Designing a Selective n-Caproate Adsorption–Recovery Process with Granular Activated Carbon and Screening of Conductive Materials in Chain Elongation

Microbial chain elongation using biomass-derived lactate can be steered to produce a variety of medium-chain carboxylates (MCC), which then need to be separated before application. In this study, we evaluated the effects of adding conductive and/or adsorbing materials to batch and continuous open-culture lactate-based chain elongation. Incubation with granular activated carbon (GAC), nickel foam (NF), and stainless steel (SS) mesh improved lactate use for chain elongation due to a ∼30% reduction in propionate formation compared to the control (no material). Isobutyrate production was stimulated in the presence of GAC and NF (up to 1.2 g·L–1, 9% electron selectivity). Adding GAC to a continuous reactor led to in situ adsorption of n-caproate. GAC showed a high affinity to n-caproate from real and artificial effluents, as well as from blends containing C2–C8 carboxylates, adsorbing 60–80% of the initial n-caproate with recoveries up to 42% after desorption. Adsorption isotherms showed that n-caproate adsorption increased with decreasing pH conditions (184–243 mg·g GAC–1). In conclusion, conductive materials changed the product spectrum and steered to isobutyrate formation in batch open-culture chain elongation. Based on the promising adsorption properties of GAC, the first design of chain elongation with in-line adsorption–recovery is proposed as well as potential direct applications of MCC-loaded porous carbons.