Kinetics of Dissolution of beta-Tricalcium Phosphate

The rate of dissolution of beta-tricalcium phosphate (beta-Ca3 (PO4 )2 ; beta-TCP) has been measured in the solution system Ca(OH)2 -H3 PO4 -NaOH-HNO3 -H2 O. 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/m2 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 (Ca5 (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.