Transport characteristics of neutralization dialysis and desalination of tap water

In neutralization dialysis, the salt solution is desalinated continuously. The cations in desalination compartment are exchanged with protons in the acid compartment across a cation-exchange membrane and the anions are exchanged with hydroxide ions in the base compartment across an anion-exchange membrane. A simple index (C/J), where C is the source phase concentration (mol cm^-3) and J is the flux (mol cm^-2 min^-1), was proposed for the estimation of the desalination properties in neutralization dialysis. The index was based on the concepts of the ion-exchange reaction rate at the membrane interface and was related to the permeation constant in the source-phase boundary layer at the membrane interface, P_a (cm min^-1), as C/J = 1/(k₁·C_ex) + 1/Pa, where k₁ is the rate constant of ion-exchange reaction (mol^-1 cm⁴ min^-1) and C_ex is the ion exchange capacity of the membrane (mol cm^-3). The desalination efficiency increased with the decrease of the desalination compartment thickness and the decrease of the linear velocity, which can be estimated well by using the index. Tap water was desalinated by neutralization dialysis and pure water was obtained at the outlet of the desalination cell.