Titania membranes for liquid phase separation: effect of surface charge on flux

Titania membranes were prepared by sol-gel processes. Molecular weight cut-offs (MWCO) were successfully controlled in the range from 500 to 1000 with pure water permeabilities of 0.6-1.5 x 10(-11) m(3) m(-2) s(-1) Pa-1. Nanofiltration experiments were carried out for various types of electrolytes (NaCl, Na2SO4, MgCl2 MgSO4) using several membranes having different MWCOs (400, 500, > 1000). Rejection showed the minimum values near the isoelectric point (IEP) which were determined by streaming potential measurements. Larger rejections were obtained for the case where electrolytes having divalent co-ions were nanofiltrated, while low rejection were observed for the case of electrolytes having divalent counterions. On the other hand, permeate volume fluxes were maximized near IEP and the fluxes were almost the same irrespective of types of electrolytes. However, permeate volume fluxes decreased at pH higher than IEP. The dependency was pronounced for the case of divalent counterions and smaller pore diameters, probably because of larger hydrodynamic resistance by ionically-adsorbed counter ions. (C) 2001 Elsevier Science BN. All rights reserved.