Biomedical Engineering Reference
In-Depth Information
“tough” dissolution conditions are common in industry of fertilizer
production [124] and have to be taken into account by the final
version of any dissolution model.
However, there are no doubts that each of the models discussed
is correct for the specific experimental conditions studied. The
different models are just devoted to diverse aspects of the same
process of apatite dissolution and are likely to complement each other.
Undoubtedly, the dissolution process of calcium apatites has all the
features elaborated within eight dissolution models. Unfortunately,
it is still impossible to obtain the complete understanding for all
the processes involved because experimental techniques able to
provide direct following after diffusion, sorption, detachment, and
transformation of the single atoms, ions, and molecules currently are
lacking. Therefore, researchers are still forced to make conclusions
about the events happening at the atomic (ionic) level based on
indirect measurements and/or calculations only. That is why,
combining the experimental and theoretical findings, obtained by
different investigators (often under hardly comparable experimental
conditions), currently, seems to be the only way to create a general
depiction of the dissolution mechanism of calcium apatites in acids.
7.4
A Reasonable Classification of the
Dissolution Models
At the fundamental level reactions between solids and liquids involve
a coupled sequence of mass transport, adsorption/desorption
phenomena, heterogeneous reaction, chemical transformations of
intermediates, etc., whose identification, separation, and kinetic
quantification are all necessary if the mechanism of the process is to
be fully understood and described [125]. It is also generally agreed
that during dissolution lattice ions undergo the following processes:
(1) detachment from a kink site, (2) surface diffusion away from
the crystal steps, (3) desorption from the surface and (4) diffusion
into the bulk solution [126]. Therefore, the steady-state conditions
of apatite dissolution in aqueous acidic media include the following
simultaneous processes or steps:
1. diffusion of chemical reagents (H
+
and anions of acid A
n−
)
from bulk solution to the solid/liquid interface;
2. adsorption of the chemicals onto the surface of apatite;
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