Environmental Engineering Reference
In-Depth Information
increases steadily. Nanotechnology provides a chance to establish next-generation water-
purifying systems. Permissible limits for safe drinking water norms have been decreasing
with time. Novel and advanced materials are required to detect and remove contaminants
at such low levels. Materials, such as graphene, possessing large surface area and high
reactivity may be made as composites with NMNs (including NPs or luminescent clusters)
to add more desirable properties. One of the dificulties in working with nanomaterials
for water puriication is the separation of materials from water. Making composite mate-
rials such as iron oxide-Ag/Au and graphene-iron oxide nanosystems deinitely would
enhance the separation of materials from treated water for reuse. It is important to con-
sider the fate of nanoscale gold and silver that is entering into water during treatment.
The quantity of metal ions released during treatment varies from one method to the other.
Designers need to keep in mind that the designed material is applicable to remediate a
large range of contaminants (instead of one or two) present in water. There must be a clear
understanding of the mechanism of interactions. It is very important to focus on methods
of preparation of materials. Methods involving easy handling, less time consumption for
synthesis, and avoiding/minimizing special experimental conditions (such as maintain-
ing very high and very low temperatures, expensive equipment, etc.) are always advanta-
geous. A large number of strategies, such as microluidics and single particle spectroscopy,
may be more sensitive but have not yet entered signiicantly in the water segment. The
strategies of sensing described above may be combined with novel materials to do simulta-
neous removal. In the drinking water segment, products of this kind have not yet appeared
in the market place.
26.5 Summary
NMNs, including NPs and QCs, have been extensively used in sensing water contam-
inants such as heavy metal ions, anions, and biologically relevant molecules. All the
detection methods are mainly based on changes in optical properties leading to color
changes and SERS. Several molecular silver and gold clusters have been studied for
metal ion detection on the basis of changes in luorescence. The mechanisms were differ-
ent for different clusters, which depend on the metal ions. In the case of NPs, detection
was on the basis of aggregation of NPs leading to color changes. Importantly, several
studies have extended the applications of materials in real samples as well. Most of the
methods and materials were successful in detecting contaminants below the maximum
contaminant levels set by the US EPA. However, the disadvantage of some of the studies
is the dificulty in recycling of the materials, which is due to chemical transformation
of materials and analytes. As a result, the original property of the material cannot be
regained.
It is very much important to consider the reuse/recyclability when working with pre-
cious materials such as silver and gold. Silver clusters and NPs react with chlorocarbon to
give silver chloride, which is one of the important ores of silver. This reaction is green as
the silver chloride can again be used for extraction of silver. Bimetallic Pd-on-Au NPs were
shown to be highly active catalysts for dechlorination of one of the groundwater contami-
nants, trichloroethane, through conversion to ethane in the presence of hydrogen. Silver
NPs and clusters were utilized for antibacterial study. It was found that silver in the form
of clusters was more eficient than that in the NP form.
