EVIDENCE OF AS ON FEOOH CARRIER IN THE BENGAL AQUIFER SEDIMENTS FROM THE XANES AND EXAFS EXPERIMENT: IMPLICATION OF AS MOBILIZATION

To investigate the arsenic binding nature, geochemical and mineralogical features of the aquifer sediments were studied in Barasat, India, a part of Gangetic Basin, for better understanding the sources and mobilization processes responsible for arsenic enrichment. High arsenic has been reported in shallow groundwater (<70 m depth) in this area and this is also consistent with our study indicating the presence of alarming levels of arsenic in groundwater as well as in subsurface sediments. Results of sequential extraction study revealed different dominant species in solid phase. Amount of extractable arsenic in different solid phases accounts for 50-70%. XRD analysis demonstrated that iron oxides/oxyhydroxides with residual magnetite and chlorite, illite, iron oxide/oxyhydroxide-coated quartz, feldspar, and ankerite are the dominant carriers of arsenic in aquifer sediments as evidenced by EDS spectra as well. The study reveals that As is released to groundwater through reduction of arsenic-bearing iron oxide/oxyhydroxides when anoxic conditions prevail in the aquifer sediments. XRD and EDS studies of sediment indicate that residual magnetite, chlorite, illite, iron-coated quartz and feldspar and ankerite are dominant carriers of As in the aquifer. Arsenic and Fe speciation in sediments were determined using XANES and the results imply that As(III) and Fe(III) are the dominant species in most sediments. The results also imply that As (V) and Fe (III) in most of the sediment samples of the three areas are the dominant species. X-ray absorption fine structure (EXAFS) analysis shows that FeOOH is the main carrier of As in the sediments of this area.