Determining Mechanism of Pu(VI) Reduction by Manganese Oxides
EMSL Project ID
2364
Abstract
As part of the EMSP project: Transuranic Interfacial Reaction Studies on Manganese Oxide Hydroxide Mineral Surfaces, it has been demonstrated that Pu(VI) and Pu(V) are reduced and adsorbed by manganese oxides. XAFS results indicate the managanese minerals containing Mn(III) reduce Pu(VI) and Pu(V) to Pu(IV) and that Pu is sorbed to manganite or hausmannite in an innersphere configuration with no evidence of PuO2 precipitation. In this reduction process, two pathways are possible. Using manganite as an example, direct reduction at the surface would occur as follows: PuO22+ + 2MnOOH + 2H2O = Pu(OH)4(s) + 2MnO2(s) + 2H+ In this case, the plutonyl ion is reduced directly at the surface with two MnOOH sites are required for the complete reduction to Pu(OH)4(s). It is possible that this reaction actually occurs as two separate single electron steps: PuO22+ + MnOOH = PuO2+ + MnO2(s) + H+ PuO2+ + MnOOH + 2H2O = Pu(OH)4(s) + MnO2(s) + H+ In the second possible reduction pathway Mn(II) reduces Pu(VI) in solution: PuO22+ + Mn2+ + 4H2O = Pu(OH)4(s) + MnO2(s) + 4H+ This reaction could also occur as two separate single electron steps: PuO22+ + Mn2+ + 2H2O = PuO2+ + MnOOH + 3H+ PuO2+ + MnOOH + 2H2O = Pu(OH)4(s) + MnO2(s) + H+ Recent advances in XPS resolution and interpretation of Mn(2p) spectra of Mn oxyhydroxide (Nesbitt and Banerjee, 1998) suggest that XPS may be applied to quantify Mn oxidation states within Mn oxyhydroxides. We propose to use XPS to study changes in oxidation state of Mn oxyhydroxides as a mean to elucidate the reaction pathway and mechanism of Pu(VI) reduction by Mn oxyhydroxides. Due to radioactive contamination issues, Pu cannot be used in the XPS system. Instead Ce4+ will be used as strongly oxidizing metal ion analog for PuO22+. Although the thermodynamic data for Ce4+ hydrolysis and solubility are poorly characterized, it is believed that Ce(OH)22+ is the dominant species at low pH values (Baes and Mesmer, 1976). Using available solubility estimates for the amorphous hydrated oxide (Baes and Mesmer, 1976), the Ce(OH)22+ concentration at pH 4.0 is estimated to be 4 x 10-5. At pH 4.5 the concentration is estimated to be 4 x 10-6 and 4 x 10-7 at pH 5.0. The Ce3+/ Ce(OH)22+ redox potential is known to be significantly greater than that of PuO2+/PuO22+ and Pu4+/PuO22+ (Pourbaix, 1966). The experimental approach of this investigation will be to prepare surfaces of manganite and birnessite that will be exposed to solutions containing known concentrations of Ce(OH)22+ and Mn2+. Experiments will be conducted at various initial Mn2+ concentrations and for various time intervals. The solution phase samples will be collected for Ce and Mn analysis by ICP and the solid surfaces will be analyzed by XPS. Solution pH will be measured at the beginning and end of the experiments. Prior to the experiments, the surface samples will be analyzed by XRD to verify the nature of the solid phase.
Project Details
Project type
Exploratory Research
Start Date
2001-10-22
End Date
2003-10-29
Status
Closed
Released Data Link
Team
Principal Investigator