Removal of Cr(VI) and electricity production by constructed wetland combined with microbial fuel cell (CW-MFC): Influence of filler media

Due to the high mobility and toxicity of Cr(VI), it is of significance to develop a low-cost and effective method to treat Cr(VI) contained wastewater. Currently, constructed wetland combined with microbial fuel cell (CW-MFC) technique has exhibited the possibilities in Cr(VI) removal. However, the influence of filler media was still unclear. In this study, the up-flow CW-MFC systems with different fillers were constructed to investigate Cr(VI) removal and electricity generation, including bio-ceramic (CW-MFC1), zeolite (CW-MFC2), calcite (CW-MFC3), and volcanic rock (CW-MFC4). Cr(VI) absorption ability of fillers was negligible during the long operation period. Over 93% COD removal was observed in all systems, and Cr(VI) removal rate was as follow: CW-MFC4 (99.0%) > CW-MFC2 (95.5%) > CW-MFC3 (89.7%) > CW-MFC1 (72.2%). Most of Cr(VI) ions were converted to Cr(III) precipitation and immobilized in the anode layer, followed by the bottom layer. There was a competitive relationship between the bottom microbes and anode layer microbes. Bio-ceramic is easy to realize immobilization of microorganisms, which as filler (CW-MFC1) showed the lowest removal rate of Cr(VI) in CW-MFC1 system, due to superabundant organic matter consumed by microorganisms of filler layer weakening the action of anodic microbes. Besides, calcite with lower specific surface area and less porous structure than volcanic rock and zeolite was unbeneficial to microbial living, leading to bottom layer lower Cr(VI) removal in CW-MFC3. Both output voltage and maximum power density of CW-MFC system were as the following order: CW-MFC3 > CW-MFC 4 > CW-MFC2 > CW-MFC1. This was mainly because higher organic matter were consumed (higher COD removal rate) in anode layers, more electricity was generated by exoelectrogenic microbes. Therefore, volcanic rock as fillers was the best choice, which could maximize Cr(VI) removal (99.0%) and obtain the preferable electricity generation (0.595 ± 0.019 V of output voltage, 0.462 W/m3 of power density). In addition, microbial communities analysis indicated that microbial richness of cathode was higher than anode due to the long Cr(VI) stress, and Acetoanaerobium and Exiguobacterium were the dominant genera in anode and cathode, respectively.