Effect of pH on boron coprecipitation by calcite: Further evidence for nonequilibrium partitioning of trace elements

摘要

The influence of pH and concentration on boron coprecipitation by calcite were evaluated under near-equilibrium conditions at 25 deg C. Calcite was precipitated by adding a metastable polymorph (vaterite or aragonite) to a solution of known boron concentration. This method maintains a nearly constant solution composition during the slow conversion of the metastable polymorph to calcite. Boron uptake in calcite was found to be strongly pH-dependent, increasing two orders of magnitude from pH 8.5 to pH 10.5. Boron incorporation into calcites precipitated from vaterite-saturated solutions was five times greater than in calcites precipitated from aragonite-saturated solutions. Ostensibly, these results suggest that the calcite precipitation rates were not low enough to attain equilibrium partitioning of boron into calcite. However, scanning electron micrograph analyses showed that the prevalent crystal forms of calcite generated from aragonite and those generated from vaterite were distinctly different. The different quantities of boron incorporated into these calcites may reflect different crystal growth mechanisms, consistent with face-dependent, nonequilibrium partitioning of trace elements in calcite. At a constant pH of 9.0 boron uptake increased from less than 15 to over 290 mg/kg CaCO_3 as the solution boron concentration was increased from 5 to 100 mg/kg. Our results agree with those of other investigators, despite differences in solution composition, and calcite precipitation techniques used. The agreement between the studies may be because the crystal growth morphology of the calcite was rhombohedral in both cases.