Structure and reactivity of the calcite-water interface

Area of Science:

• Geochemistry
• Surface Chemistry
• Materials Science

Background:

• Understanding the electrical properties of mineral-solution interfaces is crucial for various geological and industrial processes.
• Calcite, a common mineral, plays a significant role in natural systems and material applications.
• The surface charge and structure of calcite influence its reactivity and interactions with aqueous environments.

Purpose of the Study:

• To determine the zeta potential of calcite across a range of aqueous solution compositions.
• To investigate the influence of solution chemistry, specifically CO(2) partial pressure and pH, on calcite surface charge.
• To elucidate the calcite surface structure and hydration at the molecular level using surface diffraction.

Main Methods:

• Electrophoretic measurements and streaming potential measurements were used to determine calcite zeta potential.
• Surface diffraction techniques were employed to analyze the structure of calcite (104)-faces in contact with solutions.
• A Basic-Stern surface complexation model was developed and applied to interpret the zeta potential data.

Main Results:

• Carbonate and calcium ions were identified as the primary charge-determining ions for calcite.
• The isoelectric point of calcite shifts with changes in CO(2) partial pressure, while pH has a minor effect in non-equilibrium solutions.
• Surface diffraction revealed minimal ion relaxation and identified two ordered water layers at the calcite-solution interface, with no direct evidence of inner-sphere surface species.

Conclusions:

• The Basic-Stern surface complexation model, considering only outer-sphere complexes, successfully reproduces experimental zeta potential data.
• The model provides reasonable estimations for inner Helmholtz capacitance, consistent with the observed Stern layer thickness.
• This study enhances the understanding of calcite surface chemistry and its behavior in aqueous environments.