Synergistic enhancement of piezocatalysis and electrochemical oxidation for the degradation of ciprofloxacin by PbO2 intercalation material

In this study, the TiO2 nanotube array intercalated PbO2 (Ti/TiO2-NTA/PbO2-La) material was applied in the piezo-electro-catalytic removal of ciprofloxacin (CIP). The degradation rate of CIP reached up to 91.4% after the mechanical ultrasonic vibration of the Ti/TiO2-NTA/PbO2-La material for 20 min with applied current density of 0.04 mA cm−2. The degradation rate constant (kCIP) of CIP by piezo-electro-catalysis was 8.6 and 17.2 times that of the electrochemical oxidation and piezocatalysis. The degradation of CIP exhibited a rapid nearly-linear increase with increasing applied ultrasonic power (<13.6 W) at a current density of 0.04 mA cm−2 or the current density (<0.11 mA cm−2) at an ultrasonic power of 15.0 W. The enhancement mechanism of the synergistic process was due to that the dynamic piezoelectric field and the applied bias facilitate the carriers transport and accelerate the electric charges carrier separation, hence favouring the generation of homogeneous reactive species. Based on quenching experiments and electron spin resonance (ESR) results, holes (h+) and hydroxyl radical (OH) were found to be the predominant reactive species in CIP removal in the piezo-electro-catalytic process. h+ may be involved in the rate-limiting degradation steps. With the identification of the intermediate byproducts and the theoretical calculations of the frontier electron densities, three possible degradation pathways were proposed in the cleavage of the piperazine and quinolone rings, and substitution of –COOH by –OH group. The findings of this study suggest that the synergistic process of piezocatalysis and electrochemical oxidation is an effective way to promote degradation efficiency of CIP at a ultralow current density and ultrasonic power.