Study On The Screening Of High-yielding Biosurfactant Strains And Its Degradation Performance On Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons(PAHs)are widespread hydrophobic organic pollutants in the environment,which could cause carcinogenicity,teratogenicity and mutagenicity for human beings.Among the numerous strategies of remediation PAHs pollution,microbial remediation has attracted much attention due to its advantages of low cost and eco-friendly nature.However,the high hydrophobicity of PAHs results in low solubility in the environment,which in turn leads to limited degradation by microorganisms.Surfactant-enhanced remediation is an approach to increase the solubility of PAHs in the environment,which in turn enhances the degradation performance of microbial strains.In contrast to the secondary contamination resulting from chemical surfactants,biosurfactants have good degradability and biocompatibility making them more suitable for environmental remediation.However,biosurfactants have not been widely utilized in environmental remediation due to problems such as high cost.Therefore,it is of great significance to screen highly efficient biosurfactant-producing bacteria for remediation of PAHs pollution.In this study,a strain with high surface activity was screened from the preserved strains of crude oil contaminated soil,and the biosurfactant synthesized by this strain was obtained by different extraction methods.In addition,inexpensive raw materials were utilized as fermentation substrate to optimize biosurfactant yield and then characterized the physicochemical properties of biosurfactant.Finally,pyrene was used as a simulated contaminant of PAHs pollution to study the degradation performance of biosurfactant and its producing bacteria on pyrene in aqueous phase and soil system.The results of the study are as follows:(1)The strain(Pseudomonas aeruginosa AY-1)with high surface activity was screened by primary screening of crude oil medium and the rescreening by determination of liquid paraffin emulsification index and surface tension.(2)Two different extraction methods were compared for the extraction of biosurfactant produced by the AY-1 strain.The results indicated that acid precipitation-ethyl acetate was more effective in the extraction of biosurfactant compared to acid precipitation-chloroform:methanol(2:1,v:v).Moreover,the biosurfactant yield of the strain AY-1 was optimized by single-factor experiments to produce 5.86 g/L biosurfactant A by fermentation at 35°C,p H 8.0,shaker speed 180 r/min,50 g/L of food waste oil and 2 g/L of sodium nitrate for 96 h.(3)The biosurfactant A was identified as rhamnolipid by acid methylene blue method,acidic bromophenol blue method,sulfuric acid-anthraquinone experiment,ninhydrin test,Fourier Transform Infrared Spectrometer(FT-IR)and Thin-layer chromatography(TLC)analysis.In addition,it was determined by electrospray ionization mass spectrometry(ESI-MS)analysis that the rhamnolipids produced by AY-1 were a mixture of five rhamnolipid molecules,with Rha-C₁₀-C₁₀ and Rha-Rha-C₁₀-C₁₀ as the major components.(4)The critical micelle concentration(CMC)of rhamnolipid produced by AY-1 strain was 200 mg/L with high surface tension stability at temperature(4-120°C),p H(2-12),and Na Cl concentration(0-20%,w/v).Meanwhile,the rhamnolipid showed a high emulsification effect on different hydrophobic compounds(methylbenzene,n-hexane,cyclohexane,heptane and n-hexadecane),and significant solubilization performance for PAHs(phenanthrene and pyrene).(5)AY-1 was not only able to degrade polycyclic aromatic hydrocarbons(pyrene),but also the addition of certain concentrations of rhamnolipids in the aqueous phase and soil system can significantly enhance the degradation ability of AY-1 on pyrene,which makes it a potential application in bioremediation of PAHs contamination.