Retardation of arsenic transport by oxidized Holocene aquifer sediments of West Bengal, India

Abstract

• As(III) adsorption by oxidized Holocene aquifer sediments at pH 6–9 was studied. • No pH dependency was observed for As(III) adsorption by oxidized Holocene sediments. • K d values for As(III) are comparable with those for As(V) in West Bengal, India. • Low adsorption capacity of Holocene sediments demonstrates significant As transport. The transport of arsenic (As) in groundwater is known to be considerably controlled by the adsorption behavior of aquifer sediments and changes in subsurface redox conditions. Here we report wet chemical analyses of As(III) adsorption onto oxidized, low As containing Holocene aquifer sediments from West Bengal, India. Batch experiments in an open laboratory atmosphere show that the slow removal of As(III) from aqueous solution is a resulting effect of adsorption and oxidation which is not significantly influenced by the changes in pH (6.0–9.1). The isotherm data were fitted with Freundlich and Langmuir equations and maximum adsorption density ( Γ max ≈ 0.40 mmol kg −1 ) obtained is lower than that for As(V) at pH 7.5 (0.95 mmol kg −1 ). The estimated distribution coefficient maxima ( K d max ) values are within the ranges of K d values previously reported in case of Holocene aquifer sediments of West Bengal, India and Bangladesh. The low adsorption capacity of oxidized Holocene aquifer sediments in this study compared to oxidized Pleistocene sediments of Bangladesh is attributed to coarser grain sizes and low abundance of Fe-oxyhydroxides and micas which act as a potential sink of As. This study has important implications for As(III) attenuation and transport in the shallow aquifers (especially around the zone of ‘maximum’ water table fluctuations, i.e., 10–20 m depth below ground surface) when oxygen-depleted (anoxic) Holocene aquifers get in contact with oxygen (e.g., by irrigation/groundwater drawdown) and also for predicting possible future risk associated with its transport from shallow aquifers to present day “low-As” containing deeper aquifers of West Bengal, India.