Basic Research On Sulfate-rich Wastewater Treatment Using Microbial Electrolysis Cell

Low sulfate reduction efficiency and low utilization efficiency of electron donors are main factors restricting the industrial application of biological desulfurization.The study of biological desulfurization under condition of poor electron donors can not only reduces the cost of biological desulfurization process,but also has practical significance for the treatment of sulfate-rich wastewater with limited electron donors such as acid mine wastewater and flue gas desulfurization wastewater.Base on this issue,this paper proposes the treatment of sulfate-rich wastewater by bioelectrochemcal technology,using microbial electrolysis cell(MEC)reactor to enhance sulfate degradation.The optimal working condition of MEC reactor was studied in detail,and the effect of applied electric field on bacteria metabolic activity and microbial community were elucidated.The effect of microorganism on cathode properties was also investigated.Furthermore,electron transfer mechanism between microorganisms and electrode was studied in order to lay the theoretical base for electrode material development.Eventually,we developed modified electrode which can significantly improve the performance of MEC reactor.The details were demonstrated below:1.MEC coupled with SRB was used to degrade sulfate-rich wastewater which was deficient in electron donors.Results confirmed that SRB could trigger vigorous synergy with applied current.Applied electrical field of 1.5 mA resulted in highest sulfate removal,which was 14.9%higher than control reactor.In addition,organic-substance consumption decreased with the increase of applied current.High current led to plasmatorrhexis,low growth rate,and metabolic activity,subsequently reduced sulfate-reduction efficiency.Conversely,proper current enhanceed extracellular secretion of bacterials,which was conducive to biofilm formation.Electrochemical impedance spectroscopy(EIS)illustrated SRB could accelerate the rate of direct electron transfer to cathode.High-throughput sequencing analysis further revealed bacterium Desulfovibrio was dominant in microbial community.2.Under long-term operation,due to the impact of electric double layers(EDLs),salt crystals formed on the electrode of MEC reactor,thereby potentially retarding sulfate removal during the whole operation.Here,an improved MEC reactor using intermittent electric field was established.It works better in sulfate removal for a longer period,which was higher than the conventional MEC reactor by 2.18-fold after 10 days.The formation of salt crystals on the electrode led to plasmatorrhexis.Conversely,improved reactor contributed to extracellular substances production and prevented the salt crystal formation,which was conducive to biofilm formation.Biofilm can accelerate electron transfer whereas the salt crystals increased the charge transfer resistance.High-throughput sequencing analysis illustrated that improved reactor could maintain the competitiveness of SRB in the microbial community for a longer period.Moreover,the improved reactor resulted in high species diversity,thereby showing the significant resistance of the microorganisms to arduous environments.3.Neutral red(NR)is involved as electron transfer mediator to investigate whether it could contribute to MEC performance.Afterwards,influence of electrode material on MEC system was also studied.The results indicate MEC with NR shows better electrochemical activity,which means SRB can response to electron transfer mediator NR.Further study reveals graphite felt triggers a better synergy with NR to facilitate electron utilization efficiency of the system subsequently maintains bacteria metabolic activity for a longer time.Sulfate removal in this reactor reaches 79.0%with electron utilization efficiency of 54.2%.To explore the mechanism,electrode bioelectrochemical property,microorganism activity and community were investigated.Electrode morphology analysis confirms graphite felt affords abundant space for the growth of electroactive microorganisms especially SRB and promotes electron exchange through cooperating with NR.High-throughput sequencing analysis confirms improved reactor can fortify the population dominant of SRB in the community which greatly raises electron utilization efficiency of SRB.4.A well-designed method for accelerating electron transfer between microbes and electrodes,which significantly promoted performance of MEC,was introduced in this study.Functionalized multi-walled carbon nanotubes(MWCNTs)were obtained via covalent linking through ester bond and noncovalent bond ?-? stacking.2-hydroxy-1,4-naphthoquinone(HNQ)was successfully immobilized on the surface of MWCNTs.Results supported that HNQ could act as a mediator and improve bioelectrocatalytic activities.The presence of HNQ moieties significantly aided direct electron transfer.MEC reactor with modified electrode exhibited improved performance in sulfate removal and hydrogen production.Moreover,the metabolic activity of microorganisms in the reactor increased due to the enhancement of electron utilization.High-throughput sequencing analysis showed that improved MEC can strengthen the population dominance of SRB in microbial community,which considerably increases the electron utilization of these bacteria.This new reactor also demostrated increased species diversity,thereby enhancing resistance of the microorganisms to arduous environments.This method showed how smart modification of MWCNTs can be used to develop materials.The process can also be applied to bioelectrocatalysis and biofuels.