The environmental cleanup project at the U.S. Department of Energy’s Hanford site, located in Washington state, is one of the largest projects in history. (1,2) In this endeavor, dealing with approximately 200 million liters of mixed radioactive and chemical waste has emerged as a monumental challenge for Hanford. (3,4) Of the 177 subterranean storage tanks, 67 are suspected to have suffered leaks, resulting in contaminated groundwater. (1,2,5) This waste originated from various separation processes employed to extract plutonium (Pu) from the fuel. Aluminum (Al), introduced either as an aluminum nitrate during the process or as part of the fuel casing, constitutes a significant component of Hanford tank waste. (6) Al primarily exists in waste sludge as the mineral phases gibbsite [α-Al(OH)₃, but sometimes defined as γ-Al(OH)₃] and boehmite (γ-AlOOH). (6,7) This tank waste sludge will be subjected to washing and alkaline leaching to solubilize the Al so it can be removed from the tanks for ultimate transformation into a stable, glass product. (8)

However, the removal of Al minerals from the tank waste was more challenging than anticipated. (4,9,10) This can be attributed to the complex chemical composition of the waste tanks. Apart from sodium and Al, substantial quantities of iron (Fe) and chromium (Cr) are also found in the waste. (3,6,8) The Fe originates from bismuth phosphate waste treatment, while Cr is introduced as an oxidizing agent in the reduction oxidation (REDOX) process. (3,6,8) Several interactions involving Fe, Cr, and Al have been documented in waste treatment processes. For example, the coprecipitation of Al and Fe metal oxides, (11) or the use of ferrate (FeO₄^2-) to leach Cr from solids. (1) Additionally, Cr dissolution through oxidative leaching also releases Al. (6) Hence, investigation of the mechanisms of interaction between Fe/Cr and Al hydroxides would provide valuable insights for waste retrieval and processing at Hanford.

It has been proposed that Fe or Cr could bind to the surface of Al minerals in the form of nanoscale precipitates or adsorbed clusters. (12) Alternatively, Fe or Cr might be incorporated into the structure of Al minerals, acting as substitutes for Al atoms. (9) Studies have confirmed that the inclusion of Cr can hinder the dissolution of boehmite. This inhibition mechanism involves the adsorption of Cr clusters onto boehmite, potentially leading to the partial passivation of the surface. (4,12) Frost et al. synthesized up to 20% Fe- or Cr-doped boehmite nanofibers by steam-assisted solid wet-gel synthesis. (13,14) Nevertheless, evaluating the binding energy of Cr³⁺ substitution for Al³⁺ within the boehmite structure highlights a clear incompatibility of Cr³⁺, especially when concentrations surpass the 1% threshold. (9)

The substitution of Fe by Al and other elements is a well-documented phenomenon in iron (hydr)oxides in various environmental contexts. This substitution gradually transforms hematite crystals, altering their facets from (101), (112), (110), and (104) rhombohedra to (001) faceted plates, leading to a general decrease in the adsorption capacity for Cr. (15) However, it is not known if Fe³⁺ or Cr³⁺ ions effectively substitute for Al³⁺ within the gibbsite structure, and, if these substitutions of Fe³⁺ or Cr³⁺ have any discernible impact on gibbsite reactivity.

This study reveals that both Cr³⁺ and Fe³⁺ can substitute for Al³⁺ in the octahedral lattice positions in gibbsite during growth. impacting its reactivity. Gibbsite is a significant component of the legacy nuclear waste stored at the U.S. Department of Energy’s Hanford Site, where its dissolution and transformation are critical to nuclear waste processing. Previous research has shown that the adsorption of trace metals, such as Cr and Ti, can alter the solubility and reactivity of host minerals, impacting waste management practices. Our findings conclusively demonstrate that trace metals can be incorporated into gibbsite during its growth, not just adsorbed onto the surface. This incorporation may significantly affect the dissolution behavior and transformation pathways of gibbsite, with direct implications for nuclear waste processing strategies at the Hanford Site. Understanding how these trace metals interact with gibbsite is crucial for optimizing methods to manage and stabilize nuclear waste as their incorporation could either inhibit or promote dissolution under certain conditions. Additionally, this study provides insight into the broader environmental roles of metal impurities in mineral crystal growth. The ability of minerals to incorporate metal impurities during growth highlights a potential natural mechanism for the sequestration of contaminant metals in soils and sediments. Such a process could influence the mobility, bioavailability, and ultimate fate of contaminants in natural and engineered environments. Our findings suggest that understanding how metal impurities are incorporated into mineral structures could inform the design of more effective remediation strategies for contaminated sites by harnessing or replicating these natural sequestration processes. Overall, this research underscores the importance of investigating the fundamental processes underlying mineral crystal growth and dissolution, particularly in the context of environmental remediation and nuclear waste management. It highlights how metal incorporation affects the stability and reactivity of minerals, ultimately influencing the efficiency and effectiveness of remediation efforts.