Rejection of ions by NF membranes for binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2

Permeation experiments of binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 through three kinds of NF membranes (ESNA 1-LF, ESNA 1 and LES 90) were carried out to investigate the effects of the ionic species and their interactions on the rejections of the NF membranes. The experimental results indicate that the observed rejections of Ca2+ by NF membranes all slightly increased with the growth of the concentration of Na+ and the observed rejection of Na+ decreased with the growth of the concentration of Ca2+. The repulsion force of the membrane negative charges on the cation with lower charge density are weaker, thus it presents higher permeability and lower rejection. The observed rejections of Cl− and NO3− by NF membranes were almost constant and hardly changed with the equivalent fractions of Cl− and NO3− in binary electrolyte solutions. For charged species, the electrostatic mechanisms play a more important role in NF separation.