Kinetics of dissolution of beta-tricalcium phosphate

The rate of dissolution of beta-tricalcium phosphate (beta-Ca-3(PO4)(2); beta-TCP) has been measured in the solution system Ca(OH)(2)H3PO4-NaOH-HNO3-H2O. The effects of different parameters such as pH, temperature, time, and saturation have been investigated, At zero saturation, the logarithm of the dissolution rate is a linear function of the pH (log(j(0)) = 2.02 - 0.82 pH; r(2) = 0.993; in mmol/m(2)s), indicating diffusion-limited dissolution, A simple calculation of the theoretical rate of a diffusion-controlled process showed that our data are consistent with theory, Moreover, the activation energy for this process is low (E-act = 3.9 kcal/mol) also suggesting that the beta-TCP dissolution is controlled by diffusion processes, At increased saturation, the initial beta-TCP dissolution rate decreases much faster than that predicted assuming a diffusion-controlled model, However, this latter model gives a good prediction of the results if it is assumed that beta-TCP dissolution is controlled by the dissolution of an interfacial layer of hydroxyapatite (Ca-5(PO4)(3)OH;HAp): log(j)= -1.47 + 1.34 log(1 - S-HAp); r(2) = 0.959. The beta-TCP dissolution rate decreases very sharply with time, This effect increases at higher pH or saturation, Several explanations are proposed and discussed, even though none is conclusive. (C) 1997 Academic Press.