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.

Fig. 1. DLCW–MFC structure diagram (S1, S2 and S3 were sample taps, and the suffix E represents effluent tap).

Fig. 2. (a) COD, (b) 4-CP, (c) Cr(vi) and (d) TCr removal rates at different concentrations of 4-CP.

Fig. 3. SEM image of (a) raw graphite felt and (b) anode biofilm of the DLCW–MFC.

Fig. 4. (a) Output voltage, (b) cathode and anode potential, (c) power density curves and (d) CE at different concentrations of 4-CP.

Fig. 5. (a) Cr concentrations in roots, stems and leaves and TF and (b) Cr content in aboveground and underground parts of L. hexandra at different concentrations of 4-CP.

Fig. 6. (a) COD, (b) 4-CP, (c) Cr(vi) and (d) TCr removal rates at different HRT.

Fig. 7. (a) Output voltage, (b) cathode and anode potential, (c) power density curves and (d) CE at different HRT.

Fig. 8. (a) Cr concentrations in roots, stems and leaves and TF and (b) Cr content in aboveground and underground parts of L. hexandra at different HRT.