Photocatalytic Oxidative Degradation of Carbamazepine by TiO₂ Irradiated by UV Light Emitting Diode

Abstract

Here, ultraviolet light-emitting diodes (UV-LED) combined with TiO₂ was used to investigate the feasibility of carbamazepine (CBZ) degradation. The effects of various factors, like crystal form of the catalyst (anatase, rutile, and mixed phase), mass concentration of TiO₂, wavelength and irradiation intensity of the UV-LED light source, pH of the reaction system, and coexisting anions and cations, on the photocatalytic degradation of CBZ were studied. The mixed-phase (2.8 g/L) showed the best degradation efficiency at 365 nm among three kinds of TiO₂, wherein CBZ (21.1 µM) was completely oxidized within 1 h. The results of batch experiments showed that: (i) CBZ degradation efficiency under UV-LED light at 365 nm was higher than 275 nm, due to stronger penetrability of 365 nm light in solution. (ii) The degradation efficiency increased with increase in irradiation intensity and pH, whereas it decreased with increase in initial CBZ concentration. (iii) The optimal amount of mixed-phase TiO₂ catalyst was 2.8 g/L and excessive catalyst decreased the rate. (iv) The co-existence of CO₃²⁻, HCO₃⁻, and Fe³⁺ ions in water significantly accelerated the degradation rate of photocatalytic CBZ, whereas Cu²⁺ ions strongly inhibited the degradation process of CBZ. ·OH was found to be the main active species in the UV-LED photocatalytic degradation of CBZ. UV-LED is more environmentally friendly, energy efficient, and safer, whereas commercial TiO₂ is economical and readily available. Therefore, this study provides a practically viable reference method for the degradation of pharmaceuticals and personal care products (PPCPs).

1. Introduction

In recent years, pharmaceuticals and personal care products (PPCPs) have gained more attention, due to the manufacturing and application of pesticides and medicines for treating humans and livestock [1]. PPCPs and its metabolites can be found in water bodies during regular inspections in aquatic environments conducted world-wide. Since, most PPCPs have very stable and complex molecular structures they cannot be easily captured and degraded by organisms. This increases the environmental pollution and is potentially harmful to human health [2,3]. At the same time, the presence of trace amounts of chemical pollutants in the aquatic environment may lead to the antibiotic-resistance in organisms and the generation of resistant genes, posing a serious threat to the entire ecosystem [4,5,6]. Therefore, PPCP must be completely eliminated from the natural water bodies to reduce its threat to all biomes.

Carbamazepine (CBZ, C₁₅H₁₂N₂O), a dibenzazepine derivative, has molecular structure similar to that of tricyclic antidepressants. At present, it is widely used to treat trigeminal neuralgia, epilepsy, and mental illness [7].

Table 1. Molecular structure and solubility of carbamazepine (CBZ).

Chemical Formula	Molecular Weight/(g·mol−1)	Solubility (20 °C)/(µg·mL−1)	Molecular Structure
C15H12N2O	236.27	36.5

enlists the molecular structure and solubility of CBZ. Due to its stable structure, the removal efficiency of CBZ is generally less than 10% by traditional sewage treatment methods like coagulation/flocculation, chlorination, and microbial degradation [8]. The rest of it (major portion) is discharged into different aqueous environments either as a raw drug or its degradation intermediates. It is now commonly found in sewage, surface water, groundwater, and even drinking water. Its residues can trigger homologous estrogen activity and cause reproductive toxicity [9,10]. Hence, there is an urgent need to develop an environmentally friendly, college-level method to degrade or remove CBZ from the water bodies.

Various advanced oxidation process (AOP) methods are remediation techniques, effective in the removal of PPCPs. They include treatment methods using UV radiations in combination with H₂O₂, ozone, or titanium dioxide [11,12,13,14]. Most AOP methods can effectively remove CBZ residues (93–100%) from water [15,16,17,18]. One kind of AOP, semiconductor based photocatalytic oxidation technology, has the characteristics of environmental protection and shows great application prospects in the treatment of organic pollutants. Hydroxyl radicals have extremely strong oxidizing ability and, similar to most of the macromolecular organic pollutants having complex structures, can be oxidized and decomposed into smaller molecular weight and non-toxic organic compounds, or even completely decomposed into inorganic substances. Nano-semiconductor TiO₂ particles are endowed with characteristics like green environmental protection, high chemical stability, large specific surface area, and low cost. They have become increasingly popular as photocatalytic semiconductor materials, because of their ability to photocatalytically decompose water [19]. Many previous studies by scholars have shown that TiO₂ photocatalysis by ultraviolet irradiation is an efficient and environment-friendly technique for removing organic antibiotic compounds from the aquatic environment. However, the main limitation is the source of UV light in the practical applications of photocatalysis. The most commonly used source, a low and medium pressure mercury lamp, has many short-comings, such as fragility, bulkiness, serious heat radiations, high energy consumption, short life (500–2000 h), and so on [20]. In addition, mercury ultraviolet lamps emit light of only a fixed wavelength in a very narrow range and also have a risk of pollution due to mercury leakage [21].

In recent years, UV-LED technology has been widely explored for its applications, due to its unique advantages such as environmental protection, long service life, and short warm-up time. LED lights require low energy input and each LED lamp bead is very small, making it flexible and easy to install and transport [22,23]. In addition, the emission wavelength of UV-LEDs can be easily adjusted according to the material composition of the lamp beads. Today, UV-LED is a more powerful narrow-band light emitting device with shorter relaxation times [24]. Compared with traditional ultraviolet light, ultraviolet LED technology is more suitable for wastewater treatment, s as it is easy to regulate. For example, Cai et al. used UV-A type LED light to irradiate TiO₂ nanoparticles to photo-catalyze antibiotics in wastewater, and their research showed that continuous UV-LED/TiO₂ photocatalytic technology could be applied to 100 ppb concentration of sulfamethoxazole/methoxybenzyl above 320 nm. The degradation rate of pyridine was >90% [25]. Liang et al. found that an UV-LED/TiO₂ photocatalytic reaction system, using periodic lighting control, was extremely effective for the degradation and removal of PPCP. At the same time, the use of porous titanium dioxide substrate significantly reduced the energy requirements of the reaction system [26]. It was also found that under certain experimental conditions, the UV-LED/TiO₂ process could effectively decompose methylene blue (MB) in water. This indicated that UV-LED is a radiation source that effectively stimulates the photocatalytic action of TiO₂, and this technology has a high potential for photodegradation [27]. In all, the UV-LED/TiO₂ system is a valuable technology, especially for wastewater treatment containing PPCP.

At present, there is only limited research on photocatalytic degradation of CBZ, using a more energy-saving and environmentally safe UV-LED source and an economical and readily available commercial TiO₂, which inspired us to explore the feasibility of this study. Herein, single-factor experiments were conducted to study the effects of crystal forms of nano-TiO₂, the amount of TiO₂, the wavelength of UV-LED and its irradiation intensity, CBZ concentration in water, pH of aqueous solution, and coexisting anions and cations common in water, on the degradation efficiency of CBZ. Our research demonstrated the feasibility of this UV LED combined with TiO₂ photodegradation technology and explored the best operating conditions in actual water treatment project.

2. Results and Discussion

2.1. Characterization of Three Different Crystal Forms of TiO₂ Particles

Scanning electron microscopy (SEM) was employed to characterize the morphologies of the three different phases of TiO₂. Figure 1a–c showed the particle sizes of TiO₂ nanoparticles, viz. anatase, rutile, and mixed phases, to be 20–100, 50–100, and 20–100 nm, respectively. The UV-Vis diffuse reflectance spectra (UV-Vis DRS, Figure 1d) showed that all the three phases of TiO2 nanoparticles could absorb light below 420 nm. The N2 adsorption-desorption tests were used to calculate the surface areas of the three crystal forms of TiO₂. According to

Table 2. Characteristics of nano-TiO₂ nanoparticles.

Crystalline Phase	Composition	Particle Size (nm)	BET Surface Area (m2/g)
Anatase	100% anatase	20–100	78.7
Rutile	100% rutile	50–100	32.2
Mixed	83% anatase + 17% rutile	20–100	102.6

and Figure 1e, the surface areas of the anatase, rutile, and mixed phases were 78.7, 32.2, and 102.6 m²/g, respectively, by Brunner−Emmet−Teller (BET). X-ray diffraction patterns of three forms of TiO₂ nanoparticles can be seen in Figure 1f. Anatase and rutile matched with their respective standard cards (JCPDS 89-4921) and (JCPDS 21-1276). The equation, X = 1/(1 + 0.8I_A/I_R), was used to calculate the rutile content in the mixed phase, where X was the mass fraction of the rutile phase. I_A and I_R in the equation were the 2θ values of the characteristic peaks of anatase and rutile forms, which were 25.3° and 27.4°, respectively. Herein, the ratio of anatase to rutile forms in the mixed crystal TiO₂ form was about 83:17, which was similar to the common commercial P25 (80:20).

2.2. Control Experiments of CBZ Degradation

The control experiments for CBZ degradation were also conducted. Figure 2a shows that the CBZ degradation failed to proceed in the absence of any of the following: illumination or the mixed-phase TiO₂ photocatalyst. Owing to the adsorption of CBZ on the surface of TiO₂, its concentration decreased by 5%, after reaching adsorption equilibrium. The degradation rate (η_t) of CBZ was calculated according to Equation (1) (Where η_t is the conversion rate (percentage) at a given time t, C₀ is the initial concentration of the reagent, and C_t is the residual concentration at a given time t). The concentration of CBZ decreased sharply under irradiation of UV-LED light for 60 min in the presence of mixed-phase TiO₂, wherein it was almost completely degraded (CBZ degradation rate reached 98.5%). Since, the valence band potential (3.1 eV vs. NHE) was more positive than H₂O/·OH (2.74 eV vs. NHE), the TiO₂ photocatalyst excited by UV-LED, could produce highly oxidized photogenerated holes and hydroxyl radicals that could oxidize the CBZ molecules into H₂O, CO₂, and other non-toxic small molecules. It is speculated that the CBZ photocatalytic degradation step may be a common photocatalytic reaction, as shown in reaction formulae 1–4. In addition, the CBZ degradation process was consistent with the pseudo-first-order kinetic models with a degree of fit R² > 0.99 (Figure 2b). These results indicated that the photocatalytic method of UV-LED combined with TiO₂ photocatalyst is a technology that can be implemented to effectively remove CBZ molecules from an aqueous environment.

η_t = (C₀ − C_t)/C₀·100%

(1)

$${\mathrm{TiO}}_2+\mathrm{UV}-\mathrm{LED}\to {\mathrm{e}}_{\mathrm{CB}}^{-}+{\mathrm{h}}_{\mathrm{VB}}^{+}$$

(2)

$${\mathrm{H}}_2\mathrm{O}/{\mathrm{OH}}^{-}+{\mathrm{h}}_{\mathrm{VB}}^{+}\to •\mathrm{OH}+{\mathrm{H}}^{+}$$

(3)

$$\mathrm{CBZ}+{\mathrm{h}}_{\mathrm{VB}}^{+}/•\mathrm{OH}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2+\mathrm{products}$$

(4)

2.3. Effect of UV-LED Wavelength and Its Intensity

Figure 3 shows the effects of two different UV-LED emission wavelengths on CBZ degradation. The CBZ photocatalytic degradation rate (98.5%) using 365 nm irradiation was significantly higher than that of 275 nm (36.78%). The degradation rate constant at 365 nm irradiation was 0.067 min⁻¹, whereas at 275 nm it was only 0.007 min⁻¹. Generally, the energy of the shorter-wavelength ultraviolet rays is higher than longer-wavelength, however, its penetrating power is weaker [27,28] Therefore, we speculate that the difference in degradation effects could be due to the differences in penetrating power. Compared with 275 nm irradiations, the use of 365 nm UV-LED source is a more cost-effective, energy-saving, and efficient method for photocatalytic oxidation of CBZ.

The radiation intensity of UV-LED is one of the important factors affecting the photocatalytic reaction, and it directly determines the number of photo-generated electrons and holes in the photocatalytic system. The effects of UV radiations of different intensities (220, 332, 437, 551, 663, and 774 μW/cm²) on the degradation rate of CBZ at a fixed wavelength of 365 nm, were studied. Obviously, as shown in Figure 4a, the degradation rate increased with increase in light intensity. In particular, CBZ could be completely degraded at an irradiation intensity of 774 μW/cm² within 60 min. However, the degradation rate of CBZ at a light intensity of 220 μW/cm² within 60 min was only 57.23%. When the light intensity was changed from 220 μW/cm² to 774 μW/cm², the reaction rate constant for photocatalytic degradation showed a 6.7-fold increase (Figure 4b).

2.4. Effect of Crystalline Form of Nano-TiO₂

The photocatalytic performances of the three crystalline forms of TiO₂ on CBZ degradation were studied. The degradation curves and pseudo first-order kinetic models are shown in Figure 5. The degradation rate of anatase form (87.78%) was significantly higher than the rutile form (80.29%), within 60 min. The mixed-phase TiO₂ showed the highest rate and could degrade 98.5% of CBZ within 60 min. At the same time, the reaction rate constant of mixed-phase was as high as 0.067 min⁻¹, which was nearly 2 and 3 times higher than those of anatase and rutile forms, respectively. Previous studies had demonstrated that the photogenerated carriers could easily transfer at the interface between the anatase and rutile forms, which promoted charge separation efficiency and photocatalytic activity [29].

2.5. Effect of Initial CBZ Concentration

CBZ solutions of four different concentrations (10.6, 21.2, 31.7, and 42.3 µM) were used to evaluate the effect of initial concentration of CBZ on degradation. As shown in Figure 6, when the CBZ concentration in the original aqueous solution increased from 10.6 to 42.3 µM, the degradation rate decreased slightly. However, the rate constant decreased sharply from 0.084 to 0.040 min⁻¹. The reason for this phenomenon was that when the initial CBZ concentration was high, a higher number of CBZ molecules competed for the photogenerated holes and hydroxyl radicals in the system. The reduction in the main active material for photocatalytic reaction reduced the degradation rate of CBZ.

2.6. Effect of Nano-TiO₂ Dosage

The effect of amount of photocatalyst on CBZ degradation was investigated by changing the mass concentration of initial TiO₂ (mixed crystal form) in the range of 0.8–3.6 g/L. Figure 7 shows the degradation curves of CBZ with different dosages of TiO₂. When the amount of TiO₂ was 0.8 g/L, the degradation rate reached 87.78% within 60 min. The degradation rate gradually reached its maximum value (98.5%) when 2.8 g/L of TiO₂ was used (Figure 7a). Further, as the TiO₂ dosage was further increased from 2.8 to 3.6 g/L, the degradation efficiency decreased. High concentration of catalyst increases light scattering and reduces light penetration, which decreases the utilization of light. These factors were responsible for the decrease in degradation activity. CBZ underwent fastest degradation, with a rate constant of 0.058 min⁻¹, when the TiO₂ mass concentration was 2.4 g/L (Figure 7b).

2.7. Effect of pH

During the entire process of photocatalytic oxidation, the pH of reaction system also greatly influences the degradation efficiency of the target compounds, which in turn is mainly affected by the generation of hydroxyl radicals [30]. Figure 8 shows the degradation of CBZ under different pH conditions (3.0, 5.0, 7.0, 9.0, and 11.0). As the pH increased from 3.0 to 11.0, the degradation rate increased gradually. The reaction rate was the fastest when the pH was 11 and its rate constant reached 0.110 min⁻¹, which was about 4 times than that at pH = 3.0. The ·OH species was mainly formed from OH⁻, and the production efficiency of OH⁻ was much higher than that of H₂O to produce ∙OH. Therefore, an aqueous alkaline environment was more conducive for the degradation of CBZ.

2.8. Influence of Co-Existing Ions in the Aquatic Environment

Coexistence of different anions and cations in water has different effects on the photocatalytic degradation of CBZ [31]. The concentration of a fixed coexisting anion and cation in the CBZ degradation reaction system was 1 mmol/L, keeping other conditions unchanged (C(mixed-phase TiO₂) = 2.0 g/L; C(CBZ)₀ = 18.8 µM; operating temperature = 20 °C; wavelength of UV-LED = 365 nm; I₀ = 774 µW/cm²; and operating pH = 7.6). The effects of common anions and cations in water on the degradation of CBZ were investigated. As shown in Figure 9, the presence of Cl⁻ in the system had little effect on the photodegradation process of CBZ, whereas NO₃⁻ had a slight inhibitory effect. The SO₄²⁻ promoted the degradation to certain extent, while the presence of CO₃²⁻ and HCO₃⁻ significantly promoted the photocatalytic degradation of CBZ. When a large number of CO₃²⁻ and HCO₃⁻ coexisted in the solution, the solution was alkaline. An increase in the number of OH⁻ ions was useful for the formation of ·OH, which improved the photocatalytic activity of the system. Hu et al. [32] reported that CO₃²⁻ and HCO₃⁻ promoted the photocatalytic degradation of sulfamethoxazole, which was consistent with the results of this study.

The effects of four commonly coexisting cations on the degradation of CBZ in natural water are shown in Figure 10. The presence of Ca²⁺, Mg²⁺, and Cu²⁺ ions inhibited the photocatalytic degradation to a certain extent and the order of their inhibition effects was: Cu²⁺ > Ca²⁺ > Mg²⁺. The Fe³⁺ ions had a positive effect, and 100% CBZ degradation was observed in less than 20 min. The photocatalytic degradation of CBZ promoted by Fe³⁺ ions could be attributed to the fact that Fe³⁺ traps photo-generated electrons generated by TiO₂, thereby effectively reducing the chance of electron-hole recombination. In addition, the addition of Fe³⁺ ions could form a Fenton system, thereby increasing the number of free active·OH species in the reaction system, which significantly increased the degradation capacity and degradation rate of CBZ.

2.9. Influence of Free Radical Scavenging

The ·OH radical is considered to be the main active species in TiO₂ photocatalytic degradation reaction [33]. By adding different concentrations of t-butanol (TBA) as the OH scavenger to the reaction system, the effect of number of active ·OH radicals on the photocatalytic degradation was studied. As shown in Figure 11, the presence of TBA significantly inhibited the photocatalytic degradation of CBZ. With increase in TBA concentration in the reaction system, the inhibitory effect was enhanced. When the TBA concentration in the system was 10 mM, the degradation efficiency was 19.91% after 60 min of UV-LED irradiation. As the TBA concentration increased to 50 mM, the degradation efficiency was only 6.58%. These experiments also show that OH species are indeed the main active species in CBZ degradation.

3. Materials and Experimental Methods

3.1. Materials

Carbamazepine (≥98%, Shandong Xiya Chemical Co. Ltd., Linyi, Shandong, China) A stock mother liquor of CBZ, with a concentration of 211 µmol/L, was prepared and then serially diluted with ultrapure water, according to the concentration required for each experiment. Acetonitrile (HPLC grade purity, Merck KGaA, Darmstadt, Germany). HCl (0.1 mol/L), NaOH (0.01 mol/L), and phosphate buffer (0.01 mol/L) solutions were used to adjust the pH of the reaction system. The water used was ultra-pure and was prepared using Millipore Milli-Q ultra-pure water system, with a resistivity of 18.2 MΩ·cm. Three commercial nano TiO₂ catalysts having different crystal forms: pure anatase, mixed-phase, and rutile forms (>98%, Shandong Xiya Chemical Co. Ltd., Linyi, Shandong, China). All other reagents were from Damao Chemical Reagent Factory (Analytical grade, Tianjin, China).

3.2. Test Device

The photocatalytic degradations were performed in a 500 mL open plexiglass container. In order to prevent light energy, UV-LED light, and heat energy, a layer of aluminum foil was tightly wrapped around the outer wall of the reactor. The UV-LED tube was placed in a cylindrical radiation window having a diameter of about 5 cm, which was made of quartz. The light source was composed of two self-made UV-LED units, and the working wavelengths of the two units were 275 and 365 nm. Each unit contained 40 LED chips (manufactured by Shenzhen Julan Technology Co. Ltd., China). The output of 275 nm was about 2.8 mW with a current of 40 mA and the output of 365 nm was about 700 mW with a current of 500 mA. The fluence rate was measured in all experiments using a potassium ferrioxalate actinometer [34]. A low-temperature constant temperature circulating water tank was used to circulate cooling water on the outer wall of the reactor in order to control the temperature during the operation at 20 ± 0.5 °C. The intensity of the radiations irradiated into the reactor was measured using an IL-2400 irradiation photometer (International Light, USA). The diagram of UV-LED/TiO₂ photocatalytic device used in this study is shown in Figure 12.

3.3. Analytical Method

In the UV-LED/TiO₂ photocatalytic experiment, 2 mL of the sample suspension was withdrawn at regular time intervals, and the suspension was filtered through a 0.22 µm syringe filter (Tianjin Jinteng Experimental Equipment Co. Ltd., Tianjin, China). The concentration of CBZ in the test sample was measured by HPLC (Waters e2695) using a Symmetry C18 column [35]. The reaction conditions were as follows: the mobile phase consisted of 60% acetonitrile and 40% pure water. The injection volume was 100 μL, the flow rate was 1 mL/min, the column temperature was maintained at 35 °C, and the UV-LED wavelength of CBZ was below 286 nm.

4. Conclusions

The main conclusions of this study were as follows:

(1) UV-LED irradiated TiO₂ photocatalytic technology could effectively degrade CBZ in water, and the degradation rate of CBZ reached 98.5% after 60 min of reaction. The photocatalytic reaction process fit the pseudo-first-order kinetic model.

(2) Photocatalytic degradation of CBZ under LED wavelength of 365 nm was more effective than 275 nm, which could be due to the stronger permeability of 365 nm light in solution.

(3) Increasing the irradiation intensity of UV-LED could lead to increase the number of effective photons absorbed on the surface of TiO₂. Hence, it could significantly promote the degradation of CBZ.

(4) The photocatalytic activities of different crystalline forms of TiO₂ varied significantly and were in order of: mixed-phase > anatase > rutile.

(5) The optimal amount of mixed-phase TiO₂ catalyst was 2.8 g/L, at which the CBZ degradation efficiency was the highest and excessive catalyst decreased the rate.

(6) The CBZ degradation efficiency increased with increase in pH.

(7) The CBZ degradation efficiency decreased with the increase in initial CBZ concentration in the aqueous solution.

(8) The effects of SO₄²⁻, NO₃⁻, Cl⁻, Ca²⁺, and Mg²⁺ on the photocatalytic degradation of CBZ were negligible, and the presence of CO₃²⁻, HCO₃⁻, and Fe³⁺ ions in water significantly promoted CBZ degradation.

(9) Hydroxyl radical is the main active species in the degradation of CBZ by UV-LED photocatalytic reaction system.