A model for pH dependent strontium partitioning during calcite precipitation from aqueous solutions

The relationship between the strontium partition coefficient in calcite (K-sr) and calcite precipitation rate (R-p) has been found to change systematically with pH of the solution in precipitation experiments of synthetic calcite. We hypothesize that the observed pH dependence of strontium partitioning is due to the involvement of HCO3- during calcite precipitation. We use an ion-by-ion calcite growth model which considers the contribution of both HCO3- and CO32- to calcite precipitation to test this hypothesis. Three groups of experimental data of inorganic calcite precipitation at various pH at room temperature were fit successfully with one set of parameters. The results indicate that the speciation of carbonate can explain the pH dependence of the strontium partition coefficient. Our model also reproduces observed K-S(r)-R-p relationships at various temperatures of 5, 25 and 40 degrees C. This general model predicts calcite precipitation rates and strontium partition coefficients with known aqueous carbonate chemistry and provides a useful framework for understanding strontium partitioning in inorganic and biogenic calcite.

Automated summary

This study reveals the relationship between the strontium partition coefficient and the calcite precipitation rate in synthetic calcite experiments, showing that this relationship varies systematically with the pH of the solution. It is hypothesized that this pH dependence is due to the involvement of HCO3- during calcite precipitation. By using an ion-by-ion calcite growth model, the hypothesis is successfully tested, explaining the pH dependence of the strontium partition coefficient and reproducing the observed relationships at different temperatures.