definition of electro-bioremediation excludes the electrokinetic (EK) techniques (Reddy and Cameselle, 2009; Barba et al., 2018; Kaushal et al., 2020). EK is mainly aiming at extracting pollutants after transport over long distances in an electric field. But the application of weak electric fields can have a negative effect on the pollutant-degrading biofilm, electrolyze the pore water, cause pH changes at the cathode, and produce reactive oxygen or chlorine species with antimicrobial effects near the anode (Liu et al., 1997).
Our definition of electro-bioremediation only deals with techniques based on the microbial property to transfer electrons (to breath) to solid surfaces (anode) according to various transfer mechanisms (Lovley, 2012; Malvankar and Lovley, 2014; Santoro et al., 2017). The transfer mechanisms can be chained, so the electron will be transported to the anode surface along a chain of microbial chemical shuttles and conductive nanowires. The anodic surface acts as an inexhaustible TEA for the bacteria near the anode, improving their breathing and thus the oxidation of OM and associated contaminants. Some technologies use the cathode to directly inject electrons for the clean-up of halogenated solvents (Aulenta et al., 2009), nitrate (Virdis et al., 2010), in recalcitrant wastes (Huang et al., 2011).
In electro-bioremediation, instead of optimizing the collection of as many electrons as possible or injecting energy to catalyze the degradation of pollutants, electron flows are regulated via an external resistance control in order to maintain an anodic microbial consortium in optimal conditions for the biodegradation of OM. The maximal potential difference between oxygenated superficial water and anoxic sediments generally reaches approximately 800 mV (Ryckelynck et al., 2005; Donovan et al., 2008; Zhang et al., 2011; Yang et al., 2015; Gonzalez-Gamboa et al., 2017). The greater the TEA potential difference, the higher the energy gain for the bacteria. The external resistance is therefore the relevant parameter to control the performance of the anode biofilm (Ren et al., 2011). A proper anodic potential poised between 0 and 100 mV (NHE) can both enhances OM oxidation (bioremediation enhancement) and cuts noxious gas production (H2S, CH4, and N2O) generated at lower potentials (Jeon et al., 2012). Indeed, electro-bacteria competing with methanogens for OM have half-saturation coefficients lower than methanogens: e.g., Geobacter sulfurreducens: 10μM (Esteve-Nunez et al., 2005) versus Methanosaetaceae: 169μM and Methanosarcinaceae: 3.4mM (Qu et al., 2009). Electrobioremediation stimulates OM removal without chemical or energy inputs, so the operational cost can be significantly lower with other remedial methods. But it is able to lead to higher treatment efficiencies than other bioremediation technologies (Logan et al., 2006; Huang et al., 2011; Wang and Ren, 2013).
Three levels of electro-bioremediation setups exist: 1) bench tests on lab-scale microbial fuel cell (MFC) with a volume <1L; 2) pilot tests on semi-industrial devices (>1 L); and 3) in-field tests on quasi-full-scale. The two first levels deal with Sediment MFCs, and only the last one concerns actually Benthic MFCs.
1.10.4 Bench Tests
Pollutant bench tests, often referred as microcosm studies, focus on the optimization of operational electrochemical parameters. Bench tests reach high removal for contaminants such as: nitrogen salts by cathode ammonification and anode denitrification processes (Zhang et al., 2020b); crude oil and PAH contaminations (Mohan and Chandrasekhar, 2011; Morris and Jin, 2012; Li et al., 2014; Lu et al., 2014a; Viggi et al., 2015; Venkidusamy et al., 2016; Daghio et al., 2017; Kronenberg et al., 2017; Li et al., 2017b; Yan et al., 2017; Yu et al., 2017; Palma et al., 2018; Li et al., 2020; Zhang et al., 2020b); solvents (Strycharz et al., 2008; Zhang et al., 2010; Daghio et al., 2016); OM (Morris and Jin, 2012; Venkidusamy et al., 2016; Gonzalez-Gamboa et al., 2017; Zhao et al., 2017), or pesticides (Rodrigo et al., 2014; Cao et al., 2015; Domínguez-Garay et al., 2018; Quejigo et al., 2018). Electro-bacteria can promote reductive remediation of organochlorine compounds (Aulenta et al., 2011; Yu et al., 2016). On its side, cathode electrode can remove heavy metal ions by metal reducing and depositing such as for Cu2+, Ag2+, and Hg2+. But, for metal ions with negative reduction potential, such as Pb2+, Cd2+, Zn2+, and Ni2+, energy input is required for reduction. However, this technique currently works only at bench scale and on synthetic solutions, thus avoiding the problem of metals bound to OM. Only an application on actual wastewater will be able to prove the relevance of electro-bioremediation in treatment of heavy metal pollutions (Bagchi and Behera, 2020).
Since electro-bioremediation involves a microbial consortium, it takes time to become operational: prior enrichment of the electrodes (bio-augmentation) accelerates the biodegradation capacities (Venkidusamy et al., 2016). The potential difference measurement between the anode and the cathode provides a way to monitor the set up progress of the microbial consortium, and its stabilization time varies with the device size. In bench tests it stabilizes after about 10 days: it takes 2 to 3 days in marine sediment (Najafgholi and Rahimnejad, 2016); 10 days in swamp sediment (Gonzalez-Gamboa et al., 2017) and Diesel-fed sludge (Venkidusamy et al., 2016); 13 days in tidal mud (An et al., 2010); and around 20 days in waterlogged soil (Yu et al., 2017; Zhang et al., 2020b). In pilot tests, it stabilizes about some tens of days: 35 days in TF-CWs (Arends et al., 2014; Schievano et al., 2017) crude oil contaminated marine sediment (Hamdan and Salam, 2020), and waste-contaminated river sediment (Yang et al., 2015); over 40 days in FSF-CWs (Oon et al., 2016). In field-experiments, because of the size, the voltage stabilization time is supposed to be longer.
1.10.5 Pilot Tests
Pilot tests allow systems to be evaluated under more realistic field conditions such as: in waterlogged soil (Lu et al., 2014a; Lu et al., 2014b), sediment (Li et al., 2017a), and wastewater (Wu et al., 2016). The bioremediation system requires an anode in anaerobic zones and a cathode in contact with oxygen; both zones are naturally located in CWs. From merging of CWs and BMFC concepts emerges a new setup: the BMFC-CW to simultaneously improve wastewater treatment capacity and produce power (Jeon et al., 2012; Yadav et al., 2012; Doherty et al., 2015; Song et al., 2017). GHG production can be minimized and even suppressed (≈98%) by fine tuning of electron fluxes (Liu et al., 2017; Jobin et al., 2018). But electrobioremediation can also be merged with other mitigation approaches, such as using selected plants to prevent CH4 and N2O production (Timmers et al., 2012; Arends et al., 2014; Maucieri et al., 2017) or smart hydraulic management (Mander et al., 2014; Fang et al., 2015; Jahangir et al., 2016; Maucieri et al., 2017). A smartly managed electricity generation in paddy-fields can mitigate CH4 production (Kaku et al., 2008; Kouzuma et al., 2014), even if chemical fertilizer is added (Kamaraj et al., 2020).
1.10.6 Field Implementations
There are few full-scale field implementations of electro-bioremediation (Kronenberg et al., 2017). Some deal with BMFC setup in rivers (Donovan et al., 2008; Friedman et al., 2016), lagoons (Nielsen et al., 2007; Kaku et al., 2008), paddy-fields (Kamaraj et al., 2020), or ponds (Jeon et al., 2012; Schievano et al., 2017). Field data are still scarce and conflicting. But the actual CW-BMFC configuration seems site-specific and must be tailored according to its own features (Li and Yu, 2015).
Management potential needs some optimization efforts in order to finer control bioremediation processes. In particular, it requires a fine tuning of the anode potential to adjust the ohmic loss variation, demanding a three-electrode setup, to measure precise and well-controlled electrochemical potential. Microbial consortia change, according to external resistance imposed (Lyon et al., 2010; Goud and Mohan, 2013; Lu et al., 2014a; Li et al., 2017a), favoring some to the detriment of other ones, deeply
