Quantitative Evaluation Of The Effects Of Different Cathode Materials On Cd(Ⅱ)-reduced Microbial Electrolysis Cells

The heavy metal bivalent cadmium(Cd(Ⅱ)) widely discovered in the wastewater emanating from electroplating and rechargeable battery processes,has been recognized to be toxic and carcinogenic to living organisms for many decades.Compared to conventional techniques such as chemical precipitation,ion exchange or adsorption for Cd(Ⅱ) removal,microbial electrolysis cells(MECs)have been proved to be a more environmentally friendly and cost-effective technology for simultaneously consuming organic substrates and recovering Cd(Ⅱ) from wastewater.The selection of electrode materials has an important effect on MECs system.However,the commonly used methods for electrode evaluation(such as linear sweep voltammetry,electrochemical impedance spectroscopy,etc.)often have the disadvantage of a great error in the test of biological electrode or unable to obtain the proportion of electrode resistance in the system.To overcome the limitations of these methods,this study attempts to put forward a new type of electrode evaluation method called electrode potential slope method in MECs system.By using this method,quantitative evaluation of effects of different cathode materials on performance in Cd(Ⅱ)-reduced MECs are explored.It is mainly divided into the following two aspects:(1)By making full use of the linearized part of the polarization curve of the whole cell and individual electrodes,the resistance is obtained by the slope and the experimental open circuit potential is calculated by the y-intercept,the electrode potential slope method is derived.The accuracy of the electrode potential slope method was verified by testing in acetate-fed tube MECs.The results showed that the error between the internal resistance and the sum of each component resistance in the MECs is within 4%,and the difference between subtraction of cathode and anode experimental open circuit potential and whole cell experimental open circuit potential is less than 5%.The electrode potential slope method provides a new idea for the evaluation of electrode performance.By directly quantifying the resistance and experimental open circuit potential of each component(anode,cathode or solution)and whole-cell of different MECs,the electrochemical performance of the electrode itself is evaluated,and the proportional influence of the electrode on the overall system is obtained.(2)Three materials including stainless steel woven mesh(SSM),nickel foam(NF)and carbon cloth(CC)were conducted as cathode in Cd(Ⅱ)-reduced microbial electrolysis cells(MECs),respectively.By using electrode potential slope method,the experimental open circuit potentials of three cathodes were similar,while the SSM cathode showed the smallest resistance(6±1 mΩm~2),following by NF cathode(18±2 mΩm~2)and CC cathode(32±5mΩm~2).These values were analyzed to predict higher current density and more positive cathode potential in the MEC with SSM cathode under subsequent operating conditions(i>1.34 A/m~2).Electrochemical performance was more likely to be limited by current density than cathode potential.This is consistent with the better performance of the MEC with SSM cathode in terms of the performance index of Cd(Ⅱ) reduction,cathode coulomb efficiency,yields of Cd,system efficiency and energy recovery compared with NF cathode and CC cathode.