In 1931, "natural" fluoride was found to be the cause of "mottled teeth" in humans. De-fluoridation is needed when the naturally occurring fluoride level exceeds recommended limits. A 1994
World Health Organization expert committee suggested a level of fluoride from 0.5 to 1.0 mg/L (milligrams per litre).
Activated alumina, a highly porous material consisting essentially of aluminum trihydrate. It is widely used as a commercial desiccant and in many gas drying processes.
The studies, perhaps the earliest, have demonstrated the high potential of activated alumina for fluoride uptake. The removal capacity of the medium was found to be about 800 mg/L of fluoride e/L of Alumina. Many modifications of process was suggested by subsequent workers, several patents based on the use of Aluminum oxide for fluoride removal were issued. At this level the fluoride removal capacity was approximately 500 mg of fluoride per liter of alumina.
Similar studies employing activated alumina was later conducted by many workers and all these works confirmed the ability of activated alumina for higher uptake of fluoride from water. Some researchers have concluded that removal was the result of ion exchange, but investigations by others have shown that the process is one of the adsorption and follows the Langmuir isotherm model.
The ability of activated alumina to remove fluoride depends on other aspects of the chemistry of water as well. Such factors as hardness, silica and boron, etc., if present in water will interfere with fluoride removal and reduce the efficiency of the system.