Environmental Engineering Reference
In-Depth Information
were observed when the diameters of Cu nanoparticles were in the range 18-29 nm. More
recently, He and Zhao [49] synthesized the monodispersed Pd/Fe bimetallic nanoparticles
by applying a water-soluble starch as a stabilizer. The mean particle diameters were in
the range 14.1 ± 11.7 nm. The
k
SA
value for 40 mg L
−1
TCE dechlorination by 1 g L
−1
starch-
stabilized Pd/Fe nanoparticles was 0.67 L h
−1
m
−2
, which is 37× higher than that reported
by Lien and Zhang [50] who used 5 g L
−1
nonstabilized Pd/Fe for the dechlorination of
20 mg L
−1
TCE.
4.4.2 pH Values
pH value is one of the important environmental parameters inluencing the reactivity of
micro- and nanosized ZVI. Several studies have demonstrated the relation between pH
and the pseudo-irst-order constant (
k
obs
) for dechlorination of chlorinated hydrocarbons,
and found that the dechlorination eficiency and rate decreased upon increasing pH val-
ues [16,51-53]. Deng et al. [51] demonstrated that the
k
SA
values for vinyl chloride dechlori-
nation by microscale ZVI decreased less than an order of magnitude when pH increased
from 6 to 10. Chen et al. [52] found that the
k
SA
of iron corrosion linearly decreased from
0.092 to 0.018 L h
−1
m
−1
between pH 4.9 and 9.8, while it is signiicantly higher at pH 1.7
and 3.8. The dechlorination process is a consequence of direct oxidative corrosion of iron
by chlorinated compounds, resulting in the production of ferrous and hydroxyl ions. At
elevated pH values, the ferrous ions react with hydroxyl ions and form ferrous hydroxide
and ferric oxides, and subsequently precipitate on the surface of ZVI. The precipitated iron
oxides would hinder the transport of chlorinated compounds and block the reactive sites
on ZVI, resulting in the decrease in reaction rate. Lin and Lo [54] indicated that the dechlo-
rination rate of TCE by iron can be enhanced when the iron oxide shell was pretreated
with HCl. Interesting, Doong et al. [9] examined the solution pH proile in zerovalent sili-
con (Si), nZVI, and Si/Fe systems without the addition of buffer solution, and found that
the pH increased rapidly from 7.5 to 10, while the pH decreased from 8.7 to 7.8 in the irst
50 h in Si solution (Figure 4.1a). The combination of Si and Fe constituted a buffer system
(a)
(b)
25
10.5
Rate = (1.68SA)/(1+ 0.0563SA)
10.0
20
9.5
15
0.06 g Si
1.08 g Fe
0.06 g Si/1.08 g Fe
9.0
8.5
10
8.0
5
7.5
0
7.0
0
50
100
150
200
250
300
0
10 0 0 0 0 0 0
Surface area of Fe (SA, m
2
L
-1
)
Time (h)
FIGURE 4.1
(a) Changes in pH values in Si, Fe, and Si/Fe systems. (b) Relation between rate constants for PCE dechlorination
and surface area of Fe in the unbuffered Si/Fe-H
2
O system containing 0.06 g Si and 0.06-2.1 g Fe. (From Doong
RA et al.,
Environ Sci Technol
, 37, 2575, 2003.)
