Environmental Engineering Reference
In-Depth Information
Advantages of nZVI over other ZVI such as microparticles (mZVI) and iron filings
include a higher reactive surface area [22-54 m
2
g
-1
for nZVI (Ponder et al., 2000; Chen
et al., 2004; Liu et al., 2005; Li et al., 2006; Bezbaruah et al., 2008) and 1 m
2
g
-1
for
mZVI (Bezbaruah et al., 2008; Thompson, 2008)], faster and more complete reactions,
and better injectability into the aquifer (Cantrell et al., 1995; Wang and Zhang, 1997).
The effectiveness of nZVI for groundwater remediation depends upon the effective
delivery of the nanoparticles to the water/contaminate interface without flocculation and
severe oxidation. While nZVI are easier to be injected than mZVI, the nZVI still
agglomerate because of interparticle attraction due to magnetic and van der Waals
forces. The agglomerated particles behave more like mZVI and settle out. The particles
also get attached to aquifer materials such as sand grains. These characteristics of nZVI
reduce the effective surface area available for contaminant degradation.
On the basis of the relevant literature, there exists a need for an efficient nZVI
delivery vehicle that is easy to synthesize in the laboratory and mass produce when
scaled-up. The authors suggest that one of the requirements for such a delivery vehicle
be the commercial availability of the raw materials. Maintaining the stability of the
nanoparticles for a long time without agglomeration should be the primary objective (Lu
et al., 2007). The main difficulty for the use of pure metal nanoparticles (e.g., nZVI)
arises from their high reactivity towards dissolved oxygen and oxygen containing
compounds. However, oxidation is the main mechanism of decontamination by nZVI.
So, it is important to protect the nZVI from non-target oxidation while allowing
oxidation by targeted contaminants. It would further be desirable to create an affinity of
the modified nZVI towards the target contaminants (Saleh et al., 2005). There exist a
number of possible techniques for the stabilization of nZVI (Kim et al., 2003; Lu et al.,
2007). A polymer or surfactant coating on nZVI that provides electrosteric (rather than
electrostatic) repulsion to balance the magnetic and van der Waals attractive forces is
needed (Sousa et al., 2001; Shen et al., 1999; Lu et al., 2007; Saleh et al., 2008).
This chapter introduces the current status of knowledge on surface modification
of nZVI, followed by descriptions of an on-going work on nZVI stabilization using
amphiphilic polysiloxane graft copolymers (APGCs) which will be easy to mass produce
at a low cost from commercially available starting materials. Treatment of nZVI with
APGCs has been found to enhance colloidal stability of nanoparticle in water, and the
magnitude of the enhancement is a function of APGC chemical composition.
8.2
Current Status of nZVI Surface Modification
There are at least three possible approaches to achieving colloidal stability of
nanoparticles: (1) modification of the nanoparticle synthesis process, (2) post-synthesis
modification of nanoparticles, and (3) application of a physical stimulus. Among these
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