Application of Zero-valent Iron Nanoparticles for Environmental Clean Up - Advanced Materials for Agriculture, Food and Environmental Safety - page 388

nanoparticles, and hence the need to be maintained constant in the experi-
ments to produce consistent samples [16, 24-26].

14.2.3 Structure

Zero-valent iron nanoparticle (nFe 0 ) has a core shell structure, wherein core
is made of metallic iron and shell consists of iron oxides/hydroxides/oxy-
hydroxides. h e metallic iron present at the center holds reducing power,
whereas the oxides hydroxides coating exhibits sorption characteristics. h us
core shell structure bestows nFe 0 with dual characteristics of sorption and
reduction, which could be of signii cant importance in the separation and
transformation of contaminants. h e constituent of shell, i.e., iron oxides,
may have either metal-like or ligand-like coordination properties, depending
on the solution chemistry. At acidic pH, iron oxides acquire positive charge
and show ai nity towards anionic ligands, in contrast, at higher pH, they have
negative charge and form surface complexes with cationic species [16]. On
one hand the shell endows nFe 0 with the properties of sorption, on the other
hand it also acts as a passive i lm, which provides a physical barrier between
the metal and dissolved contaminants [27]. As the nFe 0 mediated reactions
are driven by oxidation of Fe 0 core, sustained reduction of contaminants
requires localized defects in the passive i lm, which could facilitate charge and
mass transport through it. h ese transport processes play a signii cant role in
contaminant reduction kinetics. h e overall reactivity of nFe 0 is a function of
the surface concentration of these reactive sites/localized defects [28].
It is always dii cult to measure the exact thickness of the shell due to the
high reactivity of iron, which is responsible for continuously increasing shell
thickness, as the oxidation reaction proceeds. However, Martin et al. [29]
attempted to determine the thickness of the shell using three methods, i.e.,
high resolution transmission electron microscopy (HR-TEM), high resolu-
tion photoelectron microscopy (HR-XPS) and complete oxidation reaction
of nFe 0 . h e HR-TEM images revealed that the shell thickness predominately
lies in the range of 2-4 nm. h e HR-XPS analysis also provides approxi-
mately similar thickness range with an average in the range of 2.3-2.8 nm.
h e complete oxidation of nFe 0 by Cu(II) indicated a shell thickness of 3.4
nm, which is consistent with results of HR-TEM and HR-XPS.

14.2.4 Environmental Application

14.2.4.1 Zero-Valent Iron Nanoparticle

Zero-valent iron nanoparticle (nFe 0 ) is widely used to detoxify and degrade
various classes of environmental contaminants. It has diminutive size, high