Sequential chemical extraction of arsenic and related elements from the Holocene sediments of Sonargaon, Bangladesh, in relation to formation of arsenic-contaminated groundwater

The mineralogy and chemistry of aquifer sediments of As-contaminated groundwater in Sonargaon, Bangladesh were analyzed to identify the host phases of As and to understand release mechanism of As into the groundwater. Sequential chemical extraction revealed that the As was mostly fixed in silicate(s). Iron oxyhydroxides/oxides were produced via chemical weathering of basic minerals at depths where the redox conditions experience seasonal changes with the groundwater level. The Fe oxyhydroxides/oxides are not effective adsorbents of dissolved As at that depth. A significant amount of As, together with Si, Al, Mg, and Fe, was extracted by oxidation-decomposition of the silicates during a sequential chemical extraction process, indicating that chemical weathering of As-bearing silicate(s), which mostly occurs under oxic aqueous conditions, results in As dissolution in nature. Since chlorite is the only silicate that includes As in the aquifer sediments (Masuda et al., 2012a), chlorite decomposition in oxic groundwater is the most plausible mechanism of As release into the groundwater from aquifer sediments in the study area.