The crystallization of calcite in the presence of orthophosphate

The kinetics of crystal growth of calcite seed crystals was investigated at pH 8.50, 25°C, in stable calcium carbonate supersaturated solutions. An apparent rate order of 2 suggests a spiral growth mechanism. The presence of inorganic orthophosphate in the supersaturated solutions at levels as low as 8 × 10−8 mole dm−3 completely inhibited crystal growth. Application to a kinetic Langmuir-type model suggested that adsorption of phosphate at the active growth sites is responsible for the reduction in the crystal growth rates. Adsorption studies revealed that phosphate adsorbs on calcite and the Langmuir equation was found to give a satisfactory fit of the data at ionic strength 0.1 mole dm−3. At lower solution ionic strengths, adsorption was enhanced, and the plateau attained still corresponded to a surface coverage lower than that anticipated for a monolayer of inorganic orthophosphate anions. The ionic strength dependence, however, suggests that the calcite—phosphate interaction is mainly electrostatic in its nature. Electrokinetic measurements yielded an isoelectric point for calcite at pH 10.4 ± 0.3. Adsorption of orthophosphate at the calcite/water interface resulted in more negative electrokinetic charges, as a consequence of the adsorption of negative phosphate species in the solution.