Model of inner-sphere adsorption of oxyanions in goethite - Why is phosphate adsorption more significant than that of sulfate?

Abstract

Phosphorus availability in soils is low due to its strong retention by inner-sphere complexation on minerals in the clay fraction with pH-dependent charges, such as goethite. On the other hand, sulfur has greater availability because it is retained mainly by electrostatic attraction. We evaluated the intensities of the inner-sphere complexation of orthophosphate and sulfate (H 2 PO 4- /HPO 42- and SO 42- - generically treated as PO 4 and SO 4 ) under different experimental conditions (pH, goethite purity, and contact times) on synthetic goethite samples to establish the mechanisms and models involved in those bonds. Inner-sphere PO 4 and SO 4 were extracted using both HNO 3 1 mol L -1 and USEPA 3051A methods. Inner-sphere complexation of PO 4 and SO 4 was highest at pH 5 in relation to pH 9. Attenuated total reflectance/Fourier transform infrared spectroscopy (ATR-FTIR) spectra showed inner-sphere complexation bands of PO 4 on goethite in the protonated binuclear bidentate (pH 5) and deprotonated binuclear bidentate (pH 9) forms. Inner-sphere complexation of PO 4 was much more expressive than that of SO 4 . Phosphorus and sulfur oxyanions displace the diprotonated ferrol ligand (-OH 2+0.5 in -FeOH 2+0.5 ), while the -OH -0.5 in the -Fe-OH -0.5 group are only displaced by PO 4 . The -O -1.5 ligand in Fe-O -1.5 group is not displaced by PO 4 or SO 4 . The high surface negative charge density of PO 4 defined its higher activation energy for exchanging -OH 2+0.5 and -OH -0.5 on the goethite surface in relation to SO 4 . The proposed model can be used to reduce inner sphere phosphate adsorption in soils and improve P fertilization efficiency for farming.