Driving forces of mineral recrystallization in aqueous solutions derived from kinetic modeling of isotope exchange data

Recrystallization in aqueous solutions is a ubiquitous process susceptible to control the entrapment and release of toxic contaminants in the subsurface. However, unraveling the underlying mechanisms and driving forces has proven to be elusive, as recrystallization frequently follows different kinetic pathways even for the same mineral, depending on its initial state and pre-treatment. To obtain a better insight, a large body of experimental data from isotope tracer experiments carried out in the past two decades on a variety of minerals (baryte, calcite, calcium-silicate hydrates, goethite, and UO 2 ) was reviewed and modeled, using the HOmogeneous Recrystallization (HOR) and the Continuous HOmogeneous Recrystallization (CHOR) models, both coupled to instantaneous reversible adsorption and denoted as a whole as C(HOR)-K d models. In the first part of this contribution, we develop the full mathematical formalism and discuss the model parameters. The second part is devoted to the review, modeling and interpretation of selected data. It is shown that the C(HOR)-K d models successfully reproduce recrystallization data for widely different minerals, including Fe-isotope data on goethite modeled elsewhere using a different approach (“back-reaction” model). In combination with microscopic characterization data, the modeling results allow us to identify the thermodynamic driving forces controlling the recrystallization kinetics. These include: (i) the reduction of surface energy excess arising from a high density of defects (kink sites, dislocations, steps) and/or a high initial specific surface area; (ii) the spontaneous tendency to increase crystallinity (increase in crystallite size, transformation to a more stable habitus); (iii) the annealing of chemical potential gradients when foreign trace elements are incorporated as solid solution into the recrystallized solid; (iv) the annealing of electric potential gradients in redox active solids (Fe oxy-hydroxides). Our findings demonstrate that mineral recrystallization in aqueous solutions is a complex phenomenon driven by multiple mechanisms correlated to the properties of the primary solid. Accurate predictions on kinetics and extent of recrystallization are possible only after detailed characterization of the solid down to the molecular scale.