Activated alumina is a form of aluminum oxide that is very porous and bonds with certain liquids and gases without its chemical or physical form changing. Due to its high porosity, it has a high ratio of surface area to weight. It is commonly used as a desiccant, for water treatment, and as a catalyst in natural gas and refining operations.
A desiccant is a substance with a high affinity for water, which has a drying effect on its local atmosphere, such as inside a sealed container. As a desiccant, activated alumina operates by the process of adsorption. This should not be confused with the more commonly known process of absorption. Absorption is a physical process by which a fluid is drawn into and fills the empty spaces in a solid without bonding to it. Adsorption, on the other hand, is a process in which the fluid drawn into the solid material’s pores actually bonds chemically with the solid material.
When exposed to activated alumina, water molecules in the air will bond to the alumina, thereby resulting in drier air. If the material is heated, it will release the water bonded to it back into the air. This process of binding to and releasing water can be repeated indefinitely, making this material one of the most common desiccants.
It can also adsorb other materials such as arsenic, fluoride, copper, and lead. This ability makes it a suitable candidate for use in water treatment operations. In many cases, it is more cost-effective than a cutting-edge, large-scale water treatment facility. Activated alumina water treatment is often the method of choice for rural areas and small municipal facilities.
Small amounts of fluoride are generally considered beneficial to dental health, and in some areas, fluoride is purposely added to the public water supply. High concentrations of fluoride, however, occur naturally in the ground water in some locations. This poses a problem locally because excess consumption of fluoride can result in fluorosis, a degenerative bone disease. In some locations, the toxin arsenic also occurs naturally in dangerously high concentrations. Both of these substances can be filtered from drinking water to acceptable levels by adsorption with activated alumina.
This material is also used to clean storm water runoff, mining waste, and for remediation of already contaminated sites. Rainwater can pick up soluble metals such as copper and zinc from industrial sites, recycling centers, and so on. Tailings piles associated with mining operations can leach substances such as lead and arsenic into the nearby water features. Sites that have long been contaminated are also being identified for remediation.
In all of these situations, activated alumina can be a cost-effective way to remove unwanted or dangerous substances from the environment. Its ability to bind to these substances makes it possible to capture them in a stable form. When the material is finally disposed of, unwanted substances do not leach back into the environment but instead remain bound to the alumina.
Natural gas operations, smelters, and refineries also frequently make use of activated alumina as what is known as a Claus catalyst. In chemical reactions, a catalyst is a substance that accelerates a reaction without itself being affected. Activated alumina works as a catalyst in a process that recovers elemental sulfur from hydrogen sulfide and sulfur dioxide produced as byproducts from industrial operations. This prevents these waste byproducts, which are damaging in sufficiently high amounts, from contaminating the environment.