Ten year performance evaluation of a field-scale zero-valent iron permeable reactive barrier installed to remediate trichloroethene contaminated groundwater

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

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

Current status and strategies for controlling hexachlorobutadiene from multiple perspectives of emission, occurrence, and disposal

Hexachlorobutadiene (HCBD), as an emerging persistent organic pollutant, poses a pressing global environmental issue concerning its reduction and control. However, the lack of systematic studies on the sources and occurrence of HCBD hinders the development of effective disposal technologies. This study addresses HCBD prevention and treatment from multiple perspectives, including source emissions, environmental contamination distribution, and control technologies. The dominant source of HCBD emissions varied by country, mainly industrial production processes of trichloroethylene, perchloroethylene, and carbon tetrachloride in China and magnesium production in Europe. Further research on the relevant generation mechanisms is necessary to develop targeted source control strategies. HCBD has been detected in various environmental media and biological organisms worldwide. Compared to sludge and soil, the concentration of HCBD in the atmosphere and water were relatively higher, particularly in China and Nigeria, with the concentration reaching up to 179 μg/m3 and 2629 μg/L, respectively. Attention should be focused on the water treatment processes to reduce HCBD levels in sludge and ensure the safety of drinking water. Additionally, studies of HCBD exposure levels in organisms should also focus on diet to further assess health risks to humans. Currently, available disposal technologies primarily focus on the treatment of contaminated environmental media, including physical thermal desorption, chemical reduction dechlorination and oxidative degradation, and biodegradation, while the development and application of source control methods remain insufficient. However, these technologies may not completely degrade HCBD, potentially causing secondary pollution. Future efforts should prioritize the development of green, efficient, and thoroughly destructive thermal catalytic technologies, with an emphasis on the integration of multiple techniques. This work provides critical insights for the development and implementation of comprehensive control strategies for HCBD regarding its source, occurrence, and pollution disposal.

Surface-bound Fe(0) and Fe(II) mediated by 2-picolinic acid functionalized zero-valent iron for highly Cr(VI) removal

Electron transfer of zero-valent iron (ZVI) is significantly impeded by its oxide layer, and limiting its removal of pollutants. In this study, 2-picolinic acid (PA) and ZVI were co-ball milled to improve electron transfer in ZVI (PA-ZVIbm), and used for the removal of heavy metal Cr(VI). Characterization analysis showed that the presence of electron-rich groups on the surface of PA-ZVIbm promoted the transfer of electrons from the Fe(0) core to the surface, and the surface Fe(0) and Fe(II) contents increased from 1.1 % to 6.3 % and from 60.2 % to 72.9 %, respectively, effectively reducing Cr(VI) through an electron transfer mechanism. Theoretical calculations showed that the modification of PA enhanced the adsorption of Cr(VI) on the ZVI surface, and the adsorption energy decreased from -3.561 eV to -5.119 eV. PA-ZVIbm showed strong advantages in the removal of Cr(VI), with a reaction rate constant and adsorption capacity 17 and 13 times that of ZVIbm, respectively, and a conversion rate of 100 %. Moreover, PA-ZVIbm showed excellent performance over a wide pH range (3-10) and under different coexisting ions, while being cost-effective and having low environmental risks. This study explored the relationship between ZVI surface modification and performance, and provided new insights into the modification of ZVI using small molecule oxygen-containing organic acids.

Chemical aging of biochar-zero-valent iron composites in groundwater: Impact on Cd(II) and Cr(VI) co-removal

Biochar (BC), zero-valent iron (ZVI), and their composites are promising materials for use in permeable reactive barriers, although further research is needed to understand how their properties change during long-term aging in groundwater. In this study, BC, ZVI and their composites (4BC-1ZVI) were subjected to the chemical aging tests in five media (deionized water, NaCl, NaHCO3, CaCl2 and a mixture of CaCl2 and NaHCO3 solutions) for 20 days. After treatment, the microscopic analysis and performance tests for the co-removal of Cd(II) and Cr(VI) were carried out. The results indicated that the removal of Cd(II) by aged 4BC-1ZVI followed a pseudo-second-order model, whereas the removal of Cr(VI) was better fitted with a pseudo-first order model. The aging mechanism of 4BC-1ZVI was primarily governed by iron corrosion/passivation, the reduction of soluble components, and the formation of carbonate minerals. Less Fe3O4/ γ-Fe2O3 was formed during aging in deionized water, NaCl and CaCl2 solutions. The corrosion products, Fe3O4/ γ-Fe2O3, FeCO3 and α/γ-FeOOH, were observed after aging in NaHCO3 and a mixture of NaHCO3 and CaCl2 solutions. The decrease in the soluble components of biochar led to a decrease in cation exchange, while carbonate minerals contributed to Cd(II) precipitation. This work provides insights into the aging processes of BC-ZVI composites for long-term groundwater remediation applications.

New insights on zero-valent iron permeable reactive barrier for Cr(VI) removal: The function of FeS reaction zone downstream in-situ generated by sulfate-reducing bacteria

The biogeochemical behavior downstream of the zero-valent iron permeable reactive barrier (ZVI-PRB) plays an enormous positive role in the remediation of contaminated-groundwater, but has been completely neglected for a long time. Therefore, this study conducted a 240-day SRB-enhanced ZVI-PRB column experiment, focusing on what exactly happens downstream of ZVI-PRB. Results show that biosulfidation of SRB inside ZVI-PRB prolonged the complete Cr(VI) removal longevity of ZVI-PRB from 38 days to at least 240 days. More importantly, unlike previous studies that focused on improving the performance of ZVI-PRB itself, this study found an in-situ generated FeS reduction reaction zone downstream of the ZVI-PRB. When the ZVI-PRB fails, the downstream reaction zone can continue to play a role in Cr(VI) removal. The maximum Cr(VI) removal capacity of the aquifer media from the reaction zone reached 155.1 mg/kg, which was 39.7 % of commercial ZVI capacity. The reduction zone was further confirmed to be predominantly FeS rather than FeS2. Biogeochemistry occurring within and downstream of ZVI-PRB leads to the formation of FeS. Gene sequencing revealed significantly higher SRB abundance downstream of ZVI-PRB than within the ZVI-PRB. The understanding of the downstream FeS reaction zone provides new insights for more effective remediation using ZVI-PRB.

Defining quality assurance guidance for effective selection of technical grade zero-valent iron production batch for groundwater remediation using permeable reactive barrier

Zero-valent iron (ZVI) applied to the remediation of contaminated groundwater (GW) in situ, especially using engineered permeable reactive barriers (PRBs), has been proven to be an effective reactive material. However, many of ZVI brands do not represent tailored reagents specifically regarding destroying pollutants in GW. Thus, their reactivity towards certain contaminants in GW may vary significantly in a wide range even with different production batches of the same ZVI brand. This issue has rarely been known and consequently not addressed to a higher extend so far. Therefore, this study implemented extensive, long-term column experiments followed by short-term batch experiments for chlorinated volatile organic compounds (cVOCs) degradation for developing a semi-empirical test methodology to thoroughly resolve this pivotal issue by achieving an improved quality assurance guidance regarding proper field-scale emplacement of different ZVI brands and their production batches. The results showed that during column experiments perchloroethylene (PCE) led to a significant degradation up to a certain period but sulfate-reducing microorganisms enhanced the dehalogenation and led approximately to 100 % PCE removal. However, the efficacy varied for different ZVI brands, i.e., Gotthart Maier (GM) and Sponge Iron (Responge®). Furthermore, it could be shown that it might even vary among different production batches of the same ZVI brand. It was also observed that evolution of sulfate-reducing microorganisms may improve the efficacy of PCE degradation vastly that occur at different intensities with different ZVI brands and their respective production batches over time. Further, comparing comprehensive long-term column (kobs = 0.0488 1/h) and short-term batch experiments (kobs = 0.07794 1/h) as well as refined kinetic analyses (kobs = 0.0424 1/h) clearly prove that an appropriate guidance protocol for successful full-scale in situ remediation is required for properly select the right ZVI brand and production batch before it is loaded to a PRB in the field.

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

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

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

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

Reduction Pathway-Dependent Formation of Reactive Fe(II) Sites in Clay Minerals

Structural Fe in clay minerals is an important, potentially renewable source of electron equivalents for contaminant reduction, yet our knowledge of how clay mineral Fe reduction pathways and Fe reduction extent affect clay mineral Fe(II) reactivity is limited. Here, we used a nitroaromatic compound (NAC) as a reactive probe molecule to assess the reactivity of chemically reduced (dithionite) and Fe(II)-reduced nontronite across a range of reduction extents. We observed biphasic transformation kinetics for all nontronite reduction extents of ≥5% Fe(II)/Fe(total) regardless of the reduction pathway, indicating that two Fe(II) sites of different reactivities form in nontronite at environmentally relevant reduction extents. At even lower reduction extents, Fe(II)-reduced nontronite completely reduced the NAC whereas dithionite-reduced nontronite could not. Our ⁵⁷Fe Mössbauer spectroscopy, ultraviolet-visible spectroscopy, and kinetic modeling results suggest that the highly reactive Fe(II) entities likely comprise di/trioctahedral Fe(II) domains in the nontronite structure regardless of the reduction mechanism. However, the second Fe(II) species, of lower reactivity, varies and for Fe(II)-reacted NAu-1 likely comprises Fe(II) associated with an Fe-bearing precipitate formed during electron transfer from aqueous to nontronite Fe. Both our observation of biphasic reduction kinetics and the nonlinear relationship of rate constant and clay mineral reduction potential E_H have major implications for contaminant fate and remediation.

The addition of nano zero-valent iron during compost maturation effectively removes intracellular and extracellular antibiotic resistance genes by reducing the abundance of potential host bacteria

Applying compost to soil may lead to the spread of antibiotic resistance genes (ARGs) in the environment. Therefore, removing ARGs from compost is critical. In this study, for the first time, nano zero-valent iron (nZVI) was added to compost during the maturation stage to remove ARGs. After adding 1 g/kg of nZVI, the abundance of total intracellular and total extracellular ARGs was decreased by 97.62% and 99.60%, and that of total intracellular and total extracellular mobile genetic elements (MGEs) was decreased by 92.39% and 99.31%, respectively. A Mantel test and network analysis indicated that the reduction in potential host bacteria and intI1 after nZVI treatment promoted the removal of intracellular and extracellular ARGs. The addition of nZVI during composting reduced the horizontal transfer of ARGs and improve the total nitrogen and germination index of compost, allowing it to meet the requirements for organic fertilizers.

Treatment of the insensitive munitions compound, 3-nitro-1,2,4-triazol-5-one (NTO), in flow-through columns packed with zero-valent iron

The need for effective technologies to remediate the insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) is emerging due to the increasing use by the US Army and environmental concerns about the toxicity and aqueous mobility of NTO. Reductive treatment is essential for the complete degradation of NTO to environmentally safe products. The objective of this study is to investigate the feasibility of applying zero-valent iron (ZVI) in a continuous-flow packed bed reactor as an effective NTO remediation technology. The ZVI-packed columns treated an acidic influent (pH 3.0) or a circumneutral influent (pH 6.0) for 6 months (ca. 11,000 pore volumes, PVs). Both columns effectively reduced NTO to the amine product, 3-amino-1,2,4-triazol-5-one (ATO). The column treating the pH-3.0 influent exhibited prolonged longevity in reducing NTO, treating 11-fold more PVs than the column treating pH-6.0 influent until the breakthrough point (defined as when 85% of NTO was removed). The exhausted columns (defined as when only 10% of NTO was removed) regained the NTO reducing capacity by reactivation using 1 M HCl, fully removing NTO. After the experiment, solid-phase analysis of the packed-bed material showed that ZVI was oxidized to iron (oxyhydr)oxide minerals such as magnetite, lepidocrocite, and goethite during NTO treatment. This is the first report on the reduction of NTO and the concomitant oxidation of ZVI in continuous-flow column experiments. The evidence indicates that treatment in a ZVI-packed bed reactor is an effective approach for the removal of NTO.

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.

Long-term performance evaluation of zero-valent iron amended permeable reactive barriers for groundwater remediation - A mechanistic approach

Permeable reactive barriers (PRBs) are used for groundwater remediation at contaminated sites worldwide. This technology has been efficient at appropriate sites for treating organic and inorganic contaminants using zero-valent iron (ZVI) as a reductant and as a reactive material. Continued development of the technology over the years suggests that a robust understanding of PRB performance and the mechanisms involved is still lacking. Conflicting information in the scientific literature downplays the critical role of ZVI corrosion in the remediation of various organic and inorganic pollutants. Additionally, there is a lack of information on how different mechanisms act in tandem to affect ZVI-groundwater systems through time. In this review paper, we describe the underlying mechanisms of PRB performance and remove isolated misconceptions. We discuss the primary mechanisms of ZVI transformation and aging in PRBs and the role of iron corrosion products. We review numerous sites to reinforce our understanding of the interactions between groundwater contaminants and ZVI and the authigenic minerals that form within PRBs. Our findings show that ZVI corrosion products and mineral precipitates play critical roles in the long-term performance of PRBs by influencing the reactivity of ZVI. Pore occlusion by mineral precipitates occurs at the influent side of PRBs and is enhanced by dissolved oxygen and groundwater rich in dissolved solids and high alkalinity, which negatively impacts hydraulic conductivity, allowing contaminants to potentially bypass the treatment zone. Further development of site characterization tools and models is needed to support effective PRB designs for groundwater remediation.

Performance of field-scale permeable reactive barriers: An overview on potentials and possible implications for in-situ groundwater remediation applications

Permeable reactive barriers (PRBs) are significant among all the promising remediation technologies for treating contaminated groundwater. Since the first commercial full field-scale PRB emplacement in Sunnyvale, California, in 1994-1995, >200 PRB systems have been installed worldwide. The main working principle of a PRB is to treat a variety of contaminants downstream from the contaminated source zone ("hot spot"). However, to accurately assess the longevity of PRBs, it is essential to know the total contaminant mass in the source area and its approximate geometry. PRBs are regarded as both a safeguarding and an advanced decontamination technique, depending on the contamination scenario and its outcome during the operational lifetime of the barrier. In the last three decades, many PRBs have performed very well, that is, met expected clean-up goals at a variety of contaminated sites. However, there is still the necessity of thoroughly evaluating the implications of the performance of different PRB designs and reactive or adsorptive materials worldwide. Therefore, this study presents a comprehensive overview of field-scale PRBs applications and their long-term performance after on-site emplacements. This paper provides in-depth insight into this passive in-situ remediation technology for treating and even eliminating a contaminated plume over a long time in the subsurface. The overview will help all stakeholders worldwide understand the implications of PRBs and guide them to take all the required measures before its on-site application to avoid any potential failure.

Coupled microscale zero valent iron-autotrophic hydrogen bacteria dechlorination system is not always superior to its standalone counterparts: A sustainable remediation perspective

The coupling of microscale zero-valent iron with autotrophic hydrogen bacteria (mZVI-AHB) are often believed to show greater potential than the single abiotic or biotic systems in remediating chlorinated aliphatic hydrocarbon-contaminated groundwater. However, our understanding of the remediation performance of this system under real field conditions, especially by incorporating the concept of sustainable remediation, remains limited. In this study, the performances of the mZVI, H₂-AHB, and mZVI-AHB systems in dechlorinating groundwater containing complex electron acceptors were compared by evaluating their removal efficiency (RE), reaction products, and electron efficiency (EE), using trichloroethylene (TCE) as the target contaminant and NO₃^- and SO₄^2- as the coexisting natural electron acceptors. Ultimately, which of these systems had TCE removal superiority was dependent on the coexisting electron acceptor. mZVI-AHB and mZVI resulted in more complete dechlorination, whereas H₂-AHB exhibited higher N₂ selectivity in reducing NO₃^-. Regardless of the coexisting electron acceptor, the mZVI-alone system showed the highest EE. Finally, the sustainability concerns and applicability of the three systems were evaluated on the basis of their TCE RE, complete dechlorination ratio, N₂ selectivity, EE, and cost, which were integrated into a comparison of overall benefits. Our findings provide comprehensive and insightful information on the factors that determine remediation scheme selection in real practice.

Resistance and resilience of soil bacterial community to zero-valent iron disposal of lindane contamination

Zero-valent iron (ZVI, Fe⁰) enables chemical reduction of environmental pollutants coupled with reactivity loss due to surface oxidation. During ZVI treatment process, however, microbial community stability in terms of resistance and resilience remains largely unclear. Here, we monitored bacterial community succession over a 4 weeks period in soil microcosms with or without 2% (w/w) Fe⁰ amendment. To simulate soil pollution, 100 μg g^-1 chlorinated pesticide lindane (γ-hexachlorocyclohexane) was added to the microcosms as a model contaminant. In addition to microbial activity as measured by soil organic carbon mineralization, bacterial abundance, diversity and composition were determined using qPCR and high-throughput sequencing of 16 S rRNA genes. Co-occurrence analysis was performed to reveal the interaction patterns within the bacterial communities. The results indicated that ZVI caused near-complete transformation of lindane, while in the microcosms without Fe⁰ amendment the pesticide was recalcitrant. ZVI strongly inhibited CO₂-efflux at the early stage of incubation, but the bacterial community appeared to be less sensitive to Fe⁰ amendment. The ratios of negative to positive correlations between network nodes suggested that Fe⁰ had marginal influence on community stability compared to the lindane treatments, which destabilized the bacterial community. Community succession occurred in the presence of ZVI, as exemplified by a dominancy transition from anaerobic to aerobic taxa. Yet, ZVI alleviated the stress of lindane on soil bacteria by improving community structure and increasing network complexity. Taken together, these findings demonstrate the stability of soil bacterial community under Fe⁰ stress, which might be conducive to functional recovery of soil microorganisms following ZVI remediation.

Synthesis and Characterization of Zero-Valent Fe-Cu and Fe-Ni Bimetals for the Dehalogenation of Trichloroethylene Vapors

In this study, zero-valent iron-copper (Fe-Cu) and iron-nickel (Fe-Ni) bimetals were prepared by disc milling for the dehalogenation of trichloroethylene vapors. For both Fe-Ni and Fe-Cu, three combinations in terms of percentage of secondary metal added were produced (1%, 5%, 20% by weight) and the formation of the bimetallic phase by milling was evaluated by X-ray diffraction (XRD) analysis. The disc milled bimetals were characterized by a homogenous distribution of Ni or Cu in the Fe phase and micrometric size visible from scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) analysis and by a relatively low specific surface area (0.2–0.7 m2/g) quantified by the Brunauer–Emmett–Teller (BET) method. The reactivity of the produced bimetals was evaluated by batch degradation tests of TCE in the gas phase with 1 day of reaction time. Fe-Ni bimetals have shown better performance in terms of TCE removal (57–75%) than Fe-Cu bimetals (41–55%). The similar specific surface area values found for the produced bimetals indicated that the enhancement in the dehalogenation achieved using bimetals is closely related to the induced catalysis. The obtained results suggest that ZVI-based bimetals produced by disc milling are effective in the dehalogenation of TCE vapors in partially saturated conditions.

Integration of in-situ chemical oxidation and permeable reactive barrier for the removal of chlorophenols by copper oxide activated peroxydisulfate

In-situ chemical oxidation (ISCO) and permeable reactive barrier (PRB) have been used in field practices for contaminated groundwater remediation. In this lab-scale study, a novel system integrating ISCO and PRB using peroxydisulfate (PDS) as the oxidant and copper oxide (CuO) as the reactive barrier material was developed for the removal of 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP). The influences of chlorophenol concentration and flow rate on the system performance were first evaluated using synthetic solutions. The removal efficiencies of target chlorophenols were greater than 90% when sufficient PDS was supplied ([PDS]/[chlorophenol]>1). It was also found that the removal efficiencies decreased with the increasing chlorophenol concentrations (10-150 μM) and flow rates (1.8-14.4 mL/min). When three real groundwaters were employed, the removal efficiencies of 2,4-DCP and 2,4,6-TCP slightly reduced to 90% and 85%, respectively. For PCP, the removal efficiency dropped to 20% in two groundwaters with relatively high levels of alkalinity. The influences of pH and TOC were found to be insignificant for the range investigated (pH 6.5-8.7 and TOC = 0.4-1.5 mgC/L). The reduced removal efficiency could be due to the formation of weaker radicals and the stronger competition between bicarbonate ions and PDS for the activation sites on the CuO surfaces.

Metallic Iron for Water Remediation: Plenty of Room for Collaboration and Convergence to Advance the Science

Scientific collaboration among various geographically scattered research groups on the broad topic of “metallic iron (Fe0) for water remediation” has evolved greatly over the past three decades. This collaboration has involved different kinds of research partners, including researchers from the same organization and domestic researchers from non-academic organizations as well as international partners. The present analysis of recent publications by some leading scientists shows that after a decade of frank collaboration in search of ways to improve the efficiency of Fe0/H2O systems, the research community has divided itself into two schools of thought since about 2007. Since then, progress in knowledge has stagnated. The first school maintains that Fe0 is a reducing agent for some relevant contaminants. The second school argues that Fe0 in-situ generates flocculants (iron hydroxides) for contaminant scavenging and reducing species (e.g., FeII, H2, and Fe3O4), but reductive transformation is not a relevant contaminant removal mechanism. The problem encountered in assessing the validity of the views of both schools arises from the quantitative dominance of the supporters of the first school, who mostly ignore the second school in their presentations. The net result is that the various derivations of the original Fe0 remediation technology may be collectively flawed by the same mistake. While recognizing that the whole research community strives for the success of a very promising but unestablished technology, annual review articles are suggested as an ingredient for successful collaboration.

Should the term 'metallic iron' appear in the title of a research paper?

Over the past three decades, groundwater remediation using permeable reactive barriers (PRBs) has proven to be effective. The majority of installed PRBs uses metallic iron (Fe(0)) as a reactive material. However, the success of implemented Fe(0) PRBs is yet to be rationalized as Fe(0) is a generator of iron oxides (contaminant scavengers) and secondary reducing agents (e.g. Fe(II), Fe₃O₄, H₂, green rust), This communication demonstrates that Fe(0) is not an environmental reducing agent. Therefore, more science-based investigations are needed to optimize the operation of Fe(0) PRBs. In particular, Fe(0) PRBs and Fe(0)-based water filters should be regarded as particular cases of "metal corrosion in porous media". A key feature of such systems is that the extent of Fe⁰ corrosion temporally depends on the residual porosity (capillarity). Thus, the functionality of any Fe⁰ PRB should be monitored in a way that the time-dependent variation of the kinetic of iron corrosion is discussed.

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.

Performance and mechanism of nickel hydroxide catalyzed reduction of N-nitrosodimethylamine by iron

Since iron (Fe) was first proven to have a strong reduction ability, it has been successfully applied to remove pollutants from water. In this study, nickel hydroxide (Ni(OH)₂), a catalyst commonly used in hydrogen evolution reactions, was added to improve the activity of Fe to remove N-nitrosodimethylamine (NDMA). The results showed that with the increasing Ni(OH)₂ dosages, the reactions accelerated. The NDMA removal rates increased when the pH value was 6 or 7. Further, when the dissolved oxygen concentration was in the range of 0-12.0 mg∙L^-1, it had little effect on the Fe/Ni(OH)₂ system, and all the reactions obeyed pseudo-first-order kinetics. 1,1-dimethylhydrazine and dimethylamine were formed during NDMA degradation. The capture of active substances and electron spin resonance method confirmed that the main active species were active hydrogen atoms, which participated in the removal of NDMA. Ni(OH)₂ acting as a catalyst was confirmed using wide-angle X-ray diffraction, X-ray photoelectron spectroscopy and Ni²⁺ dissolution. Further, catalytic hydrogenation was proposed as the main removal mechanism as Ni(OH)₂ promotes the corrosion of Fe and dissociation of water, thereby generating more active hydrogen atoms. In addition, Ni(OH)₂ may activate both Fe and NDMA. This technique could be employed as an alternative for NDMA reduction and expand the application field of Ni(OH)₂.

The key role of contact time in elucidating the mechanisms of enhanced decontamination by Fe0/MnO2/sand systems

Metallic iron (Fe0) has shown outstanding performances for water decontamination and its efficiency has been improved by the presence of sand (Fe0/sand) and manganese oxide (Fe0/MnOx). In this study, a ternary Fe0/MnOx/sand system is characterized for its discoloration efficiency of methylene blue (MB) in quiescent batch studies for 7, 18, 25 and 47 days. The objective was to understand the fundamental mechanisms of water treatment in Fe0/H2O systems using MB as an operational tracer of reactivity. The premise was that, in the short term, both MnO2 and sand delay MB discoloration by avoiding the availability of free iron corrosion products (FeCPs). Results clearly demonstrate no monotonous increase in MB discoloration with increasing contact time. As a rule, the extent of MB discoloration is influenced by the diffusive transport of MB from the solution to the aggregates at the bottom of the vessels (test-tubes). The presence of MnOx and sand enabled the long-term generation of iron hydroxides for MB discoloration by adsorption and co-precipitation. Results clearly reveal the complexity of the Fe0/MnOx/sand system, while establishing that both MnOx and sand improve the efficiency of Fe0/H2O systems in the long-term. This study establishes the mechanisms of the promotion of water decontamination by amending Fe0-based systems with reactive MnOx.

Combination of zeolite barrier and bio sparging techniques to enhance efficiency of organic hydrocarbon remediation in a model of shallow groundwater

Adsorption and bioremediation are effective processes for remediation of benzene, toluene, and ethylbenzene (BTE) through Permeable Reactive Barriers (PRBs). A few researches focus on adsorption of natural zeolite because of its hydrophilic property. On the other hand, PRBs need to be replaced by fresh materials after a while when all the possible absorption positions were filled up. We tried to find a way to increase the efficiency of PRB, elongation of its replacement period and of course decreasing the cost of remediation. Equipping of PRB with microbial degradation system was the idea. The present study describes the performances of natural Clinoptilolite-Heulandite Zeolite (CH-Z) and three new strains (safe and low-cost media) utilized in a PRB for removing BTE from contaminated shallow groundwater. First, batch tests were conducted to recognize the optimal removal conditions for utilization of C-HZ and strains to remediate BTE compounds. Then, an aerobic PRB system filled with a natural zeolite was designed and investigated in a continuous flow sand-tank model to assess the efficiency of combined PRBs (zeolite + biosparging), for BTE-contaminated groundwater. Batch experiments showed that the BTE removal of zeolite was 89%, as well as, a consortium of three bacterial strains, Variovorax sp. OT16, Pseudomonas balearica OT17, and Ornithinibacillus sp. OT18 efficiently removed the BTE mixture. The process of BTE removal in the PRB under continuous-flow condition was divided into three phases: Phase I, in which the barrier was made of the only zeolite, and in Phases II and III the reactor was fed by microorganisms. This experiment revealed that in Phases I, the concentrations of BTE decrease (92%) due to zeolite adsorption. In Phase II and III, the degradation process became the principal removal mechanism (68% and 81%, respectively). Consequently, this research showed high ability of C-HZ in the BTE treatment, and a combination of Natural Zeolite, with a biological degradation system (CH-Z -PRB) improves the efficiency of BTE remediation. However, the slow biodegradation rates and the continuous injection of BTE in the model confirmed that longer time was needed for the PRB to function optimally. Finally, the combined method of CH-Z- BIO PRB showed the great potential in the restriction of the BTE migration that can be used at the field-scale after up-scaling.

Effect of Copresence of Zerovalent Iron and Sulfate Reducing Bacteria on Reductive Dechlorination of Trichloroethylene

Sulfur amendment of zerovalent iron (ZVI) materials has been shown to improve the reactivity and selectivity of ZVI toward a select group of organohalide contaminants in groundwater, most notably trichloroethene (TCE). In previous studies, chemical or mechanochemical sulfidation methods were used; however, the potential of using sulfate-reducing bacteria (SRB) to enable sulfur amendment has not been closely examined. In this study, lab-synthesized nanoscale ZVI (nZVI) and Peerless iron particles (ZVI^PLS) were treated in a sulfate-reducing monoculture (D. desulfuricans) and an enrichment culture derived from freshwater sediments (AMR-1) prior to reactivity assessments with TCE as the model contaminant. ZVI conditioned in both cultures exhibited higher dechlorination efficiencies compared to unamended ZVIs. Remarkably, nZVI and ZVI^PLS exposed to AMR-1 attained similar TCE dechlorination rates as their counterparts receiving chemical sulfidation (i.e., S-nZVI) using previously reported method. Product distribution data show that, in the SRB-ZVI system, abiotic dechlorination is the dominant TCE reduction pathway. In addition to dissolved sulfide, biogenic or synthesized FeS particles can enhance nZVI reactivity even as nZVI and FeS were not in direct contact, implying that SRB may influence the reactivity of ZVI via multiple mechanisms in different remediation situations. A shift in Archaea abundance in AMR-1 with nZVI amendment was observed but not with ZVI^PLS. Overall, the synergy exhibited in the SRB-ZVI system may offer a valuable remediation strategy to overcome limitations of standalone biological or abiotic dechlorination approaches for chlorinated solvent abatement.

Characterizing the impact of pyrite addition on the efficiency of Fe0/H2O systems

The role of pyrite (FeS₂) in the process of water treatment using metallic iron (Fe⁰) was investigated. FeS₂ was used as a pH-shifting agent while methylene blue (MB) and methyl orange (MO) were used as an indicator of reactivity and model contaminant, respectively. The effect of the final pH value on the extent of MB discoloration was characterized using 5 g L^-1 of a Fe⁰ specimen. pH variation was achieved by adding 0 to 30 g L^-1 of FeS₂. Quiescent batch experiments with Fe⁰/FeS₂/sand systems (sand loading: 25 g L^-1) and 20 mL of MB were performed for 41 days. Final pH values varied from 3.3 to 7.0. Results demonstrated that MB discoloration is only quantitative when the final pH value was larger than 4.5 and that adsorption and co-precipitation are the fundamental mechanisms of decontamination in Fe⁰/H₂O systems. Such mechanisms are consistent with the effects of the pH value on the decontamination process.

Facile construction of Fe, N and P co-doped carbon spheres by carbothermal strategy for the adsorption and reduction of U(vi)

In this work, nitrogen and phosphorus co-doped magnetic carbon spheres encapsulating well-dispersed active Fe nanocrystals (Fe/P-CN) were fabricated via a simple copolymer pyrolysis strategy. Benefiting from heteroatoms doping, Fe/P-CN could primarily adsorb soluble U(vi) ions through abundant functional groups, and subsequently, the adsorbed U(vi) could be reduced to insoluble U(iv) by Fe nanocrystals. Fe/P-CN pyrolyzed at 800 °C (Fe/P-CN-800) exhibited excellent U(vi) removal capacity of 306.76 mg g⁻¹, surpassing nitrogen and phosphorus co-doped carbon spheres and nano zero-valent iron. In addition, the magnetic separation and thermal reactivation properties endow Fe/P-CN-800 with excellent reusability. This research, especially, provides a promising synergistic adsorption and reduction strategy to effectively remove U(vi) using heteroatom-doped composites.

The constructed novel magnetic carbon sphere co-doped by N, P, Fe (Fe/P-CN) exhibits high U(vi) removal efficiency, excellent magnetic separation and reusability, evidencing the potential practical applications in environmental remediation.

Multi-objective optimization of permeable reactive barrier design for Cr(VI) removal from groundwater

The present study aims to develop a practical approach for the optimal permeable reactive barrier (PRB) design towards Cr(VI) removal from groundwater. Batch and column experiments were performed to investigate the characteristics of the four proposed reactive materials; nanoscale zero-valent iron (Fe⁰), bimetallic nanoscale zero-valent iron (Fe⁰/Cu), activated carbon (AC) and sand/zeolite mixture (S/Z). Kinetic analysis and dynamic modeling of the experimental data were implemented to determine the controlling conditions of the reactive performance of the PRB's materials. The sensitivity index of the design parameters was examined as an indicator of their effect on the reactive responses. Moreover, the Response Surface Methodology (RSM) was considered for optimizing the design variables of the PRB based on the practical factorial analysis. Results revealed that Fe⁰ and Fe⁰/Cu showed high performance in Cr(VI) removal, with a slight superiority to Fe⁰, with final removal efficiency values of 89.7 and 84.1%, respectively. Kinetic analysis depicted that pseudo second order was the best fitting model for Cr(VI) removal in the four materials' cases. ANOVA statistical analysis revealed that quadratic polynomial model was the best model, corresponding to the highest correlation efficiency and adequate precision, to describe the relationships in the four PRB's cases between the selected dependent variables; resident time (t_R), reactive material mass per sectional area of contaminant plume (M/A) and reactive material cost (Cost_PRB) towards the independent parameters; barrier thickness (b) and permeability (K_r). Additionally, sensitivity analysis has been conducted which depicted the high sensitivity, in the four PRB's cases, of average pore water velocity within the barrier (v_r) v_r and K_r with the highest and the second-highest sensitivity index (SI) values towards t_R, respectively. The RSM-optimization revealed that Fe⁰ is the most feasible reactive material, comparing to the other considered materials, with respect to the optimal conditions regarding the long residency (t_R = 22 days) and low cost (b = 0.521 m), with around 95.2% desirability of its optimal solution. Overall, the current study represents a significant contribution and a vital step towards an accurate PRB's design based on previously determined optimal conditions.

Tracing the Scientific History of Fe0-Based Environmental Remediation Prior to the Advent of Permeable Reactive Barriers

The technology of using metallic iron (Fe0) for in situ generation of iron oxides for water treatment is a very old one. The Fe0 remediation technology has been re-discovered in the framework of groundwater remediation using permeable reactive barriers (PRBs). Despite its simplicity, the improvement of Fe0 PRBs is fraught with difficulties regarding their operating modes. The literature dealing with Fe0 remediation contains ambiguities regarding its invention and its development. The present paper examines the sequence of contributions prior to the advent of Fe0 PRBs in order to clarify the seemingly complex picture. To achieve this, the current paper addresses the following questions: (i) What were the motivations of various authors in developing their respective innovations over the years?, (ii) what are the ancient achievements which can accelerate progress in knowledge for the development of Fe0 PRBs?, and (iii) was Fe0 really used for the removal of organic species for the first time in the 1970s? A careful examination of ancient works reveals that: (i) The wrong questions were asked during the past three decades, as Fe0 was premised as a reducing agent, (ii) credit for using Fe0 for water treatment belongs to no individual scientist, and (iii) credit for the use of Fe0 in filtration systems for safe drinking water provision belongs to scientists from the 1850s, while credit for the use of Fe0 for the removal of aqueous organic species does not belong to the pioneers of the Fe0 PRB technology. However, it was these pioneers who exploited Fe0 for groundwater remediation, thereby extending its potential. Complementing recent achievements with the chemistry of the Fe0/H2O system would facilitate the design of more sustainable Fe0-remediation systems.

Graphene oxide-induced formation of a boron-doped iron oxide shell on the surface of NZVI for enhancing nitrate removal

The surface products have a significant influence on the reactivity of zero-valent iron-based materials. Although the enhancing effect of graphene on the reactivity of nanoscale zero-valent iron (NZVI)/graphene composites have been confirmed, the effect of graphene on the formation of surface products of NZVI is not well understood. In order to assess the effect of graphene on the structural of the outer iron oxide layers of NZVI, the NZVI was pre-oxidized by graphene oxide (ONZVI-GO). Compared with the NZVI oxidized by O₂ (ONZVI-O₂), ONZVI-GO was shown to be effective at NO₃^- removal with a high efficiency over a wide range of initial pH values. The results from characterization showed that GO could induce the formation of a tight iron oxide shell with dense spinel structures. The boron introduced during the preparation of NZVI was doped into iron oxides on the surface of ONZVI-GO. The B-O in adsorbed borate was transformed to B-B/B-Fe in the lattice structure of iron oxides, causing the formation of highly electron-deficient Lewis acid sites on the surface of ONZVI-GO, which could effectively gather NO₃^- and OH^-, leading to the higher efficiency removal of NO₃^- than ONZVI-O₂ over a wide range of initial pH values. This study provides new insight into the interaction between graphene and the surface species of NZVI.

A Bimetallic Fe-Mn Oxide-Activated Oxone for In Situ Chemical Oxidation (ISCO) of Trichloroethylene in Groundwater: Efficiency, Sustained Activity, and Mechanism Investigation

Bimetallic Fe-Mn oxide (BFMO) has been regarded as a promising activator of peroxysulfate (PS), the sustained activity and durability of BFMO for long-term activation of PS in situ, however, is unclear for groundwater remediation. A BFMO (i.e., Mn_1.5FeO_6.35) was prepared and explored for PS-based in situ chemical oxidation (ISCO) of trichloroethylene (TCE) in sand columns with simulated/actual groundwater (SGW/AGW). The sustained activity of BFMO, oxidant utilization efficiency, and postreaction characterization were particularly investigated. Electron spin resonance (ESR) and radical scavenging tests implied that sulfate radicals (SO₄^•-) and hydroxyl radicals (HO^•) played major roles in degrading TCE, whereas singlet oxygen (¹O₂) contributed less to TCE degradation by BFMO-activated Oxone. Fast degradation and almost complete dechlorination of TCE in AGW were obtained, with reaction stoichiometry efficiencies (RSE) of ΔTCE/ΔOxone at 3-5%, much higher than those reported RSE values in H₂O₂-based ISCO (≤0.28%). HCO₃^- did not show detrimental effect on TCE degradation, and effects of natural organic matters (NOM) were negligible at high Oxone dosage. Postreaction characterizations displayed that the BFMO was remarkably stable with sustained activity for Oxone activation after 115 days of continuous-flow test, which therefore can be promising catalyst for Oxone-based ISCO for TCE-contaminated groundwater remediation.

Remediation of BTEX plume in a continuous flow model using zeolite-PRB

Adsorption is a well-known phenomenon that causes the remediation of BTEX (Benzene, Toluene, Ethylbenzene, and Xylene). Zeolite is typically useful for the removal of BTEX from groundwater. In this study, the migration of the BTEX plume was investigated in a bench-scale tank model as a shallow aquifer. The objective of this research was to analyze the performance of a natural zeolite in-situ PRB remediation technique. Natural zeolite was applied as a physical permeable reactive barrier. In the first part of the experiment, 40 ml of BTEX as a contaminant was injected at the injection point (BI) into the sand tank. Samples were taken periodically via 14 boreholes for BTEX test for 23 days and analyzed using a GC-FID instrument. The results indicated high removal rates of BTEX by passing through the zeolite barrier. Zeolite barrier reduced the BTEX concentration up to 90% of the initial value. However, the barrier efficiency started to decrease after 132 h since pollution injection reached a minimum amount (%53 of the initial value) due to occupying the free space and grain pore where BTEX was adsorbed onto the surface of zeolite, thereby decreasing the barrier efficiency.

Metallic Iron for Environmental Remediation: Starting an Overdue Progress in Knowledge

A critical survey of the abundant literature on environmental remediation and water treatment using metallic iron (Fe0) as reactive agent raises two major concerns: (i) the peculiar properties of the used materials are not properly considered and characterized, and, (ii) the literature review in individual publications is very selective, thereby excluding some fundamental principles. Fe0 specimens for water treatment are typically small in size. Before the advent of this technology and its application for environmental remediation, such small Fe0 particles have never been allowed to freely corrode for the long-term spanning several years. As concerning the selective literature review, the root cause is that Fe0 was considered as a (strong) reducing agent under environmental conditions. Subsequent interpretation of research results was mainly directed at supporting this mistaken view. The net result is that, within three decades, the Fe0 research community has developed itself to a sort of modern knowledge system. This communication is a further attempt to bring Fe0 research back to the highway of mainstream corrosion science, where the fundamentals of Fe0 technology are rooted. The inherent errors of selected approaches, currently considered as countermeasures to address the inherent limitations of the Fe0 technology are demonstrated. The misuse of the terms “reactivity”, and “efficiency”, and adsorption kinetics and isotherm models for Fe0 systems is also elucidated. The immense importance of Fe0/H2O systems in solving the long-lasting issue of universal safe drinking water provision and wastewater treatment calls for a science-based system design.

An interpretation to Cr(Ⅵ) leaching concentration rebound phenomenon with time in ferrous-reduced Cr(Ⅵ)-bearing solid matrices

Toxicity characteristic leaching procedure (TCLP) is a prevalent way to evaluate the treatment effectiveness for Cr(Ⅵ)-bearing solid matrices (CBSM). But when a certain amount of residual reductants are present in the treated CBSM, Cr(Ⅵ) leaching concentration rebound phenomenon (CLCRP) occurs, which invalidates the TCLP. This study explores the microstructure of ferrous-reduced CBSM and proves that the residual Cr(Ⅵ), Fe_xCr_1-x(OH)₃ precipitate and residual ferrous are separately distributed in a three-layer structure. In natural scenarios, the residual ferrous in the out-layer is firstly flushed away by rainfall and groundwater or oxidized by dissolved oxygen, resulting in the decrease of ferrous with time. Residual Cr(Ⅵ), due to being blocked by precipitate layer, is less flushed away. While in TCLP, all of released residual ferrous and Cr(Ⅵ) are in the leachate and react till one of them is almost exhausted, resulting in the underestimation of Cr(Ⅵ) leaching concentrations. The longer the samples experience the natural scenarios, the less of the residual ferrous, resulting in the decline of underestimation of Cr(Ⅵ) leaching concentrations with time. This study also provides a pretreatment which can effectively reduce the residual ferrous, achieving more accurate Cr(Ⅵ) leaching concentrations and eliminating CLCRP.

Water Treatment Using Metallic Iron: A Tutorial Review

Researchers and engineers using metallic iron (Fe0) for water treatment need a tutorial review on the operating mode of the Fe0/H2O system. There are few review articles attempting to present systematic information to guide proper material selection and application conditions. However, they are full of conflicting reports. This review seeks to: (i) Summarize the state-of-the-art knowledge on the remediation Fe0/H2O system, (ii) discuss relevant contaminant removal mechanisms, and (iii) provide solutions for practical engineering application of Fe0-based systems for water treatment. Specifically, the following aspects are summarized and discussed in detail: (i) Fe0 intrinsic reactivity and material selection, (ii) main abiotic contaminant removal mechanisms, and (iii) relevance of biological and bio-chemical processes in the Fe0/H2O system. In addition, challenges for the design of the next generation Fe0/H2O systems are discussed. This paper serves as a handout to enable better practical engineering applications for environmental remediation using Fe0.

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

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

A critical review of volatile fatty acids produced from waste activated sludge: enhanced strategies and its applications

This paper reviews the recent achievements in the enhanced production of volatile fatty acids (VFAs) from waste activated sludge (WAS). The enhanced strategies are divided into two approaches. The first strategy focuses on the regulation of carbon-to-nitrogen (C/N) ratio by co-digestion of WAS with carbon-rich substrates, including municipal solid wastes (MSW), marine algae, agricultural residues, and animal manures. The other strategy is to enhance the solubilization and hydrolysis of WAS or inhibit the methanogenesis by applying various pretreatments, such as mechanical, chemical, enzymatic, and thermal pretreatment. Finally, the applications of WAS-derived VFAs are discussed. The future researches in enhancing VFAs production and wide application of the VFAs from both technical and economic perspectives are proposed.

PVA/CS and PVA/CS/Fe gel beads' synthesis mechanism and their performance in cultivating anaerobic granular sludge

Biomass washout from high-speed anaerobic suspended bed bio-reactors is still a challenge to their stable operation. Preserving active biomass to efficiently retain biomass in the reactor is one of the solutions to this problem. Herein, two carriers (polyvinyl alcohol/chitosan (PVA/CS) and PVA/CS/Fe gel beads) were prepared using the cross-linking method. The fourier transform infrared (FTIR) and ¹³C nuclear magnetic resonance (¹³C NMR) analyses showed that PVA/CS gel beads formed mainly through hydrogen-bonds (NH₂OH^-). Furthermore, FTIR, ¹³C NMR, energy dispersive spectrum (EDS), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that PVA/CS/Fe gel beads formed mainly through chelate bond (NH₂-Fe^M+OH^-). The scanning electron microscope (SEM) results affirmed that the gel beads had rough and well-developed porous structure for the attachment of microbes. Furthermore, the abilities of gel beads on the cultivation of granular sludge in an up-flow anaerobic sludge bed (UASB) reactor were effectively demonstrated while treating wastewater polluted with glucose and alkali lignin. The results showed that the gel beads-assisted reactors had a higher performance than those without the gel beads. The cultivation of granules in these reactors was accelerated, while the granules became bigger and exhibited better settling velocities. The reactor with gel beads was easier to withstand a higher organic loading rate due to dense microbial aggregates, which were caused by more humic-like substance. Particularly, the reactor with PVA/CS/Fe gel beads was able to improve the overall robustness of the system due to stronger mechanical properties of gel beads, and also prevented cells detachment.

Fe0/H2O Filtration Systems for Decentralized Safe Drinking Water: Where to from Here?

Inadequate access to safe drinking water is one of the most pervasive problems currently afflicting the developing world. Scientists and engineers are called to present affordable but efficient solutions, particularly applicable to small communities. Filtration systems based on metallic iron (Fe0) are discussed in the literature as one such viable solution, whether as a stand-alone system or as a complement to slow sand filters (SSFs). Fe0 filters can also be improved by incorporating biochar to form Fe0-biochar filtration systems with potentially higher contaminant removal efficiencies than those based on Fe0 or biochar alone. These three low-cost and chemical-free systems (Fe0, biochar, SSFs) have the potential to provide universal access to safe drinking water. However, a well-structured systematic research is needed to design robust and efficient water treatment systems based on these affordable filter materials. This communication highlights the technology being developed to use Fe0-based systems for decentralized safe drinking water provision. Future research directions for the design of the next generation Fe0-based systems are highlighted. It is shown that Fe0 enhances the efficiency of SSFs, while biochar has the potential to alleviate the loss of porosity and uncertainties arising from the non-linear kinetics of iron corrosion. Fe0-based systems are an affordable and applicable technology for small communities in low-income countries, which could contribute to attaining self-reliance in clean water supply and universal public health.

Strategies to enhance the stability of nanoscale zero-valent iron (NZVI) in continuous BrO3- reduction

The reduction of bromate to bromide was successfully achieved by bimetallic catalysts with NZVI support in continuous-flow reactors. The stability of NZVI-supported bimetallic catalysts was enhanced by decelerating the iron corrosion and sequential rapid passivation of the iron-Cu-Pd ensembles under optimized reaction conditions. Thus >99% bromate removal can be continuously achieved for 11 h. The lifetime of the bimetallic catalyst was further enhanced and tested under different hydraulic retention time, catalyst loading, and initial bromate concentrations. At the optimized operation conditions, the catalyst showed a complete bromate reduction by 24 h and then the reactivity slowly decreased to 20% over the next 100 h. X-ray diffraction and X-ray photoelectron spectroscopy showed that the reactive NZVI support was oxidized to Fe(II) and Fe(III) along with Cu(0) oxidation to CuO, while the oxidation state of Pd did not change. Therefore, bromate reduction occurred on the surface of reactive NZVI support and Cu(0) particle, while Pd played a role as a hydrogenation catalyst that prolonged the lifetime of the bimetallic catalyst.

The Impact of Selected Pretreatment Procedures on Iron Dissolution from Metallic Iron Specimens Used in Water Treatment

Studies were undertaken to determine the reasons why published information regarding the efficiency of metallic iron (Fe0) for water treatment is conflicting and even confusing. The reactivity of eight Fe0 materials was characterized by Fe dissolution in a dilute solution of ethylenediaminetetraacetate (Na2–EDTA; 2 mM). Both batch (4 days) and column (100 days) experiments were used. A total of 30 different systems were characterized for the extent of Fe release in EDTA. The effects of Fe0 type (granular iron, iron nails and steel wool) and pretreatment procedure (socking in acetone, EDTA, H2O, HCl and NaCl for 17 h) were assessed. The results roughly show an increased iron dissolution with increasing reactive sites (decreasing particle size: wool > filings > nails), but there were large differences between materials from the same group. The main output of this work is that available results are hardly comparable as they were achieved under very different experimental conditions. A conceptual framework is presented for future research directed towards a more processed understanding.

Enhanced quinoline removal by zero-valent iron-coupled novel anaerobic processes: performance and underlying function analysis

Quinoline is toxic and difficult to degrade biologically; thus, it is a serious threat to the safety of ecosystems. To promote quinoline reduction, zero-valent iron (ZVI) was introduced into an anaerobic digestion (AD) system through batch experiments. The performance of three different types of ZVI (i.e., iron powder, iron scrap and rusty iron scrap) on quinoline degradation, methane production, formation of volatile fatty acids (VFAs) and chemical oxygen demand (COD) removal were investigated systematically. Compared to the AD system alone, quinoline and COD removal as well as the production of methane and acetic acid were effectively enhanced by ZVI, especially rusty iron scrap. The removal efficiencies of quinoline and COD were increased by 28.6% and 19.9%, respectively. The enhanced effects were attributed to the high accumulation of ferrous ions and high pH self-buffering capability, which were established by ZVI addition. Furthermore, high-throughput sequencing analysis indicated that the functional microorganisms in the ZVI-AD system were higher than in the AD system, and the added types of ZVI played important roles in structuring the innate microbial community in waste activated sludge (WAS). Especially, high enrichment of microorganisms capable of degrading quinoline, such as Pseudomonas and Bacillus, in the coupled system was detected.

Geochemical and Isotope Study of Trichloroethene Degradation in a Zero-Valent Iron Permeable Reactive Barrier: A Twenty-Two-Year Performance Evaluation

This study provides a twenty-two-year record of in situ degradation of chlorinated organic compounds by a granular iron permeable reactive barrier (PRB). Groundwater concentrations of trichloroethene (TCE) entering the PRB were as high as 10670 μg/L. Treatment efficiency ranged from 81 to >99%, and TCE concentrations from <1 μg/L to 165 μg/L were detected within and hydraulically down-gradient of the PRB. After 18 years, effluent TCE concentrations were above the maximum contaminant level (MCL) along segments of the PRB exhibiting upward trending influent TCE. Degradation products included cis-dichloroethene ( cis-DCE), vinyl chloride (VC), ethene, ethane, >C4 compounds, and possibly CO_2(aq) and methane. Abiotic patterns of TCE degradation were indicated by compound-specific stable isotope data and the distribution of degradation products. δ¹³C values of methane within and down-gradient of the PRB varied widely from -94‰ to -16‰; these values cover most of the isotopic range encountered in natural methanogenic systems. Methanogenesis is a sink for inorganic carbon in zerovalent iron PRBs that competes with carbonate mineralization, and this process is important for understanding pore-space clogging and longevity of iron-based PRBs. The carbon isotope signatures of methane and inorganic carbon were consistent with open-system behavior and 22% molar conversion of CO_2(aq) to methane.

Characterization methods of zerovalent iron for water treatment and remediation

Appropriately selecting methods for characterizing the reaction system of zerovalent iron (ZVI) favors its application for water treatment and remediation. Hence, a survey of the available ZVI characterization techniques used in laboratory and field studies are presented in this review for clarifying the characteristic properties, (in-situ) corrosion processes, and corrosion products of ZVI system. The methods are generally classified into four broad categories: morphology characterization techniques, (sub-)surface and bulk analysis mainly via the spectral protocols, along with the (physio)electrochemical alternatives. Moreover, this paper provides a critical review on the scopes and applications of ZVI characterization methodologies from several perspectives including their suitable occasions, availability, (semi-)quantitative/qualitative evaluations, in/ex-situ reaction information, advantages, limitations and challenges, as well as economic and technical remarks. In particular, the characteristic spectroscopic peak locations of typical iron (oxyhydr)oxides are also systematically summarized. In view of the complexity and variety of ZVI system, this review further addresses that different characterization methods should be employed together for better assessing the performance and mechanisms of ZVI-involved systems and thereby facilitating the deployment of ZVI-based installations in real practice.

Fe0/H2O Systems for Environmental Remediation: The Scientific History and Future Research Directions

Elemental iron (Fe0) has been widely used in groundwater/soil remediation, safe drinking water provision, and wastewater treatment. It is still mostly reported that a surface-mediated reductive transformation (direct reduction) is a relevant decontamination mechanism. Thus, the expressions “contaminant removal” and “contaminant reduction” are interchangeably used in the literature for reducible species (contaminants). This contribution reviews the scientific literature leading to the advent of the Fe0 technology and shows clearly that reductive transformations in Fe0/H2O systems are mostly driven by secondary (FeII, H/H2) and tertiary/quaternary (e.g., Fe3O4, green rust) reducing agents. The incidence of this original mistake on the Fe0 technology and some consequences for its further development are discussed. It is shown, in particular, that characterizing the intrinsic reactivity of Fe0 materials should be the main focus of future research.

Advances in Surface Passivation of Nanoscale Zerovalent Iron: A Critical Review

Nanoscale zerovalent iron (NZVI) is one of the most extensively studied nanomaterials in the fields of wastewater treatment and remediation of soil and groundwater. However, rapid oxidative transformations of NZVI can result in reduced NZVI reactivity. Indeed, the surface passivation of NZVI is considered one of the most challenging aspects in successfully applying NZVI to contaminant degradation. The oxidation of NZVI can lead to the formation of Fe^II-bearing phases (e.g., Fe^IIO, Fe^II(OH)₂, Fe^IIFe^III₂O₄) on the NZVI surface or complete oxidation to ferric (oxyhydr)oxides (e.g., Fe^IIIOOH). This corrosion phenomenon is dependent upon various factors including the composition of NZVI itself, the type and concentration of aqueous species, reaction time and oxic/anoxic environments. As such, the coexistence of different Fe oxidation states on NZVI surfaces may also, in some instances, provide a unique reactive microenvironment to promote the adsorption of contaminants and their subsequent transformation via redox reactions. Thus, an understanding of passivation chemistry, and its related mechanisms, is essential not only for effective NZVI application but also for accurately assessing the positive and negative effects of NZVI surface passivation. The aim of this review is to discuss the nature of the passivation processes that occur and the passivation byproducts that form in various environments. In particular, the review presents: (i) the strengths and limitations of state-of-the-art techniques (e.g., electron microscopies and X-ray-based spectroscopies) to identify passivation byproducts; (ii) the passivation mechanisms proposed to occur in anoxic and oxic environments; and (iii) the effects arising from synthesis procedures and the presence of inorganics/organics on the nature of the passivation byproducts that form. In addition, several depassivation strategies that may assist in increasing and/or maintaining the reactivity of NZVI are considered, thereby enhancing the effectiveness of NZVI in contaminant degradation.

Effect of climate change on humic substances and associated impacts on the quality of surface water and groundwater: A review

Humic substances (HS), a highly transformed part of non-living natural organic matter (NOM), comprise up to 70% of the soil organic matter (SOM), 50-80% of dissolved organic matter (DOM) in surface water, and 25% of DOM in groundwater. They considerably contribute to climate change (CC) by generating greenhouse gases (GHG). On the other hand, CC affects HS, their structure and reactivity. HS important role in global warming has been recognized and extensively studied. However, much less attention has been paid so far to effects on the freshwater quality, which may result from the climate induced impact on HS, and HS interactions with contaminants in soil, surface water and groundwater. It is expected that an increased temperature and enhanced biodegradation of SOM will lead to an increase in the production of DOM, while the flooding and runoff will export it from soil to rivers, lakes, and groundwater. Microbial growth will be stimulated and biodegradation of pollutants in water can be enhanced. However, there may be also negative effects, including an inhibition of solar disinfection in brown lakes. The CC induced desorption from soil and sediments, as well as re-mobilization of metals and organic pollutants are anticipated. In-situ treatment of surface water and groundwater may be affected. Quality of the source freshwater is expected to deteriorate and drinking water production may become more expensive. Many of the possible effects of CC described in this article have yet to be explored and understood. Enormous potential for interesting, multidisciplinary studies in the important research areas has been presented.

Evaluating long-term patterns of decreasing groundwater discharge through a lake-bottom permeable reactive barrier

Identifying and quantifying groundwater exchange is critical when considering contaminant fate and transport at the groundwater/surface-water interface. In this paper, areally distributed temperature and point seepage measurements are used to efficiently assess spatial and temporal groundwater discharge patterns through a glacial-kettle lakebed area containing a zero-valent iron permeable reactive barrier (PRB). Concern was that the PRB was becoming less permeable with time owing to biogeochemical processes within the PRB. Patterns of groundwater discharge over an 8-year period were examined using fiber-optic distributed temperature sensing (FO-DTS) and snapshot-in-time point measurements of temperature. The resulting thermal maps show complex and uneven distributions of temperatures across the lakebed and highlight zones of rapid seepage near the shoreline and along the outer boundaries of the PRB. Repeated thermal mapping indicates an increase in lakebed temperatures over time at periods of similar stage and surface-water temperature. Flux rates in six seepage meters permanently installed on the lakebed in the PRB area decreased on average by 0.021 md^-1 (or about 4.5 percent) annually between 2004 and 2015. Modeling of diurnal temperature signals from shallow vertical profiles yielded mean flux values ranging from 0.39 to 1.15 md^-1, with stronger fluxes generally related to colder lakebed temperatures. The combination of an increase in lakebed temperatures, declines in direct seepage, and observations of increased cementation of the lakebed surface provide in situ evidence that the permeability of the PRB is declining. The presence of temporally persistent rapid seepage zones is also discussed.