Diffuse reflectance infra red spectra and ion-adsorption properties of the phosphate surface complex on goethite

Abstract

The structure of the phosphate surface compIex on goethite was investigated by means of diffuse reflectance infrared spectroscopy and Na⁺ and Cl^- adsorption associated with phosphate adsorption was measured. The differential spectra of the phosphate surface compIex were obtained by subtracting the spectra of goethite from those of phosphated goethite. The spectrum of phosphate adsorbed on the goethite surface changed with increasing pH and this change occurred in one step over the pH range from 3.3 to 11.9. The positions of absorption bands did not change with surface coverage under similar pH conditions. Change of Na⁺~P/Pads. [(Na+ adsorption of phosphated goethite- Na⁺ adsorption of goethite)/P adsorption] with pH was closely related to the spectral change with pH. It was revealed that the PO-H bond of phosphate surface complex on goethite dissociates with increasing pH and that the dissociated phosphate adsorbs Na+. The pH of half dissociation of the PO-H bond was about 6.7 under the conditions where the temperature=298 K and ionic strength (/)= 0.01. These results support the idea of the formation of the bridging binuclear-type surface complex as the mechanism of phosphate adsorption on goethite.