Simple kinetic Monte Carlo models for dissolution pitting induced by crystal defects

A Kinetic Monte Carlo Approach to Model Barite Dissolution: The Role of Reactive Site Geometry

Barite (Ba[SO4]) is one of the promising candidates for sequestration of radioactive waste. Barite can incorporate radium (Ra) and form ideal solid solutions, i.e., (Ba,Ra)[SO4]. Together with isostructural celestite (Sr[SO4]), ternary solid solutions, (Ba,Sr,Ra)[SO4], may exist in natural conditions. Our fundamental understanding of the dissolution kinetics of isostructural sulfates is critically important for a better risk assessment of nuclear waste repositories utilizing this mineral for sequestration. So far, the barite-water interface has been studied with experimental methods and atomistic computer simulations. The direct connection between the molecular scale details of the interface structure and experimental observations at the microscopic scale is not yet well understood. Here, we began to investigate this connection by using a kinetic Monte Carlo approach to simulate the barite dissolution process. We constructed a microkinetic model for the dissolution process and identified the reactive sites. Identification of these sites is important for an improved understanding of the dissolution, adsorption, and crystal growth mechanisms at the barite–water interface. We parameterized the molecular detachment rates by using the experimentally observed etch pit morphologies and atomic step velocities. Our parameterization attempts demonstrated that local lattice coordination is not sufficient to differentiate between the kinetically important sites and estimate their detachment rates. We suggest that the water structure and dynamics at identified sites should substantially influence the detachment rates. However, it will require more work to improve the parameterization of the model by means of Molecular Dynamics and ab initio calculations.

KIMERA: A Kinetic Montecarlo Code for Mineral Dissolution

KIMERA is a scientific tool for the study of mineral dissolution. It implements a reversible Kinetic Monte Carlo (KMC) method to study the time evolution of a dissolving system, obtaining the dissolution rate and information about the atomic scale dissolution mechanisms. KIMERA allows to define the dissolution process in multiple ways, using a wide diversity of event types to mimic the dissolution reactions, and define the mineral structure in great detail, including topographic defects, dislocations, and point defects. Therefore, KIMERA ensures to perform numerous studies with great versatility. In addition, it offers a good performance thanks to its parallelization and efficient algorithms within the KMC method. In this manuscript, we present the code features and show some examples of its capabilities. KIMERA is controllable via user commands, it is written in object-oriented C++, and it is distributed as open-source software.

Effect of ultrasound on the dissolution of magnesium hydroxide: pH-stat and nanoscale observation

It has been previously confirmed that the dissolution of magnesium hydroxide was a crucial procedure in its application process. Note that the ultrasound is an effective method for enhancing solid dissolution. In this study, the enhancement of ultrasound on the dissolution of magnesium hydroxide was investigated by the pH-stat method. The titrating results indicated that the promotion effect of ultrasound was only be observed at some certain cases, such as lower pH value. Meanwhile, the activation energy data obtained based on Shrinking Core Model for sonication case ranged from 6.56 to 49.13 kJ/mol, implying that magnesium hydroxide dissolution proceeds at sonication case cannot be explained well by this model. Nanoscale observation was conducted to identify the crystal surface variations during the dissolution process by using SEM and AFM. The analysis results indicated that the dissolution behaviors of magnesium hydroxide in the absence and presence of ultrasound were quite different. In the silent case, the dissolution followed the famous Stepwave model. As for the sonication case, the larger particle was broken into flake by the ultrasound firstly. Then, the heterogeneous global dissolution accompanied by the Stepwave model drove the dissolution process. The analysis results of nanoscale observation provided a reasonable explain for the kinetics research results on micro-level.

Effect of nonlinearity in hybrid kinetic Monte Carlo-continuum models

Recently there has been interest in developing efficient ways to model heterogeneous surface reactions with hybrid computational models that couple a kinetic Monte Carlo (KMC) model for a surface to a finite-difference model for bulk diffusion in a continuous domain. We consider two representative problems that validate a hybrid method and show that this method captures the combined effects of nonlinearity and stochasticity. We first validate a simple deposition-dissolution model with a linear rate showing that the KMC-continuum hybrid agrees with both a fully deterministic model and its analytical solution. We then study a deposition-dissolution model including competitive adsorption, which leads to a nonlinear rate, and show that in this case the KMC-continuum hybrid and fully deterministic simulations do not agree. However, we are able to identify the difference as a natural result of the stochasticity coming from the KMC surface process. Because KMC captures inherent fluctuations, we consider it to be more realistic than a purely deterministic model. Therefore, we consider the KMC-continuum hybrid to be more representative of a real system.