Phosphate uptake by calcite: Constraints of concentration and pH on the formation of calcium phosphate precipitates

Abstract

The uptake of phosphate by carbonates plays an important role in controlling the fate and transport of phosphorus in soil, but the mechanisms are not fully understood. In this study, 31P solid-state NMR was used to investigate the mechanisms of phosphate uptake by calcite over wide pH (6–10) and concentration (5 μM – 10 mM) ranges. It was found that calcium phosphate precipitates formed at very low phosphate concentrations (~ 50 μM, pH 8). Because of their low yield and poor crystallinity, the precipitates were not directly detected by powder X-ray diffraction (XRD) but were unequivocally identified by a single NMR peak at δP-31 = ~2.9 ppm. This peak was assigned to carbonated hydroxyapatite (CHap) based on further characterization with 2-D 31P{1H} heterogeneous correlation (HetCor) experiments and high-resolution transmission electron microscopy (HRTEM). When [PO43−] was below 30 μM at pH 8, a broad peak at δP-31 = ~4.0 ppm was observed with an FWHM of ~5 ppm; this was assigned to surface adsorbed phosphate on calcite owing to its distinct chemical shift and insignificant 31P-1H cross-polarization (CP) effect. Additionally, we found that the presence of a small amount of dissolved fluoride facilitated the formation of carbonate fluorapatite (CFap), implying that calcite dissolution plays an important role in phosphate sequestration. This study indicated that calcite may be effective for mobilization dissolved phosphate and lowering its bioavailability in aquatic and soil systems.