Volatile fatty acid selectivity during electrodialysis: effect of a competing counterion on volatile fatty acid transport mechanisms

Volatile fatty acids (VFAs) are short-chain carboxylic acids with versatile applications that can be sustainably produced through anaerobic digestion. VFAs must be separated from anaerobic digestate for downstream use. Electrodialysis (ED) is an approach to selectively separate anionic VFAs from other ions, water, and neutral solutes. Towards enhancing selective VFA separation, VFA transport mechanisms during ED need to be understood. Accordingly, our objectives were to (i) determine the relative contributions of the partition and mobility selectivities to the permeability selectivity for VFA-containing counterion pairs, and (ii) determine the effect of a competing counterion on VFA transport mechanisms. To do this, we evaluated the permeability (P_i/P_j), partition (K_i/K_j), and mobility (u_i/u_j) selectivities of six VFA-containing counterion pairs, including VFA/VFA and VFA/Cl⁻ pairs, and a chloride (Cl⁻)/bromide (Br⁻) control under (i) mixed- and (ii) single-counterion conditions. Results showed the partition selectivity drove the permeability selectivity for Cl⁻/VFA and Cl⁻/Br⁻ pairs while the mobility selectivity drove the permeability selectivity for VFA/VFA pairs. Further, for the VFA/Cl⁻ and Cl⁻/Br⁻ pairs, we observed lower mixed-counterion partition selectivities and higher mixed-counterion mobility selectivities than corresponding single-counterion selectivities. Conversely, for the VFA/VFA pairs, we observed generally lower mixed-counterion partition and mobility selectivities than single-counterion selectivities. Overall, results indicate that VFA partitioning decreased by the presence of a counterion with greater affinity for the membrane while the effect of the competing counterion on VFA mobility depended on the competing counterion physicochemical properties. This work provides mechanistic insights into electro-driven VFA transport under conditions with competing counterions which is critical given the complexity of digestates. This study is the first to systematically evaluate VFA transport properties and selectivities in an AEM and provides a robust experimental framework for determining electro-driven counterion permeabilities under mixed-counterion conditions.