wAbstract
@The dissolution and surface complexation of a non-stoichiometric
hydroxyapatite (Ca8.4(HPO4)1.6(PO4)4.4(OH)0.4), (HAP) was studied in the pH range 3.5-10.5,
at 25 in 0.1 M Na(Cl). The results from well-equilibrated batch
experiments, potentiometric titrations, and zeta-potential measurements
were combined with information provided by Attenuated Total Reflectance
Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray Photoelectron
Spectroscopy (XPS). The information from the analyses was used
to design an equilibration model that takes into account dissolution,
surface potential, solution and surface complexation, as well
as possible phase transformations. The results from the XPS measurements
clearly show that the surface of the mineral has a different composition
than the bulk and that the Ca/P ratio of the surface layer is
1.4}0.1. This ratio was also found in solution in the batches
equilibrated at low pH where the dominating reaction is dissolution.
In the batches equilibrated at near neutral pH values, however,
the Ca/P ratio in solution attains values as high as 25, which
is due to re-adsorption of phosphate ions to the HAP surface.
The total concentration of protons as well as the total concentration
of dissolved calcium and phosphate in solution were used to calculate
a model for the dissolution and surface complexation of HAP. The
constant capacitance model was applied in designing the following
surface complexation model:
OCaOH{H+ΜOCaOH2+@@@logΐ(intr.)8.41}0.16
OOPO3H2ΜOOPO3H-{H+@@@logΐ(intr.)-1.11}0.13
OCaOH{HPO42-{H+ΜOCaOPO3H-{H2O@@@logΐ(intr.)11.63}0.15
OOPO3H2{Na+ΜOOPO3Na-{2H+@@@logΐ(intr.)-11.08}0.12
@In addition a solubility product with logΐ-23.27}0.29 was obtained:
Ca8.4HPO4)1.6(PO4)4.4(OH)0.4(s){4.8H+Μ8.4Ca2+{6HPO42-{0.4H2O
@Furthermore, this model predicts a pHzpc
= 7.9, which is in agreement with pHiep =
8.1 obtained from zeta potential measurements. This model can
be used as a helpful tool to predict the reactivity of HAP in
different environments.x
1. Introduction
2. Surface characterization of HAP
@2.1. Surface layer formations
@2.2. Surface site characterization
3. Materials and methods
@3.1. Synthesis and characterization of hydroxyapatite
@3.2. Batch experiments
@3.3. Titrations
@3.4. Zeta potential measurements
@3.5. Attenuated Total Reflectance Fourier Transform InfraRed
(ATR-FTIR) spectroscopy
@3.6. X-ray Photoelectron Spectroscopy (XPS)
4. Surface complexation modeling
5. Results and discussion
@5.1. Surface analysis of HAP
@5.2. Dissolution and surface complexation
@@5.2.1. Region (I): pH4.5 (dissolution)
@@5.2.2. Region (III): pH8.2 (surface complexation)
@@5.2.3. Region (II): 4.5
pH
8.2 (dissolution and surface complexation)
@@5.2.4. Regions (I-III)
6. Conclusions
Acknowledgements
References