The Science

Particle aggregation is a common occurrence in a variety of natural and anthropogenic settings, including legacy waste storage and processing. Just as varied are the factors that can affect aggregation mechanisms, including pH, electrolyte species, concentration, and particle morphologies. This presents a particular challenge for processing waste materials at legacy sites, such as the Hanford Site in Washington State. This study investigated how electrolyte composition in radioactive materials affects the aggregation mechanisms of boehmite nanoparticles, showing that the presence of an electrolyte encourages particle aggregation.

The Impact

At the Hanford Site, waste storage tanks contain highly complex slurries with variable compositions and a tendency to aggregate, making waste processing especially difficult. Thus, developing a molecular-level understanding of aggregation mechanisms, particularly in high-pH and high-salinity environments, is vital for developing effective processing methodologies. In addition, the findings can be applicable for elucidating how salt solutions affect aggregation in other settings, such as crystal formation and wastewater treatment. 

Summary

While traditional theories used to describe particle aggregation do not explain the specific effects of electrolyte species and concentration on aggregation, this study investigates the molecular mechanisms of how alkali nitrate ions (i.e., Na⁺, K⁺, and NO₃^-) affect mineral boehmite (γ-AlOOH) nanoparticle aggregation. Classical molecular dynamics simulations were used to simulate nanoparticle aggregation, alongside the metadynamics rare event approach. As shown through calculated free energy landscapes, electrolyte ions alter particle aggregation on different crystal faces relative to pure water. Results indicated that adding electrolytes reduces the energy barrier of particle aggregation, thereby encouraging aggregate formation. The study also shows that this occurs as the ions disrupt interstitial water networks. Additionally, aggregation between (010) basal–basal surfaces is about 5 to 6 times more favorable than that between (001) edge–edge surfaces because the interfacial water densities are higher on edge surfaces. Due to these lower energy barriers, aggregation and disaggregation in salt solutions is likely more reversible. The researchers also determined that the extent of solvent ordering is likely to directly affect aggregate structure and related energy barriers. These results open doors for developing a comprehensive, molecular-level understanding at the sub-nano scale that can be used to predict how ion and crystal face interactions may encourage aggregation in different solution conditions.

Contact

Tingting Liu, Oak Ridge National Laboratory, liut@ornl.gov

Elias Nakouzi, Pacific Northwest National Laboratory, elias.nakouzi@pnnl.gov

Andrew Stack, Oak Ridge National Laboratory, stackag@ornl.gov

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

Funding

This research was supported by Interfacial 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 program (FWP 68932).