Switching selectivity of carboxylic acids and associated physico-chemical changes with pH during electrodialysis of ternary mixtures

• Published in: Separation and purification technology 2018-03, Vol.193, p.327-344
• Main Authors: Chandra, Anusha, Tadimeti, Jogi Ganesh Dattatreya, Bhuvanesh, E., Pathiwada, Darshak, Chattopadhyay, Sujay
• Format: Article
• Language: English
• Subjects: Effective diffusivity; Electrodialysis of ternary mixture of carboxylic acids; Ion-exchange equilibrium; LCD; Membrane resistance; Transport number
• ISSN: 1383-5866; 1873-3794
• DOI: 10.1016/j.seppur.2017.10.048

•Electrodialytic separation of solutions containing ternary mixture of carboxylic acids.•Switching selectivity of carboxylic acids with pH.•Interpretation of Physico-chemical phenomena during anion transport.•Theoretically estimated LCD could closely (±4–8%) predict experimental data.•Anions bearing higher size showed low transport number, high adsorption and resistance. Electrodialytic (ED) transport of carboxylic acids from their ternary mixtures (acetic, malic and citric) of different compositions were investigated through anion exchange membrane (AEM). Solution pH played crucial role in selectivity and ion transport (expressed by limiting current density, LCD). These observations were analysed based on flow hydrodynamics, ion specific properties and ionic interaction with AEM influencing water uptake and membrane resistance. Fractional hydrolysis of weak acids generated complex mixture of multiple anions of varying charges which strongly influenced LCD. Theoretically estimated LCD (from effective diffusivity, Deff and Sh number based effective mass transfer coefficient, keff) could closely match experimental LCD (±4–8% deviation). The nature of solute species, concentration and their interaction with AEM strongly influenced ion transport. Estimation of Deff, transport number, ion exchange equilibrium and membrane resistance could satisfactorily explain the observed LCD. At low pH, LCD was proportional to available free ions while, at pH > 5.2 component specific properties/interactions dominated. Water uptake indicated variation in micro-structure and ion selectivity with different compositions of ternary mixture. This was attributed to shrinkage/swelling of membrane due to ion-membrane interactions. Stronger hydrophobic interaction of acetic acid caused shrinkage of micro-structure resulted in low water uptake but got reversed at higher pH.