Effect of dissolved oxygen concentration on iron efficiency: Removal of three chloroacetic acids

Boosting the denitrification efficiency of iron-based constructed wetlands in-situ via plant biomass-derived biochar: Intensified iron redox cycle and microbial responses

Considering the unsatisfied denitrification performance of carbon-limited wastewater in iron-based constructed wetlands (ICWs) caused by low electron transfer efficiency of iron substrates, utilization of plant-based conductive materials in-situ for improving the long-term reactivity of iron substrates was proposed to boost the Fe (III)/Fe (II) redox cycle thus enhance the nitrogen elimination. Here, we investigated the effects of withered Iris Pseudacorus biomass and its derived biochar on nitrogen removal for 165 days in ICWs. Results revealed that accumulate TN removal capacity in biochar-added ICW (BC-ICW) increased by 14.7 % compared to biomass-added ICW (BM-ICW), which was mainly attributed to the synergistic strengthening of iron scraps and biochar. The denitrification efficiency of BM-ICW improved by 11.6 % compared to ICWs, while its removal capacity declined with biomass consumption. Autotrophic and heterotrophic denitrifiers were enriched in BM-ICW and BC-ICW, especially biochar increased the abundance of electroactive species (Geobacter and Shewanella, etc.). An active iron cycle exhibited in BC-ICW, which can be confirmed by the presence of more liable iron minerals on iron scraps surface, the lowest Fe (III)/Fe (II) ratio (0.51), and the improved proportions of iron cycling genes (feoABC, korB, fhuF, TC.FEV.OM, etc.). The nitrate removal efficiency was positively correlated with the nitrogen, iron metabolism functional genes and the electron transfer capacity (ETC) of carbon materials (P < 0.05), indicating that redox-active carbon materials addition improved the iron scraps bioavailability by promoting electron transfer, thus enhancing the autotrophic nitrogen removal. Our findings provided a green perspective to better understand the redox properties of plant-based carbon materials in ICWs for deep bioremediation in-situ.

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

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

Hydrated electron based photochemical processes for water treatment

Hydrated electron (e_aq^-) based photochemical processes have emerged as a promising technology for contaminant removal in water due to the mild operating conditions. This review aims to provide a comprehensive and up-to-date summary on e_aq^- based photochemical processes for the decomposition of various oxidative contaminants. Specifically, the characteristics of different photo-reductive systems are first elaborated, including the environment required to generate sufficient e_aq^-, the advantages and disadvantages of each system, and the comparison of the degradation efficiency of contaminants induced by e_aq^-. In addition, the identification methods of e_aq^- (e.g., laser flash photolysis, scavenging studies, chemical probes and electron spin resonance techniques) are summarized, and the influences of operating conditions (e.g., solution pH, dissolved oxygen, source chemical concentration and UV type) on the performance of contaminants are also discussed. Considering the complexity of contaminated water, particular attention is paid to the influence of water matrix (e.g., coexisting anions, alkalinity and humic acid). Moreover, the degradation regularities of various contaminants (e.g., perfluorinated compounds, disinfection by-products and nitrate) by e_aq^- are summarized. We finally put forward several research prospects for the decomposition of contaminants by e_aq^- based photochemical processes to promote their practical application in water treatment.

Zero-valent iron coupled plant biomass for enhancing the denitrification performance of ecological floating bed

The ecological floating bed (EFB) coupled with zero-valent iron (ZVI) is proposed to treat low carbon-to-nitrogen ratio water. However, the application of ZVI is limited by low electron transfer efficiency. Coupling ZVI with carbon materials may improve the performance. In this study, the EFB with ZVI coupled plant biomass (IB-EFB) was established to enhance denitrification performance and compared to the EFB with ZVI coupled activated carbon (IC-EFB). The results showed that higher denitrification rate was observed in IB-EFB (68.8%) than that in IC-EFB (54.40%), which attributed to the synergistic effect of ZVI and plant biomass. Plant biomass also promoted the electron transfer of ZVI which enhanced the Fe(II)-mediated denitrification. High-throughput sequencing analysis revealed that IB-EFB enriched iron-related denitrifying bacteria more effectively than IC-EFB, and obtained high abundance of phototrophic Fe(II)-oxidizing bacteria Rhodopseudomonas (19.26%). Thus coupling ZVI with plant biomass has a potential for enhanced nitrogen removal in EFB.

Simply closing the reactor improves the electron efficiency of zerovalent iron toward various metal(loid)s removal

The controlled corrosion of zerovalent iron (ZVI) is crucial for the favorable performance of ZVI toward metal(loid)s removal, and dissolved oxygen (DO) plays an important role in the process of ZVI corrosion. However, few efforts have been made to control the concentration of DO in real practice. In this study, we found that the electron efficiency and the specific removal capacity of ZVI toward the removal of four metal(loid)s were increased by 1.2-9.1 times and 1.2-3.6 times, respectively, by simply closing the reactor, while the removal kinetics of metal(loid)s was slightly influenced. The rate constants obtained under open condition were always greater than those obtained under closed condition, and the removal amounts of metal(loid)s by ZVI at the reaction equilibrium under closed condition were nearly equivalent to those under open condition. Compared with the case under open condition, the consumption-redissolution process of DO was decelerated under closed condition, and the rapid corrosion of ZVI was alleviated subsequently. Although closing the reactor is simple, it does contribute much to the favorable electron efficiency of ZVI toward metal(loid)s sequestration and can be easily adopted in real practice. © 2021 Water Environment Federation PRACTITIONER POINTS: Closing the reactor promoted the selectivity of ZVI towards four metal(loid)s removal. The consumption-redissolution process of DO and corrosion of ZVI were decelerated by closing the reactor. Metal(loid)s were reduced to lower valence by ZVI under closed condition. Effect of DO was different when ZVI was applied to remove different metal(loid)s.

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.

Denitrification using permeable reactive barriers with organic substrate or zero-valent iron fillers: controlling mechanisms, challenges, and future perspectives

Nitrate as a diffusive agricultural contaminant has been causing substantial groundwater quality deterioration worldwide. In situ groundwater remediation techniques using permeable reactive barriers (PRBs) have attracted increasing interest. Particularly, PRBs based on biological denitrification, using the organic substrate as a biostimulator, and chemical nitrate reduction, using zero-valent iron (ZVI) as a reductant, are two major PRB approaches for groundwater denitrification. This review paper analyzed the published studies over the past 10 years (2010-2020) using laboratory, modeling, and field-scale approaches to explore the performance and mechanisms of these two types of PRBs. Important factors affecting the denitrification efficiencies as well as the influential mechanisms were discussed. Several research gaps have been identified and further research needs are discussed in the end.

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

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

Enhancement of microbial redox cycling of iron in zero-valent iron oxidation coupling with deca-brominated diphenyl ether removal

Iron-redox cycling microorganisms are important for understanding the biogeochemical iron and play key roles in zero-valent iron (ZVI) mediated environmental bioremediation. Their influence on ZVI oxidation coupling with organic contaminant removal is of particular interest but is still poorly understood. The objective of this research was to study microbial redox cycles of iron in ZVI oxidation and deca-brominated diphenyl ether (deca-BDE) removal. It was found that iron-oxidizing bacteria (IOB) enhanced ZVI oxidation by using iron as the sole electron donor. Iron-reducing bacteria (IRB) with high activity of Fe (III) reduction, also significantly accelerated rather than inhibited ZVI oxidation. ZVI oxidation activity was increased from 3.42% to 24.28% by IOB and 19.49% by IRB. When deca-BDE was present in the medium, ZVI oxidation activity by IOB and IRB was increased from 2.67% to 48.33% and 64.33%, respectively. However, no co-accelerating effect of IOB and IRB occurred but rather a neutralizing influence on ZVI oxidation was detected with iron-redox cycling bacteria (IORB). ZVI oxidation activity by IORB only increased to 13.14% and 37.0% in the absence and presence of deca-BDE, respectively. Meanwhile, IRB also exhibited the highest removal activity of deca-BDE. Approximately 71.67% of deca-BDE was removed by IRB, compared to 18.91% by IOB and 43.24% by IORB. Deca-BDE significantly influenced the effects of iron-metabolizing microorganisms on ZVI oxidation by altering the composition of microbial communities. Pseudomonas, Paenibacillus, and Sporolactobacillus were the key genera influencing ZVI oxidation and deca-BDE removal. Sporolactobacillus was firstly reported to be able to stimulate both ZVI oxidation and deca-BDE removal. Pseudomonas accelerated ZVI oxidation but had no significant contribution to deca-BDE removal. However, Paenibacillus inhibited both Fe(III) reduction and deca-BDE removal. It is expected that continuous integration of ZVI oxidation and organic contaminant removal can be achieved by regulating the key genera in iron-metabolizing microbial communities.

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.

Disinfection byproduct formation during drinking water treatment and distribution: A review of unintended effects of engineering agents and materials

Unintended effects of engineering agents and materials on the formation of undesirable disinfection byproducts (DBPs) during drinking water treatment and distribution were comprehensively reviewed. Specially, coagulants, biologically active filtration biofilms, activated carbons, nanomaterials, ion-exchange resins, membrane materials in drinking water treatment and piping materials, deposits and biofilms within drinking water distribution systems were discussed, which may serve as DBP precursors, transform DBPs into more toxic species, and/or catalyze the formation of DBPs. Speciation and quantity of DBPs generated rely heavily on the material characteristics, solution chemistry conditions, and operating factors. For example, quaternary ammonium polymer coagulants can increase concentrations of N-nitrosodimethylamine (NDMA) to above the California notification level (10 ng/L). Meanwhile, the application of strong base ion-exchange resins has been associated with the formation of N-nitrosamines and trichloronitromethane up to concentrations of 400 ng/L and 9.0 μg/L, respectively. Organic compounds leaching from membranes and plastic and rubber pipes can generate high NDMA (180-450 ng/L) and chloral hydrate (∼12.4 μg/L) upon downstream disinfection. Activated carbon and membranes preferentially remove organic precursors over bromide, resulting in a higher proportion of brominated DBPs. Copper corrosion products (CCPs) accelerate the decay of disinfectants and increase the formation of halogenated DBPs. Chlorination of high bromide waters containing CCPs can form bromate at concentrations exceeding regulatory limits. Owing to the aforementioned concern for the drinking water quality, the application of these materials and reagents during drinking water treatment and distribution should be based on the removal of pollutants with consideration for balancing DBP formation during disinfection scenarios. Overall, this review highlights situations in which the use of engineering agents and materials in drinking water treatment and distribution needs balance against deleterious impacts on DBP formation.

Intercalation of Nanosized Fe3C in Iron/Carbon To Construct Multifunctional Interface with Reduction, Catalysis, Corrosion Resistance, and Immobilization Capabilities

As a robust reducing system in industrial wastewater treatment, iron/carbon (Fe/C) microelectrolysis suffers from surface passivation and low utilization efficiency. Herein, we introduced Fe₃C into the Fe/C system to develop a core-shell Fe⁰/Fe₃C/C nanorod with a multifunctional interface (Fe₃C/C) providing reduction, catalysis, adsorption, and corrosion resistance. The results proved that the fabricated Fe⁰/Fe₃C/C possesses 5.6 times higher reduction capacity (220 mg/g) for Cr(VI) reduction but a relatively lower Fe leakage (2.7 mg/L) than Fe/C. On the basis of the results of electrochemical characterization (Tafel polarization curves and electrochemical impedance spectroscopy), the corrosion-resistant Fe₃C/C shell can significantly prevent surface passivation of the Fe⁰ core, whereas Fe₃C efficiently catalyzes electron transfer from the inner Fe⁰ to the external carbon surface. Moreover, the reductive species involved in Cr(VI) removal were identified as hydrogen atoms, adsorbed Fe(II) ions, and electrons tunneling from Fe⁰. STEM, XPS, and Mössbauer spectroscopies were further adopted to characterize the interface reaction of Fe⁰/Fe₃C/C during the Cr(VI) removal process. Finally, the reaction mechanism for Cr(VI) reduction over Fe⁰/Fe₃C/C was proposed, and the distribution of active sites was inferred.

Effect of 3-D distribution of ZVI nanoparticles confined in polymeric anion exchanger on EDTA-chelated Cu(II) removal

Millispherical nanocomposites are promising for water decontamination combining the high reactivity of the confined nanoparticles and the excellent hydrodynamic properties of the supporting host. However, the effect of three-dimensional (3-D) distribution of the nanoparticles inside the host on the performance of the nanocomposite was highly dependent on the specific decontamination process. In this study, four D201-ZVI nanocomposites from peripheral to uniform 3-D distributions of nZVI were prepared to evaluate the effect of 3-D distribution of the confined nanoparticles inside the host beads on the removal of EDTA-chelated Cu(II). The performance of Cu(II) removal increased with the 3-D distribution tailoring towards the peripheral region, which was also validated under various solution chemistry conditions in terms of initial pH, DO, and coexisting sulfate. The mechanism underlying the 3-D distribution effect may be ascribed to three perspectives. First, the dissolution of Fe was also higher from the peripherally distributed nZVI nanocomposites compared with the uniform ones. In addition, SEM-EDS analysis revealed the immobilization of Cu occurred at limited depth from the outermost surface of the composite beads, leading to the low spatial utilization of the inner core region. Furthermore, XRD and XPS analyses demonstrated the higher chemical utilization of nZVI for the outer-distributed nanocomposites owing to the shortened pathway for mass transfer. This study shed new light on the design and development of tunable nanocomposites of improved reactivity for water decontamination processes.

Haloacetic acids degradation by an efficient Ferrate/UV process: Byproduct analysis, kinetic study, and application of response surface methodology for modeling and optimization

Haloacetic acids (HAAs) after trihalomethanes are the second main group of chlorination byproducts. In this study, decomposition of the two most common HAAs in drinking water was studied by an advanced oxidation process using a combination of Ferrate [Fe(VI)] and UV irradiation. The decomposition rate was measured, and the byproducts formed during the process and the mass balances were also analyzed. HAAs were quantified by GC-ECD, and the final products including acetate and chloride ions were measured by ion chromatography (IC). A central composite design was used for the experimental design, and the effect of four variables including the initial HAA concentration, pH, Fe(VI) dosage, and contact time were investigated by response surface methodology (RSM). Dichloroacetic acid decomposed more easily than TCAA. Results show that when TCAA and DCAA were studied individually, the degradation rates were 0.0179 and 0.0632 min^-1, respectively. When the HAAs were simultaneously placed in the reactor, the decomposition rates of both TCAA and DCAA decreased dramatically. In this case their decomposition rate constants decreased by 67% and 49%, respectively. In the mixture, the decomposition rate of DCAA was 2.5 times higher than that of TCAA. In summary, Fe(VI)/UV process can be used as a promising treatment option for the decomposition of recalcitrant organic pollutants such as HAAs, and RSM can be used for modeling and optimizing the process.

Mechanism and kinetics of halogenated compound removal by metallic iron: Transport in solution, diffusion and reduction within corrosion films

A detailed kinetic model comprised of mass transport (k_tra), pore diffusion (k_dif), adsorption and reduction reaction (k_rea), was developed to quantitatively evaluate the effect of corrosion films on the removal rate (k_obs) of halogenated compounds by metallic iron. Different corrosion conditions were controlled by adjusting the iron aging time (0 or 1 yr) and dissolved oxygen concentration (0-7.09 mg/L DO). The k_obs values for bromate, mono-, di- and tri-chloroacetic acids (BrO₃^-, MCAA, DCAA and TCAA) were 0.41-7.06, 0-0.16, 0.01-0.53, 0.10-0.73 h^-1, with k_tra values at 13.32, 12.12, 11.04 and 10.20 h^-1, k_dif values at 0.42-5.82, 0.36-5.04, 0.30-4.50, 0.30-3.90 h^-1, and k_rea values at 14.94-421.18, 0-0.19, 0.01-1.30, 0.10-3.98 h^-1, respectively. The variation of k_obs value with reaction conditions depended on the reactant species, while those of k_tra, k_dif and k_rea values were irrelevant to the species. The effects of corrosion films on k_dif and k_rea values were responsible for the variation of k_obs value for halogenated compounds. For a mass-transfer-limited halogenated compound such as BrO₃^-, an often-neglected k_dif value primarily determined its k_obs value when pore diffusion was the rate-limiting step of its removal. In addition, the value of k_dif might influence product composition during a consecutive dechlorination, such as for TCAA and DCAA. For a reaction-controlled compound such as MCAA, an increased k_rea value was achieved under low oxic conditions, which was favorable to improve its k_obs value. The proposed model has a potential in predicting the removal rate of halogenated compounds by metallic iron under various conditions.

Enhanced removal of EDTA-chelated Cu(II) by polymeric anion-exchanger supported nanoscale zero-valent iron

In this study, a polymeric anion exchanger (D201) was utilized as the support for nanoscale zero-valent iron (NZVI), and the resultant nanocomposite (D201-ZVI) was employed to remove EDTA-chelated Cu(II) from water. The removal of EDTA-chelated Cu(II) was significantly enhanced by D201-ZVI in comparison with NZVI over a wide pH range from 5 to 9. Most of the removed Cu (97.2%) was immobilized inside the D201-ZVI beads, implying the enhanced permeation of CuEDTA^2- by the fixated quaternary ammonium groups of the host D201. HPLC analysis revealed that the EDTA-chelated Cu(II) was gradually replaced by Fe(III) originated from Fe⁰ oxidation. Then, the released Cu(II) was in situ removed via adsorption/precipitation, or further reduced into Cu⁰, as quantified by XPS spectra. The higher removal of EDTA-chelated Cu(II) by D201-ZVI than NZVI was mainly ascribed to the enhanced permeation of the host D201 as well as to the better dispersion and higher reactivity of the confined ZVI nanoparticles. Through the combination of periodic regeneration and complete regeneration, D201-ZVI could be sustainably employed for EDTA-chelated Cu(II) removal. Also, D201-ZVI exhibited great potential for practical application in the fixed-bed column operation. Therefore, the D201-ZVI nanocomposite was promising in highly efficient removal of EDTA-chelated Cu(II) from water.

The influences of iron characteristics, operating conditions and solution chemistry on contaminants removal by zero-valent iron: A review

For successful application of a zero-valent iron (ZVI) system, of particular interest is the performance of ZVI under various conditions. The current review comprehensively summarizes the potential effects of the major influencing factors, such as iron intrinsic characteristics (e.g., surface area, iron impurities and oxide films), operating conditions (e.g., pH, dissolved oxygen, iron dosage, iron pretreatment, mixing conditions and temperature) and solution chemistry (e.g., anions, cations and natural organic matter) on the performance of ZVI reported in literature. It was demonstrated that all of the factors could exert significant effects on the ZVI performance toward contaminants removal, negatively or positively. Depending on the removal mechanisms of the respective contaminants and other environmental conditions, an individual variable may exhibit different effects. On the other hand, many of these influences have not been well understood or cannot be individually isolated in experimental or natural systems. Thus, more research is required in order to elucidate the exact roles and mechanisms of each factor in affecting the performance of ZVI. Furthermore, based on these understandings, future research may attempt to establish some feasible strategies to minimize the deteriorating effects and utilize the positive effects so as to improve the performance of ZVI.

Sequestration of chelated copper by structural Fe(II): Reductive decomplexation and transformation of Cu(II)-EDTA

Chelated coppers, such as Cu(II)-EDTA, are characteristically refractory and difficult to break down because of their high stability and solubility. Cu(II)-EDTA sequestration by structural Fe(II) (Fe(II)) was investigated intensively in this study. Up to 101.21mgCu(II)/gFe(II) was obtained by Fe(II) in chelated copper sequestration under near neutral pH condition (pH 7.70). The mechanism of Cu(II)-EDTA sequestration by Fe(II) was concluded as follows: 3Cu(II)-EDTA+7Fe(II)+9H2O → Cu(0)↓+ Cu2O↓(the major product)+2Fe2O3·H2O↓+3Fe(II)-EDTA +14H(+) Novel results strongly indicate that Cu(II) reductive transformation induced by surface Fe(II) was mainly responsible for chelated copper sequestration. Cu(0) generation was initially facilitated, and subsequent reduction of Cu(II) into Cu(I) was closely combined with the gradual increase of ORP (Oxidation-Reduction Potential). Cu-containing products were inherently stable, but Cu2O would be reoxidized to Cu(II) with extra-aeration, resulting in the release of copper, which was beneficial to Cu reclamation. Concentration diminution of Cu(II)-EDTA within the electric double layer and competitive adsorption were responsible for the negative effects of Ca(2+), Mg(2+). By generating vivianite, PO4(3-) was found to decrease surface Fe(II) content. This study is among the first ones to identify the indispensible role of reductive decomplexation in chelated copper sequestration. Given the high feasibility and reactivity, Fe(II) may provide a potential alternative in chelated metals pollution controlling.

Aqueous nickel sequestration and release during structural Fe(ii) hydroxide remediation: the roles of coprecipitation, reduction and substitution

SFH has a strong potential for reducing aqueous Ni²⁺ and the release of precipitated Ni(ii) into solution could be controlled.

Role of dissolved oxygen in metal(loid) removal by zerovalent iron at different pH: its dependence on the removal mechanisms

The role of oxygen in metal(loid)s removal by zerovalent iron (ZVI) is strongly dependent on the removal mechanisms of metal(loid)s at different pH.

The limitations of applying zero-valent iron technology in contaminants sequestration and the corresponding countermeasures: the development in zero-valent iron technology in the last two decades (1994-2014)

Over the past 20 years, zero-valent iron (ZVI) has been extensively applied for the remediation/treatment of groundwater and wastewater contaminated with various organic and inorganic pollutants. Based on the intrinsic properties of ZVI and the reactions that occur in the process of contaminants sequestration by ZVI, this review summarizes the limitations of ZVI technology and the countermeasures developed in the past two decades (1994-2014). The major limitations of ZVI include low reactivity due to its intrinsic passive layer, narrow working pH, reactivity loss with time due to the precipitation of metal hydroxides and metal carbonates, low selectivity for the target contaminant especially under oxic conditions, limited efficacy for treatment of some refractory contaminants and passivity of ZVI arising from certain contaminants. The countermeasures can be divided into seven categories: pretreatment of pristine ZVI to remove passive layer, fabrication of nano-sized ZVI to increase the surface area, synthesis of ZVI-based bimetals taking advantage of the catalytic ability of the noble metal, employing physical methods to enhance the performance of ZVI, coupling ZVI with other adsorptive materials and chemically enhanced ZVI technology, as well as methods to recover the reactivity of aged ZVI. The key to improving the rate of contaminants removal by ZVI and broadening the applicable pH range is to enhance ZVI corrosion and to enhance the mass transfer of the reactants including oxygen and H(+) to the ZVI surface. The characteristics of the ideal technology are proposed and the future research needs for ZVI technology are suggested accordingly.