1 research outputs found
Plutonium(IV) and (V) Sorption to Goethite at Sub-Femtomolar to Micromolar Concentrations: Redox Transformations and Surface Precipitation
PuÂ(IV)
and PuÂ(V) sorption to goethite was investigated over a concentration
range of 10<sup>–15</sup>–10<sup>–5</sup> M at
pH 8. Experiments with initial Pu concentrations of 10<sup>–15</sup> – 10<sup>–8</sup> M produced linear Pu sorption isotherms,
demonstrating that Pu sorption to goethite is not concentration-dependent
across this concentration range. Equivalent PuÂ(IV) and PuÂ(V) sorption <i>K</i><sub>d</sub> values obtained at 1 and 2-week sampling time
points indicated that PuÂ(V) is rapidly reduced to PuÂ(IV) on the goethite
surface. Further, it suggested that Pu surface redox transformations
are sufficiently rapid to achieve an equilibrium state within 1 week,
regardless of the initial Pu oxidation state. At initial concentrations
>10<sup>–8</sup> M, both Pu oxidation states exhibited deviations
from linear sorption behavior and less Pu was adsorbed than at lower
concentrations. NanoSIMS and HRTEM analysis of samples with initial
Pu concentrations of 10<sup>–8</sup> – 10<sup>–6</sup> M indicated that Pu surface and/or bulk precipitation was likely
responsible for this deviation. In 10<sup>–6</sup> M PuÂ(IV)
and PuÂ(V) samples, HRTEM analysis showed the formation of a body centered
cubic (bcc) Pu<sub>4</sub>O<sub>7</sub> structure on the goethite
surface, confirming that reduction of PuÂ(V) had occurred on the mineral
surface and that epitaxial distortion previously observed for PuÂ(IV)
sorption occurs with PuÂ(V) as well