Radionuclide sorption dynamics in the Rhone River: Experimental and modelling approach

Area of Science:

• Environmental Science
• Radiochemistry
• Environmental Modeling

Background:

• Radionuclide transfer in rivers is often modeled using the equilibrium distribution coefficient (Kd), assuming instantaneous and reversible solid/liquid fractionation.
• This equilibrium assumption may underestimate dissolved fractions and fail to account for environmental changes affecting radionuclide partitioning.
• Slower adsorption kinetics and environmental shifts (e.g., confluences, dams) necessitate more dynamic modeling approaches.

Purpose of the Study:

• To evaluate the accuracy of kinetic fractionation models compared to the traditional equilibrium (Kd) approach for radionuclide transfer in rivers.
• To assess the ability of different models (Kd, one-kinetic, two-kinetic) to predict radionuclide behavior under changing environmental conditions.

Main Methods:

• Experimental determination of solid/liquid fractionation kinetics for 137Cs, 60Co, 54Mn, and 110mAg using Rhone River suspended particulate matter over 2 months.
• Simulated environmental condition changes via suspension dilutions after varying adsorption periods (1h to 31 days).
• Fitting experimental data to equilibrium (Kd) and kinetic (EK, KK) models and evaluating predictive capacity for dilution scenarios.

Main Results:

• Radionuclide adsorption kinetics stabilized only after several days to two weeks, contradicting the instantaneous assumption of the Kd model.
• Dilution induced initial radionuclide release followed by unexpected re-adsorption, highlighting non-equilibrium dynamics.
• Kinetic models (EK, KK) predicted variations in solid/liquid activity ratios with time and environmental changes, unlike the constant Kd model.

Conclusions:

• Kinetic models provide more realistic estimations of radionuclide solid/liquid fractionation in rivers than the equilibrium Kd approach.
• Models incorporating kinetics are essential for accurately predicting radionuclide transfer dynamics, especially when facing environmental perturbations.
• One-step sorption kinetic models showed limitations in reproducing complex adsorption behaviors observed after dilution events.