Photolysis of chlortetracycline on a clay surface

Chlortetracycline, an antibiotic commonly used as a growth promoter in livestock, enters the environment primarily through application of animal waste to open fields. The photochemical loss of chlortetracycline in sunlight-exposed soils is a potentially important process in its environmental fate, especially because it is photochemically labile and sorbs strongly to mineral surfaces. In this study, photolysis on kaolinite clay under simulated sunlight was used as a model system to elucidate the mechanistic kinetics of chlortetracycline photolysis on soil surfaces. The results suggest that photolysis may be an important loss process for chlortetracycline sorbed to sunlight-exposed soils, as well as to suspended clays in surface waters. Under direct irradiation equivalent to noon-time, summer sunlight in the midwestern United States, chlortetracycline at the outer clay surface (before light attenuation) degraded with a rate constant of k(0)(p) = 0.65 +/- 0.30 h(-1). The depth at which photochemical action was reduced by 50% (z(0.5)), one of the parameters of the mechanistic model, was found to be 0.014 +/- 0.004 mm. The quantum yield on the clay surface was estimated to be (1.3 +/- 0.7) x 10(-4), an order of magnitude lower than the quantum yield of the aqueous chlortetracycline zwitterion [(1.3 +/- 0.3) x 10(-3); pH 5], although still significant.