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

Effects of pH on simultaneous Cr(VI) and p-chlorophenol removal and electrochemical performance in Leersia hexandra constructed wetland-microbial fuel cell

Cr(VI) and p-chlorophenol (4-CP) are common pollutants in the aquatic environment but are difficult to degrade and have complex toxic effects. A downflow Leersia hexandra microbial fuel cell (DLCW-MFC) system was constructed to purify Cr(VI) and 4-CP polluted wastewater, as well as to investigate the effects of different pHs on Cr(VI) and 4-CP removal, electrochemical performance, physiological and biochemical responses, and Cr enrichment status of L. hexandra. The results showed that the DLCW-MFC had the highest Cr(VI) and 4-CP removal rates at pH 6.5, which were 99.0% and 78.6%, respectively. At the same time, 543 mV output voltage and 72.25 mW/m2 power density of the system were generated at pH 6.5, which were better than those at pH 7.4 and pH 5.8. The electrochemical performance result showed that pH 6.5 enhanced charge transfer ability and ion diffusion ability of the system. pH 6.5 also promoted growth and photosynthesis, and enhanced the Cr enrichment capacity (4.56 mg/10 plants) of L. hexandra. These results demonstrate that pH 6.5 was the optimum pH for the DLCW-MFC synchronous treatment of Cr(VI) and 4-CP as well as the generation of electricity. The DLCW-MFC designed in this study will provide a reference for purifying polluted wastewater and generating electricity.

The electron transport mechanism of downflow Leersia hexandra Swartz constructed wetland-microbial fuel cell when used to treat Cr(VI) and p-chlorophenol

Constructed wetland-microbial fuel cells are used to treat heavy metal and/or refractory organic wastewater. However, the electron transport mechanism of downflow Leersia hexandra constructed wetland-microbial fuel cells (DLCW-MFCs) is poorly understood when used to treat composite-polluted wastewater containing Cr(VI) and p-chlorophenol (4-CP) (C&P). In this study, metagenomics and in situ electrochemical techniques were used to investigate the electrochemical properties and the electricigens and their dominant gene functions. The DLCW-MFC was used to treat C&P and single-pollutant wastewater containing Cr(VI) (SC) and 4-CP (SP). The results showed that C&P had a higher current response and charge transfer capability and lower solution resistance plus charge transfer resistance. The anode bacteria solution of C&P contained more electron carriers (RF, FMN, FAD, CoQ10, and Cyt c). Metagenomic sequencing indicated that the total relative abundance of the microorganisms associated with electricity production (Desulfovibrio, Pseudomonas, Azospirillum, Nocardia, Microbacterium, Delftia, Geobacter, Acinetobacter, Bacillus, and Clostridium) was the highest in C&P (4.24%). However, Microbacterium was abundant in SP (0.12%), which exerted antagonistic effects on other electricigens. Among the 10 electricigens based on gene annotation, C&P had a higher overall relative abundance of the Unigene gene annotated to the KO pathway and CAZy level B compared with SC and SP, which were 1.31% and 0.582% respectively. Unigene153954 (ccmC), Unigene357497 (coxB), and Unigene1033667 (ubiG) were related to the electron carrier Cyt c, electron transfer, and CoQ biosynthesis, respectively. These were annotated to Desulfovibrio, Delftia, and Pseudomonas, respectively. Unigene161312 (AA1) used phenols and other substrates as electron donors and was annotated to Pseudomonas. Other functional carbohydrate enzyme genes (e.g., GT2, GT4, and GH31) used carbohydrates as donors and were annotated to other electricigens. This study provides a theoretical basis for electron transfer to promote the development of CW-MFCs.

The in-depth revelation of the mechanism by which a downflow Leersia hexandra Swartz constructed wetland-microbial fuel cell synchronously removes Cr(VI) and p-chlorophenol and generates electricity

The composite pollution by Cr(VI) and p-chlorophenol (4-CP) has high toxicity and harms water safety. However, research on the effective removal of Cr(VI) and 4-CP composite-polluted wastewater (C&P) and efficient synchronous electricity generation with reclaimed resources is limited. In this study, a downflow Leersia hexandra constructed wetland-microbial fuel cell (DLCW-MFC) was builded to treat C&P, as well as wastewater singularly polluted by Cr(VI) (SC) and 4-CP (SP), respectively, to reveal the mechanism by which DLCW-MFC treats C&P and synchronously generates electricity. The results demonstrate that the cathode layer had a stronger removal effect on pollutants than the middle layer and anode zone layer. Moreover, SC and SP had stronger pollutant removal effects than C&P. Cr(VI) had more competitive with electrons than 4-CP, and they had a synergistic effect on efficient electricity generation. The L.hexandra in SC and SP had a better growth state and lower Cr enrichment concentration than that in C&P. Cr existed in the DLCW-MFC mainly in the form of Cr(III). Gas chromatography-mass spectrometry was used to investigate the degradation pathway of 4-CP in C&P, and indicated that Phenol, 2,4-bis(1,1-dimethylethyl)- and benzoic acid compounds were the main intermediates formed at the cathode, and further mineralized to form medium-long-chain organic compounds to form CO₂. The microbial community distribution results revealed that Simplicispira, Cloacibacterium, and Rhizobium are associated with Cr(VI) removal and 4-CP degradation, and were found to be rich in the cathode of C&P. The anode of C&P was found to have more Acinetobacter (1.34%) and Spirochaeta (4.83%) than SC and SP, and the total relative abundance of electricigens at the anode of C&P (7.46%) was higher than that at the anodes of SC and SP. This study can provide a theoretical foundation for the DLCW-MFC to treat heavy metal and chlorophenol composite-polluted wastewater and synchronously generate electricity.