OCCURRENCE AND STABILITY OF NANOCOLLOIDAL SILICA PARTICLES IN AQUEOUS SYSTEMS

Dissolved SiO₂ concentrations are often supersaturated in waters associated with siliceous sediments leading to the polymerization of SiO₂ and the formation of SiO₂ colloids. As the surfaces of these colloids are negatively charged above pH 3, they readily adsorb dissolved cations and are effective at sequestering heavy metals and transporting them within groundwaters. In order to model the interactions of SiO₂ colloids and dissolved metals in natural aqueous systems, we must know specific characteristics of the colloids, such as their surface structure, their persistence in different geological environments, and the evolution of particle sizes over time. Very little information is available regarding the kinetics of formation and the mechanism(s) of growth of SiO₂ colloids under environmentally relevant conditions (i.e., low ionic strength, relatively low degree of supersaturation, pH 3-7). The purpose of our work is to examine the kinetics of SiO₂ polymerization over a range of initial SiO₂ concentrations and pH values to understand the conditions under which SiO₂ colloids form and persist, as well as the growth mechanism leading to precipitation of amorphous SiO₂ particles.

Batch kinetic experiments have been conducted using three initial SiO₂ concentrations over a pH range of 3-7. Changes in the concentrations of monomeric, nanocolloidal and precipitated SiO₂ concentrations over time were monitored and the rate constants predicted using a kinetic model. Results indicate that at low degrees of supersaturation (2-5X) and low pH (3-4), SiO₂ colloids persist for more than 100 days. Analysis of the solutions using GFC, combined with AFM analysis of size fractionated samples, provide insight into the mechanism of growth of SiO₂ nanocolloids. Two distinct size populations within the nanocolloidal fraction were observed. Primary nanocolloids approximately 3 nm in diameter exist in conjunction with larger nanocolloids approximately 30-40 nm in diameter. These larger particles appear to be aggregates comprised of the 3 nm-sized primary particles. This work is the first attempt to model and predict the environmental conditions under which nanocolloidal SiO₂ is important and should be considered when investigating geochemical cycles such as the dissolution and precipitation of silica-based minerals.