Surface properties and aggregation of basic aluminium chloride hydrolysis products

The influence of major variables (working solution concentration, salt dosage, pH, ionic strength of the solution) on the electrokinetic potential, degree of hydration and aggregation of products of hydrolysis of basic aluminium chlorides (BAC) during water purification has been studied. The degree of hydration characterised by the spin-lattice relaxation time of the protons of adsorbed water increases with increasing basicity ([OH]/[Al] ratio) of coagulants, pH value and with decreasing the dose of salts and the concentration of their working solutions; this is reflected in the change of the xi-potential of particles. It is shown that an increase of water pH and alkalinity, as well as decrease of BAC basicity lead to the formation of primary particles of a larger diameter (3.1-8.7 nm) whose surface area sharply reduces in the first minutes of aggregate formation. The latter is explained by different size-to-density properties (measured by laser diagnostic method) when varying the conditions of their formation. Aggregation of BAC hydrolysis product particles occurs in a discrete way; aggregates of higher orders are formed not from primary particles but from the first-order aggregates. An increase in basicity of BAC specimens leads to formation of more polydisperse and larger aggregates. The role of electrostatic and hydration factors in the mechanism of aggregation of ABC hydrolysis products is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.