A PITZER MODEL FOR NA+—ND3+—SO42–—H+—OH– SYSTEM TO HIGH IONIC STRENGTHS

In this work, the measured solubilities of Nd(OH)₃(s) in Na₂SO₄ solutions up to 1.8 mol•kg^–1 from our work are combined with the literature solubility data on Nd₂(SO₄)•8H₂O in H₂SO₄ solutions to develop a Pitzer model to describe the Na⁺—Nd³⁺—SO₄^2–—H⁺—OH^– system to high ionic strengths, applicable to nuclear waste isolation and recovery of Rare earth elements (REE) from acidic mine drainage.

REE are critical elements, as they are widely used in numerous industries. At the same time, REE such as Nd(III), are used as good analogs to transuranic actinide elements in +III oxidation state in nuclear waste management. In particular, in the field of nuclear waste management, natural groundwaters such as brines associated with salt formations, may contain relatively high concentrations of sulfate, and the interactions between sulfate and actinides are important. As an example, in the Waste Isolation Pilot Plant (WIPP), a U.S. DOE geological repository for defense-related transuranic waste in the bedded salt formations in New Mexico, USA, the sulfate concentrations in the two WIPP brines that are important for performance assessment, i.e., Generic Weep Brine (GWB), and Energy Research and Development Administration (WIPP Well) 6 (ERDA-6), are 0.203 mol•kg^–1 and 0.187 mol•kg^–1, respectively. In the low level and intermediate level radioactive sulfate liquid waste in Spain, they are very rich in Na₂SO₄ with SO₄^2– concentrations up to 2.2 mol•kg^–1.

Acidic mine drainages with high sulfate concentrations are potentially the new resources for REE, as they have relatively high concentrations of REE. For instance, the acidic mine drainages from Spain have REE concentrations up to ~70 mmol•kg^–1, and the acidic mine drainages in coal mines from China have REE concentrations up to ~1 mmol•kg^–1. Therefore, the Pitzer model from this work could also be applied for recovery of REE from acidic mine drainages.

^A This research is funded by the WIPP programs administered by the Office of Environmental Management (EM) of the U.S. Department of Energy.

^B Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2016-6612A