A langmuir isotherm-based prediction of competitive sorption of Sr, Cs, and Co in Ca-montmorillonite

Abstract

An accurate determination of parameters such as the distribution coefficient K d (for low concentrations) and the maximum adsorption capacity S T (for high concentrations) is desirable, as these values can be used in contaminant transport modeling. Considering that contaminants would more likely be found as mixtures and not as single components, the effects on K d and S T values resulting from competitive interactions between the target cation and other cations present in the system were estimated. This study focused on the determination of K d and S T for suspensions of Ca-montmorillonite and mixtures of the solutes strontium (Sr), cesium (Cs), and cobalt (Co) as analogs of a possible release of contaminants from radioactive waste repositories. Batch experiments were conducted for mixtures of the above solutes at intermediate concentrations at varying pH values and for two background electrolytes, NaCl and CaCl 2. The Langmuir isotherm adequately fitted the experimental data and was used to determine K d and S T . Maximum K d values were obtained for Sr at pH 6 and for Co at pH 8, while for Cs the pH did not affect the resulting K d value. The presence of competing ions resulted in a decrease of K d values depending on the type and concentration of ions in the original mixture. For clay preconditioned with NaCl electrolyte solution, the Na + partially saturated surface attracted more solutes, leading to greater K d values than for suspensions preconditioned with a CaCl 2 solution. Although the total amount of adsorbed cations ( S cum) in either electrolyte solution was nearly constant, a difference in the adsorption of individual cations was observed as an increased percentage of adsorbed Sr and Co in systems preconditioned with NaCl solution.