Performance Of Microbial Fuel Cells For Hexavalent Chromium Reduction In Wastewater And Chromium Hemitrioxide Recovery

The development of modern industries has resulted in more and more chromium-containing wastewater,especially generated by the electroplating industry,and the overexploitation of mineral resources has also resulted in the scarcity of chromium resources,which have made the disposal of chromium-containing wastewater and the recovery of chromium resources an urgent problem to be solved.Microbial fuel cell treatment methods for chromium-containing wastewater have been widely studied in recent years.In this paper,we aim to obtain efficient Cr???reduction and recovery and stable production of electricity,and build a Cr???reduction system of microbial fuel cell combined biocathode and chemical cathode and a chromium recovery system of the five-chamber microbial fuel cell to reduce Cr???and recover chromium hemitrioxide from wastewater.And the mechanism of the Cr???reduction system of microbial fuel cell combined biocathode and chemical cathode and the precipitation transformation mechanism of the chromium recovery system of the five-chamber microbial fuel cell were systematically studied.The results indicate that the Cr???reduction system of microbial fuel cell combined biocathode and chemical cathode can complete Cr???reduction and electricity generation with wastewater containing Cr???.The system can reduce 201 mg/L Cr???to 12 mg/L,reduction degree 94.03%,under the optimal conditions that the Cr???concentration in the chromium-containing wastewater is 200 mg/L,pH=2,and the COD of anode is 2000 mg/L from nutrient solution easily degradated by the electrotrophic bacteria.The system potential is above 0.83 V,the internal resistance is 144.53?,and the maximum output power density is 5.245 W/m³.After operation,trace amounts of light green crystalline reduction products are obtained,the main constituent elements of which are chromium and oxygen.The chromium recovery system of the five-chamber microbial fuel cell can reduce Cr???,recover chromium hemitrioxide and obtain electricity.The system can reduce 208mg/L Cr???to the minimum within 114 h,and collect 190 mg/L Cr????including ion form and precipitation form?in the precipitation area under adjusted conditions that the Cr???concentration in the chromium-containing wastewater is 200 mg/L,pH=1,and the COD of anode is 2000 mg/L from nutrient solution easily degradated by the electrotrophic bacteria.91.35%Cr???has been reduced and collected in the precipitation area.The potential of the system is always maintained above 0.69 V,the internal resistance is 53.17?,and the maximum output power density is 7.91 W/m³.After operation,the gray-green precipitation is collected in the precipitation area,and the components of which are chromium hemitrioxide and a small amount of chromic anhydride.The study on the mechanism of the Cr???reduction system of microbial fuel cell combined biocathode and chemical cathode shows that when the concentration of Cr???at the cathode decreases,the biocathode avoids the accumulation of electrons at the anode,thus stabilizing the activity of anode electrogenic bacteria and system potential.The amount of electrons provided by the stable anode far exceeds the required amount of Cr???reduction at the chemical cathode,which promotes Cr???reduction to some extent.The combination of biocathode and chemical cathode promotes the stability of the amount and activity of anodic electrogenic bacteria,and increases the system potential and Cr???reduction rate to some extent.The study on the precipitation transformation mechanism of the chromium recovery system of the five-chamber microbial fuel cell shows that Cr³⁺in the precipitation zone is gradually converted to Cr?OH?²⁺,Cr₂?OH?₂⁴⁺and Cr₃?OH?₄⁵⁺after Cr???reduced to minimum.When pH>4.3,these three kinds of ions are rapidly converted to the precipitation Cr₂O₃.The amount of precipitation Cr₂O₃ accumulated to the maximum until these three ions converted completely.The chromium recovery system of the five-chamber microbial fuel cell converts Cr???from wastewater into precipitation Cr₂O₃ in the precipitation area,with a maximum conversion of 89.14%.