Natural attenuation processes of arsenic in the groundwater of the Brahmaputra floodplain of Assam, India.

Natural attenuation of higher arsenic (As) concentration in the groundwater of the Brahmaputra valley of Assam can be a viable option for its remediation. The assessment of attenuation capacities in aquifers was done using three sediment core samples (namely C_50, C_70 and C_150) representing the commonly found sediments and characterized as viz. reduced, oxidized and moderately oxidized sediments, respectively. The feasability of aquifers to release low As water depends on the sorption capacity of the sediments to directly adsorbed as As(III) and/or oxidation to As(V) on oxidative sites. The various batch sorption experiments along with competition of other anions for adsorption sites at different concentrations were investigated. The sediments were observed to adsorb both As(III) and As(V) with varying magnitudes depending on the species of As, pH and concentrations of competing anions. The Elovich kinetic model best described kinetic test data. The oxidized sediment (C_70) with the highest Fe and Mn content has the highest adsorption capacity. The competition for adsorption sites with As(V) in the sediment samples followed the order PO4(3-) > SiO2 > HCO3(-). The oxidation of As(III) to As(V) by the sediments was analyzed as it is also the most effective pathway to reduce the toxicity as well as the mobility of As in the subsurface environment. Sediment C_70 showed the highest potential to oxidize As(III) to As(V) in the oxidative sites of the sediments. The removal of As(III) in the oxidative site via As(V) was the dominant As removal mechanism for the oxidized sediment (C_70) and moderately oxidized sediment (C_150) than the direct adsorption of As(III) on the adsorptive sites as on reduced sediment sample C_50. Therefore, targeting the oxidized sediment with a high concentration of Fe-oxides and Mn-oxides might make natural attenuation of As viable in the study areas.