Environmental Engineering Reference
In-Depth Information
to a desired concentration level [58]. The Nalgonda technique (NT), a modiied coagula-
tion process, has been applied in India at different levels. NT involves the addition of lime,
aluminum salt(s), and bleaching powder to raw water and proper mixing [59]. Bleaching
powder is usually added to disinfect the water. This method is reported to be suitable
for both community and household use, and has been successfully used at both levels
[60,61]. Although this process claims high luoride removal capacity, it has been recently
reported that it removes only a small portion of luoride (18%-33%) in the form of precipi-
tates. It converts a greater portion of ionic luoride (67%-82%) into soluble ion of aluminum
luoride complex [62]. Apparao and Kartikeyan [63] reported that the soluble aluminum
luoride complex is itself toxic; hence, the adoption of NT for deluoridation of water is not
desirable. The large amount of luoride-contaminated sludge produced during this pro-
cess, like with other coagulation processes, is another factor that limits its use.
Various types of ion-exchange resins have been tested for the deluoridation process.
It is reported that luoride removal by anion-exchange resins is not effective because
of their low selectivity toward luoride [52]. Besides, strong base anion-exchange resins
impart a taste to the treated water that may not be acceptable to the consumers. Cation-
exchange resins, such as Deluoro-1 Deluoro-2, Carbon, Wasoresin-14, and polystyrene,
have shown luoride exchange capacity [64]. Many researchers have studied the luoride
exchange capacity of cation-exchange resins loaded with various metals, and the results
showed that they are more preferable than anion-exchange resins [65,66]. Membrane
separation is also employed for deluoridation of water. Among the different membrane
techniques, reverse osmosis is more popular. These processes are relatively expensive to
install and operate, especially in rural conditions. They also experience fouling, scaling
and membrane degradation, which reduce the eficiency of the process [67].
Adsorption by a solid surface is considered as one of the best technologies for removing
luoride from drinking water, especially in rural areas, because of its easy handling, ver-
satility in operation, minimal sludge production, and regeneration capability. However,
the active surface area of the adsorbent, surface energy, and pH of the solution highly
inluence the removal eficiency. At present, several sorptive media have been reported
to be successful in removing luoride from drinking water. These include several natural,
synthetic, and metal oxide-based sorbents [68]. Among the adsorbents tested, activated
alumina (AA) is reported to be the most suitable one. However, adsorption by AA is a
slow process and is only effective in a narrow pH range. These factors limit its use in
treating luoride-contaminated water in many cases in the ield [54]. Hence, the develop-
ments of new adsorbents with tailor-made or customized physical and chemical proper-
ties are important to overcome the limitations and the use in the ield.
19.4 Defluoridation of Water Using Nanoscale Materials
Since the discovery of buckyballs by Curl, Kroto, and Smalley in 1985, the ield of nano-
technology has attracted overwhelming attention and has emerged rapidly [69]. Various
kinds of nanosystems have been investigated for diverse applications, including medical,
biotechnology, and electronics [70-72]. However, its beneicial use in puriication of water
has gained momentum only recently [73-75]. The advancement in nanotechnology sug-
gests that many issues regarding water quality can be improved using nanomaterials [76].
The basic properties such as extremely small size, high surface area to volume ratio, greater
