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Iron and Phosphate Dissolution during Abiotic Reduction of Ferrihydrite-Boehmite Mixtures

^a Soil and Environmental Consultants, PA, 11010 Raven Ridge Rd., Raleigh, NC 27614
^b Dep. of Soil Science, Box 7619, North Carolina State Univ., Raleigh, NC 27695-7619

View larger version (17K): [in a new window]   Fig. 1. Adsorption isotherms for PO4 on ferrihydrite or boehmite (22°C) and Fe(II) on boehmite (25°C) at pH 6.0 in 0.01 mol L–1 KCl background. Smooth curves are Freundlich model fits to the data.

View larger version (25K): [in a new window]   Fig. 2. Selected data for temporal changes in dissolved Fe(II) during abiotic reduction of ferrihydrite–boehmite mixtures containing 0.5 g ferrihydrite kg–1 suspension, 0–0.008 g boehmite kg–1 suspension, and 750 mmol P kg–1 ferrihydrite.

View larger version (23K): [in a new window]   Fig. 3. Trends in dissolved Fe(II) during abiotic reduction (pH 6.0) of ferrihydrite after adding aqueous Al(III) at rates equivalent to Al(III) additions with 0, 0.002, 0.008, or 0.08 g boehmite kg–1 suspension. Aqueous Al(III) rates were 0–2667 mmol kg–1 ferrihydrite (0–1.33 mmol kg–1 suspension).

View larger version (21K): [in a new window]   Fig. 4. Trends in zero-order rate coefficients for Fe(II) dissolution during abiotic reduction of mixed-mineral suspensions as a function of added Al(III) (mmol kg–1 suspension) as boehmite or aqueous Al(III) (AlCl3). The inset figure shows the model fit to the data excluding inputs of 0 mmol Al kg–1, for which the inverse of the Al(III) input (1/Al) is undefined.

View larger version (21K): [in a new window]   Fig. 5. Temporal changes in dissolved PO4 for selected abiotic reduction experiments on suspensions containing 0.5 g ferrihydrite kg–1; 0, 0.002, 0.008, or 0.3 g boehmite kg–1; and 750 mmol P kg–1 ferrihydrite. Each suspension containing boehmite exhibited trends analogous to those shown.

View larger version (23K): [in a new window]   Fig. 6. Phosphorus K-XANES spectra with an inset of the white-line region (a) and the pre-edge region of XANES spectra (b) for moist samples collected at various times from an abiotic reduction experiment on a suspension containing 1:1 by mass ferrihydrite/boehmite and 750 mmol P kg–1 total solids. Spectra for standards of PO4 adsorbed on boehmite (500 mmol kg–1) or ferrihydrite (1000 mmol kg–1) and noncrystalline (non-xl) Al(III)- and Fe(III)-phosphate are included.

View larger version (21K): [in a new window]   Fig. 7. Dissolved Fe(II) as a function of acid added to maintain pH 6.0 in selected reduction reactor experiments. Suspensions contained 0.5 g ferrihydrite kg–1, 750 mmol P kg–1 ferrihydrite, and boehmite additions of 0, 0.002, 0.008, or 0.3 g kg–1 suspension.