| Polychlorinated biphenyls (PCBs), a class of typical POPs (Persistent Organic Pollutants), are widely accumulated in the soil environment. Soil contamination with PCBs poses a great threat to the production and ecological function of soil, the food quality and human health due to their widespread occurrence, persistence in soil ecosystems, carcinogenic, mutagenic and not easy degradable properties. Phytoremediation is a relatively novel approach to remove organic contaminants from soils by plants, rhizosphere microorganisms and root exudates. Investigations of the environmental behavior of PCBs in soils, including the residue, dissipation and the controlling key factors, seem urgent and effective.Firstly, a method for detecting PCBs residue in soils from the extraction, cleanup to the final detection, was modified and investigated. Based on the fast, accurate, high sensitivity method, the degradation of PCBs in the rhizosphere of the ryegrass and the corresponding responses, including soil enzyme and microbial structure were studied. A special-designed rhizobox and a novel method of phospholipid fatty acids (PLFAs) were used in this research. Rhizo environment by the addition of root exudates was simulated and dissipation of PCBs in soils was studied. The adsorption, transfer and accumulation of PCBs from soils to plants and the detoxification enzymes in ryegrass to 24-CB were investigated in the study. The main conclusions are as following:(1) The ultrasonic extraction was used for the extraction of PCBs residue in soil sample. The influence factors of solvents, temperature, and time on the extraction efficiency of were discussed. The results indicated that the optimal analytical conditions of solvent, temperature, and time were hexane-acetone (v:v=1:1),35℃and 30min respectively. The spike standard recoveries rates were between 81.4 and 90.2%, relative standard deviation was 2.8 to 3.8%. Florisil column (wet and dry filled column) for purification and separation were used. The results demonstrated that wet filled column was useful and effective. And the optimal parameters for GC-MS were modified and established.(2) A special-designed rhizobox experiment was conducted in the greenhouse. The ryegrass seedlings were grown for 135 days in soils spiked with Aroclor 1242 at concentrations of 8 and 16mg/kg soil to investigate rhizosphere effects on the dissipation of PCBs. The soil in the rhizobox was divided into six separate compartments at various distances from the root surface. Changes in Aroclor 1242 concentration with increasing distance from the root compartment of rhizobox were then assessed. In the treatment of 8 mg/kg, the Aroclor 1242 degradation gradients observed in the rhizobox of planted treatments appeared to be highest at the 3 mm zone layer, then at 2,4,1 and 5 mm sequentially. Thus the greatest decrease in Aroclor 1242 concentration could be achieved here because 3 mm zone represents balance between microbial activities and root exudates availability. The trend observed in the high spiked level was similar. The dissipation gradient followed the order:near rhizosphere>root compartment>far-rhizosphere soils zones. The results also showed that different congeners with different chlorine atoms had the various dissipation rates and the PCBs with two chlorine atoms appeared high degradation rates.(3) Further investigations were conducted using phospholipid fatty acids (PLFAs) profiles to follow the millimeter spatial response of the soil microbial community with the purpose to illustrate the mechanism of dissipation gradients of Aroclor 1242 in the rhizosphere of ryegrass (Lolium perenne L). The highest concentration of total PLFAs also occurred at 3 mm from the root zone in planted soils. Bacteria (cyl7:0,16:0), gram positive bacteria (a15:0, i16:1, a17:0) and actinomycete (18:2ω6,9c) were significantly higher in planted soils than in unplanted soils. Furthermore, individual PLFAs (i16:0,16:0 N alcohol,18:0(10Me), i16:1, a15:0, i14:1,14:02OH,18:1ω9c, a17:0,14:03OH, i14:0, a16:0,16:lω5c) were strongly correlated with the Aroclor 1242 degradation rates (%) (p<0.05) in planted treatments, whereas individual PLFAs i16:1,12:03OH,15:0, a15:0 had significant correlations with the Aroclor 1242 degradation rates (%) (p<0.05) in unplanted soils. In particular, individual PLFAs i16:1 had strong correlations with Aroclor 1242 degradation both in treatments with and without ryegrass.(4) PCB 7 (24-CB) was selected as the target pollutant, the dissipation rate in rhizosphere soils, the adsorption, transfer and accumulation of 24-CB from soils to plants, and the detoxification enzymes to in ryegrass 24-CB were conducted in this research. Two different soils which contain different DOM content were selected in this experiment. The results showed that ryegrass was able to absorb 24-CB from soils, and only very small amount could be transferred to the overground part. We found that the transfer of 24-CB in plants was quite limited. The 24-CB by ryegrass was mainly accumulated in root. The uptake and accumulation of 24-CB was not the main reason which induced the dissipation of 24-CB concentration. The physiological response related to the 24-CB metabolism in ryegrass was also investigated. We found that SOD and POD appeared different responses in two types of soils. We speculated that DOM played the key role on the 24-CB of bioavailability in this study, further impacted the detoxification enzymes in ryegrass.(5) The remediation effects and mechanisms of root exudates on rhizo-remediation in 2345-CB-spiked soils were studied in a simulated rhizo environment. The addition of root exudates improved the fast dissipation of 2345-CB at the very beginning of the experiment. Then the dissipation rates became quite slow with the increase of the time. The PLFAs profile indicated that the aerobic microbes were inhibited while the anaerobic microbes were stimulated, especially the methane oxidizing bacteria and sulfate reducing bacteria biomass. Our reports indicated that ortho-, meta-, para-dechlorination exhibited different sequence and optima. Chlorines at the position of meta-and para-were removed, result to the accumulation of 234-CB, 245-CB,25-CB.
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