Effect of TCE concentration and dissolved groundwater solutes on NZVI-promoted TCE dechlorination and H2 evolution

Quinones stimulate reactive oxygen species production from zero-valent iron over centimeter distances

Zero-valent iron (ZVI) has demonstrated high potential for in-situ remediation of contaminated groundwater and soils. Upon exposure to oxygen, ZVI generates reactive oxygen species (ROS). In parallel with the electron transfer mediated-reductive path, ROS plays a critical role in the oxidative degradation of organic pollutants during ZVI remediation of groundwater and soil. Yet, the efficiency is often constrained by the confined ROS production localized to the surface or immediate vicinity of ZVI particles. Here, we demonstrate that quinones significantly enhance ROS production from ZVI over centimeter-scale distances. H₂O₂ and •OH were detected over 1 cm from ZVI particles after 24 h of incubation, with production increasing alongside quinone concentration and incubation time, reaching 318.3 ± 50.0 µM and 1263.2 ± 143.5 nM at 2 mm, respectively. The broad applicability of quinone in promoting remote ROS generation was demonstrated for various ZVI materials. This remote ROS production is driven by sequential electron transfer from ZVI to quinone, long-distance electron transfer via quinone, and subsequent electron transfer from reduced quinone to oxygen. The resulting increase in ROS production amount and extended range improved ZVI remediation efficiency by 5- to 15-fold for organic pollutant degradation. These findings provide a promising strategy for enhancing ROS-mediated ZVI remediation in heterogeneous environments.

Reductive dechlorination of 1,1,2-trichloroethane in groundwater by zero valent iron coupled with biostimulation under sulfate stress: Differences and potential mechanisms

Zero-valent iron (ZVI) coupled with biostimulation is recognized as one of the most promising and effective dechlorination methods for chlorinated hydrocarbons in groundwater. However, the heterogeneity of the aquifer environment may affect the dechlorination efficiency of the coupled systems, and the underlying mechanisms remain unclear. In this study, we systematically explored the effect and potential mechanism of sulfate (SO42-) on the removal of 1,1,2-trichloroethane (1,1,2-TCA) by the coupled ZVI and biostimulation. Results revealed that the coupled systems enhanced the degradation rate of 1,1,2-TCA by an order of magnitude compared with that of each individual treatment under SO42- stress. However, the complete dechlorination of the main product, vinyl chloride (VC), remains challenging in the absence of obligate organohalide respiration bacteria (OHRB). SO42- dynamically altered the sulfidation degree of ZVI and microbial interactions, leading to the disappearance of non-chlorinated products in the micron ZVI (mZVI) coupled system and decreased dechlorination efficiency with increasing SO42- concentration. In the nano ZVI (nZVI) coupled system, suitable SO42- concentrations promoted continuous VC degradation, likely due to the inherent high reactivity of the nanometer-size effect. Nevertheless, excessive SO42- reduced ZVI sulfidation, causing differences in dechlorination efficiency and extent trends between mZVI and nZVI coupled systems. These findings will provide scientific support for the optimal application scenarios and limitations of the coupled strategies, thereby facilitating the regulation of technology application according to actual aquifer environmental parameters to achieve low-cost environmental safety control.

Temperature effects for tetrachloroethylene removal with NZVI between 10 and 55 °C in flow-through column experiments

Subsurface utilization for underground thermal energy storage (UTES) in urban areas can promote a sustainable and climate-neutral heat supply. Meanwhile, frequent subsoil contamination raises the question of whether the stored heat could be used for remediation, which can benefit from increased temperatures (e.g., by increased reactivity and desorption processes or enhanced microbial degradation). We focus on nanoscale zerovalent iron (NZVI), which is suitable for chlorinated hydrocarbon remediation. However, the potential benefits or drawbacks of increased temperatures for groundwater remediation, including accelerated contaminant degradation, declining long-term reactivity, or influenced passivation processes by mineral precipitation, remain largely unexplored. Herein, we investigate the effect of temperature changes (10-55 °C) on NZVI's long-term degradation of tetrachloroethylene (PCE) using column experiments to assess the approach's suitability for coupling with UTES. Using demineralized water (DW), maximum PCE removal rates (kPCE) between 10 and 55 °C increased from 0.13 to 2.3 h-1. With tap water (TW), kPCE increased between 10 and 40 °C from 0.18 to 0.75 h-1. Due to the higher PCE removal rates in combination with increasing H2 generation caused by enhanced anaerobic corrosion, long-term corrosion reactivity decreased between 10 and 55 °C from ∼275 to 14 d with DW and from 150 to 30 d between 10 and 40 °C with TW, whereby the increased H2 formation is beneficial for microbial degradation. Accelerated passivation of the NZVI due to carbonate precipitation was not observed for the examined temperatures. Therefore, the experiments revealed that combining UTES with NZVI for remediation purposes is practicable, offering clear remediation advantages.

Silicate-Confined Hydrogen on Nanoscale Zerovalent Iron for Efficient Defluorination Reactions

Defluorination reactions are increasingly vital due to the extensive use of organofluorine compounds with robust carbon-fluorine (C-F) bonds; particularly, the efficient defluorination of widespread and persistent per- and polyfluoroalkyl substances under mild conditions is crucial due to their accumulation in the environment and human body. Herein, we demonstrate that surface-modified silicate of pronounced proton affinity can confine active hydrogen (•H) onto nanoscale zerovalent iron (nZVI) by withdrawing electrons from nZVI to react with bound protons, generating confined active hydrogen (•H*) for efficient defluorination under ambient conditions. The exposed silicon cation (Siσ+) of silicate functions as a Lewis acid site to activate the C-F bond by forming Siσ+...F--C and substantially lowers the energy barrier of nucleophilic •H* attack, thereby facilitating selective C-F hydrodefluorination and subsequent fluorine immobilization. In a column flow reactor, silicate-modified nZVI efficiently removes perfluorooctanoic acid (PFOA) of concentrations ranging from 0.24 to 24 μmol/L with 75-92% defluorination efficiencies, 8 times higher than those of nZVI, generating environmentally friendly alkyl carboxylic acids as the primary products. Besides PFOA, this novel nZVI also realizes deep defluorination of other organofluorine compounds, including perfluorooctanesulfonates and fluoroquinolones, demonstrating its superior defluorination potential.

Roles of silica coating on nanosized zero-valent iron in sequential reduction-oxidation process in a system containing persulfate

A sequential reduction-oxidation process using silica-coated nanosized zero-valent iron (nZVI) particles (nZVI@SiO2) and persulfate for mineralizing recalcitrant compounds was developed, and the effects of the process on nitrobenzene were evaluated. This sequential process significantly enhanced contaminant mineralization, which could not be effectively achieved by reduction or oxidation alone. The nZVI@SiO2 rapidly reduced nitrobenzene to aniline, then the aniline concentration gradually decreased after persulfate had been added and initiated sequential oxidative degradation. The SiO2 coating on the nZVI@SiO2 limited outward mass transfer of reaction products and increased the efficiency with which nitrobenzene was converted into aniline. Slow release of Fe(II) caused by the coating caused persulfate activation and subsequent aniline oxidation to be more sustained and efficient than without the coating. The final nitrobenzene-aniline mineralization efficiency was higher for the nZVI@SiO2/persulfate system than the nZVI/persulfate system. The SiO2 coating of the nZVI@SiO2 particles was an excellent protective layer, protecting the particles from undesirable consumption through reactions with groundwater components. nZVI@SiO2 particle transformations during the sequential process were investigated, and the operating conditions were optimized to maximize the recalcitrant compound removal efficiency. The results indicated that nZVI@SiO2 and persulfate could be used to mineralize organic contaminants in groundwater through sequential reduction-oxidation.

Nonmetallic modified zero-valent iron for remediating halogenated organic compounds and heavy metals: A comprehensive review

Zero Valent Iron (ZVI), an ideal reductant treating persistent pollutants, is hampered by issues like corrosion, passivation, and suboptimal utilization. Recent advancements in nonmetallic modified ZVI (NM-ZVI) show promising potential in circumventing these challenges by modifying ZVI's surface and internal physicochemical properties. Despite its promise, a thorough synthesis of research advancements in this domain remains elusive. Here we review the innovative methodologies, regulatory principles, and reduction-centric mechanisms underpinning NM-ZVI's effectiveness against two prevalent persistent pollutants: halogenated organic compounds and heavy metals. We start by evaluating different nonmetallic modification techniques, such as liquid-phase reduction, mechanical ball milling, and pyrolysis, and their respective advantages. The discussion progresses towards a critical analysis of current strategies and mechanisms used for NM-ZVI to enhance its reactivity, electron selectivity, and electron utilization efficiency. This is achieved by optimizing the elemental compositions, content ratios, lattice constants, hydrophobicity, and conductivity. Furthermore, we propose novel approaches for augmenting NM-ZVI's capability to address complex pollution challenges. This review highlights NM-ZVI's potential as an alternative to remediate water environments contaminated with halogenated organic compounds or heavy metals, contributing to the broader discourse on green remediation technologies.

Biochar-nanoparticle combinations enhance the biogeochemical recovery of a post-mining soil

Here we addressed the capacity of distinct amendments to reduce arsenic (As), copper (Cu), selenium (Se) and zinc (Zn) associated risks and improve the biogeochemical functions of post-mining soil. To this, we examined nanoparticles (NPs) and/or biochar effects, combined with phytostabilization using Lolium perenne L. Soil samples were taken in a former metal mine surroundings. Ryegrass seeds were sown in pots containing different combinations of NPs (zero-valent iron (nZVI) or hydroxyapatite (nH)) (0 and 2 %), and biochar (0, 3 and 5 %). Plants were grown for 45 days and the plant yield and element accumulation were evaluated, also soil properties (element distribution within the soil fractions, fertility, and enzymatic activities associated with microbiota functionality and nutrient cycling) were determined. Results showed biochar-treated soil had a higher pH, and much higher organic carbon (C) content than control soil and NP-treated soils, and it revealed increased labile C, total N, and available P concentrations. Soil treatment with NP-biochar combinations increased exchangeable non-acid cation concentrations and reduced exchangeable Na%, improved soil fertility, reduced sodicity risk, and increased ryegrass biomass. Enzymatic activities, particularly dehydrogenase and glucosidase, increased upon the addition of biochar, and this effect was fostered by NPs. Most treatments led to a significant reduction of metal(loid)s contents in biomass, mitigating contamination risks. The two different NPs had similar effects in many parameters, nH outperformed nZVI in terms of increased nutrients, C content, and enzymatic activities. On the basis of our results, combined biochar-NP amendments use, specially nH, emerges as a potential post-mining soil restoration strategy.

Differential and mechanism analysis of sulfate influence on the degradation of 1,1,2- trichloroethane by nano- and micron-size zero-valent iron

The in-situ reduction of zero-valent iron (ZVI) is an effective method for removing chlorinated aliphatic hydrocarbons (CAHs) from groundwater. The heterogeneity of environmental conditions is also crucial in affecting dechlorination efficiency. Until now, the effect of Sulfate (SO42-) on ZVI activity has been debated, and the related mechanism research on SO42- behaviour during the abiotic reduction process of chlorinated alkanes is still lacking. In this study, the impacts of SO42- concentrations (0, 2, 4, 8, 80 mM) on the degradation of 1,1,2-trichloroethane (1,1,2-TCA) by micron-size ZVI (mZVI) and nano-size ZVI (nZVI) were systematically investigated. For mZVI, Kobs increased by 0.6 (2 mM), 0.5 (4 mM), 1.1 (8 mM), and 1.6 times (80 mM). For nZVI, Kobs decreased by 32% (2 mM), 39% (4 mM), 45% (8 mM), and 9% (80 mM). The results showed that SO42- increased the rate of 1,1,2-TCA degradation by mZVI but weakened the reduction performance of nZVI; however, this inhibition was reduced when the concentration reached 80 mM. SO42- controlled the degradation of 1,1,2-TCA mainly through the formation of different iron-sulfate complexes on the ZVI surface: water-soluble bidentate iron-sulfate complexes formed on the mZVI surface promoted the corrosion of the oxide layer and accelerated the reduction of 1,1,2-TCA, monodentate complexes mainly formed on the nZVI surface inhibited the reduction of 1,1,2-TCA by blocking surface sites. These results demonstrate the proof of concept to assist land managers in the field application of ZVI technology for the remediation of CAHs contaminated sites with different background concentrations of SO42-.

Effects of common dissolved anions on the efficiency of Fe0-based remediation systems

Quiescent batch experiments were conducted to evaluate the influences of Cl-, F-, HCO3-, HPO42-, and SO42- on the reactivity of metallic iron (Fe0) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe0 and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0-45 d), (ii) 4 different types of Fe0, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO3- enhanced the extent of MB discoloration, while Cl-, F-, HPO42-, and SO42- inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO3- inhibited O-II discoloration. The increased MB discoloration in the HCO3- system was justified by considering the availability of FeCPs as contaminant scavengers, pH increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe0-based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.

Evaluating the role of salinity in enhanced biogas production from two-stage anaerobic digestion of food waste by zero-valent iron

Salts including NaCl are the most common food flavoring agents so they are often accumulated in food waste (FW) and have potential impact on anaerobic digestion (AD) of FW. In this study, the enhanced biogas production from two-stage anaerobic digestion (TSAD) of FW by microscale zero-valent iron (ZVI) under different salinity (3, 6, 9, and 15 g NaCl/L) was evaluated. Under salinity stress, ZVI becomes a continue-release electron donor due to the enhanced corrosion and dissolution effect and the slow-down surface passivation, further improving the performance of TSAD. Experimental results revealed that the biogas production including H2 and CH4 from TSAD with 10 g/L ZVI addition was promoted under salinity stress. The maximum H2 and CH4 yield (303.38 mL H2/g-VS and 253.84 mL CH4/g-VS) were observed at the salinity 9 g NaCl/L. Compared with that of zero salinity, they increased by 40.94% and 318.46%, respectively. Additionally, Sedimentibacter, an exoelectrogen that can participate in the direct interspecies electron transfer, also exhibited the highest relative abundance (34.96%) at the salinity 9 g NaCl/L. These findings obtained in this study might be of great importance for understanding the influence of salinity on the enhanced AD by ZVI.

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.

One-step strategy for efficient Cr(VI) removal via phytate modified zero-valent iron: Accelerated electron transfer and enhanced coordination effect

The toxic Cr(VI) from industrial wastewater pose serious threat to the human beings and eco-systems. To reduce the operation processes and enhance the removal efficiency of Cr(VI), targeted design of functionalized material is critical in practical applications. Herein, we developed a one-step strategy for simultaneous Cr(VI) reduction and total Cr capture by a novel phytate modified zero-valent iron (PA-ZVI). The reaction kinetics of Cr(VI) removal by PA-ZVI (0.2225 min-1) was 53 times higher compared to ZVI (0.0042 min-1). The Fe(0) content on the surface of PA-ZVI increased from 2.2% to 15.6% compared to ZVI. Meanwhile, Cr(VI) was liable to adsorb on the surface of PA-ZVI due to its lower adsorption energy compared with the original ZVI (-2.09 eV vs -0.85 eV). The incorporation of the phytate ligand promoted electron transfer from iron core to Cr(VI), leading to the rapid in-situ reduction of Cr(VI) adsorbed on the surface of PA-ZVI to Cr(III). PA-ZVI exhibited a satisfactory performance for Cr(VI) removal at a broad pH range (3-11) and in the presence of coexisting ions and humic acid. Moreover, the reactor with the addition of PA-ZVI achieved more than 90% Cr(VI) removal within 72 h in continuous flow experiments. The feasibility of PA-ZVI for the removal of Cr(VI) is also validated in authentic wastewater. This work provides novel ZVI materials that can effectively address decontamination challenges from Cr(VI) pollution.

Formation of corrosion-based ZVMg nanoparticles for reductive degradation of high-level trichloroethylene in aqueous solution

This study discovered that nanosized zero valent magnesium (nZVMg) could be formed during the electrochemical corrosion of microsized ZVMg (mZVMg) in aqueous solution. It is observed that the nZVMg particle sizes were less than 50 nm with the specific surface area of 54.63 m2/g after it was corroded for 96 h (ZVMg96) at the expense of losing about 60 wt% Mg0. However, the XPS characterization indicated the thickness of Mg(OH)2 layer over ZVMg96 being less than 5 nm, accompanied by the faster electron transfer rate but slower corrosion rate than mZVMg. Most importantly, the removal efficiency of 82 % under high-level trichloroethylene (TCE) at 100 mg/L was achieved by ZVMg96 within one hour relative to 48 % by mZVMg. The rate constant normalized by surface area was 3.11 × 10-2 L/m2/h by ZVMg96 due to the high surface energy of nanoparticles. The degradation products were dependent on the initial TCE concentrations, with environmentally friendly and biodegradable degradation products being generated via hydrodechlorination, hydrogenation and polymerization pathways according to the density functional theory calculations. ZVMg corroded for 14 days illustrated a long-term chemical stability and excellent degradation performance, demonstrating significant application potential in remediating the TCE plumes in groundwater.

From liquid to solid: A novel approach for utilizing sulfate reduction effluent through phase transition - Effluent-induced nanoscale zerovalent iron sulfidation

This study investigated the use of sulfate reduction effluent (SR-effluent) to induce sulfidation on nanoscale zerovalent iron (nZVI). SR-effluent-modified nZVI achieved a 100% improvement in Cr(VI) removal from simulated groundwater, a result comparable to cases where other, more typical sulfur precursors (Na2S2O4, Na2S2O3, Na2S, K2S6, and S0) were used. Through a structural equation model analysis, amendment of nanoparticles' agglomeration (standardized path coefficient (std. path coeff.) = -0.449, p < 0.05) and hydrophobicity (std. path coeff. = 0.100, p < 0.05) and direct reaction between iron-sulfur compounds and Cr(VI) (std. path coeff. ranged from -0.195 to 0.322, p < 0.05) were primarily contributing to sulfidation-induced Cr(VI) removal enhancement. Regarding the property improvement of nZVI, the SR-effluent's corrosion radius played a crucial role in tuning the content and distribution of the iron-sulfur compounds based on the core-shell structure of the nZVI and the redox processes at the aqueous-solid interface.

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

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

Flax fiber based semicarbazide biosorbent for removal of Cr(VI) and Alizarin Red S dye from wastewater

In the present study, flax fiber based semicarbazide biosorbent was prepared in two successive steps. In the first step, flax fibers were oxidized using potassium periodate (KIO4) to yield diadehyde cellulose (DAC). Dialdehyde cellulose was, then, refluxed with semicarbazide.HCl to produce the semicarbazide functionalized dialdehyde cellulose (DAC@SC). The prepared DAC@SC biosorbent was characterized using Brunauer, Emmett and Teller (BET) and N2 adsorption isotherm, point of zero charge (pHPZC), elemental analysis (C:H:N), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses. The DAC@SC biosorbent was applied for the removal of the hexavalent chromium (Cr(VI)) ions and the alizarin red S (ARS) anionic dye (individually and in mixture). Experimental variables such as temperature, pH, and concentrations were optimized in detail. The monolayer adsorption capacities from the Langmuir isotherm model were 97.4 mg/g and 18.84 for Cr(VI) and ARS, respectively. The adsorption kinetics of DAC@SC indicated that the adsorption process fit PSO kinetic model. The obtained negative values of ΔG and ΔH indicated that the adsorption of Cr(VI) and ARS onto DAC@SC is a spontaneous and exothermic process. The DAC@SC biocomposite was successfully applied for the removal of Cr(VI) and ARS from synthetic effluents and real wastewater samples with a recovery (R, %) more than 90%. The prepared DAC@SC was regenerated using 0.1 M K2CO3 eluent. The plausible adsorption mechanism of Cr(VI) and ARS onto the surface of DAC@SC biocomposite was elucidated.

Decoupling Fe0 Application and Bioaugmentation in Space and Time Enables Microbial Reductive Dechlorination of Trichloroethene to Ethene: Evidence from Soil Columns

Fe⁰ is a powerful chemical reductant with applications for remediation of chlorinated solvents, including tetrachloroethene and trichloroethene. Its utilization efficiency at contaminated sites is limited because most of the electrons from Fe⁰ are channeled to the reduction of water to H₂ rather than to the reduction of the contaminants. Coupling Fe⁰ with H₂-utilizing organohalide-respiring bacteria (i.e., Dehalococcoides mccartyi) could enhance trichloroethene conversion to ethene while maximizing Fe⁰ utilization efficiency. Columns packed with aquifer materials have been used to assess the efficacy of a treatment combining in space and time Fe⁰ and aD. mccartyi-containing culture (bioaugmentation). To date, most column studies documented only partial conversion of the solvents to chlorinated byproducts, calling into question the feasibility of Fe⁰ to promote complete microbial reductive dechlorination. In this study, we decoupled the application of Fe⁰ in space and time from the addition of organic substrates andD. mccartyi-containing cultures. We used a column containing soil and Fe⁰ (at 15 g L^-1 in porewater) and fed it with groundwater as a proxy for an upstream Fe⁰ injection zone dominated by abiotic reactions and biostimulated/bioaugmented soil columns (Bio-columns) as proxies for downstream microbiological zones. Results showed that Bio-columns receiving reduced groundwater from the Fe⁰-column supported microbial reductive dechlorination, yielding up to 98% trichloroethene conversion to ethene. The microbial community in the Bio-columns established with Fe⁰-reduced groundwater also sustained trichloroethene reduction to ethene (up to 100%) when challenged with aerobic groundwater. This study supports a conceptual model where decoupling the application of Fe⁰ and biostimulation/bioaugmentation in space and/or time could augment microbial trichloroethene reductive dechlorination, particularly under oxic conditions.

Performance Enhancement of Biogenetic Sulfidated Zero-Valent Iron for Trichloroethylene Degradation: Role of Extracellular Polymeric Substances

Chemical sulfidation has been considered as an effective strategy to improve the reactivity of zero-valent iron (S-ZVI). However, sulfidation is a widespread biogeochemical process in nature, which inspired us to explore the biogenetic sulfidation of ZVI (BS-ZVI) with sulfate-reducing bacteria (SRB). BS-ZVI could degrade 96.3% of trichloroethylene (TCE) to acetylene, ethene, ethane, and dichloroethene, comparable to S-ZVI (97.0%) with the same S/Fe ratio (i.e., 0.1). However, S-ZVI (0.21 d^-1) exhibited a faster degradation rate than BS-ZVI (0.17 d^-1) based on pseudo-first-order kinetic fitting due to extracellular polymeric substances (EPSs) excreted from SRB. Organic components of EPSs, including polysaccharides, humic acid-like substances, and proteins in BS-ZVI, were detected with 3D-EEM spectroscopy and FT-IR analysis. The hemiacetal groups and redox-activated protein in EPS did not affect TCE degradation, while the acetylation degree of EPS increased with the concentration of ZVI and S/Fe, thus inhibiting the TCE degradation. A low concentration of HA-like substances attached to BS-ZVI materials promoted electron transport. However, EPS formed a protective layer on the surface of BS-ZVI materials, reducing its TCE reaction rate. Overall, this study showed a comparable performance enhancement of ZVI toward TCE degradation through biogenetic sulfidation and provided a new alternative method for the sulfidation of ZVI.

Iron mediated autotrophic denitrification for low C/N ratio wastewater: A review

In recent years, iron mediated autotrophic denitrification has been a concern because it overcomes the absence of organic carbon and has been successfully used in denitrification for low C/N ratio wastewater. However, there is currently a lack of a more systematic summary of iron-based materials that can be used for denitrification, and no detailed overview about the mechanism of iron mediated autotrophic denitrification has been reported. In this study, the iron materials with different valence states that can be used for denitrification were summarized, and emphasized, as well as the mechanism in different interaction systems were emphasize. In addition, the contribution of various microorganisms in nitrate reduction were analyzed and the effects of operating conditions and water quality were evaluated. Finally, the challenges and shortcomings of the denitrification process were discussed aiming to find better practical engineering applications of iron-based denitrification.

Photoelectrochemical degradation of trichloroethylene by iron modified TiO2 nanotube arrays

In this study, iron was deposited to titanium dioxide nanotube arrays (TNAs) by impregnation method to enhance its photocatalytic ability. The as-synthesized iron-modified TNAs (Fe-TNAs) was employed in a photoelectrochemical (PEC) system to degrade trichloroethylene (TCE). Results of AFE-SEM analysis showed that the iron nanoparticles (NPs) were successfully attached evenly to the nozzle of Fe-TNAs. Results of XRD analysis confirmed the findings of EDS and XPS, indicating the success of iron modification. The absorption wavelength of Fe-TNAs-27 mL red-shifts to 543 nm which corresponds to the band gap of 2.54 eV after iron modification. Mott-Schottky analysis yielded a donor density of 7.21 × 10²⁰ and 2.30 × 10²⁰/cm³ for TNAs and Fe-TNAs-27 mL, respectively. The photo-generated electrons had a lifetime (τ_el) of 21.49 and 39.19 ms for TNAs and Fe-TNAs-27 mL, respectively, illustrating the reduce of recombination of photo-generated electron-hole pairs. process. PEC methods performed the most effective way to degrade TCE with a rate constant of 0.079 min^-1 in Fe-TNAs PEC system. Mechanism of Fe-TNAs PEC system was proposed in detail.

In-depth exploration of toxicity mechanism of nanoscale zero-valent iron and its aging products toward Escherichia coli under aerobic and anaerobic conditions

The bacteria toxicity of nanoscale zero-valent iron (nZVI) can be changed during its application in water treatment but the toxicity mechanism is still not well understood, particularly under anaerobic conditions. Here, the toxicity of nZVI and its aging products towards Escherichia coli (E. coli) and the mechanisms of extracellular and intracellular reactive oxygen species (ROS) damage were deeply probed in the presence and absence of oxygen in ultrapure water. Under aerobic conditions, the ROS damage primarily caused by the generation of extracellular free •OH can be a major contributor to the toxicity of nZVI to E. coli. By contrast, in anaerobic nZVI treatment system, the intracellular •OH can be quenched by benzoic acid which is a cell permeable quencher and the electron spin resonance (ESR) signals of 5,5-dimethy-1-pyrroline (DMPO)- •OH were evidently observed in system with the addition of F^- which could desorb the surface •OH into solution. It indicated that the intracellular •OH adsorbed on the particle surface can also play an indispensable role in inactivating cells under anaerobic conditions. Moreover, nZVI can steeply decline the membrane potential, causing severe membrane disruption and therefore resulting in the stronger toxicity in anaerobic conditions. Furthermore, the chemical composition transformation of nZVI and generation of benign iron corrosion products (e.g., Fe₃O₄, γ-Fe₂O₃, γ-FeOOH) are mainly responsible for the reduced toxicity with the increasing aging time. These results provide insights into the extracellular and intracellular ROS damage occurred in aerobic and anaerobic nZVI treatment systems, offering more perspective to the risk assessment of nZVI application.

Trichloroethylene remediation using zero-valent iron with kaolin clay, activated carbon and bacteria

Nanoscale particles of zero-valent iron were used to form a permeable reactive barrier whose performance in dechlorinating a solution of trichloroethylene was compared with that of a barrier formed from limestone. The iron was combined with kaolin by calcination. The test liquid contained sewage sludge, and also added NH₄Cl and KH₂PO₄. The average removal rates of trichloroethylene and phosphorus over 365 days both exceeded 94%. Chemical oxygen demand was reduced by 92% and ammonium nitrogen by 43.6%. All were significantly greater than the removals with the limestone barrier. The ceramsite barrier retained 85% of its effectiveness even after 365 days of use. Dechloromonas sp. was the main dechlorinating bacterium, but its removal ability is limited. The removal of trichloroethylene in such a barrier mainly depends on reduction by the zero-valent iron and biodegradation. The results show that the prepared ceramsite is stable and effective in removing trichloroethylene from water. It is a promising in-situ remediation material for groundwater.

Unveiling the positive effect of mineral induced natural organic matter (NOM) on catalyst properties and catalytic dechlorination performance: An experiment and DFT study

Herein, we report the significant effects of natural organic matter contained in natural zeolite (Z-NOM) on the physicochemical characteristics of a Ni/Fe@natural zeolite (NF@NZ) catalyst and its decontamination performance toward the dechlorination of trichloroethylene (TCE). Z-NOM predominantly consists of humic-like substances and has demonstrable utility in the synthesis of bimetallic catalysts. Compared to NF@NZ_600C (devoid of Z-NOM), NF@NZ had increased dispersibility and mobility and showed significant enhancement in the catalytic dechlorination of TCE owing to the encapsulation of Ni⁰/Fe⁰ nanoparticles by Z-NOM. The results of corrosion experiments, spectroscopic analyses, and H₂ production experiments confirmed that Ni⁰ acted as an efficient cocatalyst with Fe⁰ to enhance the dechlorination of TCE to ethane, and Z-NOM-capped Ni⁰ showed improved adsorption of TCE and atomic hydrogen on their reactive sites and oxidation resistance. The density functional theory (DFT) studies have substantiated the improved adsorption of TCE due to the presence of NOM (especially by COOH structure) and the enhanced charge density at the Ni site in the Ni/Fe bimetal alloy for the stronger adsorption of hydrogen atoms that ultimately enhanced the TCE reduction reaction. These findings illustrate the efficiency of NOM containing natural minerals toward the synthesis of bimetallic catalysts for practical applications.

Metallic nanoparticles for catalytic reduction of toxic hexavalent chromium from aqueous medium: A state-of-the-art review

The ever increasing concentration of toxic and carcinogenic hexavalent chromium (Cr (VI)) in various environmental mediums including water-bodies due to anthropogenic activities with rapid civilization and industrialization have become the major issue throughout the globe during last few decades. Therefore, developing new strategies for the treatment of Cr(VI) contaminated wastewaters are in great demand and have become a topical issue in academia and industry. To date, various techniques have been used for the remediation of Cr(VI) contaminated wastewaters including solvent extraction, adsorption, catalytic reduction, membrane filtration, biological treatment, coagulation, ion exchange and photo-catalytic reduction. Among these methods, the transformation of highly toxic Cr(VI) to benign Cr(III) catalyzed by metallic nanoparticles (M-NPs) with reductant has gained increasing attention in the past few years, and is considered to be an effective approach due to the superior catalytic performance of M-NPs. Thus, it is a timely topic to review this emerging technique for Cr(VI) reduction. Herein, recent development in synthesis of M-NPs based non-supported, supported, mono-, bi- and ternary M-NPs catalysts, their characterization and performance for the reduction of Cr(VI) to Cr(III) are reviewed. The role of supporting host to stabilize the M-NPs and leading to enhance the reduction of Cr(VI) are discussed. The Cr(VI) reduction mechanism, kinetics, and factors affecting the kinetics are overviewed to collect the wealthy kinetics data. Finally, the challenges and perspective in Cr(VI) reduction catalyzed by M-NPs are proposed. We believe that this review will assist the researchers who are working to develop novel M-NPs catalysts for the reduction of Cr(VI).

The role of nitrate in simultaneous removal of nitrate and trichloroethylene by sulfidated zero-valent Iron

Sulfidated zero-valent iron (S-ZVI) is commonly used to degrade trichloroethylene (TCE). The reactivity of S-ZVI is related to not only the properties of S-ZVI but also the geochemical conditions in groundwater, such as coexisted NO₃^-. Therefore, the effect of NO₃^- on TCE degradation by S-ZVI and its mechanism were systematically studied. 95.17% of TCE was degraded to acetylene, dichloroethene, ethene, ethane and multi‑carbon products via β-elimination by fresh S-ZVI that contained 85.31% Fe⁰ and 14.69% FeS in the presence of NO₃^-, demonstrating that NO₃^- did not affect the degradation pathway of TCE. While high concentration of NO₃^- (> 10 mg/L) competed for electrons at the Fe/FeO_x interface with degradation products, leading to a continuous rising of acetylene. Moreover, the rapid reduction of NO₃^- to NH₄⁺ (89.79%) at the Fe⁰ interface contributed to the release of 5.08 mM Fe²⁺ from S-ZVI, which promoted the formation of Fe₃O₄ with excellent electron conduction properties on the surface of S-ZVI. Accordingly, NO₃^- improved the degradation and electron selectivity of TCE by 51.07% and 2.79 fold, respectively. This study demonstrated that S-ZVI could remediate the contamination of NO₃^- and TCE simultaneously and the presence of NO₃^- could effectively enhance the degradation of TCE in groundwater.

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.

Iron Nitride Nanoparticles for Enhanced Reductive Dechlorination of Trichloroethylene

Nitriding has been used for decades to improve the corrosion resistance of iron and steel materials. Moreover, iron nitrides (Fe_xN) have been shown to give an outstanding catalytic performance in a wide range of applications. We demonstrate that nitriding also substantially enhances the reactivity of zerovalent iron nanoparticles (nZVI) used for groundwater remediation, alongside reducing particle corrosion. Two different types of Fe_xN nanoparticles were synthesized by passing gaseous NH₃/N₂ mixtures over pristine nZVI at elevated temperatures. The resulting particles were composed mostly of face-centered cubic (γ'-Fe₄N) and hexagonal close-packed (ε-Fe_2-3N) arrangements. Nitriding was found to increase the particles' water contact angle and surface availability of iron in reduced forms. The two types of Fe_xN nanoparticles showed a 20- and 5-fold increase in the trichloroethylene (TCE) dechlorination rate, compared to pristine nZVI, and about a 3-fold reduction in the hydrogen evolution rate. This was related to a low energy barrier of 27.0 kJ mol^-1 for the first dechlorination step of TCE on the γ'-Fe₄N(001) surface, as revealed by density functional theory calculations with an implicit solvation model. TCE dechlorination experiments with aged particles showed that the γ'-Fe₄N nanoparticles retained high reactivity even after three months of aging. This combined theoretical-experimental study shows that Fe_xN nanoparticles represent a new and potentially important tool for TCE dechlorination.

Even Incorporation of Nitrogen into Fe0 Nanoparticles as Crystalline Fe4N for Efficient and Selective Trichloroethylene Degradation

Surface modification of microscale Fe powder with nitrogen has emerged recently to improve the reactivity of Fe⁰ for dechlorination. However, it is unclear how an even incorporation of a crystalline iron nitride phase into Fe⁰ nanoparticles affects their physicochemical properties and performance, or if Fe⁰ nanoparticles with a varied nitridation degree will act differently. Here, we synthesized nitridated Fe⁰ nanoparticles with an even distribution of N via a sol-gel and pyrolysis method. Nitridation expanded the Fe⁰ lattice and provided the Fe₄N species, making the materials more hydrophobic and accelerating the electron transfer, compared to un-nitridated Fe⁰. These properties well explain their reactivity and selectivity toward trichloroethylene (TCE). The TCE degradation rate by nitridated Fe⁰ (up to 4.8 × 10^-2 L m^-2 h^-1) was much higher (up to 27-fold) than that by un-nitridated Fe⁰, depending on the nitridation degree. The materials maintained a high electron efficiency (87-95%) due to the greatly suppressed water reactivity (109-127 times lower than un-nitridated Fe⁰). Acetylene was accumulated as the major product of TCE dechlorination via β-elimination. These findings suggest that the nitridation of Fe⁰ nanoparticles can change the materials' physicochemical properties, providing high reactivity and selectivity toward chlorinated contaminants for in situ groundwater remediation.

Removal of Organoselenium from Aqueous Solution by Nanoscale Zerovalent Iron Supported on Granular Activated Carbon

Nanoscale zerovalent iron particles (nZVI) immobilized on coconut shell-based granular activated carbon (GAC) were studied to remove organoselenium from wastewater. A chemical reduction technique that involves the application of sodium borohydride was adopted for the adsorbent preparation. The texture, morphology and chemical composition of the synthesized adsorbents were analyzed with a scanning electron microscope (SEM), nitrogen adsorption–desorption isotherms and X-ray diffraction (XRD). Batch experiment with various pHs and contact times were conducted to evaluate nZVI/GAC adsorption performance. The results showed that nZVI/GAC has a strong affinity to adsorb selenomethionine (SeMet) and selenocysteine (SeCys) from wastewaters. The maximum removal efficiency for the composite (nZVI/GAC) was 99.9% for SeCys and 78.2% for SeMet removal, which was significantly higher than that of nZVI (SeCy, 59.2%; SeMet, 10.8%). The adsorption kinetics were studied by pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models. Amongst the two, PSO seemed to have a better fit (SeCy, R2 > 0.998; SeMet, R2 > 0.999). The adsorption process was investigated using Langmuir and Freundlich isotherm models. Electrostatic attraction played a significant role in the removal of organoselenium by nZVI/GAC adsorption. Overall, the results indicated that GAC-supported nZVI can be considered a promising and efficient technology for removing organoselenium from wastewater.

An overview of in-situ remediation for nitrate in groundwater

Faced with the increasing nitrate pollution in groundwater, in-situ remediation has been widely studied and applied on field-scale as an efficient, economical and less disturbing remediation technology. In this review, we discussed various in-situ remediation for nitrate in groundwater and elaborate on biostimulation, phytoremediation, electrokinetic remediation, permeable reactive barrier and combined remediation. This review described principles of each in-situ remediation, application, the latest progress, problems and challenges on field-scale. Factors affecting the efficiency of in-situ remediation for nitrate in groundwater are also summarized. Finally, this review presented the prospect of in-situ remediation for nitrate pollution in groundwater. The objective of this review is to examine the state of knowledge on in-situ remediation for nitrate in groundwater and critically evaluate factors which affect the up-scaling of laboratory and bench-scale research to field-scale application. This helps to better understand the control mechanisms of various in-situ remediation for nitrate pollution in groundwater and the design options available for application to the field-scale.

Increasing the electron selectivity of nanoscale zero-valent iron in environmental remediation: A review

Nanoscale zero-valent iron nanoparticles (nZVI) have been used for groundwater remediation and wastewater treatment due to their high reactivity, high adsorption capacity and nontoxicity. However, side reactions generally occur in tandem with the target contaminants removal process, resulting in poor electron selectivity (ES) of nZVI, and subsequently restricting its commercial application. Major efforts to increase ES of nZVI have been made in recent years. This review's objective is to provide a progress report on the significant developments in nZVI's ES during the past decade. Firstly, the definition of ES and its quantification approaches were documented, and the intrinsic (i.e. particle size, crystallinity, and surface area) and extrinsic factors (i.e. solutions pH, target contaminant concentration, and presence of co-contaminants) affecting the ES of nZVI were reported. The latest techniques for increasing ES were summarized in detail, with reference made to sulfidation, magnetization, carbon loading and other features. Then the mechanisms of those strategies for ES enhancement were described. Finally, some constructive suggestions on future research directions concerning nZVI's ES in the future were proposed.

Reactivity characterization of SiO2-coated nano zero-valent iron for iodoacetamide degradation: The effects of SiO2 thickness, and the roles of dehalogenation, hydrolysis and adsorption

The effect of SiO₂-layer thickness in SiO₂-coated nano zero-valent iron (nZVI) particles on the reactivity characteristics of iodoacetamide (IAcAm) degradation was evaluated. SiO₂-layer thicknesses ranging from 3.6 to 27.3 nm were obtained through varying tetraethyl orthosilicate dosages of 0.001-1 M. The crystallinity, surface chemical composition, and physicochemical properties were evaluated for their effects on synergetic degradation mechanisms, dehalogenation, hydrolysis, and adsorption. At a thickness of 3.6 nm, the SiO₂ layer offered the highest observed pseudo-first-order rate (k_obs) and higher rates of IAcAm degradation were maintained under pH fluctuations (pH 5-7) and aerobic conditions compared to pristine nZVI. At this SiO₂-layer thickness (3.6 nm), the rate of iron oxide-layer formation was reduced and the migration of reactive iron species (Fe⁰ and Fe²⁺) for the dehalogenation and hydrolysis reactions was enabled. In a single-solute solution, IAcAm elimination was greater than bromoacetamide and chloroacetamide elimination due to the weak ionic I-C bond. In mixed solute conditions, the hydrophobicity of chloroacetamide played a more significant role in competitive degradation through greater adsorption. The proportion of dehalogenation relative to hydrolysis during IAcAm degradation by pristine nZVI and SiO₂-coated nZVI was approximately 0.6:0.4. Iodoacetic acid and acetic acid were detected as intermediates in the degradation pathway of IAcAm by pristine nZVI. In contrast, the SiO₂ layer on nZVI can accelerate the transformation of IAcAm to acetamide and iodoacetic acid. The electrolyte background of tap water exhibited a slight inhibitory effect on the degradation of IAcAm for both nZVI and SiO₂-coated nZVI.

Influence of several crucial groundwater components on the toxicity of nanoscale zero-valent iron towards Escherichia coli under aerobic and anaerobic conditions

In this paper, the effects of several groundwater components (heavy metals, inorganic anions, and organics) on the cytotoxicity of nanoscale zero-valent iron (NZVI) towards Escherichia coli (E. coli) under aerobic/anaerobic conditions were studied. The results showed that NZVI exhibited much higher toxicity in anaerobic conditions than aerobic conditions. Under the state of air-saturation, corrosion of NZVI occurred rapidly, at the same time, it could stably and continuously generate Fe (Ⅱ) and trigger reactive oxygen species (ROS), which led to oxidative stress in E. coli. While in the deareated state, the TEM images showed that the integrity of the cell membrane was destroyed, which validated that the main mechanism of NZVI cytotoxicity was the rapid membrane damage of E. coli. The presence of Cr (Ⅵ) reduced the toxicity of NZVI through oxidation-reduction with NZVI, especially under anaerobic conditions. In contrast, the presence of Cd (Ⅱ) could be adsorbed onto NZVI to increase the cytotoxicity of NZVI. The presence of phosphate and humic acid greatly improved the survival rate of E. coli through the complex reaction with Fe (Ⅱ), especially under aerobic conditions. On the one hand, the formed Fe (II)-phosphate/humic acid complex could reduce the production of ROS. On the other hand, the complex accumulated on the outer surface of E. coli cells could provide steric hindrance to impede the contact between NZVI and cell. These findings were crucial for practical significance to evaluate environmental risk during the groundwater remediation process by using NZVI.

A Rational Analysis on Key Parameters Ruling Zerovalent Iron-Based Treatment Trains: Towards the Separation of Reductive from Oxidative Phases

The development of treatment trains for pollutant degradation employing zerovalent iron has been attracting a lot of interest in the last few years. This approach consists of pre-treatment only with zerovalent iron, followed by a Fenton oxidation taking advantage of the iron ions released in the first step. In this work, the advantages/disadvantages of this strategy were studied employing commercial zerovalent iron microparticles (mZVI). The effect of the initial amount of mZVI, H₂O₂, pH, conductivity, anions and dissolved oxygen were analysed using p-nitrobenzoic acid (PNBA) as model pollutant. 83% reduction of PNBA 6 µM into p-aminobenzoic acid (PABA) was achieved in natural water at an initial pH 3.0 and 1.4 g/L of mZVI, under aerobic conditions, in 2 h. An evaluation of the convenience of removing mZVI after the reductive phase before the Fenton oxidation was investigated together with mZVI reusability. The Fenton step against the more reactive PABA required 50 mg/L of H₂O₂ to achieve more than 96% removal in 15 min at pH 7.5 (final pH from the reductive step). At least one complete reuse cycle (reduction/oxidation) was achieved with the separated mZVI. This approach might be interesting to treat wastewater containing pollutants initially resistant to hydroxyl radicals.

Mechanisms of metabolic performance enhancement and ARGs attenuation during nZVI-assisted anaerobic chloramphenicol wastewater treatment

Anaerobic wastewater treatment is a promising technology for refractory pollutant treatment. The nano zero-valent iron (nZVI) assisted anaerobic system could enhance contaminant removal. In this work, we added nZVI into an anaerobic system to investigate the effects on system performances and metabolic mechanism for chloramphenicol (CAP) wastewater treatment. As nZVI concentrations increased from 0 to 1 g/L, the CAP removal efficiency was appreciably improved from 46.5% to 99.2%, while the CH₄ production enhanced more than 20 times. The enhanced CAP removal resulted from the enrichments of dechlorination-related bacteria (Hyphomicrobium) and other functional bacteria (e.g., Zoogloea, Syntrophorhabdus) associated with refractory contaminants degradation. The improved CH₄ production was ascribed to the increases in fermentative-related bacteria (Smithella and Acetobacteroides), homoacetogen (Treponema), and methanogens. The increased abundances of anaerobic functional genes further verified the mechanism of CH₄ production. Furthermore, the abundances of potential hosts of antibiotic resistance genes (ARGs) were reduced under high nZVI concentration (1 g/L), contributing to ARGs attenuation. This study provides a comprehensive analysis of the mechanism in metabolic performance enhancement and ARGs attenuation during nZVI-assisted anaerobic CAP wastewater treatment.

Escherichia coli Reduction in Water by Zero-Valent Iron–Sand Filtration Is Based on Water Quality Parameters

Improving the microbial quality of agricultural water through filtration can benefit small farms globally. The incorporation of zero-valent iron (ZVI) into sand filters (ZVI–sand) has been effective in reducing E. coli, Listeria spp., and viruses from agricultural water. This study evaluated ZVI–sand filtration in reducing E. coli levels based on influent water type and the percentage of ZVI in sand filters. A ZVI–sand filter (50% ZVI/50% sand) significantly (p < 0.001) reduced E. coli levels in deionized water by more than 1.5 log CFU/mL compared to pond water over six separate trials, indicating that water type impacts E. coli removal. Overall reductions in E. coli in deionized water and pond water were 98.8 ± 1.7% and 63 ± 24.0% (mean ± standard deviation), respectively. Filters constructed from 50% ZVI/50% sand showed slightly more reduction in E. coli in pond water than filters made from a composition of 35% ZVI/65% sand; however, the difference was not statistically significant (p = 0.48). Principal component analysis identified that the turbidity and conductivity of influent water affected E. coli reductions in filtered water in this study. ZVI–sand filtration reduces Escherichia coli levels more effectively in waters that contain low turbidity values.

A comparison study of Fenton-like and Fenton reactions in dichloromethane removal

Dichloromethane (DCM), as a low-chlorinated organic compound, is hardly to be degraded through the reductive dechlorination pathway. In this study, the removal of DCM in Fenton-like system, using activated carbon fibres-supported zero-valence Fe/Ni nanoparticles (ACF-Fe/Ni) as catalysts, was investigated and compared with that of a traditional Fenton system (Fe2+/H2O2). The influence of vital parameters, including initial solution pH, DCM concentration, catalyst and H2O2 dosages, temperature and cosolute on the removal of DCM, was systematically studied. The results showed that 94.2% of DCM with an initial concentration of 5 mg/L could be removed in the Fenton-like reaction under the optimum condition: initial pH of 2.0, 0.4 g/L of ACF-Fe/Ni, 10 mM of H2O2 and a temperature of 30°C. In comparison, the removal of DCM in the Fenton-like system was faster than that of the Fenton system and the corresponding activation energies were 39.69 and 33.82 kJ/mol, respectively. The coexistence of solute was adverse to the removal of DCM in both Fenton-like and Fenton systems. Moreover, the active species for DCM removal in the Fenton-like system was confirmed as hydroxyl radical (·OH) via the quenching experiment and electron paramagnetic resonance measurement. The incomplete mineralisation (41.7%) of DCM after reaction indicated that the Fenton-like technology had the potential to realise DCM's non-toxic and harmless conversion and organic intermediates formed needed to take longer time to be decomposed.

Critical roles of sulfidation solvent in controlling surface properties and the dechlorination reactivity of S-nZVI

Sulfidation of nanoscale zero-valent iron (nZVI) has been frequently applied to enhance its reactivity, selectivity, and electron utilization efficiency. However, sulfidation of nZVI is generally carried out in aqueous solution, and formation of passivated iron (hydro)oxide species on the surface of S-nZVI due to the reaction between nZVI and water is inevitable. To mitigate this issue, sulfidation of nZVI with hydrogen sulfide dissolved in absolute ethanol was developed. The properties of the resultant S-nZVI, denoted as S-nZVI-H₂S-Ethanol, were compared with S-nZVIs prepared through sulfidation of nZVI with aqueous hydrogen sulfide (S-nZVI-H₂S-Water) and aqueous sodium sulfide (S-nZVI-Na₂S-Water). S-nZVI-H₂S-Ethanol shows increased BET specific surface, reduced susceptibility to incidental oxidation, increased reduction potential, decreased electron-transfer resistance, and improved reactivity toward the reduction of trichloroethylene, compared with S-nZVI-Na₂S-Water and S-nZVI-H₂S-Water. The results highlight the critical roles of sulfidation solvent in controlling the structure, the physicochemical and electrochemical properties, and the dechlorination reactivity of S-nZVI. In addition, these findings offer fundamental mechanistic insights into the sulfidation processes of nZVI by sulfides, suggesting that solvent-iron (hydro)oxide and sulfide-iron (hydro)oxide interactions at the solvent/nZVI interface play key roles in regulating the sulfidation of nZVI and the properties of S-nZVI.

Modeling porosity loss in Fe0-based permeable reactive barriers with Faraday's law

Solid iron corrosion products (FeCPs), continuously generated from iron corrosion in Fe⁰-based permeable reactive barriers (PRB) at pH > 4.5, can lead to significant porosity loss and possibility of system's failure. To avoid such failure and to estimate the long-term performance of PRBs, reliable models are required. In this study, a mathematical model is presented to describe the porosity change of a hypothetical Fe⁰-based PRB through-flowed by deionized water. The porosity loss is solely caused by iron corrosion process. The new model is based on Faraday's Law and considers the iron surface passivation. Experimental results from literature were used to calibrate the parameters of the model. The derived iron corrosion rates (2.60 mmol/(kg day), 2.07 mmol/(kg day) and 1.77 mmol/(kg day)) are significantly larger than the corrosion rate used in previous modeling studies (0.4 mmol/(kg day)). This suggests that the previous models have underestimated the impact of in-situ generated FeCPs on the porosity loss. The model results show that the assumptions for the iron corrosion rates on basis of a first-order dependency on iron surface area are only valid when no iron surface passivation is considered. The simulations demonstrate that volume-expansion by Fe⁰ corrosion products alone can cause a great extent of porosity loss and suggests careful evaluation of the iron corrosion process in individual Fe⁰-based PRB.

Transformation and implication of nanoparticulate zero valent iron in soils

Knowledge of nanoparticulate zero-valent iron (nZVI) transformation in soils and its relationship with the potential impacts on soil properties are crucial to evaluate the environmental implication and application of nZVI. This study investigated nZVI transformation and the effects on soil properties in eight soils with various ageing time and soil moisture content (SMC). Spherical nZVI was gradually oxidized, collapsed, and adhered to clay minerals, and crystalline maghemite and magnetite were the primary oxidation products. Compared with the flooded condition, nZVI oxidation was accelerated under 70% SMC but was limited under 30% SMC. Acidic soil with lower content of dissolved aromatic carbon was advantage to nZVI oxidation under the flooded condition, while carboxymethylcellulose coating and iron oxides on nZVI surface limited nZVI oxidation. The aged nZVI existed mainly in the form of association with soil mineral or organic matter rather than in ion-exchangeable or carbonate form. nZVI treatment promoted soil aromatic carbon sequestration and decreased soil redox potential, and the impacts of nZVI on soil pH, electrical conductivity, ζ-potential, dissolved organic carbon, and catalase and urease activities were dependent on soil type and SMC. The findings are of significance for the evaluation of the environmental risk and proper application of nZVI.

Degradation of petroleum hydrocarbons in soil via advanced oxidation process using peroxymonosulfate activated by nanoscale zero-valent iron

Recently, the use of nanoscale zero-valent iron (nZVI) for removal of organic contaminants from aqueous and soil system has increased. In this study, we employ nZVI to activate peroxymonosulfate (PMS) for the degradation of total petroleum hydrocarbons (TPHs) in aged diesel-contaminated soil. Upon PMS activation by nZVI, PMS produces more highly reactive oxygen species (ROS) in both aqueous solution and soil compared to other compounds (PMS/Co(II)), as determined by electron paramagnetic resonance spectroscopy. Thus, nZVI is an effective catalyst for PMS activation, leading to the efficient degradation of diesel oil in soil compared to other catalysts and oxidants. The optimal concentrations of PMS and nZVI were found to be 3 and 0.2%, respectively, showing the best degradation efficiency (61.2% in 2 h). The observed TPH degradation was retarded (up to 19.1-37% efficiency) in the presence of radical scavengers, such as tert-butyl alcohol, nitrobenzene, ethyl alcohol, and isopropyl alcohol. These results also demonstrate that ROS (hydroxyl and sulfate free radicals) are generated via PMS activation by nZVI. Moreover, more than 96% of TPH can be degraded by sequential applications of PMS/nZVI. Factors affecting TPH degradation, namely PMS/nZVI concentration, soil:solution ratio, soil pH, activators, and oxidants, are also analyzed. The results demonstrate that TPH is degraded to below the residential soil quality limit using PMS/nZVI based on the advanced oxidation process (AOP), which is therefore an effective option for chemical remediation of diesel-contaminated soils over a wide range of pH.

Phosphorus immobilization in water and sediment using iron-based materials: A review

As an essential element for life, phosphorus (P) is very important for organisms. However, excessive P in water and sediment can cause eutrophication, which poses a potential risk to drinking water safety and the sustainability of aquatic ecosystems. Therefore, effective phosphorus-control in water and sediment is the key strategy to control eutrophication. Iron-based materials exhibit high efficiency for P immobilization due to their strong affinity with P, low cost, easy availability, and environmentally friendliness. They are promising materials for controlling P in application. This work comprehensively summarizes the recent advances on P immobilization in water and sediment by different iron-based materials, including iron (hydr)oxides, iron salts, zero-valent iron and iron-loaded materials. This review is focused on the mechanism of the processes and how they are impacted by major influencing factors. The combination of iron-containing materials with other assisting materials is a good strategy to enhance P-fixation efficiency and selectivity. Finally, the current challenges and prospects of P-control technologies based on iron-containing materials are proposed. This review provides a systemic theoretical and experimental foundation for P-immobilization in water and sediment using iron-based materials.

Mechanistic role of nitrate anion in TCE dechlorination by ball milled ZVI and sulfidated ZVI: Experimental investigation and theoretical analysis

Mechanistic role of NO₃^- in trichloroethylene (TCE) dechlorination by ball milled, micro-scale sulfidated and unsulfidated ZVI (e.g., S-mZVI^bm and mZVI^bm) was explored through experiments and density functional theory (DFT) calculations. Sulfidation inhibited NO₃^- reduction by mZVI^bm as S weakened its interaction with NO₃^-. mZVI^bm reduced NO₃^- within 2 h. This just resulted in a short-term electron competition during the dechlorination process by mZVI^bm and hardly affected its sluggish dechlorination kinetics (complete TCE dechlorination in 11 d). On the contrary, NO₃^- suppressed TCE dechlorination by S-mZVI^bm. This was attributed to that inhibited NO₃^- reduction by S-mZVI^bm (40 % reduction in 6 h) induced continuous electron competition with TCE during the time span of its dechlorination by S-mZVI^bm. NO₃^- reduction was also observed to facilitate formation/crystallization of Fe₃O₄ on both ZVI particles, promoting dechlorination by mZVI^bm after 4 d while not taking effect to the S-mZVI^bm/TCE system, as its dechlorination time was too short for the surface of S-mZVI^bm to transform. This observation has important implication on groundwater remediation by ZVI or sulfidated ZVI PRBs under a scenario of upgradient anthropogenic release of NO₃^-.

Catalytic properties of transition metals modified nanoscale zero-valent iron for simultaneous removal of 4-chlorophenol and Cr(VI): Efficacy, descriptor and reductive mechanisms

Since chlorophenols (CPs) and Cr(VI) are two types of common pollutants in the environment, developing an effective approach to remove these contaminants has important benefits for public health. However, few efforts have been made so far. In this study, we prepared nanoscale zero-valent iron (nZVI) and a series of bimetallic nanoparticles (transition-metal modified nZVI) to investigate their catalytic properties for the simultaneous removal of 4-chlorophenol (4-CP) and Cr(VI). While nZVI enabled a fast removal of Cr(VI), it had a poor dechlorination ability. However, effective simultaneous removal of 4-CP and Cr(VI) was achieved with the transition metal modified nZVI, especially in the Pd/Fe bimetallic system. The enhanced catalytic activity of transition metal modified nZVI was primarily attributed to the formations of numerous nano-galvanic cells and atomic hydrogen species that facilitated electron transfer in the reaction system and played a key role in triggering the C-Cl bond cleavage, respectively. According to the dechlorination ability, the transition-metal catalysts examined in this study can be divided into three groups in descending order: the first being Pd and Ni, the second including Cu and Pt, while the last consisting of Au and Ag. The catalytic hydrodechlorination activity of bimetals can be well described by the volcano curve and rationally explained by the hydrogen adsorption energies on the metals, and was severely impaired by increasing Cr(VI) concentrations. Characterization results validated the formations of Fe(III)-Cr(III) hydroxide/oxyhydroxide on the bimetals surface after reacting with 4-CP and Cr(VI). This work provides the first insight into the catalytic properties of transition-metal modified nZVI for the effective removal of combined pollutants.

Groundwater geochemical constituents controlling the reductive dechlorination of TCE by nZVI: Evidence from diverse anaerobic corrosion mechanisms of nZVI

The corrosion mechanisms of nanoscale zero-valent iron (nZVI) vary with different geochemical constituents, which affect the reductive dechlorination process of trichloroethylene (TCE). In this study, the effect of nZVI anaerobic corrosion on the reductive dechlorination of TCE with different groundwater geochemical constituents (Ca²⁺-SO₄^2-, Ca²⁺-HCO₃^-, Na⁺-NO₃^-) was investigated. Microscopic characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM) combined with pH, oxidation-reduction potential (ORP) and dissolved Fe²⁺ in solutions to illustrate the corrosion mechanism of nZVI. In the four systems including ultrapure water (UPW), the reduction of TCE conformed to pseudo-first-order kinetics, the generation of Cl^- accorded with zero-order kinetics, and multi-step reaction kinetics was used to fit the generation and degradation of chlorinated byproducts (Dichloroethylene, DCEs). Compared with UPW system, the dissolution corrosion of Ca²⁺-HCO₃^- and Ca²⁺-SO₄^2- promoted the reductive dechlorination of TCE (k_{obs, TCE} = 0.658 ± 0.010 & 0.245 ± 0.028 d^-1 and k_{obs, Cl-} = 41.682 ± 1.016 & 20.623 ± 1.923 μM⋅d^-1 for Ca²⁺-HCO₃^- & Ca²⁺-SO₄^2-, respectively) and the degradation of DCEs (0.444 ± 0.036 & 0.244 ± 0.040 μM⋅d^-1 for Ca²⁺-HCO₃^- & Ca²⁺-SO₄^2-, respectively); redox-active NO₃^- competed for electrons and passivated the surface of nZVI, which limited the reductive dechlorination of TCE (k_{obs, TCE} = 0.111 ± 0.025 d^-1 & k_{obs, Cl-} = 14.943 ± 0.664 μM⋅d^-1) and the degradation of DCEs (0.078 ± 0.018 μM⋅d^-1), and the passivation layer promoted the adsorption of TCE. This study from the perspective of nZVI corrosion provides a theoretical basis for the long-term application of nZVI technology in the remediation of TCE-contaminated sites with different groundwater geochemical types.