Disinfection of indoor air using ultraviolet and titanium dioxide photocatalytic reactor

Abstract

Air contamination in confined environments can lead to severe health damage. Searching for effective and sustainable technologies that might bring quality to indoor air is necessary. Heterogeneous photocatalysis has been studied for its ability to oxidize, inactivating microorganisms in the air. In the present work, a reactor was assembled, where titanium dioxide (TiO₂) P25 was incorporated into the inner face of polyvinyl chloride (PVC) tubes and vegetable sponges (Luffa sp.). Polyester Orthophthalic (PO) resin was used to fix the TiO₂ onto the surfaces. Ultraviolet lamps (UVA) were used to activate the TiO₂ catalyst to test the inactivation capacity of microorganisms, as they are economical and present high energy efficiency and long service life. The inactivation of microorganisms was evaluated in natural and artificially contaminated atmospheres. The photocatalytic reactor proved efficient in most tests in both atmospheres. In tests 1 and 2, no bacterial colony-forming units (CFUs) were found in the photocatalysis tube. In test 3, the average of 5 CFUs of fungi in the photocatalysis tube and 12.67 in the control tube was found, indicating inactivation. Therefore, this research is essential for presenting an alternative solution for indoor air treatment.

Key words
Air disinfection; Heterogeneous Photocatalysis; Luffa; TiO2; UVA

INTRODUCTION

Airborne particles can be solid or liquid, including dust, soot, sea salt crystals, spores, microbes, and many other microscopic elements. These are conventionally called particles with aerodynamic diameters ranging between 0.001 and 10µm (Agarwal et al. 2019). These pollutants are significant inducers of Sick Building Syndrome (SBS) that can cause tiredness, headaches, skin irritation, nose, eyes, throat, and mucous membranes. This syndrome is prevalent in individuals who spend periods in residential or occupational buildings, such as offices, schools, hotels, and hospitals (Riaz et al. 2020).

Infectious indoor bioaerosols include viruses (flu, measles, and chickenpox), bacteria (Chlamydia [psittacosis], Mycobacterium [tuberculosis], Legionella [legionnaires’ disease]), and fungi (Aspergillus [aspergillosis]) (Morawska 2006). Good-quality indoor air is essential in healthcare environments, such as Basic Health Units (BHU), Family Health Strategies, and hospitals.

Advanced Oxidative Processes (AOPs) may constitute a promising alternative to improve indoor air quality. AOPs are based on highly active radical species, including hydroxyl radicals (HO•). They can be classified according to the reactive phase (homogeneous and heterogeneous) or hydroxyl radical generation methods (chemical, electrochemical, sonochemical, and photochemical) (Ortiz et al. 2019). One of the most used AOPs is the well-known photocatalysis process.

The photocatalysis process is used in several emerging fields such as energy production (photocatalytic water separation), environmental protection (self-cleaning materials and photo-reduction of atmospheric pollutants such as nitrogen oxides, volatile and halogenated hydrocarbons), water purification (photooxidation of micropollutants, volatile organic compounds, pesticides), and for inactivation of microorganisms (Magalhães et al. 2017).

A photocatalyst is a substrate that, when exposed to light, acts as a catalyst for chemical reactions. In photocatalysis, an electron-hole pair is generated. All photocatalysts are basically semiconductors. Titanium dioxide (TiO₂) is the most widely used compound (Ameta et al. 2018, Ortiz et al. 2019). Due to its absorption spectrum, TiO₂ activation requires a light source with a wavelength below 400 nm in the Ultraviolet lamps (UVA) range (Martín-Sómer et al. 2017). In order to choose the ideal lamp for a specific AOP, several factors must be taken into consideration: (1) the absorption spectrum, (2) the number of photons emitted by the lamp, (3) the geometry of the lamp related to the ideal design of the ballast, (4) manufacturing and operating costs, and (5) service life. Available UV LEDs present wavelengths ranging between 210 nm and 405 nm (Ortiz et al. 2019).

Luffas are part of a plant in the Cucurbitaceae family, which comprises 97 genera with 950 species and has a wide tropical and subtropical distribution. They are dicotyledonous, herbaceous, with a creeping stem, and generally have spiral and often branched tendrils (Silva et al. 2023, Reis et al. 2023, Brasileiro & Pessenti 2022). Luffas fruits have fibrous material in the form of a network that allows support of inorganic materials. These fibers are cheap, abundant, ecological, and renewable resources. Studies have demonstrated high performance of UVA LEDs associated with TiO₂ in dye degradation (Matafonova & Batoev 2018) and in the inactivation of microorganisms through photoreactors (Paschoalino & Jardim 2008, Pigeot-Remy et al. 2014, Rodrigues-Silva et al. 2017, Martínez-Montelongo et al. 2020, Zacarías et al. 2020).

This study used Luffas to support the catalyst to increase the contact surface area between the catalyst, UVA light and microorganisms. Luffas were chosen because they constitute an alternative and natural material, which presents important characteristics that contribute positively to the research in environmental technology. Therefore, considering the studies that point to the efficiency of heterogeneous photocatalysis photoreactors in the inactivation of microorganisms and the importance of indoor air quality, the present study aims to seek a sustainable alternative to promote human health. So, our work aimed to evaluate the efficiency of photocatalysis in reducing microorganisms from the air in a confined environment and applying the same process in a healthcare environment.

MATERIALS AND METHODS

Preparation and reactor configuration

Preparation and deposition of PO/TiO₂ film in tubes

The reactor tubes were constructed using two configurations. The first experiment was conducted with photocatalysis consisting of TiO₂ incorporated directly into the inner face of polyvinyl chloride (PVC) tubes, and the second experiment with TiO₂ also incorporated in the Luffas introduced into the tubes. The titanium dioxide used was Degussa P25 (TiO₂, Degussa, Rutile:Anatase/85:15, 99.9%, 20nm).

The Polyester Orthophthalic (PO)/TiO₂ film was first prepared to develop the photocatalytic reactor. This film consisted of a solution of polyester resin (Anjo Tintas) and 20% TiO₂ (m/m), prepared according to Paschoalino & Jardim (2008), using a mechanic stirrer (1000 rpm for 20 minutes). After homogenization, the resin catalyst was added to the solution (described previously). The resin ensured the fixation of the TiO₂ onto the surfaces. The same methodology was used to prepare the film used in Luffas.

The PO/TiO₂ solution was poured into the tubes by rotating them over the surface of a bench so that the solution covered the entire internal extension. Afterward, the tubes were washed with a KOH (potassium hydroxide)/5% ethanol (w/w) solution. The solution remained inside each tube for one hour (Paschoalino 2006). The solution was removed, then the tube was filled with deionized water and sealed, remaining for 24 hours. The final drying was conducted at room temperature. Film deposition resulted in a TiO₂-coated area of 5.024 cm².

Treatment of Luffas, preparation, and deposition of the PO/TiO₂ film

The treatment aimed to clean the fiber and facilitate the adhesion of the resin/catalyst mixture. Luffas were used to support the catalysts and increase the contact surface area between the catalyst, UVA light, and microorganisms. It is an alternative and natural material with important characteristics that positively contribute to research in environmental technology.

The Luffas used were collected directly from a Luffa sp. specimen and sent to the laboratory for prior treatment and use in the reactor. The treatment was based on immersing the Luffas parts for seven minutes in a solution with 50% acetone at room temperature in ultrasound. The solution was prepared by adding 900mL of acetone to 900mL of deionized water, which is the capacity of the ultrasonic washer. The Luffas were kept in NaOH solution (0.1mol.L^-¹) for two hours, according to the methodology adapted from El-Roz et al. (2013). Afterward, they were dried at room temperature and cut into pieces measuring about 15x4 cm.

The deposition of the films in Luffas consisted of placing them in a container and pouring the resin/catalyst mixture to cover the entire material uniformly. After removing the excess, the same methodology was used in the tubes, according to Paschoalino & Jardim (2008). The Luffas were observed through a metallographic microscope (Bel Photonics Biovideo) to evaluate the removal of the polyester resin film, and the method’s efficiency was thus confirmed.

Reactor assembly

Figure 1 presents the assembled reactor. The equipment for the atmosphere disinfection consisted of a reactor divided into a photocatalytic, photolytic, and control area. The photocatalytic system consisted of TiO₂ tubes. The first part of the equipment consisted of an acrylic box with a lid. This box is intended to receive the synthetic air from the compressor, contaminate it, and distribute it to the rest of the system through the PVC pipes. Petri dishes with microbiological cultures were placed inside the box during the tests.

Figure 1
Reactor with tubes (1) photolysis, (2) control, (3) photocatalysis, (4) acrylic box, (5) adapted Petri dishes, and (6) synthetic air inlet.

A 1-m long and 4-cm diameter PVC tube was attached to each outlet. A PVC elbow was attached to the end of the tube so that three other tubes were connected to it, creating a closed circuit of about 4 meters, aiming at increasing the time the atmosphere remains under the action of UVA + TiO₂. The same was done for the other two exit holes of the box, totaling 12 tubes. Inside two circuits, a ribbon with LED lamps was introduced. The tape used in the study has the following characteristics: purple color, black light, ultraviolet, 72 W of total power, 14.4 W/meter, 60 LEDs per meter, with a size of 10 mm x 5 m and a useful life of approximately 50.000 hours.

The first PVC circuit (Circuit 1) contained the tube impregnated with the PO/TiO₂ solution. It had a ribbon with LED-UVA lamps attached to its interior, forming a photocatalytic reactor. The second PVC circuit (Circuit 2) did not undergo catalyst deposition, only the LED-UVA strip, forming a photolytic reactor. The third circuit (Circuit 3) was used as a control for data comparison so that neither catalyst nor UV irradiation was used. Each circuit presents a useful volume of 56.3944 m³.

At the exits of the last tubes of each circuit, hoses were attached to the air outlet, directing it to a Petri dish containing culture medium for bacteria (Standard Methods Agar) or culture medium for fungi (Sabouraud Dextrose Agar) to assess the inactivation of microorganisms from the atmosphere that passed through the reactor tubes. The Petri dishes were subjected to adaptation to reduce the possibility of contamination of the agar with microorganisms present in the atmosphere outside the reactor. Two 1.5-cm-diameter holes were made in each Petri dish’s lid. A quick coupling connection with a lid fixed with silicone glue was attached to each hole. The holes allowed for the entry and exit of air from each plate. The exit was directed to the external area of the laboratory.

The system was built on a wooden support measuring 1.6 x 0.5 m and 1.5 cm thick. In the third test, the circuit 1 tubes remained in the system. However, the Luffas were introduced, and new samplings were performed to compare the results.

To perform the fourth, fifth, and sixth tests, the reactor was modified so that it could be removed from the laboratory. The air injection was adapted for this, and three small coolers replaced synthetic air and were placed on the reactor’s air inlet. The acrylic box was removed, and the atmosphere used in the disinfection tests did not undergo artificial contamination.

At the exit of the reactor, small supports were attached to support the Petri dishes containing culture medium for bacteria (Standard Methods Agar) or Culture medium for fungi (Sabouraud Dextrose Agar). These supports were installed to verify the occurrence of microorganism inactivation of the air that passed through the reactor tubes. Acetate sheets were also attached to the system to direct the air in each circuit and prevent contamination from one to the other. The plates used were standard plates without adapting the quick coupling connections. Table I shows reactor configuration data.

Table I
Different configurations of the UVA/TiO₂ reactor.

SAMPLING CAMPAIGNS	ATMOSPHERE	CIRCUIT 1 PHOTOCATALYTIC REACTOR	CIRCUIT 2 PHOTOCATALYTIC REACTOR	CIRCUIT 3 CONTROL
Sampling 1 - laboratory	artificial contamination - 1 atm	PO/TiO2 + LED-UVA incorporation in the tube	LED-UVA	only tube
Sampling 2 - laboratory	artificial contamination – 0.5 atm	PO/TiO2 + LED-UVA incorporation in the tube	LED-UVA	only tube
Sampling 3 - laboratory	artificial contamination – 0.5 atm	PO/TiO2 + LED-UVA incorporation in the tube and in Luffas	LED-UVA	only tube
Sampling 4 - laboratory	real atmosphere - cooler	PO/TiO2 + LED-UVA incorporation in the tube	LED-UVA	only tube
Sampling 5 - BHU	real atmosphere - cooler	PO/TiO2 + LED-UVA incorporation in the tube	LED-UVA	only tube
Sampling 6 - BHU	real atmosphere - cooler	PO/TiO2 + LED-UVA incorporation in the tube	LED-UVA	only tube

The air velocity was determined using an anemometer (Minipa MDA-01) at the exit of each circuit. After obtaining these values, the flow rate of each circuit was identified. Two samples were collected from each tube to assess the sterility of the equipment: one to assess contamination by bacteria and the other by fungi. Samples were collected using the surface swab method. Only one sample for bacteria and one for fungi were taken before each experiment.

Preparation of culture media

For the capture and cultivation of microorganisms, two culture media were used: Standard Methods Agar (NEOGEN ® Culture Media) for bacteria and Sabouraud Dextrose Agar (NEOGEN ® Culture Media) for fungi, both prepared according to the manufacturer’s instructions. After preparation, the media were autoclaved for 15 minutes at 121°C using a vertical autoclave (Prismatec) and then transferred, in a laminar flow hood, to sterilized Petri dishes. Petri dishes were sealed with film and kept in the refrigerator until used for collection.

Obtention of microorganism’s culture through sedimentation collection for artificial contamination

Aiming for the contamination of the atmosphere used in the study, the culture of microorganisms was collected through passive sedimentation (Nunes 2005), where three Petri dishes with each medium were placed for 30 minutes. The methodology followed the proposed by Quadros (2008), Bisognin & Marquardt (2017), and Salustiano (2002) in one of the environments of the University of Santa Cruz do Sul (UNISC). The University’s Community Center was chosen as the sample source. The Community Center is an open covered area where several people circulate daily and presents cafeterias, a bookstore, a pharmacy, and other structures that meet daily needs.

This technique aimed to expose the agar of the plates to microorganisms in the environment and thus cultivate colony-forming units (CFUs) that would later artificially contaminate the air used in the reactor. Then, the plates were closed, adequately identified, and transported to the Laboratory of General Microbiology and Parasitology, located at the Universidade de Santa Cruz do Sul. The plates with bacteria were placed in an oven at 35-37°C and the plates with fungi at 25-30°C (Andrade et al. 2003, Paschoalino & Jardim 2008, Brasil 2003) for seven days in order to guarantee the proliferation of microorganisms.

Microorganism disinfection experiment with UVA/TiO2 reactor

The atmosphere disinfection tests occurred at the UNISC’s Effluent and Solid Waste Treatment Laboratory and at a Basic Health Unit (BHU) in Santa Cruz do Sul. Ten employees work at the BHU and attend to around 50 patients per day. The room chosen for the reactor installation was the reception and waiting room (220.8 m³). It presents the unit’s main door and three corridors that lead to other rooms for various services. The air conditioning is done by a Split type (18,000 BTU/h) positioned 2.5 m from the floor. Air renewal occurs naturally. During the reactor’s operation, an intense flow of people was observed in the room, and an average of about ten patients always remained.

For the samplings, Petri dishes containing the microbiological culture of bacteria were placed in the acrylic box and kept open, aiming for the exposure of a medium containing the microorganisms. Then, the synthetic air from the compressor (White Martins) was introduced into the box and worked as a carrier gas. The gas carried the microorganisms in the air through the three tubes (photocatalysis, photolysis, and control) to the adapted Petri dishes.

In test 1, the injection of synthetic air into the system was determined through a pressure reducer that guaranteed 1 atm (atmosphere). The flow was equivalent to a 12v cooler, replacing synthetic air in subsequent samplings and being conducted outside the laboratory. The 12v cooler was also used by Paschoalino & Jardim (2008). The air residence time of each tube was calculated by dividing the volume by the volumetric flow rate. The flow rate was calculated from the air velocity data, and the results were 0.001758 m³/s in the photocatalysis, 0.002009 m³/s in the photolysis, and 0.002386 m³/s in the control tube.

In test 2, the injection of synthetic air into the system was reduced to 0.5 atm to improve results. The flow rate in the photocatalysis tube was 0.001381 m³/s, whereas in the photolysis and the control tube were 0.001381 m³/s and 0.002135 m³/s, respectively.

In test 3, the synthetic air inlet injection was kept at 0.5 atm, as test 2’s results were considered better than the results of test 1. The photocatalysis tube’s flow rate was 0.002135 m³/s; in the photolysis tube, 0.002512 m³/s; and in the control tube, 0.002512 m³/s. In this test, Luffas with TiO₂ were introduced into the photocatalysis tube. The other settings remained the same.

The experiments were carried out in triplicate, lasting 30 minutes each (Paschoalino & Jardim 2008). After the established time, the adapted Petri dishes were carefully uncoupled from the system and placed in an oven.

Test 4 was performed after adapting the reactor with the coolers, still in the laboratory. The flow rates of tests 4, 5, and 6 in the photocatalysis tube, the photolysis tube, and the control tube were 0.002009 m³/s, 0.002512 m³/s, and 0.002637 m³/s, respectively.

In all tests, the air velocity values at each circuit outlet diverged. This might be attributed to LED strips placed inside the photocatalysis and photolysis tubes, which acted as a barrier to the airflow, especially at the ends of the circuits (PVC knees). The flow rate of the last three tests remained the same, as there was no change in the reactor settings.

Tests 5 and 6 were carried out at the BHU, where the reactor was positioned 1m above the floor, on two small tables at the back of the waiting room, behind the benches used by patients to wait for care. It operated continuously for two days from 7:45 am to 5 pm.

Sampling was performed once a day, in the morning, using plates containing Standard Methods Agar and Sabouraud Dextrose Agar for 30 minutes and in triplicate (Paschoalino & Jardim 2008). After the established time, the Petri dishes were carefully uncoupled from the system, transported to the General Microbiology and Parasitology Laboratory at the Universidade de Santa Cruz do Sul, and stored in an oven.

For bacterial culture, the temperature in the oven was set to 35-37°C for 48 hours, but no microorganism growth was verified. Since the generation time can vary among microorganisms (Tortora et al. 2000), some microorganisms have a slower growth rate than others. Therefore, it was decided to define an incubation time of 7 days for bacteria.

For the fungi culture, the incubation temperature was set to 25-30°C for four days (Andrade et al. 2003, Paschoalino & Jardim 2008, Brasil 2003). The Petri dishes used to evaluate the reactor efficiency were analyzed by counting the number of CFUs present in each one of the triplicates. After the counting, a simple average calculation was performed for each of the circuits, photocatalysis, photolysis, and control, allowing us to compare the results and assess which presented the highest biocidal effect compared to the control.

Temperature and humidity were measured with a hygrometer (Cotronic Technology Limited). Before each test, the interior of the photolysis and control tubes were cleaned with 70% alcohol. It was impossible to sanitize the photocatalytic tube with alcohol due to the deposition of TiO₂. However, before the second test, a new KOH wash was performed. Also, before the tests, the lamps were turned on for about an hour, so the microorganisms in the support were inactivated. The tape was sanitized with 70% alcohol on its underside during handling and introduction into the tubes. The tape was not wholly sanitized with 70% alcohol to avoid damaging LEDs and other structures.

Statistical analysis of data

Statistical data analysis was performed by calculating the simple means of triplicates and the standard deviation using Microsoft Excel and GNU PSPP software.

RESULTS AND DISCUSSION

Reactor sterility assessment

Evaluation of the equipment sterility

The equipment’s sterility was evaluated by sampling with a swab before tests 2, 3, and 4. On the one hand, the results presented in Table II indicated a higher contamination level than in the tubes with the LED strip. Despite cleaning the strip with 70% alcohol, submitting this material to rigorous decontamination was impossible due to its roughness. On the other hand, the control tube could be easily sanitized before each test with the reactor, as it contained no material inside.

Table II
Sterility test for each tube.

	Test 1		Test 2		Test 3
Sample	Bacteria	Fungi	Bacteria	Fungi	Bacteria	Fungi
	CFU	CFU	CFU	CFU	CFU	CFU
1-Photocatalysis	4	2	15	14	3	10
2-Photolysis	5	2	1	1	5	29
3-Control	2	0	1	26	1	3

Furthermore, an increase in the level of contamination in the photocatalysis tube was also observed when Luffas were inserted. This result might be related to the impossibility of cleaning this material with 70% alcohol. Due to the high number of fungal colonies in the control tube, it was sanitized again.

The obtained results indicated that there was no pattern of contamination in the tubes. Nevertheless, in the test 2 and test 3 verification, a higher number of fungal CFUs was found compared to bacteria, possibly because these microorganisms are more sporogenic, spreading more easily (Tortora et al. 2000). Before samplings 5 and 6, no assessment of sterility was performed, as the last tests were not performed with artificial contamination.

Experiment of artificially contaminated atmosphere disinfection

The residence time results in each circuit show that the time was longer in test 2 than in test 1. This occurred because test 2 was carried out with 0.5 atm of air injection, half the time in test 1 (1 atm). This means the air remained in the system for longer. In test 3, the residence time was shorter compared to test 2, even with the same air pressure (0.5 atm). This may have occurred due to the introduction of the Luffas in the photocatalysis tube, resulting in a shorter residence time inside the acrylic box, even though more air was directed to the photocatalysis tube.

The longer residence time indicates that the microorganisms remained inside the reactor for longer, which could benefit the photocatalysis and photolysis processes. However, variations in residence time in this study did not indicate an influence on decontamination efficiency (Table III).

Table III
Residence time in seconds.

Sample	Test 1	Test 2	Test 3	Test 4, 5 and 6
1-Photocatalysis	40,28	51,26	33,17	35,24
2-Photolysis	35,24	51,26	28,19	28,19
3-Control	29,68	33,17	28,19	26,85

The results of test 1 showed that, for bacteria and within 48 hours of incubation, only one colony grew on one of the triplicate plates of the photolysis tube. On none of the other plates, bacterial colonies were observed. However, after seven days, the mean values demonstrated a greater decontamination efficiency of the photocatalysis tube (Figure 2). This is in line with the results obtained by Paschoalino & Jardim (2008) for bacteria, where photocatalysis inhibited bacterial growth more efficiently than photolysis, and both were more efficient than the control. Zacarías et al. (2020), in a similar study using TiO₂ irradiated with UVA, entirely removed a high load of microorganisms in the air stream in one hour of the experiment.

Figure 2
Results of samplings carried out in an artificially contaminated atmosphere.

As for fungal contamination, it was possible to observe that the photolysis tube was more efficient than the photocatalysis tube, which was more efficient than the control tube. Photolysis was also more efficient than photocatalysis for fungi in three of the four experiments by Paschoalino & Jardim (2008).

Both high standard deviations in experiments 1 and 3 mean that the samples varied greatly. This probably occurred because a higher number of CFUs was found in these tests (Figure 2). This considerably higher number of fungal than bacterial CFUs may have occurred due to the characteristics of these microorganisms that spread more quickly due to their morphology and the production of spores (Tortora et al. 2000).

Rodrigues-Silva et al. (2017) reported that fungal inactivation was slower than resistant bacteria in their experiments, followed by Gram-positive and Gram-negative bacteria. Nevertheless, the research described here did not aim to compare the results of bacteria with the results of fungi but rather to compare photocatalysis, photolysis, and control.

Due to the high number of fungal CFUs found in test 1, it was decided to reduce the airflow at the system’s inlet to increase the residence time. The results of test 2 showed that, for bacteria, the photocatalysis tube showed higher decontamination efficiency than photolysis and the control tube (Figure 2).

Regarding fungal contamination, it was possible to observe that the photocatalysis was more efficient than the photolysis tube, which was more efficient than the control tube. The difference between the values of bacteria and fungi was not expressive. This may have occurred because the contamination of the acrylic box was not the same. Therefore, there was probably less contamination in the injected air in test 2. Figure 2 shows the results of the means and standard deviation obtained in the tests.

Test 3 introduced Luffas with PO/TiO₂ into the photocatalysis tube. Concerning bacteria, the results did not allow us to conclude improvement in inactivation, as the bacterial concentration in the control was zero. The contamination of the air injected through the acrylic box was possibly not enough to contaminate the air in the control tube. This result disagrees with the results found by Rodrigues-Silva et al. (2017), where photocatalysts under UVA irradiation caused a significant inactivation of bacteria.

Regarding fungal contamination, it was possible to observe that the photocatalysis tube was more efficient than the photolysis tube, which was also more efficient than the control tube (Figure 2). The third experiment showed that using Luffas was positive for the inactivation of fungi, as it degraded more than half of these microorganisms compared to the control tube. This result may be directly related to the increase in the surface area of contact between the catalyst, UVA light, and microorganisms.

During the three tests, the laboratory temperature remained between 24.7°C and 28.2°C, whereas the humidity remained between 41 and 57%. Since the most commonly found microorganisms are mesophiles, that is, they have an optimal growth temperature between 25 and 40°C (Tortora et al. 2000), it is possible to assume that the UNISC laboratory presented an ideal temperature for the growth of microorganisms on the sampling days. Humidity is also necessary for growth since microorganisms remove most of their soluble nutrients from the water present in the environment (Tortora et al. 2000).

Natural-atmosphere disinfection experiment - UNISC laboratory and Basic Health Unit

Tests 4, 5, and 6 showed the same residence time. This happened because, in these tests, there was no change in air injection or any other system configuration (Table III). Variations in residence time did not indicate an influence on decontamination efficiency.

Test 4 was conducted at the Effluent and Solid Waste Treatment Laboratory of the Universidade de Santa Cruz do Sul. Air was injected into the reactor through the coolers. The results showed that, for bacteria, there was no growth on any plate (Figure 3). Concerning the fungi, no contamination was observed in the photocatalysis and photolysis tubes, and no CFU was verified in the control tube. Both the values obtained with bacteria and fungi may be related to the low contamination of the ambient air in the laboratory since there was little circulation of people. These results did not allow us to conclude if possible improvements in the inactivation of microorganisms occurred.

Figure 3
Results of sampling carried out in real atmosphere – laboratory and BHU.

The CFUs found in photocatalysis and photolysis tubes may be related to microorganisms in the reactor due to the difficulty in cleaning the LED strip. Figure 3 shows the results of the means and standard deviation obtained in test 4.

The laboratory temperature on test day 4 was 26.3°C, within the optimal microbial growth temperature (between 25 and 40°C) (Tortora et al. 2000), while the air humidity was 61%. Tests 5 and 6 were conducted at the HBU, and air injection into the reactor occurred through the coolers.

The results of test 5 for bacteria showed that photolysis was more efficient than photocatalysis. However, both presented more UFCs than the control (Figure 3).

As for fungal contamination, the photocatalysis tube showed a lower mean than the control tube, whereas photolysis presented a higher average. This result is not in line with what was expected, which would be the inactivation of fungi in the photolysis tube through the action of UVA radiation. Figure 3 shows the results of the means and standard deviation obtained in the tests.

The results of test 6 for bacteria showed the inactivation of microorganisms in the photocatalysis compared to the control. Regarding fungal contamination, the results indicated the efficiency of photocatalysis (Figure 3). The average of CFUs in the photolysis tube for bacteria and fungi was higher than in the control tube. This result did not match what was expected, as in test 5.

During tests 5 and 6, the temperature of the UBS remained between 25.5 and 26.7°C. Knowing that the optimal temperature for the growth of microorganisms varies between 25 and 40°C (Tortora et al. 2000), it is possible to state that the UBS presented the ideal temperature for the growth of microorganisms on the days that sampling occurred. Air humidity remained between 60 and 51%. The low number of CFUs in tests conducted in a natural atmosphere indicates little contamination in the environments on the sampling days.

There are numerous mechanisms elucidating how the biocidal action of heterogeneous photocatalysis using TiO₂ occurs (Rodríguez-González et al. 2020). According to Czumaj et al. (2020), cells treated with the semiconductor show a lower coenzyme A (CoA) concentration, indicating direct oxidation, inhibiting cellular respiration, and consequently killing the microorganisms.

Armaković et al. (2022) state that the oxidation caused by the TiO₂ catalyst and UV light disrupts the cell wall and plasma membrane, thereby increasing cellular permeability and leading to the efflux of intracellular content, destroying microorganisms. García-Martín et al. (2024) explain that OH• radicals act as germicides due to their high oxidation power and low selectivity.

Titanium is an excellent photocatalyst with diverse applications due to its stable chemical structure, biocompatibility, physical, optical, and electrical properties, and relatively low cost and hydroxylation power (Bejaoui et al. 2023). A drawback of TiO₂ is its low absorption in the visible light spectrum. With a bandgap energy of about 3.0 to 3.2 eV, it absorbs only in the UV range, corresponding to wavelengths below 380 nm, approximately 6% of the solar spectrum (Ortiz et al. 2019).

According to Schwarze et al. (2021), TiO₂ catalyst deposition can be applied in suspension (aqueous phase) or on internal supports in reactors (aqueous/gaseous phase). Studies by Mei et al. (2023) and Nair et al. (2023) highlighted that one of the most common processes involves suspending the catalyst in solvents, leading to its deposition on the internal walls of reactors, followed by solvent removal treatment.

Some researchers have investigated using polymeric matrices to fix TiO₂ in reactors to preserve the photocatalytic activity of the systems. Giannakis et al. (2024) developed TiO₂ films in the anatase phase by the sol-gel process and spin-cast on laser-microstructured silicon substrates to form photocatalytic surfaces of increased activity.

CONCLUSIONS

The presented study evaluated the efficiency of AOPs using heterogeneous photocatalysis in reducing microorganisms from the air in a confined environment. The experiments were conducted using a photolytic and photocatalytic reactor, applying TiO₂ as a catalyst, UVA LED lamp, and Luffas.

The efficiency of both photolytic and photocatalytic reactors was compared in artificially contaminated and natural atmospheres (UBS). In an artificially contaminated atmosphere, the photocatalytic reactor proved efficient in two of the three tests for bacteria. For fungi, the same reactor presented satisfactory results in all three experiments. In the natural atmosphere, it also proved efficient in two out of the three tests, both for bacteria and fungi. The low level of contamination found in the control circuit in the natural atmosphere caused difficulty in carrying out the inactivation comparison. Using a luffa-based support medium was positive concerning fungi; however, as for bacteria, this resource proved inefficient.

ACKNOWLEDGMENTS

The authors would like to thank all those who contributed to this work, especially for funding agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), (Process n° 427402/2016-6), Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), (Process n° 8888710390/2022-1) and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS).

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Publication Dates

• Publication in this collection
  15 Nov 2024
• Date of issue
  2024

History

• Received
  22 Mar 2024
• Accepted
  11 Aug 2024