First-principles constraints on strontium coprecipitation and partitioning in calcite

Area of Science:

• Geochemistry
• Environmental Science
• Materials Science

Background:

• Coprecipitation with calcite (CaCO3) is a promising method for immobilizing radioactive strontium (90Sr).
• Uncertainty in Sr incorporation mechanisms and partition coefficients hinders quantitative modeling of 90Sr immobilization.
• Understanding Sr2+ coordination environments in calcite is crucial for accurate reactive transport models.

Purpose of the Study:

• To elucidate the concentration-dependent coordination chemistry of Sr2+ in calcite using first-principles DFT.
• To determine the equilibrium partition coefficient (D_Sr^eq) for Sr in calcite.
• To provide a molecular-level understanding of 90Sr transport in calcite barriers for nuclear waste management.

Main Methods:

• Configurational-ensemble-based first-principles density functional theory (DFT) calculations.
• Calculation of Sr K-edge X-ray absorption near-edge structure (XANES) spectra.
• Integration of DFT-calculated D_Sr^eq into a surface reaction kinetic model.

Main Results:

• DFT revealed concentration-dependent Sr2+ coordination in calcite, shifting from six-fold at low concentrations to non-six-fold at high concentrations (12.5 mol% SrCO3).
• Calculated Sr K-edge XANES spectra validated the heterogeneous Sr coordination ensemble and reproduced experimental data.
• A DFT-predicted D_Sr^eq of 0.01 at 25°C was obtained, consistent with low-coordination equilibrium and explaining observed kinetic effects.

Conclusions:

• The study provides a molecular-level framework for understanding Sr incorporation in calcite.
• Findings clarify the kinetic mechanism controlling Sr partitioning during calcite precipitation.
• This research enhances the predictive capability for 90Sr transport in calcite barriers relevant to nuclear waste repository safety.