Abiotic transformation of hexabromocyclododecane by sulfidated nanoscale zerovalent iron: Kinetics, mechanism and influencing factors

Weak static magnetic fields facilitated highly efficient 2,4,6-trichlorophenol removal by sulfurized nanoscale zero-valent iron supported on biochar (BC-SNZVI) at neutral pH

Sulfurized nanoscale zero-valent iron supported on biochar (BC-SNZVI) has been successfully synthesized for 2,4,6-trichlorophenol (2,4,6-TCP) removal, while was only effectively under acidic conditions. To obtain highly efficient removal of 2,4,6-TCP within a broader pH range, weak static magnetic fields (WMF) was applied in BC-SNZVI/2,4,6-TCP aqueous systems. Results showed 30 mT WMF supported the most extensive 2,4,6-TCP removal, and 87.4% of 2,4,6-TCP (initial concentration of 30 mg/L) was removed by 0.5 g/L BC-SNZVI at neutral pH (pH = 6.8) within 180 min, which was increased by 54.4% compared to that without WMF. The observed rate constant (Kobs) under 30 mT WMF was 2.1-fold greater than that without WMF. Although three typical anions (NO3- (0.5-10.0 mM), H2PO4- (0.05-0.5 mM), and HCO3- (0.5-5.0 mM)) still inhibited 2,4,6-TCP removal, WMF could efficiently alleviate the inhibitory effects. Moreover, 73.1% of 2,4,6-TCP was successfully removed by BC-SNZVI under WMF in natural water. WMF remarkably boosted the dechlorination of 2,4,6-TCP, increasing the 2,4,6-TCP dechlorination efficiency from 45.2% (in the absence of WMF) to 83.8% (in the presence of WMF) by the end of 300 min. And the complete dechlorination product phenol appeared within 10 min. Force analysis confirmed the magnetic field gradient force (FB) moved paramagnetic Fe2+ at the SNZVI surface along the direction perpendicular to the external applied field, promoting the mass-transfer controlled SNZVI corrosion. Corrosion resistance analysis revealed WMF promoted the electron-transfer controlled SNZVI corrosion by decreasing its self-corrosion potential (Ecorr). With the introduction of sulfur, the magnitude of FB doubled and the Ecorr decreased comparing with NZVI. Our findings provide a facile and viable strategy for treating chlorinated phenols at neutral pH.

Integrated influence of sulfide modification on the reactivity of nanoscale zero-valent iron towards decabromodiphenyl ether under an electromagnetic field

Individual application of sulfide modification and electromagnetic field (EMF) can enhance the reactivity of nanoscale zero-valent iron (nZVI), yet the potential of both in combination is not clear. This work found that the reactivity of nZVI towards decabromodiphenyl ether was significantly enhanced by the combined effect of sulfidation and EMF. The specific reaction rate constant of nZVI increased by 7 to 10 times. A series of characterization results revealed that the sulfidation level not only affects the inherent reactivity but also the magnetic-induced heating (MIH) and corrosion (MIC) of nZVI. These collectively influence the degradation efficiency of nZVI under EMF. Sulfidation generally diminished the MIH effect. The low degree of sulfidation (S/Fe = 0.1) slightly reduced the MIC effect by 21.4%. However, the high degree of sulfidation (S/Fe = 0.4) led to significantly enhanced MIC effect by 107.1%. For S/Fe = 0.1 and 0.4, the overall enhancement in the reactivity resulting from EMF was alternately dominated by the contributions of MIH and MIC. This work provides valuable insights into the MIH and MIC effects about the sulfidation level of nZVI, which is needed for further exploration and optimization of this combined technology.

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.

Biotransformation of HBCDs by the microbial communities enriched from mangrove sediments

1,2,5,6,9,10-Hexabromocyclododecanes (HBCDs) are a sort of persistent organic pollutants (POPs). This research investigated 12 microbial communities enriched from sediments of four mangroves in China to transform HBCDs. Six microbial communities gained high transformation rates (27.5-97.7%) after 12 generations of serial transfer. Bacteria were the main contributors to transform HBCDs rather than fungi. Analyses on the bacterial compositions and binning genomes showed that Alcanivorax (55.246-84.942%) harboring haloalkane dehalogenase genes dadAH and dadBH dominated the microbial communities with high transformation rates. Moreover, expressions of dadAH and dadBH in the microbial communities and Alcanivorax isolate could be induced by HBCDs. Further, it was found that purified proteins DadAH and DadBH showed high conversion rates on HBCDs in 36 h (91.9 ± 7.4 and 101.0 ± 1.8%, respectively). The engineered Escherichia coli BL21 strains harbored two genes could convert 5.7 ± 0.4 and 35.1 ± 0.1% HBCDs, respectively, lower than their cell-free crude extracts (61.2 ± 5.2 and 56.5 ± 8.7%, respectively). The diastereoisomer-specific transforming trend by both microbial communities and enzymes were γ- > α- > β-HBCD, differed from α- > β- > γ-HBCD by the Alcanivorax isolate. The identified transformation products indicated that HBCDs were dehalogenated via HBr elimination (dehydrobromination), hydrolytic and reductive debromination pathways in the enriched cultures. Two enzymes converted HBCDs via hydrolytic debromination. The present research provided theoretical bases for the biotransformation of HBCDs by microbial community and the bioremediation of HBCDs contamination in the environment.

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.

A Review of Distribution and Profiles of HBCD in Different Environmental Media of China

Hexabromocyclododecane (HBCD) is the most important flame retardant that has been used in Expanded Polystyrene foam and Extruded Polystyrene foam in the past forty years across the world. China was the major producer and user of HBCD, and the total HBCD production was about 0.3 million tons. Although HBCD was completely banned in China in 2021 because of its long-range transport, bioaccumulation and toxicity, there is still a lot of residue in the environment. Therefore, we reviewed multiple studies concerning the distribution of HBCD in diverse environmental matrices, such as in the air, dust, soil, water, sediment, and biota. Results revealed that HBCD levels in different environments in China present geographical variation and were at a high level compared with other countries. In all environmental media, relatively high HBCD concentrations have been found in industrial and urban areas. Industrialization and urbanization are two important factors that influence the concentration and distribution of HBCD in the environment. In terms of isomer, γ-HBCD was the dominant isomer in soil, water, and sediment, while in the biota α-HBCD was the predominant isomer.

Recent advances in sulfidized nanoscale zero-valent iron materials for environmental remediation and challenges

Over the past decade, sulfidized nanoscale zero-valent iron (S-nZVI) has been developed as a promising tool for the remediation of contaminated soil, sediment, and water. Although most studies have focused on applying S-nZVI for clean-up purposes, there is still a lack of systematic summary and discussion from its synthesis, application, to toxicity assessment. This review firstly summarized and compared the properties of S-nZVI synthesized from one-step and two-step synthesis methods, and the modification protocols for obtaining better stability and reactivity. In the context of environmental remediation, this review outlined an update on the latest development of S-nZVI for removal of heavy metals, organic pollutants, antibiotic resistance genes (ARGs), and antibiotic resistant bacteria (ARB) and also discussed the underlying removal mechanisms. Environmental factors affecting the remediation performance of S-nZVI (e.g., humic acid, coexisting ions, S/Fe molar ratio, pH, and oxygen condition) were highlighted. Besides, the application potential of S-nZVI in advanced oxidation processes (AOP), especially in activating persulfate, was also evaluated. The toxicity impacts of S-nZVI on the environmental microorganism were described. Finally, the future challenges and remaining restrains to be resolved for better applicability of S-nZVI are also proposed. This review could provide guidance for the environmental remediation with S-nZVI-based technology from theoretical basis and practical perspectives.

Fabrication of Chitin microspheres supported sulfidated nano zerovalent iron and their performance in Cr (VI) removal

Sulfidated nanoscale zerovalent iron (S-nZVI) has been extensively studied for the reductive removal of Cr(VI), but its applicability is limited by agglomeration and unexpected efficiency reduction. In this study, chitin microsphere supported sulfidated nanoscale zero-valent iron (S-nZVI@Chi-M) was prepared by in-situ one-step reduction method and used to remove Cr(VI) from water. Compared to chitin and chitosan powder, Chi-M with nanofibrous structure and large surface area performed best in stabilizing S-nZVI with a Fe0 loading content of 3.01 wt%. The S-nZVI particles were homogeneously distributed on the surface of Chi-M, effectively avoiding agglomeration. Compared with bare nanoparticles and supported nZVI, S-nZVI@Chi-M showed significantly enhanced Cr(VI) removal capacity (924.5 mg Cr(VI) for per gram of effective Fe0). The influences of sulfidation degree, dosages, initial Cr(VI) concentration, pH, DO, humic acid and typical ions on Cr(VI) removal kinetics were further studied. S-nZVI@Chi-M could be recycled for at least 4 times with acceptable reactivity. The mechanism investigation results indicated that the Cr(VI) removal was a complex process of reduction, adsorption and co-precipitation under the synergistic effect of Chi-M and S-nZVI. This work provides new ideas for the continuous fabrication of highly reactive nanoparticles, hopefully expanding the application scope of biomass resources in pollution remediation.

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.

Integrated effect of bulk cations on nano-confined reactivity of clay-intercalated subnanoscale zero-valent iron in water-tetrahydrofuran mixtures

Smectite clay-intercalated subnanoscale zero-valent iron (CSZVI) exhibits superior reactivity toward contaminants due to the small iron clusters (∼0.5 nm) under nano-confinement, which however is significantly influenced by the solution chemistry e.g., various cations, of polluted soil and water. This work was undertaken to elucidate the mechanisms of solution chemistry effects on dehalogenation ability of CSZVI in water-tetrahydrofuran solution using decabromodiphenyl ether as a model contaminant. By combined spectroscopic characterization and molecular dynamics simulation, it was revealed that bulk cations, i.e., Na⁺, K⁺, Mg²⁺ and Ca²⁺ collectively affected the interlayer distance, water content and Brønsted acidity of CSZVI and thus its degradation efficiency. Although causing inter-particle aggregation, Mg²⁺ induced optimal nano-confined interlayers at concentration of 20 mM, exhibiting a superior debromination efficiency with rate constant 9.84 times larger than that by the common nano-sized ZVI. Conversely, K⁺ rendered the interlayers less reactive, but protected CSZVI from corrosion loss with higher electron utilization efficiency, which was 1.7 times higher than CSZVI in presence of Mg²⁺. The findings provide new strategies to manipulate the reactivity of nano-confined CSZVI for effective wastewater and contaminated soil remediation.

Zero-valent iron based materials selection for permeable reactive barrier using machine learning

The zero-valent iron (ZVI) based reactive materials are potential remediation reagents in permeable reactive barriers (PRB). Considering that reactive materials is the essential to determining the long-term stability of PRB and the emergence of a large number of new iron-based materials. Here, we present a new approach using machine learning to screen PRB reactive materials, which proposes to improve the efficiency and practicality of selection of ZVI-based materials. To compensate for the insufficient amount of existing machine learning source data and the real-world implementation, machine learning combines evaluation index (EI) and reactive material experimental evaluations. XGboost model is applied to estimate the kinetic data and SHAP is used to improve the accuracy of model. Batch and column tests were conducted to investigate the geochemical characteristics of groundwater. The study find that specific surface area is a fundamental factor correlated with the kinetic constants of ZVI-based materials, according to SHAP analysis. Reclassifying the data with specific surface area significantly improved prediction accuracy (reducing RMSE from 1.84 to 0.6). Experimental evaluation results showed that ZVI had 3.2 times higher anaerobic corrosion reaction kinetic constants and 3.8 times lower selectivity than AC-ZVI. Mechanistic studies revealed the transformation pathways and endpoint products of iron compounds. Overall, this study is a successful initial attempt to use machine learning for selecting reactive materials.

Efficient degradation of hexabromocyclododecane using montmorillonite supported nano-zero-valent iron and Citrobacter sp. Y3

The coupling of modified nanoscale zero-valent iron (nZVI) with organohalide-degrading bacteria provides a promising solution for the remediation of hexabromocyclododecane (HBCD)-contaminated environments. However, the interactions between modified nZVI and dehalogenase bacteria are intricate, and the mechanisms of synergistic action and electron transfer are not clear, and requires further specific investigation. In this study, HBCD was used as a model pollutant, and stable isotope analysis revealed that organic montmorillonite (OMt)-supported nZVI coupled with the degrading bacterial strain Citrobacter sp. Y3 (nZVI/OMt-Y3) can use [¹³C]HBCD as the sole carbon source and degrade or even mineralise it into ¹³CO₂ with a maximum conversion rate of 100% within approximately 5 days. Analysis of the intermediates showed that the degradation of HBCD mainly involves three different pathways: dehydrobromination, hydroxylation, and debromination. The proteomics results showed that nZVI introduction promoted the transport of electrons and debromination. Combining the results from XPS, FTIR, and Raman spectroscopy with the analysis results of proteinomics and biodegradation products, we verified the process of electron transport and proposed a metabolic mechanism of HBCD degradation by the nZVI/OMt-Y3. Moreover, this study provides insightful avenues and models for the further remediation of HBCD and other similar pollutants in the environment.

Abiotic transformation of synthetic progestins in representative soil mineral suspensions

Altrenogest (ALT), drospirenone (DRO), and melengestrol acetate (MLA) are three highly potent synthetic progestins that can be released into agricultural soils, while their fate in soil minerals remains unclear. This study explored the transformation of these progestins in MnO₂, SiO₂, and ferrihydrite suspensions and identified their transformation products (TPs) via high resolution mass spectrometry and density functional theory calculations. Transformations were only observed for DRO and MLA in SiO₂ suspension and ALT in MnO₂ suspension (half-lives = 0.86 min - 9.90 day). ALT transformation was facilitated at higher MnO₂ loadings, while DRO and MLA transformations were inhibited at higher SiO₂ loadings. These data indicated that hydrophobic partitioning interaction was dominant at higher SiO₂ loadings rather than specific interaction, which limited subsequent surface-catalyzed transformation. ALT transformation rate decreased with increasing pH because MnO₂ reduction requires proton participation. In contrast, relatively high pH facilitated MLA and DRO transformation, indicating that base-catalyzed hydrolysis occurred in SiO₂ suspension. The clustermap demonstrated the formation of abundant TPs. Lactone ring and acetoxy group hydrolysis was the major transformation pathway for DRO and MLA, with estimated yields of 57.7% and 173.2% at 6 day, respectively. ALT experienced C12 hydroxylation and formed the major TP 326g (yield of 15.4% at 8 hr). ALT also experienced allyl group oxidation and subsequent C5 hydroxylation, forming the major TP 344a (yield of 14.1% at 8 hr). This study demonstrates that TPs of metastable progestins are likely the main species in soils and that TP identification is a particular priority for risk assessment.

Mechanochemically sulfidated zero-valent iron as persulfate activation catalyst in permeable reactive barriers for groundwater remediation - A feasibility study

The technology of permeable reactive barriers is reliable and economically effective to prevent the spread of pollutants in groundwaters. Yet, it is efficacious only with easily reducible chemicals such as heavy metals and halogenated organics. In the present study, sulfidated zero-valent iron solventless synthesized by ball-milling is proposed as a possible barrier filling for activation of persulfate to achieve sound removal of reduction-resistant organic pollutants (the herbicide atrazine was used as a model pollutant). Preliminary batch experiments demonstrated rapid degradation of atrazine. Continuous experiments executed in columns proved the superior efficiency of sulfidated iron as a persulfate activator, compared to zero-valent iron, in terms of removal of both atrazine and byproducts. Optimal atrazine removal in the column was achieved with 10% sulfidated iron packing, and 9 mM persulfate at a hydraulic residence time of 6.02 h. Under such conditions, the estimated bed length of the reactive barrier for 99% atrazine removal was 8.69 cm. The morphology and surface-active species in the column demonstrated that activation of persulfate mainly occurred at the inlet of the column until the complete usage of the active species. Batch experiments with coexisting ions suggested that they have a minor influence on atrazine removal percentage, while Mg2+, Ca2+, CO2- and HCO- had a significant impact on the kinetics of the process. However, analogous column experiments demonstrated that the coexisting ions have a negative influence on both atrazine and its byproducts. The results obtained in this study corroborate the potential application of persulfate-enhanced permeable reactive barriers for in situ removal of atrazine from underground water.

Cr(VI) immobilization in soil using lignin hydrogel supported nZVI: Immobilization mechanisms and long-term simulation

A novel nanocomposite, named as nZVI@LH, was prepared by nanoscale zero-valent iron (nZVI) supported on lignin hydrogel and was used in the remediation of Cr(VI)-contaminated soil collected from an industrial site. Meanwhile, scanning electron microscopy with energy dispersive X-ray (SEM-EDX) and X-ray diffractometry (XRD) results determined that nZVI nanoparticles disperse uniformly on hydrogel. After the 14 days remediation, the immobilization efficiency of Cr(VI) could reach over 87% in the treatment of 3% (w/w%) nZVI@LH and 26% in the treatment of bare-nZVI. Leaching experiment results showed that the treatment group with 3% (w/w%) nZVI@LH was up to the national leaching toxicity identification standard, and there was no threat in simulation of acid rain over the long term. The water-soluble (WS) fraction in 3^# nZVI@LH treatment decreased 31.1%, while the Fe-Mn oxide bound (OX) fraction and organic matter-bound (OM) fraction increased 10.9% and 13.4%, respectively. Moreover, nZVI@LH had limited impact on soil properties and the capability to immobilize Cr over a long period exposure to acid rain. This work prove that nZVI@LH has the potential to remediate Cr contaminated soil. Furthermore, details of possible mechanistic insight into the Cr remediation were carefully discussed.

Transformation of sulfidized nanoscale zero-valent iron particles and its effects on microbial communities in soil ecosystems

Sulfidized nanoscale zero-valent iron (S-nZVI) is a promising material for in situ soil remediation. However, its transformation (i.e., aging) and effects on the microbial community in soil ecosystems are largely unknown. In this study, S-nZVI having low (S-nZVI (L)) and high sulfur-doping (S-nZVI (H)) were incubated in soil microcosms and bare nZVI was used as a control. Their aged products were characterized using microspectroscopic analyses and the changes in the corresponding soil microbial community were determined using high-throughput sequencing analyses. The results indicate that severe corrosion of both bare and S-nZVI occurred over 56 days of aging with significant morphological and mineral changes. Magnetite, lepidocrocite, and goethite were detected as the main aged products. In addition, sulfate ions, pyrite, and iron polysulfide were formed in the aged products of S-nZVI. Cr(VI) removal test results indicated that S-nZVI(L) achieved the best results after aging, likely because of the optimal FeS arrangement on its nanoparticle surfaces. The presence of nZVI and S-nZVI increased the abundance of some magnetotactic microorganisms and altered bacterial and fungal community structures and compositions. Moreover, the addition of S-nZVI enriched some bacterial and fungal genera related to sulfur cycling because of the presence of sulfide-bearing material. The findings reveal the transformation of S-nZVI during aging and its effects on microbial communities in soil ecosystems, thereby helping to the evaluation of S-nZVI application in soil remediation.

A win-win solution to chromate removal by sulfidated nanoscale zero-valent iron in sludge

This study investigates the reaction between sulfidated nanoscale zero valent iron (S-nZVI) and Cr(VI) in the sludge system and explores the effect of S-nZVI on microbes. Results of the batch experiments indicated that the optimal Cr(VI) removal capacity (35.3 mg/g) was reached when the S/Fe ratio was at 0.05. It was about 20-time higher than that of nanoscale zero valent iron (nZVI) (<2.0 mg/g). However, the removal efficiency decreased as the S/Fe molar ratio further increased. Solid characterizations revealed that the S-nZVI consisted of a Fe⁰ core encapsulated by a flake FeS shell and had a similar "core-shell" structure to that of the nZVI. X-ray photoelectron spectroscopy (XPS) indicated that Cr(VI) was reduced to less toxic Cr(III). In addition, the 16 S rRNA gene and cryo-scanning electron microscopy (cryo-SEM) results showed S-nZVI mildly influenced the initial microbial diversity. Some microflora including Caldiserica, Planctomycetes were promoted, while others groups such as Actinobacteria, Bacteroidetes and Chloroflexi were inhibited: specifically, bacteria such as Proteobacteria (possibly related to sulfide oxidization) began to develop after the S-nZVI feeding. The high Cr(VI) removal efficiency and the mildly influenced microbial diversity make the usage of S-nZVI a win-win solution for Cr(VI) removal in sludge.

Mechanochemical destruction and mineralization of solid-phase hexabromocyclododecane assisted by microscale zero-valent aluminum

Hexabromocyclododecane (HBCD) has been listed in Annex A of the Stockholm Convention as a persistent and bio-accumulative chemical. While HBCD is often present in the solid form for its low solubility, cost-effective technologies have been lacking for the degradation of solid-phase HBCD. In this work, mechanochemical (MC) destruction of high-energy ball milling was employed for direct destruction of solid-phase HBCD, where a strong reducer, microscale zero-valent aluminum (mZVAl), was used as the co-milling agent. The new mZVAl-assisted MC process achieved complete debromination and mineralization of HBCD within 3 h milling. The optimal operating parameters were determined, including the milling atmosphere, the milling speed, the mZVAl-to-HBCD molar ratio, and the ball-to-mZVAl mass ratio. Fourier transform infrared spectrometry and Raman analyses revealed that the organic structures of HBCD were destroyed and organic bromine was completely converted into inorganic bromide, accompanied by the generation of amorphous and graphite carbon. Analysis of the milled samples by GC-MS demonstrated the absence of obvious organic matter after MC treatment, also indicating the complete degradation and conversion of HBCD to inorganic compounds. Further X-ray photoelectron spectroscopic analysis indicates that the fresh surface of mZVAl was generated upon the MC treatment, and Al(0) served as a strong reducing agent (e-donor) for reductive debromination and destruction of the carbon skeleton. The mZVAl-assisted MC milling appears promising as a non-combustion approach for effective destruction and carbonization/mineralization of solid-phase HBCD or potentially other persistent organic pollutants.

A comprehensive evaluation of factors affecting the reactivity of FeS towards hexabromocyclododecane diastereoisomers

Reactivity of iron sulfide (FeS) towards hexabromocyclododecane (HBCD) was explored under conditions of varying temperature, pH, inorganic ion and dissolved organic matter (DOM) in this study. Results show that the reduction of HBCD by FeS has an activation energy of 29.2 kJ mol^-1, suggesting that the rate-limiting step in the reduction was a surface-mediated reaction. The reduction of HBCD by FeS was a highly pH-dependent process. The optimal rate for HBCD reduction by FeS was observed at a pH of 6.2. All the tested inorganic ions suppressed the reduction of HBCD by FeS. XPS analysis confirmed that both Fe(II)-S and bulk S(-II) on FeS surface could be impacted by solution pH and inorganic ions and were responsible for the regulation of HBCD reduction. Some DOMs (i.e., fulvic acid, humic acid, salicylic acid, catechol and sodium dodecyl sulfate) were found to hinder the reduction via competing with HBCD for active sites on FeS surface. However, the presence of 2,2'-bipyridine, triton X-100 and cetyltrimethyl ammonium bromide was able to significantly enhance the rate of HBCD reduction by 5.8, 9.0 and 20 times, respectively. Different factors could influence the reduction efficiency of HBCD diastereoisomers to different extent, but not the reaction orders of HBCD diastereoisomers (α-HBCD < γ-HBCD < β-HBCD). Moreover, FeS could completely remove HBCD diastereoisomers in sediments with multiple factors within 9 d reaction. Our results contribute to give a better understanding on the performance of FeS towards HBCD under real and complex conditions and facilitate the application of FeS in remediation sites.

Enhanced dechlorination of trichloroethene by sulfidated microscale zero-valent iron under low-frequency AC electromagnetic field

In this study an electromagnetic heating strategy is proposed for remediation of trichloroethene (TCE) by ball milled, sulfidated microscale zero valent iron (S-mZVI^bm) particles. S-mZVI^bm is ferromagnetic, which generates heat under the application of a low-frequency alternating current electromagnetic field (AC EMF). We found that the temperature reached up to ~120 ℃ during 30-min electromagnetic induction heating of 10 g/L S-mZVI^bm (with S/Fe molar ratio of 0.1), compared with ~55 ℃ and ~80 ℃ for ZVI and ball milled mZVI^bm, respectively. The application of AC EMF accelerated the TCE degradation rate (k_TCE = 5.5 × 10^-1 h^-1) by up to 4-fold without compromising or even enhancing electron efficiency of S-mZVI^bm compared to no-heating. Furthermore, this process halved the generation of chlorinated intermediate, cis-DCE. In contrast, water-bath heating only increased the dechlorination rate 2-fold with unchanged cis-DCE generation and lowered electron efficiency. This is attributed to both rising temperature by induction heating and accelerated ZVI corrosion and surface Fe⁰ exposure caused by AC EMF. In real groundwater, the AC EMF maintained the same promoting effects for TCE dechlorination by S-mZVI^bm. This study shows that combination of filed-scale available AC EMF with S-mZVI^bm provides a promising approach for remediation of chlorinated hydrocarbons in contaminated groundwater.

Environmental occurrence and remediation of emerging organohalides: A review

As replacements for "old" organohalides, such as polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs), "new" organohalides have been developed, including decabromodiphenyl ethane (DBDPE), short-chain chlorinated paraffins (SCCPs), and perfluorobutyrate (PFBA). In the past decade, these emerging organohalides (EOHs) have been extensively produced as industrial and consumer products, resulting in their widespread environmental distribution. This review comprehensively summarizes the environmental occurrence and remediation methods for typical EOHs. Based on the data collected from 2015 to 2021, these EOHs are widespread in both abiotic (e.g., dust, air, soil, sediment, and water) and biotic (e.g., bird, fish, and human serum) matrices. A significant positive correlation was found between the estimated annual production amounts of EOHs and their environmental contamination levels, suggesting the prohibition of both production and usage of EOHs as a critical pollution-source control strategy. The strengths and weaknesses, as well as the future prospects of up-to-date remediation techniques, such as photodegradation, chemical oxidation, and biodegradation, are critically discussed. Of these remediation techniques, microbial reductive dehalogenation represents a promising in situ remediation method for removal of EOHs, such as perfluoroalkyl and polyfluoroalkyl substances (PFASs) and halogenated flame retardants (HFRs).

Immobilization of cadmium in contaminated soils using sulfidated nanoscale zero-valent iron: Effectiveness and remediation mechanism

Sulfidated nanoscale zero-valent iron (S-nZVI) has shown excellent removal capacity for the removal of cadmium (Cd) in aqueous phase. Herein, the effectiveness and the mechanism of S-nZVI for the remediation of Cd contaminated soil were investigated for the first time. The results of sequential extraction procedures (SEP) showed that the exchangeable (EX) Cd was decreased by over 97.6% at the optimal dosage of 5 g kg^-1 S-nZVI during 30 d incubation and converted to less available Cd such as iron-manganese oxides-bound (OX) and organic matter-bound (OM) fractions. pH has negligible effect on the immobilization of Cd in soil, since OX fraction was stabilized in the range of 72-92% at initial soil pH range from 5.3 to 7.5. SEM-EDS analysis of the separated magnetic particles implied that Cd was successfully enriched on S-nZVI and the distribution of Cd was closely related to Fe, S, and O. CdO and CdS was confirmed as the key products for Cd immobilization in soil. Meanwhile, the S-nZVI was oxided to α-FeOOH, γ-FeOOH, and γ-Fe₂O₃. The existence of CdO was visibly related to the iron oxides, suggesting the synergetic immobilization effect by iron oxides. Overall, S-nZVI was promising for the remediation of Cd-contaminated soil.

A novel ball-milled aluminum-carbon composite for enhanced adsorption and degradation of hexabromocyclododecane

Hexabromocyclododecane (HBCD) is one of the priority persistent organic pollutants (POPs), yet a cost-effective technology has been lacking for the removal and degradation of HBCD. Zero-valent aluminum (ZVAl) is an excellent electron donor. However, the inert and hydrophilic surface oxide layer impedes the release of the electrons from the core metallic Al, resulting in poor reactivity towards HBCD. In this research, a new type of modified mZVAl particles (AC@mZVAl^bm/NaCl) were prepared through ball milling mZVAl in the presence of activated carbon (AC) and NaCl, and tested for adsorption and reductive degradation of HBCD in water. AC@mZVAl^bm/NaCl was characterized with a metallic Al core with newly created reactive surface coated with a thin layer of crushed carbon nanoparticles. AC@mZVAl^bm/NaCl was able to rapidly (within 1 h) adsorb HBCD (C₀ = 2 mg L^-1) and thus effectively enriched HBCD on the carbon surface of AC@mZVAl^bm/NaCl. The pre-enriched HBCD was subsequently degraded by the electrons from the core Al, and ∼63.44% of the pre-sorbed HBCD was completely debrominated after 62 h of the contact. A notable time lag (∼12 h) from the onset of the adsorption to the debromination was observed, signifying the importance of the solid-phase mass transfer from the initially adsorbed AC particles to the reactive Al-AC interface. Overall, AC@mZVAl^bm/NaCl synergizes the adsorptive properties of AC and the high reactivity of metallic Al, and enables a novel two-step adsorption and reductive degradation process for treating HBCD or likely other POPs.

Response surface design for removal of Cr(VI) by hydrogel-supported sulfidated nano zero-valent iron (S-nZVI@H)

In this study, a new sulfidated nanoscale zero-valent iron (S-nZVI) supported on hydrogel (S-nZVI@H) was successfully synthesized for the removal of chromium (Cr) (VI) from groundwater. The surface morphology, dispersion phenomenon and functional groups of novel S-nZVI@H were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. Box-Behnken design (BBD) optimization technology based on response surface methodology (RSM) is applied to demonstrate the influence of the interaction of S-nZVI@H dose, initial Cr(VI) concentration, contact time, and initial pH with the Cr(VI) removal efficiency. The analysis of variance results (F = 118.73, P < 0.0001, R² = 0.9916) show that the quadratic polynomial model is significant enough to reflect the close relationship between the experimental and predicted values. The predicted optimum removal conditions are: S-nZVI@H dose 9.46 g/L, initial Cr(VI) concentration 30 mg/L, contact time 40.7 min, and initial pH 5.27, and the S-nZVI@H dose is the key factor affecting the removal of Cr(VI). The predicted value (99.76%) of Cr (VI) removal efficiency is in good agreement with the experimental value (97.75%), which verifies the validity of the quadratic polynomial model. This demonstrates that RSM with appropriate BBD can be utilized to optimize the design of experiments for removal of Cr(VI).

Removal of heavy metal ions and polybrominated biphenyl ethers by sulfurized nanoscale zerovalent iron: Compound effects and removal mechanism

Sulfurized nanoscale zerovalent iron (S-nZVI) has been widely reported to be able to quickly remove heavy metals/persistent organic pollutants, but the limited understanding of the complicated removal process of heavy metals-organic combined pollutants restricts the application of S-nZVI. Here, we demonstrate that there is significant difference in the effectiveness of S-nZVI for removing single pollutant and complex pollutants. The removal kinetic constant (k_obs) of heavy metals by S-nZVI followed a sequence of Cr(VI)>Pb(II)>Ni(II)>Cd(II) with or without polybrominated diphenyl ethers (PBDEs). While the capacity of co-existing cations increasing the k_obs of PBDEs followed the order: Ni(II)>Pb(II)>Cd(II), and the co-existence of Cr(VI) anion inhibited the reduction of PBDE by S-nZVI because the generated Cr-Fe precipitate hindered the electron transfer. The de-passivation process on S-nZVI surface by Cd(II) ions slightly accelerated the transformation rate of electron. Nevertheless, the co-existing Pb(II) significantly accelerated the transformation of BDE-209 via the galvanic effect from the generated Pb⁰/Fe⁰ bimetal. Interestingly, the k_obs of BDE-47 in Ni(II)/S-nZVI system was 5.51 times higher than that of Pb(II)/S-nZVI system, implying that an atomic hydrogen mechanism dominated the reduction of BDE-47 by Ni(II)/S-nZVI. In conclusion, the results provided a deep comprehending of removal mechanism of heavy metal-organic complex pollutants by S-nZVI.

Enhanced As(III) sequestration using nanoscale zero-valent iron modified by combination of loading and sulfidation: characterizations, performance, kinetics and mechanism

Nanoscale zero-valent iron (nZVI) and sulfides have been confirmed to be effective in arsenic sequestration from aqueous solution. In this study, attapulgite supported and sulfide-modified nanoscale zero-valent iron (S-nZVI@ATP) are synthesized to realize the superposition effect of enhanced arsenic sequestration. The results indicated that nZVI clusters were well disaggregated and the BET specific surface area increased from 19.61 m²·g^-1 to 46.04 m²·g^-1 of S-nZVI@ATP, resulting in an enhanced removal efficiency of arsenic from 51.4% to 65.1% at 20 min. The sulfides in S-nZVI@ATP mainly exist as mackinawite (FeS) and this causes the spherical nanoparticles to exhibit a larger average particle size (94.6 nm) compared to bare nZVI (66.0 nm). In addition, S-nZVI@ATP exhibited a prominent ability for arsenic sequestration over a wide pH range of 3.0-6.0. The presence of anions SO₄^2- and Cl^- can enhance the arsenic removal whereas HCO₃^- inhibited it. The arsenic adsorption by S-nZVI@ATP could be explained by the pseudo-second-order kinetic model and the Langmuir model, with the maximum adsorption capacity of 193.8 mg·g^-1. The mechanism of As(III) sequestration by S-nZVI@ATP involved multiple processes, mainly including precipitation conversion from FeS to As₂S₃, surface-complexation adsorption and co-precipitation.

Sulfidation of zerovalent iron for improving the selectivity toward Cr(VI) in oxic water: Involvements of FeSx

Recognition of the general roles of FeS_x in selectivity of zerovalent iron (ZVI) toward target contaminants is of great significance but challenging, especially in oxic water system. Herein, the ZVI amended with Na₂S₂O₃ (i.e., S-ZVI^Na2S2O3) and Na₂S₂O₄ (i.e., S-ZVI^Na2S2O4) were applied for the sequestration of Cr(VI) and corresponding FeS_x involvements were explored. Results revealed that the largest effect for S-ZVI^Na2S2O3 and S-ZVI^Na2S2O4 observed at S/Fe molar ratio of 0.05 were 7.9- and 11.6- folds increase in removal rate (k_obs) of Cr(VI), respectively. respectively. Correspondingly, the electron efficiency (EE) of S-ZVI for reducing Cr(VI) were mainly from 2.1- to 2.4- folds greater than that that of the ZVI^H2O. Further, this work suggested that the improved selectivity of ZVI toward Cr(VI) by sulfidation should be mainly ascribed to the involvements of FeS_x, which could tune the reactive sites and corrosion products of ZVI for synergistically improving the mass transfer of Cr(VI) and subsequent electron transfer from iron core to Cr(VI). Overall, this work offers a new platform for improving ZVI selectivity for water decontamination.

Oxidative removal of antibiotic resistant E. coli by sulfidated zero-valent iron: Homogeneous vs heterogeneous activation

As an emerging contaminant in water, antibiotic resistant bacteria are threatening the public health gravely. In this study, sulfidated ZVI was used to activate persulfate, for antibiotic resistant E. coli and antibiotic resistant genes removal. Impressively, 7 log of antibiotic resistant E. coli was inactivated within 30 min, in sulfidated ZVI activated persulfate system (S/Fe = 0.05). Electron paramagnetic resonance and free radical quenching experiments suggested that sulfidation treatment did not change the specie of radicals. SO₄^•-and HO• were the main reactive oxygen species for the removal of antibiotic resistant E. coli and genes. Investigation on the activation mechanism of persulfate indicated that persulfate decomposition was mainly attributed to heterogeneous activation. More importantly, in-situ characterization (ATR-FTIR) indicated that the main charge transfer complex was formed on the surface of sulfidated ZVI, which would predominantly mediate the generation of SO₄^•- and HO•. Finally, the proposed system was evaluated in modeling water and secondary effluent. Results revealed that only 2.86 log and 0.84 log of antibiotic resistant E. coli were inactivated in the presence of NOM ⁽10 mg/L) and HCO₃^- (84 mg/L), respectively. Besides, sulfidated ZVI activated persulfate system could be pH-dependent in actual wastewater treatment.

Remediation of soil contaminated with organic compounds by nanoscale zero-valent iron: A review

In recent years, nanoscale zero-valent iron (nZVI) has been gradually applied in soil remediation due to its strong reducing ability and large specific surface area. Compared to conventional remediation solutions, in situ remediation using nZVI offers some unique advantages. In this review, respective merits and demerits of each approach to nZVI synthesis are summarized in detail, particularly the most commonly used aqueous-phase reduction method featuring surface modification. In order to overcome undesired oxidation and agglomeration of fresh nZVI due to its high reactivity, modifications of nZVI have been developed such as doping with transition metals, stabilization using macromolecules or surfactants, and sulfidation. Mechanisms underlying efficient removal of organic pollutants enabled by the modified nZVI lie in alleviative oxidation and agglomeration of nZVI and enhanced electron utilization efficiency. In addition to chemical modification, other assisting methods for further improving nZVI mobility and reactivity, such as electrokinetics and microbial technologies, are evaluated. The effects of different remediation technologies and soil physicochemical properties on remediation performance of nZVI are also summarized. Overall, this review offers an up-to-date comprehensive understanding of nZVI-driven soil remediation from scientific and practical perspectives.

Remarkable promotion in particle dispersion and electron transfer capacity of sulfidated nano zerovalent iron by coating alginate polymer

Alginate has been widely employed to increase the performance of nanoscale zerovalent iron (nZVI)-based materials for site remediation. Yet, the effects of alginate on reactivity of sulfidated nZVI (an efficient reductant material) towards contaminants have been understood poorly. In this study, we have developed a one-step synthesis of alginate-coated sulfidated nZVI (S-nZVI@alginate) under air atmosphere and evaluated the reactivity of S-nZVI@alginate towards tetrabromobisphenol A (TBBPA) debromination. Surface analysis shows that S-nZVI has been successfully coated by alginate through the interaction of OH and COO^- groups of alginate with Fe species. The coating of alginate increases particle stability and dispersion under various conditions and facilitates FeS precipitation on the particle surface. Reactivity experiments show that the coating of alginate significantly enhances TBBPA debromination by S-nZVI. The optimized alginate to Fe weight ratio of S-nZVI@alginate is 0.06, with ~3-fold greater TBBPA debromination rate than S-nZVI. S-nZVI@alginate can completely debrominate TBBPA into bisphenol A via a four-sequential step debromination pathway while S-nZVI not. Its superior reactivity may be attributed to that the formation of alginate-Fe complex can lower the redox potential of Fe species to accelerate electron transfer on the particle surface. The TBBPA debromination rate by S-nZVI@alginate is initially enhanced followed by a decrease with an increase in TBBPA concentration, while it can increase 3.3-, 8.9- and 5.6-fold by increasing S-nZVI@alginate dosage, decreasing pH and adding co-contaminant Cd²⁺, respectively. S-nZVI@alginate has greater performance in aging and reusability tests than S-nZVI, and achieves rapid TBBPA removal from wastewater, which may be due to the role of alginate on inhibiting surface oxidation of Fe and S species. Taken together, these results suggest that S-nZVI@alginate provides better reactivity, longevity and reusability than S-nZVI, having the great potential for application into site remediation.

Enhanced tetrabromobisphenol A debromination by nanoscale zero valent iron particles sulfidated with S0 dissolved in ethanol

Modification of nanoscale zero-valent iron (nZVI) with reducing sulfur compounds has proven to improve the reactivity of nZVI towards recalcitrant halogenated organic contaminants. In this study, we develop a novel method for the preparation of sulfidated nZVI (S-nZVI) with S0 (a low cost and available reducing sulfur agent) dissolved in ethanol under mild conditions and apply it for the transformation of tetrabromobisphenol A (TBBPA), a potential persistent organic pollutant. Surface analysis shows that S0 dissolved in ethanol has been successfully doped into nZVI via a reaction with Fe0 to form a relatively homogeneous layer of FeS/FeS2 on the nZVI surface. The H2 production test and the electrochemical analysis show that the FeS/FeS2 layer not only slows the H2 evolution reaction but also enhances the electron transfer. Debromination kinetics indicate that the resulting S-nZVI with a S/Fe ratio of 0.015-0.05 possesses higher debromination activity for TBBPA and its debromination products (i.e., tri-BBPA, di-BBPA, mono-BBPA and BPA) in comparison with nZVI. Among them, S-nZVI at a S/Fe of 0.025 (S-nZVIS-0.025) has the greatest debromination rate constant (kobs) of 1.19 ± 0.071 h-1 for TBBPA. It debrominates TBBPA at a faster rate than other conventional S-nZVI made from Na2S and Na2S2O4 and has been successfully applied in the treatment of TBBPA-spiked environmental water samples (including river water, groundwater, and tap water). The results suggest that the modification of nZVI with S0 dissolved in ethanol is a simple, safe, inexpensive, and effective sulfidation technique, which can be applied for the large-scale production of S-nZVI for treating contaminated water.

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

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

Enhanced molybdenum(VI) removal using sulfide-modified nanoscale zerovalent iron: kinetics and influencing factors

The overall goal of this study is to investigate the effect of sulfidated nanoscale zerovalent iron (S-nZVI) on the removal of hexavalent molybdate (MoO₄^2-) under different aquatic chemistry conditions. Surface analysis suggests that Mo(VI) is removed mainly by adsorption and co-precipitation onto the surface of S-nZVI and a small amount of Mo(VI) can be reduced to Mo(V) species. The results of batch tests show that Mo(VI) removal by S-nZVI are well described with the pseudo-second-order adsorption model. The removal rate increases with a decrease in solution pH (4.0-9.0) and is significantly affected by the S/Fe ratio of S-nZVI, with the optimal S/Fe ratio being 0.5. The presence of anions WO₄^2- or CrO₄^2- can reduce the Mo(VI) removal, which is likely because they compete for adsorption sites on the solid surfaces. The divalent cations Ni²⁺, Cu²⁺ and Co²⁺ also inhibit the removal of Mo(VI) whereas Zn²⁺, Ca²⁺ and Mg²⁺ enhance it. After being aged for 35 d in water, S-nZVI still exhibits high reactivity towards Mo(VI) removal (57.39%). The study demonstrates that S-nZVI can be used as an environmentally friendly material for effectively removing Mo(VI) from contaminated water.

Reductive degradation of chlorinated organophosphate esters by nanoscale zerovalent iron/cetyltrimethylammonium bromide composites: Reactivity, mechanism and new pathways

Chlorinated organophosphate esters (Cl-OPEs), e.g., tris(2-chloroethyl) phosphate (TCEP), tris(2-chloro-2-propyl) phosphate (TCPP) and tris(1,3-dichloro-2-propyl) phosphate (TDCPP), are widely used as additive flame retardants in commercial and building products. They have potential persistent organic pollutant properties and are frequently detected in various waters, especially in wastewaters. Nanoscale zerovalent iron (nZVI)-based method is an efficient reductive technology for treating waters polluted by halogenated organic pollutants (HOCs). Cetyltrimethylammonium bromide (CTAB) is a ubiquitous surfactant in wastewaters and can favorably affect the interaction between HOCs and nZVI. However, its effect on the Cl-OPEs removal by nZVI-based materials still remains unknown. Herein, the adsorption and degradation efficiencies of Cl-OPEs by nZVI and sulfidated nZVI (S-nZVI) in the presence or absence of CTAB were quantified based on the decreasing concentrations of Cl-OPEs in reaction systems. Our results showed that TDCPP and TCPP were adsorbed onto the nZVI or S-nZVI surface and subsequently degraded. In contrast, TCEP was just adsorbed onto the particle surface without further degradation. The addition of CTAB significantly enhanced the hydrophobic adsorption between Cl-OPEs and nZVI or S-nZVI, leading to increased degradation of Cl-OPEs (especially TCEP). CTAB adsorption isotherms indicated that S-nZVI had a higher adsorption capacity for CTAB than nZVI. The S-nZVI/CTAB composite exhibited a better performance than nZVI/CTAB composite. When S-nZVI was combined with 100.0 mg L^-1 CTAB, 100% of TDCPP, TCPP and TCEP was degraded within 3 hours, 5 and 14 days, respectively. As the concentration of CTAB was increased up to 335.0 mg L^-1, TCEP could be completely degraded within 3 days by S-nZVI. Five degradation products of TCEP were identified, of which O,O-di-(2-chloroethyl) O-ethyl phosphate (DCEEP) and ethane were reported for the first time. We propose that TCEP is dechlorinated by nZVI or S-nZVI through the electron attack at the ethyl-chlorine group to form bis(2-chloroethyl) phosphate, DCEEP, chloride, ethene and ethane, representing previously unknown degradation pathways.

Effects of non-reducible dissolved solutes on reductive dechlorination of trichloroethylene by ball milled zero valent irons

Non-reducible solution anions have been well recognized to affect reactivity of ZVI in dechlorinating chlorinated hydrocarbons. However, their effects and corresponding functional mechanisms on electron efficiency (ε_e) of ZVI remain unclear. In this study, mechanochemically modified microscale sulfidated and unsulfidated ZVI particles (i.e., S-mZVI^bm and mZVI^bm) and trichloroethylene (TCE) were used as model particles and contaminant to explore such effects. PO₄^3- as a corrosion promoter enhanced initial dechlorination rate by both particles. However, its passivating role as a surface complex agent became significant at the later stage of dechlorination by mZVI^bm, while sulfidation alleviated this effect without inhibition of dechlorination. Compared with enhancing dechlorination, PO₄^3- promoted hydrogen evolution reaction (HER) to a higher extent, decreasing ε_e for both particles by 17-73 %. HCO₃- negligibly affected dechlorination by both particles, while elevated HER. Thus, HCO₃- [5 mM] decreased ε_e for S-mZVI^bm and mZVI^bm by 1.9 % and 22 %. Different from PO₄^3- and HCO₃-, Cl- and SO₄^2- showed no significant effects on dechlorination, HER, and therefore ε_e for both particles. These results imply that even though some co-existing anions (i.e., PO₄^3- and HCO₃-) acting as corrosion promoters could improve the dechlorination by ZVIs, they would lead to decreased ε_e and shortened particle reactive lifetime.

Sulfidated nanoscale zero-valent iron is an efficient material for the removal and regrowth inhibition of antibiotic resistance genes

Antibiotic resistance genes (ARGs) and mobile gene elements (MGEs), the emerging genetic contaminants, are regarded as severe risks to public health for impairing the inactivation efficacy of antibiotics. Secondary effluents from wastewater treatment plants are the hotspots for spreading these menaces. Herein, sulfidated nanoscale zero-valent iron (S-nZVI) was occupied to remove ARGs and MGEs in secondary effluents and weaken the regrowth capacity of their bacterial carriers. The effects of S/Fe molar ratios (S/Fe), initial pH and dosages on 16S rRNA and ARGs removal were also investigated. Characterization, mass balance and scavenging experiments were conducted to explore the mechanisms of the gene removal. Quantitative PCR (qPCR) and high throughput fluorescence qPCR showed more than 3 log unit of 16S rRNA and seven out of 10 ARGs existed in secondary effluent could be removed after S-nZVI treatment. The mechanisms might be that DNA accepted the electron provided by the Fe⁰ core of S-nZVI after being adsorbed onto S-nZVI surface, causing the decrease of 16S rRNA, ARGs and lost their regrowth capacity, especially for typical MGE (intI1) and further inhibiting the vertical gene transfer (VGT) and intI1-induced horizontal gene transfer (HGT). Fe⁰ core was oxidized to iron oxides and hydroxides at the same time. High throughput sequencing, network analysis and variation partitioning analysis revealed the complex correlations between bacteria and ARGs in secondary effluent, S/Fe could directly influence ARGs variations, and bacterial genera made the greatest contribution to ARGs variations, followed by MGEs and operational parameters. As a result, S-nZVI could be an available reductive approach to deal with bacteria and ARGs.

Degradation of hexabromocyclododecane (HBCD) by nanoscale zero-valent aluminum (nZVAl)

Hexabromocyclododecane (HBCD) has been listed in Annex A of the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2013, but till now there is a lack of efficient methods for its degradation. In this study, nanoscale zero-valent aluminum (nZVAl), an excellent reductant with a very low redox potential of E⁰(Al³⁺/Al⁰) = -1.662 V and strong electron transfer ability, was used to reductively degrade HBCD. Nearly 100% HBCD was degraded within 8 h reaction at 25 °C in ethanol/water (v/v, 50/50) solution without pH adjustment. And about 67% cyclododecatriene (CDT) was obtained, which is the complete debromination product. What's more, the yield of Br^- could achieve nearly 100% after optimizing conditions. The reaction was strongly promoted by increasing the dosages of nZVAl or decreasing the initial concentration of HBCD. The temperature had the most significant influence and the degradation was completed in 40 min with elevating the reaction temperature to 45 °C. The reaction mechanism was further revealed through the characterization of nZVAl particles before and after the reaction by SEM-EDS, TEM, HRTEM, XRD, and XPS. It was found that, after corrosion of the oxide film on the surface of nZVAl, metallic aluminum inside was exposed. The reactive sites were provided and electrons released were transferred from nZVAl to HBCD, causing HBCD degraded to dibromocyclododecadiene (DBCD) and then CDT by reductive debromination. These findings imply that nZVAl can degrade HBCD efficiently with no extra energy input and this offers a new idea for better treatment of HBCD.

Adsorptive and Reductive Removal of Chlorophenol from Wastewater by Biomass-Derived Mesoporous Carbon-Supported Sulfide Nanoscale Zerovalent Iron

Chlorinated compounds in a water environment pose serious threats to humanity. A nanoscale zerovalent iron (nZVI) has desirable properties for water dichlorination, but its reactivity is still limited by agglomeration and oxidation. In this study, the mesoporous carbon (MC) derived from biomass waste was prepared for immobilizing nZVI, and the nZVI@MC was further modified by sulfur (S-nZVI@MC) to relieve surface oxidation. The synergistic effect between nZVI and surface modification, the reaction conditions and the removal mechanism were investigated systematically. The characterization results showed nZVI was successfully loaded on the surface of MC, and the aggregation of nZVI was prevented. Moreover, sulfidation modification resulted in the formation of FeS on the surface of nZVI, which effectively alleviated surface oxidation of nZVI and promoted the electron transfer. Batch experiments demonstrated S-nZVI@MC had greatly enhanced reactivity towards 2,4,6-trichlorphenol (TCP) as compared to MC and nZVI, and the removal rate could reach 100%, which was mainly attributed to the significant synergistic effect of MC immobilization and sulfidation modification. Furthermore, the TCP removal process was well described by a Langmuir adsorption model and pseudo-second-order model. The possible mechanism for enhanced removal of TCP is the fast adsorption onto S-nZVI@MC and effective reduction by S-nZVI. Therefore, with excellent reducing activity and antioxidation, S-nZVI@MC has the potential as a pollutant treatment.

Treatability of hexabromocyclododecane using Pd/Fe nanoparticles in the soil-plant system: Effects of humic acids

Hexabromocyclododecane (HBCD) is a persistent organic pollutant that accumulates in soil and sediments, however, it has been difficult to degrade HBCD with developed remediation technologies so far. In this study, degradation of HBCD by bimetallic iron-based nanoparticles (NPs) under both aqueous and soil conditions considering the effects of humic acids (HAs) and tobacco plant was investigated. In the aqueous solution, 99% of the total HBCD (15 mM) was transformed by Pd/nFe (1 g L-1) within 9 h of treatment and the HBCD debromination by Pd/nFe increased with the addition of HAs. In the soil system, 13%, 15%, 41% and 27% of the total HBCD were removed by treatments consisting of plant only, plant with HAs, plant with NPs and plant + NPs + HAs, respectively, compared to the HBCD removal in an unplanted soil. The 221-986 ng/g of HBCD were detected inside the plant after the treatments, and HAs showed considerable influence on the selective bioaccumulation of HBCD stereoisomers in the plant. Overall, this approach represents a meaningful attempt to develop an efficient and eco-friendly technology for HBCD removal, and it provides advantages for the sustainable remediation of recalcitrant emerging contaminants in soils.

Reductive debromination of decabromodiphenyl ether by iron sulfide-coated nanoscale zerovalent iron: mechanistic insights from Fe(II) dissolution and solvent kinetic isotope effects

The mechanism that iron sulfide-coated nanoscale zero valent iron (S-nZVI) has better reduction activity towards organic pollutants than nanoscale zero-valent iron (nZVI) has long been debated. In this work, a systematic study was investigated to compare differences of main influences, BDE-209 degradation pathway, degradation kinetics and reduction mechanism of BDE-209 between nZVI and S-nZVI systems. The observed transformation rate of BDE-209 (k_obs) by S-nZVI was 58.3 and 7.1 times greater than that by S^2- and nZVI, respectively. The valence change of Fe and S on S-nZVI surface before and after BDE-209 degradation process based on XPS characterization confirmed that both Fe⁰ and iron sulfide were the reduction entity of the surface-mediated reaction. The presence of tetrahydrofuran (THF) promoted the surface contact of BDE-209 with S-nZVI, thus accelerating the BDE-209 degradation process. Compared with nZVI, the iron sulfide coated on the Fe⁰ core surface could not only greatly reduce unnecessary electron loss via Fe⁰ corrosion with water, but also accelerate the transmission of electrons from Fe⁰ core to organic pollutants according to Fe(II) dissolution and solvent kinetic isotope effects investigations. These findings help to clarify the synergistic degradation mechanism between Fe⁰ core and iron sulfide shell layer.

Sulfidation mitigates the passivation of zero valent iron at alkaline pHs: Experimental evidences and mechanism

Groundwater pH is one of the most important geochemical parameters in controlling the interfacial reactions of zero-valent iron (ZVI) with water and contaminants. Ball milled, microscale ZVI (mZVI^bm) efficiently dechlorinated TCE at initial stage (<24 h) at pH 6-7 but got passivated at later stage due to pH rise caused by iron corrosion. At pH > 9, mZVI^bm almost completely lost its reactivity. In contrast, ball milled, sulfidated microscale ZVI (S-mZVI^bm) didn't experience any reactivity loss during the whole reaction stage across pH 6-10 and could efficiently dechlorinate TCE at pH 10 with a reaction rate of 0.03 h^-1. Increasing pH from 6 to 9 also enhanced electron utilization efficiency from 0.95% to 5.3%, and from 3.2% to 22%, for mZVI^bm and S-mZVI^bm, respectively. SEM images of the reacted particles showed that the corrosion product layer on S-mZVI^bm had a puffy/porous structure while that on mZVI^bm was dense, which may account for the mitigated passivation of S-mZVI^bm under alkaline pHs. Density functional theory calculations show that covered S atoms on the Fe(100) surface weaken the interactions of H₂O molecules with Fe surfaces, which renders the sulfidated Fe surface inefficient for H₂O dissociation and resistant to surface passivation. The observation from this study provides important implication that natural sulfidation of ZVI may largely contribute to the long-term (>10 years) efficiency of TCE decontamination by permeable reactive barriers with pore water pH above 9.

Degradation of Tetrabromobisphenol A by Sulfidated Nanoscale Zerovalent Iron in a Dynamic Two-Step Anoxic/Oxic Process

A dynamic two-step anoxic/oxic process using sulfidated nanoscale zerovalent iron (S-nZVI) was employed to degrade tetrabromobisphenol A (TBBPA). In the anoxic stage, TBBPA followed a four-step sequential debromination pathway and was completely transformed to bisphenol A (BPA) with the optimal S/Fe molar ratio of 0.3. S-nZVI inhibited H₂ evolution and preserved the reducing capacity of Fe(0). Fe(0), rather than the formed FeS in S-nZVI, was responsible for TBBPA debromination. In the oxic stage, the product BPA was attacked by ^•OH, transformed to dihydroxybenzenes and benzoquinones, and eventually, achieved mineralization via ring-opening reactions. The sulfidation process facilitated ^•OH production through a two-electron transfer pathway by surface-bound Fe(II), in which structural Fe(II) in FeS and regenerated Fe(II) from Fe(III) reduction by Fe(0) played significant roles toward total BPA degradation. S-nZVI was transformed to S₈ and α-FeOOH after the oxic treatment. After these two steps, complete degradation of TBBPA was achieved. This study demonstrated the feasibility that refractory contaminants could be completely degraded in the dynamic two-step anoxic/oxic process, thus broadening the utility of S-nZVI for environmental applications in water treatment.

Stereoisomer specific reaction of hexabromocyclododecane with reduced sulfur species in aqueous solutions

The individual degradation rates of the three dominant stereoisomers (α, β, γ) of hexabromocyclododecane (HBCDD) with bisulfide and polysulfides were investigated at pH 9 in methanol/water solutions at two different temperatures (25 °C and 40 °C). Under all conditions investigated, α-HBCDD reacts 10 to 20 times slower with bisulfide than β-HBCDD and γ-HBCDD. The difference in reactivity of HBCDD isomers can be explained by the different populations of stable conformers with large dihedral angle between the vicinal bromine atoms. It was also observed that the reaction of HBCDD with polysulfides is about six times faster than with bisulfide. The experiments performed in solvent mixtures with increased water content at 40 °C indicated that the reaction of HBCDD with bisulfide is faster with higher percentage of water. The much slower abiotic reaction of α-HBCDD compared to β-HBCDD and γ-HBCDD could potentially contribute to the fact that α-HBCDD is more persistent in the environment than γ-HBCDD. Only one isomer of tetrabromocyclododecene (TBCDe-5) was identified as a degradation product of the reaction of HBCDD with reduced sulfur species. TBCDe-5 itself reacts about ten times slower with bisulfide and twenty times slower with polysulfide than HBCDD. The study demonstrates that polysulfides and bisulfides can reduce HBCDD sufficiently in natural anoxic environments and the dominant pathway for the degradation of HBCDD by reduced sulfur species is very likely to be the reductive debromination of vicinal dibromides via concerted anti-elimination.

Thermal catalytic degradation of α-HBCD, β-HBCD and γ-HBCD over Fe3O4 micro/nanomaterial: Kinetic behavior, product analysis and mechanism hypothesis

The new persistent organic pollutant (POP), 1,2,5,6,9,10-hexabromocyclododecane (HBCD), has been widely detected in various environmental media and proved to be biotoxic. However, the research on catalytic degradation of HBCD is in its infancy. Herein, we examined the degradation of α-HBCD, β-HBCD and γ-HBCD, over Fe₃O₄ micro/nanomaterial at 200 °C. The pseudo-first-order kinetic rate constants were in the range of 0.04-0.15 min^-1, with half-life values of 5-19 min. γ-HBCD is slightly less stable than β-HBCD, but both of them readily convert into α-HBCD, as consistent with the Gibbs free energies of isomers themselves. The four products containing pentabromocyclododecene, two isomers of tetrabromocyclododecene and 1,5,9-cyclododecatriene were detected by conventional GC-MS. Interestingly, a high-throughput non-target product detection were performed by ESI-FT-ICR-MS, where up to 59 types of intermediate products were determined. It is tentatively proposed that different types of bromine-removed products (C₁₂H₁₇Br₅, C₁₂H₁₈Br₄, C₁₂H₁₈, C₁₂H₁₉Br₅, C₁₂H₂₄ and C₁₂H₁₉Br₅O) and cyclododecane ring-opened products (C₁₂H₁₉Br₇, C₁₂H₂₀Br₆O and C₁₂H₂₀Br₆) form via elimination reaction, nucleophilic substitution, hydrodebromination and addition reaction. Besides, most of the products that were detected contained oxygen. The average carbon oxidation state (OS_c¯) of the products indicate that the oxidation reaction is the dominant reaction type. Deep oxidation products, such as small molecular organic acids (formic, acetic, propionic, and butyric acids) and gas-phase oxidation products (CO₂ and CO) were further detected by ion chromatography and GC-FID, respectively. This study might provide an alternative technique for the low-cost treatment of HBCD waste.

Reactivity, Selectivity, and Long-Term Performance of Sulfidized Nanoscale Zerovalent Iron with Different Properties

Sulfidized nanoscale zerovalent iron (SNZVI) has desirable properties for in situ groundwater remediation. However, there is limited understanding of how the sulfidation type and particle properties affect the reactivity and selectivity of SNZVI toward groundwater contaminants, or how reactivity changes as the particles age. Here, SNZVI synthesized by either a one-step (SNZVI-1) or two-step (SNZVI-2) process were characterized, and the reactivity of both fresh and aged (1d to 60 d) nanoparticles was assessed. The measured S/Fe ratio was 5.4 ± 0.5 mol % for SNZVI-1 and 0.8 ± 0.1 mol % for SNZVI-2. XPS analysis indicates S^2-, S₂^2-, and S _n^2- species on the surface of both SNZVI-1 and SNZVI-2, while S₂^2- is the dominant species inside of the SNZVI nanoparticles. SNZVI-1 particles were hydrophobic (contact angle = 103 ± 3°), while the other materials were hydrophilic (contact angles were 18 ± 2° and 36 ± 3° for NZVI and SNZVI-2, respectively). SNZVI-1, with greater S content and hydrophobicity, was less reactive with water than either NZVI or SNZVI-2 over a 60 d period, resulting in less H₂ evolution. It also had the highest reactivity with TCE and the lowest reactivity with nitrate, consistent with its higher hydrophobicity. In contrast, both NZVI and SNZVI-2 were reactive with both TCE and nitrate. Both types of SNZVI remained more reactive after aging in water over 60 d than NZVI. These data suggest that the properties of the SNZVI made from a one-step synthesis procedure may provide better reactivity, selectivity, and longevity than that made from a two-step process.

Sulfidation of ZVI/AC composite leads to highly corrosion-resistant nanoremediation particles with extended life-time

Nanoscale zero-valent iron (nZVI) is a powerful reductant for many water pollutants. The lifetime of nZVI in aqueous environments is one of its limitations. Sulfidation of the nZVI surface by reduced sulfur species is known to significantly modify the particle properties. In the present study we examined various post-synthesis sulfidation methods applied on Carbo-Iron, a composite material where iron nanostructures are embedded in colloidal activated carbon (AC) particles. In such cases, where ZVI is surrounded by carbon, sulfidation largely inhibits the anaerobic corrosion of ZVI in water whereas its dechlorination activity was slightly increased. Even at a very low molar S/Fe ratio of 0.004 a strong decrease of the corrosion rate by a factor of 65 was achieved, while concurrently dechlorination of tetrachloroethene (PCE) was accelerated by a factor of three compared to the untreated particles. As a consequence, over 98% of the reduction equivalents of the sulfidated ZVI were utilized for the reduction of the target contaminant (33 mg L^-1 PCE) under simulated groundwater conditions. In a long-term experiment over 160 days the extended life-time and the preservation of the reduction capacity of the embedded ZVI were confirmed. Reasons for the modified reaction behavior of Carbo-Iron after sulfidation compared to previously studied nZVI are discussed. We hypothesize that the structure of the carbon-embedded iron is decisive for the observed reaction behavior. In addition to reaction rates, the product pattern is vastly different compared to that of sulfidated nZVI. The triple combination of ZVI, AC and sulfur makes the composite particle very suitable for practical in-situ applications.