December 19, 2018
Web Feature

Understanding Principles Controlling Mineral Dissolution Far from Equilibrium

Diagram of the three step dissolution mechanism

Three-step dissolution mechanism: 1. Displacement of Al from step atom to ledge adatom. 2. Migration from ledge to interior of terrace. 3. Detachment from adatoms sites into solution. Steps 1 and 3 are rate-limiting steps.

Researchers at the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center computed the first detailed free-energy landscape for the multistep dissolution of gibbsite – one of the mineral forms of aluminum hydroxide and an important mineral resource for industrial aluminum production. The model compares neutral to high pH, providing mechanistic clues to guide experiments aimed at evaluating dissolution rates under alkaline conditions.

The work, led by Zhizhang Shen of Pacific Northwest National Laboratory, was featured in The Journal of Physical Chemistry Letters in a paper titled, “Free-Energy Landscape of the Dissolution of Gibbsite at High pH.”

Why it matters: Mechanistic simulations based on statistical mechanics are challenging but necessary to move beyond empirically-based models of dissolution, which are not easily extended to extreme alkaline environments. The new model will also help process optimization for high-level radioactive waste treatment.

Summary: Although conceptually simple, mechanistic descriptions of the individual elementary steps involved in the dissolution of metal oxides remain speculative, especially for highly alkaline systems. Yet these reactions control rates of nutrient availability in natural systems, and overall rates of industrial processing of ore materials. Direct experimental probes to observe individual elementary reactions do not exist. In this work, atomistic simulations are used to map the free energy landscape of the dissolution of gibbsite by way of transfer of an aluminum ion from the solid phase into solution. 

The solid phase of interest in this work is gibbsite (Al(OH)3). In essence, it is a two-dimensional network of octahedrally coordinated trivalent aluminum cations. This relatively simple structure allowed focus on a single species that is transferred from the basal plane of the solid to the solution. Atomistic simulations were used to map the free energy landscape of this single species from a step edge on the basal plane, an overall reaction that combines kink formation and kink propagation. Two rate-limiting reactions for dissolution were identified: (1) displacement of aluminum from a step site to a ledge adatom, and (2) detachment of the adatom from the ledge into solution. The simulations revealed detailed information about the ligand exchange reactions that facilitate the detachment steps and the coordination change of Al3+ from six-fold in the gibbsite structure to four-fold in alkaline solution. 

What’s next? Because gibbsite is an important solid phase that must be dissolved to remove high-level radioactive wastes from storage tanks, this research provides a technical foundation for waste processing schemes that will be used to remediate high-level radioactive waste tanks at Hanford and Savannah River. 

Acknowledgments: This work is related to the Basic Research Needs for Environmental Management, specifically the Priority Research Direction entitled “Elucidating and Exploiting Complex Speciation and Reactivity Far From Equilibrium."

Sponsors: This work was supported by IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier Research Center managed for the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). The research team also gratefully acknowledges Xin Zhang for fruitful discussions regarding experimental observations of gibbsite dissolution and Micah Prange regarding DFT simulations setup.

Research Area: Physical Sciences

User Facilities: The research was performed using PNNL Institutional Computing at Pacific Northwest National Laboratory (PNNL).

Research Team: Zhizhang Shen, Sebastien N. Kerisit, and Kevin M. Rosso (Pacific Northwest National Laboratory) and Andrew G. Stack (Oak Ridge National Laboratory).

Reference: Z. Shen, S. N. Kerisit, A. G. Stack, and K. M. Rosso. 2018. Free-Energy Landscape of the Dissolution of Gibbsite at High pH.” The Journal of Physical Chemistry Letters. DOI: 10.1021/acs.jpclett.8b00484

Published: December 19, 2018