PROBING THE CHARGE OF REACTIVE SITES AT THE MINERAL-WATER INTERFACE: EFFECT OF IONIC STRENGTH ON CRYSTAL GROWTH KINETICS OF FLUORITE

Abstract

Seeded fluorite crystal growth experiments were performed at three different ionic strengths, using a constant addition method and NaNO3 as background electrolyte. By varying the fluorite seed concentration, growth rates were measured with and without aggregation taking place, hence allowing us to differentiate between the chemical effect of ionic strength on growth kinetics and possible changes in reactive surface area due to aggregation. Fluorite crystal growth followed a second order rate law consistent with a mechanism limited by the rate at which Ca2+ ions are integrated into the crystal lattice at reactive sites along surface spirals (kink sites). Aggregation did not affect the growth kinetics. The linear growth rate constant, however, decreased with increasing ionic strength. We explain this as a kinetic salt effect on the rate-limiting surface reaction and, from the magnitude of the salt effect, we estimate a local charge of anion kink sites of -2.7.