membranes Review Insights into Advancements and Electrons Transfer Mechanisms of Electrogens in Benthic Microbial Fuel Cells Mohammad Faisal Umar 1 , Syed Zaghum Abbas 2,* , Mohamad Nasir Mohamad Ibrahim 3 , Norli Ismail 1 and Mohd Rafatullah 1,* 1 Division of Environmental Technology, School of Industrial Technology, Universiti Sains Malaysia, Penang 11800, Malaysia; [email protected] (M.F.U.); [email protected] (N.I.) 2 Biofuels Institute, School of Environment, Jiangsu University, Zhenjiang 212013, China 3 School of Chemical Sciences, Universiti Sains Malaysia, Penang 11800, Malaysia; [email protected] * Correspondence: [email protected] (S.Z.A.); [email protected] or [email protected] (M.R.); Tel.: +60-4-6532111 (M.R.); Fax: +60-4-656375 (M.R.) Received: 7 August 2020; Accepted: 19 August 2020; Published: 28 August 2020 Abstract: Benthic microbial fuel cells (BMFCs) are a kind of microbial fuel cell (MFC), distinguished by the absence of a membrane. BMFCs are an ecofriendly technology with a prominent role in renewable energy harvesting and the bioremediation of organic pollutants through electrogens. Electrogens act as catalysts to increase the rate of reaction in the anodic chamber, acting in electrons transfer to the cathode. This electron transfer towards the anode can either be direct or indirect using exoelectrogens by oxidizing organic matter. The performance of a BMFC also varies with the types of substrates used, which may be sugar molasses, sucrose, rice paddy, etc. This review presents insights into the use of BMFCs for the bioremediation of pollutants and for renewable energy production via different electron pathways. Keywords: bioremediation; renewable energy; organic pollutants; electrogens; wastewater 1. Introduction Different environmental pollutants, such as organic- and inorganic-based contaminants, remain a severe challenge to the sustainability of water resources [1,2]. This poses a serious threat to living organisms, including human beings and marine organisms [3]. Due to the depletion of natural water resources, there is an imbalance in the natural ecosystem, but simultaneously the commutability of renewable pure water resources has been enhanced. There is a plethora of potential sources of pollution in water bodies (e.g., oceans, lakes, rivers and reservoirs) stemming from human activity, and notably the chemical and oil filtration industries. The chemical substances emitted from these industries contain very harmful and potentially carcinogenic inorganic and organic pollutants [4]. These pollutants have a severe impact on living organisms and pose a serious threat to the environment. Several techniques exist for the treatment of wastewater prior to irrigation, such as lagoon ponds, constructed wetlands, conventional wastewater treatment plants, membrane bioreactors and membrane filtration. Although these techniques have been shown to be effective, disadvantages remain, i.e., they require a large area for operation, along with high economic stability [5]. Recently, a novel approach was introduced for the treatment of wastewater: the microbial fuel cell. Microbial fuel cells (MFCs) are devices which utilize microbial activity to produce electricity from chemical energy stored in an organic substrate. Thus, MFCs are a promising technique for wastewater bioremediation and for generating electricity in an economical way. Membranes 2020, 10, 205; doi:10.3390/membranes10090205 www.mdpi.com/journal/membranes Membranes 2020, 10, 205 2 of 18 OrganicMembranes pollutant2020, 10, x FOR compounds PEER REVIEW are oxidized by microorganisms and the transfer of electrons2 of 18 to the anode of the MFC via exoelectrogens [6,7]. A new type of MFC, the benthic microbial fuel cell energy stored in an organic substrate. Thus, MFCs are a promising technique for wastewater (BMFC),bioremediation was designed and tofor generate generating electricity electricity from in an organiceconomical matter way. present in wastewater. As a result, like with MFCs,Organic chemical pollutant energycompounds is converted are oxidized into by electrical microorganisms energy and with the exoelectrogens transfer of electrons working to as + a catalyst,the anode i.e., electronsof the MFC (e −via) and exoelectrogens protons (H [6,7].) are A released.new type of In MFC, this way, the benthic a potential microbial difference fuel cell exists between(BMFC), the anode was designed and cathode. to generate Here, electricity we present from informationorganic matter regarding present in recent wastewater. developments As a result, using exoelectrogenslike with MFCs, on the chemical anode byenergy direct is converted and indirect into processes. electrical energy with exoelectrogens working as a catalyst, i.e., electrons (e−) and protons (H+) are released. In this way, a potential difference exists 2. Benthicbetween Microbial the anode Fuel and Cellcathode. (BMFC) Here, we present information regarding recent developments using exoelectrogens on the anode by direct and indirect processes. There is a need for sustainable and clean energy sources to meet growing energy demands. In 2014,2. Benthic the global Microbial percentage Fuel Cell of electricity(BMFC) generated via the consumption of fossil fuels was 66%; however, only 11% of this was utilized together with renewable energy [8,9]. Organic substrates are There is a need for sustainable and clean energy sources to meet growing energy demands. In used2014, as bio the sediments, global percentage and they of protectelectricity the generated microbial via ecosystem the consumption in various of fossil regions fuels and wasprovide 66%; a suitablehowever, environment only 11% for of the this bioremediation was utilized together of accumulated with renewable pollutants energy via [8,9]. the Organic electron substrates donor–acceptor are mechanismused as [ 10bio]. sediments, Currently, and physiochemical they protect the processes, microbial such ecosystem as dredging, in various ozonation regions and and electrochemical provide a degradation,suitable areenvironment used for the for bioremediation the bioremediation of pollutants. of accumulated These techniques pollutants are via eff ectivethe electron but require a lot ofdonor–acceptor energy and mechanism are costly, [10]. limiting Currently, their physiochemical application. Usually,processes, the such accumulation as dredging, ozonation of reductive substancesand electrochemical and the lack ofdegradation, electron acceptors are used for are the the bi mainoremediation limitations of pollutants. for the remediation These techniques of sediment are undereffective anaerobic but conditions. require a lot of energy and are costly, limiting their application. Usually, the Inaccumulation recent years, of microbialreductive substances fuel cells (MFC) and the have lack beenof electron considered acceptors as anare alternative, the main limitations cheap approach for the remediation of sediment under anaerobic conditions. to the bioremediation of toxic organic pollutants via power generation. Recently, BMFCs have attracted In recent years, microbial fuel cells (MFC) have been considered as an alternative, cheap the attentionapproach of to many the bioremediation researchers dueof toxic to their organic nonaggressive pollutants via and power easily generation. controllable Recently, nature. BMFCs BMFCs consisthave of attracted an anode, the which attention is embeddedof many researchers in organic due matter,to their andnonaggressive a cathode, and which easily iscontrollable placed in the overlyingnature. water. BMFCs The consist air di offfuser an anode, provides which a constant is embedded supply in organic of oxygen matter, which and playsa cathode, a vital which role is in the transferplaced of electrons in the overlying and protons water. The from air the diffuser anode provides to cathode a constant via an suppl externaly of oxygen circuit, which where plays electrons a react withvital role oxygen in the and transfer produce of electrons water [ 11and,12 protons]. from the anode to cathode via an external circuit, Reimerswhere electrons et al. [ 13react] were with theoxygen first and to employproduce water BMFCs; [11,12]. their approach included a platinum mesh for the anodeReimers and et carbon al. [13] fiberwere forthe thefirst cathode. to employ A BM uniqueFCs; their feature approach of the included BMFC isa itsplatinum membrane-less mesh assembly;for the this anode is possible and carbon thanks fiber to for the the boundary cathode.organic A unique substrate feature of used the asBMFC a substrate, is its membrane-less which itself acts assembly; this is possible thanks to the boundary organic substrate used as a substrate, which itself as a pseudo membrane. Nowadays, many researchers are working on improving ecofriendly systems, acts as a pseudo membrane. Nowadays, many researchers are working on improving ecofriendly includingsystems, BMFCs including [14]. BMFCs The prototype [14]. The ofprototype a double of chambera double chamber BMFC is BMFC shown is shown in Figure in Figure1. 1. FigureFigure 1. General 1. General prototype prototype scheme scheme of a benthic microbial microbial fuel fuel cell. cell. An airAn cathode air cathode in the in the overlaying overlaying water water connected connected with a a benthic-integrating benthic-integrating anode anode is the is most the most commoncommon BMFC BMFC model. model. In aIn saline a saline environment, environment, conductivityconductivity is is normally normally high, high, so sothe