Contributions of Abiotic and Biotic Dechlorination Following Carboxymethyl Cellulose Stabilized Nanoscale Zero Valent Iron Injection

Cr(VI) removal during cotransport of nano-iron-particles combined with iron sulfides in groundwater: Effects of D. vulgaris and S. putrefaciens

Iron-based materials such as nanoscale zerovalent iron (nZVI) are effective candidates to in situ remediate hexachromium (Cr(VI))-contaminated groundwater. The anaerobic bacteria could influence the remediation efficiency of Cr(VI) during its cotransport with nZVI in porous media. To address this issue, the present study investigated the adsorption and reduction of Cr(VI) during its cotransport with green tea (GT) modified nZVI (nZVI@GT) and iron sulfides (FeS and FeS2) in the presence of D. vulgaris or S. putrefaciens in water-saturated sand columns. Experimental results showed that the nZVI@GT preferred to heteroaggregate with FeS2 rather than FeS, forming nZVI@GT-FeS2 heteroaggregates. Although the presence of D. vulgaris further induced nZVI@GT-FeS2 heteroaggregates to form larger clusters, it pronouncedly improved the dissolution of FeS and FeS2 for more Cr(VI) reduction associated with lower Cr(VI) flux through sand. In contrast, S. putrefaciens could promote the dispersion of the heteroaggregates of nZVI@GT-FeS2 and the homoaggregates of nZVI@GT or FeS by adsorption on the extracellular polymeric substances, leading to the improved transport of Fe-based materials for a much higher Cr(VI) immobilization in sand media. Overall, our study provides the essential perspectives into a chem-biological remediation technique through the synergistic removal of Cr(VI) by nZVI@GT and FeS in contaminated groundwater. ENVIRONMENTAL IMPLICATION: The green-synthesized nano-zero-valent iron particles (nZVI@GT) using plant extracts (or iron sulfides) have been used for in situ remediation of Cr(VI) contaminated groundwater. Nevertheless, the removal of Cr(VI) (including Cr(VI) adsorption and Cr(III) generation) could be influenced by the anaerobic bacteria governing the transport of engineered nanoparticles in groundwater. This study aims to reveal the inherent mechanisms of D. vulgaris and S. putrefaciens governing the cotransport of nZVI@GT combined with FeS (or FeS2) to further influence the Cr(VI) removal in simulated complex groundwater media. Our findings provides a chemical and biological synergistic remediation strategy for nZVI@GT application in Cr(VI)-contaminated groundwater.

Polymer Coatings Affect Transport and Remobilization of Colloidal Activated Carbon in Saturated Sand Columns: Implications for In Situ Groundwater Remediation

Colloidal activated carbon (CAC) is an emerging technology for the in situ remediation of groundwater impacted by per- and polyfluoroalkyl substances (PFAS). In assessing the long-term effectiveness of a CAC barrier, it is crucial to evaluate the potential of emplaced CAC particles to be remobilized and migrate away from the sorptive barrier. We examine the effect of two polymer stabilizers, carboxymethyl cellulose (CMC) and polydiallyldimethylammonium chloride (PolyDM), on CAC deposition and remobilization in saturated sand columns. CMC-modified CAC showed high mobility in a wide ionic strength (IS) range from 0.1 to 100 mM, which is favorable for CAC delivery at a sufficient scale. Interestingly, the mobility of PolyDM-modified CAC was high at low IS (0.1 mM) but greatly reduced at high IS (100 mM). Notably, significant remobilization (release) of deposited CMC-CAC particles occurred upon the introduction of solution with low IS following deposition at high IS. In contrast, PolyDM-CAC did not undergo any remobilization following deposition due to its favorable interactions with the quartz sand. We further elucidated the CAC deposition and remobilization behaviors by analyzing colloid-collector interactions through the application of Derjaguin-Landau-Verwey-Overbeek theory, and the inclusion of a discrete representation of charge heterogeneity on the quartz sand surface. The classical colloid filtration theory was also employed to estimate the travel distance of CAC in saturated columns. Our results underscore the roles of polymer coatings and solution chemistry in CAC transport, providing valuable guidelines for the design of in situ CAC remediation with maximized delivery efficiency and barrier longevity.

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

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

Performance of field demonstration nanoscale zero-valent iron in groundwater remediation: A review

Nanoscale zero-valent iron (nZVI) has gained widespread usage in groundwater remediation due to its exceptional reactivity. Since its initial deployment in field demonstrations in 2001, nZVI has proven to be an effective nanomaterial for addressing groundwater contaminants. Subsequent research has highlighted the versatility of nZVI, showcasing its potential to overcome critical limitations associated with conventional remediation technologies. The effectiveness of nZVI in remediation varies, contingent on factors such as the type of nZVI, contaminant nature, site conditions, and injection methodologies employed. This review aims to present a comprehensive progress report on the field application of nZVI spanning 22 years across eight countries. Drawing from a database encompassing 32 pilot or full-scale remediation sites, the study delineates the various types of nZVI, modification methods, demonstration sites, and primary contaminants targeted in field tests. Specific attention is given to the application effects and mechanisms of unmodified nZVI, Pd, surfactants, and carbon-modified nZVI in diverse field demonstrations. An analysis of the key factors influencing their performance is provided, and potential future applications of nZVI in groundwater remediation are discussed.

Catalytic dechlorination of 1,2-DCA in nano Cu0-borohydride system: effects of Cu0/Cun+ ratio, surface poisoning, and regeneration of Cu0 sites

Aqueous-phase catalyzed reduction of organic contaminants via zerovalent copper nanoparticles (nCu⁰), coupled with borohydride (hydrogen donor), has shown promising results. So far, the research on nCu⁰ as a remedial treatment has focused mainly on contaminant removal efficiencies and degradation mechanisms. Our study has examined the effects of Cu⁰/Cuⁿ⁺ ratio, surface poisoning (presence of chloride, sulfides, humic acid (HA)), and regeneration of Cu⁰ sites on catalytic dechlorination of aqueous-phase 1,2-dichloroethane (1,2-DCA) via nCu⁰-borohydride. Scanning electron microscopy confirmed the nano size and quasi-spherical shape of nCu⁰ particles. X-ray diffraction confirmed the presence of Cu⁰ and Cu₂O and x-ray photoelectron spectroscopy also provided the Cu⁰/Cuⁿ⁺ ratios. Reactivity experiments showed that nCu⁰ was incapable of utilizing H₂ from borohydride left over during nCu⁰ synthesis and, hence, additional borohydride was essential for 1,2-DCA dechlorination. Washing the nCu⁰ particles improved their Cu⁰/Cuⁿ⁺ ratio (1.27) and 92% 1,2-DCA was removed in 7 h with k_obs = 0.345 h^-1 as compared to only 44% by unwashed nCu⁰ (0.158 h^-1) with Cu⁰/Cuⁿ⁺ ratio of 0.59, in the presence of borohydride. The presence of chloride (1000-2000 mg L^-1), sulfides (0.4-4 mg L^-1), and HA (10-30 mg L^-1) suppressed 1,2-DCA dechlorination; which was improved by additional borohydride probably via regeneration of Cu⁰ sites. Coating the particles decreased their catalytic dechlorination efficiency. 85-90% of the removed 1,2-DCA was recovered as chloride. Chloroethane and ethane were main dechlorination products indicating hydrogenolysis as the major pathway. Our results imply that synthesis parameters and groundwater solutes control nCu⁰ catalytic activity by altering its physico-chemical properties. Thus, these factors should be considered to develop an efficient remedial design for practical applications of nCu⁰-borohydride.

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.

Combining nanoscale zero-valent iron and anaerobic dechlorinating bacteria to degrade chlorinated methanes and 1,2-dichloroethane

Nanoscale zero-valent iron (nZVI) has the potential to degrade a diversity of chlorinated compounds, and it is widely used for remediation of contaminated groundwaters. However, some frequently detected contaminants such as dichloromethane (DCM) and 1,2-dichloroethane (1,2-DCA) have shown nearly no reactivity with nZVI. Here, we tested the feasibility of combining anaerobic dechlorinating bacteria, Dehalobacterium and Dehalogenimonas, and nZVI as a treatment train to detoxify chlorinated methanes (i.e., chloroform-CF- and DCM), and 1,2-DCA. First, we showed that CF (500 μM) was fully degraded by 1 g/L nZVI to DCM as a major by-product, which was susceptible to fermentation by Dehalobacterium to innocuous products. Our results indicate that soluble compounds released by nZVI might cause an inhibitory impact on Dehalobacterium activity, avoiding DCM depletion. The DCM dechlorination activity was recovered when transferred to a fresh medium without nZVI. The increase in H2 production and pH was discarded as potential inhibitors. Similarly, a Dehalogenimonas-containing culture was unable to dichloroeliminate 1,2-DCA when exposed to 1 g/L nZVI, but dechlorinating activity was also recovered when transferred to nZVI-free media. The recovery of the dechlorinating activity of Dehalobacterium and Dehalogenimonas suggests that combination of nZVI and bioremediation techniques can be feasible under field conditions where dilution processes can alleviate the impact of the potential inhibitory soluble compounds.

Recent advances and trends of trichloroethylene biodegradation: A critical review

Trichloroethylene (TCE) is a ubiquitous chlorinated aliphatic hydrocarbon (CAH) in the environment, which is a Group 1 carcinogen with negative impacts on human health and ecosystems. Based on a series of recent advances, the environmental behavior and biodegradation process on TCE biodegradation need to be reviewed systematically. Four main biodegradation processes leading to TCE biodegradation by isolated bacteria and mixed cultures are anaerobic reductive dechlorination, anaerobic cometabolic reductive dichlorination, aerobic co-metabolism, and aerobic direct oxidation. More attention has been paid to the aerobic co-metabolism of TCE. Laboratory and field studies have demonstrated that bacterial isolates or mixed cultures containing Dehalococcoides or Dehalogenimonas can catalyze reductive dechlorination of TCE to ethene. The mechanisms, pathways, and enzymes of TCE biodegradation were reviewed, and the factors affecting the biodegradation process were discussed. Besides, the research progress on material-mediated enhanced biodegradation technologies of TCE through the combination of zero-valent iron (ZVI) or biochar with microorganisms was introduced. Furthermore, we reviewed the current research on TCE biodegradation in field applications, and finally provided the development prospects of TCE biodegradation based on the existing challenges. We hope that this review will provide guidance and specific recommendations for future studies on CAHs biodegradation in laboratory and field applications.

Catalytic degradation of brominated flame retardants in the environment: New techniques and research highlights

Due to the extensive commercial use of brominated flame retardants (BFRs), human beings are chronically exposed to BFRs, causing great harms to human health, which imposes urgent demands to degrade them in the environment. Among various degradation techniques, catalytic degradation has been proven to be outstanding because of its rapidness and effectiveness. Therefore, much attention has been given to catalytic degradation, especially the extensively studied photocatalytic degradation and nanocatalytic reduction techniques. Recently, some novel advanced catalytic techniques have been developed and show excellent catalytic degradation efficiency for BFRs, including natural substances catalytic degradation, new Fenton catalytic degradation, new chemical reagent catalytic degradation, new material catalytic degradation, electrocatalytic degradation, plasma catalytic degradation, and composite catalytic degradation systems. In addition to the common features of traditional catalytic techniques, these novel techniques possess their own specific advantages in various aspects. Therefore, this review summarized the degradation mechanism of BFRs by the above new catalytic degradation methods under the laboratory conditions, simulated real environment, and real environment conditions, and further evaluated their advantages and disadvantages, aiming to provide some research ideas for the catalytic degradation of BFRs in the environment in the future. We suggested that more attention should focus on features of novel catalytic techniques, including eco-friendliness, cost-effectiveness, and pragmatic usefulness.

Using sequential H2O2 addition to sustain 1,2-dichloroethane detoxification by a nanoscale zerovalent iron-induced Fenton's system at a natural pH

1,2-dichloroethane (1,2-DCA) is a chlorinated hydrocarbon used for polyvinyl chloride plastic production. As such, 1,2-DCA is a common persistent contaminant in saturated zones. While nanoscale zerovalent iron (NZVI) is considered an effective reductant for removing a wide range of chlorinated hydrocarbons, 1,2-DCA is resistant to reduction by NZVI as well as by modified forms of NZVI (e.g., sulfidated-NZVI). Hydroxyl radicals produced in Fenton's reaction can effectively degrade 1,2-DCA, but Fenton's reaction requires the acidification of saturated zones to achieve a groundwater pH of 3 to facilitate the catalytic reaction. To overcome this problem, this study has developed a sequential treatment process using an NZVI-induced Fenton-like reaction that can effectively degrade 1,2-DCA at an initially neutral pH range. The experiments were conducted using a high 1,2-DCA concentration (2000 mg/L) to evaluate the feasibility of using the treatment process at source zones. The process degraded 99% of 1,2-DCA with a pseudo-first-order rate constant of 0.49 h^-1. Unlike the single-stage treatment process, the sequential treatment can control the used H₂O₂ concentration in the system, thus sustaining the reaction and resulting in more efficient 1,2-DCA degradation. To mimic subsurface conditions, batch experiments were conducted to remove 1,2-DCA sorbed in contaminated soil. The results show that 99% removal of 1,2-DCA was obtained within 16 h. Additionally, this study suggests that the NZVI can be used for at least three consecutive 1,2-DCA degradation cycles while maintaining high removal efficiency.

Strategies to enhance the reactivity of zero-valent iron for environmental remediation: A review

Application of zero-valent iron (ZVI) has become one of the most promising innovative technologies for the remediation of environmental pollutants. However, ZVI may suffer from the low intrinsic reactivity toward refractory pollutants, which seriously restricts its practical application in fields. Therefore, strategies have been developing to enhance the reactivity of ZVI. Until now, the most commonly used strategies include pretreatment of ZVI, synthesis of highly-active ZVI-based materials and additional auxiliary measures. In this review, a systematic and comprehensive summary of these commonly used strategies has been conducted for the following purposes: (1) to understand the fundamental mechanisms of the selected approaches; (2) to point out their advantages and shortcomings; (3) to illustrate the main problems of their large-scale application; (4) to forecast the future trend of developing ZVI technologies. Overall, this review is devoted to providing a fundamental understanding on the mechanism for enhancing the reactivity of ZVI and facilitating the practical application of ZVI technologies in fields.

Zero-valent iron-induced successive chemical transformation and biodegradation of lindane in historically contaminated soil: An isotope-informed metagenomic study

Zero-valent iron (ZVI) is widely used to mitigate environmental pollutants such as chlorinated pesticides through reductive reactions accompanied by extensive impacts on the soil microbial community. However, whether and how ZVI changes the biodegradation of target compounds remain poorly understood. Here, we monitor the fate of lindane using a ¹⁴C-labled tracer and evaluate the growth and functions of the bacterial community in ZVI-stressed conditions in a historically γ-hexachlorocyclohexane (lindane)-contaminated soil using a combination of isotopic (¹⁸O-H₂O) and metagenomic methods. ZVI promoted the biomineralization of lindane in a dose-dependent manner. Soil bacteria were inhibited by amendment with ZVI during the initial stages of incubation (first three days) but recovered during the subsequent six weeks. Metagenomic study indicates that the todC1/bedC1 genes involved in the oxidation of dechlorinated lindane intermediates were upregulated in the ¹⁸O-labeled bacterial community but the presence of the lin genes responsible for lindane dechlorination was not confirmed. In addition, the benzoate biodegradation pathway that links to downstream catabolism of lindane was enhanced. These findings indicate successive chemical and biological degradation mechanisms underlying ZVI-enhanced lindane mineralization and provide a scientific basis for the inclusion of an extended bioremediation stage in the environmental application of ZVI materials.

Coupling microscale zero-valent iron and autotrophic hydrogen-bacteria provides a sustainable remediation solution for trichloroethylene-contaminated groundwater: Mechanisms, regulation, and engineering implications

Coupling microscale zero-valent iron (mZVI) and autotrophic hydrogen bacteria (AHB) has gained increasing attention owing to its potential to improve dechlorination performance by bridging H₂ donors and acceptors. However, few studies have attempted to test its sustainable remediation performance and to comprehensively unveil the governing mechanisms. This study systematically compared the performances of different systems (mZVI, H₂-AHB, and mZVI-AHB) for trichloroethylene (TCE) removal, and further optimized dechlorination and H₂ evolution of mZVI-AHB synchronously by regulating the mZVI particle size and dosage to achieve a win-win remediation solution. The final removal efficiency and removal rate of TCE by mZVI-AHB were 1.67-fold and 5.30-fold of those by mZVI alone respectively, and mZVI-AHB resulted in more complete dechlorination than H₂-AHB alone. Combining H₂ evolution kinetics, material characterization data, and bacterial community analysis results, the improved dechlorination performance of mZVI-AHB was mainly due to the following mechanisms: H₂ generated by mZVI corrosion was efficiently utilized by AHB, lasting corrosion of mZVI was facilitated by AHB, and dechlorination functional bacteria were highly enriched by mZVI. Finally, the remediation performance of mZVI-AHB with different mZVI particle sizes and dosages was evaluated comprehensively in terms of dechlorination reactivity, H₂ utilization efficiency and chemical cost, and suggestions for possible engineering applications are provided.

Green synthesis of modified polyethylene packing supported tea polyphenols-NZVI for nitrate removal from wastewater: Characterization and mechanisms

Nano-zero-valent iron (NZVI), as an electron donor, performed excellence in the reduction and remove of nitrate. However, the easy agglomeration and poor antioxidation of NZVI declined the nitrate removal and limited the application in the field of wastewater treatment. Herein, a novel composite packing of tea polyphenol, NZVI and modified polyethylene carrier (TP-NZVI/PE) was prepared and characterized, the removal efficiency of nitrate was verified, and the preliminary removal mechanism was finally investigated. The results showed that the maximum iron loading on TP-NZVI/PE composite achieved under 50 °C, pH of 5.0, 4.0 g/L of Fe²⁺, and 7.2 g/L of TP, respectively, with 3.51 ± 0.12 mg/g. NZVI presented satisfactory antioxidation and anti-agglomeration via TP encapsulation. TP encapsulation of TP-NZVI/PE composite was easily degraded by microorganisms and NZVI was exposed to nitrate during wastewater treatment, which made the reduction of nitrate possible. The nitrate removal efficiency of TP-NZVI/PE composite with microorganism was 79.88 ± 0.17%, higher three times than that of TP-NZVI/PE (25.54 ± 0.21%). The oxidized NZVI was transformed to Fe²⁺/Fe³⁺, which were prone to adsorb nitrate and then co-precipitate. It was favorable for further removal of nitrate. Results suggested a novel approach for fast and eco-friendly preparation and efficient application of NZVI.

In situ nanoremediation of soils and groundwaters from the nanoparticle's standpoint: A review

Anthropogenic pollution coming from industrial processes, agricultural practices and consumer products, results in the release of toxic substances into rural and urban environments. Once released, these chemicals migrate through the atmosphere and water, and find their way into matrices such as sediments and groundwaters, thus making large areas potentially uninhabitable. Common pollutants, including heavy metal(loid)s, radionuclides, aliphatic hydrocarbons and halogenated organics, are known to adversely affect physiological systems in animal species. Pollution can be cleaned up using techniques such as coagulation, reverse osmosis, oxidation and biological methods, among others. The use of nanoparticles (NPs) extends the range of available technologies and offers particular benefits, not only by degrading, transforming and immobilizing contaminants, but also by reaching inaccessible areas and promoting biotic degradation. The development of NPs is understandably heralded as an environmentally beneficial technology; however, it is only now that the ecological risks associated with their use are being evaluated. This review presents recent developments in the use of engineered NPs for the in situ remediation of two paramount environmental matrices: soils and groundwaters. Emphasis will be placed on (i) the successful applications of nano-objects for environmental cleanup, (ii) the potential safety implications caused by the challenging requirements of [high reactivity toward pollutants] vs. [none reactivity toward biota], with a thorough view on their transport and evolution in the matrix, and (iii) the perspectives on scientific and regulatory challenges. To this end, the most promising nanomaterials will be considered, including nanoscale zerovalent iron, nano-oxides and carbonaceous materials. The purpose of the present review is to give an overview of the development of nanoremediators since they appeared in the 2000s, from their chemical modifications, mechanism of action and environmental behavior to an understanding of the problematics (technical limitations, economic constraints and institutional precautionary approaches) that will drive their future full-scale applications.

Recent Advances in Sulfidated Zerovalent Iron for Contaminant Transformation

2021 marks 10 years since controlled abiotic synthesis of sulfidated nanoscale zerovalent iron (S-nZVI) for use in site remediation and water treatment emerged as an area of active research. It was then expanded to sulfidated microscale ZVI (S-mZVI) and together with S-nZVI, they are collectively referred to as S-(n)ZVI. Heightened interest in S-(n)ZVI stemmed from its significantly higher reactivity to chlorinated solvents and heavy metals. The extremely promising research outcomes during the initial period (2011-2017) led to renewed interest in (n)ZVI-based technologies for water treatment, with an explosion in new research in the last four years (2018-2021) that is building an understanding of the novel and complex role of iron sulfides in enhancing reactivity of (n)ZVI. Numerous studies have focused on exploring different S-(n)ZVI synthesis approaches, and its colloidal, surface, and reactivity (electrochemistry, contaminant selectivity, and corrosion) properties. This review provides a critical overview of the recent milestones in S-(n)ZVI technology development: (i) clear insights into the role of iron sulfides in contaminant transformation and long-term aging, (ii) impact of sulfidation methods and particle characteristics on reactivity, (iii) broader range of treatable contaminants, (iv) synthesis for complete decontamination, (v) ecotoxicity, and (vi) field implementation. In addition, this review discusses major knowledge gaps and future avenues for research opportunities.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Remediation of Cd-Contaminated Soil by Modified Nanoscale Zero-Valent Iron: Role of Plant Root Exudates and Inner Mechanisms

In this study, the role of exogenous root exudates and microorganisms was investigated in the application of modified nanoscale zero-valent iron (nZVI) for the remediation of cadmium (Cd)-contaminated soil. In this experiment, citric acid (CA) was used to simulate root exudates, which were then added to water and soil to simulate the pore water and rhizosphere environment. In detail, the experiment in water demonstrated that low concentration of CA facilitated Cd removal by nZVI, while the high concentration achieved the opposite. Among them, CA can promote the adsorption of Cd not only by direct complexation with heavy metal ions, but also by indirect effect to promote the production of iron hydroxyl oxides which has excellent heavy metal adsorption properties. Additionally, the H⁺ dissociated from CA posed a great influence on Cd removal. The situation in soil was similar to that in water, where low concentrations of CA contributed to the immobilization of Cd by nZVI, while high concentrations promoted the desorption of Cd and the generation of CA–Cd complexes which facilitated the uptake of Cd by plants. As the reaction progressed, the soil pH and cation exchange capacity (CEC) increased, while organic matter (OM) decreased. Meanwhile, the soil microbial community structure and diversity were investigated by high-throughput sequencing after incubation with CA and nZVI. It was found that a high concentration of CA was not conducive to the growth of microorganisms, while CMC had the effect of alleviating the biological toxicity of nZVI.

Porous media transport of iron nanoparticles for site remediation application: A review of lab scale column study, transport modelling and field-scale application

Injection of surface modified zero valent iron nanoparticles for in situ remediation of soil, contaminated with an array of pollutants has attracted great attention due to the high reactivity of zero valent iron towards a broad range of contaminants, its cost effectiveness, minimal physical disruption and low toxicity. The effectiveness of this technology relies on the stability and mobility of injected iron nanoparticles. Hence the development of a modelling tool capable of predicting nZVI transport is indispensable. This review provides state of the art knowledge on the mobility of iron nanoparticles in porous media, mechanisms involved in subsurface retention of nZVI based on continuum models and field scale application. Special attention is given to the identification of the influential parameters controlling the transport potential of iron nanoparticles and the available numerical models for the simulation of laboratory scale transport data.

Biotechnology and nanotechnology for remediation of chlorinated volatile organic compounds: current perspectives

Chlorinated volatile organic compounds (CVOCs) are persistent organic pollutants which are harmful to public health and the environment. Many CVOCs occur in substantial quantities in groundwater and soil, even though their use has been more carefully managed and restricted in recent years. This review summarizes recent data on several innovative treatment solutions for CVOC-affected media including bioremediation, phytoremediation, nanoscale zero-valent iron (nZVI)-based reductive dehalogenation, and photooxidation. There is no optimally developed single technology; therefore, the possibility of using combined technologies for CVOC remediation, for example bioremediation integrated with reduction by nZVI, is presented. Some methods are still in the development stage. Advantages and disadvantages of each treatment strategy are provided. It is hoped that this paper can provide a basic framework for selection of successful CVOC remediation strategies.

Nanoscale zero-valent iron reduction coupled with anaerobic dechlorination to degrade hexachlorocyclohexane isomers in historically contaminated soil

Hexachlorocyclohexane (HCH) isomers pose potential threats to the environment and to public health due to their persistence and high toxicity. In this study, nanoscale zero-valent iron (nZVI) coupled with microbial degradation by indigenous microorganisms with and without biostimulation was employed to remediate soils highly polluted with HCH. The degradation efficiency of total HCHs in both the "nZVI-only" and "Non-amendment" treatments was approximately 50 %, while in the treatment amended with nZVI and acetate, 85 % of total HCHs was removed. Addition of nZVI and acetate resulted in enrichment of anaerobic microorganisms. The results of quantitative PCR (qPCR) and 16S rRNA gene amplicon sequencing revealed that Desulfotomaculum, Dehalobacter, Geobacter, and Desulfuromonas likely contributed to the depletion of HCH isomers. Moreover, some abiotic factors also favored this removal process, including pH, and the generation of iron sulfides as revealed by the result of Mössbauer spectrometer analysis. Our research provides an improved remediation strategy for soils polluted with HCH isomers and an understanding of the synergistic effect of nZVI and indigenous microorganisms.

Two distinct Dehalobacter strains sequentially dechlorinate 1,1,1-trichloroethane and 1,1-dichloroethane at a field site treated with granular zero valent iron and guar gum

Chlorinated ethanes are environmental pollutants found frequently at many contaminated industrial sites. 1,1,1-Trichloroethane (1,1,1-TCA) can be dechlorinated and detoxified via abiotic transformation or biologically by the action of dechlorinating microorganisms such as Dehalobacter (Dhb). At a field site, it is challenging to distinguish abiotic vs. biotic mechanisms as both processes share common transformation products. In this study, we evaluated using the Dhb 16S rRNA gene and specific reductive dehalogenase genes as biomarkers for 1,1,1-TCA and 1,1-dichloroethane (1,1-DCA) dechlorination. We analyzed samples from laboratory groundwater microcosms and from an industrial site where a mixture of granular zero valent iron (ZVI) and guar gum was injected for 1,1,1-TCA remediation. Abiotic and biotic transformation products were monitored and the changes in dechlorinating organisms were tracked using quantitative PCR (qPCR) with primers targeting the Dhb 16S rRNA gene and two functional genes cfrA and dcrA encoding enzymes that dechlorinate 1,1,1-TCA to 1,1-DCA and 1,1-DCA to chloroethane (CA), respectively. The abundance of the cfrA- and dcrA-like genes confirmed that the two dechlorination steps were carried out by two distinct Dhb populations at the site. The biomarkers used in this study proved useful for monitoring different Dhb populations responsible for step-wise dechlorination and tracking biodegradation of 1,1,1-TCA and 1,1-DCA where both abiotic (e.g., with ZVI) and biotic processes co-occur.

Bioremediation of typical chlorinated hydrocarbons by microbial reductive dechlorination and its key players: A review

Chlorinated hydrocarbon contamination in soils and groundwater has a severe negative impact on the human health. Microbial reductive dechlorination is a major degradation pathway of chlorinated hydrocarbon in anaerobic subsurface environments, has been extensively studied. Recent progress on the diversity of the reductive dechlorinators and the key enzymes of chlororespiration has been well reviewed. Here, we present a thorough overview of the studies related to bioremediation of chloroethenes and polychlorinated biphenyls based on enhanced in situ reductive dechlorination. The major part of this review is to provide an up-to-date summary of functional microorganisms which are either detected during in situ biostimulation or applied in bioaugmentation strategies. The applied biostimulants and corresponding reductive dechlorination products are also summarized and the future research needs are finally discussed.

Effects of hydrogen gas production, trapping and bubble-facilitated transport during nanoscale zero-valent iron (nZVI) injection in porous media

The injection of nanoscale zero-valent iron (nZVI) can be an effective technique for the treatment of groundwater contaminants, including chlorinated solvents. However, its effectiveness can be limited by natural reductant demand (NRD) reactions, including the reduction of water resulting in the production of hydrogen gas. This study presents results from a series of laboratory experiments to investigate gas production and mobilization following the injection of nZVI solutions, along with sodium borohydride (NaBH₄) that is used for nZVI synthesis. Experiments were performed in a thin, two-dimensional flow cell (22 × 34 × 1 cm³) to measure hydrogen gas volumes and local gas saturations, and to investigate the distribution of gas within and above the injection zone. An additional experiment was conducted in a larger flow cell (150 × 150 × 2 cm³) containing dissolved trichloroethene (TCE) to assess changes in aqueous flow pathways and enhanced vertical transport of TCE by mobilized gas. The results showed substantial gas production (60% to 740% of the injected solution volume) resulting in gas mobilization as a network of gas channels above the injection zone, with more gas produced from greater excess NaBH₄ used during nZVI synthesis. Trapped gas saturations were sufficient to cause the diversion of aqueous flow around the nZVI injection zone. In addition, gas production and mobilization resulted in the bubble-facilitated transport of TCE, and detectable concentrations of TCE and reaction products (ethane and ethene) above the target treatment zone.

Electron competition and electron selectivity in abiotic, biotic, and coupled systems for dechlorinating chlorinated aliphatic hydrocarbons in groundwater: A review

Chlorinated aliphatic hydrocarbons (CAHs) have been frequently detected in aquifers in recent years. Owing to the bioaccumulation and toxicity of CAHs, it is essential to explore high-efficiency technologies for their complete dechlorination in groundwater. At present, the most widely used abiotic and biotic remediation technologies are based on zero-valent iron (ZVI) and functional anaerobic bacteria (FAB), respectively. However, the main obstacles to the full potential of both technologies in the field include their lowered efficiencies and increased economic costs due to the co-existence of a variety of natural electron acceptors in the environment, such as dissolved oxygen (DO), nitrate (NO₃^-), sulfate (SO₄^2-), ferric iron (Fe (III)), bicarbonate (HCO₃^-), and even water, which compete for electrons with the target contaminants. Therefore, a clear understanding of the mechanisms governing electron competition and electron selectivity is significant for the accurate evaluation of the effectiveness of both technologies under natural hydrochemical conditions. We collected data from both abiotic and biotic CAH-remediation systems, summarized the dechlorination and undesired reactions in groundwater, discussed the characterization methods and general principles of electron competition, and described strategies to improve electron selectivity in both systems. Furthermore, we reviewed the emerging ZVI-FAB coupled system, which integrates abiotic and biotic processes to enhance dechlorination performance and electron utilization efficiency. Lastly, we propose future research needs to quantitatively understand the electron competition in abiotic, biotic, and coupled systems in more detail and to promote improved electron selectivity in groundwater remediation.

Sulfidation enhances stability and mobility of carboxymethyl cellulose stabilized nanoscale zero-valent iron in saturated porous media

Sulfidation can enhance the reactivity and longevity of nanoscale zero-valent iron (nZVI), but little is known about its effect on the fate and transport of nZVI in saturated porous media. This work compared the stability and mobility of carboxymethyl cellulose (CMC) stabilized nZVI (CMC-nZVI) and sulfidated nZVI (CMC-S-nZVI) particles in saturated porous media. After sulfidation, the hydrodynamic size of CMC-S-nZVI was 100-150 nm larger than CMC-nZVI due to enhanced adsorption of CMC onto the S-nZVI surface, which was facilitated by the bidentate bridging interaction between CMC and the FeS_x phase on S-nZVI. Of note is that they had a similar core size and zeta potential. In comparison to CMC-nZVI, CMC-S-nZVI exhibited less physical settling (0-5% vs. 5-73%) and chemical dissolution (2-10% vs. 3-27%) within 55 min under the same ionic conditions (Na⁺, K⁺ < 200 mM; Al³⁺ < 0.75 mM). Column breakthrough experiments showed that both CMC-S-nZVI and CMC-nZVI had relatively high mobility in saturated porous media. However, CMC-S-nZVI exhibited greater breakthrough (C/C₀ = 0.57-1.0) and corresponding greater mass recovery rates than the corresponding CMC-nZVI (C/C₀ = 0.44-1.0) under most of the experimental conditions (e.g., different ion type and concentration, flow rate, and input concentration). The fitted colloid filtration theory model was in good agreement with experiments. This work suggests that in addition to the significant reactivity and longevity improvements demonstrated in other studies, CMC-S-nZVI is also more mobile than CMC-nZVI suggesting that CMC-S-nZVI has many of the characteristics favorable for field application.

Sulfidated nano zerovalent iron (S-nZVI) for in situ treatment of chlorinated solvents: A field study

Sulfidated nano zerovalent iron (S-nZVI), stabilized with carboxymethyl cellulose (CMC), was successfully synthesized on site and injected into the subsurface at a site contaminated with a broad range of chlorinated volatile organic compounds (cVOCs). Transport of CMC-S-nZVI to the monitoring wells, both downgradient and upgradient, resulted in a significant decrease in concentrations of aqueous-phase cVOCs. Short-term (0-17 days) total boron and chloride measurements indicated dilution and displacement in these wells. Importantly however, compound specific isotope analysis (CSIA), changes in concentrations of intermediates, and increase in ethene concentrations confirmed dechlorination of cVOCs. Dissolution from the DNAPL pool into the aqueous phase at the deepest levels (4.0-4.5 m bgs) was identifiable from the increased cVOCs concentrations during long-term monitoring. However, at the uppermost levels (∼1.5 m above the source zone) a contrasting trend was observed indicating successful dechlorination. Changes in cVOCs concentrations and CSIA data suggest both sequential hydrogenolysis as well as reductive β-elimination as the possible transformation mechanisms during the short-term abiotic and long-term biotic dechlorination. One of the most positive outcomes of this CMC-S-nZVI field treatment is the non-accumulation of lower chlorinated VOCs, particularly vinyl chloride. Post-treatment soil cores also revealed significant decreases in cVOCs concentrations throughout the targeted treatment zones. Results from this field study show that sulfidation is a suitable amendment for developing more efficient nZVI-based in situ remediation technologies.

Enhanced removal of tetrachloroethylene from aqueous solutions by biodegradation coupled with nZVI modified by layered double hydroxide

Chlorinated volatile organic compounds, such as tetrachloroethylene (PCE), are the most commonly detected toxic contaminants in groundwater. In this study, the performance of PCE removal by a microbial consortium combined with nZVI modified by layered double hydroxide (nZVI-LDH) was evaluated. The enriched PCE-degrading consortium consisted of 44.49% Clostridium and other potential PCE degraders, and 0.5-2.5 mg/L PCE was completely biodegraded within 4 days. The characterization of nZVI-LDH indicated that LDH was coated on the surfaces of nZVI particles with an increased surface area. The PCE removal kinetics by nZVI-LDH was well described by a second-order model, and the removal rate constant of nZVI-LDH was 0.12 L h/mg, higher than that of native nZVI (0.02 L h/mg). Interestingly, the presence of Cu²⁺ improved the removal efficiency of PCE by nZVI-LDH, owing to its role as a catalyst or medium for charge transfer during reduction. Removal of PCE was enhanced by coupling the PCE-degrading consortium and nZVI-LDH. The initial removal of PCE was mainly dominated by the abiotic degradation and adsorption of nZVI-LDH, and biodegradation then played a major role in the exhaustion of nZVI-LDH. These results suggest that biodegradation coupled with nZVI-LDH has a great potential for applications in the remediation of chlorinated-solvent contaminated groundwater.

Quantifying the efficiency and selectivity of organohalide dechlorination by zerovalent iron

The efficiency and selectivity of zerovalent iron-based treatments for organohalide contaminated groundwater can be quantified by accounting for redistribution of electrons derived from oxidation of Fe⁰. Several types of efficiency are reviewed, including (i) the efficiency of Fe(0) utilization, ε_Fe(0), (ii) the electron efficiency of target contaminant reduction, ε_e, and (iii) the electron efficiency of natural reductant demand (NRD) involving H₂O, O₂, and co-contaminants such as nitrate, ε_NRD. Selectivity can then be calculated by using ε_e/ε_NRD. Of particular interest is ε_e and the key to its determination is measuring the total quantity of electrons provided by Fe⁰ oxidation, which can be based on either the loss of Fe(0), the formation of Fe(ii)/Fe(iii), or the composition of the total reaction products. Recently, many data have accumulated on ε_e for the treatment of various chlorinated solvents (esp. trichloroethylene, TCE) by zerovalent iron (ZVI), and analysis of these data shows that ZVI particle properties (e.g., stabilization with polymers, bimetallic modification, sulfidation, etc.) and other operational factors have variable effects on ε_e. Of particular interest is that pre-exposure of ZVI to reduced sulfur species (i.e., sulfidation) consistently improves the ε_e of contaminant reduction, mainly by suppressing the reduction of water.

Impacts of a novel strain QY-1 allied with chromium immobilizing materials on chromium availability and soil biochemical properties

In-situ passivation of soil chromium (Cr) contamination based on chemical and biological passivators has been widely concerned, however, the cooperative effect of two types of passivators on Cr passivation and soil properties was little investigated. In this study, nano zero valent iron (nZVI) and humic acid (HA) as the chemical passivators were selected and were combined with a novel Cr resistant strain QY-1 to study these two points. Results demonstrated that the combination was more effective in Cr immobilization, among which, HA + QY-1 had the highest passivation rate (82.83%), followed by nZVI + QY-1. HA + QY-1 alleviated soil Cr stress most efficiently as its soil relevant fertility indicators, microbial quantity, respiration and seed gemination rate significantly increased. On the contrary, nZVI decreased soil respiration and microbial abundance, but the addition of QY-1 could relieve this phenomenon. The results highlighted the ability of HA + QY-1 to remediate Cr contaminated soil and improve soil stability.

Sulfur Dose and Sulfidation Time Affect Reactivity and Selectivity of Post-Sulfidized Nanoscale Zerovalent Iron

Exposing nanoscale zerovalent iron (NZVI) to dissolved sulfide species improves its performance as a remediation agent. However, the impacts of sulfur dose and sulfidation time on morphology, sulfur content, reactivity, and selectivity of the resulting sulfidized NZVI (SNZVI) have not been systematically evaluated. We synthesized SNZVI using different sulfur doses and sulfidation times and measured their properties. The measured S/Fe molar ratio in the particles ([S/Fe]_particle) was 10-500 times lower than [S/Fe]_dosed but was predictable based on [S/Fe]_dosed × t_sulfidation. The low sulfur content (0.02-0.65 mol % S/Fe) inhibited the reaction of SNZVI with water (up to 13-fold) and increased its reactivity with trichloroethene (TCE) (up to 14-fold) and its electron efficiency (up to 20-fold). A higher [S/Fe]_particle (0.86-1.13 mol % S/Fe) led to complex particle structures and lowered the resistance to electron transfer but did not improve the benefits realized at the lower S/Fe ratios. Adding small amounts of sulfur into NZVI led to more accumulation of acetylene, especially for low Fe/TCE conditions, suggesting that sulfur lowers the rate of hydrogenation of acetylene to ethene. These results show that [S/Fe]_dosed × t_sulfidation can be used to predict the measured S content in the particles and that affects reactivity, longevity, and electron selectivity, for post-SNZVI.

Nanoscale zero valent iron supported on MgAl-LDH-decorated reduced graphene oxide: Enhanced performance in Cr(VI) removal, mechanism and regeneration

The scaled application of nanoscale zero-valent iron nanoparticles (nZVI or Fe° NPs) in environmental remediation is challenged by easy surface passivation and particle aggregation. To improve this situation and enhance their performance in Cr(VI) removal from water phase, we present one novel strategy to hybridize nZVI with layered double hydroxide (LDH) decorated reduced graphene oxide (rGO). The as-prepared ternary (Fe@LDH/rGO) composites possess better dispersibility, improved hydrophilicity and more positive surfaces that allows higher removal efficiency and capacity for Cr(VI) oxyanions. Composition proportion are optimized and influences of surroundings (solution pH, Cr(VI) concentration and temperature) are evaluated. Also, we demonstrate that Fe@LDH/rGO can be reused with suitable post-treatments, which combines alkaline solution desorption and NaBH₄ revivification possess. Cr desorption and Fe leaching ratio during regeneration should be critical indicators that determine the recovery efficiency. Synergistic effect within this ternary system not only contributes to its superiorities in stability, but also continuous iron corrosion via the formation of micro Fe-C batteries, where rGO acts as cathode and alternative electron conductor. The present work suggests great potentials of Fe@LDH/rGO composites in groundwater remediation.

Comparison of short-term dosing ferrous ion and nanoscale zero-valent iron for rapid recovery of anammox activity from dissolved oxygen inhibition

As obligate anaerobes, anammox bacteria are sensitive to oxygen, which might hinder the maximization of anammox activity. However, there are very few effective strategies to rapidly recover anammox activity after its deterioration under exposure of oxygen. In this study, the activity recovery of anammox bacteria encountering dissolved oxygen (DO) exposure (0.2 and 2.0 mg L^-1) were compared by three strategies in short-term experiments, nZVI, Fe(II) dosing, and N₂ purging. nZVI is more effective in recovering anammox activity with a high DO exposure (2 mg L^-1), compared to a low DO exposure (0.2 mg L^-1). After inhibiting by 2.0 mg L^-1 DO, anammox activity recovery (normalized to the control) was ranked in the order of nZVI (5 mg L^-1) addition (63 ± 8.2%) > Fe(II) (5 mg L^-1) addition (41 ± 8.0%) >N₂ purging (39 ± 4.0%). In contrast to Fe(II) ion additions, the shell structure of nZVI combined with the buffering effect of biomass-extracellular polysaccharide (EPS) prevented the sharp pH variation and excessive dissolved Fe(II)/Fe(III) in solution. Under such circumstances, nZVI addition (5 and 25 mg L^-1) increased the intracellular reactive oxygen species (ROS) to a moderate level (<200%), which might be responsible for the better activity recovery of anammox than that of Fe(II) addition and N₂ purging. Specifically, 5 mg L^-1 nZVI dosage moderately enhanced the intracellular O₂^- production (∼150% of the control) after scavenging 2.0 mg L^-1 DO, and the anammox activity recovered better than that of both 5 and 25 mg L^-1 Fe(II) ions additions. However, high dosage nZVI (75 mg L^-1) inhibited anammox activity in spite of low or high DO exposure. Our findings elucidate that appropriate amount of nZVI (short-term dosing) can rapidly recover anammox activity when anammox bacteria encountering oxygen exposure accidentally and could be useful in facilitating the robust operation of anammox-based processes.

Integration of nanoscale zero-valent iron and functional anaerobic bacteria for groundwater remediation: A review

The technology of integrating nanoscale zero-valent iron (nZVI) and functional anaerobic bacteria has broad prospects for groundwater remediation. This review focuses on the interactions between nZVI and three kinds of functional anaerobic bacteria: organohalide-respiring bacteria (OHRB), sulfate reducing bacteria (SRB) and iron reducing bacteria (IRB), which are commonly used in the anaerobic bioremediation. The coupling effects of nZVI and the functional bacteria on the contaminant removal in the integrated system are summarized. Generally, nZVI could create a suitable living condition for the growth and activity of anaerobic bacteria. OHRB and SRB could synergistically degrade organic halides and remove heavy metals with nZVI, and IRB could reactive the passivated nZVI by reducing the iron (hydr)oxides on the surface of nZVI. Moreover, the roles of these anaerobic bacteria in contaminant removal coupling with nZVI and the degradation mechanisms are illustrated. In addition, this review also discusses the main factors influencing the removal efficiency of contaminants in the integrated treatment system, including nZVI species and dosage, inorganic ions, organic matters, pH, type of pollutants, temperature, and carbon/energy sources, etc. Among these factors, the nZVI species and dosage play a fundamental role due to the potential cytotoxicity of nZVI, which might exert a negative impact on the performance of this integrated system. Lastly, the future research needs are proposed to better understand this integrated technology and effectively apply it in groundwater remediation.

A 2D tank test on remediation of nitrobenzene-contaminated aquifer using in-situ reactive zone with emulsified nanoscale zero-valent iron

Nitrobenzene (NB) is one of the most challenging pollutants for groundwater remediation due to its great harm and recalcitrance. Emulsified nanoscale zero-valent iron (EZVI) is considered as a promising agent for in-situ remediation of contaminated groundwater for its high reactivity, good durability and low cost. In this paper, 2D tank experiment was conducted to evaluate the effectiveness of enhanced remediation of NB-contaminated groundwater with EZVI. 9 L of EZVI solution was injected into aquifer to establish in-situ reactive zone (IRZ) before 40 d of NB contamination. Results indicate that injection of EZVI leads to 90% reduction of total NB, which is mainly converted to aniline (AN). NB concentration decreases along the flow path in the tank. Fe²⁺ is generated from Fe⁰ oxidation. Significant acetate and bicarbonate are released due to emulsified oil decomposition during the whole operation time. Groundwater pH maintains in neutral value (6.6-8.2) owing to the balance between organic acids and OH^- released after iron oxidation. Drastic decrease of ORP and DO indicates the transformation from oxidizing to reducing condition, leading to the reduction of oxidative species (e.g. sulfate, nitrate) in subsurface. Calculation of reducing equivalents suggests that microbial breakdown of emulsified oil provides more electrons than Fe⁰ oxidation does to the system. Both biotic and abiotic processes are involved in the enhanced degradation of NB.

Dechlorination of Excess Trichloroethene by Bimetallic and Sulfidated Nanoscale Zero-Valent Iron

Nanoscale zerovalent iron (nZVI) likely finds its application in source zone remediation. Two approaches to modify nZVI have been reported: bimetal (Fe-Me) and sulfidated nZVI (S-nZVI). However, previous research has primarily focused on enhancing particle reactivity with these two modifications under more plume-like conditions. In this study, we systematically compared the trichloroethene (TCE) dechlorination pathway, rate, and electron selectivity of Fe-Me (Me: Pd, Ni, Cu, and Ag), S-nZVI, and nZVI with excess TCE simulating source zone conditions. TCE dechlorination on Fe-Me was primarily via hydrogenolysis while that on S-nZVI and nZVI was mainly via β-elimination. The surface-area normalized TCE reduction rate ( k^'_SA) of Fe-Pd, S-nZVI, Fe-Ni, Fe-Cu, and Fe-Ag were ∼6800-, 190-, 130-, 20-, and 8-fold greater than nZVI. All bimetallic modification enhanced the competing hydrogen evolution reaction (HER) while sulfidation inhibited HER. Fe-Cu and Fe-Ag negligibly enhanced electron utilization efficiency (ε_e) while Fe-Pd, Fe-Ni, and S-nZVI dramatically increased ε_e from 2% to ∼100%, 69%, and 72%, respectively. Adsorbed atomic hydrogen was identified to be responsible for the TCE dechlorination on Fe-Me but not on S-nZVI. The enhanced dechlorination rate along with the reduced HER of S-nZVI can be explained by that FeS conducting major electrons mediated TCE dechlorination while Fe oxides conducting minor electrons mediated HER.

Combined chemical and microbiological degradation of tetrachloroethene during the application of Carbo-Iron at a contaminated field site

After the injection of Carbo-Iron® into an aquifer contaminated with tetrachloroethene (PCE), combined chemical and microbiological contaminant degradation processes were found in a long-term study of the field site in Lower Saxony (Germany). The applied composite material Carbo-Iron, which consists of colloidal activated carbon and embedded nanoscale zero-valent iron (ZVI) structures, functioned as intended: accumulating the pollutants and promoting their reductive dechlorination. Furthermore, the particles decreased the redox potential of the groundwater due to their reaction with oxygen and to the ZVI-corrosion-induced formation of molecular hydrogen up to 190 days after the injection, the latter promoting sulphate-reducing conditions. The emergence of cis-dichloroethene (cis-DCE), which was only found in trace quantities before the injection of Carbo-Iron, together with the presence of organisms related to Sulfospirillum multivorans, Desulfitobacterium spp. and Dehalococcoides mccartyi, indicate that Carbo-Iron is also able to support microbial degradation of PCE. However, cis-DCE did not accumulate in the present case study, although it is often observed at sites with active microbial dechlorination. The results of compound-specific isotope analysis in combination with pyrosequencing data suggested the oxidative degradation of cis-DCE by an organism related to Polaromonas sp. strain JS666. Consequently, the formation of the carcinogenic degradation intermediate vinyl chloride was circumvented. Overall, the moderate and slow change of environmental conditions mediated by Carbo-Iron not only supported organohalide-respiring bacteria, but also created the basis for a subsequent microbial oxidation step.

The effect of zerovalent iron on the microbial degradation of hexabromocyclododecane

Hexabromocyclododecane (HBCD), a commonly used brominated flame retardant (BFR), has been listed as a persistent organic pollutant (POP). In order to remediate HBCD in the environment, the influence of microscale zerovalent iron (MZVI) on the HBCD degrading microcosm was evaluated. In the acclimated microcosm collected from river sediment, 49% of HBCD was initially removed through adsorption and then 30% of HBCD was biodegraded through non-debromination processes. In contrast to MZVI only, over 60% of HBCD was gradually degraded by MZVI through a debromination reaction. In the microcosm-MZVI combined system, the biodegradation ability of the microcosm was inhibited. The aqueous chemistry was changed by the addition of MZVI, which led to the alteration of microbial composition and biodegradation ability. These better understandings can facilitate an evaluation of the impact of MZVI on HBCD biodegradation when ZVI was used to remediate this BFR.

In situ remediation of chlorinated solvent-contaminated groundwater using ZVI/organic carbon amendment in China: field pilot test and full-scale application

Chlorinated solvents in groundwater pose threats to human health and the environment due to their carcinogenesis and bioaccumulation. These problems are often more severe in developing countries such as China. Thus, methods for chlorinated solvent-contaminated groundwater remediation are urgently needed. This study presents a technique of in situ remediation via the direct-push amendment injection that enhances the reductive dechlorination of chlorinated solvents in groundwater in the low-permeability aquifer. A field-based pilot test and a following real-world, full-scale application were conducted at an active manufacturing facility in Shanghai, China. The chlorinated solvents found at the clay till site included 1,1,1-trichloroethane (1,1,1-TCA), 1,1-dichloroethane (1,1-DCA), 1,1-dichloroethylene (1,1-DCE), vinyl chloride (VC), and chloroethane (CA). A commercially available amendment (EHC^®, Peroxychem, Philadelphia, PA) combining zero-valent iron and organic carbon was used to treat the above pollutants. Pilot test results showed that direct-push EHC injection efficiently facilitated the in situ reductive remediation of groundwater contaminated with chlorinated solvents. The mean removal rates of 1,1,1-TCA, 1,1-DCA, and 1,1-DCE at 270 days post-injection were 99.6, 99.3, and 73.3%, respectively, which were obviously higher than those of VC and CA (42.3 and 37.1%, respectively). Clear decreases in oxidation-reduction potential and dissolved oxygen concentration, and increases in Fe²⁺ and total organic carbon concentration, were also observed during the monitoring period. These indicate that EHC promotes the anaerobic degradation of chlorinated hydrocarbons primarily via long-term biological reductive dechlorination, with instant chemical reductive dechlorination acting as a secondary pathway. The optimal effective time of EHC injection was 0-90 days, and its radius of influence was 1.5 m. In full-scale application, the maximum concentrations of 1,1,1-TCA and 1,1-DCA in the contaminate plume fell below the relevant Dutch Intervention Values at 180 days post-injection. Moreover, the dynamics of the target pollutant concentrations mirrored those of the pilot test. Thus, we have demonstrated that the direct-push injection of EHC successfully leads to the remediation of chlorinated solvent-contaminated groundwater in a real-world scenario. The parameters determined by this study (e.g., effectiveness, injection amount, injection depth, injection pressures, and radius of influence) are applicable to other low-permeability contaminated sites where in situ remediation by enhanced reductive dechlorination is required.

Environmental transformations and ecological effects of iron-based nanoparticles

The increasing application of iron-based nanoparticles (NPs), especially high concentrations of zero-valent iron nanoparticles (nZVI), has raised concerns regarding their environmental behavior and potential ecological effects. In the environment, iron-based NPs undergo physical, chemical, and/or biological transformations as influenced by environmental factors such as pH, ions, dissolved oxygen, natural organic matter (NOM), and biotas. This review presents recent research advances on environmental transformations of iron-based NPs, and articulates their relationships with the observed toxicities. The type and extent of physical, chemical, and biological transformations, including aggregation, oxidation, and bio-reduction, depend on the properties of NPs and the receiving environment. Toxicities of iron-based NPs to bacteria, algae, fish, and plants are increasingly observed, which are evaluated with a particular focus on the underlying mechanisms. The toxicity of iron-based NPs is a function of their properties, tolerance of test organisms, and environmental conditions. Oxidative stress induced by reactive oxygen species is considered as the primary toxic mechanism of iron-based NPs. Factors influencing the toxicity of iron-based NPs are addressed and environmental transformations play a significant role, for example, surface oxidation or coating by NOM generally lowers the toxicity of nZVI. Research gaps and future directions are suggested with an aim to boost concerted research efforts on environmental transformations and toxicity of iron-based NPs, e.g., toxicity studies of transformed NPs in field, expansion of toxicity endpoints, and roles of laden contaminants and surface coating. This review will enhance our understanding of potential risks of iron-based NPs and proper uses of environmentally benign NPs.

Sulfidation of Iron-Based Materials: A Review of Processes and Implications for Water Treatment and Remediation

Iron-based materials used in water treatment and groundwater remediation-especially micro- and nanosized zerovalent iron (nZVI)-can be more effective when modified with lower-valent forms of sulfur (i.e., "sulfidated"). Controlled sulfidation for this purpose (using sulfide, dithionite, etc.) is the main topic of this review, but insights are derived by comparison with related and comparatively well-characterized processes such as corrosion of iron in sulfidic waters and abiotic natural attenuation by iron sulfide minerals. Material characterization shows that varying sulfidation protocols (e.g., concerted or sequential) and key operational variables (e.g., S/Fe ratio and sulfidation duration) result in materials with structures and morphologies ranging from core-shell to multiphase. A meta-analysis of available kinetic data for dechlorination under anoxic conditions, shows that sulfidation usually increases dechlorination rates, and simultaneously hydrogen production is suppressed. Therefore, sulfidation can greatly improve the efficiency of utilization of reducing equivalents for contaminant removal. This benefit is most likely due to inhibited corrosion as a result of sulfidation. Sulfidation may also favor desirable pathways of contaminant removal, such as (i) dechlorination by reductive elimination rather than hydrogenolysis and (ii) sequestration of metals as sulfides that could be resistant to reoxidation. Under oxic conditions, sulfidation is shown to enhance heterogeneous catalytic oxidation of contaminants. These net effects of sulfidation on contaminant removal by iron-based materials may substantially improve their practical utility for water treatment and remediation of contaminated groundwater.

Electromagnetic induction of foam-based nanoscale zerovalent iron (NZVI) particles to thermally enhance non-aqueous phase liquid (NAPL) volatilization in unsaturated porous media: Proof of concept

Nanoscale zerovalent iron (NZVI) is a promising remediation agent for volatile organic compound (VOC) contamination in saturated sub-surfaces, but is rarely applied to the vadose zone as there are not enough water molecules in the unsaturated zone to participate in reductive dechlorination. In this study, we evaluated the possibility of using foam as a carrying vehicle to emplace NZVI in unsaturated porous media followed by the application of low frequency-electromagnetic field (LF-EMF) to enhance VOC volatilization in laboratory batch reactors. We found that the optimal condition for generating foam-based NZVI (F-NZVI) was using sodium lauryl ether sulfate (SLES) at a concentration of 3% (w/w) and a N₂ flow rate of 500 mL/min. Also, F-NZVI could carry as much as 41.31 g/L of NZVI in the liquid phase of the foam and generate heat to raise ΔT to 77 °C in 15 min under an applied LF-EMF (150 kHz and 13 A). Under these conditions, F-NZVI together with LF-EMF enhanced trichloroethylene (TCE) volatilization from TCE-dense non-aqueous phase liquid (DNAPL) in unsaturated sand by 39.51 ± 6.59-fold compared to reactors without LF-EMF application. This suggested that using F-NZVI together with LF-EMF could theoretically be an alternative to radio frequency heating (RFH) as it requires a much lower irradiation frequency (336-fold lower), which should result in significantly lower capital and operational costs compared to RFH.

Environmental Electrokinetics for a sustainable subsurface

Soil and groundwater are key components in the sustainable management of the subsurface environment. Source contamination is one of its main threats and is commonly addressed using established remediation techniques such as in-situ chemical oxidation (ISCO), in-situ chemical reduction (ISCR; most notably using zero-valent iron [ZVI]), enhanced in-situ bioremediation (EISB), phytoremediation, soil-washing, pump-and-treat, soil vapour extraction (SVE), thermal treatment, and excavation and disposal. Decades of field applications have shown that these techniques can successfully treat or control contaminants in higher permeability subsurface materials such as sands, but achieve only limited success at sites where low permeability soils, such as silts and clays, prevail. Electrokinetics (EK), a soil remediation technique mostly recognized in in-situ treatment of low permeability soils, has, for the last decade, been combined with more conventional techniques and can significantly enhance the performance of several of these remediation technologies, including ISCO, ISCR, EISB and phytoremediation. Herein, we discuss the use of emerging EK techniques in tandem with conventional remediation techniques, to achieve improved remediation performance. Furthermore, we highlight new EK applications that may come to play a role in the sustainable treatment of the contaminated subsurface.

The interactions between nanoscale zero-valent iron and microbes in the subsurface environment: A review

Nanoscale zero-valent iron (NZVI) particles, applied for in-situ subsurface remediation, are inevitable to interact with various microbes in the remediation sites directly or indirectly. This review summarizes their interactions, including the effects of NZVI on microbial activity and growth, the synergistic effect of NZVI and microbes on the contaminant removal, and the effects of microbes on the aging of NZVI. NZVI could exert either inhibitive or stimulative effects on the growth of microbes. The mechanisms of NZVI cytotoxicity (i.e., the inhibitive effect) include physical damage and biochemical destruction. The stimulative effects of NZVI on certain bacteria are associated with the creation of appropriate living environment, either through providing electron donor (e.g., H₂) or carbon sources (e.g., the engineered organic surface modifiers), or through eliminating the noxious substances that can cause bactericidal consequence. As a result of the positive interaction, the combination of NZVI and some microbes shows synergistic effect on contaminant removal. Additionally, the aged NZVI can be utilized by some iron-reducing bacteria, resulting in the transformation of Fe(III) to Fe(II), which can further contribute to the contaminant reduction. However, the Fe(III)-reduction process can probably induce environmental risks, such as environmental methylation and remobilization of the previously entrapped heavy metals.

Electromagnetic induction of nanoscale zerovalent iron particles accelerates the degradation of chlorinated dense non-aqueous phase liquid: Proof of concept

In this study, a novel electromagnetically enhanced treatment concept is proposed for in situ remediation of a source zone of chlorinated dense non-aqueous phase liquid (DNAPL) that is slowly dissolved, causing contaminated groundwater for centuries. Here, we used polystyrene sulfonate (PSS)-modified nanoscale zerovalent iron (NZVI) particles (ferromagnetic) in combination with a low frequency (LF) (150 kHz) AC electromagnetic field (EMF) to accelerate the degradation of the DNAPLs via enhanced dissolution and reductive dechlorination. Trichloroethylene (TCE) and tetrachloroethylene (PCE) were used in a bench-scaled evaluation. The PSS-modified NZVI successfully targeted the DNAPL/water interface, as evidenced by the Pickering emulsion formation. Dechlorination of TCE- and PCE-DNAPL was measured by quantifying the by-product formation (acetylene, ethene, and ethane). Without magnetic induction heating (MIH) by LF EMF, PSS-modified NZVI transformed TCE- and PCE-DNAPL to ethene and ethane at the rate constants of 12.19 × 10^-3 and 1.00 × 10^-3 μmol/h/m², respectively, following pseudo zero-order reactions. However, four MIH cycles of PSS-NZVI increased the temperature up to 87 °C and increased the rate constants of TCE-DNAPL and PCE-DNAPL up to 14.58 and 58.01 times, respectively, in comparison to the dechlorination rate without MIH. Theoretical analysis suggested that the MIH of the PSS-modified NZVI enhanced the dechlorination of TCE- and PCE-DNAPL via the combination of the enhanced thermal dissolution of DNAPL, the effect of increasing the temperature on the rate constant (the Arrhenius equation), and the accelerated NZVI corrosion. Nevertheless, the effect of the Arrhenius equation was dominant. For the first time, this proof-of-concept study reveals the potential for using polyelectrolyte-modified NZVI coupled with LF EMF as a combined remediation technique for increasing the rate and completeness of in situ chlorinated DNAPL source remediation.

Ecotoxicity and environmental safety related to nano-scale zerovalent iron remediation applications

This mini-review summarizes the current information that has been published on the various effects of nano-scale zerovalent iron (nZVI) on microbial biota, with an emphasis on reports that highlight the positive aspects of its application or its stimulatory effects on microbiota. By nature, nZVI is a highly reactive substance; thus, the possibility of nZVI being toxic is commonly suspected. Accordingly, the cytotoxicity of nZVI and the toxicity of nZVI-related products have been detected by laboratory tests and documented in the literature. However, there are numerous other published studies on its useful nature, which are usually skipped in reviews that deal only with the phenomenon of toxicity. Therefore, the objective of this article is to review both recent publications reporting the toxic effects of nZVI on microbiota and studies documenting the positive effects of nZVI on various environmental remediation processes. Although cytotoxicity is an issue of general importance and relevance, nZVI can reduce the overall toxicity of a contaminated site, which ultimately results in the creation of better living conditions for the autochthonous microflora. Moreover, nZVI changes the properties of the site in a manner such that it can also be used as a tool in a tailor-made approach to support a specific microbial community for the decontamination of a particular polluted site.