Heavy metal removal with Mexican clinoptilolite: Multi-component ionic exchange

This paper describes the interactions of Pb(II), Cd(II), and Cr(VI) competi ng for ion-exchange sites in naturally occurring clinoptilolite. Dissolved Pb and Cd were effectively removed within Ish in batch reactors, with highe r removal efficiencies (> 95%) in the acidic pH range. The presence of Cr(V I); which can interact with these metals to form anionic complexes, signifi cantly diminished the Pb and Cd removal efficiencies. A decrease in the eff iciency of clinoptilolite to remove Pb was also observed in the high (great er than or equal to 10) pH range. This was attributed to the formation of a nionic hydroxo-complexes with little affinity for cationic ion exchange sit es. Pb outcompeted Cd for ion exchange sites in a flow-through column packe d with clinoptilolite (contact time = 10 s). The preferential removal of Pb in column, but not in batch reactors, reflects that competitive retention can be affected by contact time because diffusion kinetics may influence th e removal efficiency to a greater extent than equilibrium partitioning. Phe nol, which was tested as a representative organic co-contaminant, slightly hindered heavy metal removal in batch reactors. This was attributed to the formation of organometallic complexes that cannot penetrate the zeolite exc hange channels. Altogether, these results show that natural zeolites hold g reat potential to remove cationic heavy metal species from industrial waste water. Nevertheless, process efficiency can be hindered by the presence of ligands that form complexes with reduced accessibility and/or affinity for ion exchange. (C) 2000 Elsevier Science Ltd. All rights reserved.