Photocatalytic Degradation of Pollutants in Air Streams Using Luminous Textiles Under Ultraviolet Light Illumination: A Pilot-Scale Remediation Study

Abstract

Air pollution from volatile organic compounds poses significant environmental and public health issues due to their toxicity and persistence in the environment. In this context, this experimental study explored photocatalytic degradation as a promising approach for the degradation of two polluting fatty acids, butyraldehyde (BUTY) and isovaleraldehyde, utilizing a TiO₂ photocatalyst-supported nonluminous textile within a continuous planar reactor. The impact of varying airflow rates (2 to 6 m³/h), initial pollutant concentrations (10 to 60 mg/m³), and air relative humidity (5 to 90%) on oxidation performance and removal efficiency were systematically investigated. The following optimal conditions were identified: an inlet concentration of 10 mg/m³, an airflow rate of 2 m³/h, a catalyst mass of 25 g/m², a UV intensity of 2 W/m², and 50% RH. The luminous textile photocatalytic degradation exhibited notable effectiveness for BUTY removal. To enhance our understanding, a mass transfer model using the Langmuir–Hinshelwood approach as a kinetic model was developed. This modeling approach allowed us to determine kinetic adsorption and degradation constants, reasonably agreeing with the experimental data. This study provides valuable insights into applying nonluminous textile-supported TiO₂ photocatalysts for environmental pollutant removal in continuous planar reactors.

1. Introduction

Air pollution, primarily caused by human activities, has become a serious health challenge to humans and other living organisms [1]. Several studies have shown that myriad volatile organic compounds (VOCs) with distinct concentrations are emitted by various industrial processes, agricultural activities, and animal husbandry industries [2]. VOCs have caused fatal diseases, including chronic respiratory and pulmonary problems, ischemic heart disease, and lung cancers. Emitted VOCs are also responsible for diverse environmental issues, such as global warming and ozone layer depletion [3,4]. Hence, their mineralization and/or removal has become vital. Several technologies, such as adsorption [5], thermal and catalytic incineration [6], ozonation, and biofiltration [6,7], have been employed to improve air quality and solve the problems caused by these pollutants. Nonetheless, these techniques have limitations, like secondary VOC emissions, solid waste generation, and considerably low energy efficiencies. Researchers are innovating a novel procedure to comprehensively remove volatile organic compounds (VOCs) from the air followed by efficient mineralization, ensuring high process-energy efficiency.

Advanced oxidation processes (AOPs), such as photocatalysis, non-thermal plasma (NTP) catalysis, and catalytic oxidation technologies, have been employed as promising alternatives for efficient and low-cost VOC removal from gaseous effluents to overcome the demerits of usual processes. These advanced oxidation processes (AOPs), generally employed in situ, generate highly reactive species such as hydroxyl (°OH) radicals at room temperature. Among these AOPs, photocatalysis is the most widely used air purification technique as it mineralizes VOCs into CO₂ and H₂O under ambient conditions [8]. It is based on UV irradiation on a semiconductor catalyst, mainly TiO₂, to convert the pollutants into less harmful products with a series of reactions involving oxidizing agents like reactive oxygen species (O₂°⁻ and °OH radicals) [9,10].

Table 1 summarizes the studies investigating the removal of volatile organic compounds (VOCs) using various advanced oxidation process (AOP) combinations. The synergy of AOPs with diverse physicochemical methods demonstrates their effectiveness at degrading VOCs. Notably, the research findings consistently highlight the superior efficacy of photocatalytic treatments for achieving comprehensive VOC removal. Photocatalytic oxidation-based air purification relies on semiconductors as photocatalysts, including TiO₂, ZnO, ZrO₂, and SnO₂. Among these, due to its physicochemical properties, TiO₂ is the most efficient and widely used catalyst for VOC removal in photocatalysis [11,12]. Hence, exploring novel TiO₂ photocatalysts with varied textures and chemical characteristics is a thriving area of research. This dynamic field has seen the application of diverse, innovative catalysts, encompassing nanoparticle suspensions [13], nanotubes [14], and nanoparticles deposited on disparate substrates [13,14].

However, TiO₂ powder is associated with many problems; for example, the photocatalytic efficiency of TiO₂ increases with a decrease in particle size [15]. Notably, a decline in the TiO₂ particle size causes difficulty in the post-treatment separation of particles from a solution [16,17]. Therefore, developing photocatalytic films is essential to avoid nanoparticle leaching in the environment. Many techniques, such as sol–gel and impregnation, have been adopted to prepare TiO₂ films on various substrates. Materials exhibiting mechanical and chemical stability have been employed as TiO₂ supports [18]. Various substrates, including carbonaceous materials [19,20], polymer supports [21], cellulosic paper [22], glass-fiber tissue [23,24], and polyester, have been investigated. However, the purification capacity of the effluent under visible light has not been proven to be significantly effective. Further enhancements are required to achieve more substantial levels of mineralization. Using an external UV lamp in conventional reactors diminishes catalyst efficiency due to the dissipation of UV radiation when traversing the gas layer. Employing an external lamp poses a challenge in reactor design, requiring substantial energy consumption and occupying a larger space. The development of a novel reactor utilizing optical fibers to support the photocatalyst has been introduced to address this issue. Placing the catalyst on the surface of the light source ameliorates their contact, overcoming material transfer limitations [25]. The specific treatment of the luminous textile triggers intense lighting thanks to the low energy consumption of LED technology. Nonetheless, a few studies on luminous textiles, concentrating on their characterization aspects, have been produced [17,26,27,28]. Our study focused on the photocatalytic oxidation of butyraldehyde (BUTY) and isovaleraldehyde (ISOVAL) using a groundbreaking catalytic planar reactor that integrated textile fibers (high-tech) and operated without an external light source. Factors such as the concentration of pollutants, air humidity, the amount of TiO₂, airflow, and light intensity were investigated in terms of how they influenced the efficiency of the photocatalytic process. Additionally, this research delved into the impact of different pollutants on removal efficiency and the mineralization process. The experimental observations and theoretical modeling were integrated to comprehensively understand the process. This is the first investigation to enhance aldehyde treatment using TiO₂-coated luminous textiles in a planar configuration for a pilot-scale setup.

2. Results

2.1. Degradation of Butyraldehyde

2.1.1. Effect of Inlet Concentration and Flow Rate on the Photocatalytic Process Efficiency

The BUTY photocatalytic oxidation was studied using a planar reactor to investigate the effect of inlet concentrations and the feed flow rates on the process performance. Figure 1 shows the BUTY removal efficiency of the planar reactor deposited with 7 g/m² of TiO₂. The initial concentrations ranged between 10 and 60 mg/m³, the feed flow rate varied between 2 and 6 m³/h, and the relative humidity was 50%.

At a constant flow rate, when the BUTY initial concentration increased, the degradation rate decreased owing to a decrease in the number of available active sites on the catalyst surface [29]. Moreover, an increase in the feed flow led to a decrease in the pollutant removal performance. The reason for such a result was that at high flow rates, the contact time of the pollutants with the active species, such as °OH and O₂°⁻, which formed on the catalyst surface, became shorter, leading to a significant decline in the efficiency of the pollutant degradation [30]. Previous studies reported similar results [31]. The highest degradation efficiency occurred at a feed flow rate of 2 m³/h, which was attributed to an ideal equilibrium between adequate contact time and efficient pollutant diffusion onto the catalyst surface.

As shown in Table 1, various photocatalysts have been developed and used for the photocatalytic degradation of air pollution over the past five years. Mamaghani and collaborators [32] evaluated the effect of the operating parameters (inlet VOC concentration, residence time, and relative humidity) on pollutant degradation efficiency using various types of commercially available TiO₂. They concluded that regardless of the catalyst type, the photocatalytic efficiency always declined with an increase in the airflow rate and/or inlet concentration [32,33]. El Falleh et al. [34] studied the photocatalytic degradation of BUTY and ISOVAL using TiO₂-impregnated polyester (PES) and glass-fiber tissue (GFT) in a 1 L batch reactor. They found that TiO₂/GFT exhibited more significant photocatalytic activity than TiO₂/PES. Moreover, they found that BUTY removal on GFT was about twice as high as PES. The enhanced BUTY removal was ascribable to the accessibility of TiO₂ at lower depths on the GFT, leading to a higher number of active sites per TiO₂ nanoparticle on GFT than on PES.

Saoud et al. [35] investigated BUTY photocatalytic removal using a supported TiO₂–glass-fiber tissue (GFT) continuous planar reactor. They reached a removal capacity of 25 mg/m³ (with 12 g/m² of TiO₂ and a UV intensity of 20 W/m²) of about 4.615 mg/(gTiO₂) h⁻¹.W⁻¹. The removal capacity of the same concentration through a luminous textile (with a TiO₂ mass of 7 g/m² and UV intensity of 0.5 w/m²) was about 123.15 mg/(gTiO₂) h⁻¹.W⁻¹. The removal capacity using the luminous textile was 26 times higher than that of TiO₂ on glass fiber, with high energy efficiency. The photocatalytic removal efficiency of luminous textile composites improved under UV LED irradiation compared with TiO₂ on glass-fiber tissue. They reported that the chloroform removal capacity of 25 mg/m³ was enhanced more than four times when using luminous textiles (Table 1).

Table 1. Comparison of VOC removal efficiency of various photocatalytic reactors.

Catalyst	VOC	Operating Parameters	Studied Parameters	Optimal Results	Ref.
TiO2-deposited polyester (PES) and TiO2-deposited glass-fiber tissue (GFT)	Butyraldehyde	Batch reactor, UV intensity = 9 W, RH = 40–50%, and T = 20 °C	Inlet concentration, RH, Q, and residence time	90% removal in 30 min on GFT. BUTY removal on GFT was about twice as fast as PES	[34]
Commercialized TiO2 (P25, PC500, and UV100)	Toluene; methyl ethyl ketone	Continuous reactor, UV at 5 mw/cm2, and T = 20 °C	Inlet concentration, Q, RH, and UV	Between 72% and 100% degradation	[32]
TiO2/diatomite	Formaldehyde, acetone, and p-xylene	Continuous reactor, RH = 20%, I = 5 w/m2. Composite dosage = 3.76 mg/cm2, light intensity = 1.33 mW/cm2, RH = 0%, and Q = 1 l.mn−1	Pollutant inlet concentration, RH, and Q,	>90% of degradation for a concentration of 10 ppm	[36]
TiO2 coating on GFT	Ammonia and butyraldehyde	Continuous reactor, UV lamp = 9 W, T = 20 °C, Q = 1 m3/h, and RH = 50%	Inlet concentration, RH, and flow rate	High selectivity for photodegradation at a rate of 72%	[35]
TiO2-incorporated KIT6	Ethylene and propylene	A mixture of UVA (320–400 nm) and UVB (290–320 nm), 100 ppm, and RH 60%	Pollutant conversion	90% conversion for 30% TiO2 KIT6	[37]
Glass-fiber tissue (GFT)	Butyraldehyde and dimethyl disulfide (DMDS)	Continuous reactor, UV intensity = 9 W, T = 20 °C, and Q = 4 m3/h	Inlet concentration and flow rate	Around 70% removal	[38]

Table 1. Comparison of VOC removal efficiency of various photocatalytic reactors.

Catalyst	VOC	Operating Parameters	Studied Parameters	Optimal Results	Ref.
TiO2-deposited polyester (PES) and TiO2-deposited glass-fiber tissue (GFT)	Butyraldehyde	Batch reactor, UV intensity = 9 W, RH = 40–50%, and T = 20 °C	Inlet concentration, RH, Q, and residence time	90% removal in 30 min on GFT. BUTY removal on GFT was about twice as fast as PES	[34]
Commercialized TiO2 (P25, PC500, and UV100)	Toluene; methyl ethyl ketone	Continuous reactor, UV at 5 mw/cm2, and T = 20 °C	Inlet concentration, Q, RH, and UV	Between 72% and 100% degradation	[32]
TiO2/diatomite	Formaldehyde, acetone, and p-xylene	Continuous reactor, RH = 20%, I = 5 w/m2. Composite dosage = 3.76 mg/cm2, light intensity = 1.33 mW/cm2, RH = 0%, and Q = 1 l.mn−1	Pollutant inlet concentration, RH, and Q,	>90% of degradation for a concentration of 10 ppm	[36]
TiO2 coating on GFT	Ammonia and butyraldehyde	Continuous reactor, UV lamp = 9 W, T = 20 °C, Q = 1 m3/h, and RH = 50%	Inlet concentration, RH, and flow rate	High selectivity for photodegradation at a rate of 72%	[35]
TiO2-incorporated KIT6	Ethylene and propylene	A mixture of UVA (320–400 nm) and UVB (290–320 nm), 100 ppm, and RH 60%	Pollutant conversion	90% conversion for 30% TiO2 KIT6	[37]
Glass-fiber tissue (GFT)	Butyraldehyde and dimethyl disulfide (DMDS)	Continuous reactor, UV intensity = 9 W, T = 20 °C, and Q = 4 m3/h	Inlet concentration and flow rate	Around 70% removal	[38]

2.1.2. Effect of VOC Inlet Concentration and Relative Humidity on the Photocatalytic Process Efficiency

The moisture in the air is an important parameter that can directly affect the efficiency of gas-phase photocatalytic oxidation. Numerous research studies have investigated the impact of relative humidity (RH) on photocatalytic oxidation [39,40,41]. In this context, the effect of relative humidity (5, 50, and 90%) on the elimination of BUTY was studied at a fixed flow rate of 2 m³/h for various BUTY inlet concentrations. The results are reported in Figure 2; the presence of water molecules in the airflow improved BUTY oxidation for the different inlet concentrations studied. By varying the relative humidity from 5 to 50%, the BUTY removal efficiency attained 27.8% and reached an optimum of 52.3% at an inlet concentration of 10 mg/m³. This finding indicated that low moisture in the gas stream promoted the BUTY photocatalytic degradation, probably due to OH° hydroxyl radical formation on the TiO₂ surface [41,42] produced by the adsorbed water molecules, which are known to be very reactive chemical species for pollutant mineralization [43]. Moreover, as the humidity of the effluent to be treated continued to increase, reaching an additional increment in RH up to 50%, the removal efficiency exhibited a notable decline. It was noticed that at 60 mg/m³ and with an RH of 90%, the degradation rate decreased by around 40% compared with 50% RH. This decline was due to the competitive adsorption of BUTY and water molecules on TiO₂ adsorption sites. Moreover, on the TiO₂ surface, water molecules adsorbed and formed a thin film by creating strong hydrogen bonds with OH groups. Additionally, at high values of RH%, the adsorbed water acted as a physical barrier, blocking effective contact between the pollutant and the catalyst surface.

The BUTY molecules should have traversed the water layer to adhere to the photocatalyst surface [44]. Additionally, the relative humidity and catalyst properties affected the quantity and nature of the byproducts resulting from the reaction pathway. As can be seen in Figure 2, the best photocatalytic activities were achieved with 50% RH regardless of the inlet pollutant concentration. Consistent results were observed in previous studies. Our previous study demonstrated the impact of relative humidity on the photocatalytic degradation of BUTY and ammonia using GFT/TiO₂, highlighting that moisture contributes to an increased degradation rate, particularly at an average humidity of 50%. Thus, the mechanism of reaction can be explained as follows: Under light irradiation, TiO₂ deposed on optical fibers generates electron–hole pairs (e⁻/h⁺). The trapped electrons can reduce molecular oxygen (O₂) to generate reactive oxygen species and, in the presence of humid air, the H₂O can be reduced to °OH. These ROS are highly reactive and can effectively degrade fatty acids, resulting in enhanced indoor air quality.

The effect of relative humidity on the photocatalytic degradation of chloroform and glutaraldehyde over a luminous textile with a UV LED lamp has been studied, reporting that better chloroform degradation efficiency varied between 50 and 60% relative humidity. Zhang et al. [36] studied the photocatalytic degradation of acetone and p-xylene on TiO₂/diatomite composites. They showed that the highest degradation rate could be acquired when the relative humidity reached 70%. Mamaghani et al. [32] determined that the maximum removal efficiency for toluene photodegradation using various types of commercial TiO₂ occurred at approximately 20% relative humidity.

2.1.3. Effect of Pollutant Concentration and UV Radiation Intensity on the Photocatalytic Removal Efficiency

Numerous experiments were conducted using BUTY in a photocatalytic reactor involving varying input concentrations and UV radiation intensities to evaluate the impact of UV radiation on the performance of the planar reactor. Diverse light intensities were achieved through variations in the UV LED output, spanning values of 0.5, 1, and 2 W/m². Figure 3 reports the BUTY removal efficiency for various concentrations under different UV radiation intensities. The results revealed that for a BUTY inlet concentration of 10 mg/m³, the removal efficiency was 28% when the light intensity was 0.5 W/m². As the light intensity increased to 2 W/m², the removal efficiency reached 65.7%. This increment occurred because raising the intensity generated more electrons and holes, produced more reactive species such as °OH and O₂°⁻ on the TiO₂ surface, and ultimately increased the BUTY degradation rate [36]. This non-linearity was due to the reduction in quantum efficiency at high intensities owing to the elevation of the recombination rate. When light intensities are too high, the recombination of electron–hole pairs become the dominant process, leading to a drop in the energy efficiency of the photoreaction. It is worth noting that, regardless of the UV radiation intensity, an increase in the inlet concentration of BUTY reduced the removal efficiency. Specifically, the efficiency of BUTY removal decreased by approximately 40% as the inlet concentration increased from 10 to 60 mg/m³.

2.1.4. Effect of Inlet Concentration and TiO₂ Mass on the Photocatalytic Process

Several experiments were conducted, varying the amount of TiO₂ from 7 to 25 g/m² under different initial BUTY concentrations at a fixed flow rate of 2 m³/h, RH of 50%, and UV light intensity of 2 W/m². The aim was to investigate the impact of TiO₂ mass on BUTY degradation and reactor performance, and the result is reported in Figure 4. The trend of the removal efficiency vs. the catalyst mass remained consistent across different inlet BUTY concentrations. The experimental findings indicated that as the catalyst mass increased from 7 to 13 g/m², the BUTY removal efficiency rose from 42% to 59% at an inlet concentration of 10 mg/m³. A subsequent 1.85-fold increase in the catalyst mass resulted in approximately a 1.5-fold enhancement in removal efficiency. The maximum removal efficiency was achieved at a catalyst mass of 13 g/m², which was attributed to increased active sites on the catalyst surface [32,33]. The removal efficiency, however, dropped significantly when the catalyst mass increased to 25 g/m². The degradation rate became 1.85-fold lower, reaching 32% when the catalyst mass was about 3.5 times higher. Similar trends were observed for the other inlet BUTY concentrations. Notably, the BUTY degradation rate decreased when the inlet concentration for the same catalyst mass was raised. This behavior could be attributed to the decrease in available catalyst active sites. Researchers have observed similar results for different reactor types and photocatalytic materials [33,36]. An increase in the catalyst mass can yield two counterproductive effects. Firstly, the mass transfer step becomes more significant, driven by enhanced site accessibility. Secondly, not all catalyst particles receive illumination owing to the aggregation of catalyst particles.

2.2. Degradation of a Mixture of Compounds

This section describes our aim to individually assess the removal efficiency of butyraldehyde (C₄H₈O) and isovaleraldehyde (C₅H₁₀O) and compare it with their degradation rates when both pollutants were mixed in equal proportions (50% butyraldehyde–50% isovaleraldehyde). The photocatalytic degradation of BUTY and ISOVAL and their mixture at different inlet pollutant concentrations (between 0.138 and 0.832 mmol/m³) were conducted at a flow rate of 2 m³/h, relative humidity of 50%, UV radiation intensity of 2 W/m², and TiO₂ mass of 25 g/m². Figure 5 shows that the C₄H₈O and C₅H₁₀O photocatalytic removal efficiency decreased with the gas mixture (C₄H₈O and C₅H₁₀O). Moreover, for the different concentrations tested, the mineralization of BUTY and ISOVAL was affected when these two contaminants were mixed. When each contaminant was tested alone, the removal efficiency was 65.7% for BUTY and 58.7% for ISOVAL for an inlet pollutant concentration of 0.138 mmol/m³. The removal efficiency in the mixture was 53% for BUTY and 49% for ISOVAL because the mixed compounds seemed easier to eliminate than the pure compounds. Such a competition was traceable to the difference in volatility between the relevant ingredients. In our case, it was noticed that BUTY was the most volatile compound (P_sat,vap = 12.2 KPa at 20 °C), which is why it was more vulnerable to the competition than ISOVAL (P_sat,vap = 6.1 KPa at 20 °C). This result was very likely to be closely related to the competitive adsorption of these two compounds on the free active sites of the catalyst. Mixing the pollutants decreased the BUTY removal efficiency by 12% compared with a single VOC; however, mixed ISOVAL degradation declined by a factor of 1.2 compared with the single pollutant. Other works have reported similar behavior; for instance, Debono et al. (2017) noted an inhibitive effect when they investigated the photocatalytic degradation of the ternary mixture of toluene, decane, and trichloroethylene [45].

2.3. CO₂ Selectivity

The CO₂ selectivity of the photocatalytic removal of BUTY, ISOVAL, and their mixture was monitored and is reported in Figure 6. CO₂ selectivity was negatively affected by increasing the initial concentration of the pollutants. The highest CO₂ selectivity, 80.8%, was obtained for BUTY degradation at an inlet concentration of 0.14 mmol/m³. During the photocatalytic degradation of the contaminants, the carbonaceous intermediates could have accumulated on the catalyst surface, which could have blocked the active sites and prevented the access of substrate molecules. Even though the pollutant concentration in the reactor increased, the amount of photogenerated reactive species remained constant. Therefore, a small fraction of active species could be utilized to completely mineralize the adsorbed pollutants on the TiO₂ surface.

The highest CO₂ selectivity was obtained at a low input concentration because there was better accessibility to available active sites and, therefore, the intermediate byproducts could be mineralized entirely to CO₂. When the inlet concentration increased, the ratio between the pollutant and the availability of catalyst active sites decreased, i.e., the byproducts produced could not be mineralized [46]. A competitive phenomenon, influenced by the representative concentrations of each entity and the affinity between the species and the catalytic support [47], manifested among BUTY, ISOVAL, and their respective byproducts. Figure 6 reveals that the CO₂ selectivity of the mixture was lower than that obtained for each pollutant, likely due to the competition between the contaminants and their generated byproducts.

2.4. Kinetic Modeling of the Degradation with the Contribution of Mass Transfer in the Planar Reactor

Both the mass transfer and chemical reactions must be considered during the photocatalytic reaction over an immobilized catalyst. Indeed, both external and internal mass transfers could play significant roles. Several researchers [48,49] have developed the Langmuir–Hinshelwood expression for modeling photocatalytic reactions on catalyst surfaces. The L–H ratio equation is widely recognized to deliver good correlations for heterogeneous reactions [50]. This work stands out, given that the results obtained when studying the impact of the air gap on the removal efficiency proved that the mass transfer stage should be considered.

The pertinent photocatalyst exhibited minimal intraparticle transport resistance, attributed to the non-porous configuration of the catalytic thin film. Consequently, only the mass transfer between the catalyst and the bulk fluid could be considered. The limitation of the internal mass transfer step was not considered in the mass transfer model because photocatalysis implies superficial reaction sites, i.e., the internal diffusion, either molecular or Knudsen, is negligible. Many researchers [51,52] have studied photocatalysis kinetics and mass transfer considering Equations (1) and (2).

The gaseous phase is as follows:

$$u{\displaystyle \frac{dCm}{dz}}+k_m\times a_v(C_m-C_s)=0$$

(1)

The solid phase is as follows:

$$k_m\ast a_v\left(c_m-C_s\right)=k_{obs}\times {\displaystyle \frac{k_m\times C_s}{1+k_m\times C_s}}$$

(2)

where u and k_obs are gas flow velocities (m/s) and the observed kinetic constant, respectively; C_s and C_m are the bulk and the medium surface gas-phase concentrations of VOCs (mmol/m³), respectively; k_m is the mass transfer coefficient (m/s); and a_v is the medium area per unit volume of the reactor (m²/m³_reactor). The BUTY and ISOVAL concentrations at the inlet were significantly low. Thus, their byproducts would also be minimal, and their competitive effect would be overlooked.

The viscosity and density of the fluid used to calculate the mass transfer were assumed to be like air. By altering the flow rate from 2 to 6 m³/h, the Reynolds number soared from 248.52 to 745.57, respectively, implying that the mass transfer step could play a role in the degradation process due to laminar flow (Re < 2000) [53]. Subsequently, it was suggested that the mass transfer stage be considered. The external coefficient of mass transfer, k_m, is a function of the gas phase, the fluid flow regime, and the nature of the gas. It can be set using a semi-empirical correlation (Equation (3)) [54].

$$Sh=0.664{\left(Re\right)}^{{\displaystyle \frac{1}{2}}}\times {\left(SC\right)}^{{\displaystyle \frac{1}{3}}}$$

(3)

where Sh, Sc, and Re are the dimensionless numbers of Sherwood, Schmidt, and Reynolds, respectively.

The pollutant’s molecular diffusivity (D_m) of the pollutant is gauged using the Fuller, Schettler, and Giddings correlation (Equation (4)) [53].

$$D_m={\displaystyle \frac{{10}^{-3}{T}^{1.75}P}{({{(\sum V_A)}^{\frac{1}{3}}+{(\sum V_B)}^{\frac{1}{3}})}^2\times {(\frac{1}{M_A}+\frac{1}{M_B})}^{1/2}}}$$

(4)

where P is the pressure of the gas stream (Pa), T is the absolute temperature (K), (Ʃν)_A and (Ʃν )_B are the molecular volumes of gases A and B, and M_A and M_B are the molecular weights of A and B (g/mol). The index “A” represents the air stream and “B” is the pollutant. This expression (4) permitted the evaluation of the diffusivity coefficients for both pollutants (

Table 2. Diffusivity values of BUTY and ISOVAL in air.

Pollutant	Formula	Diffusivity in the Air (Dm) (cm2/s)
Butyraldehyde	C4H8O	0.0892
Isovaleraldehyde	C5H10O	0.0797

).

From Equation (5), the Sherwood number, the diffusion coefficient D_m, and the mass transfer coefficient (k_m) can be determined.

$$Sh={\displaystyle \frac{km\times d}{D_m}}$$

(5)

Here, d is the equivalent diameter.

Table 3. Reynolds number and mass transfer coefficients.

Flow Rate (Q) (m3/h)	Reynolds Number	Mass Transfer Coefficient (km) (m/s)
		Butyraldehyde	Isovaleraldehyde
2	248	7.7	7.2
4	497	10.9	10.1
6	745	13.4	12.4

gives the Reynolds number and mass transfer coefficients in the planar reactor.

Maple software (2020.2) was used to develop a system using the Method series to solve Equations (1) and (2). Previous works showed that a second-order development is enough to correlate the experimental results [50]. The second-order development is as follows [55]:

$$C_{in}-C_{out}={\displaystyle \frac{L{k}_m{a}_v}{2u}}{\left[C_{in}+{\displaystyle \frac{1}{K}}+{\displaystyle \frac{k_{obs}}{k_m\times a_v}}–{\left(-C_{in}+{\displaystyle \frac{1}{K}}+{\displaystyle \frac{k_{obs}}{k_m\times av}}\right)}^2+4{\displaystyle \frac{C_{in}}{K}}\right]}^{0.5}$$

(6)

In Equation (6), the unknown parameters are K and k_obs. Both were determined by a digital resolution using the Solver function in Excel. A target cell was made for each experimental point as the difference between the theoretical and empirical outlet concentrations. The sum of the squares of the target cell mean difference for each compound was minimized by optimizing the variable cell values of K and k_obs, helping to find the best figures for both butyraldehyde and isovaleraldehyde.

Table 4. k_obs and K values for butyraldehyde and isovaleraldehyde.

	Butyraldehyde	Isovaleraldehyde
kobs (mmol/(m 3.s))	0.018	0.081
K (m3/mmol)	1.159	0.246
R2 (%)	98.6	92

summarizes the K and k_obs values.

It is noteworthy that these constants were not flow-rate-dependent. The influence of the flow-rate regime was completely incorporated through the parameter k_m, and the mass transfer and the chemical reaction step were separated. The experimental results gave a more reasonable description than that delivered by this model for both VOCs. For instance, the theoretical removal efficiency (Figure 7a,b) correlated well with the experimental results for all the flow rates investigated.

Figure 7 illustrates the variation in the calculated removal efficiency for both pollutants concerning the removal efficiency achieved for different BUTY and ISOVAL inlet concentrations across various flow rates. The lower removal efficiency observed at high flow rates was attributed to the inadequate residence time for the pollutants to effectively transfer from the gaseous phase onto the catalytic surface. Notably, the removal efficiency declined when the pollutant inlet concentration rose at a constant airflow. This figure proves that this model was well suited to the experimental results. Thus, it was possible to estimate the removal efficiency at different inlet concentrations and distinct flow rates for both VOCs.

3. Materials and Methods

3.1. Reagents

In this study, two compounds, butyraldehyde (C₄H₈O) and isovaleraldehyde (C₅H₁₀O), were purchased from SIGMA-ALDRICH, Germany, and used without purification. TiO₂ was supported on luminous textiles, i.e., a combination of textile fibers and optical fibers was used as a photocatalyst.

3.2. Luminous Textiles

The luminous textile used a combination of textile fibers made from polyester (Triwra CSTM fibers) and a luminous textile (optical fibers) made from polymethylmethacrylate resin (produced by PMMA CK-20 EskaTMfibers), interlaced and simultaneously woven according to a Jacquard loom with an average diameter of 480 µm, and covered with 10 µm thick fluoropolymer [6], which was supplied by Brochier Technologies (UVtex^®). The luminous textile was collected to one extremity in the aluminum connector for one textile sample. The tissue was covered by an intermediate layer of silica, shielding the luminous textile degradation by photocatalysis. TiO₂ Degussa P25 was used as a photocatalyst. The textile fiber was soaked for 60 min in a TiO₂ suspension (50 g/L TiO₂ powder) and then dried for 2 h at 70 °C. Eventually, 12 g/m² of TiO₂ was uniformly dispersed only on the textile fiber. These luminous textiles underwent treatment to enable the transmission and emission of light laterally from their external surface. The optical fiber and textile fiber consisted of polymethyl methacrylate, made by Mitsubishi (Tokyo, Japan) (PMMA CK-20 Eska™ fiber), and polyester (Trévira CS™ fiber), respectively. Figure 8a illustrates a scanning electron microscopy image characterizing the photocatalyst. Figure 8b highlights the luminous textiles placed in the planar reactor with the UV LEDs. Each UV LED illuminated the luminous textiles.

3.3. Experimental Setup

Figure 9 portrays the schematic of the experimental setup, which consisted of the following three sections: (i) gas stream preparation, (ii) photocatalytic reactor, and (iii) analytical systems. The VOC removal process operated using a single-pass tangential flow configuration. Before beginning the photocatalytic process, the photoreactor (the glass chamber + the photocatalyst) was air-cleaned under UV light for 60 min to eliminate contamination. The VOCs were continuously injected into the gas stream using syringe pumps (Kd Scientific Model 100, Holliston, MA, USA) and thoroughly mixed before entering the reactor. The heating tape was wrapped around the pipe from the injection port to the reactor inlet to guarantee adequate evaporation of the pollutant. The airflow rate was controlled using a calibrated flow meter (Bronkhorst, Montigny-lès-Cormeilles, France), and the airflow varied between 2 and 6 m³/h.

The experimental reactor was a continuous-operation rectangular chamber. It had the flexibility to be supplied with either an airflow containing a single pure compound (BUTY or ISOVAL) or an equimolar binary mixture of contaminants. An analytical system facilitated the measurement of both the initial and final concentrations of the pollutants. Regular gas samples were extracted throughout the experiment and promptly subjected to gas chromatography (GC) analyses. The gas stream was directed towards a CO₂ analyzer at the installation outlet, enabling continuous monitoring of the mineralization rate. The reactor, depicted in Figure 8, comprised a rectangular glass chamber (length = 1000 mm; square section = 145 mm × 145 mm). Two parallel glass plates, each with a length of 800 mm and a thickness of 1 mm, were installed within the chamber. Luminous textiles, equipped with an ultraviolet LED, showed even radiation distribution across the catalyst’s surface. All experiments were conducted under ambient conditions, maintaining atmospheric pressure and room temperature. A column humidifier was employed to achieve relative humidity (RH) values of 5%, 50%, and 90%.

3.4. Apparatus and Analysis

The concentrations of BUTY and ISOVAL were studied using gas chromatography (GC) equipped with a flame ionization detector (FID). The analysis was performed with a chromatograph (Fisons GC9000) fitted with a 25 m Chrompact FFAP-CB capillary column. Next, 500 μL of the sample was manually injected at least three times using a gas-tight syringe. The analysis conditions were as follows: injector and detector temperatures were fixed at 200 and 190 °C, respectively. The temporal evolution of CO₂ during the photocatalytic treatment was monitored using an infrared (IR) multi-gas CO₂ analyzer (MIR 9000 Environment SA Gas Filters Correlation).

The photocatalytic process efficiency was measured based on the VOC removal efficiency (RE) and CO₂ selectivity (S_CO2) using Equations (7) and (8).

$$\mathrm{R}\mathrm{E}\left(\mathrm{\%}\right)={\displaystyle \frac{\left[\mathrm{C}\right]\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}-\left[\mathrm{C}\right]\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}{\left[\mathrm{C}\right]\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}}\times 100$$

(7)

where [C]_inlet and [C]_outlet are the pollutant inlet and outlet concentrations, respectively.

$$S_{{CO}_2}(\%)={\displaystyle \frac{{\left[{\mathrm{C}\mathrm{O}}_2\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}-{\left[{\mathrm{C}\mathrm{O}}_2\right]}_{\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}}{{\mathrm{n}}_{\mathrm{c},\mathrm{v}\mathrm{o}\mathrm{c}}\mathrm{x}\mathrm{R}\mathrm{E}\left(\mathrm{\%}\right)\times {\left[\mathrm{V}\mathrm{O}\mathrm{C}\right]}_{\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}}}\times {10}^4$$

(8)

where [VOC]_inlet and n_c,voc are the VOC inlet concentration and the number of carbon atoms in the pollutant, respectively.

4. Conclusions

The degradation of butyraldehyde and isovaleraldehyde through photocatalysis was investigated in a TiO₂-supported luminous textile planar reactor. The degradation rates of BUTY and ISOVAL decreased with high pollutant inlet concentrations and feed flow rates, while an increase in relative humidity initially enhanced pollutant photodegradation. The presence of water vapor proved beneficial at low concentrations due to the formation of OH° radicals. However, at high relative humidity (90%), degradation diminished due to competition between the water and pollutants for active sites.

The impact of the inlet concentration and UV intensity on the removal efficiency was explored, revealing that regardless of the UV light intensity, an increase in inlet concentration led to a 40% reduction in the BUTY degradation rate, particularly from 10 to 60 mg/m³. The influence of the catalyst quantity was also studied for various BUTY inlet concentrations, and high removal efficiency was achieved under the following specific conditions: a flow rate of 2 m³/h, a concentration of 10 mg.m⁻³, relative humidity of 50%, and a TiO₂ amount of 2 g/m² on the luminous textile. Notably, the BUTY removal capacity using the luminous textile surpassed that achieved for the same TiO₂ concentration on fiberglass more than 26 times, with reduced energy consumption. Additionally, photocatalysis of the BUTY and ISOVAL mixture and the effect of the inlet concentration on removal efficiency and overall CO₂ selectivity were studied. A mass transfer model was developed using the Langmuir–Hinshelwood approach to determine kinetic adsorption and degradation constants. The external mass transfer was estimated using semi-empirical correlations, yielding results consistent with the experimental findings.