First-principles constraints on strontium coprecipitation and partitioning in calcite

Coprecipitation with calcite (CaCO₃) is a potential pathway for immobilizing radioactive isotopic strontium (⁹⁰Sr) in the environment. However, uncertainty in Sr incorporation mechanism-particularly Sr²⁺ coordination environments and its equilibrium partition coefficient (D_Sr^eq)-has limited the quantitative implementation of coprecipitation processes into reactive transport modeling. In this study, configurational-ensemble-based first-principles density functional theory (DFT) revealed concentration-dependent coordination chemistry, in which Sr²⁺ was exclusively six-fold coordinated in dilute calcite, whereas non-six-fold Sr coordination became thermodynamically favored at high concentrations (12.5 mol% SrCO₃), with a Boltzmann-weighted effective coordination number of 6.6. Calculated Sr K-edge X-ray absorption near-edge structure spectra reproduced experimental spectra of high-Sr calcite, validating the heterogeneous Sr coordination ensemble. DFT predicted a D_Sr^eq value of 0.01 at 25°C, representing the lower bound of reported literature values and corresponding to low-coordination equilibrium. Using this DFT-calculated D_Sr^eq value in a surface reaction kinetic model satisfactorily captured the observed precipitation rate-dependent enhancement of apparent partition coefficients, revealing the kinetic mechanism controlling access to thermodynamically favored non-six-fold coordination configurations. These findings provide a molecular-level mechanistic framework for predicting ⁹⁰Sr transport in natural and engineered calcite barriers that are relevant to nuclear waste repository safety.