[Sorption kinetics of fluoride by artificial and natural hydroxyapatite]

The nature and kinetics of fluoride uptake by hydroxyapatite under various conditions remain the object of interest. This problem was now investigated with an experimental model reproducing as closely as possible the conditions in vivo. The aim of this work was: (1) to study the kinetics of fluoride uptake by natural and artificial hydroxyapatite depending on experimental conditions and to determine the reaction rate constants; (2) to describe the equilibrium of fluoride uptake with adsorption isotherms and develop a best fit mathematical model for the process taking place under various experimental conditions; (3) to determine and compare the capacity for fluoride uptake by natural and artificial hydroxyapatite depending on experimental conditions. Attention has focused on the equilibrium and kinetics of the process of fluoride uptake under conditions as similar to those in a living organism as it is possible to reproduce in vitro. Those conditions represented just one of sixteen various experimental setups differing as to process parameters of the experimental system. The equilibria and kinetics of the experimental system were determined basing on measurements of selected parameters. Adsorption isotherms were obtained experimentally and a best fit mathematical model was developed to describe the process. Additionally, maximal capacity for fluoride uptake was calculated, as well as equilibrium constants, adsorption and desorption rate constants for both hydroxyapatite types. The following conclusions were drawn: 1. Fluoride uptake by artificial and natural hydroxyapatite is a biphasic process essentially independent of conditions in the reaction environment adopted in the present work. The first phase is rapid and does not exceed some 15 minutes. The second phase is much slower and takes place over a period of several dozen hours. 2. Fluoride sorption by both hydroxyapatites is essentially a physico-chemical process which can mathematically be best described with Langmuir and Langmuir-Freundlich adsorption isotherms. Under conditions of equilibrium, the adsorbed substance forms a monolayer on the surface of the sorbent. 3. The binding of fluoride by natural hydroxyapatite is stronger than by its artificial counterpart. 4. The stability of fluoroapatite formed during fluoride uptake by natural hydroxyapatite is greater than that of its artificial counterpart. 5. Natural hydroxyapatite has a relatively large capacity for fluoride ions under experimental conditions adopted in the present work. This capacity exceeds that of artificial hydroxyapatite in spite of smaller specific surface of the natural substance.