Donnan dialysis with anion-exchange membranes as a pretreatment step before electrodialytic desalination

Donnan dialysis is used for the removal of harmful anions (fluorides, nitrates) from drinking water. It can also be applied to the recovery of valuable metals (chromium, aluminium, titanium) from industrial wastewater streams. We have evaluated the applicability of Donnan dialysis with an anion-exchange membrane (AEM) as a pretreatment process before electrodialytic desalination. We have examined the rate and the efficiency of anions removal (NO3-, SO42-, HCO3-) from a multi-component solution with three types of AEMs: Selemion AMV, Neosepta AMX and Neosepta AFN. The best effects of anion removal were obtained with the Neosepta AFN membrane: flux of the removed anions was about two times higher than that with other membranes, and the efficiency of bicarbonates and sulphates removal equaled, respectively, 78% and 98%. Then, we determined limiting current densities for the raw multi-component solution and for the same solution pretreated with Donnan dialysis. A significant increase in the limiting current density for the solution after preliminary anion exchange was observed. Finally, we compared the efficiency of electrodialytic desalination of the raw solution and the solution after Donnan dialysis with AEM. We observed that as a result of the exchange of sulphates and bicarbonates for chlorides, average salt flux during electrodialysis was higher - up to 21% - than that obtained for the raw solution. Moreover, electrodialysis of the solution after preliminary anion exchange required half the energy for the removal of the same amount of salt.