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

Seeded fluorite crystal growth experiments were performed at three dif ferent ionic strengths, using a constant addition method and NaNO3 as background electrolyte. By varying the fluorite seed concentration, gr owth rates were measured with and without aggregation taking place, he nce allowing us to differentiate between the chemical effect of ionic strength on growth kinetics and possible changes in reactive surface a rea due to aggregation. Fluorite crystal growth followed a second orde r rate law consistent with a mechanism limited by the rate at which Ca 2+ ions are integrated into the crystal lattice at reactive sites alon g surface spirals (kink sites). Aggregation did not affect the growth kinetics. The linear growth rate constant, however, decreased with inc reasing ionic strength. We explain this as a kinetic salt effect on th e rate-limiting surface reaction and, from the magnitude of the salt e ffect, we estimate a local charge of anion kink sites of -2.7. Copyrig ht (C) 1997 Elsevier Science Ltd.