Competitive adsorption interactions of strontium, cobalt and cesium in suspensions of calcium montmorillonite: Electrochemical behavior and modeling

In this study, the capacity of calcium montmorillonite to adsorb stable Sr, Cs, and Co (analogs of typical low-level waste radionuclides), under a variety of operational conditions, was investigated. Specifically, the objectives of this study were to determine the factors governing adsorption, the reversibility of the adsorption process, and to model variations of adsorption with respect to pH, ionic strength and presence of competitive cations.

The adsorption of Sr and Cs is insensitive to pH in a range of 4 to 9, while Co adsorption increases in alkaline conditions (pH $>$ 8), suggesting that the complexating behavior of cobalt is reflected in its adsorption behavior. The adsorption was more reversible for Cs than for Sr and Co, implying that hydrated Sr and Co adsorb in the interlayer, while Cs probably adsorbs in the broken edges of the particles. The presence of competing cations causes a decrease in adsorption of Sr, Cs, and Co.

Adsorption of these cations at intermediate concentrations was modeled using a logarithmic function, in both single-component and mixture systems. Deviations from the logarithmic behavior at low (C$\sb{e}<50$ $\mu$eq/L) and high (C$\sb{e}>1000$ $\mu$eq/L) concentrations suggest linear adsorption for the former and other reactions, such as precipitation and multiple-layer adsorption, for the latter. The comparable magnitude of the competition coefficients obtained for Sr, Cs and Co in binary mixtures implies that these cations exert similar degree of competiveness.

Adsorption data was also modeled by surface complexation models (SCM), based on a hypothesis of four mechanisms: protonation and deprotonation of surface groups, adsorption of target cations in OH-sites, and ion-exchange of the targeted cation with Ca ions of the surface.

Complete analysis of the chemical species present in the solution was found to be useful to determine the surface-solution interphase interactions occurring in a particular system, by allowing the calculation of mass- and charge-balances in each phase. Variations in adsorption results due to the type of electrolyte solution used as background suggests the use of a standardized procedure for adsorption batch experiments, so that results obtained by different researchers can be compared. A full description of the response of the system to parameters that affect adsorption is essential to predict contaminant transport, in case of soil contamination events. (Abstract shortened by UMI.)