Environmental Engineering Reference
In-Depth Information
hematite [96]. The preparation process probably plays an important role in increasing or decreasing the PeCh activity of Fe 2 O 3 /
TiO 2 composite electrodes [85g]. researchers have shown that nanoscale zero-valent iron (nZvi) can be implicated as a hetero-
geneous PeF-like catalyst for the removal of environmental contaminants [85e].
in Table  24.2, we outline additional examples of reported environmental pollutants and their respective nanocatalysts in
heterogeneous Fenton-like systems.
24.6
soNocatalytic processes
24.6.1
introduction
Since the past few decades, there has been considerable interest in the use of ultrasound for the treatment of water and waste-
water [97]. Nevertheless, sonolysis alone is not generally considered to be attractive for application in large-scale treatment
processes because for many organic pollutants the sonochemical degradation rate is very low, and for some stable and compli-
cated organic pollutants, incomplete degradation may occur. moreover, it requires costly equipment and consumes a high
amount of energy. in order to solve these problems, a method using ultrasonic irradiation coupled with Fe 2+ , H 2 O 2 , and Fenton
reagent has been attempted. much effort has also been devoted to heterogeneous sonochemical degradation, namely, sonocata-
lytic degradation, in the presence of various types of catalysts such as metal oxides.
it is presumed that the efficiency achieved using ultrasound irradiation is due to the cavitation phenomenon. An ultrasonic
wave is a pressure wave with alternate compression and rarefaction, which is able to break the intermolecular forces maintain-
ing the cohesion of the liquid and produce a cavity in the rarefaction section of the wave. The chemical and physical effects of
ultrasound derive primarily from acoustic cavitation, which includes formation, growth, and collapse of the cavity. Bubble
collapse in liquids results in an enormous concentration of energy from the conversion of kinetic energy of liquid motion into
heating of the contents of the bubble [98]. Water sonolysis will induce the splitting of water molecules into free radical species
(H · , HO · ) in a cavitation bubble. in the absence of any solutes, these primary radicals could recombine to form H 2 O or H 2 O 2 and
then be released into the bulk solution (eqs. 24.4-24.7).
ultrasonic waves
2
HO
+
OH
H
(24.4)
OH HHO
+→ 2
(24.5)
OH
+ → 22
OH HO
(24.6)
OH HO O
2
→+
(24.7)
2
The presence of dissolved oxygen is reported to improve sonochemical reactions. if the solution is saturated with oxygen,
additional radicals are produced in the gas phase (eqs. 24.8-24.11) [99]:
ultrasonic waves
O
2
O
(24.8)
2
OHO H
+ →
2
2
(24.9)
OHOOH
2 +→
(24.10)
OOH HO O
2
→+
(24.11)
22 2
The two proposed sonodegradation mechanisms are pyrolysis and hydroxyl radical generation in the cavitations bubble,
which subsequently oxidize the organic compounds [100].
24.6.2
heterogeneous sonocatalytic degradation
24.6.2.1 Sonochemical Degradation of Organic Compounds by TiO 2 -Based Catalysts Sonodegradation efficiency can
be  improved by adding a catalyst. The amount of heterogeneous catalyst plays a crucial role in the rates of sonochemical
degradation reactions. The presence of an excessive amount of catalyst could result in the scattering of ultrasound waves that
may decrease the rates of reactions. However, an appropriate amount of catalyst promotes the formation of free radicals, thereby
increasing the rate of degradation of the organic compound. The most common catalyst under ultrasonic irradiation of an
organic compound is TiO 2 . it is now well known that the particle size and crystalline phase of TiO 2 powders determine the
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