Nanoscale Porous Materials for Water Treatment: Advances and Challenges - Nanotechnologies for Water Environment Applications - page 351

11.4 Discussion
NPM have been used for water treatment for a wide variety of contaminants
(e.g., organics, inorganics, heavy metals). In recent days effort has been given to
synthesize new and novel NPM that have high surface areas for water treatment. The
effort has also been given to synthesize single and multi component NPM composites.
For example, porous TiO 2 and INPs have been synthesized and used for water treatment.
The surface area of NPM can be increased by coating catalytically active and efficient
adsorbents on the substrate (e.g., silica and alumina). For example, iron particles can be
coated on the silicate and alumina substrate to get NPM of a surface area > 1000 m 2 /g
whereas the surface area of the iron particles themselves is < 1 m 2 /g.
The highly porous material with a large surface area is very effective to remove
contaminants from aqueous phase. A bottle neck to the application of NPM to water
treatment will be: (a) the availability of large quantities of NPM at a cost-effective price;
and (b) integration of nanomaterials into existing water treatment systems (Savage and
Diallo, 2005). However, more laboratory-, pilot- and field-scale tests will be required.
The scale up of TiO 2 -based photocatalytic reactors is also poorly understood (Adesina,
2004) even though a significant progress has been made towards the development of
visible light activated TiO 2 nanoparticles. The experiments reported on literature are
focused on batch experiments, which are good for understanding kinetics mechanisms
and isotherms of contaminants adsorption on NPM. However more studies are needed in
column experimental mode so that NPM can be used as a PRB material for in-situ
groundwater treatment. In addition, there should be more adsorption/desorption studies
on these materials one the short- and long-term basis. As shown in Figure 11.15,
nanosorbents (e.g., carboneous nanoparticles and inorganic nanocrystals), redox active
nanoparticles (e.g., Fe 0 ) and bioactive nanoparticles (e.g., Ag 0 ) can be readily integrated
into existing water treatment plants for removal of organic and inorganic contaminants,
heavy metals, and microbes (Savage and Diallo, 2005).
PRB materials is suitable for shallow groundwater (about < 30 m depth)
treatment. For deep groundwater treatment, NPM should be used as slurry so that they
will be mobile and be able to reach the contaminant plum. Recently, Kanel et al. (2007)
have studied the transport of S-INPs and their application for arsenic removal in column
studies (Kanel and Choi, 2007; Kanel et al., 2007b). Similarly, Shrick et al. (2004) also
reported transport of INPs and their application for TCE removal. The NPM are tested as
a PRB material, but there are fewer attempts to use it as a CRB material. Though detail
adsorption/ desorption studies should be carried out in the laboratory before applying
this technique for groundwater treatment, there should be more research in this direction.
Use of NPM as a PRB and CRB materials will increase in the future for ex-situ
as well as in-situ treatment. The fate and transport of these materials in 2-D and 3-D