Two-dimensional solid-state NMR spectroscopy investigations of surface precipitation of phosphate onto calcite

The interaction of phosphate with typical soil minerals is important for understanding P cycling in natural and agricultural systems. We investigated the mechanisms of kinetics of phosphate uptake onto calcite using solid-state NMR spectroscopy. At a low phosphate concentration of 0.5 mM, the ³¹P single-pulse solid-state NMR peak revealed the formation of amorphous calcium phosphate (ACP) within the initial 30 min, which transformed to carbonated hydroxyapatite (CHAP) after 12 d. At a high phosphate concentration (5 mM), the results showed transformation from ACP to OCP, later to brushite, and eventually to CHAP. The formation of brushite is further supported by ³¹P{¹H} heteronuclear correlation (HETCOR) spectra via a correlation of δ_P-31 = 1.7 ppm and the ¹H peak at δ_H-1 = 6.4 ppm, which denotes the structure water of brushite. Furthermore, ¹³C NMR directly revealed both A-type and B-type CHAP. Generally, this work provides a detailed understanding of the aging effect on the phase transition scale of phosphate surface precipitation onto calcite in soil environments.