Investigating the efficiency of microscale zero valent iron-based in situ reactive zone (mZVI-IRZ) for TCE removal in fresh and saline groundwater

In situ remediation of mercury-contaminated groundwater through an in situ created reactive zone enabled by carboxymethyl cellulose stabilized FeS nanoparticles

Faced with worldwide mercury (Hg) contamination in groundwater, efficient in situ remediation technologies are urgently needed. Carboxymethyl cellulose (CMC) stabilized iron sulfide (CMC-FeS) nanoparticles have been found effective for immobilizing mercury in water and soil. Yet, the potential use of the nanoparticles for creating an in situ reactive zone (ISRZ) in porous geo-media has not been explored. This study assessed the transport and deliverability of CMC-FeS in sand media towards creating an ISRZ. The nanoparticles were deliverable through the saturated sand bed and the particle breakthrough/deposition profiles depended on the injection pore velocity, initial CMC-FeS concentration, and ionic strength. The transport data were well interpreted using an advection-dispersion transport model combined with the classical filtration theory. The resulting ISRZ effectively removed mercury from contaminated groundwater under typical subsurface conditions. While the operating conditions are yet to be optimized, the Hg breakthrough time can be affected by groundwater velocity, influent mercury concentration, dissolved organic matter, and co-existing metals/metalloids. The one-dimensional advection-dispersion equation well simulated the Hg breakthrough data. CMC-FeS-laden ISRZ effectively converted the more easily available Hg species to stable species. These findings reveal the potential of creating an ISRZ using CMC-FeS for in situ remediation of Hg contaminated soil and groundwater.

Rapid and efficient removal of organophosphorus pollutant and recovery of valuable elements: A boosted strategy for eliminating organophosphorus from wastewater

Considering the significant hazards of organophosphorus compounds (OPs) and the potential crisis of phosphorus (P) resource shortage, there is a great necessity to develop economically feasible, highly effective, and sustainable strategies to remove OPs and recover P resources. In this study, low-cost microscale zero-valent iron (mZVI) was used to activate hydrogen peroxide for the rapid and efficient elimination of Tetrakis(hydroxymethyl)phosphonium sulfate (THPS) from the aquatic environment. Compared to the conventional Fenton reaction and commercial mZVI, mZVI/H2O2-based Fenton-like reaction exhibited superior removal performance for THPS. The removal mechanism of the mZVI/H2O2 system for THPS was thoroughly elucidated through the identification of reactive oxygen species, characterization analysis, and theoretical calculation. Furthermore, the valuable components of the degradation products were successfully recovered through thermally induced precipitation of the sample followed by high-temperature calcination. The mZVI/H2O2 system has demonstrated significant advantages in removing organic compounds from various types of actual wastewater and improving the biodegradability of the wastewater. This study presented an environmentally friendly and highly efficient strategy to eliminate OPs pollution and recover P resources. It also provided an easy-to-operate method for remediating actual industrial wastewater.

Removal of Cr(VI) by glutaraldehyde-crosslinked chitosan encapsulating microscale zero-valent iron: Synthesis, mechanism, and longevity

Microscale zero-valent iron (mZVI) has shown great potential for groundwater Cr(VI) remediation. However, low Cr(VI) removal capacity caused by passivation restricted the wide use of mZVI. We prepared mZVI/GCS by encapsulating mZVI in a porous glutaraldehyde-crosslinked chitosan matrix, and the formation of the passivation layer was alleviated by reducing the contact between zero-valent iron particles. The average pore diameter of mZVI/GCS was 8.775 nm, which confirmed the mesoporous characteristic of this material. Results of batch experiments demonstrated that mZVI/GCS exhibited high Cr(VI) removal efficiency in a wide range of pH (2-10) and temperature (5-35°C). Common groundwater coexisting ions slightly affected mZVI/GCS. The material showed great reusability, and the average Cr(VI) removal efficiency was 90.41% during eight cycles. In this study, we also conducted kinetics and isotherms analysis. Pseudo-second-order model was the most matched kinetics model. The Cr(VI) adsorption process was fitted by both Langmuir and Freundlich isotherms models, and the maximum Langmuir adsorption capacity of mZVI/GCS reached 243.63 mg/g, which is higher than the adsorption capacities of materials reported in most of the previous studies. Notably, the column capacity for Cr(VI) removal of a mZVI/GCS-packed column was 6.4 times higher than that of a mZVI-packed column in a 50-day experiment. Therefore, mZVI/GCS with a porous structure effectively relieved passivation problems of mZVI and showed practical application prospects as groundwater Cr(VI) remediation material with practical application prospects.

Efficient remediation of mercury-contaminated groundwater using MoS2 nanosheets in an in situ reactive zone

Mercury contamination in groundwater is a serious global environmental issue that poses threats to human and environmental health. While MoS2 nanosheets have been proven promising in removing Hg from groundwater, an effective tool for in situ groundwater remediation is still needed. In this study, we investigated the transport and retention behavior of MoS2 nanosheets in sand column, and employed the formed MoS2in situ reactive zone (IRZ) for the remediation of Hg-contaminated groundwater. Breakthrough test revealed that high flow velocity and MoS2 initial concentration promoted the transport of MoS2 in sand column, while the addition of Ca ions increased the retention of MoS2. In Hg removal experiments, the groundwater flow velocity did not influence the Hg removal capacity due to the fast reaction rate between MoS2 and Hg. With an optimized MoS2 loading, MoS2IRZ effectively reduced the Hg effluent concentration down to <1 μg/L without apparent Hg remobilization. Additionally, flake-like MoS2 employed in this study showed much better Hg removal performance than flower-like and bulk MoS2, as well as other reported materials, with the Hg removal capacity a few to tens of times higher than those materials. These results suggest that MoS2 nanosheets have the potential to be an efficient IRZ reactive material for in situ remediation of Hg in contaminated groundwater.

Evaluating groundwater ecosystem dynamics in response to post in-situ remediation of mixed chlorinated volatile organic compounds (CVOCs): An insight into microbial community resilience, adaptability, and metabolic functionality for sustainable remediation and ecosystem restoration

The in-situ remediation of groundwater contaminated with mixed chlorinated volatile organic compounds (CVOCs) has become a significant global research interest. However, limited attention has been given in understanding the effects of these remediation efforts on the groundwater microbial communities, which are vital for maintaining ecosystem health through their involvement in biogeochemical cycles. Hence, this study aimed to provide valuable insights into the impacts of in-situ remediation methods on groundwater microbial communities and ecosystem functionality, employing high-throughput sequencing coupled with functional and physiological assays. The results showed that both bioremediation and chemical remediation methods adversely affected microbial diversity and abundance compared to non-polluted sites. Certain taxa such as Pseudomonas, Acinetobacter, and Vogesella were sensitive to these remediation methods, while Aquabacterium exhibited greater adaptability. Functional annotation unveiled the beneficial impact of bioremediation on the sulfur cycle and specific taxa such as Cellvibrio, Massilia, Algoriphagus, and Flavobacterium which showed a significant positive relationship with dark oxidation of sulfur compounds. In contrast, chemical remediation showed adverse impacts on the nitrogen cycle with a reduced abundance of nitrogen and nitrate respiration along with a reduced utilization of amines (nitrogen rich substrate). The findings of this study offer valuable insights into the potential impacts of in-situ remediation methods on groundwater microbial communities and ecosystem functionality, emphasizing the need for meticulous consideration to ensure the implementation of effective and sustainable remediation strategies that safeguard ecosystem health and function.

Reductive dechlorination of trichloroethene by sulfided microscale zero-valent iron in fresh and saline groundwater: Reactivity, pathways, and selectivity

S/mZVI is a promising material for groundwater remediation due to its excellent properties. However, the reactivity and electron selectivity toward target contaminant are critical. Thus, this study investigated the effect of complex groundwater chemistries (Milli-Q water, fresh groundwater and saline groundwater) on the reactivity of S/mZVI toward trichloroethylene (TCE), dechlorination pathway, hydrogen evolution kinetic, electron efficiency and aging behaviors. Results showed that sulfidation appreciably increased the reactivity and electron selectivity. The major degradation product of TCE dechlorination by S/mZVI was acetylene, which was consistent with TCE dechlorination by β-elimination. Moreover, reductive β-elimination was still the dominant dechlorination pathway for the application of S/mZVI in three groundwater conditions. However, the rates and the quantities of major products from TCE degradation varied significantly. S/mZVI in saline groundwater can maintain the reactivity towardTCE due to the protection of Fe0 by Fe3O4 deposited on the surface. Thus, the higher TCE removal efficiency and less hydrogen accumulation resulted in the greatest electron efficiency (4.3-79.2%). Overall, S/mZVI was more effective for the application in saline groundwater. This study proved insight into the comprehensive evaluation and implications for the application of S/mZVI based technologies in saline contaminated groundwater.

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.

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.

Agar-stabilized sulfidated microscale zero-valent iron: Its stability and performance in chromate reduction

Sulfidated microscale zero-valent iron (SmZVI) attracts much attention recently in remediation of contaminated groundwater, but whether polymer coating on SmZVI would impact on its reactivity and capacity is yet to be understood. In this work, SmZVI was prepared by milling mZVI with elemental sulfur, and its stability in agar solution was evaluated. The impact of polymer coating on SmZVI grains' capacity and reactivity for chromate reduction was then examined. Experimental results indicated that SmZVI having the best overall performance was attained by grinding mZVI with elemental sulfur at 0.05 S/Fe molar ratio for 10 h. SmZVI's stability can be substantially improved if dispersed in 2.0 g/L agar solution. Existence of agar films on the SmZVI grain (A-SmZVI) lowered the material's capacity for chromate reduction by 56%, and the associated reaction kinetics by 70.4%, as estimated by pseudo first-order reaction model using the early-stage experimental data. Analysis of XPS spectra of A-SmZVI post reaction with chromate indicated that multiple reductive species including Fe⁰, Fe(II), FeS, and S(-II) may have jointly participated in the redox reaction taking place on the A-SmZVI-water interface. Fitting of XPS data supported that S(-II) was oxidized to SO₄^2-, S₂O₃^2-, and S⁰, in order of decreasing surface concentration.

Remediation of trichloroethylene by microscale zero-valent iron aged under various groundwater conditions: Removal mechanism and physicochemical transformation

Microscale zero-valent iron (mZVI) has been widely used for the in-situ groundwater remediation of various pollutants. However, the aging behavior of injected mZVI particles limits the widespread application in groundwater remediation projects. To assess the long-term reactivity of mZVI particles, the mechanism of trichloroethylene (TCE) degradation by various aged mZVI particles (A-mZVI) was determined by quantitatively evaluating the contributions of chemical reduction and adsorption. Further, this study investigated the physicochemical transformation of mZVI particles aged under various hydraulic conditions (static and dynamic), redox conditions (anoxic and aerobic) and aging durations (152 d and 455 d). The results show that the removal of TCE by different A-mZVI particles increased the sorption capacity in the initial period (0-6 h). However, in the long term, a significant inhibition of TCE removal was observed because of the decreased TCE reduction capacity caused by the hindrance of electron transfer, which was generated by corrosion precipitates. Furthermore, the characterization results demonstrated that despite the significant differences in the apparent morphology of the A-mZVI particles in various groundwater conditions, the final crystal corrosion products were mainly Fe₃O₄. Thus, the aging and inactivation of mZVI particles on TCE removal were promoted under the aerobic conditions. In addition, the structure of mZVI particles collapsed from the micro- to nanoscale under anaerobic dynamic over 455 d. No substantial impact on the final TCE removal was observed for the A-mZVI particles prepared under various hydraulic conditions and aging times. These findings provide insights regarding the impact mechanisms of corrosion precipitates on the removal of target contaminant and provide implications for long-term mZVI application under various target aquifer conditions.

Hydroxyapatite Coatings on Calcite Powder for the Removal of Heavy Metals from Contaminated Water

An approach for the remediation of heavy metal-contaminated wastewater that is gaining increasing attention is the application of hydroxyapatite (HAP)-based particles. HAP is conventionally synthesized through wet chemical precipitation of calcium and phosphate ions, although later studies have focused on HAP synthesis from solid calcite contacted with a phosphate solution under ambient conditions. This synthesis route can allow saving soluble Ca-chemicals and, thus, make the process more cost-efficient. The aim of this study was to coat natural calcite powder with a layer of HAP for the removal of Zn and Cu from contaminated water. For this purpose, a HAP layer was synthesized on calcite particles, characterized using several complementary techniques and evaluated for the removal of Zn and Cu from synthetic solutions. Sorption kinetics and equilibrium isotherms, as well as the effect of sonication of the synthesized sample on its sorption performance, were determined. The results showed that calcite particles were efficiently coated with a HAP layer with high capacity in removing Zn and Cu from acidic solutions, with a qmax of 34.97 mg/g for Zn (increased to 37.88 g/mg after sonication of the sample) and 60.24 mg/g for Cu (which hardly varied with sonication). The mechanisms behind the sorption of Zn and Cu onto HAP, inferred from pH changes, the relation between metal uptake and Ca2+ release and XRD analysis, included surface complexation, ion exchange and precipitation of new Zn- and Cu-containing phases.

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

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

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.

Biological synthesis of iron nanoparticles using hydrolysates from a waste-based biorefinery

The purpose of this work was to produce iron nanoparticles (Fe-NP) by microbial pathway from anaerobic bacteria grown in anaerobic fluidized bed reactors (AnFBRs) that constitute a new stage of a waste-based biorefinery. Bioparticles from biological fluidized bed reactors from a biorefinery of organic fraction of municipal solid wastes (that produces hydrolysates rich in reducing sugars) were nanodecorated (embedded nanobioparticle or nanodecorated bioparticle, ENBP) by biological reduction of iron salts. Factors "origin of bioparticles" (either from hydrogenogenic or methanogenic fluidized bed reactor) and "type of iron precursor salt" (iron chloride or iron citrate) were explored. SEM and high-resolution transmission electron microscopy (HRTEM) showed amorphous distribution of nanoparticles (NP) on the bioparticles surface, although small structures that are nanoparticle-like could be seen in the SEM micrographs. Some agglomeration of NPs was confirmed by DLS. Average NP size was lower in general for NP in ENBP-M than ENBP-H according to HRTEM. The factors did not have a significant influence on the specific surface area of NPs, which was high and in the range 490 to 650 m² g^-1. Analysis by EDS displayed consistent iron concentration 60-65% iron in nanoparticles present in ENBP-M (bioparticles previously grown in methanogenic bioreactor), whereas the iron concentration in NPs present in ENBP-H (bioparticles previously grown in hydrogenogenic bioreactor) was more variable in a range from 8.5 to 62%, depending on the iron salt. X-ray diffraction patterns showed the typical peaks for magnetite at 35° (3 1 1), 43° (4 0 0), and 62° (4 0 0); moreover, siderite diffraction pattern was found at 26° (0 1 2), 38° (1 1 0), and 42° (1 1 3). Results of infrared analysis of ENBP in our work were congruent with presence of magnetite and occasionally siderite determined by XRD analysis as well as presence of both Fe⁺² and F⁺³ (and selected satellite signal peaks) observed by XPS. Our results on the ENBPs hold promise for water treatment, since iron NPs are commonly used in wastewater technologies that treat a wide variety of pollutants. Finally, the biological production of ENBP coupled to a biorefinery could become an environmentally friendly platform for nanomaterial biosynthesis as well as an additional source of revenues for a waste-based biorefinery.

Cr(VI) removal from groundwater using double surfactant-modified nanoscale zero-valent iron (nZVI): Effects of materials in different status

The excellent potential of nanoscale zero-valent iron (nZVI) makes it a promising remedy for contaminated aquifers. More efficient remediation modes with nZVI have been investigated recently to overcome the inherent drawbacks of materials. In this study, a double surfactant-modified synthesis method is established to make the removal of Cr(VI) more efficiency. A specific focus of the materials status (suspension or powder) is devoted to explore the best application condition, especially for groundwater remediation. A non-ionic surfactant, polyvinylpyrrolidone (PVP), and an anionic surfactant, sodium oleate (NaOA), were selected to modify nZVI simultaneously. The kinetics and isotherm experiments, reactions at different pHs, initial concentrations, gas conditions, and coexisting ion conditions were conducted to analyse the removal mechanism. The characterizations before and after the reaction were used to further explain the results. From the batch experiments, a synergistic effect could be recognized in Cr(VI) elimination when PVP and NaOA were both used for nZVI modification. The materials in suspension (without drying process) exhibited higher removal efficiency in comparison with powder ones. These reactions happened in acidic condition demonstrated higher reactivity. The anaerobic condition facilitated the reaction, which showed prospect application in groundwater. Equilibrium could be reached within 2 min using the suspension sample with a removal efficiency above 99.5% and a maximum removal amount of 231.75 mg g^-1. The reaction process was well-fitted with pseudo-second-order kinetics and the Langmuir model. Cr(VI) was fully transformed into Cr(III), a safer status. These results show this is a promising in-situ method to eliminate Cr(VI) pollution in groundwater.

Effects of co-existing nitrate on TCE removal by mZVI under different pollution load scenarios: Kinetics, electron efficiency and mechanisms

Microscale zero-valent iron in situ reaction zone (mZVI-IRZ) has proved to be effective and efficient for the removal of chlorinated aliphatic hydrocarbons (CAHs) from groundwater. However, nitrate (NO₃^-), which is ubiquitous in groundwater, affects the mZVI-based attenuation of CAHs in a complicated manner. Both the reaction rate constant (k) and electron efficiency (EE) of mZVI must be considered to comprehensively reflect the effects of NO₃^- on the short and long-term remediation performances of mZVI. Therefore, the influence of NO₃^- on trichloroethylene (TCE) removal under high-pollution-load (iron limited) and low-pollution-load (iron excess) conditions was investigated. Low concentrations of NO₃^- (10 and 50 mg N L^-1) were found to enhance the TCE removal rate and efficiency, whereas high concentrations of NO₃^- (100 mg N L^-1) inhibited the reaction. Although TCE removal was increased at low concentrations of NO₃^-, the EE of mZVI was dramatically decreased in the presence of NO₃^- at all concentration levels. Therefore, both the short-term TCE removal characteristics and the EE of mZVI should be considered when evaluating the long-term remediation effectiveness of mZVI-IRZ technology. The effects of NO₃^- on the TCE removal trends under high- and low-pollution-load scenarios were similar, but had different magnitudes. NO₃^- affected the TCE removal mainly by promoting mZVI corrosion, competing for electrons and affecting passivation product evolution. Our results provide guidance for the practical application of mZVI-IRZ technology.

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.

Redirecting Research on Fe0 for Environmental Remediation: The Search for Synergy

A survey of the literature on using metallic iron (Fe⁰) for environmental remediation suggests that the time is ripe to center research on the basic relationship between iron corrosion and contaminant removal. This communication identifies the main problem, which is based on the consideration that contaminant reductive transformation is the cathodic reaction of iron oxidative dissolution. Properly considering the inherent complexities of the Fe⁰/H₂O system will favor an appropriate research design that will enable more efficient and sustainable remediation systems. Successful applications of Fe⁰/H₂O systems require the collective consideration of progress achieved in understanding these systems. More efforts should be made to decipher the long-term kinetics of iron corrosion, so as to provide better approaches to accurately predict the performance of the next generation Fe⁰-based water treatment systems.

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.

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.