Kinetics of Oxytetracycline Reaction with a Hydrous Manganese Oxide^†

Tetracycline antibiotics comprise a class of broad spectrum antimicrobial agents finding application in human therapy, animal husbandry, aquaculture, and fruit crop production. To better understand the processes affecting these antibiotics in soils and sediments, the kinetics of oxytetracycline transformation by a hydrous manganese oxide (MnO₂) were investigated as a function of reactant concentration, pH, and temperature. Oxytetracycline was rapidly degraded by MnO₂. Initial reaction rates exhibited pronounced pH-dependence, increasing as pH decreased. Reaction of oxytetracycline with MnO₂ was accompanied by generation of Mn^II ions, suggesting oxidative transformation of the antibiotic. At pH 5.6, apparent reaction orders for oxytetracycline and MnO₂ were 0.7 and 0.8. Reaction order with respect to H⁺ was 0.6 between pH 4 and 9. Initial reaction rates increased by a factor of 2.4 for 10 °C temperature increases; the apparent activation energy (60 kJ·mol^-1) was consistent with a surface-controlled reaction. Reactivity of tetracycline antibiotics toward MnO₂ increased in the following order:  rolitetracyline ≈ oxytetracycline ≤ tetracycline ≈ meclocycline < chlortetracycline. The initial rate of chlortetracycline degradation by MnO₂ was substantially larger than that of the other tetracycline antibiotics investigated. MnO₂ reactivity toward oxytetracycline decreased with time; a retarded rate equation was used to describe oxytetracycline reaction with MnO₂ under declining rate conditions. This study indicates that natural manganese oxides in soils and sediments are likely to promote appreciable degradation of tetracycline antibiotics, and that reaction rates are strongly dependent on reaction time scale and solution conditions.