Nanofiltration membranes have been used to study their selectivity properti
es employing different feed solutions such as aqueous solutions of HCl, NaC
l, CaCl2 and Na2SO4 in single and binary mixed systems. A marked change in
the rejection of chloride due to the addition of a second solute, sometimes
approaching negative values, has been observed. The rejection curves at hi
gh and low volume fluxes obtained at various operating pressures have been
generated so as to cover the shifting of positive to negative rejection. Ph
enomenological equations based on the irreversible thermodynamics approach
and Donnan equilibrium theory have been used to derive an expression for ch
loride rejection for a binary mixed system. The applicability of the model
has been tested by comparing the computed and experimental chloride rejecti
on data.