Wetland plants selection and electrode optimization for constructed wetland-microbial fuel cell treatment of Cr(VI)-containing wastewater

Abstract

This study evaluated the effects of electrode arrangement and aquatic plant selection on the performance of single-chamber constructed wetland-microbial fuel cell (CW-MFC) in the treatment of wastewater containing Cr(VI). The CW-MFC with the electrode distance of 10 cm obtained the highest Cr(VI) removal efficiency of 98.8 % at 9 h and the largest power density of 37.8 mW/m2. Short electrode distance would destroy anodic anoxic atmosphere by oxygen secretion of plant roots, while large electrode distance would increase the internal resistance of the system. In the three sets of CW-MFCs with different cathode areas of 25 cm2, 75 cm2 and 100 cm2, the system with 75 cm2 cathode area achieved the highest maximum power density of 39.9 mW/m2 and corresponding Cr(VI) removal efficiency of 90.9 %. Cr(VI) removal by the CW-MFC conformed to first-order kinetics, and the main Cr(VI) removal mechanisms involved cathodic reduction of Cr(VI) to Cr(III), Cr(III) deposition and plant uptake. High-throughput analysis indicates that Lactococcus and Streptococcus were key electroactive bacteria for bioelectricity generation, while Rhodobacter and Hydrogenophaga were responsible for cathodic Cr(VI) reduction. To select optimal wetland plant with the characteristics of both high Cr enrichment capacity and high bioenergy output performance, six CW-MFCs with different plant species were established. Scirpus Validus CW-MFC exhibited better comprehensive performance with maximum power density of 40.6 mW/m2 and Cr(VI) removal rate constant of 0.346 h−1. For ensuring high electricity yield and efficient Cr(VI) removal, Scirpus validus can be selected as the optimal aquatic plant of the CW-MFC.