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DIVISION S-2—SOIL CHEMISTRY

XANES Determination of Adsorbed Phosphate Distribution between Ferrihydrite and Boehmite in Mixtures

^a Dep. of Soil Science, Box 7619, North Carolina State University, Raleigh, NC 27695-7619
^b Natural Resources Canada, CANMET, 555 Booth St., Office 332A, Ottawa, ON, KIA 0G1
^c Dep. of Soil and Environ. Sci., National Chung Hsing University, Taichung, 402, Taiwan
^d Dep. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071

^* Corresponding author (dean_hesterberg{at}ncsu.edu).

Iron- and Al-(hydr)oxide minerals are important sorbents for retaining PO₄ in soils. Our objective was to determine the distribution of adsorbed PO₄ between ferrihydrite and boehmite in aqueous mixtures of these minerals. Phosphate was adsorbed in aqueous suspensions up to maximum concentrations of 1860, 850, and 1420 mmol kg^–1 for ferrihydrite, boehmite, and 1:1 (by mass) mixtures of these minerals at pH 6. The solids were analyzed as moist pastes using P K-XANES (X-ray absorption near edge structure) spectroscopy. The adsorption isotherm for the mixed-mineral suspensions could essentially be described as a linear combination of Freundlich isotherm models for each single-mineral system, indicating negligible mineral interactive effects on PO₄ adsorption in the mixtures. X-ray absorption near edge structure spectra for PO₄ adsorbed on ferrihydrite or in ferrihydrite/boehmite mixtures showed a pre-edge feature at approximately 2146 eV that was absent in boehmite systems. Linear combination fitting of the pre-edge region of XANES spectra for mixtures with average spectra for PO₄ adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO₄ was adsorbed on ferrihydrite in the mixtures. With increasing concentration of adsorbed PO₄ in the mineral mixtures, the concentration adsorbed on the ferrihydrite component increased linearly. Phosphate distribution trends in the mixtures suggested an affinity preference for ferrihydrite at the lowest adsorbed PO₄ concentration (100 mmol kg^–1 minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO₄ kg^–1), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO₄ kg^–1).

Abbreviations: LCF, linear combination fitting • XANES, X-ray absorption near-edge structure

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