Sodium chloride rejection by a UF ceramic membrane in relation to its surface electrical properties

The rejection of salt (NaCl) by a ceramic (titanium dioxide–alumina layered) ultrafiltration membrane having a nominal pore size of 5 nm and being operated in the cross flow mode is investigated. Measurements were undertaken using different concentrations of salt over range of pH at relatively low transmembrane pressure (TMP). The rejection was assessed by determining the concentrations of sodium and chloride ions in the permeate and retentate and comparing them with the feed concentration. Rejections in the range 15–40% were found and at the same pH lower rejections are found at higher salt concentrations. Increased concentration reduces the rejection by up to 15%. Overall the rejection is seen to be a mainly function of pH which causes the rejection to vary by up to 50%. The observed rejection characteristics were compared with both filtration potential and ζ-potential data over the same range of pH and salt concentration. The filtration potential data was obtained in situ whilst the ζ-potentials were determined from electrophoretic measurements ground membrane material. The membranes are positively charged at low pH and negatively charged at high pH, the isoelectric points being pH 3.1 and 3.8 from electrophoretic and filtration potential measurements, respectively, the difference being attributed to differences in surface characteristics. The minimum rejection occurs at around pH 4 which corresponds to the isoelectric point (i.e.p.) found using filtration potentials showing that the electrostatic interactions between ions and the membrane surface are an important factor in salt rejection.