ENERGY, RESOURCES AND ENVIRONMENTAL TECHNOLOGY
Chinese Journal of Chemical Engineering, 20(4) 754—759 (2012)
Removal of Organic Matter and Ammonia Nitrogen in
Azodicarbonamide Wastewater by a Combination of Power
Ultrasound Radiation and Hydrogen Peroxide*
LI Wenjun (李文军)1,2, WU Di (吴笛)1, SHI Xin (石鑫)1, WEN Lixiong (文利雄)1,** and
SHAO Lei (邵磊)1
1
2
State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing
100029, China
Chemical Industry Productivity Promotion Center of China, Beijing 100723, China
Abstract A simple and efficient sonochemical method was developed for the degradation of organic matter and
ammonia nitrogen in azodicarbonamide wastewater. The effects of initial pH, ultrasound format and peripheral water
level on the sonolysis of hydrazine, urea, COD and ammonia nitrogen were investigated. It is found that the initial
pH has a significant influence on the degradation of hydrazine and ammonia nitrogen, whereas this impact to urea is
relatively small. It also shows that a noticeable enhancement of ammonia nitrogen removal could be achieved in a
proper intermittent ultrasound operation mode, i.e., 1/1 min on/off mode. The height difference between the peripheral water level and the inner water level of the flask affects the efficiency of ultrasonic treatment as well.
Keywords azodicarbonamide wastewater, organic matter, ammonia nitrogen, ultrasound radiation, hydrogen peroxide
1
INTRODUCTION
The last twenty years has witnessed a swiftly deteriorating water environment as the result of rapid
development of industry and extravagant use of complex
organic compounds, in which the eutrophication problem caused by the discharge of ammonia-containing
water, especially in lakes and touring areas, is one of
the major concerns. The removal of hazardous substances from industrial waste streams containing ammonia nitrogen becomes a big challenge to the related
industries, and effective and economical treatment
technologies to remove ammonia-nitrogen are highly
demanded. Biological processes, air stripping and
breakpoint chlorination methods are well developed
and have been applied to remove ammonia from some
industrial or domestic wastewaters [1]. In the azodicarbonamide (ADC) industry, air/steam stripping technology is widely used for treating the wastewater that
contains ammonia nitrogen at remarkably high concentration. However, it is only effective for removing
the inorganic ammonia nitrogen, while not sufficiently
workable on decomposing the organic wastes including hydrazine, urea and COD introduced into the
wastewater in the production process. To remove such
organic components, a pretreatment step is usually
applied before air/steam stripping. Different oxidation
methods, including supercritical water oxidation and
catalytic wet oxidation, and some biological processes
are normally used for treating organic hazards [1-3].
However, new pretreatment methods are needed for
treating ADC ammonia nitrogen wastewater, due to
the complicated organic species and inorganic ammonia
nitrogen at remarkably high concentration.
Ultrasound has been proven to be a very useful
tool in enhancing the reaction rates in a variety of reaction systems and is widely used to degrade a number of water pollutants [4-6]. Ultrasound is one of advanced oxidation processes (AOPs), which are defined
as those technologies that utilize the hydroxyl radical
(•OH) for oxidation with extremely low selectivity of
contaminants. In an aqueous liquid, the chemical effects
of ultrasound result from the almost adiabatic collapse
of cavitation bubbles. Cavitation bubble collapse produces extremely high local temperatures, as high as
4000 K and pressures over 101.325 MPa, resulting in
high-energy chemical reactions [7, 8]. This phenomenon
is capable of initiating and promoting chemical reactivity through thermolysis, supercritical water oxidation, and free radical oxidation [9]. Thermolysis can
take place in the bubble cavity and interfacial layer
surrounding the cavity and is the core mechanism for
generating free radicals. Furthermore, sonication improves mass transfer and chemical reaction and is expected to reduce or eliminate chemical usage, resulting in minimal sludge and disposal problems [10].
A number of studies have been published for the
sonochemical degradation of individual organic compounds. However, there have been few reports about
the sonochemical treatment on heavily contaminated
wastewater such as ADC wastewater and the removal
of dual organic matters as well as inorganic ammonia
simultaneously by ultrasound irradiation.
The objective of this paper is to investigate the
removal efficiency of urea, hydrazine and ammonia
nitrogen by ultrasound radiation, and to enhance the
Received 2011-11-28, accepted 2012-06-18.
* Supported by the National Natural Science Foundation of China (21121064, 20990224) and National Science and Technology
Ministry of China (2008BAE64B02).
** To whom correspondence should be addressed. E-mail: [email protected]
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Chin. J. Chem. Eng., Vol. 20, No. 4, August 2012
oxidation effects by combining ultrasound with hydrogen peroxide. The influence of initial pH, ultrasound
format and peripheral water level on the oxidation
processes is examined and the oxidation mechanism is
explored.
2
2.1
EXPERIMENTAL
Materials and chemicals
The effluent of ADC industry wastewater typically
contains inorganic ammonia nitrogen at high concentration and two organic ammonia nitrogen contaminants,
urea and hydrazine, which were chosen as model organic ammonia pollutants in our study. Urea, hydrazine
(volume fraction, 80%), hydrogen peroxide (volume
fraction, 30%), potassium iodide, sodium sulfite, ammonium sulfate, sodium hydroxide, sulfuric acid, biurea, and p-dimethylaminobenzaldehyde (PDAB) were
of analytical grade reagents and purchased from commercial source. All solutions were prepared with deionized water.
2.2
Experimental set-up
The schematic diagram of the ultrasound reactor
is shown in Fig. 1. The ultrasonic source is a KQ-100B
ultrasound generator (KUNSHAN Ultrasonic Equipment Co., Ltd.) with two planar transducers under the
rectangle water tank of 4 L capacity. The ultrasound
reactor can generate a maximum output power of 100
W at the frequency of 40 kHz. The temperature of
ultrasound bath can be maintained from room temperature to 80 °C and controlled by circulating cooling
water. The inner shell of the ultrasound reactor is covered by stainless steel in order to resist corrosion.
Figure 1 Scheme of ultrasound experimental setup
1—radiation time control; 2—ultrasonic transducer; 3—water
tank; 4—ADC wastewater; 5—temperature control
2.3 Wastewater treating procedures and concentration measurement
All experiments were carried out in a 100 ml
glass conical flask containing 50 ml wastewater with
flat bottom and glass plug, positioned right upon one
ultrasound transducer of the water tank and partially
emerged in the water tank, as illustrated in Fig. 1. The
height of the inner wastewater in the flask was h and
the height of the surrounding water in the water tank
was H. The temperature for all experiments was 60 °C.
The conical flask was filled with simulated ammonia
nitrogen wastewater containing urea and hydrazine at
specific initial concentrations, which are close to the
industrial wastewater from ADC plant. The urea and
hydrazine solution was adjusted to the required pH
with 2 mol·L−1 HCl or NaOH solution. COD and ammonia nitrogen concentrations of the sonicated sample
were measured by a 5B-3(B) analyzer (Lian-Hua Tech
Co., Ltd.) based on the K2Cr2O7 and Nessler’s reagent
method, respectively. Urea and hydrazine concentrations were measured by a UV-2550 UV-vis spectrophotometer (Shimadzu Co., Ltd. Japan) at the maximum
wavelength (λmax). pH was measured by a pHS-3C pH
meter (Shanghai Leici Instrument Factory, China).
3
RESULTS AND DISCUSSION
3.1 Determination of actual ultrasonic energy
dissipated into wastewater
When a liquid is exposed to an acoustic field, the
ultrasonic energy (high frequency sound waves) produces an alternating adiabatic compression and rarefaction of the liquid medium being irradiated [11]. The
chemical effects of sonochemistry of ultrasound are
due to the phenomenon of acoustic cavitation, which
involves the formation and subsequent collapse of
micro-bubbles from the acoustical wave induced
compression/rarefaction. The ultrasonic power or intensity has been considered as an important factor. It is
generally believed that ultrasound intensity of 0.3-10
W·cm−2 could generate noticeable cavitation bubbles
in liquid medium [12]. In many papers, only the input
power was given as a measure of the ultrasonic power.
However, the input power is not always instructive to
be a sign of ultrasonic power for sonochemistry, because the energy conversion of ultrasound is primarily
transducer-dependent. In addition, when an ultrasound
wave travels through the liquid medium, wave energy
is absorbed by the medium and converted into the
form of heat, which causes the energy loss between
the actual power input (Pin) and energy output quoted
by the manufacture (Pout). Measurement of sound
pressure with a hydrophone is useful to determine the
distribution of ultrasonic intensity in a reaction vessel.
However, it is not suitable to specify the mean ultrasonic power dissipated into the reaction system, because
the sound field is not distributed uniformly. One of the
most prevailing methods of measuring Pin is based on
calorimetry and assumed that the energy entering the
water tank is dissipated into solutions as heat [13, 14].
The ultrasonic power dissipated into the treated
wastewater is calculated by the following equation:
Pin =
dT
dt
∑ mi C pi
(1)
where mi and Cpi are the mass and heat capacity of the
materials present in the ammonia-nitrogen wastewater,
respectively, and dT/dt is the initial slope of the graph
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Chin. J. Chem. Eng., Vol. 20, No. 4, August 2012
of temperature versus time in the ultrasonic bath, as
shown in Fig. 2. In this work, the sample volume of
the wastewater was 50 cm3. The temperature rise was
measured at room temperature by using a thermocouple, which was immersed and held at the half height of
the solution. The actual ultrasonic power dissipated in
the wastewater was determined by adopting the procedure reported by Hagenson and Doraiswamy [15].
On basis of Eq. (1), the energy actually input to the
water bath was about 68.7 W, though the maximum
available ultrasonic output power quoted from the
manufacturer was 100 W. The ultrasound intensity, based
on the actual energy input, reached 0.359 W·cm−2 in
our wastewater solution, which was high enough to
cause cavitation effect in the solution [12]. Due to the
limit of the ultrasound generator, the effects of ultrasound intensity on the waste removal were not examined in this work, but the influence of treating time
was explored.
Table 1 Synergetic effects of ultrasonic and hydrogen
peroxide on the removal of hydrazine hydrate (H = 1.5 h;
ultrasonic: 60 °C, 40 kHz, 100 W; initial N2H4·H2O concentration:
20 mg·L−1; treating time: 90 min; amount of 30% H2O2: 1 ml)
N2H4·H2O sonolysis rate/%
Initial pH
H2O2 only
3
11
Ultrasonic only
Ultra-H2O2
30.4
28.8
35.2
38.7
34.6
60.3
O2 ⎯⎯
→O+ O
(3)
O + H2O ⎯⎯
→ •OH + •OH
(4)
→ H2O2 + O2
(5)
•O2H + •O2H ⎯⎯
In the presence of oxygen acting as a scavenger of
hydrogen atom (and thus suppressing the recombination of •H and •OH), the hydroperoxyl radical (•HO2)
is formed additionally, which is an oxidizing agent.
Therefore, the addition of hydrogen peroxide not only
improves the oxidation ability of the overall homogenous ultrasonic system, but also favors the cavitation
event to maximize chemical reaction yields and/or
reaction rates by providing excessive nuclei in order to
lower the cavitation threshold and speed up the initialization of cavity formation [18], hence strengthening the sonolysis reduction.
3.3 Effects of initial pH on the removal of organic
waste
Figure 2 Temperature rise in ultrasonic bath with ultrasound assistance (H = 1.5 h; ultrasonic: 40 kHz, 100 W)
3.2 Effects of adding hydrogen peroxide on the
sonolysis reduction of organic waste
Since intense oxidation-reduction reactions occur
between hydrazine and hydrogen peroxide, the synergetic effect of ultrasonic and hydrogen peroxide on the
removal rate of hydrazine hydrate for ADC wastewater was investigated first in this work, as shown in
Table 1. It demonstrates that the introduction of hydrogen peroxide facilitates sonolysis reduction of hydrazine hydrate, especially when the initial pH is high,
which proves the combinative effect of ultrasonic and
hydrogen peroxide. The addition of hydrogen peroxide can generate oxygen via its own decomposition
reaction, particularly in basic environments. If the
solution is saturated with oxygen, the dissolved oxygen may act as nucleation sites for acoustic cavitation.
Moreover, peroxyl and more hydroxyl radicals may be
created in the gas phase (upon the decomposition of
molecular oxygen), and the recombination of the former at the cooler sites (interface or the solution bulk)
produces additional hydrogen peroxide, as shown in
Eqs. (2)-(5) [16, 17]:
→ •O2H
O2 + •H ⎯⎯
(2)
The effects of initial pH in the wastewater on the
sonolysis reduction of urea and hydrazine with the
help of H2O2 are shown in Figs. 3 and 4. The initial
pH has different influences on the removal efficiency
in urea and hydrazine solutions. The hydrazine reduction efficiency is enhanced significantly at a basic environment as compared to acidic or neutral environment, while the effects of initial pH on urea decomposition are much less appreciable. This large difference
is due to the different properties between hydrazine and
urea. After 90 min ultrasound irradiation, hydrazine
removal efficiency achieves 60.3% at initial pH = 11.0,
while only 35.2% at initial pH = 3.0. The chemical
property of hydrazine is quite close to that of ammonia of high volatility and its solution is basic, and the
fraction in the molecular state of hydrazine is larger at
higher pH. Under neutral and acidic conditions, the
hydrazine ions can not vaporize into the cavitation
bubbles, so the major degradation is mainly achieved
by •OH and •H free radicals in the interfacial sheath
between the gaseous bubble, surrounding liquid and
bulk of solution. However, in the molecular state,
gaseous hydrazine can vaporize and migrate into cavitation bubbles and degrade inside by pyrolytic reactions and radical reactions simultaneously. According
to the sonochemistry theory, the efficiency of thermal
cleavage is normally much superior to radical reaction.
Therefore, the sonolysis reduction of hydrazine is
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Chin. J. Chem. Eng., Vol. 20, No. 4, August 2012
Figure 3 Effects of initial pH on the decomposition of
hydrazine within 90 min (H = 1.5 h; ultrasonic: 60 °C, 40
kHz, 100 W; initial N2H4·H2O concentration: 20 mg·L−1;
amount of 30% H2O2: 1 ml)
■ pH = 3; ● pH = 7; ▲ pH = 11
Figure 4 Effects of initial pH on the decomposition of
urea within 90 min (H = 1.5 h; ultrasonic: 60 °C, 40 kHz,
100 W; initial urea concentration: 5 mg·ml−1; amount of 30%
H2O2: 1 ml)
■ pH = 3; ● pH = 7; ▲ pH = 11
more effective at higher pH. However, the reduction
efficiency of urea solution is all around 25% with
relatively minute variation at all three initial pH values,
as shown in Fig. 4. This may be because that urea remains molecular state at these initial pH values and its
solution is non-volatile. During ultrasonic irradiation,
it tends to accumulate within the aqueous phase and
cavitation bubble membrane, where it is degraded
predominately via hydroxyl radical reactions. Thus,
the initial pH has a minor influence on the sonolysis
rate of urea as compared to that of hydrazine.
The COD reduction efficiency by the combined
treatment of ultrasonic and hydrogen peroxide at different initial pH is shown in Fig. 5. The wastewater
samples contain urea, hydrazine and minute quantity
of biurea. The COD reduction efficiency increases with
increasing initial pH. The highest COD removal rate is
at initial pH = 11. After 90 min ultrasonic radiation, it
reaches a COD removal rate of 68.45%.
3.4 Effects of ultrasonic format on the removal of
organic waste
During sonolysis process using ultrasonic, some
Figure 5 COD reduction efficiency under the combined
treatment of ultrasonic and hydrogen peroxide (H = 1.5 h;
ultrasonic: 60 °C, 40 kHz, 100 W; initial COD: 1500 mg·L−1;
amount of 30% H2O2: 1 ml)
■ pH = 3; ● pH = 7; ▲ pH = 11; ▼ pH = 11, no H2O2
of the input ultrasonic energy is dissipated as heat to
decrease the ultrasonic efficiency. At the same time
the noises of ultrasonic would have negative effects on
operation surroundings. To improve the cavitation
efficiency and reduce the ultrasonic time, an operation
technique of intermittent ultrasonic was investigated
in this work. The ultrasonic generator worked at 5/5,
3/3 or 1/1 min of on/off alternative mode separately,
for a total treatment time of 90 min. Table 2 demonstrates the NH4-N removal rate difference between
continuous and intermittent ultrasonic radiation after
90 min. It is interesting to find that the 1/1 min intermittent ultrasonic operation achieves higher ammonianitrogen removal rate than continuous mode, though
the other intermittent modes with longer on/off switch
time does not show the same result. Therefore, a
proper intermittent ultrasonic operating format can not
only enhance the sonolysis reduction of the waste, but
also be more energy efficient and environment benign.
Table 2 Comparison of NH4-N removal rate in continuous
and intermittent ultrasonic radiation (H = 1.5 h; ultrasonic:
60 °C, 40 kHz, 100 W; initial NH4-N concentration:
2250 mg·L−1; amount of 30% H2O2: 1 ml)
NH4-N removal rate/%
Operating condition
(all combined with H2O2)
pH = 3
pH = 7
pH = 11
continuous radiation
43.6
28.5
85.6
1/1 min intermittent
46.7
31.5
90.4
3/3 min intermittent
37.5
22.5
71.4
5/5 min intermittent
25.8
18.8
36.9
The main and possible reason may involve the
change of the collapse of micro-bubbles in cavitation
and the enhancement of cavitation efficiency by intermittent ultrasound. The effective region of sonochemical reaction is spatially restricted when using
continuous ultrasound, because many degassing bubbles are formed under such conditions of sonication
[19]. These degassing bubbles are ineffective for
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Chin. J. Chem. Eng., Vol. 20, No. 4, August 2012
sonochemical reaction, due to their small volumetric
oscillation, and they restrict the propagation of sound
by absorption and spreading of the sound, leading to a
decrease in the sound pressure amplitude and limited
sonochemical reaction. An appropriate pulse-off time
that is specific to the ultrasound pulsing operation
suppresses the generation of degassing bubbles by the
coalescence of bubbles, promoting high amplitude of
sound propagation that is responsible for the expansion and contraction of the bubbles. Therefore, the
number of bubbles that are effective for sonochemical
reaction is increased [20]. This suggests that ammonianitrogen removal process in a proper intermittent ultrasound field would make more efficient use of ultrasound energy due to the higher cavitation efficiency
of intermittent ultrasound.
3.5 Effects of peripheral water level on the removal
of ammonia-nitrogen
Ultrasonic bath systems are widely used in
sonochemical research because they are readily available and relatively inexpensive. As shown in Fig. 1,
the glass conical flask is typically immersed in the
coupling fluid contained in the bath, which forms the
indirect sonication system. Kawase et al. [21] reported
that there is a strong relationship between the inner
liquid height and the ammonia removal rate. However,
there has been little reported about the effect of the
peripheral water level on the sonochemical efficiency.
To explore these effects on ammonia-nitrogen degradation efficiency, the sonochemical experiments were
conducted in an ultrasonic bath with different peripheral water levels, as shown in Fig. 1. The inner water
height h was fixed by adding 50 ml (NH4)2SO4 solution and 1 ml 30% H2O2 to the flask. Fig. 6 depicts the
removal percentage of ammonia-nitrogen with three
different peripheral water levels at initial pH = 7.3.
The best ammonia-nitrogen removal rate was obtained
when the peripheral water level was 1.5 times of inner
water height. Further increase of the peripheral water
level would lower the removal efficiency, while the
least removal efficiency occurred when the peripheral
water level was below the inner water height. A possible reason is that a relatively higher peripheral water
height would be beneficial to uniform energy distributions within both liquids in the flask and in the bath,
resulting in more efficient utilization of the input
ultrasound energy by the inner ammonia-nitrogen
sonolysis reaction system. However, too much excessive water in the ultrasonic bath would absorb more
ultrasound input energy, while insufficient water in the
bath might not be able to transfer the input ultrasound
energy to the liquid in the flask which was above the
peripheral water level, both reducing energy utilization
of the input ultrasound energy by the sonolysis reaction system within the flask. Therefore, an appropriate
height difference between the surrounding water level
and the inner water level of the flask is also an important
factor for optimizing the indirect sonication system.
4
CONCLUSIONS
The removal of organic matter and ammonia nitrogen in ADC wastewater by a combination of power
ultrasound radiation and hydrogen peroxide was investigated and the effects of reaction time, initial pH,
ultrasonic format and peripheral water level on the
COD reduction and ammonia removal efficiency were
explored. It is found that the initial solution pH has a
significant influence on the degradation of water contaminants. Higher initial pH is favorable to the sonolysis of hydrazine, whereas this effect on the degradation
of urea is relatively small. Adding hydrogen peroxide
to the sonolysis reaction can enhance the sonolysis
reduction of organic waste as well, especially at high
pH. In addition, a proper intermittent ultrasonic operating format and/or an appropriate height difference
between the surrounding water level and the inner
water level of the flask can further improve the utilization efficiency of the input ultrasound energy by the
inner ammonia-nitrogen sonolysis reaction system
within the flask, and therefore, boosting the sonolysis
reduction of the waste in ADC wastewater by ultrasonic treatment.
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Figure 6 Effects of peripheral water level on the removal
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H2O2: 1 ml)
■ H = 0.5 h; ● H = 1.5 h; ▲ H = 2 h
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