Removal mechanism of high concentration borate by co-precipitation with hydroxyapatite

Co-precipitation of borate in a wide range of concentration with hydroxyapatite (HAp) was investigated using Ca(OH)₂ as a mineralizer in the presence of phosphate. The sorption data of borate was fitted to Freundlich model. The maximum sorption density of B/Ca to maintain a mono-phase of HAp was found around 0.40. In higher B concentrations, borate was still immobilized, however, the crystalization of hydroxyapatite was inhibited, where the solid residues were accompanied with amorphous CaB₂O₄, as well as HAp. Based on ¹¹B-NMR and elemental analysis for solid residues in addition to solution chemistry, the removal mechanism of high concentration borate can be explained by the surface complexation of triborate on Ca(OH)₂, subsequently decomposition of triborate into monoborate to release [CaB(OH)₄]⁺ and B(OH)₄^-, followed by co-precipitation with HAp. These tetragonal B species were immobilized in the solid residues including amorphous HAp. During the process there was a trend to eliminate carbonate from the solid phase. TEM images suggested that the HAp particles precipitated at room temperatures were in a fibrous shape consisting of a number of short rods when borate species are not added. When borate species were immobilized, the HAp particles have gotten swelled with losing fibrous shapes. When further higher borate concentrations were encapsulated in co-precipitated products, the morphologies were dramatically changed, that is, nano-sized and less crystalline HAp particles were enveloped by possibly amorphous CaB₂O₄.