Carbon dioxide to bio-oil in a bioelectrochemical system-assisted microalgae biorefinery process

Microbial electrosynthesis (MES) for bioelectro carbon dioxide (CO₂) recycling is an interesting and sustainable opportunity to exploit off gases from industrial facilities and convert them into valuable energy sources. In the present study, a two-step process based on coupling a bioelectrochemical system (BES) and heterotrophic microalgaeAuxenochlorella protothecoidesis proposed to convert carbon dioxide into a biodiesel compatible oil. The MES effluent was further processed in a heterotrophic microalgae batch reactor, where the acetate previously synthesized from CO₂was converted into bio-oil in a subsequent, extraction-free step. Two MES reactors were operated in batch mode at an applied cathodic potential of −0.8 Vvs.SHE (standard hydrogen electrode) for 95 days. The system reached a concentration of up to 13 g L⁻¹of acetate (at a maximum production rate of 0.29 g L⁻¹d⁻¹). Microbial community analysis revealed the presence ofClostridiumspp. in both reactors. In a second stage, the effluent from the biocathode was transferred to microalgae reactors containingA. protothecoidesto assess oil production. The bio-oil content was up to 22% w/w (dry weight), sufficient to further explore the feasibility of microalgae-to-oil recovery in the future. According to our estimations, 7.59 kg CO₂can be converted into 1 kg acetate, which can be used to grow heterotrophically 1.11 kg dry algae; an overall balance of 0.03 kg bio-oil produced per kg CO₂captured was assessed. The oil obtained can be further processed to produce a biodiesel compatible with EU requirements for biofuels.