SORPTION KINETICS AND SURFACE COMPLEXATION AS FACTORS CONTROLLING RELEASE AND MOBILIZATION OF ARSENIC IN PARTS OF THE BRAHMAPUTRA FLOODPLAINS, NORTHEASTERN INDIA

Selective Sequential Extraction (SSE) studies carried out with sediment samples collected from a part of the Upper Brahmaputra floodplains demonstrated that reductive dissolution of Fe-oxides phase indeed accounts to highest As extraction and substantial amount of As extracted in specific sorption phase. The SSE, hydrogeochemical conditions, minerals saturation and the redox environment suggests reductive dissolution of Fe-oxides as the dominant As releasing mechanism in the study area. Competition of adsorption sites by anions viz. PO₄^3- and HCO₃^- to a minor extent was also might be responsible for the release of As in some affected areas. The total sediment As concentration was found to be in the range 6.5 to 45 mg/kg with a mean of 14.5 mg/kg, which is not unusually higher than global normal range. Thus, the amount extractable As fractions (%) of the sediment plays a major role in the enrichment of As in the groundwater.The oxidized sediment sample (C_70) has highest As adsorption capacity and lesser effect of competing anions viz. PO₄^3-, HCO^3- and Si (silica). The reduced sediment sample (C_50) has lowest As adsorption capacity and high reduction in adsorption due to competitive anions.The batch kinetics study revealed that the main mechanism of adsorption take place through chemisorptions, and fitted well for Elovich model. The oxidation of As(III) to As(V) experiment showed that As(III) oxidation was the dominant removal mechanism for oxidized sediment (C_70) and moderate sediment (C_150) whereas in reduced sediment (C_50) As(III) adsorption was main process. Thus suggested that natural aquifer materials might make natural attenuation of As viable. A Surface Complexation Model (SCM) using Component Additivity (CA) predicted the arsenic distribution from surface-site densities, chemical extraction data, published sorbent-site densities, measured rock-water ratios and detailed chemical analysis of groundwater using two phases i.e. Ferrihydrite phase (Hfo_) and Goethite phase (Goe_) satisfactorily.