Remediation Of PAHs Contaminated Soil By Immobilized Microorganisms From Distiller’s Lees

Polycyclic Aromatic Hydrocarbons(PAHs)are a group of persistent organic pollutants that are present in large quantities in soil and can adversely affect the environment.The biodegradation of PAHs in soil is often limited due to the following reasons:low bioavailability of PAHs,limited degradation ability of indigenous bacteria and low activity of exogenous PAHs degrading bacteria.Immobilized microorganism technology can protect microorganisms from harsh environment.Biomass materials can adsorb PAHs and have advantages such as high affinity with microorganisms,easy biodegradation,and improved oxygen diffusion in soil.Distiller’s lees has the potential to be used as microbial immobilized carrier.At present,the removal ability,range and mechanism of PAHs from soil by immobilized microorganisms in distiller’s lees remain unclear.This study intends to use distiller’s lees as a carrier of microorganisms to study the feasibility and distance effect of the remediation of PAHs contaminated soil by immobilized microorganisms,explore the relationship between soil nutrient content,consumption and PAHs degradation rate,and reveal the mechanism of its bioreaction from the perspective of soil enzyme activity and microbial community composition.Fresh distiller’s lees can be used to immobilize the carrier after air drying,repeatedly rinsed and dried dozens of times,until the residual alcohol is removed and no toxic effect on microorganisms.According to the characterization of distiller’s lees,rice husk in distiller’s lees had large specific surface area(0.85 m~2/g),pore volume(0.0052 cm~3/g)and pore size(24.28 nm).Functional groups including methyl,amide and C=C in distiller’s lees affected the adsorption capacity of PAHs on the carrier.The results showed that although the microorganisms fixed on the surface of distiller’s lees could adsorb certain phenanthrene and pyrene through biological adsorption,the adsorption contribution of lees to phenanthrene and pyrene was much greater than that of biological adsorption.The adsorption behavior of lees was consistent with the quasi-second-order kinetic model(R~2>0.99),and the chemical action was the main factor controlling the adsorption behavior.The feasibility of remediation of PAHs contaminated soil by immobilized microorganisms in distiller’s lees was determined through comparative experiments of biostimulation and bioaugmentation.The optimal degradation conditions were determined by single factor and response surface optimization experiments.Under the conditions of 1.98 g/kg sulfate addition,10.26%inoculation amount of immobilized microorganisms in distiller’s lees and 20.69%moisture content,the simulated in-situ soil remediation was performed.After 72 days,The degradation rates of phenanthrene and pyrene reached 91.78%and 58.59%,respectively.The addition of distiller’s lees increased the proportion of small and medium particles in soil(particle size<2mm),increased the ventilation capacity of soil,and provided nutrients for microorganisms,thus increasing the degradation rate of PAHs.The distance effect of immobilized microbial degradation of PAHs in distiller’s lees was studied through a multi-compartment box.The results showed that the degradation rate of PAHs(phenanthrene,pyrene),soil nutrient consumption(organic matter,total nitrogen,total phosphorus),peak enzyme activity(dehydrogenase,polyphenol oxidase,urease,hydrogen peroxide enzyme)decreased with the increase of distance,and the maximum range of immobilized microbial remediation was 8 mm.From the perspective of correlation,the degradation rates of phenanthrene and pyrene are related to organic matter consumption(r=0.87**,r=0.94**),total nitrogen consumption(r=0.60*,r=0.68*),total phosphorus consumption(r=0.93**,r=0.96**),and related enzyme activities(dehydrogenase activity(r=0.87**,r=0.90**),polyphenol oxidase activity(r=0.98**,r=0.99**),urease activity(r=0.89**,r=0.93**)The activity of catalase(r=0.73**,r=0.73**))showed a significant positive correlation.Through microbial community analysis,it was found that the structure and abundance of domesticated and immobilized microbial communities were similar,with the dominant genera being PAHs degrading bacteria(Methylobacillus,Acrobacter,Raoultella,etc.),indicating that the immobilization process did not change the microbial community structure and composition.After 72 days of remediation,the initially screened PAHs degrading bacteria were still detected in the immobilized microbiome of distiller’s lees,indicating that immobilizing microorganisms on distiller’s lees is a remediation strategy to reduce the mortality rate of exogenous microorganisms and improve the removal rate of PAHs.From the perspective of distance,in the later stage of bioremediation,the dominant bacteria in the middle compartment,the 4mm soil compartment and the 8mm soil compartment of the immobilized microbial group of distillers’lees are similar.There may be PAHs degrading bacteria migrating to the 4mm and 8mm soil compartments,and under the stimulation of nitrogen and phosphorus,some phenanthrene and pyrene were degraded.Overall,as a food industry waste,distiller’s lees can be used as immobilized carriers after simple treatment.It has the function of protecting microorganisms,providing nutrients for microorganisms,adsorbing PAHs,improving soil ventilation and PAHs bioavailability,and has good application prospects in in-situ remediation of PAHs contaminated soil.There are two possible ways of degradation of phenanthrene by immobilized microorganisms in distiller’s lees.Phenanthrene is decomposed into naphthalene in some way,and then enters tricarboxylic acid cycle to generate carbon dioxide and water through salicylic acid pathway.Phenanthrene may also be transformed into naphtho[1,2-b]thiophene and then into 4-ethylphenol,which is finally decomposed by microorganisms.The thesis has 53 figures,17 tables,and 182 references.