Biomedical Engineering Reference
In-Depth Information
A number of bimetallic systems in which various metals are plated onto
nZVI particles have shown the ability to reduce chlorinated organic com-
pounds at rates significantly faster than the nZVI alone [25]. Some combina-
tions available today are Fe/Pt, Fe/Ag, Fe/Ni, Fe/Co, and Fe/Cu [25]. Among
them, the palladized iron (Fe/Pd) has the fastest reaction kinetics. Laboratory
studies showed that TCE is destroyed twice as fast by Fe/Pd than by nZVI
alone [25]. Unfortunately, owing to the high cost of palladium, the
in situ
use
of Fe/Pd NPs is infeasible.
As it was shown in this section, nZVI represent a powerful means of
groundwater and site remediation. Along with the benefits associated with
its utilization, however, scientists are afraid that exposure to the iron NPs
can potentially cause environmental and/or health problems.
Most analysts of this issue suggest that the current nZVI-based remedia-
tion methods do not appear to pose environmental and human health risks
[26-29]. Modeled transport data indicate that NP plumes may travel only
slightly faster than most contaminant plumes and as a consequence the
human exposure is likely to be minimal [27]. However, there are still many
uncertain aspects, for example, the inherent toxicity of the particles, the envi-
ronmental behavior, and the fate of their transformation by-products. More
research is necessary before final conclusions can be drawn about the health
risks of nZVI particles.
7.4.1.2 Nanoscale Semiconductor Photocatalysts (nSPs)
Semiconductor photocatalysts (SPs) are catalysts built from semiconducting
materials (e.g., titanium dioxide [TiO
2
], zinc oxide, iron oxide, tungsten tri-
oxide), which obtain their activation energy from the light in the UV spec-
trum [25,30]. It was recognized that building SP materials from NPs greatly
enhances their photocatalytic activity. The nanoscale SPs are able to degrade
a great variety of inorganic and organic contaminants, which makes them
useful for environmental remediation. A tentative list of these contaminants
is provided in Table 7.2.
Nanoparticulate TiO
2
has been traditionally used in environmental reme-
diation because of its low toxicity, high photoconductivity, high photosta-
bility, availability, and low cost [25,30]. Novel technologies and improved
processes have enabled the development of a variety of photocatalytic deriv-
atives. Metals such as copper, silver, gold, and platinum have been used to
modify the TiO
2
particle surfaces and improve their decontamination activ-
ity. The combination of TiO
2
with copper, for example, results in a “synergis-
tic photocatalytic effect” for the remediation of hexavalent chromium [32].
Coupling the TiO
2
NPs with gold or silver results in similar effects [32].
The TiO
2
-based p-n junction nanotubes represent a novel, fascinating
development in the field of nanoscale SPs. The nanotubes contain platinum
(in the inside) and TiO
2
(on the outside). The TiO
2
coating of the tube acts as
an oxidizing surface, while the inside of the tube is reductive [33]. The ability
