An investigation of phosphate adsorbed on aluminium oxyhydroxide and oxide phases by nuclear magnetic resonance

The adsorption of phosphate by soil minerals controls availability of P to plants, but the chemical environments of adsorbed phosphate are poorly known. We used ³¹P MAS NMR to study the adsorption of phosphate on to boehmite (γ-AlOOH) and γ-Al₂O₃ with large surface areas. The solid phases were reacted in 0.1 M phosphate solutions at pH from 3 to 11 and in solutions with pH 5 at concentrations from 10^-1 M to 10^-4 M. The spectra suggested three different phosphate environments: (i) orthophosphate precipitated from the residual solution after vacuum filtering, (ii) surface-adsorbed phosphate in inner-sphere complexes, and (iii) Al-phosphate precipitates on the surfaces of the minerals. The chemical shifts of both the inner-sphere complexes and surface precipitates became progressively less shielded with increasing pH and decreasing concentration of phosphate solution. For the inner-sphere complexes, we interpret these changes to be the result of decreasing phosphate protonation combined with rapid proton exchange among phosphate tetrahedra with different numbers of protons, which causes peak averaging. The chemical shifts of ³¹P of the Al-phosphate precipitates were more negative than those of the surface phosphates at a given pH and solution concentration, probably because of a larger number of P-O-Al linkages per tetrahedron. The observed trend of decreasing shielding is probably due to the decreasing average number of P-O-Al linkages per tetrahedron combined with decreasing protonation and an increasing number of K⁺ next-nearest neighbours. Even at small concentrations of phosphate solution, a significant amount of Al-phosphate precipitate was present.