Studies on Parametric Optimization of Pentachlorophenol Degradation using Box-Behnken Design

Abstract

Due to the presence of biorecalcitrant organic contaminants in the environment resulting from the ever increasing industrial growth is of primary 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), which involves the in situ generation of highly potent chemical oxidants such as OH radicals, have emerged as an important class of technologies to accelerate the non selective oxidation and thus leads to the destruction of wide range of recalcitrant organic contaminants in wastewater which cannot be eliminated biologically. 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, Pentachlorophenol (PCP), which has been listed third in the priority pollutants list of EPA, was chosen as the model compound. PCP is an important industrial raw material frequently used in the manufacture of pesticide, herbicides and fungicides. The photocatalytic degradation of PCP (25 mg/l) was carried out in slurry mode in the specially designed photoreactor equipped with UV tubes at constant temperature. The UV/Vis Spectrophotometer was used for analyzing the concentration of PCP in solution at different time intervals during the experiment. The rate of degradation has been examined in terms of change in the absorbance at λmax of 219 nm. The effect of varying various photocatalytic process parameters such as catalyst dose, pH, UV intensity and light source (UV/solar) has been investigated. 83.45% and 74.3% degradation efficiency was achieved after 6 hr under UV light employing ZnO and TiO2 under optimized conditions (0.5 g/L ZnO; pH 8.0 and 0.5 g/L TiO2; pH 8.0) respectively. The results demonstrated that ZnO exhibited better photocatalytic activity as compared to TiO2 for the degradation of PCP. 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 PCP with ZnO (25 mg/l). The variables examined in this study included ZnO dose, pH and time. The significant variables and optimum conditions were identified (ZnO dose .35 g/L, pH 9.5 and time 5.2) from statistical analysis of the experimental results. 89.5% degradation efficiency was achieved under these conditions which were in agreement with the predicted value. The degradation of PCP shows better results in solar light under optimized conditions after 6 hrs with ZnO as compared to UV irradiation. The rate of degradation of PCP was also studied in terms of TOC. Maximum TOC reduction 86.89% was achieved on optimized conditions (0.5 g/l ZnO dose, pH 8 after 6 hrs) The results depicted that heterogeneous photocatalysis employing solar light can be efficiently and cost effectively used for the degradation of PCP.