Photocatalytic Oxidation of Biorecalcitrant Compound and its Optimization

Abstract

The presence of highly biorecalcitrant organic contaminants in the environment due to industrial sector is of prime concern for the preservation of aquatic ecosystems. Biorecalcitrant compounds are basically non-treatable in conventional biological wastewater treatment plants, so the development of new technologies that pursue the degradation of such substances is of practical interest. Advanced oxidation processes (AOPs), a group of chemical reactions characterized by the “in situ” production of OH radicals under mild experimental conditions, have shown to rapidly degrade many different biorecalcitrant compounds. Among AOPs, heterogeneous photocatalysis has emerged as an efficient method for degrading the organic contaminants. Heterogeneous photocatalysis involves the acceleration of photoreaction in presence of semiconductor photocatalyst and UV irradiation. In the present study, Acrylonitrile (ACN), which has been listed third in the priority pollutants list of EPA, was chosen as the model compound. ACN is an important industrial raw material frequently used in the manufacture of acrylic and modacrylic fibres, resins and rubber and as a chemical intermediate. The photocatalytic degradation of ACN (50 mg/L) was carried out in slurry mode in the specially designed photoreactor equipped with UV tubes at constant temperature. The rate of degradation has been examined in terms of change in the absorbance at λmax of 204 nm. The effect of varying various photocatalytic process parameters such as catalyst dose, pH, UV intensity, area to volume ratio, oxidant concentration, light source (UV/solar) and mixed catalyst has been investigated. 78.9% and 71.1% degradation efficiency was achieved in 30 min under UV light employing ZnO and TiO2 under optimized conditions (1.5 g/L ZnO; pH 8.0; 5mM/L H2O2 and 1.5 g/L TiO2; pH 4.0; 5mM/L NaOCl), respectively. The results demonstrated that ZnO exhibited better photocatalytic activity as compared to TiO2 for the degradation of ACN. Experiments were also conducted in solar light under optimized conditions which showed 82% and 78.4% degradation efficiency after 30 min with ZnO and TiO2, respectively. The results depicted that heterogeneous photocatalysis could be used as an efficient and eco-friendly technique for the complete degradation of biorecalcitrant organic compounds. In order to minutely evaluate the effects of individual operating variables and optimization of process parameters, Box Behnken design of experiments was applied for the degradation of ACN with ZnO (50 mg/L). The variables examined in this study included ZnO dose, pH and H2O2 concentration. The significant variables and optimum conditions were identified (ZnO dose 1.48 g/L, pH 7.0 and H2O2 concentration of 4.22 mM/L) from statistical analysis of the experimental results using response surface methodology (RSM). 89.6% degradation efficiency was achieved under these conditions which was in agreement with the predicted value. The results depicted that heterogenous photocatalysis employing solar light can be efficiently and cost effectively used for the degradation of ACN.