key: cord-0839066-db9kpfeu
authors: Chanklom, Pattamaphon; Kreetachat, Torpong; Chotigawin, Rotruedee; Suwannahong, Kowit
title: Photocatalytic Oxidation of PLA/TiO(2)-Composite Films for Indoor Air Purification
date: 2021-04-19
journal: ACS Omega
DOI: 10.1021/acsomega.0c06194
sha: f5a75c196d1add00407c1374a531362cfb25fe1b
doc_id: 839066
cord_uid: db9kpfeu

[Image: see text] Non-decomposable plastic has been replaced with polylactic acid, which is a biodegradable aliphatic polyester stationary phase, in composite films embedded with a TiO(2) photocatalyst for mitigation of indoor air pollution. PLA has superior properties relative to those of other biopolymers, such as a relatively high melting point, crystallinity, and rigidity. This study aimed to incorporate TiO(2)-anatase into PLA for use as a photocatalyst using the blown film method. Photocatalytic oxidation, an advanced oxidative process, has been recognized as an economical technique providing convenience and efficiency with indoor air treatment. Therefore, the use of new environmentally friendly biodegradable polymers provides an alternative way to address the severe environmental concerns caused by non-decomposable plastics. UV–vis spectrophotometry and scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDX) were used to observe the dispersibility and mixing capacity of the TiO(2)-anatase PLA matrix. TiO(2) dosages were 5, 10, and 15% (wt/wt), and they were incorporated with a twin-screw extruder. SEM–EDX images demonstrated the homogeneity of TiO(2) distribution in the PLA matrix. The energy band gaps of TiO(2) in the PLA/TiO(2)-composite films were between 3.14 and 3.22 eV. The relationship between the photocatalytic oxidation rate and the TiO(2) dosage in the PLA/TiO(2)-composite films was determined. A prototype reactor model is geared toward the development of air purifiers for indoor air conditioning. Rate constants for benzene degradation were obtained using first-order kinetics to find rate constants matching experimental findings. In the PLA/TiO(2)-composite film, the TiO(2)-anatase photocatalyst was able to degrade 5 ppm benzene. This work contributes to the use of ecoefficient photocatalytic oxidation.

Adults inspire and expire 400−500 mL of air in each respiratory cycle. 1 People spend approximately 90% of their time indoors, in offices, homes, malls, gyms, and schools. 2 Therefore, indoor air quality (IAQ) can significantly affect the health and quality of life. 3 Poor indoor air quality, particularly involving indoor air microbiomes, e.g., Mycobacterium tuberculosis 4 and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), 5 seriously affects human health. 6 The coronavirus spreads through liquid droplets when an infected person coughs, 7 sneezes, 8 speaks, 9 sings, 10 and/or shouts. 7 During the coronavirus (COVID-19) pandemic, people spend almost all of their time at home; as a result, they are exposed to high levels of indoor air pollution, not only bioaerosols but also volatile organic compounds (VOCs). Sources of VOCs that cause poor indoor air quality are tobacco, 11 cooking, 12 solvents, 13 wood preservatives, 14 painting, 15 cleansers, and disinfectants. 16 The main indoor and outdoor environmental problems caused by VOCs are increased tropospheric ozone, 17 adverse health effects, 18 and carcinogenicity. 19 VOCs increase the risk of obstructive pulmonary diseases, skin cancers, birth defects, and neurocognitive impairment. 20 Benzene is one of the VOCs identified as carcinogenic to humans by the International Agency for Cancer Research (IARC) (Group I) on the basis of ample evidence that it causes leukemia. 21 The main industrial hygiene principles to follow to reduce exposure to hazards are recognition, measurement, evaluation, and control. 22 An air purifier is an affordable, user-friendly, and easily accessible way to control chemical hazards. 23 Technologies for VOC reduction include adsorption, 24 liquid absorption, 25 photolysis, 26 membrane separation, 27 bio-filtration, 28 ozonation, 29 plasma catalysis, 30 biological degradation, 31 and advanced oxidation processes such as photocatalytic oxidation. 32 Photocatalytic oxidation (PCO) has been widely used and is well known for the treatment of VOCs because it can transform not only VOCs but also a wide range of pollutants to end products obtained from processes involving H 2 O and CO 2 ; it exhibits various advantages, such as operation at low pressure and temperature. 28 The authors of this paper chose titanium dioxide (TiO 2 ) as a photocatalyst due to its abundance, lack of toxicity, low cost, effectiveness, and high chemical stability. 33 Many types of TiO 2 composites with polymers, such as polyethylene, 34 polypropylene, 35 polyvinyl chloride, 36 and nylon-6, 37 have been successfully synthesized; however, after treatment, environmental issues arise. As a result, non-decomposable plastics have been replaced by polylactic acids, which are biodegradable aliphatic polyesters that serve as stationary phases for composite films containing TiO 2 ; these compositions can be used as photocatalysts to test the photocatalytic activity of biodegradable composite films for the removal of VOCs. A plant-based biodegradable polylactic acid (PLA) was used in this work as a biosupport for TiO 2 catalysts. 28 Benzene is a major indoor and industrial air pollutant 38 and has been chosen as a representative indoor VOC. 39 The Langmuir−Hinshelwood model has been used extensively to successfully explain the photocatalytic oxidation rate of various contaminants, such as VOCs. This research uses the Langmuir adsorption isotherm to explain the rate of pollutant removal.

This research aims to incorporate TiO 2 -anatase into PLA for photocatalytic applications. Characterization of PLA/TiO 2composite films was performed by analyzing SEM−EDX data, determining the functional groups of chemical structures by ATR-FTIR and using UV−visible absorbance spectra to determine band gap energies. Finally, the efficiencies of PLA/TiO 2 -composite films in benzene photocatalytic oxidative degradation in simulated indoor air were analyzed using the films in indoor air purifiers.

TiO 2 (anatase phase) was purchased from Dupont Company. Food-grade polylactic acid was purchased from NatureWorks.

Preparation of the Photocatalyst; PLA/TiO 2 -Composite Film. Titanium dioxide (TiO 2 ) dosages of 5, 10, and 15% (wt/wt) were mixed with polylactic acid (PLA) and incorporated utilizing a twin-screw extruder (Lab Tech, LTE16-40). TiO 2 anatase levels were altered to increase TiO 2 dispersion and boost the compatibility between the TiO 2 and the PLA matrix. 40 PLA/TiO 2 -composite films with a thickness of 30 μm were obtained at operating temperatures ranging from 100 to 190°C and at 200 rpm using the blown film technique. 41 Characterization of the Photocatalyst; PLA/TiO 2 -Composite Film. Scanning electron microscopy (SEM) (LEO 1455 VP, 20 kV) was used to study the morphologies of the films. Energy-dispersive X-ray spectroscopy (EDX) (LEO 1455 5VP, SEM−EDX 20 kV) was used for qualitative and quantitative analysis of elements in films. A UV−visible spectrophotometer (UV−vis) (Analytik Jena SPECORD 210 PLUS) was used to measure the photocatalyst absorptions in the ultraviolet−visible spectrum with a scan range of 200−800 nm, a scan speed of 50 nm/s, a slit of 1 nm, and a delta lambda of 1 nm. All spectra were monitored in absorbance mode under ambient conditions. The band gap energy of TiO 2 on a film was calculated from the absorbance results. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (PerkinElmer Frontier) was used to determine the functional groups of the chemicals. 42 Spectra were recorded in the range of 400−4000 cm −1 with a scan step of 1 cm −1 ; the functional groups of the films were determined before and after photocatalytic experiments (360 h duration).

Photocatalytic Oxidation Degradation with Scale-Up of the Photoreactor for Air Purification. A UVC 254 nm lamp was placed in the middle of a cylindrical annular reactor 43 containing the photocatalyst. The annular reactor was set up in a commercial air purifier, as shown in Figure 1 . Photocatalytic oxidative degradation was conducted in an annular closedsystem reactor. In a 785 L chamber cabinet, a scaled-up photoreactor was used to simulate indoor air conditions, and the total reactor volume was approximately 2 L. The PLA/ TiO 2 -composite film and a type-C ultraviolet lamp with an output wavelength of 254 nm were mounted inside the reactor. The electrical power of the UVC lamp was 18 W, which is equivalent to a light intensity of approximately 5 mW·cm −2 . The reactor was linked to an air compressor that maintained an air flow rate of 300 L·min −1 . The relative humidity in the reactor was changed to 55% by initially circulating air with a particular humidity through the reactor. The humidity and temperature inside the reactor were continuously measured with a thermohygrometer. The original benzene concentration was 5.21 ppm. The UVC lamp was switched on to start the photocatalytic oxidation after a steady state was reached.

Quantitative research was performed using gas chromatography (GC) with an instrument fitted with a flame ionization detector (FID) 44 to track the concentration of benzene in the reactor throughout the experiment. Heterogeneous catalytic processes, including adsorption and photodegradation, were defined in the model. The Langmuir−Hinshelwood model represented by eq 1 was used to determine the bulk photodegradation rate.

where k is the apparent kinetic constant (ppm·min −1 ), K is the adsorption equilibrium constant (ppm −1 ), and C is the concentration of VOCs on the photocatalyst surface in the gas phase (ppm). The adsorption equilibrium and reaction rate constants are intrinsic properties of the photocatalyst for the decomposition of VOCs. Other variables were measured by experimentation.

For determination of the oxidation rate and CO 2 yield, 45 gas samples were collected at the sampling port of the reactor. The gas samples were then studied using gas chromatography− flame ionization detection (GC-FID) and gas chromatography−thermal conductivity detection (GC-TCD) for the analyses of benzene and CO 2 , respectively. The analyses of benzene were performed using gas chromatography−mass spectroscopy (GC−MS) as described earlier, whereas the CO 2 concentration was analyzed using GC-TCD with a Shimadzu model GC-8A TCD instrument equipped with a polar pack Q 5 M in a glass column. The analytical conditions were set as follows: an injection temperature of 50°C, a column temperature of 100°C, a TCD temperature of 100°C, a current of 90 mA, an injection volume of 0.2 mL, and a gas pressure of 2 kg·cm −3 .

Characterization of the PLA/TiO 2 -Composite Film. The compounding of the virgin PLA film and PLA/TiO 2composite film by the extrusion method is demonstrated in Figure 2 . The virgin-PLA film and PLA/TiO 2 -composite film were produced by compounding using a blow machine, as shown in Figure 2 . SEM−EDX was used to classify the morphology of the PLA/TiO 2 -composite film. The dispersion of TiO 2 on the surface of PLA/TiO 2 -composite films is shown in Figure 3 , and a virgin PLA film is shown for reference in Figure 3a . TiO 2 was well dispersed on the inner surface of the film, especially for 5 and 10% TiO 2 loadings, as shown in Figure 3 . In comparison with prior research, better results were obtained during this work. 46 However, a few clumps of TiO 2 particles were observed on the inner surface of the film with high dosages of TiO 2 . During the heating phase of the blown film method, aggregation of the TiO 2 particles could occur. 47 EDX mapping of the PLA and the PLA/TiO 2 -composite films is described in Table 1 .

Optical Properties of the PLA/TiO 2 Composite Film Determined Using a UV−Visible Spectrophotometer. The UV−vis absorbance spectra of the PLA/TiO 2 -composite films with different concentrations of TiO 2 -anatase were examined. As seen from the spectra in Figure 4 , the virgin PLA film absorbed little UV−visible light in the range of 200− 800 nm, while the films with 5 and 10% wt/wt TiO 2 -anatase absorbed UV light with wavelengths less than 350 nm. The energy band gap (E g ) of the TiO 2 -anatase in PLA composite films can be calculated from the absorption spectra. A range of 3.14−3.22 eV was determined for the band gaps, as shown in Table 2 . The band gap energy is provided when this material is irradiated with photons with wavelengths less than 350 nm. An electron from the valence band is promoted to the conduction band under these conditions. In the space-charge field, the resulting electron−hole pair has a lifetime that allows its participation in chemical reactions. 36 Chemical Structure of PLA/TiO 2 -Composite Films Determined by ATR-FTIR. The functional groups contained in the PLA/TiO 2 composite films were defined by ATR-FTIR. The same findings were obtained for all samples. The ATR-FTIR spectra are shown in Figure 5 . The characteristic bands corresponding to HCO::C−H aliphatic hydrocarbons, aldehydes, 48 and COOR groups are at 2850−3000, 2830− 2695, and 1640−1690 cm −1 , respectively. Additionally, C−C aromatic stretching 49 vibrations gave peaks at 1400−1500 cm −1 . The band width at 719 cm −1 should be attributable to the band envelope for Ti−O−Ti groups. 50 After photocatalytic oxidation experiments, all FTIR spectra of the PLA/TiO 2 composite films recorded decreases in the intensities of all adsorption bands. Moreover, after photocatalytic oxidation, the adsorption peak at 1640−1690 cm −1 was shifted to 1710− 1760 cm −1 . Furthermore, as mentioned in general texts, the photocatalytic oxidation of PLA under UV light generates various radicals during decomposition. 51 In addition, the photocatalytic oxidation of PLA was performed at ambient temperature and has the potential for degradation of PLA into small organic acids or compounds such as HCOOH, CH 3 COOH, (COOH) 2 , or CH 3 CHO. 52 Photocatalytic Oxidation of Benzene in Simulated Indoor Air. Benzene photocatalytic oxidation was studied by using PLA/TiO 2 -composite films with different TiO 2 dosages. The relationship of the photocatalytic oxidation rate to the catalyst dosage level is presented in Figure 6 . Benzene degradation rate constants were obtained using first-order kinetics, and rate constants were determined by fitting experimental results. Table 3 shows the apparent kinetic rate constant for photocatalytic oxidation, obtained by using a simplified Langmuir−Hinshelwood approach. The apparent kinetic constant of the 5% PLA/TiO 2 -composite film was the highest. The plots demonstrate that the slopes were determined from straight lines by following first-order kinetic methods and using the least-squares procedure. A first-order reaction at low concentrations was confirmed by previous research, and a linear increase in the reaction rate at low initial benzene concentrations was reported. In this study, benzene was the only pollutant available for degradation, and a single site Langmuir−Hinshelwood model was applied to evaluate the photocatalytic oxidation rates. 45 In addition, Figure 6 shows that the film with a TiO 2 dosage of 5% wt/wt yielded the highest removal efficiency (44%); the 10 and 15% wt/wt films exhibited removal efficiencies of 42 and 32%, respectively. On the other hand, there was a small decrease when the dose of TiO 2 rose from 5 to 15% wt/wt. This could be due to the higher doses of TiO 2 and higher abundance of hydroxyl radicals interacting with benzene, resulting in faster film removal. However, over a certain dose of TiO 2 , the rate of degradation began to decrease due to mass transfer 53 Mass transfer affects the rate of degradation, which is explained by the processes of photocatalytic oxidation. These processes are as follows: first, benzene is transferred to the photocatalyst surface; second, benzene is adsorbed on the photocatalyst surface; third, a photocatalytic oxidation reaction occurs on the surface; then, byproducts are desorbed from the surface of the photocatalyst; finally, byproducts are transferred from the surface into the air. 55 TiO 2 dosages of 10 and 15% may be too high for PLA/TiO 2 -composite films and may cause aggregation, which affects mass transfer and lowers the degradation rate. 56 Moreover, the photocatalytic oxidation reaction occurs at the liquid−catalyst interface, and therefore, when the catalyst is immobilized, both external and internal mass transfers play significant roles in the overall photocatalytic process.

Intermediate Products in the Photocatalytic Oxidation of Benzene in Simulated Indoor Air. Intermediate products were produced in the process of photocatalytic degradation of benzene by the PLA/TiO 2 -composite film, as shown in Figure 7 . The mechanism of the photocatalytic degradation of benzene by TiO 2 has been shown in a previous study. 57 Benzene was converted into phenol, then into other intermediates, and finally into carbon dioxide and water. 57 The proportions of residual intermediates increased as the oxidation rate and time increased. The CO 2 yield was probably due to a surface reaction. The CO 2 yield rate was related to the apparent oxidation rate of benzene or a secondary intermediate. Hypothetically, benzene can be transformed into CO 2 and H 2 O; however, residual intermediates can be found for incomplete mineralization. The amounts of residual intermediates increased rapidly. After this, the concentrations of intermediates increased slowly at rates corresponding to the oxidation rates. For PCO, accumulation of residual intermediates can occur at the active site and in the benzene gas Simplified Langmuir−Hinshelwood form is ln(C/Co) = −kKt. b k is the apparent kinetic constant (ppm·min −1 ). c K is the adsorption equilibrium constant (ppm −1 ). phase, which subsequently increases the proportions of residual intermediates. 45 

In summary, TiO 2 -anatase incorporated into a PLA/TiO 2composite film was found to be a highly promising photocatalyst for the degradation of benzene at a 5% wt/wt level of TiO 2 -anatase. The benzene removal efficiencies for photocatalytic oxidation in simulated indoor air suggest that PLA/TiO 2 -composite films can be used for indoor air purifiers.

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Figure 7. Intermediate products formed in the photocatalytic process using the 5% PLA/TiO 2 composite film

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