1. Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea, Department of Chemistry and Biochemistry, San Francisco State University, 1600 Holloway Avenue, San Francisco, California 94132, and Environmental Geochemistry Group, LGIT, University of Grenoble I/CNRS, BP 53, F-38041 Grenoble Cedex, France