Research Of Bioslurry Treatment For Polychlorinated Biphenyls Contaminated Soil

Polychlorinated biphenyls (PCBs) are typical persistent organic pollutants (POPs), which are long-term residual, have carcinogenic, teratogenic and mutagenic effects, and are able to accumulate in organisms. Although PCBs have been banned for a long time, now they still can be detected in soils, sediments, water, air and organisms worldwide and thus their ecological risk can not be neglected. Bioremediation technology has become a research hotspot in the field of soil remediation recent years. However, the low remediation efficiency of bioremediation technology limit its development and application. Bioslurry treatment has preferable research and application value because of its controllability and higher remediation efficiency. Therefore, this thesis focused on improving the biodegradation efficiency of PCBs in bioslurry treatment process, confirming the major factors which affected PCBs degradation, and illuminating the potential and bottleneck of bioslurry treatment for PCBs-contaminated soil remediation. The main results are as follows:A bacterial mixed-culture MH1 and two PCBs-degrading bacteria, which were identified as Rhodococcus ruber SS1 and Rhodococcus pyridinivorans SS2, were obtained from PCBs-contaminated soil. SSI, SS2 and Sphingobium fuliginis HC3 (previously obtained) have strong degradation ability of lower chlorinated (mono-, di-and tri-) PCBs in aerobic condition. It had been found that SS1 and HC3 could also degrade PCBs metabolites (benzoate and mono-CBAs).PCBs were spiked into the contaminated soil and then aerobic bioslurry systems were constructed. It was found that PCBs degradation capacity of the native microorganisms were very weak, and the reduction of PCBs were mainly caused by their volatilization in these systems. There were a certain amount of PCBs-and CBAs (PCBs metabolites)-degrading bacteria in PCBs-contaminated soil. However, they only constituted approximately 0.1% of the culturable bacteria in soil, which might be a possible reason for restricting in situ PCBs biodegradation.In the contaminated soil or aerobic slurry with high concentration of PCBs, both the bioavailability of PCBs and PCBs degradation capacity of the native microflora affect PCBs biodegradation. The bioavailability of tri-CBs were significantly increased by changing soil into slurry and adding RAMEB and assisting mechanical mixing, but these methods had unobvious effects on the bioavailability of tetra-CBs and high chlorinated PCBs (Cl> 4). The removal of tri-CBs in the soil and aerobic slurry mesocosms reached 44% in 100 d while other PCBs changed little, which showed the weak PCBs degradation capacity of the native microflora. Based on the analysis of bacterial community structure, it was found that the richness and diversity of bacterial community presented an overall downward trend over time in the soil and slurry mesocosms, but the slurry mesocosms were favorable for the survival of several PCBs-degrading bacteria.The orthogonal experiment was adopted to study the effects of glucose (A), biphenyl (B), RAMEB (C), soil-to-water ratio (D) and degrading bacteria (E) on the aerobic degradation of PCBs in bioslurry. The removal amount of tri-CBs reached 42.1% in 20 d under the best degradation condition (A,0.5 g/kg; B,0 g/kg; C,20 g/kg; D,1:6; E,15×108cfu/mL), but the amount of tetra-CBs and high chlorinated PCBs (Cl>4) still changed little. The importance of the five factors were D>C>A>E>B. The aerobic degradation efficiency of tri-CBs in slurry might be improved by increasing the levels of C, D, and E, or by decreasing the levels of A.Anaerobic reductive dechlorination and aerobic degradation of PCBs were implemented in the anaerobic-aerobic bioslurry. In anaerobic stage, the dechlorination of PCBs could conduct by inoculating the dechlorinating bacterial mixed-cultures. Dechlorination characteristics of the PCBs congeners in these groups were determined by the dechlorinating bacterial mixed-cultures and the removal percentage of some tetra-to hepta-CBs congeners reached 8% to 15% in 100 d. In aerobic stage, the aerobic degradation rates of tri-CBs and tetra-CBs were significantly enhanced by inoculating exogenous PCBs-degrading bacteria (SSI, SS2 and HC3), which could colonize in slurry. After 30 d of aerobic degradation, the removal percentage of tri-CBs and tetra-CBs were 59.1% and 14.1%, respectively. However, in the single aerobic slurry systems only tri-CBs could be degraded, and the removal percentages were 43.8%(100 d, Chapter 4) and 42.1%(20 d, Chapter 5). Thus, anaerobic-aerobic bioslurry treatment has more advantages for restoring PCBs-contaminated soil.In conclusion, our research recognize that bioslurry treatment can be further developed and applied in PCBs-contaminated soil remediation because of its higher and stable PCBs removal capacity. And the remediation efficiency is mainly affected by the content of high chlorinated PCBs, the bioavailability of PCBs and PCBs biodegradation capacity of the microorganisms. Further research should focus on improving the reductive dechlorination efficiency of high chlorinated PCBs, increasing the bioavailability of high chlorinated PCBs and obtaining high-efficiency PCBs-degrading microbes.