Pu(V) and Pu(IV) sorption to montmorillonite

Area of Science:

• Environmental Geochemistry
• Radiochemistry
• Materials Science

Background:

• Plutonium (Pu) mobility in the environment is governed by its adsorption/desorption to mineral phases.
• Environmental concentrations of Pu in subsurface waters (<10⁻¹² M) are significantly lower than typical laboratory study concentrations (≥10⁻¹⁰–10⁻⁶ M).
• Understanding Pu adsorption behavior across concentration gradients is crucial for environmental risk assessment.

Purpose of the Study:

• To determine if Pu adsorption behavior observed in laboratory studies is representative of environmentally relevant concentrations.
• To compare the adsorption rates of different oxidation states of Pu (Pu(V) and Pu(IV)) to Na-montmorillonite.
• To investigate the influence of mineral composition, particularly iron and manganese content, on Pu(V) adsorption rates.

Main Methods:

• Sorption experiments of Pu(V) to Na-montmorillonite across initial concentrations ranging from 10⁻⁶ M to 10⁻¹⁶ M over 30 days and 1 year.
• Sorption experiments of Pu(IV) to montmorillonite at initial concentrations from 10⁻⁶ M to 10⁻¹¹ M over 30 days.
• Comparative adsorption studies of Pu(V) to various minerals including hematite, goethite, magnetite, groutite, corundum, diaspore, and quartz.

Main Results:

• Pu(V) adsorption to Na-montmorillonite exhibited linear behavior across the entire tested concentration range (10⁻⁶–10⁻¹⁶ M) after 30 days.
• Pu(IV) sorption to montmorillonite was faster than Pu(V) sorption within 30 days, but Pu(V) adsorption approached similar levels after one year due to slow reduction to Pu(IV).
• Minerals rich in iron and manganese (hematite, goethite, magnetite, groutite) demonstrated faster Pu(V) adsorption compared to minerals lacking these elements.

Conclusions:

• The linear adsorption behavior of Pu(V) to Na-montmorillonite supports extrapolating laboratory findings to low, environmentally relevant concentrations.
• Surface-mediated reduction of Pu(V) to Pu(IV) is a significant process influencing long-term Pu sequestration.
• The presence of redox-active minerals containing Fe and Mn can enhance the rate of Pu(V) removal from aqueous solutions.