April 2009

Modified bentonites are of widespread technological importance. Common modifications include acid activation and organic treatment. Acid activation has been used for decades to prepare bleaching earths for adsorbing impurities from edible and industrial oils. Organic treatment has sparked an explosive interest in a class of materials called polymer–clay nanocomposites (PCNs). The most commonly used clay mineral in PCNs is montmorillonite, which is the main constituent of bentonite. PCN materials are used for structural reinforcement and mechanical strength, for gas permeability barriers, as flame retardants, and to minimize surface erosion (ablation). Other specialty applications include use as conducting nanocomposites and bionanocomposites.

Increasingly stringent regulation of pollution and waste production worldwide drives the need to isolate contaminants that pose a threat to human and environmental health by using engineered barrier systems. The relatively low cost and wide versatility of reactive barriers favour the use of bentonite as an important component in barrier systems. Current research aims to optimise the performance of bentonite-based barriers under the effects of coupled thermal, mechanical, hydraulic and chemical stresses, for a wide range of pollutants and over long time periods – tens of thousands of years in the case of nuclear waste.

The practice of eating clay for gastrointestinal ailments and applying clay topically as bandaids for skin infections is as old as mankind. Bentonites in particular have been used in traditional medicines, where their function has been established empirically. With modern techniques for nanoscale investigations, we are now exploring the interactions of clay minerals and human pathogens to learn the lessons that Mother Nature has used for healing. The vast surface area and chemical variability of hydrothermally altered bentonites may provide a natural pharmacy of antibacterial agents.

Bentonites are clay rocks consisting predominantly of smectite. They form mainly from alteration of pyroclastic and/or volcaniclastic rocks. Extensive deposits, linked to large eruptions, have formed repeatedly in the past. Bentonite layers are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration, favoured by fluids that leach alkali elements and by high Mg content. Smectite composition is partly controlled by parent rock chemistry. Recent studies have shown that bentonite deposits may display cryptic variations in layer charge – i.e. the variations are not visible at the macroscopic scale – and these correlate with physical properties.

Smectites are the main components of bentonites; their characteristics define the remarkable range for their uses in industry and technology. Their application depends on their fundamental properties, namely, their atomic structure with a rather flexible crystal lattice, their variable chemical composition, their particle size, and their morphology. The interlayer region of smectites is a favorable target for molecular engineering to design organic and inorganic hybrid materials, including smectite–polymer nanocomposites and pillared complexes of smectite–metal oxides.

Although bentonite has been used for thousands of years, most of its current uses were developed after 1900. Today it is a key raw material in the production of energy and steel, and in numerous other common products and applications that are critical to the world economy. This article reviews several of these important bentonite uses and describes some basic principles of bentonite exploration, mining, and processing.