Hydroxyapatite (HAp) is a multi-purpose inorganic material with applications in the field of medicine, wastewater treatment, sensor, catalysis, and chromatography1,2. Among the various morphologies of HAp, the microsphere morphology is preferred because of its unique physical characteristics such as large surface area, better protein adsorption capacity, and superior flowability 3,4. Therefore, several fabrication methods have been proposed for microsphere preparation. HAp (Ca5(PO4)3OH) is a well studied calcium phosphate (CaP) mineral with a hexagonal crystalline structure. However, the production of HAp microspheres with uniform properties remains a challenge due to the notable dissimilarity in crystallographic structure, chemical composition, phase stability, dissolution behavior, crystallization thermodynamics,, and kinetics of the different forms of CaPs 5,6.
Various methods have been developed to prepare HAp, such as biomimetic technique, solid − solid reaction, spray − drying, sol − gel, wet precipitation, emulsion, microwave-assisted synthesis, hydrothermal and template-assisted synthesis, etc. 1,4. The precipitation process with aqueous orthophosphate and calcium sources appears to be the most common technique to prepare HAp particles. In this route, water-soluble calcium sources such as calcium chloride, calcium acetate, or calcium nitrate are preferred because of the resultant homogeneity of the reaction medium. Subsequently, highly pure HAp can be obtained for better quality materials in biomedical and chromatographic applications. However, these soluble calcium precursors present some disadvantages regarding their relative expense compared to the less water-soluble calcium precursors like calcium carbonate or calcium hydroxide and the labor-intensive processes involved during HAp formation. They also need to be treated to eliminate waste contaminants from counterions 7,8. This work has focused on relatively less expensive materials such as calcium carbonate for HAp synthesis. In environmental and purification applications, such inexpensive HAp materials are also preferred as sorbents.
Calcite naturally exists in many biological wastes such as mussel shells, eggshells, kina shells, oyster shells, snail shells, etc.9–11. Utilizing inexpensive calcite with reduced energy consumption protocols for HAp preparation will be a valuable research challenge 4,10. Most methods reported in literature do not directly utilize calcium carbonate for HAp synthesis. The difficulty in directly utilizing calcium carbonate is that it has a long reaction time and cannot be wholly decomposed into HAp at room temperature. Calcite in waste materials are usually converted into calcium oxide by thermal treatment or into water-soluble calcium salt by acid solubilization route 2. The hydrothermal method is a highly reported technique for converting calcium carbonate into HAp3. However, in the hydrothermally-treated mixtures of calcium carbonate and (NH4)2HPO4, it is observed that the conversion is slow and incomplete for a ball-and-block structure of calcium carbonate when compared to a rod-shaped carbonate 7,12. Highly crystalline particles with uniform morphology can be synthesized by the hydrothermal method but the outcome is in small scale and associated with high costs. Moreover the hydrothermal method is time-consuming and involves complicated steps 13. The literature suggests that the calcium carbonate conversion reaction depends on various parameters like liquid-solid (L/S) ratio, precursor shape, reaction time, and temperature 14. There is not much literature on the effect of stirring conditions on calcite into HAp conversion. Even though a lot of information has been obtained regarding converting calcium carbonate to HAp, it is essential to optimize the parameters affecting the quantitative conversion of calcite into HAp. The current work focuses on the reaction parameter optimization for a single-step process for the direct and complete transformation of calcite to HAp. Different process parameters which influence conversion were examined, including reaction time, temperature, and stirring rate 12.
Finally, the HAp microspheres prepared by this method were used as matrices for chromatographic separation of protein mixtures.