Anion vs cation exchange membrane strongly affect mechanisms and yield of CO2 fixation in a microbial electrolysis cell
Introduction
The rapidly developing microbial electrochemical technology represents an innovative route to stimulate and control microbial metabolism [1]. In a microbial electrolysis cell (MEC), as an example, provided the addition of an external power supply it is possible to convert CO2 into methane and the process is commonly referred to as “bio-electromethanogenesis” [2]. The latter occurs at a bio-cathode, where the reducing power necessary for CO2 reduction is given by a solid state electrode [3] through microorganism-electrode interactions. Two main mechanisms underlying these interactions have been identified over the last years, which are based on a direct contact between the cathode and microorganisms [4], or on an hydrogen mediated electron exchange [5]. Also, a direct interspecies electron transfer (DIET) has been recently found to drive the synthrophic interactions between methanogens and other microbial species involved in the anaerobic digestion process [6], giving new insights in the understanding of bio-electromethanogenesis. However, regardless the mechanism involved, the utilization of mixed autothrophic methanogenic bacteria as sustainable and renewable bio-catalysts for CO2 reduction offers several advantages over chemical catalysis, which typically requires noble and heavy metals as well as the need to operate at high temperature and pressure, as occurs for the reaction of CO2 methanation, also known as the Sabatier reaction [7].
The biocatalysis could offer an efficient route to enhance the environmental sustainability of the CO2 reduction process. Furthermore, the utilization of no cost inocula (i.e. mixed cultures of anaerobic sludge) brings the economic advantage to avoid sterilization and to operate at neutral pH, low pressure and low temperature. A key aspect of the bio-electromethanogenesis reaction is the possibility to offer a new approach for energy storage from the surplus of electricity production deriving from renewable energy sources (e.g. photovoltaic, eolic, etc.), since this energy surplus can be exploited to reduce CO2 into methane, a well storable energy vector which can be easily distributed to the grid or used in automotive engines [8]. In this frame, a rich renewable source of CO2 is offered by the biogas that is the product of the anaerobic digestion process (AD) [9]. Biogas is a gas mixture mainly composed of CH4 (50–70%) and CO2 (50–30%) besides other impurities, such as H2S, NH3, siloxane, and H2O, and the final percentage of each component depends on the composition of the raw materials used as feedstock. Biogas has been for decades considered as a byproduct but, over the last years, it has become the main target product of the AD process. Indeed, thanks to the development of mini and micronized combined heat power (CHP) units [10], the produced raw biogas can be used for the in situ energy recovery, especially in small plants (mainly used to treat agro-zootechnical effluents). Moreover, upon purification and upgrading, biogas can be turned into biomethane (BM), that is a carbon neutral footprint substitute of compressed natural gas (CNG) originated from fossil resources with an added value on the market higher than biogas. In particular, while the biogas purification step is aimed at eliminating impurities from the gas mixture in order to avoid corrosion or other problems related to downstream applications [11], the biogas upgrading process consists of an efficient CO2 removal with a consequent significant increase of the methane content up to, at least, 95% [12]. Technologies mainly based on a physical chemical separation of CO2, such as the water scrubbing (WS) and the pressure swing adsorption (PSA), are typically used at industrial scale for biogas upgrading [13]. From an economical point of view, however, CO2 removal is feasible only for biogas produced in large-plants unless novel low cost upgrading approaches are developed. In this context, microbial electrochemical technology has been recently proposed as an innovative and promising tool to upgrade the AD biogas [14], [15], [16], [17].
Here, mechanisms involved in CO2 removal in a fully biocatalyzed MEC have been deeply analyzed. The MEC was assessed to couple the bio-anodic COD oxidation to CO2 removal and methane generation at the cathode and two configurations with either an anion or a proton exchange membrane were assembled in order to test the effect of ionic transport phenomena on the overall process performance.
Based on literature, MEC can be used to convert CO2 into methane so offering a way to both purify and upgrade biogas from anaerobic digestion. The MEC effectiveness is improved by the alkalinity generation in the cathodic chamber, due to ion transport across separation membranes which is needed to counterbalance the external electron flow, in order to maintain system electroneutrality [18], [19], [20], [21]. This mechanism is strongly depending on the separation membrane (either anionic vs protonic) which establishes type and ratio of transported ions. As an additional consideration, alkalinity generation is in turn sustained by the bio-anode exploitation of the chemical energy contained in the COD source and in the electron scavenging due to biological reduction of CO2 into methane, which both contribute to lower the energy demand of the overall process. Thus, MEC performance is a complex function of several mechanisms, which include anodic and cathodic biological reactions as well as mass and ion transfer phenomena.
This study aims to give a complete and quantitative picture of all relevant mechanisms of CO2 removal in an MEC bio-cathode, and to compare their relative importance as function of different separation membranes (protonic vs anionic). Main reference is given to the role of ionic mobility and membrane-related transport phenomena on overall CO2 sorption and removal. The study also includes the determination of the mass and energy balance of the process as determined by different membrane types.
Section snippets
Microbial electrolysis cell design and setup
Throughout the study, two identical microbial electrolysis cells (MEC) were set up. Each MEC consisted of two identical Plexiglas frames, with internal dimensions of 17 cm × 17 cm × 3 cm, bolted together between two Plexiglas plates. A Nafion® 117 proton exchange membrane (PEM) or a Fumasep FAD anion exchange membrane (AEM) was placed between the frames (Fig. 1). Prior to being used, both PEM and AEM were pretreated as reported elsewhere [22]. The total empty volume of each frame (i.e., of the anodic
MEC continuous operation with the anode poised at +0.2 V (vs SHE)
The two fully microbially-catalyzed MECs, equipped with either an AEM or a PEM, were operated under continuous feeding of the synthetic organic mixture with the potential of the anode poised at +0.2 V. The oxidation of the fed COD was linked in both configurations to current generation, indicating the capability of the anodic biomass to use the graphite granules as electron acceptors. The profile of COD concentration in the influent and effluent streams of the anode compartments is reported in
Conclusions
One of the most attractive features of the MEC technology is certainly represented by the possibility to couple several positive actions in one process (e.g. COD removal, methane generation, low microbial growth) and, in this study, the performance of an MEC equipped with either an AEM or a PEM has been deeply analyzed. In particular, most of equivalents deriving from COD oxidation at the anode were diverted into current with a CE between 53 ± 9% and 83 ± 15%, with only a little fraction being used
Acknowledgments
This work has been carried out with the financial support of the project PRIN 2012 WISE “Advanced process to sustainable useful innovative products from organic waste”.
Authors also wish to thank Ilaria Ceccarelli and Alessandro Mattia for their skilful assistance with the experimental work.
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