Characteristic Sorption of H₃BO₃/B(OH) ₄^- on magnesium oxide

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 H₃BO ₃ 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 H₃BO ₃/B(OH)₄^-.