An iron-modified silica nanofiltration membrane: Effect of solution composition on salt rejection

Electrolyte rejection measurements with single salt solutions of NaCl, Na2SO4, MgCl2, and MgSO4 were performed to determine the effects of solution composition on the separation performance of an iron-modified silica nanofiltration ceramic membrane. The unsupported membrane material was characterized with respect to pore structure and zeta potential, whereas the supported membrane was utilized to evaluate gas diffusion properties, water flux and salt rejection. Charge exclusion mechanisms were determined to be the basic mode of salt rejection, and a qualitative relationship was constructed to relate salt rejection to the zeta potential of the membrane. The results obtained agree with the Donnan exclusion principle of electrically charged membranes. The data shows that the membrane zeta potential is not constant but depends extensively on the type of electrolyte in solution. For example, there is a dramatic drop in zeta potential when the membrane is exposed to magnesium salts as opposed to sodium salts. This drop in potential at the membrane surface leads to little rejection of magnesium salts. Results also show a charge reversal of the membrane zeta potential at high pH values due to specific adsorption of magnesium.