A mechanistic modeling of montmorillonite contamination by cesium sorption

The triple layer electrochemical adsorption model (TLM) was utilized to model adsorption of Cs+ onto Ca-montmorillonite. Input data for the model were obtained via batch adsorption experiments in which Cs+ and mixtures of Cs+ and Sr2+ were added to a Ca-montmorillonite suspension at various conditions of pH and background electrolytes. A best fit of the model was achieved after gradually adjusting the values of the equilibrium constants and adsorption site concentrations. Two types of adsorption sites were considered to be responsible for adsorption in the clay: interlayer and frayed edges. The dominant mechanism of adsorption was identified as specific adsorption in the frayed edge sites, which although composing only 5% of the adsorption sites, they accounted for about 94% of total Cs adsorption. The model predicted precipitation of CsOH at concentrations above 1 mmol Cs/l, this value depending strongly on pH. The TLM successfully represented the adsorption of Cs over a wide range of concentrations, pH, nature of background electrolytes, and presence of competing cations.