Chemical Oxidation Treatment Of Petroleum-contaminated Soil And Its Effect On The Diversity Of Bacterial Community Structur

Petroluem contamination in soil has an important impact on the safety of the ecological environment,which seriously restricts the sustainable development of the social,economic and environmental aspects of the oil field.The combined use of in situ chemical oxidation(ISCO)and in situ microbial remediation(ISB)is one of the main means of remediation of petroluem contaminated soil.Chemical oxidation can reduce the toxicity of pollutants,increase the availability of bacteria,and promote the ISB process.However,the chemical oxidation process also has a certain inactivation effect on soil bacteria,which is not conducive to ISB treatment.In view of the "double" effect of ISCO on soil bacteria during the remediation process of oilcontaminated soil,different oxidants(H2O2,Na2S2O8)were selected to study the effects of different oxidant dosages and dosage methods on the removal of total petroleum hydrocarbons(TPH)in petroleum-contaminated soil and the changes in bacterial community structure.During the reaction,the changes of oxidant remaining amount,pH,dissolved organic carbon(DOC),dissolved total nitrogen(TN),and total dissolved phosphorus(TP)contents were analyzed and compared.TPH content,the remaining oxidant amount,DOC,TN,TP,pH and other environmental factors have been discussed in relation to bacterial species richness and changes in community structure.The main findings are as follows:(1)Na2S2O8 has good sustained effectiveness.Adding H2O2 in batches can increase the sustained effectiveness of oxidants and the release of nutrients,and can effectively control the amount of H2O2 added,but it is completely consumed on the fourth day.When Na2S2O8 is used as an oxidant,it lasts longer in the soil.After oxidizing petroleum contaminated soil with low initial concentration of Na2S2O8 for 10 days,9.5 mM oxidant remains in the system.After 10 days of oxidation reaction,the final TPH removal rates were 38.09%,46.20%,and 47.95%in the Na2S2O8 initial concentration of 21 mM,63 mM,and 105 mM,respectively,which were 0.46%,12.41%,and 14.21%higher than the H2O2 treatment.H2O2 decomposes faster in soil and has less inhibitory effect on microorganisms,but at the same time it has a lower oxidation removal rate of TPH in petroleum contaminated soil.The removal rate of TPH by Na2S2O8 is relatively high,but due to the continuous existence of oxidants,it has a certain inhibitory effect on soil microorganisms.(2)When Na2S2O8 is activated by different iron-based catalysts to oxidize and repair petroleum-contaminated soil,the TPH degradation rate is related to the remaining amount of Na2S2O8.Compared with ferrous citrate and Fe3O4,Fe0 catalytically activates Na2S2O8 to oxidize TPH with the best effect.After 10 days of reaction,the final removal rate of TPH reached 53.39%in the treatments with initial Na2S2O8 concentrations of 21 mM,63 mM,and 105 mM.59.63%,64.71%.Compared with the treatment group without catalyst addition,the TPH degradation rate increased significantly.The addition of homogeneous catalyst ferrous citrate is more conducive to the precipitation of nitrogen in the soil,and also provides a sufficient nitrogen source for the subsequent growth of bacteria.(3)In the H2O2 system,the DOC concentration shows an upward trend.Since Na2S2O8 is more capable of oxidizing macromolecular organics in oily soil to small molecules than H2O2,the DOC growth rate in the Na2S2O8 treatment group at the early stage of the reaction(54.88%)It is 34%higher than the H2O2 treatment group(20.88%),but with the decrease of organic pollutants in the soil,the DOC concentration in the Na2S2O8 treatment group gradually decreased.The concentration of DOC in the system of dosing H2O2 in batches is less than one dosing process.The change tendency of the total nitrogen content in the soil is basically the same.The TN content in the CK group increased first and then decreased.The TN content in the H2O2 system fluctuated within the range of 100～150 mg·kg-1,while the TN content in the Na2S2O8 treatment group There is an upward trend,and the TN concentration curve in the H2O2 system added in batches shows a trend of increasing first and then decreasing.Compared with the H2O2 pre-oxidation treatment group,the pH value in the Na2S2O8 treatment group was significantly lower,maintained at pH 2～4,and the higher the initial oxidant concentration,the lower the pH.(4)The Chao1,Shannon and ACE indexes in the alpha diversity index showed an upward trend with the oxidation time,while the Simpson index values between the treatment groups were not significantly different.Among them,in the treatment of adding oxygen H2O2(HT)three times,medium initial concentration Na2S2O8(PM)and zero-valent iron activated high concentration Na2S2O8(ZH),the species diversity is the most abundant.The Venn diagram reflects that the bacterial aggregation is more abundant in the oxidation treatment group with low initial concentration,and the pre-oxidized petroleum-contaminated soil produces a large number of new bacterial species for the bacterial oil-contaminated environment.The heterogeneous catalyst(Fe0)activates Na2S2O8 to repair oil-contaminated soil,which is beneficial to the recovery of bacteria in the soil,and the higher the initial concentration of oxidant,the greater the number of bacteria in the soil.(5)The results of species abundance changes indicate that the predominant bacterial groups in the soil change after chemical oxidation,and the increase in the removal rate of petroleum hydrocarbons is related to the increase in hydrocarbon-degrading bacteria in the soil.In the initial stage of oxidation,Firmicutes accounted for a relatively high proportion.As the reaction time increased,the abundance of Firmicutes decreased,and the abundance of Proteobacteria increased in each oxidation treatment group.After the 50th day of the reaction,Proteobacteria accounted for 64.41%in the low-concentration H2O2 treatment group,which was significantly higher than that in the other H2O2 treatment groups(CK:32.73%,highconcentration H2O2:38.23%,and H2O2:29.56%was added in three doses).In the Na2S2O8 treatment group,the relative abundance of dominant phylums was Proteobacteria,Firmicutes,and Bacteroidetes in descending order.Firmicutes have strong resistance to soil and better resistance to chemical oxidation;as the oxidant is gradually consumed,the abundance of Proteobacteria in the soil increases,which may be due to bacteria degrading petroleum hydrocarbon pollution the main contributor to matter.The low concentration of Na2S2O8 is more conducive to the growth of Actinobacteria.With the increase of reaction time,the relative abundance of Firmicutes increased in the Fe(Ⅱ)and Fe0 activated low concentration Na2S2O8 treatment groups(FL,ZL),which stimulated the degradation of petroleum hydrocarbons by bacteria.At the genus level,the dominant bacteria in the different oxidation treatment groups are Bacillus and Paenibacillus.(6)The correlation analysis results of the changes in the bacterial diversity level of the reaction system and the main indicators of the physical and chemical properties of the soil and the changes in the total petroleum hydrocarbon content show that TPH is the factor that has the greatest impact on the community structure in all treatments,and soils with similar TPH content have Similar species composition structure.TP had a significant effect on the distribution of bacterial communities in Na2S2O8 oxidized soil,but no significant effect on the distribution of bacterial communities in H2O2 oxidized soil.Through VPA analysis of different chemical oxidation treatments,the changes of nutrients in each treatment group contributed more to the differences in bacterial community distribution.When two iron-based catalysts(Fe(II),FeO)activate different concentrations of Na2S2O8 to pre-oxidize petroleum-contaminated soil,nutrients are used as a group of environmental factors,and their interpretation is 31.04%,which is different from that of different treatments of H2O2 and different initial concentrations of Na2S2O8 treatment group.They were 6.68%and 17.19%higher,respectively.In conclusion,different chemical oxidation treatments of petroleum-contaminated soil have showed various influences on TPH removal rate,the contents of disolved nutrients,and the bacterial community structure and diversity in soil.The results provided further insight into remediation technologies based on ISCO-ISB of petroluem-contaminated soil.