Environmental Engineering Reference
In-Depth Information
methods and starting materials used for synthesis. Choi et al. [24] used a chemical vapor
condensation method to synthesize iron nanoparticles using iron pentacarbonyl (Fe(CO)
5
)
as the precursor under a helium atmosphere. Iron pentacarbonyl would decompose to
ZVI and carbon monoxide (CO) at a temperature >300°C. The typical particle sizes were
found on the order of 5−13 nm. Makela et al. [25] used a liquid lame spray process for the
generation of single-component Ag, Pd, and Fe nanoparticles. The average diameters of
synthesized nanoparticles were in the range 10−60 nm.
Although several methods are available to chemically synthesize nZVI, particles syn-
thesized by gas-phase reduction and aqueous-phase reduction of ferrous or ferric iron by
sodium borohydride (NaBH
4
) have been the most thoroughly examined technology [26-29].
The most often studied nanomaterial synthesized by the gas-phase reduction method is
reactive nanoscale iron particles (RNIP). RNIP is a commercial product from Toda Kogyo
Corp. (Onoda, Japan), which is produced by the reduction of goethite (α-FeOOH) or hema-
tite (Fe
2
O
3
) particles with H
2
at high temperatures of 350°C−650°C. The particle sizes and
speciic surface areas are in the range 50−300 nm and 7−55 m
2
g
−1
, respectively. The liquid-
phase reduction of metal cations by a strong reducing agent such as NaBH
4
and hydrazine
dihydrochloride (N
2
H
4
·2HCl) in aqueous or nonaqueous solution is the most widely used
method in the laboratory for preparation of ZVMs [30]. For nZVI, nanoparticles can be
prepared by using NaBH
4
as the key reductant to reduce ferric chloride (FeCl
3
·6H
2
O) or fer-
rous sulfate (FeSO
4
·7 H
2
O) to form iron metal (Fe
BH
) [26,31]. Zhang [31] reported that exces-
sive borohydride is typically needed to accelerate the synthesis reaction and ensure the
uniform growth of iron nanoparticles. Usually the particle sizes of the freshly prepared
nZVI range between 8 and 15 nm and then aggregate to around 20−100 nm.
Several studies have compared the characteristics of the nZVI prepared using different
chemical methods, especially the gas- and liquid-phase reduction methods. Nurmi et al.
[32] conducted a systematic investigation for comparing the physicochemical properties
and reactivity of two nZVI, RNIP, and Fe
BH
, prepared under different conditions. Table 4.2
compares the characteristics and reactivity of iron nanoparticles prepared by hydrogen
reduction in gas phase and NaBH
4
in aqueous solution. RNIP is a two-phase material con-
sisting of 40−70 nm α-Fe
0
and magnetite (Fe
3
O
4
) as the core and shell coating, respectively,
TABLE 4.2
Comparison of Physical and Chemical Properties of Fe
BH
and RNIP
Parameters
Fe
BH
RNIP
Primary particle size (nm)
20-80
40-70
BET surface area (m
2
g
−1
)
33.5-36.5
23-39
Major phase (core layer)
Fe
0
α-Fe
0
Shell coating
Magnetite (Fe
3
O
4
)
Goethite (α-FeOOH), wustile (FeO)
Surface composition
a
Fe: 20.0%, O: 49.1%, B: 16.0%, Na: 14.5%,
S: 10.5%
Initial Fe
0
content (wt%)
97 ± 8% 26.9 ± 0.3
Crystallinity Highly disordered Crystalline
Use of hydrogen Ye s No
End product Mostly saturated Unsaturated
Source:
Nurmi JT et al.,
Environ Sci Technol
, 39, 1221,
2005; Liu YQ et al.,
Environ Sci Technol
, 39, 1338,
2005; Liu
YQ, Lowry GV,
Environ Sci Technol
, 40, 6085, 2006.
a
Determined by XPS.
Fe: 50.9%, O: 44.2%, B: 0.0%,
Na: 3.0%, S: 1.9%
