[Oxidative kinetics of chlortetracycline by manganese dioxide]

To better understand the transformation of chlortetracycline (CTC) under the oxidation of manganese dioxide (MnO2), kinetics of CTC reaction with MnO2 were investigated and the effects of MnO2 initial concentration, pH, and temperature on kinetics were also discussed. Results showed that loss of CTC was not detected in the absence of MnO2. But CTC was rapidly degraded along with the generation of Mn2+ in the presence of MnO2, which indicated that this process was an oxidation-reduction reaction. In order to explain the adsorption of CTC on MnO2 surface, two different approaches (filtration and oxalic acid addition) were used to quench the reactions. Oxidation was observed dominantly in the initial stage compared with adsorption, and adsorption become noticeable after the reaction reached an equilibrium level. The reaction rate of CTC increased with the increase of MnO2 initial concentration and temperature of the reaction system. In addition, the reaction rate of CTC also decreased significantly as the pH of reaction mixture increased from 3.77 to 7.41, which was caused by the decreased adsorption of CTC on MnO2 with the increase of pH. Over the entire pH range (3.77-7.41), the speciation of CTC transited from cation and then zwitterion to anion, however, the MnO2 surface exhibited all with negative charges because its zeta-potential (pH point of zero charge) was 2.25. Thus, pH increase would result in the decline of the adsorption force between CTC speciation and the MnO2 surface.