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

Abstract

Changes in aqueous- and solid-phase plutonium oxidation state were monitored over time in hematite (alpha-Fe2O3) and goethite (alpha-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 (k(rxn)), including both adsorption and reduction of Pu(V), was 1.75+/-2.05 x 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 x 10(-5) and 1.5 x 10(-4) s(-1), respectively. For hematite, the reaction rate increased by 3 orders of magnitude across the same pH range.

Development of a Composite Non-Electrostatic Surface Complexation Model Describing Plutonium Sorption to Aluminosilicates

“…This is consistent with the observation of Pu(V) as the stable aqueous oxidation state in slightly acidic to neutral solutions including ocean and lake water and suspensions from Pu(IV) solubility experiments. Additionally, in all Pu sorption experiments where the oxidation state of the aqueous Pu fraction in a solid suspension was measured, Pu(V) was found to be the dominant oxidation state (Banik et al, 2007;Keeney-Kennicutt and Morse, 1985;Penrose et al, 1987;Powell et al, 2006;Powell et al, 2005;Powell et al, 2008;Sanchez et al, 1985). This was found to be the case for this work as well.…”

“…Surface mediated reduction of Pu(V) to Pu(IV) as discussed above is affected by pH. Powell et al (2005) found a -0.2 and -0.4 order dependence on the hydrogen ion concentration for the rate of Pu(V) reduction on goethite and hematite, respectively. Therefore, reduction of Pu(V) was expected to be more favored with increasing pH.…”

“…An alternate hypothesis is that light or alpha particle induced peroxide formation may result in the reduction of Pu(V) to Pu(IV). Reduction of Pu(V) to Pu(IV) on a variety of mineral surfaces has been observed (Keeney-Kennicutt and Morse, 1985;Penrose et al, 1987;Powell et al, 2006;Powell et al, 2004;Powell et al, 2005;Sanchez et al, 1985). Most notably, Pu(V) reduction to Pu(IV) was observed on surfaces expected to be either oxidizing or non-reactive such as pyrolusite (-MnO 2 ), tuff (Pu predominantly associated with ranciete/Mn(IV) phase), hematite (-Fe 2 O 3 ), goethite (-FeOOH), and silica (SiO 2 ) (Powell et al, 2006;Powell et al, 2005).…”

EBS Radionuclide Transport Abstraction

“…They found that the sorption rate was strongly pH-dependent for hematite, but only weakly pH-dependent for goethite. For both minerals, they found that, at pH values greater than pH 4.5, Pu(V) reduced to Pu(IV) on the surface of the goethite, with nearly complete adsorption and conversion within days or weeks, depending on the pH (Powell et al 2005 [DIRS 174726], Figures 2 and 3). For hematite, sorption of Pu(V) onto the surface was the rate-limiting step in the transformation; for goethite, reduction was the rate-limiting step.…”

Pu(IV) Intrinsic Colloid Stability in the Presence of Montmorillonite at 25 & 80 C

“…Among the most ubiquitous inorganic colloids are aluminosilicate clays that are commonly observed as mobile colloids due to their inherently small particle size and prevalence as alteration minerals from original host rock material (Kersting et al, 1999). Clays are known to sorb Pu Keeney-Kennicutt and Morse, 1985;Kozai et al, 1996;Kozai et al, 1993;Lujaniene et al, 2007;Powell et al, 2004Powell et al, , 2005Powell et al, 2008;Sabodina et al, 2006;Sanchez et al, 1985;Turner et al, 1998). In addition, the proposed use of bentonite within some engineered barrier systems scenarios for high-level nuclear waste repositories provides additional importance to understanding Pu interaction with smectite aluminosilicate clays (Sabodina et al, 2006).…”