Exchange and reduction of Cu2+ ions in clinoptilolite

The ion-exchange and reduction processes for Cu2+ ions in clinoptilolite from the Caimanes deposit (Moa, Cuba) were studied at different temperatures. The ion-exchange studies were done to determine the kinetic parameters of Cu2+ removal from solution by this clinoptilolite modified previously to NH4+ form, and thermodynamic parameters of Cu2+ elution from zeolite using NH4Cl solution. The results show that temperature increase favors the exchange and that it is a reversible process. The external diffusion rate appreciably increases with temperature, while, the internal diffusion coefficient rises relatively little. This means that besides ion exchange other processes (such as precipitation of the low-solubility phase and/or salt adsorption) occur, which cause copper removal from solution and affect the intracrystalline diffusion of the ions. For steric reasons the exchange of [Cu(H2O)(6)](2+) ions from a solution must occur with a number of water molecules n smaller than 6 (6 > n >= 0). Cu2+ reduction by hydrogen and the formation of Cu-particles in the clinoptilolite were verified. The Cu2+ reduction mechanism is complex, should be present in the reduced indirect, and sensitive to reduction temperature; consequently, Cu-n(+) states intermediate between Cu2+ and Cu-0 samples. (C) 2007 Elsevier Inc. All rights reserved.