| Trichloroethylene (TCE) is a kind of widely used chlorinated organic solvent. The contamination of TCE is widespread in soil and groundwater. In this thesis, TCE co-metabolic biodegradation was studied in soil and groundwater using gasoline and phenol as growth substrates under the aerobic conditions. Immobilized microorganism technique was developed by using biochar as a carrier material in the remediation of TCE contaminated soil. Study results are as follows:(1) Relatively good results were achieved on the co-metabolic biodegradation of TCE in water phase by using Ralstonia eutropha as degradation bacterium and the93#gasoline as growth substrate. It is generally considered that phenol and toluene are good substrates for removing TCE by co-metabolism, the study proved the potential of gasoline to be used as substrate. The best initial concentration of gasoline was10mg/L,which was less than optimal initial concentration of phenol and toluene, maybe due to the toxicity of gasoline. The TCE degradation rate was optimal under the neutral to slightly acidic (pH=5.0) environment system, decreased slightly by adding additional NaCl. The initial dissolved oxygen in aqueous phase was saturated by pre-aeration of oxygen, which greatly improved the TCE degradation rate. The optimum TCE degradation rate could be66.8%after24h when the initial concentrations of TCE and gasoline were1mg/L and10mg/L, respectively. Preliminary analysis of chlorinated ethylenes in the reaction system did not find these intermediates. TCE-Cl was completely converted to the Cl", and the degradation of organic compounds was completely.(2) To enhance the degradation rate of TCE, TCE degradation in aqueous phase by Pseudomonas fluorescens aerobic co-metabolism using phenol as growth substrate was studied. The TCE co-metabolism efficiency was up to80.1%when the initial optical density (OD600) of bacterium suspension was0.14, the initial concentration of phenol was100mg/L, and the TCE concentration was0.1mg/L. The method can be used in the area of coastal saline-alkali soil since the TCE degradation rate could reach a satisfactory level when the circumstance was changed in a wide range of pH (pH=5.0-9.0) and salinity (0.045-3%). Although nutrient broth could not be the co-metabolic substrate of TCE, it could promote TCE co-metabolism degradation in the presence of phenol which used as the growth substrate. The fundamental reason was that the nutrient broth could enhance the proliferation of bacteria, which generated more active enzyme in the presence of phenol. Hence, the addition of nutrient broth could improve the transformation yield of TCE, while reduce the transformation capacity. Catechol1,2-dioxygenase (C12O) was critical to the process of TCE co-metabolism degradation, and it only could be generated in the presence of phenol; but the activity of the C12O would remain for some time after the exhaustion of phenol. TCE was completely dechlorinated during the degradation process and the chlorine atom was completely conversed to Cl". Analysis of intermediate products during the preliminary stage (7h) of reaction found the existence of C2organic acid at2h and3h, and they disappeared at4h. The Cl organic acid was also found at the time of1-5h, and they disappeared at6h. The degradation was slow when the gasoline was used as growth substrate, however, the addition of phenol can enhance the removal of both gasoline and TCE.(3) TCE and gasoline degradation in soil slurry by co-metabolism of microbes was studied. TCE degradation achieved good results when using Pseudomonas fluorescens as the active strain and phenol as growth substrate. The results showed that the type of soil did not show obvious effects on TCE degradation rate, which might be due to the soil samples used in this study did not have much difference in their physical-chemical properties and the mass diffusion in the slurry system was not limited greatly. Kinetic studies found that TCE degradation rate was fast within the first8h, and then became slower. The degradation rate increased and then reduced with the rise of bacterial content of the system, and it would be optimal at the initial bacterial content of0.008mg/g in soil slurry. TCE degradation increased with phenol concentration in the range of0-100mg/L, while the rate would be drastically reduced when the phenol concentration was greater than500mg/L. TCE degradation rate decreased with the increase of its initial concentration. The co-metabolism efficiency was up to100%when TCE concentration was0.1mg/L. The soil used in this study is coastal greening soil, which was rich in organic matter. The soil has good buffering capacity, and the pH of soil slurry remained in the range of5.3-7.0though the initial pH of the inorganic salt medium was changed between1.0-12.0, so that the TCE degradation rate remained satisfying. The degradation rate was proportion to the salinity due to the high cation exchange capacity of the soil, and being up to88.3%by the initial salinity of2.5%. The degradation rate would be optimal when the ratio of water with soil was between2-20. If the ratio was below1, the degradation rate decreased significantly because the soil and water could not mix as a homogeneous slurry and the mass diffusion was retarded. TCE could be degraded efficiently in the presence of a wide range of gasoline when phenol was added. Nutrient broth had no significant effect on TCE degradation. The addition of phenol also enahnced the degradation of gasoline. This indicated that P. fluorescens may co-metabolize some components of gasoline under phenol’s stimulation. Adding nutrient broth singly or together with phenol played a significant role in promoting the degradation of gasoline.(4) TCE removal in slurry using P. fluorescens as degradation strain, biochar as the carrier for immobilization and phenol as growth substrate was studied. Five immobilized carrier materials was tested, and the best removal rate was obtained when biochar was used as immobilizing carrier. Comparing with the free system, although TCE bio-degradation rate was reduced, the overall removal rate increased due to absorption. Short contacting time of biochar powder and the bacterial suspension benefited for the continuous removal of TCE. On the contrary, Long-term of contacting was not good for TCE removal. Changing pH between7.0-10.0did not affect the removal rate which maintained at47.5-54.7%. While, TCE removal would be inhibited at pH of11.0-13.0. This is due to the inhibition of cell growth and the weakening of absorption at high pH value. Immobilization could enhance the tolerance of bacteria to the toxicity of TCE and the immobilized microorganism may keep relatively stable biological activity under adverse conditions. This is an advantage of immobilized organisms applied in soil remediation field. Changing salinity did not show obvious effect on co-metabolic degradation of TCE, and this is due to salt ions combined to surface and interior cavity of biochar with electrostatic force and chemical bond, which are much stronger than physical absorption. To a certain extent, the degradation rate of TCE increased with microbes cell amount, but tended to become stable when OD600>0.3. Comparing with the free system, immobilization system may load more microorganism, and did not decrease degradation rate of TCE. This is another advantage of immobilizing technique applied in microbial environmental restoration field.The results of this thesis elucidated the mechanism of TCE co-metabolism and achieved satisfactory degradation rates for the co-removal of TCE and gasoline, the microbial immobilization technique was developed by using biochar as a carrier material, which provided theoretical foundation and technology for the remediation of TCE pollution in soil and groundwater. |
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