Environmental Engineering Reference
In-Depth Information
inexpensive, abundant, and easy to prepare and apply to a variety of
systems, and devoid of any known toxicity induced by its usage. h e
concept of using metals, such as iron, as remediation agents is based on
reduction-oxidation or “redox” reactions, in which a neutral electron
donor (a metal) chemically reduces an electron acceptor (a contami-
nant). Nanoscale iron particles have surface areas signii cantly greater
than larger-sized powders or granular iron, which leads to enhanced
reactivity for the redox process. As a result, iron nanoparticles have
been extensively investigated for the decomposition of halogenated
hydrocarbons to benign hydrocarbons and the remediation of many
other contaminants, including anions and heavy metals [44]. Zero-
valent iron nanoparticles are highly reactive and react rapidly with
surrounding media in the subsurface [45]. A signii cant loss of reac-
tivity can occur before the particles are able to reach the target con-
taminant. In addition, zero-valent iron nanoparticles tend to l occulate
when added to water, resulting in a reduction in ef ective surface area
of the metal. h erefore, the ef ectiveness of a remediation depends
on the accessibility of the contaminants to the nanoparticles; and the
maximum ei ciency of remediation will be achieved only if the metal
nanoparticles can ef ectively migrate without oxidation to the con-
taminant or the water/contaminant interface. To overcome such dif-
i culties, a commonly used strategy is to incorporate iron nanoparticles
within support materials, such as polymers, porous carbon, and poly-
electrolytes [46, 47]. Zero-valent iron removes aqueous contaminants
by reductive dechlorination, in the case of chlorinated solvents, or by
reduction to an insoluble form, in the case of aqueous metal ions [48,
49] (Figure 7.1). Zero-valent iron removes aqueous contaminants by
the reduction of nitrate compounds (Figure 7.2). Increasing the surface
area of zero-valent iron nanoparticles results in an increased rate of
remediation. In general, chlorinated organics (C x H y Cl z) and iron in
aqueous solutions can be expressed by equation:
C
x
H
y
Cl
z
+
zH
+
+
zFe
0
C
x
H
y
+
z
+
zFe
2+
+ zCl
-
(7.11)
Iron undergoes classical electrochemical/corrosion redox reactions in
which iron is oxidized from exposure to oxygen and water:
2Fe
2+
(aq) + 4OH
-
(aq)
(7.12)
2Fe
0
(s)
+
O
2
(g)
+
2H
2
O
Fe
2+
(aq) + H
2
(g) + 2OH
-
(aq)
(7.13)
Fe
0
(
s
)+ 2H
2
O (g)
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