Photocatalytic degradation of reactive black 5 from synthetic and real wastewater under visible light with TiO₂ coated PET photocatalysts

Abstract

The photocatalytic removal of Reactive Black 5 (RB5) was investigated using a titanium dioxide-polyethylene terephthalate (TiO₂-PET) catalyst under visible (VIS) light. The sol-gel method was employed for the fabrication of the TiO₂-coated PET catalyst, which was then characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and elemental mapping (MAP) analysis. This study examined the reaction kinetics using the one-factor-at-a-time (OFAT) approach and evaluates the effects of various parameters, including pH (3–11), catalyst dosage (0.1–1 g L^− 1), contact time (15–120 min), RB5 concentration (10–50 mg L^− 1), hydrogen peroxide (H₂O₂) content (2–100 mM), purging gases, organic compound types, and ionic strength, on the photocatalytic removal of RB5. Under optimal conditions (pH= 3, [RB5]_°= 20 mg L^− 1, nanocatalyst dosage= 0.5 g L^− 1), 99.99% of the dye was removed after 120 min. Increasing the RB5 concentration (10–50 mg L^− 1) resulted in a decrease in the observed reaction rate constant (k_obs) from 0.052 to 0.0017 min^− 1, while the calculated electrical energy per order (EEO) increased from 11.08 to 338.82 kWh m^− 3. Furthermore, the total operating cost of the light emitting diode (LED)/TiO₂-PET process (3 USD kg^− 1) was lower than that of other photocatalytic processes, including LED/TiO₂ (4.73 USD kg^− 1), LED/PET (40 USD kg^− 1), and LED (63.16 USD kg^− 1). The removal of RB5 was negatively affected by the presence of H₂O₂, O₂ and N₂ gases, organic compounds, and ionic species. Radical quenching experiments confirmed that hydroxyl radicals (^·OH) were the dominant reactive species responsible for RB5 degradation. The RB5 removal efficiency using the LED/TiO₂-PET method (99.99%) was significantly higher than that of the LED/TiO₂ method (63.42%). Desorption experiments demonstrated excellent catalyst stability, maintaining catalytic activity for up to five sequential cycles. GC-MS analysis identified several intermediate degradation products, including 1,2-benzenedicarboxylic acid, benzoic acid (2-amino-, methyl ester), benzene [(methylsulfonyl) methyl], phenol, 4-naphthalenedione, acetic acid, and propionic acid. Moreover, the removal efficiency in drinking water samples was approximately 63.31%, whereas for real textile wastewater samples, it reached 96.66%. Toxicity tests conducted on the final treated solutions confirmed no toxicity toward Daphnia magna, demonstrating the effectiveness of the LED/TiO₂-PET method in degrading both RB5 dye and its toxic by-products.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-95091-x.

Introduction

The global issue of water pollution is driven by the increasing production of chemicals across various industries and the rapid growth of the world’s population^1–5. Wastewater discharged from textiles, leather, pharmaceuticals, and paper industries contains various dyes^6–8. Textile production is a major source of color-causing pollutants, as even trace amounts of dyes (less than 1 part per million) can significantly alter water clarity, gas solubility, and other characteristics of rivers, lakes, and other water bodies^9–11. Azo dyes dominate the dye industry, accounting for over 70% of total annual production. Characterized by nitrogen double bonds (N = N) in their chemical structure, they are among the most widely used commercial reactive dyes^12,13. The vivid coloration of wastewater not only creates an unpleasant appearance but also poses a threat to both the environment and human health^14,15. Remazol Black B (Reactive Black 5) is a toxic, mutagenic, and poorly biodegradable compound with potential carcinogenic effects^16,17. Reducing or eliminating the release of colored wastewater is essential^16,17. Traditional methods for treating dye-contaminated water have historically relied on adsorption¹⁸, filtration¹⁹, photo degradation, biodegradation²⁰ and coagulation²¹. However, due to the chemical stability of dyes, these treatments often have drawbacks, including high costs, limited effectiveness, and an inability to fully degrade contaminants^9,22,23. Instead, they typically transfer pollutants from liquid to solid form, generating secondary waste that requires further processing^22,23. To overcome these challenges, advanced oxidation processes (AOPs) have gained popularity as effective alternatives^24–28. AOPs facilitate the oxidation and degradation of organic contaminants through methods such as photocatalysis, ultraviolet (UV)/H₂O₂, UV/O₃, UV/ZnO, UV/TiO₂, and Fenton-based processes, including UV/Fenton^29–34. The utilization of TiO₂ photocatalysis as an AOPs for the degradation of hazardous organic compounds is gaining popularity due to its non-toxic nature, photochemical stability, and cost-effectiveness^35–38. Although TiO₂ photocatalysts offer several advantages, there are also some drawbacks associated with TiO₂ photocatalysis, including low efficiency in utilizing light irradiation and difficulties in separating the photocatalyst from the treated solution after the process³⁹. To tackle these limitations and enhance photocatalytic efficiency, loading photocatalysts onto suitable fine adsorbents can be advantageous. This approach helps concentrate pollutants around the photocatalysts, facilitating the rapid and efficient decomposition of organic pollutants, while also simplifying the overall photocatalytic process^39,40. Due to its widespread use in the production of single-use mineral water bottles and its non-biodegradable properties, PET has become a major environmental concern. However, since thermoplastic polymers can be melted and reformed, they are easily recyclable. In addition, PET can be used as a support for nanoparticles, enabling the production of nanocomposites with enhanced physicochemical properties^36,41,42. During the previous study, visible-light photocatalysts were irradiated using a Xenon (Xe) lamp equipped with a UV cut-off filter. To prevent overheating, the lamp was cooled with water. It is important to note that UV light leakage from the filter may impact the performance of the photocatalysts⁹. Recent research has shown promising results by utilizing energy-efficient LED sources to design compact photocatalytic reactors for industrial effluent treatment. The use of LEDs allows for the emission of pure visible light and enables the development of small, space-saving equipment for various applications⁴³. There are many reports on the removal of RB5 using TiO₂ nanoparticles as a photocatalyst^9,44,45. However, limited information is available regarding the removal efficiency and kinetics of RB5 using illuminated TiO₂-PET. In the field of environmental photocatalysis, most studies have focused on TiO₂ nanoparticles, where higher photocatalytic efficiency is observed due to the large surface-to-volume ratio resulting from reduced particle size. However, in practical photocatalytic processes, separating these finely powdered photocatalysts from photoreactors is challenging. Ongoing efforts have aimed at finding low-cost, highly active, and easily recyclable TiO₂-based photocatalysts. TiO₂-PET nanocomposites can be easily separated from the solution, overcoming the disadvantages of TiO₂ nanoparticles as nanocatalysts. This study examined the efficacy of using TiO₂ immobilized on PET in an LED system for the removal of RB5 from aqueous solutions. Various operational variables, including pH, catalyst dosage, RB5 concentration, different gases, H₂O₂ amount, organic and anion compounds, and radical scavengers, were investigated to determine their effects. The efficiency of various systems, reusability, kinetic models, and energy consumption were evaluated under constant conditions. The effectiveness of RB5 removal from both drinking water and textile effluent was assessed under optimized conditions. A cost evaluation was conducted using Operational Cost (OC), intermediate analysis was performed using gas chromatography-mass spectroscopy (GC–MS, Varian 4000), and toxicity bioassays were conducted with Daphnia magna.

Materials and methods

Materials

All the materials used in this study were obtained from Merck (Darmstadt, Germany). RB5 (C₂₆H₂₁N₅O₁₉S₆Na₄), with a purity of more than 95%, was acquired from Alvan Sabet Co., Iran. The chemical structure has been presented in Table S1. A Namanoorasia Co. LB 15/A80 model LED lamp with a 15 W power rating was used as the light source for the photocatalytic removal of RB5. The same photocatalytic reactor used in a prior experiment was employed in this process⁴⁶.The pH of the solution was measured using a Metrohm (Switzerland) pH meter, and adjustments were made with either HCl (0.1 N) or NaOH (0.1 N).

Synthesis of the TiO₂-PET nanocomposite and characterizations

The process began by collecting PET bottles from waste recycling facilities, washing them with deionized water, and allowing them to air-dry at room temperature⁴⁰. Subsequently, the bottles were broken into small pieces measuring 1–3 mm. Ten grams of shredded PET were combined with 2 weight% hydrogen peroxide and placed in a hydrothermal autoclave reactor. The reactor was then placed in an oven and heated to a temperature of 180 °C, which was maintained for 7 h. Afterward, the reactor was allowed to cool overnight, and the resulting PET products were removed, dried, powdered, and used to create a TiO₂-PET nanocomposite. A colloidal suspension of TiO₂ was prepared by dissolving titanium tetra-isopropoxide (TTIP, Sigma Aldrich) in isopropanol at a 1:13 molar ratio³⁷. This solution was placed into a beaker containing 50 mL of 0.1 M HNO₃. The PET samples were then submerged in the acid TiO₂ precursor and heated in a reflux condenser for 120 min at 80 °C while stirring. After the samples were removed and rinsed, they were placed in an Elma sonic ultrasound bath at low frequency (37 kHz) for two minutes to remove any unbound TiO₂. A schematic diagram of the synthesis of the TiO₂ nanoparticles-coated PET is shown in Fig. S1.

Characterizing and analytical methods

To determine the crystal structure of PET, TiO₂, and the TiO₂-PET nanocomposite at room temperature, XRD was employed. The average particle size of the catalyst was calculated using the Debye–Scherrer formula. The identification of functional groups on the surface of PET, TiO₂ nanoparticles, and TiO₂-PET nanocomposite was performed using FT-IR analysis. SEM was used to determine the surface morphology of the samples, while EDX-MAP was employed to determine the material composition of the samples. The pH_pzc of the samples was evaluated using the technique described by Shirzad-Siboni et al.⁴⁷. To assess the surface charge properties of the TiO₂-PET nanocomposites, the pH_pzc was measured by preparing a 1 L solution of 0.1 M NaNO₃ and dividing it into 10 separate solutions with pH values ranging from 2 to 11. HCl and NaOH of the appropriate molarity were added to adjust the pH. Then, 0.2 g of TiO₂-PET nanocomposites was added to the solutions and shaken at 170 rpm for 48 h. After centrifuging the mixtures, the final pHs were measured and plotted against the initial pHs. The pH_pzc was determined at the point where the line of final pH intersected the line of initial pH. The transformation products of RB5 were investigated to understand the reaction pathways. GC-MS was used to identify the products. Samples were pretreated using the liquid–liquid extraction method with trichloroethylene solvent to extract possible intermediate products of the dye oxidation. A HP-5 column (30 m length, 0.25 mm internal diameter, 0.25 μm film thickness) was employed. The temperature was initially set to 100 °C for 5 min, then incrementally raised to 280 °C at a rate of 10 °C per minute, with helium as the carrier gas at a rate of 1 mL min^− 1. The ionization mode was electron impact (70 eV), and data were collected in full scan mode (m/z 50–1000). The injection temperature and MS source temperature were set to 280 °C and 230 °C, respectively.

Experimental method

This study employed the OFAT approach to explore the influence of various variables on the photocatalytic removal of RB5. The parameters investigated included pH (3–11), catalyst dosage (0.1–1 g L^− 1), initial RB5 concentration ([RB5]₀) (10–50 mg L^− 1), O₂ and N₂ gases at a 2 L min^− 1 flow rate, H₂O₂ concentration (2–100 mM), type of organic compounds, and type of anions at concentrations of 20 mg L^− 1. To conduct the experiment, a stock solution of RB5 with a concentration of 1000 mg L^− 1 was prepared by dissolving 1 g of RB5 in distilled water. The removal efficiency of RB5 was compared when exposed to PET-alone, TiO₂-alone, TiO₂-PET, LED-alone, LED/PET, LED/TiO₂, and LED/TiO₂-PET under the same reaction conditions. To evaluate the reusability of the catalyst, it was exposed to a 2 M NaOH solution, and the experiment was repeated five times. The pH of the solution was found to be optimum at 3, while the catalyst dosage and [RB5]_° were held constant at 0.5 g L^− 1 and 20 mg L^− 1, respectively. With these conditions fixed, the effects of other parameters were then examined. Experiments were conducted in batches, with constant stirring, at a temperature of 25 ± 1 °C over a period of two hours, under ambient conditions. A solution was prepared by combining RB5 and the TiO₂-PET nanocomposite, which was then left in the dark environment for 30 min to reach equilibrium. Afterward, the solution was irradiated with a 15 W LED lamp to conduct the photocatalytic removal of RB5. The TiO₂-PET nanocomposite was removed by centrifuging the suspension for 10 min. Subsequently, the concentration of residual RB5 in the supernatant was measured using a HACH-DR 5000 (USA) at λmax = 597 nm⁴⁸. The kinetic behavior of the process was investigated and modeled using zero-order, first-order, second-order, and Langmuir–Hinshelwood equations. To assess the cost-effectiveness, the EEO was calculated. A summary of the equations and constants employed is presented in Table S2. At the end of the reaction, intermediate products in the photocatalytic degradation of RB5 were identified through GC-MS analysis, and the toxicity in the photocatalytic removal of RB5 was assessed using Daphnia magna. The experiments were conducted under optimal conditions (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]_°= 20 mg L^− 1, and contact time= 120 min) to determine the toxicity of the treated effluent. Samples were collected, and the results of the bioassay were determined using Daphnia magna. The Daphnia were fed 5 mg L^− 1 of rough rice every 2 days⁴⁰. The assessment followed the guidelines outlined in NBR 12.713 regulations⁴⁰. Several solutions were selected for testing, subjected to volumetric dilution, and evaluated using 10 samples of Daphnia magna cultured in the laboratory between 2 and 26 h. The diluted solutions were maintained at temperatures between 20 and 25 °C, with exposure times ranging from 12 to 96 h. The number of immobilized organisms was calculated based on their concentrations, and the ratio of immobilized organisms was determined using the gathered data. A Daphnia was considered immobile or deceased if it did not move for 15 s after gentle shaking. The samples selected for examination were characterized by the following attributes:

1. Different levels of concentration for the PET, TiO₂, and TiO₂-PET (50, 100, 250, 500, 1000, 2000, 4000, 6000, and 8000 mg L^− 1).

2. Different levels of RB5 (0.5, 1, 3, 5, 15, 20, 30, 40, and 50 mg L^− 1).

3. Different levels of the synthetic effluent and textile industry outlet effluent after treatment (0.5, 1, 3, 6, 10, 15, 25, 50, 75, and 100 volume percentage).

To ensure the accuracy of the evaluation results, control sets of the experiments were conducted. The LC₅₀ and its 95% confidence limits were determined using the probit analysis feature in SPSS software. The toxicity of the substance was measured as LC₅₀ and toxicity units (TU). The toxicity unit value was calculated as 100/LC₅₀, described as the volume proportion.

Results and discussion

Catalyst properties

XRD and FTIR

Figure 1 displays the XRD patterns of the PET, TiO₂ and TiO₂/PET composite. The XRD study for the PET revealed 2θ values of 18.1°, 23.4° and 25.8° in relation to the (010), (110), and (100) planes, respectively. This indicates that the PET employed had a low crystallinity and a triclinic structure⁴⁹. Also, the XRD study for the TiO₂ nanoparticles revealed 2θ values of 25.33°, 36.01°, 37.90°, 48.06°, 53.96°, 55.06°, 62.70°, 69.01°, and 70.41° in relation to the (101), (103), (004), (200), (105), (211), (204), (116), and (220) planes, respectively^50,51. The diffraction patterns revealed that the nanoparticles synthesized were of anatase phase, which was further confirmed using the JCPDS card No. 84-1285. The peak at 25.33° was the most prominent diffraction peak, which indicated the (101) plane of anatase TiO₂. The peaks at 53.96 and 55.06° are associated with the rutile structure, while other peaks are attributable to the anatase structure. The evidence suggests that the primary crystal phase of the TiO₂ nanoparticles was anatase, with only a small amount of rutile present. The findings of this work are consistent with the research carried out by Liu et al.⁵². The presence of distinct peaks observed in Fig. 1 confirmed the successful growth of the TiO₂ nanoparticles on the PET substrate, thus demonstrating the effectiveness of the TiO₂-PET nanocomposite. Based on the Debye–Scherrer formula⁵³, the average sizes of TiO₂ and TiO₂-PET were calculated to be 12.13 and 19.83 nm, respectively. Figure 2 displays the FT-IR patterns of the PET, TiO₂ and TiO₂/PET composite. The TiO₂ nanoparticles absorption peaks observed at 690.5, 1638.71 and 3454.51 cm^− 150. The vibrational energy in the range of 500–600 cm^− 1 is attributed to the bending of titanium-oxygen-titanium bands in the TiO₂ lattice. The broad band in the range of 3400–3600 cm^− 1 is due to the interaction between the hydroxyl groups of water molecules and the TiO₂ surface. The peak at 1650 cm^− 1 is associated with the bending of the ⁻OH group. There were multiple noticeable absorption peaks at various wavenumbers, as indicated by an analysis of the FT-IR spectrum such as 435.50, 504.64, 726.73, 791.86, 846.54, 872.25, 969.85, 1019.45, 1127.70, 1253.47, 1342.70, 1411.12, 1471.54, 1505.79, 1577.32, 1717.27, 2968.49 and 3430.93 cm^− 1 for the PET⁴⁰. The following vibrations were found to be responsible for the peak at various bands: vibrations of the C–H benzene ring (726.73 cm^− 1), ester group (1127.70 and 1253.47 cm^− 1), C=O band vibrations (1717.27 cm^− 1), C–H vibrations (2968.49 cm^− 1), and O–H group (3430.93 cm^− 1). The TiO₂-PET nanocomposite absorption peaks observed at 513.69, 645.71, 726.8, 783.41, 926.30, 968.93, 1019.41, 1124.04, 1252.93, 1341.46, 1413.48, 1574.10, 1638.84, 1715.75 and 3431.13 cm^− 1. The presence of peaks observed in Fig. 2 confirmed the successful growth of the TiO₂ nanoparticles on the PET substrate, thus demonstrating the effectiveness of the TiO₂-PET nanocomposite.

Fig. 1.

XRD image of the samples.

Fig. 2.

FT-IR image of the samples.

SEM, EDX and MAP

The shape of the samples was examined using SEM (Fig. 3a–c). The TiO₂ and TiO₂–PET nanocomposite showed a spherical shape, with some particles adhering to one another. The aggregation of nanoparticles results from their mutual attraction through weak interactions, leading to the formation of micron-sized clusters with high surface energy⁵⁴. The synthesis conditions—such as the surrounding environment, temperature, and surface chemistry—can influence whether nanoparticles form soft or hard agglomerates. To determine the material composition of the samples, EDX-MAP analysis was performed. As shown in Fig. 4a, PET contained 54.96% carbon and 45.04% oxygen, TiO₂ nanoparticles comprised 50.83% oxygen and 49.17% titanium, and the TiO₂–PET nanocomposite consisted of 32.72% carbon, 41.13% oxygen, and 26.14% titanium. This indicates that the nanocomposite produced was pure, as only carbon, oxygen, and titanium were detected. The elemental mapping results from energy-dispersive X-ray spectroscopy (EDX), depicted in Fig. 4b, further confirm that titanium and carbon elements are present in the same regions within the samples, suggesting that TiO₂ is effectively coating the surface of PET.

Fig. 3.

SEM image of the samples: (a) PET, (b) TiO₂ and (c) TiO₂-PET.

Fig. 4.

EDX (a) and MAP (b) image of the samples.

The influence of operating parameters

pH

The pH of the solution plays a crucial role in determining the surface charge characteristics of catalysts, which subsequently influences the adsorption and degradation processes of RB5. The photocatalyst’s effectiveness in removing RB5 was evaluated at pH levels ranging from 3 to 11 under identical conditions: a catalyst dosage of 0.5 g L⁻¹ and an initial RB5 concentration of 20 mg L⁻¹. As illustrated in Fig. 5, an increase in pH from 3 to 11 resulted in a decrease in removal efficiency from 99.99 to 14.8%. TiO₂, TiO₂-PET, and PET were found to have pH_pzc values of 6.8, 4.385, and 5.7, respectively. At pH 3 (a strongly acidic environment), the maximum degradation rate constant of RB5 was 0.0328 min⁻¹. However, as the pH increased from 5 (slightly acidic) to 11 (strongly basic), the first-order degradation rate constant of RB5 decreased from 0.0105 to 0.0013 min⁻¹. This decline is attributed to the influence of pH on the surface charge of the nanocomposite. Specifically, TiO₂ exhibits a positive charge at pH values below 6.8 and a negative charge at higher pH levels (Eqs. 1–2)⁹.

Fig. 5.

Effect of initial pH on the photocatalytic removal of RB5 (catalyst dosage= 0.5 g L^− 1, [RB5]₀= 20 mg L^− 1).

1

2

At the pH range studied, it is recognized that RB5 predominantly existed in its ionic form because of the low pK_a value of the sulfonic group in its structure^37,55. It was anticipated that, at basic pH, the negative charges of the catalyst would push away the dye molecules, thereby reducing the effectiveness of degradation. The surface of the catalyst contained a greater amount of positive charges below pH_pzc. As a result, the removal of RB5 was enhanced due to the presence of negatively charged RB5 species. Additionally, the generation of hydroxyl radicals by the nanocomposite increased with rising solution pH. However, no significant change in process efficiency was observed when the pH was increased from 9 to 11. A pH of 3 was found to be the most favorable condition for the photocatalytic removal of RB5 using TiO₂-PET.

Nanocatalyst dosage

The graph in Fig. 6 illustrates the effectiveness of varying nanocatalyst dosages (0.1–1 g L^− 1) in the photocatalytic elimination of RB5, while other parameters, such as pH and initial RB5 concentration, were kept constant at 3 and 20 mg L^− 1, respectively. The results indicated that the photodegradation process was most effective at a catalyst dosage of 0.5 g L^− 1, after which the performance began to decline. This trend can be attributed to the relationship between the number of active sites available on the catalyst surface and the penetration depth of LED light into the suspension^56–58. As the catalyst dosage increased, the number of active sites also went up; however, this is accompanied by increased turbidity, which reduces LED light penetration^9,17. Consequently, the portion of the suspension that is photo-activated decreases. Similar observations have been reported by other researchers^39,59. A dosage of 0.5 g L^− 1 was found to be the most favorable for the photocatalytic removal of RB5 using TiO₂-PET.

Fig. 6.

Effect of catalyst dosage on the photocatalytic removal of RB5 (pH= 3, [RB5]₀=20 mg L^− 1).

Initial RB5 concentration

In this context, the initial RB5 concentrations was changed from 10 to 50 mg L^− 1 in the photocatalytic elimination of RB5, while other parameters, such as pH and catalyst dosage, were kept constant at 3 and 0.5 g L^− 1, respectively. As shown in Fig. 7, the removal efficiency of RB5 was nearly 100% at an initial concentration of 10 mg L^− 1 within the first 90 min. However, as the concentration increased to 20, 30, 40, and 50 mg L^− 1, the efficiency declined from 99.99 to 72.01%, 49.75%, and 22.48%, respectively. This decrease in efficiency has been attributed to the increased adsorption of dye molecules on the catalyst surface, which can reduce its photocatalytic activity^60–62. Besides, Silva and coworkers⁹ reported that an excessive amount of dye molecules can absorb UV radiation, preventing the effective stimulation of the TiO₂– brass structured particles. As a result, the production of ^·OH radicals decreases due to reduced radiation exposure on the catalyst surface. Based on these findings, experiments were conducted using 20 mg L^− 1 of RB5, as this concentration was found to yield the optimal decomposition rate.

Fig. 7.

Effect of initial RB5 concentration on the photocatalytic removal of RB5 (catalyst dosage= 0.5 g L^− 1, pH= 3).

Kinetics study of RB5 degradation

The results of the experiments, taken at various reaction times and different concentrations of the RB5 (10–50 mg L^− 1) under the optimum condition (pH= 3 and dosage= 0.5 g L^− 1), were evaluated through zero, first, and second-order equations to find the best fit. A summary of the equations and constants employed has been presented in Table S2. The values of k for zero-order, first-order and second-order reactions were ascertained by plotting graphs of , , against interaction duration, respectively^60,63. The photocatalytic degradation of RB5 resulted in a strong linear correlation that passed the first-order kinetic model with an impressive coefficient of regression (R² > 0.9931). The RB5 concentration was increased from 10 to 50 mg L^− 1, which consequently led to a considerable decrease in the reaction rate constant (K_obs) from 0.052 to 0.0017 min^− 1 (Table 1). Using the Langmuir–Hinshelwood (L–H) model, it is possible to predict the adsorption and photocatalytic characteristics of the photocatalyst. By plotting 1/k_obs versus the initial concentration of RB5⁶³, the values of K_RB5 and k_c obtained were 0.1636 L mg^− 1 and 0.1309 mg L^− 1min^− 1, respectively. Aguedach et al.⁶⁴ reported that Langmuir adsorption constant of RB5 was K_RB5= 0.027 L mg^− 1 and k_c= 0.757 mg L^− 1min^− 1, respectively, from the photocatalytic degradation RB5 by UV-irradiated TiO₂ coated on non-woven paper. Alaton and Balcioglu⁶⁵ reported that the Langmuir adsorption constant of RB5 was K_RB5= 2.01 L mg^− 1 and k_c= 4.47 mg L^− 1 min^− 1, respectively, from the photocatalytic degradation RB5 by TiO₂/UV-A process. Soltani and Entezari⁶⁶ stated that the Langmuir adsorption constant of RB5 was K_RB5= 0.416 L mg^− 1 and k_c= 0.936^− 1 min^− 1, respectively, from the solar photocatalytic removal of RB5 by BiFeO₃.

Table 1.

Kinetic parameters and electrical energy per order obtained at different RB5 concentration (catalyst dosage= 0.5 g L^− 1 and pH= 3).

[RB5]0 (mg L− 1)	Zero-order			First-order			Second-order
	k0 (mol L− 1 min− 1)	R 2	kobs (min− 1)	1/kobs (min)	R 2	EEO (kWh m− 3)	k2 (Lmol− 1 min− 1)	R 2
10	0.0502	0.5023	0.052	19.2308	0.9936	11.08	254.46	0.6449
20	0.117	0.6785	0.0328	30.4878	0.9931	17.561	44.292	0.2533
30	0.1396	0.8185	0.0096	104.167	0.9532	60	0.0007	0.9879
40	0.1328	0.8994	0.0048	208.333	0.9517	120	0.0002	0.769
50	0.0754	0.8812	0.0017	588.235	0.9057	338.824	0.0001	0.9266

Hydrogen peroxide

The effectiveness of the photocatalyst in eliminating RB5 was evaluated at H₂O₂ concentrations ranging from 2 to 100 mM, while other parameters, including pH, catalyst dosage, and initial RB5 concentration, were kept constant at 3, 0.5 g L^− 1, and 20 mg L^− 1, respectively. As shown in Fig. 8, RB5 photo-degradation efficiency reached its maximum at an H₂O₂ concentration of 10 mM. However, beyond this threshold, degradation efficiency declined. Specifically, as H₂O₂ concentrations increased from 10 to 100 mM, removal efficiency decreased from 99.99 to 55.86%. The initial enhancement in reaction rate following the addition of H₂O₂ was attributed to the increased generation of hydroxyl radicals⁶⁷. Equation (3) suggests that at low concentrations, hydrogen peroxide effectively inhibits electron-hole recombination. Compared to oxygen, hydrogen peroxide has a greater electron-accepting capacity, making it a potential alternative to oxygen as an electron acceptor in photocatalytic reactions.

Fig. 8.

Effect of hydrogen peroxide on the photocatalytic removal of RB5 (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]₀= 20 mg L^− 1).

3

At high concentrations, H₂O₂ has been shown to act as a strong scavenger of hydroxyl radicals (^·OH), leading to a reduction in the efficiency of RB5 photocatalytic degradation, as outlined in Eqs. (4–6)⁶⁷. Similar observations have also been reported in previous studies on dye removal^43,45.

4

5

6

Gases

The graph in Fig. 9 illustrates the effectiveness of different gases (bubbling of O₂ and N₂ gas) in the photocatalytic elimination of RB5 while the other parameters like pH, [RB5]₀ and catalyst dosage were kept constant at 3, 20 mg L^− 1 and 0.5 g L^− 1, respectively. It is evident that removing RB5 from ambient condition was more successful than when exposed to O₂ and N₂ gas, with the efficiency dropping from 99.99 to 98.33% and 68.19%, respectively. The removal efficiency was higher in an oxygen-rich environment compared to nitrogen exposure. When the catalyst is irradiated with photons of appropriate energy, electrons in the valence band gain sufficient energy to transition to the conduction band, generating electron-hole pairs that migrate to the photocatalyst surface^31,43,44. These separated charges initiate oxidation-reduction reactions between oxygen and water molecules, leading to the production of reactive oxygen species such as hydrogen peroxide. Photogenerated holes on the catalyst surface interact with hydroxyl ions, forming highly reactive oxygen species, including OH^·, OH₂^·, and H₂O₂. Meanwhile, electrons from the reaction are captured by oxygen molecules introduced via bubbling, generating additional oxygen-active species such as O₂^·−, which further enhances photocatalytic efficiency. In contrast, when nitrogen is bubbled into the reaction suspension, the photocatalytic removal efficiency of RB5 decreases. Unlike oxygen, nitrogen cannot act as an electron acceptor, resulting in the depletion of oxygen molecules from the solution. These findings align with previous studies, which also reported a reduction in RB5 removal efficiency in the presence of nitrogen and oxygen gases³¹.

Fig. 9.

Effect of purging of different gases on the photocatalytic removal of RB5 (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]₀= 20 mg L^− 1).

Organic compounds

The graph in Fig. 10 illustrates the effectiveness of different organic compounds (humic acid, oxalic acid, ethylene diamine tetra acetic acid (EDTA), citric acid, folic acid, formic acid, phenol, benzoic acid) in the photocatalytic elimination of RB5 while the other parameters like pH, [RB5]₀ and catalyst dosage were kept constant at 3, 20 mg L^− 1, and 0.5 g L^− 1, respectively. As the different organic compounds were added to system, the rate of photocatalytic degradation typically decreased. As shown in Fig. 10, by adding humic acid, oxalic acid, EDTA, citric acid, folic acid, phenol, formic acid and benzoic acid to the solution, the efficiency reduced from 99.99 to 91.56, 69.56, 79.23, 52.3, 92.3, 78.36, 43.56 and 32.36%, respectively. The decrease in catalytic activity is thought to be caused by organic compounds taking up space on the catalyst’s active sites, limiting the amount of space available for the desired compound⁴³. A potential cause for the decrease may be that the organic compounds act as electron scavengers. These compounds may react with ^·OH radicals, impeding the recombination of holes and electrons. Organic compounds also absorb light within a specific range of wavelengths near the maximum emission of the lamp, hindering photons from reaching the catalyst’s surface. The findings of previous studies support ours, which also observed a decrease in the removal efficiency of reactive dye when humic acid was present⁶⁸.

Fig. 10.

Effect of different organic compounds on the photocatalytic removal of RB5 (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]₀= 20 mg L^− 1, organic compounds= 20 mg L^− 1).

Ionic strength

The graph in Fig. 11 indicates the effectiveness of common inorganic anions (NaOH, NaCl, Na₂SO₄, Na₂CO₃ and NaHCO₃) in the photocatalytic elimination of RB5 while the other parameters like pH, [RB5]₀ and catalyst dosage were kept constant at 3, 20 mg L^− 1, and 0.5 g L^− 1, respectively. It is clear that all anions lead to a reduction in the rate of photodegradation. As illustrated in Fig. 11, the removal efficiency RB5 decreased from 99.99% to 58.36, 50.23, 45.36, 55.69, and 40.23%, respectively, when NaOH, NaCl, Na₂SO₄, Na₂CO₃ and NaHCO₃ were added to the solution. These species could inhibit the speed of oxidation by obscuring the active regions on the catalyst or through contesting the photo-oxidizing entities^43,69. The inhibitory influence of anions can be attributed to their reaction with positive holes and hydroxyl radicals (Eqs. 7–14), acting as h^· and ^·OH scavengers, thereby delaying color elimination.

Fig. 11.

Effect of different anions on the photocatalytic removal of RB5 (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]₀= 20 mg L^− 1, anions= 20 mg L^− 1).

7

8

9

10

11

12

13

14

Previous studies have suggested that anions are more effective in hindering reaction rates due to their ability to act as scavengers of h⁺ and ^·OH, a finding consistent with the results observed for RB5⁶⁸.

Possible photocatalytic mechanism for RB5 removal by LED/TiO₂-PET

The graph in Fig. 12 presents the effectiveness of different processes in the photocatalytic elimination of RB5 while the other parameters like pH, [RB5]₀ and catalyst dosage were kept constant at 3, 20 mg L^− 1 and 0.5 g L^− 1, respectively. The processes included PET-alone, LED-alone, LED/PET, TiO₂₋alone, TiO₂-PET, LED/TiO₂ and LED/TiO₂-PET. It is evident that LED/TiO₂-PET was the most effective in terms degradation RB5, with a rate of 99.99%. Moreover, in the presence of other processes such as PET-alone, LED-alone, LED/PET, TiO₂₋alone, TiO₂-PET and LED/TiO₂ the removal efficiency of RB5 decreased to 2.74, 4.7, 7.2, 16.53, 27.16, 63.42%, respectively. To investigate the types of radicals produced during the photocatalytic activity, radical quenching experiments were conducted in optimized conditions parameters like pH, [RB5]₀ and catalyst dosage were kept constant at 3, 20 mg L^− 1, and 0.5 g L^− 1, respectively. Ammonium oxalate, benzoquinone, and tert-butyl alcohol was used as h⁺, and ^·OH, respectively⁷⁰. As shown in Fig. 13, in the present of ammonium oxalate, benzoquinone, and tert-butyl alcohol scavengers, the removal efficiency of RB5 went up from 48.029 to 98.17%, 31.14–74.34% and 19.86–58.059% in irradiation time from 10 to 120 min, respectively. While, at the same contact time, the removal efficiency increased from to 49.85–99.99% in the absence of scavenger. The findings suggest that ^·OH and were formed in the LED/TiO₂-PET process and were the main reactive oxygen species (ROSs) source for the removal of RB5 (Fig. 13). It was found that there was no significant difference in the removal of RB5 when ammonium oxalate was used, suggesting that the reaction of RB5 with h⁺ generated from the reaction of electrons with the TiO₂-PET nanocomposite may not be the primary pathway for RB5 removal. Figure 14 illustrates the changes in the absorption spectra of RB5 solutions by the LED/TiO₂-PET system at various exposure times. The spectral characteristics of RB5 in the visible region show a significant band at 597 nm. The visible spectrum of RB5 revealed a decrease in the absorption peak at 597 nm, demonstrating the rapid decomposition of the dye molecule. Results showed that the nitrogen double band (–N=N–) of the azo dye was particularly susceptible to oxidation, leading to near complete breakdown over the course of two hours under optimized conditions. A schematic of the proposed pathway for the degradation of RB5 in the LED/TiO₂-PET system was generated based on the observed intermediate products (Fig. 15). The hydroxyl (^·OH) radicals generated during the decomposition of the RB5 dye resulted in the cleavage of the –N=N– (azo) bands, producing 1,4-naphthalenedione, benzene, [(methylsulfonyl) methyl], and benzoic acid, 2-amino-, methyl ester. The oxidation of these compounds led to the formation of phenol and 1,2-benzene dicarboxylic acid. In conclusion, the aromatic rings of these compounds underwent further decomposition and degradation, yielding propionic and acetic acids. The illustration in Fig. 16 demonstrates the mechanism of RB5 degradation using the LED/TiO₂-PET system. Based on our results, we propose a possible photocatalytic mechanism for the degradation of RB5 by the TiO₂-PET hybrid. The UV–Vis spectra and band gap of the TiO₂ nanoparticles and TiO₂-PET nanocomposites were calculated using Tauc plots and are shown in Fig. S2a, b. The band gap energy values of TiO₂ nanoparticles and TiO₂-PET nanocomposites were observed to be 3.19 eV and 3 eV, respectively. This reduction in the band gap of the TiO₂-PET nanocomposites was attributed to the presence of stacked and porous carbon (according to the EDX results) coated with TiO₂ nanoparticles, which develop intermediate states between the conduction band and the valence band across the interface, thus reducing the band gap. The TiO₂-PET nanocomposites generate more e⁻ and h⁺ due to the lower band gap, which can produce additional ^·OH radicals following the oxidation reaction. Therefore, the increased production of ^·OH radicals allows for more degradation of RB5. The enhanced photocatalytic activity of the TiO₂-PET nanocomposites is due to the shorter e⁻/h⁺ recombination time, narrower band gap (3 eV), and higher surface area compared to pristine TiO₂. When the TiO₂-PET nanoparticles absorb energy greater than the band gap between the valence and conduction bands, electrons in the VB are excited to the conduction band, leaving behind holes in the VB. These electron-hole pairs have a very short lifespan, typically in the picosecond range, and can migrate to the surface of the semiconductor, facilitating charge transfer (Eq. 15)^35,71.

Fig. 12.

Contribution of each process involved in the photocatalytic removal of RB5 (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]₀= 20 mg L^− 1).

Fig. 13.

Effect of active scavenger species on the photocatalytic removal of RB5 (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]₀= 20 mg L^− 1).

Fig. 14.

₂ Spectral changes of RB5 solution during illumination in the presence of TiO₂-PET (catalyst dosage= 0.5 g L^-1, pH=3, [RB5]₀=20 mg L^-1.

Fig. 15.

RB5 decomposition pathways determined by GC-MS data at 280 °C (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]₀= 20 mg L^− 1).

Fig. 16.

 Mechanism of the photocatalytic removal of RB5 using the TiO₂-PET under LED visible light irradiation. t  = ^− 1, 5₀= ^− 1).

15

Electrons and holes can move within and between semiconductor particles, recombine, and become inactive, releasing energy in the form of heat or light (Eq. 16).

16

Generally, the presence of oxygen in the atmosphere will lead to the formation of superoxide radical anions (O⁻₂²⁻). In an aqueous surrounding, the absence of electrons (holes) will cause water molecules or hydroxyl ions (OH⁻) that have adsorbed on the surface of titanium dioxide to react and form highly reactive hydroxyl radicals (OH) (Eqs. 17–24).

17

18

19

20

21

22

23

24

Radicals with high redox capacity can break down the chemical bands in organic compounds, such as C–C and C–O bands, leading to decomposition and oxidation into CO₂, H₂O, and other inorganic molecules (Eq. 25). Due to their strong oxidative power, they can be used to treat various types of chemicals without producing intermediate products.

25

A comparison of RB5 degradation using different catalysts, based on removal efficiency and kinetic constants, has been shown in Table S3.

Reusability

The stability and recyclability of TiO₂-PET catalysts in the photodegradation of RB5 dye were evaluated by conducting five successive cycles of RB5 dye removal under LED irradiation in optimal conditions (catalyst dosage= 0.5 g L^− 1, [RB5]₀= 20 mg L^− 1, pH= 3). The samples from the photocatalytic reactions were thoroughly rinsed multiple times with deionized water and a 2 M NaOH solution as a desorbing agent, followed by heating in an oven until completely dry, and then set aside for use in future operations. The results shown in Fig. 17 indicate that, even after 120 min, the removal efficiency of RB5 remained consistent after five repetitions of the experiment. The crystalline structure of TiO₂-PET was also analyzed using XRD measurements taken before and after five cycles. The structure of the catalyst remained unchanged and was comparable to the initial sample (Fig. 1). The study demonstrates that TiO₂-PET could be a practical material for the elimination of RB5 dye through photocatalysis, due to its excellent photosensitivity and high reusability. The durability of the catalyst was further assessed at different pH levels of the solution. The pH_PZC indicated that the nanocomposite was stable under acidic conditions (pH > 2).

Fig. 17.

i₂Reusability test for the photocatalytic removal of RB5 within five repeated cycles (catalyst dosage= 0.5 g L^− 1, pH= 3, [RB5]₀=20 mg L^− 1).

Treatment of an actual water sample containing RB5

To assess the efficacy of the LED/TiO₂-PET technique for removing RB5 from actual water, 20 mg L^− 1 of RB5 was added to a water sample collected from Rasht City’s distribution system in Iran. The properties of the real water sample are described in Table S4⁷². As shown in Fig. 18, synthetic water demonstrated better removal efficiency for RB5 than drinking water. The scavenging of hydroxyl radicals by anions such as sulfate, chloride, carbonate, and bicarbonate is responsible for the inhibition, which occurs through Eqs. (7–14)⁷³. These ions could also interfere with the active sites on the TiO₂-PET surface, hindering the catalyst’s ability to interact with RB5 and the associated molecules. Evidence suggests that the oxidizing power of the radical anions generated is weaker than that of hydroxyl radicals. The pH level of the real water, influenced by the presence of carbonate and bicarbonate ions, may be higher than that of distilled water, which could contribute to the reduced photodegradation efficiency in real water compared to distilled water. The solution’s specific conductivity increased from 0.76 to 0.92 after the LED/TiO₂-PET process, likely due to the formation of sulfate, phosphate, and nitrate during the photocatalytic removal of RB5.

Fig. 18.

Investigation of the efficiency of LED/TiO₂-PET process in removal of RB5 from real drinking water (catalyst dosage= 0.5 g L^− 1, [RB5]₀= 20 mg L^− 1).

RB5 removal from textile industry wastewater

This study assessed the effectiveness of TiO₂-PET photocatalysis in removing RB5 from wastewater samples collected from the textile industry in Rasht, Iran. Table 2 provides an overview of the composition of the textile effluent before and after treatment. Prior to the experiments, the wastewater sample was allowed to settle for 24 h, after which it was filtered. The filtered sample was then used in the photocatalytic removal of RB5. The initial measurements of the textile wastewater showed a COD of 1480 mg L^− 1, BOD of 325 mg L^− 1, TSS of 1250 mg L^− 1, color of 90 PCU, and a pH value of 8.5. After treatment through LED/TiO₂-PET, the COD, BOD, TSS, and color of the final product were reduced to 115 mg L^− 1, 45 mg L^− 1, 129 mg L^− 1, and 3 PCU, respectively, all of which are below the National Environmental Quality Standards (NEQS) established for textile effluent. The effectiveness of LED/TiO₂-PET in removing color from actual textile wastewater is presented in Fig. 19; the process was found to be 96.66% successful in decreasing the color parameter from 90 PCU to 3 PCU. The removal of color from textile industry wastewater also contributed to a reduction in COD and BOD, as the complex dyes were broken down into simpler molecules that are less harmful and more easily biodegraded during wastewater treatment by the LED/TiO₂-PET process. As a result of this research, the levels of COD, BOD, and TSS were reduced to 115 mg L^− 1, 45 mg L^− 1, and 129 mg L^− 1, respectively, which correspond to removal rates of 92.22, 86.15, and 89.68%. Similar observations have also been reported for the removal of dyes^23,74.

Table 2.

Characteristics of real textile effluent before and after the treatment at LED/TiO₂-PET process (dose= 0.5 g L⁻¹ within 120 min).

Parameters (mg L− 1)	Before treatment	After treatment	National environmental quality standards (NEQS) of textile effluent
pH	8.5	6.8	7
COD	1480	115	150
BOD	325	45	80
TSS	1250	129	150
TDS	2850	2770	3500
Color (PCU)	90	3	7

Fig. 19.

Decolonization using of the LED/TiO₂-PET process and its comparison with National Environmental Quality Standards (NEQS) of Textile Effluent (catalyst dosage= 0.5 g L^− 1).

Toxicity

The Table 3 displays the results of an acute toxicity test, including the 12 to 96-h LC₅₀ values and confidence limits, as well as the TU value. After the treatment, the 48-h LC₅₀ values were 2.991% for the textile industry outlet effluent, 102.87% for the synthetic effluent, 5238.05 mg L^− 1 for TiO₂-PET, 11.19 mg L^− 1 for RB5, 2081.19 mg L^− 1 for PET, and 1210.75 mg L^− 1 for TiO₂. Table 3 shows that TiO₂-PET nanocomposite are not hazardous to aquatic life, suggesting that this catalyst could be employed in photocatalytic processes without adversely affecting aquatic species. The result illustrates that the photocatalytic process by means of the LED/TiO₂-PET caused the decrease of toxicity of effluent significantly. Nunes et al. stated that Sr_0.95Bi_0.05TiO₃ is an effective photocatalyst for decomposing the AO7 dye under visible light, and it can likewise effectively eliminate the toxic metabolites, thus yielding a final solution that is not harmful to D. magna. Mengelizadeh et al.¹⁰ reported GO-CoFe₂O₄ for the degradation of RB5, results from Daphnia pulex toxicity tests indicated that the treated effluent needed additional catalytic treatment and better stabilization because of the presence of cobalt ions and intermediates. Kunz et al.⁷⁵ explained that ozonation was used for the degradation of RB5. The results from Vibrio fischeri toxicity tests indicated that the increase in toxicity after treatment was due to the presence of copper ions formed by the intermediates following oxidation.

Table 3.

The findings of the acute toxicity test 12 to 96-h LC₅₀values, confidence limits and TU.

Samples	PET						TiO2						TiO2-PET
Time (h)	LC50 (mg L− 1)	95% Confidence Limits		TU	NOEC (mg L− 1)	100% mortality	LC50 (mg L− 1)	95% Confidence Limits		TU	NOEC (mg L− 1)	100% mortality	LC50 (%)	95% Confidence Limits		TU	NOEC (%)	100% mortality
		Lower Bound	Upper Bound					Lower Bound	Upper Bound					Lower Bound	Upper Bound
12	7811.35	4665.92	31,808.6	0.012	324.33	188,132.15	4249.51	2614.603	9094.171	0.0235	153.140	117,921	12,452	6939.159	2,023,332.785	0.008	984.836	157,441
24	4249.51	2614.6	9049.17	0.023	153.139	117,921.21	2233.45	1382.749	3888.415	0.0447	77.668	64,226.6	9987.35	5329.034	79,326.809	0.01	320.046	311,665
48	2081.19	1299.41	3512.55	0.048	81.31	53,266.14	1210.75	765.922	1890.262	0.0825	64.999	22,552.7	5238.05	3041.115	14,309.278	0.019	129.815	211,356
72	921.91	559.52	1480.06	0.108	35.23	24,121.3	921.65	544.541	1730.052	0.108	31.369	27,078.5	3229.54	1836.278	7442.097	0.03	47.658	218,849
96	519.51	291.04	894.51	0.192	12.73	21,186.39	526.688	337.52	807.396	0.189	48.784	5686.33	1606.92	957.702	2809.524	0.0622	39.445	65,463.4

Samples	RB5						Synthetic effluent						Textile industry outlet effluent after treatment
Time (h)	LC50 (mg L− 1)	95% confidence limits		TU	NOEC (mg L− 1)	100% mortality	LC50 mg L− 1)	95% confidence limits		TU	NOEC (mg L− 1)	100% mortality	LC50 (%)	95% confidence limits		TU	NOEC (%)	100% mortality
		Lower bound	Upper bound					Lower bound	Upper bound					Lower bound	Upper bound
12	20.72	17.14	25.45	4.82	− 3.56	45	185.409	–	–	0.539	35.421	970.502	7.809	4.97	11.674	12.805	0.448	135.983
24	13.58	11.17	16.9	7.36	− 1.6	28.77	142.708	–	–	0.7	29.952	679.904	4.589	2.893	6.858	21.791	0.346	60.805
48	11.19	8.99	14.22	8.93	− 3.4	25.78	102.87	76.045	669.571	0.972	24.121	437.714	2.991	1.84	4.561	33.433	0.263	34.07
72	8.87	6.91	11.5	11.27	− 4.41	22.17	80.603	60.941	141.926	1.24	20.393	318.586	2.337	1.379	3.789	42.789	0.192	28.441
96	7.18	5.22	10	13.92	− 6.37	20.74	64.827	48.144	99.131	1.542	17.489	240.299	1.757	–	–	56.91	0.167	18.465

Cost analysis

It is essential to assess the economics of the RB5 degradation approach to determine its feasibility for large-scale commercialization. The costs associated with this technique include capital, operating, and maintenance expenses. The operating cost is influenced by factors such as the characteristics of the contaminant, its concentration, contact time, amount of catalyst used, and the arrangement of the reactor. An economic assessment was conducted to evaluate the operational expenditures related to the photocatalytic degradation of RB5 from aqueous solutions using various processes (LED, LED/PET, LED/TiO₂, LED/TiO₂-PET) under optimal conditions (pH= 3, TiO₂-PET= 0.5 g L^− 1, contact time= 120 min, [RB5]₀= 20 mg L^− 1, LED light power = 15 W). The operating cost of removing RB5 from aqueous solutions was calculated in US Dollars (USD) per kilogram (kg) based on Eqs. (26) and (27)⁶⁰.

26

27

The results of a cost comparison and energy efficiency calculated for the photocatalytic degradation of RB5 are presented in Table 4. The total energy consumed for 120 min of RB5 treatment using LED-based system is 0.03 kWh. In addition, the total operating cost for the LED/TiO₂-PET (3 USD kg^− 1) process was less than that for other photocatalytic processes including LED/TiO₂ (4.73 USD kg^− 1), LED/PET (40 USD kg^− 1) and LED (63.16 USD kg^− 1). The energy efficiency calculated for the LED/TiO₂-PET (1.18 × 10^− 7 mg J^− 1) process was higher than other photocatalytic processes including LED/TiO₂ (7.49 × 10^− 8 mg J^− 1), LED/PET (8.57 × 10^− 9 mg J^− 1) and LED (5.5 × 10^− 9 mg J^− 1). The findings for the EEO of the various concentration and processes studied in this work have been shown in Tables 1 and 4, respectively. The EEO rose drastically from 11.08 to 338.824 kWh m^− 3 as the RB5 concentration was increased from 10 to 50 mg L^− 1. EEO (kWh m^− 3) value for LED/TiO₂-PET (5561.26) process was lower that for LED/TiO₂ (68689.4), LED/PET (923153.2) and LED (1431898.7) processes. Thus, the LED/TiO₂-PET process is more economical than LED/TiO₂, LED/PET and LED. LED/TiO₂-PET is seen to be an economical and efficacious catalyst for the removal of RB5, based on cost-effectiveness analysis and its capacity for degradation. Khan et al.¹⁷ reported that EEO for visible light/Fe-TiO₂ was 207 KWh m^− 3, while the cost was 33 USD for removal of RB5. Mohagheghian et al.⁷⁶ reported that EEO for visible light/ZnO-Perlite was 20.65 KWh m^− 3 while the cost was 3.11 USD kg^− 1 for removal of RB5. Akbarzadeh et al.⁷⁷ reported operational cost 8.7205 USD kg^− 1 for removal of methylene blue in the UV/Vanadium-TiO₂ process.

Table 4.

Comparison of cost analysis and energy efficiency for photocatalytic removal of RB5.

System	RB5 removal (%)	EEO (kWh m− 3)	Total power consumed (kWh)	Total operation cost (USD kg− 1)	Energy delivered/volume (J L− 1)	Energy efficiency (mg J− 1)
LED	4.7	1,431,898.7	0.03	63.16	1.692 × 108	5.5 × 10− 9
LED/PET	7.2	923,153.2	0.03	40	1.692 × 108	8.57 × 10− 9
LED/TiO2	63.41	68,689.4	0.03	4.73	1.692 × 108	7.94 × 10− 8
LED/TiO2-PET	99.99	5561.26	0.03	3	1.692 × 108	1.18 × 10− 7

Conclusions

The TiO₂ coated on PET was successfully synthesized using a simple co-precipitation method. The formation of TiO₂ on PET was confirmed through XRD, FT-IR, SEM, and EDX-MAP analysis. Maximum RB5 removal (99.99%) was achieved under the following conditions: pH 3, irradiation time of 120 min, catalyst dosage of 0.5 g L^− 1, and RB5 dye concentration of 20 mg L^− 1. The presence of H₂O₂, purging gases, organic compounds, and ionic strength negatively affected RB5 removal. According to the Langmuir–Hinshelwood kinetic model, the values for K_RB5 and K_C were 0.1636 L mg^− 1 and 0.1309 mg L^− 1 min^− 1, respectively. Radical quenching tests identified OH^· as the most active radical in RB5 removal. Additionally, the total operating cost for the LED/TiO₂-PET process was 3 USD kg^− 1, which is lower than that of other photocatalytic processes. The reusability of TiO₂ coated on PET was evaluated, showing successful reuse up to 5 cycles. The major photocatalytic pathway for the azo-dye Reactive Black 5 was proposed based on the identification of by-products using GC-MS. Acute toxicity tests carried out using Daphnia magna demonstrated that the produced nanocatalyst did not exhibit harmful effects on aquatic organisms, indicating its safe use for wastewater treatment. The TiO₂-coated PET-based photocatalytic treatment was efficient for RB5 removal under visible light irradiation, making it a viable option for treating textile effluents containing dyes.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Author contributions

M.M.G.: Supervisor, design of the project, visualization, conceptualization, investigation, methodology, software, data curation, writing- original draft, responding to reviewers and revising the manuscript. S.-K.M.: Design of the project, methodology, software, data curation. M.S.-S.: Supervisor, design of the project, visualization, conceptualization, investigation, methodology, software, data curation, writing-original draft, responding to reviewers and revising the manuscript.

Data availability

All data generated or analyzed during this study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.