Transformation of Ferrihydrite to Goethite and the Fate of Plutonium

Abstract

Understanding interactions between plutonium and iron (oxy)hydroxide minerals is necessary to gain a predictive understanding of plutonium environmental mobility and ensuring long-term performance of nuclear waste repositories. Plutonium sorption and desorption reactions with mineral surfaces, formation of PuO2 nanoparticles, and coprecipitation processes are all likely important processes under a range of geochemical conditions. We investigated plutonium fate during the formation of ferrihydrite and its subsequent recrystallization to goethite. Ferrihydrite was synthesized with varying quantities of Pu(IV) following either a sorption or coprecipitation process; the ferrihydrite was then aged hydrothermally to yield goethite. The synthesized materials were characterized via extended x-ray absorption fine structure spectroscopy, transmission electron microscopy, and acid leaching to elucidate the nature of plutonium association with ferrihydrite and goethite. In samples prepared following the sorption method, plutonium was identified in two different forms: a PuO2 precipitate and a surface sorbed plutonium complex. For the samples prepared via coprecipitation the data demonstrate that no PuO2 formation occurs in the ferrihydrite precursor and in the goethite experiments where plutonium concentration is ≤ 1000 ppm (mg Kg -1 ). In these coprecipitation experiments plutonium extended x-ray absorption fine structure data indicate the plutonium is strongly bound to the minerals either via formation of a strong inner sphere complex, or via an incorporation process. In the coprecipitation experiments, PuO2 formation only occurs at the highest plutonium concentration (3000 ppm), suggesting that during ferrihydrite recrystallization part of the plutonium can be remobilized to form PuO2 nanoparticles if the plutonium concentration is sufficiently elevated. In a series of acid leaching 11/16/21 3 experiments, less plutonium was removed from the goethite surface when formed via the coprecipitation process compared to the sorption process. Collectively, our results demonstrate that the nature of plutonium associated with the precursor ferrihydrite (adsorbed versus coprecipitated) will have a direct impact on the association of plutonium with its recrystallization product (goethite). Furthermore, the data illustrate that some properties of plutonium association with the precursor ferrihydrite are retained through recrystallization process to goethite. These findings show that plutonium strongly associates to iron (oxy)hydroxides formed through coprecipitation processes and in these materials plutonium can be strongly retained by the iron minerals.

Weathering of stannite–kësterite [Cu2(Fe,Zn)SnS4] and the environmental mobility of the released elements

“…The absence of Sn in goethite confirms the remobilization of Sn during the transformation. The released Sn is deposited in the secondary cassiterite (Balboni et al, 2020), as the ultimate sink of Sn, the most common and stable Sn mineral at the Earth's surface (Lottermoser and Ashley, 2006;Rai et al, 2011). The resistance of cassiterite to weathering was also observed from oxide zones of supergene deposits (Nickel, 1984) and supports the findings of this study.…”

Synthesis and multi-scale properties of PuO2 nanoparticles: recent advances and open questions

“…muscovite, quartz, hematite) has been demonstrated to occur by the precipitation of PuO 2 -like nanoparticles thus strengthening the interest of the community. 39,[54][55][56][57][58][59][60][61] Because the migration ability of An(IV) colloids is highly related to the particle size, stability, and chemical reactivity, there is an obvious interest in their rigorous characterization. 53 Evidence of Pu nanoparticles as colloidal or particle fractions has also been reported in oceans, seawaters and marine sediments.…”

Spatiotemporal Studies of Soluble Inorganic Nanostructures with X‐rays and Neutrons

“…It was found that Pu sorption on ferrihydrite followed by transformation into goethite produces PuO 2 and Pu is incorporated into the Fe mineral. Differently, co‐precipitation of ferrihydrite and Pu only results in Pu incorporation into the structure of the Fe minerals [129] . In another study, the same group investigated the interaction of Pu with calcite with EXAFS.…”

“…Differently, coprecipitation of ferrihydrite and Pu only results in Pu incorporation into the structure of the Fe minerals. [129] In another study, the same group investigated the interaction of Pu with calcite with EXAFS. They found that Pu(VI) is incorporated as plutonyl into the calcite structure and undergoes partial reduction to Pu(V).…”