What Is Bentonite?
Bentonite is a general term for clay consisting of different compositions, such as quartz, mica, and calcite. It has montmorillonite (a type of mineral) as the main constituent and is produced in Japan and other parts of the world.
Bentonite is characterized by its high water absorbency due to the layered structure of its main component, montmorillonite, and by its high viscosity after swelling. For this reason, it is widely used in ceramics, industry, and construction. It is also commonly used as an additive in pharmaceuticals, cosmetics, detergents, and other products closely related to daily life.
Uses of Bentonite
Bentonite is a clay characterized by high water absorbency and high viscosity after swelling. It has a wide range of applications, from ceramics, civil engineering, and construction to daily necessities and pharmaceuticals. Below are some examples of major uses.
- Ceramics: material for ceramics
- Civil engineering: waterproofing material for civil engineering works, mud during excavation
- Household products: additives to increase dispersibility, viscosity, and moisture retention in cosmetics, detergents, soaps, and agrochemicals
- Pharmaceuticals: base for pharmaceutical ointments
- Food: turbidity remover for wine and plum wine
- Other: Mixed with sand to form molds, cat litter, and impurity adsorbent in oil refining process
Principle of Bentonite
Bentonite is a clay composed mainly of a type of mineral (montmorillonite). Other constituents include silicate minerals such as quartz and opal, mica and zeolite, and carbonate and sulfate minerals such as calcite.
Bentonite deposits are found in Aomori, Miyagi, Yamagata, Niigata, Gunma, Okayama, and Shimane in Japan and outside Japan in the United States, China, Greece, and Turkey. The significant deposits mined in Japan and abroad are thought to have arisen because sediments derived from volcanic eruptions that occurred approximately 200 million to several million years ago underwent physical and chemical metamorphism due to temperature changes and pressure increases while buried deep underground.
Montmorillonite, the main component of bentonite, is a fine particle structure consisting of thin sheets stacked in layers, with water molecules and ionic components retained between the layers at intervals of approximately 1 nm. It is well known that the high water absorbency of bentonite is mainly due to the interlayer structure of montmorillonite and that the ionic components within montmorillonite play an essential role in its swelling action.
Types of Bentonite
The structure of bentonite is determined by the layer structure of its main component, montmorillonite. Montmorillonite is composed of tetrahedral sheets, which consist of silicon, oxygen, and octahedral sheets (sheets composed of aluminum and hydroxyl groups). One octahedral sheet is sandwiched between two tetrahedral sheets.
The octahedral sheet has a structure in which a portion of the aluminum is replaced by magnesium. Since aluminum is trivalent while magnesium is divalent, interlayer cations are incorporated within the layers to balance the electrical charges.
There are four main types of interlayer cations: Na+, Ca2+, K+, and Mg2+. The types of bentonite are divided according to the type of interlayer cations, and there are two types: “Na-type bentonite” and “Ca-type bentonite.
1. Na-Type Bentonite
Na-bentonite is a bentonite that contains a large amount of Na+ ions in the interlayer cations of montmorillonite. It has excellent swelling, thickening, and suspension stability.
2. Ca-Type Bentonite
Ca-type bentonite is a bentonite with high Ca2+ ion content in the interlayer cations of montmorillonite. Ca-type bentonite is inferior to Na-type bentonite in terms of swelling, thickening, and suspension stability but has excellent water absorption properties.
Some Ca-bentonite artificially becomes Na-bentonite by adding a few sodium carbonates to Ca-bentonite. It will then become activated bentonite. The properties of this activated bentonite are similar to those of Na-bentonite.
Other Information on Bentonite
1. Toxicity of Bentonite
Bentonite has no reported toxicity in regular use, as evidenced by its use in household products and pharmaceuticals. It is a common ingredient as a food additive because it causes minor skin irritation and is safe to ingest orally. In particular, fine particles of bentonite can adsorb impurities, which is used as a filtration aid in food production. For example, it is used to filter beer and other alcoholic beverages and soft drinks.
According to the safety data sheet for bentonite, the LD50 (half lethal dose: a measure of acute toxicity) value for oral intake in rats is 5,000 mg/kg. Based on this value, it can be concluded that bentonite belongs to the class of common substances under the Poisonous and Deleterious Substances Control Law.
On the other hand, inhaling large amounts of bentonite dust through the nose or mouth can cause pneumoconiosis. Pneumoconiosis is a disease caused by dust accumulation in the respiratory system.
Initially, there are no symptoms, but over time, symptoms such as coughing, phlegm, shortness of breath, and difficulty breathing may occur. Once a person suffers from pneumoconiosis, there is no cure, and the disease never returns to normal lungs. For this reason, bentonite is classified as a Class I dust, along with talc and other dusts, in the classification of dust hazard levels by the Japan Society for Occupational Health.
2. Bentonite in Civil Engineering Works
Bentonite is used as a raw material for mud to protect the walls of excavated ground in building foundations and infrastructure reinforcement projects. This is because simply excavating the ground may cause the side walls of the excavated area to collapse. Specifically, mud prepared by mixing water and bentonite is used to fill the hole while excavating to prevent the collapse of the sides.
This is done by taking advantage of the fact that adding water to bentonite causes it to swell and increase in viscosity, which penetrates the excavation surface to stabilize the sidewalls and, at the same time, forms a robust and thin mud layer on the surface, preventing a partial collapse of the excavation. It also prevents sediment generated during excavation from settling to the bottom. Because of these properties, it is used for safe and efficient excavation.
Bentonite mud is also inexpensive and has the advantage of being easy to work with. On the other hand, its disadvantages are that it is vulnerable to soil salinity and unstable to temperature.
Since it is not very effective in preventing the collapse of side walls, it is suitable for use when shallow excavation is required. Various additives, such as dispersants and lubricants, may be added to the bentonite mud to suit the excavation conditions.