Environmental Engineering Reference
In-Depth Information
XPS analysis, these edges can be confidently considered as a homogeneous
silica shell around the Fe
3
O
4
cores. Figure 6.11b also shows that no more than
one spherical core is included in each composite particle, suggesting that the
aggregation of primary particles during synthesis is minimal.
Other Properties
To determine the porous nature of template-assisted silica coatings on the
magnetite particles, standard nitrogen adsorption-desorption tests were per-
formed. A large hysteresis existing between the adsorption and desorption
branches, which is characteristic of highly porous materials, confirms the
formation of mesopores on the magnetite particles. The detailed analysis of
the results using the Brunauer-Emmett-Teller (BET) adsorption equation
resulted in an increase in specific surface area from 0.07 to 52.3 m
2
/g of total
material by the deposition of a mesoporous silica film on magnetite cores [122].
To obtain a uniform and well-covered mesoporous silica layer on magnetic
particles of high specific surface areas, the control of sol-gel conditions is extremely
important. Many synthesis conditions, such as reaction time, reaction tempe-
rature, reactant concentration, type and amount of catalyst, and the quality
of solvent, could affect the properties of the resultant silica coatings. Thus,
optimizing sol-gel conditions by factorial design to minimize the numbers
of synthesis while analyzing synergetic effects among different factors were
studied [123]. The specific surface area of produced mesoporous silica-coated
Fe
3
O
4
was improved from 52.3 to 150 m
2
/g, which provides three times more
chances of the prevalence of functional groups, so the efficiency of the synthe-
sized magnetic sorbent will be increased.
The magnetization characteristics of synthesized particles is a major concern
for potential industrial applications. A strong magnetization is required for the
collection by magnet from a complex, multiphase system. As shown in Fig. 6.12
the room temperature saturation magnetization of bare magnetite is 85.5 emu/g,
which reflects the properties of Fe
3
O
4
without any oxidation. For the final
mesoporous magnetic nanocomposite particles, the saturation magnetization
remains strong at 73.0 emu/g. The observed decrease of 15% in saturation
magnetization is attributed to the coating of diamagnetic silica. Such a reduc-
tion does not hinder the subsequent magnetic separation after the resultant
mesoporous magnetic particles are loaded with heavy metal ions. More impor-
tantly, magnetite particles coated with mesoporous silica remain fairly para-
magnetic as shown by minimal coercivity and hysteresis on the magnetization
curves (Fig. 6.12). This magnetization characteristic ensures that the magnetite
particles do not become permanently magnetized after exposure to an external
magnetic field, which in turn permits the particles to be re-dispersed without
significant aggregation when the external magnetic field is removed.
Our study above clearly shows that with the concept outlined in Fig. 6.7,
mesoporous magnetic nanocomposite particles can be successfully synthe-
sized. The synthesized particles possess the necessary attributes of strong
