Nano-MnOx on activated carbon prepared by hydrothermal process for fast and highly efficient degradation of azo dyes

Article abstract of DOI:10.1016/j.apcata.2014.07.041

Azo dyes are recalcitrant and refractory pollutants that pose a threat to environmental safety. Therefore, it is of significance to develop fast, cheap and highly effective materials for removal of azo dyes in water. Herein, fast oxidative removal of azo dyes by highly active nano-MnO_x on activated carbon prepared via hydrothermal process was explored. Active manganese oxide was supported on coconut shell activated carbon (CSAC) via hydrothermal method and was characterized. The hydrothermal treatment changed the morphology of the MnO_x@CSAC and led to the in situ synthesis of nano-MnO_x onto CSAC surfaces with the incorporation of the multi-valence oxidation states (Mn⁴⁺ and Mn³⁺) of manganese. The decolorization reactions by MnO_x@CSAC were fast and the dye removal was most pronounced at pH 3 (99.1%). Through GC-MS analyses, the most detectable degradation products were identified and possible degradation pathway was proposed. Additionally, the MnO_x@CSAC also could effectively degrade other anionic dyes. The oxidative degradation of C. I. Acid Red 73 was still strong after 20 usage cycles, with a maximum removal rate of 99.1% and a minimum removal rate of 89.7% from the same sample. Overall, these results indicate that highly active nano-MnO_x@CSAC prepared via a hydrothermal method is very attractive, implying that this material has potential practical applications for treating azo dyeing effluent.

Full text of DOI:10.1016/j.apcata.2014.07.041

Accepted Manuscript
Title: Nano-MnO on activated carbon prepared by
x
hydrothermal process for fast and highly efficient degradation
of azo dyes
Author: Lili Xu Xingfa Li Jianqing Ma Yuezhong Wen
Weiping Liu
PII:
S0926-860X(14)00471-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.07.041
APCATA 14934
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
21-5-2014
16-7-2014
18-7-2014
Please cite this article as: L. Xu, X. Li, J. Ma, Y. Wen, W. Liu, Nano-
MnOx on activated carbon prepared by hydrothermal process for fast and
highly efficient degradation of azo dyes, Applied Catalysis A, General (2014),
http://dx.doi.org/10.1016/j.apcata.2014.07.041
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Nano-MnO on activated carbon prepared by hydrothermal
x
process for fast and highly efficient degradation of azo dyes
a
a
b
a, c, a*
b
Lili Xu , Xingfa Li , Jianqing Ma , Yuezhong Wen
, Weiping Liu
a
Institute of Environmental Science, College of Environmental and Resource Sciences, Zhejiang
University, Hangzhou 310058, China
b
MOE Key Laboratory of Environmental Remediation & Ecosystem Health, College of
Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China
c
Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Zhejiang
University, Hangzhou 310058, China
*
Corresponding author: Tel.: +86-571-8898-2421; Fax: +86-571-8898-2421.
E-mail address: wenyuezhong@zju.edu.cn (Y.Z. Wen).
1
Page 1 of 33

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Research highlights
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λ
A facile and green method for carbon supported nanostructure MnO
proposed.
X
was
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8
9
λ
λ
λ
λ
MnO
The mixed-valent MnO
MnO @actived carbon exhibit good stability after twenty consecutive cycles.
X
@actived carbon could efficiently and rapidly degrade azo dyes.
X
system could cause the high degradation reactivity.
X
1
0
1
The detectable degradation products are mainly biologically degradable.
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Abstract
Azo dyes are recalcitrant and refractory pollutants that pose a threat to
environmental safety. Therefore, it is of significance to develop fast, cheap and highly
effective materials for removal of azo dyes in water. Herein, fast oxidative removal of
x
azo dyes by highly active nano-MnO on activated carbon prepared via hydrothermal
process was explored. Active manganese oxide was supported on coconut shell
activated carbon (CSAC) via hydrothermal method and was characterized. The
hydrothermal treatment changed the morphology of the MnO
x
@CSAC and led to the
in situ synthesis of nano-MnO onto CSAC surfaces with the incorporation of the
x
Page 2 of 33

4
+
3+
23
24
25
26
27
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30
31
32
multi-valence oxidation states (Mn and Mn ) of manganese. The decolorization
reactions by MnO @CSAC were fast and the dye removal was most pronounced at
x
pH 3 (99.1%). Through GC-MS analyses, the most detectable degradation products
were identified and possible degradation pathway was proposed. Additionally, the
x
MnO @CSAC also could effectively degrade other anionic dyes. The oxidative
degradation of C. I. Acid Red 73 was still strong after twenty usage cycles, with a
maximum removal rate of 99.1% and a minimum removal rate of 89.7% from the
x
same sample. Overall, these results indicate that highly active nano-MnO @CSAC
prepared via a hydrothermal method is very attractive, implying that this material has
potential practical applications for treating azo dyeing effluent.
3
3
4
x
Keywords: nano-MnO ; activated carbon; hydrothermal process; oxidative removal;
3
azo dye
Page 3 of 33

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1. Introduction
Textile industries produce large volumes of wastewater polluted with dyes. It has
been estimated that approximately 10-15% of manufactured dyes are discharged into
wastewater streams [1, 2]. Among these dyes, azo dyes represent the largest class of
dyes [3]. Azo dyes are not readily biodegradable under aerobic conditions; in addition,
under anaerobic conditions, they can be reduced to potentially hazardous aromatic
amines and other compounds. Furthermore, the degradation by-products of azo dyes
can be mutagenic, carcinogenic, or teratogenic and can cause long-term health effects
[4-7]. Therefore, developing inexpensive and highly effective materials for the rapid
removal of azo dyes in water is of importance.
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2
Recently, manganese oxides, which are naturally present in sediments and soils,
have attracted considerable interest and have demonstrated rapid oxidation properties
suitable for the removal of organic pollutants in water without the addition of other
oxidizing agents, such an ozone or H
2
O
2
. For example, Huguet et al. proposed that the
) as a bed filter can be very efficient
use of natural manganese oxide alone (79% MnO
2
for the chemical oxidation of diclofenac [8]. AquaMandix, a natural Mn material, has
already been found to be efficient for the removal of estrogenic hormones in
wastewater treatment plants [9]. In addition, a Mn oxide containing mine tailings has
been shown to promote the complete oxidative decolorization of acid azo dyes after a
reaction period of 24 hours [10]. However, the quantity of natural manganese is
limited. Thus, synthetic manganese oxides have been used for the oxidative removal
of organic contaminants, such as phenols and pharmaceuticals, in water. However,
Page 4 of 33

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both natural and synthetic manganese oxides suffer from low efficacies. For example,
Forrez et al. tested the oxidative removal of diclofenac using synthetic MnO and
BioMnO and found that both oxides required approximately 20 hours to completely
remove organic contaminants [11]. Thus, to enhance efficacy, more MnO was added
to the reaction solutions. Lin et al. reported the nearly oxidative removal of bisphenol
A in 6 minutes in a pH 4.5 solution that initially contained 800 µM MnO and 4.4 µM
2
x
2
2
bisphenol A [12]. It is known that the oxidative activity of metal oxides depends on
the size distributions and morphologies of the particles. Hence, reducing the diameter
of the material to the nanometer scale may result in enhanced its efficacy [13]. Thus,
synthetic nanostructured manganese oxides have been reported to oxidize organic
contaminants [14, 15]. Nevertheless, nano-materials are difficult to separate from
water, which becomes the major limitation for the application of nano-materials in
this field [16, 17]. Therefore, it is very desirable to develop a strategy to prepare nano
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2
x
MnO on a suitable support to obtain good separability and remarkable oxidation
properties for wastewater treatment.
Granular activated carbon has been widely used as a reaction support or
promoter in wastewater purification due to its high surface area to volume ratio and
good adsorption capacity for a large number of organic contaminants, which prompted
x
us to investigate whether suitable active MnO on activated carbon are accessible.
Supported manganese oxides are generally prepared using the wet impregnation
method with a metal salt followed by calcinations under an air or nitrogen atmosphere
x
[18-20]. However, preparing active MnO @activated carbon by calcination under an
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air atmosphere is difficult due to carbon ashing. Furthermore, to the best of our
knowledge, little work has been published in the literature concerning the preparation
of this material under a nitrogen atmosphere. The majority of previous studies focused
on carbon-supported manganese oxides as catalysts to assist oxidants, such as ozone,
in degrading organic contaminants in water [20, 21]. To date, the direct synthesis of
x
active MnO with a defined crystal structure on activated carbon remains a formidable
challenge.
Hydrothermal treatment has been intensively studied over the past few years as a
sustainable synthesis method with numerous applications. This synthesis method is
conducted in and requires water, and it is also conducted under self-generated
pressures in closed systems and with mild temperatures (i.e., 120-250 °C). From an
energetical perspective, hydrothermal treatment is an exothermic process in contrast
to the standard high-temperature treatment. Therefore, hydrothermal treatment is
considered to be a green route for the synthesis of advanced materials [22-24]. Thus,
in this study, the in situ combination of coconut shell activated carbon (CSAC) with
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nanoscale MnO
x
produced by hydrothermal treatment and its application in the
@CSAC
degradation of azo dyes were investigated. It was determined that MnO
x
exhibited the best degradation activity for C. I. Acid Red 73 (AR 73) as well as good
stability, which can be suitable for environmental applications. Compared with
manganese-supported activated carbon materials obtained through conventional
preparation methods, the in situ synthesis of nanostructured MnO
x
on CSAC
(MnO @CSAC) through the hydrothermal route is a facile and green approach.
x
Page 6 of 33

1
2
2. Experimental
x
2.1. Preparation of MnO @CSAC
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The raw CSAC (iodine value: 900 mg/g; particle size: 1-2 mm) was ground using
an agate mortar, and the resulting material was then sieved through a uniform sieve
with a mesh of 100. In our study, a lot of preliminary experiments were done to
optimize the synthesis condition of MnO
procedure for the synthesis of MnO @CSAC, 0.3 g of CSAC was added
progressively to 10 mL of a 0.0040 M MnSO solution with vigorous stirring at room
temperature to obtain a well-mixed solution, which was left to stand for 12 h in a 100
mL Teflon-lined stainless steel autoclave. Then, 0.0048 M (NH was placed
x
@CSAC. In an optima experimental
x
4
1
1
1
1
4 2 2 8
) S O
into 10 mL of distilled water to form a homogeneous solution, which was also
transferred into the autoclave and maintained at 120 °C for 12 h. The resulting product
was centrifuged, decanted, washed with deionized water and dried at 80 °C in air.
14
x
2.2. Characterization of MnO @CSAC
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Surface morphology was studied with an electron microscope. The scanning
electron microscopy (SEM) micrographs were recorded using a field emission
scanning electron microscope (FEI, SIRON) at a voltage of 25.0 kV. The nitrogen
adsorption and desorption isotherms at 77 K were measured using a NOVA2000e
surface area & pore size analyzer (Quantachrome, USA). The specific surface area
was calculated by Brunauer-Emmet-Teller (BET) equation. The sample surfaces were
gold coated before analysis. The transmission electron microscopy (TEM)
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micrographs were obtained with a JEM-1230 (JEOL, Japan) transmission electron
microscope at an accelerating voltage of 200 kV. Synchrotron-based scanning
transmission X-ray microscopy (STXM) was performed at beamline BL08U1 of the
Shanghai Synchrotron Radiation Facility (SSRF). The materials were ultrasonically
dispersed in ethanol and dropped directly onto a TEM grid. After evaporation of
excess ethanol, the grids were fixed to the sample holder of the STXM device.
Single-energy images at energies of L edge and pre-edge of Mn were scanned and
recorded as raw data. Then, image differences were calculated and analyzed for
mapping chemical species over the scanned areas using the dual-energy ratio contrast
analysis method. The 2D spatial distribution of Mn element can be mapped
quantitatively from its absorption difference at two photon energies, 636.35 eV and
642 eV, which are just on pre-peak and peak of Mn absorption edge, respectively.
Furthermore, the crystalline structure and the valence of manganese were obtained
from X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS),
respectively. Detailed procedures are presented in the SI Text S1-2.
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16
x
2.3. General Procedure for degradation of dyes by the MnO @CSAC system
17
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All of the dye degradation experiments were performed by placing 0.1 g of
MnO @CSAC in contact with 100 mL of the dye solution (50 mg/L) in 250 mL flasks
during 2 minutes of constant stirring at 25 ºC. The pH of the dye solution was varied by
adding 2.0 M H SO or 0.1 M NaOH. For the MnO @CSAC reuse studies, the
x
2
4
x
materials were collected by centrifugation and were dried at 80 °C following the
degradation reaction. The chemical structures of dyes are shown in Figure S1 in the
Page 8 of 33

1
2
Supporting Information. The dye concentrations were analyzed using a Shimadzu
UV-2401PC UV-Vis spectrometer (Tokyo, Japan) at the absorbance maximum of the
C − C
0
t
3
dyes. The decolorization rate η (%) of dyes can be calculated as: η=
×100%
C0
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0
1
2
3
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5
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7
8
0 t
where C is the initial concentration of dye solution (mg/L), C is the concentration of
dye solution at the desired time, t (mg/L). GC-MS analysis of the degradation products
was performed using an Agilent 7890-5975C (American Agilent Technologies Co. Ltd.)
instrument equipped with a HP-5 capillary column (30 m × 0.25 mm × 0.25 µm). The
carrier gas was helium, and the temperature of the injection port was 250 °C. The
temperature program of the column was set to an initial oven temperature of 50 °C
and was increased at a rate of 10 °C/min to 250 °C, and the oven was held at 250 °C
for 10 min. The sample for GC-MS analysis was prepared as follows [25]: after
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1
x
reaction with MnO @CSAC, the AR 73 solution was withdrawn, and the water was
removed using a vacuum rotatory evaporator, which left a pale yellow residue. The
residue was dissolved using a sufficient volume of 3 M HCl, and the resulting solution
was saturated with NaCl, extracted with diethyl ether, and then dried with anhydrous
sodium sulfate; the ether was evaporated under vacuum. The residue was dissolved in
+
1 mL of methanol, and 0.5 mL of 0.2 M trimethylsulfonium hydroxide (Me
3
S OH) in
methanol was added to the methanol solution before GC-MS analysis.
19
3. Results and discussion
2
0
1
3.1. Hydrothermal synthesis of MnO
x
@CSAC: morphology and structure analysis
2
The morphology of the raw CSAC and MnO
x
@CSAC was examined by SEM
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(Figures 1a, 1b). Based on images Figure 1a and Figure 1b, the two materials exhibit
different particle sizes and pore structures. For the raw CSAC (Figures 1a),
well-dispersed particles were observed and the particle sizes were small in the range
x
of 5-15 micrometers. However, the MnO @CSAC particles were in an agglomerated
state with a relatively broader size distribution (about 100 micrometers) and
noticeable scattering of pores compared to the raw CSAC, which might be a
consequence of the hydrothermal treatment. Thus, to explain this phenomenon, the
raw CSAC was treated using the hydrothermal method (HT-CSAC) without the
addition of MnSO
As shown in Figure 1c, HT-CSAC also exhibits agglomeration behavior, which is
consistent with the result of MnO @CSAC. This phenomenon may be attributed to
4 2
·H O and the corresponding SEM image is presented in Figure 1c.
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x
the following two reasons: (1) the effect of high-temperature. During the
hydrothermal process, the surface of activated carbon was modified by OH groups,
which resulted in a hydrogen bonding interaction between activated carbon. Then, the
activated carbon was promoted to further assemble the low-energy close-packed
mesostructure with further cross-linking of the OH groups because the cross-linking
process occurs actively at the high temperature. Simultaneously, the high-temperature
hydrothermal treatment drives the small activated carbon to accumulate and rearrange
[26]. Consequently, a large activated carbon with a mesostructure is formed. (2) The
effect of stirring. The hydrothermal process was performed without stirring; thus, the
raw CSAC could be promoted to further assemble into a close-packed mesoporous
structure. Additionally, the specific surface area of CSAC, HT-CSAC and
MnO
of the raw CSAC, HT-CSAC and MnO
CSAC, HT-CSAC and MnO @CSAC exhibited type IV adsorption/desorption
x
2
@CSAC were determined in this study. The N adsorption/desorption isotherms
x
@CSAC are presented in Figure S2. The raw
X
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isotherms with hysteresis loops, which indicates that the three materials had a
2
mesoporous structure. CSAC presented the highest BET surface area (723.168 m /g),
2
whereas MnO
x
@CSAC presented the lowest surface area (504.530 m /g). The
2
specific surface area of HT-CSAC (703.001 m /g) was slightly smaller than that of
raw CSAC because of substantial aggregation. The BET surface area of
MnO
the CSAC.
The morphology and structure of different product are further elucidated by TEM.
As shown in Figure 1d, the needlelike MnO nanowires with diameter 10-50 nm and
x x
@CSAC decreased because of the in situ synthesis of MnO nano-particles into
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length up to several micrometer can be clearly seen, which are in good agreement
with the SEM observation (the inset of Figure 1d). Compared with the raw CSAC
x
(Figure 1e), TEM image of MnO @CSAC (Figure 1f) demonstrates that ribbon-like
nanowires of MnO nanocrystals anchored randomly into the amorphous CSAC, as
x
well as the higher-magnification TEM image depicted in the inset of Figure 1f.
The as-synthesized products are also characterized by X-ray diffraction (XRD) to
identify their crystallographic structures (Figure 2). According to a previous report
2 2
[27], the pattern of the as-prepared MnO can be assigned to the MnO phase (JCPDS
No. 72-1982). In this work, the two obvious diffraction peaks were observed at
approximately 2θ=22° and 43°, which are common to activated carbon, correspond to
(002) and (110) reflections due to the stacking structure of aromatic layers. The XRD
patterns of MnO
of the small dimensions of crystallites perpendicular to the aromatic layers. In
addition, weakly crystallized structure signals corresponding to MnO were observed
x
@CSAC show that the FWHM of the 22° signal broadened because
2
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because the MnO @CSAC contained a considerable amount of amorphous
carbonaceous organic compounds.
x
In addition, the valence of manganese in MnO @CSAC after hydrothermal
treatment was determined by XPS analysis (Figure S3). The Mn 2p3/2 and Mn 2p1/2
binding energies are 641.9 and 653.7 eV, respectively, with a spin-energy separation
of 11.8 eV. This doublet can be assigned to a mixed-valent manganese system, most
4
+
3+
likely Mn and Mn [28].
From the above results, the hydrothermal treatment changed the morphology of
x x
the MnO @CSAC. Meanwhile, the in situ synthesis of nano-MnO onto CSAC
1
surfaces led to the incorporation of the multi-valence oxidation states of manganese.
11
3.2. Oxidative degradation of AR 73
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22
The removal of AR 73 by different MnO
x
@CSAC prepared using different
approach was studied in this work. As shown in Figure 3, the removal rates of AR 73
by MnO
x
@CSAC (obtained via the hydrothermal method), N
2
-MnO
x
@CSAC
@CSAC
(obtained by calcination under a nitrogen atmosphere) and Air-MnO
x
(obtained by calcination under an air atmosphere) were 99.1%, 31.6% and 24.0% after
10 minutes treatment, respectively. The CSAC, N -CSAC and Air-CSAC exhibited
almost the same decolorization activity, approximately 43-45%. Based on the above
data, the color removal ability of MnO @CSAC was much stronger than the CSAC.
Conversely, comparing with the corresponding CSAC, dye removal efficiencies by
both N -MnO @CSAC and Air-MnO @CSAC obtained through calcination method
did not enhance. These results demonstrated that MnO @CSAC prepared via
2
x
2
x
x
x
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hydrothermal method is a highly active material for the removal of AR 73.
Decolorization reactions by MnO @CSAC were investigated within the pH
range of 3-7 at 25 °C. This pH range was chosen because the pH values of the raw
CSAC and MnO @CSAC were close to 7 and also acid dyes are often applied in
x
x
organic acid solutions resulting in acidity of textile effluents. The effect of the initial
solution pH on the decolorization of the dye at concentrations of 50 mg/L AR 73 and
x
1.0 g/L MnO @CSAC is shown in Figure 4a. The results indicated that the removal
rate of AR73 in solution increased as the pH decreased from 7.0 to 3.0, and the dye
removal was most pronounced at pH 3 (99.1%). The reason for this large variation
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most likely stems from the surface charge of MnO
solution pH. The point of zero-charge (pHpzc) for MnO
(below pHpzc) will increase the proportion of positively charged sites on the
MnO @CSAC surface, thereby increasing the potential for AR 73 (anionic dye)
molecules electrostatic attraction to the active MnO surface and further oxidation by
MnO @CSAC. When increasing the solution pH above pHpzc, MnO @CSAC surface
x
, which strongly depends upon the
x
is pH 4-5. Decreasing the pH
x
x
x
x
is negatively charged while AR 73 exists as an anion. Consequently, interactions at
the dye-active surface are not expected. In addition, the measured UV-Vis spectra of
the color change (pH 3) at each reaction time are shown in Figure 4b. Notably, the
characteristic adsorption band of the dye in the visible region (λ509nm) almost
disappeared after 4 minutes of contact with MnO
decolorization of azo dye. In the meantime, as a reference, the removal of dye by the
MnO without CSAC was studied. The result showed that MnO prepared via
x
@CSAC, indicating a rapid
x
x
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hydrothermal method is also a highly active material for the removal of AR 73
(Figure S4), but the nano-MnO is very difficult to separate from water, which
x
becomes the major limitation for the application of nano-materials in field of
wastewater treatment.
To test whether the dye decolorization rate increasing with decreasing pH was
as a result of oxidative reaction, the raw CSAC, which was used as a reference
material, was investigated to adsorb AR 73. Figure 4c shows the removal rate of AR
73 by raw CSAC. As the pH was decreased, the removal rate of AR 73 increased.
However, the raw CSAC had a low affinity for AR 73 (48.1%), even after 10 minutes
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1
2
2
x
of contact with the dye solution at pH 3. When using MnO @CSAC for the
decolorization reaction, the removal rate was 99.1% after 10 minutes at pH 3 without
the use of any additional oxygen sources in the solution. Therefore, the data from the
x
pH experiments with the raw CSAC indicate that MnO @CSAC can degrade dye.
Furthermore, the initial concentration of dyes is an important parameter in
practical applications. The influence of the initial dye concentration, ranging from 50
to 200 mg/L, on the degradation efficiencies of AR 73 was investigated at pH 3
(Figure S5). The AR 73 solutions with initial concentrations of 50, 100, 150 and 200
mg/L resulted in degradation efficiencies of 99.1, 97.5, 87.3 and 84.3%, respectively,
after 10 minutes of contact. Although the color removal efficiency gradually
decreased as the initial concentration increased, the MnO
for the oxidative removal of dyes in water.
x
@CSAC had high reactivity
22
3.3. Oxidation products and degradation pathway of AR 73
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Based on the data from the adsorption experiment of raw CSAC, it is clearly that
the dye was oxidized by using MnO @CSAC. To further identify and establish the
x
degradation intermediates and products of AR 73, the pH 3 treatment was analyzed by
GC-MS (Detailed results are presented in Figure S6-8 of the Supporting Information).
Figure S9 shows that the intermediates and products were mostly acetic acid (1),
propionic acid (2), butyric acid (3), pentanoic acids (4), phthalic acid (5) and
4-(diphenyldiazenyl) phenol (6). The results indicated that the most detectable
degradation products were mainly low-molecular-weight organic acids, which are
known to be biologically degradable.
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Based on the degradation products identified from the GC-MS analysis, a
reaction pathway scheme for the degradation of AR 73 by MnO
in Figure 5. First, electron transfer from the phenolic group of AR 73 to
MnO @CSAC generates phenoxy radical, which undergoes resonance
rearrangement and additional electron transfer to generate a carbocation on the C-1
carbon of the naphthol ring. Subsequently, nucleophilic attack by H O generates an
unstable tetrahedral complex, which breaks down to produce
x
@CSAC is proposed
x
a
2
1-(4-diazenylphenyl)-2-phenyldiazene and 1,2-naphthoquinone. One-electron
oxidation of 1-(4-diazenylphenyl)-2-phenyldiazene, an unstable intermediate product,
by O
unstable, it cleaves to produce a 1,2-diphenyldiazene radical and N
1,2-diphenyldiazene radical can undergo further oxidation followed by nucleophilic
attack by H O to generate 4-(diphenyldiazenyl) phenol. Another degradation pathway
2
yields a 1-(4-diazenylphenyl)-2-phenyldiazene radical. Since this radical is also
2
[29]. The
2
Page 15 of 33

1
2
3
4
is from 1,2-naphthoquinone to phthalic acid [30], which is also determined by GC-MS,
but not a final degradation product. Some of them can be further oxidized to
low-molecular-weight organic acids. This reaction mechanism suggests the
asymmetric cleavage of the azo bond [10].
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
3.4. Oxidative degradation of other anionic dyes
It is well known that different chemical structures with different functional
groups directly impact the degradation behaviors of dyes [31, 32]. The same reaction
conditions used for AR 73 were applied to the removal of a variety of other anionic
dyes. Acid dyes, including C. I. Acid Red 1 (AR 1), C. I. Acid Blue 1 (AB 1), C. I.
Acid Blue 25 (AB 25), C. I. Acid Blue 40 (AB 40), C. I. Acid Blue 62 (AB 62) and C.
I. Acid Blue 193 (AB 193), as well as reactive dyes, including C. I. Reactive Red 11
(RR11), C. I. Reactive Blue 194 (RB 194) and C. I. Reactive Blue 74 (RB 74), were
chosen as typical dye pollutants. The removal rates of the nine dyes after reaction with
1
1
1
1
1
1
1
1
1
1
MnO
x
x
@CSAC for 10 minutes are presented in Table 1. The result suggested that the
@CSAC could degrade both the acid and reactive dyes at pH 3. The anionic
MnO
dyes AR 1, AB 25, AB 40, AB 62, AB 193, RR 11 and RB74 all showed almost
complete degradation (>90%) after reaction with MnO @CSAC for 10 minutes.
x
These nine dyes have different chemical structures with different functional groups
that influence the reactivity of the molecules in oxidative degradation processes.
20
21
22
3.5. Stability test
The stability is an important property for an effective oxidant in effluent
treatment technologies. To evaluate the longevity of the oxidative degradation of
Page 16 of 33

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
MnO
solution at 50 mg/L. At the first cycle use, the degradation rate of AR 73 using
MnO @CSAC was almost 100% after 10 minutes. However, some loss of Mn in the
x
@CSAC, it was examined through the recycling potential with 100 mL of dye
x
remnant solution was observed (Figure 6a). In this particular case, we performed
synchrotron-based scanning transmission X-ray microscopy (STXM) imaging
analysis to determine Mn distribution on the MnO
(Figure 6b, 6c). This analysis suggested that MnO
both the surface and pores of the mesoporous CSAC treated by hydrothermal method,
but there was a slightly decrease of Mn from MnO @CSAC sample. To determine the
contribution of adsorption by MnO @CSAC, the adsorption isotherms of the raw
x
@CSAC before and after reaction
x
was successfully deposited on
x
1
1
1
1
1
1
1
1
1
1
2
2
2
x
CSAC were investigated. It can be observed from Figure 7a that the Langmuir
2
isotherm fits best with the experimental data (correlation coefficient R =0.9906),
2
while the low correlation coefficients (R =0.8360) show less agreement of Freundlich
isotherm with the experimental data. The adsorption capacity of the raw CSAC was
found to be 62.89 mg/g according to the fitted parameter of Langmuir equation
(Figure 7b). Therefore, under the above mentioned conditions, the raw CSAC
theoretically need to be reused thirteen continuous cycles to reach its maximum
adsorption capacity. Therefore, in order to determine the stability of MnO
and the oxidation degradation of AR 73 dye, the twenty continuous cycles of
MnO @CSAC were investigated (Figure 8). It is found that the oxidative degradation
of AR 73 by MnO @CSAC was still strong after twenty usage cycles, with a
maximum removal rate of 99.1% and a minimum removal rate of 89.7% from the
x
@CSAC
x
x
Page 17 of 33

1
2
same sample. The results indicated that the oxidation activity of the MnO
x
@CSAC
was relatively stable and efficient.
3
4. Conclusions
4
5
6
7
8
9
0
1
In this study, a facile and convenient method was proposed for the in situ
synthesis of nanostructured MnO on raw CSAC. MnO @CSAC obtained via a
x
x
hydrothermal method could efficiently and rapidly degrade AR 73 at pH 3 with a
maximum decolorization efficiency of 99.1% after just 10 minutes of contact. The
degradation intermediates and products were identified by GC-MS analysis. The
x
MnO @CSAC also could efficiently degrade other anionic dyes. Additionally, after
1
1
twenty continuous cycles 89.7% decolorization was still being achieved illustrating
x
the longevity of the MnO @CSAC oxidative capacity.
12
Acknowledgments
13
14
15
16
The authors acknowledge financial support from Zhejiang University-Focused
Photonics United Foundation, Zhejiang Provincial Natural Science Foundation of
China under Grant No. LY12B07006 and Intercross-Science Research Seed
Foundation for the Youth Scholars of Zhejiang University.
17
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Page 21 of 33

Table 1
Degradation efficiencies of common anionic dyes
Dye
Removal Rate (%)
Pictures of Experiment
AR1
RB194
RB74
98.8
72.0
98.2
Control
AB40
95.4
AB62
AB193
AB25
99.7
96.8
90.1
x
MnO @CSAC
AB1
79.4
92.0
RR11
The initial concentration of anionic dyes is 50 mg/L, volume is 10 mL, pH=3, T=25
C, and the dosage of MnO @CSAC is 0.01 g, and the reaction time is 10 minutes.
°
x
Page 22 of 33

Figure legends
x
Figure 1. (a) SEM image of the raw CSAC. (b) SEM image of MnO @CSAC (the
nanostructured MnO
the HT-CSAC (the raw CSAC treated by a hydrothermal method). (d) TEM image of
MnO nanowires. (e) TEM image of the raw CSAC. (f) TEM image of MnO @CSAC:
the green arrows designate the MnO nanowires.
Figure 2. XRD patterns of the raw CSAC, MnO
Figure 3. Decolorization of AR 73 with different materials (Initial concentration 50
mg/L, 10 mL, pH 3, T=25 °C, product dosage 0.01 g, including MnO @CSAC: the
nanostructured MnO -loaded CSAC by a hydrothermal method; N -MnO @CSAC:
the MnO -loaded CSAC by calcination under nitrogen atmosphere;
Air-MnO @CSAC: the MnO -loaded CSAC by calcination under an air atmosphere;
-CSAC obtained by calcination under a nitrogen atmosphere; Air-CSAC obtained
by calcination under an air atmosphere).
Figure 4. (a) pH sensitivity of MnO @CSAC (Initial concentration 50 mg/L, 100 mL,
T=25 °C, the MnO @CSAC dosage 0.1 g). (b) UV-Vis spectra of MnO @CSAC in
pH 3 treatment. (c) pH sensitivity of the raw CSAC (Initial concentration 50 mg/L,
00 mL, T=25 °C, the raw CSAC dosage 0.1 g).
Figure 5. Proposed pathway of AR 73 degradation under MnO
Figure 6. (a) Soluble Mn measured for pH 3 after 10 minutes in the decolorized dye
and blank solutions. (b) The qualitative distribution of Mn in MnO @CSAC before
reaction in a STXM image. (c) The qualitative distribution of Mn in MnO @CSAC
x
-loaded CSAC through a hydrothermal route). (c) SEM image of
2
x
x
2
x
and MnO @CSAC.
x
x
2
x
x
a
x
x
N
2
x
x
x
1
x
@CSAC system.
x
x
Page 23 of 33

after reaction in a STXM image.
Figure 7. (a) Adsorption isotherms fitted to Langmuir model and Freundlich model
for the raw CSAC. (b) The linear fitting curve of Langmuir model for the raw CSAC.
Figure 8. Degradation of AR 73 in the recycle experiment (Initial concentration 50
x
mg/L, 10 mL, pH 3, T=25 °C, MnO @CSAC dosage 0.01 g)
Page 24 of 33

Figure 1
Page 25 of 33

MnO @CSAC
x
MnO2
CSAC
10
20
30
40
50
60
70
2
Theta (degree)
Figure 2
Page 26 of 33

100
80
60
40
20
0
C
C
C
A
C
A
C
C
A
A
A
S
A
S
S
C
S
S
S
C
C
C
-
C
-
C
x@
x@
x@
ir
O
N 2
n
O
n
O
A
n
M
-M
N 2
-M
ir
A
Different products
Figure 3
Page 27 of 33

100
pH 3
pH 4
pH 5
pH 6
pH 7
80
60
40
20
0
0
2
4
6
8
10
Time (min)
2.5
2.0
1.5
1.0
0.5
0.0
(
b)
1
8
6
4min
2
0
0min
min
min
min
min
200
300
400
500
600
700
800
Wavelength(nm)
pH 3
(
c)
1
00
pH 4
pH 5
pH 6
pH 7
8
6
4
2
0
0
0
0
0
0
2
4
6
8
10
Time (min)
Figure 4
Page 28 of 33

HO
O
N
3+
-
2+
N
Mn
Mn
N
N
N
N
N
N
-
+
O3S
-e
-H
-
O3S
3
+
+
Mn
SO3-
SO -
3
-
-
e
2
Mn
N
O
O
H
2
O
N
N
N
N
N
N
H
O3S
+
-
H
N
O
-
-O3S
SO3-
SO3-
O
O
2
H
2
O
O
N
N
N NH
+
N
N
N
N
-
N
2
2
+
3+
Mn
Mn
N
N
N
N
-
-
e
O
OH
OH
HO
N
N
O
m/z: 166
m/z: 198
Figure 5
Page 29 of 33