Effect of temperature on single and competitive adsorptions of Cu(II) and Zn(II) onto natural clays.

This study conducted a combined adsorption-sequential extraction analysis (CASA), by which five phases (i.e., exchangeable, carbonate, Mn-Oxide, organic, and Fe-Oxide phases) of adsorbed heavy metals were analyzed, to investigate temperature effects on single and competitive adsorptions of Zn(II) and Cu(II) onto natural clays. In the case of single adsorption of Zn, the exchangeable phase adsorption decreased from 65 to 40%, but the carbonate phase adsorption increased from 30 to 40%, with an increase in temperature from 15 to 55 degrees C. However, in its competitive adsorption with Cu, Zn was mostly present in the exchangeable phase (over 90%), and with an increase in temperature, the exchangeable phase adsorption decreased only 10%. In the case of Cu, over 50% among the total amount of adsorption was present in the carbonate phase in both cases of single and competitive adsorptions. The carbonate phase adsorption of Cu increased from 56 to 61% and from 60 to 66% in single and competitive adsorptions, respectively, with a temperature increase. These results show that in the case of Zn, the major mechanism of retention in natural clay soils might be exchangeable phase adsorption, especially in the case of competitive adsorption with Cu. However, in the case of Cu, the major mechanism might be carbonate phase adsorption, which is known to be a more immobile phase than exchangeable phase adsorption. It seems that the adsorption of Zn and Cu onto natural clays is an endothermic reaction, which represents that the adsorption equilibrium constants and capacities increase with a temperature increase, with the exception of exchangeable phase adsorption.