Aqueous U(VI) interaction with magnetite nanoparticles in a mixed flow reactor system: HR-XANES study

I Pidchenko,Ko Kvashnina,D Schild,T Vitova,E Bohnert,H Geckeis,F Heberling,N Finck,T Schäfer,J Rothe

doi:10.1088/1742-6596/712/1/012086

Abstract

The redox variations and changes in local atomic environment of uranium (U) interacted with the magnetite nanoparticles were studied in a proof of principle experiment by the U L3 and M4 edges high energy resolution X-ray absorption near edge structure (HR-XANES) technique. We designed and applied a mixed flow reactor (MFR) set-up to maintain dynamic flow conditions during U-magnetite interactions. Formation of hydrolyzed, bi- and poly-nuclear U species were excluded by slow continuous injection of U(VI) (10-6 M) and pH control integrated in the MFR set-up. The applied U HR-XANES technique is more sensitive to minor changes in the U redox states and bonding compared to the conventional XANES method. Major U(VI) contribution in uranyl type of bonding is found in the magnetite nanoparticles after three days operation time of the MFR. Indications for shortening of the U-Oaxial bond length for the magnetite compared to the maghemite system are present too.

Highlights

The safety assessment of long-term repositories for high-level radioactive waste (HLW) motivates speciation studies of actinides (An) and fission products in relevant systems
There are still open questions which need to be addressed: 1) What is the speciation of U after continuous redox and phase transformation processes induced by the interaction of U with magnetite; 2) Are the reduced U species long term stable; 3) Are these species incorporated in the vacancies created by the removal of Fe(II), which is favourable at about pH 5.5? We present benchmark experiment with a mixed flow reactor (MFR) set-up designed for U(VI) interaction with magnetite nanoparticles in aqueous media
The energy position of the multiple scattering H resonance is sensitive to the U-Oaxial bond distance. It does not change significantly for the two spectra. This is a clear indication that the U L3 edge has lower sensitivity to small variations in the U-Oaxial bond length compared to the U M4 edge HR-XANES spectrum

Summary

Introduction

The safety assessment of long-term repositories for high-level radioactive waste (HLW) motivates speciation studies of actinides (An) and fission products in relevant systems. To model the U interaction mechanisms with one of the main iron corrosion products, magnetite, various laboratory experiments were performed mainly in static conditions. Such static experiments facilitate formation of secondary phases and might not be representative for potential dynamic conditions in a HLW repository. There are still open questions which need to be addressed: 1) What is the speciation of U after continuous redox and phase transformation processes induced by the interaction of U with magnetite; 2) Are the reduced U species long term stable; 3) Are these species incorporated in the vacancies created by the removal of Fe(II), which is favourable at about pH 5.5? For example the U M4 edge HR-XANES was successfully applied to characterize U(V) in U4O9 [3]

Experimental part

Results and discussion

Conclusions