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

The influence of pH and concentration on boron coprecipitation by calcite w ere evaluated under near-equilibrium conditions at 25 degrees C. Calcite wa s precipitated by adding a metastable polymorph (vaterite or aragonite) to a solution of known boron concentration. This method maintains a nearly con stant solution composition during the slow conversion of the metastable pol ymorph to calcite. Boron uptake in calcite was found to be strongly pH-dependent, increasing t wo orders of magnitude from pH 8.5 to pH 10.5. Boron incorporation into cal cites precipitated from vaterite-saturated solutions was five times greater than in calcites precipitated from aragonite-saturated solutions. Ostensib ly, 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 for ms of calcite generated from aragonite and those generated from vaterite we re distinctly different. The different quantities of boron incorporated int o these calcites may reflect different crystal growth mechanisms, consisten t with face-dependent, nonequilibrium partitioning of trace elements in cal cite. At a constant pH of 9.0, boron uptake increased from less than 15 to over 2 90 mg/kg CaCO3 as the solution boron concentration was increased from 5 to 100 mg/kg. Our results agree with those of other investigators, despite dif ferences in solution composition, and calcite precipitation techniques used . The agreement between the studies may be because the crystal growth morph ology of the calcite was rhombohedral in both cases. Copyright (C) 1999 Els evier Science Ltd.