Photolytic transformation of trace organic compounds: Roles of direct photolysis and indirect photolysis by singlet oxygen

Both direct and indirect solar photolysis contribute to the in-situ attenuation of trace organic compounds (TOrCs) in surface waters, including those that are impacted by treated wastewater. In particular, the formation of singlet oxygen (¹O₂) from photosensitizers may play a role in the degradation of specific TOrCs. Quantification of the kinetics of photolytic processes is essential for anticipation of TOrC attenuation in sunlit waters. In this work, quantum yields for direct photolysis in sunlight and ultraviolet A light (290–400 nm), and second-order rate constants for TOrC reactions with ¹O₂ were determined for sixteen TOrCs that are ubiquitous in effluent-receiving surface waters. Six of the sixteen TOrCs (prednisone, dexamethasone, benzophenone, hydrocortisone, hydrochlorothiazide, and furosemide) were transformed via direct photolysis. Compounds that reacted readily with ¹O₂ included furosemide, propylparaben, and diltiazem. Second-order rate constants for reactions with ¹O₂ were determined using a kinetic model applied to a batch reactor and confirmed by measurement of kinetic solvent isotope effects (KSIE) in deuterated water (D₂O). A reactor model that combined direct and indirect photolysis was used to predict TOrC in chemically complex solutions containing natural organic matter (NOM) or effluent organic matter (EfOM), using the measured direct photolysis quantum yield and reaction rate constant with ¹O₂.