Removal of troublesome anions from water by means of Donnan dialysis

Donnan dialysis is a simple and one of the most economic mebrane processes. The driving force of the process is the chemical potential gradient of components of two solutions separated by amembrane. During Donnan dialysis there is a stechiometric exchange of ions of the same charge (so called counter-ions) through an ion-exchange membrane, and the process ends when Donnan equilibrium is reached. Donnan dialysis can be used in the recovery of valuable metals or their complexes from diluted solutions (e.g. rinse waters after metal plating). Good results were also obtained in the process application for organic acids separation from fermentative solutions. An important field of Donnan dialysis application is undesired ions removal from water. The research has shown that efficient defluoridation and denitrification of drinking water is possible. A new direction for the process is bioreactor with ion-exchange membrane (IEMB), where nitrates removed from water are subject to biological denitrification. This paper presents the results of our experiments on selected anions removal from water. Preliminary removal of some ions, especially 'troublesome' anions (sulphates and bicarbonates), would enable high degree of water desalination by means of electrodialysis. In the experiments we used Selemion (AMV, DMV) and Neosepta (AFN, AMX) anion-exchange membranes (AEMs). We investigated the effects of salt concentration in the receiving solution, and of volume ratio of the feed and the receiver on the efficiency and on the rate of ions removal. The phenomenon of salt leakage from the receiver to the treated water (that is related to co-ions transport through the membrane) was also analysed. For each of four examined membranes high efficiency of anions removal was obtained: 87-98% removal of nitrates, 94-100% removal of sulphates, whereas bicarbonates were removed with the efficiency of 77-99%. We found out that with higher concentrations of NaCl in the receiving solution (300 and 500 mM), equilibrium concentration of the exchanged ion in the feed was lower. However, with the higer ratio of feed and receiver volumes (4:1), the concentration at the point of Donnan equilibrium was set on higher level. The best transport properties for the examined anions has shown the Neosepta AFN membrane. An average flux of nitrates and sulphates was up to two times higher than with the other membranes, and bicarbonate flux-even three times higher. This membrane also performed greater NaCl leakage from the receiving solution, reaching 0.33 mol/m2h, with the receiver concentration of 500 mM NaCl.