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Preliminary EXAFS studies of solid phase speciation of As in a West Bengali sediment

Preliminary EXAFS studies of solid phase speciation of As in a West Bengali sediment Abstract Knowledge of the solid-phase speciation of As in Bengali sediments associated with hazardous As-rich groundwaters is crucial to understanding the processes controlling As release. The local coordination environment of As in such a sediment has been probed using K-edge As EXAFS. This revealed that As exists predominantly in its oxidized form, As(V), probably adsorbed as bidentate arsenate tetrahedra on metal (Fe and/or Al) oxide/hydroxide surfaces, although incorporation of As into a metal oxide structure cannot be ruled out. Arsenic was found to occur in several different coordination environments and this, together with the low concentration (<10 μg g -1 ) of As in the sediment prevented the unambiguous assignment of the second coordination sphere. The EXAFS analysis of the sediment after incubation under anaerobic conditions in the presence of added electron donor for metal reduction indicated changes in the relative concentrations of different solid-phase As species, providing circumstantial evidence for differential susceptibility to microbial action. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Mineralogical Magazine de Gruyter

Preliminary EXAFS studies of solid phase speciation of As in a West Bengali sediment

Preliminary EXAFS studies of solid phase speciation of As in a West Bengali sediment


groundwater contaminated with As, some with concentrations exceeding 800 mg l ­1 (Bandyopadhyay, 2002; Gault et al., 2003b). The sediment was collected from a depth of 12.80 ­ 13.15 m in a nitrogen- ushed plastic tube and the sediment and associated porewaters were sealed tightly to prevent the ingress of air. The samples were refrigerated prior to shipment to Manchester. The sample received was a dark-coloured, wet sandy sediment. Some reddish colouration, indicative of oxidation during storage, was observed, but was limited to a narrow zone around the margins of the sample. Material used in this work was taken carefully from the interior of the core where no oxidation was evident. An amended sediment was prepared by mixing a portion of the sediment with synthetic arti cial groundwater (prepared based on a typical composition of Bengali groundwater), amended with acetate to stimulate microbial activity and incubated at 20ºC under anaerobic conditions for 2 months. Further details of these microcosm experiments are given by Islam et al. (2003). A sample of sediment was withdrawn immediately prior to EXAFS analysis. A similar approach has been used successfully to stimulate metal reduction in previous studies using sediment samples from the subsurface, e.g. in Finneran et al. (2002). Sequential extraction A six-stage sequential extraction, adapted from the procedure speci cally designed for As by Wenzel et al. (2001), was performed on both the 1 184 unamended and acetate-amended sediment after incubation under anaerobic conditions for 2 months. All the equipment and reagents used in this extraction scheme were acid-washed and of analytical grade (Analar, BDH UK). Extractions were performed in triplicate at room temperature (~20ºC), unless otherwise stated. After each extraction stage, the samples were centrifuged at 3000 rpm for 15 min and the supernatant removed for ICP-AES (Horizon, VG Elemental) and/or ICP-MS (PlasmaQuad 2, VG Elemental)...
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References (35)

Publisher
de Gruyter
Copyright
Copyright © 2003 by the
ISSN
0026-461X
eISSN
1471-8022
DOI
10.1180/0026461036760157
Publisher site
See Article on Publisher Site

Abstract

Abstract Knowledge of the solid-phase speciation of As in Bengali sediments associated with hazardous As-rich groundwaters is crucial to understanding the processes controlling As release. The local coordination environment of As in such a sediment has been probed using K-edge As EXAFS. This revealed that As exists predominantly in its oxidized form, As(V), probably adsorbed as bidentate arsenate tetrahedra on metal (Fe and/or Al) oxide/hydroxide surfaces, although incorporation of As into a metal oxide structure cannot be ruled out. Arsenic was found to occur in several different coordination environments and this, together with the low concentration (<10 μg g -1 ) of As in the sediment prevented the unambiguous assignment of the second coordination sphere. The EXAFS analysis of the sediment after incubation under anaerobic conditions in the presence of added electron donor for metal reduction indicated changes in the relative concentrations of different solid-phase As species, providing circumstantial evidence for differential susceptibility to microbial action.

Journal

Mineralogical Magazinede Gruyter

Published: Dec 1, 2003

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