Effect of polarisation layers on salt permeability across membranes with different structures

Abstract

The influence of concentration-polarisation on salt transport across three membranes with different structures, an ultrafiltration (PPS), an asymmetric nanofiltration (PES-10) and a symmetric for dialysis (C-5), was studied. Salt diffusion measurements were carried out in a dead-end cell, at different stirring rates of the solutions at both sides of the membrane (0 ≤ n (rpm) ≤ 1100) under the same NaCl concentration gradient (Δ C ext = 5×10 −2 M). Salt permeability for the membrane system ( P s ) at each stirring rate, which includes both the membrane itself and the polarisation layers, was determined by the fitting of the corresponding concentration vs. time curves. The permeability of the solute in the membrane ( P s m ) and the thickness of the solution layers at a given stirring celerity, d, were estimated by the fit of P s as a function of the solution stirring rate. Differences found between P s and P s m for the studied samples clearly show the different influence of concentration-polarisation layers on salt diffusion depending on the membrane structure. In fact, salt permeability through the C-5 membrane was at least one order of magnitude higher than for the two other samples, while P s m is around 2.5 times higher for the ultrafiltration membrane than for the nanofiltration one. A comparison of the thickness of polarisation layers at a given stirring celerity shows the following values: d(C-5)=25 μm, d(PPS) = 1 mm and (PES-10) = 6 mm. On the other hand, a comparison of the membrane permeability obtained for the cellophane membrane in the dead-end cell (3×10 −6 m/s) and that obtained in a tangential flow cell (5×10 −6 m/s), shows rather good agreement, which can be considered as a proof of validity of the results.