Colloid Forces between Silica Surfaces in Alum Solutions Relevant for Dinking Water Treatment
Interaction forces acting at nanometre level are the foundation of many sub-microscopic phenomena which determine the dynamic behaviour of a number of the important industrial processes. The ubiquitous presence of colloidal particles in natural water resources makes the study of their collisions and aggregation significant. In this work, colloid forces between pure silica surfaces were measured by atomic force microscopy (AFM). In addition to the force measurements, the surfaces were analysed to determine the electrical surface properties by performing the streaming potential experiments. The macroscopic effect of the interaction forces was studied by observing the coagulation by jar testing. Specifically, an atomic force microscope (MFP3D) from Asylum Research (USA) was used to measure the normal interactions between silica beads and atomically smooth silica substrates in solutions of aluminium sulphate (alum) which is widely used as coagulant in the drinking water treatment. Maintaining the same conditions as in the AFM experiment, an Anton Paar (Austria) Electrokinetic Analyser was used to determine the streaming potential and an Aquagenics microFLOC jar tester was used for the coagulation investigation. The study aimed at better understanding the role of hydrated aluminium species in the coagulation of negatively charged oxide colloids in drinking water treatment at pH = 6.3 which is specific for the industrial water treatment. The coagulant concentrations used in this study were in the range of the industrial process. The alum solutions were prepared from analytical grade Al2(SO4)3·16H2O. It was found that the presence of aluminium sulphate at concentration close to the values typically used in industrial scale water treatment applications generally induced strong, long-range repulsive forces between the studied surfaces. At these alum concentrations streaming potential measurements indicated reversal in the sign of the surface charge and no sign of jar coagulation. A mechanism was proposed to explain the observed coagulation phenomena, based on the deduced microstructure of the adsorbed surface layers.