Effects of Fe on the Absorbance, Fluorescence, and Molecular Weight of Natural Organic Matter

Natural organic matter (NOM) is ubiquitous in aquatic environments. It plays an important role in many biogeochemical processes, including metal binding. This study investigated the effects of Fe binding by NOM on the molecular weight, fluorescence, and absorbance of NOM. Various forms of Fe were added to XAD-8 NOM isolate. It was found that addition of Fe caused an increase in visible absorbance, a decrease in fluorescence, and a shift from intermediate molecular weight components to higher molecular weight components. Changes occurred within the first 2 hours, and Fe(III) affected NOM more than Fe(II). Reaction of the XAD-8 isolate with the siderophore desferrioxamine-B (DFO-B), caused an increase in one of the fluorescence peaks, due to Fe removal and a decrease in a second peak, perhaps due to Al removal. Comparison of XAD-8 and XAD-4 isolates showed similar changes in absorbance and fluorescence, but the XAD-4 showed little change in molecular weight distribution. These results have important implications for the concept of 'molecular weight' as applied to NOM, and for NOM reactivity including bioavailability, adsorption to minerals, and other processes.