Cesium migration in Hanford sediment: a multisite cation exchange model based on laboratory transport experiments

Understanding cesium migration in subsurface environments is critical for assessing risks from nuclear waste sites. Prior research has shown that cation exchange plays a major role in controlling the mobility of radionuclides like cesium. However, the behavior of cesium in Hanford vadose zone sediments has not been fully characterized under conditions that mimic leaking tank fluids. This gap motivated the need to evaluate how cesium interacts with Hanford sediments in the presence of high concentrations of sodium and other cations. Previous studies have proposed two exchange sites for cesium in these sediments, but their predictive power under field conditions remains uncertain. The Hanford site presents a unique challenge due to the high variability in cation concentrations from tank leaks. No prior work had resolved the extent to which these conditions influence cesium retention and desorption. The complexity of cation exchange in heterogeneous sediments requires a more detailed model that accounts for multiple exchange sites and their relative affinities for different ions. This uncertainty drives the need to refine the cation exchange framework to better predict cesium behavior in the subsurface.

This study aimed to evaluate the transport behavior of cesium in Hanford formation sediments under conditions that reflect leaking tank fluids. The primary goal was to determine whether a multisite cation exchange model could accurately describe cesium migration in the vadose zone. Researchers focused on how cesium retention and desorption are affected by the presence of high concentrations of sodium and calcium. The motivation for this work stems from the need to improve predictive models for cesium transport in heterogeneous soils. By simulating cesium movement in laboratory columns, the study sought to identify the number and characteristics of cation exchange sites. The researchers also aimed to test the reversibility of cesium sorption under different ionic conditions. The study's design allowed for the comparison of batch and column experiments to assess model accuracy. This approach helps to bridge the gap between laboratory-scale findings and field-scale predictions.

The study used laboratory column experiments to simulate cesium transport in Hanford sediments. Columns were packed with uncontaminated sediment from the SX tank farm and exposed to cesium solutions with varying concentrations. Researchers monitored cesium breakthrough to assess retention and desorption dynamics. They also measured the total cation exchange capacity using 22Na isotopic equilibrium exchange. A flow-through column experiment provided data on the sediment's capacity to exchange cations. The study compared results from batch and column experiments to evaluate model performance. Researchers tested the fit of a multisite cation exchange model against experimental data. They proposed three distinct exchange sites based on the observed behavior of cesium and sodium. The model was calibrated using data from both binary batch and column transport experiments. This approach allowed for the identification of site-specific selectivity coefficients and their contribution to overall cesium retention.

The study found that cesium retention in Hanford sediments strongly depends on the concentration of other cations, especially sodium. Cesium retardation increased from 41 to 282 as its concentration decreased from 10^-4 M to 5x10^-7 M in 1 M NaNO3. The total cation exchange capacity was measured at 120 microeq/g, consistent with values from multi-cation elution. The proposed model included three exchange sites, with two representing frayed edge sites on micas. These sites showed extremely high selectivity for cesium over sodium, with K(Na-Cs) values of 10^7.22 and 10^4.93. The third site, associated with planar clay surfaces, had a lower selectivity coefficient of 10^1.99. Desorption experiments revealed that 72–90% of cesium sorbed on frayed edge sites was released over 10–24 days. At high cesium concentrations, 95% desorption was observed, indicating greater reversibility. The model accurately predicted cesium desorption curves using equilibrium reactive transport simulations.

The study supports the use of a multisite cation exchange model to describe cesium migration in Hanford vadose zone sediments. The model accounts for three distinct exchange sites, each with different selectivity for cesium and sodium. The high selectivity of frayed edge sites for cesium implies that desorption under natural recharge conditions is unlikely. The researchers observed that cesium sorption on these sites is largely reversible in a thermodynamic sense. The model's ability to match desorption curves with simulations confirms its predictive power. The study also highlights the importance of considering multiple exchange sites when modeling cation behavior in heterogeneous soils. The results suggest that cesium migration is strongly influenced by the ionic composition of leaking tank fluids. The findings provide a framework for improving predictions of cesium transport in the subsurface.