Insights into pH-Dependent CuII-Mediated Transformation of Oxytetracycline: Evidence from CuI Formation Kinetics

As animal feed additives, both veterinary tetracycline antibiotics (TCs) and heavy metals, such as CuII ions, often coexisted in aquaculture wastewater and could cotransport as CuII–TCs complexes into groundwater. Previous study emphasized the oxidative role of CuII on the transformation of TCs in the presence of oxygen and the excess CuII, but the transformation of TCs with limited CuII in dark and anoxic groundwater remained unclear. Hence, this study systematically investigated the CuII-mediated transformation of oxytetracycline (OTC), as the most widely used veterinary TCs, under limited CuII and anoxic conditions. The results indicated that OTC degradation was significantly pH-dependent in relation with the speciation of CuII–OTC complexes at pH 3–8.5, and the presence of humic acid (1–10 mg/L) or CaII (10–50 times of CuII) did not change the transformation rate of OTC with CuII significantly. Linking OTC degradation kinetics and CuI formation kinetics, the oxidative role of CuII could be quantitatively evaluated to 6.99–20.2% at varying pH and CuII/OTC ratios from 1:1 to 4:1. A redox site model was built to describe the kinetics of CuI formation, which indicated that CuII-mediated oxidative transformation of OTC is subject to nonidentical specific CuII-binding sites at varying pH. This study is among the first to focus on the CuI formation kinetics to further evaluate the oxidative role of CuII in the catalyzed transformation of TCs, which provides novel insights into the natural attenuation of CuII–TCs complexes as an overlooked class of emerging contaminants in anoxic groundwater.