| Polycyclic aromatic hydrocarbons (PAHs), a unique class of persistent organic pollutants, are one of 129 kinds of priority pollutants, which widely exist in the environment of human life such as air, water, soil, crops and food. Due to the nature of physical and chemical stability, strongly recalcitrant degradablity and high carcinogenicity, the study of PAHs has become a hot topic. Strongly hydrophobic nature makes them bind to soil or sediment tightly, so, the remediation of PAHs contaminated soils is always a challenge problem. Bioremediation has become one of the most potential methods due to high efficiency and bioavailability (the main factor which limits the bioremediation). Surfactants can effectively enhance mobilization, desorption and bioavailability of PAHs, however, the chemical surfactant is limited in the application of bioremediation due to the toxicity and difficult to degradation. Biosurfactants have gained increasing attention because of its biodegradability and low toxicity with comparison with chemical surfactant. So, in this paper, the representative of biosurfactant was produced by strain D3 which was isolated and preserved by the group of Nanjing Agricultural University when vegetable oil was selected as sole carbon source, phenanthrene was selected as the model of PAHs, biosurfactant application in remediation of soil contaminated by phenanthrene through a series of physical chemistry, spectroscopy, biological and toxicological methods were investigated.After the extracellular secretion of D3 strain was precipitated by acetone, its crude product was prepared by freeze-drying, purified by thin layer chromatography and measured by infrared and mass spectrum, it was determined primarily as glycolipid and its critical micelle concentration (CMC) was:10.0 mg·L-1, the surface tension of CMC was 49.2 mN·m-1. The average molecular weight of the biosurfactant was 353. Its adsorption and biodegradation on black and red soil were investigated through sequencing batch experiments. The results showed that the biosurfactant could be sorbed on black and red soil, the largest sorption capacity of black soil was 58.8±0.008μg·g-1, which of red soil was 76.9±0.007μg·g-1 on the basis of the Langmiur equation. In addition, biosurfactant could be degraded by the indigenous microbe in black or red soil. At the day of 1,3 and 7, the degradation efficiency was 10,20 and 92% respectively in black soil, however 5,16 and 92% in red soil.The effects of biosurfactant on phenanthrene apparent solubility and octanol water distribution coefficient (KoW) were investigated by shake flask experiments. The effects of biosurfactant on sorption and desorption of phenanthrene on different soil were studied by sequencing batch. At the same time, "soft carbon" of the soil was removed, so that the mechanism of the effect of biosurfactant on sorption of phenanthrene on the soil was investigated. The experimental results demonstrated that:apparent solubility of phenanthrene increased linearly with the surfactant concentration increase. Kow reduced with the surfactant concentration increase. When the concentration of biosurfactant was 30 CMC, logKow was 3.51, which was significantly lower (P<0.05) than that in pure water. Soil organic matter and mineral content played a key role in sorption of phenanthrene onto the soils. "Hard carbon" dominated nonlinear sorption. The effect of biosurfactants on sorption of the phenanthrene onto soil was relevant to soil organic matter (quantity and type) and biosurfactant. Phenanthrene sorptions onto black and red soil were affected differently by the selected biosurfactants. Surfactant and the nature of the soil were two factors which cause the different effects on phenanthrene sorption. The selected biosurfactant can significantly enhance desorption of phenanthrene from the black soil.The bacteria GF2B, isolated from garden soil near Nanjing Agricultural University when phenanthrene was selected as the sole carbon source, was identified as Sphingomonas sp through further physiological and biochemical measured and 16S rRNA sequencing test. Degradations of phenanthrene in culture medium and black soil were studied. The results suggested that:phenanthrene was degraded by Sphingomonas sp. GF2B through the salicylic acid pathway. At the day of 10, degradation efficiency in culture medium was reached to 83.6%. Sphingomonas sp. GF2B was able to degrade phenanthrene in black soil, at the day of 7,14,21 and 28; degradation efficiencies were 27.5%,41.7%,66.6% and 72.2% respectively.The effects of biosurfactants on the degradation of phenanthrene by Sphingomonas sp. GF2B in PAHs-contaminated black soil and culture medium were investigated. Biosurfactant enhanced degradation rate and efficiency in culture medium. At the day of 10, degradation efficiency of phenanthrene was 99.5% with the addition of biosurfactant. In addition, biosurfactant promoted degradation efficiency of phenanthrene in black soil, at the day of 7,14 and 21, the degradation efficiency were 47.9%,59.9% and 73.3% respectively. But at the day of 28, there were no significant discrepancies in degradation of phenanthrene among all the experiments.The impact of biological toxicity of phenanthrene by biosurfactant was investigated in black soil though biological cultivation and physical chemistry experiment. Phenanthrene sensitive range of poison ryegrass root is 0～100 mg·kg-1. Biosurfactant existence (0-26.9 mg·kg-1) will reduce significantly the biological toxicity of the phenanthrene.The effects of different bioremediation methods on the degradation of phenanthrene were investigated. The results displayed that:ryegrass can significantly promote the degradation of phenanthrene on the soil. At the day of 50, the different removal efficiency of phenanthrene was displayed as the following order:plant+biosurfactant+bacteria> plant+Tween+bacteria> plant+bacteria> plant+Tween> plant+biosurfactant> plant+ H2O> CK. Surfactants can promote the active of indigenous and exogenous microorganisms which could speed up the degradation efficiency. |
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