Oxidation of sulfamethoxazole and related antimicrobial agents by TiO₂ photocatalysis

The widespread detection of pharmaceutically active compounds, including many synthetic antimicrobial agents, in aquatic environments is raising public health concerns. As a result, there is growing interest in the development of innovative technologies to efficiently transform these compounds to non-toxic and pharmaceutically inactive byproducts. This work examines the photocatalytic degradation of sulfamethoxazole (SMX) and related sulfonamide antimicrobial agents in aqueous suspensions of nanophase titanium dioxide (TiO₂). Experimental results demonstrate that SMX is mineralized by TiO₂ irradiated with ultraviolet-A light (UVA: 324≤λ≤400 nm). Rates of UVA-TiO₂ photocatalyzed SMX degradation are dependent upon several variables, including the initial SMX concentration, catalyst phase identity and concentration, electron acceptor identity and concentration, and the presence of non-target water constituents. In contrast, reaction rates are not sensitive to changes in sulfonamide structure. Although pH has little direct effect on reaction rates, the presence of natural organic matter (NOM) inhibits photocatalytic degradation of SMX to a much greater extent at pH 5 than pH 9. In addition, the presence of bicarbonate leads to enhanced SMX photocatalysis at pH 9. Kinetic trends are consistent with a mechanism involving sulfonamide oxidation by hydroxyl radicals (^{{radical dot}}OH) generated via TiO₂ band gap excitation by UVA radiation. Identified transformation intermediates and products are consistent with SMX mineralization initiated by ^{{radical dot}}OH attack on either the aromatic or heterocyclic rings or the sulfonamide bond. Results demonstrate that UVA-TiO₂ photocatalysis can be a very effective approach for degrading sulfonamide micropollutants, particularly in natural waters exhibiting either alkaline pH or low concentrations of NOM, or both conditions.