| The polycyclic aromatic hydrocarbons (PAHs) are one of the most dangerous pollutants released into the soil environment by human activity. They come from sewage irrigation, burning fossil fuel, and so on. PAHs are a class of persistent organic pollutants (POPs), and have high toxic with carcinogenicity, teratogenicity, and mutagenicity, which poses a great threat to Ihe ecological security of farm and population health.In recent decades, bioremediation has been highly concerned about removal PAHs contaminated farmland soil by the researchers of environmental science. Although the technology has gradually come into in-situ experiment level by laboratory method, the bioremediation technology system which did not affect agricultural productivity has not been formed, and we have had a shortage of some research results, including how to enhance effect of microbes and plants to remove the pollutants in the natural medium.Therefore, this paper focused on the technology for in-situ bioremediation for, the PAHs contaminated farmland soil, using the methods of indoor imitated experiments and field trials. The effect and mechanisms, including degradation of PAHs with microbe, activation and absorption of PAHs with functional plants, plant-microorganism combined bioremediation, were researched and discussed. Furthermore, through the site trials of PAHs contaminated soil and the integration of some measures, a low cost, green and ecological technology method about in-situ bioremediation for the PAHs contaminated farmed soil was primarily formed. The main results were following:Firstly, the degradation results were studied in the PAHs contaminated soil with immobilized microbial agents, and the application methods, regulatory and control measures of nutrition was confirmed, which will lay a foundation for large-scale in-situ bioremediation. The immobilized microbe was made of target microbe which was separated and screened from PAHs contaminated soil and was applied in the degradation of pyrene and benzopyrene, which are the representative pollutants of PAHs. The results showed the degradation efficiency by immobilized mixed microbial agents was significantly greater than free microbe and the immobilized mixed microbe, which has the highest removal effect followed by immobilized fungi and immobilized bacteria. And immobilized microbe has a good adaptability in natural soil because it could avoid competition between the indigenous floras and maintain a relatively long period activity. The removal rate of pyrene and benzopyrene was95.32%and64.98%, which was15.75%and16.75%higher than the degradation of free microbe. Furthermore, under the action of agents, the degradation of total PAHs was39.97%of the PAHs aged soils and that of pyrene and benzopyrene was45.73%and31.93%respectively. There was significant positive correlation between degradation efficiency and the number of PAHs’rings.Secondly, a pot experiment in greenhouse and in-situ trial was carried out to research the action of plants on the PAHs contaminated soil and the advantage functional plants were screened. The experimental results demonstrated that plant concentration factors (PCFs) were very small. SCF (shoot concentration factors) were0.0075-0.067and RCF (Root concentrations factors) were0.013-0.090, which provided a possibility for interaction of phytoremediation-microorganism. The capability of plants for phytoremediation of PAHs contaminated soil was ryegrass, alfalfa, soybean, clover, marigold and castor. The field trial also showed ryegrass and alfalfa had better remediation efficiency, which were23.54%-26.38%and19.65%-23.50%. And the average removal ratio of PAHs in rhizosphere soils was2.1%-7.2%higher than that in non-rhizoshpere soils, which was consistent with the quantity of soil microorganism.Thirdly, the method of plant-microorganism combined bioremediation was preliminarily pointed out and the basic conditional parameters were optimized by the pot experiments in the greenhouse. The results showed that PAHs removal rate could reach37.57%-41.30%, which was higher than phytoremediation (18.72%-20.15%) and remediation by immobilized microbial agents (16.05%-17.34%) separately. And the removal rate of5-6rings PAHs was18.07%～21.68%by the combined remediation, which had met the basic requirements for in-situ bioremediation. When the using amount of immobilized microbe was3%(mass ratios), the removal effect was the best. The further research showed mycorrhizal fungi was an effective strengthen measure to improve the removal of PAHs. And microbe and plants combined remediation also could improve the quality of micro-environment in surface soil, along with the increase of the number of microorganisms. At the same time, there was significant positive correlation between the number of microorganisms and dehydrogenase activity in the soil (p<0.01). The removal rate of PAHs had significant positive correlation with dehydrogenase activity (p<0.01) and positive correlation with polyphenol oxidase activity (P<0.05). Therefore dehydrogenase activity and polyphenol oxidase activity might accomplish the aim of micro-level indicators of the degree of bioremediation.Finally, the technique system suitable for northern region of bioremediation on PAHs contaminated soil was basically established by the field trial in Shenfu wastewater irrigation area, which used to be the largest area of petroleum wastewater irrigation and PAHs pollution was serious. The technological and designed processes were put forward to be based on dry land and dry land changed by paddy. Various agronomic measures were integrated, such as plowing soil, fertilizing organic fertilizer or inorganic fertilizer, planting crop infected by mycorrhizal fungi. The bioremediation efficiency could attain to63.46%after a remediation cycle. |
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