Environmental Engineering Reference
In-Depth Information
generally, photocatalysis includes (i) photogeneration of electrons in the conduction band trapped by a recipient
(e.g., superoxide/hydroxyl radicals) and (ii) photogenerated holes in the valence band consumed by the donors, that is, organic
pollutants [24]. Similar to the photocatalytic process, sonolysis of aqueous solution is known to produce hydroxyl radicals and
other radical species that can rapidly initiate the degradation of 1,4-DCB through the direct attack on the aromatic ring [23].
Additionally, under ultrasound, VOCs such as 1,4-DCB may undergo degradation via another path, that is, by direct pyrolysis
in the vapor phase of pulsating or collapsing cavitation bubbles within the hot interfacial region between the vapor and the
surrounding liquid phases.
5.3.2
cNts for chlorobenzene sorption
in a study by liu et al. [25], MWCNTs were used as solid-phase extraction (SPE) sorbents for chlorobenzenes. Chromatographic
techniques are the most widely used methods for analyzing chlorobenzenes, but preliminary separation and enrichment of trace
chlorobenzenes is usually necessary in order to increase chlorobenzene concentration and to eliminate matrix effects [25].
Therefore, liu et al. [25] utilized MNCNTs as a solid phase for extracting 1,2-chlorobenzene from aqueous solution and com-
pared its adsorption characteristics with commonly used SPE sorbents such as C
18
silica and activated carbon. The adsorption
capacities of MWCNTs, C
18
silica, and activated carbon for 1,2-chlorobenzene at an equilibrium concentration of 90 µg/ml were
237, 189, and 150 mg/g, respectively. These results show that MWCNTs can be efficiently applied for the determination of
chlorobenzenes and other VOCs in natural and polluted water. Peng et al. [26] investigated the use of CNTs as an adsorbent for
1,2-DCB in aqueous solution. Their results showed that it took only 40 min for the 1,2-DCB sorption onto CNTs to reach
equilibrium with a maximum sorption capacity of 30.8 mg/g.
5.4
Nms for chloriNated alkeNes removal
Hazardous chlorinated compounds, especially chlorinated alkenes, contaminate groundwater and continue to be a significant
environmental problem [27]. Examples of volatile chlorinated alkenes include trichloroethylene (TCE), tetrachloroethylene
(PCE), and 1,1,1-trichloroethane (1,1,1-TCA). More attention is given to TCE as it is a common contaminant in soils and
groundwater. TCE can also remain in the subsurface as dense nonaqueous-phase liquids (DNAPl) and remain a continuous
long-term threat to the environment [28, 29]. TCE is a possible carcinogen at low concentrations. The Safe Drinking Water Act
issued by the u.S. Environmental Protection Agency determined the maximum contaminant level of TCE at 5 µg/l [7].
Conventional treatment methods for the removal of chlorinated compounds include activated carbon adsorption, air stripping,
and catalysis. While adsorption and air stripping successfully remove these compounds, they only replace the contaminant to
another phase rather than convert it to nonhazardous products. Catalysis is a better approach to completely remove chlorinated
compounds from the environment since it converts them into safer, nonchlorinated compounds [27]. NMs have also been
employed as new adsorbents or catalysts to remove chlorinated alkenes from aqueous solution.
5.4.1
metallic Nps for the dechlorination of chlorinated alkenes
laboratory studies have shown that nano zero-valent iron (nzVi) is a powerful reductant for targeting chlorinated ethylenes
(e.g., PCE and TCE) (sometimes referred to as chlorinated ethenes) [30]. The electrons transferred from the metallic nzVi par-
ticles to the chlorinated ethenes convert them to environmentally benign chloride and nonchlorinated organic compounds.
Typically, nzVi particles have ~30 times higher surface area than their larger-sized granular iron counterparts and can degrade
chlorinated ethylenes at rates orders of magnitude faster than granular zVi.
Despite the advantages of using nzVi particles for environmental remediation of chlorinated ethylenes, bare nzVi forms larger
aggregates of particles shortly after production. This aggregation limits the ability of nzVi particles to migrate in aquifers and there-
fore cannot treat a large volume of the subsurface in situ by direct injection. To overcome this drawback, carboxymethyl cellulose
(CMC) was effectively used as a stabilizing agent to generate highly dispersed nzVi particles [31]. Bennett et al. [30] investigated
the transport of CMC-stabilized nzVi particles (CMC-zVi) in saturated sediments and their reactivity toward chlorinated ethenes
in a series of single-well push-pull tests. The results indicated that CMC-nzVi particles were mobile in the aquifer but appeared to
lose mobility with time as a result of the interactions of the particles and the aquifer sediments. The total mass of destroyed ethenes
from groundwater was low because the injected solutions “pushed” the dissolved chlorinated ethenes away from the injection well.
These results indicate that for in situ remediation programs using metallic NPs, high advective groundwater velocities are needed to
deliver the NP substantial distances from the point of injection. For example, groundwater recirculation would likely be an efficient
method for enhancing NP migration by maintaining high postinjection groundwater velocities.
