Removal Of Hexavalent Chromium From Microbial Fuel Cells Based On Three-dimensional Activated Carbon Cathode

Microbial fuel cell（MFC）is a technology that combines wastewater treatment and energy recovery.The electrochemically active bacteria at anode can decompose the organic matter in the wastewater and transfer the generated electrons to the cathode through an external circuit.The organic matters can be oxidized at the anode,and the oxidizing pollutants can also be removed by accepting electrons at the cathode in MFC.Cr（Ⅵ）can be reduced to Cr（III）with low toxicity at the cathode.Then the generated Cr（III）was precipitated with OH^-to form Cr（OH）₃.The Cr（Ⅵ）will be removed in this way.In addition,the theoretical reduction potential of Cr（Ⅵ）was 1.33V,which was higher than the reduction potential of O₂（1.23V）which commonly was used as the electron acceptor at cathode in MFC.If Cr（Ⅵ）can be rapidly reduced at the cathode of MFC,the Cr（Ⅵ）will be removed and the electricity will be generated simultaneously.At present,the removal performance Cr（Ⅵ）in MFC was poor,and the power output of MFC was low,which hinders the application of MFC in the treatment of actual wastewater containing Cr（Ⅵ）.The main work in this paper was as following:（1）the removal performance of Cr（Ⅵ）was improved by constructing a three-dimensional cathode MFC（3D-MFC）.The 3D-MFC was added an activated carbon layer on the side of the cathode catalytic layer based on normal MFC reactor（2D-MFC）;（2）the Cr（Ⅵ）concentration in anode was restricted by using appropriate ion exchange membrane;（3）the Cr（Ⅵ）removal efficiency was further improved by modifying the activated carbon with trace N and Fe atoms.Firstly,the Cr（Ⅵ）removal efficiency and power output of 3D-MFC was compared with those of 2D-MFC.The results showed that the removal efficiency of Cr（Ⅵ）by3D-MFC reached 93.75%,while the removal rate of Cr（Ⅵ）by 2D-MFC was only56.25%.In terms of electricity generation,the cell voltage of 3D-MFC（external resistance is 1000Ω）was up to 468 m V,which was about 40m V higher than that of2D-MFC.The maximum power density of 3D-MFC was 900±10m W·m^-2,which was55.6%higher than inactive carbon 2D-MFC(400±10m W·m^-2).The polarization curve showed that the high cathode potential played a key role for increasing power output.These results indicated that 3D-MFC can improve the Cr（Ⅵ）removal performance and electricity generation.When used an ion-exchange membrane CMI-7000,Cr（Ⅵ）easily diffused from the cathode to the anode,which resulted in the inactivation of the electrochemically active bacteria.The ion-exchange membrane N117 was tried to limited the amount of Cr（Ⅵ）from the cathode chamber to the anode chamber.The results showed that when the MFC uses the ion exchange membrane N117,the maximum concentration of Cr（Ⅵ）in the anode chamber solution was less than 0.6 mg/L.When the ion exchange membrane CMI-7000 was used,the Cr（Ⅵ）in the anode chamber solution was as high as 8mg/L,which is 12 times higher than the value when using membrane N117.This shows that the membrane N117 can effectively prevent the penetration of Cr（Ⅵ）from cathode to the anode.However,under this system,the removal effect of Cr（Ⅵ）by 3D-MFC was relatively low,which was about 50%.In order to further improve the removal efficiency of Cr（Ⅵ）by 3D-MFC,we doped a small amount of N and Fe into the activated carbon and carbon black poweder by thermal treatment.The normal activated carbon was replaced by N/Fe doped activated carbon to form a new 3D-MFC.The results showed that the removal efficiency of Cr（Ⅵ）by the new 3D-MFC was as high as 90%.In addition,the removal of Cr（Ⅵ）by this 3D-NFe-MFC is affected by p H and external resistance factors.The removal rate of Cr（Ⅵ）under neutral conditions（external resistance is 1000Ω）reached95%.Microscopic characterization results show that the doped activated carbon is composed of nano-sized spheres and has rich pores,which may be the necessary conditions for the adsorption and diffusion of Cr（Ⅵ）.STEM results showed that N and Fe were successfully incorporated into activated carbon,which may be the key reason for the faster reduction rate of Cr（Ⅵ）.