Improved chromium reduction and removal from wastewater in continuous flow bioelectrochemical systems

Bioelectrochemical systems (BESs) including microbial electrolysis cells (MECs) and microbial fuel cells (MFCs) are promising for hexavalent chromium [Cr(VI)] reduction and total chromium (Cr) removal from wastewater. This study assessed the performance of simple, inexpensive, and continuous flow BESs with neither cathode catalyst nor proton exchange membrane for Cr(VI) reduction and total Cr removal. The effect of bioreactor configuration and wastewater feed mode on the performance of the BESs was investigated. Biological Cr(VI) reduction in the MEC followed a first-order kinetics with a rate constant of 0.103 d^-1, significantly higher than that of the control (0.033 d^-1). For comparison, the first-order reduction rate constants in the MFCs with the Cr(VI) fed to the anodic and the cathodic zones were 0.072 and 0.064 d^-1, respectively. The BESs improved total Cr removal through coprecipitating Cr(III) and phosphors as evidenced from the scanning electron microscopy energy-dispersive X-ray spectroscopy analysis. The total Cr removal efficiencies in the control, MFCs, and MEC were 26.1%, 56.7%, and 66.2%, respectively. Only 25.1% to 26.7% of total Cr was present intracellularly in the BESs (both MFCs and MEC), whereas 31.8% ± 1.4% and 38.0% ± 0.9% of total Cr in the anodic and cathodic zones of the control were present intracellularly. Overall, the BESs demonstrated a great potential to reduce Cr(VI) and remove total Cr with the MEC having the fastest Cr(VI) reduction and most efficient total Cr removal. Furthermore, the BESs significantly reduced the intracellular total Cr content.