Solution chemistry effects on sorption behavior of 109Cd(II) on Ca-montmorillonite

The sorption of radiocadmium on Ca-montmorillonite as a function of contact time, pH, ionic strength, foreign ions, humic acid (HA) and fulvic acid (FA) was studied using batch technique. The results demonstrated that the sorption of Cd(II) was dependent on ionic strength at pH 9. Outer-sphere surface complexation and/or ion exchange were the main mechanism of Cd(II) sorption on Ca-montmorillonite at low pH, whereas the sorption at high pH was mainly dominated via inner-sphere surface complexation. The sorption of Cd(II) on Ca-montmorillonite was dependent on foreign ions at low pH values, but was independent of foreign ions at high pH values. A positive effect of HA/FA on Cd(II) sorption was found at low pH values, whereas a negative effect was observed at high pH values. The thermodynamic parameters (i.e., ΔH 0, ΔS 0, ΔG 0) were calculated from the temperature dependent sorption isotherms, and the results indicated that the sorption process of Cd(II) on Ca-montmorillonite was spontaneous and endothermic.