Influence of iron redox transformations on plutonium sorption to sediments

Abstract

Plutonium / Subsurface transport / Redox / XANES / Iron / KineticsSummary. Plutonium subsurface mobility is primarily controlled by its oxidation state, which in turn is loosely coupled to the oxidation state of iron in the system. Experiments were conducted to examine the effect of sediment iron mineral composition and oxidation state on plutonium sorption and reduction. A pH 6.3 vadose zone sediment containing iron oxides and iron-containing phyllosilicates was treated with various complexants (ammonium oxalate) and reductants (hydroxylamine hydrochloride and dithionite-citrate-bicarbonate (DCB)) to selectively leach and/or reduce iron oxide and phyllosilicate/clay Fe(III). 57 Fe-Mössbauer spectroscopy was used to identify initial iron mineral composition of the sediment and monitor dissolution and reduction of iron oxides and reduction of phyllosilicate Fe(III). 57 Fe-Mössbauer spectroscopy showed that the Fe-mineral composition of the untreated sediment is: 25-30% hematite, 60-65% smallparticle/Al-goethite, and < 10% Fe(III) in phyllosilicate; there was no detectable Fe(II). Upon reduction with a strong chemical reductant (dithionite-citrate-bicarbonate buffer), much of the hematite and goethite was removed. Partial reduction of phyllosilicate Fe(III) was evident in the sediments subjected to DCB treatment. Sorption of Pu(V) was monitored over one week for the untreated and each of five treated sediment fractions. Plutonium oxidation state speciation in the aqueous and solid phases was monitored using solvent extraction, coprecipitation, and XANES. The rate of sorption appears to correlate with the fraction of Fe(II) in the sediment (untreated or treated). Pu(V) was the only oxidation state measured in the aqueous phase, irrespective of treatment, whereas Pu(IV) and much smaller amounts of Pu(V) and Pu(VI) were measured in the solid phase. Surface-mediated reduction of Pu(V) to Pu(IV) occurred in treated and untreated sediment samples; Pu(V) remained on untreated sediment surface for two days before reducing to Pu(IV). Similar to the sorption kinetics, the reduction rate appears to be correlated with sediment Fe(II) concentration. The correlation between Fe(II) concentrations and Pu(V) reduction demonstrates the potential impact of changing iron mineralogy on plutonium subsurface transport through redox transition areas. These findings should influence the conceptual models of long-term stewardship of Pu

Literature Review on the Sorption of Plutonium, Uranium, Neptunium, Americium and Technetium to Corrosion Products on Waste Tank Liners

“…Thus, increasing the Fe(II) content of the system should correlate to an increase in reaction (sorption + reduction) kinetics. This expected trend was confirmed by Hixon et al (2010), who observed a decrease in the rate of Pu(V) reduction to Pu(IV) when the concentration of Fe(II) in SRS sediments was decreased. Therefore, magnetite should exhibit faster kinetics than hematite or goethite.…”

Influence of humic acid on plutonium sorption to gibbsite: Determination of Pu-humic acid complexation constants and ternary sorption studies

“…Previous sorption experiments indicated that changes in the sorption behavior were continuously observed over a 60 day period and were attributed to reduction of Pu(V) to Pu(IV) [34]. Similar surface mediated reduction of Pu(V) has been observed on a variety of pure minerals and soils [35][36][37][38][39][40][41] and sorption of Pu(IV) is considerably stronger than Pu(V). A double layer surface complexation model fit to the data is shown as the solid lines in Figure 8 and the aqueous and surface complexation constants are given in Table 3.…”

Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: A review

“…At approximately 50 m 2 /L of each mineral and pH 8, the kinetic reaction rate for magnetite (1.97 g/L) was an order of magnitude greater than hematite (1.38 g/L) or goethite (0.30 g/L) [32,33]. Hixon et al [40] studied the influence of iron redox transformations on Pu sorption to SRS sediments that consisted of goethite-coated sands and found that native Fe(II) in the sediments was responsible for the reduction of trace levels of Pu, the rate of Pu(V) reduction to Pu(IV) decreased as the concentration of Fe(II) in SRS sediments was decreased, in agreement with the iron oxide surface-mediated reduction mechanism of Pu(V).…”