Some Interesting Chemistry Associated with the Radioactive Hanford Waste Tanks.

[NNadir]

Last edited Sun Nov 17, 2024, 11:24 AM - Edit history (1)

The paper I'll discuss in this post is this one: Understanding Trace Iron and Chromium Incorporation During Gibbsite Crystallization and Effects on Mineral Dissolution Yatong Zhao, Micah P. Prange, Meirong Zong, Yining Wang, Eric D. Walter, Ying Chen, Zihua Zhu, Mark H. Engelhard, Xiang Wang, Xiaodong Zhao, Carolyn I. Pearce, Aijun Miao, Zheming Wang, Kevin M. Rosso, and Xin Zhang Environmental Science & Technology 2024 58 (45), 20125-20136.

I will only discuss this paper, because of time constraints, very, very briefly. It concerns efforts to address chemical byproducts of nuclear fuel reprocessing in the 20th century to get plutonium for nuclear weapons and efforts to "clean up" the waste tanks left from that rather sloppy effort. I once went on a rather long, and personally satisfying, intellectually satisfying, rant here on the Hanford situation in response to a dumb shit antinuke here who carried on about the collapse of a tunnel at the Hanford site, this on an entire planet literally burning up from the accumulation of the dangerous fossil fuel waste carbon dioxide. I won't have the displeasure of hearing from that person, because of the very wonderful DU ignore list function, but as the inspiration to look into what should have been a short dismissal led me down a path I very much enjoyed, since getting carried away with research into the Hanford situation taught me a lot I didn't know.

828 Underground Nuclear Tests, Plutonium Migration in Nevada, Dunning, Kruger, Strawmen, and Tunnels

Whatever.

The paper above concerns chemistry that might be covered - at least when I was a kid - in an introductory chemistry lab course, specifically the dissolution of aluminum hydroxide Al(OH)₃ and the apparent effect of co-precipitated metals. (The paper also gives some insight into the historic chemical practices utilized in nuclear fuel reprocessing, as any investigation into the literature associated with these tanks does.)

From the paper's introductory paragraphs:

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.

The question the paper asks is whether the chromium and iron impurities reside on the surface of the gibbsite formed in the tanks, impacting their ease of dissolution, or whether the iron and chromium are incorporated into the crystals when they form.

The authors, after some cool sophisticated science discussed in the paper, conclude that the chromium and iron are incorporated within the crystals, and that this does in fact affect the dissolution rates. They note that these chemical findings can have implications for other types of contaminated sites where heavy metal chemistry is involved.

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.

None of this dissuades me from my oft stated opinion that the money spent "cleaning up" Hanford to a standard to which we "clean up" nothing else, say for instance, the planetary atmosphere, will save very few lives, if any, because very few lives are at risk from the Hanford leaking tanks.

The paper itself is, however, interesting I think. If the money spent at Hanford saves few lives, the science learned might, in a better world than the one in which we live, may advance science.

[QED]

1. I think I've been retired too long - I read the formula for gibbsite as the same as for corundum.

Corundum is Al2O3. Or maybe I just need more coffee this morning.

I wrote a stoichiometry problem for my chemistry students "the conundrum of corundum". Add a Cr impurity and a ruby forms. Add one of a number of different transition metals and a sapphire forms. The context rich problems always confused them - they were learning to separate important details from extraneous info (much like my post here).

[NNadir]

2. The DU editor doesn't allow for Greek letters designating crystal polymorphs.

It appears that in the Hanford tanks the aluminum oxide phase is partially the gamma phase, not the pure alpha associated with rubies and sapphires.

If Rubies formed in the tanks - they would probably be radioactive.

That's an interesting observation though, that hadn't occurred to me. Thanks for the reminder.