2013 Volume 54 Issue 9 Pages 1809-1817
The reaction mechanism of H3BO3/B(OH)4− with MgO in aqueous phase was investigated using sorption isotherm, XRD, 11B-NMR and FTIR. Release of Mg2+ was observed soon after contact of MgO with H3BO3 and maximum released Mg2+ was proportional to the initial boron concentration, suggesting ligand-promoted dissolution of MgO by H3BO3. The molecular form of H3BO3 was more reactive with MgO in releasing Mg2+ ions than B(OH)4−. 11B-NMR results indicated that trigonal B ([3]B) was predominant over tetrahedral B ([4]B) in solid residues after sorption of H3BO3. The molar ratio of [4]B/[3]B increased with H3BO3 sorption density. XRD patterns for the solid residues were assigned to Mg(OH)2 and peaks broadened with increasing H3BO3 sorption density, except for (hk0) planes due to c-axis lattice strain induced by incorporation of H3BO3 between layers. These results indicated that H3BO3 interfered in the c-axis stacking of in Mg(OH)2. Molecular H3BO3 acted as a trigger when reacting with the MgO surface, releasing Mg2+ to produce an unstable complex leading to the precipitation formation of Mg(OH)2, which is a sink for the immobilization of H3BO3/B(OH)4−.
