Transport Behaviour of Asymmetric Cellulose Acetate Membranes in Dialysis and Hyperfiltration

The transport of salt solutions across asymmetric cellulose acetate membranes is discussed using the linear relationships of the thermodynamics of irreversible processes. The corresponding transport coefficients such as the mechanical permeability; 1p, the osmotic permeability, 1π, and the reflection coefficient, σ, are determined by dialysis experiments for different salt solutions. The experimental results manifest a strong dependence of 1p and 1π on the salt concentration. This is shown to be due to concentration polarization within the porous sublayer of the asymmetric membrane. The dependence of the mechanical permeability, 1p, on concentration is then estimated using a Nernst-Planck equation for the salt transport within the porous sublayer of the asymmetric membrane. The influence of the concentration profile within the porous sublayer, caused by concentration polarization effects, on measured membrane potentials is also discussed. Furthermore, a relationship between salt rejection, r, measured in hyperfiltration and the three transport coefficients, the salt concentration of the brine, as well as the pressure difference, ΔP, across the membrane is derived and compared to experimental findings