Quantifying photo-production of triplet excited states and singlet oxygen from effluent organic matter

The organic matter present in wastewater effluents (EfOM), is likely to have different properties than the organic matter present in the receiving water. The properties of EfOM will affect the fate of contaminants of emerging concern because EfOM is a source of photochemically produced reactive intermediates (PPRIs) capable of transforming contaminants. Effluent water samples were taken seasonally from sixteen wastewater treatment plants in Minnesota and two effluent dominated rivers in California and Arizona. Samples (n = 94) were tested for water chemistry, light absorption characteristics, and excited state triplet organic matter (³EfOM^∗) and singlet oxygen (¹O₂) production. Based on analysis of spectral parameters, EfOM had higher molecular weight and lower aromatic content than organic matter present in stormwaters from Minnesota, which are representative of human-impacted natural organic matter (NOM) containing waters. The second order rate constant for the reaction of the ³EfOM^∗ probe 2,4,6-trimethylephenol (k_T,TMP) for the effluents was 7.79 (±3.03) × 10⁸ M^-1s^-1, and this value was used to calculate apparent quantum yields for ³EfOM^∗ production, which ranged from 0.006 to 0.114. The quantum yield for the production of singlet oxygen ranged from 0.007 to 0.064. Processes in the wastewater treatment train, season, and water chemistry parameters did not serve as predictors of ³EfOM^∗ or ¹O₂ production. Among the parameters measured, Spearman rank correlations were strongest between quantum yields of ¹O₂ and ³EfOM^∗ and E2/E3 (absorption at 250 nm/absorption at 365 nm). This relationship, however, is weaker than that previously observed for NOM. The efficiency of ¹O₂ production from ³EfOM^∗ was 54%. Results indicate that 2,4,6-trimethylphenol samples nearly all of the triplets in EfOM that have sufficient energy to produce ¹O₂, which may not be the case for NOM.