Activated alumina is a porous aluminum oxide used as a desiccant, water treatment, and catalyst. It works by adsorption, binding to substances like water, arsenic, and fluoride. It is cost-effective for rural and small municipal water treatment and can also clean stormwater runoff and contaminated sites. It is used as a Claus catalyst in natural gas operations, refineries, and smelters to prevent waste by-products from contaminating the environment.
Activated alumina is a form of aluminum oxide that is very porous and binds to certain liquids and gases without changing its chemical or physical form. Due to its high porosity, it has a high surface area to weight ratio. It is commonly used as a desiccant, for water treatment, and as a catalyst in refining and natural gas 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 works by the adsorption process. This is not to be confused with the more commonly known absorption process. Absorption is a physical process by which a fluid is drawn in and fills the empty spaces in a solid without binding to it. Adsorption, on the other hand, is a process where the fluid drawn into the pores of the solid material actually bonds chemically with the solid material.
When exposed to activated alumina, water molecules in the air will bind to the alumina, resulting in drier air. If the material is heated, it will release the water bound to it into the air. This process of binding 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 capability makes it a suitable candidate for use in water treatment operations. In many cases, it’s more cost-effective than a large-scale, state-of-the-art water treatment plant. Water treatment with activated alumina 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 added to public water supplies on purpose. High concentrations of fluoride, however, occur naturally in groundwater in some locations. This poses a local problem because excessive fluoride consumption can lead to fluorosis, a degenerative bone disease. In some localities, the arsenic toxin 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 stormwater runoff, mining waste, and for remediation of already contaminated sites. Rainwater can pick up soluble metals like 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 nearby water features. Sites that have been contaminated for some time to be reclaimed are also being identified.
In all of these situations, activated alumina can be a cost-effective way to remove unwanted or hazardous substances from the environment. Its ability to bind to these substances allows it to capture them in a stable form. When the material is finally disposed of, the unwanted substances do not flow back into the environment but instead remain bound to the alumina.
Natural gas operations, smelters and refineries also frequently use activated alumina as a so-called Claus catalyst. In chemical reactions, a catalyst is a substance that speeds up a reaction without being affected itself. The activated alumina serves as a catalyst in a process that recovers elemental sulfur from the hydrogen sulfide and sulfur dioxide produced as by-products of industrial operations. This prevents these waste by-products, which are harmful in large enough quantities, from contaminating the environment.
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