Environmental Engineering Reference
In-Depth Information
FIGURE 17.5
Electro-deluoridation units installed at different luoride affected regions.
17.7 Adsorption- and Ion-Exchange-Based Defluoridation Methods
A wide variety of adsorbent materials have been reported in the past few years for the efi-
cient and economical deluoridation of water. The most commonly used adsorbent for water
treatment, particularly for deluoridation of water, is activated alumina (AA), which is cur-
rently used in the ield for deluoridation of water in many parts of the world. The various
sorbents studied and investigated for luoride removal are reviewed in detail as follows.
17.7.1 Activated Alumina
Various alumina-based adsorbents have been reported in the past few decades for the
effective removal of luoride from drinking water, and clearly AA is the most studied
adsorbent for ield application. The luoride removal eficiency of these materials mainly
depends on the nature of the adsorbent. The most commonly used one for the deluori-
dation of drinking water is adsorption by AA, which has a high afinity for luoride and
can be regenerated with mild acid or alkali. However, luoride removal by AA is strongly
pH dependent and the presence of other co-ions also signiicantly interferes in the luo-
ride removal. The use of AA in a continuous-low luidized system is an economical and
eficient method for deluoridating water supplies. The process could reduce the luoride
levels down to 0.1 mg/L. The operational, control, and maintenance problems, mainly
clogging of bed, may be averted in this method.
Different researchers have reported different luoride adsorption capacities for AA. Ku
and Chiou et al. (2002) have reported luoride adsorption capacity of 16.3 mg/g at pH rang-
ing from 4.0 to 6.0. Ghorai and Pant (2005) investigated the adsorption of luoride by AA in a
ixed-bed column showing 2.41 mg/g of luoride adsorption capacity at neutral pH. Tripathy
et al. (2006) have reported the luoride removal performance of alum-impregnated alu-
mina signiicantly high at 40.38 mg/g at pH of 6.5 and luoride concentration of 1-35 mg/L.
To overcome the limitations of AA, several modiied alumina have also been reported.
Tripathy and Raichur (2008) have found manganese dioxide-coated AA as a promising
adsorbent for enhanced deluoridation of water as compared with uncoated AA. Coated
AA adsorbed more luoride than uncoated over a wider pH range. More importantly, max-
imum luoride adsorption occurred at pH ~7.0 in the case of coated alumina, which makes
it a potential adsorbent for treating drinking water. Maliyekkal et al. (2008) described the
