Dissolved organic matter (DOM) is a heterogenous mixture of organic molecules that are present in all natural bodies of water. Photochemically produced reactive intermediates (PPRIs) including excited triplet state DOM (3DOM*), singlet state oxygen (1O2), and hydroxyl radicals (●OH) are produced when DOM absorbs light. PPRIs are of interest in sunlit surface waters due to their ability to degrade certain organic contaminants through indirect photolysis. In this study, water samples were collected throughout Minnesota and Wisconsin from various rural, agricultural, and urban surface waters, with varying DOM composition. Water samples were spiked with one of five contaminants (atorvastatin, sulfadiazine, carbamazepine, cimetidine, or benzotriazole) and irradiated in a photoreactor (λ = 350 nm) to quantify direct and indirect photodegradation rate constants. Quenching experiments were then conducted to determine which PPRI(s) primarily contribute to contaminant degradation. Quenching molecules included sorbic acid (3DOM* quencher), 1,4-diazabicyclo[2.2.2]octane (DABCO) and L-histidine (1O2 quenchers), and isopropanol (radical quencher) to determine the contributions of PPRIs to degradation rate. Furthermore, anoxic experiments were used to further elucidate PPRI contributions. 3DOM* and 1O2 appear to be the primary PPRIs responsible for degradation in atorvastatin, sulfadiazine, and carbamazepine. Quenching data suggest cimetidine degrades primarily via reaction with 1O2 and benzotriazole through reaction with ●OH. Results from this study will allow for a better prediction of the rates of contaminant degradation in natural waters.

Influence of dissolved organic matter composition and photochemically-produced reactive intermediates on contaminant photodegradation rates

ACS Spring 2022

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