Sorption of Am(III) onto 6-Line-Ferrihydrite and Its Alteration Products:  Investigations by EXAFS

Abstract

For the long-term performance assessment of nuclear waste repositories, knowledge about the interactions of actinide ions with mineral surfaces such as iron oxides is imperative. The mobility of released radionuclides is strongly dependent on the sorption/desorption processes at these surfaces and on their incorporation into the mineral structure. In this study the interaction of Am(III) with 6-line-ferrihydrite (6LFh) was investigated by EXAFS spectroscopy. At low pH values (pH 5.5), as well at higher pH values (pH 8.0), Am(III) sorbs as a bidentate corner-sharing species onto the surface. Investigations of the interaction of Am(III) with Fh coated silica colloids prove the sorption onto the iron coating and not onto the silica substrate. Hence, the presence of Fh, even as sediment coating, is the dominant sorption surface. Upon heating, Fh is transformed into goethite and hematite as shown by TEM and IR measurements. The results of the fit to the EXAFS data indicate the release of sorbed Am(III) at pH 5.5 during the transformation and likely a partial incorporation of Am into the Fh transformation products at pH 8.0.

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

“…At pH 5.5 or 8.0, Am(III) sorbed onto ferrihydrite surfaces as a bidentate corner-sharing species [125]. Bidentate bonding indicates two bonds are formed between the Am(III) species and the surface.…”

“…The implications of a bidentate speciation is that the bonding is strong and desorption is less likely to occur than in a monodentate system. Upon heating, ferrihydrite transforms to goethite or hematite, the sorbed Am(III) at pH 5 was released from the sorbed samples during the transformation, but the sorbed Am(III) at pH 8 was partially incorporated into the ferrihydrite transformation products [125].…”

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

“…The implication of this to the PA is that the bond is strong and desorption is less likely to occur than in a monodentate system. Upon heating, ferrihydrite transforms to goethite or hematite, the sorbed Am(III) at pH 5 is released from the sorbed samples during the transformation, but the sorbed Am(III) at pH 8 is partially incorporated into the ferrihydrite transformation products (Stumpf et al 2006). …”

Production and properties of transuranium elements

“…The sorption behavior of Am 3+ onto 6-line-ferrihydrite was studied using EXAFS at the ESRF by Stumpf et al [94]. They obtained EXAFS signals both at pH 5.5 and at pH 8.0 and well explained the difference of sorption behavior of Am 3+ by changing the pH value.…”