Fluorite dissolution at acidic pH: In situ AFM and ex situ VSI experiments and Monte Carlo simulations

Abstract

Dissolution of the fluorite (1 1 1) cleavage surface was investigated by means of in situ atomic force microscopy (AFM) and ex situ vertical scanning interferometry (VSI) experiments at pH range 1–3 in HCl solutions. Surface retreat was quantified at different pH values, yielding dissolution rates that were used to derive an empirical rate law for fluorite dissolution: Rate fluorite ( mol m - 2 s - 1 ) = 2.0 ± 0.4 × 10 - 6 · ( a H + ) 0.38 ± 0.07 where a H + is the proton activity. The influence of Δ G on fluorite dissolution rate at pH 2 was investigated by means of AFM and VSI surface measurements and flow-through experiments with powdered fluorite. The fluorite dissolution rate decreases non-linearly with increasing Gibbs energy (Δ G) and a dissolution plateau is obtained at Δ G ⩽ −7 kcal mol −1. This Δ G effect can be expressed with a rate law of the form Rate fluorite mol m - 2 s - 1 = k · a H + n 1 - exp - 4.7 × 10 - 2 · | Δ G | RT 1.41 ± 0.24 An alternative form based on a formulation making use of a Temkin number is also possible Rate fluorite mol m - 2 s - 1 = k · a H + n 1 - exp Δ G 8 RT Dissolution proceeds by formation of equilateral triangular etch pits with trigonal pyramidal morphology and emanation of stepwaves that are responsible for the surface retreat. The results of Monte Carlo simulations are consistent with this reaction mechanism.