Solubility behavior of apatites in seawater1

The solubilities of apatitcs of varying compositions were measured as a function of pH in seawater at 10 ” and 25°C. Wide ranges of solubilities were observed; at pH, = 7.4 they ranged from 0.4 to 11 PM Poe-P liter-l, depending on the nature of the samples. Results are interpreted in terms of complex coatings formed on the surface of the apatitcs. The goals of our work were to define the solubility of apatites in seawater better and to examine the differences in solubility of apatites of different composition, We found that the solubility behavior is best described in terms of complex reactions on the apatite surface rather than by simple solubility theory. Studies of the solubility oE apatites in sea-water have heretofore been based on the assumption of simple phases equilibrating with the seawater. Sill&r (1961) suggested that hydroxyapatite controls the equilib-rium concentration of phosphate in seawa-ter. Kramer (1964) and Roberson (1966) postulated that francolite, a carbonate fluor-apatite, rather than hydroxyapatite is the solid phase which occurs in seawater and that its solubility is the one that should be studied. Differences in their results and those of Smirnov et al. (1962), and the relatively poor precision obtained by Ro-berson, made it impossible to determine accurately the saturation state of seawater with respect to apatite. The apatite surface is very susceptible to rearrangement or to complex formation in distilled water solutions (Dictz et al. 1964; Smith ct al. 1974). Greenwald ( 1942) reported surface area effects on the solu-bility oE calcium phosphates, although he used a poorly defined solid phase in his work. Levinskas and Neumann (1955) found a decrease in the solubility of hy-l This work was supported by the Oceanograph