The Science

Researchers obtained molecular-level insights into the coordination behavior of sulfate ligands with ytterbium (Yb³⁺) in hydrothermal fluids at elevated temperatures, revealing a temperature-dependent transition from hydration-dominated to sulfate-dominated complexes. At room temperature, Yb³⁺ in aqueous solution is coordinated by five water molecules and two sulfate ligands. As the temperature rises to 200 °C, the hydration shell begins to break down, while sulfate ligands bind more strongly. By 300 °C, sulfate coordination dominates and Yb³⁺ begins to precipitate, forming a solid phase. This progression directly demonstrates that sulfate ligands not only facilitate rare earth element (REE) transport in hydrothermal fluids but also play a critical role in triggering their deposition at high temperatures.

The Impact

REEs are vital to high technologies, advanced electronics, and national defense systems, yet their global supply remains highly concentrated and vulnerable to disruption. This research provides a critical step forward in understanding how REEs behave at the molecular level in Earth's crust—specifically, how they are transported and deposited in sulfate-rich hydrothermal systems, which host many economically important REE resources. This work directly supports the identification and development of domestic REE deposits, offering a scientific foundation to better predict where REEs concentrate underground and how they might be efficiently extracted.

Summary

A fundamental gap in scientists’ understanding of REE ore formation lies in the lack of molecular-level insight into how heavy REEs—such as ytterbium (Yb³⁺)—interact with sulfate ligands in hydrothermal fluids under geologically realistic conditions. This knowledge is essential for modeling REE mineralization processes and informing the design of extraction and separation technologies. Researchers performed the first detailed characterization of Yb³⁺ coordination and ligand-binding behavior in sulfate-rich hydrothermal fluids across a broad temperature range. Using a novel combination of in situ synchrotron-based extended X-ray absorption fine structure spectroscopy and ab initio molecular dynamics simulations, they tracked the evolution of Yb³⁺ coordination geometry from room temperature up to 300 °C. Their results revealed a surprising and systematic shift from hydration-dominated complexes at low temperatures to mixed hydration–sulfate coordination near 200 °C to strongly sulfate-dominated environments that drive Yb³⁺ precipitation at 300 °C. This temperature-driven transformation overturns the long-held assumption that sulfate solely facilitates REE transport, demonstrating that sulfate can actively induce REE deposition by dominating coordination at elevated temperatures. This dual role, which first enhanced solubility at low temperatures and later triggered precipitation at high temperatures, was previously unrecognized.

Contact

Xin Zhang, Pacific Northwest National Laboratory, xin.zhang@pnnl.gov 

Kevin Rosso, Pacific Northwest National Laboratory, kevin.rosso@pnnl.gov 

Funding

This work was primarily supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division through its Geosciences Program at Pacific Northwest National Laboratory (PNNL) (FWP #56674). Experimental capabilities developed to perform this work were supported by Laboratory Directed Research and Development program at PNNL, under the Non-Equilibrium Transport Driven Separations Initiative, and the Advanced Research Projects Agency-Energy’s Mining Innovations for Negative Emissions Resource Recovery (MINER) program with award number 22CJ0000901. Work was performed in part at the Advanced Photon Source and Environmental Molecular Sciences Laboratory DOE user facilities.