Adsorption of heavy metal cations in montmorillonite

Abstract

Adsorption behavior of Ca, Ni, and Cu by a montmorillonite at pH 4 through 7 was analyzed within a wide range of divalent cation concentrations (M2+=1.25 to 4.5 mi/liter). The data were interpreted in terms of the following Langmuir equation which was derived in assuming that the adsorption process of divalent metal cations by Na-montmorillonite does not involve more than simple ion exchange: All the figures for Ca adsorption at each pH value could be plotted on a straight line, indicating that Ca was adsorbed onto the montmorillonite by a simple ion-exchange reaction. Langmuir plots of Ni and Cu adsorption revealed that the montmorillonite surface seems to provide the heavy metal cations with three types of adsorption sites of widely different reactivities, that is, Sites I, II, and III. Adsorption onto Site I corresponded to the ion-exchange reaction and both of the divalent cations were adsorbed up to 70-85% of the adsorption maximum through this reaction. Adsorption onto Site III was attributed to the precipitation of the metal hydroxides in the bulk solution. Adsorption onto Site II also seemed to be c10sely related to the formation of hydroxy species of the metal cations although it occurred even at pH 4. Adsorption maximum of Site II exceeded the CEC-value of the montmorillonite. On the basis of these finding, the excess adsorption on Site II can be interpreted as follows. Since the metal cations adsorbed on the clay surface were partially hydroIyzed there, further adsorption of the divalent metal cations from the bulk solution could proceed. The mechanism of adsorption onto Site II corresponded to the second adsorption pattern in Site I. © 1983 Taylor & Francis Group, LLC.