Role of electrolyte type on TiO2–ZrO2 nanofiltration membranes performances

Separation efficiency of ceramic NF membranes manufactured by Tami Industries (Nyons, France) is investigated for the separation of aqueous solutions containing strong inorganic electrolytes.The membrane used is multi-layer structured with a mixed titania and zirconia selective layer (1000 Da MWCO) supported on α-Al2O3, TiO2 and ZrO2. Experimentation was performed testing the membrane with binary aqueous solutions containing NaCl, Na2SO4 or CaCl2 in a wide range of compositions, at two pH values kept in the feed side, at various recirculation rates. pH and electrolyte concentration effects were studied both on salt rejection and on total flux. Total transmembrane flux was measured as a function of time until steady conditions were approached. No sensible time effect was observed in the salt rejection.The effect of the electrolyte type and the role of concentration polarisation on membrane permeability were investigated. Although the role of concentration polarisation phenomena in the feed side is not negligible for all the cases investigated, the role of the electrolyte type is dominant in determining a flux decline as a function of time. The membrane requires a long-time stabilisation to the electrolyte solution (40–60 h), which leads to a decrease of the membrane permeability with respect to the values measured with pure water and washed membrane. The reduction of membrane permeability is essentially related to specific interactions between dissolved ions and the amphoteric membrane material. It mainly depends on pH conditions: at alkaline pH, salts with common cation (NaCl and Na2SO4) approach similar asymptotic values, whereas CaCl2 solutions give lower permeabilities; on the contrary, at acidic pH all the salts investigated approach the same asymptotic values.