Environmental Engineering Reference
In-Depth Information
size in determining adsorption affinities, resulting in a higher affinity of Cu than Ni for
the oxides (Kebew, 2001).
Competition from Common Ions.
In chrome-plating wastewater, the main
coexisting cations are Cu
2+
, Ni
2+
, Na
+
, Ca
2+
and Mg
2+
, and the main anions are NO
3
-
, Cl
-
,
SO
4
2-
, PO
4
3-
, CN
-
and F
-
. Batch tests were performed to evaluate competing effects of
inorganic ions for Cr(VI) adsorption onto -Fe
2
O
3
nanoparticles.
Effect of Cations.
In cationic competition experiments, the aforementioned
adsorption procedure was used except that 80 or 160 mg/L Cu
2+
, Ni
2+
, Na
+
, Ca
2+
and
Mg
2+
were added into 20 mL of 80 mg/L Cr(VI) solution at pH 2.5, individually. From
Table 9.3, it can be observed that the influence of Cu
2+
, Ni
2+
, Na
+
, Ca
2+
and Mg
2+
on the
adsorption of Cr(VI) is rather insignificant since they do not compete on the active
surface with CrO
4
2-
under the testing conditions. There is no more competitive influence
on Cr(VI) with the further increase of cationic concentration.
Table 9.3
Competition from various cations with Cr(VI).
Removal Efficiency (%)
Solutions
80 mg/L
Cr(VI)
80 mg/L
Cation
80 mg/L
Cr(VI)
160 mg/L
Cation
Cr(VI)
95.6
/
95.3
/
Cr(VI) + Cu(II)
93.7
0.4
93.1
0.5
Cr(VI) + Ni(II)
94.5
0.2
94.2
0.4
Cr(VI)+ Mg(II)
94.9
0.3
94.2
0.5
Cr(VI) + Ca(II)
95.0
0.2
95.3
0.5
Cr(VI) + Na(I)
95.3
0.1
95.0
0.2
Effect of Anions.
20 mL of 0.5 mM Cr(VI) as CrO
4
2-
was shaken with each of
various anions (i.e., PO
4
3-
, SO
4
2-
, F
-
, CN
-
, NO
3
-
, and Cl
-
) with a concentration of 0.5 mM
or 1 mM to evaluate the effect of anions on the removal of Cr(VI). As shown in Figure
9.18, the influence of NO
3
-
, Cl
-
and CN
-
is very small. But the competition of F
-
, PO
4
3-
and SO
4
2-
is significant and becomes stronger with an increase in anion concentration.
Cr(VI) species (i.e., CrO
4
2-
) are Lewis bases or ligands with an ability to donate
one pair of electrons. In addition, CrO
4
2-
is a stronger bidentate ligand with two oxygen
donor atoms and may form stronger inner-sphere complexes with Lewis acids
(Weerasooriya and Wickramarathma, 1999). Comparing with Cr(VI) species, NO
3
-
, Cl
-
and CN
-
are poor ligands and suggest a weak adsorption mechanism via outer-sphere
complexation. The H-bond formation readily occurs on a solid surface due to the
reduction of anion mobility. Thus, Lewis acid-base interaction may be used as the
underlying sorption mechanism to separate dissolved Cr(VI) species from relatively high
background concentrations of nitrate or chloride.
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