DISSOLUTION KINETICS OF BIOAPATITE FROM PH 2 TO 8 AT 4° TO 38°C

Dissolution experiments were conducted on bioapatite at 4°C, 21°C and 38°C and solution pH values between 2 and 8 in a stirred tank reactor. The bioapatite was obtained from a modern white-tailed deer scapula, crushed, and cleaned in 30% hydrogen peroxide to remove organic matter. Bioapatite particles from 75-106 and 106-246 µm yielded an average BET specific surface area of 255 m² g^-1. At 21°C, the following dissolution rate law was derived from the experimental results:

R = k₁(H⁺)ⁿ + k₂

in which R is the dissolution rate (mol bioapatite m^-2 s^-1) based on the bioapatite stoichiometry, n = 1.01 ± 0.15, k₁ = 1.84 x 10^-6 mol m^-2 s^-1, and k₂ =4.29 x 10^-10 mol m^-2 s^-1, and H⁺ is the hydrogen ion activity. From 2 < pH < 4, the dissolution rate is dependent on pH and becomes independent of pH from 4 < pH < 8. These results for modern bone may be compared with previous investigations of igneous (FAP) and phosphorite-derived sedimentary carbonate fluorapatites (CFA)(e.g., Guidry and Mackenzie, 2003). At pH = 6, in the pH-independent region, dissolution of the modern bioapatite was ca. 7 times faster than FAP and 100 times faster than CFA. The acid transition pH of the bioapatite is lower than that for FAP. The apparent activation energy of bioapatite dissolution, 18.1 kJ mol^-1, suggests a diffusion-controlled reaction. Previous studies of dissolution reactions for other forms of apatite have yielded higher activation energies, indicating that their dissolution reactions are surface controlled. BET surface area analyses of bioapatite particles are approximately 5 orders of magnitude greater than calculated geometric surface areas. BET surface areas are therefore critical for calculating accurate bioapatite dissolution rates. Bioapatite has been suggested for use as reactive barriers to control pollutants and nuclear waste isolation and as a feedstock mineral in CO₂ sequestration processes. Modeling the dissolution of bioapatite is necessary to determine its potential for these uses.