the antibiotic resistance and detrimental effects of florfenicol (C12H14Cl2FNO4S, FF), which is achievable by sulfidated nanoscale zerovalent iron (S-nZVI), yet a comprehensive understanding of the mechanism is lacking. Herein, we used experimental data and density functional theory calculations to demonstrate four dechlorination-promoted defluorination pathways of FF, depending on S-nZVI or not. FF was defluorinated in a rapid and then slow but continuous manner, accompanying a consecutive dechlorination to deschloro (dFF) and dideschloro FF (ddFF). Unexpectedly, the predominant defluorination occurs by spontaneous hydrolysis of ddFF to form the hydrolyzed byproduct (HO-ddFF), i.e., independent of S-nZVI, which is initiated by intramolecular attack from carbonyl O to alkyl F and is thus limited for FF and dFF owing to the diminished nucleophilicity by electron-withdrawing Cl. The removal of Cl also makes the reductive defluorination of ddFF by S-nZVI amenable. The other two minor but more rapid defluorination pathways occur in synergy with the dechlorination of FF and dFF, which are mediated by the reactive carbanion intermediates and generate HO-dFF and HO-ddFF, respectively. The reliability of these dechlorination-facilitated defluorination pathways was verified by the consistency of theoretical calculations with experimental data, providing valuable insights into the degradation of fluorinated contaminants.

Minor chromium passivation of S-ZVI enhanced the long-term dechlorination performance of trichlorethylene: Effects of corrosion and passivation on the reactivity and selectivity

The corrosion and surface passivation of sulfidized zero-valent iron (S-ZVI) by common groundwater ions and contaminants are considered to be the most challenging aspects in the application of S-ZVI for remediation of chlorinated contaminants. This study investigated the impacts of corrosive chloride (Cl-) and passivation of hexavalent chromium (Cr(VI)) on the long-term reactivity, selectivity, corrosion behavior, and physicochemical properties during the 60-day aging process of S-ZVI. Although the co-existing of Cl- promoted the initial reactivity of S-ZVI, the rapid consumption of Fe° content shortened the reactive lifetime owing to the insufficient electron capacity. Severe passivation by Cr(VI) (30 mg L-1) preserved the Fe° content but significantly interfered with the reductive sulfur species, resulting in an increase in electron transfer resistance. In comparison, minor passivated S-ZVI (5.0 mg L-1 Cr(VI)) inhibited the hydrogen evolution while concurrently mitigating the further oxidation of the reductive iron and sulfur species, which significantly enhanced the long-term reactivity and selectivity of S-ZVI. Furthermore, the enhancement effect of minor passivation could be detected in the aging processes of one-step, two-step, and mechanochemically synthesized S-ZVI particles with different S/Fe ratios and precursors, which further verified the advantages of minor passivation. This observation is inspirable for the development of innovative strategies for environmental remediation by S-ZVI-based materials.

Regulating the FeSx assembly pattern of sulfidated zero-valent iron: All-in-one interface modulation with activated carbon

Specifically designing the heterogeneous interface in sulfidated zero-valent iron (S-ZVI) has been an effective, yet usually overlooked method to improve the decontamination ability. However, the mechanism behind FeSx assembly remains elusive and the lack of modulating strategies that can essentially tune the applicability of S-ZVI further imposes difficulties in creating better-performing S-ZVI with heterogeneous interface. In this study, by introducing powdered activated carbon (PAC) during S-ZVI preparation, S-ZVI/PAC microparticles were prepared to modulate the assembly pattern of FeSx for the applicability and reactivity of the material. S-ZVI/PAC showed robust performance in Cr(VI) sequestration, with 11.16 and 1.78 fold increase in Cr(VI) reactivity compared to ZVI and S-ZVI, respectively. This was attributed to the fact that the introduced PAC could acquire FeSx to enhance the electron transfer capacity matching its adsorption threshold, thus helping to accommodate the transfer of the reduction center to PAC in S-ZVI/PAC. In optimizing the FeSx allocation between ZVI and PAC, the chemical assembly of FeSx on S-ZVI was superior to physical adsorption. Critically, we found that isolated FeSx in the prepared solution was physically adsorbed by the PAC, allowing chemically assembled FeSx on the S-ZVI. This was achieved by controlling the addition sequence of Na2S and PAC, as it effectively controlled the release rate and content of Fe(II) in the preparation solution. S-ZVI/PAC was demonstrated to be extremely effective in simulated wastewater and electrokinetics-permeable reactive barrier (EK-PRB) treatments. Introducing PAC enriches the diversity of sulfidation mechanisms and may realize the universality of the S-ZVI/PAC application scenarios. This study provides a new interface optimization strategy for S-ZVI targeted design towards environmental applications.

Corrosion behaviors and kinetics of nanoscale zero-valent iron in water: A review

Knowledge on corrosion behaviors and kinetics of nanoscale zero-valent iron (nZVI) in aquatic environment is particularly significant for understanding the reactivity, longevity and stability of nZVI, as well as providing theoretical guidance for developing a cost-effective nZVI-based technology and designing large-scale applications. Herein, this review gives a holistic overview on the corrosion behaviors and kinetics of nZVI in water. Firstly, Eh-pH diagram is introduced to predict the thermodynamics trend of iron corrosion. The morphological, structural, and compositional evolution of (modified-) nZVI under different environmental conditions, assisted with microscopic and spectroscopic evidence, is then summarized. Afterwards, common analytical methods and characterization technologies are categorized to establish time-resolved corrosion kinetics of nZVI in water. Specifically, stable models for calculating the corrosion rate constant of nZVI as well as electrochemical methods for monitoring the redox reaction are discussed, emphasizing their capabilities in studying the dynamic iron corrosion processes. Finally, in the future, more efforts are encouraged to study the corrosion behaviors of nZVI in long-term practical application and further build nanoparticles with precisely tailored properties. We expect that our work can deepen the understanding of the nZVI chemistry in aquatic environment.

Enhanced Biogenic Sulfidation of Zero-Valent Iron in Columns: Implications for Promoting Dechlorination in Permeable Reactive Barriers

Biogenic sulfidation of zero-valent iron (ZVI) using sulfate reducing bacteria (SRB) has shown enhanced dechlorination rates comparable to those produced by chemical sulfidation. However, controlling and sustaining biogenic sulfidation to enhance in situ dechlorination are poorly understood. Detailed interactions between SRB and ZVI were examined for 4 months in column experiments under enhanced biogenic sulfidation conditions. SRB proliferation and changes in ZVI surface properties were characterized along the flow paths. The results show that ZVI can stimulate SRB activity by removing excessive free sulfide (S2-), in addition to lowering reduction potential. ZVI also hinders downgradient movement of SRB via electrostatic repulsion, restricting SRB presence near the upgradient interface. Dissolved organic carbon (e.g., >2.2 mM) was essential for intense biogenic sulfidation in ZVI columns. The presence of SRB in the upgradient zone appeared to promote the formation of iron polysulfides. Biogenic FeSx deposition increased the S content on ZVI surfaces ∼3-fold, corresponding to 3-fold and 2-fold improvements in the trichloroethylene degradation rate and electron efficiency in batch tests. Elucidation of SRB and ZVI interactions enhances sustained sulfidation in ZVI permeable reactive barrier.

Tailoring Fe0 Nanoparticles via Lattice Engineering for Environmental Remediation

Lattice engineering of nanomaterials holds promise in simultaneously regulating their geometric and electronic effects to promote their performance. However, local microenvironment engineering of Fe0 nanoparticles (nFe0) for efficient and selective environmental remediation is still in its infancy and lacks deep understanding. Here, we present the design principles and characterization techniques of lattice-doped nFe0 from the point of view of microenvironment chemistry at both atomic and elemental levels, revealing their crystalline structure, electronic effects, and physicochemical properties. We summarize the current knowledge about the impacts of doping nonmetal p-block elements, transition-metal d-block elements, and hybrid elements into nFe0 crystals on their local coordination environment, which largely determines their structure-property-activity relationships. The materials' reactivity-selectivity trade-off can be altered via facile and feasible approaches, e.g., controlling doping elements' amounts, types, and speciation. We also discuss the remaining challenges and future outlooks of using lattice-doped nFe0 materials in real applications. This perspective provides an intuitive interpretation for the rational design of lattice-doped nFe0, which is conducive to real practice for efficient and selective environmental remediation.

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.

The removal of antibiotic resistant bacteria and antibiotic resistance genes by sulfidated nanoscale zero-valent iron activating periodate: Efficacy and mechanism

Antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) have drawn much more attention due to their high risk on human health and ecosystem. In this study, the performance of sulfidated nanoscale zero-valent iron (S-nZVI)/periodate (PI) system toward ARB inactivation and ARGs removal was systematically investigated. The S-nZVI/PI system could realize the complete inactivation of 1 × 108 CFU/mL kanamycin, ampicillin, and tetracycline-resistant E. coli HB101 within 40 min, meanwhile, possessed the ability to remove the intracellular ARGs (iARGs) (including aphA, tetA, and tnpA) carried by E. coli HB101. Specifically, the removal of aphA, tetA, and tnpA by S-nZVI/PI system after 40 min reaction was 0.31, 0.47, and 0.39 log10copies/mL, respectively. The reactive species attributed to the E. coli HB101 inactivation were HO• and O2•-, which could cause the destruction of E. coli HB101 morphology and enzyme system (such as superoxide dismutase and catalase), the loss of intracellular substances, and the damage of iARGs. Moreover, the influence of the dosage of PI and S-nZVI, the initial concentration of E. coli HB101, as well as the co-existing substance (such as HCO3-, NO3-, and humic acid (HA)) on the inactivation of E. coli HB101 and its corresponding iARGs removal was also conducted. It was found that the high dosage of PI and S-nZVI and the low concentration of E. coli HB101 could enhance the disinfection performance of S-nZVI/PI system. The presence of HCO3-, NO3-, and HA in S-nZVI/PI system showed inhibiting role on the inactivation of E. coli HB101 and its corresponding iARGs removal. Overall, this study demonstrates the superiority of S-nZVI/PI system toward ARB inactivation and ARGs removal.