GEOC 198 |
| Arsenic contamination of soil and groundwater resulted from the application of arsenolite (As2O3) to soils as an herbicide in areas of North America during the 1950s and 1960s. The mechanisms by which arsenic was leached from the herbicide, transported through the underlying soil and partially immobilized in secondary solid phases are poorly understood. Column experiments were conducted using background soil collected near one of the contaminated sites and spiked with a surface application of arsenolite, with and without carbonate gravel cover, to simulate the contamination process and better understand the transformations of arsenic during weathering of the arsenolite. Synthetic acid rain was applied at the tops of the soil columns to simulate natural leaching. Effluent samples were taken at the bottoms of the columns and analyzed by ICP-OES and ion chromatography. The spiked soil column with carbonate gravel produced higher peak arsenic effluent concentrations and released arsenic earlier than the spiked column without gravel, but released significantly lower levels of arsenic during long-term leaching. Peak release of arsenic was related to removal of fine-grained and angular portions of the arsenolite, as documented by SEM images of the leached arsenic residue. The observed lower level of long-term arsenic leaching from the graveled column is likely due to precipitation of calcium arsenate in the soil, which was identified using XAS synchrotron techniques in similar soil contaminated with arsenolite herbicide more than 50 years ago. Early in the leaching process, aqueous arsenic was dominated by As(III); however, after 180 pore volumes of acid rain leaching, As(V) began to dominate the effluent arsenic chemistry for all of the spiked columns. Speciation studies indicate that approximately one third of the As(V) is produced at the top of the column, and the remaining two thirds is oxidized in contact with soil grains. |
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Speciation of Arsenic and Other Trace Elements in Soils and Sediments
8:35 AM-11:40 AM, Thursday, April 10, 2008 Morial Convention Center -- Rm. 211, Oral
Division of Geochemistry |