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Pt-catalyzed Ozonation of Aqueous Phenol Solution Using Highgravity Rotating Packed Bed
C. C. Chang*, C. Y. Chiu**, C. Y. Chang*, D. R. Ji*, J. Y. Tseng*, W. K. Tu*, C. F. Chang***, Y. H.
Chen**** and Y. H. Yu*
* Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan
(E-mail: [email protected]; [email protected]; [email protected]; [email protected];
[email protected]; [email protected])
** Department of Cosmetic Science and Application, Lan-Yang Insititute of Technology, I-Lan 261, Taiwan
(E-mail: [email protected])
*** Department of Environmental Science and Engineering, Tunghai University, Taichung 407, Taiwan
(E-mail: [email protected])
**** Department of Chemical and Material Engineering, National Kaohsiung University of Applied Science,
Kaohsiung City 807, Taiwan
(E-mail: [email protected])
Abstract
In this study, the high-gravity rotating packed bed (HGRPB) was used as a catalytic ozonation (OZ)
reactor to decompose phenol. The operation of HGRPB system was carried out in semi-batch
apparatus which combines two major parts, rotating packed bed (RPB) and photo-reactor (PR).
The RPB consists of a rotor and a stationary housing. The platinum-containing catalyst (Dash
220N) is packed in the RPB to adsorb molecular ozone and the target pollutant of phenol on the
surface to catalyze the oxidation. An ultra violet (UV) lamp (applicable wavelength λ = 200-280
nm) is installed in the PR to enhance the self-decomposition of molecular ozone in water to form
high reactive radical species. The phenol solution flows outward from the inner edge of the rotor
due to the centrifugal force. Gaseous ozone flows inward countercurrently from the outer edge of
the packed bed via the pressure-driving force. Different combinations of advance oxidation
processes (AOPs) with HGRPB for the degradation of phenol were tested. These include highgravity OZ (HG-OZ), HG catalytic OZ (HG-Pt-OZ), HG photolysis OZ (HG-UV-OZ) and HGUV/Pt-OZ (HG-UV-Pt-OZ). The addition of Dash 220N enhances the mineralization efficiency
(ηTOC) of phenol significantly.
Keywords
Ozone; catalyst; high-gravity rotating packed, phenol
INTRODUCTION
The ozonation (OZ) process has been used in drinking water and wastewater treatment science
last century. However, the OZ of organic compounds in solution is limited by gas-liquid mass
transfer efficiency and its selective reactivity. The high-gravity rotating packed bed (HGRPB, HG
or Higee) was used in this study as a gas-liquid contactor to enhance the mass transfer rate between
the phases. The HGRPB can increase the interfacial area while decrease the mass transfer resistance
between gas and liquid phases by decreasing the thickness of the liquid film and the size of the
droplets [1]. The HGRBP is being developed science 1979 [2]. Recently, the HGRPB is applicable
to absorption, adsorption, desorption, distillation, polymer devolatilization, bio-oxidation, reactive
crystallization, stripping, extraction and other separation processes [3]. HGRPB also has been used
for micro-mixing and emulsion [4].
The values of volumetric gas-liquid mass transfer coefficients achievable in HGRPB are one or
two orders of magnitude higher than those in conventional packed beds [5]. Recent studies [6-8]
showed that HGRPB has some advantageous characteristics such as 1) fast renewable rate on the
surface of packed materials, 2) high gas-liquid mass transfer coefficients, 3) short time to reach
steady state, 4) low overflow rate, 5) short hydraulic retention time (HRT) and 6) thin liquid film.
Previous studies point out that HGRPB has a higher mass transfer rate in gas-liquid system
than conventional gas-liquid contactor. Chen et al. [9] used RPB as well as completely stirred tank
reactor (CSTR) as ozone gas-liquid contactor to treat CI Reactive Black 5 (RB5). The results
indicated that the mass transfer coefficient of RPB (kLAa = 0.0975 s-1) is significantly higher than
that of CSTR (kLAa = 0.0248 s-1).Thus, the ozonation process can be combined with HGRPB to
enhance the ozone mass transfer from gas phase to liquid phase. Lin and Liu [1] found that the
centrifugal force can facilitate the decolorization efficiency of dyestuff Reactive Blue 19 (RB19)
because the mass transfer rate of ozone is enhanced via increasing the interfacial area and
decreasing the resistance of ozone transfer. By using HGRPB as ozone mass transfer apparatus can
improve the gas-liquid mass transfer efficiency and increase the oxidation ability of the OZ system.
In this research, the HGRPB was applied to the catalytic ozonation processes using Pt/γ-Al2O3
to decompose the organic compound of phenol in aqueous solution. System performances for the
degradation of phenol via various advanced oxidation processes (AOPs) using HGRPB were
examined and compared. Different combinations of advance oxidation processes (AOPs) with
HGRPB for the degradation of phenol were tested. These include high-gravity OZ (HG-OZ), HG
catalytic OZ (HG-Pt-OZ), HG photolysis OZ (HG-UV-OZ) and HG-UV/Pt-OZ (HG-UV-Pt-OZ).
EXPERIMENTAL
Chemicals
The phenol (Sigma-Aldrich Co., St. Louis, MO, USA) was subjected to DI water prior to the
use with the concentration (CBL0) of 100 mg L-1. The initial pH of the phenol solution is about 7.2.
The potassium iodine (KI) and acetonitrile (CH3CN) use were analytical grade (J. T. Baker,
Phillipsburg, NJ, USA). Ozone was decomposed to form radicals via a commercial Pt/γ-Al2O3
catalyst Dash 220N (N. E. Chemical Co, Japan), which is a platinum-containing catalyst.
Characteristic of the Dash 220N is shown in Table 1. Dash 220N is a spherical catalyst in 3-4 mm
diameter. The bulk density of the Dash 220N is 0.77 g cm-3. The content of platinum in the alumina
sphere is about 0.23 wt%.
Instrumentation
The HGRPB system (Fig. 1) was carried out via semi-batch operation. The experiments were
performed with the volume (VL) of phenol solution of 1 L and total reaction time (t) of 80 min. The
HGRPB system has two major parts, namely RPB and photo-reactor. The RPB consists of a
packing chamber, a rotor and a stationary housing. The phenol solution flows outward from the
inner edge of the packing chamber subjected to the centrifugal force. Gaseous ozone flows inward
countercurrently from the outer edge of the packed bed by the pressure-driving force. The spherical
glass beads and Dash-220N catalyst were used as packing materials and tested in the packed bed.
The diameter of glass beads is about the same as that of Dash 220N. The RPB is 2 cm in high and
5.9 cm in diameter. In the experiments, the RPB was operated with rotating speed (Nr) 1,500 rpm,
which provided gravitational force of 148.55 g (1455 m s-2). The phenol solution was fed to RPB by
a peristaltic pump at 462 mL min-1 recycle rate (QLR) and contacted with gaseous ozone and
catalysts in the bed.
The photoreactor is made of Pyrex glass with the dimensions of diameter and height of 5 and
30 cm, respectively. A UV lamp (TUV-16W, Phillps, Tokyo, Japan) was placed at the center of the
UV reactor and shielded by a quartz jacket. The UV irradiation was introduced to enhance the
decomposition of ozone to form the OH˙ radical. All the experiments were controlled at 25 ℃ with
water jacket around the reactor. The phenol solution was pumped and recycled continuously
through a closed loop connected with all sensors of pH, oxidation reduction potential (ORP) and
dissolved ozone (DO3) monitors. Liquid samples were taken from a sampling port for chemical
analyses.
The ozone was generated from dried oxygen with purity of 99.99 % via corona discharge using
an ozone generator (Model LAB2B, Ozonia, Duebendorf, Switzerland). The ozone inlet flow rate
(QG) was controlled via a mass flow controller (MFC, Model 5850E, Brooks, Hatfield, PA, USA) at
1 L min-1. The inlet and outlet gaseous ozone concentrations were measured via a UV/visible
spectrophotometer (UV mini-1240, Shimadzu, Kyoto, Japan) with the absorbance of ozone
measured in a 2 mm flow-through quartz cell at the wavelength 258 nm. An extinction coefficient
of 3,000 M-1 s-1 was used to convert absorbances into concentration units [10]. The inlet ozone
concentration (CAG,in) was controlled at 60 mg L-1. Ozone concentration in water was determined by
DO3 meter (Model 3600, Orbisphere, Trasadingen, Swizerland). The UV-vis irradiation intensity of
UV lamp was measured by diffraction grating spectrometer (Model EPP 2000, StellarNet, Oldsmar,
FL, USA).
Fig. 1. Schematic diagram of HGRPB ozonation system.
The analyses of phenol concentration were performed using the high performance liquid
chromatography (HPLC, Model 500, Viscotek, Houston, TX, USA) with 250 mm x 4.6 mm C18
column (LC-18, Supelco, Bellefonte, PA, USA) to separate the phenol and by-products of
ozonation. The wavelength of UV/visible detector (Model 1706, Bio-Rad, Hercules, CA, USA) was
set at 270 nm. Effluent is composed of CH3CN and DI water (CH3CN/DI water = 50/50) with flow
rate controlled at 1.0 mL min-1. Total organic carbon (TOC) was analyzed by TOC analyzer (Model
1010, O.I. analytical, College Station, TX, USA).
RESULTS AND DISCUSSION
Decomposition of Phenol via HGRPB System
The decomposition of phenol in HGRPB systems of HG, HG-UV, HG-OZ, HG-UV-OZ and
HG-UV-Pt-OZ at QG = 1 L min-1, QLR = 100 mL min-1 and Nr = 1,500 rpm are shown in Fig. 2. In
this part of experiment, the HGRPB was packed with glass spheres and Dash 220N of 4 mm in
diameter. For the ozonation, the applied ozone doasage (mA,in) and decomposition efficiency of
phenol (ηphenol) are defined as follows:
(1)
mA ,in  QG  C AG ,in  t VL1
 phenol  C BL0  C BL  / C BL
(2)
where QG, CAG,in and VL are gas flow rate, concentration of inlet ozone and volume of liquid sample,
while CBL0 and CBL are concentrations of phenol at time t = 0 and t.
Fig. 2. Variations of CBL/CBL0 with applied ozone dosage (mA,in) of different advanced oxidation processes in HGRPB
(High gravity rotating packed bed). Initial concentration of phenol CBL0 = 100 mg L-1, VL = 1 L, Nr = 1,500 rpm,
reaction temperature T = 25 ℃, QG = 1 L min-1, power of light source of UV = 16 W. : HG ozonation (HGOZ); ○: HG catalytic ozonation (HG-Pt-OZ), ms = 42 g; ∆: HG catalytic ozonation (HG-Pt-OZ), ms = 77 g; ＊:
HG-UV-OZ; ×: HG-UV-Pt-OZ, ms = 42 g.
Fig. 3. Variations of pH value with applied ozone dosage (mA,in) of different advanced oxidation processes in HGRPB.
Initial concentration of phenol CBL0 = 100 mg L-1, VL = 1 L, Nr = 1,500 rpm, reaction temperature T = 25 ℃, QG
= 1 L min-1, power of light source of UV = 16 W. : HG-OZ; ○: HG-Pt-OZ, ms = 42 g; ∆: HG-Pt-OZ, ms = 77 g;
＊: HG-UV-OZ; ×: HG-UV-Pt-OZ, ms = 42 g.
The values of ηphenol for HG-OZ process reached about 89% in 5 min reaction time (mA,in = 300
mg L-1-sample) and 99% in 10 min (mA,in = 600 mg L-1-sample), respectively, revealing effective
decomposition of phenol HG-OZ process. All the above combined processes of HG and AOPs can
decompose phenol completely with mA,in of 1,200 mg L-1 sample. No significant reduction for HG
and HG-UV processes were observed in the studied conditions. Jorge et al [11] used manganese
dioxide supported on titania as catalysts for the phenol photodegradation and used ozone as the
oxidant agent. The results show that in absence of O3 and/or catalysts, the UV does not produce a
significant degradation of the phenol.
The variation of pH value with mA,in of the combined HG-OZ processes are shown in Fig. 3.
The initial pH value of sample solution was about 7.2. For all the AOPs in HGRPB, the pH value of
phenol solution decreased rapidly in 5 min (mA,in = 300 mg L-1-sample) because of the formation of
acid intermediates such as oxalic acids and methanoic acids [12]. In the HG-OZ process, the pH
value decreased from 7.2 to 3.4 in 5 min and maintained until 60 min (mA,in = 3,600 mg L-1-sample)
then increase to 7 at the end of reaction (80 min, mA,in = 4,800 mg L-1-sample). For the HG-UV-OZ,
HG-Pt-OZ and HG-UV-Pt-OZ process, the pH value changed from 7.2 to 3-4 in 5 min then
increased slightly to 5-7 at the end of the reaction. Comparing with the results of TOC
decomposition efficiency, the rising phenomenon of pH value resulted in further decomposition of
organic acids.
TOC Decomposition via Ozonation and Photolysis Ozonation in HGRPB
The TOC decomposition rate (CTOC / CTOC0) of HG, HG-OZ, HG-UV and HG-UV-OZ at QG =
1 L min-1, QLR = 100 mL min-1 and Nr = 1,500 rpm are shown in Fig. 4. The glass beads were used
as packing material. For the cases without ozone, oxygen gas was introduced to maintain the same
hydraulic conditions. The value of ηTOC for HG-OZ process reaches 50% at mA,in = 2,400 mg L-1sample (in 40 min) and 82% at mA,in = 4,800 mg L-1-sample (in 80 min). The phenol and byproducts are highly reactive with ozone.
Combination of HG-OZ with UV irradiation (HG-UV-OZ) can further enhance the
decomposition of TOC. As shown in Fig. 4, after 20 min oxidation (mA,in = 1,200 mg L-1-sample),
ηTOC of HG-UV-OZ is 54%, while that of HG-OZ without UV irradiation is 24%. After 80 min
oxidation, the ηTOC is 94%. The low-pressure UV lamp used in this research is a multi-wavelengths
light source with three regions of UV-A (315-400 nm), UV-B (280-315 nm) and UV-C (200-280
nm). Counting the whole wavelength range of the respective regions, the irradiation intensities of
the UV lamp in UV-A, UV-B and UV-C are 3.99, 1.59 and 3.73 W m-2. Molecular phenol has
obvious UV absorption below 280 nm, while molecular ozone absorbs short-UV light in 200-300
nm with the maximum absorption coefficient at 253.7 nm. For the decomposition of phenol via HGUV-OZ, UV irradiation of UV-C region has significant absorption. The molecules absorbing UV
irradiation of UV-C region in HG-UV-OZ include: 1) molecular ozone – enhancing the
decomposition of ozone in water and generating highly reactive hydroxyl radicals; 2) phenol
molecule - absorbing UV-C via the aromatic ring and 3) intermediates from the decomposition of
phenol - absorbing UV and reacting with ozone and hydroxyl radicals to yield other by-products
(such as phenols-quinones-aromatic acids, short-chain aliphatic acids and short-chain aliphatic
aldehydes ).
The mechanism of UV-OZ may be described as follows. In aqueous phase, the dissolved ozone
absorbs UV radiation to product hydrogen peroxide (H2O2) via the following reactions [13]:
h
O3  H 2 O 
O2  H 2 O2
(3)
h

O3  H 2O  O2  2OH
(4)
The second step of UV-OZ process is to form hydroxyl radical. The H2O2 may be decomposed
further to form hydroxyl radical via photolysis or ozonation as follows.
Photolysis:
h
H 2O2 
2OH 
Ozone enhanced decomposition:
H 2 O2  HO2  H 
(5)
(6)
HO2  O3  HO2  O3
(7)
O3   H   HO3  OH   O2
(8)
Comparing the reactions in the above pathway, the reaction rate of H2O2 via photolysis is
quite slow (Eq. 5). Therefore, the decomposition of H2O2 via ozone to form hydroxyl radical is the
main pathway (Eqs. 6 to 8) in this experiment.
The results of Fig. 4 for the photolysis of phenol were obtained using the low-pressure UV
lamp, yielding ηTOC of 1%. Thus, phenol is hard to be destructed via UV irradiation without oxidant
agent (O3) or catalysts.
1.0
CTOC / CTOC0 , -
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
Time, min
50
60
70
80
Fig. 4. Variations of CTOC/CTOC0 with time for the mineralization of phenol using ozonation and photolysis ozonation in
HGRPB. Initial concentration of phenol CBL0 = 100 mg L-1, VL = 1 L, Nr = 1,500 rpm, reaction temperature T =
25℃, QG = 1 L min-1, power of light source of UV = 16 W. ◆: HG; ●: HG-UV; : HG-OZ; ＊: HG-UV-OZ.
TOC Decomposition via Catalytic Ozonation in HGRPB
Figure 5 presents the decomposition of TOC with and without Pt/γ-Al2O3 (Dash 220N) during
the ozonation in HGRPB. A comparison with HG-OZ indicates that phenol and intermediates are
decomposed rapidly because Dash 220N promotes the dissolved O3 to form high reactive OH˙
radicals. For the case without catalyst, glass beads were used. The value of ηTOC for HG-Pt-OZ
process with ms = 42 g reaches 56% at mA,in = 1,192 mg L-1-sample (in 30 min)and 87% at mA,in =
4,800 mg L-1-sample (in 80 min). By increasing the catalyst mass to 77 g, the ηTOC reaches 71% at
mA,in = 1,192 mg L-1-sample (in 30 min) and 90% at mA,in = 4,800 mg L-1-sample (in 80 min). The
ηTOC increases with ms because of more active sites can adsorb O3 to form OH˙ radical to destroy
organic compounds.
The Pt contented catalyst is effective for the decomposition of O3 in aqueous phase. The
mechanism of the heterogeneous catalytic ozonation in water was explained by Legube and Vel
Leitner [14]. Base on the mechanism, the Pt on the Dash 220N surface assists the generation of OH˙
radicals. The organic molecules (phenol and intermediates) are adsorbed on the Dash 220N surface
and oxidized via an electron-transfer reaction. Then the organic radical species are desorbed from
the Dash 220N and oxidized by OH˙ radical or O3.
A simplified scheme of the decomposition pathways of phenol for the catalytic ozonation with
UV is described as follow. Both the phenol and intermediates are attacked by O3 and OH˙ radical.
The initial attack of oxidants on phenol during ozonation is via the electrophilic addition to form
aromatic acids. Then the aromatic rings are broken down by O3 and OH˙ radical to form short-chain
organic acids such as oxalic acid, glyoxalic acid and formic acid. These short-chain organic acids
are further decomposed to CO2 and H2O eventually.
Fig. 5. Variations of CTOC/CTOC0 with applied ozone dosage (mA,in) for the mineralization of phenol using different
advanced oxidation processes in HGRPB. Initial concentration of phenol CBL0 = 100 mg L-1, VL = 1 L, Nr =
1,500 rpm, reaction temperature T = 25 ℃, QG = 1 L min-1, power of light source of UV = 16 W. : HG-Pt-OZ,
ms = 0 g; ○: HG-Pt-OZ, ms = 42 g; ∆: HG-Pt-OZ, ms = 77 g; ×: HG-UV-Pt-OZ, ms = 42 g.
Combined UV irradiation in the HG-Pt-OZ process with (HG-UV-Pt-OZ) can further enhance
the decomposition of phenol. As shown in Fig. 5, at the same catalyst packing mass (ms = 42 g)
after 40 min oxidation (mA,in = 2,400 mg L-1-sample), ηTOC of HG-UV-Pt-OZ is 91%, while that of
HG-Pt-OZ without UV irradiation is 74%. The value ηTOC of HG-UV-Pt-OZ and HG-Pt-OZ at 80
min (mA,in = 4,800 mg L-1-sample) are 94 and 92%. The combination of UV irradiation of HG-PtOZ can reach the same TOC decomposition efficiency at less catalyst packing mass.
CONCLUSIONS
The decomposition of phenol via HG-OZ, HG-UV-OZ, HG-Pt-OZ and HG-UV-Pt-OZ is
investigated in this paper. Several conclusions can be drawn as follows.
(1) The introduction of UV irradiation in HG-OZ can also enhance both decomposition and
mineralization of phenol significantly. The molecular ozone absorb UV-C energy results in
self-decomposition to form high active radicals. The value of ηTOC employing HG-UV-OZ
process is 92% at mA,in = 4,800 mg L-1-sample.
(2) Packing Pt/γ-Al2O3 catalyst (Dash 220N) in HG-OZ process can effectively enhance the
decomposition of phenol and intermediates under the experimental conditions of this study.
The ηTOC of phenol increases from 82% of HG-OZ to 90% of HG-Pt-OZ (ms = 77 g) because
of the formation of OH˙ radicals.
(3) The use of UV irradiation in HG-Pt-OZ process significanlty enhances both the
decomposition and mineralization rate of phenol. The HG-UV-OZ process mainly absorbs
UV-C (λ = 200-280 nm) energy to decompose ozone to form high active radicals. The
photolysis of phenol via UV irradiation (HG-UV) only is negligible.
ACKNOWLEDGMENTS
The authors would like to thank the National Science Council of Taiwan for the financial
support of the work (NSC 932211E267005).
NOMENCLATURE AND UNITS
a: Specific area of gas-liquid interface per unit volume of contactor or packed bed, m2 m-3
C BL: Concentration of phenol, mg L-1.
C BL0: Initial concentration of phenol, mg L-1.
CAG,in: Concentration of feed O3, mg L-1.
HGRPB: High-gravity rotating packed bed.
HG: High-gravity process.
HG-OZ: HGRPB-ozonation.
HG-Pt-OZ: Pt catalytic HG-OZ.
HG-UV: Ultra violet HG.
HG-UV-OZ: Ultra violet HG-OZ.
HG-UV-Pt-OZ: Pt catalytic HG-UV-OZ.
HRT: Hydraulic retention time, s.
kLA: Physical liquid-phase mass transfer coefficient of ozone, m s-1.
mA,in: Applied dosage of ozone, mg L-1-sample.
mS: Mass of catalyst, g.
Nr: Rotating speed of HGRPB, rpm.
QG: Flow rate of feed O3, L min-1.
QLR: Flow rate of recycled liquid, L min-1
T: Temperature, .
TOC: Total organic carbon.
t: Reaction time, min.
VL: Sample volume, L.
ηTOC: Mineralization efficiency of TOC.
εB: Void fraction of packed bed.
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