Chemical aspects of coagulation using aluminum salts—I. Hydrolytic reactions of alum and polyaluminum chloride

This is the first of a two-part series of papers investigating the chemistry of Al coagulants. This paper examines hydrolysis reactions of alum and polyaluminum chloride (PAC1). Part II of the series addresses the coagulation of fulvic acid by these coagulants.

Monomeric, polymeric and precipitated Al were identified based on a timed spectrophotometric analysis. At typical A1 doses used in water treatment, alum showed no evidence of polymer formation. PAC1 consists of preformed polymers which are stable upon dilution below pH 6 and over the time frames encountered in water treatment. Solubility studies showed that alum and PAC1 precipitate to form different solid phases. Alum precipitates are adequately described by amorphous A1(OH)₃(s) solubility. The polymeric structure of PAC1 is retained upon precipitation yielding a solid phase with different light scattering characteristics, electrophoretic mobility and solubility than alum floc. A1 hydrolysis is interpreted as a coordination reaction between A1 and OH⁻. Effects of low temperature on alum are shown to be a result of changes in OH⁻ caused, in part, by the temperature dependence of the ion product of water. Hydrolysis products in PAC1 are preformed and therefore less sensitive to in situ hydrolysis than alum. Results suggest that when using alum, some of the adverse effects of low temperatures may be mitigated by an increase in pH, thereby maintaining a constant concentration of the complexing ligand, OH⁻.