Inhibition of the Dissolution of Calcium Carbonate Filler
DISCUSSIONS
Based on the above results, it is certain that when GCC was treated with H3PO4, new solid phase which composed of Ca, P and O were formed on the surface of GCC particles which are serve as "centers of crystallization". However, the TEM scans and XPS which showed a high C signal indicated that full coverage of GCC surfaces by the fragments was not likely. One of the possible reasons why full coverage was not achieved is that these fragments are fragile with poor adhesion to the CaCO3 surfaces. Detachment and breakage could also occurred during filtration and drying of the samples.
At this point, it is difficult to determine the stoichiometric composition of the newly formed solids. The Ca/P ratio for treated GCC from XPS did not match with any common low solubility Ca-P compounds. However, it is important to note that the thin surface layer that XPS analyzed may consist of both original CaCO3 and Ca-P phases. Since the intensity of Ca peak may origin from both of these phases, it may be misleading to determine the stoichiometry of the Ca-P compound from Ca/P ratio alone.
Although the exact composition of the precipitated solid phase is not known, it is still possible to speculate how H3PO4 plays a role on the inhibition of CaCO3 dissolution. When CaCO3 is initially added to the water which consisted of H3PO4, CaCO3 dissolves rapidly due to the acidic condition. In the absence of H3PO4, the dissolution introduces carbonate into the solution and causes the pH to rise rapidly to pH above 9 before falling back to an equilibrium pH of 8.3. However, in the presence of H3PO4, the pH rose slowly and steadily to the equilibrium pH due to the strong buffering ability of the polyprotic acid.
The increase in the dissolved Ca concentration causes Ca-P compound to precipitate when its solubility is exceeded. The presence of CaCO3 particles in the system provide a favorable place for the crystallization to occur. As the Ca-P compound is precipitated on the surface of CaCO3 particles, it acts as a barrier and hinders the dissolution. With time, Ca2+ continuously precipitates with P onto the CaCO3 surfaces until the Ca-P compound fully covers the CaCO3 particles and prevents further dissolution. At this point, Ca2+ cannot be replenished and at the same time it will continuous to precipitate with P and reach an equilibrium concentration which depends on the solubility of the Ca-P compound.