On the use of dead-end ultrafiltration for the study of the influence of sulfate and/or silicate on particle size distribution in concentrated and partially prehydrolyzed Al(III) flocculants

Abstract

Because of their great tendency towards hydrolysis, concentrated and partially prehydrolyzed Al(III) flocculants cannot be diluted as required by many physico-chemical characterization methods. In this work, the complete dead-end ultrafiltration has proven to be a useful characterization method for these products. The results presented have shown that the behavior of the membrane/solution interphase could be described by general ultrafiltration (UF) theories. In the first part of this work, the most discriminating UF method (serial UF) is chosen for the characterization of concentrated flocculants. Next, prehydrolyzed flocculants with an increasing silicate and/or sulfate content were synthesized and the influence of silicate and/or sulfate on particle size distribution was resolved by serial UF. The silicate content is correlated to the size increase of the colloidal particles in such flocculants. This is explained by the reduced effectiveness of fragmentation of aluminosilicate particles formed during flocculant synthesis as the silicate content increases. Sulfate also increases the mean size of the particles, but the mean size is not correlated to the sulfate content. The role of sulfate is apparently to prevent the dissolution of the Al hydroxide particles formed in the initial step of flocculant synthesis. UF in presence of silicate and sulfate shows that, in such cases, sulfate could also limit the aluminosilicate fragmentation leading to a greater increase of the particle size than that observed with only silicate addition.