Construction Optimization And Biochar Improvement Of Soil Microbial Electrochemical System To Remediate The Petroleum Hydrocarbon Contamination

The modern world is in high demand for the use of petroleum hydrocarbons for energy consumption,therefore this has put pressure on finding a benign remediation technology.Microbial electrochemical systems have gained attention,herein,Microbial electrochemical systems（MESs）that combine electrochemistry with biological methods have gained attention in the remediation of polluted environments,including wastewater,sludge,sediments,and soils.The most attractive advantage of MESs is that the solid electrode is used as an inexhaustible electron acceptor or donor,and biocurrent directly converted from organics can afford the reaction energy of contaminant breakdown,crossing the internal energy barrier of endothermic degradation,which achieves a continuous biodegradation process without the simultaneous use of exogenetic chemicals and bioelectricity recovery.However,soil MESs are hindered by various factors to note expensive electrode materials,difficult in pollutant and electron transfer,low microbial competitive activity,and biocompatibility in contamination remediation.The current study investigated the construction optimization and biochar improvement of microbial electrochemical systems to remediate petroleum hydrocarbon contaminations.Specifically,we first decided the best electrodes（i.e.,between stainless steel activated carbon air cathode,Titanium mesh cathode,and Carbon felt cathode marked as ST,TM,and CF）based on chemical,electrochemical,and biological properties analysis.In addition,by utilizing the best electrode,we then decided the best water seal level were investigated(i.e.,between MES₀,MES₃,MES₅,MES_0-3-5),and later incorporate biochar for further investigating the processes,mechanisms,and effects of biochar-bioelectric field combination in enhancing petroleum hydrocarbon degradation for better functionality of MES system.The main study conclusions are as follows:（1）In the constructed soil MES,the ST generated high electricity generation compared to TM and CF after 32 days of experimental period,with the maximum voltage record of 350mv,210m V,and 177m V on the 3^rd day.Likewise,ST enhanced the TPH removal（6.5%）compared to the OS（80980.2±843）,which was is 5.3%and 3.2%higher than TM and CF respectively.Again,the biological analysis showed that Proteobacteria,Firmicutes,Actinobacteriota,Desulfobacterota,and Chloroflexi were the top five phyla accounting for 87%of the total bacterial.Whereas Proteobacteria improved in ST than the rests of treatments（which is 72%,38%,and 218%higher than those of TM,CF,and OS）（2）After showing that ST outperformed the rest of the electrodes in terms of voltage generation and TPH removal,it was used in the constructed soil MES for the water levels experiment.After65 days of operation,the accumulated charge was 1282C,840C,630C,and 151C in MES₀,MES_0-3-5,MES₃,and MES₅ respectively.Likewise,a TPH removal of 28.3%:cathode,29.7%:anode was achieved in MES₀ relative to OS（original soil:07575±6318.6）which was 3.5%:cathode and 11%:anode higher than that of MES₅（P<0.05）.Through the analysis of microbial community structure and interactions,it was revealed that the co-occurrence networks analysis showed dominance in positive than negative links within and between bacterial,Fungal,and Archaeal communities in MES₀ than in MES₅ and OS,it also confirmed that they were more closely related.Additionally,Anaerobacillus,and Clostridium＿sensu＿stricto and Clostridium＿sensu＿stricto＿13;Salinimicrobium;Halomonas;Marinobacter were the taxa hub in MES_0.Assumed to have played a bigger role in the enhancement of petroleum hydrocarbon degradation.Additionally,the reason for better performance in MES₀ compared to the rest of the treatments is linked to high dissolved oxygen diffusion in their layers,redox potential,and reduction of resistance（i.e.,both internally and externally）,which shifted the electron acceptors hence contributing in the process of electrons and mass transfer of substrates and metabolites in the electrode chambers which in turn dictated the biogeochemical processes which affected the types of predominant soil bacterial,fungal,and archaeal communities.（3）Biochar introduction into soil MESs revealed a high capability in addressing MES inadequacies thereby improving its performance.This is made possible because of the characteristics of biochar acquired from the feedstock used and pyrolysis temperature.Because of biochar’s high conductivity,transferability,high specific surface area,high porosity,large functional groups,and biocompatibility.Biochar was also revealed to not only carry electrons but also transfer nutrients,pollutants,and even bacteria by facilitating transmission in the bioelectric field of MESs.Consequently,the abilities of biochar are beneficial for better functionality of MESs.Additionally,the roles of biochar in MES might can be divided into the following four main categories.（a）Adsorbing and/or remediating pollutants to assist removal,（b）Electron transfer mediator,（c）Electrode materials and proton exchange membrane（PEM）,and（d）Conductive network and amendment,and pollutants（i.e.,organic and/or inorganic）removal mechanisms involved vary based on the above-mentioned categories.（4）By utilizing fresh biochar（FB）obtained from corncobs（700 ^oC,1.5 hours）and aged biochar（AB）oxidized using strong acids（HNO₃/H₂SO4 at 1:3（v/v）diluted at 40%）used as amendments to assess the spatial-temporal hydrocarbon removal,bacterial community shifts and functional genes change in the electrode vicinities（cathode and anode）,After 78 days of operation,an accumulated charge of 750 C for FB was gained,this was 12%and 98%higher than that of AB and the no-biochar control（CK）,respectively.Moreover,the removal of TPH removal was enhanced in the order of CK（17-26%）<AB（22-28%）<FB（24-29%）in the cathodic area,while in the anodic region,the TPH removal was enhanced in the order CK（22-31%）<FB（24-30%）<AB（28-30%）on days 43-78.Additionally,the top six phyla were Proteobacteria,Desulfobacterota,Firmicutes,Chloroflexi,Actinobacteriota,and Bacteroidota,accounting for>92%of the total bacterial communities,with a declining tendency over time.Additionally,there were Proteobacteria-cathode domination and Firmicutes-Desulfobacterota-anode domination.furthermore,functional genes emphasized petroleum hydrocarbons as the carbon source and relationships with nitrogen,sulfur,and iron compounds in soil.The difference in the performance of fresh and aged biochar is strongly attributed to variation in properties acquired after pyrolysis vis a vis to carbon（i.e.,with a decrement of 26%）,hydrogen,oxygen,and nitrogen（i.e.,an increment of 18%;214%and 148%in AB respectively compared to FB）,and this was confirmed by the FTIR,SEM,and 2D-COS map as well.To sum up,study results validated the aptitudes and application effects of fresh and aged biochar in MES in a meticulously constructed and optimized MES system.