Degradation of oxytetracycline in the aquatic environment: a proposed steady state kinetic model that takes into account hydrolysis, photolysis, microbial degradation and adsorption by colloidal and sediment particles

Environmental context Pollution of the aquatic environment by oxytetracycline can lead to microbial resistance thereby compromising the efficacy of current medication regimes. Adsorption by colloidal and sediment particles reduces the rate at which oxytetracycline degrades, whereas the longer the antimicrobial remains in the aquatic environment, the greater the danger of microbial resistance. There is need therefore for a fuller understanding of the kinetics of degradation of oxytetracycline in aquatic ecosystems before measures for mitigating pollution by the antimicrobial can be designed. Abstract The persistence of oxytetracycline in an aquatic microcosm and distilled water control experiments, was studied over a period of 90 days. An immediate 35% loss as a result of adsorption by the sediment was observed in the microcosm experiment soon after charging. Subsequently triphasic linear rates of oxytetracycline degradation were observed for both the water phase (3.1×10–2, 5.8×10–3 and 1×10–3µgg–1day–1) and sediment phase (4.8×10–2, 6.5×10–3 and 2×10–4µgg–1day–1). Degradation is attributed to photolysis and microbial degradation of the free oxytetracycline in solution, and microbial degradation of the colloidal and sediment particle adsorbed speciation forms. The distilled water control exhibited biphasic zero order kinetics attributed to hydrolysis (2×10–6µgg–1day–1) and microbial degradation (2.7×10–3µgg–1day–1) under dark conditions, and monophasic zero order kinetics attributed to photolysis (6.9×10–3µgg–1day–1) under sunlight exposure. A kinetic model that takes into account hydrolysis, photolysis, microbial degradation and adsorption of the antibiotic by colloidal and sediment particles, is presented to account for the monophasic, biphasic and triphasic zero order kinetics observed in the control and microcosm experiments. Possible remediation strategies for mitigating aquatic environments polluted by the antimicrobial are discussed.