TiO2

Journal of Environmental Chemical Engineering 7 (2019) 103455

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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Photocatalytic degradation of gaseous toluene using self-assembled air filter based on chitosan/activated carbon/TiO2

T

Lekshmi Mohan Va, S.M. Shiva Nagendraa,⁎, M.P. Maiyab a b

EWRE Laboratory, Department of Civil Engineering, IIT Madras, Chennai, 600036, India R&AC Laboratory, Department of Mechanical Engineering, IIT Madras, Chennai, 600036, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Self-assembly Volatile organic compounds Titanium dioxide Activated carbon Photocatalytic oxidation Air filter

Photocatalytic filters are integrated with air purifiers and ventilation systems to remove volatile organic compounds (VOCs) from indoor air. However, the filters, generally prepared by dip or spray coating, are less stable as the deposited TiO2 NPs are prone to be blown off by treated air. In the present study, we propose a novel filter preparation method based on self-assembly of activated carbon (AC) and TiO2 on non-woven polyethylene terephthalate (PET) fabric using chitosan (CS). The prepared filter (CSAT-PET) was characterized for morphological and chemical properties which revealed the homogeneity and chemical bonding of deposited TiO2. Photocatalytic activity of the prepared filter was evaluated for toluene degradation (200–600 ppb) under dark and UV illumination. Toluene removal efficiency reached 91% over CSAT-PET, while it was only 62% over pure TiO2 filter. About 40% of toluene removal was achieved by adsorption on CSAT-PET. Furthermore, CSAT-PET showed robust performance for selected face velocity ranging from 0.5 to 1.5 m/s and, the reaction rate followed Langmuir-Hinshelwood model. FTIR study identified benzaldehyde and benzoic acid as adsorbed intermediates. A UV-induced filter regeneration partially released the adsorbed species. CSAT-PET showed consistent toluene removal and intact morphology over five degradation cycles. The Box-Behnken design (BBD) in RSM was applied to optimize the filter preparation method. The optimum values of TiO2, AC and CS loading were 38.3 g/m2, 52.6 g/m2 and 2.06% (w/v), respectively which exhibited highest removal efficiency for toluene (93%). The proposed methodology can be adapted to fabricate low-cost, stable, and reusable photocatalytic filter for air purification applications.

1. Introduction Volatile organic compounds (VOCs) are harmful pollutants in indoor air that affect human health and productivity. Prolonged exposure to VOCs can induce eye and throat irritation, central nervous system damage, and increased risk of cancer [1]. Various VOC removal techniques have been developed, such as adsorption [2], ozonation [3], non-thermal plasma [4], and photocatalytic oxidation (PCO) [5]. Among them, adsorption based on activated carbon (AC) filter is a particularly popular method to remove the low concentrations (i.e., parts per billion level) of VOCs typically present indoors [6,7]. However, the application of AC filters for adsorption of VOCs is often a

failure due to its limited adsorption capacity [8,9]. In the last two decades, photocatalytic oxidation (PCO) has emerged as a promising technology for air pollution abatement due to ease of integrating with ventilation systems and portable air purifiers [10]. Principally, PCO employs a photocatalyst (i.e., TiO2) and UV light to mineralize pollutants. On UV illumination, electron (e−) in the valence band (VB) is excited to the conduction band (CB) leaving behind a hole (h+). The e−/h+ pair assists in the generation of reactive oxygen species (i.e., OH˙, O2˙‫ )־‬which decomposes the adsorbed pollutants into less harmful products (i.e., CO2 and H2O) along with byproducts like formaldehyde (HCHO). However, the oxidation step in PCO doesn’t occur unless pollutants are adsorbed on the catalyst surface making

Abbreviations: AC, activated carbon; ANOVA, analysis of variance; ASHRAE, American Society of Heating, Refrigeration and Air conditioning Engineers; BBD, BoxBehnken design; BET, Brunauer-Emmett-Teller; CS, chitosan; CSAT-PET, chitosan-activated carbon and TiO2 coated PET filter; DOE, design of experiment; DRS, diffuse reflectance spectroscopy; EDX, energy dispersive X-ray spectroscopy; FTIR, fourier transform infrared spectroscopy; IPA, iso-propanol; HVAC, heating, ventilation, and air conditioning; L-H, Langmuir-Hinshelwood model; LbL, layer-by-layer assembly; PCO, photocatalytic oxidation; PET, polyethylene terephthalate; PID, photoionization detector; RE, removal efficiency (%); RSM, response surface methodology; SEM, scanning electron microscopy; TGA, thermo gravimetric analysis; TiO2, titanium dioxide; UV, ultravoilet; VOCs, volatile organic compounds; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction ⁎ Corresponding author. E-mail address: [email protected] (S.N. S.M.). https://doi.org/10.1016/j.jece.2019.103455 Received 16 July 2019; Received in revised form 1 September 2019; Accepted 4 October 2019 Available online 07 October 2019 2213-3437/ © 2019 Published by Elsevier Ltd.

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Fig. 1. Schematic representation of the experimental reactor.

filter paper using CS as a binding agent, and the coated filter exhibited good photoactivity, stability, and reusability. Apart from the antimicrobial, self-cleaning, film-forming properties of CS, it is an exceptionally good adsorbent [22]. Coincidently, CS has excellent adsorption ability for HCHO and its permanent arrestance through Schiff base formation [23,24], which is beneficial for reducing byproduct formation in the PCO process. Therefore, it was expected that TiO2, CS and AC can complement each other with their advantages and provide a comprehensive method for VOC abatement. Herein, CS is used as a bridging layer due to its good adsorption capacity and ability to bind TiO2 NPs to the filter surface through partial chemical bonding. To the best of author’s knowledge, no studies have reported the application of composites based on TiO2, AC, and CS for air pollution control applications. The major aim of the study is to investigate the applicability of self-assembled CSAT-PET filter for VOC removal at real indoor conditions based on parameters such as, (i) morphology, bonding properties and photostability of coating using various characterization techniques, (ii) efficiency of CSAT-PET in VOC removal using toluene as model pollutant in comparison with pure TiO2 filter and its reusability (iii) propose a reaction mechanism based on the identification of adsorbed intermediates using FTIR, and (iv) optimization of component loading using response surface methodology (RSM) analysis.

adsorption a vital step in VOC removal. Practically, VOC adsorption on photocatalyst surface from the dilute air stream would be difficult considering the high volumetric airflow rates in ventilation systems. Moreover, the inherent low surface area and fast e−/h+ recombination in TiO2 results in lower removal efficiency. Use of composite photocatalysts with porous materials such as AC enhances VOC removal due to synergy between the large surface area and electron storage capacity of AC with the high oxidation ability of TiO2 [11,12]. Aggregation of TiO2 nanoparticles (NPs) and poor adherence of TiO2 with the substrate are other main issues existing in PCO. For application in VOC removal, TiO2 NPs must be immobilized on a suitable substrate, and a strong adherence of TiO2 with substrate is crucial for achieving a uniform coating [13]. Poor adherence of TiO2 NPs causes them to be blown away into indoor spaces with the treated air and pose health risks. In the context of preventing TiO2 aggregation and obtaining a uniform coating on substrates, coating methods such as dip, spray, sol-gel process have been used. It would be inevitably difficult to achieve a good adherence of NPs with dip and spray coating alone. Therefore, binders are used in coating which often enwraps TiO2 within the binder matrix resulting in reduced photoactivity [14]. On the contrary, sol-gel technique generally produces a homogenous TiO2 coating but requires high-temperature post-treatment that alters the surface/chemical properties of the fabrics and transform the active anatase TiO2 into stable rutile phase which is less photoactive [15]. Moreover, the homogeneity of coating, TiO2 adhesion and photoactivity depends upon the selection of substrate material. Non-woven fabric has been used for TiO2 immobilization given the availability of enough active sites and good adhesion of TiO2 [16]. Non-woven polyethylene terephthalate (PET) fabric was selected as it is one of the low cost and stable artificial fabric widely used as air cleaning filter in building and automobile ventilation systems due to its good surface area, hydrophobicity and wear properties [17]. Recently, the self-assembly method has been applied to produce well-defined multilayer coatings on fabrics [18]. In self-assembly, the filters are alternatively dipped in oppositely charged aqueous solutions, and the individual layers are held by electrostatic interaction forces [19]. Qian et al. [13] used the self-assembly technique to obtain a uniform layer of mesoporous TiO2 over carbon form using Pluronic P123 copolymer and reported a high photocatalytic removal of acetone and toluene. Here, chitosan (CS), a natural polysaccharide, rich in amino (eNH2) and hydroxyl (eOH) group is used for binding metal ions through electrostatic interaction, thanks to its cationic nature in acidic medium [20]. Kamal et al. [21] fabricated nickel NPs coated

2. Materials and methods 2.1. Chemicals and materials Chitosan (SRL Private Limited, India) with deacetylation degree of ∼90% and molecular weight of 186 kDa was used as binder. TiO2 P25 (Evonik) nanopowder (80% anatase and 20% rutile) with a nominal particle size of ∼21 nm was used as photocatalyst. DARCO G60 (Sigma Aldrich Inc., UK) activated carbon with a particle size of 150–45 μm (100–325 U.S. mesh size), and a specific surface area of 600 m2/g was used as adsorbent. Acetic acid and iso-proponal (IPA) were used as dispersing agents for CS and TiO2, respectively and was obtained from Merck, India. A commercially available 1.7 mm thick non-woven PET filter with an air permeability of 380 L/m2/s and surface density of 60 g/m2 was used as substrate and was supplied by Impec Filters Private Limited, India.

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potentials, to achieve stability of particles in suspension via electrostatic repulsion [18]. The measured zeta potential values (Table S1) revealed that the aqueous suspensions were stable [26]. The coated filters were dried at 60 °C overnight and placed in a desiccator for 12 h to eliminate moisture [27]. The weight of the filter before and after coating was recorded to calculate AC and TiO2 loading. The obtained filter had an AC, and TiO2 loading of 60 and 30 g/m2, respectively and hereafter will be referred to as CSAT-PET filter, which was used for the characterization and experimental study. The same procedure was used to prepare the pure TiO2-PET filter except that there was only one dipping, i.e., in TiO2 suspension.

2.2. Photocatalytic apparatus The closed-loop experimental setup (Fig. 1) was made of acrylic and consisted of an annular PCO reactor, air mixing chamber, and air pump. The cylindrical PCO reactor had a capacity of 1.6 L and housed 3 UV lamps (4 W UVA lamps; 365 nm; Philips, Holland) sandwiched between two circular filters (10 cm dia.). UV lamps were positioned 5 cm away from each filter. The air mixing chamber had a volume of 8 L and was provided with septa for injecting VOC. In the air mixing chamber, the injected VOC was mixed with laboratory air, which was then passed through the PCO reactor in multiple passes. Real-time VOC monitoring at the upstream and downstream of the reaction chamber was carried out using VOC monitor (PhoCheck Tiger, Ion Science Ltd., UK) and commercially available photoionization detector (PID) sensor (PIDAH2, Alphasense Pvt. Ltd., UK). Air circulation through the PCO reactor was enabled by an air pump with mass flow regulators. A hot-wire anemometer (Testo 450i, Testo Pvt. Ltd., India) was used to measure the face velocity as an average of five measurements across the face of filter (Fig. 1). The measured face velocity through the filter ranged from 0.5 to 1.5 m/s.

2.3.3. Filter characterization The prepared CSAT-PET was analyzed with attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectrometer (Perkin Elmer Spectrum 100, USA) at a resolution of 4 cm−1 in the wavelength range of 4000–550 cm−1. Raman spectrum in the wavelength range of 4000–50 cm−1 at 4 cm−1 resolution was obtained by Bruker RFS 27 spectrophotometer that used an Nd:YAG laser source (1064 nm) for excitation. X-ray diffraction (XRD) spectra were recorded using Philips X’pert diffractometer with Bragg-Brentano geometry and Cu-Kα radiation (λ = 1.5418 Å) at a scanning range of 10° < 2θ < 90° (0.1°/min) to analyze the crystalline structure. X-ray photoelectron spectroscopy (XPS) was obtained using Specs X-ray photoelectron spectrometer with MgKα as the X-ray source and equipped with PHOIBOS 100MCD analyzer to characterize the chemical composition of the coating. The light absorption spectra of samples were obtained by Ultraviolet-visible (UV–vis) spectrometer (Perkin Elmer, Lambda 950 UV-VIS-NIR Spectrophotometer, USA) at a wavelength range of 200–800 nm using diffuse reflectance spectroscopy (DRS) method and BaSO4 as reflectance standard. Thermogravimetric analyses (TGA) of the filters were performed under N2 atmosphere at temperature from 25 °C to 700 °C (heating rate of 10 °C/min) using SDT Q600 TGA (T.A.) instrument. Scanning electron microscopy (SEM, FEI Quanta 200) combined with energy-dispersive X-ray spectroscopy (EDX, EDAX) was used to understand the morphology and to obtain the elemental mapping of the samples. Micrometrics ASAP 2020 porosimeter was employed to obtain the specific surface area of the coating at 77 K by N2 adsorption-desorption isotherm employing the Brunauer-Emmett-Teller (BET) method. The filter samples were degassed at 150 °C overnight before the analysis.

2.3. Filter coating and characterization 2.3.1. Preparation of coating solutions The dip-coating solutions were prepared as follows: (i) CS solution was prepared by dispersing 2% w/v CS powder in 2% (v/v) acetic acid solution with continuous stirring at 60 °C for 4 h, (ii) AC-CS solution by dispersing 4% w/v AC powder in 1% CS solution at 60 °C for 4 h, and (iii) 2% w/v TiO2 suspension prepared in 20% (v/v) IPA with sonication for 2 h. Zeta potentials (ξ) of the prepared solutions were determined by dynamic light scattering principle (SZ-100, Horiba Scientific, Japan) and are given in Table S1. 2.3.2. Layer-by-layer (LbL) assembly The non-woven PET substrate was soaked in 1 M NaOH (at 70 °C) for 30 min [25]. The alkali treatment was done to achieve eOH and eCOOH active sites on the surface of PET fiber for depositing AC and TiO2. The pretreated PET fabric was immersed successively in CS, ACCS, CS, and TiO2-IPA solutions, each for 5 min (Fig. 2). Before each dipping, the filter was washed for 2 min to remove the unbounded particles from the previous step. Self-assembly of components driven by electrostatic interaction requires that the solutions exhibit opposite zeta

Fig. 2. Schematic of layer-by-layer self-assembly of chitosan/activated carbon and TiO2 on PET filter. 3

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2.4. Experimental study

Table 2 Values and levels of chosen factors.

2.4.1. Photodegradation studies Toluene (C7H8) was used as a model pollutant, and the vapor was generated by evaporating a known volume of solvent in a 500 mL Pyrex glass bottle at 60 °C [28]. When the temperature returned to room level, a known volume of vapor was collected using a 50 mL airtight glass syringe (Hamilton Company, Nevada, U.S.) and injected via the septum in the air mixing unit. The blower was switched on, and the mixture of toluene and carrier air was allowed to tranquilize for 30 min to get a steady and uniform concentration (Cinitial). Later, the filter was placed, and experiment was carried out under dark condition for 60 min. The filter was then irradiated with UV light, and the experiment was continued for another 100 min, the final concentration (Cfinal) was noted. The toluene removal efficiency which is the sum of adsorption and photodegradation was then calculated according to Eq. 1.

Removal efficiency (%) = (1 −

Cfinal ) × 100% Cinitial

Factors

2

TiO2 loading (g/m ) AC loading (g/m2) Chitosan content (% w/v)

(2)

(3)

where Kp (mol/m /s) is rate constant, C (ppb) is the concentration of toluene and Kad (ppb−1) is adsorption constant, which represents the ratio of toluene adsorbed to gaseous concentration. The reaction constants could be obtained by regression analysis of experimental data [29]. The toluene degradation rate, r (mol/m2/s), was calculated according to Eq. 4. (4)

where Cin and Cout (ppb) were the upstream and downstream toluene concentration, respectively, Q (m3/s) is the flow rate and, A (m2) is the surface area of the filter. Table 1 Experimental conditions of the study. Parameter

Experimental range/condition

VOC Initial concentration (ppb) Face velocity (m/s) Relative humidity (%) Temperature (°C)

Toluene 200–600 0.5–1.5 51–55 28–30

10 30 1

30 60 2

50 90 3

The FTIR spectra of CSAT-PET, as well as pristine CS, TiO2, AC, and PET, are presented in Fig. 3A. For pristine CS, primary OeH and NeH stretching peak occurred at 3380 cm−1. In addition, the typical peaks of CS at 2877 cm−1 (CeH stretching peak), 1650 cm−1 (CeO stretching of primary amide), 1565 cm−1 (NeH bending of secondary amide group), 1375 cm−1 (CeH and OeH bending vibration), and 1044 cm−1 (CeO stretching skeletal vibration) were also observed. The characteristic peak of pristine TiO2 (Fig. 3A) was found at 500–650 cm−1, caused by the OeTieO bond in anatase structure together with a weak peak at 1630 cm−1. Also, a broad band at 3380 cm−1 was observed, which could be due to the bending vibration of adsorbed water and OeH groups [30]. The characteristic band of AC appeared at 588 cm−1 which could be attributed to the CeH vibrations (Fig. 3A). Other bands appeared at 1240, 1632 and 2320 cm−1 which could be due to CeO stretching in lactones, C]C stretching, and alkyne groups, respectively. Furthermore, the broad band at 3200 cm−1 could be ascribed to the OeH stretching of alcohols, phenols and carboxylic groups [31]. For pristine PET (Fig. 3A), the strong peak at 1712 cm−1 could be attributed to carbonyl (eC]O) stretching of eCOOH. Further, the peaks at 1240 and 1130 cm−1 could be due to ester group vibrations [32]. The presence of peak at 716 cm−1 is due to CeH(CH2) bending vibration of benzene ring [33]. Similarly, the out-of-plane CeH bands (1017, 875, 715 cm−1) and overtone peaks (1400-1600 cm−1) due to benzene ring were also observed [32]. In addition, weak peaks at 3676 cm−1 (end of OeH group), 3450 cm−1 (overtone vibration of eC]O group) and 2917 cm−1 (CeH stretching) were noticed [34,35]. It can be seen that few new peaks appeared in the self-assembled CSAT-PET. In specific, the new peak at 2877 cm−1 and broadening of 3380, 1375, 1044 cm−1 peaks indicated that the bonding between CS, TiO2 and PET substrate has occurred. More importantly, the broadening of the peak at 1478 and 1556 cm−1 corresponding to symmetric and asymmetric vibration of TieOeC (bidentate carboxylic group with Ti) suggested that chemical bonding between substrate PET and the catalyst was achieved [36]. The results of the FTIR analysis was further confirmed by Raman spectroscopy analysis. The Raman spectra of CSAT-PET filter before and

Kp K ad C

Q (Cin − Cout ) 24.5 × A × 1000

High (+1)

3.1. Characterization of CSAT-PET filter

2

r=

Centre (0)

3. Results and discussion

2.4.2. Reaction kinetics study Toluene degradation data were fitted with Langmuir-Hinshelwood (L-H) model (Eq. 2), based on the assumptions that mass transfer was not the rate-limiting step, and the formation of the intermediates was negligible. The linearized form of L-H is given in Eq. 3.

1 1 1 C −1 + = r K ad Kp Kp

Low (-1)

2.5.1. Box Behnken design in RSM An experimental design approach was performed using the Box Behnken design (BBD) to optimize the filter preparation method. The analysis of experimental data was performed using the Design-Expert software (Trial version 11, Stat-Ease Inc., MN, USA). The effect of 3 independent factors namely, TiO2 (g/m2), AC (g/m2) and CS loading (% w/v) on toluene removal efficiency was investigated using BBD with response surface methodology (RSM). The chosen variables were converted into dimensionless form (A, B, C) and coded as -1, 0, +1 levels as seen in Table 2. The values of selected factors were determined based on preliminary experiments. A three-factorial, threelevel BBD consisting of 17 experimental runs was performed in the present study, and the center point was repeated five times.

(1)

1 + K ad C

A B C

Coded levels

2.5. Optimizing filter preparation

where Cinitial and Cfinal are the initial and final toluene concentrations. The experiments were conducted at room conditions for different experimental conditions presented in Table 1. Furthermore, control experiments were performed with only UV light illumination to determine the effect of photolysis on toluene removal. The reusability of CSAT-PET was investigated by repeating the photodegradation experiments for five cycles. FTIR analysis was conducted to identify reaction intermediates deposited on CSAT-PET. The characteristics of CSAT-PET before (CSAT-PET0) and after (CSAT-PET5) reaction were evaluated by SEM, XRD and Raman analysis. Additionally, regeneration tests were performed by exposing CSATPET5 to UV light irradiation for 120 min, and the corresponding sample CSAT-PETR was analyzed using FTIR.

r=

Symbol

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Fig. 3. Characteristics of CSAT-PET (A) FTIR spectra of pristine CS; TiO2; AC; PET and CSAT-PET filter; (B) Raman spectra before and after 5 degradation cycles; (C) XRD patterns of TiO2; AC; CS; PET; CSAT-PET before and after 5 reaction cycles and, (D) Diffuse reflectance UV–vis spectra of TiO2; CS; AC; pure TiO2-PET and CSATPET.

after five photodegradation cycles is shown in Fig. 3B. The characteristics peaks of TiO2 anatase phase could be observed at 143, 394, 519, and 631 cm−1 [37]. Also, characteristic PET peaks were observed at 1728, 1612 cm−1. Several other bands of PET [38] along with the characteristic peaks of CS [39] and amorphous carbon [40] can be seen in Fig. 3B. It has to be mentioned that, overlapping of the Raman peaks of PET at 143 and 631 cm−1 with that of characteristic TiO2 peaks, confirmed the presence of chemically bonded TiO2 NPs on CSAT-PET [38,41]. To determine the crystal phase composition of CSAT-PET, XRD analysis was performed. The XRD spectra of pristine TiO2, AC, CS, PET together with CSAT-PET filter before and after five reaction cycles are presented in Fig. 3C. The XRD pattern of pristine TiO2 (Fig. 3C) showed that the catalyst was predominantly comprised of anatase phase. After TiO2 immobilization, the characteristic TiO2 anatase phase peaks at 25.32°, 37.82°, 48.08°, 53.93°, 55.11° and 62.74° which corresponded to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) planes of anatase phase were formed (Fig. 3C) as per the JCPDS card no. 900-9086 (Fig. S1). The characteristic peaks of PET at 2θ values of 17.90°, 22.20° and 25.26° were also observed after coating (Fig. 3C) [36]. In AC (Fig. 3C), distinct peaks were observed together with broad peaks in the 2θ range of 20–30° and 45–50° which could be attributed to the reflections of (0 0 2) and (1 0 0) peaks of crystalline graphitic structure, respectively [42]. Furthermore, very small sharp peaks can be seen in the diffraction pattern of AC indicating the presence of SiO2. From the diffraction pattern of CS (Fig. 3C), it was seen that the broad characteristic peaks at 2θ values of 10° and 20° due to its crystalline structure disappeared after coating (Fig. 3C) [43]. It was inferred that TiO2 having anatase phase structure, graphitic carbon and amorphously adsorbed CS was successfully coated over fiber.

The DRS spectra of TiO2, CS, AC, pure TiO2-PET, and CSAT-PET are shown in Fig. 3D. It can be seen that the absorbance spectrum of filter consisted of a band at 220 nm and the other around 300 nm (Fig. 3D). While the band at 220 nm could be ascribed to tetrahedral Ti atoms, the one at 300 nm could be due to the charge transfer from 2p orbital of oxide (VB) to the 3d t2g orbital of Ti4+ (CB). These findings further ascertain the presence of anatase TiO2 in CSAT-PET [44]. Also, it was seen that the strong absorbance of visible light by CSAT-PET beyond the wavelength of 380 nm was due to the presence of AC [45]. In addition, the band gap of pure TiO2-PET and CSAT-PET was calculated to be 3.36 and 2.95 eV by the application of Kubelka-Munk function (Fig. S2). Further, the specific surface area of pure TiO2-PET and CSAT-PET filters were 9.6 and 85.7 m2/g, respectively, as obtained by the BET surface area measurements (Table S2). It was inferred that both the surface area and pore volume of CSAT-PET filter was ten times higher than that of pure TiO2-PET filter, which could be attributed to the presence of porous AC (Fig. 4d). The N2 adsorption-desorption isotherm plot (Fig. S3) of CSAT-PET exhibited H4 type hysteresis loop which is associated with narrow slit-shaped pores whereas pure TiO2-PET exhibited an H3 type hysteresis loop indicating slit-shaped pores. Accordingly, it can be supposed that CSAT-PET would provide excellent adsorption capacity and smaller mean pore size compared to pure TiO2PET. The surface morphology of CSAT-PET was observed using high-resolution SEM. The low-resolution SEM image of pristine PET and CSATPET showed microfilaments with a diameter of ∼25 μm (Fig. 4a and f). The macropores formed by these microfilaments had a pore size of 100–150 μm (Fig. 4a and f). The changes in surface morphology of PET filter following each coating can be seen in Fig. 4b–e. Under high-resolution image, PET showed smooth surface without any defects 5

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Fig. 4. SEM-EDX of CSAT-PET (a) SEM image of pristine PET; (b)-(e) High magnification (60,000x) image of fiber surface before and after coating CS, AC and TiO2, respectively; (f) SEM image of CSAT-PET; (g) EDX spectra and, (h) Elemental mapping of CSAT-PET along with those of C, O, N and Ti in inset.

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Fig. 5. XPS spectra of CSAT-PET (a) Ti 2p; (b) N 1s; (c) C 1s and (d) O 1s narrow scans.

Fig. 6. Photodegradation of toluene a) concentration profile before (Dark condition represented by the grey area) and after UV illumination using Pure TiO2-PET and CSAT-PET. The experimental conditions are C0 = 400 ppb; v = 1 m/s; b) Influence of initial concentration and face velocity on toluene removal.

(Fig. 4g) of CSAT-PET revealed the presence of C, N, O, and Ti as main elements, suggesting the successful immobilization of CS, AC, and TiO2 on PET. The SEM image and EDX spectra showing the intact surface morphology of CSAT-PET after five photodegradation cycles are given in Fig. S4. The valence states of four elements (Ti, N, C, O) in CSAT-PET was characterized by XPS spectroscopy (Fig. 5), and the survey spectrum is shown in Fig. S5. The binding energies of Ti 2p (Fig. 5a) exhibited two distinct peaks at 457.5 and 463.5 eV suggesting that Ti in CSAT-PET exist as Ti4+ ions. The N 1s spectra (Fig. 5b), showed a peak at 399 eV

(Fig. 4b). A rough surface with few surface defects was formed after CS coating (Fig. 4c). After AC coating, the surface became more porous and rougher (Fig. 4d). As shown in Fig. 4e, the introduction of TiO2 NPs resulted in a smoother, compact and uniform layer. The low-resolution image of CSAT-PET (Fig. 4f) showed the uniform coating of different components on the fiber. Similarly, the elemental mapping of CSATPET (Fig. 4h) revealed the uniform distribution of TiO2 on the fibers. The inset image of Fig. 4h shows the distribution of C, O, N, and Ti on the filter. Comparing the images in Fig. 4f and h, intimate contact between Ti in TiO2, N in CS and C in AC could be seen. The EDX spectra 7

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due to the protonated amine groups (NH3+). Furthermore, the peak at 397.8 eV which could be ascribed to NeC]O and eNH2 bonds [12]. The chemical states of C 1s level could be ascribed to the AC, CS or PET substrate. The C1s peaks at 284.8 and 287.05 eV could be from CeC and CeO groups, respectively [46]. Further, the peak at 531.2 eV in O 1s spectra revealed the presence of TieO (metal oxide) on the coating surface [37]. The observations were in line with the elemental mapping results. Also, the thermal stability of coated PET filters was analyzed by TGA measurement (Fig. S6). It was observed that even though pure TiO2-PET was thermally more stable, only a slight weight loss (4.2%) occurred in the range of 100–200 °C for CSAT-PET. Therefore, CSATPET is thermally stable and suitable for practical applications.

velocity range (0.5–1.5 m/s) with only a 5% reduction in removal efficiency as the face velocity increased from 0.5 to 1.5 m/s. This could be due to the high surface area of the filter and good adsorption capacity of AC and CS for toluene (Fig. S3 and Table S2). Contrastingly, the reaction rate (Fig. 6b) increased from 2.5 to 3.2 μmol/m2/s (C0 = 400 ppb) when the face velocity increased from 0.5 to 1 m/s whereas, further increase in face velocity to 1.5 m/s resulted in lower reaction rate (1.84 μmol/m2/s). Therefore, here, two antagonistic effects of flow velocity must be taken into account: a decrease in residence time inside the reactor and an increase in mass transfer from the gas to the catalyst surface. At lower face velocities (v = 0.5–1 m/s), the degree of gas mixing enhanced the interfacial mass transfer and thereby promoting the mobility of e−/h+ from TiO2 surface to the boundary layer, where the oxidation takes place [54]. However, at high face velocities (v = 1–1.5 m/s), the insufficient residence time of toluene near the catalyst surface caused lower photodegradation. Furthermore, a non-linear least-squares regression analysis was applied to the experimental data. It was seen that the photodegradation data of toluene fitted well with the L-H model [55]. The obtained values of the adsorption and photodegradation rate constants were Kad = 0.015 ppb−1 and Kp = 1.39 μmol/m2/s (C0 = 400 ppb, v = 1 m/ s). The values of the constants were comparable to those obtained by Tang and Yang [56] for toluene degradation.

3.2. Photocatalytic performance Toluene adsorption on the surface of filter is an important parameter for efficient photodegradation. The effect of adsorption and photodegradation on toluene removal performance is shown in Fig. 6a. During the initial dark reaction (indicated by a negative time scale), the disappearance of toluene could be attributed to the adsorption on the surface of the filter. As seen in Fig. 6a, the adsorption capacity of CSATPET filter was higher than the pure TiO2-PET filter as per the surface area measurements from BET analysis (Table S2 and Fig. S3). Besides, the adsorption capacity of CSAT-PET filter is dependent on AC and CS loading (Fig. S7). It was seen that higher the AC or CS loading, higher was the adsorption. However, excess adsorbent loading was seen to reduce the photodegradation efficiency possibly due to the UV shielding effect [47]. For example, when the AC loading was increased from 30 to 90 g/m2, the adsorption efficiency increased by 15% meanwhile, the photodegradation efficiency reduced by 17%. As seen in Fig. 6a, after UV light was switched on, the concentration of toluene decreased according to the Langmuir-Hinshelwood (L-H) reaction (the corresponding rate constants of CSAT-PET is discussed in Section 3.3). It was seen that (Fig. 6a) CSAT-PET showed superior toluene removal efficiency (91.4%) than pure TiO2-PET (62.5%). A similar study by Hequet et al. [48] using TiO2/SiO2 coated filter reported a toluene removal efficiency of 65% (C0 = 700 ppb). Another study using TiO2 coated activated carbon fiber felt (ACFF) reported a removal efficiency of 86% (C0 = 500 ppb) [49]. The synergistic adsorption and photodegradation mechanism could be the reason for the higher efficiency using CSAT-PET when compared to TiO2/SiO2 and TiO2/ACFF for toluene removal. A comprehensive review of previous studies using adsorption-photodegradation for toluene removal is given in Table S3. Control experiments were conducted to determine the effect of photolysis (only UV lamps) on VOCs removal [50]. It was observed that the toluene degradation efficiency due to photolysis was about 10% (Data is not shown here). Similar observations were reported by Tasbihi et al. [51] under UVA illumination for toluene degradation.

3.4. Reaction pathway of toluene degradation Catalyst deactivation is a commonly reported phenomenon in toluene PCO [9,57]. In the present study, to determine the reaction intermediates formed on the filter, a series of FTIR spectra were collected, as shown in Fig. 7. The declining peak at 3676 cm−1 indicated that the surface eOH groups were consumed during toluene degradation. At the same time, the broadening of the band at 3450 cm−1 indicated the formation of surface water [58]. Meanwhile, comparing the FTIR spectra of CSAT-PET before and after five degradation cycles, the intensification of peaks in the range of 2800 to 3100 cm−1 could be ascribed to the CeH stretching modes of CH‒containing aliphatic or aromatic intermediates [9]. Further, the pattern in the wavelength range of 1425–1650 cm−1 was due to the formation of reaction intermediates containing benzene ring. More specifically, the flattening of peaks at 1638 and 1605 cm−1 indicate the presence of benzaldehyde [59]. Similarly, the peak at 1464 cm−1 may be due to the formation of benzoic acid [60]. A reaction scheme was proposed for toluene degradation (Fig. S8) based on the identified intermediates. The first step involved H-abstraction from the methyl group of toluene and OH˙ radical inclusion in

3.3. Effect of operating parameters on PCO The effect of initial concentration and face velocity on toluene removal performance of CSAT-PET is presented in Fig. 6b. It was seen that the removal efficiency exhibited only a marginal decrease from 96 to 92% as initial concentration increased from 200 to 600 ppb; this applied at all face velocities (0.5, 1, and 1.5 m/s). Therefore, in this range of toluene concentration (200–600 ppb), there was no competitive adsorption between the adsorbed intermediates and toluene [52] and the filter can be used satisfactorily for degradation of VOCs at indoor (ppb) levels. On the other hand, the reaction rate (Fig. 6b) doubled with initial concentration from 200 to 600 ppb at all face velocities (0.5, 1, and 1.5 m/s). A higher number of toluene molecules resulted in the better transport of pollutants from gas bulk to catalyst surface, causing faster removal [53]. The effect of face velocity on toluene removal is shown in Fig. 6b. The filter showed robust performance in the selected face

Fig. 7. FTIR spectra of the sample before and after reaction and after regeneration (BZH:-Benzaldehyde, BZAc:-Benzoic acid). 8

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the aromatic ring leading to the formation of benzyl alcohol. In the subsequent reactions, benzyl alcohol was oxidized to produce benzaldehyde benzoic acid and then to CH containing intermediates and CO2 through photo-Kolbe reaction [52]. Augugliaro et al. [61] proposed a similar reaction pathway for toluene degradation through the formation of benzaldehyde, benzoic acid, and benzene, which readily oxidized to form carbon dioxide. Benzene, being a less hydrophilic and volatile compound desorbed immediately and could not be detected in the adsorbed phase. A similar reaction pathway was proposed by Bianchi et al. [60] who found irreversible adsorption of intermediates to the catalyst surface and eventual catalyst deactivation. However, in the present study, the adsorbed species did not cause saturation of the active sites in the catalyst as the PCO efficiency did not show any reduction after five reaction cycles (Fig. S9). An earlier study demonstrated that UV-C induced regeneration could partially restore catalytic performance [62]. We investigated the possibilities of increasing filter lifetime using a UV-induced regeneration method where, the CSAT-PET5 was placed in the reactor with UV illumination for a duration of 2 h, and the corresponding FTIR spectra of regenerated filter (CSAT-PETR) is shown in Fig. 7. Before the regeneration study, the reactor was purged with fresh air for 30 min. It was observed that the surface hydroxyl groups (3676 cm−1), which decreased significantly following the degradation reaction improved after regeneration. The surface eOH radicals could be regenerated from the produced water on the surface of the catalyst (3450 cm−1) [58]. At the same time, decrease in narrow bands corresponding to 1425–1650 and 2800-3100 cm−1 indicated partial regeneration of the filter by the release of the adsorbed intermediates [63]. Therefore, under the ppb level VOC concentrations normally encountered in indoors, CSAT-PET would have a long lifetime. A brief schematic of the proposed reaction mechanism of toluene degradation over CSAT-PET is shown in Fig. 8.

Table 3 Experimental design matrix and the determined values of the response. Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Factor 1

Factor 2

Factor 3

A: TiO2 loading (g/ m2)

B:AC loading (g/ m2)

C:CS content (% w/v)

10 50 30 50 30 10 30 30 10 30 50 30 50 10 30 30 30

90 60 90 60 60 60 90 60 30 60 90 30 30 60 60 60 30

2 3 3 1 2 3 1 2 2 2 2 3 2 1 2 2 1

Experimental values Removal efficiency (%)

Predicted values

67.2 82.1 74.5 78.4 93.0 69.9 77.4 92.5 76.1 92.5 83.3 81.1 81.3 69.3 93.0 93.0 77.1

67.3 81.7 74.8 78.8 92.8 69.5 76.9 92.8 76.0 92.8 83.4 81.6 81.2 69.7 92.8 92.8 76.8

R2 = 0.9987, R2adjusted = 0.9970, and R2predicted = 0.9827.

suggested that CSAT-PET can be applied for indoor conditions. 3.6. Optimization of filter preparation The prepared filter comprised three components, namely, the catalyst TiO2, adsorbent AC, and binder/adsorbent CS. Previous studies have reported that the component loading rate could influence the photocatalytic efficiency of the filter [64,65]. Therefore, we evaluated the effect of individual component loading on toluene removal efficiency using the BBD in RSM to optimize the filter preparation method. A semi-empirical equation (Eq. 5) comprising of 9 statistically significant coefficients was obtained using statistical software Design Expert 11 at a 95% confidence level (p < 0.05). The model fit statistics (Table S4) and ANOVA Fischer F-test (Table S5) showed the significance and adequacy of the obtained model.

3.5. Stability and reusability tests The stability and reusability of CSAT-PET were analyzed by the recycle tests (5 cycles), as shown in Fig. S9 (C0 = 400 ppb, v = 1 m/s). The results showed no significant loss in photocatalytic activity during the five cycles illustrating the stability of filter. Further, to evaluate the effect of PCO on the morphological and chemical characteristics of the filter, Raman, XRD spectra, SEM, and EDX were collected for CSAT-PET0 and CSAT-PET5 samples, as shown in Fig. 3B, C and Fig. S9, respectively. The SEM-EDX results (Fig. S9) showed that no changes in CSAT-PET surface structure and chemical composition occurred after the recycle tests. The results further confirmed that the intermediates did not remain adsorbed on the catalyst surface [53]. Furthermore, Raman spectra and XRD of CSAT-PET indicated the presence of TiO2 in anatase phase on the filter surface after five cycles of toluene PCO. The results suggested that toluene degradation did not significantly affect the filter morphology or its chemical properties. Thus, the stability, recycle, and regeneration studies

Removal efficiency (%) = 8.5 + 1.3[TiO2] + 0.81[AC] + 37.6[CS] + 0.005[TiO2 ][AC] + 0.04[TiO2 ][CS] − 0.06[AC][CS] − 0.02[TiO22] − 0.007[AC2] − 8.7[CS2]

(5) 2

2

where TiO2, AC and CS are TiO2 (g/m ), AC (g/m ) and CS loading (% w/v), respectively. Eq. 5 can be used to predict the removal efficiency of toluene by CSAT-PET within the selected range of factors (Table 2). The R2 values (Table 3) indicated that the predicted values of toluene removal efficiency matched well with the experimental data.

Fig. 8. Schematic of proposed removal mechanism on self-assembled CSAT-PET. 9

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Fig. 9. Removal efficiency as a function of (a) and (b) the interaction effect of AC and TiO2 loading (2% CS content); (c) the interaction effect of TiO2 and CS loading (AC loading of 60 g/m2); and (d) CS content for toluene PCO.

removal efficiency based on the experimental results. The optimization runs were performed by keeping TiO2, AC and CS loading in the experimental range, i.e., 10–50 g/m2, 30–90 g/m2 and 1–3% (w/v) and the desirable degradation efficiency was set as "maximum" with an upper limit of 100% (Fig S11). The obtained optimum values of factors with highest toluene degradation efficiency (93.3%) were 38.3 g/m2, 52.6 g/m2 and 2.06% (w/v) for TiO2, AC and CS loading, respectively.

The two-factor interaction effect of the component loading on removal efficiency is illustrated in Fig. 9a and c whereas, the effect of individual factors on the response is presented in Fig. 9b and d. The downward parabola shaped response surface (Fig. 9a and b) indicated that an increase in TiO2 and AC loading caused higher removal efficiency till a point of inflection, due to the synergetic adsorption-photodegradation effect. Beyond this point, higher AC loading could cause a reduction in UV light absorption by TiO2, whereas higher TiO2 loading could lead to the aggregation of TiO2 NPs [66]. A similar trend was noticed with the effect of CS content on toluene removal efficiency (Fig. 9c and d). When the CS content increased, removal efficiency increased due to higher adsorption. Furthermore, CS dissolves in acidic media to become polyelectrolyte by the protonation of the eNH2 groups. It has been reported that enhanced protonation and higher charge density of CS occurs in the pH range of 2.2–2.5, resulting in a stable solution by electrostatic repulsion [67]. The highest removal occurred at a CS content of 2% (pH = 2.4) due to the formation of optimum eOH and eNH2 radicals in aqueous solution leading to proper bonding of TiO2 NPs. A stable solution exhibits sufficiently high zeta potential, which initiates self-assembly of components through electrostatic interaction [68]. At higher CS content, the thickness of film increased, and the UV irradiation incident on TiO2 reduced due to blocking. The peak removal efficiency (93.6%) was achieved at TiO2, AC and CS loading of 35 g/m2, 60 g/m2 and 2% (w/v), respectively while it reduced to 70–85% at either side on approaching away from the inflection points (Fig. 9). The two-factor interaction effect of CS and AC loading on toluene removal efficiency (TiO2 loading =30 g/m2) is given in Fig. S10. Finally, filter preparation parameters were optimized for high

4. Conclusions Activated carbon and TiO2 coatings were successfully fabricated by a facile layer-by-layer self-assembly approach on non-woven PET fabric filter, and the filter was characterized by FTIR, Raman, XRD, UV–vis DRS, XPS, and SEM. In this approach, the electrostatic interaction between the oppositely charged aqueous suspensions (CS, AC-CS, and TiO2-IPA) was responsible for the bonding of TiO2 and AC on the fiber. Photodegradation tests were performed using toluene as model pollutant at an inlet concentration of 200–600 ppb. The enhanced efficiency of CSAT-PET was attributed to the synergy between the adsorption ability of AC/CS and the photodegradation ability exerted by the immobilized TiO2 NPs. The results showed robust filter performance at the selected face velocity range (0.5–1.5 m/s), and the effect of initial concentration was ascertained by the Langmuir-Hinshelwood kinetic model. Benzaldehyde and benzoic acid were detected as adsorbed intermediates of toluene degradation, and the filter could be regenerated through UV irradiation. The stability tests showed that CSAT-PET did not undergo significant change in morphology and chemical composition following the repeated toluene degradation cycles. Finally, the preparation of CSAT-PET filter with the highest photocatalytic 10

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degradation efficiency for toluene was optimized by applying RSM. The optimum values of coating parameters were 38.3 g/m2, 52.6 g/m2 and 2.06% (w/v) for TiO2, AC and chitosan loading, respectively. The fabricated low cost, stable and reusable CSAT-PET filter seems to have great potential for building and automotive indoor air cleaning applications.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to express their gratitude to the Department of Civil Engineering, Department of Chemistry, Department of Physics (MSRC & NFMTC), CFF, CEC and SAIF at IIT Madras for their kind support to this research work. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2019.103455. References [1] L.A. Wallace, Comparison of risks from outdoor and indoor exposure to toxic chemicals, Environ. Health Perspect. 95 (1991) 7–13, https://doi.org/10.1289/ehp. 91957. [2] M. Jahandar Lashaki, J.D. Atkinson, Z. Hashisho, J.H. Phillips, J.E. Anderson, M. Nichols, The role of beaded activated carbon’s pore size distribution on heel formation during cyclic adsorption/desorption of organic vapors, J. Hazard. Mater. 315 (2016) 42–51, https://doi.org/10.1016/J.JHAZMAT.2016.04.071. [3] Z. Fan, P. Lioy, C. Weschler, N. Fiedler, H. Kipen, J. Zhang, Ozone-initiated reactions with mixtures of volatile organic compounds under simulated indoor conditions, Environ. Sci. Technol. 37 (2003) 1811–1821, https://doi.org/10.1021/ es026231i. [4] M. Bahri, F. Haghighat, S. Rohani, H. Kazemian, Impact of design parameters on the performance of non-thermal plasma air purification system, Chem. Eng. J. 302 (2016) 204–212, https://doi.org/10.1016/J.CEJ.2016.05.035. [5] L. Zhong, F. Haghighat, C.S. Lee, N. Lakdawala, Performance of ultraviolet photocatalytic oxidation for indoor air applications: systematic experimental evaluation, J. Hazard. Mater. 261 (2013) 130–138, https://doi.org/10.1016/j.jhazmat.2013. 07.014. [6] Y. El-Sayed, T.J. Bandosz, A study of acetaldehyde adsorption on activated carbons, J. Colloid Interface Sci. 242 (2001) 44–51, https://doi.org/10.1006/JCIS.2001. 7772. [7] M. Popescu, J.P. Joly, J. Carré, C. Danatoiu, Dynamical adsorption and temperature-programmed desorption of VOCs (toluene, butyl acetate and butanol) on activated carbons, Carbon N. Y. 41 (2003) 739–748, https://doi.org/10.1016/S00086223(02)00391-3. [8] L. Luo, D. Ramirez, M.J. Rood, G. Grevillot, K.J. Hay, D.L. Thurston, Adsorption and electrothermal desorption of organic vapors using activated carbon adsorbents with novel morphologies, Carbon N. Y. 44 (2006) 2715–2723, https://doi.org/10.1016/ j.carbon.2006.04.007. [9] W. Den, C.C. Wang, Enhancement of adsorptive chemical filters via titania photocatalysts to remove vapor-phase toluene and isopropanol, Sep. Purif. Technol. 85 (2012) 101–111, https://doi.org/10.1016/j.seppur.2011.09.054. [10] K.P. Yu, G. Whei-May Lee, W.M. Huang, C.C. Wu, C.I. Lou, S. Yang, Effectiveness of photocatalytic filter for removing volatile organic compounds in the heating, ventilation, and air conditioning system, J. Air Waste Manag. Assoc. 56 (2006) 666–674, https://doi.org/10.1080/10473289.2006.10464482. [11] Y. Lu, D. Wang, C. Ma, H. Yang, The effect of activated carbon adsorption on the photocatalytic removal of formaldehyde, Build. Environ. 45 (2010) 615–621, https://doi.org/10.1016/j.buildenv.2009.07.019. [12] M. Li, B. Lu, Q.F. Ke, Y.J. Guo, Y.P. Guo, Synergetic effect between adsorption and photodegradation on nanostructured TiO2/activated carbon fiber felt porous composites for toluene removal, J. Hazard. Mater. 333 (2017) 88–98, https://doi.org/ 10.1016/j.jhazmat.2017.03.019. [13] X. Qian, M. Ren, D. Yue, Y. Zhu, Y. Han, Z. Bian, Mesoporous TiO2 films coated on carbon foam based on waste polyurethane for enhanced photocatalytic oxidation of VOCs, Appl. Catal. B Environ. 212 (2020) 1–6, https://doi.org/10.1016/j.apcatb. 2017.04.059. [14] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima, A film-type photocatalyst incorporating highly active TiO2 powder and fluororesin binder: photocatalytic activity and long-term stability, J. Electroanal. Chem. 415 (1996)

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