Environmental Engineering Reference
In-Depth Information
reduces the number of surface sites available to each. However, it is interesting to
observe that the adsorption of CrO
4
2-
was promoted by the presence of SO
4
2-
at pH>
pH
zpc
. It can be explained by considering the fact that the ligand exchange has more
influence on CrO
4
2-
than SO
4
2-
in this pH range and the complexation between SO
4
2-
and
CrO
4
2-
in the mixture strengthens the ligand exchange. To have a quantitative
comparison, the competitive adsorption of SO
4
2
and CrO
4
2-
onto the -Fe
2
O
3
nanoparticles was studied at pH 2.5 in systems containing these two anions varying from
10 to 200 mg/L (as S, Cr) under a shaking speed of 200 rpm. Figure 9.20 shows that the
competitive inhibition occurred to coexisting anions due to the similar configuration of
molecular structures of these species. The experimental data for SO
4
2-
and CrO
4
2-
onto
the -Fe
2
O
3
nanoparticles were fitted by the Langmuir isotherm. From the constants
listed in Table 9.4, in binary system, SO
4
2-
gives lower q
m
values indicating the weaker
affinity to the -Fe
2
O
3
in comparison with that of CrO
4
2-
under the same condition.
20
18
16
14
12
10
8
Cr(VI) alone
Cr(VI) from Cr(VI) +SO4
SO4 alone
SO4 from Cr(VI) +SO4
6
4
2
0
0
30
60
90
120
150
180
Ce (mg/L)
Figure 9.20
Adsorption isotherms for SO
4
2-
and CrO
4
2-
in single and binary systems.
Table 9.4
Langmuir constants for SO
4
2-
and CrO
4
2-
adsorption (at pH 2.5 and room
temperature of 22.5
o
C).
Langmuir Constants
R
2
Adsorbate
q
m
(mg/g)
b (L/mg)
CrO
4
2-
(binary)
13.89
0.229
0.992
SO
4
2-
(binary)
9.81
0.315
0.991
CrO
4
2-
(single)
19.42
0.319
0.996
SO
4
2-
(single)
12.21
0.286
0.996
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