Degradation Performance And Mechanism Study Of Antibiotic Levofloxacin Treated By Electrochemical Advanced Oxidation Processes

The abuse of antibiotics has caused serious environmental pollution which has become one of major environmental problems in China and even all over the world. The antibiotic levofloxacin (LVX), which was most widely used in fluoroquinolone drug, has lead to residues and seriously exceeded, so it is of great significance to degrade LVX efficiently. In this paper, we use the electro-Fenton (EF), one of efficient advanced oxidation processes, to degrade the target pollutant LVX from two aspects:complete degradation (incomplete mineralization) and complete mineralization. The degradation behavior and reaction mechanism of LVX in EF system was investigated and its different operation conditions were optimized. The degradation intermediates were detected and its possible degradation pathways were pruposed. At the same time, the chelation between LVX and Fe2+/Fe3+ and its effect on the degradation of LVX by EF and BDD-EF processes were elucidated. The main results can be drawn as follows:(1) In order to reduce the treatment cost, using EF to improve the biodegradability:The optimal reaction conditions for LVX EF treatment were identified, i.e. I=0.36 A, pH=3.0,1.00 mM Fe2+ and 100 mL min-1 O2. Under this optimum condition,61.0% TOC was removed after 360 min EF electrolysis when a 500 mL 200 mg-L-1 LVX solution was treated. The BOD5/COD of the solution increased from 0 to 0.24 after 360 min treatment, showing the biodegradability was improved significantly. Increments in EF treatment time had a positive effect on biodegradability, for instance BOD5/COD ratios of 0.32,0.41 and 0.55 were obtain at 480,600 and 720 min, respectively. Thus LVX solution became more easily biodegrabdable and could be futher treated by low-cost biochemical methods. The LVX degradation by EF followed a pseudo-first-order kinetic with the apparent rate constant 2.37×10-2 min-1. Eight degradation intermediates of LVX were detected by the UPLC-QTOF-MS/MS system, while three carboxylic acids and F-, NO3-, NH4+ were analysed by HPLC and IC, respectively. On this basis, the possible degradation pathway of LVX in EF process was proposed.(2) Chelation mechanism between LVX and Fe2+/Fe3+ and its effect on LVX degradation by EF:The results indicated that Fe3+likely complex with LVX, and a better saturation complexation ratio of [LVX]:[Fe3+]= 2:1 was determined by Job’s method. With the same Fe2+ or Fe3+ concentration, more TOC abatement could be reached in EF when Fe3+ was used as catalyst, showing that the complexation between Fe3+ and LVX was beneficial to the mineralization of LVX by EF. The conversion of Fe2+/Fe3+ in EF process stated clearly that Fe3+ can be effectively reduced by ACF cathode, which can maintain a suitable Fe2+ concentration for EF reaction continuously and efficiently.(3) To achieve the purpose of complete mineralization, BDD was used in EF process for mineralizing LVX efficiently. Under the optimum BDD-EF conditions of I=0.36 A, pH=3.0,1.00 mM Fe2+ and 100 mL min-1 O2,200 mg·L-1 LVX could be completely degraded only within 60 min, and 92.0% TOC removal was obtained after 360 min electrolysis. The results showed that BDD-EF could not only decompose LVX quickly, but also mineralize LVX and its intermediates efficiently. Five degradation intermediates of LVX were detected, as well as three carboxylic acids and F-,NO3-, NH4+, a possible mineralization pathway of LVX in BDD-EF process was proposed.(4) In order to obtain optimal treatment process, the degradation abilities and their reaction mechanisms of RuO2/Ti-AO, EF, BDD-AO and BDD-EF processes were compared. Results demonstrated that LVX was degraded in the order BDD-AO< RuO2/Ti-AO<EF<BDD-EF, while the mineralization abilities followed the order of RuO2/Ti-AO<BDD-AO<EF<BDD-EF, their TOC removals after 360 min treatment were 14.6%,79.4%,90.9% and 92.0%, respectively. No H2O2 or OH was generated in RuO2/Ti-AO system, while there were H2O2 and OH formed in other oxidation systms. After 120 min electrolysis, the H2O2 concentrations were 215.3 μM for AO-H2O2,237.3 μM for BDD-AO and 515.3 μM for BDD-AO-H2O2, respectively. The formed OH concentrations were 78.0 μM for EF,85.8 μM for BDD-AO and 147.9 μM for BDD-EF, respectively. These results illustrated that the introduction of BDD can obviously increase the productivity of H2O2 and OH. In BDD-EF process, the BDD and ACF electrodes have a synergistic effect, so the oxidation ability of BDD-EF system is greater than the simple addition of BDD anoede and ACF cathode alone.