Distribution and Mobilization of Arsenic in the Ganges plain sedimentary deposits of South-western Bangladesh; implications from field and laboratory observations

The variation of arsenic concentration depending on sediment size and its depositional age in a variety of sediments extracted from four As contaminated sites of the southwestern Bangladesh were studied to elucidate the aquifer geological parameters that controls the vertical As distribution and mobilization in the sediment-water interface. It was found that sediment size, reactive surface area, relative depositional age and presence of other carrier minerals having higher affinity to adsorb As, may greatly dominate the arsenic accumulation. Sorption of As onto sediment surfaces was found to vary based on the variation of the particle diameters (2 to 250 μm), which eventually reflects the role of geological materials in controlling the As distribution in various depositional layers. Medium sands commonly found in the deeper aquifer (~150m), being older in age (> 7000 yrs BP) and having relatively larger diameter (φ~250 μm) were found to contain relatively low amount of As (0.8 μg/g) whereas higher As (5 to 25 μg/g) was identified noticeably in the recently deposited and reasonably younger (100 to 1000 yrs BP) sediment particles including clay and finer sands that commonly have moderately smaller diameter (φ~2 to 90 μm). These observations were supported strongly by the findings obtained from the laboratory batch adsorption tests conducted with those sediments. Presence of As was also observed to be greatly dependent on the availability of its carrier minerals particularly Fe and Al oxide/hydroxide along the aquifer depths. Clay particles with relatively moderate Fe and Al oxide minerals was found to adsorb as much as 70 μg/g As whereas medium sand with less Fe and Al oxide minerals were noticed to capture only 4 μg/g of As in the batch adsorption test. In laboratory leaching test, significant amount of As (12 μg/g) coupled with Fe (4.8 mg/g) were found to be leached out from the shallower brown clay by using sodium bicarbonate (pH~9) as the leaching agent. This implies that bicarbonate may play a key role in triggering the desorption of As in the upper shallower aquifers that explains the high As pollution there, whereas insignificant As that was desorbed from the deeper medium sand suggests that the As mobilization is not an usual phenomena in the later one.