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Combined membrane photocatalytic ozonation and wet absorption of elemental mercury
Atmospheric Pollution Research xxx (2017) 1e8
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Combined membrane photocatalytic ozonation and wet absorption of elemental mercury Z.S. Huang, Z.S. Wei*, Y.M. He, J.L. Pei, X.L. Xiao, M.R. Tang, S. Yu School of Environmental Science and Engineering, Sun Yat-sen University, Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2017 Received in revised form 17 September 2017 Accepted 18 September 2017 Available online xxx
Membrane photocatalytic ozonization coupled with wet absorption offers potential for elemental mercury (Hg0) removal. This study reports on a novel FeeTiO2-coated polyvinylidene fluoride (PVDF) wet photocatalytic membrane reactor (WPCMR) for mercury removal in flue gas. Hg0 removal efficiency in the WPCMR reached up to 93.3%. Ozone could enhance mercury oxidation in WPCMR. Wet absorption helps to increases mercury removal efficiency. FeeTiO2 catalyst was synthesized by sol-gel method and characterized by XRD, FTIR, UVeVis, XPS and SEM. XPS analysis confirmed Hg0 oxidation to divalent mercury (Hg (II)). Elemental mercury was oxidized to mercuric oxide followed by wet absorption in the presence of OH free radical and ozone. Wet photocatalytic membrane reactor and photocatalytic membrane reactor (PCMR) of elemental mercury reaction with the FeeTiO2/PVDF catalyst all follow Langmuir-Hinshelwood kinetics. © 2017 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Elemental mercury FeeTiO2/PVDF Photocatalytic oxidation Catalyst characterization Kinetics
1. Introduction Mercury emission has become a global issue due its toxicity, volatility, persistence and bioaccumulation (Driscoll et al., 2013), which generated from coal-fired power plants, industrial boilers, metal smelting and cement kiln (Pirrone et al., 2010). Mercury emission from coal-fired flue gas often presents in three main forms: elemental mercury (Hg0), oxidized mercury (Hg (II)), and particulate mercury (HgP) (Zygarlicke, 2000). HgP can be effectively removed by particle matter capture device, Hg (II) can be absorbed by aqueous solution (Helble, 2000). HgP and Hg (II) can be captured by wet flue gas desulfurization (WFGD). (Pavlish et al., 2003). However, Hg0, accounting for over 64e90% in flue gas, is the most difficult to be captured with the existing air pollution control devices (Wang et al., 2010). Thus, Hg0 oxidation to water soluble Hg (II) followed by flue gas desulfurization scrubber was a promising solution to control Hg0 emissions (Pavlish et al., 2003). Extensive research have been undertaken using traditional catalytic oxidation (Zhao et al., 2015), membrane catalytic
* Corresponding author. E-mail address: [email protected] (Z.S. Wei). Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.
oxidation (Guo et al., 2012) and photocatalytic oxidation to reduce Hg0 emission (Zhuang et al., 2014). Photocatalytic oxidation is a promising technology for at high removal efficiency, and low cost of maintenance (Zhou et al., 2017), which utilizes semiconductors like TiO2, WO3, FeTiO3, and metallic ions-doped TiO2 photocatalysts to carry out a photo-induced redox process (Kityakarn et al., 2013). Metal elements doping narrowed TiO2 band gap and enhance its visible light activity (Ma et al., 2015). Catalytic membrane reactor combine chemical reaction and separation in a single unit, in which a porous membrane, rendered catalytic by impregnation with catalysts, the reactants flow convectively through the membrane pores where the catalyst is located, thus resulting in intensive contact between the reactants and the catalytic sites, and in high catalytic activity with negligible mass transport resistance (Kajama et al., 2016). Membrane functioned as selective barrier for mixed contaminants, carrier for catalyst and provided spacious area for reaction (Coronas and Santamar, 1999). A novel membrane delivery catalytic oxidation system was set up for Hg0 oxidation and shown phenomenal performance (Guo et al., 2011). Hg0 was oxidized to mercuric chloride oxidation by advanced oxidation process (AOPs) (Zhan et al., 2013). Ozonation can efficiently oxidize many recalcitrant organic compounds (Hassani et al., 2017). The objective of this work is to study elemental mercury (Hg0) removal by coupling membrane photocatalytic oxidation and wet
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absorption with a novel FeeTiO2/PVDF wet photocatalytic membrane reactor (WPCMR). This study evaluates the influence of Hg0 inlet concentration, gas residence time (GRT), water spray rate, UV illuminance, ozone concentration, the coupling role of membrane and catalysis on mercury oxidation. XRD, FTIR, UVeVis, XPS and SEM method are utilized in this study to figure out the formation of the intermediate products and their mechanistic involvement in membrane catalytic oxidation of Hg0, and the mechanistic and kinetic analysis of membrane catalytic oxidation were elicited, which is believed to provide impetus for the application of the membrane catalytic oxidation. 2. Materials and methods 2.1. Preparation of FeeTiO2/PDVF FeeTiO2 catalyst was prepared by solegel method. 25 mL Ti(OC4H9)4 and 10 mL acetic acid was dissolved in 350 mL absolute ethyl alcohol in a beaker. The mixture was stirred for 45 min and designated as solution A. 7.26 g ferric nitrate was dissolved in 30 mL deionized water, 150 mL absolute ethyl alcohol and 20 mL acetic acid. The mixture was stirred for 20 min and designated as solution B. Solution B was dropped to solution A by peristaltic pump at the speed of 1 drop per second. The mixture solution A and solution B
was stirred for 24 h at room temperature. The FeeTiO2/PVDF was synthesized by soaking the entire PVDF hollow fiber membrane in the mixed solution for 48 h then dehydrated for 2 d in thermostat at 80 C. 2.2. Experimental procedure The experimental schematic of wet photocatalytic membrane bioreactor (WPCMR) was shown in Fig. 1. PVDF hollow fiber membrane was purchased from Blue cross Company, Tianjin, China. The dimensional parameters of the PVDF hollow fiber membrane were as follow: 0.38 mm fiber internal diameter; 0.5 mm fiber external diameter; 2 400 fibers; 0.1e0.01 mm pore size; 60% porosity; 200 mm column length; 40 mm column diameter; 2.2 m2 total area. Hg0 was generated by heating a mercury permeation tube (supplied by Qingan scientific instruments Co., Ltd) in a 50e70 C water bath. Airstreams of Hg0 and compressed air were dehydrated in a glass jar loaded with calcium chloride granules, mixed (21% O2 content) in a buffer jar then pumped into FeeTiO2/PVDF hollow fiber membrane reactor. Water was sprayed onto the membrane surface using a subaqueous pump in WPCMR. Outlet gas was absorbed by saturate potassium chloride solution and potassium permanganate solution (10%, m/V).
Fig. 1. The experimental schematic of wet photocatalytic membrane bioreactor (WPCMR):1. Air compressor; 2. Flowmeter; 3. Water bath; 4. Elemental mercury permeation tube; 5. Gas drying jar; 6. Gas mixing jar; 7. Sampling point; 8. UV lamp; 9. FeeTiO2/PVDF; 10. Water tank; 11. Subaqueous pump; 12. Saturate KCl solution; 13. KMnO4 solution.
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2.3. Characterization FeeTiO2 was characterized by XRD, FTIR, UVeVis spectroscopy, XPS and SEM. X-ray diffraction analysis was carried out using a RIKAGU D/max 2200v X-Ray Diffractometer (CuKa radiation, 40 mA, 40 kV). Fourier Transform Infrared Spectroscopy analysis was carried out using a Nicolet 5 700 FTIR spectrometer (Thermo Fisher Scientific USA) in a spectra range of 4 000 cm1 to 400 cm1. UltravioleteVisible light spectroscopy was carried out using a Lambda 950 UV-Vis-NIR Spectrophotometer (Perkin Elmer UK). Xray photoelectron spectroscopy was carried out using an ESCALab250 X-ray photoelectron spectrometer (Thermo Fisher Scientific USA). Scanning electron microscopy was carried out using a JSM6330F Field Emission Scanning Electron Microscope (Japan Electron Optics Laboratory Co, Ltd). 2.4. Analytical methods Hg0 was sampled by ontario hydro method (OHM) set by U. S. environmental protection agency (EPA). Hg0 concentration was determined by atomic fluorescence spectrometer (AFS). Flow rates of inlet gas, outlet gas as well as and water flow were measured by Model LZB-1 flow meters (Purchased from Yuyao instrument company, Zhejiang, China). Gas residence time was calculated by dividing membrane volume by gas flowrate. UV illuminance was determined by LX1330B illuminometer (Sampo scientific instrument Company, Shenzhen, China). Ozone was generated by HY018-150A air source ozone generator (Guangzhou Jia Huan Electric Appliance Co.,Ltd). Determinations of all parameters were conducted in triplicate and the averages were adopted as the consequent values. 3. Results and discussion 3.1. Characterization of FeeTiO2 3.1.1. X-ray diffraction As shown in Fig. 2, major diffraction peaks located at 2q ¼ 25.3 , 37.8 , 38.6 , 48.1, 53.9 , 55.1, 62.8 were characteristics for (101), (103), (004), (112), (200), (105), (211) and (204) planes of anatase phase (JCPDS NO. 21-1 272). Mean sizes calculated by Scherrer
Fig. 2. XRD pattern of FeeTiO2 catalyst.
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formula were 3.5120 Å, 2.4276 Å, 2.3768 Å, 2.3294 Å, 1.8908 Å, 1.6984 Å, 1.6654 Å and 1.4784 Å accordingly. The result was in agreement with previous work (Radoi et al., 2017). However, no obvious crystalline phase of Fe2O3 or FexTiOy was observed possibly due to the following reasons: (1) Fe content was below the detection limits of the XRD instrument; (2) Fe evenly and finely dispersed over the sample surface; (3) Fe ions substituted Ti atoms in the TiO2 matrix (Ma et al., 2015). Besides, the lattice parameters were as follow: a ¼ 3.7212, b ¼ 3.5289, c ¼ 11.6049, a ¼ b ¼ g ¼ 90 . 3.1.2. Fourier transform infrared spectroscopy As shown in Fig. 3, peak at 3 425.98 cm1 was as assigned to eOH stretching vibration of H2O (Anwar and Mulyadi, 2015). The peak was weak suggesting that only very little H2O was attached onto the sample surface after 2-day 80 C dehydration. A broad absorption band ranging from 1734.20 cm1 to 1 457.30 cm1 was probably associated with C]O stretching vibration (17251740 cm1), carboxyl (1 565 cm1) and methyl bending vibration (1 456 cm1) (Kamnev et al., 2017) which were possibly the residues of organic reagents used in 2.1. No prominent peaks but only broad absorption band were assigned to organic compounds (e.g. CeH (3 296 cm1), methylene (2 926.84 cm1) and CeC (1 396.32 cm1) stretching vibration) possibly because organic residues (ethanol and acetic acid used in synthesis) were largely removed after 2-day 80 C dehydration. The peak at 488.48 cm1 was ascribed to TieO stretching vibration bond (Jiang et al., 2003). 3.1.3. UVevis spectroscopy As shown in Fig. 4, FeeTiO2 sample manifested greater absorbance at the wavelength region from 200 nm to 400 nm. Absorbance sharply decreased to 0.4% at the wavelength region over 400 nm. The band gap of newly-synthesized FeeTiO2 was 3.102 eV similar to that in previous finding (Anwar and Mulyadi, 2015). 3.1.4. X-ray photoelectron spectroscopy XPS core-level spectra of FeeTiO2 samples before and after photocatalytic reaction were shown in Fig. 5. As shown in Fig. 5(A), the peak at 458.4 eV corresponded to Ti4þ of TieO bond (Zhang et al., 2010). The peak at 529.8 eV corresponded to TieO bond of lattice oxygen (Olatt), while the peak at 531.4 eV corresponded to eOH bond of surface oxygen (Osurf) (Reszczy Ska et al., 2015). As
Fig. 3. FT-IR spectrum of FeeTiO2 catalyst.
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HgeO bond and no peak relevant to Hg0 (99.8 eV) was found (Vieira et al., 2011) suggesting Hg0 oxidation to Hg (II). The atomic concentrations of each element in the two FeeTiO2 samples were quantified. The outcome showed that proportion of Ti4þ slightly decreased from 100% to 99.05%, while Ti3þ slightly increased from 0% to 0.95%, suggesting the occurrence of photocatalytic redox reaction. Furthermore, proportion of surface oxygen slightly declined from 32.67% to 8.70%, while lattice oxygen slightly increased from 67.33% to 91.30%. Mostly important, after reaction, mercury took up 1.78% mass of detected elements, confirming Hg0 oxidation to Hg (II). In both samples, no peak corresponding to Fe 2p was detected due to the same reasons illustrated in 3.1.1. 3.1.5. Scanning electron microscopy As shown in Fig. 6, FeeTiO2 photo-catalyst had a granular microstructure and an irregular texture. The average diameter of the discernible micro-particles was approximately 100 nm by observation. Particles of 200 nm average diameter evenly dispersed over the sample surface. No nanostructure of specific shape was observed indicating agglomeration of the catalyst particles. Fig. 4. UVeVis diffuse reflectance spectrum of FeeTiO2 catalyst.
shown in Fig. 5(C), the peak at 288.5 eV, 286.1 eV and 284.7 eV were attributed to C]O bond, CeO bond and CeC bond respectively (Okpalugo et al., 2005). With regard to sample after reaction, the peak at 457.3 eV corresponded to Ti3þand was attributed to TieO bond. As shown in Fig. 5(D), the peak at 101.7 eV was attributed to
3.2. Performance of the PCMR and WPCMR system 3.2.1. Influence of inlet concentration As shown in Fig. 7, gradual increase of Hg0 inlet concentration from 37.1 mg m3 to 102.3 mg m3 resulted in the gradual decrease of RE from 82.9% to 46.0%. Subsequent increase of Hg0 inlet concentration from 108.2 mg m3 to 126.3 mg m3 caused a sharper decrease of RE from 31.5% to 10.88%. In comparison, Hg0 removal by
Fig. 5. XPS core-level spectra of Ti2p(A), O1s(B), C1s(C), Hg4f(D) of FeeTiO2 catalysts before and after reaction.
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Fig. 6. SEM images of FeeTiO2 catalyst.
wet photocatalytic membrane reactor (WPCMR) under identical conditions (water flow rate: 40 mL min1). As is shown in Fig. 7, the gradual increase of Hg0 inlet concentration from 39.8 mg m3 to 80.0 mg m3 resulted in the sharp decrease of RE from 87.0% to 57.0%. Subsequent increase of Hg0 inlet concentration from 80.0 mg m3 to 127.7 mg m3 caused a slight decrease of RE from 57.0% to 45.9%. In comparison to removal performance of PCMR, Hg0 RE of WPCMR is averagely higher. Increasing Hg0 inlet concentration meant fewer and fewer activation sites of FeeTiO2 catalyst to capture Hg0 (Seo and Yun, 2017). Photocatalytic reaction rate reduced as fewer available activation sites on catalyst surface.
Fig. 7. Influence of Hg0 inlet concentration in PCMR and WPCMR.
Thus, Hg0 RE in PCMR and WPCMR declined as Hg0 inlet concentration increased. On one hand, Hg0 RE in WPCMR was higher than that in PCMR presumably due to Hg0 oxidation to water soluble Hg (II) which was absorbed by water and was flushed away from the membrane reactor. Dynamic equilibrium was established in photocatalytic reaction in which both reverse reaction and forward reaction took place at the same time. However, reverse reaction can be restricted by removing final products and thereby forward reaction was promoted (Zheng et al., 2016). On the other hand, as previous study theorized (Zhou et al., 2017), TiO2 excitation by UV irradiation yielded photoelectron (e) and electron hole (hþ). The e reacted with O2 adsorbed onto the catalyst surface to produce þ O 2 , while h reacted with OH or H2O to produce OH . Water flowed across the catalyst surface in WPCMR and provided more OH or H2O to react with hþ to generate OH which was highly reactive free radical and oxidized Hg0 to Hg (II).
3.2.2. The influence of GRT As shown in Fig. 8, in the case of PCMR, gradual increase of GRT from 5 s to 10 s resulted in the gradual increase of Hg0 RE from 52.9% to 66.1%. Subsequent increase of GRT from 10 s to 30 s had no noticeable impact on the Hg0 RE. Longer GRT provided sufficient reaction time for complete Hg0 photocatalytic oxidation (Jansson et al., 2017). However, once maximum reaction time required for dynamic equilibrium was reached, further increase of GRT did not promote Hg0 RE anymore. The estimated maximum reaction time in PCMR was 15 s. As shown in Fig. 8, with respect to WPCMR, gradual increase of GRT from 5.0 s to 10.0 s resulted in the proportionate increase of Hg0 RE from 59.7% to 69.2%. Subsequent increase of GRT from 10.0 s to 30.0 s had no significant impact on the Hg0 RE. The estimated reaction time of the system was 11.0 s. In comparison to PCMR, Hg0 RE was averagely higher in WPCMR and maximum reaction time was shorter suggesting faster photocatalytic reaction rate in WPCMR.
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Hg0 RE from 68.5% to 69.3%. Further increase of water spraying density from 55 mL min1 to 70 mL min1 caused sharp decrease of Hg0 RE from 69.4% to 60.3%. The principle of Hg0 RE enhancement by water spray was illustrated in 3.2.1. However excessive water spray possibly caused dissociation of catalyst with the membrane and thus resulted in lower removal performance.
Fig. 8. Influence of GRT in PCMR and WPCMR.
3.2.3. The influence of illuminance As shown in Fig. 9, the gradual increase of illuminance from 85 lux to 1 280 lux resulted in the gradual increase of Hg0 RE from 72.3% to 87.3%, while further increase of illuminance from 1 280 lux to 1939 lux caused little enhancement of Hg0 RE. Likewise, with respect to WPCMR, gradual increase of illuminance from 87 lux to 1 277 lux resulted in the gradual increase of Hg0 RE from 72.6% to 90.6%, while further increase of illuminance from 1 277 lux to 1944 lux hardly affected Hg0 RE. Larger quantities of photons were generated under higher UV illuminance thereby more catalysts were activated for reaction (Subramanian and Kannan, 2008).
3.2.5. Effect of ozone concentration As shown in Fig. 11, with regard to PCMR, as ozone concentration increased from 50 mg m3 to 200 mg m3, Hg0 RE increased from 71.4% to 91.7%. Likewise, in the case of WPCMR, as ozone concentration increased from 50 mg m3 to 300 mg m3, Hg0 RE increased from 81.4% to 93.3%. The Hg0 RE almost remained unchanged with subsequent increase of ozone concentration from 250 mg m3 to 300 mg m3 in both PCMR and WPCMR. Presence of ozone remarkably enhanced the photocatalytic performance of both PCMR and WPCMR. The enhancement was chiefly due to the syn ergistic effect of photocatalytic ozonation including (1) H2O2, HO 2 and OH generation by ozone photolysis on catalyst surface, OH free radicals were detected by HPLC methods; (2) Hg0 oxidation by dissolved molecular ozone; (3) Hg0 oxidation by adsorbed molecn et al., ular ozone on catalyst surface (Gomes et al., 2017; Beltra 2009). Furthermore, ozone trapped photoelectrons efficiently and prevented electron-hole recombination (Solís et al., 2016) which also promoted photocatalytic performance. Hg0 RE in WPCMR was averagely higher than that of PCMR possibly due to more efficient ozone decomposition to OH in liquid matrix (Roth and Sullivan, 1983). As shown in Fig. 7 and Fig. S1, average Hg0 RE in PCMR was 47.9% while that in WPCMR was 59.6%. Average Hg0 RE in adsorption was 9.6%. As shown in Fig. 11, average Hg0 RE by PCMR and WPCMR coupled with ozone was 81.6% and 87.4%. Thus contribution of adsorption, PCMR, wet scrubber and ozonation to Hg0 removal were as follow: 9.6%, 38.3%, 11.7% and 27.8%. 3.3. Kinetic evaluation
3.2.4. Influence of water spray rate As shown in Fig. 10, the gradual increase of water spray rate from 40 mL min1 to 55 mL min1 resulted in the very slight increase of
Langmuir-Hinshelwood (L-H) kinetic model was widely applied in photocatalytic oxidation. Grounded on different assumptions, mathematical expression of L-H model can be modified to various forms (Hoffmann et al., 1995). Theoretical assumptions of
Fig. 9. Influence of illuminance in PCMR and WPCMR.
Fig. 10. Influence of water spray rate in WPCMR.
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for PCMR and WPCMR were 0.03971 mg1 m3 and 0.01804 mg1 m3. Reaction rate k for PCMR and WPCMR were 11.3903 mg m3 s1 and 17.6076 mg m3 s1. 4. Conclusions The present work focused on Hg0 removal by PCMR and WPCMR. Anatase FeeTiO2 photocatalyst was synthesized and coated onto PVDF hollow fiber membrane. Hg0 RE decreased as Hg0 inlet concentration increased while increased as UV illuminance, GRT increased in PCMR and WPCMR. Introduction of ozone significantly promoted Hg0 removal by at most. The optimal conditions for Hg0 removal in PCMR and WPCMR were: 10e11 s GRT, 1 277e1 280 lux UV illuminance. XPS confirmed Hg0 oxidation to Hg (II). L-H kinetic model was successfully applied to describe this process on Hg0 removal in PCMR and WPCMR. This provides a new way of flue gas demercuration using a wet photocatalytic membrane reactor, serves the purpose to build mercury emission control strategy. Acknowledgements The authors gratefully acknowledge the financial support from the Nation Nature Scientific Research Foundation of China (21677178, 21377171).
Fig. 11. Influence of ozone concentration in PCMR and WPCMR.
heterogeneous catalysis described by L-H model were thoroughly elaborated in previous work (Khuzwayo and Chirwa, 2015). Eq. (1) was obtained from previous theoretical study on L-H model (Kumar et al., 2008):
In
Cin Cout
ðCin Cout Þ
¼
kKðV=Q Þ K ðCin Cout Þ
(1)
where k and K were the reaction rate constant and the adsorption equilibrium constant, mg m3 s1 and mg1 m3; Cin and Cout were the inlet and outlet Hg0 concentration respectively, mg$m3; V was the volume of the hollow fiber membranes reactor, m3; Q was the flow rate through the reactor; V/Q equaled to GRT, s; profile of ln [(Cin/Cout)/(Cin - Cout)] versus 1/(Cin - Cout) was plotted verify the linear fitness of L-H model. As shown in Fig. 12, Hg0 removal kinetics in PCMR and WPCMR exhibited good fitness for L-H model (R2 ¼ 0.98 and 0.99 respectively). By further calculation, adsorption equilibrium constant K
Fig. 12. Kinetic evaluation of PCMR and WPCMR.
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light intensity on photocatalytic degradation of phenol, 25, 1300e1308. Vieira, R.S., Oliveira, M.L.M., Guibal, E., Rodr Guez-Castell, N.E., Beppu, M.M., 2011. Copper, mercury and chromium adsorption on natural and crosslinked chitosan films: an XPS investigation of mechanism. Colloids Surf. A Physicochem. Eng. Asp. 374, 108e114. Wang, S., Zhang, L., Li, G., Wu, Y., Hao, J., Pirrone, N., Sprovieri, F., Ancora, M.P., 2010. Mercury emission and speciation of coal-fired power plants in China. Atmos. Chem. Phys. Discuss. 10, 1183e1192. Zhan, F., Li, C., Zeng, G., Tao, S., Xiao, Y., Zhang, X., Zhao, L., Zhang, J., Ma, J., 2013. Experimental study on oxidation of elemental mercury by UV/Fenton system. Chem. Eng. J. 232, 81e88. Zhang, Y., Tang, Z., Fu, X., Xu, Y., 2010. TiO2 Graphene nanocomposites for gasphase photocatalytic degradation of volatile aromatic pollutant: is TiO2 Graphene truly different from other TiO2 Carbon composite materials? ACS Nano 4, 7303e7314. Zhao, S., Li, Z., Qu, Z., Yan, N., Huang, W., Chen, W., Xu, H., 2015. Co-benefit of Ag and Mo for the catalytic oxidation of elemental mercury. Fuel 158, 891e897. Zheng, Q., Tan, S., Feng, H., Cui, X., Zhao, J., Wang, B., 2016. Dynamic equilibrium of reversible reactions and migration of hydrogen atoms mediated by diffusive methanol on rutile TiO2(110)-(1 1) surface. J. Phys. Chem. C 120, 7728e7735. Zhou, R., Wu, J., Zhang, J., Tian, H., Liang, P., Zeng, T., Lu, P., Ren, J., Huang, T., Zhou, X., Sheng, P., 2017. Photocatalytic oxidation of gas-phase Hg 0 on the exposed reactive facets of BiOI/BiOIO 3 heterostructures. Appl. Catal. B Environ. 204, 465e474. Zhuang, Z., Yang, Z., Zhou, S., Wang, H., Sun, C., Wu, Z., 2014. Synergistic photocatalytic oxidation and adsorption of elemental mercury by carbon modified titanium dioxide nanotubes under visible light LED irradiation. Chem. Eng. J. 253, 16e23. Zygarlicke, C.J., 2000. Mercury transformations in coal combustion flue gas. Fuel Process. Technol. 65e66, 289e310.
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Combined membrane photocatalytic ozonation and wet absorption of elemental mercury Z.S. Huang, Z.S. Wei*, Y.M. He, J.L. Pei, X.L. Xiao, M.R. Tang, S. Yu School of Environmental Science and Engineering, Sun Yat-sen University, Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2017 Received in revised form 17 September 2017 Accepted 18 September 2017 Available online xxx
Membrane photocatalytic ozonization coupled with wet absorption offers potential for elemental mercury (Hg0) removal. This study reports on a novel FeeTiO2-coated polyvinylidene fluoride (PVDF) wet photocatalytic membrane reactor (WPCMR) for mercury removal in flue gas. Hg0 removal efficiency in the WPCMR reached up to 93.3%. Ozone could enhance mercury oxidation in WPCMR. Wet absorption helps to increases mercury removal efficiency. FeeTiO2 catalyst was synthesized by sol-gel method and characterized by XRD, FTIR, UVeVis, XPS and SEM. XPS analysis confirmed Hg0 oxidation to divalent mercury (Hg (II)). Elemental mercury was oxidized to mercuric oxide followed by wet absorption in the presence of OH free radical and ozone. Wet photocatalytic membrane reactor and photocatalytic membrane reactor (PCMR) of elemental mercury reaction with the FeeTiO2/PVDF catalyst all follow Langmuir-Hinshelwood kinetics. © 2017 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Elemental mercury FeeTiO2/PVDF Photocatalytic oxidation Catalyst characterization Kinetics
1. Introduction Mercury emission has become a global issue due its toxicity, volatility, persistence and bioaccumulation (Driscoll et al., 2013), which generated from coal-fired power plants, industrial boilers, metal smelting and cement kiln (Pirrone et al., 2010). Mercury emission from coal-fired flue gas often presents in three main forms: elemental mercury (Hg0), oxidized mercury (Hg (II)), and particulate mercury (HgP) (Zygarlicke, 2000). HgP can be effectively removed by particle matter capture device, Hg (II) can be absorbed by aqueous solution (Helble, 2000). HgP and Hg (II) can be captured by wet flue gas desulfurization (WFGD). (Pavlish et al., 2003). However, Hg0, accounting for over 64e90% in flue gas, is the most difficult to be captured with the existing air pollution control devices (Wang et al., 2010). Thus, Hg0 oxidation to water soluble Hg (II) followed by flue gas desulfurization scrubber was a promising solution to control Hg0 emissions (Pavlish et al., 2003). Extensive research have been undertaken using traditional catalytic oxidation (Zhao et al., 2015), membrane catalytic
* Corresponding author. E-mail address: [email protected] (Z.S. Wei). Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.
oxidation (Guo et al., 2012) and photocatalytic oxidation to reduce Hg0 emission (Zhuang et al., 2014). Photocatalytic oxidation is a promising technology for at high removal efficiency, and low cost of maintenance (Zhou et al., 2017), which utilizes semiconductors like TiO2, WO3, FeTiO3, and metallic ions-doped TiO2 photocatalysts to carry out a photo-induced redox process (Kityakarn et al., 2013). Metal elements doping narrowed TiO2 band gap and enhance its visible light activity (Ma et al., 2015). Catalytic membrane reactor combine chemical reaction and separation in a single unit, in which a porous membrane, rendered catalytic by impregnation with catalysts, the reactants flow convectively through the membrane pores where the catalyst is located, thus resulting in intensive contact between the reactants and the catalytic sites, and in high catalytic activity with negligible mass transport resistance (Kajama et al., 2016). Membrane functioned as selective barrier for mixed contaminants, carrier for catalyst and provided spacious area for reaction (Coronas and Santamar, 1999). A novel membrane delivery catalytic oxidation system was set up for Hg0 oxidation and shown phenomenal performance (Guo et al., 2011). Hg0 was oxidized to mercuric chloride oxidation by advanced oxidation process (AOPs) (Zhan et al., 2013). Ozonation can efficiently oxidize many recalcitrant organic compounds (Hassani et al., 2017). The objective of this work is to study elemental mercury (Hg0) removal by coupling membrane photocatalytic oxidation and wet
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absorption with a novel FeeTiO2/PVDF wet photocatalytic membrane reactor (WPCMR). This study evaluates the influence of Hg0 inlet concentration, gas residence time (GRT), water spray rate, UV illuminance, ozone concentration, the coupling role of membrane and catalysis on mercury oxidation. XRD, FTIR, UVeVis, XPS and SEM method are utilized in this study to figure out the formation of the intermediate products and their mechanistic involvement in membrane catalytic oxidation of Hg0, and the mechanistic and kinetic analysis of membrane catalytic oxidation were elicited, which is believed to provide impetus for the application of the membrane catalytic oxidation. 2. Materials and methods 2.1. Preparation of FeeTiO2/PDVF FeeTiO2 catalyst was prepared by solegel method. 25 mL Ti(OC4H9)4 and 10 mL acetic acid was dissolved in 350 mL absolute ethyl alcohol in a beaker. The mixture was stirred for 45 min and designated as solution A. 7.26 g ferric nitrate was dissolved in 30 mL deionized water, 150 mL absolute ethyl alcohol and 20 mL acetic acid. The mixture was stirred for 20 min and designated as solution B. Solution B was dropped to solution A by peristaltic pump at the speed of 1 drop per second. The mixture solution A and solution B
was stirred for 24 h at room temperature. The FeeTiO2/PVDF was synthesized by soaking the entire PVDF hollow fiber membrane in the mixed solution for 48 h then dehydrated for 2 d in thermostat at 80 C. 2.2. Experimental procedure The experimental schematic of wet photocatalytic membrane bioreactor (WPCMR) was shown in Fig. 1. PVDF hollow fiber membrane was purchased from Blue cross Company, Tianjin, China. The dimensional parameters of the PVDF hollow fiber membrane were as follow: 0.38 mm fiber internal diameter; 0.5 mm fiber external diameter; 2 400 fibers; 0.1e0.01 mm pore size; 60% porosity; 200 mm column length; 40 mm column diameter; 2.2 m2 total area. Hg0 was generated by heating a mercury permeation tube (supplied by Qingan scientific instruments Co., Ltd) in a 50e70 C water bath. Airstreams of Hg0 and compressed air were dehydrated in a glass jar loaded with calcium chloride granules, mixed (21% O2 content) in a buffer jar then pumped into FeeTiO2/PVDF hollow fiber membrane reactor. Water was sprayed onto the membrane surface using a subaqueous pump in WPCMR. Outlet gas was absorbed by saturate potassium chloride solution and potassium permanganate solution (10%, m/V).
Fig. 1. The experimental schematic of wet photocatalytic membrane bioreactor (WPCMR):1. Air compressor; 2. Flowmeter; 3. Water bath; 4. Elemental mercury permeation tube; 5. Gas drying jar; 6. Gas mixing jar; 7. Sampling point; 8. UV lamp; 9. FeeTiO2/PVDF; 10. Water tank; 11. Subaqueous pump; 12. Saturate KCl solution; 13. KMnO4 solution.
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2.3. Characterization FeeTiO2 was characterized by XRD, FTIR, UVeVis spectroscopy, XPS and SEM. X-ray diffraction analysis was carried out using a RIKAGU D/max 2200v X-Ray Diffractometer (CuKa radiation, 40 mA, 40 kV). Fourier Transform Infrared Spectroscopy analysis was carried out using a Nicolet 5 700 FTIR spectrometer (Thermo Fisher Scientific USA) in a spectra range of 4 000 cm1 to 400 cm1. UltravioleteVisible light spectroscopy was carried out using a Lambda 950 UV-Vis-NIR Spectrophotometer (Perkin Elmer UK). Xray photoelectron spectroscopy was carried out using an ESCALab250 X-ray photoelectron spectrometer (Thermo Fisher Scientific USA). Scanning electron microscopy was carried out using a JSM6330F Field Emission Scanning Electron Microscope (Japan Electron Optics Laboratory Co, Ltd). 2.4. Analytical methods Hg0 was sampled by ontario hydro method (OHM) set by U. S. environmental protection agency (EPA). Hg0 concentration was determined by atomic fluorescence spectrometer (AFS). Flow rates of inlet gas, outlet gas as well as and water flow were measured by Model LZB-1 flow meters (Purchased from Yuyao instrument company, Zhejiang, China). Gas residence time was calculated by dividing membrane volume by gas flowrate. UV illuminance was determined by LX1330B illuminometer (Sampo scientific instrument Company, Shenzhen, China). Ozone was generated by HY018-150A air source ozone generator (Guangzhou Jia Huan Electric Appliance Co.,Ltd). Determinations of all parameters were conducted in triplicate and the averages were adopted as the consequent values. 3. Results and discussion 3.1. Characterization of FeeTiO2 3.1.1. X-ray diffraction As shown in Fig. 2, major diffraction peaks located at 2q ¼ 25.3 , 37.8 , 38.6 , 48.1, 53.9 , 55.1, 62.8 were characteristics for (101), (103), (004), (112), (200), (105), (211) and (204) planes of anatase phase (JCPDS NO. 21-1 272). Mean sizes calculated by Scherrer
Fig. 2. XRD pattern of FeeTiO2 catalyst.
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formula were 3.5120 Å, 2.4276 Å, 2.3768 Å, 2.3294 Å, 1.8908 Å, 1.6984 Å, 1.6654 Å and 1.4784 Å accordingly. The result was in agreement with previous work (Radoi et al., 2017). However, no obvious crystalline phase of Fe2O3 or FexTiOy was observed possibly due to the following reasons: (1) Fe content was below the detection limits of the XRD instrument; (2) Fe evenly and finely dispersed over the sample surface; (3) Fe ions substituted Ti atoms in the TiO2 matrix (Ma et al., 2015). Besides, the lattice parameters were as follow: a ¼ 3.7212, b ¼ 3.5289, c ¼ 11.6049, a ¼ b ¼ g ¼ 90 . 3.1.2. Fourier transform infrared spectroscopy As shown in Fig. 3, peak at 3 425.98 cm1 was as assigned to eOH stretching vibration of H2O (Anwar and Mulyadi, 2015). The peak was weak suggesting that only very little H2O was attached onto the sample surface after 2-day 80 C dehydration. A broad absorption band ranging from 1734.20 cm1 to 1 457.30 cm1 was probably associated with C]O stretching vibration (17251740 cm1), carboxyl (1 565 cm1) and methyl bending vibration (1 456 cm1) (Kamnev et al., 2017) which were possibly the residues of organic reagents used in 2.1. No prominent peaks but only broad absorption band were assigned to organic compounds (e.g. CeH (3 296 cm1), methylene (2 926.84 cm1) and CeC (1 396.32 cm1) stretching vibration) possibly because organic residues (ethanol and acetic acid used in synthesis) were largely removed after 2-day 80 C dehydration. The peak at 488.48 cm1 was ascribed to TieO stretching vibration bond (Jiang et al., 2003). 3.1.3. UVevis spectroscopy As shown in Fig. 4, FeeTiO2 sample manifested greater absorbance at the wavelength region from 200 nm to 400 nm. Absorbance sharply decreased to 0.4% at the wavelength region over 400 nm. The band gap of newly-synthesized FeeTiO2 was 3.102 eV similar to that in previous finding (Anwar and Mulyadi, 2015). 3.1.4. X-ray photoelectron spectroscopy XPS core-level spectra of FeeTiO2 samples before and after photocatalytic reaction were shown in Fig. 5. As shown in Fig. 5(A), the peak at 458.4 eV corresponded to Ti4þ of TieO bond (Zhang et al., 2010). The peak at 529.8 eV corresponded to TieO bond of lattice oxygen (Olatt), while the peak at 531.4 eV corresponded to eOH bond of surface oxygen (Osurf) (Reszczy Ska et al., 2015). As
Fig. 3. FT-IR spectrum of FeeTiO2 catalyst.
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HgeO bond and no peak relevant to Hg0 (99.8 eV) was found (Vieira et al., 2011) suggesting Hg0 oxidation to Hg (II). The atomic concentrations of each element in the two FeeTiO2 samples were quantified. The outcome showed that proportion of Ti4þ slightly decreased from 100% to 99.05%, while Ti3þ slightly increased from 0% to 0.95%, suggesting the occurrence of photocatalytic redox reaction. Furthermore, proportion of surface oxygen slightly declined from 32.67% to 8.70%, while lattice oxygen slightly increased from 67.33% to 91.30%. Mostly important, after reaction, mercury took up 1.78% mass of detected elements, confirming Hg0 oxidation to Hg (II). In both samples, no peak corresponding to Fe 2p was detected due to the same reasons illustrated in 3.1.1. 3.1.5. Scanning electron microscopy As shown in Fig. 6, FeeTiO2 photo-catalyst had a granular microstructure and an irregular texture. The average diameter of the discernible micro-particles was approximately 100 nm by observation. Particles of 200 nm average diameter evenly dispersed over the sample surface. No nanostructure of specific shape was observed indicating agglomeration of the catalyst particles. Fig. 4. UVeVis diffuse reflectance spectrum of FeeTiO2 catalyst.
shown in Fig. 5(C), the peak at 288.5 eV, 286.1 eV and 284.7 eV were attributed to C]O bond, CeO bond and CeC bond respectively (Okpalugo et al., 2005). With regard to sample after reaction, the peak at 457.3 eV corresponded to Ti3þand was attributed to TieO bond. As shown in Fig. 5(D), the peak at 101.7 eV was attributed to
3.2. Performance of the PCMR and WPCMR system 3.2.1. Influence of inlet concentration As shown in Fig. 7, gradual increase of Hg0 inlet concentration from 37.1 mg m3 to 102.3 mg m3 resulted in the gradual decrease of RE from 82.9% to 46.0%. Subsequent increase of Hg0 inlet concentration from 108.2 mg m3 to 126.3 mg m3 caused a sharper decrease of RE from 31.5% to 10.88%. In comparison, Hg0 removal by
Fig. 5. XPS core-level spectra of Ti2p(A), O1s(B), C1s(C), Hg4f(D) of FeeTiO2 catalysts before and after reaction.
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Fig. 6. SEM images of FeeTiO2 catalyst.
wet photocatalytic membrane reactor (WPCMR) under identical conditions (water flow rate: 40 mL min1). As is shown in Fig. 7, the gradual increase of Hg0 inlet concentration from 39.8 mg m3 to 80.0 mg m3 resulted in the sharp decrease of RE from 87.0% to 57.0%. Subsequent increase of Hg0 inlet concentration from 80.0 mg m3 to 127.7 mg m3 caused a slight decrease of RE from 57.0% to 45.9%. In comparison to removal performance of PCMR, Hg0 RE of WPCMR is averagely higher. Increasing Hg0 inlet concentration meant fewer and fewer activation sites of FeeTiO2 catalyst to capture Hg0 (Seo and Yun, 2017). Photocatalytic reaction rate reduced as fewer available activation sites on catalyst surface.
Fig. 7. Influence of Hg0 inlet concentration in PCMR and WPCMR.
Thus, Hg0 RE in PCMR and WPCMR declined as Hg0 inlet concentration increased. On one hand, Hg0 RE in WPCMR was higher than that in PCMR presumably due to Hg0 oxidation to water soluble Hg (II) which was absorbed by water and was flushed away from the membrane reactor. Dynamic equilibrium was established in photocatalytic reaction in which both reverse reaction and forward reaction took place at the same time. However, reverse reaction can be restricted by removing final products and thereby forward reaction was promoted (Zheng et al., 2016). On the other hand, as previous study theorized (Zhou et al., 2017), TiO2 excitation by UV irradiation yielded photoelectron (e) and electron hole (hþ). The e reacted with O2 adsorbed onto the catalyst surface to produce þ O 2 , while h reacted with OH or H2O to produce OH . Water flowed across the catalyst surface in WPCMR and provided more OH or H2O to react with hþ to generate OH which was highly reactive free radical and oxidized Hg0 to Hg (II).
3.2.2. The influence of GRT As shown in Fig. 8, in the case of PCMR, gradual increase of GRT from 5 s to 10 s resulted in the gradual increase of Hg0 RE from 52.9% to 66.1%. Subsequent increase of GRT from 10 s to 30 s had no noticeable impact on the Hg0 RE. Longer GRT provided sufficient reaction time for complete Hg0 photocatalytic oxidation (Jansson et al., 2017). However, once maximum reaction time required for dynamic equilibrium was reached, further increase of GRT did not promote Hg0 RE anymore. The estimated maximum reaction time in PCMR was 15 s. As shown in Fig. 8, with respect to WPCMR, gradual increase of GRT from 5.0 s to 10.0 s resulted in the proportionate increase of Hg0 RE from 59.7% to 69.2%. Subsequent increase of GRT from 10.0 s to 30.0 s had no significant impact on the Hg0 RE. The estimated reaction time of the system was 11.0 s. In comparison to PCMR, Hg0 RE was averagely higher in WPCMR and maximum reaction time was shorter suggesting faster photocatalytic reaction rate in WPCMR.
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Hg0 RE from 68.5% to 69.3%. Further increase of water spraying density from 55 mL min1 to 70 mL min1 caused sharp decrease of Hg0 RE from 69.4% to 60.3%. The principle of Hg0 RE enhancement by water spray was illustrated in 3.2.1. However excessive water spray possibly caused dissociation of catalyst with the membrane and thus resulted in lower removal performance.
Fig. 8. Influence of GRT in PCMR and WPCMR.
3.2.3. The influence of illuminance As shown in Fig. 9, the gradual increase of illuminance from 85 lux to 1 280 lux resulted in the gradual increase of Hg0 RE from 72.3% to 87.3%, while further increase of illuminance from 1 280 lux to 1939 lux caused little enhancement of Hg0 RE. Likewise, with respect to WPCMR, gradual increase of illuminance from 87 lux to 1 277 lux resulted in the gradual increase of Hg0 RE from 72.6% to 90.6%, while further increase of illuminance from 1 277 lux to 1944 lux hardly affected Hg0 RE. Larger quantities of photons were generated under higher UV illuminance thereby more catalysts were activated for reaction (Subramanian and Kannan, 2008).
3.2.5. Effect of ozone concentration As shown in Fig. 11, with regard to PCMR, as ozone concentration increased from 50 mg m3 to 200 mg m3, Hg0 RE increased from 71.4% to 91.7%. Likewise, in the case of WPCMR, as ozone concentration increased from 50 mg m3 to 300 mg m3, Hg0 RE increased from 81.4% to 93.3%. The Hg0 RE almost remained unchanged with subsequent increase of ozone concentration from 250 mg m3 to 300 mg m3 in both PCMR and WPCMR. Presence of ozone remarkably enhanced the photocatalytic performance of both PCMR and WPCMR. The enhancement was chiefly due to the syn ergistic effect of photocatalytic ozonation including (1) H2O2, HO 2 and OH generation by ozone photolysis on catalyst surface, OH free radicals were detected by HPLC methods; (2) Hg0 oxidation by dissolved molecular ozone; (3) Hg0 oxidation by adsorbed molecn et al., ular ozone on catalyst surface (Gomes et al., 2017; Beltra 2009). Furthermore, ozone trapped photoelectrons efficiently and prevented electron-hole recombination (Solís et al., 2016) which also promoted photocatalytic performance. Hg0 RE in WPCMR was averagely higher than that of PCMR possibly due to more efficient ozone decomposition to OH in liquid matrix (Roth and Sullivan, 1983). As shown in Fig. 7 and Fig. S1, average Hg0 RE in PCMR was 47.9% while that in WPCMR was 59.6%. Average Hg0 RE in adsorption was 9.6%. As shown in Fig. 11, average Hg0 RE by PCMR and WPCMR coupled with ozone was 81.6% and 87.4%. Thus contribution of adsorption, PCMR, wet scrubber and ozonation to Hg0 removal were as follow: 9.6%, 38.3%, 11.7% and 27.8%. 3.3. Kinetic evaluation
3.2.4. Influence of water spray rate As shown in Fig. 10, the gradual increase of water spray rate from 40 mL min1 to 55 mL min1 resulted in the very slight increase of
Langmuir-Hinshelwood (L-H) kinetic model was widely applied in photocatalytic oxidation. Grounded on different assumptions, mathematical expression of L-H model can be modified to various forms (Hoffmann et al., 1995). Theoretical assumptions of
Fig. 9. Influence of illuminance in PCMR and WPCMR.
Fig. 10. Influence of water spray rate in WPCMR.
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for PCMR and WPCMR were 0.03971 mg1 m3 and 0.01804 mg1 m3. Reaction rate k for PCMR and WPCMR were 11.3903 mg m3 s1 and 17.6076 mg m3 s1. 4. Conclusions The present work focused on Hg0 removal by PCMR and WPCMR. Anatase FeeTiO2 photocatalyst was synthesized and coated onto PVDF hollow fiber membrane. Hg0 RE decreased as Hg0 inlet concentration increased while increased as UV illuminance, GRT increased in PCMR and WPCMR. Introduction of ozone significantly promoted Hg0 removal by at most. The optimal conditions for Hg0 removal in PCMR and WPCMR were: 10e11 s GRT, 1 277e1 280 lux UV illuminance. XPS confirmed Hg0 oxidation to Hg (II). L-H kinetic model was successfully applied to describe this process on Hg0 removal in PCMR and WPCMR. This provides a new way of flue gas demercuration using a wet photocatalytic membrane reactor, serves the purpose to build mercury emission control strategy. Acknowledgements The authors gratefully acknowledge the financial support from the Nation Nature Scientific Research Foundation of China (21677178, 21377171).
Fig. 11. Influence of ozone concentration in PCMR and WPCMR.
heterogeneous catalysis described by L-H model were thoroughly elaborated in previous work (Khuzwayo and Chirwa, 2015). Eq. (1) was obtained from previous theoretical study on L-H model (Kumar et al., 2008):
In
Cin Cout
ðCin Cout Þ
¼
kKðV=Q Þ K ðCin Cout Þ
(1)
where k and K were the reaction rate constant and the adsorption equilibrium constant, mg m3 s1 and mg1 m3; Cin and Cout were the inlet and outlet Hg0 concentration respectively, mg$m3; V was the volume of the hollow fiber membranes reactor, m3; Q was the flow rate through the reactor; V/Q equaled to GRT, s; profile of ln [(Cin/Cout)/(Cin - Cout)] versus 1/(Cin - Cout) was plotted verify the linear fitness of L-H model. As shown in Fig. 12, Hg0 removal kinetics in PCMR and WPCMR exhibited good fitness for L-H model (R2 ¼ 0.98 and 0.99 respectively). By further calculation, adsorption equilibrium constant K
Fig. 12. Kinetic evaluation of PCMR and WPCMR.
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