Environmental Engineering Reference
In-Depth Information
14.2.4.1.2 Inorganic Contaminants
Not only can nFe
0
reduce organic contaminants, but also the inorganic
contaminants such as perchlorate, chromate, nitrate, arsenic [49, 50-53],
etc. h e ability of nFe
0
to reduce redox-sensitive elements has been demon-
strated at both bench-scale and i eld-scale tests [11, 14, 15]. h e degrada-
tion mechanism is based on the transformation of the toxic contaminant
to nontoxic or less toxic form. For example, nFe
0
transforms highly solu-
ble, highly mobile, extremely toxic Cr(VI) into relatively less soluble, less
mobile and less toxic Cr(VI) [20, 54]. Arsenic is another example, whose
+3 and +5 oxidation states are both reported to be ef ectively removed
from groundwater by nFe
0
[53, 55]. Also, nFe
0
has been proven ef ective
for degradation of alkaline earth metals such as barium, transition metals
- copper, silver, lead, etc., and radioactive elements such as uranium and
technetium [56-59]. Table 14.1 lists the organic and inorganic contami-
nants which are reported to be successfully degraded or transformed into
less toxic or nontoxic entities by nFe
0
.
As signii cant variations exist in the contaminant chemistry, there are
numerous possible pathways for contaminant removal in nFe
0
mediated
reactions such as sorption, complexation, precipitation/co-precipitation
and surface-mediated chemical reduction. Generally the contaminant
is removed via a combination of two or more processes. For example,
the removal mechanism of Cr(VI) involve instantaneous adsorption of
Cr(VI) on nFe
0
surface where electron transfer takes place, and Cr(VI) is
reduced to Cr(III)
with oxidation of Fe
0
to Fe(III). Subsequently, Cr(III)
precipitates as Cr(III) hydroxides and/or mixed Fe(III) /Cr(III)hydroxides/
oxyhydroxides as per the following Eqs. (14.9-14.11).
3Fe
0
+ Cr
2
O
7
-
+ 7H
2
O
3Fe
2+
+ 2Cr(OH)
3
+ 8OH
-
(14.9)
(1-x)
Fe
3+
(aq)
+
(x)
Cr
3+
(aq)
+ 3H
2
O
Cr
x
Fe
1-x
(OH)
3 (s)
+ 3H
+
(aq)
14.10)
(1-x)
Fe
3+
(aq)
+
(x)
Cr
3+
(aq)
+ 2H
2
O
Cr
x
Fe
1-x
(OOH)
(s)
+ 3H
+
(aq)
(14.11)
where
x
varies from 0 to 1. Mixed hydroxides of Cr(III) and Fe(III) get
incorporated into the iron oxy hydroxide shell of nFe
0
forming (Cr
x
Fe
1-x
)
(OH)
3
or Cr
x
Fe
1-x
OOH at the surface; in this way Cr(III) gets stabilized/
immobilized on nanoparticle surface [20, 60]. Similarly, the removal mech-
anism of As(V) and As(III) involves spontaneous adsorption and co-pre-
cipitation with iron(II) and iron(III) oxides and hydroxides [55, 61]. Yan
et al.
[62]
studied the removal mechanism of Hg(II), Zn(II) and hydrogen
sulphide and reported that Hg(II) sequestrates via chemical reduction to
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