Salt effect on the performances of TiO2-ZrO2 NF membranes

Nanofiltration of aqueous solutions through inorganic membranes is a promising technique for all the applications requiring high stability and harsh washing procedures. Aim of this work is to investigate the separation performances of ceramic NF membranes manufactured by Tami Industry (Nyons, France) with aqueous solutions containing strong inorganic electrolytes. The membrane material exhibits a typical amphoteric behaviour: the point of zero charge was measured at a pH value close to 6 with 1 mol/m3 KCl-water . A wide experimental investigation was performed testing the membrane with binary aqueous solutions containing NaCl, Na2SO4 or CaCl2 in a wide range of compositions and pH values kept in the feed side, at various recirculation rates. PH and electrolyte concentration effects were studied both on salt rejection and on proton rejection. Total transmembrane flux was measured as a function of time until steady conditions were approached. No variation of salt rejection was measured as a function of time. However, the effect of the electrolyte type on the membrane permeability is also relevant and pH dependent. At pH = 7.6, at which the membrane is supposed to be negatively charged, the salts with common cation approach similar asymptotic values, whereas CaCl2 solutions give lower permeabilities. On the contrary, at pH = 4.6, at which the membrane is supposed to be positively charged, the influence of the salt is quite negligible. That behaviour can be ascribed to a stronger calcium adsorption on the negatively charged sites of the membrane. Correspondingly, also a rejection increase with the salt concentration is obtained.