Study On The Joint Remediation Of Pyrene-contaminated Soil By Low-temperature Thermal Treatment And Microbial Degradation

Pollution of petroleum hydrocarbons(PHCs)has attracted a lot of attention in recent years.As a typical class of PHCs,polycyclic aromatic hydrocarbons(PAHs)are difficult to be degraded,so they can permanently exist in soil,causing a constant damage to the soil environment.Pyrene is composed of four benzene rings,which is identified as a model compound for studying the degradation of PAHs.As an environmentally friendly treatment technique,microbial remediation can effectively remediate PHCs-contaminated soils.However,it is difficult to maintain the high activity of functional microbes in the practical applications,resulting in a slow degradation rate and a low efficiency.The microbial immobilization technology and joint remediation technology can effectively solve these problems.Immobilization technology can maintain the microbial activity,while joint remediation technology can improve the microbial degradation efficiency.In this study,we firstly screened and isolated a functional strain with high-capacity of pyrene degradation from the contaminated soils adjacent to a coking plant.Based on the 16S r DNA gene sequencing and physico-chemical property analysis of the isolated strain,the degradation performance and potential pathways degradation mechanism of pyrene were investigated.In addition,using acid-modified kaolin as a carrier,an immobilized microbial agent was obtained by an adsorption method.And the impacts on the remediation efficiency of pyrene-contaminated soil were investigated,as well as the enzyme activities and microbe in soil.Finally,the joint remediation technology combined low-temperature thermal desorption technology and microbial degradation technology was studied,the impact on soil dissolved organic matter(DOM)and small molecule acid were analyzed,the mechanism of joint remediation was discussed,and corresponding technical support for joint remediation of petroleum hydrocarbon contaminated soil was provided.The main conclusions are listed as follows:(1)The functional degradation microbe of pyrene was separated from contaminated soil adjacent to a coking plant.The strain was identified as Ochrobactrum anthropic by 16S r DNA sequecing,and named as P1.The appearance of strain P1 showed white round spots with flat plaque edges,and the colony size was uniform.The shape of the strain observed by SEM was ellipsoid without flagella.The length of the strain was about 0.5?m.P1 grew best under neutral conditions(p H=7.0)and 37?and showed high temperature resistance,it reached the logarithmic growth phase at 12th hour,the value of OD₆₀₀ could reach to 1.6 at the stationary phase.The degradation rate of pyrene was 79%in 180 hours in 50 mg/L pyrene solution under best condition.(2)The P1-acid-modified kaolin immobilized microbe agent was prepared by adsorption procedure,the microbe load was calculated as 6.1×10⁷ CFU/g.Compared with the degradation efficiency of free functional strain in pyrene contaminated soil,it was found that the efficiency of immobilized bacteria was significantly higher with 58.4%net removal rate at 30th day.Which were 57.7% higher than that of free functional strain and 76.9%higher than that of the blank control.(3)The immobilized agent could improve the dehydrogenase activity,invertase activity and bacterial community number,which reached 1012.6?g/g/d,249.3 mg/g/d,1.5×10⁷ CFU/g,repectively at 30th day.By analyzing the correlation between the degradation rate of pyrene and dehydrogenase activity,invertase activity,the number of bacterial colonies in the soil,a significant positive correlation was found between the pyrene remove efficiency and the dehydrogenase activity in group M and BK.Which means the remove efficiency of pyrene grew with the rising dehydrogenase activity in soil.There was a correlation between pyrene degradation rate and soil sucrase activity only in group M,indicating that immobilization technology could effectively protect the activity of microbes.In groups B and M,a significant positive correlation could also be found between the pyrene remove efficiency and soil bacterial colonies counts,indicating that the addition of exogenous microbes could increase the number of soil bacteria and enhance the microbial remediation effect of pyrene.(4)Through GC-MS analysis of pyrene metabolites,it was found that there were some low-cyclic PAHs(phenanthrene,naphthalene)and the final metabolites of styrene,toluene and benzene in the soil.It was inferred that the pyrene metabolized by strain P1 was mainly degraded through phthalic acid pattern.(5)Through the low-temperature thermal treatment of contaminated soil,the changes in soil moisture content,pyrene extraction rate,DOM and small molecule acids before and after heating were characterized.It was found that the moisture content of the soil after thermal treatment decreased rapidly within 30 minutes,and the minimum moisture content of the soil was 60% at 12th hour.The extraction rate of pyrene is also affected by the low-temperature thermal treatment,reduced by 21.4%to 32.7%.Low temperature thermal treatment can significantly enhance the production of soil humus,soluble microbial products,protein-like substances,and their content increases with the increase of heating time.Moreover,the concentration of small molecular organic acids improved as well while adding up the duration time,and the content of acetic acid in soil could reach 44.7 mg/L at 12th hour.(6)The low-temperature thermal treatment could significantly improve the degradation rate of pyrene in contaminated soil by functional strain.After 12 hours of low-temperature thermal treatment,the degradation rate of pyrene in soil by group M reached 77.2%at 14th day.After 6hours of low-temperature thermal treatment,the degradation rate of pyrene in soil by group B reached 79.8%at 14th day.