James A.Davis - Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

Adsorption of inorganic and organic solutes by clay minerals, including competitive or synergetic effects in multisolute systems, has direct impacts on the rates and mechanisms of chemical transfer through clay barriers. Adsorption of ions, molecules and low- and high-molecular mass compounds may influence (1) clay mineral hydration and swelling, (2) mineral porosity and surface coating, (3) the development of bioorganic phases associated with clay minerals, and (4) surface tension at the clay mineral–water interface. The goal of this chapter is to summarize the major mechanisms controlling clay mineral interactions with inorganic and organic species (cations, anions, and molecules) and to discuss their relationships with the surfaces and interfaces of clay minerals. Regarding inorganic solutes, the chapter discusses cation exchange, surface complexation, and metal substitution reactions in the clay mineral structure, emphasizing the role of the interlayer space and edges, outer- and innersphere complex formation, and the contributions of electrostatic and covalent interactions to inorganic solute adsorption by clay minerals. The role of solute–bulk water interactions and the various effects of surface-associated water on adsorption of organic solutes by clay minerals are discussed, with an emphasis on the possible use of linear free energy relationships for the analysis of adsorption interactions. The interactions of clay mineral surfaces in soils and sediments with natural organic matter (NOM) and natural nanoparticles of other minerals are briefly reviewed, as these are relevant for engineered and environmental applications of clay minerals. In addition, the interactions of metal and metalloid ions and organic compounds with clay minerals in natural and engineered environments, the interfering effects of NOM, metal oxides, dissolved organic matter, and the impact of wetting–drying processes on the ability of clay minerals to interact with solutes are reviewed. Finally, some gaps in the current knowledge and critical research priorities are identified.