Colloids and Surfaces A: Physicochem. Eng. Aspects 298 (2007) 34–41 The effect of mixing on stability and break-up of aggregates formed from aluminum sulfate hydrolysis products Irina Solomentseva a,∗ , S´ andor B´ ar´ any b , John Gregory c a Institute of Colloid and Water Chemistry, 42 Vernadsky Blvd., 03680 Kiev, Ukraine b Institute of Chemistry, University of Miskolc, 3515 Miskolc-Egyetemvaros, Hungary c Department of Civil and Environmental Engineering, University College London, Gower Street, London WC1E 6BT, UK Available online 15 December 2006 Abstract The structure of aggregates formed from aluminum sulfate hydrolysis product particles (HPP), their stability, break-up and re-growth, under the action of shear (stirring the dispersion at different rates) have been studied. The effect of main variables such as the hydrolyzing salt dose, pH, alkalinity and ionic strength of the water on the growth, breakage and re-growth of HPP aggregates has been investigated. It has been shown that the aluminum sulfate hydrolysis product particles formed in a model water with parameters that facilitate appearance of small particles with relatively high charge and -potential values, low degree of hydration and interaction of particles predominantly in the primary minimum, are stronger and more easily reformed after their breakage by intense stirring, than aggregates built of particles with lower charge, higher initial particle size and hydration, aggregating in the secondary minimum. © 2006 Elsevier B.V. All rights reserved. Keywords: Aluminum sulfate; Hydrolysis products; Aggregates; Stability; Break-up; Re-growth; Shear; Salt dose; pH; Alkalinity; Ionic strength 1. Introduction Aggregation of dispersed particles plays a key role dur- ing phase separation processes, in particular in the course of water conditioning and treatment [1]. Typically, these processes are initiated or enhanced by adding hydrolyzing salts or high- molecular polymeric flocculants to dispersion or water to be treated. Depending on the balance of main variables operating in the system, the size of aggregates formed, their packing density as well as the changes of these parameters during the aggregation process can be varied in a broad interval [2–4]. There is special interest concerning the laws and kinetics of aggregate formation from the aluminum salts hydrolysis prod- uct particles (HPP) widely used in water treatment [4–8]. It has been shown that the HPP aggregation occurs in a step- wise manner and the “discreteness” of the process increases in series: basic aluminum chloride (BAC) > basic aluminum sulfate (BAS) > aluminum sulfate (AS). The course of these processes as well as the size and density of aggregates depend not only on the coagulant nature but also on the system variables like pH, ∗ Corresponding author. E-mail address: irinasol@i.com.ua (I. Solomentseva). ionic strength and alkalinity. By changing these parameters, the colloid-chemical properties of the hydrolysis product primary particles, the size and density of aggregates formed and their strength, can be regulated. Recently several papers have been published on the problem of breakage and re-growth of aggregates formed under the action of coagulants or polymeric flocculants [9–13]. Different ideas about the mechanisms of these processes have been proposed but without satisfactory experimental confirmation. Mixing conditions can have a very significant effect on the performance of coagulants and flocculants. The first require- ment is for the additive to be distributed uniformly throughout the suspension and this should be achieved by some form of rapid mixing. The particles then need to collide in order to form aggregates and this process can be assisted by some form of agitation. With hydrolyzing metal salts, the formation of hydrolysis products occurs very rapidly and competing processes, such as adsorption and precipitation, could depend on mixing conditions [1]. After coagulant dosing and mixing, flocs grow initially at a rate that is determined mainly by the applied shear, the parti- cle concentration and the collision efficiency. As flocs become larger, further growth is restricted by the applied shear for essen- tially two reasons. Existing flocs may be broken as a result of 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.12.016