Boron incorporation into precipitated calcium carbonates affected by aqueous pH and boron concentration.

The objectives of this study were to investigate the amount of B incorporation into precipitated calcium carbonate (PCC) in the coprecipitation process, and to determine specific mineral phases (calcite or vaterite) and the mode of B coordination (trigonal or tetrahedral) in PCC under different pH and B concentrations. The amount of B incorporation into PCC increased in general with increasing aqueous B (Baq) concentrations in the pH range from 8 to 12. The B removal by PCC reached maximum (∼200 mmol kg-1) at pH 10 with Baq concentrations between 30 and 50 mM. The transformation of vaterite to calcite was promoted with increasing Baq at pH 8 and 10, whereas an excess concentration of aqueous (poly)borate anions (100 mM) inhibited crystal growth of calcite. As determined by B K-edge X-ray absorption fine structure spectroscopy, the coordination of B incorporated in PCC was preferentially tetrahedral (IVB, 55-70%) over trigonal (IIIB, 30-45%) at Baq <75 mM. In contrast, the preferential incorporation of IVB into PCC was not observed in the solution with a high B concentration (i.e., 100 mM). The amount of B incorporation, the morphology of PCC and B coordination in PCC were remarkably changed in high Baq concentrations.