Environmental Engineering Reference
In-Depth Information
the preparation of nZVI, and found that precursor concentration controls the diameter and
reactivity of nZVI. In addition, the nitrate reduction rate was signiicantly enhanced by
nZVI without acidiication and the irst-order rate constants for nitrate reduction followed
the order nZVI
0.01M
> nZVI
0.1M
> nZVI
1M
. Chen et al. [121] proposed a novel technology com-
bining electrochemical and ultrasonic methods to produce nZVI for the removal of nitrate.
The nZVI with a diameter of 1-20 nm was successfully fabricated by placing platinum in
the cathode and adding cetylpyridinium chloride as the dispersion agent in the solution.
When using the synthesized nZVI to remove nitrate, the pseudo-irst-order rate constants
for nitrate removal decreased linearly upon increasing pH from 4 to 7. In addition, only
around 36%-45% of nitrate was converted into ammonium in solution, while 5%-64% of
nitrate was transformed to nitrogen or other nitrogen-containing gases, clearing showing
that nZVI not only accelerated the nitrate reduction rate at near-neutral pH but also altered
the reaction pathways for producing environmentally friendly end products.
4.6.3 Reduction of Bromate and Perchlorate
Bromate is a refractory by-product during disinfection/oxidation processes in water treat-
ment. This compound is now classiied as a possible carcinogen with the current maxi-
mum contamination level of 10 μg L
−1
in the national primary drinking water standard
of the United States. Only a few studies have applied ZVI technology to the treatment
of bromate in aqueous solution. Xie and Shang [122] found that bromate can be reduced
to bromide ion by microscale ZVI and the reduction reaction is a surface-mediated reac-
tion. The precipitation of iron oxyhydroxide onto the ZVI surface resulted in the reduction
of the reactivity of ZVI to bromate reduction. Several environmental parameters inluence
the eficiency and rate of bromate reduction by ZVI. The incorporation of copper ion led
to the increase in the reduction rate of borate by ZVI, while incorporation of a palladium
ion had little effect on bromate reduction [122]. The enhanced bromate reduction rate in
the presence of copper is most likely the result of the newly formed active Cu(I) on the
iron surface. On the contrary, the reactivity of ZVI toward bromate reduction declined by
a factor of 1.3-2.0× in the presence of 5-35 mg DOC L
−1
humic acid because of the quick
complexation of humic acid with iron species [79].
Perchlorate, a commonly used agent as an energetics booster and oxidant in a variety of
munitions and ireworks, has also been frequently found in groundwater. Recently, per-
chlorate has been recognized to be an endocrine-disrupting chemical and promulgated by
the US Environmental Protection Agency in the drinking water contaminant candidate
list. Several technologies, including adsorption, biological, and electrochemical processes,
have been used to reduce perchlorate to chlorate or to chloride via the sequential reduction
reaction [123,124]. Recently, a few studies suggested that perchlorate can also be reduced by
ZVI. However, the reduction rate of perchlorate by nZVI is considerably slower than many
other contaminants [125-127]. Although little degradation of perchlorate by bare ZVI was
observed during the 57 days of experimental course, perchlorate was found to be partially
reduced (4.1%-82%) by acid-washed ZVI after 14 days [125,128].
Several methods such as the use of nZVI and the combination of biological reduction
with ZVI have been developed to enhance the removal eficiency and rate of perchlorate
reduction. Cao et al. [126] have demonstrated a nearly complete reduction of perchlorate
to chloride by nZVI. nZVI also can reduce chlorate, chlorite, and hypochlorite to chloride
under anaerobic conditions. In addition, the activation energy of perchlorate-iron reaction
was calculated to be 79.02 ± 7.75 kJ mol
−1
. This large activation energy implies that per-
chlorate reduction is a kinetic-controlled reaction. Recently, the combination of microbial
