PNNL

The Science

The dissolution of minerals is a complex process, key to models at the molecular and macroscopic scales. Researchers studied the dissolution of gibbsite, an aluminum-based mineral that is an important component of legacy radioactive tank waste. The typical assumption about a mineral dissolution process is that monomers, which contain a single unit of material, detach from the mineral surface and move into the solution. However, the team determined that gibbsite dissolves via dimeric complexes, which contain two material units. This has significant implications for modeling dissolution processes at the molecular and macroscopic levels.

The Impact

Mineral dissolution is central to natural and industrial processes such as weathering, reactive flow and transport, and mineral resource refining. Developing a picture of the dissolution process from observed rates involves making assumptions about the chemical species detaching from the mineral surface. By identifying that aluminum detaches from gibbsite as aluminate dimers rather than monomers, this work offers critical information for developing molecularly accurate models of dissolution across a range of conditions. These results run counter to previous assumptions about the simplicity of the dissolving species and offer a new perspective on mineral behavior, helping effectively manage legacy nuclear tank waste.

Summary

A collaborative team of scientists discovered how dimeric metal–hydroxyl complexes control mineral dissolution across molecular and macroscopic scales. Using high-speed, high-resolution atomic force microscopy, machine learning, kinetic experiments, and molecular dynamics simulations, they found that aluminum hydroxide dissolves through the formation of transient dimeric complexes that weaken surface bonds and release structural units into solution. This work provides the first molecular-level evidence linking solution speciation with measurable dissolution rates, bridging decades of disconnected experimental and theoretical studies. The proposed mechanism establishes a unified picture of how reactive aqueous species promote mineral transformation. The results have broad implications for predicting mineral reactivity in natural and engineered systems, including corrosion, soil weathering, and the mobilization of aluminum-bearing sludge in radioactive tank waste at the Hanford Site and other locations.

Contact

Xiaoxu Li, Pacific Northwest National Laboratory, xiaoxu.li@pnnl.gov

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

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

Carolyn Pearce, Pacific Northwest National Laboratory, carolyn.pearce@pnnl.gov 

Funding

This research was supported by Ion Dynamics in Radioactive Environments and Materials (IDREAM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under FWP 68932. A portion of the work was carried out in the Environmental Molecular Sciences Laboratory, a national scientific user facility at Pacific Northwest National Laboratory sponsored by the Department of Energy’s Biological and Environmental Research program.