Environmental Engineering Reference
In-Depth Information
compared with other graphitic forms. Compared with other allotropes of carbon, gra-
phene has some special properties such as excellent electrical and thermal conduc-
tivity, high planar surface area, transparency to visible light, exceptional mechanical
strength, and hence many possible applications. However, most of the reported works
in graphene-related materials deal with catalytic or electronic processes [125]. Its pos-
sible beneicial use in water puriication has attained some momentum recently [126].
The recent investigation of Li et al. [127] showed that graphene has excellent luoride
adsorption ability with a saturated monolayer adsorption capacity of 17.65 mg/g at pH
7.0 and 25°C. The study reported that adsorption is a speciic process and luoride ions
are predominantly removed by the surface exchange reaction between luoride ions in
solution and hydroxyl ions on the adsorbent. The reported capacity was signiicantly
higher than that of CNTs [120]. A composite of graphene oxide and manganese oxide
(MOGO) has also been investigated for luoride removal. The studies revealed that
optimum removal of luoride occurred in a pH range of 5.5-6.7. The composite showed
maximum adsorption capacity of 11.93 mg of F
−
/g of MOGO and is 8.34 times higher
than that of GO [128].
19.5 Summary
Fluorosis is a chronic menace affecting a large population worldwide. Consumption of
drinking water obtained from luoride-rich ground strata is the major mechanism for luo-
ride intake in India and other luoride-affected areas. An immediate and viable solution
to alleviate the problem is to remove the pollutant from the contaminated groundwater
resources. Many treatment methods have been developed and practiced over the years
for deluoridation of water. However, many of them did not produce the desired results in
the affected areas where high rural poverty prevails. Among the technologies, adsorption
seems to be the most appropriate one. Surprisingly, among the numerous adsorbents that
were tested for removing luoride, AA is still the most preferred adsorbent. However, poor
sorption kinetics and effectiveness in a narrow pH range, competition from other ions,
poor stability, especially in acidic and alkaline pH, and energy-intensive synthesis limit
the use of AA in many ield situations.
Nanomaterials offer great opportunity in developing adsorbents of high capacity and
faster kinetics. Many nanomaterials, especially metal oxide-based materials, showed
enhanced luoride uptake. However, most of these materials are available only as ine
powders or are dispersed in aqueous suspension. Using such materials has practical
limitations, such as dificulty in solid-liquid separation and low hydraulic conductiv-
ity in packed bed. To overcome these limitations, NPs were anchored on inorganic and
organic matrices and tested for luoride uptake. However, the practical utility of the
nanomaterials are not well demonstrated. In short, the review indicates that although
nanomaterials offer better eficacy in terms of capacity and kinetics, every technology
has signiicant shortcomings and no single process can yet serve the purpose in all
the conditions. In the future, the success of the process for down-to-earth applications
such as water deluoridation is going to be largely dependent on, besides eficiency,
ease of large-scale synthesis, solid-liquid separation, and posttreatment handling. It
is also important to understand the local condition and the technology should be tai-
lored according to the local needs.
