Pu(V)O₂⁺ Adsorption and Reduction by Synthetic Magnetite (Fe₃O₄)

Abstract

Changes in aqueous- and solid-phase Pu oxidation state were monitored over time in magnetite (Fe3O4) suspensions containing 239Pu(V)-amended 0.01 M NaCl. Oxidation state distribution was determined by leaching of Pu into an aqueous phase followed by an ultrafiltration/solvent extraction technique. The capability of the technique to measure Pu oxidation state distribution was verified using 230Th(IV), 237Np(V), and 233U(VI) as oxidation state analogues. Reduction of Pu(V) was observed at all pH values (pH 3 to 8) and magnetite concentrations (10 to 100 m2 L(-1)). In the pH range 5 to 8, adsorption was a rate-limiting step, and reduction was mediated by the solid phase; at pH 3 reduction occurred in the aqueous phase. The overall reaction (describing both adsorption and reduction of Pu(V)) was found to be approximately first order with respect to the magnetite concentration and of order -0.34+/-0.02 with respect to the hydrogen ion concentration. Assuming first order dependence with respect to Pu, the overall reaction rate constant was calculated as k(rxn) = 4.79+/-0.62 x 10(-8) (m(-2) L)0.99(mol(-1) L)-0.34(s(-1)). The Pu(IV) solid-phase species became more stable over time.

A Discussion of Reversible and Irreversible Sorption for Sr, Cs, Np, and Pu

“…This suggests that Pu became more strongly associated with the solids phase over time. This "aging effect" was also observed by Powell et al (2004Powell et al ( , 2005 in similar experiments with goethite, hematite, and magnetite.…”

“…Reduction of Pu(V) to Pu(IV) promotes sorption due to the higher affinity of Pu(IV) for mineral surfaces, relative to Pu(V). Powell et al (2004) determined an overall rate expression describing Pu(V) reduction by hematite and goethite as a function of solution pH. In continued studies, Powell et al, (2006) found that Pu(IV) was the dominant sorbed oxidation state for all synthetic minerals tested, even on oxidizing minerals such as pyrolysite (β-MnO 2 ).…”

Molecular interactions of plutonium(VI) with synthetic manganese-substituted goethite

“…Similar sorption studies of Pu(V) on magnetite (Fe II Fe III 2 O 4 ) [6,7] using solvent extraction also showed reduction to Pu(IV). However, previous XAS measurements involving goethite, Fe III O(OH) [8], a mineral devoid of Fe(II), showed reduction of Pu(VI) to Pu(V) at micromolar to millimolar concentrations, but no evidence of further reduction to Pu(IV).…”

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

“…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).…”