Environmental Engineering Reference
In-Depth Information
capture gold and silver ions from their leachates. The metal-loaded magnetic
particles can then be readily separated from leaching solutions using magnetic
separation. In addition, the approach can be readily extended to the fabrication
of magnetic particles with other customized functional groups by controlling the
reactivity of functional groups of a bolaamphiphile with magnetic particles. The
functionalized magnetic particles can be used to recover secondary resources or
to remove toxic species from industrial effluents by controlling the reactivity of
functional groups with targeted species in the effluents. In addition, the special
affinity of thio with antibodies makes fabrication of thio-containing magnetic
nanoparticles of special interest in the biological applications.
6.4.2 Direct Silanation
Although self-assembled bolaamphiphile monolayer on magnetic particles is
densely packed and stable in acidic and basic environments, the control of the
reactivity between the two functional groups and the substrate is crucial in
determining the quality of the coatings. The ideal case is that one functional
group anchors chemically to the surface while the other is unreactive to the
surface. This requirement limits the type of functional groups that can be
directly introduced onto the magnetic particles.
A more general approach to fabricating magnetic nanocomposites is direct
silanation. Silanation is to use silane-coupling agents to tailor surface properties of
metal oxides [75]. A typical silane-coupling agent has the structure of
Y-(CH
2
)
n
-Si-X
3
, where X represents the alkoxy or halide groups and Y, the
organic functional groups, including amine, thiol, carboxylic, phosphate, vinyl,
cyanide, and methacrylate. The Si-X
3
group hydrolyzes readily in the presence of
water and catalyst to form silanol groups which couples with surface metal hydroxyl
groups, forming Si-O-M bonds upon dehydration. As a result, the organic func-
tional groups (Y) remain reactive on the surface. This unique feature of silane-
coupling agents has made silanation a widely used method in modifying surface
properties. A large volume of literature is available in this area [76, 77, 78, 79, 80, 81].
In the patent of Whitehead et al. [82], they described the procedures of direct
silanation of functional groups, including aminophenyl, amino, hydroxyl, sul-
phydryl, aliphatic, and mixed functional groups, on paramagnetic particles and
pointed out the potential applications of the resultant magnetic particles.
However, no fundamental characterizations of the silanized films are available,
in particular the stability of the coatings and possible leaching of iron from the
substrate.
It should be noted that coating of silane-coupling agents by silanation is a
multistep process. A triangular relationship among substrate, silane, and
solvent, along with the parameters that need to be considered, is shown in
Fig. 6.4 [6]. Evidently, hydrolysis is necessary but must be controlled to avoid
intramolecular condensation of silane-coupling molecules. It is also important
