Structure, stability, and synthesis of copper autunite minerals with implications for uranium remediation

Abstract

The copper-containing autunite minerals zeunerite [Cu(UO₂)₂(AsO₄)₂•12H₂O], metatorbernite [Cu(UO₂)₂(PO₄)₂•8H₂O] (CuUP), and metazeunerite [Cu(UO₂)₂(AsO₄)₂•8H₂O] (CuUAs) are promising remediation materials for environmental U (and As) contamination. In this thesis, we examine the structure, stability, and synthesis of these mineral phases in order to better understand their long-term stability in the environment.

Firstly, we present new structural models for zeunerite, CuUP, and CuUAs to better understand material properties and predict phase behaviour in the environment. In contrast to the existing structural models, which have been exclusively determined via X-ray diffraction, we have conducted structural studies using a combination of neutron powder diffraction (NPD) and ab initio random structure searching (AIRSS). This combined approach is the first of its kind and has allowed us to accurately locate H₂O molecules within the crystal structure and describe a detailed network of hydrogen bonds that contribute towards overall phase stability. Our findings have been published in the International Union of Crystallographers Journal (IUCrJ) and show exciting potential for locating H₂O (and other light elements) in other (trans)uranium materials.

In addition to the room temperature structures, we also present new structural models for the high temperature phases that occurred during the dehydration of CuUP and CuUAs (up to 450°C), with applications for predicting the long-term stability of Cu-autunites in hot/ arid climates. We present three high temperature structural models for each material, which were refined from variable temperature NPD data. Our models for CuUP agree with the only previous study but contain more favourable atomic sites. No previous structural models for the high temperature CuUAs phases exist, and we therefore present the first. We also show, for the first time, that high-temperature phase changes for CuUP are reversible, but that the high-temperature phase changes for CuUAs are not reversible under the same conditions. The high temperature treatment did not cause the release of U, P, As, or Cu, and thus heating alone would not likely cause the re-release of remediated U into the environment; instead, the potential re-release of U will hinge on whether the high temperature phases are more or less stable than CuUP and CuUAs under environmental conditions.

We then describe the stability of CuUP, CuUAs, and the intermediate composition Cu(UO₂)₂(PO₄)(AsO₄)•8H₂O (CuUPAs) following immersion in bicarbonate solutions (KHCO₃, NH₄HCO₃, and NaHCO₃; 0.5 M) as a function of time. In all cases, the bicarbonate solutions (KHCO₃, NH₄HCO₃, and NaHCO₃) promoted mineral dissolution, with the most extreme effects observed from NaHCO₃. We build on these findings to suggest a U recovery method, with the aim of using recycled U for the fabrication of nuclear fuel.

Finally, we describe a novel mechanochemical method for the solid-state synthesis of CuUP and CuUAs, with applications for the remediation of solid waste streams. The mechanochemical method eliminates the need for a solvent in line with the United Nations’ 10 principles of green chemistry, and significantly reduces the risks and costs involved in producing a hazardous liquid waste by-products. We have proven the effectiveness of this technique using both laboratory reagents (proof-of-concept) and a real-world As waste sample received from a smelter plant based in Namibia.

Ultimately, we aim for the results presented in this thesis to be used towards stability assessments to determine where autunite remediation materials can be safely used/ stored, without risking the re-release of U (and As) back into the environment.