(714d) The Influence of Temperature Gradients on Selective Charge Transport in Electrodialysis Systems

Electrodialysis (ED) is an established technique for the desalination of brackish water or the concentration of industrial streams. An electric field is applied over a stack of alternating cation and anion exchange membranes to selectively transport ions, resulting in a concentrated and a diluted product stream. The number of ions transported through the system, indicated by the measured current, increases linearly (following Ohm's law) with increasing driving potential difference, until there is a mismatch between the supply and removal rates of ions at the membrane interface. Concentration polarization, the development of a depleted boundary layer at one side of the membrane and a concentrated boundary layer on the opposite site occurs at a critical potential. Transport through the depleted boundary layer, which is dictated by diffusion and electromigration limits the overall transport through the system. As a result, a system specific so-called ``limiting current density" is observed for all electrodialysis systems. Increasing the transport through the boundary layer can enhance the total transport through the system and increase the efficiency of ED at higher potentials.

In this research [1-2], we experimentally investigate the influence of temperature and temperature gradients on the transport of various ions in electrodialysis systems. Temperature has a large influence on the physical properties of the ionic solutions in the system, as the viscosity decreases with increasing temperature and the diffusivity and mobility of the ions increases with increasing temperature. Increasing the temperature of the fluid on the depleted side of the membrane interface can thus enhance the transport of ions through the depleted boundary layer. After initial theoretical work [3] indicating the possible enhancement of selective transport through a charge selective interface in the presence of a temperature gradient, experiments were conducted using a commercially available, lab-scale electrodialysis stack. The influence of temperature and temperature gradients on the transport of different ions was measured in both the Ohmic and limiting current regimes.

For measurements in the Ohmic regime using a 1;1 electrolyte (NaCl), a clear increase in total ion transport was measured when increasing the temperature of either the depleted or enriched stream by 20ËšC when compared to having both streams at a lower temperature. The energy required for the ED process was reduced by 9% when heating one of the feed streams. Increasing the temperature in both streams enhanced the total transport even further and reduced the required energy input for the ED process by 15%. However, in contrast to our expectations, no significant change in selective transport was measured for any of the different temperature configurations in the Ohmic regime. The reduced power input is mainly attributed to the increasing diffusivity of the ions at elevated temperatures, and the viscosity of the solutions was found to be of less influence.

In the limiting current regime, where the depletion boundary layer is dictating the overall transport through the stack, the influence of a temperature gradient is more pronounced. We find that the direction of the temperature gradient influences the total transport of ions and the selectivity towards transport. A significant improvement of the total transport is found when the depleted stream is heated by 20 ËšC, when compared to heating the concentrated stream. Additionally, for systems containing a mixture of monovalent (Na⁺) and divalent (Mg²⁺) ions, a change in selectivity was found when applying a temperature gradient. The selectivity favored the separation of divalent ions when the dilute stream was heated when compared to isothermal systems. This is attributed to competitive transport of the mono- and divalent ions that is influenced by their different response on temperature.

The results obtained in these experiments indicate that the application of temperature gradients has a positive influence on the required power for electrodialysis systems and can be of use in enhancing the selective transport between mono- and divalent ions. This is of potential use in industrial systems where heavy metals have to be selectively removed and in water softening technologies exchanging divalent for monovalent ions. The utilization of industrial waste heat, available from various processes and at various locations, for the application of a temperature gradient is a promising method for the improvement of overall process efficiency.

[1] Benneker, Anne M., et al. "Influence of temperature gradients on mono-and divalent ion transport in electrodialysis at limiting currents." Desalination 443 (2018): 62-69.

[2] Benneker, Anne M., et al. "Effect of temperature gradients in (reverse) electrodialysis in the Ohmic regime." Journal of membrane science 548 (2018): 421-428.

[3] Wood, Jeffery A., Anne M. Benneker, and Rob G.H. Lammertink. "Temperature effects on the electrohydrodynamic and electrokinetic behaviour of ion-selective nanochannels." Journal of physics: Condensed matter 28.11 (2016): 114002.