Environmental Engineering Reference
In-Depth Information
eficiency of priority pollutants by a core-shell structured bimetallic system is usually
better than that of the alloy bimetallic structure when the same ratio of catalytic metal to
reductive metal was compared.
4.5.2 Reduction of Pollutants by Bimetallic Systems
Bimetallic systems have received signiicant attention because of their considerably faster
reduction rates of priority pollutants. Several bimetallic particles, such as Pd/Fe, Ni/Fe,
Cu/Al, and Cu/Fe, have been synthesized and applied to the reduction of a wide variety
of priority pollutants, including chlorinated hydrocarbons [44,55,82,85,86], nitrobenzenes
[87], pentachlorophenol [88], and anions [79,89]. Nutt et al. [81] showed that a bimetallic
treatment approach involving palladium supported on gold (Pd-on-Au) increased the reac-
tion rate in the dechlorination of TCE. A rapid and complete dechlorination of chlorinated
solvents with the production of nonchlorinated hydrocarbons was also reported by using
nanoscale bimetallic Pd/Fe particles [22,90,91]. In addition, bimetallic Ni/Fe nanoparti-
cles have been found to rapidly dechlorinate chlorinated ethylenes with the formation
of ethane as the main products [82,85], showing that the bimetallic system is an effective
technology for accelerating the dechlorination processes and converting the chlorinated
hydrocarbons to the nontoxic end products.
The deposition of a catalytic second metal such as Pd, Ni, and Cu onto the surface of a
reductive metal could enhance the dechlorination eficiency and rate of chlorinated hydro-
carbons and prevent the formation of toxic products by dechlorinating the chlorinated
hydrocarbons via hydrogen reduction rather than through electron transfer [8,85]. In addi-
tion, the type of products obtained during the dechlorination reaction is dependent on the
identity and mass loading of the second metal employed [85]. When the bimetallic system
was employed to decompose the chlorinated hydrocarbons, the dechlorination eficiency
could be enhanced by increasing the loading of the second metal [55,81,82]. However,
higher loading of catalytic metal on the reductant metal surface inhibits the dechlorination
eficiency and an optimal loading usually exists. Several plausible explanations, includ-
ing the formation of a galvanic cell [85], the surface coverage of catalytic metal on the
reductive metal [82], and the absorbed atomic hydrogen [86,92], have been proposed to
explain this phenomenon. More recently, Parshetti and Doong [93] immobilized nZVI onto
TiO
2
nanoparticles and investigated the coupled removal of TCE and 2,4-dichlorophenol
(DCP) in aqueous solutions under anoxic conditions in the presence of nickel ions and
UV light at 365 nm. They found that both TCE and DCP were effectively dechlorinated by
Fe/TiO
2
nanocomposites, which were higher than by nZVI alone. Addition of nickel ions
signiicantly enhanced the simultaneous photodechlorination eficiency of TCE and DCP
under the illumination of UV light. The pseudo-irst-order rate constants for DCP and
TCE photodechlorination by Fe/TiO
2
in the presence of 20-100 μM Ni(II) was 30.4-136×
and 13.2-192×, respectively, higher than those in the dark. The reaction mechanism was
also proposed. As shown in Figure 4.3, the TiO
2
photocatalysts can be photoexcited by
UV light to generate electron-hole pairs, while the metallization of TiO
2
with Fe prevents
the recombination of holes with electrons, leading to the enhancement of the oxidizing
capability of TiO
2
. In addition, the Ni(II) and produced Fe(II) ions from the anaerobic cor-
rosion of nZVI can react with photogenerated holes to form Ni(III)/Fe(III) ions and then be
converted back to Ni(II)/Fe(II) ions again when reacted with electrons or hydroxyl anions,
resulting in the prevention of hole-electron recombination and the increase in the total
amounts of hydroxyl radicals.
