Environmental Engineering Reference
In-Depth Information
efficiency for Cu
2+
increased with increasing solution pH. The adsorption of
copper ions on Mag-Im
18
is not significant below pH 3. The competitive
adsorption of hydrogen ions with metal ions for imidazolyl groups at low pH
values accounts for the observed low removal efficiency. Since the imidazolyl
groups are most likely protonated at a low pH, the magnetic particles are
positively charged, resulting in a strong electrostatic repulsive force between
the poly(1-vinylimidazole) on the magnetic particles and positively charged
metal ions. This long-range repulsive force also contributes to the observed
low copper removal efficiency at solution pH below 3. On the other hand, Co
2+
cannot be removed by Mag-Im
18
over the entire pH range studied. The Mag-
Im
18
showed an effective and selective separation of Cu
2+
from a mixture of
Cu
2+
and Co
2+
solution. Given the wide selection of polymer functionality,
polymer-grafted magnetic particles can provide vast potential applications.
6.5.4 Functionalized Mesoporous Silica-Coated Magnetic
Particles
6.5.4.1 Separation of Transition Metals
Cupric ion adsorption tests were performed on magnetic composite particles
with varying surface treatments. To illustrate the role of molecular templating
in synthesis of the silica coating and subsequent capture of target species, the
results obtained with the micrometer-sized magnetite coated with silica under
the identical conditions but without templating are included for comparison.
From the results shown in Fig. 6.20, the following general conclusions can be
made: (1) The capability of the particles to capture copper ions increased when
mesoporous films were formed on the magnetite particles to produce a higher
specific surface area (curves (b) and (d) are higher than curves (a) and (c),
respectively). (2) Surface functionalization with amine groups by silanation
increased the capability of the particles to capture metal ions, arisen from a
stronger chemical affinity of the immobilized amine groups for copper ions
(curves (c) and (d) are higher than curves (a) and (b), respectively). (3) The
functionalized mesoporous surfaces showed the highest loading capacity for
copper ions (curve (d)), suitable for detoxification or recovery of copper ions
from industrial effluents.
Mesoporous magnetic particles functionalized with APTES were tested to
determine the loadings of other soluble heavy metals. The extractability of
soluble metals was examined by adding 50 mg of particles to 25-mL samples
of aqueous solutions, each containing 0.5 mmol/L Cu
2+
,Zn
2+
, and/or Ni
2+
ions. In this set of tests, the solution pH varied from 2 to 6. The upper pH limit
was set at 6 to avoid precipitation of metal hydroxides, which would complicate
the interpretation of results.
Figure 6.21A shows the loading distribution coefficients, K
d
, of different metal
ions from the corresponding single element solutions as a function of the equilibrium
