Mechanisms Of Microbial Electrocatalysis For Treating Sulfate-containing Wastewater

Sulfate,a typical component in many industrial wastewaters,which can stimulate the growth of sulfate reducing bacteria(SRB)to inhibit methanogenesis,further decreases the performance of anaerobic digestion in sulfate-rich organic wastewater.It is crucial to improve the removal efficiency of organic matter for treating sulfate-rich organic wastewater.Recently,Bioelectrochemistry system such as microbial electrolysis cell(MEC)has received considerable attentions.Its primary mechanism may be ascribed that electrodes(extracellular solid conductive materials)can serve as electron donors or acceptors for microbial energy metabolisms,which forms a new pathway for the decomposition of organic substrates.The application of MEC for organic matter removal and sulfate reduction has been reported.However,few studies investigated the mechanism for organic matter removal and sulfate reduction,especially for the mechanism of MEC on the regulation of the relationship between sulfate reduction and methanogenesis.Furthermore,conductive materials could form the connection between the exoelectrogenic bacteria and other electron acceptors,such as direct interspecies electron transfer(DIET)for methanogenesis.It provided a possibility for enhancing the degradation of organic substrates.However,connection of exoelectrogenic bacteria and other electron acceptors in sulfate wastewater has been less investigated.Based on these considerations,in the presence of applied electric field and conductive materials,the establishment and enhancement of electron transfer for exoelectrogenic bacteria and other electron acceptors had been proposed in this study to accelerate the decomposition of organic matter.Furthermore,the mechanism of sulfate reduction and methanogenesis by MEC was discussed.The main results are as follows:(1)A pair of electrodes(carbon brush and stainless steel)was put into anaerobic acidogenic reactor to form a MEC-acidogenesis to investigate its COD removal and sulfate reduction.The results showed that MEC obviously improved anaerobic acidogenesis for treating sulfate-containing wastewater(10.2%-21.3%),more volatile organic acids were converted into acetate.Fluorescence in situ hybridization(FISH)analysis showed that exoelectrogenic bacteria were enriched on anodic biofilm.However,the coulombic efficiency of anodic oxidation coupling cathodic sulfate reduction was slow.The reason of MEC accelerating the decomposition of organic substrates was that the anodic exoelectrogenic bacteria were enriched on the anodic biofilm.It could enhance the conversion of volatile organic acids.Then it improved pH and provided a positive environment,and reduced the inhibition such as hydrogen sulfide and acetate concentration.The results showed that the performance of MEC reactor was always higher than control reactor.COD/sulfate ratio affected the reduction of sulfate by MEC.When the COD/sulfate ratio decreased from 3 to 0.4,contribution of cathodic sulfate reduction increased from 4.6%to 14.7%.The effect of anodic oxidation coupling cathodic sulfate reduction was limited.The main reason was MEC promoted the heterotrophic sulfate reduction,and further accelerated organic matter degradation by sulfate reduction.(2)In order to investigate the possibility of enhancement methanogenesis during sulfate conditions,effects of anodic potential were investigated on the organic matter removal,sulfate reduction and methanogenesis.The COD removal rate in the-0.3V(vs NHE),+0.3V(vs NHE)and control reactors were 75.5%,64.4%and 59.5%,respectively.Further results showed that more organic matter was converted into methane.Methane production in the-0.3V reactor was 15 and 6 folds higher than control and +0.3V reactors respectively.Furthermore,the conductivity of sludge in the-0.3V reactor was 3 and 1.2 folds higher than control and +0.3V reactor.In addition,VFAs,hydrogen production and FISH image also illustrated that the potential direct interspecies electron transfer(DIET)of methanogenesis between Geobacter and methanogens was established on anodic biofilm by-0.3V potential.However,+0.3V reactor could not form the DIET of methanogenesis for its higher potential.Based on the results,electrode was used to maintain the acidic balance of anaerobic system.Effluent COD concentration by Adding electrode was obviously decreased.The results showed that the method could quickly recover the treatment efficiency of acidification reactor.(3)Based on the mechanism of conductive materials promoting methanogenesis in MEC reactor,DIET was constructed by conductive materials for enhancing the methane production in sulfate wastewater.Results showed that methane production of the digesters increased by 7.5%-24.6%by adding stainless steel.EDX and Raman spectra showed that iron oxides(Fe2O3)were formed on the surface of stainless steel.FISH analysis showed that the dissimilatory Fe(III)reducers(like Geobacter sp.)were enriched on the surface of the stainless steel.Stoichiometry revealed that DIET of methanogenesis was more favorable in kinetics(108 folds)than interspecies H2 transfer(IHT)of sulfate reduction.Thus,the main reason of enhancing methanogenesis was that DIET between Geobacter and methanogens accelerated the degradation of organic matter and production of methane.DIET associating with stainless steel had an advantage of kinetics of methanogenesis,which could reduce the contribution of sulfate receiving electron,and make more electrons guiding to methanogenesis.Then potential DIET of methanogenesis associating with stainless steel could outcompete the sulfate reduction.Furthermore,in order to investigate the range of application in DIET of methanogenesis for sulfate wastewater,iron oxides(Fe2O3)were used to study the effect of DIET for methanogenesis under different sulfate conditions(ethanol as electron donors).Results showed that in the presence of 0 mmol-40 mmol sulfate,all methanogenesis were enhanced by DIET.When the Fe2O3/sulfate was 1:1,the production of methane was the highest.In order to enhance the methanogenesis,DIET was established in two phase anaerobic reactor.High-throughput sequencing showed that Methanothrix(75.4%)was largely enriched in methanogenic digesters,and PICRUSt analysis showed functional genes of Pili and cytochrome C were high expression.The reason of increasing methane production in two phase anaerobic reactor was that DIET between Clostridium and methanogens was established in methanogenic digesters.