Calcination effect of borate-bearing hydroxyapatite on the mobility of borate

Discharge from accidental nuclear power plants includes boric acid, which is used as a neutron absorbent in nuclear reactors. Co-precipitation of borate with hydroxyapatite (HAp), using Ca(OH)₂, is known to be an effectively fast method for stabilization of borate as well as coexisting radioactive nuclides. To reduce bulky volume of solid residues after co-precipitation, calcination is necessary to investigate the chemical stability of targets. Calcination at 850 °C resulted in the high crystalization of HAp with formation of xCaO·B₂O₃ as a by-phase in which x increased with a decrease in the borate contents. After calcination, the lattice parameter a of HAp showed a reentrant curve and c showed a convex curve with an increase in borate contents. A dissolution assay revealed that calcination sometimes increases the borate moiety and that the acceptable B contents in HAp are lower than 1.59 mmol/g-calcined HAp. These results imply that during calcination of HAp, some borate is excluded to form the by-phase xCaO·B₂O₃, which is relatively insoluble in water, but some other fractions might be additionally emitted from the amorphous phase to weakly bind the calcined products.