Salt rejection in nanofiltration for single and binary salt mixtures in view of sulphate removal

Three commercial membranes (NF70, NF90 and TFC-SR) were firstly characterized in terms of pure water flux and the rejection of uncharged (alcohols and sugars) compounds. Subsequently, the rejection of monovalent (sodium and chloride) and divalent (calcium and sulphate) ions in single (NaCl, CaCl2 and Na2SO4) and binary (NaCl/Na2SO4, CaCl2/CaSO4, NaCl/CaCl2 and Na2SO4/CaSO4) salt mixtures was studied. According to the pure water permeability the TFC-SR membrane is a loosely packed NF membrane (12.3 L.m!2.h!1.bar!1), while both NF70 and NF90 are tightly packed (2.6 and 3.6 L.m!2.h!1.bar!1). According to the uncharged solute rejection, the MWCONF70 = 60, MWCONF90= 200 and MWCOTFC-SR > 500. NF70 and NF90 were equally efficient in rejecting 1-2, 1-1 and 2-1 salts (>90%), while TFC-SR showed typical negatively charged surface behaviour, i.e., R (1-2) salt > R (1-1) salt > R (2-1). Sulphate rejection decreased in the presence of sodium chloride more significantly than in the presence of calcium chloride due to the more efficient retention of the bivalent calcium.

It was shown that, although all three membranes tested were able to reject sulphate, the amount of sulphate rejection is highly dependent on the counter ion and other salts in the solution. The permeability of the membranes ranged from 2.6 (NF70) to 16.5 (TFC-SR) L.m!2.h!1.bar!1. According to the uncharged rejection, NF70 and NF90 have MWCO of 60 and 200, respectively, while the TFC-SR membrane’s MWCO o500. For the single-salt experiments all membranes effectively rejected sulphate (R > 92%). While both NF70 and NF90 rejected NaCl and CaCl2 equally well, TFC-SR rejection clearly followed the pattern R (1-2) salt > R (1-1) salt > R (2-1) salt, which is indicative of a negatively charged membrane. The solvent permeability in the presence of charged solutes remained approximately the same as the pure water permeability. From the binary salt studies it was shown that sulphate rejection decreased in the presence of sodium chloride, but not in the presence of calcium chloride. The higher retention of the divalent calcium thus ensured the higher retention of the sulphate in an attempt to ensure electron neutrality. Chloride retention decreased in the presence of sulphate especially sodium sulphate. Since sodium retention is low, the chloride has to permeate with the sodium while the sulphate is retained. In terms of the cations (sodium and calcium), it was shown that sulphate improved their retention.