Environmental Engineering Reference
In-Depth Information
and maintained the pH at a stable value (7.5-7.8), resulting in acceleration of the dechlori-
nation rate of PCE. As shown in Figure 4.1b, the dechlorination rate of PCE was a function
of Fe concentration in Si/Fe solutions without addition of buffer solution. The dechlorina-
tion rate increased linearly from 6.0 × 10
−3
μM h
−1
at 1.85 m
2
L
−1
of Fe to 12.8 × 10
−3
μM h
−1
at
7. 4 m
2
L
−1
of Fe, and then leveled off to 23.2 × 10
−3
μM h
−1
at 64.7 m
2
L
−1
of Fe. In addition, the
inal solution pH increased from 7.4 at 0.06 g of Fe to 8.1 at 2.1 g of Fe. Only 0.3 units of pH
were changed as the Fe amount increased by a factor of 35, clearly showing that the com-
bination of Si and Fe can form a natural buffering system to achieve an environmentally
friendly condition near pH 7-8.
4.4.3 Transition Metal Ions
In addition to the solution pH, the impact of transition metal ions on the reductive
dechlorination of chlorinated compounds and nitro-aromatics by both structural Fe(II)
species and ZVI has been addressed. The deposition of small amounts of second metals
such as Ni and Pd onto the iron surface has been demonstrated to effectively enhance
the dechlorination eficiency and rate of chlorinated hydrocarbons [12,55,56]. The coexis-
tence of other heavy metal ions in aqueous solution also inluences the reactivity of iron
nanoparticles toward contaminants. The core-shell structures of iron nanoparticles can
serve as reductants as well as adsorbents to immobilize metal ions onto the surface. The
adsorbed metal ions may further receive electrons produced from the core layer of ZVI.
This would enhance the reduction eficiency of reducible contaminants by converting
the metal ion into its low valence state form or decrease the eficiency by competing for
electrons with organic contaminants. Lee and Doong [57] have shown that the
k
obs
for
PCE dechlorination by zerovalent silicon in the presence of 0.1 mM Fe(II) and Ni(II) were
1.5-3.8× higher than in the absence of metal ions. Schlicker et al. [58] have shown that the
existence of Cr(VI) signiicantly hindered the dechlorination of chloroethanes by ZVI.
On the contrary, addition of catalytic ions such as Ni(II), Cu(II), Ag(I), Pd(II), and Pt(II)
may enhance the reduction rate and eficiency of chloroethanes. Dries et al. [59] reported
that the addition of 5-100 mg L
−1
Ni(II) enhanced TCE reduction by 100 g L
−1
ZVI owing
to the catalytic hydrodechlorination by bimetallic Ni
0
/Fe
0
. However, Cr(VI) and Zn(II)
lowered the TCE dechlorination rate by a factor of 2-13×. Lien et al. [60] depicted that
Cu(II) enhanced the CT dechlorination by nZVI, while Pb(II) only increased the reduc-
tion rate slightly. However, Cr(VI) decreased the dechlorination rate by a factor of 2.
Xie and Shang [61] investigated the impact of metal ions on the reduction of bromate by
nZVI and found that the incorporation of Cu(II) led to an increase in the bromate reduc-
tion rate. Addition of Pd(VI), on the contrary, had little effect on bromate reduction by
microscale ZVI.
The addition of metal ions also inluences the reactivity of the Fe(II)-Fe(III) system,
and could alter the long-term reactivity of ZVI in aqueous solutions. Jeong and Hayes [62]
showed that the addition of transition metal ions, including Cu(II), Ni(II), Zn(II), Cd(II),
and Hg(II), increased the dechlorination rates of hexachloroethane in the presence of
iron-bearing minerals. The catalytic activities of Cu(II), Au(III), and Ag(I) in the reduc-
tion of chlorinated alkanes by green rust were also demonstrated [63,64]. Maithreepala
and Doong [65,66] demonstrated that that addition of Ni(II), Co(II), and Zn(II) lowered the
k
obs
for CT dechlorination by structural Fe(II)-Fe(III) minerals, whereas the amendment of
0.5 mM Cu(II) into the Fe(II)-Fe(III) system signiicantly enhanced the eficiency and the
