1. Chemistry Department, Faculty of Science, Alexandria University, P. O. Box 426, Ibrahimia, 21321 Alexandria, Egypt, Fax: +20‐3‐391‐1794
2. Departament de Química Inorgànica, Universitat de Barcelona, Martí I Franqués 1‐11, 08028‐Barcelona, Spain, Fax: +34‐93‐490‐7725
3. Department of Chemical and Biological Engineering, Division of Materials and Surface Chemistry, Subdivision of Environmental Inorganic Chemistry, Chalmers University of Technology, 41296 Göteborg, Sweden, Fax: +46‐31‐772‐2853