Environmental Engineering Reference
In-Depth Information
density of reactive sites on the particle surfaces, and short diffusion route, and hence better
kinetics, make them better candidates for puriication of water including water deluori-
dation [11,74,77]. This section reviews some of the recent developments in deluoridation
of water using nanoscale materials. Various classes of nanoscale materials considered for
the discussion are as follows: (i) metals and metal oxides/hydroxide/oxyhydroxide NPs,
(ii) embedded metals and metal oxides/hydroxide NPs, and (iii) carbon-based materials.
19.4.1 Metals and Metal Oxides/Hydroxides/Oxyhydroxide
Metals and their oxides/hydroxides/oxyhydroxide are important classes of materials
widely used in water puriication [74,77]. Recently, studies have been directed toward the
controlled synthesis of these materials with respect to their size, morphology, phase, and
crystallinity, expecting enhanced performance. The application of such materials in deluo-
ridation of water is discussed in this section.
19.4.1.1 Alkaline Earth Metal-Based Oxides
The potential of magnesium oxide to scavenge luoride from water has been known for
>70 years [78,79]. Nagappa and Chandrappa [80] studied the luoride retention potential of
mesoporous MgO nanocrystals prepared through the combustion route using magnesium
nitrate as the oxidizer and glycine as the fuel. NPs of size 12-23 nm prepared by this route
showed a six times increase in the luoride uptake compared with the commercial MgO
and could reduce the sludge volume by 90%. However, an increase in pH was observed
after treating the water using magnesium oxide. The mechanism for luoride removal sug-
gested is by chemisorption and the formation of MgF
2
. A modiied combustion route for
the synthesis of nanomagnesia (NM) was proposed by Maliyekkal and coworkers [81]. The
synthesis is based on the self-propagated combustion of the magnesium nitrate trapped
in cellulose ibers, using urea and glycine as fuels. Various characterization studies con-
irmed that NM formed after combustion is crystalline and porous, with NP sizes vary-
ing from 3 to 7 nm (Figure 19.2). The proposed synthetic approach claims high product
recovery, owing to reduced loss of NPs during combustion. The method also claims 20%
reduction in the synthesis cost in terms of the raw materials used. The NM has been found
to be superior to conventional MgO in removing luoride from water. On the basis of the
microscopic and macroscopic studies, the authors ascribed the mechanism of luoride
removal to isomorphic substitution of luoride in brucite lattice (Figure 19.3). They did
not observe evidence of MgF
2
formation under the experimental conditions (temperature
30 ± 2°C; luoride concentration <50 mg/L). It is clear that MgF
2
(Ksp = 5.16 × 10
−11
) has a
higher-solubility product than Mg(OH)
2
(Ksp = 5.61 × 10
−12
), and hence, Mg(OH)
2
will be
a preferred product than MgF
2
at a lower concentration of luoride. NPs of CaO were also
investigated for the purpose [82]. The Langmuir maximum sorption capacity of the mate-
rial was found to be 163.3 mg/g. The luoride removal by the material was attributed to the
formation of insoluble CaF
2
. Although these compounds are eficient in removing luoride
and insensitive to normal pH variations and coexisting ions, the pH of the treated water
being alkaline is a concern and needs further attention.
19.4.1.2 Alumina and Other Aluminum-Based Materials
Alumina and aluminium-based oxides and oxyhydroxides are key materials in many appli-
cations, including catalysis and molecular adsorption [83,84]. Although several adsorbents
