Studies On Method And Mechanism Of Organic Pollutants Contaminated Soil Remediation By Pulsed Discharge Plasma

With the quick development of industrial production and urbanization, lots of relocated or closed industrial enterprises caused serious soil pollution, especially organic pollutants contaminated soil. These polluted sites need quick commercial utilization, but lots of organic pollutants in soil bring dangers to people’s living. Therefore, rapid and high efficient remediation for these sites is becoming a necessary. Lots of methods had been proposed, such as physical remediation, traditional chemical remediation, and bioremediation; however, there exist some drawbacks, and can not meet remediation requirements of high efficiency and speed, and thus development of new methods for soil remediation is important. Discharge plasma, one of advanced oxidation processes, has been received great attention in pollution control, showing advantages of high efficiency, high speed, no selective for pollutants, and no secondary pollution. Therefore, pulsed discharge plasma was proposed to remediate organic pollutants contaminated soil in this paper. The main work was conducted in terms of pollutants degradation characteristics, generation and roles of active species, pollutants degradation mechanisms, transport of pollutants and active species in soil, synthetic effects of discharge plasma and catalyst for pollutants removal. The detailed work and the summarized results are as follows:1. Pulsed discharge plasma was used to remediate pentachlorophenol (PCP) contaminated soil. PCP degradation characteristics in soil were studied by evaluating the effects of electrical parameters, reactor configuration parameters, carrier gas atmospheres, and soil properties on its degradation. The experimental results showed that pulsed discharge plasma was an efficient and quick method for PCP contaminated soil remediation.83.5%of PCP could be removed within60min’s discharge treatment. Increasing pulsed discharge voltage and pulsed frequency, and decreasing electrode distance could improve PCP removal. The greatest PCP removal efficiency was obtained in O2atmosphere, followed by air atmosphere, and the lowest in N2atmosphere. There existed an optimal value for gas flow rate to obtain high PCP removal. Soil organic matter inhibited PCP removal. Alkaline soil and certain water content in soil benefited PCP removal.2. By qualitative and quantitative analysis of active species, and analysis of PCP degradation intermediates by TOC, UV-VIS, FT-IR, IC, and HPLC-MS, PCP degradation mechanisms were proposed. O-containing active species oxidation (such as O3and·OH) was the main process for PCP degradation. The attacks of these active species to C-Cl bonds in PCP molecule initiated PCP degradation process, generating hydroxylated intermediates, accompanied by dechlorination reactions, and then the hydroxylated intermediates were further degraded into small organic acids, CO2and H2O.3.p-Nitrophenol (PNP) degradation in soil by pulsed discharge plasma was investigated, and the transfer of O3in soil was also studied. The effects of matrix, soil properties, discharge voltage, and PNP concentration on PNP degradation and O3breakthrough were evaluated. The results presented that due to the different oxidative conditions in soil layers, PNP degradation, mineralization, and intermediates were quite different in different soil layers. PNP degradation efficiency decreased with increase of soil depth. Soil properties, discharge voltage, and PNP concentration affect PNP degradation. Soil organic matter and metal oxides, soil properties, and O3concentration affect its diffusion in soil layer.4.Pulsed discharge plasma was used to remediate PCP and PNP mixtures contaminated soil. The mutual effects between reactants were discussed. The results showed that pulsed discharge plasma presented great performance for mixtures contaminated soil remediation.86.0%of PNP and94.1%of PCP could be removed in certain conditions. There existed mutual suppressant effects between PCP and PNP during soil remediation. Alkaline soil and high discharge voltage were both beneficial for mixtures contaminated soil remediation.5. Pulsed discharge plasma and TiO2catalyst were combined to remediate PNP contaminated soil. The effects of discharge voltage, soil pH and gas moisture on PNP degradation were studied. The synthetic mechanisms were discussed by comparing PNP degradation, mineralization, active species generation, and evolution of intermediates with single discharge system. The results showed that more active species were generated in pulsed discharge plasma/TiO2system, resulting in higher PNP degradation and mineralization, and then accelerated generation and mineralization of intermediates.