Researchers at KAUST have devised a bioreactor using chemolithoautotrophs—microbes that consume CO₂ through their natural metabolism, spitting out small organic molecules as a byproduct—to convert industrial CO₂ emissions into valuable chemicals.

Chemolithoautotrophs are commonly found in the deep sea, in caves and hydrothermal vents, where conventional energy sources, such as sunlight and organic carbon, are lacking. The microbes obtain their energy from the oxidation of inorganic compounds, such as hydrogen, iron and sulfur. The microbes strip the inorganic compounds of electrons while taking up CO₂ and reducing it to organic products as part of the process.

To harness chemolithoautotroph capabilities for recycling CO₂ emissions into useful chemicals, researchers supply electrons to the microbes in a process called microbial electrosynthesis (MES). Typically, MES reactors have grown chemolithoautotrophs on a submerged flat-sheet cathode and bubbled CO₂ gas into the solution. However, flat-sheet cathodes are difficult to scale up and CO₂ gas has poor solubility.

The team developed an alternative MES reactor using cathodes made from stackable, cylindrical porous nickel fibers that Pascal Saikaly’s group had previously applied to recover water and energy from wastewater. CO₂ is pumped through each cylinder, and electrons flow along it.

Using this architecture, we directly deliver CO₂ gas to chemolithoautotrophs through the pores in the hollow fibers. We provided electrons and CO₂ simultaneously to chemolithoautotrophs on the cathode surface.

— Manal Alqahtani, a Ph.D student in the team

In an initial study, methane-producing microbes were able to convert CO₂ to methane with 77% efficiency, compared to 3% efficiency with a conventional design.

A follow-up study improved performance further by coating the electrodes with carbon nanotubes. These offered a more biocompatible surface for microbial growth, and improved the hollow fibers’ CO₂ adsorption capability 11-fold.

In tests using acetate-producing microbes, production of the chemical almost doubled when the nanotube coating was applied.

Ongoing work includes investigating easier approaches to develop porous cylindrical cathodes, optimizing CO₂ flow rates and investing renewable MES energy sources, such as solar.

Resources

• Bian, B., Alqahtani, M.F., Katuri, K.P., Liu, D., Bajracharya, S., Lai, Z. & Saikaly, P.E. (2018) “Porous nickel hollow fiber cathodes coated with CNTs for efficient microbial electrosynthesis of acetate from CO₂ using Sporomusa ovata”. Journal of Materials Chemistry A 6, 17201 doi: 10.1039/C8TA05322G