Environmental Engineering Reference
In-Depth Information
Another issue with the utilization of pure nzVi for the dehalogenation of chloroethylenes is that nzVi particles are pyrophoric
and react spontaneously with atmospheric oxygen [32]. in order to prevent nzVi from oxidation, Tiehm et al. [32] synthesized
air-stable nzVi particles (20-25 nm) by applying ultrasound to a solution of Fe(CO)
5
in edible oil (corn oil) and dispersing the
resulting nzVi particles in a carbon matrix. This synthesis approach resulted in the formation of air-stable nzVi particles with
a protecting layer composed of graphitic and disordered carbon. The authors utilized the air-stable nzVi for the dehalogenation
of chloroethylenes in synthetic aqueous medium as well as polluted groundwater. The results indicated faster mass-normalized
degradation kinetics of chloroethylenes (trichlororethylene,
cis
-dichloroethylene, vinyl chloride, ethane, and ethane) using the
air-stable nzVi, in both synthetic media and groundwater samples, as compared to conventional nzVi.
Choi and lee [7] utilized the nzVi Fenton system with Cu(ii) (Cu/nzVi) for the degradation of TCE. Among the available
methods for TCE degradation, Fenton reaction has attracted attention because of its strong oxidative capacity for organic contam-
inants. in the conventional Fenton process, the decomposition of H
2
O
2
to hydroxyl radicals (OH·) in the presence of Fe(ii) is the
main mechanism for producing a representative oxidant. The main drawback of this conventional process is that the pH should
be kept in the acidic range to avoid the precipitation of Fe(OH)
3(s)
[33]. When using the Cu(ii)/nzVi Fenton system, the degrada-
tion of TCE was not influenced by the initial pH [7]. The Cu(ii)/nzVi Fenton system degraded 95% of the TCE in 10 min with
20 mM Cu(ii) at an initial pH of 3.0. This was significantly higher than the TCE degradation (25%) using the conventional nzVi
Fenton process without Cu(ii) under the same experimental conditions.
The authors [7] suggested that oxidative TCE degradation in Cu(ii)/nzVi Fenton systems involves the following four steps:
(1) Fe(ii) is released from nzVi surfaces via oxidative substitution of Fe(0) by Cu(ii); (2) Cu species are then reduced by Fe(0)
on the nzVi surface; (3) the OH· are generated by aqueous Fe(ii) oxidation with H
2
O
2
, which results in the degradation of TCE
by the OH· in Cu(ii)/nzVi suspension; and (4) the Fe(ii) are released by direct oxidation with H
2
O
2
on the nzVi surface, and
the full degradation of TCE occurs because of the continuous production of OH·. The results of this study imply that the Cu(ii)/
nzVi Fenton system can be applied to systems contaminated with TCE at circum-neutral pH.
5.4.2
Biometallic Nps for the dechlorination of chlorinated alkenes
As discussed earlier, several approaches utilizing nzVi NPs are tested for the dechlorination of chlorinated alkenes.
Nonetheless, these approaches have some disadvantages such as the formation of more toxic by-products (e.g., vinyl chlo-
ride), slow reaction rates, and the difficulty to treat several halogenated organic compounds together [34, 35]. As a result of
its superior catalytic capacity in hydrodehalogination reactions, palladium can overcome these shortcomings. Hennebel
et al. [29] utilized microbially produced palladium NPs (bio-Pd) for the remediation of TCE-contaminated groundwater. The
authors [29] utilized a method to produce a nano bio-Pd catalyst by precipitating palladium on
Shewanella oneidensis
in
order to form NPs. Bacteria, in the presence of a hydrogen donor, can reduce Pd(ii) and subsequently precipitate it as Pd
nanocrystals on their cell wall and in their periplasmatic space. The bacteria in this case serve as a biological carrier for the
Pd NPs, keeping them in suspension and preventing their aggregation. The bacteria can also interact with the contaminant
and contribute toward more efficient removal.
The use of
S. oneidensis
in this study [29] has a major advantage over other bacteria because it the only known bacteria that
can optimally reduce Pd when it is grown under controlled conditions of oxygen, carbon source, and hydrogen donor. in their
experiment, Hennebel et al. [29] used hydrogen gas, formate, and formic acid as hydrogen donors. The results showed that
TCE was efficiently removed and ethane was the only organic degradation product and that no intermediate chlorinated reac-
tion products were detected. Hydrogen gas was the most appropriate hydrogen donor used in the process. The authors also
impregnated the bio-Pd in a continuous plate membrane reactor (MRr) for the treatment of TCE-contaminated water. The MRr
system achieved removal rates of up to 2515 mg TCE/day/g Pd using H
2
gas as the hydrogen donor. The authors concluded
that biocatalysis with bio-Pd resulted in complete, efficient, and rapid removal of TCE.
in another study, Xiu et al. [28] investigated the effect of nzVi particles on a mixed culture dechlorinating TCE. Biological
degradation by dechlorinating bacteria is a promising technology for DNAPl remediation. Examples of anaerobic bacteria
used for the reductive dechlorination of TCE and PCE include
Desulfuromonas
,
Sulfurospirillum multivorans
, and
Dehalobacter
.
However, several field studies identified many challenges facing bioremediation within DNAPl source zones such as low
dechlorination rates, insufficient supply of suitable electron donors such as H
2
and acetate to the dechlorinating bacteria, and
toxicity because of high concentrations of PCE or TCE [36].
Because corrosion of nzVi produces H
2
, which is a highly favorable electron donor for anaerobic dechlorinating bacteria, Xiu
et al. [28] investigated the effect of synergetic use of nzVi particles with microbial dechlorinating ability of bacteria on the dechlo-
rination efficiency of TCE. The results showed that the presence of nzVi initially inhibited the dechlorinating organisms. However,
dechlorination activity showed recovery after a lag period. During the active period of the dechlorinating bacteria, the H
2
resulting
from the cathodic corrosion of nzVi was utilized as an electron donor by the bacteria, which resulted in the recovery of the bacterial
