Structural Characterization of and Plutonium Sorption on Mesoporous and Nanoparticulate Ferrihydrite

The interactions of plutonium with reactive metal oxide surfaces, such as iron oxide and oxyhydroxide minerals, is an area of great interest for scientists studying both environmental contaminant transport and actinide separation and sensing applications. The work presented here addresses both fields with the synthesis and characterization of two iron oxyhydroxide minerals./nThe first mineral is manganese-substituted goethite, for which the synthesis and characterization were the primary concern. A wide range of percent substitutions were prepared, spanning from nearly zero percent up to twenty percent. These minerals were then characterized with respect to their structure, morphology, percent substitution, and solubility. Although smaller percent substitutions could be made, the minimum with a reliable concentration was around 0.5 %. Additionally, the manganese seemed to be more concentrated at the surface of the particles than at the center, suggesting an uneven distribution during particle formation. This trend was more pronounced for the lower substitutions. The 0.5 % and 1 % Mn-substituted goethite showed Fe solubility that matched well with published goethite solubility. The Mn solubility is less than other Mn oxides, but still significantly higher than Fe, which is expected./nThe second mineral is ferrihydrite. For these studies three different 2-line ferrihydrite materials were synthesized and characterized: a suspended colloidal ferrihydrite, a dried and ground version of that suspension, and a nanocast mesoporous ferrihydrite. The data presented is an extended characterization of the structural and surface properties of these three materials, culminating in a study of the interactions of PuO22+ with the three materials as a function of pH, concentration, and light exposure./nAll three materials were measured by X-ray diffraction to be 2-line ferrihydrite. The suspended ferrihydrite was measured to be in the expected 2-5 nm size range. The dried was considerably more aggregated, showing sizes from 20 nm up to 100 μm, and the mesoporous ferrihydrite ranged from 100 nm to 200 μm. The surface areas were also in agreement with the literature, with the dried ferrihydrite measuring at 300-350 m2/g and the mesoporous ferrihydrite near 200 m2/g. The point of zero charge, however, was very different from literature values at around pH 5.5 for the dried ferrihydrite and near pH 9 for the mesoporous ferrihydrite. These values were supported by the plutonium sorption data though and suggest that although all three materials are 2-line ferrihydrite that the surfaces are dissimilar./n/nIn batch sorption studies with Pu(VI), the suspended and dried ferrihydrite proved to be better plutonium scavengers than the mesoporous ferrihydrite, sorbing up to 22 milligrams of Pu per gram of solid compared to only 12 milligrams per gram. The mesoporous released the plutonium more reliably in desorption studies, further supporting the determination that the binding of Pu(VI) to the mesoporous ferrihydrite is not as strong as the binding to the dried and suspended ferrihydrite. All three materials were shown to be photocatalysts for the reduction of Pu(VI) to Pu(IV) using both batch sorption and X-ray absorption spectroscopy measurements, but under different conditions. Two mechanisms were suggested; one through the light-induced creation of Fe(II) in solution and the other through a direct surface electron transfer to the plutonium. Only the dried ferrihydrite exhibited this behavior under all conditions measured, but also demonstrated the weakest overall photocatalytic activity. The suspended ferrihydrite was only a weak photocatalyst at low pH, but was strong at higher pH conditions, likely due to high surface area and greater particle aggregation. The mesoporous ferrihydrite showed increasing photocatalytic activity with increasing pH, suggesting that the mesoporous ferrihydrite can only achieve a direct electron transfer and not the solution-mediated Fe(II) mechanism that the nanoparticulate ferrihydrite materials will undergo. These results highlight the differences between the surface chemistry and structure of these materials despite the fact that they are all 2-line ferrihydrite. This is particularly important for the understanding of the suspended and dried ferrihydrite, as distinctions between these two materials are not often considered.

Advisor: Nitsche, Heino