The Ultrafiltration of Very Dilute Colloidal Gold Suspensions

The ultrafiltration behaviour of very dilute colloidal suspensions has been investigated in terms of transmembrane pressure and pH in a batch cell with and without stirring using PM30 and SKIP membranes. For small (10 nm) colloidal gold particles the unstirred flux was higher than the stirred. An increase in concentration polarization due to the lack of stirring shields the electrostatic repulsion between particles, resulting in aggregation of particles that are retained at the membrane surface. This loosely packed layer can be responsible for the higher flux in the unstirred condition. For large (50 nm) gold particles, greater flux was achieved in stirred condition due to the decrease in concentration polarization. While the SKIP membranes showed a complete retention for larger colloidal gold particles, for smaller particles the retention was complete after around 1 minute. The PM30 membrane completely retained colloidal gold particles of both sizes. Lower flux with higher flux decline was obtained for the smaller colloidal gold particles compared to the larger ones. The cross section micrographs show that the larger sol forms a less densely packed deposit layer on the membrane surface. Changing the pH of the colloidal gold suspension resulted in a substantial change to the flux and retention. However the level of local ionic concentration at the membrane surface appears to be of utmost importance as it affects the degree of colloidal aggregation and packing of the deposit layer, thus influencing flux and retention. Analysis of the filtration data coupled with electron microscopy showed that cake filtration was the dominant mechanism during the course of ultrafiltration as well as microfiltration of very dilute colloidal gold suspensions.