Removal of ciprofloxacin by persulfate activation with CuO: A pH-dependent mechanism

CuO synthesized by a hydrothermal method was used to activate persulfate (PS) for removing ciprofloxacin (CIP) in water. The influences of reaction parameters and coexisting substances on this process were studied. Under the optimal conditions ([CuO]0 = 6.3 mmol L−1, [PS]0 = 1 mmol L−1, pH 8), CIP was completely degraded and the total organic carbon removal efficiency was 84.2%. Ca2+, Mg2+ and Cl− had little effect on CIP removal, while fulvic acid showed a slight inhibitory effect. By contrast, bicarbonate enhanced CIP degradation and significantly suppressed Cu leaching. Subsequently, the reactive species for CIP degradation were identified by electron paramagnetic resonance and radical scavenging experiments. Although the decomposition efficiency of PS was significantly accelerated by increasing pH, similar amounts of O2−, OH, SO4− and 1O2 were detected at different pH, suggesting the formation of more surface activated PS at higher pH. The reactive species for CIP degradation were found to be dependent on solution pH. CIP was mainly degraded by SO4− and 1O2 at acidic conditions. The activated PS might have high selectivity towards CIP and its amount might be higher than that of radicals at pH 8. In addition, the uncharged surface of CuO at pH 8 facilitated the adsorption of CIP, which is beneficial to the surface reaction between CIP and activated PS. Hence, the activated PS was the dominated species for CIP degradation at this pH.