Simulation of FeCO3 ion clusters in aqueous solution: Implications for crystal growth
Abstract
Recent studies suggest the deviation of M(II)CO3 solutions from ideality arrises not only from the formation of ion-pairs and reductions in ion-activity upon concentration, but from the appearance of sub-nanometer sized ion clusters. These species are apparently present in undersaturated solutions and have a strikingly monodisperse size distribution according to ultra centrifugation, which indicates the clusters are thermodynamically stable. The existence of these pre-nucleation clusters has significant implications for crystal nucleation and growth in natural and engineered environments, especially those where fluids can become highly-supersaturated with respect to M(II)CO3 phases (e.g. soil pore waters, CO2 storage reservoirs, etc.). Indeed, the presence of transient disordered precursors has already been documented in the CaCO3 system; however, little is known about the growth and transformation of these phases into their crystalline counterparts. The nature of the amorphous to crystalline transition also remains controversial, as both solid state and dissolution / re-precipitation mechanisms have been proposed. Yet, an understanding of the underlying processes driving the structural reorganization is essential to interpretation of compositional signatures that are frequently used as indicators of Earth's climate history. This study uses molecular dynamics simulations to investigate the formation and structural evolution of iron(II) carbonate clusters in aqueous solution. A model is presented for the Fe(II) and CO32- which produces measured free energies and enthalpies of hydration for these ions, water exchange rates about Fe(II) that are in good agreement with available experimental rates obtained from NMR spectroscopy, and a free energy of association for the ions that is consistent with reported stability constants for the FeCO3(aq) ion pair. The potential of mean force for association of Fe(II) and CO32- presents a single minimum in the contact ion pair region that corresponds to a monodentate interaction. This is consistent with the lack of polymorphism in the FeCO3 system, in which the only known anhydrous phase (siderite) is isostructural with calcite. Unbiased simulations on concentrated solutions indicate that the initial clusters are disordered, and that they grow by forming linear chains up to four ions in length. Four-membered chains are long enough to self-interact and can collapse to form more condensed clusters. These observations are supported by umbrella sampling simulations which indicate similar behavior at lower concentrations.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMPP22A..06W
- Keywords:
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- 0419 BIOGEOSCIENCES / Biomineralization;
- 1009 GEOCHEMISTRY / Geochemical modeling;
- 1011 GEOCHEMISTRY / Thermodynamics;
- 1012 GEOCHEMISTRY / Reactions and phase equilibria