Mechanism of Enhanced Strontium Uptake into Calcite via an Amorphous Calcium Carbonate Crystallization Pathway

Calcite formation via an amorphous calcium carbonate (ACC) precursor phase potentially offers a method for enhanced incorporation of incompatible trace metals, including Sr²⁺. In batch crystallization experiments where CaCl₂ was rapidly mixed with Na₂CO₃ solutions, the Sr²⁺/Me²⁺ ratio was varied from 0.001 to 0.1, and the pathway of calcite precipitation was directed by either the presence or absence of high Mg²⁺ concentrations (i.e., using a Mg²⁺/total Me²⁺ ratio of 0.1). In the Mg-free experiments crystallization proceeded via ACC → vaterite → calcite, and average K_{d Sr} values were between 0.44 and 0.74. At low Sr²⁺ concentrations (Sr²⁺/Me²⁺ ratio ≤ 0.01), extended X-ray absorption fine structure analysis revealed that the Sr²⁺ was incorporated into calcite in the 6-fold coordinate Ca²⁺ site. However, at higher Sr²⁺ concentrations (Sr²⁺/Me²⁺ ratio = 0.1), Sr²⁺ was incorporated into calcite in a 9-fold site with a local coordination similar to Ca²⁺ in aragonite, but calcite-like at longer distances (i.e., >3.5 Å). In the high-Mg experiments, the reaction proceeded via an ACC → calcite pathway with higher K_{d Sr} of 0.90-0.97 due to the presence of Mg²⁺ stabilizing the ACC phase and promoting rapid calcite nucleation in conjunction with higher Sr²⁺ incorporation. Increased Sr²⁺ concentrations also coincided with higher Mg²⁺ uptake in these experiments. Sr²⁺ was incorporated into calcite in a 9-fold coordinate site in all the high-Mg experiments regardless of initial Sr²⁺ concentrations, likely as a result of very rapid crystallization kinetics and the presence of smaller Mg²⁺ ions compensating for the addition of larger Sr²⁺ ions in the calcite lattice. These experiments show that the enhanced uptake of Sr²⁺ ions can be achieved by calcite precipitation via ACC, and may offer a rapid, low temperature, low-cost method for removal of several incompatible Me²⁺ ions (e.g., Pb²⁺, Ba²⁺, Sr²⁺) during effluent treatment.