Comparison of transport of alkali halides and nitrates across asymmetric cellulose acetate and polyamide, composite poly(etherurea), and cation exchange membranes

Using typical synthetic membranes (asymmetric CA and polyamide, composite poly (ether—urea), strong cation exchange membrane), hydrodynamic permeabilties, lp, normalized salt permeabilities, Ps/d, and asymptotic rejections, r∞, for alkali chlorides and nitrates as well as for sodium halides were obtained from hyperfiltration experiments with 0.01 m and partially with 0.2 m brine solutions. Using the measured transport parameters, partition coefficients, Ks, and diffusion coefficients, Dsm, within the membranes were estimated. In general, the partition coefficients of the asymmetric CA membranes for the alkali chlorides increase in the following order CsCl s/d (d = effective membrane thickness), of sodium salts increase in the order Cl− 3− − − for all membranes except the CK—1 membrane which exhibits a reversed sequence. Moreover, varying the cation or anion, respectively, the normalized salt permeabilities of the CK—1 membrane vary according to Walden's rule Λ·η = const.·T (Λ = equivalent conductivity, η = viscosity, T = absolute temperature). It is tried to correlate the variations of the cation and anion partition coefficients, Ks = Kco = 1 — r∞, as well as of the normalized permeabilities, Ps/d, with the Gibbs free energy of hydration of the alkali ions, Δ Ghy