Inhibitory effect of cadmium(II) ion on anodic electrochemically active biofilms performance in bioelectrochemical systems

Cadmium(II) ion can affect the anode performance of bioelectrochemical systems (BES); however, how the presence of Cd2+ affect the extracellular electron transfer of anodic electrochemically active biofilms (EABs), the microbial viability and species composition of microorganism on the anode remain poorly understood. Here, we investigated the inhibitory effect of Cd2+ at different concentrations on the electrochemical performance and the biofilm community in mixed-culture enriched BES. The electrochemical performance of the BES was not inhibited at 2 mg L−1 Cd2+, while higher concentrations of 5-20 mg L−1 resulted in the decrease in the maximum power density, with 0.34 ± 0.01 W m−2 at 5 mg L−1, 0.28 ± 0.01 W m−2 at 10 mg L−1, and 0.17 ± 0 W m−2 at 20 mg L−1, respectively. When adding 30 mg/L Cd2+, there was almost no power output. The decline of the power output was possibly ascribed to the suppressed viability and the change of species richness as evident from confocal laser scanning microscopy and microbial community analysis. Cyclic voltammogram and electrochemical impedance spectroscopy revealed that high concentration of Cd2+ exceeding 5 mg L−1 can inhibit the secretion of outer membrane cytochromes, thus reducing the electron transfer between the EABs and the anode surface. Analysis of bacterial structures showed a decrease in Geobacter accompanied by an increase in Stenotrophomonas and Azospira in response to Cd2+ at 10 and 20 mg L−1. This study added to the in-depth analysis of the inhibition of Cd2+ on EABs, and provided new insights into the removing Cd2+ and organics simultaneously in BES.