Degradation of p-nitrophenol using CuO/Al2O3 as a Fenton-like catalyst under microwave irradiation

Article abstract of DOI:10.1039/c4ra14516j

CuO/Al₂O₃ was synthesized successfully by the impregnation-deposition method and used as a heterogeneous catalyst in a microwave assisted Fenton-like process (MW/FL). The morphology and physico-chemical properties of the CuO/Al₂O₃ catalyst were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) analysis. The degradation of p-nitrophenol (PNP) was investigated by different processes, including microwave irradiation (MW) alone, hydrogen peroxide (H₂O₂) oxidation under microwave irradiation without the catalyst (MW/H₂O₂), a Fenton-like (FL) process, a MW/FL process and a thermally-assisted Fenton-like (TH/FL) process. The results showed that the CuO/Al₂O₃ catalyzed MW/FL could generate more hydroxyl radicals (OH) and remove PNP more effectively compared to other processes. The effects of initial pH, dosage of H₂O₂ and catalyst, microwave power and radiation time, and PNP concentration on the removal efficiency were also studied. The results showed that 93% removal efficiency of PNP was obtained within 6 min under optimized conditions. Moreover, the catalyst had a good stability and reusability, thus expanding the scope of non-iron based catalysts in the Fenton-like reactions.

Full text of DOI:10.1039/c4ra14516j

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DOI: 10.1039/C4RA14516J
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Degradation of p-nitrophenol using CuO/Al2O3 as Fenton-like catalyst under
microwave irradiation
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Weiqian Pan,a Guangshan Zhang,a Tong Zheng,a Peng Wang*ab
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aSchool of Municipal and Environmental Engineering, Harbin Institute of
Technology, Harbin 150090, China
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bState Key Laboratory of Urban Water Resource and Environment, Harbin Institute of
Technology, Harbin 150090, China
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*Corresponding authors. Tel.: 86ꢀ451ꢀ86283109; Fax: 86ꢀ451ꢀ86283109.
Eꢀmail addresses: pwang73@vip.sina.com (Peng Wang)
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Abstract
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CuO/Al2O3 was synthesized successfully by impregnationꢀdeposition method and used as a
heterogeneous catalyst in microwave assisted Fentonꢀlike process (MW/FL). The morphology
and physicoꢀchemical properties of the CuO/Al2O3 catalyst were characterized using field
emission scanning electron microscopy (FESEM), Xꢀray diffraction (XRD) and
BrunauerꢀEmmettꢀTeller (BET) analyzer. The degradation of pꢀnitrophenol (PNP) was
investigated by different processes, including microwave irradiation (MW) alone, H2O2
oxidation under microwave irradiation without the catalyst (MW/H2O2), Fentonꢀlike (FL)
process, MW/FL process and thermalꢀassisted Fentonꢀlike (TH/FL) process. The results
showed that the CuO/Al2O3 catalyzed MW/FL could generate more hydroxyl radicals and
remove PNP more effectively compared to other processes. The effects of initial pH, dosage
of H2O2 and catalyst, microwave power and radiation time, and PNP concentration on the
removal efficiency were also studied. The results showed that 93% removal efficiency of PNP
solution was obtained within 6 min under the optimized conditions. Moreover, the catalyst
had a good stability and reusability, thus expanding the scope of nonꢀiron based catalyst in the
Fentonꢀlike reactions.
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Keywords: microwave, Fentonꢀlike, CuO/Al2O3, PNP
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1. Introduction
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pꢀNitrophenol (PNP) is one of the most important nitroaromatic compounds which are a
major class of environmental contaminants and widely used in the manufacture of
agrochemicals, dyes and explosives[1ꢀ3]. Because of its acute toxicity and mutagenic potential,
PNP contamination can pose a significant environmental and public health risk[2, 4, 5]. The
United States Environmental Protection Agency has listed PNP as the priority pollutant[1].
Due to its stability to chemical and biological degradation, it is difficult to purify
PNPꢀcontaminated wastewater[5ꢀ7]
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The advanced oxidation processes (AOPs) based on hydroxyl radicals (•OH) have attracted
considerable attention to remove toxic and persistent contaminants[8ꢀ12]. The Fenton process,
as one of the most popular AOPs, has been investigated extensively in the treatment of
nonꢀbiodegradable organic pollutants, in which the iron (II) catalyzed decomposition of
hydrogen peroxide (H2O2) can produce •OH easily[8, 13, 14]. Nevertheless, the homogeneous
Fenton systems have a couple of disadvantages with which the overall efficiency is limited: (i)
the presence of large quantities of dissolved iron in the solution after the treatment may
demand a secondary treatment[15, 16] and (ii) Fenton reactions need a limiting pH range (2.5–
3.5), which requires extra conditioning before and after the treatment[17ꢀ20]
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To overcome the drawbacks raised from the homogeneous Fenton process, more and more
reports have focused on the development of a heterogeneous catalyst as an alternative for the
degradation of organic pollutants, which is a soꢀcalled heterogeneous Fentonꢀlike process[21]
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During the last decades, it has been extensively reported that transitional metals or metal
oxides as a heterogeneous catalyst pose high activity in Fentonꢀlike oxidation of hazardous
pollutants[22]. And among a variety of heterogeneous catalysts, copperꢀbased catalysts have
attracted significant interest because of their outstanding catalytic properties and reaction can
be performed at near neutral pH value[11, 23ꢀ25]
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On the other hand, researchers used additional assistance such as photo[26ꢀ28], ultrasound[29]
and microwave[30ꢀ33] to promote the Fenton oxidative degradation of wastewater, because it’s
swift, high efficient, and environmental friendly. The applications of microwave energy to
enhance chemical reactions are well known[34]. There are researches suggested that in AOPs
with microwave irradiation a better degradation efficiency could be obtained compare with
traditional treatment methods[35ꢀ37]
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In this work, the applicability of CuO/Al2O3 as a heterogeneous catalyst for MW/FL
degradation of PNP was investigated. The influence of various parameters on the degradation
efficiency was investigated and the beneficial effect of microwave field in promoting the
reaction rate of the Fentonꢀlike processes was emphasized.
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2. Experimental
2.1. Materials
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The γꢀAl2O3 particles (2-3 mm in diameter) used as carriers were purchased from Zibo
Henghuan Chemical Co., Ltd.. All other chemicals used in the experiments were purchased
from Aladdin Reagent Co., Ltd. They were all of analytical grade and used without further
purification. All solutions were prepared using deionized (DI) water at room temperature.
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2.2. Preparation of catalyst
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The CuO/Al2O3 catalyst was prepared by impregnationꢀdeposition method[38]. Briefly,
Cu(NO3)2·3H2O salt was used as the source of CuO. Before the preparation, Al2O3 particles
used as carriers were pretreated by the following steps: Al2O3 particles were washed
repeatedly with DI water at first, dried in an air drying oven at 383 K, and then calcined at
623 K for 3 h. In a typical synthesis, 20 g of the activated Al2O3 particles were immersed into
100 mL of 0.6 M Cu(NO3)2 solution in a constant temperature water bath at 328 K for 24 h.
Then, they were filtered and immersed into 100 mL of 0.4 M NaOH solution to react at 328 K
for 24 h. After the reaction was completed, the resulting blue granulated product was filtered,
washed with DI water to remove possibly remaining NaOH, and then dried overnight at 383
K in the air drying oven. The CuO/Al2O3 catalyst was finally obtained after being calcined in
air atmosphere at 623 K for 4 h using a muffle furnace with the heating ramp rate of 10
K/min.
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To study the XRD, the unsupported CuO was prepared as follows: 0.06 mol of Cu(NO3)2
and 0.04 mol of NaOH were mixed with 100 mL of DI water and the mixture continued to be
stirred at 328 K for 24 h. Then the resulting products were filtered and washed with DI water
repeatedly and dried overnight at 383 K. Finally, CuO powders were obtained after calcining
at 623 K for 4 h.
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2.3. Characterization
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The surface morphology of the CuO/Al2O3 catalyst was observed by field emission scanning
electron microscopy (FESEM, FEI Helios Nanolab 600i) equipped with Oxford Instruments
Xꢀray energy dispersion spectrograph (EDS). The XRD analyses for the CuO/Al2O3 and CuO
were obtained by Xꢀray diffractometer (Rigaku, DꢀMAXꢀRB) with Cu K alpha radiation
source (λ = 1.5418 Å) at 40 kV and 40 mA. The data were collected for 2θ from 20 o to 80 o
o
with a scan speed of 0.02 /s. The BrunauerꢀEmmettꢀTeller (BET) specific surface area, pore
volume and pore size distribution of the CuO/Al2O3 catalyst were measured by BET analyzer
(Micromeritics, ASAP2020) using N2 adsorption/desorption at 77 K.
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2.4. Procedures and analyses
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All the experiments were carried out in a 250ꢀmL glass conical flask containing 100mL of
PNP solution. The microwave radiation source was a chemical reaction microwave oven
(EXCEL, with a frequency of 2.45GHz), which was purchased from PreeKem Scientific
Instruments Co., Ltd. (Figure 1). The initial pH of PNP solution were adjusted to the desired
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values using 0.4 M HCl and 0.4 M NaOH for the experiments at different pH conditions. A
given amount of CuO/Al2O3 catalyst was added into the PNP solution, and the suspension was
stirred for 30 min to reach adsorptionꢀdesorption equilibrium between the catalyst and PNP
before the reaction. Then the appropriate amount of H2O2 was also added into the solution.
The temperature of the PNP solution after 6 min of microwave irradiation of 100 W power
reached about 343 K. A thermalꢀassisted Fentonꢀlike (TH/FL) process was also carried out as
a reference condition, and a constant temperature water bath (358 K) was used as the thermal
source. The removal efficiencies in other different processes were also investigated at ambient
temperature (∼298 K). For the stability and reusability test of the catalyst, after each
degradation experiment, the resulting suspension was centrifugalized and the catalyst was
recycled. The experiments for the comparison of different processes and the catalyst’s
reusability test were performed three times to confirm their reproducibility. The presented
data were mean values with standard deviations (SD) as error bars.
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Samples (1.0 ml) taken at selected time intervals were immediately filtered through a 0.45
ꢁm syringe microꢀfilter to separate the catalyst from the solution and diluted 1:25 with 0.4 M
NaOH solution to stop any further reaction. Subsequently, the concentrations of residual PNP
were determined by light absorbance measurement at 400 nm using a UVꢀvis scanning
spectrophotometer (Shimadzu, UV2550). The TOC of the sample was measured by a TOC
analyzer (Shimadzu, 5000A). The indirectly determination of •OH was conducted by a
fluorescence method, as reported in our previous studies[39, 40]. The dissolved copper
concentration in the solution after reaction was determined using
flame atom absorption spectrophotometer (PerkinElmer, AAnalyst200).
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Figure 1. Schematic diagram of experimental setup.
26 3. Results and Discussion
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3.1. Characterization of catalyst
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Figure 2 shows the EDS spectra and FESEM images of Al2O3 support and the synthesized
CuO/Al2O3 catalyst. It could be found obviously that, comparing with the pure Al2O3 (Figure
2a), a large number of particles attached to outer surface after the CuO loading (Figure b).
According to the chemical EDS analysis, Al and O are the only elements in Al2O3 support,
while the resulting CuO/Al2O3 catalyst was composed of copper, aluminium and oxygen
without other elements, indicating that the CuO/Al2O3 catalyst was synthesized successfully
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and not contaminated with other elements during the preparation process. CuO particles with
the flake structure are uniform and well defined, and about 50 nm thick and 150–400 nm in
diameter (Figure 2c).
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The XRD patterns of the CuO/Al2O3 catalyst and unsupported CuO sample (prepared by
the same method) are presented in Figure 3. The XRD pattern of the CuO/Al2O3 catalyst
showed no obvious diffraction peak of crystalline CuO, indicating the high dispersivity of
CuO on the surface of Al2O3. It also can be found that the pattern of the CuO sample show
XRD reflections in the 2θ region of 20–80 °, which were characteristic of the monoclinic
phase of CuO (JCPDS 45ꢀ0937). These peaks exhibit narrow and intense characteristics,
indicating a good crystallinity.
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Figure 4 shows the nitrogen adsorption/desorption profiles obtained for the Al2O3 support
and the synthesized CuO/Al2O3 catalyst. As shown in Figure 3, both the Al2O3 and the
CuO/Al2O3 have significant hysteresis loops that exhibited typical isotherms of mesoporous
structure, where the curve obtained from the CuO/Al2O3 catalyst are found to shift down
along the Yꢀaxis. This suggested the decrease of pore size after the CuO loading, and also
proved that the loaded CuO entered into channels of the Al2O3 support[18]
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The BET surface area, the average pore volume and the average pore diameter derived
from nitrogen physisorption are listed in Table 1. It can be found that the loaded CuO on the
Al2O3 leads to an increase in the specific surface area and a slight decrease in the average
pore volume and the average pore diameter. A specific surface area of 262 m2/g, an average
pore volume of 0.37 cm3/g, and an average pore diameter of 5.3 nm were obtained for the
resulting CuO/Al2O3 catalyst.
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c)
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Figure 2
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EDS spectra and FESEM images of (a) the Al2O3 support and (b) the synthesized CuO/Al2O3
catalyst; (c) FESEM image of the synthesized CuO/Al2O3 catalyst.
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Figure 3
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XRD patterns of the synthesized CuO/Al2O3 catalyst and unsupported CuO sample (prepared
by the same method).
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Figure 4. N2 adsorption–desorption isotherms obtained for Al2O3 support and the synthesized
CuO/Al2O3 catalyst.
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Table 1
Physicoꢀchemical properties of Al2O3 support and the synthesized CuO/Al2O3 catalyst
Sample
Al2O3
SBET (m2/g)
246
Vpore (cm3/g)
0.40
Dp (nm)
5.8
5.3
CuO/Al2O3
262
0.37
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3.2. Microwave-Fenton-like (MW/FL) oxidation of PNP
3.2.1. Performance of different processes
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To compare the PNP removal efficiency in different processes, the PNP degradation
experiments by microwave irradiation (MW) alone, H2O2 oxidation under microwave
irradiation without the catalyst (MW/H2O2), Fentonꢀlike (FL) process, MW/FL process and
TH/FL process were carried out. The results (Figure 5) show that the removal of PNP was
hardly observed by MW alone, which indicated that the energy of microwave irradiation is
too weak to break the bonds of the PNP, and the volatilization of PNP in our experiments
could be ignored. Furthermore, there was only 15% PNP removal in MW/H2O2 process. The
PNP removal efficiency exceeded 90% in MW/FL process, which was nearly 80% higher than
that in FL process. In addition, in TH/FL process the removal efficiency of PNP reached 71%,
in which the temperature of water bath (358 K) is higher than the final temperature of the
solution in MW/FL process (343 K). The order of the PNP removal efficiency was
MW/FL>TH/FL>>MW/H2O2>FL. This indicated that MW/FL process had a synergistic
effect for degrading PNP. Compared with the conventional heating method, an obvious
improvement of the removal efficiency was observed in the case of microwave irradiation.
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Figure 5. PNP removal in different processes. (PNP initial concentration = 50 mg/L; initial pH = 6.0;
H2O2 dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W).
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Fluorescence spectra of 2ꢀhydroxyterephthalic acid generated in different processes were
shown in Figure 6. It could be observed that the order of the amount of •OH generated in
different processes was similar with that of the PNP removal efficiency (Figure 5). The
amount of •OH generated in the MW/FL process was much larger than that in FL process,
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indicating the generation of •OH in MW/FL process was enhanced by the addition of
microwave irradiation. There was nearly no •OH generated in MW process. Therefore, it
could be confirmed that the synergistic effect of microwave irradiation and Fentonꢀlike
reaction promoted the generation of •OH. The MW/H2O2 process had some contribution to
the generation of •OH because microwave irradiation improves the generation of •OH by
[41]
decomposition of H2O2
:
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H2O2 + Microwave energy → 2•OH
(1)
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It could be found that, although the temperature of the PNP solution after reaction in TH/FL
process was higher than that in MW/FL process, the promoting effect of constant temperature
water bath on the generation of •OH was not as much as that of microwave irradiation. This
indicates that the heat effect is not the only cause of the promoting effect of microwave
irradiation.
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Figure 6. •OH generated in different processes. (Terephthalic acid concentration = 800 mg/L; initial
pH = 6.0; H2O2 dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W).
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3.2.2. Effect of various parameters on PNP removal
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In MW/FL process the removal efficiency of PNP depends on several factors such as initial
pH, dosage of H2O2 and catalyst, microwave power and radiation time, and PNP
concentration.
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Conventional Fenton processes are influenced greatly by solution pH and can only achieve
ideal removal efficiency when initial pH is 2.5–3.5[18]. The effect of the initial pH on PNP
removal under microwave irradiation was investigated first by varying the initial pH in the
range of 2.0–10.0. The results (Figure 7) show that PNP was almost completely removed from
initial pH 2.0 to 3.0. And the removal efficiency of PNP decreased slowly when the initial pH
increased from 3.0 to 8.0, which still remained more than 85%; A further increase in initial pH
resulted in a faster decrease of the removal efficiency of PNP. At the initial pH of 10.0, the
removal of PNP dropped to 62.2%. This phenomenon is similar with the reported literature[42,
43]
and can be attributed to several aspects. First, at lower pH, the H2O2 decomposition
reaction was promoted to yield •OH in the solution whereas the recombination reaction of the
free radicals decreases[42, 43]. Furthermore, in alkaline solution the deprotonation of hydroxyl
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groups increases the stability of the C–NO2 bond and consequently the PNP molecule[42]
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Obviously, MW/FL could greatly expand the pH range of reaction compared with
conventional Fenton system. In particular, the removal of PNP reached 93% when the initial
pH of the PNP solution was 6 (without adjustment)
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Figure 7. Effect of initial pH on PNP removal. (PNP initial concentration = 50 mg/L; H2O2 dosage = 25
mM; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).
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The effect of the H2O2 concentration on the PNP removal was investigated for the H2O2
concentration from 5 to 100 mM. The data illustrated in Figure 8 show the similar influences
of the H2O2 concentration on the processes with and without CuO/Al2O3. For both processes,
the removal efficiency of PNP increased significantly with the increase of the H2O2
concentration until a concentration of 25 mM was reached. When the H2O2 concentration
exceeded 25 mM, the increase of the removal efficiency slowed down greatly. At low
concentrations, there was not enough H2O2 that generated enough •OH[44]. At rather high
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concentrations, H2O2 can act as a scavenger of •OH[45, 46]
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H2O2+HO• → HO2• + H2O
(2)
(3)
HO2• + HO• → H2O + O2
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Nevertheless, at the same H2O2 concentration of 25 mM, the removal efficiency of PNP was
93% in the process with CuO/Al2O3 while that was just below 15% in the process without
CuO/Al2O3.
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Figure 8. Effect of H2O2 concentration on PNP removal. (PNP initial concentration = 50 mg/L; initial
pH = 6.0; catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).
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Figure 9 shows the effect of the catalyst dosage on PNP removal. It can be observed that
the PNP removal was largely enhanced by increasing the catalyst dosage from 0 to 4.0 g,
mainly because higher dosage of catalyst can offer more active sites that can expedite
generation of •OH and thereby promote the removal efficiency of PNP[47]. When the dosage
of CuO/Al2O3 reached to 4.0 g, PNP could be removed 93% in 6 min. While further
enhancing the catalyst amount, the PNP removal efficiency was hardly enhanced. The change
occurred probably because the concentration of H2O2 is constant and there was not enough
H2O2 to be decomposed by excess catalyst. Thus, it can be concluded that the optimum
catalyst dosage was about 4.0 g under the present MW/FL system.
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Figure 9. Effect of catalyst dosage on PNP removal. (PNP initial concentration = 50 mg/L; initial pH =
6.0; H2O2 dosage = 25 mM; microwave power = 100 W; radiation time = 6 min).
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PNP removal can be enhanced by increasing microwave power. As mentioned before, H2O2
in solution generates extra •OH when it is exposed to MW irradiation (Eq. (1)). Violent
motion of H2O2 molecules (which are polar compounds) induced by microwave irradiation,
can result in an increase of collision numbers and excitation of molecules to a higherꢀexcited
state (higher vibrational and rotational energy levels), resulting that the strength of the
molecular bonds reduce and consequently rupture[48]. Moreover, the excitation of PNP
molecules also contributed to the higher PNP removal in MW/FL. Besides, localized
superheating (hot spots) with high temperature above 1473 K on the surface of CuO/Al2O3
catalyst can be caused by MaxwellꢀWagner effect of microwaves[49]. These hot spots could
accelerate the reactions of H2O2 and PNP molecules which are adsorbed on the surface of
CuO/Al2O3 catalyst. Since microwave energy efficiency is an important parameter in the
MW/FL process, the effects of microwave energy consumption on the PNP removal were
studied under different microwave powers. As shown in Figure 10, under the same microwave
energy consumption, the different microwave powers got the different PNP removal
efficiencies. The PNP removal efficiency was enhanced in different degrees by increasing
microwave energy consumption, and increased rapidly at low microwave powers (50 and 100
W) while slowly at high microwave powers (200, 300 and 400 W). Under a constant
microwave energy consumption, smaller microwave power means longer radiation time and
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consequently more completed PNP removal. Consequently, the removal efficiency increased
by prolonging reaction time under the same microwave energy consumption and reached a
peak of 93% after 6 min of reaction time under microwave radiation of 100 W.
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Figure 10. Effect of microwave energy consumption on PNP removal. (PNP initial concentration = 50
mg/L; initial pH = 6.0; H2O2 dosage = 25 mM; catalyst dosage = 4.0 g)
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The effect of initial PNP concentration on the MW/FL processes was investigated with
different concentrations of PNP and the results were shown in Figure 11. From the results it is
possible to see that the increase of the initial concentration of PNP from 50 to 300 mg/L
decreases the PNP removal efficiency from 93% to 67.9%. This phenomenon might be due to
the fact that with a constant concentration of •OH the increase of PNP concentration
decreases the relative concentration of •OH, which led to reduced PNP removal efficiency[47,
50]. However, 67.9% of PNP removal efficiency was still obtained even when the initial PNP
concentration was as high as 300 mg/L.
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Figure 11. Effect of PNP concentration on PNP removal. (initial pH = 6.0; H2O2 dosage = 25 mM;
catalyst dosage = 4.0 g; microwave power = 100 W; radiation time = 6 min).
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Figure 12 shows the change of the UVꢀvis absorption spectrum for the PNP solution during
the MW/FL degradation process in a typical experiment. The gradual decrease of absorption
peak at λmax (400 nm) indicated that PNP was removed gradually with the degradation time.
To get the degree of mineralization of the PNP, the TOC removal efficiency was measured at
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the end of degradation process. The TOC removal efficiency reached 68% within 6 min in
MW/FL process. The final pH of the solution is 5.3, and this decrease may be due to the
generation of intermediate degradation products of PNP.
4
5
6
7
Figure 12. The UV–vis absorption spectra of PNP with the reaction time, as indicated in the legend.
(PNP initial concentration = 50 mg/L; initial pH = 6.0; H2O2 dosage = 25 mM; catalyst dosage = 4.0 g;
microwave power = 100 W; radiation time = 6 min).
8
9
The AOPs are mainly based on the generation of the •OH, which has a great oxidation
power and is able to oxidize almost all organic compounds to carbon dioxide and water[51]
.
10
11
12
13
Fluorescence spectra data given in Figure 6 show that there were a large amount of •OH
generated during the MW/FL process. In the MW/FL process the •OH that oxidize PNP are
produced through the decomposition of H2O2 under microwave irradiation (Eq. (1)) and the
reaction between CuO and H2O2 adsorbed on the surface of CuO/Al2O3 catalyst[23]
.
14 ≡Cu2++ H2O2→≡Cu++ HO2•+ H+
15 ≡Cu++ H2O2→≡Cu2++ OH• + OHꢀ
(4)
(5)
16
17
18
19
20
21
22
23
24
25
26
The PNP degradation pathway by •OH oxidation has been well discussed previously[4, 5, 7]
.
The intermediate degradation products of PNP mainly include 4ꢀnitropyrocatechol,
hydroquinone, 1,2,4ꢀbenzenetriol and benzoquinone, and the possible formation pathway of
intermediate products in PNP degradation process is illustrated in Figure 13. The
electronꢀdonor property of hydroxyl group on the aromatic ring favors the electrophilic attack
of the OH• on the orthoꢀ and paraꢀpositions[7, 35]. Attack of •OH at orthoꢀposition leads to the
formation of 4ꢀnitropyrocatechol. Meanwhile the paraꢀposition of PNP may be attacked by
•OH and nitrite ion is eliminated from PNP to yield hydroquinone, which subsequently
changes into 1,2,4ꢀbenzenetriol and benzoquinone. Subsequently, these intermediates further
react with •OH leading to ring cleavage and final formation of carbon dioxide and water, and
nitrous acid is oxidized into nitrate ion.
27
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Figure 13. Possible producing pathway of the intermediate degradation products of PNP.
3
3.3. Stability tests of CuO/Al2O3 catalyst
4
5
In order to evaluate the stability of catalyst, the PNP removal efficiencies over CuO/Al2O3 in
MW/FL process were assessed and the results were showed in Figure 14. After each cycle, the
catalyst was just recycled by filtration after the former four cycles. Furthermore, the catalyst
was regenerated by washing with DI water and drying in oven at 110 ℃ after the fifth, sixth
and seventh cycles. It can be found that the decay of catalyst activity was quite slight between
the former four cycles. When the catalyst was reused four times, the PNP removal efficiency
still remained more than 90%. Although a significant decline in the PNP removal efficiency
was observed at the fifth reuse, high removal efficiency was restored by regeneration. The
PNP removal efficiency remained more than 90% in 3 more times of reuse with regeneration.
The concentrations of dissolved copper in the solution after each reaction were determined
and the results indicated that the total amount of dissolved copper after five cycles of reaction
was less than 1%. So the decrease of the catalyst activity may be due to the occupying the
6
7
8
9
10
11
12
13
14
15
16
active sites of CuO/Al2O3 by some organic intermediates[45]
.
17
18
19
20
Figure 14. Oxidation of PNP on CuO/Al2O3 in eight batch reaction cycles. (PNP initial concentration =
50 mg/L; initial pH = 6.0; H2O2 dosage = 25 mM; catalyst dosage = 4.0 g; microwave power = 100 W;
radiation time = 6 min).
14

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4. Conclusion
2
3
In this study, the CuO/Al2O3 catalyst was synthesized and used at the heterogeneous catalytic
system for degradation of PNP in MW/FL process. The catalyst was characterized by FESEM,
XRD and BET analyzer. The results showed that, the CuO attached to the surface and pores of
Al2O3 with the flake structure and there is an increase of specific surface area. The results also
showed that the MW/FL process could degrade PNP quickly. In contrast to FL process, the
MW/FL could generate larger amount of hydroxyl radicals, and remove PNP more effectively.
The initial pH, dosage of H2O2 and catalyst, microwave power and radiation time, and PNP
concentration had strong effect on the removal efficiency of PNP in MW/FL process, and the
optimum parameters were a H2O2 dosage of 25 mM, a catalyst dosage of 4.0 g, a microwave
power of 100 W without adjustment of the initial pH (pH=6). Under these conditions, the PNP
removal efficiency reached 93% after a reaction time of 6 min for 100 ml of 50 mg/L initial
concentration. The results also showed the catalyst had a good stability and reusability by
catalyst regeneration. Notably, this study proved that the MW/FL process catalyzed over
CuO/Al2O3 could overcome the drawback of the homogeneous catalyst, widen the applicable
initial pH range of reaction and improve greatly the degradation efficiency.
4
5
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7
8
9
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12
13
14
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16
17
Acknowledgements
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19
20
21
22
The authors sincerely thank the National Water Pollution Control and Management
Technology Major Projects (2012ZX07205ꢀ005) and Creative Research Groups of the
National Science Foundation of China (5112106) for the financial support. We also
acknowledge the technical support of State Key Laboratory of Urban Water Resource and
Environment at Harbin Institute of Technology.
23 Notes and references
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