Pt-catalyzed Ozonation of Aqueous Phenol Solution Using Highgravity Rotating Packed Bed

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1 Manuscript Click here to download Manuscript: LET2008_IWA-324_ doc 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 ( d @ntu.edu.tw; cychang3@ntu.edu.tw; jdr0826@gmail.com; f @ntu.edu.tw; f @ntu.edu.tw; yuehwayu@ntu.edu.tw) ** Department of Cosmetic Science and Application, Lan-Yang Insititute of Technology, I-Lan 261, Taiwan ( cychiu@mail.fit.edu.tw) *** Department of Environmental Science and Engineering, Tunghai University, Taichung 407, Taiwan ( cfchang@thu.edu.tw) **** Department of Chemical and Material Engineering, National Kaohsiung University of Applied Science, Kaohsiung City 807, Taiwan ( yhchen@cc.kuas.edu.tw) 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 = 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 HG- UV/Pt-OZ (HG-UV-Pt-OZ). The addition of Dash 220N enhances the mineralization efficiency (η TOC ) 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

2 indicated that the mass transfer coefficient of RPB (k LA a = s -1 ) is significantly higher than that of CSTR (k LA a = 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-Al 2 O 3 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 (C BL0 ) of 100 mg L -1. The initial ph of the phenol solution is about 7.2. The potassium iodine (KI) and acetonitrile (CH 3 CN) use were analytical grade (J. T. Baker, Phillipsburg, NJ, USA). Ozone was decomposed to form radicals via a commercial Pt/γ-Al 2 O 3 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 (V L ) 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 g (1455 m s -2 ). The phenol solution was fed to RPB by a peristaltic pump at 462 ml min -1 recycle rate (Q LR ) 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 (DO 3 ) monitors. Liquid samples were taken from a sampling port for chemical analyses. The ozone was generated from dried oxygen with purity of % via corona discharge using an ozone generator (Model LAB2B, Ozonia, Duebendorf, Switzerland). The ozone inlet flow rate

3 (Q G ) 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 (C AG,in ) was controlled at 60 mg L -1. Ozone concentration in water was determined by DO 3 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 CH 3 CN and DI water (CH 3 CN/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 Q G = 1 L min -1, Q LR = 100 ml min -1 and N r = 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 (m A,in ) and decomposition efficiency of phenol (η phenol ) are defined as follows: 1 m Q C t V (1) A,in phenol G AG,in C BL0 CBL / CBL L (2) where Q G, C AG,in and V L are gas flow rate, concentration of inlet ozone and volume of liquid sample, while C BL0 and C BL are concentrations of phenol at time t = 0 and t.

4 Fig. 2. Variations of C BL /C BL0 with applied ozone dosage (m A,in ) of different advanced oxidation processes in HGRPB (High gravity rotating packed bed). Initial concentration of phenol C BL0 = 100 mg L -1, V L = 1 L, N r = 1,500 rpm, reaction temperature T = 25, Q G = 1 L min -1, power of light source of UV = 16 W. : HG ozonation (HG- OZ); : HG catalytic ozonation (HG-Pt-OZ), m s = 42 g; : HG catalytic ozonation (HG-Pt-OZ), m s = 77 g; *: HG-UV-OZ; : HG-UV-Pt-OZ, m s = 42 g. Fig. 3. Variations of ph value with applied ozone dosage (m A,in ) of different advanced oxidation processes in HGRPB. Initial concentration of phenol C BL0 = 100 mg L -1, V L = 1 L, N r = 1,500 rpm, reaction temperature T = 25, Q G = 1 L min -1, power of light source of UV = 16 W. : HG-OZ; : HG-Pt-OZ, m s = 42 g; : HG-Pt-OZ, m s = 77 g; *: HG-UV-OZ; : HG-UV-Pt-OZ, m s = 42 g. The values of η phenol for HG-OZ process reached about 89% in 5 min reaction time (m A,in = 300 mg L -1 -sample) and 99% in 10 min (m A,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 m A,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

5 dioxide supported on titania as catalysts for the phenol photodegradation and used ozone as the oxidant agent. The results show that in absence of O 3 and/or catalysts, the UV does not produce a significant degradation of the phenol. The variation of ph value with m A,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 (m A,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 (m A,in = 3,600 mg L -1 -sample) then increase to 7 at the end of reaction (80 min, m A,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 (C TOC / C TOC0 ) of HG, HG-OZ, HG-UV and HG-UV-OZ at Q G = 1 L min -1, Q LR = 100 ml min -1 and N r = 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 η TOC for HG-OZ process reaches 50% at m A,in = 2,400 mg L -1 - sample (in 40 min) and 82% at m A,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 (m A,in = 1,200 mg L -1 -sample), η TOC of HG-UV-OZ is 54%, while that of HG-OZ without UV irradiation is 24%. After 80 min oxidation, the η TOC is 94%. The low-pressure UV lamp used in this research is a multi-wavelengths light source with three regions of UV-A ( nm), UV-B ( nm) and UV-C ( 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 nm with the maximum absorption coefficient at nm. For the decomposition of phenol via HG- UV-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 (H 2 O 2 ) via the following reactions [13]: h O3 H 2O O2 H 2O2 (3) h H O O 2OH O3 2 2 (4) The second step of UV-OZ process is to form hydroxyl radical. The H 2 O 2 may be decomposed further to form hydroxyl radical via photolysis or ozonation as follows. Photolysis: h H2O2 2OH (5) Ozone enhanced decomposition: H 2 O2 HO2 H (6)

6 HO O 2 O3 HO2 O3 3 H HO3 OH O2 (8) Comparing the reactions in the above pathway, the reaction rate of H 2 O 2 via photolysis is quite slow (Eq. 5). Therefore, the decomposition of H 2 O 2 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 η TOC of 1%. Thus, phenol is hard to be destructed via UV irradiation without oxidant agent (O 3 ) or catalysts. (7) CTOC / CTOC0, Time, min Fig. 4. Variations of C TOC /C TOC0 with time for the mineralization of phenol using ozonation and photolysis ozonation in HGRPB. Initial concentration of phenol C BL0 = 100 mg L -1, V L = 1 L, N r = 1,500 rpm, reaction temperature T = 25, Q G = 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-Al 2 O 3 (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 O 3 to form high reactive OH radicals. For the case without catalyst, glass beads were used. The value of η TOC for HG-Pt-OZ process with m s = 42 g reaches 56% at m A,in = 1,192 mg L -1 -sample (in 30 min)and 87% at m A,in = 4,800 mg L -1 -sample (in 80 min). By increasing the catalyst mass to 77 g, the η TOC reaches 71% at m A,in = 1,192 mg L -1 -sample (in 30 min) and 90% at m A,in = 4,800 mg L -1 -sample (in 80 min). The η TOC increases with m s because of more active sites can adsorb O 3 to form OH radical to destroy organic compounds. The Pt contented catalyst is effective for the decomposition of O 3 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 O 3. A simplified scheme of the decomposition pathways of phenol for the catalytic ozonation with

7 UV is described as follow. Both the phenol and intermediates are attacked by O 3 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 O 3 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 CO 2 and H 2 O eventually. Fig. 5. Variations of C TOC /C TOC0 with applied ozone dosage (m A,in ) for the mineralization of phenol using different advanced oxidation processes in HGRPB. Initial concentration of phenol C BL0 = 100 mg L -1, V L = 1 L, N r = 1,500 rpm, reaction temperature T = 25, Q G = 1 L min -1, power of light source of UV = 16 W. : HG-Pt-OZ, m s = 0 g; : HG-Pt-OZ, m s = 42 g; : HG-Pt-OZ, m s = 77 g; : HG-UV-Pt-OZ, m s = 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 (m s = 42 g) after 40 min oxidation (m A,in = 2,400 mg L -1 -sample), η TOC of HG-UV-Pt-OZ is 91%, while that of HG-Pt-OZ without UV irradiation is 74%. The value η TOC of HG-UV-Pt-OZ and HG-Pt-OZ at 80 min (m A,in = 4,800 mg L -1 -sample) are 94 and 92%. The combination of UV irradiation of HG-Pt- OZ 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 η TOC employing HG-UV-OZ process is 92% at m A,in = 4,800 mg L -1 -sample. (2) Packing Pt/γ-Al 2 O 3 catalyst (Dash 220N) in HG-OZ process can effectively enhance the decomposition of phenol and intermediates under the experimental conditions of this study. The η TOC of phenol increases from 82% of HG-OZ to 90% of HG-Pt-OZ (m s = 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 = nm) energy to decompose ozone to form high active radicals. The photolysis of phenol via UV irradiation (HG-UV) only is negligible.

8 ACKNOWLEDGMENTS The authors would like to thank the National Science Council of Taiwan for the financial support of the work (NSC E267005). NOMENCLATURE AND UNITS a: Specific area of gas-liquid interface per unit volume of contactor or packed bed, m 2 m -3 C BL: Concentration of phenol, mg L -1. C BL0: Initial concentration of phenol, mg L -1. C AG,in : Concentration of feed O 3, 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. k LA : Physical liquid-phase mass transfer coefficient of ozone, m s -1. m A,in : Applied dosage of ozone, mg L -1 -sample. m S : Mass of catalyst, g. N r : Rotating speed of HGRPB, rpm. Q G : Flow rate of feed O 3, L min -1. Q LR : Flow rate of recycled liquid, L min -1 T: Temperature,. TOC: Total organic carbon. t: Reaction time, min. V L : Sample volume, L. η TOC : Mineralization efficiency of TOC. ε B : Void fraction of packed bed. REFERENCES 1. Lin, C.C. and Liu, W.T. (2003) Ozone oxidation in a rotating packed bed. Journal of Chemical Technology and Biotechnology, 78(2/3), Ramshaw, C. (1979) Mass transfer process. European Patent No Guo, K., Guo, F., Feng, Y.D., Chen, J.F., Zheng, C., Gardner, N.C. (2000) Synchronous visual and RTD study on liquid flow in rotating packed-bed contactor. Chemical Engineering Science, 55(11), Chen, Y. S., Liu, H. S., Lin, C. C., Liu, W. T. (2004) Micromixing in a Rotating Packed Bed. Journal of Chemical Engineering of Japan, 37(9), Ramshaw, C., Mallinson, R. H. (1981) Mass Transfer Process, U.S. Patent 4, 283, Lin, C.C. (1999) The Study of Higee, PhD dissertation, Dept. Chem. Eng., National Taiwan University, Taipei, Taiwan. 7. Chen, Y.H., Chang, C.Y., Su, W.L., Chen, C.C., Chiu, C.Y., Yu, Y.H., Chiang, P.C., Chiang, S. (2004) Modeling ozone contacting process in a rotating packed bed, Industrial and Engineering Chemistry Research, 43(1), Chen, Y.H., Chiu, C.Y., Chang, C.Y., Huang, Yu, Y.H., Chiang, P.C., Shie, J.L., Chiou,C.S. (2005) Modeling ozonation process with pollutant in a rotating packed bed, Industrial and

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