A study of concentration polarization and solution-membrane interactions in the electrodialytic desalination process
The problem of concentration polarization in electrodialytic desalination occurs due to mass transfer limitations within thin film layers adjacent to anion and cation exchange membranes. Each type of membrane behaves quite differently in electrodialysis. Anion exchange membranes promote water ionization while cation exchange membranes do not. Control of water ionization is desirable since it limits process efficiency. Studies have been conducted to better understand the nature of each solution-membrane interface and suggest why each behaves differently.^ Experiments designed to promote concentration polarization (interfacial ionic depletion) have been conducted for characterization of several different ion exchange membranes by their limiting current behavior as a function of flow velocity in a small 9 x 10 membrane stack. The Nernst film model was employed along with an analogy between heat and mass transfer for a flat plate for comparison of experiment and theory. Results for anion exchange membranes were in good agreement with theory whereas cation exchange membranes exhibited limiting currents well beyond those theoretically expected.^ Theoretical quantum mechanical calculations indicate that stronger ion-solvent interactions exist for ions present at cation exchange membrane-solution interfaces ($-$SO$\sb3\sp-$ affixed to the membrane and Na$\sp+$ moving from solution into membrane) relative to anion exchange membrane-solution interfaces ($-$N(CH$\sb3$)$\sb3\sp+$ affixed to the membrane and Cl$\sp-$ moving from solution into membrane).^ Spectroscopic experiments in the 1300-1800 nm region of the near infrared suggest that interactions having a significantly greater effect on water structure are present in the case of concentrated (2N) solutions containing a cation exchange membrane solution analogue (p-toluenesulfonic acid, sodium salt) relative to similar solutions containing an anion exchange analogue (benzyl-trimethylammonium chloride).^ In the final analysis, modifications to solvent (water) structure are believed due to the nature of ion hydration in the vicinity of the cation exchange membrane-solution interface. Along with the presence of high electric fields induced by polarization in the interface region that increase exchange rates of primary hydration water with bulk water due to accelerated dipole reorientation (75), the combined effects may enhance cation mobility/diffusivity under these conditions. The result then being a suppression of water ionization at cation exchange membranes due to the enhanced flux, versus absence of a similar effect at anion exchange membranes. ^
Kenneth James Irwin,
"A study of concentration polarization and solution-membrane interactions in the electrodialytic desalination process"
Dissertations and Master's Theses (Campus Access).