Environmental Engineering Reference
In-Depth Information
pollution. The main sources of these contaminants are wastewater from crude oil and petro-
leum reineries, waste released from manufacturing sites, pesticide and mosquito larvicide,
and pharmaceutical intermediates. Scott et al
.
48
developed an Au NPs/PDMS (polydimeth-
ylsiloxane) composite for the removal of the above contaminants from water. The principle
of contaminant removal was based on the ability of PDMS to undergo large expansion in
nonpolar solvents. The composite was prepared as ilms and foams among which foams
exhibit large absorption of contaminants. This was attributed to the presence of a large den-
sity of micropores. Incorporation of Au NPs into a PDMS matrix led to the increase of surface
area. Gupta and
Kulkarni
49
have used similar composite foams for the removal of solvents
such as BTEX (benzene, toluene, ethylbenzene, and xylene) and oil spills. They also showed
the decolorization of garlic extract by this composite, which demonstrates promise as a food
packaging material. Regeneration of composite was done by simple heat treatment and it
works for several cycles. Other organic compounds, such as phenols and anilines, are being
used mainly as disinfectants, antiseptics, and intermediates of dyes in organic synthesis. The
“Integrated wastewater discharge standard” of the Ministry of Environmental Protection
of China has set the maximum levels of aniline and phenolic compounds in discharged
wastewater as <1.0 and 0.3 mg/L, respectively. Chen et al
.
50
used β-CD-modiied Ag NPs for
sensitive and selective recognition of isomers of substituted anilines (
o
-,
m
-,
p
-phenylenedi-
amine) and phenols (pyrocatechin, hydroquinone, and resorcinol). There were color changes
when pyrocatechin, resorcinol, or hydroquinone was added to the β-CD-modiied AgNP
solutions, from apricot to yellow, dark brown, and red, respectively. These color changes
were due to the different degrees of aggregation of Ag NPs, which, in turn, was due to the
varying association constants between an aromatic isomer and β-CD. Binding strengths of
β-CD with the above phenolic compounds decrease as follows: resorcinol > hydroquinone >
pyrocatechin > phenol. The main reason for the above order is match (which follows the
above order) of size of the guest, phenol isomer, and the pore volume of the host, β-CD. In
the case of phenylenediamine (
o
,
m
, and
p
) isomers, more obvious color changes were seen.
The color changes were due to the varying association constants with the guest and the hosts
along with coulombic interactions between amino group and Ag NPs.
26.2.4 Biological Contamination/Sensing
Silver and copper were found to exhibit antimicrobial properties. As discussed in Section
26.1, in ancient times, people stored drinking water in utensils made of copper/silver.
Nanosilver is more eficient as an antibacterial agent compared with bulk Ag due to the
large surface area. The exact mechanism of antibacterial activity of Ag NPs is not known.
Researchers propose the uptake of Ag NPs by bacteria, which leads to the leaching of
Ag
+
ions from particles under such conditions as found inside bacteria. As a result, there
is damage to DNA/cell lyses leading to the death of the bacteria. Using luorescence of
clusters and SPR peak shifts and color changes of NPs, several biological molecules were
detected. A few of these are discussed below.
Dankovich and Gray
51
demonstrated the practical use of silver NPs in an antibacterial
study (Figure 26.7a). In this experiment, silver NPs were incorporated into the blotting
paper sheet. Water containing pathogenic bacteria,
Escherichia coli
and
Enterococcus faecalis
,
was percolated through the blotting paper, and the log reduction values in the efluent
were found to be about log 6 and log 3, respectively. Bacteria were found to be dead in the
collected water. The quantity of silver that was leached from the paper was estimated to be
<0.1 ppm (the current US EPA and WHO limits for silver in drinking water). This particu-
lar design demonstrates the effectiveness of silver NPs for emergency water puriication.
