Activated alumina

Activated alumina is manufactured from aluminium hydroxide by dehydroxylating it in a way that produces a highly porous material; this material can have a surface area significantly over 200 square metres/g. The compound is used as a desiccant (to keep things dry by absorbing water from the air) and as a filter of fluoride, arsenic and selenium in drinking water. It is made of aluminium oxide (alumina; Al₂O₃), the same chemical substance as sapphire and rubies (but without the impurities that give those gems their color). It has a very high surface-area-to-weight ratio. That means it has a lot of very small pores, almost like tunnels, that run throughout it.

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Uses

Catalyst applications

Activated alumina is used for a wide range of adsorbent and catalyst applications including the adsorption of catalysts in polyethylene production, in hydrogen peroxide production, as a selective adsorbent for many chemicals including arsenic, fluoride, in sulphur removal from gas streams (Claus Catalyst process)etc.

Desiccant

Use as a desiccant, it works by a process called adsorption. The water in the air actually sticks to the alumina itself in between the tiny passages, as the air passes through them. The water molecules become trapped so that the air is dried out as it passes through the filter. This process is reversible, and if the alumina desiccant is heated to about 200°C it will release all of the water stored in it. This process is called regenerating the desiccant. Air always has some water in it; one way to measure the amount of water in the air is the air's dew point.

Fluoride absorbent

Activated alumina is also widely used to remove fluoride from drinking water. In the US, there are widespread programs to fluoridate drinking water. However, in certain regions, such as the Jaipur region of India, there is enough fluorine in the water to cause fluorosis. Activated alumina filters can easily reduce fluoride levels from .5 ppm to less than .1 ppm. The amount of fluoride leached from the water being filtered depends on how long the water is actually touching the alumina filter media. Basically, the more alumina in the filter, the less fluoride will be in the final, filtered water. Lower temperature water, and lower pH water (acidic water) are filtered more effectively too.

Activated alumina, when used as a fluoride filter, can be regenerated by a solution of lye (sodium hydroxide; NaOH), sulfuric acid (H₂SO₄), or alum (KAl(SO₄)₂).

See also

• Activated carbon
• Silica gel