• Energy Research

AbstractWetlands are an important carbon sink for greenhouse gases (GHGs), and embedding microbial fuel cell (MFC) into constructed wetland (CW) has become a new technology to control methane (CH4) emission. Rhizosphere anode CW–MFC was constructed by selecting rhizome-type wetland plants with strong hypoxia tolerance, which could provide photosynthetic organics as alternative fuel. Compared with non-planted system, CH4 emission flux and power output from the planted CW–MFC increased by approximately 0.48 ± 0.02 mg/(m2·h) and 1.07 W/m3, respectively. The CH4 emission flux of the CW–MFC operated under open-circuit condition was approximately 0.46 ± 0.02 mg/(m2·h) higher than that under closed-circuit condition. The results indicated that plants contributed to the CH4 emission from the CW–MFC, especially under open-circuit mode conditions. The CH4 emission from the CW–MFC was proportional to external resistance, and it increased by 0.67 ± 0.01 mg/(m2·h) when the external resistance was adjusted from 100 to 1000 Ω. High throughput sequencing further showed that there was a competitive relationship between electrogenic bacteria and methanogens. The flora abundance of electrogenic bacteria was high, while methanogens mainly consisted of Methanothrix, Methanobacterium and Methanolinea. The form and content of element C were analysed from solid phase, liquid phase and gas phase. It was found that a large amount of carbon source (TC = 254.70 mg/L) was consumed mostly through microbial migration and conversion, and carbon storage and GHGs emission accounted for 60.38% and 35.80%, respectively. In conclusion, carbon transformation in the CW–MFC can be properly regulated via competition of microorganisms driven by environmental factors, which provides a new direction and idea for the control of CH4 emission from wetlands. Graphical Abstract

A novel ecological-microbial fuel cell (E-MFC) was constructed based on the mutualistic symbiosis relationship among wetland plants Ipomoea aquatic, benthic fauna Tubifex tubifex (T. tubifex) and microorganisms. The maximum power densities of sediment MFC (S-MFC), wetland plant MFC (WP-MFC) and E-MFC were 6.80 mW/m2, 10.60 mW/m2 and 15.59 mW/m2, respectively. Ipomoea aquatic roots secreted organic matter as electricigens' fuel for electricity generation, while T. tubifex decomposed decaying leaves and roots into soluble organic matter and plant nutrients, forming a co-dependent and mutually beneficial system, which was conducive to bioelectricity production. The E-MFC obtained the highest nitrogen removal, and the removal efficiencies of NH4+-N and NO3--N were 90.4% and 96.5%, respectively. Hydraulic retention time (HRT), cathodic aeration and T. tubifex abundance had significant effects on E-MFC power generation. The performeance boost of E-MFC was closely related to anodic microbial community change caused by the introduction of T. tubifex.

To harvest electricity and mitigate methane emissions from wetlands, a novel microbial fuel cell coupled constructed wetland (MFC-CW) was assembled with an anode placing in the rhizosphere and a cathode on the water surface. Plant-mediated methane accounted for 71-82% of the total methane fluxes. The bioanode served as an inexhaustible source of electron acceptors and resulted in reduced substantial methane emissions owing to electricigens outcompeting methanogens for carbon and electrons when substrate was deficient. However, when supplying sufficient organic carbon, both electricity and methane increased, indicating that electrogenesis and methanogenesis could co-exist in harmony. Direct methane emission (diffusion/ebullition) and plant-mediated methane emission were affected by operating conditions. Methanogenesis was significantly suppressed (∼98%) at HRT of 96h and with external resistance of 200Ω, accompanied with improved coulombic efficiency of 14.9% and current density of 187mA/m2. Contrarily, change of electrode polarity in the rhizosphere led to more methane efflux.

Microbial fuel cells (MFCs) can remove and recover metals in wastewater; however, there are relatively few studies of metal removal from soil by MFCs. In this study, we developed a three-chamber soil MFC consisting of an anode, contaminated soil, and cathode chamber to remove heavy metals from soil. The performance of the soil MFC was investigated by assessing the relationships among current, voltage, and Cu migration, and reduction. The developed soil MFC successfully reduced and removed Cu, and the Cu removal efficiency in the cathode surpassed 90% after only 7 days of operation. External resistance had a remarkable effect on the performance of the soil MFC which was depended on cathodic polarization. The pH in the cathode also depended on the external resistance. Lower external resistance were associated with lower pH values, higher Cu removal efficiencies, and greater amounts removed in the cathode. Based on sequential fractionation, the acid-extractable and reducible fractions were the main fractions that migrated within the three-chamber soil MFC. Enhancing the voltage output in the three-chamber soil MFC by increasing the external resistance promoted Cu migration, enriched Cu near the cathode, and facilitated Cu removal. Therefore, the developed three-chamber soil MFC not only supports heavy metal migration from soil towards the cathode, but can also realize reduction of heavy metals in the cathode by adjusting the current or voltage generated by the soil MFC.

To optimize the performance of a vertical subsurface flow constructed wetland-microbial fuel cell (CW-MFC), studies of bio-cathode materials and reactor configurations were carried out. Three commonly used bio-cathode materials including stainless steel mesh (SSM), carbon cloth (CC) and granular activated carbon (GAC) were compared and evaluated. GAC-SSM bio-cathode achieved the highest maximum power density of 55.05 mWm(-2), and it was most suitable for CW-MFCs application because of its large surface area and helpful capillary water absorption. Two types of CW-MFCs with roots were constructed, one was placed in the anode and the other was placed in the cathode. Both planted CW-MFCs obtained higher power output than non-planted CW-MFC. Periodic voltage fluctuations of planted CW-MFCs were caused by light/dark cycles, and the influent substrate concentration significantly affected the amplitude of oscillation. The coulombic efficiencies of CW-MFCs decreased greatly with the increase of the influent substrate concentration.

In the present study, a new technology that coupled constructed wetland (CW) with microbial fuel cell (MFC) (CW-MFC) was developed to convert solar energy into electricity on the principles of photosynthetic MFC by utilizing root exudates of Ipomoea aquatica as part of fuel. The maximum power density of 12.42 mW m−2 produced from the CW-MFC planted with Ipomoea aquatica was 142% higher than that of 5.13 mW m−2 obtained from the unplanted CW-MFC. The maximum power output for the planted CW-MFC could be divided into two parts: the maximum power yield from in the water body was 66.05 KJ Kg−1 , and the maximum power transformation from plant photosynthesis was 2.31 GJ ha−1 year−1. The average COD removal efficiencies were 92.1% and 94.8% in the unplanted CW-MFC and planted CW-MFC, respectively; the average TN removal efficiencies amounted to 54.4% and 90.8% in the unplanted CW-MFC and planted CW-MFC. This research demonstrates that planting Ipomoea aquatica in the CW-MFC achieved a higher power density and nutrient removal of nitrogen simultaneously.