Grantee Research Project Results
1999 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, 1998 through September 30, 1999
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; and (3) determine the effectiveness of direct cathodic reduction for removing chlorinated organic compounds from dilute aqueous solutions.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 10 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 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 the rate 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.
Future Activities:
During the next year, we will investigate arsenic and chromium removal in zerovalent iron remedial systems, and the mechanisms involved in reduction of chlorinated organic compounds by iron, nickel, and alloy electrodes.Journal Articles:
No journal articles submitted with this report: View all 11 publications for this projectSupplemental Keywords:
remediation, solvents, electrolysis, dechlorination, chlorinated organics, groundwater, TCE, PCE, perchloroethylene, trichloroethylene, chemistry., 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.