Variations in the redox state of As and Fe measured by X-ray absorption spectroscopy in aquifers of Bangladesh and their effect on As adsorption

Abstract

Groundwater enrichment with naturally occurring As is a very serious problem. In Bangladesh, many thousands of people are drinking such groundwater. Household well waters generally show a large variation of concentration of As even within a small village. This variation creates difficulty in the design of mitigation strategies. It was hypothesized that the concentration of As in each well is controlled by an adsorption–desorption equilibrium between sediment and groundwater. To verify the hypothesis, two factors are focused upon in this study: (i) speciation of As and Fe in the solid phase, and (ii) the adsorption properties of As(III) and As(V) to sediment. By this approach, it is possible to understand the variation of aqueous As in groundwater. Sediment core samples were collected from an As-enriched aquifer at Sonargaon, central eastern Bangladesh. The oxidation states of As and Fe in the sediments determined by X-ray absorption near edge structure (XANES) showed a distinct redox boundary below 5 m from the ground surface, whereas the peak of the concentration of dissolved As is observed below 15 m. The apparent distribution coefficient (Kd = Csolid/Csolution) of As(V) is always larger than that of As(III) at all the depths. Simulated concentrations of As in the groundwater obtained by multiplying the amount of P-extracted As and Kd-1 with consideration of the oxidation state of As is consistent with the depth profile of As in the groundwater. This suggests that the concentration of dissolved As is controlled by an adsorption–desorption equilibrium between sediment and groundwater. Variation in Kd is primarily controlled by the concentration of Fe oxyhydroxides, whereas surface area is also important for As(III). The discrepancy between the depth of the redox boundary and the peak of dissolved As is attributed to the difference in the abundance of P-extracted As rather than to a variation of Kd. The adsorption equilibrium model proposed in this study can be applied to the evaluation of the large variation in aqueous As concentration in groundwater from Holocene aquifers.