Study on Photocatalytic Degradation of Unsym-Dimethylhydrazine
Wastewater by Bi
2
O
3
Thin Films
Jing Qi
*
, Zelong Xu, Yi Wu, Ziwen Hou, Rimei Cong and Tianhu Guo
China Jiuquan Satellite Launch Centre, Gansu Jiuquan 732750
Keywords: unsym-dimethylhydrazine (UDMH); Bi
2
O
3
; photocatalysis
Abstract: The Bi
2
O
3
thin films were prepared by sol-gel method for photocatalytic degradation of unsym-
dimethylhydrazine (UDMH) wastewater. The effects of initial concentration of UDMH, initial pH value of
solution and H
2
O
2
addition were investigated. Besides, the reaction kinetics and mechanism were studied.
The results showed that when the initial concentration of UDMH was between 100 mg/L and 500mg/L, the
concentration had little effect on the degradation efficiency; initial pH value of solution had noticeable
impact on the degradation of UDMH and higher removal efficiency can be achieved in neutral or weakly
alkaline condition; the addition of H
2
O
2
can effectively improve the degradation. The kinetic study showed
that the reaction kinetics were well fitted by the pseudo first-order rate model and Langmuir-Hinshelwood
model.
1 INTRODUCTION
Unsym-dimethylhydrazine (UDMH) is a high-
energy fuel and its wastewater is usually produced in
the engine test, propellant transfer and rail tank
cleaning process, which contains a variety of toxic
and hazardous substances (Liang et al., 2016). In
recent years, with the increase of spacecraft
launching, the use of UDMH has increased
significantly, and the pollution of UDMH
wastewater has attracted extensive attention (Angaji
and Ghiaee, 2015). Advanced oxidation processes
(AOPs) are a set of chemical treatment procedures
designed to remove organic materials in wastewater
by oxidation through reactions with hydroxyl
radicals (·OH). Photocatalytic oxidation is one of the
AOPs which began in the 1970s (Fujishima and
Honda, 1972). It has the unique advantages of high
efficiency, low cost and environmental friendliness,
and is considered to be an ideal wastewater
treatment method (Hoffmann et al., 1995). In recent
years, many researchers have used photocatalysts to
degrade UDMH wastewater to obtain satisfactory
results (Khalid et al., 2013; Bessekhouad et al.,
2005; Zhu et al., 2011).
Bi
2
O
3
is an effective and stable photocatalyst
with a narrow band gap (about 2.8 eV), many
oxygen vacancies, strong redox capability, and high
photocatalytic activity. Since 1988, Bi
2
O
3
was
introduced into the field of photocatalysis (Anthony
et al., 1988), many scholars have achieved good
results in treating organic wastewater with Bi
2
O
3
as
photocatalyst (Zhang et al., 2006). However, most of
the current researches focus on the suspension
photocatalysts (Li et al., 2013), which have the
disadvantages of easy agglomeration, difficult
separation, and low photon utilization. Therefore,
scholars turn to the study of supported Bi
2
O
3
film
(Fruth et al., 2005; Leontie et al., 2005; Qin et al.,
2014). In this study, the supported Bi
2
O
3
thin films
were prepared on the quartz glass substrate by sol-
gel method (Qin et al., 2014), and the photocatalytic
degradation of UDMH wastewater was studied.
2 EXPERIMENT
All chemicals were of reagent grade and used
without further purification. Distilled water was
used throughout this experiment.
2.1 Photocatalyst Preparation
The Bi
2
O
3
thin films were prepared according to the
following procedure: 5.0 g Bi (NO
3
)
3
·5H
2
O was
dissolved in 20.0 mL HNO
3
aqueous solution (HNO
3
38
Qi, J., Xu, Z., Wu, Y., Hou, Z., Cong, R. and Guo, T.
Study on Photocatalytic Degradation of Unsym-Dimethylhydrazine Wastewater by Bi2O3 Thin Films.
DOI: 10.5220/0008185100380041
In The Second International Conference on Materials Chemistry and Environmental Protection (MEEP 2018), pages 38-41
ISBN: 978-989-758-360-5
Copyright
c
 2019 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

3.3 mL, H
2
O 16.7 mL). Then 4.0 mL PEG200 and
2.0 g citric acid were added in the above solution,
and 3.5 mL t-Oct-C
6
H
4
-(OCH
2
CH
2
)
x
OH (x=9-10)
was added as surfactant. The above solution was
stirred for 10 h at room temperature. After that,
pieces of 50×100 mm
2
quartz glass were dipped into
the sol solution where they would be kept for 3 min,
and then withdrawn at a rate of 4.0 cm/min. All the
films were preliminarily treated at a heating rate of 1
°C/min to 390 °C, during the process of which two
30 min plateaus at 275 °C and 390 °C were
maintained, respectively. The films were then
annealed at 550 °C with a heating rate of 1 °C/min,
and at this temperature 2 h plateau was maintained.
2.2 Photocatalytic Experiments
A certain concentration of UDMH aqueous solution
(100 mg/L, 200 mg/L, 300 mg/L, 400 mg/L, 500
mg/L) was prepared as simulated wastewater. The
photocatalytic activity of Bi
2
O
3
was evaluated by
degrading UDMH wastewater under irradiation of
an UV-light source (Philips, 8 W, wavelength 253.7
nm). The UV-light irradiated perpendicularly to the
surfaces of the samples and the distance from the
UV-light source to the samples was 10 cm. The
ambient temperature was 20~25°C and the humidity
was 20~30%. 5 mL UDMH solution was taken
every 30 minutes and the concentration changes of
UDMH with the irradiating time were measured by
spectrophotometry (GB18063-2000).
3 RESULTS AND DISCUSSION
3.1 Initial Concentration of UDMH
The influence of initial concentration of UDMH on
photocatalytic degrading efficiency was studied and
the results were shown in Figure 1. It showed that
the removal efficiency of UDMH increased rapidly
with the extension of reaction time, and the removal
efficiency increased slowly after 60 minutes. It can
be easily concluded from the figure that there was no
obvious difference among the photocatalytic
degrading when the initial concentration of UDMH
was between 100 mg/L and 500 mg/L. The removal
efficiency of UDMH with different initial
concentrations were above 90% (120 min). It can be
seen from the above that the initial concentration of
UDMH has little effect on the degradation
efficiency.
Figure 1: Influence of initial concentration on removal
efficiency of UDMH.
3.2 Initial pH Value of the Solution
In order to investigate the degradation effect of
initial pH value on UDMH, H
2
SO
4
/NaOH was added
into the UDMH aqueous solution (300 mg/L) to
adjust the pH of the solution to 4, 6, 8, 10, and the
reaction time was 60 min. The results were shown in
Figure 2.
Figure 2: Influence of initial pH value on removal
efficiency of UDMH.
It can be inferred that the degradation efficiency
of UDMH increased first and then decreased with
the increase of pH value, and reached a maximum
when the pH value was about 8. This is mainly
related to the existence state of UDMH under acidic
or strongly alkaline conditions and the surface
electrical properties of the catalyst. UDMH is an
organic weak base which is mainly present as
(CH
3
)
2
NNH
3+
in acidic medium. At this time, the
surface of the catalyst is positively charged. Under
strong alkaline conditions, it is mainly in the
presence of (CH
3
)
2
NNH
2
OH
-
and the surface of the
catalyst is negative. So UDMH can not be adsorbed
effectively under acidic or strongly alkaline
Study on Photocatalytic Degradation of Unsym-Dimethylhydrazine Wastewater by Bi2O3 Thin Films
39

conditions. The ideal adsorption effect be achieved
in neutral or weakly alkaline condition.
3.3 Addition of H
2
O
2
The unequal amount of H
2
O
2
(30 wt%) was added to
the reaction system to make the concentration of
H
2
O
2
0.5, 1.0 and 1.5 g/L, respectively. The
influence of H
2
O
2
on removal efficiency of UDMH
were shown in Figure 3.
Figure 3: Influence of H
2
O
2
on removal efficiency of
UDMH with Bi
2
O
3
as photocatalyst.
Obviously the addition of H
2
O
2
can significantly
improve the photocatalytic degradation efficiency of
UDMH. To further understand the role of H
2
O
2
in
photocatalytic reaction, the blank photodegradation
experiments without Bi
2
O
3
were conducted and the
results were shown in Table 1.
Table 1: Influence of H
2
O
2
concentration on removals of
UDMH without B
2
O
3
(120 min).
H
2
O
2
concentration (g/L)
Removals (%)
0.5
32.5
1.0
38.7
1.5
36.5
It can be seen from Table 1 that the removal
efficiency of UDMH in the presence of H
2
O
2
without Bi
2
O
3
were less than 40%, which mainly
attributed to the strong oxidizing property of H
2
O
2
.
As a comparison, the UDMH removals in solutions
with different concentrations H
2
O
2
with Bi
2
O
3
as
photocatalyst exceeded 90% (120 min). When the
concentration of H
2
O
2
reached 1.0 g/L, the removal
efficiency of UDMH was 95% in 60 min and 100%
in 120 min. The significantly improved
photocatalytic efficiency can be attributed to the
electron scavenger role of H
2
O
2
. As an effective
electron trapping agent, H
2
O
2
can be act as electron
scavenger during the photocatalytic reaction which
can reduce the recombination rate of photogenerated
electrons/holes and prolong the effective life of
photogenerated holes.
3.4 Reaction Kinetics
To quantitatively understand the reaction kinetics of
UDMH degradation, the pseudo first-order kinetic
equation (equation 1) with a simplified Langmuir-
Hinshelwood model can be applied because of the
low reactant concentration.
ln(C
0
/C) = k
app
t
(1)
where C
0
and C are the reactant concentrations of
dyes in solution at times 0 and t, respectively, and
k
app
(min
-1
) is the apparent phtocatalytic reaction rate
constant determined from a linear fit to the data as
shown in Figure 4. The results indicated that the
reaction kinetics of all the samples were well fitted
by the pseudo first-order rate model and the
photocatalytic reaction was carried out at the
interface between Bi
2
O
3
and the solution.
Figure 4: Photocatalytic kinetic linear fitting of UDMH.
3.5 Reaction Mechanism
At present, for the mechanism of semiconductor
photocatalysis, the theory of hydroxyl radicals is
generally accepted. Bi
2
O
3
is a p-type semiconductor
with an energy band structure, and it can be used as
photocatalyst in connection with its semiconductor
optoelectronic characteristics.
MEEP 2018 - The Second International Conference on Materials Chemistry and Environmental Protection
40

Figure 5: Schematic diagram of photocatalytic reaction.
As shown in Figure 5, the energy band of Bi
2
O
3
is discontinuous, and there is a forbidden band
between the electron-filled valence band (VB) and
the empty conduction band (CB). When the
photocatalyst is exposed to the light with an energy
greater than the band gap, the photogenerated
electrons (e
-
) on the valence band are excited to
transition to the conduction band, and at the same
time, corresponding photogenerated holes (h
+
) are
generated on the valence band (equation 2).
Bi
2
O
3
+ hυ → h
+
+ e
–
(2)
Under the action of the electric field, the
photogenerated electrons and holes are separated to
form oxidation-reduction system on the surface of
the semiconductor. The holes can react with OH
-
or
H
2
O to produce OH• (equation 3 and 4).
OH
–
+ h
+
→ OH•
(3)
H
2
O + h
+
→ OH• + H
+
(4)
4 CONCLUSIONS
(1) The Bi
2
O
3
thin films which were prepared by
sol-gel method showed high photocatalytic activity
by degrading UDMH (above 90% in 120 min). The
initial concentration of UDMH had little effect on
the removal efficiency when the concentration was
among 100~500 mg/L; in neutral or weakly alkaline
conditions, the photocatalytic degradation efficiency
of UDMH was higher; the addition of a small
amount of H
2
O
2
in the reaction system can
significantly improve the degradation rate of
UDMH.
(2) Reaction Kinetic studies showed that the
reaction kinetics were well fitted by the pseudo first-
order rate model and the photocatalytic reaction was
carried out at the interface between Bi
2
O
3
and the
solution.
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Study on Photocatalytic Degradation of Unsym-Dimethylhydrazine Wastewater by Bi2O3 Thin Films
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