HR: 0800h
AN: B31A-0957    [Abstracts]
TI: Mechanism of Arsenic Sequestration in High-Iron Sediments
AU: * Root, R A
EM: rroot@ucmerced.edu
AF: University of California, Merced, P.O. Box 2039, Merced, CA 95344 United States
AU: Campbell, K M
EM: katec@caltech.edu
AF: California Institute of Technology, 1200 E California Blvd, MC 138-78, Pasadena, CA 91125 United States
AU: Hering, J G
EM: poday jhering@caltech.edu
AF: California Institute of Technology, 1200 E California Blvd, MC 138-78, Pasadena, CA 91125 United States
AU: O'Day, P A
EM: poday@ucmerced.edu
AF: University of California, Merced, P.O. Box 2039, Merced, CA 95344 United States
AB: Naturally occurring elevated concentrations of arsenic in the runoff of the eastern Sierra Nevada and feed waters of the Los Angeles Aqueduct are remediated by the Los Angeles Department of Power and Water (LADPW) up stream of the Haiwee Reservoir (Olancha, CA). To reduce total arsenic in drinking water supplies, the LADPW adds ferric chloride and a cationic polymer coagulant to the aqueduct. The treatment precipitates as an amorphous iron oxide, spectrally similar to 6-line ferrihydrite, that adsorbs and sequesters arsenic as arsenate. As the channeled flow enters North Haiwee Reservoir, the As(V)-enriched iron floc settles as sediments in the inlet channel. Buried As(V) is reduced to As(III) near the sediment-water interface (0-10cm), and only As(III) is observed at depths below the steep (1-2cm) near-surface redox gradient. Sediment samples from 30-cm push cores were collected from the edge of the reservoir along the inlet channel in tandem with in situ porewater measurements using an inert polyacrylamide gel probe sampler. Sediments were analyzed to characterize the redox gradient, host mineralogy, and variation in bulk elemental composition with depth. X-ray absorption spectroscopy (XAS) was used to determine the depth of the microbially driven redox boundary where As (V) is reduced to As (III) and to investigate the molecular bonding of arsenic adsorbed to iron hydroxide surfaces. Specific and characteristic iron and arsenic phases were isolated by sequential extraction; extracted and bulk concentrations were determined by ICP-MS. Splits of specific extraction steps were analyzed by synchrotron EXAFS and XRD to determine the identity of separated phases. The primary mineralogy of sediments along the inlet channel is detrital quartz, plagioclase feldspar, and phyllosilicates weathered from the Sierra Nevada granitic batholith. Notably, crystalline magnetite, hematite, and goethite, phases that would indicate transformation of hydrous iron phases to more stable iron(III) oxides or reduced iron(II,III) oxide, were not observed in the sediments. EXAFS data showed that the arsenic within sediments is bonded directly to iron octahedra as binuclear bidentate complexes. Within the sediments, arsenic reduces from As(V) to As(III) at or just below the sediment/water interface, but there is no significant change in local bonding of arsenic observed in the EXAFS. Likewise, there are no apparent changes in iron mineralogy or oxidation state as a function of depth. Based on the mineralogy and surface complexation observed, the potential for release of arsenic into the sediment porewaters should be small. However, gel probe analysis has shown that arsenic is released to the porewaters at depth. Previous studies at this site, and the new sequential extraction and XAS data, indicate that the mechanism of arsenic release is not a consequence of the transformation of As(V) to As(III), but is a result of reductive dissolution of the host iron phase.
DE: 1000 GEOCHEMISTRY
DE: 1857 Reservoirs (surface)
DE: 1871 Surface water quality
DE: 3947 Surfaces and interfaces
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Biogeosciences [B]
MN: Fall Meeting 2005