Insight into the humic acid/antibiotic complexation for boosting tetracycline degradation by heterogeneous persulfate activation

Little attention has been paid to the degradation of antibiotics in their bound states, although numerous ionizable organic micro-pollutants possessed a strong tendency towards complexing with natural organic matters (NOM) in aquatic ecosystems. This study systematically investigated the degradation kinetics of tetracycline (TCH) bound with humic acid (HA) versus free TCH in heterogeneous persulfate activation system. Contrary to the homo-geneous UV/PS oxidation processes documented, the complexation with HA promoted the removal of TCH due to the adsorption behavior of TCH on the heterogeneous catalyst. The results displayed that higher first-order rate constants of HA-bound TCH degradation (up to 3.8 folds) were obtained compared to free TCH at varying TCH and HA concentrations. The presence of ionic strength, alkalinity and hardness had an inhibitory effect on the degradation of bound TCH, and the effect of pH was more significant due to the change of electrostatic complexation of TCH/HA. Differential absorbance spectra further identified that the degradation sensitivity followed the order of bound TCH > free TCH in TCH/HA mixed systems. In addition, the enhanced interfacial electron transfer that played a dominant role in TCH oxidation may be responsible for the efficient removal of TCH in the bound state. Results of this study provide new insights into the NOM/antibiotics complexation for boosting the antibiotics degradation in heterogeneous catalysis.