The Influence of Incorporated and Adsorbed Fluoride on the Dissolution of Powdered and Pelletized Hydroxyapatite in Fluoridated and Non-fluoridated Acid Buffers

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Abstract
The effects of fluoride (F) incorporated into hydroxyapatite (HA), adsorbed onto the HA surface, and fluoride in solution were studied during HA dissolution. Dissolution rates at pH 5. and 25°C were determined in fluoridated (0.1, 5.0 μg/mL) and non-fluoridated buffers for the following powdered and pelletized synthesized HAs: non-fluoridated HA, partially-fluoridated (100, 1000, 37,000 μglg) HA, and surface-adsorbed fluoridated (100, 1000 μg/g) HA. The dissolution rate curves were used to derive two components we have called 'diffusion' and 'surface chemical reactivity'. With powders, the rate-determining factor was surface chemical reactivity during the first 60 min. Diffusion was reduced by increasing the F levels in the apatite or buffer. In non-fluoridated buffers, diffusion was reduced 29% for apatite with 100 μg/g incorporated F and 99% with 37,000 μg/g-Diffusion was reduced by 37% and 81 % by 0.1 and 5.0 μg/mL F, respectively, in the buffer. With pelletized HA, the rate-determining factor during the first 10 min was surface chemical reactivity. Diffusion then became relatively more important, but the diffusion rate was independent of F content. During the dissolution of powdered apatites, F was taken up by the remaining apatite solid, as evidenced by the lower F concentration in solution than that calculated to be released during dissolution. There was also a decrease in the F concentration of the 0.1 μg/mL buffer as reaction progressed. Pelletized apatites did not release enough F to influence dissolution significantly. Surface-adsorbed F was more effective than incorporated F in reducing HA dissolution. However, solution F present at the apatite surface in small concentrations (