Salt retention in nanofiltration with multilayer ceramic TiO2 membranes

This paper reports the characterization of a multilayer TiO2 membrane, consisting of two mesoporous anatase interlayers and a microporous anatase top-layer, prepared by a sol–gel procedure. A molecular weight cut-off (MWCO) in the range 500–600 and a permeability of 20 l h−1 m−2 bar−1 was demonstrated. In order to provide information about the membrane charge, a detailed study was made of zeta potential and salt retention. Five types of salts were considered: NaCl, KCl, LiCl, Na2SO4, CaCl2. Experiments with mono-monovalent salts revealed the amphoteric character of the membrane material with an IEP around pH 6. The membrane showed a minimal salt retention at pH 6 and a fairly high retention at alkaline pH (R(NaCl) = 85%, R(KCl) = 87%, R(LiCl) = 90%). For salts containing divalent ions, zeta potential measurements showed the effect of selective SO42− or Ca2+ adsorption on the membrane charge. In retention experiments with these salts, the correlation between experimentally determined zeta potential and retention behaviour is demonstrated. Depending on the pH of the salt solution and the acquired membrane charge, high retentions were again achieved (R(Na2SO4) > 95%; R(CaCl2) = 78%).