Phosphate Sorption in Single and Mixed Fe- and Al-oxide Systems

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The interaction of phosphate with Fe(III) and Al(III) is important in soils, wastes and other systems of environmental significance. The goal of this research was to characterize phosphate sorption in single- and mixed Fe- and Al-oxide systems using XANES (X-ray absorption near edge structure spectroscopy). The specific objectives of this research were: 1) To determine the quantitative distribution of phosphate between Fe-and Al-oxide minerals in mixtures containing these minerals; 2) To assign XANES spectral features for phosphate associated with Fe(III) or Al(III) to specific electronic transitions; and 3) To characterize adsorption versus surface precipitation in single- and binary mixtures of Fe- and Al-oxide minerals. Phosphate was sorbed in single-mineral aqueous suspensions of ferrihydrite (ferric hydroxide), boehmite (aluminum oxyhydroxide), goethite (iron oxyhydroxide), or non-crystalline (non-xl) Al-hydroxide, and mixtures of ferrihydrite/boehmite, goethite/boehmite, and ferrihydrite/non-xl Al-hydroxide at pH 6. Samples were reacted at 22 degrees Celsius for 42 h. Phosphate sorption isotherm trends for mixed-mineral systems were L-curves and were intermediate to those of the respective minerals in the mixture. Phosphorus K-XANES spectra for phosphate on Fe- vs. Al-oxide minerals differed in that a weak doublet peak was observed for Fe-oxides on the low-energy side of the P K-edge, i.e., in the pre-edge region. The quantitative distribution of phosphate between ferrihydrite and boehmite in mixtures of these minerals was determined using linear combination fitting (LCF) analysis of the XANES pre-edge region. Results showed that phosphate essentially distributed itself in proportion to the maximum phosphate sorption capacity of each of these minerals. Using a XANES fitting procedure, phosphate was found to show a greater apparent preference for boehmite and non-xl Al-hydroxide minerals in goethite/boehmite and ferrihydrite/non-xl Al-hydroxide mixtures, respectively. To interpret XANES spectra based on molecular bonding configuration, spectral features were assigned to specific electronic transitions using bonding arguments supported by extended Huckel (EH) model computations of molecular orbital energies (projected density of states-PDOS). Experimental evidence (both XANES and UV-visible spectroscopy) was given for the white-line peak in Fe(III)/phosphate systems being caused by a dipole allowed transition of a P 1s electron to a P(3p)-O(2p) antibonding molecular orbital. Similarly, the white-line peak in Al-phosphate systems was assigned to a dipole allowed transition into a Al(3p)-O(2p)-P(3p) antibonding molecular orbital. The pre-edge feature in XANES spectra was assigned to a dipole allowed transition into a Fe(4p)-O(2p) antibonding molecular orbital. Using these XANES spectral assignments, the increase in FWHM (full width at half maximum height) of the white-line peak in XANES spectra indicated precipitation. Based on a linear increase in FWHM with increasing sorbed phosphate concentration, Al-phosphate surface precipitation occurred in boehmite and non-xl Al-hydroxide systems. On the contrary, no evidence was found for Fe-phosphate precipitation in single-mineral systems of goethite and ferrihydrite. Surface precipitation occurred in goethite/boehmite mixtures following similar trends as in boehmite, but no evidence for surface precipitation was found in ferrihydrite/non-xl Al-hydroxide mixtures over the range of phosphate studied (up to 1230 mmol/ kg). In these mixtures, mineral interactive effects apparently inhibited Al-phosphate precipitation as occurred when phosphate was reacted with non-xl Al-hydroxide alone. Furthermore, phosphate showed a trend of affinity preference for non-xl Al-hydroxide with increasing adsorbed P concentrations in the ferrihydrite/non-xl Al-hydroxide mixtures.



non-crystalline aluminum hydroxide, boehmite, adsorption, XANES, XAFS, goethite, ferrihydrite, phosphorus





Soil Science