Infra-red study of phosphate adsorption on goethite

Abstract

Infra-red spectra of phosphate adsorbed on synthetic goethite (α-FeOOH) show only PO^3–₄ ions to be present. Phosphate bands observed at about 1125, 1050 and 1000 cm^–1 are shifted to 1190, 1100 and 1000 cm^–1 following the removal of physically adsorbed H₂O or D₂O. Comparison of the spectra with model systems suggests that the high frequencies observed for the ν₃ vibration of phosphate in evacuated samples arise from binuclear coordination (coordination of two oxygen atoms of a phosphate group to two surface Fe ions) to Fe³⁺ on the surface. Binuclear sites on the predominant (100) and (010) faces are due to the unshared apices of adjacent Fe octahedra. Spectral changes due to physically adsorbed water in non-evacuated samples are attributed to strong hydrogen bonding to one of the free phosphate oxygens.