Surface properties of ceramic ultrafiltration TiO2 membranes: Effects of surface equilibriums on salt retention Review Article

Commercial ceramic low cut-off (1 kDa) ultrafiltration TiO2 membrane behavior is studied in the presence of single salt solutions (NaCl and MgCl2: 5.10− 2 M). Surface property characterizations of the membrane layer are carried out on TiO2 powder. Zetametry and streaming induced potential experiments show that the isoelectric point of TiO2 remains stable in the presence of NaCl, while it shifts to a higher pH value in the presence of MgCl2. At native pH (close to 7), the adsorption of Mg2+ on TiO2 surface leads to an inversion of the surface charge (negative to positive) and to an important increase of the surface charge density. Filtration tests are carried out on a laboratory pilot using NaCl and MgCl2 5.10− 2 M solutions. Contrary to the sodium chloride solution, retentions of MgCl2 solution show some peculiarities. First, the permeation flux of pure water decreases after each experimental test in an irreversible way. Second, in particular conditions, the adsorption equilibriums are reached after several days, which significantly affect membrane performances. Finally, punctual pH modification (acid or basic) affects the surface charge of the active layer and thus the retentions of magnesium chloride, but the amphoteric behavior of the TiO2 surface tends to bring back the pH to the initial value.