title: Role of effluent organic matter in the photochemical degradation of compounds of wastewater origin.
abstract: The photoreactivity of treated wastewater effluent organic matter differs from that of natural organic matter, and the indirect phototransformation rates of micropollutants originating in wastewater are expected to depend on the fractional contribution of wastewater to total stream flow. Photodegradation rates of four common compounds of wastewater origin (sulfamethoxazole, sulfadimethoxine, cimetidine and caffeine) were measured in river water, treated municipal wastewater effluent and mixtures of both to simulate various effluent-stream water mixing conditions that could occur in environmental systems. Compounds were chosen for their unique photodegradation pathways with the photochemically produced reactive intermediates, triplet-state excited organic matter (3OM*), singlet oxygen (1O2), and hydroxyl radicals (OH). For all compounds, higher rates of photodegradation were observed in effluent relative to upstream river water. Sulfamethoxazole degraded primarily via direct photolysis, with some contribution from OH and possibly from carbonate radicals and other unidentified reactive intermediates in effluent-containing samples. Sulfadimethoxine also degraded mainly by direct photolysis, and natural organic matter appeared to inhibit this process to a greater extent than predicted by light screening. In the presence of effluent organic matter, sulfadimethoxine showed additional reactions with OH and1O2. In all water samples, cimetidine degraded by reaction with1O2(>95%) and caffeine by reaction with OH (>95%). In river water mixtures, photodegradation rate constants for all compounds increased with increasing fractions of effluent. A conservative mixing model was able to predict reaction rate constants in the case of hydroxyl radical reactions, but it overestimated rate constants in the case of3OM* and1O2pathways. Finally, compound degradation rate constants normalized to the rate of light absorption by water correlated with E2/E3ratios (sample absorbance at 254 nm divided by sample absorbance at 365 nm), suggesting that organic matter optical properties may hold promise to predict indirect compound photodegradation rates for various effluent mixing ratios.
authors.affiliation: Department of Chemistry, University of Connecticut, Storrs, CT, USA.
authors.forename: Laleen C
authors.initials: LC
authors.lastname: Bodhipaksha
authors.affiliation: Department of Chemistry, University of Mary Washington, Fredericksburg, VA, USA. Electronic address: csharple@umw.edu.
authors.forename: Charles M
authors.initials: CM
authors.lastname: Sharpless
authors.affiliation: School of Earth Sciences, The Ohio State University, Columbus, OH, USA.
authors.forename: Yu-Ping
authors.initials: YP
authors.lastname: Chin
authors.affiliation: Department of Chemistry, University of Connecticut, Storrs, CT, USA; Department of Civil, Environmental and Geodetic Engineering, The Ohio State University, Columbus, OH, USA. Electronic address: mackay.49@osu.edu.
authors.forename: Allison A
authors.initials: AA
authors.lastname: MacKay
mesh_terms: D005618:Fresh Water / Q000737:chemistry; D010782:Photolysis; D013420:Sulfamethoxazole; D062065:Wastewater; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D013420:Sulfamethoxazole; D062065:Wastewater; D014874:Water Pollutants, Chemical
keywords: DOM; Dissolved organic matter; Emerging contaminants and environmental fate; Pharmaceuticals; Photochemistry; Wastewater treatment