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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

View larger version (32K): [in a new window]   Fig. 1. Edge-normalized, stacked P K-XANES spectra for PO4 adsorbed on boehmite, ferrihydrite (ferri.) or mixed-mineral systems (June 2002 at pH 6.0 ± 0.1) at selected concentrations. Numbers in the legend denote adsorbed PO4 in mmol kg–1.

View larger version (25K): [in a new window]   Fig. 2. Adsorption isotherms for PO4 on boehmite, ferrihydrite, and mixed boehmite/ferrihydrite (1:1 mass basis) at pH 6.0 ± 0.1, along with Freundlich isotherm models as solid lines for the June 2002 data. Data for the mixed mineral isotherm for June 2002 are fit using a mass weighted (1:1) linear combination of Freundlich models from the single-mineral systems (dashed line, see text). Some additional data collected in October 2002 for PO4 adsorbed on ferrihydrite and mixed-mineral systems are shown. qf, qm, qb, denote the Freundlich model predicted PO4 adsorption for ferrihydrite (f), mineral mixtures (m), and boehmite (b) as a function of dissolved PO4 concentration (c).

View larger version (23K): [in a new window]   Fig. 3. Edge-normalized P K-XANES spectra for strengite versus variscite and ensemble-averaged spectra for PO4 adsorbed on ferrihydrite (ferri.) versus boehmite at pH 6.0 ± 0.1, showing a pre-edge feature for PO4 associated with Fe(III).

View larger version (28K): [in a new window]   Fig. 4. Comparison of the pre-edge region for edge-normalized XANES spectra for PO4 adsorbed in mixed-mineral systems (June 2002) at selected concentrations with spectra for averaged standards for PO4 adsorbed on boehmite or ferrihydrite at pH 6.0 ± 0.1. Least square linear fits to the mixed-mineral systems using a combination of the averaged standards for PO4 adsorbed on boehmite or ferrihydrite are also included. Numbers in the legend denote adsorbed PO4 concentrations in mmol kg–1.

View larger version (31K): [in a new window]   Fig. 5. Pre-edge region in edge-normalized P K-XANES spectra for PO4 adsorbed on ferrihydrite (ferri.—June 2002 and October 2002 data) or boehmite at various concentrations at pH 6.0 ± 0.1 along with averaged standards for PO4 adsorbed on each mineral. Numbers in the legend denote adsorbed PO4 concentrations in mmol kg–1.

View larger version (21K): [in a new window]   Fig. 6. Concentration of PO4 adsorbed on ferrihydrite (as calculated from XANES fitting analysis) plotted as a function of total adsorbed PO4 concentration in ferrihydrite-boehmite mixtures. (A) Linear regression fits for June 2002 and October 2002 data; (B) Comparison of June 2002 data with a no-preference line based on distribution of adsorbed PO4 between the minerals in direct proportion to their relative contributions to the maximum adsorption capacity of the mixed systems.