This monograph is intended to provide a systematic presentation of theories concerning the adsorption of metal ions from aqueous solutions onto surfaces of natural and synthetic substances and to outline methods and procedures to estimate the extent and progress ofadsorption. As heavy metals and the problems associated with their transport and distribution are of serious concern to human health and the environment, the materials presented in this volume have both theoretical and practical significance. In writing this monograph, one ofour goals was to prepare a book useful to environmental workers and practicing engineers. For this reason, our presentation relies heavily on concepts commonly used in the environmental engineering literature. In fact, the volume was prepared for readers with a basic understanding of environmental engineering principles and some knowledge of adsorption processes. No prior familiarity with the ionic solute adsorption at solid-solution interfaces is assumed. Instead, introduction of the necessary background information was included. Generally speaking, metal ion adsorption may be studied in terms of three distinct but interrelated phenomena: surface ionization, complex formation, and the formation and presence of an electrostatic double layer adjacent to adsorbent surfaces. Analyses of these phenomena with various degrees of sophistication are xviii ADSORPTION OF METAL IONS FROM AQUEOUS SOLUTIONS presented, and their various combinations yield different models that describe metal ion adsorption.