Ultrafiltration of Colloidal Silver Particles: Flux, Rejection, and Fouling

The ultrafiltration and microfiltration of dilute fine colloidal silver particles have been studied for a range of membranes in a batch cell with and without stirring. Unstirred conditions gave greater fluxes (with lower rejection for permeable membranes) than stirred. This is attributed to the different degrees of local concentration of silver particles at the membrane surface, and hence different levels of aggregation in the retained layer. Analysis of unstirred filtration results by the constant pressure filtration theory coupled with transmission electron micrographs indicates that the filtration involves a two-step process; internal deposition, where pores are blocked or filled by particles, followed by cake accumulation on the membrane surface. The extent of each stage depended on the "retentiveness" of the membrane. Deposit resistance increased with filtrate volume and was greatest for partially permeable membranes that allowed significant internal deposition. Resistance was lower for the retentive "cake-forming" membrane and for the most open (low retention) membrane. Flux and rejection varied dramatically following salt addition when a partially permeable membrane was used. In the absence of salt most of the solids were retained as a cake due to membrane/particle interactions. For low salt concentrations (0.001 and 0.01 M NaCl) particle penetration occurred. At higher concentration (0.1 M NaCl) the particles aggregated again, forming a surface cake.