Environmental Engineering Reference
In-Depth Information
The shell layer also plays an important role in removing heavy metals. Recent studies
have demonstrated that iron nanoparticles have a superior metal removal capacity than
that of conventional adsorbents such as zeolites and ion exchange [23,38-40]. Shokes and
Moller [41] used ZVI to remove heavy metals from acid rock drainage and found that
copper (Cu) and cadmium (Cd) cemented onto the surface of the iron as ZVMs, presum-
ably due to the positive standard potential than that of ZVI. Li and Zhang [39] used high-
resolution XPS to investigate the removal mechanism for metal sequestration by nZVI.
They found that dissolved metals can be removed by several mechanisms. For metal ions
with standard potential very close to or more negative to that of nZVI, such as zinc (Zn(II))
and cadmium (Cd(II)), the removal mechanism is sorption/surface complex formation. On
the contrary, the metal ions would be reduced to the ZVMs when the metals have greater
positive standard potential than that of nZVI. Meanwhile, Ni(II) and Pb(II) can be immo-
bilized at the nanoparticle surface by both sorption and reduction.
4.4 Effect of Environmental Parameters on the Reactivity of nZVI
Although nZVI is effective for the removal of chlorinated compounds, heavy metals, and
other inorganics, the reactivity of nZVI is highly controlled by the surface characteristics
and water chemistry of groundwater. The iron particle crystallizes in a body-centered
cubic structure. It is estimated that about 4% of iron atoms are exposed onto the surface
when the diameter of iron nanoparticles is at 50 nm. However, the ratio of iron atoms on
the surface area of nanoparticles decreases to 0.001% when the particle size of the iron
nanoparticles increases to 1 mm, which means that the reactivity of nZVI particles will be
dramatically increased when the particle size decreases to the nanoscale. Several studies
have also depicted that the reduction rate by nZVI is dependent on the speciic surface
area of iron. A liner relation between reduction rate and surface area of iron was observed
[15,42]. Surface area-normalized pseudo-irst-order rate constant (
k
SA
) for contaminant
degradation between metallic nanoparticles and reducible contaminant, including chlo-
rinated methanes, chlorinated ethanes, chlorinated aromatics, and nitrate, have been
reported to be signiicantly inluenced by the nanoparticle size, which may vary by as
much as 1-2 orders of magnitude higher than those of commercial or microscale iron
powders [43-48].
4.4.1 Particle Size of nZVI
The reduction eficiency and rate of the individual contaminant by nZVI at different par-
ticle sizes are varied. Liu et al. [33] reported that the
k
SA
value for TCE dechlorination by
iron nanoparticles with diameters of 30-40 nm was 1.4 × 10
−2
L h
−1
m
−2
, which is higher
than that reported by Wang and Zhang [26] (3 × 10
−3
L h
−1
m
−2
) who used 1-100 nm nZVI for
TCE dechlorination. Liou et al. [47] indicate that the reactivity of iron nanoparticles with
diameters of 9-10 nm for the denitriication of nitrate in aqueous solution was higher than
that of iron nanoparticles with diameters of 20-60 nm. They further investigated the size
effect of copper nanoparticles on the dechlorination toward CT and found that the
k
SA
of
copper nanoparticles at 10 nm on resin was 110-120× higher than that of powdered copper
particles, while only 10- to 20-fold increase in
k
SA
relative to that of powder copper particles
