Chemistry Reference
In-Depth Information
The use of ultrasound for the destruction of dilute
aqueous solutions of low-molecular-weight organic
compounds (alcohols, ketones and aldehydes) at
ambient temperature has been reported. An aqueous
flow cell system was assembled to measure the
oxidatively degraded formate and acetate products
by on-line ion chromatography [111].
Ultrasonic irradiation of aqueous solutions results
in the formation of free radicals due to the homoge-
neous sonolysis of water. These radicals attack and
degrade numerous organic compounds. A study of
sonochemical treatment of chlorinated hydrocarbons
in water demonstrated the homogeneous destruc-
tion of CH
2
Cl
2
, CCl
4
, MeCCl
3
and ClCH : CCl
2
in solu-
tion at concentrations of 100-1000 ppm by volume
[112]. The method appears to be quite powerful
potentially for the purification of contaminated
water. In a separate study, when saturated aqueous
solutions of CH
2
Cl
4
(110 ppm) and MeCCl
3
(1300
ppm) were sonicated for 20 min some 75% of the
contaminant was degraded [113].
The sonochemical removal of chlorinated aromatic
compounds from water is attracting considerable
attention. By using phenol itself as a model substrate,
Berlan
et al
. have shown that degradation takes place
via sequential oxidation and the intermediate for-
mation of hydroquinone and catechol [114]. The
final products of the degradation at 541 kHz are low-
molecular-weight carboxylic acids. The process for
the degradation of phenol (100 mg l
-1
) requires long
reaction times of 1-3 h, depending on the entrained
gas used.
The degradation of a number of 2-, 3- and 4-
chlorophenols has been examined under pulsed
sonolytic conditions (20 kHz, power = 50Wcm
-2
) in
air-equilibrated aqueous media [115]. These phenols
are totally transformed to dechlorinated and hyd-
roxylated intermediate products via first-order
kinetics in a time of 10-15 h. The process involves
hydroxyl radical attack and at low concentrations
of chlorophenol the reaction takes place in the bulk
solution, whereas at the higher concentrations
the reaction occurs predominantly at the gas bubble/
liquid interface.
Up to a few years ago the majority of sonochem-
istry research used a narrow range of frequencies
(generally 40 kHz using a bath and 20 kHz for a
probe). Only a few studies have been performed at
higher frequencies (around 1 MHz) [116]. Recently
it has been shown that the insonation frequency
Table 16.5
Comparison between H
2
O
2
production and the
rates of phenol and carbon tetrachloride disappearance at
different frequencies (
m
Mmin
-
1
)
Frequency (kHz)
20
200
500
800
H
2
O
2
formation
0.7
5.0
2.1
1.4
Phenol degradation
0.5
4.9
1.9
1.0
CCl
4
degradation
19
33
37
50
employed affects radical production, with higher
frequencies generating more radicals for the same
power input [14,117,118].
The question as to where the sonochemical degra-
dation of chemicals occurs with relation to the
collapsing bubble has been investigated. In a com-
parative study of the decomposition of phenol (to
carboxylic acids) and carbon tetrachloride (to CO
2
and Cl
-
) in water saturated with oxygen at different
frequencies (Table 16.5) [119], the results clearly
show a difference in the behaviour of the chemical
contaminants (original concentration 10
-3
M), with
the phenol degradation mirroring the peroxide for-
mation, indicating that this reaction is proceeding
at the bubble interface or outside of the bubble.
The volatile CCl
4
, however, is decomposed within
the bubble and increased frequency slightly acceler-
ates the process. An intriguing calculation for these
reactions shows that the efficiency for each of these
processes over one ultrasonic cycle decreases as the
frequency increases.
The occurrence of an optimum frequency at
200 kHz was explained through a two-step reaction
pathway. In the first step, water sonolysis produces
radicals within the bubble. In the second step the
radicals must migrate to the bubble interface or into
the bulk aqueous medium to form peroxide or react
with the phenolic substrate. The authors suggest that
the lower frequencies are the most efficient for the
decomposition of molecules inside the bubble, but
a proportion of the radicals recombine inside the
bubble at high temperature to form water, thereby
reducing the overall yield of H
2
O
2
(Equations 16.1
and 16.2):
2HO
•
1
/
2
O
2
+
ÆH
2
O +
(16.1)
HO
•
+ HOO
•
Æ H
2
O + O
2
(16.2)
