Direct and indirect photolysis of sulfamethoxazole and trimethoprim in wastewater treatment plant effluent

Christopher C. Ryan, David T. Tan, William A. Arnold

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218 Scopus citations


The photolysis of two antibacterial compounds, sulfamethoxazole and trimethoprim, was studied in wastewater effluent. The rate of loss of sulfamethoxazole was enhanced in wastewater effluent due to indirect photolysis reactions, specifically reactions with hydroxyl radicals and triplet excited state effluent organic matter. Photolysis in the presence of natural organic matter, however, did not lead to enhanced degradation of sulfamethoxazole. Trimethoprim was also found to be susceptible to indirect photolysis in wastewater effluents, with hydroxyl radical and triplet excited effluent organic matter being the responsible species. Deoxygenation of solutions led to more rapid direct photolysis of sulfamethoxazole and trimethoprim, indicating that direct photolysis proceeds through a triplet excited state, which was verified by demonstrating that trimethoprim is a singlet oxygen sensitizer. In the wastewater effluents tested, photolysis could be apportioned into direct photolysis (48% for sulfamethoxazole, 18% for trimethoprim), reaction with hydroxyl radicals (36% and 62%, respectively) and reaction with triplet excited effluent organic matter (16% and 20%, respectively). These results indicate that allowing photolysis in wastewater stabilization ponds or wastewater treatment wetlands may lead to enhanced pharmaceutical removal prior to discharge and that effluent organic matter has different photoreactivity than natural organic matter.

Original languageEnglish (US)
Pages (from-to)1280-1286
Number of pages7
JournalWater Research
Issue number3
StatePublished - Jan 2011

Bibliographical note

Funding Information:
This work was funded by the Minnesota Environment and Natural Resources Trust Fund . Thanks to Peter Steen and Kristopher McNeill for valuable discussions. The reviewers are also thanked for their valuable input.


  • Dissolved oxygen
  • Nitrate
  • Pharmaceuticals
  • Photolysis
  • Triplet organic matter
  • Wastewater


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