Environmental Engineering Reference
In-Depth Information
follows: coating temperature, 65°C; latex/FAC, 0.5:1; and coating amount, 27.8%. The FAC@
glass beads showed a slightly higher luoride adsorption capacity (2.77 mg/g) compared
with FAC@sand (2.22 mg/g).
19.4.3 Carbon-Based Materials
The use of carbon in water puriication dates back to the Harappan civilization. Today, car-
bon, particularly activated carbon, has become one of the most common and trusted means
for removing contaminants from water [8]. Large surface area, reactive surface functional
groups, pore volume, and pore size distribution are some of the key factors that deter-
mine the success of any adsorbent in adsorption process. Over the years, various forms
of carbon and their composites have been developed, expecting enhanced performance,
and investigated as adsorbents for removing diverse pollutants from water [116,117]. Few
recent efforts speciic to deluoridation of water using carbon-based nanoscale materials
are reviewed below.
Carbon nanotubes (CNTs), allotropes of carbon with a cylindrical nanostructure, have
gained huge interest since their discovery. Their large surface area, small sizes, and
high mechanical strength make them attractive material for water puriication includ-
ing deluoridation. A composite of Al
2
O
3
and CNTs was prepared by depositing Al
2
O
3
on
CNTs, and its application for luoride uptake from water was studied [118]. The results
revealed that the adsorption process is very effective over a broad pH range (5.0-9.0)
unlike AA, which is effective only at a narrow pH range of 5-6. The adsorption capacity
of Al
2
O
3
@CNTs was found to be about 13.5 times higher than that of AC-300 carbon and
4 times higher than that of γ-Al
2
O
3
at an equilibrium luoride concentration of 12 mg/L.
The mass of luoride adsorption for Al
2
O
3
@CNTs at pH 6.0 reached 28.7 mg/g at an equi-
librium concentration of 50 mg/L. The same group has studied the effect of calcination
temperature, alumina loading, and pH on the preparation of Al
2
O
3
@CNTs and its ability
to remove luoride from water [119]. The optimum calcination temperature for prepar-
ing the adsorbents was observed to be 450°C. The maximum adsorption capacity was
observed at an alumina loading of 30% and a pH range of 6-9. Aligned CNTs (ACNTs)
were synthesized by catalytic decomposition of xylene using ferrocene as catalyst, and
their performance was evaluated for luoride removal from water [120]. Both the surface
and inner cavities of ACNTs were reported to be readily accessible for luoride adsorp-
tion. ACNTs have shown a maximum adsorption capacity of 4.5 mg/g at an equilibrium
luoride concentration of 15 mg/L. The adsorbent performed well over a broad range
of pH (5-8) unlike activated carbon, which is effective only at pH 3 [121,122]. Ansari et
al. [123] investigated the luoride-scavenging capacity of multiwalled CNTs (MWCNT)
from drinking water. MWCNTs showed a saturation capacity of 3.5 mg/g. Unlike ACNTs
and Al
2
O
3
@CNTs, MWCNTs showed high sensitivity to solution pH and the optimum
pH was found to be at 5 (~94% luoride removal in 18 min). Upon increasing the pH, the
adsorption capacity decreased gradually and reached 41.2% at pH 9.0. This reduction in
the luoride adsorption capacity in the alkaline pH range was ascribed to the competi-
tion for adsorption sites between hydroxyl ions and luoride. MWCNTs exhibited a satu-
ration capacity of 3.5 mg of luoride per gram. The presence of coexisting anions such as
chloride, nitrate, hydrogen carbonate, sulfate, and perchlorate showed a negligible effect
on the adsorption of F
−
onto MWCNTs.
Graphene, a one-atom-thick sheet of carbon, is a recent addition to the carbon fam-
ily. Since its discovery in 2004 [124], the material has gained overwhelming attention
owing to its unique physical properties, chemical properties, and low production cost
