ReviewBidirectional extracellular electron transfers of electrode-biofilm: Mechanism and application
Introduction
Microbial electrochemical technology (MET) employs electrochemical active bacteria (EAB) as the catalysts for at least one of the two electrodes (anode and cathode) in an electrochemical cell to conduct the oxidation and reduction reactions (Zou and He, 2018). Microbial fuel cell (MFC) is the original and fundamental case of MET, which shows the promising of directly electric energy recovery from the degradation of pollutants in wastewater. Within the past decade, the steady-state volumetric power density of miniaturized MFCs was reported to over 10,000 W/m3 (Ren et al., 2016). Meanwhile, the scale of MFC reactors have also advanced from the bench scale with couples to dozens of milliliters, to the pilot scale with a maximum volume of 250 L in an individual reactor (Feng et al., 2014), and a total volume of 1000 L in a modularized system (Liang et al., 2018). However, the practical application of MFCs is largely restricted by the inevitable diminished power in large scales when feeding with actual wastewater with a poor conductivity and a limited buffer ability, which makes it generally insufficient to justify the high capital cost (Jiang et al., 2018b, Jiang and Zeng, 2018, Logan et al., 2015, Wu et al., 2016). With the integration of microbiology, electrochemistry, materials science, and many other research areas of relevance together, the MET recently has advanced as such a flexible platform that dozens of functions have been invented and explored (Champigneux et al., 2018, Choudhury et al., 2017, Wang and Ren, 2013, Yu et al., 2018b). Among these, the emerging MET without the requirement for large numbers of electrode assemblies or high current/power density outputs, such as electro-fermentation, electric syntrophy, electro-stimulation, microbial photo-electrosynthesis, and environmental biosensing, is recently highlighted by some outstanding reviews (Jiang et al., 2018c, Kracke et al., 2018, Sasaki et al., 2018). In addition, the synergy between MET and conventional wastewater treatment processes (e.g., anaerobic digestion (AD), membrane bioreactor (MBR), constructed wetland (CW), capacitive deionization (CDI), and Fenton process) has shown the promising of practical application in large scale, rather than using the MET as a standalone technology (Li and Yu, 2016, Tee et al., 2016, Xu et al., 2015).
Despite the variation in constructions and purposes, the basis of MET is the unique electrical communication, i.e., extracellular electron transfer (EET) process, between the EAB and electrodes. The EET process could be divided into the outward EET (from the EAB to electrode) and the inward EET (from the electrode to EAB), according to the direction. The outward EET is generally conducted at the bioanode, where substrate like carbohydrate is served as the electron donor while the electrode is served as the electron acceptor, and it could be adopted for the production of electricity, bioremediation, and wastewater treatment (Choudhury et al., 2017, Pandey et al., 2016). In contrast, the inward EET is generally conducted at the biocathode, where the electrode is served as the solid electron donor while different types of final electron donors could be used for various applications, including the production of valuable products, wastewater treatment, and heavy metal or nutrient recovery (Kracke et al., 2015, Lovley and Nevin, 2011, Rosenbaum et al., 2011, Tremblay et al., 2017, Yu et al., 2018b).
Recently, the using of a single biofilm-electrode to comparatively understand the mechanism of the bidirectional EET process, and explore its various function in water-energy nexus has received a growing research interest. Biofilm-electrode with the bidirectional EET process shares unique community structure, operational mode, and multiple functions, which all distinguish it from conventional cases where only the unidirectional EET is considered (Kumar et al., 2017). There are some excellent reviews focused on the EAB in the anode (Kumar et al., 2016, Logan, 2009), the electroactive autotrophic in the cathode (Rosenbaum et al., 2011, Tremblay and Zhang, 2015), the using of electrode modification to boost the outward EET (Sonawane et al., 2017, Yazdi et al., 2016), the mechanism of EET conducted separately by bioanode and biocathode (Choi and Sang, 2016, Kumar et al., 2017, Nealson and Rowe, 2016), and also the assessment of electron donor (Pandey et al., 2016), and acceptors (Ucar et al., 2017) in MET. The objectives of this review are, for the first time, to provide an overview of current status of research in biofilm-electrode with the bidirectional EET process, and to analyze the challenges and perspectives of this biotechnology. Being more specific and focused, this review is mainly limited to investigate and elucidate: (1) the EAB in pure culture reported with the capability of bidirectional EET; (2) the problems and applications of bidirectional EET using electrode-biofilm without polarity reversal; (3) the applications of bidirectional EET with polarity reversal for pollutant removal, biotemplated material development, and bioelectronic device design; (4) the challenges faced by the practical application of bidirectional EET in future research endeavors and also the promising technical solutions.
Section snippets
EET process and involved EABs
Although some debates in this research field still exist, the mechanisms of the outward EET are well studied, whereas, while the intricacies of inward EET has not been fully elucidated yet (Nealson and Rowe, 2016). In this section, the outward EET and inward EET are briefly introduced, and then those EABs capable of bidirectional EET and their various applications are summarized.
Bidirectional EET without polarity reversal
There are only few microorganisms tested in pure culture with the capability of bidirectional EET, however, the mixed culture based bidirectional EET offers great prospects for pollutant removal, biotemplated material development, and bioelectronic device design. Actually, the problems and applications of bidirectional EET are directly associated with where polarity reversal is applied to a MET based device. In this section, the bidirectional EET without polarity reversal will be firstly
Bidirectional EET with polarity reversal once only
The operational mode of polarity reversal once only in microbial electrolysis cell (MEC) reactors is generally achieved by first enriching a bioanode in anodic condition with an outward EET process, followed by direct inversion of the bioanode to function as a biocathode, providing a cathodic condition with an inward EET process. The polarity reversal can be achieved by switching the setting of electrode potentials and/or the feeding of substrates. The main goals of polarity reversal once only
Periodic polarity reversal (PPR)
In the operational mode of periodic polarity reversal (PPR), the roles of bioanode and biocathode is switched periodically due to the capability of bidirectional EET of a same piece of biofilm-electrode. Typically, electrode reactions with products such as easily separated gases (hydrogen, nitrogen, and methane) and water, are suitable for the operational mode of periodic polarity reversal. With such periodic polarity reversal (Fig. 5), the pH degradant across the ion exchange membrane can be
Challenges and future prospects
The characters of bidirectional EET of a same biofilm-electrode provide a unique opportunity to elucidate various routes of EET pathways. The bidirectional EET of a single electrode-biofilm also holds great prospects for pollutant removal, biotemplated electrode material development, and bioelectronic design. However, some biotechnological barriers are yet to be solved for a widely utilization of such a unique bidirectional EET capability in the advanced MET platform.
There is a need to identify
Conclusions
This review mainly emphasizes on the fundamentals and applications of bidirectional EET of a single electrode-biofilm. The characters of bidirectional EET of a same biofilm-electrode provide a unique opportunity to elucidate the EET mechanism, despite that the microorganisms tested in pure culture with bidirectional EET are so limited. The bidirectional EET carries great prospects for biocathode enrichment, pollutant complete mineralization, biotemplated material development, pH stabilization,
Acknowledgments
This work was supported by the National Science Foundation of China (51478447, 51878175) and Central Guidance on the Development of Local Science and Technology (Fujian, 2017L3003).
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