Environmental Engineering Reference
In-Depth Information
In these studies, the adsorption capacity of Al
2
O
3
/CNTs (14.9 mg/g) for fluoride at an
equilibrium fluoride concentration of 12 mg/L was about 13.5 times higher than that of
AC-300 (1.1 mg/g), 4 times higher than that of
-Al
2
O
3
(3.6 mg/g), and significantly
higher than that of IRA-410 polymeric resin (13.2 mg/g), an excellent adsorbent for
fluoride removal (Li et al., 2001). The adsorption isotherm of fluoride onto Al
2
O
3
/CNTs
was estimated to follow the Freundlich isotherm. The high removal efficiency was
achieved in a broad range of solution pHs, ranging from 5 to 9, making it suitable for
removing fluoride in environmental water. The aligned carbon nanotubes (ACNTs) did
not possess an adsorption capacity as high as for Al
2
O
3
/CNTs, though they showed
faster adsorption kinetics for the removal of fluoride (Li et al., 2003c). Here, the highest
adsorption capacity of ACNTs occurred at pH 7 and reached 4.5 mg/g at an equilibrium
concentration of fluoride of 15 mg/L, which was higher than that of γ-Al
2
O
3
, activated
carbon, and soil. The adsorption of fluoride onto ACNTs increased rapidly in the first 60
minutes and then slowly reached equilibrium at 180 minutes, with adsorption isotherms
belonging to both the Langmuir and Freundlich isotherms. The marginal difference in
the adsorption capacity of ACNTs in a broad pH range from 3 to 9 makes them feasible
for environmental applications.
γ
Since ceria has been reported to have a good adsorption capacity for the removal
of some anions, ceria supported carbon nanotubes were developed for the removal of
arsenate and Cr(VI) (Peng et al., 2005b; Di et al., 2006). It was found that the adsorption
of As(V) to CeO
2
-CNTs decreased with an increase in pH from 3 to 10, which was due
to electrostatic repulsion between the negatively charged As(V) ions and the negatively
charged adsorbent surface at high pH values (Peng et al., 2005b). Interestingly, the
presence of Ca
2+
and Mg
2+
in the solution significantly promoted the adsorption capacity
of arsenate. An increase in the concentration of Ca
2+
and Mg
2+
from 0 - 10 mg/L
resulted in an increase in the amount of As(V) adsorbed from 10 to 81.9 and 10 to 78.8
mg/g, respectively. This is because the cations Ca
2+
and Mg
2+
can act as a bridging ion
between the negative surfaces of adsorbent and negative As(V) ions. These adsorbents
can be successfully regenerated with a regeneration efficiency of 94% by using a 0.1 M
NaOH solution. Unexpectedly, the adsorption of Cr(VI) onto CeO
2
/CNTs showed the
best efficiency in the pH range between 3 and 7.4 (Di et al., 2006). Out of this range, a
sharp decline in the adsorption capacity of Cr(VI) was observed, due to the fact that at
low pH values Cr(VI) species are quickly reduced to Cr(III) ions that are not sorbed at
these values. At a high pH (> 7.4) there was electrostatic repulsion between the surface
adsorbent and the Cr(VI) species, and consequently competition between OH
-
and
Cr(VI) occurred. The adsorption capacity of CeO
2
/CNTs (28.3 mg/g) for Cr(VI) was
about 1.5, 2.0 and 1.8 times higher than that of activated carbon, Al
2
O
3
, and ball-milled
ACNTs, respectively, at an equilibrium Cr(VI) concentration of 33 mg/L. Hence,
CeO
2
/CNTs are promising materials for the removal of anion contaminants such as
Cr(VI) and As(V).
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