Book excerpt: The Ion Exchange and Solvent Extraction series treats ion exchange and solvent extraction both as discrete topics and as a unified, multidisciplinary study - presenting new insights for researchers in many chemical and related field. Containing current knowledge and results in ion exchange, this text: presents an overview of the chemical thermodynamics of cation-exchange reactions, with particular emphasis placed on liquid-phase- and solid-phase-activity coefficient models; describes the development of surface complexation theory and its application to the ion exchange phenomenon; discusses metal-natural colloid surface reactions and their consideration by surface complexation modelling complements; and covers the influence of humic substances on the uptake of metal ions by naturally occurring materials.