Nanoscale Porous Materials for Water Treatment: Advances and Challenges - Nanotechnologies for Water Environment Applications - page 319

Self-assembled monolayers on mesoporous supports (SAMMS), a novel form of
zeolites, have been developed for effective adsorption of toxic metal ions (Lin et al.,
2005b), anions (Kelly et al., 2001) and radionuclides (Fryxell et al., 2005; Lin et al.,
2005a). These sorbents are made via surfactant templated synthesis of mesoporous
ceramics. The process produces nanoporous ceramic oxides with very large surface
areas (1000 m 2 /g) and high density of sorption sites that can be functionalized to
increase their selectivity toward target pollutants (Savage and Diallo, 2005).
In general, synthetic zeolites are known to be good adsorbents/catalyst. However
due to their small pore sizes, the adsorption of bigger size contaminants by these
synthetic zeolites has been limited. Natural zeolites, though contain bigger pore sizes,
exhibit lower adsorption capacity. To achieve best properties of both natural and
synthetic zeolites, a new synthetic route has been reported by manufacturing synthetic
zeolites upon an expanded lamella matrix (vermiculite). Properties of hydraulic
conductivity, adsorption capacity and the rate of attainment of equilibrium of the
synthetic composite have been studies in comparison to both natural and synthetic
zeolites. The results demonstrate that the vermiculite-based composite shows the same
hydraulic properties as a natural clinoptilolite with a similar grain size (25 mm);
however, the rate of adsorption and maximum coverage were improved by a factor of 4
(Johnson and Worrall, 2007). These studies confirm that zeolites have great potential to
be used for water treatment. However, most of these studies are limited to the bench
scale only; more research is necessary in 1-D, 2-D and 3-D column systems. More
details will be discussed in Section 11.4 and 11.5.
11.2.1.2 Silica-Based NPM
Work in mesoporous silica-based materials was reported for the first time in the early
1990s by Mobil Oil Corporation (Bennett et al., 1992). They described the successful
synthesis of mesoporous silica with hexagonal and cubic symmetry with surface areas of
7001100 m 2 /g and pore sizes ranging from 2 to 10 nm using surfactants as organizing
agent (Figure 11.2). It is important to note that the efficient adsorption of a certain target
ion is not expected on silica surfaces. Therefore, the surface of silica is mostly
functionalized with different catalysts, making it being activated to interact with a
variety of contaminants and its porosity being helpful for adsorption. For example silica
containing mesoporous material such as MCM-41 has a record Brunauer-Emmett-Teller
(BET) surface area of 1040 m 2 /g (Cooper and Burch, 1999).
There are several reports on application of functionalized mesoporous silica for
contaminants removal from water. The adsorption of toxic metal cations has been
explored for functional groups fixed on mesoporous silica. The divalent cations (e.g.,
Cu, Zn, Cr, and Ni) present in wastewater are adsorbed to a better extent on amino-
functionalized SBA-15 than on thiol-functionalized SBA-15, whereas Hg 2+ is preferably