Environmental Engineering Reference
In-Depth Information
on the ecosystem. h is chapter gives an overview of existing applications
of nFe
0
in facilitating waste management and reducing the environmental
burden of hazardous/toxic substances, factors governing nFe
0
reactivity,
practices adopted for improving the performance of nFe
0
, recent progress
made in laboratory studies and i eld-scale studies. In addition, various
concerns related with the application of nFe
0
have also been discussed at
the end.
14.2
Zero-Valent Iron Nanoparticles: A Versatile Tool
for Environmental Clean Up
14.2.1 Iron Chemistry
Iron is the fourth most abundant element in the earth's crust which pri-
marily exists in two valence states—one is water-soluble ferrous iron (Fe
2+
)
and the other is water-insoluble ferric iron (Fe
3+
). h e zero-valent or ele-
mental state of iron is a highly reactive species and is rarely formed on the
earth's surface. In the environment, Fe
0
undergoes rapid oxidation/corro-
sion owing to its high unstability. h is occurs through an electrochemical
process, whereby Fe
0
dissolves at anode and reduction of redox amenable
species take place at cathode. Under aerobic conditions, usually dissolved
oxygen is the primary electron acceptor (Eq. 14.1), whereas water predom-
inately accepts electron under anaerobic conditions (Eq. 14.2).
2Fe
0
(s)
+ 4H
+
(aq)
+ O
2(aq)
2Fe
2+
+ 2H
2
O
(l)
(14.1)
2Fe
0
(s)
+ 2H
2
O
(l)
2Fe
2+
+ H
2(g)
+ 2OH
-
(aq)
(14.2)
h e ferrous ions formed during corrosion undergo further oxidation to
form ferric ions (Eqs. 14.3 and 14.4);
2Fe
2+
(s)
+ 2H
+
(aq)
+ 1/2O
2(aq)
2Fe
3+
+ H
2
O
(l)
(14.3)
2Fe
2+
(s)
+ 2H
2
O
(l)
2Fe
3+
+ H
2(g)
+ 2OH
-
(aq)
(14.4)
As evident from the above results, nFe
0
mediated redox reactions pro-
duce a signii cant increase in solution pH and a concomitant decline
in solution potential (Eh). In other words, introduction of nFe
0
leads
to development of highly reducing conditions in the system. Sun
et al.
[16] reported an increase from pH ~6 to 8-9 in nFe
0
containing distilled
water.
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