Chemical controls on the magnesium content of amorphous calcium carbonate

The primary control on ACMC composition is the steady state solution composition that develops in the reactor during precipitation. Analysis of the data shows the Mg content of ACMC is regulated by the interplay of three factors at steady state conditions: (1) Mg/Ca ratio; (2) total carbonate concentration; and (3) solution pH. Using the Henderson-Kracek model to estimate the partition coefficients for the Mg content of ACMC, we find KD is approximately constant at 0.047 ± 0.003 when steady state pH is less than 9.5, but values of KD triple as steady state pH increases from 9.5 to 10.3. Our KD values are lower than previous estimates that are based upon initial solution composition. In contrast, our estimates of KD are determined from the solution chemistry at steady state conditions and for pH conditions that are less extreme than previous experimental studies. We suggest the approach of using steady state composition to estimate KD gives a more accurate representation of relationships between ACMC composition and local conditions. The findings demonstrate local pH and total carbonate concentration can be regulated at the time of formation to produce Mg amorphous carbonates of a designated composition.