Application of Zero-valent Iron Nanoparticles for Environmental Clean Up - Advanced Materials for Agriculture, Food and Environmental Safety - page 394

elemental mercury, Zn(II) undergoes sorption to iron oxide shell followed
by zinc hydroxide precipitation and hydrogen suli de gets immobilized on
nFe 0 surface as disuli de (S 2 2− ) and monosuli de (S 2− ) species. h e removal
mechanism of metal ions is mainly dependent on the electrode potential of
the metal [57]. h e detailed explanation is given in Section 14.3.

14.2.4.1.3 Factors Af ecting nFe 0 Reactivity
When nFe 0 is introduced into the contaminated sites, it gets exposed to
dif erent environmental factors. h ese factors all together determine the
overall ei cacy of nFe 0 for the contaminant in the environment. h e fol-
lowing section deals with the factors which play a crucial role in determin-
ing the potential ei cacy of nFe 0 for remediation of contaminants present
in dif erent environmental matrices.

• Size: h e smaller particle size is accountable for greater

density of reactive surface sites or surface sites of higher
intrinsic reactivity. As the size of particle decreases, parti-
cle dimensions approach the size of certain physical length
scales, such as the electron mean-free path and electron
wavelength, which in turn results in quantum ef ects. h ese
ef ects cause changes in the Fermi level and the band gap,
which ultimately leads to an increase in reactivity [63].

• Specii c surface area: It is an important factor which con-

trols the physicochemical properties of nanoparticles. h e
surface area of a nanoparticle can be determined through
BET gas adsorption isotherms. Alternatively, it can be calcu-
lated using Eq. 14.12.

r = 3[ρ * S] -1 (14.12)

where r is the radius of the nanoparticle, ρ is the density of
Fe (7,870 kg m -3 ) and S is the specii c surface area. Specii c
surface area shares a direct relationship with the reactivity
of nFe 0 . Increasing the specii c surface area results in an
increase in the fraction of iron atom present on the parti-
cle surface, thereby creating a greater reductive capacity per
unit of nanoparticles [64]. Several investigators have demon-
strated the supremacy of nFe 0 over microscale Fe 0 in terms
of reactivity and reaction rate constants [35, 40, 60, 64].

• Aggregation and oxidation: Generally, because of high

reactivity of nFe 0 , it reacts rapidly with the surrounding