Investigation of kinetics and mechanism for the degradation of antibiotic norfloxacin in wastewater by UV/H2O2

This study investigates the degradation of norfloxacin by using UV combined with hydrogen peroxide (UV/H2O2) with different cations and anions. The inhibition effect order of cations for degradation of norfloxacin was Cu2+ > Ca2+ > Mg2+, attributed to the forms of the coordination compounds between norfloxacin and the cations. The inhibition effect order of anions for degradation of norfloxacin was CO32− > NO3− > SO42−, attributed to the ability and efficiency of the anions to quench ·OH or H2O2. The order of the reactive oxygen species (ROS) contribution rate to the degradation of norfloxacin under UV/H2O2 was ·OH (72.56%) > 1O2 (24.24%) > other ROS (3.20%) based on the trapping experiments and electron spin resonance (ESR) measurements. There were at least 8 kinds of intermediate products, with charge-to-mass ratio (m/z) of 294, 267, 333a, 333b, 347, 335, 351 and 317, which were formed by attacks at the piperazine ring and F atom active sites. The F atom was replaced with -OH via a substitution reaction, resulting in P1 (m/z = 317) formation. The inhibition effect of P1 on DNA gyrase A was stronger than that of norfloxacin due to the binding free energy (the binding free energy of P1-DNA gyrase A was -12.8457 kJ/mol, and the binding free energy of norfloxacin-DNA gyrase A was -11.8488 kJ/mol).