DISSOLUTION KINETICS OF BIOAPATITE AT pH 2 TO 6.5 AND 21 C

Dissolution experiments were conducted on bioapatite at 21°C and solution pH values between 2 and 6.5 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. The BET surface area was 255 m² g^-1. During experiments the ratio of Ca/P released became stoichiometric at ca. 1.38. 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 ≈ 0.75, log K₁ ≈ -6.32, and log K₂ ≈ -9.36, and H⁺ is the hydrogen ion activity. Above pH 4 dissolution rate is independent of pH. 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. Bioapatite dissolution rates have implications for fossilization, use of bone as fertilizer, and as reactive barriers to control pollutants and nuclear waste isolation.