The Removal Characteristics And Mechanism Of Typical Soil Pollutants By Microbial Fuel Cell

Soil is a significantly important component of natural environment factors.Recently,the area of soil polluted by organic substance and heavy metal has been expending due to the development of industry and agriculture.There are many technologies,like physical,chemical and biological technologies to deal with these pollutants.However,most of these physical and chemical technologies are high energy consumption and emit greenhouse gases into the atmosphere.Although biological treatment technologies are economical and friendly,they can be greatly restricted in the treatment of pollutants with low biological utilization.Therefore,some more advanced technologies have been developed for pollution control,such as microbial fuel cells(MFCs).MFC technique becomes an alternative worthy of attention while it can generate electricity.The removal of pollutants by MFCs has been extensively investigated,besides the performance and mechanism of electrogenesis.In this study,two types of soil MFCs have been constructed.The hexachlorobenzene(HCB)and atrazine were degraded and copper was migrated in soil MFCs.In soil MFCs,a microbially catalyzed oxidation of reduced compounds(acetate)at the anode was responsible for the delivery of electrons to the anodic electrode.The electrons passed through an electrical circuit containing a power user.On arrival at the cathodic electrode,they reacted with available electron acceptors,such as oxygen.In this study,the performance of electricity generation was investigated,the effect of external resistance,electrode spacing,buffer concentration on HCB and atrazine degradation were analyzed,the performance of soil MFCs in the copper removal was assessed,the mechanisms for organic pollutions degradation and copper removal were evaluated.The main results were as followed:(1)The performance of electricity generation was studied.The voltage and maximum power density increased with higher phosphate buffer(PB)concentration and external resistance.With the PB concentration of 0.2 M,the voltage and maximum power density was170 mV and 103 mW/m~2.Altering the spacing of the reducing electrode resulted in a lower ohmic resistance in the soil MFCs.The performance of soil MFCs did not enhance by shorting electrode spacing but there was an optimum range.When the electrode spacing was 8cm,the soil MFCs obtained the high voltage and maximum with the value of 90 mV and 33mW/m~2.When the initial atrazine concentration was 100-750 mg/kg,the soil MFCs were under proper functioning,and those performances were depressed with higher atrazine concentration.(2)The effects of external resistance,initial concentration,electrode spacing and PB concentration on HCB and atrazine degradation were evaluated,the performance of soil MFCs in the degradation of mixed pollutants was assessed.The removal efficiency,degradation rate increased with decreasing external resistance,while the half-life decreased.With an external resistance of 20?,the removal efficiencies of HCB and atrazine were 64.68%and 95%,respectively.The removal efficiency of HCB and atrazine was declined while removal content and degradation rate were inhibited when the initial concentration increased.The removal efficiency of HCB increased firstly and then decreased with increasing electrode spacing.The highest removal efficiency was 64.09%when the electrode spacing was 8 cm.However,the highest removal efficiency of atrazine was 91.69%when the electrode spacing was 4 cm.The removal efficiency of atrazine increased firstly and then decreased when the electrode spacing ranged from 6 cm to 12 cm.The removal efficiency of HCB was enhanced by increasing PB concentration.The mixed pollutants did not affect HCB and atrazine degradation significantly.(3)The electrode effectiveness played a significant role in HCB and atrazine degradation.There were different degradation trends for different pollutants under different soil layers.The HCB degradation rate followed the order of lower>middle>upper MFCs sections,and that trend of atrazine followed the order of upper>lower>middle.This difference depended on the characteristics of the pollutants and soil redox conditions generated by soil MFCs.The HCB and atrazine was degraded by a reductive dechlorination process in soil MFCs.The fate of HCB was reduced to pentachlorobenzene(PeCB),tetrachlorobenzene(TeCB),and trichlorobenzene(TCB)in sequence.The atrazine was degraded to hydroxyatrazine and then to urea.(4)The microbial community analysis demonstrated that Geobacter spp.(24.3%)involved in current generation was extremely abundant on the anode and the lower layer in the soil MFCs.Atrazine concentration had a profound negative effect on the development of the microbial community composition in soil MFCs.The percentage abundances of HCB and atrazine degrading bacterial were considerably lower.These results suggested that soil MFCs can enhance the degradation of atrazine and HCB,and bioelectrochemical reduction was the main mechanism for the pollutants degradation.Current generated by soil MFCs was the major influence on the degradation of HCB and atrazine.The removal efficiency could be enhanced by adjusting external resistance,electrode spacing,PB concentration to increasing current.(5)A single-chamber and membrane-less soil MFCs was constructed to remediate the contaminated soil,and the electric field was generated from the oxidation of the acetate at the anode.We demonstrated the copper migration,the power generation,and the pH variation in the soil and the electrodes.The maximal voltage and the power density of 539 mV and 65.77m W/m~2 were obtained in the soil MFC.The chemical fractionation of copper(Cu)was analyzed with a modified BCR sequential extraction method.The soluble Cu form and the total Cu contents from the anode to the cathode increased,and the difference between them kept growing over time.The Cu fractions in the soil and the electrodes were converted with the change of the dramatic pH from the anode to the cathode.There was a focusing effect leading to the change of the Cu forms,and the extractable acid form content increased in the three fifths where the acid and the alkali fronts met.(6)A three-chamber soil MFCs was constructed to remediate the contaminated soil.The external resistance and electrode spacing had a remarkable effect on the performance of soil MFCs and that influence was depended on cathodic polarization.The pH in cathode changed slightly and Cu removal efficiency in cathode increased when the external resistance decreased and electrode spacing shortened.Copper was enriched near the cathode chamber and enrichment amount increased with external resistance increasing.The enrichment amount was a minimum of 268 mg/kg when the electrode spacing was 12 cm.(7)Copper was removed in the soil by the electricity field generated by soil MFCs.Copper was migrated from the anode to cathode by electromigration and electroosmosis.Furthermore,copper was enriched in cathode area and the soil section near the cathode,and the enrichment amount was enhanced when the voltage increased.The two fractions,acid extractable fraction and reducible fraction,were migrated easily compared with other fractions.The copper could be reduced to elemental copper owing to cathodic reductive in the cathode in three-chamber soil MFCs.the efficiency of cathodic reductive was depended on the current rather than voltage generated by soil MFCs.