**6.1.12. Cadmium-substituted apatites**

The absorption of cadmium cations in apatites is relevant both from the medical standpoint of cadmium uptake by human bones, as well as since cadmium migration in nature involves

the absorption and desorption equilibria with natural minerals, including apatites. Cadmi‐ um has a slight preference for Ca(I) site in fluorapatite and for Ca(II) site in hydroxyapatite [7], [52]. The interactions between these two ions (Cd and Ca) during absorption and ionic change processes in apatites present therefore considerable practical and theoretical interest. Cadmium is also a frequent heavy toxic pollutant element in water [7].

Calculated energy differences (*E*) between these sites are of 12 and 8 kJ·mol−1 for fluorapatite and hydroxylapatite, respectively. The preference is not strong, and however, a part of the sites of the other type is also occupied by cadmium ions. The relative site occupation can be expressed by the equation [7]:

$$\text{P} = \frac{\text{probability of substitution at Ca} \text{(l) site}}{\text{probability of substitution at Ca} \text{(2) site}} = \frac{4}{6} \text{ } \exp\left(-\frac{\text{E}}{\text{kT}}\right) \tag{21}$$

where *E* = *E*(Cd2+ or Zn2+ on Ca(1)) – *E*(Cd2+ or Zn2+ on Ca(2)). At *T* = 298 K, *P* = 85 and 17 Cd2+ in fluorapatite and hydroxylapatite, respectively. From the value of *P* and from the fact that the sum of the two probabilities is 1, one can calculate that the probability of the lower-energy site occupancy is of 99% and 94%, respectively.

BADRAOUI et al [53] reported that the maximum amount of cadmium substitution for stronti‐ um in the system Sr10−xCdx(PO4)6Z2 (Z = OH and F) accounts for about 40 at% in HAP and for 60 at% in FAP. The increase of cadmium content induces stronger decrease of the c-axis with respect to the a-axis. The structure refinements evidence found a statistical distribution of Cd atoms in Sr10−xCdx(PO4)6(OH)2 and a light preference for M(1) site in Sr10−xCdx(PO4)6(F)2. The stability ofthe system M10−xM'x(PO4)6Z2 (M and M' = Ca, Pb, Cd, Sr andZ = OH and F) is strongly affected by the polarizability. As a matter of fact, complete miscibility is possible even when the cations exhibit great size differences, provided they are not both soft acids. Otherwise, the presence of two cations with quite different radii and relevant polarizabilities induces important distortions of the apatite unit cell and PO4 tetrahedra and consequently limits the possibility of mutual substitution.
