The chemical formula of hydroxyapatite is Ca10(PO4)6(OH)2, abbreviated as HAP, which is the most widely used calcium phosphate salt crystal phase. Calcium phosphate salts are the main mineral component of vertebrate bones and teeth. Among calcium phosphate salts, hydroxyapatite is the thermodynamically most stable crystalline phase of calcium phosphate salts in body fluids and is most similar to the mineral parts of human bones and teeth. The ratio of calcium and phosphorus in hydroxyapatite is affected by the synthesis method. The calcium-phosphorus ratio is not fixed and its composition is complicated.
Properties
HAP has a high specific surface area, strong water absorption and release ability, and is in the air. moisture absorption capacity. The density of hydroxyapatite is 3.16g/cm3, the specific surface area is 26m2/g, the solubility product constant Ksp(25℃)=(6.3±2.1)×10-59, and it is weakly alkaline (pH=7-9). Hardly soluble in alkali and easily soluble in acid. The hydroxyapatite crystal has a hexagonal crystal structure and belongs to the P63/m space group. The unit cell parameters are a=b=0.943nm and c=0.688nm.
Applications
Due to its unique physical and chemical structure, hydroxyapatite is widely used in the following fields: (1) Application in sewage treatment; (2) In Application in contaminated soil remediation; (3) Application in medicine; (4) Application in other aspects.
Main synthesis method
Dry method
The dry preparation of HAP is to select finely ground precursors, mix them, and then heat-treat the mixed precursors. Way. Dry preparation requires the mixture to be completely homogeneous in order to ensure that the mixture can react completely. The purity of the final product depends on accurate weighing during preparation, ensuring that the mixture is completely reacted. Therefore, dry preparation has very strict requirements on the purity and dosage of reactants, and these variable factors may limit the production of the final compound.
Tromel et al. determined the optimal conditions for the formation of HAP by calcining a mixture of calcium phosphate and calcium oxide at 1050°C. The advantage of using the dry method is that the crystallinity of the product is generally better, but the dry method requires relatively high temperatures, which may affect the porosity of the product.
Wet method
Wet method preparation is widely used due to its simple operation, which mainly includes chemical co-precipitation method. Preparation methods include emulsification method, hydrolysis method, sol-gel method and hydrothermal method. Wet preparation can better control the structure and morphology of HAP and increase the yield of HAP.
Wet preparation can be carried out in water or other organic solvents, various catalysts can be added and it is suitable for a variety of devices. Wet preparation has relatively low requirements on the reaction environment and is suitable for room temperature and normal pressure environments. The main disadvantages of the wet method are that sometimes the product is not pure enough and has low crystallinity, and other phosphate crystals may exist in the product.
(1) Co-precipitation method
Co-precipitation method is the most direct and commonly used method to prepare HAP. The chemical reaction process involves the joint reaction of phosphate (PO43-) ions and calcium ions (Ca2+) in the reactants to generate HAP.
Depending on the sources of phosphate and calcium ions, the conditions of the co-precipitation method are variable, but the usual reaction conditions are pH 3-12 and temperature from room temperature to 100°C. Co-precipitation methods can use reaction industrial additive templates to tailor the product to specific needs.
(2) Hydrothermal method
The technology of hydrothermal synthesis of HAP crystals is now very mature. The reaction process occurs in a closed environment of high temperature and high pressure, using water or organic solution as the The reaction medium causes originally insoluble substances to dissolve and recrystallize. Qiu Mande and others conducted a systematic study on the synthesis of HAP crystals with different calcium sources in a hydrothermal system. The study found that the calcium source has a great influence on the resulting morphology. Calcium carbonate and calcium hydroxide can be used to produce a spherical structure. Nano-HAP and HAP prepared using calcium chloride and calcium nitrate are in the shape of short rods.
(3) Microemulsion method
The advantage of the emulsion method for preparing HAP is that it can accurately control the morphology and distribution of grain size and better avoid the problem of particle agglomeration.
W.Y. Zhou et al. used nanoemulsion technology to synthesize carbonated hydroxyapatite nanospheres, which can be used to produce composite medical tissue scaffolds.
(4) Sol-gel method
The sol-gel method is to hydrolyze or alcoholize inorganic salts or alkoxides in a solution and then polymerize and gel the solute to form a sol. , then the sol is dehydrated to form a gel, and finally the gel is dried and calcined. At present, the sol-gel method is not yet capable of large-scale production. The main disadvantages are twofold:
① Commonly used alkoxide materials are more expensive; antioxidants ② The synthesis process is delicate and usually takes more time. Jingdi Chen et al. prepared hydroxyapatite (HAP), β-tricalcium phosphate (β-TCP) and biphasic calcium phosphate (BCP) nanocrystal powders through a simple sol-gel method.
Source: Wang Yuxuan, preparation of waste putty powder-hydroxyapatite composite powder
