Ion hydration number and electro-osmosis during electrodialysis of mixed salt solution

Abstract

Water transfer is an important aspect to be considered in electrodialysis since it fixes the performances of the process. It is due to electro-osmosis, i.e. the water carried by the migrating species and is thus related to their hydration. Few results were reported about the hydration number of solutes transferring through ion-exchange membranes. In this work, a methodology is proposed to calculate the hydration numbers of ions transferring through ion exchange membranes during electrodialysis. It is based on the experimental measurements of ion and water transfer under different conditions, like salt compositions and current. Salt hydration is first obtained, and then the hydration numbers of 4 transferring ions (Na+, Mg2+, Cl−, SO42−) are calculated simultaneously. It is shown that these hydration numbers are constant, independent from the salt composition and current. The hydration number for monovalent ions is found to be lower than that of divalent ones, which is in agreement with the values of the hydration free energy. Further comparison with the reported values concerning the hydration of the same ions in solution shows that for monovalent ions the hydration numbers are close to those reported for the 1st hydration shell while much higher values are obtained for divalent ions.