ADSORPTION OF SOME HEAVY METALS BY NATURAL ZEOLITES: XPS BATCH STUDIES

摘要

The adsorption of some heavy metal ions by mordenite and clinoptilolite has been investigated using batch techniques and X-ray photoelectron spectroscopy (XPS). At 10~-5 M of Pb, Cu, Cd, and Zn, and in the presence of 10~3 M Ca as a competing cation, the preferential sequence of adsorption was Pb>Cu>Cd>Zn for mordenite, and Pb>Cu>Zn>Cd for clinoptilolite. The adsorption capacities and removal efficienices of the zeolites for the two highly toxic heavy metals, Cd and Pb, were also determined at a low-to-medium concentration rage (10~-6 to10~-3 M) and at a zeolite: solution ratio of 50:30 (mg:mL), in competition with 10~-3 M of Ca. Below 10~-4 M, both mordenite and clinoptiloite removed Pb from the solution almost completely. As Pb concentration increased, removal efficiency decreased. However, the zeolite samples (particularly mordenite) were not effective in reducing Cd concentrations to the legal μg/L level. Solution pHs and the photoelectron binding energies of adsorbed Pb(4f~7/2) and Cd(3d_5/2) gave no evidence of precipitation of Pb and Cd as hydroxides or carbonates during adsorption. Up to one-third fo the cation exchange capacity (CEC) of mordenite, and half that of clinoptilolite, were used for Pb adsorption. A much smaller fraciton of the CEC was used in Cd adsorption. XPS showed that Na ion was the principal cation involved in the exchange adsorption of Pb and Cd. As such, clinoptilolite with 676 mmol/kg of exchangeable Na adsorbed more Pb and Cd than mordenite with only 326 mmol/kg of exchangeable Na, comparing surface (XPS) with bulk (average) concentrations o