Quaternary ammonium compounds (QACs) are not completely removed during wastewater treatment and are frequently detected in surface waters and sediments. The photochemical transformation of QACs has not been thoroughly investigated as a potential degradation pathway affecting their fate in the environment. Kinetic studies of common QACs with and without aromatic groups under simulated and natural sunlight conditions were performed with model sensitizers and dissolved organic matter to estimate photochemical half-lives in the aquatic environment. All QACs investigated react with hydroxyl radicals at diffusion-controlled rates (∼2.9 × 109 to 1.2 × 1010 M-1 s-1). Benzethonium reacted via direct photolysis (ΦBZT,outdoor = 1.7 × 10-2 (mol Ei-1)). Benzethonium also reacted with the triplet excited state model sensitizer 2-acetylnaphthalene, but evidence suggests this reaction pathway is unimportant in natural waters due to faster quenching of the triplet 2-acetylnapthalene by oxygen. Reactivity with singlet oxygen for the QACs was minimal. Overall, reactions with hydroxyl radicals will dominate over direct photolysis due to limited spectral overlap of sunlight emission and QAC absorbance. Photolysis half-lives are predicted to be 12 to 94 days, indicating slow abiotic degradation in surface water.
Bibliographical noteFunding Information:
We would like to acknowledge Dr Sarah Pati for help with LC-MS/MS measurements as well as Makenzie Pillsbury. Mass spectrometry was carried out in the Analytical Biochemistry Shared Resources of the Masonic Cancer Center, University of Minnesota, funded in part by Cancer Center Support Grant CA-77598. The authors thank Annika Heaps and Bryanna Lopez-Moran for assistance with laboratory work. This work was funded by the Minnesota Environmental and Natural Resources Trust fund (M.L. 2017, Chp. 96, Sec. 2, Subd. 04a) as recommended by the Legislative and Citizen Commission on Minnesota Resources.
© 2020 The Royal Society of Chemistry.
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