Bentonite is the most widely used clay soil in the industrialized world. Currently, 20 million tons of bentonite are used worldwide each year, at an annual cost of $4 billion. Most people come into contact with materials containing or manufactured with bentonite on a daily basis. Foundries employ bentonite when casting automobile parts, paints include bentonite as a rheological agent and for pigment suspension, paper relies on bentonite to provide opacity, water treatment plants use bentonite as a catalyst, pharmaceuticals rely on bentonite as a carrier and neutralization agent, and the plastics industry uses bentonite to enhance the properties of polymers. Other applications for bentonite include fertilizers, pesticides, animal feeds, foods (extenders) and beverages (filtration), detergents, waxes, petroleum exploration, and dust control. In each of these applications, nanoscale phenomena in the bentonite affect behavior.
Because bentonite swells extensively in the presence of water, bentonite has a characteristic "tight" porous structure that tends to impede liquid migration when in a stable condition. For this reason, bentonite also is often used to control liquid flow and aqueous contaminant transport in geoenvironmental applications, such as in groundwater cutoff walls, barriers for waste containment (e.g., landfills, wastewater ponds, nuclear storage, etc.), secondary containment in tank farms, and seals in monitoring and water supply wells. These applications are ubiquitous in the United States; nearly every community has a waste containment facility, a petroleum storage facility, a groundwater remediation or treatment project, or sealed monitoring or water supply wells. However, in many of these applications, bentonite is exposed to conditions that can lead to instability and poor performance. Thus, modification of conventional bentonite to overcome such tendencies towards instability represents an important area of research.
Accordingly, this research focuses on modifying bentonite at the nanoscale to improve its stability for sustainable performance in a variety of geoenvironmental applications. Modification will involve inserting large organic molecules between crystalline montmorillonite layers comprising the bentonite at the nanoscale, and then polymerizing these molecules after insertion. This process will yield a more rigid structure that retains the large organic molecules thereby providing permanence. The modified material, known as a bentonite-polymer nanocomposite (BPN), is expected to retain the useful advantages of conventional bentonites, while being more resistant to long-term instability due to factors commonly encountered in geoenvironmental applications. Aside from resulting in superior barriers, seals, and sorbents that can provide considerable reduction in the risk to human health and the environment, BPNs also could revolutionize the way bentonite is used worldwide and impact a wide range of industries. The research project also represents an interdisciplinary, collaborative effort among researchers at three universities and an industrial partner (CETCO, or Colloidal Environmental Technologies Corporation), and will stimulate cross-fertilization among industry researchers, faculty, and students. Efforts also are planned to involve undergraduate students in the research as well as women and minorities.
Bentonite is a clay mineral used in environmental applications to prevent the flow of water and other contaminants. Applications include ground water barrier walls, seals around well seals, and liners for waste containment systems such as landfills. In some cases, interactions with contaminants or other constituents in the surrounding environment can affect bentonite adversely, affecting its ability to function effectively as a barrier. In this study we evaluated how these adverse impacts can be ameliorated by creating composite materials comprised of bentonite and polymers. The most successful composite was created by dissolving acrylic acid in a bentonite slurry, which was polymerized to create a bentonite-polymer composite (BPC). Testing indicated that the BPC swells more and maintains exceptionally low fluid transmission characteristics (low "hydraulic conductivity") compared to bentonite alone even when contacted with aggressive solutions that are known to have a severe adverse impact on conventional bentonites. Swelling of BPC is nearly four times larger than conventional bentonite in water. Thin layers of BPC simulating geosynthetic clay liners (GCLs) used for waste containment maintained very low hydraulic conductivity even when permeated directly with solutions having very high calcium concentration or extreme pH. Conventional bentonite and the polymer alone were at least three orders-of-magnitude more permeable than the BPC under the same conditions. The hydraulic conductivity of BPC does not follow the classical hydraulic conductivity-swell relationship typical of conventional bentonite, indicating that mechanisms other than internal swelling are responsible for the hydraulic conductivity of BPC. Clogging of pore spaces by the polymer is believed to the most significant mechanism responsible for the effectiveness of BPC under a very wide range of chemical conditions. Commercial bentonite-polymer composite products have been developed based on the principles identified in this study. These bentonite-polymer composite products have been deployed at full scale as environmental barrier materials to contain aggressive liquids and prevent environmental contamination at large industrial sites.
