Environmental Engineering Reference
In-Depth Information
may be influenced by solution pH. Furthermore, to the extent to which adsorption differs
between the conjugate forms of acidic or basic compounds, pH affects adsorption in that
it governs degree of protolysis or “ionization” of such compounds. Both adsorbent and
adsorbate may have chemical characteristics which are affected by the concentration of
hydrogen ions in the solution.
The effect of solution pH on the removal of 20 mL of 80 mg/L Cr(VI), Cu(II)
and Ni(II) from a ternary component system during the adsorption process is shown in
Figure 9.17. As far as the metals are concerned, the removal efficiency was strongly pH-
dependent. The uptake of Cr(VI) decreased gradually with increasing pH, whereas the
removal of Cu(II) and Ni(II) increased with an increase in pH. As observed, the
maximum removal of Cr occurred at about pH 2.5, while Cu and Ni were barely
removed at this pH. At pH 6.5, the removal efficiency of Cu reached 96.2%; while only
5.1% of Ni was removed. When pH further reached 9.5, almost 100% of the Ni was
removed. Thus, the selective removal of these three metals can be achieved by
controlling solution pH. For the adsorption of Cr(VI) and Cu(II), the optimal pH was
found to be 2.5 and 6.5, respectively. Regarding the adsorption of Ni(II), the maximum
adsorption occurred at pH 9.5, but heterogeneous precipitation of Ni is suspected to
emerge on the particle surface when pH is higher than 9.0 (Feitknecht and Schindler,
1963). To ensure only adsorption reaction exists for Ni, the operating pH is therefore
designed to be 8.5.
100
90
80
70
Cu(II)
Ni(II)
Cr(VI)
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
11
pH
Figure 9.17
Effect of pH on the removal of Cr(VI), Cu(II) and Ni(II) (Hu et al., 2006).
The pH dependence of metal removal can be explained from the perspective of
surface chemistry in aqueous phase; the surfaces of metal oxides are generally covered
with hydroxyl groups that vary in form at different pH. The surface charge is neutral at
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