Electrolyte retention of supported bi-layered nanofiltration membranes

The electrolyte separation behaviour of a supported bi-layered ceramic membrane is investigated experimentally and the measured ion retentions are compared with the predictions of a site-binding transport model with no adjustable parameters. Due to the difference in iso-electric point between its two separating layers, the bi-layered system is expected to perform better over a large pH range compared with a membrane with only one type of selective layer. The separating layers in the membrane are a microporous silica and a mesoporous γ-alumina (pore sizes of 0.8 and 2 nm, respectively) and their retention is studied for a binary electrolyte solution of NaCl at 1 mol/m3 for pH values between 4 and 10. Because of its smaller pores and high charge, the silica layer mainly determines the membrane retention at neutral and alkaline pH, while the γ-alumina layer has a significant impact on the NaCl retention at 4 < pH < 5. The model predictions are in good agreement with the experimental data for Na+ at 4 < pH < 9 and for Cl− at the whole pH range. For a pH of 4, the predicted chloride retention is lower than the sodium retention while the experimental data show the opposite effect.