The Effect Of Thermal Treatment On The Sequestration Of Cations On Montmorillonite

Montmorillonite (Mt) is a natural and stable two-dimensional layered materials, with great adsorption capacity, surface area and swelling capacity. So it always be used for a effective adsorbent material for various of pollutants in waste water, especially for some heavy metal cations. However, how to dispose the spent montmorillonite after adsorption for heavy metal cations, is still needed further researches. In this study, Cu2+ and Cd2+ions with different ionic radius, Co2+ and Sr2+ (as radionuclide)?Cr3+ (with high charge) were chosen to explore the migration and transformation adsorbed in montmorillonite after thermal treatment. And the sequestration efficiency of these cations after heating was also studied. This research has achieved the following results:(1) This study explores the sequestration effect of heavy metal cations (Cu2+ and Cd2+) on montmorillonite by thermal treatment, and the sequestrate sites. The results of this work demonstrated that Cu2+ could migrate into both the ditrigonal cavities on the tetrahedral sheet and vacant sites on the octahedral sheet of Mt after thermal treatment due to its small ion radius. In this case, Cu2+ could be efficiently sequestrated on Mt layers, e.g., less than 3.2% desorption by ammonium acetate washing and less than 5% desorption by HCl washing for the 400? treated Cu-Mt. As for Cd2+ cations, they could hardly migrate into Mt layers, and thus much higher calcination temperature (e.g.,700?) was needed to efficiently sequestrate them on Mt.(2) Also, the results of this work demonstrated that Mt could sequestrate Co2+ and Sr adsorbed into Mt layers. Co2+ could migrate into the sheet of Mt after thermal treatment due to its small ion radius. In this case, Co2+ could be efficiently sequestrated on Mt layers. As for Sr2+ cations, they could hardly migrate into Mt layers, and thus much higher calcination temperature (e.g.,700?) was needed to efficiently sequestrate them on Mt.(3) In this work, the heating temperature can significantly affect the sequestration efficiency. For the samples heated in air atmosphere, Cr can be effectively sequestered at a relatively low temperature (e.g.,200?), as Cr3+ ions can migrate into the Mt layers because of its small ion radius. As the temperature increased to 300?, however, the oxidation of Cr3+ into Cr6+ would promote the desorption of Cr. As the temperature further increased, the collapse of Mt layers in combination with the formation of Cr2O3 then can help in sequestering Cr. Only after heating at 700?, efficient sequestration of Cr on Mt could be achieved by the significant damage of the layered structure. Heated in nitrogen atmosphere, the sequestration efficiency continually increased with rising temperature.