Ultrafiltration of very dilute colloidal mixtures

The ultrafiltration behaviour of very dilute (ca. 10 ppm) mixtures of gold (50 nm) and latex (1 mu m) colloidal suspensions has been investigated in terms of transmembrane pressure and pH in a batch cell with and without stirring. The hydrophobic PM30 and the hydrophilic XM300 membranes were used as ultrafilters. Both membranes gave higher flux and lower retention in the absence of stirring and increasing Delta P. For the PM30 membrane, increasing pH caused higher flux. For both types of membrane, a lower pressure produced a slower flux decline, lower solute resistance and higher retention. The PM30 membranes showed better performance (complete retention, slower flux decline and lower solute resistance) than the XM300 membranes due to the differences in membrane pore size and material. Pore blocking is more likely in the case of the XM300 membrane with its larger molecular weight cut-off. The dominant mechanism was cake formation after initial pore plugging. The estimated cake specific resistance showed a small increase with pressure. The cake porosity was smaller than the reported porosity for the random packing of a binary mixture due to the effect of applied pressure. These studies have shown that membrane filtration of dilute suspensions is a complex process. Clearly, the deposition on and within the membrane, and within the cake will be dependent on a number of parameters, including membrane type, relative particle size, species concentrations, particle charges (dependent on pH and ionic environment) and flux (dependent on pressure). The performance also varies with time. (C) 1998 Elsevier Science B.V.