1. Lab. for Inorganic Materials Sci., Dep. of Chem. Technol.; MESA Res. Inst., Univ. of Twente; P.O. Box 217 7500 AE Enschede The Netherlands
2. Dep. of Environ. Sci.; Subdep. Water Resour., Wageningen Univ.; Nieuwe Kanaal 11 6709 PA Wageningen The Netherlands