Impact of environmental conditions on the sorption behavior of radionuclide 90Sr(II) on Na-montmorillonite

Abstract

Clay minerals have been studied extensively due to their strong sorption and complexation ability toward various environmental pollutants. In this study, the sorption of 90Sr(II) on Na-montmorillonite was studied as a function of various environmental conditions such as pH, ionic strength, humic acid (HA) and temperature. The results indicated that the sorption of 90Sr(II) on Na-montmorillonite was strongly dependent on pH and ionic strength. The experimental data of 90Sr(II) sorption was simulated by the diffuse-layer model (DLM) well with the aid of Visual Minteq 3.0. At low pH, the sorption of 90Sr(II) was dominated by outer-sphere surface complexation and ion exchange with Na+/H+ on Na-montmorillonite surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The presence of HA enhanced 90Sr(II) sorption at pH < 7.0 but decreased 90Sr(II) sorption at pH > 7.0. Langmuir and Freundlich models were used to simulate the sorption isotherms of 90Sr(II) at three different temperatures of 303, 318 and 333 K. The thermodynamic parameters (ΔH, ΔS and ΔG) calculated from the temperature-dependent sorption isotherms indicated that the sorption of 90Sr(II) on Na-montmorillonite was an endothermic and spontaneous process. The thermodynamic parameters calculated from temperature-dependent sorption data were crucial to understand the interaction behavior of 90Sr(II) with Na-montmorillonite.