Harnessing nature for cleaner water and energy: A review of electrode innovations in constructed wetland-microbial fuel cells

In an era demanding sustainable solutions for water and energy scarcity, constructed wetland-microbial fuel cell (CW-MFC) systems present a compelling integrated technology. These systems combine the natural purification capabilities of wetlands with the bioelectrochemical energy generation of microbial fuel cells, offering a dual benefit of wastewater treatment and bioelectricity production. A recent comprehensive review, published in Carbon Research, synthesizes the advancements in electrode strategies crucial for maximizing the performance of CW-MFCs, providing a vital roadmap for future development and broader application.

Advancing Electrode Design for Enhanced Performance

The research explores the pivotal role of electrodes in CW-MFC operation, noting that nearly 60% of literature on these systems focuses on electrode strategies. The review systematically categorizes electrodes based on their material and quantity, distinguishing between unified, composited, modified, and multi-electrode models, incorporating both non-conductive and conductive particles. This detailed classification highlights how strategic electrode selection significantly influences electricity generation, pollutant removal efficiency, and the crucial control of greenhouse gas emissions within these bioelectrochemical systems.

The investigation reveals that carbon-based electrode materials, such as carbon felt, generally exhibit superior properties compared to metal-based counterparts, primarily due to their excellent specific surface area, chemical stability, and biocompatibility for microbial attachment. Further enhancements arise from modifying these carbon-based electronic receivers with novel catalysts like titanium dioxide (TiO₂)/graphene or nano zero valent iron (nZVI). These modifications augment the oxygen reduction reaction (ORR), conductivity, and surface area, leading to improved removal of refractory pollutants and increased power output.

Innovations in Conductive Particles and Multi-Electrode Systems

The inclusion of various conductive particles around electronic receivers marks another significant area of progress. Materials like activated carbon (AC), graphite granule, biochar (BC), manganese ore (MO), and iron compounds (e.g., zero-valent iron, pyrite) have been shown to drastically boost both electricity generation and pollutant removal. Biochar, manganese, and iron minerals are particularly promising due to their low cost, reusability, high electron transport capacity, and ability to foster the growth of functional bacteria, offering more sustainable and economical alternatives compared to conventional AC or graphite granules. These materials facilitate enhanced microbial activity, electron transfer, and adsorption mechanisms for a wide range of contaminants, including heavy metals, antibiotics, and polyfluoroalkyl substances (PFASs).

For scaling up CW-MFC applications, multi-electrode configurations, encompassing multi-anode and multi-cathode systems, are extensively reviewed. These designs, often incorporating three-dimensional structures like ring or U-shaped electrodes, prove effective in increasing contact surface areas, optimizing electron distribution, reducing ohmic resistance, and fostering anoxic zones crucial for processes like nitrification-denitrification. Such strategies demonstrate significant improvements in the removal of contaminants like chemical oxygen demand (COD), nitrogen, and phosphorus, while simultaneously boosting bioelectricity harvest.

Navigating Challenges and Future Pathways

Despite the promising advancements, the review acknowledges several persistent challenges that demand future attention. Issues such as the inherently low power output of CW-MFCs, the complexity of biofilm formation and electron transport pathways on electrode surfaces, the influence of plant root exudates on microbial communities, and the competition for electrons between pollutants and electrodes remain areas for deeper investigation. Operational problems like clogging, stemming from organic/inorganic particle retention and root system growth, also require innovative mitigation strategies, such as the use of air-photocathodes.

Future research directions emphasize exploring super-capacitive electrodes and hybrid CW-MFC systems coupled with external energy harvesting technologies to overcome power limitations. A more profound understanding of the specific interaction mechanisms among macrophytes, substrates, and microbes is essential. Moreover, investigations into the recycling and reuse of spent electrodes, the fate of nutrients and heavy metals in treated wastewater, and the biochemical removal pathways of emerging contaminants, including microplastics, are critical. Extending CW-MFC application zones to colder climates by developing hardy plants and microorganisms also presents a significant opportunity.

Dr. Nan Lu, a corresponding author from Northeast Normal University, states, "Our comprehensive review consolidates the diverse electrode strategies that define the performance of CW-MFC systems. We firmly believe that a deeper, more integrated understanding of these strategies, coupled with innovative material science, will be instrumental in translating these promising lab-scale systems into robust, large-scale solutions for environmental remediation and sustainable energy generation."

Corresponding Author: Hongfeng Bian or Nan Lu

Original Source: https://doi.org/10.1007/s44246-023-00092-y

 Contributions: All authors contributed to the study conception and design. Writing-original draft, conceptualization and data curation were performed by Rongdi An. Writing-review & editing and methodology were performed by Jiunian Guan and Nan Lu. Visualization and data curation were performed by Gaoxiang Li and Zhuoyu Li. Funding acquisition and project administration were performed by Jiunian Guan, Hongfeng Bian and Lianxi Sheng. All authors read and approved the final manuscript.