Thursday, April 15, 2010, 4:30PM - 5:30PM
Full title: "Chemical characteristics of dissolved humic substances from arsenic-rich shallow groundwater in Bangladesh."
The release of dissolved arsenic into groundwater from geogenic arsenic bound to sediments is thought to be influenced by dissolved organic matter (DOM) through several biogeochemical processes. It has been shown that the DOM pool in a groundwater system in Bangladesh includes both reactive humic substances and labile substrates for microbial growth. Humic substances may promote the release of sediment bound arsenic through direct chemical processes, specifically the formation of humic-As complexes and competition between As and humics for sorption sites. Microbial degradation of the labile DOM substrates may enhance arsenic mobilization by creating reducing conditions with the ferric iron in the Fe-oxides of the sediments serving as the ultimate electron acceptor for microbial growth. Dissolved humics in the groundwater may enhance this process by acting as electron shuttles, and the additional release of sorbed humics by reductive dissolution of Fe-oxides may fuel a redox cascade. I will discuss the chemical characteristics of dissolved humics from groundwater, and surface water in an As-rich region in Bangladesh. Chemical characterization included fluorescence and carbon-13 nuclear magnetic resonance (13C-NMR) spectroscopy, elemental and lignin phenol content and 14C analysis. In general, the groundwater humics had characteristics similar to wetland fulvic acid, indicative being derived from plant and soil precursor materials. Whereas, the surface water humic sample had a characteristics showing a greater contribution of precursor material from microbial biomass. Taken together these results provide chemical evidence that microbially mediated reductive dissolution of Fe-oxides in As-rich groundwaters releases both As and humics, which may in turn enhance electron shuttling capacity in the groundwater system.