Nanoparticulate FeS as an Effective Redox Buffer to Prevent Uraninite (UO₂) Oxidation

A major concern in the nuclear age is the contamination of soils and groundwater with radionuclides from nuclear weapons and fuel production as well as other human activities. One of the most prevalent contaminants is uranium (U), whose mobility greatly depends on its oxidation state. Oxidized uranium, U(VI), is soluble and can spread into groundwater. In contrast, reduced uranium, U(IV), is often found in sparingly soluble phases such as solid uraninite, UO₂, and its low solubility reduces the environmental risk. Naturally-occurring iron sulfide (FeS) is known to be an important electron source for the reduction of uranium and oxidant scavenging. Researchers from the University of Michigan and the Pacific Northwest National Laboratory have recently used SSRL’s x-rays to shed light on the role of FeS in protecting UO₂ from reoxidation. The study, published in the journal Geochimica et Cosmochimica Acta, may help evaluate the long-term stability of uranium contaminations in the subsurface.

Simulating groundwater conditions, the scientists studied the oxidation of UO₂ nanoparticles by oxygen in the presence of nanoscale FeS in a batch reactor. A time series of the reaction products were examined by X-ray absorption spectroscopy (XAS) of the local uranium and sulfur environments using SSRL’s Beam Lines 11-2 and 4-3, respectively. The team found that the solid-phase uranium remained as U(IV)O₂ while FeS rapidly oxidized to elemental sulfur and Fe(III) oxyhydroxides.

Once all FeS had been oxidized, the researchers observed the subsequent oxidative dissolution of UO₂ into U(VI) species. The oxidized U(VI) formed multinuclear carbonato complexes that adsorbed onto the Fe(III) oxyhydroxide surfaces, indicating that adsorption is a major retention mechanism for U(VI) species formed during UO₂ oxidation. Thus, the research reveals a dual role for FeS in immobilizing uranium in an environmentally less harmful state: (1) FeS is an oxygen scavenger for protecting UO₂ against oxidation, and (2) its oxidation products are adsorbents for U(VI) after FeS depletion.

 

Primary Citation

Y. Bi, S. P. Hyun, R. Kukkadapu and K. F. Hayes, "Oxidative Dissolution of UO₂ in a Simulated Groundwater Containing Synthetic Nanocrystalline Mackinawite", Geochim. Cosmochim. Acta 102, 175 (2013) doi: 10.1016/j.gca.2012.10.032

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Contact

Contacts: Yuqiang Bi and Kim F. Hayes, University of Michigan