Kinetics and pathway of levofloxacin degradation by ferrate(VI) and reaction mechanism of catalytic degradation by copper sulfide

Abstract

In this study, the effect of pH and oxidant dosage on the removal rate of levofloxacin (LEVO) was investigated using potassium ferrate (Fe(VI)) as an oxidant, and the degradation kinetics of LEVO was studied accordingly. CuS, as a catalyst, was subsequently used to activate Fe(VI) for LEVO degradation. The experimental results showed that the degradation rate of LEVO by Fe(VI) accords with the second-order kinetic equation. The more Fe(VI) dosage or the lower the pH value, the faster the degradation rate of Fe(VI) to LEVO. In addition, CuS could significantly improve the Fe(VI)-mediated degradation of LEVO; the Fe(VI)-CuS system achieved a removal rate of 96.9% in 30 s (which was only 24.8% in Fe(VI) system). Then, the surface characteristics of fresh and used CuS were analyzed by scanning electron microscope, Brunauer-Emmett-Teller, X-ray diffraction and X-ray photoelectron spectroscopy. Subsequently, under similar experimental conditions, different systems were established to investigate the effects of Cu2+ and S2- leached from CuS in Fe(VI)-CuS system. Furthermore, the radical scavengers and methyl phenyl sulfoxide probe experiments proved that Fe(V)/Fe(IV) rather than radicals play a key role in the Fe(VI)-CuS system. The reaction mechanism of CuS catalyzing Fe(VI)-mediated degradation of LEVO can be summarized as follows: Fe(VI) adsorbed on the active site oxidizes the ionic bond of CuS to form Cu2+ and S2-; as an electron donor, S2- can promote the adsorbed Fe(VI) to produce more Fe(V)/Fe(IV), thus accelerating the degradation of LEVO in Fe(VI)-CuS system. Meanwhile, the leached Cu2+ and LEVO formed an organic complex Cu-LEVO, which is easily degraded by Fe(VI)/Fe(V)/Fe(IV) in the system. Finally, three possible degradation pathways were proposed based on 10 intermediates identified by Q-TOF LC/MS. This study provides the experimental data and theoretical support for the treatment of LEVO-containing wastewater using Fe(VI) and Fe(VI)-CuS systems.