Using Box Modeling to Determine Photodegradation Coefficients Describing the Removal of Gaseous Formaldehyde from Indoor Air

Home , Photodegradation

Ming-Wei Lin1, Ching-Song Jwo1, Hsin-Jr Ho1, Liang-Yü Chen2*

1 Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei 106, Taiwan 2 Department of Biotechnology, Ming Chuan University, Taoyuan 33343, Taiwan

ABSTRACT

Human exposure to volatile organic compounds (VOCs) indoors is receiving increasing attention. Formaldehyde (HCHO) is the most common VOC emitted from household materials and is associated with many health risks, including sick building syndrome. In this study, a simple box model was developed and applied to help understand the fate and degradation mechanisms of HCHO in the indoor environment. The model was validated using observations from an air handling system under different conditions. A UV/TiO2 filter reactor was installed in a closed box with the air conditioning unit. Three parameters, temperature, relative humidity, and circulation wind speed, were investigated for their effects on the performance of the air handling system. Our results show that the operation mode of the air handling system has a greater effect on the removal of HCHO than any of the air conditioning parameters. From a kinetic perspective, the removal of gaseous HCHO from a constant-volume box clearly represents a zero-order reaction. After UV irradiation with a TiO2 filter for 2 hours, the removal efficiency of gaseous HCHO increases to approximately 90%. Contributions to the removal of gaseous HCHO from natural dissipation, photodegradation, and photocatalytic oxidation decomposition are 12%, 30%, and 58%, respectively. Our results have implications for reducing indoor air pollution and reducing stress on air conditioning systems. Meeting these goals is beneficial for human health and energy conservation in modern society.

Keywords: Heterogeneous catalysis; Titanium dioxide; Air cleaning; Reaction kinetics.

INTRODUCTION Organization (WHO) has set a 30-minute exposure limit for HCHO of 0.08 ppm (Salthammer et al., 2010; Golden, 2011). Human exposure to indoor concentrations of volatile The main exposure pathways for formaldehyde are inhalation organic compounds (VOCs) is receiving more and more of gaseous HCHO from the air or absorption of aqueous attention from scientists working in environmental medicine HCHO through the skin (Gelbke et al., 2014). Workplaces, and public health (Hellweg et al., 2009; Collinge et al., including chemical plants and places where industrial 2013; Rosenbaum et al., 2015). In urban areas, people are chemicals are used, professional health care facilities, more likely to ingest, absorb, or breathe in large amounts forensics laboratories, and mortuaries may present elevated of harmful chemicals because these chemicals are more risks of HCHO exposure (Demou et al., 2009). prevalent indoors than in outdoor environments (Geiss et Ventilation is a generally effective means of ameliorating al., 2011; Kim et al., 2015). air quality that works by diluting the concentrations of As air pollutants, VOCs are potentially hazardous indoor pollutants, but it causes energy use by indoor air byproducts or outgassing products from sources including handling systems to increase (Bari et al., 2011; Ciuzas et upholstery, smoking, candle and incense burning, household al., 2016). Specialized air cleaners can be used to address air cleaning activities, cooking, furnishings, printers, building pollution problems in office buildings or factory workshops. materials, and electronic devices (Panagopoulos et al., 2011; Air filtration can remove specific chemicals or particles Chaudhary and Hellweg, 2014). Formaldehyde (HCHO) is from the air and accompanying aerosols. The resulting one of the most common VOCs, and the World Health residues on filters can then be aggregated, collected or transformed. The use of air purifying apparatus to filter out and decompose pollutants is simple in principle and more efficient than ventilation as a means of resolving indoor air * Corresponding author. quality issues (Schleibinger and Ruden, 2009; Han et al., Tel.: +886-3-3507001 ext. 3773; Fax: +886-3-3593878 2012, Ciuzas et al., 2016). E-mail address: [email protected] Heterogeneous photocatalysis on titanium dioxide (TiO2)

Lin et al., Aerosol and Air Quality Research, 17: 330–339, 2017 331 in the presence of ultraviolet (UV) light is an emerging Mass balance provides an approach for identifying technique for chemical removal and microbe inactivation unknown concentrations and reaction rates. The initial (Peral and Ollis, 1992; Chen et al., 2012; Ortega Méndez mass of HCHO in a closed system is the sum of the initial et al., 2015). This technique is based on the generation of mass of formaldehyde and the additional mass that can be reactive radical species that are destructive to organic derived from chemical transformation of other chemical pollutants and microorganisms (Liang et al., 2013). The species present in the system, expressed as HCHOinput = + use of TiO2 nanoparticles as photocatalytic reactors has Ʃ (HCHO) + CHOtransf. The transformation of HCHO many advantages, including high photocatalytic activity and involves decomposition, polymerization, and oxidation to reactive surface area, abrasion resistance, low operating other chemicals (Salthammer et al., 2010). The photocatalytic temperature, and good thermal stability (Yang et al., 2010; oxidation of HCHO first forms the intermediate product Zhou et al., 2014; Pramod and Pandey, 2015). carboxylic acid (HCOOH), which then oxidizes to carbon Filters incorporating TiO2 and activated carbon have dioxide (CO2). The amounts of oxygen and water in the air been used to remove nitrogen oxide and toluene at ppb levels have important impacts on the overall photocatalytic reaction from indoor air (Ao and Lee, 2004, 2005). A photochemical efficiency of air cleaner units containing TiO2 catalysts reactor incorporating TiO2 was applied in a continuous (Yang et al., 2010). Therefore, the aerosol term of HCHO laminar flow that forced air to pass over the reactor (Chen et is emphasized in our model to reflect the effect of water al., 2012). These studies provide useful experimental models molecules in the photochemical reactions. Based on other for studying the effectiveness of air conditioning systems studies (Han et al., 2012; Lo et al., 2015; Yu et al., 2015), in reducing air pollution at moderate energy cost. However, three air conditions of temperature, relative humidity and the competition between adsorption onto and reemission from wind speed are considered as the principal factors on the activated carbon air purifiers results in poor overall removal removal of gaseous VOCs indoors. efficiencies (Obee and Brown, 1995). Frequent adjustments To appraise the cleaning performance of modern air to the temperature and humidity of the environment are handling system needs to model the air purifier units either necessary to avoid secondary pollution caused by reemission by the data driven or physics based approaches. Our aim is from activated carbon fabric (Niu et al., 1998). to investigate the effects of air conditioning parameters on Box modeling techniques are applied widely on large the removal of HCHO by heating, ventilation, and air scale environmental issues and engineering fields (Liu et conditioning (HVAC) systems. Thus, we develop and assess a al., 2000; Afram and Janabi-Sharifi, 2014; Sossan et al., simple box model representing an air handling chamber 2016). Indoor air quality can be affected by VOCs, tinny with a photocatalytic air purifier. The model includes a set particulates, microbes, or various types of hazardous of air conditioning parameters and describes the behavior pollutants which present a health risk to people. Rigorous of HCHO species using the principles of mass balance and mathematical modeling of fluid dynamics leads to a chemical kinetics. Three processes of HCHO removal system of partial differential equations describing chemical could be distinguished and evaluated as a natural dissipation, behavior (Sanongraj et al., 2007). To understand the fate a photodegradation, and photocatalytic oxidation and dynamics of a chemical pollutant entering an air- decomposition within our experimental validation. This conditioned environment, a conceptual box model is proposed, study presents results that describe the heterogeneous as shown in Fig. 1. Simplifying assumptions are made with catalytic reaction of these species in detail and helps respect to the relative concentrations of species in our model. engineers to design air handling units to achieve improved

Fig. 1. Mass balance and transformations of HCHO for the photoreaction experiments with box modeling. Three processes were proposed to remove the gaseous HCHO in this conceptual box.

332 Lin et al., Aerosol and Air Quality Research, 17: 330–339, 2017 control strategies on the indoor air quality and the energy Philips Lighting) were placed on the upper layer of filters consumption efficiency (Afram and Janabi-Sharifi, 2014; to achieve maximal irradiation area. The UV lamps are Sossan et al., 2016). designed to meet a specification of 28(D) × 331(L) mm, with a maximum wavelength of 253.7 nm (UVC germicidal). EXPERIMENTAL METHODS The irradiation intensity was validated using a multi-sense optical radiometer (MS-100, Ultra-Violet Products Ltd., UK). Materials and Chemicals Titanium dioxide (TiO2-1125A) was purchased from Preparation of the TiO2 Filter Yong-Zhen Technomaterial Co. LTD (Taiwan). Commercial- Part of the stainless steel filters were selected and grade polyvinyl acetate (PVA-666) binder was purchased cleaned before they were coated with TiO2 film. Commercial from Gunway Co. (Taiwan). Formaldehyde was purchased oily binder (PVA-666) was mixed with water in a 1:9 from Sigma Aldrich (USA). All chemicals were ultrapure (volume/volume) ratio, and used to evenly coat the stainless grade and were used without further purification. Ultrapure steel filter, which was then set to dry at 50°C. The nano- water produced by a Milli-Q system (Millipore, USA) was TiO2 solution was diluted to 3%, placed in an ultrasonic used as a solvent. oscillator for 10 to 20 minutes to ensure uniformity, and A stainless steel filter (21 × 21 × 1.5 mm) was used as a then placed into a solvent tank for 5 to 10 minutes. The TiO2 mesh supporting substrate in the air purifier unit to nanoparticles were evenly attached to the surface. Then, the decrease chemical adsorption. The mesh filters were used stainless steel filter was dried in a 100°C oven for 10 minutes, in our experiments without film modification, except in those until completely dry. The ignition technique was used to experiments that examined the photocatalytic oxidation ensure that the modified films were evenly distributed. The mode of the air handling system. TiO2 nanoparticles were completely immobilized on the surfaces of the stainless steel filters. Setup of Air Conditioning Box with Photoreactor As shown in Fig. 2, a recirculating air system with a Experimental Design photoreaction unit was used to examine the removal The experimental variables were grouped with the air kinetics of formaldehyde pollution in indoor environments. conditioning parameters and the operation modes. The air The air conditioning recirculation tank used in this study was conditioning parameters were temperature, relative humidity, constructed from existing laboratory equipment, including an and internal wind speed, as described in Table 1. In total, air conditioning unit (P.A. Hilton Ltd., UK) with a vapor 18 sets of experimental conditions were investigated. The compression cooling device, heating coil, fan, and humidifier. air purifier unit could be operated in one of three modes, The mixing chamber’s surface is polished and chemically and these modes are listed and coded in Table 2. inert, and inhibits adsorption. The system also allowed These operation modes are designed as cumulative operation at various wind speeds, temperatures and humidity hierarchy sets in our experimental model. The efficacies of levels. operation modes in gaseous HCHO removal (Δ[HCHOg]) A photoreactor with four mesh filters was designed as an are counted with the relationship that is F-D-S (100) ⊂ F- air purifier for photodegradation and photocatalytic oxidation L-S (110) ⊂ F-L-T (111). operations in this study. Five UV lamps (TUV 10W SLV/25,

Fig. 2. Schematic illustration of the air handling system, which includes a recirculation air tank, an air conditioning unit, and an air purifier unit.

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Table 1. Air conditioning parameters used in box chamber. 10 ppb to 10 ppm and a resolution of 1 ppb. This instrument Control Variables Control Constants was thoroughly recalibrated less than a month before our Temperature (°C) RH = 50%, WS = 0.7 m s–1 experiments. The concentration of gaseous HCHO within 18, 20, 22, 24, 26,28 the apparatus was measured every 15 minutes and recorded Relative Humidity (%) Temp. 22°C Temp. 25°C for kinetic calculations. 40, 50, 60 WS = 0.7 m s–1 Wind Speed (m s–1) Temp. 22°C Temp. 25°C RESULTS AND DISCUSSION 0.4, 0.7, 1.4 RH = 50% Microstructure and Morphology of Nano-TiO2 Film

TiO2 has a number of characteristics that make it desirable Gaseous HCHO Measurement for air pollution reduction, including high activity, chemical Five milliliters of 37% HCHO aqueous solution were stability and abundance in commercial photocatalysts. injected into the recirculation air system as shown in Fig. 2. TiO2 exists mainly in two crystallographic phases, anatase The liquid HCHO was then released from the solution as and rutile. The lattice structure of anatase is considered to gaseous HCHO and was introduced into the air purifier be particularly appropriate for encouraging photocatalytic unit and the mixing chamber by the recirculating air flow. activity. Fig. 3 shows the X-ray diffraction (XRD) patterns The residual gaseous HCHO in the air was detected by an of the TiO2 powder (1125A). Two peaks corresponding to electrochemical sensor (FormaldemeterTM 400, PPM anatase were observed at 25.3° and 48.1°, ignoring diffraction Technology Ltd., Wales, UK). The Formaldemeter contains peaks associated with amorphous titanium oxides. The an internal sampling system that has a detection range of morphology of the TiO2 film was found to have a porous

Table 2. Operation modes for experimental design. CODE Fan UV Filter F-D-S (100) Circulation Dark Stainless Steel F-L-S (110) Circulation Irradiation Stainless Steel F-L-T (111) Circulation Irradiation TiO2 coating

Fig. 3. X-ray powder diffractogram of photocatalytic nano-TiO2 1125A. The inset shows a SEM image of the nano-TiO2 film coating on the stainless steel mesh filter.

334 Lin et al., Aerosol and Air Quality Research, 17: 330–339, 2017 microstructure via scanning electron microscopy (SEM), model, the temperature dependency of reactions is strongly which should enhance the capacity of this material to affected by pore resistance. Generally, higher temperatures encourage VOC reaction. enhanced the efficiency of gaseous HCHO removal with or TiO2 displays photocatalysis and photoinduced without TiO2 filters in this study. superhydrophilicity upon irradiation with light of particular wavelengths (Lee et al., 2007). Superhydrophilicity means Effects of Relative Humidity and Wind Speed that the surface is extremely wetted by water, and the For multivariate testing, the effects of relative humidity and contact angle of a water droplet is less than 10 degrees. wind speed were evaluated by subgrouping the experiments Therefore, water molecules play a key role in determining according to whether they were conducted at high the photodegradation and/or photocatalytic oxidation temperature (25°C) or moderate temperature (22°C). decomposition of HCHO. The effect of relative humidity (RH) on gaseous HCHO removal is shown in Fig. 5. Notably, high relative Effects of Temperature on HCHO Removal humidity promoted the removal of gaseous HCHO Temperature is a key function of air conditioning systems removal at high temperatures under the F-L-T (111) and is a major factor that determines the rate of HCHO operation mode. This result indicates that high relative removal. As shown in Fig. 4, the concentration of gaseous humidity and high temperature enhance the photocatalytic HCHO decreased linearly in each operation mode. In the oxidation decomposition of HCHO in the presence of TiO2 F-D-S (100) operation mode, which is the background mode, nanoparticles. Furthermore, the water molecules are a the effectiveness of HCHO removal does not depend on dominant factor in this reaction by the photoinduced temperature. However, the efficacy of HCHO removal was superhydrophilicity of TiO2. significantly enhanced when the F-L-S (110) and F-L-T The effect of recirculation wind speed (WS) on gaseous (111) operation modes were active. In these modes, higher HCHO removal is shown in Fig. 6. High wind speed temperatures promoted increased reaction rates. According promoted the efficiency of gaseous HCHO removal at high to Arrhenius’ law, higher temperatures accelerate chemical and moderate temperatures under the F-L-T (111) operation reaction rates and increase the adsorption of reactants on mode. This result indicates that the photocatalytic oxidation catalyst surfaces. According to the solid catalyzed reaction decomposition of HCHO was dominated by diffusion into

Fig. 4. Temperature effects on time series of gaseous HCHO concentrations according to operation mode. Air conditioning constants included 50% relative humidity and a wind speed of 0.7 m s–1. Three operation modes, F-D-S (100), F-L-S (110), and F-L-T (111) are described in Table 2. The terms of F, D/L, and S/T mean the fan, the dark or lighting, and the stainless steel or TiO2 coating filters used in the referred operation mode.

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Fig. 5. Relative humidity effects on time series of gaseous HCHO concentrations according to operation mode. Air conditioning constants included a wind speed of 0.7 m s–1 and temperatures of 25°C or 22°C. The coded operation modes, F-D-S (100), F-L-S (110), and F-L-T (111) are defined as the same as in Fig. 4.

the porous surface of TiO2 nanoparticles. However, although parameters were calculated from the measured gaseous high temperatures also enhance HCHO desorption from HCHO concentrations; these values are listed in Table 3. solid surfaces, which is the reverse reaction, this effect The initial concentrations of gaseous HCHO, referred as does not appear to be significant. [HCHOg]0, are substantial and their values occupy a narrow range. The amounts of HCHO introduced into our system Kinetics of Heterogeneous Reactions were precise and accurate, that is, they were validated for The experimental system in this study is designed as a each experiment. This result also suggests that the reaction batch reaction in a box with limited volume. Based on the kinetics were not sensitive to the air conditioning parameters, time series of gaseous HCHO concentration, the removal such as temperature, relative humidity, and wind speed. of HCHO appears to follow a zero order reaction (Yang et However, the efficiencies of the three operation modes are al., 2012). The reaction rate is approximately constant over significantly different and distinguished by their inferred the monitoring period. The integrated rate equation becomes kinetic parameters. Since the apparent rate constant for the + as follows, F-D-S (100) operation mode k 100 is controlled by natural decomposition, the apparent rate constants are 2.255-fold + dC k C and 5.020-fold of k 100 for operation modes F-L-S (110) AA1 (1) and F-L-T (111), respectively. dt1 k C 2 A Effects of UV Light Irradiation + At high CA or k2CA >> 1, the apparent rate constant k = Gaseous HCHO can be removed by many pathways, k1/k2 becomes the slope of the regression line. The reactant, including the natural decomposition, adsorption onto other gaseous HCHO, was entirely introduced at the beginning materials, photolysis, polymerization and photocatalytic of reaction monitoring. Thus, the initial concentration of oxidation decomposition (Li et al., 2014). Without TiO2 gaseous HCHO was difficult to measure. Extrapolating the filters, the F-L-S (110) and F-D-S (100) operation modes regression line to the beginning of the experiment, the initial can be considered to mimic indoor environments with air concentration of gaseous HCHO can be approximated handling equipment, but without a photocatalytic air purifier from the intercept of the regression line. The regression unit. Irradiation with UV light noticeably enhanced the lines fit the data well and have R2 values > 0.95. Kinetic removal of gaseous HCHO without photocatalysts. We

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Fig. 6. Wind speed effects on time series of gaseous HCHO concentrations according to operation mode. Air conditioning constants included 50% relative humidity and temperatures of 25°C or 22°C. The coded operation modes, F-D-S (100), F- L-S (110), and F-L-T (111) are defined as the same as in Fig. 4.

Table 3. Kinetic and quantitative measurements for gaseous HCHO removal. The concentrations of gaseous HCHO at initial and after 2 hr., [HCHOg]0 and [HCHOg]2hr, are presented as the mean ± standard deviation (coefficient of variance) + in ppm. The apparent rate constants, k = k1/k2, are presented as the mean ± standard deviation (coefficient of variance) in ppb per minute. Operation Overall Data (18 × 3 sets) + Mode [HCHOg]0 k = k1/k2 [HCHOg]2hr F-D-S (100) 5.34 ± 0.05 (0.95%) 5.1 ± 0.5 (9.28%) 4.78 ± 0.09 (1.95%) F-L-S (110) 4.71 ± 0.07 (1.42%) 11.5 ± 2.0 (17.39%) 3.41 ± 0.20 (5.82%) F-L-T (111) 3.77 ± 0.19 (4.98%) 25.6 ± 1.6 (6.14%) 0.75 ± 0.26 (34.5%) suggest that the direct photolysis of gaseous HCHO and for the F-L-T (111) and F-L-S (110) operation modes. The liquid HCHO in aerosol should be the mainly contributors. photocatalytic effectiveness coefficient is 2.226, and the However, utility of filters coated with TiO2 nanoparticle decrease in the energy barrier is approximately 2.0 kilojoule film could promote the removal efficacy of gaseous HCHO per mole at 301-291 K in this photoreactor with TiO2 to approximately 90%. filters and UV illumination. In the F-L-T (111) operation mode, photocatalytic According to the results presented in Fig. 4, high oxidation decomposition is well known as a predominant relative humidity also promotes the removal of HCHO. pathway for achieving highly efficient HCHO removal. The photoinduced superhydrophilicity of TiO2 film might Arrhenius' equation can be used to calculate the change in enhance the adsorption of gaseous HCHO and liquid HCHO activation energy (Ea) associated with a chemical reaction onto the porous film. as the ratio of the rate constants: Contributions of Each Processing on HCHO Removal  For quantitative opinions, the performance of an air k111 EEaa,111 ,110 ln   (2) cleaner could be evaluated by the total amount of gaseous k110 RT HCHO removal, Δ[HCHOg] = [HCHOg]0 – [HCHOg]2hr, that are shown in Table 3. However, the measured data is derived + + Here, k 111/k 110 is the ratio of apparent rate constants from accumulated removals of all processes in our system.

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Therefore, removal of gaseous HCHO in F-L-T (111) specifically temperature, relative humidity, and recirculation operation mode (Δ[HCHOg](111) = 4.95 ppm with TiO2 filters wind speed, were investigated, as well as three air handler and UV irradiation) for 2 hours is considered as the full operation modes. Filter reactors with photocatalytic TiO2 efficacy,. On the same estimation base, the removals of nanoparticles were installed as air purifiers in this air gaseous HCHO after 2 hours are 1.93 ppm and 0.56 ppm for conditioning system. Higher temperatures encourage F-L-S (110) and F-D-S (100) operation modes, respectively. photolysis and effectively promote the removal of HCHO. With the logical implication of set theory, the Higher circulation wind speeds also enhance HCHO removal Δ[HCHOg](110) is the result of excluding the process of in the presence of porous photocatalytic materials. However, photocatalytic oxidation decomposition; the Δ[HCHOg](100) is higher relative humidity values attenuate the removal of the result of excluding the both processes of photodegradation HCHO by UV light degradation. Aerosol or suspended and photocatalytic oxidation decomposition. If particles might participate in these photochemical reactions. photodegradation is the dominant pathway in the F-L-S Our results demonstrate that the removal of gaseous HCHO is (110) operation mode, the additional removal of gaseous a zero order reaction, which is a feature of heterogeneous HCHO in the F-L-T (111) operation mode should be mainly catalytic reactions. Furthermore, kinetic analysis reveals contributed by the photocatalytic oxidation decomposition of that several different mechanisms participate in the TiO2. Considering all cases examined in this study, the removal of gaseous HCHO. In our experiments, natural fraction of photocatalytic oxidation decomposition of TiO2 dissipation, photodegradation, and photocatalytic oxidation achieved a level of 58%. Then, the 12% and 30% on decomposition remove 12%, 30%, and 58% of gaseous removal of gaseous HCHO are contributed by the natural HCHO, respectively. Finally, our results provide a baseline dissipation and the photodegradation, respectively. for evaluating the effectiveness of air handling systems for indoor air quality management. Prediction and Large Scale Application of the Box Modeling ACKNOWLEDGMENTS The development of control approaches and applied strategies need to assess the interactions in detail for the The authors thank the Ministry of Science and Technology, different chemicals, the efficacies and the living-related Taiwan, under Contracts No. MOST 103-2113-M-130-001 activities (Hunger et al., 2010). Langmuir-Hinshelwood (grant to L-Y Chen) for supporting this work financially. model is often adopted on kinetics of HCHO removal. We especially thank Professor Kon-Kee Liu in the Institute Because of the initial rapid adsorption of HCHO onto the of Oceanography of National Taiwan University for his TiO2 photoreactor, the gaseous HCHO concentration jumps suggestion to use numerical modeling. to the fixed value and the kinetic reaction is presumed as a first-order reaction (Langmuir isotherm). However, the REFERENCES results of data driven observation in most studies present an approximate zero-order reaction except for low concentration Afram, A. and Janabi-Sharifi F. (2014). Review of modeling of HCHO (Lo et al., 2015; Yu et al., 2015). We suggest methods for HVAC systems. Appl. Therm. Eng. 67: that the measurement uncertainties of rate-dependency are 507–519. trade-off the accuracies of processing contributions in Ao, C.H. and Lee, S.C. (2004). Combination effect of quantitation. 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on modified TiO2/AC composite photocatalyst. Chinese benzene and formaldehyde in air by cross J. Chem. Eng. 20: 572–576. cataluminescence on nano-3TiO2–2BiVO4. Sens. Yu, K.P., Whei-May Grace Lee, W.M.G. and Lin, G.Y. Actuators B 202: 721–726. (2015). Removal of low-concentration formaldehyde by a fiber optic illuminated honeycomb monolith photocatalytic reactor. Aerosol Air Qual. Res. 15: 1008–1016. Received for review, October 6, 2016 Zhou, K.W., Cheng, Y.L., Yang, H.W., Gu, C.X., Xiao, Y. Revised, December 3, 2016 and Zhao, M.H. (2014). Simultaneous determination of Accepted, December 5, 2016