Ion Separation by Charged Membranes in Reverse Osmosis.

Tsuru Toshinori

doi:10.5360/membrane.20.423

The present paper reviews ion separation by charged reverse osmosis and nanofiltration membranes on the basis of the author's research work. The separation mechanism of charged reverse osmosis and nanofiltration membranes is based mainly considered to be on both electrostatic (charged) and sieve effects. First, ion separation ability by charge effect was investigated in the system where sieve effect is negligible. In single electrolyte solutions, electrolytes having divalent coions were rejected more than other types of electrolytes such as mono-monovalent electrolytes. In mixed electrolytes having common counterions, divalent coions were rejected much more than monovalent coions therefore, the separation of coions by charged membranes in reverse osmosis was found to be possible. On the other hand, the separation of counterions was not satisfactory based on the valence types. The experimental results were well explained by using the extended Nernst-Planck equation which considers the contribution of volume flow to the ion transport through charged membranes. Secondly, bipolar membranes were proposed in order to improve ion selectivity based on the valence types of ions. Positively charged layer was formed on a negatively charged layer. Experimental and theoretical investigation revealed that the separation of mono- and divalent ions was achieved by bipolar membranes in reverse osmosis. The bipolar membrane was also found experimentally and theoretically to be successfully applied to separate ions of sea water. Finally, a new model is proposed by considering both charge and sieve effects. Ion distribution, the diffusion and the coupling with volume flow are restricted due to steric-hindrance effect (sieve effect) in the system where the sieve effect cannot be negligible. The electrostatic and steric-hindrance model was formulated by modifying the extended Nernst-Planck equation, and supported by permeation experiments of organic electrolytes with a supporting electrolyte salts (sodium chloride.)

Ion Separation by Charged Membranes in Reverse Osmosis.

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