Introduction

Recently, many pharmaceutical and personal care products (PPCPs) have been released into the environment, which is a growing global concern. These PPCPs and their metabolites pose a potential danger to human health and the ecosystem, even at very low concentration levels [1], [2]. Among these, acetaminophen is widely used to relieve mild or moderate headaches, back pain, arthritis, and postoperative pain. It has been detected in concentrations on the order of µg/L in water sources worldwide, as it is one of the most common prescription drugs [3].

The physicochemical properties make acetaminophen very difficult to degrade in drinking water and wastewater treatment plants. To solve this environmental problem, technologies for the elimination of acetaminophen in water sources have been explored, such as constructed wetlands and advanced oxidation processes (ozonation, photoFenton, sonolysis, H₂O₂/UV, heterogeneous photocatalytic processes, and their combinations) [4]–[6]. Heterogeneous photocatalysis is based on the absorption of radiant energy by a semiconductor material, leading to redox reactions and promoting the degradation of organic pollutants. The most commonly used photocatalysts are metal oxides, highlighting titanium dioxide (TiO₂) at the nanoscale since it presents high chemical stability, making it suitable for application over a wide pH range and allowing the production of electronic transitions by light absorption in the near-ultraviolet spectrum [7]. TiO₂ nanoparticles can be synthesized through methods based on the green chemistry paradigm, in which biological sources replace the traditional synthetic and corrosive compounds aiming at facile and environmentally friendly procedures. Green synthesis involves the use of plants [8], agricultural waste [9], fruit shells, among others, which contain different phytochemicals acting as capping, reducing, and stabilizing agents [10].

The high probability of recombination is the aspect that most affects the performance of photocatalytic reactions, as well as the separation between the loads photogenerated by the electron/hole pairs (e^-/h⁺) [11]. The resulting redox reactions of material activation are due to the migration of electrons from the valence band to the conduction band, generating e^-/h⁺ pairs and promoting reactions to produce hydroxyl radicals. When these reactions are not carried out, either by the absence of dissolved oxygen or adsorbed water, these radicals are recombined by releasing energy attributed to the effect of the absence of dissolved oxygen or adsorbed water during the reaction [12], [13]. Despite the remarkable progress in the development of TiO₂ as a photocatalyst, the practical application is challenged by two limitations inherent in the structure. The first is the low quantum performance, which is determined primarily by the recombination of photogenerated e-/h+ pairs and the bandgap of all TiO₂ phases (3.0–3.2 eV) and essentially responds to the ultraviolet spectrum (4–5% of the solar spectrum) [14], [15]. Second, TiO₂ doping with noble metals, including platinum (Pt), palladium (Pd), gold (Au), and silver (Ag), has been the subject of different studies [16], [17]. Among these, Ag represents an economical option, acting as an electron trap and promoting interfacial load transfer processes in composite systems. This decreases the recombination of photoinduced e^-/h⁺ pairs, improving the photocatalytic activity of TiO₂ [18], [19].

In this study, the photocatalytic degradation of acetaminophen was evaluated using Ag-doped TiO₂ nanoparticles in a cylindrical-parabolic composed photoreactor (CPC). The TiO₂ nanoparticles were synthesized via green chemistry using a Cymbopogon citratus (C. citratus) leaf extract, followed by surface doping with metallic Ag by photodeposition. The physicochemical properties of the Ag-doped TiO₂ nanostructures were determined using different characterization techniques, including the outstanding polycrystalline nature and the reduction in the diameter size due to Ag doping on the surface. Moreover, experiments related to the photodegradation of acetaminophen showed promising results compared to the performance observed for commercial TiO₂ (P-25) nanoparticles. Finally, this research contributes to the development of novel and eco-friendly methods for materials preparation with photocatalytic activity; in addition, this study represents a starting point for scaling up water resource decontamination processes.

Methodology

Materials

Titanium isopropoxide (95%) and hydrochloric acid (HCl, 37%) were purchased from Alfa Aesar. Silver nitrate (AgNO₃, 99.99%) was acquired from Merck, acetaminophen (150 mg/5 mL) from American Generics, reagent alcohol (EtOH, 95%) from “Productos Químicos del Caribe,” and sodium hydroxide (NaOH, 99%) from Panreac. Commercial TiO₂ (Degussa P-25) nanoparticles were purchased from Evonik®. Fresh Cymbopogon citratus (C.citratus) leaves were collected from the rural population of Puerto Escondido, Córdoba, Colombia. Distilled water was used in all experiments.

Cymbopogon citratus extract preparation

C. citratus leaves were washed with distilled water three times to remove other organic content and impurities. The leaves were cut into small pieces, dried in an oven at 40 °C for 6 h, and ground to obtain a powder. Afterward, an infusion was prepared by adding 100 g of the powder to 500 mL of distilled water previously heated at 100 °C. The infusion was then cooled to 60 °C and filtered using a vacuum pump. The concentrated C.citratus extract was obtained by reducing the infusion volume up to 100 mL at 80 °C. Finally, the extract was cooled down at room temperature and stored at 5 °C [20].

Green synthesis of titanium dioxide nanoparticles

TiO₂ nanoparticles were obtained via green synthesis using the C.citratus extract, which acts as a capping agent for the controlled growth of the rutile phase [20]. Here, a 5 mM solution of titanium isopropoxide was prepared in 85 mL of distilled water under stirring at 175 rpm for 12 h. The solution was then added to 15 mL of the C. citratus extract and allowed to react under stirring at 175 rpm for 24 h. Then, the TiO₂ nanoparticles were collected through centrifugation at 5,000 rpm for 10 min. The nanoparticles were purified with several washes with distilled water followed by centrifugation. The crystallization to the TiO₂ anatase phase was completed by drying at 100 °C for 1 h and then annealing at 550 °C for 3 h under airflow.

Synthesis of silver-doped titanium dioxide nanostructures

The Ag-doped TiO₂ nanostructures were obtained through the photodeposition of reduced Ag metal ions on the TiO₂ surface [21]. Three grams of TiO2 was initially added to 100 mL of deionized water, and the pH was adjusted to 3 using a 0.1 M HCl solution. A concentration of AgNO₃ (0.5 at% and 0.75 at%) was then added into the TiO₂ suspension, in which the atomic percentage was previously calculated using Equation 1. The suspension was UV-irradiated using a UV-light lamp (30 W G4W T5 Sylvania) under constant stirring for 3 h, aiming to reduce the Ag⁺ ions from AgNO₃ to metallic Ag. Afterward, the suspension was dried in an oven at 100 °C for 12 h. Finally, TiO₂ doped with 0.5 at% (TiO₂-A) and 0.75 at% (TiO₂-B) Ag was obtained by grinding the particulate matter and annealing at 400 °C for 6 h under airflow.

where and are the total molar amounts of Ag and Ti ions, respectively [22].

Characterization

The morphology was studied using scanning electron microscopy (SEM) images, which were collected in a Quanta FEG 650 with a field emission gun and an acceleration voltage of 20 kV. Energy dispersed electron (EDX) was used to analyze the elemental composition using an EDAX APOLO X detector coupled to the SEM microscope. The particle size histograms were acquired using the available ImageJ software. The crystal structure was determined from the X-ray diffraction (XRD) pattern collected in an XPert PANalytical Empyrean Series II – Alpha1 using Cu-Kα radiation at room temperature in the 2ϴ range of 8-70° with a scan step size of 0.015°. The acetaminophen concentration was determined in a Labomed Inc. UV-2650 spectrophotometer. The UV‒Vis diffuse reflectance (UV‒Vis/DRS) technique was used to obtain the bandgap using a UV‒Visible Shimadzu 2600 spectrophotometer with a wavelength range of 190 nm-900 nm and a polytetrafluorethylene Spectralon® as a reference.

Photolysis of acetaminophen

Photolysis was used to determine the self-photodegradation and sensitivity of acetaminophen due to sunlight exposure. Initially, a 40 ppm solution of acetaminophen was prepared in 5 L and introduced into a cylindrical parabolic composed photoreactor (CPC). Figure 1a-cshows the CPC, consisting of two continuous coupled SCHOTT Duran® borosilicate tubes with a length, external diameter, and thickness of 42 cm, 32 mm, and 1.4 mm, respectively. The acetaminophen solution was pumped using a ½ centrifugal pump horsepower, with a constant recirculation (turbulent regime flow, Re = ~21517) of 6.45 L/min. An aluminum reflective sheet was used inside the CPC to increase the incident sunlight radiation and to be redirected to the borosilicate tubes, aiming to achieve cumulative radiation of 8,000 J/m². The cumulative radiation was measured using a photoradiometer (Delta Ohm HD 2102) with an LP-UVB (300 nm-600 nm) probe, and the degradation percentage was calculated from the data collected in a UV‒Vis spectrophotometer at 245 nm (absorption maximum wavelength).

Acetaminophen adsorption on silver-doped titanium dioxide

The potential adsorption of TiO₂-A and TiO₂-B was tested by adding catalyst doses of 0.2 g/L and 0.3 g/L in 2 L of the acetaminophen solution, and commercial TiO₂ was also used to compare the results. The experiments were carried out under constant stirring and covered to avoid exposure to natural light. Aliquots were taken every 15 min for 1 h, each 30 min for 2 h, and one last sample at 3 h. After the samples were centrifuged, the supernatant was stored for UV‒Vis analysis (245 nm). The adsorption capacity of Ag-doped TiO₂ was determined to optimize the acetaminophen photodegradation test.

Photodegradation of acetaminophen

The photodegradation of acetaminophen using Ag-doped TiO₂ was evaluated using a CPC (see Figure 1a-c). Here, 5 L of the as-prepared acetaminophen solution was introduced into the CPC under the same operational conditions described in the photolysis section. The pH was adjusted to 3 using the HCl solution, reported as the optimum pH_zpc for the photodegradation of acetaminophen using TiO₂ nanomaterials [23]. Afterward, TiO2-A and TiO2-B were added at catalyst doses of 0.2 g/L and 0.3 g/L, and the CPC was exposed to sunlight irradiation. Commercial TiO₂ was also used to evaluate and compare the photocatalytic performance of Ag-doped TiO₂. Samples were removed every 20 min to calculate the instantaneous radiation (W/m²) and the cumulative radiation, considering a maximum value of 8,000 J/m². The supernatant was collected by centrifugation at 5,000 rpm for 45 min and stored at 4 °C for 24 h. UV‒Vis analysis at a wavelength of 245 nm was used to measure the acetaminophen concentration. Finally, Equation 2[24] was used to standardize the radiation time ( ), considering the sunlight UV radiation flux to have a value of 30 W/m².

where is the total volume, is the irradiated volume, is the average sunlight UV radiation, and is the irradiated average time.

Results

Characterization

The crystalline information of the titanium dioxide nanoparticles synthesized via green chemistry and those doped with metallic Ag ions at 0.5 at% (TiO₂-A) and 0.75 at% (TiO₂-B) are shown in Figure 2. The TiO₂ nanoparticle pattern shows the characteristic peaks for the tetragonal crystal structure (space Group I4₁/amd) of the pure anatase phase. These peaks located at approximately 25°, 38°, 48°, 55°, and 62° correspond to the (101), (004), (200), (105), and (204) planes, respectively, according to the Joint Committee on Powder Diffraction Standards card (JCPDS card no. 00-002-0387) [25]. In the case of Ag-doped TiO2 nanostructures, three additional peaks (marked with asterisks) can be observed at approximately 28°, 32°, and 46°, which are ascribed to doping with metallic Ag ions and indexed to the (131), (111), and (200) planes with JCPDS card no. 1-1164 [26]. Moreover, these peaks exhibited a significant increase in intensity as the concentration of metallic Ag ions increased on the TiO₂ nanoparticles and showed no effect on the crystal structure of the TiO₂ anatase phase.

The average crystallite size was estimated using the sharp and intense peaks in the patterns, which corresponded to the different crystalline domains calculated from the Debye-Scherrer Equation 3 [27].

where is the average crystallite size (nm), is the Scherrer constant value (0.9), is the wavelength in the X-ray (0.1541 nm), is the full width at half maximum (FWHM, rad), and is the Bragg diffraction angle (rad). Accordingly, the average crystallite sizes were 38 nm, 35 nm, and 34 nm with a standard deviation of ± 19 nm for TiO₂ nanoparticles, TiO₂-A, and TiO₂-B nanostructures, respectively. These values exhibited a low accuracy in the average crystallite size attributed to a possible surface effect, which can lead to a strong agglomeration and a polycrystalline nature. According to previous results, the crystallite size of the anatase phase tends to increase rapidly after 60 min of annealing at high temperature, which promotes an increase in the kinetic energy. This causes the Ostwald ripening effect consisting of the growth and deposition of small crystallites onto large crystallites over time, promoting the formation of polycrystalline TiO₂ nanoparticles with different sizes [28].

The morphology information of the nanomaterials was analyzed from the scanning electron microscopy (SEM) images shown in Figure 3a-f. Figure 3a,b shows the formation of small agglomerates of TiO₂ nanoparticles with a semispherical shape and a wide diameter size distribution of approximately 49 ± 20 nm (seeFigure 4). Although the C. citratus extract played an important role as a capping agent to control growth, the Ostwald ripening effect promoted the formation of large polycrystalline grains from smaller TiO₂ nanoparticles. In the case of the Ag-doped TiO₂ nanoparticles, the presence of smaller agglomerates can be observed. Figure 3c, d shows TiO₂ nanoparticles with the photodeposition of 0.5 at% metallic Ag on the surface, obtaining nanostructures with a diameter size of 37 ± 18 nm (see Figure 4). The enhanced dispersion is attributed to the presence of Ag on the surface, leading to the reduction of the surface effect and electrostatic interactions between TiO₂ nanoparticles. Similar results are observed in Figure 3e, f regarding the TiO₂ nanoparticles doped with metallic Ag at 0.75 at%. Here, a slight decrease in the diameter size of approximately 31 ± 17 nm was evidenced from Figure 4, corroborating the contribution of metallic Ag in the dispersion of TiO₂ nanoparticles. Despite the small difference in the concentrations used for Ag doping, EDX analysis exhibited a significant increase in the weight content from 14% to 29% when using 0.75 at% instead of 0.5 at%. This is a promising result since the higher the content of metallic Ag on the surface is, the better the photocatalytic activity on the nanostructures by providing more active sites for the photodegradation of acetaminophen.

The diffuse reflectance spectra of the Ag-doped TiO₂ nanostructures are shown in Figure 5. The visible light absorption showed no improvements after the photodeposition of Ag metal ions on the surface of TiO₂. The absorbance was calculated using Equation 4 based on the Kubelka-Munk function [29], in which the light absorption decreased due to the inverse relationship with the reflectance. Additionally, the indirect bandgap values were estimated following the Wood and Tauc method described in Equation 5 [30].

where is the absorbance, is the absolute reflectance, is the absorption coefficient, is the scattering coefficient, is the Planck constant, is the frequency, and is the transition value mode. The transition values of 1/2, 2, 3/2, or 3 correspond to the permissible direct, permissible indirect, direct prohibited, and indirect prohibited transition modes, respectively [31]. The value is 2 for pure and doped TiO₂ nanoparticles, ascribed to a permissible indirect transition mode [32]. According to Figure 6, the band gap energies were 2.94 eV, 3.08 eV, and 3.10 eV for TiO₂ nanoparticles TiO₂-A and TiO₂-B, respectively.

Currently, the literature reports low bandgap energies of TiO₂ nanoparticles doped with Ag ions obtained from different synthesis methods as a result of an electron-hole recombination delay due to the energy states located above the valence band [33]. In this study, the bandgap energy values were significantly higher (see Table 1), attributed to the reduction of the electron-hole recombination delay. However, there was no difference between the value for TiO₂ nanoparticles compared to TiO₂-A and TiO₂-B, since the photodeposition method reduced the optical properties through the reduction of Ag⁺ ions to metallic Ag [19].

Photocatalytic degradation

The direct photolysis process was performed to determine the sensitivity degree of a 40 mg/L acetaminophen solution exposed to solar irradiation in the absence of the catalyst until a minimum equivalent radiation of 8,000 J/m² was reached. Equation 6was used to calculate the degradation percentage in the photolysis, adsorption, and heterogeneous photocatalysis processes [40].

where is the initial concentration (mg/L) and is the concentration as a function of time/cumulative radiation. In the results, the photolysis process achieved a degradation rate of 14.5%, showing that the pollutant is slightly susceptible to self-degrade due to exposure to solar radiation. This is an advantage since it represents a savings in terms of photocatalyst material and increases the overall rate during the photocatalytic process. This self-degradation percentage was similar to that reported in other research, which found a reduction of 12% of acetaminophen after irradiation with a mercury lamp (500 W) for 60 min with an initial concentration of 50 mg/L [41].

On the other hand, Table 2 shows the results obtained from the adsorption experiments avoiding exposure to natural light. Adsorption of acetaminophen with the commercial photocatalyst P-25 was found to be negligible, whereas TiO₂ nanoparticles synthesized using the C. citratus extract and TiO₂-B (0.75 Ag at%) nanostructures showed higher removal percentages due to improvements in their surface area properties. A total surface area of 64.4 m²/g for TiO₂ nanoparticles synthesized via green chemistry using C. citratus extract has been previously reported [32], which is 41% higher than that reported for commercial P-25 (45.7 m²/g) [42]. Moreover, an increased photocatalyst dose decreases the adsorption removal percentage attributed to the surface interaction between the nanoparticles (aggregation), leading to a reduction in the number of available adsorption sites [43], [44].

The photodegradation tests were performed on consecutive days and at the same time to reduce the variation due to the environmental conditions.Figure 7a,bshows the experimental results of acetaminophen photodegradation using photocatalyst doses of 0.2 g/L and 0.3 g/L of commercial P-25, TiO₂ nanoparticles, and TiO₂-B nanostructures. Additionally, the behavior of the relative C_t/C₀ concentration was evaluated regarding the standardized radiation time (t_30W). At both photocatalyst doses, commercial P-25 presented the lowest degradation efficiency attributed to the adsorption capacity of acetaminophen, whereas TiO₂-B (0.75 Ag at%) showed an intermediate performance due to the photodeposition of metallic Ag ions. In this case, the reduction of Ag⁺ ions to metallic Ag promotes surface poisoning of the photocatalyst, leading to a reduction in the photocatalytic activity compared to TiO₂ nanoparticles synthesized via green chemistry. Moreover, the reaction rate decreases considering the high concentration of the photocatalyst (0.3 g/L), which was attributed to a shielding effect from the reduced light irradiation in the innermost part of the reactor [45], [46].

The mechanism for the photodegradation of acetaminophen is displayed in Figure 8, which was based on information previously reported in the literature [47, 48]. The mechanism initially consisted of the electron/hollow pair (e^-/h⁺) generation from the activation of the photocatalyst using simulated or natural UV‒Vis radiation, as detailed in Equations 7-12. Then, the hydroxyl radicals attack the ortho- or para-positions of the aromatic ring in the acetaminophen structure to produce N-Methyl formamide (C₂H₅NO), hydroquinone, and 1,4-benzoquinone. Finally, the opening ring/ring cleavage and the oxidation of species, such as maleic acid (C₄H₄O₄), formic acid (CH₂O₂), and glycolic acid (C₂H₄O₃), are generated until their mineralization (CO₂ and H₂O).

Conclusions

This study reports the green synthesis of titanium dioxide (TiO₂) nanoparticles using a Cymbopogon citratus (C. citratus) extract as a capping agent and surface modification with metallic Ag via the photodeposition method. Although C. citratus acted as a capping agent to control the growth and particle size, the TiO₂ nanoparticles exhibited polycrystallinity attributed to a strong surface effect, leading to the formation of agglomerates with large diameter sizes. The large agglomerates reduce the available active sites for the adsorption of naphthalene since its removal is expected from the photocatalytic degradation instead. On the other hand, the reduction from Ag ions to metallic Ag on the surface of the TiO₂ nanoparticles promoted a direct effect on the optical properties, showing no differences compared to the unmodified TiO2 nanoparticles. Although this effect caused surface poisoning on the photocatalyst, decreasing the photodegradation of acetaminophen, the results showed a better performance compared to commercial P-25. Therefore, the green synthesis of TiO₂ nanoparticles and photodeposition of metallic Ag on the surface provides an enhanced photocatalytic activity toward the degradation of organic pollutants, in which further research work can involve optimization aiming at better optical properties.

Acknowledgment

This work was performed by the Nanomaterials and Computer-Aided Process Engineering Research Group in the Laboratories of the Chemical Engineering Program at Universidad de Cartagena. The authors are grateful to Universidad de Cartagena for financial support through the Strengthening Plan with Grant Number 062-2018.

References

Author notes

^a Corresponding author. E-mail: aherrerab2@unicartagena.edu.co

Additional information

How to Cite: M. Coronell, G. Toscano-Lucas, R. Solano y A. Herrera, “Green Synthesis of Silver-Doped Titanium Dioxide Nanostructures for Acetaminophen Degradation Under Solar Radiation” Ing. Univ. vol. 26, 2022. https://doi.org/10.11144/Javeriana.iued26.gsst

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