THERMODYNAMIC MODELLING AND EXPERIMENTAL INVESTIGATIONS OF SULFATE INTERACTION WITH AM(III)/ND(III) HYDROXYL SPECIES TO HIGH IONIC STRENGTHS

The interactions of hydroxyl species of rare earth elements (REEs) with sulfate are important to the extraction of REEs, as the conversion of REEs sulfates to hydroxides is often involved in REEs extraction. Since REE and actinides(III) are generally used as mutual chemical analogs, such interactions are also important to nuclear waste disposal. In the performance assessment of geological repositories, the interactions of hydroxyl species of trivalent actinides with sulfate are important to accurately predict the actinide solubility, as sulfate can be present in waste streams or relevant groundwaters. Unfortunately, such interactions are generally missing in the existing REE(III)/Am(III)-sulfate interaction models, which only account for the REE³⁺/Am³⁺—SO₄^2– interaction without consideration of any interaction of REE(III)/Am(III) hydroxyl species with sulfate, thus resulting in a significant under-prediction of REE(III) represented by Nd(III) and Am(III) solubilities in sulfate-bearing solutions over a neutral-to-alkaline pH range. In this work, we present the solubility measurements of Nd₂(C₂O₄)₃•10H₂O in Na₂SO₄ solutions at a room temperature and develop two fixes and a thermodynamic model to describe the interaction of sulfate with Am(III)/Nd(III) hydroxyl species. We first develop a patch fix for one of the existing Am(III)-sulfate interaction models. Then, we develop a global fix and the thermodynamic model with Pitzer equations, which entirely replaces the Am(III)/Nd(III)-sulfate interaction model. In our thermodynamic model, we fit both osmotic coefficient data and solubility data. The resulting model is able to predict the solubilities of trivalent actinide and lanthanide solids in sulfate-bearing solutions over a wide pH range, especially in a neutral to alkaline range.

Sandia National Laboratories is a multi-mission laboratory operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This research is funded by the SFWST programs administered by the Office of Nuclear Energy (NE) of the U.S. Department of Energy, and an LDRD project (Project Number 222400). SAND2023-06800A.