oa South African Journal of Chemical Engineering - The ozone technology to reduce the concentration of polychlorinated biphenyls (PCBs) in contaminated soil-order of chemical reaction
Polychlorinated Biphenyls (PCBs) are hazardous and toxic chemicals to both humans and animals. In the late 1960's PCBs were discovered to be a harmful pollutant which could cause environmental contamination due to its slow degradation. PCBs were added into transformer oil in the past to increase its insulating properties. Oil spills are a frequent occurrence at Eskom substations which may contain PCBs. The contaminated sediments which are easily carried away by rain and wind can further contaminate the environment and aquatic bodies. Eskom, the world's 11th largest power utility has put a phase out plan in place to remove PCBs from the environment by the year 2025 since South Africa is a party to the Stockholm Convention. Eskom currently disposes of soil contaminated with PCBs by thermal destruction. This is a costly process as PCBs are regarded as hazardous materials and needs to be safely transported and disposed of at a licensed disposal facility. Based on literature, ozone has been used on a laboratory scale to treat soil contaminated with PCBs with the addition of chemicals such as hexane and acetone which assist in breaking down PCBs. The objective of this study was to evaluate whether ozone without the addition of chemicals could reduce the levels of PCBs present in contaminated soil. In this study, two soil samples were contaminated with 5ml of transformer oil which contained different PCB ranges (50 ppm, 200 ppm and 600 ppm) of PCBs and then exposed to 0.4, 0.5 and 0.6 l/min of ozone for a period of 60 minutes to examine the effects of increasing ozone flow rates on PCB destruction in soil. The results of the experimental tests showed that ozone gas reduced the concentration of PCBs in the soil samples for the different total gas flow rates. The literature study identified that the final products of the ozonation of PCBs are carbon dioxide and water and that any products formed after this process could possibly be degraded by the soil natural microorganisms. Calculations based on the Shin et al. (2004) model proved that ozone was in excess after the ozonation process. The results of the experiments also confirmed this, as the PCB residuals were similar for all three total gas flows, which showed that there was no dependence on the gas flow rates hence, ozone was in excess. The experimental data was then trended with zero, first and second order reaction equations, which showed that the best fit was obtained with the first-order reaction equation.
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