Measuring zeta potential of ceramic TiO2 NF membrane for different salts by filtration potential technique using innovated silver electrodes

Nanofiltration (NF) and zeta potential are being increasingly used in water, wastewater, and pharmaceutical applications. In this study, the zeta potential behaviors of eight different 0.01 M salts (NaCl, KCl, NaHCO3, MgCl2, CaCl2, Na2CO3, Na2SO4, and MgSO4) for 0.9 nm tubular ceramic titanium dioxide NF membrane were measured for the first time using a filtration potential technique by conducting in-situ two electrodes made from a composite material that consisted of pure silver and 4% gauge 21 gold. The measurements were conducted under pH 3-9 at an applied transmembrane pressure (TMP) of 0.25-1.5 bar. Experimental results showed that the membranes of the salts were negatively charged at neutral pH and had an isoelectric point (IEP) of pH 3.4-3.8. Membrane zeta potential results were compared and justified using the electrophoresis method for the same membrane. The powder dispersions of the pulverized membrane in this measuring technique were a suspension at a certain salt concentration. Experimental results from the electrophoresis method agreed with those of the filtration potential method, whose IEP was at pH 3.3-3.5. The rejection experiments of the reference solutions (0.1 and 0.01 M NaCl) were conducted at a constant applied TMP of 12 bar. Rejections results showed that both NaCl concentrations were a function of pH. The low rejections were 17% and 21% at pH 3.8 for 0.01 and 0.001 M NaCl, respectively, and the high rejections were 34.2% and 38% at pH 9. This work also investigated the effects of the measured membrane zeta potential on the ion rejection of the standard solution of NaCl (0.01 and 0.001 M) using a similar pH range.