Preservation and XANES determination of the oxidation state of solid-phase arsenic in shallow sedimentary aquifers in Bengal and Cambodia

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Abstract
Determination of the solid-phase arsenic speciation in sediments hosting high-arsenic groundwaters, utilized for drinking and irrigation in Bengal, SE Asia and elsewhere is important in order to understand the biogeochemistry of arsenic. Despite this, there is a relative paucity of speciation data for solid-phase arsenic in such systems, due to preservation difficulties, low arsenic concentrations in the sediments, multiple coordination environments and sample heterogeneity. In this study, X-ray absorption near edge structure spectroscopy was used in conjunction with linear least-squares fitting of model compounds to determine the oxidation state of arsenic in sediments from West Bengal and Cambodia. Whatever the collection and storage method used, substantial oxidation of arsenic was commonly observed over periods of weeks to several months. Sands were particularly susceptible to changes in arsenic oxidation state during storage. Analysis within two or three weeks of collection is therefore recommended, whilst on-site storage under a nitrogen atmosphere immediately after collection is particularly recommended for the preservation of sandy samples. Both muds and sands from West Bengal and Cambodia were dominated by arsenite (As(III)) with <35±10% arsenate (As(V)). Complete oxidation to arsenate was never observed suggesting that a significant proportion of the sedimentary arsenic is inaccessible within crystalline phases. Centrifuging under anaerobic conditions enabled more detailed information about a variety of arsenic coordination environments to be determined.