Effects of flow pattern, Leersia hexandra, and circuit mode on the Cr(Ⅵ) removal capacity, electricity generation performance, and microbial community of constructed wetland-microbial fuel cells

Abstract

Cr(Ⅵ) can harm the environment and human health due to its high toxicity. Constructed wetland-microbial fuel cells (CW-MFCs) systems are cheap and effective tools for wastewater purification and electricity generation with broad application prospects. This study investigated the Cr(VI) removal performance, electricity generation performance, and microbial community of up-flow, up-down flow, and down-flow CW-MFCs with the hyperaccumulator Leersia hexandra and open and closed circuit modes. The power density of the down-flow CW-MFC with L. hexandra was higher (64.2 mW/m2) than that of the CW-MFC without plants (52.05 mW/m2) and CW-MFCs with other flow patterns under exposure to 40 mg/L Cr(Ⅵ). The down-flow CW-MFC with L. hexandra had a higher current response and charge transfer capability, which enhanced the abundance of electricigens (e.g., Acinetobacter) in the anode. The Cr(VI) wastewater treatment performance of the down-flow CW-MFC with L. hexandra was superior to that of CW-MFCs with other flow patterns. L. hexandra and the closed circuit mode can enhance the Cr(VI) wastewater treatment performance of the CW-MFC, which stems from the photosynthesis and enrichment capacity of L. hexandra and the positive effect of the closed circuit on the performance of L. hexandra. Cr(Ⅵ)-reducing bacteria (e.g., Acinetobacter, Simplicispira, Propionivibrio, Comamonas, and Rhizobium) were abundant in the cathode. The higher wastewater purification and electricity generation performance of the down-flow CW-MFC stems from the stronger charge transferability and current response associated with its unique flow pattern.