The optimization of enzymatic oxidation of levofloxacin, a fluoroquinolone antibiotic for wastetwater treatment.

The global presence of antibiotics in the environment has created concerns about the emergence of antibiotic resistance bacteria and potential hazard to humans and the ecosystem. This work aims to study the removal of levofloxacin, a new generation fluoroquinolone antibiotic from aqueous solutions by enzyme mediated oxidation process and optimization of the conditions thereof by response surface methodology (RSM) using Box-Behnken design (BBD). For this study, experiments were conducted to analyze the effect of independent variables namely, pH, temperature, mediator concentration and antibiotic concentration on the degradation percentage of levofloxacin antibiotic using laccase enzyme derived from Trametes versicolor. The residual levofloxacin concentration was determined using high performance liquid chromatography (HPLC). On applying the quadratic regression analysis, among the main parameters, it was found that the percentage degradation was significantly affected by all the four variables. The predicted values for percentage degradation of levofloxacin were close to the experimental values obtained and the R2 (0.95) indicated that the regression was able to give a good prediction of response for the percentage degradation of levofloxacin in the studied range. The optimal conditions for the maximum degradation (99.9%) as predicted by the BBD were: temperature of 37 °C, pH of 4.5, mediator concentration of 0.1 mM and levofloxacin concentration of 5 μg mL-1. The findings of the study were further extended to study the effect of partially purified enzymes isolated from Pleurotus eryngii, Pleurotus florida and Pleurotus sajor caju on the degradation of levofloxacin at concentrations ranging from as low as 0.1 to as high as 50 µg mL-1 in synthetic wastewater utilizing the optimized conditions generated by BBD. A maximum degradation of 88.8% was achieved with the partially purified enzyme isolated from Pleurotus eryngii at 1 µg mL-1 levofloxacin concentration which was at par with the commercial laccase which showed 89% degradation in synthetic wastewater at the optimized conditions. The biodegradation studies were conducted using only 2 units of laccase. Thus, the expensive commercial laccase can be effectively replaced by crude laccase isolated from indigenous macrofungi such as P. eryngii, P. florida and P. sajor caju as a cost effective alternative to degrade levofloxacin present in contaminated wastewater using as low as 2 units of enzyme for a 72 h treatment period.