It is a compound with a large surface area and is used as a desiccant and retaining filter related compounds in water or air.
It activated alumina in a form of highly porous aluminum dioxide and creates links to some liquids and gases without physical changes. With this porosity commonly used as a drying agent; also it used for water treatment and as a catalyst in refinery operations.
Also it has the ability to adsorb other molecules such as arsenic, fluorine, lead and copper. That is why it is used in water treatment.
For example excess fluoride in drinking water can cause health damage as arsenic, so that these substances are filtered through activated alumina to control the presence of these molecules.
It is very common use for treatment of contaminated water as activated alumina retains molecules such as lead, copper and zinc; where they remain linked to the alumina without risking to be released back to the fluid already filtered.
In the case of refining the used as reaction catalyst for the recovery of sulfur elements sulfur dioxide from industrial reactions. Which are polluting the environment.