Environmental Engineering Reference
In-Depth Information
From the data shown in Figure 36.9, it may be concluded that the equilibrium loading of
luoride is larger on Al-CNF than on Al-ACF. The equilibrium loading of luoride is also
larger (approximately 10×) on Al-CNF than CNF (the sonicated samples without metals). In
addition, sonication enhanced the equilibrium capacity of the prepared nanoibers because
of the dislodging of Ni catalysts from the tips and other surfaces of the grown CNF, thereby
creating additional sites for the incorporation of Al in the subsequent impregnation step.
It is important to compare the loading (mg/g) of luoride ions on Al-CNF to those reported
in the literature for luoride ions on different adsorbents such as AC granules, activated
alumina, and natural clays. From Figure 36.9, it may be noted that the loading of luoride
ions on Al-CNF is 0.25-17 mg/g corresponding to aqueous-phase luoride concentrations
between 0.06 and 50 ppm. These values compare well with 0.58 mg/g on activated alumina
corresponding to 1 ppm of luoride in water, 1-5 mg/g on the ZICFC corresponding to
20-100 ppm of luoride in water, and 2 mg/g on clays and 2.7 mg/g on activated titanium-
rich bauxite, both corresponding to 10 ppm of luoride ions in the solution. In another study,
magnetic chitosan particles saw loading of luoride ions at 16.5 mg/g, corresponding to an
aqueous-phase concentration of 100 ppm [40]. From the data presented in this section, it
may be concluded that surface loading (equilibrium concentration) of luoride ions on Al-
CNF is larger than those reported in the literature, either on carbon or alumina.
A
breakthrough curve
is essentially the time history of the efluent concentration from a
packed bed subject to a step change in the inlet concentration, which is inluenced by the
transport processes in the voids between the ibers and within the pores of the ibers. In
general, an adsorbate of relatively small size and large BET area exhibits a breakthrough
curve suppressed for a longer duration. Consequently, such a breakthrough characteristic
is relective of a good adsorbate used under dynamic conditions. In such a case, the inter-
particle and intraparticle diffusion resistances are usually small, and the process is likely
to be controlled by the adsorption/desorption kinetic rate. On the other hand, an immedi-
ate breakthrough in the packed bed means large interparticle and/or intraparticle diffu-
sion resistance, and consequently small uptake of the solute by the adsorbent.
The total uptake of the solute during the adsorption time is determined from the break-
through curve (i.e., hatched area marked in Figure 36.10 for the uppermost curve) by cal-
culating the integral along the curve between the two time limits: the incipience of the
adsorption test and the instance when the bed is saturated. Mathematically, the uptake of
the solute (mg) is determined as follows:
T
∫
Uptake (mg)
=
QCTCt
−
d
(36.1)
in
exit
0
where
T
is the total time of adsorption until the bed is saturated with the solute in the
inluent stream. It may be mentioned that the speciic uptake (mg/g of the adsorbent) must
be approximately equal to the equilibrium loading of the solute obtained under the batch
conditions, indicating the complete utilization of the adsorbent under low conditions. In
the typical experiments performed to study breakthrough characteristics using the perfo-
rated tubular reactor wrapped with fresh (unsaturated) carbon ibers, the aqueous solution
of luoride at a predetermined concentration is fed at a constant low rate to the reactor
and the concentrations at the outlet of the tube are measured. Figure 36.10 describes the
breakthrough curves obtained for three different inlet concentration levels of luoride in
the aqueous solution. It is important to mention that the luoride uptakes calculated from
the respective breakthrough curves shown in the igure matched approximately with
