Influence of soluble silica on coagulation by aluminium sulphate

Interactions between aluminium species and dissolved silica in aqueous solution have been shown to play a large part in the coagulation of suspensions by aluminium salts. These effects are quite distinct from those associated with polymeric or colloidal silica. At a fixed aluminium concentration, coagulation may be enhanced by low levels of dissolved silica, especially at pH values around 6. At higher pH values, a strong inhibition of coagulation by aluminium sulphate is found at silica levels of around 10 mg l(-1), which are typical of those found in many natural waters. Electrophoretic mobility measurements of clay particles in the presence of aluminium and dissolved silica show that the latter can modify the charge-neutralising behaviour of hydrolysed aluminium species, providing at least a partial explanation of the observed effects. Experiments have been carried out using a simple continuous monitoring procedure, which allows dynamic aspects of the coagulation process to be studied in a systematic manner. This gives far more information than standard jar test methods, which have been widely used in studies of coagulation by hydrolysing metal salts.