SREL Reprint #3704

Vadose-zone alteration of metaschoepite and ceramic UO2 in Savannah River Site field lysimeters

Connaugh M. Fallon1,2, William R. Bower1,3, Brian A. Powell4, Francis R. Livens1,2, Ian C. Lyon2, Alana E. McNulty2, Kathryn Peruski4, J. Frederick W. Mosselmans5, Daniel I. Kaplan6, Daniel Grolimund7, Peter Warnicke7, Dario Ferreira-Sanchez7, Marja Siitari Kauppi3, Gianni F. Vettese3, Samuel Shaw2, Katherine Morris2, and Gareth T.W. Law3

1Centre for Radiochemistry Research, Department of Chemistry, The University of Manchester,
Manchester M13 9PL, UK
2Research Centre for Radwaste and Decommissioning and Williamson Research Centre, Department of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK
3Radiochemistry Unit, Department of Chemistry, University of Helsinki, Helsinki 00014, Finland
4Department of Environmental Engineering and Earth Sciences, Department of Chemistry,
Clemson University, Clemson, SC 29634, USA
5Diamond Light Source, Harwell Science and Innovations Campus, Didcot, Oxfordshire OX11 0DE, UK
6Savannah River Ecology Laboratory, University of Georgia, Aiken, SC 29808, USA
7Swiss Light Source, Paul Scherrer Institute, Villigen CH-5232, Switzerland

Abstract: Uranium dioxide (UO2) and metaschoepite (UO3•nH2O) particles have been identified as contaminants at nuclear sites. Understanding their behavior and impact is crucial for safe management of radioactively contaminated land and to fully understand U biogeochemistry. The Savannah River Site (SRS) (South Carolina, USA), is one such contaminated site, following historical releases of U-containing wastes to the vadose zone. Here, we present an insight into the behavior of these two particle types under dynamic conditions representative of the SRS, using field lysimeters (15 cm D x 72 cm L). Discrete horizons containing the different particle types were placed at two depths in each lysimeter (25 cm and 50 cm) and exposed to ambient rainfall for 1 year, with an aim of understanding the impact of dynamic, shallow subsurface conditions on U particle behavior and U migration. The dissolution and migration of U from the particle sources and the speciation of U throughout the lysimeters was assessed after 1 year using a combination of sediment digests, sequential extractions, and bulk and µ-focus X-ray spectroscopy. In the UO2 lysimeter, oxidative dissolution of UO2 and subsequent migration of U was observed over 1–2 cm in the direction of waterflow and against it. Sequential extractions of the UO2 sources suggest they were significantly altered over 1 year. The metaschoepite particles also showed significant dissolution with marginally enhanced U migration (several cm) from the sources. However, in both particle systems the released U was quantitively retained in sediment as a range of different U(IV) and U(VI) phases, and no detectable U was measured in the lysimeter effluent. The study provides a useful insight into U particle behavior in representative, real-world conditions relevant to the SRS, and highlights limited U migration from particle sources due to secondary reactions with vadose zone sediments over 1 year.

Keywords: Uranium, X-ray absorption spectroscopy, Speciation, Groundwater, Surface water

SREL Reprint #3704

Fallon, C. M., W. R. Bower, B. A. Powell, F. R. Livens, I. C. Lyon, A. E. McNutty, K. Peruski, J. F. W. Mosselmans, D. I. Kaplan, D. Grolimund, P. Warnicke, D. Ferreira-Sanchez, M. S. Kauppi, G. F. Vettese, S. Shaw, K. Morris, and G. T. W. Law. 2023. Vadose-zone alteration of metaschoepite and ceramic UO2 in Savannah River Site field lysimeters. Science of the Total Environment 862(160862).

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).