Nanoscale Porous Materials for Water Treatment: Advances and Challenges - Nanotechnologies for Water Environment Applications - page 336

have also been successfully demonstrated to reduce redox active metal ions such as
Cr(VI) to less toxic and mobile Cr(III) species (Cao and Zhang, 2006; Manning et al.,
2006).
Synthesis and application of hollow and nanoporous particles of Fe 0 is also
reported (Cao et al., 2005a). It was prepared by synthesizing INP on the surface of
polymer resin beads (0.4 mm diameter) followed by calcination to produce hollow and
nanoporous iron spheres and finally reduced to metallic iron by hydrogen at 500 °C.
This led to a highly porous structure with the shell thickness of ~5 nm and a BET
specific surface area of 2100 m 2 /kg. In comparison, the theoretical specific surface area
of solid iron particles of the same size was only 1.9 m 2 /kg. Batch tests showed that the
surface area normalized reactivity of the porous particles was 1431% higher than
microscale iron particles with similar surface areas for the transformation of hexavalent
chromium [Cr(VI)], azo dye Orange II {[4-[(2-hydroxyl-1-naphthalenyl)azo]-
benzenesulfonic acid monosodium]}, and trichloroethylene. The combined performance
enhancement (due to the larger surface area and higher surface activity) is significant (>
1200 times more than that of micron size zerovalent iron, (Cao et al., 2005b). The
immobilization of metalloporphyrinogens in sol-gel matrices has also been successfully
used to prepare redox and catalytically active nanoparticles for the reductive
dehalogenation of chlorinated organic compounds (PCE, TCE and carbon tetrachloride)
in aqueous solutions (Dror et al., 2005).
Although INP has attracted a wide attention due to its large surface area and
higher reactivity, its direct application is limited by its naturally existence as an
aggregated state. This prevents their movement through porous media such as sand and
soil which is essential for groundwater treatment. Hence, significant effort has been
placed to make it mobile, which is the most important prerequisite for in-situ application,
especially for subsurface remediation. In this perspective, Cantrell et al. (1997) reported
mobilization of polymer stabilized micron zero-valent iron particles in coarse-grained
porous media. Schrick et al. (2004) has introduced the delivery vehicle concept for the
first time in environmental remediation; their concept shows great promise for direct
remediation of contaminated sites without excavation, which is not possible by
conventional treatment and PRB. Recently, Kanel et al. (2007b) have synthesized
surfactant stabilized INP (S-INP). This not only made S-INPs to be mobile in the
subsurface environment but also increased the surface area of INPs by almost 3 fold as
compare to that of bare INPs. Transport studies was conducted using these S-INPs in
unsaturated porous media in 1-D and 2-D tanks for arsenic remediation (Kanel and Choi,
2007; Kanel et al., 2007b). Zero-valent iron nanoparticles (INP) were synthesized and
stabilized using poly acrylic acid (PAA) to yield stabilized INP (S-INP). A two-
dimensional physical model was used to study the fate and transport of the INP and S-
INP in porous media under saturated, steady-state flow conditions. Transport data for a
nonreactive tracer, INP, and S-INP were collected under similar flow conditions. The