Phosphate adsorption on the iron oxyhydroxides goethite (α-FeOOH), akaganeite (β-FeOOH), and lepidocrocite (γ-FeOOH): a 31P NMR Study

Phosphate adsorption on the surfaces of the iron oxyhydroxide polymorphs goethite, akaganeite, and lepidocrocite were studied by using 31P static spin-echo mapping NMR experiments to determine how this environmentally-important anion binds to common soil minerals. The large 31P hyperfine shifts confirm the formation of inner-sphere complexes between the phosphate anion and the iron oxyhydroxide surface, the large shifts indicating the presence of Fe3+–O–P covalent bonds. Binding was explored as a function of pH and phosphate concentrations, the phosphate ion binding via two oxygen ions to the oxyhydroxide surface under all conditions and for all the surfaces. To support our analysis of the NMR spectra, adsorption of dimethyl phosphinic acid (DPA) on iron oxyhydroxides was also investigated, since this ion can only bond via one Fe–O–P interaction to the surface. The 31P hyperfine shifts observed for this anion were 50% of those seen for the phosphate anions, confirming that the phosphate ions bind to the surface via two P–O–Fe linkages.