Removal of Cr(vi) and p-chlorophenol and generation of electricity using constructed wetland-microbial fuel cells based on Leersia hexandra Swartz: p-chlorophenol concentration and hydraulic retention time effects

Abstract

Heavy metals and phenolic compounds existing in polluted wastewater are a threat to the environment and human safety. A downflow Leersia hexandra Swartz constructed wetland-microbial fuel cell (DLCW-MFC) was designed to treat polluted wastewater containing Cr(vi) and p-chlorophenol (4-CP). To determine the effect of 4-CP concentration and hydraulic retention time (HRT) on the performance of the DLCW-MFC system, the wastewater purification, electricity generation, electrochemical performance, and L. hexandra growth status were studied. Addition of 17.9 mg L−1 4-CP improved the power density (72.04 mW m−2) and the charge transfer capacity (exchange current, 4.72 × 10−3 A) of DLCW-MFC. The removal rates of Cr(vi) and 4-CP at a 4-CP concentration of 17.9 mg L−1 were 98.8% and 38.1%, respectively. The Cr content in L. hexandra was 17.66 mg/10 plants. However, a 4-CP concentration of 35.7 mg L−1 inhibited the removal of Cr(vi) and the growth of L. hexandra, and decreased the electricity generation (2.5 mW m−2) as well as exchange current (1.21 × 10−3 A) of DLCW-MFC. An increase in power density and removal of Cr(vi) and 4-CP, along with an enhanced transport coefficient of L. hexandra, was observed with HRT. At an optimal HRT of 6.5 d, the power density, coulomb efficiency, and exchange current of DLCW-MFC were 72.25 mW m−2, 2.38%, and 4.99 × 10−3 A, respectively. The removal rates of Cr(vi) and 4-CP were 99.0% and 78.6%, respectively. The Cr content and transport coefficient of L. hexandra were 4.56 mg/10 plants and 0.451, respectively. Thus, DLCW-MFC is a promising technology that can be used to detoxify polluted wastewater containing composite mixtures and synchronously generate electricity.