Pu(V)O2+ Adsorption and Reduction by Synthetic Hematite and Goethite

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Abstract
Changes in aqueous- and solid-phase plutonium oxidation state were monitored over time in hematite (α-Fe2O3) and goethite (α-FeOOH) suspensions containing 239Pu(V)-amended 0.01 M NaCl. Solid-phase oxidation state distribution was quantified by leaching plutonium into the aqueous phase and applying an ultrafiltration/solvent extraction technique. The technique was verified using oxidation state analogues of plutonium and sediment-free controls of known Pu oxidation state. Batch kinetic experiments were conducted at hematite and goethite concentrations between 10 and 500 m2 L-1 in the pH range of 3−8. Surface-mediated reduction of Pu(V) was observed for both minerals at pH values of 4.5 and greater. At pH 3 no adsorption of Pu(V) was observed on either goethite or hematite; consequently, no reduction was observed. For hematite, adsorption of Pu(V) was the rate-limiting step in the adsorption/reduction process. In the pH range of 5−8, the overall removal of Pu(V) from the system (solid and aqueous phases) was found to be approximately second order with respect to hematite concentration and of order −0.39 with respect to the hydrogen ion concentration. The overall reaction rate constant (krxn), including both adsorption and reduction of Pu(V), was 1.75 ± 2.05 × 10-10 (m-2 L)-2.08 (mol-1 L)-0.39 (s-1). In contrast to hematite, Pu(V) adsorption to goethite occurred rapidly relative to reduction. At a given pH, the reduction rate was approximately independent of the goethite concentration, although the hydrogen ion concentration (pH) had only a slight effect on the overall reaction rate. For goethite, the overall reaction rates at pH 5 and pH 8 were 6.0 × 10-5 and 1.5 × 10-4 s-1, respectively. For hematite, the reaction rate increased by 3 orders of magnitude across the same pH range.

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