Characterization of surface mediated plutonium oxidation state transformations by synthetic iron and manganese (oxyhydr)oxides.

摘要

A study was performed to test the hypothesis that the Pu oxidation state in mineral solutions depends on the oxidation state of Fe or Mn in the mineral structure. A series of batch experiments, initially containing either Pu(IV) or Pu(V), were used to monitor changes in aqueous and solid phase Pu oxidation state over time in magnetite (Fe3O4), hematite (alpha-Fe2O3), goethite (alpha-FeOOH), hausmannite (Mn3O4), manganite (gamma-MnOOH), and pyrolusite (beta-MnO2) suspensions containing Pu amended 0.01 M NaCl. These minerals were selected because they have a wide range of Fe and Mn oxidation states. Additionally, 30-day pH adsorption edge experiments were performed to monitor the oxidation state distribution of Pu in hausmannite (Mn3O4), manganite (gamma-MnOOH), and pyrolusite (beta-MnO 2) suspensions in the pH range 2 to 8.; Reduction of Pu(V) was observed in all magnetite, hematite, and goethite solutions at pH greater than 3. The overall reaction consisted of both an adsorption step and a reduction step. Adsorption was the rate-limiting step in magnetite and hematite systems. In goethite systems, adsorption was sufficiently rapid that it did not appear to affect the reduction rate. Reduction in these systems is proposed to be promoted by the stability of Pu(IV) on mineral surfaces, likely as a hydroxide species. Additionally, the rate of Pu(V) reduction was found to increase when hematite and goethite samples were exposed to fluorescent light. The iron (oxyhydr)oxide minerals are semi-conductors. Therefore, electron transfer through conduction bands of the iron (oxyhydr)oxide minerals may have facilitated Pu(V) reduction.; Pu(V) was both oxidized and reduced upon interaction with the manganese (oxyhydr)oxide minerals hausmannite, manganese, and pyrolusite. Initially, Pu(IV), Pu(V), and Pu(VI) were all observed on the solid phase. However, over time the fraction of Pu(IV) on the solid phase increased, with concurrent decreases in the fraction of Pu(V) and Pu(VI). Reduction of Pu(V) was also observed on the surface of glass beads. Batch pH adsorption edge experiments indicated that, after 30 days, Pu(IV) was the predominant solid phase species but significant fractions of Pu(V) and Pu(VI) were observed. The fraction of Pu(V) and Pu(VI) increased as the pH increased. However, based upon the results of kinetic experiments, which showed a continually increasing fraction of solid phase Pu(IV) over time, it is expected that Pu remaining on the solid phase will become Pu(IV) given sufficient reaction time. (Abstract shortened by UMI.)