Abstract
In separation of organic solutes from aqueous solution by reverse osmosis, a solute which is strongly sorbed by the cellulose acetate membrane exhibits poor reverse osmosis rejection. Therefore, the separation is determined primarily by the distribution of a solute between the membrane and the aqueous solution, or the quantity defined as distribution coefficient. In the present study, the regular solution theory was applied to the treatment of this distribution under the condition that the membrane is an immiscible organic phase. From this approach, the following expression was derived.
RTln(1-R')=vi{(δw2-δm2)-2δi(δw-δm)}+lnvw/vm
where R' is solute rejection; vi, molar volume of solute; δ, solubility parameter; vw, molar volume of water; vm, molar volume of gel membrane.
The suffixes i, w and m denote solute, aqueous phase and membrane, respectively. In this treatment, no specific mechanism was assumed, and the standard chemical potentials were chosen for the pure solutes in both phases. For the dissociated solute, the dissociation constant Ka was incorporated in the treatment of distribution coefficient.
As the result, in the case of high Ka, the relation of log(1-R')∝pKa was expected to hold. The order of solute separation corresponds to the order of solute dissociation in the feed solution. The relations described above were confirmed experimentally.
