Adsorption of cesium by synthetic clay-organic matter complexes: Effect of the nature of organic polymers

The strong correlation observed in situ between the bioavailability of radiocesium and soil organic matter content is paradoxical given the lack of coordination chemistry of cesium and its strong affinity for clay minerals. The aim of this work is to determine whether the adsorption of soil polymers, a protein and a polysaccharide, on reference clay minerals decreases cesium adsorption. Measurements were carried out in dilute suspensions of montmorillonite or illite, varying the initial polymer concentration, pH, and ionic strength. Planar clay surfaces were also blocked to elucidate the nature of the Cs adsorption sites affected. Both polymers caused a decrease in Cs adsorption, by up to an order of magnitude, particularly on illite and for trace concentrations of Cs. The effect increased with increasing polymer adsorbed on the clay surface but varied with experimental conditions so that there is no correlation between the quantity of polymer adsorbed and decreased Cs adsorption. Polymers appear to effect various Cs adsorption sites in different ways, by preventing clay layer collapse at frayed edge sites and by modifying the properties of regular exchange sites associated with planar surfaces. This indirect effect of adsorbed polymers may also modify the adsorption of other metals on clay minerals, in addition to the solubilizing effect of their coordination chemistry.