Study Of Pb²⁺, Cu²⁺, Cd²⁺ Adsorbed On Clay Minerals Under Aqueous Solution And Adsorption Mechanism

The features of three clay minerals (montmorillonte, illite, kaolinite) adsorbing three single heavy metal cations (Pb2+, Cu2+, Cd2+) in acid aqueous solution (pH≈4.3) under low concentration were investigated by Stir-Balance Method, and the features of montmorillonte adsorbing more than one heavy metal cation under multi compounds systems were also researched using the same method. ICP-OES was used to monitor the concentration of metal heavy cations and those maybe dissolved out from clay minerals. All these adsorption data were modeled using Lagergren Pseudo first order kinetics model and second order kinetics model. X-Ray photoelectron spectrometer was used to investigate the binding energy of Si, Al, O and binding form of metal cations adsorbed on clay minerals. D001surface of montmorillonite was analyzed by means of X-Ray diffraction method. Conclusions are as follows:1. All the three clay minerals can adsorb the heavy metal cations, montmorillonite has the largest adsorption capacity among those three minerals, the order of adsorption capacity to Cd2+is illite>kaolinite, while the capacity of kaolinite adsorbing Pb2+and Cu2+is larger than illite. Mineral clays have selective adsorption for different cations. The capacity rank of heavy metal cations adsorbed on montmorillonite and illite is: Cu2+>Cd2+>Pb2+, while Cu2+≈Pb2+>Cd2+on kaolinite.2. The adsorption of three heavy metal cations on these three kinds of clay minerals under single or multi compounds systems meets Lagergren pseudo second order kinetics model, which means the rate controlling step is chemical adsorption.3. In the Pb2+-Cd2+-H2O system and Cu2+-Cd2+-H2O system, the existence of Cd2+improve adsorption rate of Pb2+and Cu2+4. The result of X-ray diffraction suggests that the interlayer spacings of montmorillonite after adsorption nearly remained the same compared with nature montmorillonite. The XPS results indicate that three heavy metal cations were adsorbed on clay minerals as metallic oxides. Characteristic peaks of Pb3O4and PbO2were only detected on montmorillonite in low intensity, while other heavy metal cations were adsorbed on clay minerals as divalent metal oxides.