Environmental Engineering Reference
In-Depth Information
when the C-Cl bond is broken, and subsequently undergo the hydrodechlorination reac-
tion by replacing the chlorine atom with hydrogen [85].
4.6.2 Reduction of Nitrate
The treatment of nitrate and nitrite contaminants in groundwater and drinking water by
microscale and nanoscale ZVI has recently become one of the innovative technologies
for environment remediation [35,47,89,107-110]. Usually the nitrate reduction reaction by
ZVI is relatively pH sensitive, and nitrate is a well-known oxidizing inhibitor to iron cor-
rosion owing to the formation of an overlying oxide layer [47]. The pH value controls the
reduction rate and eficiency of nitrate by ZVI and affects the formation of the passive
oxide layer of ZVI. Huang et al. [111] reported that microscale ZVI powder could effec-
tively reduce nitrate only when the pH was <4, which was low enough to dissolve the pas-
sive oxide layers. It has also been demonstrated that nitrate reduction by microscale ZVI
was very limited at near-neutral pH because of the formation of a black oxide ilm onto
the surface of ZVI [112,113]. Several strategies have been employed to enhance the reduc-
tion eficiency and rate of nitrate at a near-neutral pH value. The introduction of carbon
dioxide to the ZVI system for lowering solution pH was found to be effective for nitrate
reduction. Li et al. [114] used a pressurized CO
2
/ZVI system for the reduction of nitrate
under an anoxic condition. The pressurized system has potential advantages of using less
CO
2
gas and reaching equilibrium pH faster than a CO
2
-bubbled system. However, the pH
increased gradually with increasing oxidation of ZVI, resulting in the decrease in nitrate
reduction rate.
In addition to lowering the pH value by adding CO
2
or organic buffer, the reduction
eficiency and rate can be augmented by addition of ferrous (Fe
2+
) or copper (Cu
2+
) ions
[109,112,115,116]. The major role of the surface-adsorbed Fe
2+
is more likely to induce or
accelerate phase evolutions of the iron corrosion coatings toward more active ones that
could sustain higher ZVI reactivity [117]. Nitrate reduction usually consists of three stages.
At the irst stage, a proton directly participates in the corrosion of ZVI. The second stage is
very slow because of the formation of amorphous oxides on the surface of ZVI, while the
third stage is characterized by a rapid nitrate reduction concurrent with the disappearance
of aqueous Fe
2+
. Addition of Fe
2+
would adsorb onto the surface of ZVI to form surface-
bound Fe(II), resulting in the formation of structural Fe(III) and the increase in the Fe(III)/
Fe(II) ratio. The transformation of amorphous iron oxides into crystalline structural mag-
netite triggers the rapid nitrate removal rate in the presence of Fe
2+
.
Different from the role of Fe
2+
in a ZVI/nitrate/water system, the addition of copper ion
signiicantly enhances the nitrate reduction rate at near-neutral pH. Liou et al. [118] inves-
tigated the effect of three noble metals, Pd, Pt, and Cu, on the reduction eficiency and rate
for nitrate [119]. They found that the nitrate reduction rate by ZVI in the presence of 0.44%
Cu (atomic ratio) was three times higher than those in the presence of Pd and Pt. This
may probably be due to the nitrate adsorption onto the Cu surface and rapid reduction to
ammonia by a neighboring adsorbed hydrogen atom.
Another available strategy for effective reduction of nitrate at near-neutral pH is the use
of nZVI. Choe et al. [120] indicated that the use of nZVI for nitrate reduction has several
advantages. These include the increase in reduction rate, decrease in reductant dosage, and
production of nontoxic end products. Sohn and Fruehan [35] compared the reaction kinet-
ics of nitrate reduction by three different sizes of ZVI. Results clearly showed that both the
k
m
and
k
SA
values are higher with nanosized iron than with micro- and millisized iron.
Liou et al. [47] used different precursor concentrations (0.01, 0.1, and 1 M FeCl
3
ยท6H
2
O) for
