Environmental Engineering Reference
In-Depth Information
Moreover, the saturation magnetization is comparable for particles coated with
silica using the sol-gel and two-step processes, suggesting that the amount of
silica on the surface is virtually the same and confirming the important role
of the film structure in protecting the matrix component of magnetic sorbents
for biological and environmental applications. It is evident that the two-step
silica-coating process is successful in making base materials that can be further
functionalized by the conventional silanation process to produce magnetic
sorbents of a desired functionality.
6.4.4 Mesoporous Silica Coating
Despite the advantage of easy separation from complex multiphase systems by
using MCS, the limited specific surface area of existing magnetic sorbents
presents a major challenge to these applications of many technological impor-
tances. Even for very fine maghemite (g-Fe
2
O
3
) particles (
30 nm), for instance,
a specific surface area of only ca. 40 m
2
/g is reported [48]. A further decrease in
particle size to increase specific surface area is not desirable, because the
magnetic forces exerted on these tiny magnetic particles are extremely weak
for any substantial migration of the particles to a desired location for separa-
tion. Since the specific surface area for a given surface functionality determines
separation capacity, there is an urgent need for an innovative approach to
increase the surface area of magnetic sorbents.
Surfactant-templated mesostructure materials have played a prominent role
in materials chemistry during the last decade. The excitement began with the
discovery of hexagonally ordered mesoporous silicate structure by Mobil Corp.
(M41S materials) [90, 91, 92] and by Kuroda, Inagaki, and co-worker (FSM-16
materials) [93, 94]. Having extremely high surface areas, these materials are
easily accessible and of uniform nanopore structures and specific pore volume.
Most importantly, the pore sizes exceeded those attainable in zeolites and they
could be tuned in the nanometer scale by choosing an appropriate surfactant
templating system, sometimes with a co-solvent or swelling agent [95, 96, 97, 98,
99, 100]. As a result, the applications of mesoporous materials in a wide range
such as adsorption, separation, catalysis, biological sensing, medical diagnosis
and treatment, molecular engineering, and nanotechnology were envisioned
[101, 102, 103, 104, 105, 106, 107, 108].
However, the use of bulk mesoporous siliceous materials in many science and
technological applications has inherent limitations, especially for adsorption
and separation of targets or contaminants from multiphase industrial effluents
of complex nature. One noticeable challenge is to separate the loaded fine
particles from industrial effluents for safe disposal or recovery of the adsorbed
valuables and recycle of the sorbents.
The above application limitations inherent in mesoporous materials and
magnetic sorbents led us to develop a mesoporous material-based MCS. It is
