Photodegradation of natural organic matter from diverse freshwater sources

Natural organic matter (NOM) is significant in determining fate, transport and toxicity of metals in aqueous systems but NOM is not a static component; NOM can undergo photochemical changes in chemical structure. These changes will modify NOM quality and in turn influence how metals are transported in the environment, as well as their toxicity to aquatic organisms. Natural organic matter was collected from five freshwater sources using a portable reverse osmosis unit, diluted to about 10 mg C L−1, then exposed for 13 days to summer temperatures either in the dark or exposed to sunlight. Light exposed NOM had decreases in total organic carbon (TOC) of 8–35% compared to samples kept in a refrigerator (dark, 4 °C), and the NOM became optically lighter, as shown by specific absorbance coefficients (SAC) taken at 340 nm (55–76% decreases in SAC340). In contrast, dark exposed NOM showed much smaller decreases in TOC (≤3%) or SAC340 (≤32%). For light exposed samples fluorescence excitation–emission matrix spectroscopy showed loss of fluorescence mainly in the 330 nm excitation, 420–450 nm emission range, consistent with loss of fulvic- and humic-like, aromatic groups in the NOM. Commercial (Aldrich) humic acid showed similar changes when exposed to light, with decreases in fulvic and humic fluorescence peaks. Finely ground fish food, used as a source of protein-rich organic matter, showed similar decreases in TOC and SAC340 when exposed to light, but showed increased SAC340 in the foil wrapped samples, presumably because bacteria were involved in decomposition of the food organic matter. Overall, these results indicate photodegradation of terrestrially derived, aromatic functional groups (e.g., from lignin) in natural organic matter from various freshwater sources.