Grantee Research Project Results
2000 Progress Report: Iron Mediated Reductive Transformation of Chlorinated Organic Compounds and Oxidized Metal Species in Groundwater Environments
EPA Grant Number: R825223Title: Iron Mediated Reductive Transformation of Chlorinated Organic Compounds and Oxidized Metal Species in Groundwater Environments
Investigators: Farrell, James
Institution: University of Arizona
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 2001
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $500,000
RFA: Exploratory Research - Early Career Awards (1996) RFA Text | Recipients Lists
Research Category: Early Career Awards
Objective:
The objectives of this research are to: (1) investigate the mechanisms involved in reductive dehalogenation of chlorinated organic compounds by zerovalent iron; (2) assess the long-term effectiveness of zerovalent iron for use in permeable barrier groundwater treatment systems; (3) determine the effectiveness of direct cathodic reduction for removing chlorinated organic compounds from dilute aqueous solutions; and (4) determine the mechanisms involved in removing redox active metals from solution using zerovalent iron media.Progress Summary:
Reductive Dechlorination by Zerovalent Iron. Reductive dechlorination rates of trichloroethylene (TCE), perchloroethylene (PCE), and carbon tetrachloride (CT) were measured in zerovalent column reactors over a 3-year period. At early elapsed times, reaction rates were pseudo-first order in halocarbon concentration, and were independent of the solution pH. With increasing elapsed time, reaction rates deviated from pseudo-first order behavior due to reactive site saturation, and increased iron surface passivation towards the influent end of each column. The extent of passivation was dependent on both the halocarbon concentration and the background electrolyte solution. For most of the investigation, reaction rates in 3 mM CaSO4 and 5 mM CaCl2 solutions were statistically identical at the 0.05 confidence level. However, reaction rates in 5 mM Ca(NO3)2 were slower. In columns operated using chloride and sulfate containing waters, the effective half-life for TCE dechlorination increased from approximately 400 minutes after 10 days elapsed, to approximately 2,500 minutes after 667 days. The effective TCE half-life in the nitrate-containing water increased from approximately 1,500 minutes after ten days, to approximately 3,500 minutes after 667 days. Measurements of iron corrosion rates in nitrate and chloride solutions showed that nitrate contributed to increased iron surface passivation and decreased rates of iron corrosion. Corrosion current measurements indicated that halocarbon reduction on fresh iron surfaces was cathodically controlled, whereas on aged iron surfaces, iron corrosion was anodically controlled. Anodic control of iron corrosion contributed to the development of reactive site saturation with time, and to similar reaction rates for TCE and PCE. Passivation of the iron surfaces was found to be dependent on the adhering tendency of the corrosion products, and not on the overall mass of corrosion products in the columns. The decrease in TCE reaction rates over time can be attributed to anodic control of iron corrosion, and not to increasing reactant mass transfer limitations associated with diffusion through porous corrosion products.In a column reactor operated using a groundwater obtained from a contaminated field site, reaction rates for TCE and PCE gradually increased with elapsed time of operation. Throughout the course of the 3-year investigation, the reaction rate for TCE was approximately two times faster than that for PCE. However, amperometrically measured reaction rates indicated that direct PCE reduction was faster than that for TCE. This disparity between amperometrically measured reaction rates with those measured in the column reactor suggested that halocarbon reduction may occur via direct electron transfer, or may occur indirectly through reaction with atomic hydrogen absorbed in the iron. Comparison of halocarbon reduction rates by aged and fresh iron indicates that the indirect reduction mechanism appears to predominate on aged iron surfaces due to the role of water as the primary oxidant for iron corrosion. The faster reaction rate for TCE in the column was attributed to a faster rate of indirect reduction for TCE compared to PCE.
Reductive Dechlorination using Iron and Palladized Iron Cathodes. The kinetics, reaction mechanisms, and current efficiencies for TCE and CT reduction were investigated using flow- through, iron electrode reactors, and with amperometric measurements of reduction rates. The electrode reactors were operated over a range of flow rates, pH, ionic strength, dissolved oxygen concentration, and working electrode potentials. Typical reduction half-lives for TCE and CT in the iron reactor were 9.4 and 3.7 minutes, respectively. The addition of palladium as an electrocatalyst at a level of 1 mg-Pd per square meter of electrode surface area increased the reaction rates by a factor of three. When operated continuously, reaction rates in the palladized iron reactor were stable over a 9-month period of operation, indicating that there was no loss of palladium from the electrode. In both the iron and Pd-iron reactors, TCE was reduced primarily to ethane and ethene, while CT was reduced almost exclusively to methane. Under all operating conditions, chlorinated compounds accounted for less than 2 percent of the total reaction byproducts. Comparisons of amperometrically measured current efficiencies with those measured in the flow-through reactors, and the weak effect of electrode potential on TCE reaction rates, indicated that the primary pathway for TCE reduction by iron and palladized iron electrodes is indirect, and involves atomic hydrogen as the reducing agent. Direct reduction of TCE appeared to be inhibited by the preferential reduction of water. The finding that electrodes coated with a hydrophobic polymer to inhibit water reduction showed current efficiencies greater than 90 percent for direct TCE reduction supports this hypothesis. For CT, similar amperometric and analytically measured current efficiencies indicated that the primary mechanism for CT reduction is direct electron transfer. Carbon dioxide and bisulfide, which have been found to foul palladium in other catalytic systems, did not deactivate the catalyst.
Reductive dechlorination of CT and TCE at iron surfaces was studied to determine if their reaction rates were limited by the rate of electron transfer. Chronoamperometry (CA) and chronopotentiometry (CP) analyses were used to determine the kinetics of CT and TCE reduction by a rotating disk electrode in solutions of constant halocarbon concentration. Rate constants for CT and TCE dechlorination were measured as a function of the electrode potential over a temperature range from 2 to 42? C. Changes in the rate constants with electrode potential were used to determine the apparent electron transfer coefficients at each temperature. The transfer coefficients for both CT and TCE were found to be less than that for water reduction, resulting in a decreasing current efficiency for dechlorination with decreasing potential. The transfer coefficient for CT was independent of temperature, while that for TCE was temperature dependent. This indicates that CT reduction involved an outer sphere electron transfer reaction, and that CT reduction was limited by the rate of electron transfer. The temperature dependent transfer coefficient for TCE indicates that its reduction was not limited by the rate of electron transfer. In accord with a rate limiting mechanism involving an outer sphere electron transfer reaction, the apparent activation energy (Ea) for CT reduction was found to decrease with decreasing electrode potential. Conversely, the Ea for TCE reduction showed a slight increase with decreasing electrode potential, supporting the conclusion that its reaction rate was not limited by the rate of electron transfer. The small potential dependence of TCE reaction rates, and the increasing Ea with decreasing potential can be explained by an electrocatalytic hydrodechlorination mechanism in which chemisorbed TCE reacts with atomic hydrogen adsorbed at the iron surface.
Removal of Chromate and Arsenate from Solution Using Zerovalent Iron Media. Soluble chromate removal by iron wires was measured in batch experiments for initial chromium concentrations ranging from 100 to 10,000 µg/L. Chromate removal also was measured in columns packed with zerovalent iron filings over this same concentration range. Electrochemical measurements were made to determine the free corrosion potential and corrosion rate of the iron reactants. In both the batch and column reactors, chromium removal rates declined with increasing chromate concentration. This indicates that simple first or fractional order kinetic models are not useful for describing chromate removal kinetics. Corrosion current measurements indicated that the rate of iron corrosion decreased with increasing chromium concentrations between 0 and 5,000 µg/L. Analysis of Tafel polarization diagrams indicated that iron corrosion was hindered by both cathodic and anodic inhibition. Cathodic inhibition was indicated by a decrease in the exchange current for the water reduction reaction. Anodic inhibition was attributed to the buildup of Cr(III)/Fe(III) oxides at anodic sites on the iron surfaces. Even at the most passivating concentration of 10,000 µg/L, chromate removal in the column reactors reached a steady state, indicating that the passivation had also reached a steady state. Although chromate contributes to iron surface passivation, the removal rates are still sufficiently fast for iron barriers to be effective for chromate removal at most environmentally relevant concentrations.
Batch experiments utilizing iron wires suspended in anaerobic arsenate solutions were performed to determine arsenate removal rates as a function of the arsenate solution concentration. Corrosion rates of the iron wires were determined as a function of elapsed time using Tafel analysis. X-ray absorption spectroscopy (XAS) was performed to determine the oxidation state and binding environment of arsenic compounds associated with the iron surfaces. The removal kinetics in the batch reactors were best described by a dual rate model in which arsenate removal was pseudo-first order at low concentrations and zeroth order at high arsenic concentrations. The presence of arsenate decreased iron corrosion rates compared to those in blank 3 mM CaSO4 background electrolyte solutions. However, constant corrosion rates were attained after approximately 10 days, indicating that the passivation processes had reached steady state. The cathodic Tafel slopes were the same in the arsenate and blank electrolyte solutions. This indicates that water was the primary oxidant for iron corrosion, and that arsenate did not directly oxidize the iron wires. The anodic Tafel slopes were greater in the arsenate solutions, indicating that arsenate formed complexes with iron corrosion products released at anodic sites on the iron surfaces. Ion chromatography analysis indicated that there was no measurable reduction of As(V) to As(III). XAS analysis indicated that all arsenic associated with the zerovalent iron surfaces was in the +5 oxidation state. Interatomic distances for arsenate on the iron surfaces were consistent with bidentate corner sharing among arsenate tetrahedra and iron octahedra. Results from this study show that under conditions applicable to drinking water treatment, arsenate removal by zerovalent iron media involves surface complexation only, and does not involve reduction to metallic arsenic.
Future Activities:
During the next year, we will investigate the effect of hydrophobic surface modification on halocarbon reduction by iron, nickel, and stainless steel electrodes. We also will investigate the effect of water chemistry on rates of arsenate and chromate removal by iron media.Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 11 publications | 1 publications in selected types | All 1 journal articles |
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Melitas N, Chuffe-Muscoso O, Farrell J. Kinetics of soluble chromium removal from contaminated water by zerovalent iron media: corrosion inhibition and passive oxide effects. Environmental Science and Technology. 2001;35(19):3948-3953. |
R825223 (2000) |
not available |
Supplemental Keywords:
remediation, solvents, electrolysis., RFA, Scientific Discipline, Toxics, Waste, Water, POLLUTANTS/TOXICS, National Recommended Water Quality, Remediation, Environmental Chemistry, Arsenic, Water Pollutants, Groundwater remediation, Environmental Engineering, dechlorination, contaminant transport, oxidized metal species, chlorinated organic compounds, copper, dissolved metal complexes, heavy metal contamination, iron mediated reductive transformation, groundwater, nickel, electrochemical methodsProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.