The aggregation of silica and haematite particles dispersed in natural water samples

Abstract

Aggregation rates for quartz and haematite particles dispersed in two different river waters have been determined using a direct particle counting technique. The river water samples were obtained from the Sherston Avon (a basic, hard water) and Whitray Beck (an acidic, soft water), and the influence of ionic strength on the aggregation processes in these waters was investigated using samples to which indifferent electrolyte had been added. The homoaggregation characteristics of quartz and haematite differ substantially between the different river waters. For both oxides, electrostatic interactions play a role in determining the particle aggregation behaviour in both river waters, although in the case of quartz particles dispersed in Sherston Avon water, non-electrostatic interactions assume more significance. Experimental evidence indicates that for a particular oxide in a particular river water, the magnitude of the electrophoretic mobility of the particles provides a reasonable qualitative assessment of aggregation rate. When comparing the aggregation behaviour of a particular oxide in the different river waters, or the different oxides in the same river water, however, results obtained in this work indicate that electrophoretic mobility does not necessarily provide a good indication of colloidal stability. Results obtained for mixed dispersions of quartz and haematite provide a clear indication that heteroaggregation of the two oxides occurs in both river water systems. Particle aggregation in these systems can be described in terms of the appropriate homoaggregation rate constants together with a single heteroaggregation rate constant. Heteroaggregation occurs at a significant rate even under conditions where the quartz particles are essentially colloidally stable.