Development of a UV-LED Photoreactor for Colorant Degradation in Water

Abstract

This work analyzes the performance of a photoreactor built with UV-LED technology. For this task, a UV-LED wavelength of 365 nm was used as an irradiation source, and it was electrically and spectrally characterized to ensure correct operation. To evaluate the functionality, the photoreactor was tested on the degradation of Rhodamine B (Rh B), a dye commonly used in the textile industry. The experiment was conducted under optimal conditions, using a concentration of 17 ppm of Rh B and 100 mg of zinc oxide (ZnO) as a photocatalyst in a glass reactor. The mixture was continuously stirred for 120 min, achieving 99.42% efficiency. The results showed that the UV-LED photoreactor performs well in activating ZnO for the removal of Rh B from the solution, highlighting its potential for treating textile industry wastewater. The use of LEDs offers advantages such as energy efficiency and lower environmental impact compared to traditional UV lamps. ZnO, known for its reactivity under UV light, acted as a stable photocatalyst, ensuring complete degradation of the dye without producing harmful by-products. This method provides an efficient approach to dye removal in wastewater treatment, promoting cleaner and more sustainable industrial practices.

1. Introduction

According to the National Water Commission (CONAGUA), in 2021, the industrial sector was identified as one of the primary drivers of economic growth and development [1]. Around 19% of extracted water is globally allocated to industrial activities, with more than half of this volume used by thermoelectric plants in cooling processes. Between 2015 and 2020, the industrial water usage increased by 25.2%, rising from 2197.55 to 2751.45 hm³ nationwide [2].

Various industries, including textile, paper, cosmetics, and pharmaceuticals, discharge significant amounts of dye-contaminated wastewater into the environment without adequate treatment. The textile industry is the primary source of these harmful compounds [3]. This wastewater is produced by different industrial activities, such as poultry processing, pharmaceuticals, tanning, and paper pulp manufacturing, and it typically contains persistent organic pollutants, heavy metals, and toxic substances. These discharges can lead to the eutrophication of groundwater and may contain precursors to carcinogenic compounds [4]. Among these pollutants, dyes such as Rhodamine B stand out due to their persistence in aquatic environments, their potential to cause long-term harmful effects, and their resistance to degradation, which allows them to accumulate in ecosystems for extended periods [5].

The United Nations established the 17th Sustainable Development Goals (SDGs) in 2015 as an international plan to promote global cooperation in achieving shared sustainability targets [6]. There are specific technologies that have been developed for wastewater treatment, and selecting the most appropriate one depends on several factors, such as the type of dye, the variety and quantity of contaminants, the wastewater flow, and the production regime. Evaluating these aspects, it is critical to ensure efficient and effective treatment [7]. Conventional water treatment methods, such as activated carbon adsorption and coagulation/flocculation, are effective but are more suitable for treating contaminants at a higher concentration rate [8].

During the previous years, advanced oxidation processes (AOPs), such as photocatalysis, ozonization, ultrasonic oxidation, Fenton, and photo-Fenton processes, have been used as techniques to treat hazardous wastewater. Currently, they have been applied to remove non-biodegradable or toxic substances from water [9]. Process photocatalysis is a type of treatment for degrading organic pollutants, which is low-cost, high efficiency, has complete mineralization capability, and an absence of secondary pollution [10]. Photocatalysts, including this process, are biochar and titanium dioxide (TiO₂), which are graphene-based materials. The latter has been used for removing dyes during water purification. In addition to TiO₂, other metal oxide semiconductors have also demonstrated notable photocatalytic performance in degrading harmful organic compounds into less toxic substances under light exposure [11]. Among them, ZnO stands out for its impact on water remediation due to its distinctive charge carrier behavior upon band gap excitation and its efficiency in generating reactive oxygen species (ROS) in aqueous environments [12,13].

The chemical reactors enable the interactions of photons, photocatalysis, and reactants under controlled conditions while also ensuring the efficient collection of reaction products generated through the physicochemical transformations occurring during photocatalysis [14]. To ensure uniform and controlled irradiation of the catalysts, several considerations must be taken into account when integrating all components into a single system [15]. These include the arrangement of light-driven sources, their emission wavelength and intensity, the distance between the light source and the catalyst surface, and the geometry photoreactor. All of these factors contribute to maximizing light utilization and enhancing the efficiency of light-driven oxidation and reduction processes using artificial sources [16].

Therefore, there are various possible photoreactor designs, which typically require a light source with wavelengths ranging from 250 to 385 nm, either from solar radiation or lamps [17]. However, traditional photoreactors are equipped with a conventional UV mercury lamp, which has low efficiency, a short lifetime, and is highly polluting. In the last decade, the development of new light-emitting diodes in the UV region (UV-LEDs) with high powers has experienced rapid growth, offering multiple advantages over traditional UV lamps. Due to their characteristics, UV-LEDs have been implemented in several applications, from the removal of organic compounds present in water [18] to food decontamination [19].

In water treatment, the implementation of UV-LEDs has gained importance, with a growing number of studies demonstrating their effectiveness in disinfection [20]. Eskandarian et al. [21] reported a design with arrays of multiple UV-LED chips (UVCLEAN LED) emitting different wavelengths: 365 ± 10 nm for UVA, 365 ± 5 nm for high-intensity UVA (UVAH), 300 ± 5 nm for UVB, and 260 ± 10 nm for UVC. A 140 mm crystallizer was used as a photoreactor, placed beneath the UV-LED lamps. To ensure homogeneous mixing during the process, the reactor was continuously stirred using a magnetic stirrer. In the reported study [22], the optimal conditions for the degradation of Rhodamine B (Rh B), which ensure an optimum outcome, are 20 mg/L Rh B and 0.3 g ZnO/100 mL; however, this study was conducted using sunlight and modifying other variables. Piras et al. [23] achieved a photodegradation efficiency of 95.9 ± 0.1% using a Philips mercury lamp at 368 nm, with an Rh B concentration of 4 mg/L and 5 mg of ZnO catalyst in 60 min. Nagaraja et al. [24] also used sunlight irradiation and reached 95% degradation after 80 min, using 10 ppm of Rh B and 0.2 g of ZnO. Zhuang et al. [25] reported 85.5% degradation of a 10 ppm Rh B solution using 50 mg of a thiocyanate-templated catalyst synthesized with benzyl viologen under a 300 W xenon arc lamp after 240 min of irradiation. In contrast, Armaković et al. [26] evaluated five different TiO₂/Al surfaces and found a maximum degradation of only 16% for Rh B after 300 min under a 5 W UV-LED lamp. Other studies report different efficiencies using different light sources, as seen in

Table 1. Photocatalytic degradation of Rhodamine B using ZnO under different light sources.

Catalyst	Light Source	Experimental Conditions	RhB Degradation (%)	Reference
T700-ZnO (calcined at 700 °C)	UV light	10 mg/L RhB, 120 min	99.12%	[27]
Ag–ZnOsf (flower-like ZnO + Ag)	UV + SPR Ag	35 min	RhB: 94.2% (ZnO sf), 99.7% (Ag–ZnOsf)	[28]
ZnO NS (nanosheets, 800 °C annealed)	UV light	120 min	98%	[29]

.

The primary aim of this work is to present and validate the design and configuration of a novel photoreactor equipped with high-power UV-LEDs. Rather than conducting an exhaustive photocatalytic study, the focus is placed on demonstrating the functionality and potential applicability of the photoreactor in environmental remediation. The design considerations, operational parameters, and modular configuration are highlighted as key aspects of its novelty. The photoreactor was characterized electrically and spectrally to achieve better operating conditions.

2. Materials and Methods

2.1. Keyword Co-Occurrence Analysis

A keyword co-occurrence map was generated using the Fruchterman-Reingold algorithm to explore prevailing research trends related to light sources in photocatalytic reactors. This bibliometric analysis revealed a growing interest in the use of high-power UV-LEDs due to their energy efficiency, design flexibility, and environmental advantages over conventional mercury-based systems.

2.2. Photoreactor

The direct photoreactor, designed and built by the research team (Figure 1a), is a compact and versatile module with several key features. It operates within an input voltage range from 4.5 V to 36 V and offers an adjustable output voltage between 2 V and 35 V, providing flexibility for various applications. With a current output rating of 0 to 1 A, it is suitable for low-to-moderate power uses of 12 W. The photoreactor has dimensions 18 × 30 cm with a thickness of just 6 mm. The photoreactor is slim and easy to integrate into setups with limited space. Its body is constructed from durable black acrylic, which gives it a sleek appearance while effectively blocking UV light. The photoreactor is equipped with a UV light source operating at a wavelength of 365 nm, making it ideal for applications such as UV curing, photochemical reactions, and sterilization processes. The photoreactor includes an LED with an aluminum heatsink, an LCD interface for tracking reaction time and solution temperature, a cooling fan to maintain system stability, and a temperature sensor for monitoring (Figure 1b). This equipment is likely suited for scientific, industrial, or technical applications where controlled voltage, current, and UV illumination are required.

2.3. Photoreactor Characterization

For the characterization of the photoreactor, the following equipment was used: a benchtop digital multimeter (34450A, Keysight Technologies, Inc., Santa Rosa, CA, USA), a DC power supply (2231A-30-3, Keithley Instruments, Inc., Solon, OH, USA), and a USB4000 UV-VIS spectrometer (Ocean Optics Corp., Orlando, FL, USA). The measurement of emission wavelength was obtained by placing the spectrometer inside the photoreactor and, using the Spectra Suite software version 2.0.162 (Ocean Optics Corp.), the spectrum was recorded. Additionally, for the electrical characterization of the photoreactor, a PM100D radiometer (Thorlabs Corp., Newton, NJ, USA) was used with an S425C thermal sensor (Thorlabs Corp.) to measure the radiant flux with respect to the polarization current of the LED. The sensor was placed inside the photoreactor, and the current intensity was adjusted in 50 mA increments to record accurate measurements, this using the DC power supply and the multimeter described above.

Likewise, the measurement of the irradiation pattern was carried out at distances of 0.5 cm along the X and Y axes until the illumination surface of the photoreactor was fully mapped. A PM100D radiometer with an S425C sensor was used to measure the radiant flux.

2.4. Evaluation of System with Rhodamine B

Chemicals of reactive grade were used, including Rhodamine B (Sigma Aldrich, St. Louis, MO, USA, HPLC), for the experiments. For the quantification of Rhodamine B, a calibration curve was performed preparing concentrations of 20 ppm, 15 ppm, 10 ppm, 7.5 ppm, 5 ppm, 3 ppm, 1 ppm, 0.5 ppm, and 0.1 ppm. Additionally, prior to the photocatalysis test, commercial ZnO (Fagalab, Sinaloa, NM, USA) was dried at 100 °C for 1 h [30] to ensure proper activation and performance during the experiment. The material exhibits the typical hexagonal wurtzite structure with an estimated crystallite size of 33.3 nm. For the photocatalytic reaction, a concentration of 17 ppm of RhB was prepared and 100 mL of the solution was used to which 100 mg of ZnO was added. The mixture was magnetically stirred in the dark for 30 min to reach equilibrium [5].

After that, the irradiation was initiated using an LED light source placed 5 cm away from the mixture. Aliquots were collected every 20 min until the experiment reached a total duration of 120 min. The experiment was conducted under ambient conditions. Aliquots of 2.5 mL of reaction product were taken, filtered (0.45 μm, Nylon syringe filters, Lab Instruments), and analyzed to maximum band at 555 nm using quartz cells in a JENWAY 6850 UV/Vis spectrophotometer. The dye removal efficiency was calculated using the following equation:

$$\mathrm{Removal}\mathrm{Efficency}\left(\%\right)={\displaystyle \frac{C_o - C_t}{C_o}}\times 100$$

(1)

where $C_o$ and $C_t$ represent the dye concentrations in the solution (ppm) at reaction times 0 and t min, respectively [31].

The data of −ln (C/C₀) vs. t for the rhodamine B concentration were determined using the pseudo-first-order kinetic model as follows:

$$\ln \left({\displaystyle \frac{C}{C_0}}\right)=kt$$

(2)

where C and C₀ are the concentration at time t (minutes) and the initial concentration, respectively; and k (min⁻¹) is the apparent rate constant [32].

3. Results and Discussion

3.1. Co-Occurrence Map

The co-occurrence map highlights that research in photocatalysis focuses significantly on the use of different light sources for photoreactors (Figure 2), with light-emitting diodes (LEDs) standing out as the central node of the analysis. This term is strongly linked to “photoreactors”, “photocatalytic degradation”, “photodegradation”, and “titanium dioxide”, indicating that UV-LEDs are currently among the most widely used and studied light sources in advanced oxidation processes for water treatment.

In the map, different colors represent thematic clusters that group related concepts. For instance, the blue cluster emphasizes terms associated with water treatment and photoreactors, while the green cluster highlights topics related to light, photocatalytic activity, and materials. The red cluster contains terms such as “catalysis”, “chemistry”, and “photochemistry”, reflecting fundamental chemical processes. The yellow cluster groups terms related to irradiation and efficiency, and the purple and cyan clusters are associated with wastewater treatment and photocatalyst types.

Additionally, terms such as “irradiation”, “UV lamp”, and “xenon lamp” appear in distinct thematic clusters, suggesting the coexistence and comparison with traditional light sources such as mercury lamps (Hg lamps) and xenon lamps (Xe lamps). These are associated with topics such as efficiency, types of pollutants treated (e.g., bisphenol A or antibiotics), and the design of the illumination system within photoreactors.

3.2. System Characterization

In the behavior of radiant flux, the current intensity was varied in increments of 50 mA to register the radiant point of the LED. In Figure 3a, it is observed that the current increase is proportional to the radiant flux; for example, at 1 A, a radiant flux of 1.45 W is produced.

In Figure 3b, the spectral characterization is shown, which was based on measuring the emission wavelength and selecting the maximum emission point. This ensures that as the current increases in 100 mA intervals, the emission peak remains stable at 366.47 nm, with a standard deviation of ±0.55 nm. In this way, the current increment does not cause a significant variation in the emission peak, contributing to the device’s stability under different operating conditions.

The purpose of mapping the illumination of the photoreactor is to identify the area with the highest radiant flux to position the glass reactor and conduct the experiments. As shown in Figure 4, a graphical representation of the radiant flux is presented in both 2D and 3D, making it easier to visualize the areas with greater irradiation intensity.

3.3. Photodegradation of Rhodamine B

The absorption spectrum of rhodamine B shows a characteristic absorption band at 555 nm [33]. From the calibration curve of Rh B, the equation of the straight line was obtained from the absorption spectrum at various concentrations (Figure 5), which allowed the determination of the sample concentrations over time and confirmed analyte adsorption onto ZnO. A shift of the absorption band from 555 nm to 530 nm was observed, attributed to N-deethylation a degradative reaction that occurs selectively depending on the molecule’s adsorption state on the catalyst surface [34,35].

As shown in Figure 6, degradation is observed every 20 min until reaching 120 min of UV exposure. This result indicates that ZnO, having a band gap of 3.25 eV [36], was activated at an excitation wavelength of 365 nm [37,38], facilitating the photodegradation of Rh B. This visual transition, from the initial intense magenta hue to a nearly colorless solution, reflects the gradual photodegradation of the dye. In both conditions—(a) without air bubbling and (b) with air bubbling—decolorization is evident, yet the difference in degradation efficiency between the two setups is clearly distinguishable.

The clear reduction in color intensity at each time interval validates the role of UV light in activating the photocatalyst and demonstrates the stepwise nature of the degradation process. Although visual observations alone do not provide molecular-level information, they offer an immediate and intuitive confirmation of photocatalytic activity. These results align with UV-Vis spectroscopic data and further support the conclusion that the ZnO-based system, especially under air-assisted conditions, holds promise for effective photodegradation of organic contaminants.

Figure 7a shows the degradation efficiency of Rhodamine B under different conditions: UV light alone, UV with ZnO, and UV with ZnO plus air bubbling. The efficiency reaches 94.45% when using UV with ZnO and further improves to 99.42% upon incorporating air bubbling within just 120 min. In contrast, the system with only UV irradiation (photolysis) exhibits a much lower efficiency of only 16.95%, confirming that the presence of the catalyst and the additional supply of dissolved oxygen via air bubbling are key factors in enhancing the photocatalytic activity. This suggests that the direct photolysis of Rh B under UV light is minimal, and that no reactive species are generated in the absence of a catalyst; therefore, no active photocatalytic reaction is considered to occur under these conditions. When ZnO is incorporated as a photocatalyst, a significant improvement in the degradation efficiency is observed. This is due to the activation of ZnO by UV radiation (λ = 365 nm), which generates electron-hole pairs capable of producing oxidative species such as hydroxyl radicals and superoxide radicals that attack the dye molecule.

In Figure 7b, the degradation rates are constant at 23.8 × 10⁻³ min⁻¹ for UV with ZnO and 27 × 10⁻³ min⁻¹ when air bubbling is added. These values demonstrate a significant improvement in the degradation rate. However, the process follows a pseudo-first-order kinetic model, as evidenced by the high correlation coefficients obtained (

Table 2. Rate constants (k) for photodegradation of Rhodamine B in the two systems.

System	k (min−1)	R2
ZnO + UV	23.8	0.89
ZnO + UV + Air	27.8	0.92

). This suggests that the reaction kinetics are adequately described by this model, although slight deviations may still occur due to factors such as charge recombination or limited dye adsorption on the catalyst surface.

When comparing these results with those reported by other authors, the system evaluated in this study shows superior performance. Araujo et. al. [39] reported a degradation efficiency of 83% for Rhodamine B (5 mg/L, 500 mL) using 0.28 g of TiO₂ and an 11 W UV lamp in quartz tube at 20 °C, achieving this efficiency in 5 h (300 min) with a rate constant of 6.17 × 10⁻³ min⁻¹. Meanwhile, Natarajan et. al. [40] achieved 95% degradation in 360 min using a 2.08 × 10⁻⁵ M Rhodamine solution and 1.6 g/L of TiO₂-P25, employing five UV-LEDs (390–410 nm) mounted on a circular acrylic plate at an intensity of 10–12 mW. Zhang et. al. [41] reported that 100 mg of Au-ZnO in Rhodamine B (1 mg/L, 200 mL) in a quartz beaker, using a 300 W xenon lamp as a visible light source (420–780 nm), obtained a degradation of 92% under visible light irradiation after 300 min.

In comparison, the system developed in this work reaches similar or even higher efficiency in a considerably shorter time (120 min) and with a significantly higher rate constant (up to 27.8 × 10⁻³ min⁻¹) using UV-LED (365 nm) at 700 mW, highlighting a remarkable enhancement in photocatalytic activity. Although only one fixed Rhodamine B concentration (17 ppm) was used, along with 100 mg of catalyst, these parameters were selected based on previously reported studies, where this catalyst loading is generally considered optimal to avoid solution cloudiness and ensure uniform light distribution [40,42]. The use of this single concentration aimed to validate the performance of the photoreactor as an effective and sustainable alternative, yielding favorable results. This improved performance can be attributed to the synergy between the use of the catalyst and the air bubbling, which increases the availability of oxygen in the solution and promotes the generation of reactive species responsible for the degradation of the contaminant. However, the effect is mainly attributed to the cleavage through N-deethylation of Rhodamine B, although significant results were achieved in the removal of pollutants present in water. Although no reuse experiments were conducted in this study, previous research has demonstrated that pure ZnO can maintain its photocatalytic activity over multiple cycles when applied to Rhodamine B degradation. For instance, Kedruk et al. [43] reported that the catalyst performance remained almost unchanged after five cycles, while De Souza et al. [44] achieved over 90% degradation efficiency even after five reuse cycles using ZnO with RhB dye solutions. UV mercury lamps have a lifespan of 1000 to 2000 h, an initial cost ranging from $15 to $30, and require frequent maintenance due to constant replacement and the handling of hazardous waste such as mercury. In comparison, high-power UV-LEDs have a much longer lifespan, ranging from 10,000 to 50,000 h, with an initial cost ranging from $40 to $100, and require minimal maintenance, as they contain no hazardous materials and are long-lasting. Although a TOC analysis was not performed in the present study due to the lack of such equipment, a study reported by Ramanathan et al. [45] showed that during the photodegradation of Rhodamine B, successive deethylation of the aromatic ring leads to intermediates exhibiting hypsochromic shifts in the UV-Vis spectra, without the appearance of new absorption bands in the visible region. Additionally, a gradual decrease in TOC values was observed, indicating the possible complete mineralization of RhB during the reaction.

Various photoreactors have been reported in the literature, each offering specific advantages depending on their design, light source, and intended application. The following is a comparative analysis of selected systems. Roibu et al. [46] developed a microreactor system equipped with visible LEDs arranged to ensure uniform light distribution through optimized emission angles and distances. Although their design enhances irradiance homogeneity, it is intended for micro-scale applications and lacks flexibility in light source replacement or power modulation. Levine et al. [47] evaluated the performance of UV-A LEDs compared to mercury vapor lamps in a bench-scale reactor using ethanol as a model pollutant. Their results demonstrated energy efficiency and comparable photocatalytic activity; however, the system relied on fixed LED configurations with limited programmability and lacked safety features for UV exposure. Aubineau et al. [48] proposed a standardized, home-built photoreactor using Kessil PR160L LED lamps (427 nm). While this design is reproducible and suitable for photoredox chemistry, it does not include radiant flux control or interchangeability of light sources. Moreover, it is not intended for photocatalytic wastewater treatment. Casado et al. [49] developed an annular photoreactor optimized using CFD simulations, incorporating multiple UV-LEDs (LZ1-00UV00, LED Engin Inc., San Jose, CA, USA) to enhance light distribution. The system provides precise control over the radiation field and was experimentally validated using cinnamic acid. However, its practical application is limited by the need for advanced simulation tools and specific geometric adjustments, which reduce its scalability and adaptability compared to simpler configurations. Tugaoen et al. [50] designed a cylindrical quartz photoreactor for degrading sulfamethoxazole using TiO₂-coated glass fibers and four 15 W low-pressure mercury lamps (254 nm) arranged symmetrically. The setup ensures consistent UV exposure and supports continuous treatment of real wastewater. However, the reactor lacks modularity and does not support interchangeable light sources, limiting its adaptability.

Compared to these systems, the photoreactor developed in this study offers a balance of simplicity, flexibility, and safety. It employs a single high-power UV-A LED (365 nm) with programmable control of radiant flux, eliminating the need for complex LED matrices or external cooling systems. The light source is interchangeable, allowing adaptation to different photocatalytic processes. Unlike most of the aforementioned systems, the proposed reactor is scalable, maintaining geometry and photonic characteristics across volumes, and it includes a built-in safety sensor that disables the LED when opened improperly, preventing accidental UV exposure. These features make the proposed system a cost-effective, modular, and secure platform for photocatalytic studies in environmental applications.

4. Conclusions

This study makes a significant contribution to research on low-cost photoreactors equipped with high-power LEDs for the photodegradation of dyes under UV radiation. The implementation of these LEDs not only represents a cost-effective option but also proves to be effective in the degradation of compounds such as Rhodamine B, reinforcing the viability of their large-scale use.

Additionally, UV light stands out as an effective alternative for wastewater treatment, especially in industrial applications requiring efficient contaminant removal. This approach not only helps reduce environmental impact but also encourages the development of sustainable technologies, which could be incorporated into broader water treatment strategies in the future.