Plutonium sorption and desorption behavior on bentonite.

Area of Science:

• Geochemistry
• Environmental Science
• Nuclear Chemistry

Background:

• Understanding plutonium (Pu) sorption and desorption is crucial for predicting its subsurface transport.
• Sorption behavior can vary significantly with plutonium concentration, impacting experimental results.

Purpose of the Study:

• To investigate Pu(IV) and Pu(V) sorption to FEBEX bentonite across a wide concentration range.
• To compare sorption at environmental concentrations (≤10⁻¹² M) with typical laboratory concentrations (10⁻⁷–10⁻¹¹ M).
• To assess the role of montmorillonite in Pu sorption/desorption on bentonite.

Main Methods:

• Sorption experiments with Pu(IV) and Pu(V) on FEBEX bentonite over 120 days and 10 months, respectively.
• Flow cell experiments to study Pu desorption dynamics.
• Comparison of results with SWy-1 Na-montmorillonite data.

Main Results:

• Pu(IV) sorption to FEBEX bentonite was broadly linear from 10⁻⁷ to 10⁻¹⁶ M, reaching equilibrium in up to 100 days.
• At concentrations ≥10⁻⁸ M, Pu(IV) precipitation/polymerization reactions likely influenced sorption.
• Pu(V) reduction rates on smectite-rich clay were estimated over a 10-month period.
• Pu desorption from FEBEX bentonite continued over a 12-day flow period, similar to SWy-1 montmorillonite.

Conclusions:

• Montmorillonite phases significantly control Pu sorption and desorption reactions on FEBEX bentonite.
• Sorption behavior at environmental concentrations appears consistent with higher laboratory concentrations, though equilibrium times vary.
• The study highlights the importance of mineral phases in subsurface plutonium transport.