The geochemical cycle of heavy metal ion driven by microbes is widespread in nature. Previous studies are focused on the removal efficiency in the treatment of Cd in the bioelectrochemical systems; however, little is reported regarding the reduction mechanism of Cd on the electrode surface in the neutral physiological environment. In this work, we investigated the microbiological influence of Shewanella oneidensis MR-1 wild type and its mutant △omcA-△mtrc for Cd electrodeposition on a glassy carbon electrode (GCE) surface by using cyclic voltammetric (CV) and chronoamperometric methods. The CVs and I-t curves were carried out in a three-electrode system in the present of MR-1 cells (the value of optical density at 600 nm was 0.5) under nitrogen atmosphere. Much results were found in the present of MR-1 wild type:(1) the reducing peak potentials for Cd electrodeposition obviously negative shifted from CVs; (2) when the scan rate was comparatively slow (20 mV·s^-1 vs. saturated calomel electrode), the Cd electrodeposition process contained two steps in the second scan in CVs which were Cd(Ⅱ)-Cd(I)-Cd; (3) the average diffusion coefficient of Cd(Ⅱ) from bulk solution to GCE surface (0.93×10^-6 cm·s^-1), calculated from I-t curves, was slightly slower than that without MR-1 wild type (1.1×10^-6 cm·s^-1); (4) the progressive nucleation mechanism for Cd electrodeposition changed into an instantaneous three-dimensional nucleation by compared with their actual nucleation curves. Once the Cd electrodeposition process was performed in the solution with △omcA-△mtrc mutant, the diffusion of Cd(Ⅱ) from bulk solution to GCE surface (the average diffusion coefficient was 0.84×10^-6 cm·s^-1) changed much slower than before; nonetheless, the Cd electrodeposition was also consistent with the instantaneous three-dimensional nucleation. On the other hand, inhomogeneous Cd particles were observed on GCE surfaces at different stepping potentials from scanning electron microscopy (SEM) images. In contrast, the homogeneous Cd particles were found in the present of MR-1 wild type and △omcA-△mtrc mutant when the reduction potentials were higher than -0.9 V. These SEM results regarding the surface morphology of electrodeposited Cd particles also well agreed with three-dimensional nucleation mechanisms.

Key words: Shewanella oneidensis MR-1, electrodeposition, Cd, nucleation mechanism