Triplet Photochemistry of Effluent and Natural Organic Matter in Whole Water and Isolates from Effluent-Receiving Rivers

Effluent organic matter (EfOM), contained in treated municipal wastewater, differs in composition from naturally occurring dissolved organic matter (DOM). The presence of EfOM may thus alter the photochemical production of reactive intermediates in rivers that receive measurable contributions of treated municipal wastewater. Quantum yield coefficients for excited triplet-state OM ((OM)-O-3*) and apparent quantum yields for singlet oxygen (O-1(2)) were measured for both whole water samples and OM isolated by solid phase extraction from whole water samples collected upstream and downstream of municipal wastewater treatment plant discharges in three rivers receiving differing effluent contributions: Hockanum R., CT (22% (v/v) effluent flow), E. Fork Little Miami R., OH (11%), and Pomperaug R., CT (6%). While only small differences in production of these reactive intermediates were observed between upstream and downstream whole water samples collected from the same river, yields of (OM)-O-3* and O-1(2) varied by 30-50% between the rivers. Apparent quantum yields of O-1(2) followed similar trends to those of (OM)-O-3*, consistent with (OM)-O-3* as a precursor to O-1(2) formation. Higher (OM)-O-3* reactivity was observed for whole water samples than for OM isolates of the same water, suggesting differential recoveries of photoreactive moieties by solid phase extraction. (OM)-O-3* and O-1(2) yields increased with increasing E-2/E-3 ratio (A(254 nm) divided by A(365 nm)) and decreased with increasing electron donating capacities of the samples, thus exhibiting trends also observed for reference hurnic and fulvic acid isolates. Mixing experiments with EfOM and DOM isolates showed evidence of quenching of triplet DOM by EfOM when measured yields were compared to theoretical yields. Together, the results suggest that effluent contributions of up to 25% (v/v) to river systems have a negligible influence on photochemical production of (OM)-O-3* and O-1(2) apparently because of quenching of triplet DOM by EfOM. Furthermore, the results highlight the importance of whole water studies for quantifying in situ photoreactivity, particularly for (OM)-O-3*.