Recovery Of Typical Dimetals In Cathodes Of BESs And In-situ Catalysis For Degradation Of Recalcitrant Organics

Bioelectrochemical systems(BESs)that use microorganisms as catalysts are promising technologies to capture the energy in organic wastewaters for the prodcuction of electricity,hydrogen or value-added chemicals,including microbial fuel cells(MFCs)and microbial electrolysis cells(MECs).Based on the oxidative characteristic of heavy metal ions,recovery of single metals in the cathodes of BESs has been extensively studied along with critical parameters including cathode matterial,solution chemistry,cathodic electrode potential,current and so forth.However,the recovery of dimetals is rarely researched,and the performance of dimetal recovery in BESs is poor due to the competition resulting from simultaneous reduction of metal ions.Therefore,in this paper,considering the fact of co-existence of Cu(II)and Cd(II),Fe(?)and Cr(?),W(?)and Mo(?)in electroplating,metallurgy and acid mine wastewaters,the performances of recovery of these typical dimetals in cathodes of BESs were systematically investigated,including the catalysis of copper on Cd(?)recovery,the mediator role of Fe(?)in Cr(?)reduction,and the effect of in-situ generated H2O2 and light irradiation on W(?)and Mo(?)deposition.In addition,further collection and application of these metals deposited on the cathode of BESs have not yet been previously reported.Mo/W deposits on the cathodes of BESs were in-situ utilized for catalyzing degradation and mineralizaiton of metronidazole(MNZ)and methly orange(MO).The detailed results are as follows:(1)BESs were first operated in a MFC mode for Cu(?)recovery from mixed Cu(?)and Cd(?),and then shifted to a MEC mode for simultaneous Cd(?)recovery and hydrogen production.Cathode electrodes including Titanium sheet,nickel foam,and carbon cloth were explored for system performance.Insignificant Cu(?)recovery(4.79-4.88 mg/L/h)was observed for all materials whereas carbon cloth exhibited the highest Cd(?)recovery(5.86 ±0.25 mg/L/h),compared to the lowest(4.96 ± 0.48 mg/L/h)for nickel foam.This Cd(II)recovery was 1.8-4.2 times and hydrogen generation was 2.0-7.0 folds as high as those on no Cu controls.Cyclic voltammetry analysis verified the catalysis of in-situ deposited Cu in MFC mode for subsequent enhanced Cd(II)recovery and hydrogen generation in MEC mode.(2)The impact of Fe(III)as an electron-shuttle mediator for enhanced Cr(VI)reduction in MFCs was investigated.The presence of 150 mg/L Fe(III)achieved Cr(VI)reduciton of 65.6 ±2.2%,1.6 times as that in the absence of Fe(?).Linear sweep voltammetry and tafel plot tests implied that Fe(?)played a critical role in decreasing both the diffusional resistance of Cr(?)species and the overpotential for Cr(?)reduction.Cyclic voltammetry analysis confirmed the mediator role of Fe(?)in Cr(?)reduction in this system.(3)W(VI)and Mo(VI)was efficiently recovered in cathode of MFCs through the formation of more easily deposited peroxo-tungstate or peroxo-molybdate with the in-situ generated H2O2.Deposition of 67.2 ± 3.1%(W)was 2.4 times and 86.4 ± 1.9%(Mo)was 1.3 folds higher than those in the absence of H2O2.In addition,W(VI)and Mo(VI)recovery increased with the increase in the in-situ generated H2O2.The cooperative light irradiation and in-situ generated H2O2 achieved recovery of 98.1 ± 0.7%(W)and 100 ± 0.4%(Mo)in MECs.(4)The Mo/W deposits on the cathodes of BESs can be in-situ employed to establish a photo-electro-Fenton system for degradation and mineralization of MNZ and MO.The effect of Mo/W catalyst loading and ratio on degradation and mineralization of MNZ was investigated.The optimum loadings were 0.24 mg/cm2 for anaerobic conditions and 0.18 mg/cm2 for aerobic conditions.Under both anaerobic and aerobic conditions,the Mo/W molar ratio of 0.17:1.0 invariably achieved the highest rates of MNZ degradation and mineralization.Under aerobic conditions and in the co-presence of 10 mg/L Fe(?)and light,MNZ degradation of 94.5 ± 1.4%and mineralization of 89.5 ? 1.1%were obtained,1.2 and 1.3 folds as those in no light control,and 1.1 and 1.2 folds as those in no Fenton control,respectively.The impact of anaerobic and aerobic conditions on decolorization and mineralization of MO was explored.Anaerobic conditions favored MO decolorization,but were unsuitable for the subsequent mineralization.Conversely,aerobic conditions favored MO mineralization but were disadvantageous for its decolorization.Complete decolorization and mineralization of 96.8 ± 3.5%(9.7 mg/L/h)were achieved with a sequential operational strategy of first anaerobic conditions(20 min)and then aerobic conditions with addition of 10 mg/L Fe(III)(100 min).This mineralizaiton rate was 2.9 times as that under entirely anaerobic conditions,1.9 times as that under entirely aerobic conditions and 1.3 times as that under entirely aerobic conditions with addition of Fe(?).These results provide alternative methods for efficiently recovering typical multiple metals in wastewaters,not only enriching the conceptual metal recovery in BESs,and expanding the applicable metal recovery in BESs,but also deepening the further application of the cathodes of BESs with deposited metals.