X-RAY ABSORPTION SPECTROSCOPIC INVESTIGATION OF ARSENIC SPECIATION AND COORDINATION CHEMISTRY IN SMELTER-CONTAMINATED SOIL PROFILES, VASHON ISLAND, WA

We used X-ray absorption near-edge structure spectroscopy (XANES) and extended X-Ray absorption fine structure spectroscopy (EXAFS) to examine the fate of arsenic emitted from a copper smelter and deposited in the glacial, forest soils of a Puget Sound island. XANES and EXAFS were employed to reveal arsenic oxidation state and the coordination chemistry of arsenic complexes on soil particles. Illuminating this atomic geometry is fundamental to assessing and mitigating health risks because these bonding environments control arsenic toxicity, mobility, and bioavailability in the vadose zone. Results provide insight into arsenic sorption and contribute to the small quantity of existing EXAFS data on arsenic in natural materials, arsenic-aluminum complexes, and arsenic with overlapping backscattering shells.

XANES results confirm that As(III) released from the smelter stack has been uniformly oxidized to As(V). EXAFS indicate backscattering from both Al and Fe atoms in two distinct combinations. Arsenic in a sandy, organic-rich sample is associated with Fe at an inter-atomic distance of approximately 3.01 Å, with a slight curve fit improvement attained by including Al at approximately 3.29 Å. Aluminum is the main backscattering element in a sample with a larger clay fraction, at an approximate distance of 3.24 Å, with a contribution from Fe at approximately 2.96 Å. The short Fe distances are consistent with the controversial bidentate, mononuclear (edge-sharing) complexes on Fe- oxyhydroxide surfaces suggested by Manceau (1995) and reported by others. Soil mineralogy and the bidentate, binuclear (bridging) complexes suggested by As-Al bond lengths together point to clay mineral surfaces as likely complexation sites (Foster, et al., 1998).