Recycling And Transformation Of Sulfur In Microbial Fuel Cells Treating High-salinity Wastewater

The composition of high-salinity organic wastewater is quite complicated,and the current treatment technologies have high energy consumption,complex process and low treatment efficiency.It’s long been a focus point for researchers in the environmental areas to seek resources recovery for high salinity wastewater.Microbial fuel cells（MFC）have attracted many attentions due to its dual functions of wastewater purification and electricity generation.The study has shown that MFC treatment is suitable for treating high salinity organic wastewater.The high salinity is conducive to the electron transport and the high organic matter is beneficial to obtain high productive power efficiency.However,the transformation of carbon and nitrogen with the participation of sulfur cycle,together with research on electricity productive efficiency are still one of the blinded areas that haven’t been explored during the practical application of theory and technologies.Sulfur metabolism process plays an important role in the sulfur-containing bio-electrochemical system,which involves two main functional categories of microbes,that are sulfate-reducing bacteria（SRB）and sulfur-oxidizing bacteria（SOB）.The paper takes up the MFC treatment for high salinity wastewater as the research objective,explores the sulfur cycle and transformation in MFC,so as to realize the simultaneous carbon and nitrogen removal and energy recovery in the sulfur-containing bio-electrochemical system.It investigates the coupling transformation channel of carbon,nitrogen and sulfur in the system,which provides a new thinking for the resources recovery of high salinity organic wastewater.The major research achievement attained are as follows:（1）In the single-chamber MFC system,the participation of sulfur cycle had a positive effect.From the perspective of electricity generation,the participation of sulfur could enhance the energy output for the system.For the group of synthetic wastewater containing SO₄^2-,the maximum power density,open circuit voltage and internal resistance were 6.81±0.28 W·m^-3,615±5 m V and 24.58±0.71Ωrespectively.Compared with the group without SO₄^2-which were 4.93±0.37 W·m^-3,605±6 m V and 50.21±0.63Ω,they manifested better electricity production performance.In the view of pollutants,under the circumstances of different water qualities,it showed a good chemical oxygen demand（COD）removal effect,with the removal rate amounting to above 80%;the participation of sulfur created an obvious impact on the removal of total nitrogen（TN）,with the removal rate hitting to 92.16±0.94%containing SO₄^2-,and removal rate achieving 66.58±1.43%without SO₄^2-;At the same time,the electrochemical process would also facilitate the denitrification.The TN removal rate of the open circuit group containing SO₄^2-was only 82.24±0.95%.The combined action of electrochemical process and metabolism of microbes accelerated the sulfur cycle,improving the removal rate of pollutants.The change of colonial structure for microbes under different environments of water quality was quite obvious,showing a consistency with its systematic performance.The distribution of the electrogenic bacteria was as follows:Thauera、Lentimicrobium、Paracoccus.The genera unclassified＿f＿Rhodocyclaceae、Vitellibacter、unclassified＿f＿Rhodobacteraceae、Thauera were the main nitrogen removal bacteria.（2）The results of batch activity test showed that there were multiple pathways existing for coupling transformation of sulfur,carbon and nitrogen in single chamber MFC system.The removal of nitrogen mainly consisted of heterotrophic nitrification-aerobic/anoxic denitrification,the participation of sulfur had optimized in small magnitude for denitrification.As for the transformation of carbon:the carbon existed in the form of organic carbon,the organism was used for the metabolism for heterotrophic microorganisms to complete the microbial process of electricity production,hydrolytic fermentation,nitrogen removal,sulfate reduction.As for the transformation of nitrogen,nitrogen entered into the system in the form of NH₄⁺,and was discharged out after being transformed into N₂.First,heterotrophic nitrification（76.9±5.2%）,autotrophic nitrification（17.3±3.1%）and other processes were carried out at the cathode.The denitrification process was completed jointly by anode and cathode,and nitrogen removal was completed by anoxic heterotrophic denitrification（51.1±3.5%）,aerobic heterotrophic denitrification（27.0±2.7%）,sulfur autotrophic denitrification（12.1±2.1%）and electrode denitrification（9.8±1.9%）.As for the transformation of sulfur,SO₄^2-was the primitive form,the reduction of SO₄^2-was completed by SRB,and the S^2-formed was oxidized to S⁰ by electrochemical reaction and accumulated in the anode,or oxidized to higher valence state by SOB;S^2-could also form into the substances of SO₄^2-,SO₃^2-,R-SH,S^2-,S_n^2-and S⁰ through the sulfur metabolism of cathode and sulfur autotrophic denitrification process,and the generated high valence sulfur can enter the system circulation again.（3）In the single-chamber MFC system containing sulfur,the various aspects for the system had all changed after the operation for multiple cycles,which mainly reflected in below various respects.Increase in electricity generation performance:the length for electricity product had elongated from 42 hours to 114 hours from the 2^nd cycle to 18^thcycle,the maximum power density had increased from 5.21±0.22 W·m^-3 to 8.43±0.21W·m^-3 respectively.The coulomb efficiency had raised from 6.51±0.41%to 14.38±0.52%.The worsening of substance transformation:due to that the nitrification process was inhibited and hindered,the removal effect for TN had dropped from 98.75±1.11%at the2^nd cycle to 34.08±1.30%at the 18^th cycle.The recycling and transformation of sulfur had been weakened,the average reduction rate of SO₄^2-had decreased from 2.53 mg·L^-1·h^-1to 0.34 mg·L^-1·h^-1.However,all COD removal rate retained at above 80%.After the long-term operation,the transformation of nitrogen and sulfur was blocked,however the carbon was not affected seemingly.The microbial succession was obvious:after long-term operation,the species and total abundance of bacteria related to denitrification and sulfur transformation in anode decreased sharply,but the species and abundance of hydrolytic fermentation bacteria increased substantially.The genus in the cathode such as unclassified＿f＿Rhodobacteraceae,Thaurea,Paracoccus,which were responsible for heterotrophic nitrification and those total abundance dropped from 17.18%to 4.47%.The structure of microorganisms is related to the transformation of various substances.