Photochemistry of Effluent and Natural Organic Matter and Photochemical Degradation of Micropollutants

The photoreactivity of treated wastewater effluent organic matter (EfOM) has received recent attention because it is an important fraction of the dissolved organic matter (DOM) pool in wastewater-receiving rivers. Dissolved organic matter contains chromophore moieties that absorb energy from sunlight and, in turn, produce reactive intermediates through secondary reactions of these excited-state moieties. Photochemically produced reactive intermediates (PPRIs) include excited state triplet organic matter (3DOM*), singlet oxygen (1O2), hydroxyl radicals (OH•) and others. Apparent quantum yields of production of PPRIs appear to be higher for EfOM than for natural organic materials (NOM), which implied that contributions of EfOM may enhance the photoproduction of reactive intermediates in EfOM-receiving rivers. Our evaluations with EfOM and NOM mixtures showed higher photoreactivity in mixtures having more than 25% (v/v) of EfOM; however, evidence of 3DOM* and 1O2 quenching in organic matter mixtures was found when measured yields were compared to theoretical yields. These results suggest that effluent contributions of greater than 25% to rivers have a lower than expected contribution to photochemical production of 3DOM* and 1O2 because of quenching of 3DOM* by DOM. To examine how trends of reactive species productions translate to indirect photodegradation of micropollutants, we studied photodegradation of organic compounds in wastewater effluent, river water and mixtures thereof. The compounds were selected considering their photodegradation pathways. Greater contributions of oxidants in the degradation of compounds were observed in EfOM compared to NOM. Comparisons of measured rate constants with model calculations based on end member reaction rate constants showed model overestimations of photodegradation in the case of 3DOM* and 1O2 pathways, and conserved mixing behavior in the case of OH• reaction pathways. Finally, we found that amino acids in EfOM involve in quenching of triplets