Pau Batlle-Vilanova

Deciphering the electron transfer mechanisms for biogas upgrading to biomethane within a mixed culture biocathode

Biogas upgrading is an expanding field dealing with the increase in methane content of the biogas to produce biomethane. Biomethane has a high calorific content and can be used as a vehicle fuel or directly injected into the gas grid. Bioelectrochemical systems (BES) could become an alternative for biogas upgrading, by which the yield of the process in terms of carbon utilisation could be increased. The simulated effluent from a water scrubbing-like unit was used to feed a BES. The BES was operated with the biocathode poised at −800 mV vs. SHE to drive the reduction of the CO2 fraction of the biogas into methane. The BES was operated in batch mode to characterise methane production and under continuous flow to demonstrate its long-term viability. The maximum methane production rate obtained during batch tests was 5.12 ± 0.16 mmol m−2 per day with a coulombic efficiency (CE) of 75.3 ± 5.2%. The production rate increased to 15.35 mmol m−2 per day (CE of 68.9 ± 0.8%) during the continuous operation. Microbial community analyses and cyclic voltammograms showed that the main mechanism for methane production in the biocathode was hydrogenotrophic methanogenesis by Methanobacterium sp., and that electromethanogenesis occurred to a minor extent. The presence of other microorganisms in the biocathode, such as Methylocystis sp. revealed the presence of side reactions, such as oxygen diffusion from the anode compartment, which decreased the efficiency of the BES. The results of the present work offer the first experimental report on the application of BES in the field of biogas upgrading processes.

Microbial electrosynthesis of butyrate from carbon dioxide: Production and extraction

To date acetate is the main product of microbial electrosynthesis (MES) from carbon dioxide (CO2). In this work a tubular bioelectrochemical system was used to carry out MES and enhance butyrate production over the other organic products. Batch tests were performed at a fixed cathode potential of -0.8V vs SHE. The reproducibility of the results according to previous experiments was validated in a preliminary test. According to the literature butyrate production could take place by chain elongation reactions at low pH and high hydrogen partial pressure (pH2). During the experiment, CO2 supply was limited to build up pH2 and trigger the production of compounds with a higher degree of reduction. In test 1 butyrate became the predominant end-product, with a concentration of 59.7mMC versus 20.3mMC of acetate, but limitation on CO2 supply resulted in low product titers. CO2 limitation was relaxed in test 2 to increase the bioelectrochemical activity but increase pH2 and promote the production of butyrate, what resulted in the production of 87.5mMC of butyrate and 34.7mMC of acetate. The consumption of ethanol, and the presence of other products in the biocathode (i.e. caproate) suggested that butyrate production took place through chain elongation reactions, likely driven by Megasphaera sueciensis (>39% relative abundance). Extraction and concentration of butyrate was performed by liquid membrane extraction. A concentration phase with 252.4mMC of butyrate was obtained, increasing also butyrate/acetate ratio to 16.4. The results are promising for further research on expanding the product portfolio of MES.

Tracking bio-hydrogen-mediated production of commodity chemicals from carbon dioxide and renewable electricity.

This study reveals that reduction of carbon dioxide (CO2) to commodity chemicals can be functionally compartmentalized in bioelectrochemical systems. In the present example, a syntrophic consortium composed by H2-producers (Rhodobacter sp.) in the biofilm is combined with carboxidotrophic Clostridium species, mainly found in the bulk liquid. The performance of these H2-mediated electricity-driven systems could be tracked by the activity of a biological H2 sensory protein identified at cathode potentials between -0.2V and -0.3V vs SHE. This seems to point out that such signal is not strain specific, but could be detected in any organism containing hydrogenases. Thus, the findings of this work open the door to the development of a biosensor application or soft sensors for monitoring such systems.

Mixed Culture Biocathodes for Production of Hydrogen, Methane, and Carboxylates

Formation of hydrogen, methane, and organics at biocathodes is an attractive new application of bioelectrochemical systems (BESs). Using mixed cultures, these products can be formed at certain cathode potentials using specific operating conditions, of which pH is important. Thermodynamically, the reduction of CO2 to methane is the most favorable reaction, followed by reduction of CO2 to acetate and ethanol, and hydrogen. In practice, however, the cathode potential at which these reactions occur is more negative, meaning that more energy is required to drive the reaction. Therefore, hydrogen is often found as a second product or intermediate in the conversion of CO2 to both methane and carboxylates. In this chapter we summarize the inocula used for biocathode processes and discuss the achieved conversion rates and cathode potentials for formation of hydrogen, methane, and carboxylates. Although this overview reveals that BESs offer new opportunities for the bioproduction of different compounds, there are still challenges that need to be overcome before these systems can be applied on a larger scale. Graphical Abstract.

Bio-electrorecycling of carbon dioxide into bioplastics

The rise of carbon dioxide (CO2) emissions and the accumulation of non-biodegradable plastics in the environment are leading to an environmental crisis. Thus, the bio-electro recycling of recalcitrant CO2 as feedstock to produce bioplastics could be an interesting solution to explore. In this work, a bioelectrochemical reactor was used to carry out microbial electrosynthesis (MES) of volatile fatty acids (VFAs) from CO2 and then, those VFAs were used to produce polyhydroxybutyrate (PHB) by using a pre-selected mixed microbial culture (MMC). During MES (cathode potential at −0.8 V vs. SHE), CO2 fixation efficiency, i.e. carbon (C) transferred to final products was of 73% CCO2, with a final values of 43.7 and 103 mmol of C produced for acetate and butyrate. The VFAs obtained were extracted and concentrated by liquid membrane extraction getting a broth with a C concentration of approximately 400 mmol C L−1 (∼65% butyrate), to be used as feeding for PHA producing bacteria. During the PHA accumulation a maximum of 74.4 ± 0 g PHA per 100 g VSS was obtained with a PHA yield (Ytot) of 0.77 ± 0.18 mmol CPHA mmol−1 Cfed. The process efficiency calculated taking into account the PHA yield on C inlet as CO2 was of 0.50 ± 0.07 mmol CPHA mmol−1 CCO2. In terms of C conversion, 0.41 kg of carbon as PHA were obtained per 1 kg of carbon as CCO2 inlet to the entire system. These results establish a sustainable way to convert a greenhouse gas as CO2 into environmental friendly bioplastics.

Microbial Community Pathways for the Production of Volatile Fatty Acids From CO2 and Electricity

This study aims at elucidating the metabolic pathways involved in the production of volatile fatty acids from CO2 and electricity. Two bioelectrochemical systems (BES) were fed with pure CO2 (cells A and B). The cathode potential was first poised at -574 mV vs. standard hydrogen electrode (SHE) and then at -756 mV vs. SHE in order to ensure the required reducing power. Despite of applying similar operation conditions to both BES, they responded differently. A mixture of organic compounds (1.87 mM acetic acid, 2.30 mM formic acid, 0.43 mM propionic acid, 0.15 mM butyric acid, 0.55 mM valeric acid and 0.62 mM ethanol) was produced in cell A while mainly 1.82 mM acetic acid and 0.23 mM propionic acid were produced in cell B. The microbial community analysis performed by 16S rRNA gene pyrosequencing showed a predominance of Clostridium sp. and Serratia sp. in cell A whereas Burkholderia sp. and Xanthobacter sp. predominated in cell B. The coexistence of three metabolic pathways involved in carbon fixation was predicted. Calvin cycle was predicted in both cells during the whole experiment while Wood-Ljungdahl and Arnon-Buchanan pathways predominated in the period with higher coulombic efficiency (CE). Metabolic pathways which transform organic acids into anabolic intermediaries were also predicted, indicating the occurrence of complex trophic interactions. These results further complicate the understanding of these mixed culture microbial processes but also expand the expectation of compounds that could potentially be produced with this technology.