1
68
M.Q. Cai et al. / Ultrasonics Sonochemistry 22 (2015) 167–173
higher investment costs [34,35]. Moreover, the process of ultra-
sonic cavitation usually takes a long processing time in order to
achieve complete mineralization of pollutants.
The combination of ultrasound irradiation and other different
AOPs such as photocatalytic oxidation [36], electrochemical [37],
ozonation [38,39], and particularly advanced Fenton process and
Fenton like processes [40] has been reported to be much more effi-
cient and promising for the oxidative removal of organic pollutants
than the treatment alone. In the recent past, less attention has been
which is not suitable in practice [39]. As mentioned above, ultra-
sonic irradiation provides a straightforward method of using AlP
instead of ZVAl due to the chemical and physical effects of ultra-
sound irradiation. This prompted us first to study decolorization
of azo dye Orange G using AlP-UI. The results are expected to pro-
vide fundamental knowledge for the treatment of wastewater con-
taining hydrophilic pollutants via AlP-UI. The effects of various
operating operational parameters including the initial pH, initial
OG concentration, AlP dosage, ultrasound power and added H
concentration were studied.
2 2
O
paid to the reductive degradation using the zero-valent aluminum
0
(
ZVAl). The ZVAl is a strong reducing agent (E = ꢀ1.662 V) that has
a standard reduction potential more negative than zero-valent iron
2
. Experimental
0
(
E = ꢀ0.43 V) [41,42]. As well as the zero-valent iron, the degrada-
tion capable of ZVAl also rely on the amounts of in situ generation
of reactive oxygen species (ROS) including hydrogen peroxide
2.1. Chemicals and solvents
Åꢀ
Å
(H O ), superoxide radical (O ), and hydroxyl radical ( OH), which
2 2 2
Orange G and hydrogen peroxide were purchased from Huipu
Chemical&Apparatus Co. (Hangzhou, China). Aluminum powder
purity >99%, particle size 38–48 m, surface covered with native
are capable of oxidizing contaminants that cannot be well removed
by ZVAl reductively. The mechanism of the ZVAl to form hydroxyl
radical could be described as follows [43,44]:
(
l
aluminum oxide layer) were obtained from Kelon Chem. (Shang-
hai, China). Other reagents with analytical grade or high perfor-
mance liquid chromatography (HPLC) grade were obtained from
Huipu Chemical&Apparatus Co. (Hangzhou, China). The concentra-
0
þ 6Hþ ! 2Al3þ þ 3H
2
Al þ 3O
2
2
O
2
ð1Þ
0
! Al3þ þ 3 OH þ 3OH
Å
ꢀ
Al þ 3H
2
O
2
ð2Þ
2 2
tion of the purchased hydrogen peroxide (H O ) solution (30 wt.%)
First, it can be seen that the reduction of O
2
on zero valent alu-
was calibrated by titration with potassium permanganate [52]. All
chemicals were used as received. The water employed was sup-
plied by a Milli-Q water purification system from Millipore (Mols-
heim, France).
0
minum (Al ) in the acid conditions leads to form H
2
O
2
and subse-
Å
quently generate OH via Fenton like reactions (Eqs. (1) and (2)).
Surprisingly, to the best of our knowledge, no reports on the pollu-
tant degradation using the combination of ZVAl and ultrasonic irra-
diation (ZVAl–UI) can be found. As expected, the formed H
2
O
2
both
2.2. Experimental methodology
in the interface of gaseous bubble and in solution (Eqs. (3) and (4))
due to the pyrolysis of water can be utilized to produce more
amounts of hydroxyl radicals by reaction with ZVAl. Consequently,
the ZVAl–UI process should be a suitable method to increase the
efficiency of degradation process:
The decolorization of OG using the UI, AlP-acid and AlP-UI were
carried out under different conditions. An ultrasonic immersion-
probe liquid processor (TJS-3000, Fuyang Chenggong Ultrasonic
Co., China) with tapered type titanium probe (1.6 cm diameter)
operating at 20 kHz was used for the decolorization experiments
by the UI and AlP-UI. The ultrasonic electrical power was in the
range of 0–900 W. All reactions with a total suspension volume
of 250 mL for 180 min were carried out in 500 mL double walled-
type reactor and water was circulated between the walls in order
to keep the temperature constant at 25 ± 1 °C. Samples were col-
lected at designated time intervals and filtered using membrane
Å
Å
H
2
OþÞÞÞ ! OH þ H
ð3Þ
Å
Å
OH þ OH ! H
2 2
O
ð4Þ
Orange G (OG) is a typical reactive azo dye and extensively used
in the dyeing of textile fabrics. The chemical structure of OG is
shown in Fig. 1. The decolorization methods of OG in wastewater
including adsorption [45,46], sonolysis [47], biotransformation
filters (0.45 lm pore size). The concentration of OG was analyzed
[
48], UV/TiO
2
[49,50] and Fenton processes [51] had been reported.
by measuring the absorbance of dye solution at 478 nm using a
TU-1901 UV-spectrophotometer (Persee General Co., Beijing,
China). All experiments were repeated three times and the average
values are reported. The temperature of reaction systems was con-
tinuously monitored at about 25 ± 1 °C by a thermocouple con-
nected to a digital thermometer (DS18B20, Dallas Crop, USA).
Hydrogen production experiments of the reaction of aluminum
powder, sodium hydroxide and water were conducted to check the
peel effect of ultrasonic irradiation. The method and equipment
used to quantify hydrogen production yields were similar to the
reference [53]. In these experiments, the used aluminum powder
were washed thrice for 5 min in 0.1 mol/L of hydrochloric acid
However, there are no reports about the decolorization of OG via
the ZVAl under ultrasonic irradiation. In general, the surface of Al
is covered with oxide film due to oxidizability of ZVAl, and the
hydrochloric acid solution is usually used to remove the oxide film,
ONa
N
O
S O
N
(
HCl) in order to remove the surface oxidation layers, rinsed with
deionized water, and dried at 30 °C under an Ar atmosphere.
OH
2
.3. Detection technique
O
S
The OG decolorization products were determined by an Agilent
100 series LC/MSD Trap SL System (Agilent Technologies Inc., Ger-
NaO
1
many), consisting of a quaternary pump (G1311A), a column ther-
mostat (G1316A), a degasser unit (G1379A), an autosampler
O
(
G1313A), and an ion trap mass spectrometer with an electrospray
Fig. 1. Molecular structure of azo dye Orange G.
ion (ESI) source. The LC/MSD Trap SL system was controlled, and