Combination of bioanode and biocathode for the conversion of wastes into biocommodities using microbial electrosynthesis
Résumé
Bioelectrochemical systems (BES) as microbial fuel cells take advantages of microorganisms to convert the chemical energy of organic waste into electricity. Recently, the discovery that BES can also be used for the synthesis of biocommoditiesvia microbial electrosynthesis (MES) has greatly expanded the horizons for their applications. Indeed, some microbes are able to use electrons and molecules such as CO2 to synthesize reduced products: volatile fatty acids, alcohols etc... By combining both these processes, it should thus theoretically be possible to use the electrons of organic waste to synthesize bio-based chemicals in a clean and controlled compartment. However, these technologies are only few years old and required scientific data before they can be practically applied. In this context, we developed a dual-chamber reactor with both biotic anode (carbon cloth) and cathode (stainless steel) separated by a cation-exchange membrane. Bioanode was inoculated using an anodic biofilm sample formed in biological wastes and biocathode by injecting a suspension of a homoacetogen-enriched culture. Acetate (600mg/l) was used as electron donor in the anodic compartment. Chronoamperometry experiments were carried out with a multi-channel potentiostat in order to monitor electroactivity of the microbial communities. Anode potential was poised at +0.158 volts versus saturated calomel electrode (SCE) for startup. Chemical analyses (volatile fatty acids (VFAs), cations/anions, chemical oxygen demand, and total organic carbon) were performed to evaluate the metabolic pathways. Microbial community diversity was investigated by 16S rDNA pyrosequencing (MiSeq sequencer, Illumina®). After the total consumption of acetate at anode (run 1), a second run (run 2) was launched by re-injecting 600 mg/L of acetate in anodic compartment and also 2-bromo-ethane sulfonate (2-BES) at the cathode to inhibit methanogenesis. Current density of 5 A/m² was reached after 24h of experiment. During run 1 and 2 78% and 89% of electrons from acetate were respectively transferred in the system revealing satisfying coulombic efficiency at the bioanode. During run 1, incoming electrons at biocathode were mainly used to produce methane (53% of total incoming electrons) and only traces of VFAs were detected. However, at the end of the second run, VFAs accumulated with a production rate of acetate reaching 11 g.m².d-1 and corresponding to 29% of the electrons coming from the anode. Using gas chromatography coupled with mass spectrometry, caprylate (C8H16O2) was also detected in reactors showing the ability of the MES system to produce molecules with an elongated carbon chain. Microbial diversity profiles showed a switch of archaeal and bacterial populations in cathodic compartment between run 1 and run 2 suggesting that VFAs production resulted from microbial adaptation to the addition of BES in the cathodic compartment. Overall this work constitutes a first step toward the utilization of MES systems for the conversion of organic wastes into biofuels and chemicals using coupled bioanode and biocathodes