Chain length and acidity of carboxylic acids influencing adsorption/desorption mechanism and kinetics over anion exchange membrane

Selective recovery of carboxylic acids through anion exchange membrane (AEM) during electrodialysis (ED) involves bulk transport/diffusion/adsorption/desorption steps. Combined effect of size, structure, percentage dissociation, interactions alter each step of transport. In this study, we focused on interpretation of role of functional groups (number of [sbnd]COOH groups) and hydrocarbon chain length of different carboxylic acids (formic (FA), acetic (AA), propionic (PA), butyric (BA), malic (MA) and citric (CA) acids) during their transport in terms of ion interaction with membrane. Quantitative estimate of interactions between acids – AEM, adsorption mechanism and solute uptake rate were inferred from adsorption isotherms (Langmuir, Freundlich and Redlich-Peterson) and kinetic models (pseudo-first-order, pseudo-second-order and Elovich). Adsorption was rate limiting and Freundlich isotherm was more appropriate. Adsorption capacities, K_f followed an order: 0.139 > 0.121 > 0.072 > 0.062 > 0.049 > 0.025 mmol·g⁻¹ respectively with CA, MA, FA, AA, PA, BA. Pseudo-second order rate constants, k₂ followed same order of adsorption, highest with CA (0.0187 g·mmol⁻¹ min⁻¹) and lowest with BA (0.0124 g·mmol⁻¹ min⁻¹) and dominance of heterogeneous mode of chemical/ionic interactions. pK_a, chain length and molecular sieve effects were used to support the adsorption pattern.