Aquatic photochemistry of the sulfonamide antibiotic sulfapyridine

The photolytic behavior of the sulfonamide antibiotic sulfapyridine in water was investigated using a laboratory photoreactor approximating full-spectrum sunlight. Direct photolysis of sulfapyridine was rapid, with a half-life of 2.6 h and 31 min, and an observed quantum yield of 0.0013 ± 0.0002 and 0.013 ± 0.001, for the neutral species and fully deprotonated species, respectively. Direct photolysis rates increased dramatically with degree of deprotonation, with measured pKa1 and pKa2 values of 2.22 ± 0.03 and 8.58 ± 0.02, respectively. Indirect photolysis was assessed using water from constructed wetland mesocosms. A four-fold increase in the dissipation rate of sulfapyridine was observed due to the influence of high levels of dissolved organic carbon, after accounting for light screening by such materials. Nitrates had no observable effect on indirect photolysis rates. Major photoproducts identified were SO2 extrusion and OH addition products. These results show that photolytic processes are a major removal mechanism of sulfonamide drugs in aquatic systems.