| Coal is the main primary energy source in China,and various industrial activities about coal promote economic development,but generating a large number of pollutants.Through atmospheric deposition,leaching and percolation different types of pollutants enter the soil which become a sink for multiple pollution.Among them,heavy metals and PAHs,as the most typical inorganic and organic types of pollutants,usually co-exist in soil,thus posing different characteristic of pollutions.In this thesis,the contaminated soil of the coke plant was studied.By identifying its co-contaminated characteristics,the pollution status of heavy metals and PAHs was revealed and the ecological risk was evaluated.And the level of genotoxicity was discussed measured by magnetic nano-whole cell bioreporter.After testing the samples collected at five points and four depths,it was found that the concentration of total PAHs in the surface soil samples ranged from 190.20 mg/kg to 1207.00 mg/kg,which first increased and then decreased with depth at the 0m and 50 m points,and the highest concentration occurred at the 40-70 cm soil layer.At 100 m,150m and 200 m points,the concentration of PAHs decreased with depth.Of all soil samples the most common ones are tetracyclic polycyclic aromatic hydrocarbons.The concentrations of five heavy metals(lead,arsenic,chromium,mercury and cadmium)in the soil decreased with increasing distance from the boundary,and their concentrations decreased with increasing depth.The higher concentrations were lead(245.83 mg/kg),arsenic(19.65 mg/kg)and chromium(169.60 mg/kg).The ecological risks of heavy metals and PAHs in soil were evaluated using the potential ecological risk evaluation method and the risk quotient method(with the low value of effect range and the median value of effect range as controls)respectively.The results showed that the potential ecological risk index of heavy metals in 70% of soil samples exceeded 150,which meant moderate ecological risk,while the ecological risk of PAHs was at high levels as a result of the risk quotient was above1,except one sample.The genotoxicity of the soil was characterized by the relative bioluminescence response rate,and the results showed that the relative bioluminescence response rates of the soil samples ranged from 1.76 to 4.66,all of which had some genotoxicity.For the remediation of contaminated soil,some commonly used physicochemical remediation methods remove pollutants,but they also caused large changes to the the physiological and biochemical environment of soil,which can easily cause degradation of the soil.Therefore,the use of green phytoremediation methods can not only achieve the purpose of removing pollutants,but also improve the soil environment.Chromium(Cr)and arsenic(As)were identified as the target inorganic pollutants by analyzing the soil contamination characteristics of the coke plant,and three different concentrations of Cr and As additions were set for soil preparation(As+Cr,low: 20+5,medium: 60+40,high: 80+80,mg/kg).The remediation plant which can tolerate and enrich heavy metals was selected from ryegrass,maize and alfalfa through pot tests and soil enzyme activities were measured.The results showed that low concentrations contaminated soil promoted the growth of ryegrass.Ryegrass and maize could tolerate medium concentrations of contamination.The biomass of all three plants was strongly inhibited at high concentration of heavy metals.The extraction of Cr by ryegrass ranged from 5.54% to11.79%,the restoration of Cr by maize at three concentrations ranged from 5.46% to10.44%,and by alfalfa from 2.58% to 12.42%.The concentration of As decreased little throughout the phytoremediation(0.60% to 8.66% removal),and ryegrass and corn were more effective than alfalfa for remediation.Soil FDA enzymes showed a trend of first decreasing and then increasing,while urease activity decreased faster at the beginning and slower at the end.And dehydrogenase activity was sensitive to pollutants which showed a continuous decreasing trend.However,the three enzyme activities of the remediation group were always higher than those of the contaminated soil blank group.In the contaminated soil,the presence of plants alleviated the inhibition of some enzyme activities by heavy metals and improved the soil environment.Combining the degree of plant tolerance and heavy metal enrichment effects,ryegrass and maize were selected as plants for phytoremediation.A large number of microorganisms exist in the soil,and when the soil was contaminated,microorganisms capable of tolerating pollutants will be present under the stress of pollutants.After identifying the composite contamination characteristics of the coke plant,the target organic pollutants were set as phenanthrene(Phe)and benzo[a]pyrene(Bap).Phe and Bap were used as the only carbon source to formulate inorganic salt medium,and bacteria capable of growing in this medium were screened from the contaminated soil of the coke plant.Through enrichment and domestication,a strain of bacteria was finally isolated by plate scribing,which had 100% homology with Rhodococcus aetherivorans.The strain was able to tolerate low concentrations of Cr and As and was heavy metal tolerant.The degradation rates of the bacteria in inorganic salt medium containing Phe 50 mg/L and Bap 15 mg/L were 29.62% and 43.34% at 8 days,respectively.Three factors,temperature,p H and inoculum,were selected for response surface analysis on the degradation of Phe and Bap by the bacterium,and it was found that temperature had a significant effect on the degradation of Phe,and the interaction of temperature and p H had a significant effect on the degradation of Bap.To address the problem of composite pollution,single remediation technique may have lower efficiency and longer period.The plants and microorganisms screened from the results of the previous experiments were combined and formulated the composite contaminated soil(Cr40+As60+Bap25+Phe75,mg/kg)for potting experiments to investigate the effect of combined ryegrass-corn-Rhodococcus aetherivorans remediation of composite contaminated soil.The process mechanism of the combined remediation was also investigated in terms of abiotic factors(changes in heavy metal morphology and removal of pollutions)and biotic factors(enzyme activity and microbial community).The results showed that the injection of degrading bacteria did not affect the biomass of plants,but increased the proportion of exchangeable state heavy metals in the soil and enhanced the enrichment of heavy metals by plants.The soil Cr concentration in the combined remediation group decreased by 9.63% and As decreased by 5.28%.Phe and Bap in the soil were reduced by 45.14% and 26.87%,respectively,in the combined remediation group.The removal of PAHs from the soil was affected by the combined effect of plants and microorganisms,and the remediation ability of plants was stronger than that of degrading bacteria.The changes in enzyme activity in the soil again verified that the negative effects of pollutants on the soil could be reduced by bioremediation,with higher enzyme activity in the remediation test group than in the non-remediated group.The structure of the bacterial community in the soil was influenced by the extractable state of arsenic in the soil,and this influence was reflected in the dehydrogenase activity in the soil.In conclusion,the combined phyto-microbial remediation approach could be applied to the composite contaminated soil,where microorganisms could enhance the enrichment capacity of plants for heavy metals,and plants also participated in the removal of PAHs at the same time.Plants and microorganisms jointly improved the soil enzyme activity,influenced the microbial community,and improved the soil environment. |
PDF Full Text Request