Synthesis of morphologically controlled hydroxyapatite from fish bone by urea-assisted hydrothermal treatment and its Sr²⁺ sorption capacity

Needle- and sponge-shaped hydroxyapatite (HAp) particles were synthesized from calcined tuna fish bone (Tn1000) and commercially available HAp reagent (ChemHAP) by a urea-assisted hydrothermal treatment using 0.1-1.0 M urea solution at 160 °C for 3 h. The Sr²⁺ sorption capacity of the synthesized HAp was also investigated using 0-50 mM Sr²⁺ solution at 25 °C for 72 h to evaluate its performance as a sorbent for environmental remediation. Sponge-shaped HAp was formed under hydrothermal conditions with a urea concentration of 1.0 M. With decreasing urea concentration, the morphology of HAp changed from sponge-shaped to needle-shaped crystals, regardless of the starting material. Some calcium carbonate and/or β-tricalcium phosphate impurities were formed from Tn1000 at 0.1-0.5 M urea concentration. The Sr²⁺ sorption mechanism of the synthesized HAp was estimated using Ca ion-exchange reaction and precipitation of SrCO₃. The sponge-shaped HAp crystals, which had high specific surface area and CO₃^2- content, exhibited a large ion-exchange capacity with Sr²⁺. In contrast, the ion-exchange ratio of needle-shaped HAp dramatically decreased with increasing initial Sr concentration. No clear difference in Sr²⁺ sorption behaviour caused by the choice of HAp synthesis starting material was observed. These results indicate that the Sr sorption mechanism of HAp is influenced by not only its composition but also its crystal morphology.