Environmental Engineering Reference
In-Depth Information
ZnO. This result is consistent with that obtained by Anandan et al. [69], who found that the degradation rate of Tcp over lan-
thanum (la)-doped ZnO is much higher than that of pure ZnO and TiO
2
. The photocatalytic degradation rate of methyl orange
dyes on nanosized coupled oxides (ZnO/SnO
2
) is also faster than that on ZnO by about 66% [70]. The former is higher because
modified ZnO photocatalysts have higher charge separation efficiency. Sobana et al. [41] modified ZnO with Ac to enhance the
photocatalytic activity of pure ZnO. Direct blue 53 dye molecules were initially adsorbed on Ac, and then diffused to ZnO,
which was deposited within the Ac pores and degraded. As mentioned earlier, alkaline pH enhances photocatalytic degradation
compared with acidic pH. Therefore, ZnO can be modified with metals or metal oxides to enhance its efficiency. Modification
reduces the band-gap energy and improves charge separation between photogenerated electrons and holes, thereby enhancing
the efficiency of ZnO in photocatalytically degrading contaminants.
17.2.3
Water remediation by nanodisinfection agents via disinfection
conventional chlorination, ozonation, advanced filtration, and UV radiation processes are effective water disinfection tech-
niques. However, these techniques have several limitations, including the formation of toxic by-products and high energy con-
sumption [71]. Therefore, new methods for water disinfection, such as nanotechnology, that can overcome such limitations are
being explored. The photocatalysis of heterogeneous nanosized particles, such as TiO
2
and metallic Ag nanoparticles, degrades
hazardous contaminants in water, as discussed in previous sections. Moreover, these nanoparticles kill bacteria and inactivate
various harmful microorganisms [18-20, 72]. These nanoparticles can do so because their small size and high surface to volume
ratio enable them to interact closely and directly with pathogens [73].
Ag nanoparticles are effective bactericides, exhibiting strong toxicity against various microorganisms [20]. Morones et al.
[73] proposed the general route of bio-Ag
0
against
Escherichia coli
,
Pseudomonas aeruginosa
,
Vibrio cholerae
, and
Salmonella
typhi
. The bactericidal process begins by disrupting the functions of the bacteria, such as respiration, after the nanoparticles
attach to the bacteria cell membrane. The penetration of nanoparticles into the bacteria damages the DnA, a major sulfur- and
phosphorus-containing compound. Ag ions (Ag
+
) also contribute to such bactericidal effect. The bactericidal properties of the
nanoparticles are size- and surface area-dependent, and only nanoparticles of about 1-10 nm interact well with Gram-negative
bacteria. Figure 17.3 illustrates the pathway of Ag
+
ions against bacteria.
According to lalueza et al. [74], the bactericidal effect of Ag nanoparticles is weak because of the slow generation rate of
Ag
+
ions resulting from the small number of Ag nanoparticles. Thus, the number of Ag
+
ions should be sufficient to ensure full
interaction with thiol groups in proteins and interfere with DnA replication in bacteria as one disinfection pathway [75]. The
bactericidal effects with the same total silver content are ranked as follows: AgnO
3
> Ag-ZSM-5 > Ag
2
O > pellets > granular >
Ag nanoparticles. Thus, bactericidal efficiency largely depends on the easy release of Ag
+
ions from the material to yield high
silver concentration. De Gusseme et al. [20] found that biologically produced bio-Ag
0
exhibits a faster inactivation rate of
Enterobacteraerogenes
-infecting bacteriophage (UZ1) than AgnO
3
and chemically produced bio-Ag
0
with the same Ag
Ag
+
Ag
+
DNA
FigUre 17.3
pathway of Ag
+
ions against bacteria.
