Infrared spectra of phosphate sorbed on iron hydroxide gel and the sorption products

Infrared absorption spectra of phosphate sorbed on iron hydroxide gel were obtained by applying the differential diffuse reflectance method. Absorption bands due to P-O stretching vibration were observed at 1,110 and 1,010 cm^-1 at pH 12.3. With decreasing pH, these absorption bands gradually shifted to 1,100 and 1,020 cm^-1 at pH 4.9. At pH 2.3, they became a broad single absorption band at 1,060 cm^-1. At pH 11 or above, the difference in the Na⁺ adsorption between phosphated iron hydroxide gel and iron hydroxide gel was almost equal to the amount of phosphate sorption. This finding shows that phosphate was retained on the iron hydroxide gel surface as a bidentate ligand at a high pH. It was concluded that at a high pH phosphate was sorbed on iron hydroxide gel as a binuclear surface complex similar to that on goethite; the change in spectra for P-O stretching vibration with decreasing pH value was mainly caused by an increase in the fraction of amorphous iron phosphate; at pH 2.3, the phosphate sorption product consisted of amorphous iron phosphate. (author)