APPLICATION OF FLUORESCENCE SPECTROSCOPY IN THE DETECTION OF FE-AS-DOM COMPLEXES

Dissolved organic matter (DOM) influences the fate of arsenic (As), a naturally occurring element of major health concern to millions of people in the world. Size exclusion chromatography and dialysis experiments have shown that in the presence of iron (Fe), DOM may either compete with As for adsorption sites and/or bind to form ternary complexes. In both cases, it enhances the mobility of As from aquifer materials, soils and sediments. The purpose of our research is to evaluate the quenching of fluorescent components in the excitation-emission matrix (EEM) as a tool to track Fe-As-DOM complex formation under different conditions. The use of fluorescence spectroscopy with excitation – emission matrix has proven useful in the characterization of natural organic matter and its properties. Fluorescence quenching of DOM has also been used to evaluate the complexation of DOM by metal cations and to calculate stability constants associated with the reactions. However, it is not known whether ternary complexation, as in the case of Fe-As-DOM complexes, can be evaluated with fluorescence. We tracked the quenching of fluorescence peaks in EEMs in laboratory experiments with commercial humic acid and Suwannee River Fulvic Acid (SRFA) and additions of both As (1 ppb – 100 ppm) and Fe (1 ppm to 50 ppm) under oxic and anoxic condition. We also analyzed fluorescence profiles to calculate the complexation parameters, conditional stability constants and binding capacities of the complexation reactions. Our results indicate clear evidence of fluorescence quenching and, therefore, formation of Fe-DOM complexes under all conditions. By contrast in experiments with Fe, As, and DOM, and As and DOM alone, there was no evidence of additional fluorescence quenching. These results indicate that either fluorescence quenching is not sensitive enough to monitor the formation of Fe-As-DOM complexes or that the formation of such complexes did not occur in our experiment.