Environmental Engineering Reference
In-Depth Information
activity following partial oxidation and most likely passivation of the nzVi. This study showed that the overall dechlorination
efficiency of TCE was enhanced by the concurrent or subsequent addition of nzVi particles to the media. The authors [28] also sug-
gested that, in environmental systems, nzVi can be used to abiotically degrade a large fraction of DNAPl mass in the source zone
directly, while subsequent biological dechlorination can serve as a polishing step to remove residual chloroethylenes.
5.4.3
Bimetallic Nps for the dechlorination of chlorinated alkenes
The oxidation of nzVi by molecular oxygen and the formation of a metal hydroxide surface layer results in the decrease of
the surface reactivity of the NPs over time [4]. By using bimetallic NPs such as Pd/Fe and Ni/Fe, this problem can be avoided.
Cho and Choi [37] employed nanosized palladium-iron (Pd/Fe) bimetallic particles for the degradation of TCE, PCE, and
1,1,1-TCA (1,1,1-trichloroethane) from aqueous solution. The results showed that the reactivity of (nzVi) particles was
enhanced by modifying its surface using Pd and CMC to form CMC-stabilized Pd/Fe bimetallic NPs (CMC-Pd/Fe). The
bimetallic NPs were spherical in shape with an average diameter of 98.5 nm. CMC prevented individual NPs from forming
aggregates and thus enhanced the remediation efficiency of chlorinated organic compounds.
The CMC-Pd/Fe NPs achieved a significant increase in the removal of TCE (~85%) as compared to the relatively low
removal (~15%) when conventional nzVi was used. Removal efficiencies of approximately 80 and 56% were achieved for
PCE and 1,1,1-TCA, respectively, when CMC-Pd/Fe was used. The authors [37] identified reductive dechlorination as the
main degradation mechanism for the investigated chlorinated compounds. The results of their study also suggested that the
position of the chlorine in chlorinated organic compounds has a significant effect on the degradation kinetics.
Bimetallic nickel-iron (Ni/Fe) NPs (3- to 30-nm diameter) have been tested as a reagent for the dechlorination of TCE in
aqueous solution [38]; 0.1 g of Ni/Fe NPs reduced the concentration of TCE from a 40 ml saturated aqueous solution
(24 mg/l) to <6 mg/l in 20 min. The use of Ni/Fe NPs resulted in dechlorination surface area-normalized reaction rate that
was 50 times faster than that using nzVi. This is because of the nickel-catalyzed hydrodechlorination of the TCE. in this
process, as the iron corrodes, the cathodically protected nickel chemisorbs hydrogen ions, and thus TCE adsorbed on the
nickel surface is hydrogenated. Toxic dechlorination by-products such as vinyl chloride are formed in trace amounts and do
not persist. This study suggested that bimetallic NPs containing good catalysts (e.g., Ni and Pd) for hydrogenation can be
used for the dehalogenation of chlorinated organics at faster rates and with the absence of toxic by-products.
The authors [38] noted that even though Ni/Fe NPs did not leach nickel in a 33-day time period, there is still a possibility
that nickel will leach once the iron has been exhausted after long burial times. This suggests that palladium may be a better
choice as a catalyst for most applications because it is more stable than nickel. However, when a more active catalyst such as
Pd is used, the initial corrosion rate of the iron increases because Pd catalyzes both the hydrogenation reaction and the evolution
of molecular hydrogen. As the iron surface becomes passivated, the difference between catalytic metals becomes less distinct.
Moreover, because of the fast corrosion rate, the useful time of nzVi is more limited than that of lower surface area iron, and
thus Ni/Fe and Pd/Fe NPs are suitable for shorter-term remediation applications such as injection into contaminated ground-
water than for long-term reactive barriers. More research is needed to synthesize bimetallic NPs with slower corrosion rates
while retaining the appropriate degree of reactivity for remediation applications [38].
Other studies in the literature also showed that bimetallic NPs exhibit high dechlorination efficiency for chlorinated alkenes.
Shih et al. [39] reported that Pd/Fe bimetallic NPs achieved a more rapid dechlorination rate for HCB than Fe NPs. Meyer et al.
[40] have successfully prepared reactive membranes by incorporating bimetallic Fe/Pt NPs (24 nm) into cellulose acetate films.
The prepared membranes successfully reduced TCE with ethane as the only observed by-product. zhao and Nagy [41] synthe-
sized a nanosorbent for the removal of TCE and PCE by incorporating SDS into magnesium-aluminum layered double
hydroxide (lDHs), and their results showed higher adsorption capacity of the SDS-Mg/Al lDHs toward both TCE and PCE in
aqueous solution as compared to organoclays.
5.5
Nms for pheNol removal
5.5.1
metal oxide Nps for phenol degradation
Phenol is among the phenolic compounds that are considered as priority pollutants since they are harmful to organisms even at
low concentrations, and many of them have been classified as hazardous pollutants because of their potential harm to human
health [24]. NMs were also tested for the removal of phenol from aqueous solutions, which is an important challenge facing
environmental protection. Chitose et al. [6] investigated the radiolysis of an aqueous phenol solution containing TiO
2
NPs (30-nm
diameter). Titanium dioxide NPs with high surface area and band-gap energy of 3.0 eV were used. This makes TiO
2
NPs an effec-
tive photocatalyst. The tested irradiation techniques were uV, γ-ray, and electron beam irradiation. The results showed that
