Remediation Of PCBs-contaminated Soil By Low-temperature Thermal Dechlorination

PCBs(polychlorinated biphenyls,PCBs) are a class of synthetic and the most representative POPs(persistent organic pollutants,POPs), widely used as insulating oil of power capacitor and pressure transmitting of hydraulic systems and so on. In May2011, the Stockholm Convention required that PCBs were priority controllers of the12persistent organic pollutants. According to the survey results of PCBs inventory of Zhejiang and Liaoning Province, the total amount of high concentrations of PCBs (more than500mg/kg) was about50,000t, low concentrations of PCBs (50-500mg/kg) was about500,000t, the concentrations of PCBs contaminated soil less than50mg/kg was at least of millions of t. PCBs have environmental persistence, bioaccumulation and toxicity, long-distance transmission globally, soil contaminated will lead PCBs accumulate in living organism and transfer and transform through the food chain, ultimately harm plants and animals as well as human beings.Low-temperature thermal dechlorination was proved to be effective in UPOPs and HCB elimination, but present study reported few about low-temperature thermal dechlorination of PCBs. The dechlorination rate of PCBs, dechlorination pathway and ultimate dechlorination products were unknown yet. The aim of this study was to investigate the effects of PCBs removal rate under treated temperature, time and calcium oxide/magnesium oxide addition proportion, reveal the dechlorination rate and changes of pollutant composition in soil from a storage point of used capacitors containing PCBs oil and PCB77, elucidate the ultimate substances fate and the dechlorination pathway of the dioxin dechlorination.The PCBs concentration of numberl soil sample was107.7mg/g, belonging to high concentration PCBs solid waste. The results showed that PCBs dechlorination rate improved with the increase of reaction temperature and time. The optical reaction conditions for dechlorination of PCBs in soil sample were at400℃for4h with10%CaO while the removal rate and dechlorination rate of PCBs reached87.7%and85.3%respectively. Under different temperature and reaction time, PCBs dechlorination rate showed a decreasing trend after the first increase with the amount of added CaO.Increasing reaction temperature could effectively improve the dechlorination efficiency of PCBs, temperature was the main affecting factor of low-temperature thermal dechlorination of PCBs.Pentachlorobiphenyl and Tetrachlorobiphenyl in origin soil samples decreased or disappeared under the condition of350℃and400℃for4h, while monochlorinated biphenyl and biphenyl were detected after reaction, which indicating that PCBs dechlorination/hydrogenation pathway induced by CaO existed. PCB105meta-dechlorination-/hydrogenation directly generated PCB66, PCB105ortho-dechlorination/hydrogenation directly generated PCB66.The PCBs concentration of number2soil sample was5.4mg/g, belonging to high concentration PCBs contaminated soil. The dechlorination rate of PCBs reached68.2%and68.0%respectively at400℃for4h with10%CaO and10%MgO, the catalytic activity of CaO and MgO were quite the same during the low-temperature thermal dechlorination of PCBs.Organics of soil samples were not detected after reaction, which means that the removal efficiency of PCBs reached99.0%,indicating that low-temperature thermal dechlorination technology has a high removal efficiency of high concentration of contaminated soils.l0mg PCB77with200mg CaO under400℃reacted for0.5,1,2,4and6h respectively, the dechlorination rate of PCB77were34.8%,39.9%,41.5%,44.2%and44.9%, the removal rate were45.6%,49.0%,50.1%,52.4%and61.3%.After low-temperature thermal dechlorination, PCB77generated PCB8and PCB31through dechlorination/hydrogenation, while PCB77also transformated into PCB80and PCB70.The species of dechlorination products were identified by Raman spectroscopy, amorphous carbon by Raman spectroscopy. The amorphous carbon containing crystalline graphite plane structure proved that dechlorination/polymerization pathway existed. Raman spectra detected the peak of carbonyl at1769cm-1of PCB77, indicated that PCB77generated carbonyl group-containing high polymer product through dechlorination/polymerization.Three tested samples showed a phenomenon of certain degree of non-conservation mass of chlorine after low-temperature thermal dechlorination, and accordingly with higher dechlorination rate of PCBs, the extent of losses of chlorine mass was greater. The dechlorination/polymerization pathway proved to be a reasonable explanation for the non-conservation mass of chlorine after low-temperature thermal dechlorination. PCBs generated high polymer products(disordered carbon) through dechlorination/polymerization pathway was the ultimate substances fate of PCBs.PCBs existed dechlorination/hydrogenation and dechlorination/polymerization pathways through low-temperature thermal dechlorination induced by CaO, inferred that the non-conservation mass of chlorine after low-temperature thermal dechlorination was because of the generation of high polymer products and not detected intermediates through dechlorination/polymerization.