Environmental Engineering Reference
In-Depth Information
As aforementioned, a higher pH is favored for the adsorption of Cu and Ni, and
thus concentrated HCl was more effective for desorption of these two metals, whereas
the possible nanoparticle dissolution may occur at a very low pH. Therefore, to decide
the optimal HCl concentration for an efficient desorption of adsorbed Cu and Ni without
causing adsorbent dissolution, 0.001~0.2 M HCl solutions were added to the Cu/Ni-
loaded nanoparticles. By comparisons of the experimental data shown in Figure 9.25, the
desorption percentage of adsorbed Cu/Ni increased with an increase in the concentration
of HCl from 0.001 M to 0.05 M, while an insignificant increase was observed when the
HCl concentration further increased from 0.05 M to 0.2 M. Unless a higher desorption
capacity is required, highly concentrated acid eluent will not be used because of the
possible dissolution of -Fe
2
O
3
nanoparticles. Thus, 0.05 M HCl was chosen for the
desorption of Cu/Ni and the desorption efficiency for adsorbed Cu and Ni was 94.1%
and 93.4%, respectively. It was also noted that the concentration of the iron in solution
measured by ICP was found to be lower than 1 ppm, indicating that the 0.05 M HCl
dissolved the -Fe
2
O
3
nanoparticles insignificantly. As reported by Stumm (1992),
adsorbed metal ions have a capacity to inhibit the proton-promoted dissolution of oxides
or silicates. This inhibition might be due to a competition in the binding of H
+
by the
binding of metal ions, i.e., a surface metal center is no longer dissolution-active if bound
to a metal ion (Blesa et al., 1994). Alternatively, the inhibitory effect can be interpreted
as being due, in part, to a lowering of surface protonation which occurs as a consequence
of metal binding.
100
90
80
70
60
50
40
30
20
10
0
0.001M
0.01M
0.1M
0.2M
0.4M
0.6M
0.8M
1.0M
Concentration of NaOH
Figure 9.24
Effect of NaOH concentration on Cr(VI) desorption.
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