Carbonates dissolution mechanisms in the presence of electrolytes revealed by Grand Canonical and Kinetic Monte Carlo modelling

Kurganskaya, Inna; Churakov, Sergey (2018). Carbonates dissolution mechanisms in the presence of electrolytes revealed by Grand Canonical and Kinetic Monte Carlo modelling. Journal of physical chemistry. C, 122(51), pp. 29285-29297. American Chemical Society 10.1021/acs.jpcc.8b08986

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Dissolution of carbonate minerals is a complex multistep process, characterized by the particular sequence of steps dependent on pH and background electrolyte concentration. Currently, available dissolution models for carbonates do not consider dependence of the surface speciation on the local surface topography. We have developed a new approach combining grand canonical Monte Carlo (GCMC) and kinetic Monte Carlo (KMC) methods to investigate the influence of water pH and electrolyte concentration onto processes of surface charging and dissolution of carbonates. GCMC simulations of the calcite–electrolyte system are used to calculate populations of protonated sites. We consider two basic speciation models characterized by different spatial charge distributions at the surface: “ionic”, where surface >CO32– sites are represented by “–2” charges at the corresponding lattice positions; and “oxygen”, where surface >CO32– sites are represented by triplets of “–2/3” charges at the positions of oxygen atoms. The speciation of carbonate ion protonation probabilities is found to be controlled by local charge densities and the presence of electrolyte species. In all simulation results, protonation affinity of the surface >CO32– sites followed the trend kink (most acidic) > step > terrace (least acidic), with the same trend observed with respect to adsorption probabilities of Cl– ions. The influence of protonated site concentrations obtained in GCMC simulations was investigated in KMC simulations. The direct comparison of simulated and experimental data showed that the oxygen model, with an assumption of congruent dissolution, reproduces both the pH dependence of the calcite dissolution rate and the morphology of the calcite surface. On the basis of the considered model, we could identify four key factors that define pH-dependent dissolution mechanisms of calcite: (1) increase of the kink site propagation rate at pH < 10; (2) increase of kink site generation frequency at pH 4–7; (3) increase of monolayer pit generation frequency at pH = 2–4; and (4) acceleration of kink site propagation and generation at pH 2–4 due to the second protonation step. The combined GCMC + KMC approach shows great potential in resolving surface speciation of carbonates as functions of solvent composition and surface geometry and their influence on the dissolution mechanisms and rates. Generally, this approach could potentially be applied to any other mineral–fluid system.

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