Duan, Jimming; (1997) Influence of dissolved silica on flocculation of clay suspensions with hydrolysing metal salts. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Silicic acid or dissolved silica exists abundantly in natural waters. Hydrolysing metal salts such as aluminium sulphate and ferric chloride are widely used coagulants in the water industry. Interactions between the metal ions, such as Al3+ and Fe3+, and silicic acid in aqueous solution have been reported over wide concentration ranges. At typical concentrations in water, the presence of dissolved silica could exert some effects on flocculation of particles by these hydrolysing metal salt coagulants. Using a standard flocculation set-up equipped with a Photometric Dispersion Analyser (PDA), coupled with Electrophoretic Mobility (EM) measurements of the suspended particles, the influence of dissolved silica on the flocculation of kaolin suspension has been investigated. The precipitation of aluminium has also been examined with and without the presence of dissolved silica in order to explain the observed effect. The results showed that the presence of dissolved silica could either improve or prevent the flocculation depending on silica concentration, solution pH and coagulant concentration. These effects can be attributed to the interactions between aluminium and dissolved silica, which alters electric charge of dissolved aluminium hydrolysis species, modifies the colloidal properties of the precipitate and hence affects the aggregation of the primary precipitate particles. Therefore, at low coagulant dosage, the influence of dissolved silica will be characterised by the charge modification of aluminium hydrolysis products; while at high coagulant dosage, the influence will depend on the precipitation. When the interactions promote the aggregation of primary precipitate the flocculation will be improved; while the interactions prevent such precipitation process the flocculation will be prevented. It has been found that the effect of dissolved silica is much more sensitive at high pH (e.g., 8), which corresponds the isoelectric point of aluminium hydroxide. At this pH, flocculation can be readily prevented at low levels of dissolved silica (e.g., 10-20 mg/L). The effect of common ions, such as calcium, on aggregation of kaolin with aluminium salts in presence of dissolved silica is also examined.

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