The Role Of Electron-Shuttling By Dissolved Humics In The Reduction Of Iron Oxide And Release Of Bound Arsenic In A Shallow Aquifer In Bangladesh
Hageman, Clarissa 1 ; Mladenov, Natalie 2 ; McKnight, Diane 3
1 University of babyÖ±²¥app at Boulder
2 University of babyÖ±²¥app at Boulder
3 University of babyÖ±²¥app at Boulder
Arsenic (As) is a highly toxic and ubiquitous metalloid that poses a significant threat to human and ecosystem health, particularly water-borne arsenic from geogenic sources in reducing groundwater. The partitioning of arsenic between solute and solid phases is strongly redox dependent, generally occurs under reduced conditions, and is often coupled to Fe redox reactions. Past research indicates that one possible mechanism for microbial Fe(III)-oxide reduction in groundwater is the shuttling of electrons by soluble organic compounds, such as naturally occurring humic substances, from microbes to Fe(III)-oxide.
The objective of the current study is to determine the environmental importance of electron-shuttling by dissolved humics in accelerating the reduction of iron oxide and release of bound arsenic present in material from the shallow Holocene aquifer of the Ganges Brahmaputra Delta in Araihazar, Bangladesh. Fluorescence spectroscopy has been employed as the primary laboratory method to examine this issue. Recent advances in fluorescence spectroscopy for the characterization of humic substances have made it possible to evaluate the role of humic substances in Fe(III)-oxide reduction in nature for waters with dissolved organic carbon concentrations (1-5 mg/L). In this study, we use fluorescence spectroscopy and PARAFAC modeling (Cory and McKnight, 2005) of specific reduced and oxidized fluorescent components to ascertain the extent of this humic-enhanced electron transport cascade during both the initial groundwater recharge and further down the flow path at depth in the aquifer at this site. Our preliminary results suggest that Fe(III) reduction by electron shuttling of humics becomes less important down the flow path from the recharge as organic substrates are depleted.