Selectivity of nanofiltration membranes: Effect of pH, nature and concentration of solutions.

Selectivity mechanisms of 2 nanofiltration (NF) membranes were studied with sodium and calcium salt solutions. In spite of similar ion exchange capacity (7.3 10(-3) mol m(-2)) and apparent isoelectric points (4.2 +/- 0.2), both membranes showed differences in selectivity originating from a significant proportion (10% for membrane B) of A-NH2 groups. pH, nature and concentration of solutions strongly affect the NF membrane selectivity. For pH higher than 5, anions of the solution, ie co-ions of the membrane, govern the global salt retention because of the total dissociation of R-COO- membrane groups. The higher their ionic mobility U and the counter-ions valency, the lower their retention: R(Cl-) = 60% with NaCl and 40% with CaCl2 (membrane A). For pH between 4 and 5, net charge or NF membrane is close to zero, ion transmission is based on steric exclusion. For pH lower than 4, divalent ions show high retention, R(Ca2+) greater than or equal to 90%, inducing a negative rate of retention of monovalent co-ions in order to balance the so-created disequilibrium in ionic charge between membrane phase and solution. The increase in solution concentration lowers retention of all monovalent ions because of the shielding of membrane ion exchange groups: R(NaCl) = 85% for a ionic strength l = 1 10(-3) mol l(-1) and 30% for l = 200 10(-3) mol l(-1). The selectivity mechanisms of NF membrane appear to be governed by the Donnan effect except in the pH zone close to the membrane isoelectric point.