Phosphate sorption on monohydrocalcite

Abstract

The sorption behavior and the mechanism of phosphate sorption on monohydrocalcite (CaCO3·H2O; MHC) were analyzed on the basis of sorption isotherms corresponding to three Mg concentrations. MHC is known to be metastable, and it quickly transforms to calcite or aragonite. However, a small amount of phosphate (30 μM) can significantly inhibit the transformation of MHC. At low phosphate loading, the sorption isotherms obtained are Langmuir adsorption isotherms. The mode of phosphate sorption is most likely to be adsorption on MHC surface at the low phosphate loadings. The Langmuir parameters indicate that the mass basis sorption capacity of MHC is significantly higher than those of aragonite and calcite. At higher phosphate loadings, the slopes of the obtained isotherms rapidly increase and deviate from that of a Langmuir isotherm. The slopes of the isotherms for low Mg concentrations are higher than those for high Mg concentrations. It is well documented that Mg inhibits the formation of calcium phosphate minerals. The saturation calculations performed in this study showed that the reacted solutions become supersaturated with respect to amorphous calcium phosphate at corresponding high phosphate loadings. The mode of phosphate sorption at higher phosphate loadings is considered to correspond to the formation of secondary calcium phosphate minerals.