Photodegradation of pharmaceutical antibiotics on slurry and soil surfaces

Pharmaceutical antibiotics are emerging soil environmental contaminants while knowledge about their fate and degradation is insufficient. Thus, the dissipation of selected antibiotics and its kinetics through photodegradation and other abiotic processes on substrate surfaces was investigated. Oxytetracycline, chlorotetracycline, sulfanilamide, sulfadimidine, sulfadiazine, sulfadimethoxine, sulfapyridine, fenbendazole, and p-aminobenzoic acid were either spread on sterilized layers (<= 0.8 mm) of quartz sand, two topsoil samples, and pig slurry or dissolved in water. Samples were either irradiated using arc light or, in parallel, kept in the dark for 28 d. All antibiotics were directly photodegraded in water with first order rate coefficients (k(p)) from 0.005 to 0.12 d(-1). Without irradiation, the proportions recovered after incubation from soils, sand and slurry ranged from 98% to a complete decrease. Photodegradation further decreased the recovered concentration by a median factor of 2. However, for the sulfonamides and fenbendazole average k(p) in soils and sand was 0.01 d(-1), and thus 2.4 times smaller compared to water. Tetracycline photodegradation followed biphasic kinetics indicating two fractions of different photodegradability. Enhanced photodegradation of the first fraction (k(p), 10.19-1.43 d(-1)) was attributed to additional indirect photodegradation processes. The second fraction was practically unavailable for photodegradation (k(p,2) <= 0.0003 d(-1)); between 55 to 94% of the initial tetracycline concentration were not photodegraded. In pig slurry, photodegradation of most antibiotics was considerably increased with k(p) of 0.10 to 3.33 d(-1) and went along with a photobleaching of the slurry itself.