Infra-red Study of Phosphate Adsorption on Goethite

Abstract

Infra-red spectra of phosphate adsorbed on synthetic goethite (a-FcOOH) show only PO^- ions to be present. Phosphate bands observed at about 1125, 1050 and 1000 cm-1 are shifted to 1J90, 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 v3 vibration of phos phate 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.