Selective Extraction of Medium-Chain Carboxylic Acids by Electrodialysis and Phase Separation

Abstract

Carboxylic acids obtained via the microbial electrochemical conversion of waste gases containing carbon dioxide (i.e., microbial electrosynthesis) can be used in lieu of nonrenewable building-block chemicals in the manufacture of a variety of products. When targeting valuable medium-chain carboxylic acids such as caproic acid, electricity-driven fermentations can be limited by the accumulation of fermentation products in the culturing media, often resulting in low volumetric productivities and titers due to direct toxicity or inhibition of the biocatalyst. In this study, we tested the effectiveness of a simple electrodialysis system in upconcentrating carboxylic acids from a model solution mimicking the effluent of a microbial electrochemical system producing short- and medium-chain carboxylic acids. Under batch extraction conditions, the electrodialysis scheme enabled the recovery of 60% (mol mol^-1) of the total carboxylic acids present in the model fermentation broth. The particular arrangement of conventional monopolar ion exchange membranes and hydraulic recirculation loops allowed the progressive acidification of the extraction solution, enabling phase separation of caproic acid as an immiscible oil with 76% purity.

Figure 1

Schematic representation of the ED cell system and relevant fluxes. Cell’s compartments were obtained by sandwitching together two CEMs and one AEM to yield a feed chamber (containing the diluate solution), an extraction chamber (containing the concentrate solution), and the two electrode chambers, cathode and anode, hosting the respective electrodes. The diluate solution was recycled through its reservoir, whereas the concentrate solution was pumped through an oil trap to allow phase separation and accumulation of caproic acid at the top of the trap, while the aqueous concentrate solution was recycled to the concentrate chamber in the ED unit. Anode and cathode solutions were recycled through the same reservoir.

Figure 2

Concentration of carboxylic acids in aqueous solutions measured in (A) the diluate solution and (B) in the concentrate solution during batch extraction tests.

Figure 3

Measurements of conductivity and pH in (A) the diluate solution, (B) in the concentrate solution, and (C) in the combined anode and cathode solutions during batch extraction tests. (D) Measurements of the applied voltage required to deliver a constant current of 0.8 A.

Figure 4

Breakdown of carboxylic acids in the concentrate compartment (aqueous solution and oily phase) at the end of the batch extraction tests.

Figure 5

Breakdown of carboxylic acids measured in the concentrate solution at the end of the batch extraction tests, expressed as a percentage (g g^–1) of their respective initial levels measured in the diluate compartment at the beginning of the batch extraction tests.

Figure 6

Schematic representation of the ED stack used to recover carboxylic acids. Black solid arrows indicated desired fluxes; red dashed arrows indicate competing fluxes.