| Photo-assisted microbial electrolysis cells（MECs）shows promising for simultaneous production of gaseous clean energy（e.g.,H2）with recovery of value-added products（e.g.,heavy metals）from wastewater.ZnFe2O4/g-C3N4is one type-II heterojunction semiconductor photo-catalysis with merits of efficiency,sustainability and low-cost,and has been explored for hydrogen production or degradation of recalcitrant organics such as Rhodamine in conventional photo-catalysis processes.However,this ZnFe2O4/g-C3N4in photo-assisted MECs has been scarcely reported.Herein,this ZnFe2O4/g-C3N4 photocathode was purposely incorporated in single-chamber MECs for hydrogen production.This ZnFe2O4/g-C3N4photocathode single-chamber MECs was then explored for treatment of etching terminal wastewater（ETW）,one typical effluent of electroplating industrial wastewaters treated by flocculation,ion exchange and other processes,and still containing a certain amount of heavy metals such as Ni（II）,and a variety of recalcitrant organics.Main results are as the following:（1）ZnFe2O4 and g-C3N4 formed the type-II heterojunction,and thus effectively inhibited the recombination of photogenerated electrons and holes,achieving a photocurrent of 6.5μA,4.3-fold of the single g-C3N4.Circuital current of 11.6±0.5 A/m2 and hydrogen production of1.70±0.03 m3/m3/d in this single-chamber ZnFe2O4/g-C3N4photo-cathode MEC were 1.3-fold and1.5-time in the controls in the absence of light.The energy recovery rate based on electrical energy input was 233±5%whereas the total energy recovery rate based on electrical energy and substrate and the solar-to-hydrogen conversion efficiency were 73±3%and 4.01±0.01%,respectively.After a total of 12 batch cycles operation,microbial communities on the anode and on the cathode of this single-chamber ZnFe2O4/g-C3N4photo-cathode MEC,as well as in the controls in the absence of either light irradiation or circuital current were collected and analyzed.Light irradiation decreased the diversity of bacterial community on the cathode with an appreciable high abundance of genus Rhodopseudomonas（6.9-fold）of the no light control,and negligibly affected that on the anode.However,the presence of circuital current increased the diversities of bacterial communities on both electrodes.Kyoto Encyclopedia of Genes and Genomes（KEGG）analysis showed light irradiation increased the levels of light related energy metabolism and the hydrogen-related enzymes and cytochromes.Economic analysis proved the appreciable advantageous ZnFe2O4/g-C3N4（102$/m2）over the reported Mo S2/Cu2O（1074$/m2）,among which the total cost of ZnFe2O4/g-C3N4（102$/m2）included 23$/m2（ZnFe2O4）,4$/m2（g-C3N4）,62$/m2（graphite felt）,and 13$/m2（dispersant ethanol）.（2）ETW was efficiently treated in this single-chamber ZnFe2O4/g-C3N4 photocathode MEC,achieving Ni（II）removal of 99.4±0.4%with simultaneous recalcitrant organics mineralization of 70.4±1.2%and hydrogen production of 0.43±0.02 m3/m3/d with the effluent meeting China wastewater discharge standard（GB-39731-2020）,1.3-fold（Ni（II）removal）,1.6-time（recalcitrant organics mineralization）and 1.8-fold（hydrogen production）of the controls in the absence of light irradiation.Oxidative active species（·OH,3EPS*and holes）variably contributed to recalcitrant organics mineralization whereas heavy metals recovery was dominantly ascribed to circuital current.Radical capture experiments confirmed the dominant contribution of the hole（h+）（57.5±1.8%）over the excited triplet state extracellular polymeric substances（3EPS*）（31.0±0.6%）and hydroxyl radical（·OH）（10.1±0.7%）for the recalcitrant organics mineralization.From an energy point of view and based on per cubic meter ETW,this single-chamber ZnFe2O4/g-C3N4photo-cathode MEC consumed energy of 0.18 k Wh/m3,compared to the 0.22 k Wh/m3 in the controls in the absence of light.The study provides an sustainable,low-cost and energy-saving approach for simultaneous efficient mineralization of recalcitrant organics and complete recovery of Ni（II）from the ETW. |
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