Characterization of transport across cellulose acetate membranes in the presence of strong solute–membrane interactions

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Abstract
Certain organic solutes, including phenol, undergo anomalous enrichment when hyperfiltered through cellulose acetate membranes: the solute concentration is higher in the permeate than in the feed solution. A number of existing theoretical approaches describing hyperfiltration phenomena are presented and their merits and limitations upon application to the transport of phenol discussed. A new two‐parameter transport relationship is derived based on an extension of the solution–diffusion model. The enrichment, or negative solute rejection by the membrane, is predicted to occur whenever the pressure‐induced solute permeation velocity exceeds that of water. By acknowledging and incorporating the effect of pressure on the chemical potential of the solute, the present extended solution–diffusion model relationship successfully describes hyperfiltration data of phenol in homogeneous and asymmetric cellulose acetate membranes provided the contribution of convective flow to the overall solute transport is insignificant. In addition to the transport parameters of the extended solution–diffusion model, the transport parameters of the phenomenological, Kedem–Spiegler, and combined viscous flow–frictional relationship are evaluated from hyperfiltration data obtained with 0.05 and 0.1 wt % phenol feed solutions and homogeneous cellulose acetate membranes of different acetyl content.

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