Study On Remendation Of PCBs Contaminated Soil In Subcritical And Supercritical Water

Polychlorinated biphenyls (PCBs) is one of the typical kinds of persistent organic pollutants(POPs), which has carcinogenic, teratogenic and mutagenic effect. PCBs can accumulate in body fat and pursue level enrichment through food-chain. It poses a serious threat to animals and human beings. Soil in waste transformer conservation points is becoming more and more seriously polluted by leakage, which could pose a threat to the surroundings at any time. However, there is no safe, green and efficient repair technology to deal with it. Subcritical and supercritical water reaction medium exhibits unique advantage, such as high ion product, low dielectric constant and miscible with organic matters and gas. It serves an safe, green and efficient way in dealing with persistent organic pollutant such as PCBs by forming a homogeneous reaction system. Therefore, subcritical and supercritical water disposal technology has been considered as one of the most potential effective technologies for POPs disposal. This study focuses on the disposal of high concentration PCBs contaminated soil, and all the experiments were carried on batch SCWO system. The mail contents of this paper are as follows:1) The key influence factors, including temperature, retention time and with or without oxygen excess, of PCBs removal in subcritical and supercritical water were studied. The results showed that after10min retention time at450℃, the removal efficiency of PCBs can reach as high as91.83%. The extension of retention time for the promotion of PCBs removal effect is obvious at300℃and400℃. The use of oxidant plays an obvious role in the removal of PCBs. What’s more, at350℃,400℃and450℃, the removal efficency was increased by7.12%,2.51%and7.55%respectively under aerobic conditions after10min retention time. With the increase of temperature, the percentage of low chlorinated PCBs increases. After processing, dl-PCBs content in the soil was removed effectively. The removal efficiency of WHO-TEQ can reach up to98.56%at450℃with oxidant. In addition, the concentraion of PCDD/Fs was reduced from55.017ng/g to7.20ng/g, and the WHO-TEQ was also reduced to below0.1ng TEQ/g. 2) The addition of NaOH contributed to the removal of PCBs in soil obviously, especially in supercritical conditions. The PCBs removal efficiency can reach up to99.44%at450℃with NaOH. After treatment, the residual PCBs concentration decreased to36.76mg/kg, which can meet the control standard of PCBs waste pollution in our country. In addition, the addition of NaOH made the percentage of low chlorinated PCBs increase while high chlorinated PCBs decrease. After treatment, the WHO-TEQ of PCBs and PCDD/Fs got effectively removed. At350℃and450℃, the removal efficiency of WHO-TEQ reached up to96.97%and99.22%, which reduced to0.862ng TEQ/g and0.221ng TEQ/g respectively.3) As the most highly chlorinated polychlorinated biphenyls (PCBs), deca-chlorobiphenyl (D10CB) was chosen to investigate the effects of temperature, oxygen amount, retention time and NaOH on its degradation in subcritical and supercritical water. The results showed that after10min retention time with50mL oxygen at350～450℃, the removal efficiency of D10CB was all above96%and the removal efficiency of total PCBs was77.2%-81.2%. At450℃, the removal efficiency of PCBs was improved by10.5%with200mL oxygen compared to the condition without oxygen. In addition, the presence of sodium hydroxide could not only improve the removal efficiency of D10CB and total PCBs, but also contribute to the even distribution of PCBs with different chlorinated levels. According to the distribution of PCBs and the existence of other organics after treated, we predicted that the degradation of D10CB in subcritical and supercritical water was firstly dominated by dechlorination and then a series of complicated reaction such as opening benzene ring, removal of molecule groups, oxygenolysis and so on.According to my study, key factors of PCBs contaminated soil in subcritical and supercritical water were preliminarily matered and the optimum reaction conditions were got. And after treatment, the amount of PCBs in soil could satisfy the control standard of China. What’s more, accroding to the decomposition products of D10CB, we predicted the reaction path of D10CB in subercritical and supercritical water. We’ve made preliminary achievements, which may provide a reference for business application and future project promotion.