Physical and chemical characterization of selected natural apatites in synthetic and natural aqueous solutions

Several natural calcium-phosphate apatites, chosen to be representative of the apatite which enters aquatic environments in ground water, runoff and erosion material, were characterized by X-ray diffraction and chemical analyses. Samples were selected to include apatites with widely different structural stabilities and ionic compositions. The solubility of apatite increased as the pH of water and the particle size of apatite crystals were decreased. The concentrations of dissolved ortho-PO₄ ^3? increased in proportion to the amount of apatite added to water and apatite generally increased the pH of water at low slurry densities. The solubility of apatite decreased as the concentrations of ions in water were increased and apatite increased the pH and levels of ortho-PO₄ ^3? in lake water containing relatively high endogenous concentrations of Ca²⁺ and ortho-PO₄ ^3?. Increases in soluble ortho-PO₄ ^3? concentrations in limnetic solutions following apatite addition suggest that partial dissolution of apatite crystals can contribute dissolved ortho-PO₄ ^3? to the P-cycle of aquatic ecosystems. The contribution would probably be of significance under oligotrophic conditions, in areas where anthropogenic P loadings are reduced or in regions receiving high inputs of apatite in the form of erosion material.