Activation Of Persulfate By Fe-N Co-doped Carbonaceous Nanocomposites For Remediation Of Petroleum-contaminated Soil

The use of oil is an important driver of economic development,however,during the extraction,transportation and production of oil,the leakage of oil causes serious pollution to the soil environment.Oil pollution can damage the physical and chemical properties of soil,hinder the growth of plants,change the structure of soil microbial communities and endanger human health.Therefore,the treatment of oil-contaminated soils has become a major concern for domestic and foreign scholars.Advanced oxidation processes based on persulfate(PS)are considered to be an effective method for remediation of petroleum-contaminated soils,but to achieve better remediation results often require the use of higher PS concentrations,which lead to soil acidification and affect soil microbial community structure.Among the various ways to activate PS,iron-based catalysts have received much attention because of their low price and non-toxicity.Carbon materials such as biochar can be a carrier for metals due to their large specific surface area and high biocompatibility,and their application to soil can improve the catalytic performance of the material while achieving protection against microbial communities.In addition,biogas residue is the solid residue of anaerobic digestion of biogas,which contains a large amount of organic matter,nitrogen and phosphorus,etc.,and is an important biomass resource.The preparation of metal-loaded carbon-based materials using biogas residue as a precursor not only allows the use of functional groups such as hydroxyl groups in its skeleton to immobilize metals,but also enables the simultaneous synthesis of metal-loaded carbon,which has practical significance for the resource utilization of biogas residue.In this thesis,based on a large amount of relevant research literature at home and abroad,the Fe-N co-doped carbonaceous nanocomposites were synthesized simultaneously using biogas residue as a template to address the problems of existing PS advanced oxidation technology for remediation of petroleum-contaminated soils.The performance and influencing factors of Fe-N@CN activated PS(Fe-N@CN/PS)remediation of petroleum-contaminated soil were investigated,the mechanism of degradation of total petroleum hydrocarbons(TPHs)in soil by Fe-N@CN/PS system was explored,and the effects of this remediation system on soil physicochemical properties and microbial community structure were evaluated.The specific research contents and results are as follows:1、The organic functional groups rich in biogas residue were used to achieve the immobilization of Fe,while urea was used as a nitrogen source to achieve the nitrogen doping of the composites,and Fe-N co-doped carbonaceous nanocomposites(Fe-N@CN)were synthesized simultaneously.The synthesized materials were characterized using XRD,XPS,SEM and TEM.XRD results showed that there were Fe-based nanoparticles with Fe3C and FeN0.0760 as the main bonding modes in Fe-N@CN prepared at high pyrolysis temperatures.SEM and TEM showed that N was successfully doped into the composite and uniformly dispersed in the material,and the iron-based particles were encapsulated in the carbon nanosheets.The studies on the degradation of TPHs by Fe-N@CN activated PS obtained under different preparation conditions showed that the removal efficiency of TPHs by Fe-N@CN/PS system was the first to increase and then decrease with the increase of iron concentration;while the removal efficiency of TPHs by Fe-N@CN/PS system gradually increased with the increase of calcination temperature and urea doping.In addition,the graphitic nitrogen content in Fe-N@CN was highly positively correlated with the degradation rate of TPHs in Fe-N@CN synthesized at different urea doping amounts.The best catalytic performance of the synthesized composite was achieved at an iron concentration of 0.2 M,a calcination temperature of 1 100℃,and a mass of doped urea equal to 5 times the mass of biogas residue,which could activate PS to degrade 78%of TPHs.2、Fe-N@CN prepared under optimal synthesis conditions was applied to the remediation of actual petroleum-contaminated soil,and the performance,influencing factors and action mechanism of Fe-N@CN activated PS remediation of petroleum-contaminated soil were discussed.The results showed that the removal efficiency of TPHs from contaminated soil reached 73.14%within 3 days at a PS to catalyst dosage ratio of 5:1,optimal dosages of 20 g·kg-1 and 4 g·kg-1 for PS and catalyst,a water-soil ratio of 1:1,and a soil particle size of 40 mesh or more.Cyclodextrins(CD)can enhance the water solubility of contaminants by forming inclusion compounds with hydrophobic organic contaminants,thereby facilitating their removal.The experiments showed that α-、β-and γ-CD all contributed to the removal of TPHs from soil.The removal efficiency of TPHs in contaminated soil was increased by 15.8%when 30 mmol/kg of β-CD was injected.Quenching experiments and electron paramagnetic resonance(EPR)studies showed that the degradation of TPHs in soil is a combination of SO4·-、·OH、O2·-and 1O2.Petroleum hydrocarbons with different carbon chains in the soil before and after degradation were determined by gas chromatography,and it was found that the petroleum hydrocarbons of C11-C33 were relatively easy to be degraded,but the long-chain macromolecules above C33 were difficult to be removed due to their hydrophobicity and strong adsorption.The specific substances in the soil before and after degradation were further identified by Gas Chromatography-Mass Spectrometer(GC-MS)to determine the transformation and degradation pathways of TPHs.3、The long-term(28 days)effects of the Fe-N@CN/PS remediation process on the soil environment and soil microorganisms were evaluated and it was found that the Fe-N@CN/PS remediation process did not cause significant changes in soil pH.After PS application,the counts of operational taxonomic units(OTU),which reflect the diversity of soil bacteria,decreased to different degrees in the soils of four remediation systems,PS alone,mZVI/PS system,Fe-N@CN/PS and Fe-N@CN/β-CD/PS,but the OTU values of Fe-N@CN/PS system recovered slowly during the monitoring time.In Fe-N@CN-added soils,soil catalase activity first increased during the remediation process due to oxidative stress and returned to the initial level at the end of the oxidation phase,whereas in the PS alone and mZVI/PS systems,soil catalase activity continued to decrease.There was no significant change in the dominant phyla and genus of soil microorganisms in both PS alone and mZVI/PS systems.In contrast,the microbial community structures of the Fe-N@CN/PS and Fe-N@CN/β-CD/PS groups changed significantly.The dominant phyla in Fe-N@CN/PS changed from Actinobacteria and Chloroflexi to TPHs degradation-related phylum(Firmicutes),and the dominant genus shifted from Mycobacterium to Acinetobacter,which degrades long-chain alkanes,and Bacillus,which degrades polycyclic aromatic hydrocarbons.Firmicutes and Bacteroidetes became the dominant phyla in the late stages of Fe-N@CN/β-CD/PS group repair,and Sphingobacterium,a bacterium associated with petroleum hydrocarbon degradation,became the dominant genus.