Environmental Engineering Reference
In-Depth Information
4.4.2 Application of NZVI in TCE and PCE Degradation
4.4.2.1 TCE Degradation by Monometallic Nanoscale Iron Particles
The feasibility of using NZVI particles to reductively degrade TCE was recently
evaluated in anaerobic batch experiments (Choe et al., 2001). The NZVI particles were
synthesized by the solution method with a diameter range of 1 to 200 nm. The specific
surface area of the synthesized NZVI nanoparticles was 31.4 m
2
/g, which is much
greater than the value of commercially available iron particles (0.0634 m
2
/g). Within 30
minutes, 90% of TCE was degraded, and 70% of TCE was converted into ethane as a
final product. Because of high conversion efficiency of TCE to non-toxic final products,
the toxic intermediates such as 1,2-dichloroethylene represented only a small percent of
the final dechlorination products. These results demonstrated that NZVI particles have
high capacity of transforming chlorinated organic compounds into harmless compounds
largely because NZVI possesses higher specific surface areas than microscale ZVI
particles.
Hydrogen production involved in NZVI applications appears to be important in
TCE degradation. In a recent study, the capability of NZVI of different particle surface
properties to activate and use H
2
for TCE dechlorination was investigated (Liu et al.,
2005a). H
2
formed from water dissociation in the presence of NZVI can be used for TCE
removal through the hydrodechlorination pathway, where H
2
acted as a reducing agent
to dechlorinate TCE. It was hypothesized that the ability of dissociative hydrogen
adsorption to NZVI particles may increase the efficiency of TCE dechlorination. To
testify this hypothesis, fresh NZVI particles were prepared by the solution method. In
other treatments, the NZVI particles were partially oxidized by exposure to air for
around 3 days. Alternatively, the NZVI particles were annealed at 400
o
C for 3 hours to
facilitate the formation of crystal structure. The particle characterization study showed
that the freshly made NZVI particles were amorphous with poor order and had a specific
surface area of 33.7 m
2
/g. The partially oxidized particles were also amorphous with a
Brunauer-Emmett-Teller (BET) surface area of 19.7 m
2
/g. In contrast, the annealed
nanoiron particles showed a high degree of crystallization based on XRD confirmation.
The freshly made amorphous NZVI produced H
2
which was easily consumed
during the reaction, resulting in a high amount of saturated reaction products (Liu et al.,
2005a). The partially oxidized NZVI particles have showed similar reaction product
distribution and the H
2
utilization pattern compared to the freshly made particles. For
annealed NZVI particles, however, the similar pattern of H
2
production and utilization
was not observed and the final products of TCE reduction were predominantly
unsaturated at a lower TCE degradation rate. It appeared that the large degree of
crystallinity of the annealed NZVI might have lower reactivity with H
2
O because of
their highly ordered surface nature. The differences in TCE degradation and reaction
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