**6.1.4. Nickel-substituted apatites**

According to MOBASHERPOU et al [30], the reaction mechanism corresponding to equimolar exchange of nickel and calcium and yielding to Ca10−xNix(PO4)6(OH)2, where *x* varies from 0 to 10, could be described by the following equation:

$$\mathrm{Ca\_{10}(PO\_4)\_6(OH)\_2 + x \ Ni \to Ca\_{10-x}Ni\_x(PO\_4)\_6(OH)\_2 + x \ Ca^{2+}}\tag{2}$$

In this process, Ni2+ ions are first adsorbed onto the surface of hydroxyapatite (surface complexation, **Section 6.5.2**) and then the substitution of Ca2+ for Ni2+ ions takes place.

#### **6.1.5. Zinc-substituted apatites**

Zinc is a common bioelement. The zinc content in human bones ranges from 0.0126% to 0.0217% by weight [7]. Zinc as a cationic substituent in hydroxyapatite provides the option to counteract the effects of osteoporosis [31]. The incorporation of zinc into the HAP structure (Zn-HAP) was abundantly studied, owing to the key effect of Zn2+ cations in several metabol‐ ic processes that makes zinc eligible for use in many biomedical applications and to its possible antimicrobial activity [3].

The results of structure analysis indicated that Zn ions substituted partially for Ca ions in the apatite structure and the upper limit of Zn substitution for Ca in HA was about 20 mol.%. In general, the HAP lattice parameters, *a* and *c*, decreased with Zn addition [32].

Zn-substituted apatite was synthesized by the precipitation method as follows [33]:

$$\begin{array}{l} \text{10x } \text{Ca}^{2+} + \text{10(l-x) } \text{Zn}^{2+} + \text{6 } \text{PO}\_4^{3-} + \text{2 } \text{OH}^- \rightarrow\\ \text{Ca}\_{\text{10x}} \text{Zn}\_{\text{10(l-x)}} \text{(PO}\_4\text{)}\_6 \text{(OH)}\_2 \end{array} \tag{3}$$

where 0 ≤ *x* ≤ 1. The pH of the solution was adjusted to 8 by aqueous solution of NH3, and the reaction mixture was kept at 90°C for 5 h with stirring. The resulting suspension was then subjected to suction filtration, and the powdery product was dried at 100°C for 10 h. It is known that the usage of chloride or nitrate of calcium as a starting reagent may cause the incorpora‐ tion of Cl<sup>−</sup> or NO3 <sup>−</sup> into the structure of apatite. This can be avoided by the utilization of acetate salts, because acetate ions are not incorporated into the apatite, i.e. they would not affect the apatite structure.
