Environmental Engineering Reference
In-Depth Information
pathway for dechlorination. The second pathway involves the reduction by ferrous ion,
which is an immediate product of corrosion in aqueous systems. Dissolved ferrous ion is a
mild reductant capable of causing dehalogenation of some chlorinated hydrocarbons. The
importance of this process is the formation of a surface-bound ferrous iron, which may be
attributed to the long-term stability of ZVI [65,66,97-100]. In addition, the third model for
dechlorination by ZVI involves the indirect electron transfer by atomic hydrogen produced
from water reduction when catalysts such as Ni, Pd, and Pt are present in solution.
Although ZVI have been successfully employed as the reactive material for treatment of
soil and groundwater contaminated with chlorinated hydrocarbons, several challenges,
including the production of chlorinated by-products, slow reaction rate, and inability to
dechlorinate polychlorinated biphenyls (PCBs), still exist. One of the advantages for using
nanoscale iron materials is that nZVI can degrade contaminants that cannot react with
microscale ZVI such as PCBs and chlorinated aromatics [7,46]. Zhang [31] have demon-
strated that nZVI can effectively decompose >30 organic compounds, including haloge-
nated hydrocarbons, chlorinated aromatics, nitroaromatics, pesticides, and organic dyes.
Wang and Zhang [26] used the nZVI and Pd/Fe for dechlorination of TCE and Arochlor
1254. The Pd/Fe resulted in the rapid and complete dechlorination of TCE and Arochlor
1254 after 17 h, while only <25% of PCBs with biphenyl as the inal product was dechlo-
rinated by unpalladized nZVI. The
k
SA
values for TCE and PCB dechlorination are calcu-
lated to be 10-100× higher than those of commercially available microscale ZVI particles.
Lowry and Johnson [46] compared the initial rate of dechlorination of six PCBs by bare
and palladized nZVI particles in a 30% ethanol/water mixture under ambient conditions.
The observed
k
SA
for PCB dechlorination by nZVI and 0.05 wt% Pd/Fe were in the range
(5.5 -100) × 10
−4
L yr
−1
m
−2
and (3.8-17) × 10
−4
L yr
−1
m
−2
, respectively. Shih et al. [101] also
reported that hexachlorobenzene can be decomposed by nanosized Pd/Fe bimetal. For
bare nZVI, non-
ortho
-substituted congeners were found to have faster initial dechlorina-
tion rates than
ortho
-substituted congeners in the same homolog group, and chlorines in
the
para
and
meta
positions were predominantly removed over chlorines in the
ortho
posi-
tion, which suggests that more toxic coplanar PCB congeners are not likely to form from
less toxic noncoplanar,
ortho
-substituted congeners [46].
Another advantage is that nZVI particles have a larger speciic surface area and have
higher surface reactivity than microscale ZVI [31,46]. However, increased surface area
alone is not adequate to explain the high reactivity of nZVI. Higher density of reactive sur-
face sites and greater intrinsic reactivity of surface sites may also be the possible reasons
for the enhanced reactivity of nZVI [32]. The reduction of chlorinated hydrocarbons by iron
particles has been generally believed to be a surface-mediated reaction, and Langmuir-
Hinshelwood kinetics can be used to model the reaction kinetics of chlorinated hydro-
carbons by nZVI [33,96]. At low concentrations of chlorinated hydrocarbons, the reaction
rate can be simpliied to the pseudo-irst-order dechlorination reaction, while the reaction
rate would change to zero-order kinetics when the surface-active sites are occupied by the
overloaded substrate [42,102]. It is believed that the pseudo-irst-order rate constants for
dechlorination of chlorinated hydrocarbons by nZVI were higher than that by microscale
ZVI. Nurmi et al. [32] depicted that the mass-normalized pseudo-irst-order rate constants
(
k
m
) for CT reduction is larger with nZVI than with microscale ZVI. However, it is unclear
whether there is a nanosized effect on the
k
SA
for CT reduction. In contrast, the
k
SA
for
TCE dechlorination by nZVI were found to be in the range (2-3) × 10
−3
L h
−1
m
−2
[26,85],
which are higher than those for normal iron ilings ((3.9 ± 3.6) × 10
−4
L h
−1
m
−2
) [42,44]. The
k
SA
values for dechlorination of CT and chloroform by nZVI were 9.22 × 10
−3
and 5.13 ×
10
−4
L h
−1
m
−2
, respectively, which are 1.7-8× higher than those by microscale ZVI.
