Environmental Engineering Reference
In-Depth Information
significant. PO
4
3-
is a known inner-sphere complex-forming anion that has shown a
higher affinity for metal oxides (Violante and Gianfreda, 1993). Since the PO
4
3-
concentration in the electroplating wastewater is lower than 20 mg/L, its inhibiting effect
on the Cr(VI) adsorption can be compensated by using more adsorbents.
SO
4
2-
exists more commonly in electroplating wastewater as compared to PO
4
3-
.
Therefore, a more detailed study was given in relation with the influence of SO
4
2-
on the
removal of Cr(VI). It was found that, with an increase in the SO
4
2-
concentration, the
interference of SO
4
2-
became stronger. SO
4
2-
consumes surface sites of the adsorbent and
forms a charged surface complex that reduces positive interfacial charge and decreases
electrostatic attraction of CrO
4
2-
. Preliminary evidence (Figure 9.18) suggests that solute
interactions may be significant. The adsorption efficiency of 0.5 mM CrO
4
2-
and 0.5 mM
SO
4
2-
in single and binary solution as a function of pH is shown in Figure 9.19. The
adsorption efficiency of SO
4
2-
and CrO
4
2-
decreased sharply with increasing pH for all
the cases tested.
100
90
80
70
60
50
40
CrO4 alone
CrO4 from CrO4+SO4
SO4 alone
SO4 from CrO4+SO4
30
20
10
0
1
2
3
4
5
6
7
8
9
10
pH
Figure 9.19
Removal efficiency of SO
4
2-
and CrO
4
2-
in single component, binary systems
at different pH.
In the solutions with only Cr(VI) or SO
4
2-
and at pH >5, the removal efficiency
of CrO
4
2-
is much higher than that of SO
4
2-
, indicating the much higher affinity of -
Fe
2
O
3
to CrO
4
2-
than to SO
4
2-
. The components of a mixture of adsorbates may mutually
enhance adsorption, act relatively independently, or interfere with one another. In the
solutions involving both Cr(VI) and SO
4
2-
, the removal efficiency of SO
4
2-
was inhibited
by the presence of CrO
4
2-
over the entire pH range studied; the removal efficiency of
CrO
4
2-
was inhibited by the presence of SO
4
2-
at pH lower than pH
zpc
. Competitive
inhibition of adsorption capacity can be expected if the affinities of the adsorbates do not
differ by orders of magnitude and if there are no specific interactions among them which
enhance adsorption; this is because the adsorption of more than one substance generally
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