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FIELD STUDY OF ARSENIC REMOVAL FROM GROUNDWATER BY ZEROVALENT IRON
WILKIN, R. T., S. D. ACREE, R. R. ROSS, T. R. LEE, AND D. G. BEAK. FIELD STUDY OF ARSENIC REMOVAL FROM GROUNDWATER BY ZEROVALENT IRON. Presented at The 6th International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, May 19 - 22, 2008.
Present information at the 6th International Conference Remediation of Chlorinated and Recalcitrant Compounds
Contamination of ground-water resources by arsenic is a widespread environmental problem; consequently, there is a need for developments and improvements of remedial technologies to effectively manage arsenic contamination in ground water and soils. In June 2005, a 7 m long, 14 m deep, and 1.8 to 2.4 m wide (in the direction of ground-water flow) pilot-scale permeable reactive barrier (PRB) was installed at a former metal smelting facility, located near Helena, Montana. The reactive barrier was designed to treat ground water contaminated with moderately high concentrations of both arsenite and arsenate. The reactive barrier was installed over a 3-day period using bio-polymer slurry methods and modified excavating equipment for deep trenching. The reactive medium was composed entirely of granular iron which was selected based on long-term laboratory column experiments. In laboratory experiments, arsenic removal by zero-valent iron is controlled by adsorption and co-precipitation with iron corrosion products. Laboratory results indicated removal capacities on the order of 10 mg arsenic per gram of granular iron. After installation of the pilot-scale PRB, a monitoring network of approximately 50 ground-water sampling points was installed in July 2005. Monitoring results indicate arsenic concentrations >25,000 µg/L in wells located hydraulically upgradient of the PRB. Within the PRB, arsenic concentrations are reduced to 1,500 to <10 µg/L. After 2 years of operation, monitoring points located within 1 m of the downgradient edge of the PRB show significant decreases in arsenic concentrations. Conditions within the PRB support sulfate-reduction which is evidenced by reductions in influent concentrations of sulfate and the presence of dissolved sulfide within and downgradient of the PRB. Core samples were retrieved from the PRB after 1 year and were analyzed using a variety of methods including X-ray absorption spectroscopy (XANES and EXAFS) to determine the oxidation state and local bonding environment around arsenic. Spectroscopic results show that arsenic removed by zerovalent iron is present in at least three coordination environments: 1) As(III) associated with reduced sulfur, 2) As(III) associated with oxygen, and 3) As(V) associated with oxygen. This distribution of solid-phase arsenic is markedly different from that developed during laboratory column tests (As(III) and As(V) associated with oxygen). The field pilot-study shows biogeochemical uptake processes for arsenic that are not operative in laboratory column tests. The presentation will cover aspects of site characterization, remedial design and implementation, and monitoring results. .