POSSIBLE ROLE OF ORGANIC MATTER AND HUMIC SUBSTANCES IN BINDING AND MOBILITY OF ARSENIC IN GANGETIC AQUIFER, WEST BENGAL, INDIA

Arsenic-contaminated water has caused severe health risk in many areas of world including the Gangetic basin. It is imperative to understand the chemistry of As interacting with environmental components like organic carbon and humic substances. Dissolved organic carbon (DOC) of groundwater and total organic carbon (TOC) contents of the bulk sediments, collected from Barasat, West Bengal, a part of Gangetic basin, were measured by C/N Analyser. Moderate correlation (R²=0.49, p<0.05) between DOC and As suggests that the microbial degradation of organic matter in the sediment results in an overall reducing environment and facilitates the release of As in the groundwater. The mean relative fluorescent intensity of humic substances is 62 for Indian groundwater. Some portion of DOC is reactive which produces HCO₃⁻ through microbial oxidation. A significant portion of the refractory DOC remains for a longer time in the aqueous phase. Bacteria mediate the release of As in younger sediments into groundwater under reducing conditions. Enhanced microbial activity accelerates the diagenetic process, involving mobilization of As from sediments with high organic matter. It is well known that humic substances (HS) particularly humic and fulvic acid might serve as a complexing agent for metal ions like As leached in subsurface environment. Humic acids were extracted from core sediments using acid-alkaline method. Infra-red spectra of humic acid powders were determined using FT-IR Microscopy at the National Synchroton Radiation Research Centre, Hsinchu, Taiwan. Fe and Mn and As minerals were incubated with HS in a serum bottle for 3 days to form the As-Fe-Mn-HS complexation. FT-IR spectra were compared with different metal complexes. FT-IR spectra show COO-, C=O, OH, and C=C (aromatic ring) functional groups in Bengal aquifer, which might have acted as a chelating agents binded with the elements. Naturally occurring complexes of As, Fe and Mn with HS in Bengal sediments exhibit several distinct peaks (at 2850 and 2915 cm^-1 for Humic acid; at 1610, 1183 and 828 cm^-1 for metal complexation). Ability of As to bind with HS via cation bridging is possibly the main binding mechanism along with other functional groups to form organometallic complexes.