Zero-valent Iron Surface Research Articles

Synergistic effects of acclimated bacterial community and zero valent iron for removing 1,1,1‐trichloroethane and 1,4‐dioxane co‐contaminants in groundwater

AbstractBACKGROUNDTtrichloroethane acid (TCA) and 1,4‐dioxane have caused significantly combined pollution in soil and groundwater. By establishing the zero valent iron (ZVI) and the acclimated bacterial community coupled system, this study aimed to improve the treatment of 1,1,1‐trichloroethane (TCA) and 1,4‐dioxaneco‐contamination in groundwater and to investigate the synergistic mechanism of ZVI and the acclimated bacterial community coupled system for transformation of TCA and 1,4‐dioxane.RESULTSThe results showed that the ZVI effectively strengthened the acclimated bacterial community to degrade 70 mg L‐1 TCA and 1,4‐dioxane with enhancement factors (Q) of 1.20 and 2.20. The removal percentages reached 97.8% and 92.5%, respectively. The degradation rate was consistent with the first‐order reaction kinetics. The reductive dechlorination byproducts of TCA were 1,1‐dichloroethane (DCA) and chloroethane (CA), while 1,4‐dioxane was mainly converted to CO2 in the system. The corrosion products on the ZVI surface were mainly FeCO3 and FeS. Phylogenetic analysis revealed that the dominant bacteria were dechlorinating bacteria (DhB), sulfate reducing bacteria (SRB) and iron‐reducing bacteria (IRB) in the microbial community.CONCLUSIONZVI coupled with an acclimated bacterial community is a potential technique to apply to deal with TCA and 1,4‐dioxane co‐contaminants in groundwater. © 2018 Society of Chemical Industry

A phenomenological reaction kinetic model for Cu removal from aqueous solutions by zero-valent iron (ZVI)

Zero-valent iron (ZVI) being an inexpensive and eco-friendly catalyst has drawn great attention in removal of heavy metals from wastewaters. However, quantitative understandings of ZVI processes are significantly deficient. To compensate for the lack of quantitative analyses of removal of heavy metals by ZVI, a phenomenological reaction kinetic model was newly developed for removal of Cu chosen as a typical heavy metal from acidic aqueous solutions by ZVI. The novel kinetic model is based on the adsorption of Cu2+ and H+ onto ZVI surface and subsequent Cu2+ reduction on ZVI surface and Fe2+ elution from ZVI. Batch experiments were conducted to elucidate effects of pH and Cu loading on Cu removal by ZVI in acidic aqueous solutions and to validate the proposed phenomenological reaction kinetic model. The quick and complete removals of 1.57 mM Cu were established in the rage of pH 2–5. Although the maximum Cu removal rate was obtained at pH 4, effects of pH were insignificant. In the range of Cu loading from 0.393 to 4.72 mM, almost complete Cu removals were obtained at pH 4 within 35 min. The changes in concentrations of Cu2+, Fe2+, H+ and dissolved oxygen were strongly linked with each other. They could be successfully simulated by the proposed model with the average correlation coefficient of 0.979. The capability of the phenomenological reaction kinetic model for dynamic simulation of Cu removal by ZVI under acidic conditions was confirmed.

Wastewater degradation by iron/copper nanoparticles and the microorganism growth rate

Nowadays, trends in wastewater treatment by zero-valent iron (ZVI) were turned to use bimetallic NZVI particles by planting another metal onto the ZVI surface to increase its reactivity. Nano size zero-valent iron/copper (NZVI/Cu0) bimetallic particles were synthesized in order to examine its toxicity effects on the wastewater microbial life, kinetics of phosphorus, ammonia stripping and the reduction of chemical oxygen demand (COD). Various concentrations of NZVI/Cu0 and operation conditions both aerobic and anaerobic were investigated and compared with pure NZVI experiment. The results showed that addition 10mg/L of NZVI/Cu0 significantly increased the numbers of bacteria colonies under anaerobic condition, conversely it inhibited bacteria activity with the presence of oxygen. Furthermore, the impact of nanoparticles on ammonia stripping and phosphorus removal was also linked to the emitted iron ions electrons. It was found that dosing high concentration of bimetallic NZVI/Cu0 has a negative effect on ammonia stripping regardless of the aeration condition. In comparison to control, dosing only 10mg/L NZVI/Cu0, the phosphorus removal increased sharply both under aerobic and anaerobic conditions, these outcomes were obtained as a result of complete dissolution of bimetallic nanoparticles which formed copper-iron oxides components that are attributed to increasing the phosphorus adsorption rate.

Degradation of organic compounds during the corrosion of ZVI by hydrogen peroxide at neutral pH: Kinetics, mechanisms and effect of corrosion promoting and inhibiting ions

The corrosion of zero valent iron (ZVI) by hydrogen peroxide (H2O2) generates hydroxyl (⋅OH) and other radical oxygen species (ROS) that degrade organic materials. To better understand the factors that govern the ROS formation during the H2O2-induced corrosion, we investigated the degradation of an organic probe compound (acesulfame (ACE)) in slurries of ZVI powder in unbuffered laboratory water at pH 6.5 ± 0.5. Chloride ions accelerated the corrosion of ZVI by H2O2 and the formation ROS and, therefore, the degradation of organic materials. Conversely, slowing corrosion by phosphate buffer inhibited ROS formation and the degradation of organic compounds. The rate of H2O2 decomposition was correlated with the liberation of Fe2+(aq) and the ACE degradation rate. The kinetics of H2O2 decomposition was pseudo-first-order and zero-order at low (<0.04 mM/mg) and high [H2O2]/[ZVI] initial ratios, respectively, and was consistent with Langmuir kinetics. The H2O2 decomposition rate was proportional to the ZVI reactive surface area (SA) and nearly independent of the extent of ZVI oxidation, the presence of a Fe2+(aq) chelating agent, and ⋅OH quenchers (methanol and tert-butanol). Kinetic data suggest a mechanism involving rapid cathodic reduction of H2O2 at the metallic ZVI surface which causes the liberation of Fe2+(aq) that generate ⋅OH via the homogeneous Fenton reaction. The stoichiometric efficiency (SE) of organics degradation ranged from 0.0008% to 0.014% and increased with decreasing H2O2 decomposition rate.

Effect of carboxylic acids on the properties of zerovalent iron toward adsorption and degradation of trichloroethylene

Zerovalent iron (ZVI) based technology has been applied to remediate contaminated groundwater and has been paid great attention as an economic alternative. But it is still remains highly challenging to remove chlorinated pollutants such as trichloroethylene (TCE) with ZVI. Low molecular weight carboxylic ligands (formic acid (FA), oxalic acid (OA), and citric acid (CA)) were chosen to study the influence on the performance of ZVI in groundwater, including the morphology of Fe surface and the Fe dissolution. The removal rate of TCE with ZVI in the presence of 30 mM carboxylic groups followed an order of FA > OA > pure water ≅ CA. FA provides protons to promote the surface corrosion and generated more magnetite on the ZVI surface, which was further responsible for a high adsorption of TCE. With the strong complexing ability of OA and CA, passive layers could form dissoluble complexes via a ligand-promoted dissolution process. However, high concentration of OA resulted in Fe oxalate reprecipitated back onto the ZVI surface then inhibited the reactivity of ZVI. The Fe-ligand complexes also have ability to transform TCE depending on their redox properties. It is expected that effectiveness of carboxylic ligands on the ZVI: those low molecular weight carboxylic ligands in groundwater and soil may enhance the reaction efficiency of ZVI by altering the surface characteristics of ZVI. Therefore, the carboxylic ligands could increase the reactivity and the longevity of ZVI.

Hydroxyl radical generation by zero-valent iron/Cu (ZVI/Cu) bimetallic catalyst in wastewater treatment: Heterogeneous Fenton/Fenton-like reactions by Fenton reagents formed in-situ under oxic conditions

Zero-valent iron (ZVI) has been recognized as a heterogeneous Fenton/Fenton-like catalyst generating hydroxyl radical (OH radical). ZVI bimetallic catalysts modified by the deposition of transition metals on the ZVI surface have been proposed as alternatives to enhance the reactivity of ZVI catalyst. However, it is not clear from the literature whether OH radical is actively generated via the Fenton/Fenton-like reactions in ZVI bimetallic catalyst systems. To quantify the generation of OH radical by ZVI/Cu bimetallic catalysts, the amount of generated OH radical was measured under the oxic condition. Although the maximum amount of generated OH radical by ZVI/Cu bimetallic catalysts was obtained at pH 3 being an optimal pH for the Fenton reaction, OH radical generation by ZVI/Cu bimetallic catalysts was considerably restricted by the inadequate in-situ generation of H2O2 and the generation of OH radical was inhibited due to Cu deposition on ZVI surface. While the deposition of Cu on the ZVI surface enhanced the removal of Orange II via the facilitation of reductive degradation and the increase in adsorption capability on the surface of ZVI/Cu bimetallic catalysts, it could not stimulate the OH radical generation or the oxidative reactivity of heterogeneous Fenton/Fenton-like catalyst. The present study confirmed that the passivation of ZVI surface accelerated by the deposition of Cu on ZVI surface inhibited the OH radical generation. The kinetic models for OH radical generation by ZVI/Cu bimetallic catalysts were developed by considering the linkage of OH radical generation with eluted Fe and Cu ions. The model predictions could simulate the experimental results reasonably.

Reusability of zero-valent iron particles for zinc ion separation

Abstract Zero-valent iron (ZVI) is known to be effective in separating heavy metals from aqueous solution. However, used ZVI particles are usually discarded in heavy metal-containing sludge. Therefore, an appropriate technique for the reactivation of used ZVI for reuse is discussed in this work. A series of zinc desorption tests from zinc-adsorbed ZVI revealed that zinc desorption required both acidic conditions to dissolve zinc-containing iron hydroxide layers on ZVI and reductants to remove insoluble iron oxide layers from the ZVI surface. Organic acid reductants, such as citric acid, satisfy both these requirements. The ZVI particles that were reactivated by the desorption of zinc using 10.0-mM citric acid were applied repeatedly for zinc removal. Reusing the ZVI particles four times did not affect their zinc removal performance relative to that of virgin ZVI. Furthermore, the used 10.0-mM citric acid could be reused to desorb zinc from the used ZVI particles after separating the zinc and iron ions by hydroxide precipitation at pH 12 and re-acidifying the solution to pH 2.5 by the addition of sulfuric acid. Thus, both the ZVI particles and the 10.0-mM citric acid cleansing liquid were found to be reusable for zinc separation from aqueous solution.

Application of zero valent iron coupling with biological process for wastewater treatment: a review

As a promising technology, zero valent iron (ZVI) coupling with microorganisms has attracted extensive attention for contaminants removal from wastewater. The current paper provides a comprehensive review on recent developments in: (1) the chemical behavior of ZVI and potential mechanisms of integrated bio-ZVI technology in contaminants removal; (2) synergistic effects of bio-ZVI towards various common environmental pollutants in wastewater, including inorganic oxyanions, organic compounds, heavy metals and dyes; (3) promotion effects of ZVI on the biologically anaerobic digestion of waste sludge; (4) operating factors affecting the effectiveness of bio-ZVI process; (5) measures developed to enhance the long-term performance of the bio-ZVI technology. The chemical behavior and stimulating roles of ZVI playing in the growth and diversity of microorganisms is reasonable for the synergistic effects of the combined system. It was demonstrated that combined bio-ZVI system showed appreciable removal efficiencies for several types of contaminants. Additionally, the formation of passive layer on the ZVI surface can be avoided by the means of electrochemical and microbial method. Lastly, this review highlighted the research gaps to improve the sustainability of this technology. Based on these understandings, further efforts should be made to expand the applications of this combined technology and establish some feasible strategies to provide opportunities for the engineering applications.

Removal of antibiotic sulfamethoxazole by zero-valent iron under oxic and anoxic conditions: Removal mechanisms in acidic, neutral and alkaline solutions

Removal of antibiotic sulfamethoxazole (SMX) by zero-valent iron (ZVI) was examined in the range of pH from 3.0 to 11.0 under oxic and anoxic conditions to clarify mechanisms of SMX removal in acidic, neutral and alkaline solutions. SMX removal was affected by solution pH and related to the speciation of SMX. Under the oxic condition, the maximums of SMX removal efficiency and rate were obtained at pH 3.0. The SMX removal efficiency decreased from 100 to 32% with increasing pH in the acidic solutions (3 ≦ pH ≦ 5) and increased to 88% in neutral and moderately alkaline solutions (6 ≦ pH ≦ 10). In highly alkaline solution (pH = 11), the SMX removal was significantly suppressed due to the formation of passive layer on ZVI surface. The removal rate of SMX under the oxic condition significantly declined with increasing pH. Under the anoxic condition, SMX removal was completed within 300 min in the acidic solutions and remained to less than 70% after 300 min in neutral and moderately alkaline solutions. For pH ≧ 10, no SMX removal practically occurred. The removal rate of SMX under the anoxic condition approximately remained constant in the acidic solution and largely decreased in neutral and moderately alkaline solutions. SMX removal by ZVI was found to be dominated by the reductive degradation and adsorption under both the oxic and anoxic conditions. It was concluded that ZVI has the potential for effective removal of antibiotic SMX under the oxic and anoxic conditions. A kinetic model could reasonably simulate the dynamic profiles of SMX removal.

Coupled Effect of Ferrous Ion and Oxygen on the Electron Selectivity of Zerovalent Iron for Selenate Sequestration

Although the electron selectivity (ES) of zerovalent iron (ZVI) for target contaminant and its utilization ratio (UR) decide the removal capacity of ZVI, little effort has been made to improve them. Taking selenate [Se(VI)] as a target contaminant, this study investigated the coupled influence of aeration gas and Fe(II) on the ES and UR of ZVI. Oxygen was necessary for effective removal of Se(VI) by ZVI without Fe(II) addition. Due to the application of 1.0 mM Fe(II), the ES of ZVI was increased from 3.2-3.6% to 6.2-6.8% and the UR of ZVI was improved by 5.0-19.4% under aerobic conditions, which resulted in a 100-180% increase in the Se(VI) removal capacity by ZVI. Se(VI) reduction by Fe0 was a heterogeneous redox reaction, and the enrichment of Se(VI) on ZVI surface was the first step of electron transfer from Fe0 core to Se(VI). Oxygen promoted the generation of iron (hydr)oxides, which facilitated the enrichment of Se(VI) on the ZVI particle surface. Therefore, the high oxygen fraction (25-50%) in the purging gas resulted in only a slight decrease in the ES of ZVI. Fe(II) addition resulted in a pH drop and promoted the generation of lepidocrocite and magnetite, which benefited Se(VI) adsorption and the following electron transfer from underlying Fe0 to surface-located Se(VI).

Ligand effects on nitrate reduction by zero-valent iron: Role of surface complexation

Surface passivation is a key limiting factor in the application of zero-valent iron (ZVI) for water remediation. Addition of ligands is a useful approach to overcome this issue. In this work, a small amount of acetylacetone (AA) (0.5 mM) was found highly efficient to enhance the reduction of nitrate by ZVI at near neutral conditions (pH 6.0) with the formation of considerable black coating on ZVI. At an initial nitrate concentration of 20 mg N/L, the pseudo first-order reduction rate constant of nitrate in the ZVI-AA-NO3− system was 0.0991 h−1, which was 52 times higher than that in the ZVI-NO3− system. Under otherwise identical conditions, the other five ligands, including EDTA, formate, acetate, oxalate, and phosphate, had negligible effects. Based on the pKa values of these ligands and the final species of iron, the ligand effects on nitrate reduction by ZVI were summarized from three aspects: (1) the ability to offer potentially dissociable protons from the ligands; (2) the complexation ability to eliminate iron (hydr)oxide precipitates from the surface of ZVI; and (3) the ability to lower down the redox potentials of iron species. The good performance of AA in these three aspects makes it advantage over the other ligands. A cycle test up to six runs demonstrates that AA could continuously take effect in the ZVI system. The results here point out the potential of AA as an effective ligand in ZVI system for enhanced contaminant transformation.

Removal of anionic surfactant sodium dodecyl benzene sulfonate (SDBS) from wastewaters by zero-valent iron (ZVI): predominant removal mechanism for effective SDBS removal.

Mechanisms for removal of anionic surfactant sodium dodecyl benzene sulfonate (SDBS) in wastewaters by zero-valent iron (ZVI) were systematically examined. The contributions of four removal mechanisms, i.e., reductive degradation, oxidative degradation, adsorption, and precipitation, changed significantly with solution pH were quantified and the effective removal of SDBS by ZVI was found to be attributed to the adsorption capability of iron oxides/hydroxides on ZVI surface at nearly neutral pH instead of the degradation at acidic condition. The fastest SDBS removal rate and the maximum TOC (total organic carbon) removal efficiency were obtained at pH6.0. The maximum TOC removal at pH6.0 was 77.8%, and the contributions of degradation, precipitation, and adsorption to TOC removal were 4.6, 14.9, and 58.3%, respectively. At pH3.0, which is an optimal pH for oxidative degradation by the Fenton reaction, the TOC removal was only 9.8% and the contributions of degradation, precipitation, and adsorption to TOC removal were 2.3, 4.6, and 2.9%, respectively. The electrostatic attraction between dodecyl benzene sulfate anion and the iron oxide/hydroxide layer controlled the TOC removal of SDBS. The kinetic model based on the Langmuir-Hinshelwood/Eley-Rideal approach could successfully describe the experimental results for SDBS removal by ZVI with the averaged correlation coefficient of 0.994. ZVI was found to be an efficient material toward the removal of anionic surfactant at nearly neutral pH under the oxic condition.

Phosphate removal from aqueous solutions using zero valent iron (ZVI): Influence of solution composition and ZVI aging

This study investigates phosphate removal from water by ZVI and its oxidation products. The effects of ZVI aging, solution ionic strength, and co-existing anions, on the removal efficiency of phosphate were evaluated. X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, and scanning electron microscopy (SEM) were used to investigate the ZVI oxidation products and to delineate the phosphate removal mechanisms. ZVI showed the highest phosphate removal efficiency after 8h aging time (35 mgP/gZVI). Colloidal ZVI oxidation products showed very high phosphate removal rates especially the newly oxidized iron products (aged for 2h) with a capacity of 27.8 mgP/gFe. This capacity decreased to 2 mgP/gFe as aging time increased to 6 days. The characterization of the ZVI oxidation products revealed a mixture of crystalline lepidocrocite and goethite at the surface of ZVI however broader peaks were obtained in the supernatant fraction with aging time of 2h indicating the presence of poorly crystalline iron products. Nitrate showed no effects on the removal efficiency, whereas ionic strength effects were critical. Phosphate maximum uptake rate (Qe) increased from 8.7 to 15.6 mgP/gFe as ionic strength increased from 0.001M to 0.5M.

Zero-valent iron treatment of dark brown colored coffee effluent: Contributions of a core-shell structure to pollutant removals

The decolorization and total organic carbon (TOC) removal of dark brown colored coffee effluent by zero-valent iron (ZVI) have been systematically examined with solution pH of 3.0, 4.0, 6.0 and 8.0 under oxic and anoxic conditions. The optimal decolorization and TOC removal were obtained at pH 8.0 with oxic condition. The maximum efficiencies of decolorization and TOC removal were 92.6 and 60.2%, respectively. ZVI presented potential properties for pollutant removal at nearly neutral pH because of its core-shell structure in which shell or iron oxide/hydroxide layer on ZVI surface dominated the decolorization and TOC removal of coffee effluent. To elucidate the contribution of the core-shell structure to removals of color and TOC at the optimal condition, the characterization of ZVI surface by scanning electron microscopy (SEM) with an energy dispersive X-ray spectroscope (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) was conducted. It was confirmed that the core-shell structure was formed and the shell on ZVI particulate surface and the precipitates formed during the course of ZVI treatment consisted of iron oxides and hydroxides. They were significantly responsible for decolorization and TOC removal of coffee effluent via adsorption to shell on ZVI surface and inclusion into the precipitates rather than the oxidative degradation by OH radicals and the reduction by emitted electrons. The presence of dissolved oxygen (DO) enhanced the formation of the core-shell structure and as a result improved the efficiency of ZVI treatment for the removal of colored components in coffee effluents. ZVI was found to be an efficient material toward the treatment of coffee effluents.

Performance enhancement of zero valent iron based systems using depassivators: Optimization and kinetic mechanisms

The long-term ability of Zero-Valent Iron (ZVI) in contaminant removal relies on the effectiveness of iron to serve as electron donor, which makes it a versatile remediation material. However, the formation of oxide and hydroxide layers results in passive layer on ZVI surface during contaminant removal hinders its reactivity. The focus of this research was to evaluate the performance of corrosive agents such as acetic acid (HAc), aluminium sulphate (Alum) and potassium chloride (KCl) as depassivators to overcome passivation for sustainability and longevity. Batch experiments using seven combinations of the above chemicals were conducted to optimize the dosage of depassivators based on passive layer removal. The influence of depassivators in catalytic activity of ZVI in removing Cr6+ was evaluated. The passive layer on ZVI particles was characterized using Scanning Electron Microscopy (SEM) and confirmed by Energy-Dispersive X-ray spectroscopy (EDAX) analysis. The major mechanisms in passive layer removal was found to be H+ ion embrittlement followed by uniform depassivation when [HAc] was used and pitting corrosion when [Alum] and [KCl]were used. All the seven sets of chemicals enabled depassivation, but considering the criteria of maximum depassivation, catalytic activity and long term reactivity the depassivation treatments were effective in order as [HAc-Alum] > [HAc-Alum-KCl] >[HAc] > [Alum] > [HAc-KCl] > [KCl] > [Alum-KCl]. The kinetic rate of ZVI using [HAc-Alum] and [Alum] was relatively unchanged over the pH range of 4–10, made it suitable for ex-situ remediation. This insignificant influence of initial pH in catalytic activity of ZVI along with the improvement in longevity and sustainability makes it suitable for effective water treatment applications. The present work has successfully demonstrated that chemical depassivation can restore considerable reactivity of ZVI in the existing permeable reactive barriers.

Hydroxyl radical generation linked with iron dissolution and dissolved oxygen consumption in zero-valent iron wastewater treatment process

In zero-valent iron (ZVI) wastewater treatments, organic pollutants are degraded by hydroxyl radical (OH radical) generated via the Fenton reaction besides by the reductive reaction. The generation of OH radical is initiated by the dissolution of ZVI. The strong linkage of OH radical generation by ZVI with eluted ferrous ion and dissolved oxygen (DO) through the iron dissolution and the formation of passive iron oxide/hydroxide layer on the ZVI surface was found. While OH radicals were gradually generated by microscale ZVI (mZVI) until the termination of iron dissolution, the OH radical generation by nanoscale ZVI (nZVI) was very quick at the initial phase, subsequently slowed down and terminated. Although the rate of OH radical generation with nZVI was much faster than that with mZVI, the amount of generated OH radical with nZVI was less than mZVI under the same iron dosage conditions. With increasing ZVI dosage and controlled solution pH value, the amount of generated OH radical for both mZVI and nZVI increased and decreased, respectively. For nZVI under the oxic condition, the quick depletion of DO at the initial phase accompanied with the sequential recovery to the saturation concentration was found. A reaction kinetic model was developed to quantify a linkage of OH radical generation with the iron dissolution and the formation of passive iron oxide/hydroxide layer on the ZVI surface. The linkage could be reasonably simulated by the proposed kinetic model with the correlation coefficient of >0.828.

An insight in magnetic field enhanced zero-valent iron/H2O2 Fenton-like systems: Critical role and evolution of the pristine iron oxides layer.

This study demonstrated the synergistic degradation of 4-chlorophenol (4-CP) achieved in a magnetic field (MF) enhanced zero-valent iron (ZVI)/H2O2 Fenton-like (FL) system and revealed an interesting correlative dependence relationship between MF and the pristine iron oxides layer (FexOy) on ZVI particles. First, a comparative investigation between the FL and MF-FL systems was conducted under different experimental conditions. The MF-FL system could suppress the duration of initial lag degradation phase one order of magnitude in addition of the significant enhancement in overall 4-CP degradation. Monitoring of intermediates/products indicated that MF would just accelerate the Fenton reactions to produce hydroxyl radical more rapidly. Evolutions of simultaneously released dissolved iron species suggested that MF would not only improve mass-transfer of the initial heterogeneous reactions, but also modify the pristine ZVI surface. Characterizations of the specific prepared ZVI samples evidenced that MF would induce a special evolution mechanism of the ZVI particles surface depending on the existence of FexOy layer. It comprised of an initial rapid point dissolution of FexOy and a following pitting corrosion of the exposed Fe0 reactive sites, finally leading to appearance of a particular rugged surface topography with numerous adjacent Fe0 pits and FexOy tubercles.

Electron shuttling catalytic effect of mellitic acid in zero-valent iron induced oxidative degradation

The enhanced oxidation capacity of zero-valent iron (ZVI) using mellitic acid (MA) as an electron shuttle catalyst was investigated using 4-chlorophenol (4-CP) as a model pollutant. In the presence of MA, enhanced electron transfer from ZVI surface to molecular oxygen resulted in higher production of hydrogen peroxide (H2O2), which subsequently increased the effective concentration of hydroxyl radical (HO) generated through the Fenton-type reaction. The possible role of MA as an efficient electron shuttle was supported by cyclic voltammetric estimation of MA reduction potential (E0=−0.184VNHE) and corroborated with photocurrent measurements in the ZVI suspension. Control experiments using Fe(II) ions instead of ZVI demonstrated that the presence of MA in the Fe(II)/H2O2 homogeneous system had no significant effect on the 4-CP oxidation efficiency. This indicates that the formation of a Fe(II)-MA complex does not contribute to the 4-CP oxidation pathway. The primary role of MA in the ZVI/O2 system seems to mediate the electron transfer from the ZVI surface to dioxygen. It implies that organic species containing multiple carboxylic ligands (species like MA or its structural analogues) may function as an electron shuttle in the ZVI/O2 catalytic system.

Kinetic study for phenol degradation by ZVI-assisted Fenton reaction and related iron corrosion investigated by X-ray absorption spectroscopy.

In this study, we investigated phenol degradation via zero-valent iron (ZVI)-assisted Fenton reaction through kinetic and spectroscopic analysis. In batch experiments, 100 mg/L of phenol was completely degraded, and 75% of TOC was removed within 3 min under an optimal hydrogen peroxide (H2O2) concentration (50 mM) via the Fenton reaction. In the absence of H2O2, oxygen (O2) was dissolved into the solution and produced H2O2, which resulted in phenol degradation. However, phenol removal efficiency was not very high compared to external H2O2 input. The Fenton reaction rapidly occurred at the surface of ZVI, and then phenol mobility from the solution to the ZVI surface was the rate determining step of the whole reaction. The pseudo-second order adsorption kinetic model well describes phenol removal, and its rate increased according to the H2O2 concentration. X-ray absorption spectroscopic analysis revealed that iron oxide (Fe-O bonding) was formed on ZVI with [H2O2] > 50 mM. A high concentration of H2O2 led to rapid degradation of phenol and caused corrosion on the ZVI surface, indicating that Fe(2+) ions were rapidly oxidized to Fe(3+) ions due to the Fenton reaction and that Fe(3+) was precipitated as iron oxide on the ZVI surface. However, ZVI did not show corroded characteristics in the absence of H2O2 due to the insufficient ZVI-assisted Fenton reaction and oxidation of Fe(2+) to Fe(3+).

Electrochemically enhanced reduction of trichloroethene by passivated zero-valent iron

Zero-valent iron (ZVI) is commonly used for the in situ remediation of groundwater aquifers contaminated with trichloroethylene (TCE); however, ZVI is susceptible to passivation over time, which can significantly reduce its treatment efficiency. Recent studies have suggested that electrically-induced reduction (EIR) or "E-Redox" technology, which applies a low-level direct current (DC), can restore the reactivity of passivated ZVI in situ. In this study, a continuous-flow column reactor was used to assess the effects of low-voltage DC on the abiotic reduction of TCE in groundwater and on the performance of passivated ZVI. In experiments with partially passivated ZVI, the application of DC increased the rate and the extent of TCE reduction; both were correlated with the level of applied voltage (0–12V). The results provide evidence that EIR may partially recover the reduction potential of passivated ZVI and shows promise as a technique to extend the longevity of zero-valent iron. Several mechanisms, including the reduction of ferric iron minerals to ferrous iron at the ZVI surface, likely results in the enhanced TCE reduction.