Review of the fundamental geochemical and physical behaviors of organoclays in barrier applications

Organoclays are organic-rich clay minerals, synthesized under controlled laboratory conditions, with engineering properties enhanced for use in containment applications. Organoclays can be formed using a variety of clay minerals as the base material; however, montmorillonite is used most commonly due to its high cation exchange capacity. The cationic organic phase is exchanged onto the mineral surface through ion exchange, with the organic cations displacing the clay's naturally occurring inorganic cations (e.g., Na+, Ca2+). Most commonly, quaternary ammonium cations are used as the organic phase, and organoclay behavior can be engineered through careful selection of the organic cation structure, size, and density of loading. Organoclays are promising materials for geoenvironmental applications due to their high sorption affinity for nonpolar organic contaminants. Sorption mechanisms can be engineered to be competitive or non-competitive, linear or non-linear, depending on the organic cation that is exchanged into the clay interlayer. Exchange of an organic phase for the naturally occurring inorganic interlayer cations also allows control of engineering properties, and clays can be synthesized with increased strength and decreased compressibility. This review of organoclays examines the fundamental behavior of organoclays, with an emphasis on geochemical properties including electroldnetic potential, hydrophobicity, sorptivity, attenuation of nonpolar compounds and electrical conductivity; physical properties including hydraulic conductivity, strength, dynamic properties and thermal stability and thermal conductivity. Relevance of organoclays to different barriers is discussed. Emerging polymer clay hybrid barrier materials are also summarized. (C) 2016 Elsevier B.V. All rights reserved.