EVALUATION OF REDOX PROCESSES IN GROUNDWATER BY FLUORESCENCE SPECTROSCOPY: APPLICATIONS IN BANGLADESH

The partitioning of arsenic between solute and solid phases is strongly redox dependent, with mobilization generally occurring under reduced conditions, often coupled to Fe redox reactions. Past research indicates that one possible mechanism for microbial Fe(III)-oxide reduction in aquifer is the shuttling of electrons by humic substances, if metals and dissolved organic matter (DOM) (substrate, electron donor) are present in sufficient concentration. However, analyses of redox state of humic substances in Fe-rich aquifers are confounded by the complexation of DOM with Fe and other metals. In this study, we test the influence of different sampling pretreatment methods on humics redox state, by examining the chemical and spectroscopic properties of 1) GFF filtered, 2) GFF filtered-acidified, and 3) in-line cation exchange treated samples from the shallow Holocene aquifer of the Ganges Brahmaputra Delta in Araihazar, Bangladesh. Results from fluorescence analyses confirmed the reduced state of fulvic acids in groundwater. In the filtered-acidified treatment,3D fluorescence spectra and reduced quinone content remained unaltered in both oxygen-free and oxygen-exposed experiments. Use of cation exchange resin had high Fe removal, but is not recommended for fluorescence analyses due to leaching of proteins from the resin. Using only GFF filtration, fluorescence spectra were heavily influenced by iron quenching. Fluorescence index and decomposition of fluorescence spectra into individual components using PARAFAC revealed differences between surface water, shallow groundwater, and deep groundwater in terms of DOM sources and reduced quinone fluorophore content. These results show that fluorescence can be a useful tool to examine redox state of fulvic acids in Bangladesh groundwater, even at high Fe concentrations.