Study On Hydrocarbon Degradation Coupling With Nitrogen Conversion Enhanced By The Soil Bioelectric Field

With the increasing of oil use and exploitation,the problem of soil petroleum hydrocarbon pollution is more and more serious.Soil microbial electrochemical remediation?MER?utilize artificially added solid electrodes as a continuous flow of electron receptors to promote the metabolism of pollutants and generate biocurrent/electric field synchronously.It overcomes the technical problem of lack of electron acceptor in organic-contaminated soil remediation and shows great potential in the removal of refractory organic compounds such as petroleum hydrocarbons and aromatics.While degrading organic pollutants,soil MES was also accompanied with the conversion of nitrogen,exerting a great influence on the electron transfer in the system and the microbial metabolism of petroleum hydrocarbons.This study investigated the removal of petroleum hydrocarbons and the nitrogen-coupled transformation process in the constructed horizontal bioelectric field,and revealed the niche differentiation and interaction mechanism of functional bacteria?degradation bacteria,electrogenic bacteria,and nitrogen-transforming bacteria?in the system.In addition,different types of nitrogen sources?nitrate nitrogen,ammonium nitrogen and urea?were added to the system to further investigate the process and mechanism of bioelectric field enhancing petroleum hydrocarbon degradation and nitrogen conversion.And the results showed that:?1?In the constructed soil MES?no additional nitrogen source?,the highest bioelectric field of2021 mV?m^-11 was generated,and the carbon conversion and nitrogen conversion processes in the system were promoted:the removal rates of total petroleum hydrocarbon,alkanes and aromatics were increased by 52%,38%and 136%respectively compared with the control group.Cathode-dominated ammoniation and anod-dominated denitrification were also promoted.Through the analysis of microbial community structure,it was found that the enhancement of the degradation ability of cathode and anode in closed-circuit system depended on different functional bacteria:cathode,Alcanivorax;anode,Marinobacter.The significant increase of the denitrifying bacteria Pontibacillus at the anode demonstrated the enhancement of denitrification.?2?In the system with the addition of nitrate nitrogen,the electrical performance was greatly reduced and the degradation efficiency of petroleum hydrocarbons was enhanced.This was because NO₃^-,as a thermodynamic electron acceptor,could cause electron competition between anode reduction and NO₃^-reduction.Through the analysis of microbial community structure,it was found that nitrate addition significantly increased the total abundance of potential degrading bacteria and denitrifying bacteria in the system,while the electrogenic bacteria almost disappeared.Using FAPROTAX to predict microbial functions,it was found that the abundance of carbon-conversion function in the system increased significantly,indicating that nitrate addition may improve the abundance and expression of hydrocarbon degrading genes in the system.In addition,the microbial network relationship revealed the synergistic relationship between potential degrading bacteria and denitrifying bacteria in soil MES,indicating that the denitrification process involving NO₃^-may promote the degradation of hydrocarbons.?3?The effective forms of ammonia chloride and urea are NH₄⁺,but they had different effects on the power generation and degradation performance of the system.Ammonia chloride in the form of ions added to the soil produced a high concentration of NH₄⁺which was toxic to functional microorganisms,resulting in the electricity generation and degradation performance decline of system in the early stage.With the consumption of NH₄⁺its power generation and degradation capacity gradually increased.The addition of urea to the soil in molecular form increased the electrical power output capacity of the system and the removal capacity of total petroleum hydrocarbons and alkanes,as urea hydrolysis provided available nitrogen sources for microorganisms and avoided the production of high concentrations of NH₄⁺.The microbial community analysis showed that the addition of chlorinated ammonia increased the total abundance of denitrifying bacteria in the system,while the addition of urea was beneficial to the proliferation of degrading bacteria Alcanivorax and electricity-producing bacteria Bacillus.Microbial annotation function analysis found that the abundance of carbon-conversion function was higher in the urea group,but significantly decreased in the ammonia chloride group.The abundance of urea hydrolysis function was the highest in the first stage of urea group,indicating that the hydrolysis of urea mainly occurred in the early stage.This study investigated the effect of bioelectric field on microbial carbon-nitrogen conversion in the system without external nitrogen source and with external nitrogen source,aiming to reveal the electron transfer mechanism and nitrogen cycle mechanism of soil MES to repair petroleum hydrocarbon pollution.This study can provide a basis for the study of the mechanism of microbial electrochemical remediation,and at the same time provide a technical reference for its field application.