catalysts
Article
Comparison of Efficiencies and Mechanisms of
Catalytic Ozonation of Recalcitrant Petroleum
Refinery Wastewater by Ce, Mg, and Ce-Mg Oxides
Loaded Al2O3
Chunmao Chen 1,† , Yu Chen 1,† , Brandon A. Yoza 2 , Yuhao Du 1 , Yuxian Wang 1 , Qing X. Li 3 ,
Lanping Yi 1 , Shaohui Guo 1 and Qinghong Wang 1, *
1
2
3
*
†
State Key Laboratory of Heavy Oil Processing, State Key Laboratory of Petroleum Pollution Control, China
University of Petroleum, Beijing 102249, China; [email protected] (C.C.);
[email protected] (Y.C.); [email protected] (Y.D.); [email protected] (Y.W.);
[email protected] (L.Y.); [email protected] (S.G.)
Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, HI 96822, USA;
[email protected]
Department of Molecular Biosciences and Bioengineering, University of Hawaii at Manoa, Honolulu,
HI 96822, USA; [email protected]
Correspondence: [email protected]; Tel.: +86-10-8973-3771
These authors contributed equally to this work.
Academic Editors: Shaobin Wang and Xiaoguang Duan
Received: 21 December 2016; Accepted: 21 February 2017; Published: 24 February 2017
Abstract: The use of catalytic ozonation processes (COPs) for the advanced treatment of recalcitrant
petroleum refinery wastewater (RPRW) is rapidly expanding. In this study, magnesium (Mg),
cerium (Ce), and Mg-Ce oxide-loaded alumina (Al2 O3 ) were developed as cost efficient catalysts
for ozonation treatment of RPRW, having performance metrics that meet new discharge standards.
Interactions between the metal oxides and the Al2 O3 support influence the catalytic properties, as well
as the efficiency and mechanism. Mg-Ce/Al2 O3 (Mg-Ce/Al2 O3 -COP) reduced the chemical oxygen
demand by 4.7%, 4.1%, 6.0%, and 17.5% relative to Mg/Al2 O3 -COP, Ce/Al2 O3 -COP, Al2 O3 -COP,
and single ozonation, respectively. The loaded composite metal oxides significantly increased the
hydroxyl radical-mediated oxidation. Surface hydroxyl groups (–OHs) are the dominant catalytic
active sites on Al2 O3 . These active surface –OHs along with the deposited metal oxides (Mg2+ and/or
Ce4+ ) increased the catalytic activity. The Mg-Ce/Al2 O3 catalyst can be economically produced,
has high efficiency, and is stable under acidic and alkaline conditions.
Keywords: petroleum refinery wastewater; catalytic ozonation; metal oxide; hydroxyl group
1. Introduction
Petroleum refinery wastewater (PRW) contains high levels of oil and petroleum-derived
chemicals [1]. Treatment of PRW prior to discharge is needed to minimize adverse impacts on human
and environmental health. After physico-chemical and biological treatments, PRW effluents from
treatment plants previously met the Wastewater Discharge Standard of China (GB 8978-1996). However,
due to increased regulations, they currently do not meet the recently revised Emission Standard of
Pollutants for the Petroleum Refining Industry of China (GB 31570-2015), and they now require
additional treatments. Biological treatment strategies are not applicable toward PRW reclamation,
as the chemicals of concern are often present in low concentration and are recalcitrant [2,3].
Catalytic ozonation processes (COPs) have been identified as efficient methods for the removal
of recalcitrant chemicals [4,5]. A wide variety of catalysts are used during COPs for this
Catalysts 2017, 7, 72; doi:10.3390/catal7030072
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purpose. However, synthesis and preparation are cost-intensive, limiting industrial applications.
As economical supports, activated carbon (AC) and alumina (Al2 O3 ) have good adsorption and
catalytic performances [6–9]. Active components loaded onto AC and Al2 O3 can further enhance the
catalytic efficiency and result in better affordability. Metal oxides including manganese (Mn) [10], iron
(Fe) [11], magnesium (Mg) [12], and cerium (Ce) [7,13] were found to significantly enhance chemical
removal in wastewaters when deposited onto AC. The use of low-cost reclaimed carbon-containing
wastes such as sewage sludge [14] and plastics [15] can make AC catalyst use cost-efficient. However,
AC catalysts lack durability, strength, and stability, which reduce their application potential.
Al2 O3 catalysts have shown excellent mechanical properties and catalytic efficiency [16,17].
The loading of metal oxides including Mn [18], Fe [19,20], nickel [21], and ruthenium [22] on Al2 O3 has
significantly increased the efficiency of ozonation for the treatment of recalcitrant compounds including
pharmaceuticals, dimethyl phthalate, 4-dichlorophenoxy, propionic acid, nitrobenzene, and oxalic
acid. Composite metal oxides loaded onto Al2 O3 can also enhance catalytic activity relative to single
metal oxides [23,24]. The use of Al-containing wastes such as red mud [25] and spent catalysts [26] as
raw materials, and the development of efficient metallic catalysts can further increase cost efficiency.
Ce oxides are widely used in COPs owing to their biological and chemical inertness, strong oxidation
power, and cyclic usability [13,27]. Ce oxides doped or loaded on AC [13], red mud [25], SBA-15 [28],
and MCM-41 [29] zeolites were effective at reducing pharmaceutical compounds, bezafibrate, dimethyl
phthalate, and p-chlorobenzoic. MgO nanocrystals [30] and MgO-loaded granular AC [12] have also
shown potential use for ozonation treatment and are effective for the treatment of azo dyes and catechol.
However, Mg and/or Ce oxide-loaded Al2 O3 as catalysts have not been previously reported for COP
treatment of recalcitrant compounds. Furthermore, few studies have actually applied COPs toward the
treatment of real recalcitrant PRW (RPRW) samples. Natural manganese sand ore was economical, but
had low efficiency (35.7% removal of chemical oxygen demand (COD) for 90 min) [31]. The spent fluid
catalytic cracking catalyst was cost-efficient, but it was difficult to maintain the catalytically active
components [26].
Catalytic ozonation often involves a variety of oxidation mechanisms. Decomposition of ozone
to more active hydroxyl radical (•OH) species and/or adsorbing chemicals to proximally react
with dissolved ozone is the predominant catalytic mechanism that has the greatest impact [16,32].
Protonated surface hydroxyl groups (–OHs) on MnOx /SBA-15 promote •OHs generation, and
thus enhance the ozonation of norfloxacin [33]. The formation of surface complexes such as
cobalt(II)-carboxylic acid accelerates the removal of p-chlorobenzoic in cobalt(II) oxide-aided
ozonation [34]. Lewis acid sites on β-FeOOH/Al2 O3 are reactive centers for the ozonation of
pharmaceuticals [35]. The surface basic groups on MgO enhance the transformation rate of ozone
to •OHs during the ozonation of 4-chlorophenol [36]. TiO2 /Al2 O3 promotes the ozonation of oxalic
acid by adsorbing oxalic acid and allowing it to directly react with ozone in solution in contrast to
•OH-mediated oxidation [37]. Studies of COPs have currently been focused on model compounds.
Few studies have used actual RPRW.
In this study, Mg, Ce, and Mg-Ce oxides loaded onto Al2 O3 (Mg/Al2 O3 , Ce/Al2 O3 , and
Mg-Ce/Al2 O3 , respectively) catalysts were prepared and characterized. The catalytic efficiency,
mechanism, and potential of the prepared catalysts for COP treatment of RPRW were investigated.
2. Results and Discussion
2.1. Characteristics of Catalysts
X-ray diffraction (XRD) patterns for Al2 O3 , Mg/Al2 O3 , Ce/Al2 O3 , Mg-Ce/Al2 O3 (I), and
Mg-Ce/Al2 O3 (II) all showed similar peak characteristics that are associated with Al2 O3 at about 38◦ ,
46◦ and 67◦ (Figure 1). Obvious XRD diffraction peaks of metal oxides were not observed due to the
high dispersion and low concentration of the metal oxides.
Catalysts 2017, 7, 72 Catalysts 2017, 7, 72
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Intensity
Intensity(a.u.)
(a.u.)
Mg-Ce/Al2O
O3 (II)
(II)
Mg-Ce/Al
2 3
Mg-Ce/Al2O
O3 (I)
(I)
Mg-Ce/Al
2 3
O3
Ce/Al2O
Ce/Al
2 3
Mg/Al2O
O3
Mg/Al
2 3
Al2O
O3
Al
2 3
20
20
40
60
40
60
2
Theta
(degree)
2 Theta (degree)
80
80
Figure
1. XRD patterns of Al22O
and metal oxide-loaded Al22O
O33 catalysts. catalysts.
Figure 1. XRD patterns of Al
O33 and metal oxide‐loaded Al
Figure 1. XRD patterns of Al
2O3 and metal oxide‐loaded Al2O3 catalysts. Deposited Mg and/or Ce oxides formed micro‐agglomerates in irregular shapes and sizes Deposited Mg Mg and/or and/or Ce Ce oxides oxides formed formed micro‐agglomerates micro-agglomerates in in irregular irregular shapes shapes and and sizes sizes
Deposited (Figure 2). The adsorption‐desorption isotherms (Figure 3a) and pore distributions (Figure 3b) varied (Figure
2).
The
adsorption-desorption
isotherms
(Figure
3a)
and
pore
distributions
(Figure
3b)
varied
(Figure 2). The adsorption‐desorption isotherms (Figure 3a) and pore distributions (Figure 3b) varied slightly among the different examined catalysts. According to the International Union of Pure and slightly among the different examined catalysts. According to the International Union of Pure and
slightly among the different examined catalysts. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the isotherms suggest the presence of a typical type IV Applied
Chemistry (IUPAC) classification, the isotherms suggest the presence of a typical type IV
Applied Chemistry (IUPAC) classification, the isotherms suggest the presence of a typical type IV mesopore structure [38]. The surface areas (S
BET), pore volumes (V
), and average pore sizes (D
) were mesopore
structure [38]. The surface areas (SBET
), pore volumes (VPPP), and average pore sizes (D
), and average pore sizes (Daaa) were ) were
mesopore structure [38]. The surface areas (S
BET), pore volumes (V
3
22/g, 0.36–0.38 m3/g, and 5.9–6.1 nm, respectively (Table 1). The contents of in the range of 245–250 m
3/g, and 5.9–6.1 nm, respectively (Table 1). The contents of in the range of 245–250 m2/g, 0.36–0.38 m
/g, 0.36–0.38 m
/g, and 5.9–6.1 nm, respectively (Table 1). The contents
in the range of 245–250 m
MgO and CeO
2 in Mg/Al
2
O
3
or Ce/Al
2
O
3
were 0.3 wt wt %. The contents of MgO MgO and CeO
CeO
in Mg‐
Mg‐
of
MgO
and
CeO
in
Mg/Al
O
or
Ce/Al
0.3%. wtThe %. contents The contents
of MgO
and 22CeO
2 O3 were
2 in
MgO and CeO2 in 2 Mg/Al2O3 2or 3Ce/Al2O3 were 0.3 of and in Ce/Al
2O3 (I) were both 0.3 wt % and those in Mg‐Ce/Al
2
O
3
(II) were both 0.9 wt % (Table 1). The low Mg-Ce/Al
O
(I)
were
both
0.3
wt
%
and
those
in
Mg-Ce/Al
O
(II)
were
both
0.9
wt
%
(Table
1).
2 3
2 3
Ce/Al2O3 (I) were both 0.3 wt % and those in Mg‐Ce/Al
2O3 (II) were both 0.9 wt % (Table 1). The low loadings of Mg and/or Ce oxides decreased the intensity of the functional surface groups (Figure 4a). The low loadings of Mg and/or Ce oxides decreased the intensity of the functional surface groups
loadings of Mg and/or Ce oxides decreased the intensity of the functional surface groups (Figure 4a). The point zero charges (pH
pzc) of unloaded and loaded Al22loaded
O33 had had Al
only small differences, ranging (Figure
4a).zero Thecharges point zero
charges
(pHpzc ) of unloaded
and
had only
small differences,
2 O3 small The point (pH
pzc) of unloaded and loaded Al
O
only differences, ranging from 8.1 to 8.3 (Figure 4b).
ranging from 8.1 to 8.3 (Figure
from 8.1 to 8.3 (Figure 4b).
4b).
(a) (a) (b) (b) (c) (c) (d) (d)
(e)
(e)
Figure 2. Scanning electron microscope (SEM) image of Al22O
O3 substrate (a) and transmission electron Figure 2. Scanning electron microscope (SEM) image of Al
Figure 2. Scanning electron microscope (SEM) image of Al2 O33 substrate (a) and transmission electron substrate (a) and transmission electron
microscope micrographs of loaded Al
2O3 with inset SEM images: (b) Mg/Al2O3; (c) Ce/Al2O3; (d) Mg‐
microscope micrographs of loaded Al
2O3 with inset SEM images: (b) Mg/Al2O3; (c) Ce/Al2O3; (d) Mg‐
microscope micrographs of loaded Al2 O3 with inset SEM images: (b) Mg/Al2 O3 ; (c) Ce/Al2 O3 ;
O33 (I); (e) Mg‐Ce/Al
(I); (e) Mg‐Ce/Al22O
O33 (II). (II). Ce/Al22O
Ce/Al
(d) Mg-Ce/Al
O (I); (e) Mg-Ce/Al
O (II).
2
3
2
3
Catalysts 2017, 7, 72 Catalysts 2017, 7, 72
Catalysts 2017, 7, 72 3
3
Quantity
Adsorbed
(cm /g(cm
STP)
Quantity
Adsorbed
/g STP)
250
a
Mg/Al2O3
Ce/Al O
Al2O3 2 3 Mg/Al2O3
Mg-Ce/Al O (I)
Ce/Al2O3 2 3
Mg-Ce/Al2O3 (II)
Mg-Ce/Al2O
(I)
3
250
200
a
1.2
b
b
Al2O3
Mg/Al
Al
O 2O3
2 3
Ce/Al2O
O3
Mg/Al
0.9
2
150
3
Mg-Ce/Al
Ce/Al
O 2O3 (I)
2 3
Mg-Ce/Al
O (II)
2 3(I)
Mg-Ce/Al O
0.9
Mg-Ce/Al2O3 (II)
200
1.2
BJH BJH
Adsorption
dV/dlog(D)
Pore Pore
Volume
Adsorption
dV/dlog(D)
Volume
Al2O3
4 of 14
4 of 14
4 of 14
2
0.6
3
Mg-Ce/Al2O3 (II)
0.6
150
100
100
0.3
0.3
50
50
0.0
0.0
0.2
0.4
0.6
0.8
1.0
10
100
0.0 1
Relative Pressure (P/P0)
Pore diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
1
10
100 Relative Pressure (P/P0)
Pore diameter (nm)
Figure 3. Isotherms (a) and pore distribution curves (b) using N
2 adsorption‐desorption of Al2O3 and metal oxide‐loaded Al2O3 catalysts. Figure 3. Isotherms (a) and pore distribution curves (b) using N
3 and Figure 3. Isotherms (a) and pore distribution curves (b) using N2 adsorption‐desorption of Al
2 adsorption-desorption of Al2O
2O
3 and
2OO
3 catalysts. metal oxide‐loaded Al
metal oxide-loaded Al
catalysts.
2 pore structures, 3
Table 1. Surface areas, and metal oxide contents of Al2O3 and metal oxide‐loaded Al2O3 catalysts. Table Table1. Surface 1. Surfaceareas, areas,pore structures, pore structures,and andmetal metaloxide oxidecontents contentsof ofAl
Al22OO3 3and andmetal oxide‐loaded metal oxide-loaded
Al
2OO
Surface Areas and Pore Structures Metal Oxide Contents (wt %) Al
2 3 catalysts. 3 catalysts.
Catalysts SSurface Areas and Pore Structures BET (m2/g) VP (cm3/g)
Da (nm)
MgO
CeO2 Metal Oxide Contents (wt %) Surface Areas and Pore Structures
Metal Oxide Contents (wt %)
Catalysts Al
2
O
3
250 0.36 6.1 ‐ ‐ 2 Catalysts
SBET (m2/g) VP (cm3/g)
Da (nm)
MgO
CeO
Mg/Al
Al2Al
O32O
O3 2 3
Ce/Al
2O3 Mg/Al
Mg/Al
2O23 O3
SBET (m2 /g)
250 250 250
246 250
250 V P (cm3 /g)
0.38 0.36 0.36
0.38 0.38
0.38 Ce/Al2 O33 (I) Mg‐Ce/Al
Ce/Al2O2O
3 Mg-Ce/Al
2 O3 (I)
Mg‐Ce/Al
2O3 (II) Mg-Ce/Al
3 (II)
Mg‐Ce/Al
2O23O
(I) 245 246
246 245
246 246
245 0.38 0.38
0.38 0.38
0.37 0.37
0.38 Mg‐Ce/Al2O3 (II) 246 0.37 a:IR
Da (nm)
0.31
0.33 0.31 0.33
0.90 0.90
0.33 6.1 0.92 0.90 14
pH values
pH values
CeO2
‐ 0.31 -
0.31 0.31
0.92 0.92
0.31 12
10
Transmittance
(a.u.)(a.u.)
Transmittance
0.30 ‐ 0.30
0.30 6.0 6.1
6.1 6.0
6.1 6.1
6.0 14
12
a:IR
MgO
5.9 6.1 6.1
6.1 5.9
5.9 b: pHPZC
b: pHPZC
10
8
Al2O3
Mg/Al2O3
Ce/Al O
Al2O3 2 3 Mg/Al2O3
Mg-Ce/Al O (I)
Ce/Al2O3 2 3
Mg-Ce/Al2O3 (II)
Mg-Ce/Al2O
(I)
3
Mg-Ce/Al O3 (II)
1500 2000 2 2500
3000
8
6
Al2O3
6
4
Mg/Al
Al
O 2O3
2 3
Ce/Al
O3
Mg/Al 2O
4
2
Mg-Ce/Al
Ce/Al
O 2O3 (I)
2 3
Mg-Ce/AlO
O (II)
2 3(I)
Mg-Ce/Al
2
3
2
3
(II) 14
8 Mg-Ce/Al
10 2O12
3
-1
pH values
Wavenumber (cm )
500 1000 1500 2000 2500 3000 3500
2
4
6
8
10
12
14
-1
pH values
Wavenumber (cm )
Figure 4. Infrared (IR) spectra (a) and pH
pzc values (b) of Al2O3 and metal oxide‐loaded Al2O3 catalysts. 500
1000
3500
2
2
4
6
Figure 4. Infrared (IR) spectra (a) and pHpzc values (b) of Al2 O3 and metal oxide-loaded
Al2 O3 catalysts.
Figure 4. Infrared (IR) spectra (a) and pHpzc values (b) of Al2O3 and metal oxide‐loaded Al2O3 catalysts. Figure 5 illustrates the UV‐vis spectra for the prepared catalysts. Al2O3 showed an absorption peak at around 260 nm. Mg/Al
2O3 exhibited a strong absorption centered at 200 nm and a wider low‐
Figure 5 illustrates the UV-vis
spectra for the prepared catalysts. Al22OO3 showed an absorption 3 showed an absorption
Figure 5 illustrates the UV‐vis spectra for the prepared catalysts. Al
intensity absorption profile at 220–380 nm. The large initial absorption peak is associated with micro‐
peak
at
around
260
nm.
Mg/Al
O
exhibited
a
strong
absorption
centered
at 200 nm and a wider
2 3
peak at around 260 nm. Mg/Al2O3 exhibited a strong absorption centered at 200 nm and a wider low‐
2+ on the Al2O3 surface and the lower broad profile due to the aggregates from highly dispersed Mg
low-intensity absorption profile at 220–380 nm. The large initial absorption peak is associated with
intensity absorption profile at 220–380 nm. The large initial absorption peak is associated with micro‐
aggregates from highly dispersed Mg2+ on the Al2O3 surface and the lower broad profile due to the Catalysts 2017, 7, 72
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Catalysts 2017, 7, 72 5 of 14
micro-aggregates from highly dispersed Mg2+ on the Al2 O3 surface and the lower broad profile
Al2O3 substrate [12]. Ce/Al2O3 had a broad 200–380 nm absorption profile with a peak at 260 nm, due to the Al2 O3 substrate [12]. Ce/Al2 O3 had a broad 200–380 nm absorption profile with
a peak
which can be associated with overlapping transitional charge transfers that occur between O2− → Ce4+ at 260 nm, which can be associated with overlapping transitional charge transfers that occur between
[39]. Mg‐Ce/Al
2O3 (I) and Mg‐Ce/Al2O3 (II) both exhibited stronger absorption profiles between 200 O2− → Ce4+ [39]. Mg-Ce/Al2 O3 (I) and Mg-Ce/Al2 O3 (II) both exhibited stronger absorption profiles
and 380 nm and had a similar peak at 260 nm, which can be attributed to increased concentrations of between 200 and 380 nm and had a similar peak at 260 nm, which can be attributed to increased
deposited metal oxides and also to the interactions between them. concentrations of deposited metal oxides and also to the interactions between them.
Al2O3
Mg/Al2O3
Absorbance (a.u.)
Ce/Al2O3
200
Mg-Ce/Al2O3 (I)
Mg-Ce/Al2O3 (II)
300
400
500
600
Wavelength (nm)
700
800
Figure 5. UV-vis patterns of Al O and metal oxide-loaded Al O3 catalysts.
Figure 5. UV‐vis patterns of Al22O33 and metal oxide‐loaded Al22O3 catalysts. Figure 6a 6a presents presents the the Mg1s Mg1s spectra spectra of of catalysts catalysts based based on on X‐ray X-ray photoelectron photoelectron spectroscopy spectroscopy
Figure 2+ oxides was centered at 1303 eV [40]. The asymmetrical
(XPS). The binding energy of Mg1s from Mg2+
(XPS). The binding energy of Mg1s from Mg
oxides was centered at 1303 eV [40]. The asymmetrical distribution of the Mg1s peak for Mg/Al
O3 was likely due to strong interactions between Mg and
distribution of the Mg1s peak for Mg/Al
2O2
3 was likely due to strong interactions between Mg and the the
For Mg/Al
the surface
content
of MgO
(0.71
was
greaterthan thanthe thebulk bulk
Al
2OAl
3 support. For Mg/Al
2O3, the content of MgO (0.71 wt wt
%) %)
was greater 2 O3 support.
2 O3 ,surface content
(0.30
wt
%),
suggesting
surface
enrichment.
Mg-Ce/Al
O
(I)
displayed
a
similar
Mg1s
peak
content (0.30 wt %), suggesting surface enrichment. Mg‐Ce/Al22O3 (I) displayed a similar Mg1s peak 3
compared to Mg/Al2O
O33 interactions. The increased interactions. The increased
compared to Mg/Al
3, having only a slight influence of Mg and Al
2O
3 , having only a slight influence of Mg and Al22O
intensity of of the the Mg1s Mg1s peak peak for for Mg‐Ce/Al
Mg-Ce/Al2O
O
(I)
suggests
that
the
surface
distributionof ofMg Mg was was
3
(I) suggests that the surface distribution intensity 2 3
enhanced through the introduction of Ce. The surface content of MgO (0.78 wt %) for Mg-Ce/Al2 O23O(I)
enhanced through the introduction of Ce. The surface content of MgO (0.78 wt %) for Mg‐Ce/Al
3 waswas higher
than than that (0.71
%) for
symmetrical
distribution
(I) higher that wt
(0.71 wt Mg/Al
%) for 2O3. Mg‐Ce/Al
2O3 (II) a showed a symmetrical 2 OMg/Al
3 . Mg-Ce/Al
2 O3 (II) showed
of the Mg1s peak relative to Mg/Al2 O3 and Mg-Ce/Al
a reduction of the Mg and
distribution of the Mg1s peak relative to Mg/Al
2O3 and Mg‐Ce/Al
2O3 (I), suggesting a reduction of 2 O3 (I), suggesting
Al2 O3 interaction.
The surface content of MgO for Mg-Ce/Al2 O3 (II) was2O0.75
wt %, similar to those of
the Mg and Al
2O3 interaction. The surface content of MgO for Mg‐Ce/Al
3 (II) was 0.75 wt %, similar Mg/Al2 O3 (0.71 wt
and Mg-Ce/Al2 O3 (I) (0.78
%), although the loading of MgO of the former
to those of Mg/Al
2O3%)
(0.71 wt %) and Mg‐Ce/Al
2O3wt
(I) (0.78 wt %), although the loading of MgO of was three-fold of the latter. This demonstrates that greater concentrations of MgO exist in bulk rather
the former was three‐fold of the latter. This demonstrates that greater concentrations of MgO exist in than being highly dispersed on the surface of Mg-Ce/Al2 O3 (II). 2O3 (II). bulk rather than being highly dispersed on the surface of Mg‐Ce/Al
The 3d XPS spectra for Ce are complex and usually exhibit 10 peaks having a range of binding
The 3d XPS spectra for Ce are complex and usually exhibit 10 peaks having a range of binding energy between 880–920 eV: five spin-orbit split doublets corresponding to the 3d3/2 (high binding
energy between 880–920 eV: five spin‐orbit split doublets corresponding to the 3d3/2 (high binding energy component) and 3d5/2 (low binding energy component) states. Figure 6b presents the 3d
energy component) and 3d5/2 (low binding energy component) states. Figure 6b presents the 3d XPS XPS spectra from Ce catalysts. The2OCe/Al
spectra from Ce catalysts. The Ce/Al
3 catalyst did not exhibit noticeable 3d peaks that are associated 2 O3 catalyst did not exhibit noticeable 3d peaks that are
associated
with CeO2 , likely due to the low surface distribution.
Mg-Ce/Al2 O3
with CeO
2, likely due to the low surface distribution. Mg‐Ce/Al
2OMg-Ce/Al
3 (I) and Mg‐Ce/Al
2O3 (II), however, 2 O3 (I) and
4+ oxides [41]. The surface content of CeO
(II), however, exhibited well-developed 3d peaks
from the Ce4+ oxides [41]. The surface
content of
2 (0.67 wt %) exhibited well‐developed 3d peaks from the Ce
CeO
(0.67
wt
%)
on
Mg-Ce/Al
O
(I)
was
greater
(0.40
wt
%)
relative
to
Ce/Al
O
only
(Table 2).
on Mg‐Ce/Al
2O3 (I) was greater (0.40 wt %) relative to Ce/Al
2O3 only (Table 2). The introduction of 2
2 3
2 3
The introduction of Mg further promoted the surface enrichment of Ce. This was also observed
with
Mg further promoted the surface enrichment of Ce. This was also observed with Mg‐Ce/Al
2O3 (II). Mg-Ce/Al2 O3 (II). The surface
content of CeO
on
Mg-Ce/Al
O
(II)
increased
to
2.57
wt
%
after
triple
The surface content of CeO
2 on Mg‐Ce/Al
2O3 (II) increased to 2.57 wt % after triple CeO
2
loadings. 2
2 3
CeOsurface Theratios surface
molar+ ratios
(Mg1s +of Ce3d/Al2p)
(II)
were
0.012
The molar (Mg1s Ce3d/Al2p) Mg‐Ce/Al2Oof3 Mg-Ce/Al
(I) and (II) 2 O
were 0.012 and 0.0198, 2 loadings.
3 (I) and
and 0.0198, respectively, suggesting a highly disperse surface distribution of metal oxides, which is
respectively, suggesting a highly disperse surface distribution of metal oxides, which is supported by the XRD results. The surface molar ratio of Ce3d to Mg1s of Mg‐Ce/Al2O3 (II) (0.8) increased in Catalysts 2017, 7, 72
6 of 14
Catalysts 2017, 7, 72 6 of 14
supported by the XRD results. The surface molar ratio of Ce3d to Mg1s of Mg-Ce/Al2 O3 (II) (0.8) comparison to Mg‐Ce/Al2O3 (I) (0.2). The increases likely enhanced the interactions between the Ce
increased in comparison to Mg-Ce/Al2 O3 (I) (0.2). The increases likely enhanced the interactions
and Mg oxides on the surface of Mg‐Ce/Al2O3 (II), relative to Mg‐Ce/Al2O3 (I). between the Ce and Mg oxides on the surface of Mg-Ce/Al2 O3 (II), relative to Mg-Ce/Al2 O3 (I).
a
Mg1s
54000
48000
Mg-Ce/Al2O3 (I)
45000
40000
38000
36000
42000
b
Ce3d
-1
51000
39000
42000
Mg-Ce/Al2O3 (II)
-1
Counts (s )
57000
Counts (s )
60000
Mg/Al2O3
1299 1302 1305 1308
Binding Energy (eV)
34000
Mg-Ce/Al2O3 (II)
Mg-Ce/Al2O3 (I)
Ce/Al2O3
880 890 900 910 920
Binding Energy (eV)
Figure
6. X-ray photoelectron spectroscopy (XPS) spectra of Mg1s (a) and Ce3d (b) of metal
Figure 6. X‐ray photoelectron spectroscopy (XPS) spectra of Mg1s (a) and Ce3d (b) of metal oxide‐
oxide-loaded
Al2 O3 catalysts.
loaded Al2O3 catalysts. Table
2. Molar ratios of Mg and Ce to Al, and surface contents of MgO and CeO22 for catalysts by for catalysts by Table 2. Molar ratios of Mg and Ce to Al, and surface contents of MgO and CeO
XPS
analysis.
XPS analysis. Items
Items Mg/Al
Mg/Al
Ce/Al
2 O3 2O3 Ce/Al
2 O23O3 Mg-Ce/Al22O
(I)
Mg‐Ce/Al
O33 (I) Mg-Ce/Al22O
(II)
Mg‐Ce/Al
O33 (II) 0.0090.009 ‐ 0.009
- ‐ 0.0012
0.0012 0.0012
0.010
0.010 0.002
0.2
0.002 0.012
0.0099
0.0099 MgO
0.71
CeO2
- 0.009 Mg1s + Ce3d/Al2p 0.40
0.0012 0.78
0.67
0.012 0.75
2.57
0.0198 Mg1s/Al2p
Mg1s/Al2p Ce3d/Al2p
Surface molar ratios
Ce3d/Mg1s
Ce3d/Al2p Surface molar ratios Mg1s + Ce3d/Al2p
Ce3d/Mg1s Surface contents (wt %)
Surface contents (wt %) 2.2. Efficiencies
of Catalysts
‐ ‐ 0.2 0.0079
0.8
0.0079 0.0198
0.8 MgO 0.71 ‐ 0.78 0.75 CeO2 ‐ 0.40 0.67 2.57 The catalytic COD reduction efficiency of RPRW was investigated by measuring the effect
2.2. Efficiencies of Catalysts of
adsorption, single ozonation and COPs. Adsorption on these catalysts was found to reach
saturation
within 40 min. The catalysts only weakly adsorbed target chemicals in RPRW and
The catalytic COD reduction efficiency of RPRW was investigated by measuring the effect of increased
COD
removal by only 6.3%–8.5% after 40 min (Figure 7a). Due to similar surface areas
adsorption, single ozonation and COPs. Adsorption on these catalysts was found to reach saturation and
pore
structures,
no significant differences in adsorption capacity among catalysts were observed
within 40 min. The catalysts only weakly adsorbed target chemicals in RPRW and increased COD (Table
1). After 40 min, COD removal by single ozonation, Al2 O3 -catalyzed ozonation (Al2 O3 -COP),
removal by only 6.3%–8.5% after 40 min (Figure 7a). Due to similar surface areas and pore structures, Mg/Al
2 O3 -COP, Ce/Al2 O3 -COP, Mg-Ce/Al2 O3 (I)-COP, and Mg-Ce/Al2 O3 (II)-COP was 34.3%,
no significant differences in adsorption capacity among catalysts were observed (Table 1). After 40 45.9%,
47.2%,
47.8%, 51.9%,
andozonation, 52.7%, respectively
(Figure 7b).
These results
fact
min, COD removal by single Al2O3‐catalyzed ozonation (Al2O3further
‐COP), support
Mg/Al2Othe
3‐COP, that
the2Ocatalytic
process is driven primarily by ozonation rather than simple adsorption. Different COP
Ce/Al
3‐COP, Mg‐Ce/Al2O3 (I)‐COP, and Mg‐Ce/Al2O3 (II)‐COP was 34.3%, 45.9%, 47.2%, 47.8%, treatments
increased
COD removal by 18.4%–11.6% compared with single ozonation. These differences
51.9%, and 52.7%, respectively (Figure 7b). These results further support the fact that the catalytic are
the results of catalytic metal oxide-driven ozonation processes. Mg-Ce/Al2 O3 (I) exhibited a better
process is driven primarily by ozonation rather than simple adsorption. Different COP treatments performance
compared with Mg/Al2 O3 and Ce/Al2 O3 , while greater loadings of Mg and Ce were only
increased COD removal by 18.4%–11.6% compared with single ozonation. These differences are the slightly
more
effective.
The surface
areas and
pore structures
were
comparable
for these catalysts and
results of catalytic metal oxide‐driven ozonation processes. Mg‐Ce/Al
2O3 (I) exhibited a better they
had only negligible differences2on
the results.2OThe
enhanced catalytic activity of Mg-Ce/Al2 O3
performance compared with Mg/Al
O3 and Ce/Al
3, while greater loadings of Mg and Ce were only (I)
was
due
to
interactions
between
the
metal
oxides,
and
between the metal oxides and the Al2 O3
slightly more effective. The surface areas and pore structures were comparable for these catalysts and support,
as
well
as
their
environments.
The
limited
enhancement
of catalytic activity for Mg-Ce/Al
O3
they had only negligible differences on the results. The enhanced catalytic activity of Mg‐Ce/Al
2O23 (I) (II)
was
related to between the presence
of more
MgOand in bulk,
as well
the partial
of3 was due likely
to interactions the metal oxides, between the as
metal oxides occupation
and the Al2O
support, as well as their environments. The limited enhancement of catalytic activity for Mg‐Ce/Al2O3 (II) was likely related to the presence of more MgO in bulk, as well as the partial occupation of Al2O3 Catalysts 2017, 7, 72
Catalysts 2017, 7, 72 7 of 14
7 of 14
active surface sites by Mg and Ce oxides. No obvious leaching of Ce and Mg elements was detected Al
2 O3 active surface sites by Mg and Ce oxides. No obvious leaching of Ce and Mg elements was
in Mg‐Ce/Al
2O3 (I)‐and (II)‐COPs, suggesting good stability of the catalysts. COD removals of RPRW detected
in Mg-Ce/Al
2 O3 (I)-and (II)-COPs, suggesting good stability of the catalysts. COD removals
after 10 repeated uses of three COPs were monitored (Figure 7c). The Mg‐Ce/Al
2O3 (I)‐COP (51.9%–
of RPRW after 10 repeated uses of three COPs were monitored (Figure 7c). The Mg-Ce/Al
2 O3 (I)-COP
50.7%) and Mg‐Ce/Al
2O3 (II)‐COP (52.7%–49.6%) remained stable, suggesting high reusability and (51.9%–50.7%) and Mg-Ce/Al2 O3 (II)-COP (52.7%–49.6%) remained stable, suggesting high reusability
stability of the catalysts. The COD value from effluents after Mg‐Ce/Al
2O3 (I)‐COP met the Emission and
stability of the catalysts. The COD value from effluents after Mg-Ce/Al
2 O3 (I)-COP met the
Standard of Pollutants for the Petroleum Refining Industry of China (GB 31570‐2015). Emission Standard of Pollutants for the Petroleum Refining Industry of China (GB 31570-2015).
a
b
50
8
COD removals (%)
COD removals (%)
10
6
Al2O3
4
Mg/Al2O3
Ce/Al2O3
2
Mg-Ce/Al2O3 (I)
40
30
Single ozonation
Al2O3-COP
Mg/Al2O3-COP
Ce/Al2O3-COP
20
Mg-Ce/Al2O3 (I)-COP
Mg-Ce/Al2O3 (II)-COP
Mg-Ce/Al2O3 (II)
0
0
10
20
30
Time (min)
c
COD removals (%)
54
10
40
0
10
20
30
Time (min)
40
Mg-Ce/Al2O3 (II) -COP
52
50
Mg-Ce/Al2O3 (I) -COP
48
46
44
42
Al2O3-COP
0
2
4
6
Runs
8
10
Figure
7. COD removals for RPRW by adsorption (a); by single ozonation and COPs (b); and by 10
Figure 7. COD removals for RPRW by adsorption (a); by single ozonation and COPs (b); and by 10 repeated
uses of COPs (c) (note: 0.5 g catalyst, 5 mg/min ozone, and 30 ◦ C).
repeated uses of COPs (c) (note: 0.5 g catalyst, 5 mg/min ozone, and 30 °C). 2.3.
Mechanisms of Catalytic Ozonation
2.3. Mechanisms of Catalytic Ozonation Numerous
have
suggested
thatthat •OH•OH generation
induced
by various
catalystscatalysts can promote
Numerous reports
reports have suggested generation induced by various can the
removal of chemicals during COPs. In order to determine whether •OH generation was occurring
promote the removal of chemicals during COPs. In order to determine whether •OH generation was during
COP treatment, tert-butanol (tBA) and sodium bicarbonate (NaHCO3 ) were used
as inhibitors
occurring during COP treatment, tert‐butanol (tBA) and sodium bicarbonate (NaHCO
3) were used as during
testing.
The testing. introduction
tBA and NaHCO
decreased
the
COD
reduction
efficiency
(Figure 8).
inhibitors during The of
introduction of tBA and NaHCO
3
decreased the COD reduction 3
The
results suggest that COD removal occurred primarily via •OH-mediated oxidation. The decreased
efficiency (Figure 8). The results suggest that COD removal occurred primarily via •OH‐mediated extent
of COD removal by both tBA and NaHCO3 in Mg-Ce/Al2 O3 (I)-COP
and Mg-Ce/Al
oxidation. The decreased extent of COD removal by both tBA and NaHCO
3 in Mg‐Ce/Al
2O3 (I)‐COP 2 O3
(II)-COP
was greater
than that of the other COPs. The results again suggest that the use of low
and Mg‐Ce/Al
2O3 (II)‐COP was greater than that of the other COPs. The results again suggest that the concentrations
of composite Mg and Ce deposited onto Al2 O3 can further
enhance •OH generation
use of low concentrations of composite Mg and Ce deposited onto Al
2O3 can further enhance •OH Catalysts
2017, 7, 72
Catalysts 2017, 7, 72 Catalysts 2017, 7, 72 8 of 14
8 of 14
8 of 14
generation compared to Mg/Al
generation compared to Mg/Al22O
O33 and Ce/Al
and Ce/Al22O
O33. COD reduction still occurred in spite of the addition . COD reduction still occurred in spite of the addition compared to Mg/Al2 O3 and Ce/Al2 O3 . COD reduction still occurred in spite of the addition of •OH
of •OH scavengers, and was a result of direct ozonation. of •OH scavengers, and was a result of direct ozonation. scavengers, and was a result of direct ozonation.
70
70
No
No OH
OH scavenger
scavenger
tBA
tBA
NaHCO
NaHCO33
COD removals
removals (%)
(%)
COD
60
60
50
50
40
40
30
30
20
20
10
10
Al
Al22O
O33
Mg/Al
Mg/Al22O
O33
Ce/Al
O33 Mg‐Ce/Al
Ce/Al22O
Mg‐Ce/Al22O
O33 (I)
(I)
Catalysts
Catalysts
Mg‐Ce/Al
Mg‐Ce/Al22O
O33 (II)
(II)
Figure 8. Influence of •OH scavengers (1 g/L) on COD removals of RPRW over COPs (note: 0.5 g Figure
8. Influence of •OH scavengers (1 g/L) on COD removals of RPRW over COPs (note: 0.5 g
Figure 8. Influence of •OH scavengers (1 g/L) on COD removals of RPRW over COPs (note: 0.5 g catalyst, 5 mg/min ozone, 30 °C and 40 min; 1g/L •OH scavengers). catalyst,
5 mg/min ozone, 30 ◦ C and 40 min; 1g/L •OH scavengers).
catalyst, 5 mg/min ozone, 30 °C and 40 min; 1g/L •OH scavengers). It is believed that active catalytic surfaces can induce ozone to form •OH, accelerating chemical It is believed that active catalytic surfaces can induce ozone to form •OH, accelerating chemical It
is believed that active catalytic surfaces can induce ozone to form •OH, accelerating chemical
degradation. Surface –OHs on the catalytic surface are known to have an impact during this process degradation. Surface –OHs on the catalytic surface are known to have an impact during this process degradation.
Surface –OHs on the catalytic surface are known to have an impact during this
[42–44]. Solution pH values near the pH
pzc [42–44]. Solution pH values near the pH
pzc of a catalyst can result in accelerated •OH generation [45]. of a catalyst can result in accelerated •OH generation [45]. process [42–44]. Solution pH values near
the pHpzc of a catalyst can result in accelerated •OH
Initial pH values (about 4.06, 8.15, and 10.21) were examined and found to have a significant impact Initial pH values (about 4.06, 8.15, and 10.21) were examined and found to have a significant impact generation [45]. Initial pH values (about 4.06, 8.15, and 10.21) were examined and found to have a
on COD removal efficiency during COP treatment (Figure 9). on COD removal efficiency during COP treatment (Figure 9). significant
impact on COD removal efficiency during COP treatment (Figure 9).
70
70
pH=4.06
pH=4.06
pH=8.15
pH=8.15
pH=10.21
pH=10.21
CODremovals
removals(%)
(%)
COD
60
60
50
50
40
40
30
30
20
20
10
10
00
Al
Al22O
O33
Mg/Al
Mg/Al22O
O33
Ce/Al
O33
Ce/Al22O
Mg-Ce/Al
Mg-Ce/Al22O
O33 (I)
(I)
Catalysts
Catalysts
Mg-Ce/Al
Mg-Ce/Al22O
O33(II)
(II)
Figure
9. Influence of initial pH values on COD removals of RPRW over COPs (note: 0.5 g catalyst, 5
Figure 9. Influence of initial pH values on COD removals of RPRW over COPs (note: 0.5 g catalyst, 5 Figure 9. Influence of initial pH values on COD removals of RPRW over COPs (note: 0.5 g catalyst, 5 mg/min
ozone, 30 ◦ C and 40 min).
mg/min ozone, 30 °C and 40 min). mg/min ozone, 30 °C and 40 min). Low initial pH values around 4.06 resulted in low treatment efficiency, likely due to the direct Low initial pH values around 4.06 resulted in low treatment efficiency, likely due to the direct Low
initial pH values around 4.06 resulted in low treatment efficiency, likely due to the direct
oxidation of molecular ozone. Indirect oxidation was found to also occur at alkaline pH and was a oxidation of molecular ozone. Indirect oxidation was found to also occur at alkaline pH and was a oxidation
of molecular ozone. Indirect oxidation was found to also occur at alkaline pH and was
result of the •OH produced during ozone decomposition. Efficient COD removal was, however, high result of the •OH produced during ozone decomposition. Efficient COD removal was, however, high Catalysts 2017, 7, 72
9 of 14
Catalysts 2017, 7, 72 9 of 14
a result of the •OH produced during ozone decomposition. Efficient COD removal was, however,
for five of the COP treatments, having an initial pH value around 8.15. Increasing such a pH value high for five of the COP treatments, having an initial pH value around 8.15. Increasing such a pH
from 8.15 to 10.21 resulted in a decreased COD reduction efficiency for Al2O3‐COP, but it was value from 8.15 to 10.21 resulted in a decreased COD reduction efficiency for Al2 O3 -COP, but it was
increased for Mg/Al2O3‐COP, Ce/Al2O3‐COP, Mg‐Ce/Al2O3 (I)‐COP, and Mg‐Ce/Al2O3 (II)‐COP. The increased for Mg/Al2 O3 -COP, Ce/Al2 O3 -COP, Mg-Ce/Al2 O3 (I)-COP, and Mg-Ce/Al2 O3 (II)-COP.
magnitudes of surface –OHs from the greatest to smallest for the catalysts were Al2O3 > Mg/Al2O3 ≈ The magnitudes of surface –OHs from the greatest to smallest for the catalysts were Al2 O3 > Mg/Al2 O3
Mg‐Ce/Al2O3 (I) > Ce/Al2O3 > Mg‐Ce/Al2O3 (II), according to the O1s XPS spectra (Figure 10) [46]. ≈ Mg-Ce/Al2 O3 (I) > Ce/Al2 O3 > Mg-Ce/Al2 O3 (II), according to the O1s XPS spectra (Figure 10) [46].
200000
-
OH
100000
50000
200000
150000
-
OH
100000
50000
200000
-1
-1
Counts (s )
2-
OH
100000
50000
0
528 530 532 534
Binding Energy (eV)
O
-
OH
0
528 530 532 534
Binding Energy (eV) e: Mg-Ce/Al2O3 (II)
150000
Counts (s )
200000
O
2-
50000
d: Mg-Ce/Al2O3 (I)
150000
100000
0
528 530 532 534
Binding Energy (eV)
0
528 530 532 534
Binding Energy (eV)
c: Ce/Al2O3
150000
2-
O
-1
2-
O
Counts (s )
150000
-1
Counts (s )
200000
b: Mg/Al2O3
Counts (s-1)
a: Al2O3
100000
50000
-1
2-
O
OH
0
528 530 532 534
Binding Energy (eV) Figure 10. XPS spectra of O1s of Al
O3 and metal oxide‐loaded Al
2O3 catalysts. (a) Al
2O3; (b) Mg/Al
3; Figure
10. XPS spectra of O1s 2of
Al2 O3 and metal oxide-loaded
Al2 O3 catalysts.
(a) Al22O3;
2O
; (d) Mg‐Ce/Al
2O
(I); (e) Mg‐Ce/Al
3 (II). (c) Ce/Al
(b)
Mg/Al
Mg-Ce/Al2 O32O
(I);
(e) Mg-Ce/Al2 O3 (II).
2 3O
3 ; (c) Ce/Al2 O
3 ;3(d)
Al2O
3‐COP was the most efficient at an initial pH value of 8.15, close to its pHpzc value (8.19), due Al
2 O3 -COP was the most efficient at an initial pH value of 8.15, close to its pHpzc value (8.19),
to highly active surface –OHs. The other COPs were the most efficient at an initial pH value of 10.21, due to highly active surface –OHs. The other COPs were the most efficient at an initial pH value
away from their pHtheir
pzc values (8.1~8.3). This suggests that the active sites on the catalysts were of
10.21,
away
from
pHpzc values (8.1~8.3). This suggests that the active sites on the catalysts
modified due to the introduction of Mg and/or oxides on Al
O3. Mg/Al
3 and Ce/Al2O3 exhibited were modified due to the introduction of Mg and/or oxides 2on
Al2 O3 . 2O
Mg/Al
2 O3 and Ce/Al2 O3
higher COD removal efficiency in comparison to Al
2O3, despite the lower surface –OHs. This exhibited higher COD removal efficiency in comparison to Al2 O3 , despite the lower surface –OHs. This
illustrates the increased effect due to an increase in the dispersion of Mg
illustrates
the increased effect due to an increase in the dispersion of Mg or Ce oxides on the surface. or Ce oxides on the surface.
Ce/Al2OO3 had lower surface –OHs compared to Mg/Al
2O3. Furthermore, the surface molar ratio of Ce/Al
had
lower
surface
–OHs
compared
to
Mg/Al
O
.
Furthermore,
the surface molar ratio of
2 3
2 3
Ce3d/Al2p (0.0012) on Ce/Al
2O3 was lower than that of Mg1s/Al2p (0.009) on Mg/Al2O3. However, a Ce3d/Al2p (0.0012) on Ce/Al2 O3 was lower than that of Mg1s/Al2p (0.009) on Mg/Al2 O3 . However,
using Ce/Al
Ce/Al2OO3 was observed when compared with asmall smallincrease increasein inefficiency efficiencyfor for COD COD removal removal using
2 3 was observed when compared with
2+ oxide. The increased COD removal Mg/Al
2O3, suggesting higher Ce oxide catalytic activity over Mg2+
Mg/Al2 O3 , suggesting higher Ce oxide catalytic activity over Mg oxide. The increased COD removal
for Mg‐Ce/Al
for
Mg-Ce/Al22OO3 (I) and (II) was due to an enhanced surface distribution of the metal oxides when 3 (I) and (II) was due to an enhanced surface distribution of the metal oxides when
compared to Mg/Al2O3 and Ce/Al2O3, despite the lower surface –OHs. For Mg/Al2O3, Ce/Al2O3, Mg‐
Ce/Al2O3 (I) and (II), dispersion of the Ce and/or Mg oxides increased available active sites and Catalysts 2017, 7, 72
Catalysts 2017, 7, 72 10 of 14
10 of 14
accelerated COD removal during the COP treatment. However, the activity contribution from surface compared to Mg/Al2 O3 and Ce/Al2 O3 , despite the lower surface –OHs. For Mg/Al2 O3 , Ce/Al2 O3 ,
–OHs is still important for the COP treatment, especially when the low surface loading of Ce and/or Mg-Ce/Al2 O3 (I) and (II), dispersion of the Ce and/or Mg oxides increased available active sites and
Mg oxides is considered. Therefore, the Ce and/or Mg oxides the
and surface –OHs likely have a co‐
accelerated
COD
removal during
the COP
treatment.
However,
activity
contribution
from
surface
functional activity on metal oxide‐loaded Al
2O3. At an initial pH value of 4.06, Mg‐Ce/Al2O3 (II) –OHs is still important for the COP treatment, especially when the low surface loading of Ce and/or Mg
exhibited low COD Therefore,
removal efficiency, suggesting poor For have
Mg‐Ce/Al
2O3 (II), oxides is considered.
the Ce and/or
Mg oxides
andacidic surfacetolerance. –OHs likely
a co-functional
interactions between MgO and the Al
2O3 support are weak, causing the dissolution of MgO and activity on metal oxide-loaded Al2 O3 . At an initial pH value of 4.06, Mg-Ce/Al2 O3 (II) exhibited
4+ oxides under these acidic conditions. For Mg/Al2O3, Ce/Al2O3, and Mg‐Ce/Al2O3 (I), nearby Ceremoval
low COD
efficiency, suggesting poor acidic tolerance. For Mg-Ce/Al2 O3 (II), interactions
interactions between the metal oxides and the Al
2O3 support are strong, maintaining higher stability. between MgO and the Al2 O3 support are weak, causing
the dissolution of MgO and nearby Ce4+ oxides
The acid‐resistant metal oxide‐loaded Al
2O3 catalysts exhibited higher COD removal efficiency under these acidic conditions. For Mg/Al2 O3 , Ce/Al2 O3 , and Mg-Ce/Al2 O3 (I), interactions between
compared to Al2O
3 from an increase in direct oxidation. The metal oxide‐loaded Al2O3 catalysts the metal oxides
and
the Al2 O3 support are strong, maintaining higher stability. The acid-resistant
provided a platform for adsorption of chemicals and/or ozone molecules, resulting in higher metal oxide-loaded Al2 Othe 3 catalysts exhibited higher COD removal efficiency compared to Al2 O3
reactivities. These are strongly influenced by the surface metal oxide composition, the coordinated from an increase in direct oxidation. The metal oxide-loaded Al2 O3 catalysts provided a platform
environmental distribution, and the interactions between the metal oxides and reactivities.
the Al2O3 support for the adsorption
of chemicals
and/or
ozone molecules,
resulting
in higher
These
(Figure 11). are strongly influenced by the surface metal oxide composition, the coordinated environmental
distribution, and the interactions between the metal oxides and the Al2 O3 support (Figure 11).
Figure 11. 11. Proposed Proposed ozonation ozonation mechanisms mechanisms of of chemicals chemicals in in RPRW RPRW upon upon metal metal oxide‐loaded oxide-loaded Figure Al
O
catalysts.
Al22O33 catalysts. 3. Materials and Methods 3. Materials and Methods 3.1. Preparation of Catalysts
3.1. Preparation of Catalysts Commercial pseudo bohemite (65.6 wt % of Al22O
O33) was purchased from Chalco Shandong Co., ) was purchased from Chalco Shandong Co.,
Commercial pseudo bohemite (65.6 wt % of Al
Ltd., Zibo, China. Mg(NO3)32)∙6H
6H2 O (≥99.0 wt %) were obtained
2 ·6H
2 O (≥99.0 wt %) and Ce(NO
3 )32·O (≥99.0 wt %) were obtained from Ltd., Zibo, China. Mg(NO
2O (≥99.0 wt %) and Ce(NO
3)3∙6H
from
Beijing
Chemical
Reagents
Co.,
Beijing,
China.
The
catalysts
were prepared according to the
Beijing Chemical Reagents Co., Beijing, China. The catalysts were prepared according to the incipient incipient wetness impregnation method. Impregnation of 60.00 g boehmite with the mixture solution
wetness impregnation method. Impregnation of 60.00 g boehmite with the mixture solution of 0.76 g of 0.76 3g)2∙6H
Mg(NO
g Ce(NO3 )3 ·6H2 O yielded
Mg-Ce/Al2 O3 (I) catalyst. 3)Tripled
3 )2 ·6H2 O and 0.33
Mg(NO
2O and 0.33 g Ce(NO
3)3∙6H2O yielded Mg‐Ce/Al
2O3 (I) catalyst. Tripled Mg(NO
2∙6H2O Mg(NO
)
·
6H
O
and
Ce(NO
)
·
6H
O
yielded
Mg-Ce/Al
O
(II)
catalyst.
Impregnation
of
60.00
g
3
2
2
3
3
2
2
3
and Ce(NO3)3∙6H2O yielded Mg‐Ce/Al2O3 (II) catalyst. Impregnation of 60.00 g boehmite with 0.76 g boehmite
with 0.76 g Mg(NO3 )2 ·36H
or 0.33 g Ce(NO23O
)33· or Ce/Al
6H2 O yielded
Mg/Al2 O3 or Ce/Al2 O3
2 O2O yielded Mg/Al
Mg(NO
3)2∙6H2O or 0.33 g Ce(NO
)3∙6H
2O3 catalysts, respectively. The ◦ C for 4 h in air after drying at
catalysts,
respectively.
The
impregnated
samples
were
calcinated
at
550
impregnated samples were calcinated at 550 °C for 4 h in air after drying at 90 °C for 12 h. Al2O3 was 90 ◦ C for 12 h. Al2 O3 was prepared from pseudo bohemite by calcination at 550 ◦ C for 4 h in air.
prepared from pseudo bohemite by calcination at 550 °C for 4 h in air. 3.2. Characterization of Catalysts
3.2. Characterization of Catalysts XRD was performed with a XRD-6000 powder diffraction instrument (Shimadzu, Kyoto, Japan)
XRD was performed with a XRD‐6000 powder diffraction instrument (Shimadzu, Kyoto, Japan) with a 40.0 kV working voltage and 40.0 mA electric current. The surface area and pore volume
with a 40.0 kV working voltage and 40.0 mA electric current. The surface area and pore volume were were determined with an ASAP2000 accelerated surface area and porosimetry system (Micromeritics,
determined with an ASAP2000 accelerated surface area and porosimetry system (Micromeritics, Norcross, GA, USA). The composition was determined with a ZSX‐100E X‐ray fluorospectrometer Catalysts 2017, 7, 72
11 of 14
Norcross, GA, USA). The composition was determined with a ZSX-100E X-ray fluorospectrometer
(Rigaku, Tokyo, Japan). Surface element distribution was recorded with a PHI Quantera SXM X-ray
photoelectron spectrometer (ULVACPHI, Chanhassen, MN, USA). The surface morphology was
observed under a Quanta 200 F scanning electron microscope (FEI, Hillsboro, OR, USA) and a Tecnai G2
F20 transmission electron microscope (FEI). IR spectroscopy was determined on a MAGNA-IR560ESP
FT-IR spectrometer (Nicolet, Madison, WI, USA). The diffuse reflectance spectra were recorded on a
U-4100 UV-vis spectrophotometer (Hitachi, Tokyo, Japan). The pHpzc of catalysts was determined
according to the pH-drift procedure [47]. The leaching of Ce and Mg elements was measured with
an AAnalyst atomic absorption spectrometer (PerkinElmer, Waltham, MA, USA) using a nitrous
oxide/oxygen-acetylene flames.
3.3. Ozonation of RPRW
The RPRW was collected directly from the effluent of a bio-treatment unit of a wastewater
treatment plant in CNOOC Huizhou Refining & Chemical Co., Ltd, Huizhou, China. The pH value,
electric conductivity at 25 ◦ C, BOD5 , and COD were 8.15, 3418 µs/cm, 20.24 mg/L, and 101.3 mg/L,
respectively. The oils and biodegradable chemicals in RPRW have been removed after a long
physicochemical and biological treatment. The chemical compositions in RPRW mainly were organic
acids (27.42%), heterocyclic compounds (24.49%), alkanes (19.56%), esters (19.24%), and alcohols
(4.79%) according to gas chromatography mass spectrometry analysis. Due to low biodegradability
(BOD5 /COD ratio at 0.2), the COP was determined as an efficient advanced treatment method for
the RPRW.
The experimental system was consisted of an oxygen tank, RQ-02 ozone generator (Ruiqing,
China), 200 mL quartz column reactor, flow meter, and an exhaust gas collector. An aliquot of 100 mL
of RPRW and 0.5 g catalyst were added in the reactor at 30 ◦ C. The gaseous ozone was then introduced
through a porous diffuser at the bottom of the reactor with a flow rate of 5 mg/min. The experiments
were carried out using varying initial pH values (adjusted with 1 N NaOH or HCl) and reaction
times. After treatment, dried oxygen was blown into the RPRW at a rate of 3.0 L/min to quench the
reaction and eliminate the residual ozone. The resulting suspension was filtered (Whatman Qualitative
No. 5) to separate catalyst particles prior to a further analysis. The •OH quenching experiments were
performed to study the oxidation mechanism. The •OH scavengers, tBA and NaHCO3 were added
into RPRW (1.0 g/L) prior to experiments. All experiments were performed in triplicate.
The pH and electric conductivity were measured with a MP 220 pH meter (Mettler Toledo,
Greifensee, Switzerland) and a CD400 electrical conductivity meter (Alalis, Shanghai, China),
respectively. The BOD5 was measured with a BODTrak II BOD meter (HACH, Loveland, CO, USA).
The COD was measured with a CTL-12 COD meter (HATO, Chengde, China). The COD removal was
calculated using the flowing equation:
COD removal = ([COD]0 − [COD]1 )/[COD]0
(1)
4. Conclusions
Metal oxide-loaded Al2 O3 catalysts were prepared, characterized and successfully evaluated
for ozonation treatment of RPRW. Interactions between metal oxides and between the metal
oxides and the Al2 O3 support greatly influenced the surface structures and catalytic properties.
The surface –OHs and metal oxides both function as active catalytic sites that promote •OH generation.
Composite metal oxide-loaded Al2 O3 exhibited higher efficiencies compared with single metal oxides.
Mg-Ce/Al2 O3 -COP enhanced the COD removal by 4.7%, 4.1%, 6.0%, and 17.5%, respectively, in
comparison with Mg/Al2 O3 -COP, Ce/Al2 O3 -COP, Al2 O3 -COP, and single ozonation. Our results
suggest that an Mg-Ce/Al2 O3 catalyst is commercially feasible, has high stability and can be
cost-effective for ozonation treatment of RPRW.
Catalysts 2017, 7, 72
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Acknowledgments: This project was supported in part by the National Science and Technology Major Project
of China (No. 2016ZX05040-003) and the National Natural Science Foundation of China (No. 21576287 and
No. 21306229).
Author Contributions: Qinghong Wang and Chunmao Chen conceived and designed the experiments; Yu Chen
and Yuhao Du performed the experiments; Yu Chen, Qing X. Li, Yuxian Wang, and Brandon A. Yoza
interpreted and analyzed the data; Lanping Yi and Shaohui Guo contributed reagents/materials/analysis tools;
Brandon A. Yoza, Qing X. Li, and Chunmao Chen wrote the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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