Environmental Engineering Reference
In-Depth Information
9.5.2.4 Effect of Adsorbent Properties
Since the -Fe
2
O
3
nanoparticles synthesized using high-temperature
decomposition of organics easily aggregate in the aqueous solution without any surface
treatment, the nanoparticles need to be pretreated before the adsorption experiments or
dispersed using some solvent. Some methods were applied to handle the -Fe
2
O
3
nanoparticles involving strong base washing or ethanol washing before Cr(VI)
adsorption and solvent dispersion during adsorption. Undergoing NaOH or ethanol
washing and freeze-drying, the 5 g/L -Fe
2
O
3
nanoparticles were shaken with 20 mL of
100 mg/L Cr(VI) at optimal conditions (i.e., pH 2.5 and shaking speed of 200 rpm) for
10 minutes. The Cr(VI) adsorption results are shown in Table 9.5. Alternatively,
trichloroethylene (TCE) was used for the dispersion of the untreated -Fe
2
O
3
nanoparticles during adsorption tests. Since TCE is a common solvent existing in
electroplating wastewater, 2 mL of TCE was added into 20 mL of 100 mg/L Cr(VI) for
the dispersion of the untreated -Fe
2
O
3
nanoparticles during adsorption tests. The results
are shown in Table 9.5. The nanoparticles washed by ethanol had the highest Cr
adsorption efficiency as compared to the nanoparticles washed by NaOH or dispersed
using TCE, which is perhaps due to some partial aggregations of nanoparticles that
existed in the solution of NaOH or TCE. Therefore, ethanol is more suitable for
nanoparticle washing to avoid possible aggregation of nanosized particles and hence was
used for the pretreatment of -Fe
2
O
3
nanoparticles synthesized by decomposition of
organic precursors before adsorption.
Table 9.5
Cr(VI) adsorption by different sized -Fe
2
O
3
nanoparticles.
Size
Equilibrium Concentration
(mg/L)
Adsorption Efficiency
(%)
(nm)
NaOH
Ethanol
TCE
NaOH
Ethanol
TCE
3
5.83
3.42
4.99
94.17
96.58
95.01
7
7.96
6.01
6.92
92.04
93.99
93.08
11
10.82
8.64
9.85
89.18
91.36
90.15
The -Fe
2
O
3
nanoparticles prepared by decomposition of organic precursors and
the sol-gel methods were used in the Cr(VI) adsorption studies. The adsorption
efficiencies of Cr(VI) as a function of contact time are shown in Figure 9.21. The
adsorption equilibrium time for the nanoparticles of 3, 7, 11 and 15 nm was found to be
2, 4, 5 and 10 minutes, respectively, measured at pH of 2.5 and a shaking speed of 200
rpm. The -Fe
2
O
3
nanoparticles prepared by
high-temperature decomposition of organic
precursors showed a higher adsorption capacity (over 90%) as compared to those
prepared by the sol-gel method (about 81%). Furthermore, the adsorption equilibrium
time increased with an increase in the nanoparticle size. For a fixed amount of Cr(VI)
and nanoparticles with a relatively higher external surface area, the ratio of the initial
amount of Cr(VI) to the adsorption sites of the adsorbent becomes lower; hence, most of
Cr(VI) can be adsorbed onto the exposed active sites faster, thereby shortening
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