NEGATIVE REJECTION OF ANIONS IN THE LOOSE REVERSE-OSMOSIS SEPARATION OF MONOVALENT AND DIVALENT ION MIXTURES

Single and mixed electrolyte solutions were separated with loose reverse osmosis membranes which had negative charge. For mixed electrolyte solutions having the same counterion as the membrane fixed charge, the rejections of the divalent ions were much larger than those of the monovalent ions, and in some cases were negative (concentrated in the permeate). The negative rejection was dependent on both concentration and mole fraction of the feed solution and on the applied pressure. The extended Nernst-Planck equation was applied to the analysis of the experimental data by considering the effective charge density. Rejections of ions in mixed electrolyte solutions were predicted by using the transport parameters obtained from the single electrolyte experiments. Moreover, the loose reverse osmosis membranes were applied to artificial seawater, and the separation of mono- and divalent anion was found to be effective.