The effect of shear rate on controlling the concentration polarization and membrane fouling

Membranes exhibiting separation properties similar to those of conventional UF membranes can be dynamically prepared on a porous substrate by in-situ deposition of particles from a circulating colloidal suspension of certain hydrous oxides, notably Zr hydroxide. The objective of this work is to elucidate parameters affecting Zr hydroxide colloidal properties in order to determine optimal conditions for dynamic membrane (DM) formation on substrates of different porosity. The main parameters studied were the effects of pH, Zr concentration and ionic strength on colloidal properties. The main results were as follows. Over the pH range from 3 to 4, a stable mono-disperse colloidal dispersion was obtained, composed of sub-micron particles. The particle size increased systematically with the pH level of the suspension. At a pH value of near 4, a bi-modal distribution was obtained, composed of both sub-micron particles and very large agglomerates. At pH values higher than about 4.5, large agglomerates formed instantly. Particle agglomeration was also affected by the ionic strength of the solution and the procedure of the colloid preparation. The above results find full support in the flow and separation properties displayed by DM prepared with Zr suspensions of different chemistries (notably the pH level) on two types of substrate: a porous fabric having a pore size of tens of microns and a microfiltration membrane having a pore size of a few microns.