Mechanism of enhanced strontium uptake into calcite via an amorphous calcium carbonate (ACC) crystallisation pathway

Littlewood, Janice L. and Shaw, Samuel and Peacock, Caroline L. and Bots, Pieter and Trivedi, Divyesh and Burke, Ian T. (2017) Mechanism of enhanced strontium uptake into calcite via an amorphous calcium carbonate (ACC) crystallisation pathway. Crystal Growth and Design, 17 (3). 1214–1223. ISSN 1528-7483

[img]
Preview
Text (Littlewood-etal-CGD2017-Mechanism-of-enhanced-strontium-uptake-into-calcite)
Littlewood_etal_CGD2017_Mechanism_of_enhanced_strontium_uptake_into_calcite.pdf
Accepted Author Manuscript

Download (1MB)| Preview

    Abstract

    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 crystallisation 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 crystallisation proceeded via ACC → vaterite → calcite and average Kd Sr values were between 0.44-0.74. At low Sr2+ concentrations (Sr2+ : Me2+ ratio ≤ 0.01) EXAFS 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 Kd Sr of 0.90-0.97 due to the presence of Mg2+ stabilising 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 crystallisation 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.