Environmental Engineering Reference
In-Depth Information
the HCl eluent with different concentrations (i.e., 0.001-0.2 M) for 30 minutes,
respectively.
Regeneration of Adsorbents.
To recover adsorbents, tests pertaining to
magnetic regeneration and metal re-adsorption were carried out in six consecutive cycles.
For each cycle, 0.1 g magnetic nanoparticles were mixed with 20 mL of 100 mg/L of
each metal solution in glass vials to reach equilibrium. To achieve complete desorption
of all the adsorbed Cr(VI), the nanoparticles were separated using a magnet after the
adsorption process, and then added into another 2 mL of NaOH with the same
concentration and shaken for 30 minutes to reach desportion equilibrium. After each
adsorption-desorption cycle, the magnetic nanoparticles were washed thoroughly with
ultrapure water to neutrality (recondition) and were ready for the succeeding cycle.
Consequently, the regenerated nanoparticles were again mixed with 20 mL of 100 mg/L
of metal solutions and the adsorption/desorption procedures were repeated. During the
adsorption or desportion process of each cycle, once equilibrium had been reached, the
nanoparticles were separated with a magnet and the supernatant was diluted and
acidified using 0.2% HNO
3
for metal analysis.
9.5.2 -Fe
2
O
3
Nanoparticles for Cr(VI), Cu(II) and Ni(II) Removal
9.5.2.1 Adsorption Kinetics
The effect of pH on heavy metals removal was evaluated. The maximum
removal efficiency for Cr(VI), Cu(II) and Ni(II) by -Fe
2
O
3
nanoparticles occurred at pH
2.5, 6.5 and 8.5, respectively. The effect of contact time on the removal of 20 mL of 160
mg/L Cr(VI), Cu(II) and Ni(II) at their optimal pH is shown in Figure 9.14. It can be
seen that the rate of metal uptake was initially quite high, followed by a much slower
subsequent removal rate leading gradually to an equilibrium condition. About 90% of
the heavy metal was removed during the first minute of reaction, while only a very small
part of the additional removal occurred during the following nine minutes of contact. For
these three metals studied herein, adsorption equilibrium was achieved within 10
minutes (Figure 9.14). The rapid adsorption of the heavy metal by -Fe
2
O
3
nanoparticles
is perhaps attributed to the external surface adsorption. Since nearly all of the adsorption
sites of -Fe
2
O
3
nanoparticle exist in the exterior of the adsorbent, it is easy for the
adsorbate to access the active sites and thereby a rapid approach to equilibrium as
compared to the porous adsorbent. At equilibrium, the amounts of Cr(VI), Cu(II) and
Ni(II) adsorbed were 16.9, 26.9 and 23.5 mg/g, respectively.
9.5.2.2 Effect of Operational Parameters
Temperature.
Figure 9.15 shows the influence of temperature on the adsorption
of Cr(VI). The removal efficiency of Cr(VI) from 20 mL of 100 mg/L K
2
CrO
4
solution
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