AbstractThe mechanism of boron uptake by clays, especially illitic clays, and the factors which control such uptake have long been debated issues. In an attempt to answer some of the questions of the controversy, three illites were treated in solutions containing boron. In the study, boron concentration, salinity, temperature, and time were varied independently over rather wide ranges.For the illites studied, irreversible uptake of boron was increased by increasing boron concentration, salt concentration, temperature, and time of treatment. The amount of boron which was fixed also varied with the type of illite treated. The amount of fixation was controlled primarily by the specific surface area of the clay and also by the crystallinity, K content, and/or amount of mixed-layer material (“frayed-edge” development?), and apparently was independent of the original boron content of the clay.A two-step mechanism is proposed for boron fixation by illite, consisting of rapid chemical adsorption of the tetrahedral B(OH)4− anion at the “frayed-edge” of the illite flake followed by much slower diffusion of boron into the tetrahedral part of the structure.