Effects of Photoirradiation on the Physicochemical Properties of Natural Organic Matter: Considerations of Nom Isolation Technique and Iron Content

The objective of this study was to determine overall average kinetic rate constants for changes to the physicochemical properties of natural organic matter (NOM) brought about by photodegradation. The study compared the photo-degradation of samples from three of the most commonly used NOM concentration/ isolation methods: reverse osmosis, XAD-8, and XAD-4, with a natural surface water sample collected from the same site. Kinetic data provided insight into the photodegradation process. Physicochemical properties monitored included: light absorbance, dissolved organic carbon concentration, fluorescence, average molecular weight, low molecular weight carboxylic acid concentration, and iron speciation. When compared, the kinetic rate constants for the SW, RO, and XAD-8 were similar whereas the XAD-4 reacted much faster for most of the measured parameters. An initial increase followed by a decrease in the fluorescence likely explains why previous studies produced apparently conflicting results regarding the effects of photoirradiation on NOM fluorescence. Iron(II) production was only observed in two of the four NOM samples, the SW and the RO. This indicates that the XAD isolation process may remove constituents in natural surface water which act as oxidant scavengers.