Degradation of polychlorinated biphenyls in water matrix using UV/H2O2

This study investigates the effectiveness of advanced oxidation process (AOPs) for the degradation of polychlorinated biphenyls (PCBs). PCBs are a class of 209 individual chlorinated compounds whose high chemical stability and toxicity make them some of the most serious and persistent organic pollutants. Preliminary experimentations involved the application of selected advanced oxidation processes (AOPs) - UV radiation, UV/H2O2 and UV/TiO2 systems - to degrade PCBs in both oil and water matrices. Results show that there was no significant difference in the degree of degradation of PCBs for the UV/H2O2 and UV/TiO2 systems in both oil and water matrices. In oil matrix, UV/TiO2 and UV/H2O2 exhibited 50.47 and 46.5% PCB degradation after six hours of irradiation, respectively. For PCBs in water matrix, 70% degradation was obtained for the UV/H2O2 system while 67% PCB degradation was observed for UV/TiO2. From the experimental results and considering the ease of operation of the process, the UV/H2O2 system was selected for PCB degradation. However, further experiments were carried out for PCBs in water matrix only due to the low degradation efficiencies obtained for PCBs found in oil matrix. The UV/H2O2 process was used to degrade PCBs in water matrix. This system involves the generation of the highly reactive hydoxyl radicals (OH.) by irradiating a pollutant solution containing H2O2 with UV light lower than 280 nm. The effect of the initial H2O2 concentrations was determined using a tubular photoreactor with recycling to allow complete mixing of the aqueous PCB solution. Sampling was done at desired time intervals. Temperature and pH of the solution were continuously monitored. Results show that there is an optimum H2O2 concentration that may be used beyond which no significant increase in the degree of degradation was achieved. The highest PCB degradation efficiencies of 87.58% after 60 minutes were obtained at a peroxide concentration of 24.71 mM. The initial pH of the solution also proved to have an effect on PCB degradation. Highest oxidation efficiency of 88% was obtained at pH 7 after 60 minutes at peroxide concentration of 24.71 mM. The effect of the initial concentration of PCBs at a given peroxide concentration of 2800 ml per liter was also evaluated. Up to a certain concentration, degradation was found to increase at increasing initial concentration. Highest degradation of 53% and 87% at 15 and 60 minutes, respectively, was obtained at PCB concentration of 40 ppm. Decreasing percentage of efficiency was already observed at 80 ppm which gave a lower degradation efficiency of 79% after 60 minutes. The role of reaction intermediates in evaluating the effect of this parameter may prove to be critical. Preliminary kinetic analysis shows that there is variation in the order of the reaction for low and high initial PCB concentrations. The degradation, using the UV/H2O2 system, at low initial PCB concentration is described by a first order reaction while that for high initial PCB concentration was defined by a second order kinetics. Reaction rate constants for the degradation of PCB with initial concentration of 1, 5 and 10 ppm were evaluated to be 0.0145, 0.0152 and 0.0192 min-1 while for the 40 and 80 ppm runs, reaction rate constants obtained were 0.0013 and 0.0006 L.mg-1min-1. From among the initial concentration tested, the 40 ppm run reached its half-life at 19.2 minutes