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 (NaC1, CaC12 and Na2SO4) and binary (NaC1/Na2SO4, CaClJCaSO4, NaC1/CaC12 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.m2.h1.bar1 ), while both NF70 and NF90 are tightly packed (2.6 and 3.6 L.m-2.h" 1.barI). According to the uncharged solute rejection, the MWCO~70 = 60, MWCO~9o = 200 and MWCOrFc.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 rejectkm 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.