Environmental Engineering Reference
In-Depth Information
ultimate catalytic activity. For example, it has been demonstrated that the activity of nanometer anatase TiO
2
powder is better
than that of ordinary anatase TiO
2
powder in the sonocatalytic degradation of methyl parathion [101]. in addition, the degrada-
tion of the methyl orange in the presence of nanometer anatase and rutile TiO
2
powders was compared. According to Wang
et al., the sonocatalytic activity of the nanometer anatase TiO
2
powder is obviously better than that of the nanometer rutile TiO
2
powder [102], while previous research works showed that the sonocatalytic activity of ordinary rutile TiO
2
powder was remark-
ably higher than that of the ordinary anatase TiO
2
powder [103]. The sonocatalytic degradation mechanisms were found to be
different in nanometer anatase TiO
2
and nanometer rutile TiO
2
. Organic compounds undergo direct oxidation by holes on the
surface of nanometer anatase TiO
2
particles. Thus, the surface adsorption of the organic compounds increase as the size of ana-
tase TiO
2
particles decreases [104]. However, the holes on the surface of nanometer rutile TiO
2
particles first react with water
molecules and then produce the
·
OH radicals. That is, the holes indirectly degrade the organic pollutants through
·
OH radicals
[105]. The effect of ultrasonic power on the degradation of organic compounds using anatase and rutile nanometer TiO
2
cata-
lysts is also different because of these different degradation mechanisms. in general, in the case where onefold ultrasonic irra-
diation is used in the presence of a rutile TiO
2
catalyst, the degradation of organic compounds gradually increases as the output
power increases due to the increased
·
OH generation. Contrarily, the degradation ratio is in inverse proportion to the output
power of ultrasound in the presence of an anatase TiO
2
catalyst, which is a consequence of destruction of anatase TiO
2
structure
and desorption of organic compounds on the surface of nanoparticles [102].
The differences in the crystal structures between nanometer anatase and rutile TiO
2
particles caused the different reusability
behaviors of the catalysts. it was found that the degradation of organic compounds in the presence of reused anatase TiO
2
nanoparticles declines gradually, but the sonocatalytic activity of reused rutile TiO
2
nanoparticles is hardly reduced compared
with the fresh one. As mentioned earlier, the metastable structure of anatase TiO
2
particles changes under ultrasonic irradiation,
while the nanometer rutile TiO
2
particles possess a stable crystal structure [102].
recent research attention is focused on the improvement of the catalytic activity of TiO
2
catalysts. in the past few decades,
doping with various transitions, anchoring TiO
2
particles onto materials with a large surface area, such as carbon-based mate-
rials [106], and using TiO
2
nanotubes [107] or metal-doped TiO
2
nanotubes [108] have all been used to improve the sonocata-
lytic activity of TiO
2
(Table 24.3).
24.6.2.2 Sonochemical Degradation of Organic Compounds by Other Nanocatalysts
Among nanoparticles, nanosized
ZnO as a sonocatalyst has been widely used for its high efficiency, nontoxic nature, and low cost. Previously, the sonocatalytic
degradation of some organic pollutants in aqueous solution using nanosized ZnO powder has been reported [109] (Table 24.3).
it has been found that some inorganic oxidants such as KClO
4
, KClO
3
, and Ca(ClO)
2
can effectively assist sonocatalytic degra-
dation in the presence of nanosized ZnO powder [109b].
Sonocatalysis using magnetic nanoparticles, such as Zvi, also enhances the overall degradation of the organic compounds
[110]. The increase in efficiency is related to factors such as enhanced mass transfer rates, enlarged surface area of Fe
0
due to
the shockwave created by cavitational collapse, which may cause direct erosion on the particle's surface and deaggregation of
particles to hinder agglomeration, and ultrasonic cleaning and decontamination of surfaces by the shockwave that remove the
precipitation of iron oxides/hydroxides on the iron surface. Additionally, the degradation also rises via Fenton's reaction
between Fe(ii) that forms on the iron and ultrasonically produced H
2
O
2
[111].
Table 24.3 [100, 102, 105-110, 112] summarizes reported findings on ultrasonic conditions, important conclusions, and the
performance of catalysts in the enhancement of the degradation of organic compounds.
24.6.3
combination of sonocatalytic degradation with other technologies
it is very difficult to treat non- or low-transparent wastewaters using the photocatalytic degradation method due to the low pen-
etrating ability of any kind of light source, whereas the penetrating ability of ultrasound is very strong through any water
medium [113]. The simultaneous use of sonocatalytic and photocatalytic degradation is called sonophotocatalytic degradation.
in comparison to photolysis, sonolysis, and sono-photolysis, sonophotocatalytic degradation generally occurs faster than during
the respective individual processes. Sonolysis might increase the photocatalytic reaction rate by increasing the amounts of
hydroxyl radicals, enhancing the mass transfer of organics between the liquid phase and the catalyst surface, and increasing the
surface area of photocatalyst particles and thus the catalytic activity of the semiconductor catalyst [114].
ultrasound is also used to enhance the efficiency of photocatalytic-assisted electrochemical oxidation and the sono-
photoelectrocatalysis process [115].
The combination of ultrasound with the adsorption process was found to be more promising in the elimination of organic
pollutants. ultrasound not only promoted desorption but also enhanced the rate of mass transfer in the sorption process and
increased/facilitated pore diffusion [116].
