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, CaCl, and Na2SO4) and binary (NaCI/Na2,SO4 CaCl2/CaSO4, NaCI/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 Lm−2.h−1.bar-). 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 (11) 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.