Dissolution of Iron-based Minerals Furthers Understanding of Iron Redox Cycling

The Science 

The dissolution mechanism of iron-based metal oxides is poorly understood at the nanoscale, particularly the interplay between acidic and reductive processes. To clarify parts of the process, scientists at Pacific Northwest National Laboratory (PNNL) applied a new combination of techniques to detail the dissolution of metal oxides at the nanoscale level, examining the dissolution mechanism through variations in solution chemistry. They demonstrated control over acidic and reductive dissolution driven by a combination of electron beam radiolysis, pH buffer, and chloride anions. They found that buffers, such as bis-tris, effectively inhibited dissolution by consuming radiolytic acidic and reducing species. In addition, chloride anions simultaneously suppressed dissolution at rod tips by stabilizing structural elements while promoting dissolution at rod sides through surface complexation. 

The Impact 

Iron-based, redox-active minerals are found nearly everywhere in both soils and aquatic environments. The dissolution of iron-based minerals has important microbial impacts on carbon cycling and the biogeochemistry of a variety of environments, including the lithosphere and hydrosphere. This work deepens the understanding of facet-dependent dissolution through acidic and reductive attack, providing new clues into processes controlling the dissolution rate and morphologies of iron oxide minerals in natural environments.  

Summary 

A combination of liquid-phase transmission electron microscopy, solution chemistry, and radiolytic speciation simulations informed by knowledge of various possible dissolution mechanisms was used successfully to both manipulate the dissolution behavior and rate of akageneite nanorods and quantitatively account for electron-beam-induced radiolytic speciation. To understand which dissolution mechanism was at work, a team of PNNL applied liquid-phase transmission electron microscopy to akageneite nanorods to investigate the dissolution dynamics of the material. The team conducted simulations of radiolysis to probe and control acidic versus reductive dissolution of the akageneite nanorods. They varied the balance between acidic dissolution at rod tips and reductive dissolutions at rod sides using pH buffers, chloride anions, and electron beam dose. Simulations of chemical equilibria and dissolution indicate that organic buffers such as bis-tris can selectively reduce the availability of superoxides and aqueous electrons, thereby suppressing dissolution rate. These insights suggest a possible route to reconstruction of those conditions during deposition or diagenesis or subsequent alteration via analysis of specific crystallite morphologies. By enhancing the understanding of interfacial processes governing iron oxide dissolution, the findings provide a unique basis for interpreting geochemical and biogeochemical signatures governing iron redox cycling across a diversity of crustal and aquatic environments. 

Contacts 

Xin Zhang, PNNL, xin.zhang@pnnl.gov 

Lili Liu, PNNL, lili.liu@pnnl.gov 

Jim De Yoreo, PNNL, james.deyoreo@pnnl.gov 

Kevin Rosso, PNNL, kevin.rosso@pnnl.gov 

Funding 

The project was supported by the Department of Energy (DOE) Office of Science, Basic Energy Sciences program, Chemical Sciences, Geosciences, and Biosciences Division through the geosciences program at Pacific Northwest National Laboratory. A portion of the research was performed on a project award from the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility sponsored by the Biological and Environmental Research program. 

Publication

L. Liu, et al., “Understanding the mechanisms of anisotropic dissolution in metal oxides by applying radiolysis simulations to liquid-phase TEM.” Proceedings of the National Academy of Sciences 120(23) e2101243120 (2023). [DOI: 10.1073/pnas.210124312]