Removal of binary Cr(VI) and Cd(II) from the catholyte of MFCs and determining their fate in EAB using fluorescence probes

Electrochemically active bacteria (EAB) on the cathodes of microbial fuel cells (MFCs) can remove metals from the catholyte, but the fate of metals in the cells has not been examined in the presence of multiple metals. To study the relative uptake and fate of Cr(VI) and Cd(II) in cells, fluorescence probes were used to determine the amount and location of these metals in four different EAB on the biocathodes of MFCs. When both metals were present, less Cr(VI) was removed but Cd(II) uptake was not appreciably affected. As a consequence, the imaging of Cr(III) ions was lower than that using individual fluorescence probes for single Cr(III) ions in each EAB, compared to negligible changes in images for Cd(II) ions in the presence of either both Cr(VI) and Cd(II) or Cd(II) alone. The concentration of Cr(III) ions in the cells consistently increased over time, while that of Cd(II) ions decreased following an initial increase. Cr or Cd uptake could not be detected using a scanning electron microscope coupled with an energy dispersive spectrometer, reflecting the high sensitivities of the fluorescence probes to these metals. More chromium was found in the cytoplasm while cadmium preferentially accumulated in the cell envelope. These results demonstrate that the fate of chromium and cadmium in EAB was different when both metals were present, compared to controls containing a single metal. These results provide direct and visible results on the fate of the metals in the EAB when these metals are co-present in the catholyte of MFCs.