Environmental Engineering Reference
In-Depth Information
methods and structure-property relationships. The reliable performance and wide
acceptance has rendered iron-based nanoparticles to be the model metallic nanoparticles
for environmental applications, including over 20 actual field remediation projects using
nanoscale zero-valent iron (nZVI) (Li et al
.
, 2006).
Generally, iron rapidly oxidizes in air or corrodes in water. This high reactivity
of iron with respect to water and oxygen gives yield to passivation of the iron
nanoparticles as the passivation layer serves as a protective layer to the nanoparticles in
the aqueous environment. Hence it is natural that the iron nanoparticles often consist of
core/shell structures, where the shell consists of iron oxide/hydroxide and beneath lies
the zero-valent iron core. This would not only lead to their stability to passive corrosion
as well as dissolution-related processes, but also complicate the elucidation of their
decontamination mechanism(s), such as the competition between electrostatic attraction
(Fe(OH)
2
, Fe(OH)
3
, Fe
2
O
3
) and reduction (Fe
0
).
The oxide shells of iron-based nanoparticles, in general, can be expressed
stoichiometrically as FeOOH (Li and Zhang, 2006), which is similar to that of goethite
(-FeOOH). The surface binding sites commonly found on the FeOOH surface are
singly-coordinated (Fe(OH)H) and triply-coordinated (Fe
3
O(H)) oxygens (Hiemstra
et al
.
, 1996). Like other mineral oxides, these surface binding sites undergo protonation
or deprotonation, and become either negatively-charged (FeOH
-1/2
, Fe
3
O
-1/2
) or
positively-charged (FeOH
2
+1/2
, Fe
3
OH
+1/2
), depending on the solution pH and the
intrinsic point-of-zero-charge pH (pH
pzc
). The approaching heavy metal ions will adsorb
to these binding sites and be removed from the bulk phase, according to the general
schemes as shown in Eqs. 6.1 to 6.4 (Benjamin et al
.
, 1982). These processes, taking
place on the nanoparticle surface, are mainly based on electrostatic interaction by nature,
and hence, no specific selectivity could be derived. As a result, the binding sites could
easily be occupied by other environmental species of abundance, e.g. phosphates or
sodium ions.
Protonation:
(Eq.6.1)
+
Fe
−
OH
+
H
+
=
Fe
−
OH
2
Deprotonation:
(Eq. 6.2)
−
+
Fe
−
OH
=
Fe
−
O
+
H
Cation adsorption:
reaction
(Eq. 6.3)
Fe
−
OH
+
M
n
+
+
mH
O
=
2
( )
n
−
m
−
1
+
Fe
−
M
OH
+
(
m
+
1
H
m
Anion adsorption:
reaction
(Eq. 6.4)
n
−
+
m
−
n
Fe
−
OH
+
A
+
mH
=
Fe
−
OH
A
m
+
1
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