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Iron Mediated Reductive Transformation of Chlorinated Organic Compounds and Oxidized Metal Species in Groundwater Environments

EPA Grant Number: R825223
Title: Iron Mediated Reductive Transformation of Chlorinated Organic Compounds and Oxidized Metal Species in Groundwater Environments
Investigators: Farrell, James
Institution: University of Arizona
Current Institution: University of Arizona
EPA Project Officer: Manty, Dale
Project Period: October 1, 1996 through September 30, 2001
Project Amount: $500,000
RFA: Exploratory Research - Early Career Awards (1996)
Research Category: Early Career Awards

Description:

This research investigates several factors affecting the long-term performance of zero valent iron for in-situ treatment of contaminated groundwaters. One aspect of this research will investigate passivation of the iron surfaces by precipitate films. The thermodynamic instability of zero valent iron in water leads to considerable corrosion which contributes to unfavorable pH changes and surface passivation. However, a magnetite coating on the iron granules may reduce the free corrosion potential of the iron sufficiently to eliminate oxidation by anaerobic groundwater. The effect of a magnetite surface coating on the free corrosion potential of iron filings will be investigated using electrochemical methods. The relative rates of iron oxidation by water and the target species will be determined as a function of the magnetite content, groundwater chemistry, and thermodynamic favorability of target compound reduction.

Plating trace amounts of metals higher in the galvanic series onto the iron surfaces leads to substantially increased rates of reductive dechlorination. In short-term column experiments, plating 0.05% (w/w) palladium or 0.20% (w/w) copper onto iron particles increased TCE transformation rates by up to a factor of 15 over untreated iron. The mechanism through which bimetals increase reaction rates, as well as their long-term effectiveness, will be investigated for palladium, nickel, and copper bimetal systems.

Effective in-situ treatment systems must be of sufficient hydraulic conductivity to avoid plume bypassing during natural gradient flow, and of sufficient reactivity to achieve the required level of remediation. Porous iron particles may be used to achieve a high reactive surface area without sacrificing hydraulic conductivity. The adsorption/absorption capacity of iron foams and porous iron silica gels for metal hydroxide precipitates will be investigated using batch systems. Mass transfer limitations associated with intragranular diffusion will be investigated for both dissolved metal complexes and chlorinated organic compounds using column systems.

The ability of an oxide layer to passivate the iron surfaces is dependent on the porosity of the metal oxides. Both the groundwater chemistry and impurities in the iron will determine the composition and porosity of surface oxide layers. The effects of groundwater composition and iron impurities on surface passivation will be investigated using both thermodynamic solution modeling and experimental analyses. MINTEQ2 will be used to model the groundwater chemistry and identify possible precipitated or adsorbed species to target for analysis. After an extended period of operation, the quantity and composition of the surface precipitates will be determined using spectroscopic methods for metals analyses and wet chemistry methods for anion analyses.

Publications and Presentations:

Publications have been submitted on this project: View all 11 publications for this project

Journal Articles:

Journal Articles have been submitted on this project: View all 1 journal articles for this project

Supplemental Keywords:

Toxics, POLLUTANTS/TOXICS, Water, Scientific Discipline, Waste, RFA, Arsenic, Remediation, Environmental Engineering, Water Pollutants, Environmental Chemistry, Groundwater remediation, National Recommended Water Quality, copper, heavy metal contamination, nickel, iron mediated reductive transformation, oxidized metal species, dissolved metal complexes, groundwater, contaminant transport, electrochemical methods, surface passivation, dechlorination

Progress and Final Reports:
1999 Progress Report
2000 Progress Report

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The 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.

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