Biomedical Engineering Reference
In-Depth Information
mention on a study, in which release of calcium into the bulk solution
was always less, when compared with release of orthophosphate
[63]. On the other hand, having followed the release kinetics of
calcium and orthophosphate ions into solution, apatites were found
to dissolve stoichiometricaly (congruently) [21, 68, 86].
Interestingly, but apatites might be dissolved stoichiometrically
or non-stoichiometrically depending on the experimental conditions.
For example, CDHA was found to dissolve congruently at the solution
pH < 4.5, while an incongruent dissolution with Ca/P ratio in solution
attaining values as high as ~25 was detected in solutions within 4.5
< pH < 8.2. Furthermore, almost negligible dissolution with another
type of a non-stoichiometry with Ca/P ratio close to zero in solutions
was found at solution pH > 8.2 [82]. For 4.5 < pH < 8.2 conditions,
the authors explained the incongruent dissolution by re-adsorption
of already dissolved orthophosphate anions from the solution onto
the positively charged CDHA surface. While in more basic solutions,
the surface of CDHA was charged negatively and, thus, the authors
suggested a possibility of both re-adsorption of already dissolved
calcium cations onto CDHA surface and desorption of orthophosphate
ions from the CDHA surface. However, it has to be realized that at
solution pH > 8.2 the uncertainties in the chemical analysis of Ca
(in particular) and P were relatively high, making any quantitative
interpretation of this re-adsorption/desorption difficult [82]. The
same effect was discovered in another study [84]. Furthermore,
dissolution of both FA and natural sedimentary carbonated FA
was found to be incongruent in the early stages but tended toward
congruency as the dissolution reaction proceeded to steady state
[36, 63, 87]. According to the authors, this initial incongruency was
likely a result of the exchange of solution H
+
ion for mineral bound
Ca [36]. Besides, a presence of simple inorganic salts (Na
SO
,
2
4
CH
COONa, or NaCl) in solution appeared to have an influence on
the equilibrium concentrations of orthophosphate and calcium ions
released from apatite: the order of salts increasing orthophosphate
in solution was Na
3
COONa > NaCl, while that increasing
calcium was opposite [79]. This effect was explained by a specific
affinity of some ions (e.g., sulfate) for apatite and might be a reason
for transition of congruent to incongruent dissolution in some cases
[88]. Similarly, presence of other compounds might influence the
dissolution congruence [89]. Other reasonable explanations might
be due to either experimental difficulties in precise following of Ca/P
SO
> CH
2
4
3
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