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 angstrom). 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.