Relative Contributions of Mg Hydration and Molecular Structural Restraints to the Barrier of Dolomite Crystallization: A Comparison of Aqueous and Non-Aqueous Crystallization in (BaMg)CO<sub>3</sub> and (CaMg)CO<sub>3</sub> Systems
Carbonate mineralization is reasonably well-understood in the Ca–CO<sub>2</sub>–H<sub>2</sub>O system but continuously poses difficulties to grasp when Mg is present. One of the outstanding questions is the lack of success in dolomite MgCa(CO<sub>3</sub>)<sub&g...
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| Autores principales: | , , |
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| Formato: | article |
| Lenguaje: | EN |
| Publicado: |
MDPI AG
2021
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| Materias: | |
| Acceso en línea: | https://doaj.org/article/0b83ddaf676c449eba3d84e89579bbd1 |
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| Sumario: | Carbonate mineralization is reasonably well-understood in the Ca–CO<sub>2</sub>–H<sub>2</sub>O system but continuously poses difficulties to grasp when Mg is present. One of the outstanding questions is the lack of success in dolomite MgCa(CO<sub>3</sub>)<sub>2</sub> crystallization at atmospheric conditions. The conventional view holds that hydration retards the reactivity of Mg<sup>2+</sup> and is supported by solvation shell chemistry. This theory however is at odds with the easy formation of norsethite MgBa(CO<sub>3</sub>)<sub>2</sub>, a structural analogue of dolomite, leading to the premise that crystal or molecular structural constrains may also be at play. The present study represents our attempts to evaluate the separate contributions of the two barriers. Crystallization in the Mg–Ba–CO<sub>2</sub> system was examined in a non-aqueous environment and in H<sub>2</sub>O to isolate the effect of hydration by determining the minimal relative abundance of Mg required for norsethite formation. The results, showing an increase from 1:5 to 6:4 in the solution Mg/Ba ratio, represented a ~88% reduction in Mg<sup>2+</sup> reactivity, presumably due to the hydration effect. Further analyses in the context of transition state theory indicated that the decreased Mg<sup>2+</sup> reactivity in aqueous solutions was equivalent to an approximately 5 kJ/mol energy penalty for the formation of the activated complex. Assuming the inability of dolomite to crystallizes in aqueous solutions originates from the ~40 kJ/mol higher (relative to norsethite) Gibbs energy of formation for the activated complex, a hydration effect was estimated to account for ~12% of the energy barrier. The analyses present here may be simplistic but nevertheless consistent with the available thermodynamic data that show the activated complex of dolomite crystallization reaction is entropically favored in comparison with that of norsethite formation but is significantly less stable due to the weak chemical bonding state. |
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