Kinetics and energetics of pharmacolite mineralization via the classic crystallization pathway

Abstract

Pharmacolite (CaHAsO4·2H2O) mineralization is an important process controlling arsenate mobility in carbonate areas but is poorly understood in terms of crystallization mechanism and kinetics. The present study intends to investigate pharmacolite formation initiated by heterogenous nucleation through in situ observations of the mineral–water interface. Experiments were performed by exposing gypsum substrate to solutions supersaturated with respect to pharmacolite followed by atomic force microscopy imaging to record changes in surface morphology and topography. The data are used to determine growth mode, step speed, and the rate of step birth. Experimental results show the crystallization is achieved by direction-specific mono-molecular layer growth via 2D surface nucleation or spiral hillock development in accord with the classical crystallization model. Subsequent theoretical analyses allow to determine step energies and kinetic coefficients in major growth directions on (010) and indicate that [001] steps are energetically the most stable and morphologically the most prominent while [100] and [101] steps are controlled either by thermodynamic or kinetics. Gypsum plays an important role in aiding pharmacolite formation through epitaxy as the two minerals share a range of structural commonalities. The notably reduced supersaturation needed in substrate-assisted pharmacolite crystallization relative to bulk solution nucleation suggests gypsum may significantly reduce the energy barrier for the mineralization reactions and hence may find applications in As remediation practice.