Reverse osmosis transport phenomena in the presence of strong solute-membrane affinity

The reverse osmosis performance of cellulose acetate membranes has been examined and analyzed for several aqueous systems where there is a strong attraction between the organic solute and the membrane material.

The systems investigated included the aromatic hydrocarbons benzene, toluene, and cumene in single-solute aqueous solutions. Six cellulose acetate membranes, modified by annealing at different temperatures, were studied.

Experiments were performed at four pressures (690, 1725, 3450, and 6900 kPa) and at several concentrations (in the range 5 to 260 ppm). The results were found to be markedly different than those observed in the absence of strong solute-membrane affinity. In particular, the solute-water separation decreased rather than increased with increasing pressure and the flux decreased with increasing concentration even though low concentrations, with low osmotic pressures, were studied. Qualitatively, the behavior was explained in terms of a porous membrane mechanism with both solute-membrane affinity and solute mobility varying as a function of solute position with respect to the membrane. The observed reduction in flux was expressed by an empirical equation as a function of concentration of solute in the boundary layer.

The experimental results were analyzed quantitatively by several transport models. The irreversible thermodynamics phenomenological transport, solution-diffusion imperfection and extended solution-diffusion relationships generated parameters that were inconsistent with the original formulations of the models. The irreversible thermodynamics Kedem-Spiegler model, solution diffusion model, Kimura-Sourirajan analysis, and the three parameter finely-porous model were functionally unable to represent the data. Only the four parameter finely-porous model and the surface force-pore flow model were consistent with experimental results. From the finely-porous model the partition coefficient was found to be different on the high and low pressure sides of the membrane and this difference was a function of both pore size and solute. For the surface force-pore flow model, the agreement between the model and data was excellent. However, the surface force-pore flow model was considerably more difficult to use.