How anatase TiO₂ with {101} {001} and {100} surfaces affect the photooxidation process of roxithromycin

Antibiotics, widely used to treat various diseases due to their abilities of effectively treating bacterial infections and preventing bacterial infections, can be processed into anti-inflammatory drugs, animal feed additives, animal growth promoting agents and sterilizations (Hang et al. 2007; Ding et al. 2015; Gothwal & Shashidhar 2015). However, more than 30 antibiotics have been found in sewage, surface water and even underground drinking water environment, which were mainly derived from industrial wastewater, medical antibiotics and veterinary antibiotics (Dodd et al. 2006; Mahmoud et al. 2017). Antibiotics include macrolide antibiotics, tetracycline and its derivative antibiotics, glycoside antibiotics, amino acid and its derivative antibiotics, β-lactam antibiotics and so on (Zhang et al. 2004). Roxithromycin (ROX), classified as a semi-synthetic 14-membered macrolide antibiotic, has antibacterial activity against Mycoplasma pneumoniae, Chlamydia pneumoniae, and venom (Xu et al. 2017). ROX is relatively stable in physical and chemical properties, has strong stability to light, heat and humidity, and has a half-life of 130 days to 180 days in water (Massé et al. 2014). It has serious adverse effects on animals, plants and microorganisms in the natural environment, such as the emergence of super bacteria and inhibiting the growth of plant roots and the emergence of drug resistance in humans and animals (Alok et al. 2008). Because of their high toxicity and high residues, antibiotics have attracted widespread attention from environmental scholars (Grujić et al. 2009). In the report of Gao et al., the concentration of ROX detected at the outlet of the sewage treatment plant was at least 1 μg/L (Gao et al. 2012). Peng et al. (Peng et al. 2011; Yang et al. 2011) measured the concentration of ROX in the Guangzhou section of the Pearl River as more than 1 μg/L. Zhou et al. (2011) detected the maximum concentration of ROX in the bottom mud of the Haihe River in China, which was as high as 67.2 μg/L. It is shown that currently a large number of ROX has been detected in water bodies and sediments, which are threatening human health. Therefore, research about the degradation of ROX is important and necessary. The chemical structure of ROX is in Figure S1 of the appendix.

TiO₂ crystals are widely used in photocatalytic processes due to their low cost and fabulous catalytic performance (Babić et al. 2017). Experiments have shown that controlling the specific exposed surface of TiO₂ can improve the catalytic efficiency of titanium dioxide. Liu et al. (2010a) proposed a kind of nanosized anatase TiO₂ single crystal with the {001} facet of 18%, which had a lower band gap and exhibited excellent photocatalytic activity both for generating OH radicals and for splitting water into hydrogen. In 2016, Yan et al. (2016) investigated the effect of the distribution of exposed facets in adsorption, and the result showed that anatase {001} facets had stronger adsorption affinity than anatase {101} facets to reduce arsenic. These investigations into facet effect were based on the controlled synthesis methods of TiO₂. In the recent years, there have been some methods to control the mainly exposing facets of TiO₂ such as {101}, {001} and {100} facets (Han et al. 2009; Li & Xu 2010; Pan et al. 2011). Amano et al. (Amano et al. 2009a, 2009b) used P25 and potassium hydroxide to perform a high-temperature and high-pressure hydrothermal reaction in a reactor to prepare potassium titanate nanowires. The synthesized potassium titanate nanowires were used as precursors and successfully synthesized octahedral-shaped anatase titanium dioxide during the hydrothermal reaction, which mostly exposed {101} crystal planes. In 2010, Liu's group (Liu et al. 2010b) used titanium powder as a titanium source and hydrofluoric acid solution as a solvent to hydrothermally synthesize the flower-like structure of exposed {001} face titanium dioxide. The experimental results showed that, compared with P25, the flower-like structure {001} facet TiO₂ had a higher photocatalytic degradation performance for methylene blue. In 2012, Li et al. (2012) used P25 as a titanium source to hydrothermally synthesize sodium titanate nanotubes in NaOH solvent, and then used sodium titanate nanotubes as precursors to hydrothermally synthesize anatase titania square facet nanometer rods (TFNRs), which mainly exposed the {100} surface, and the result indicated that the {100} surface may play an important role in the photocatalytic reaction. In our recent report (Wei et al. 2019), three kinds of anatase TiO₂ crystals with the main facets of {101}, {001} and {100} were synthesized and used to investigate the facet effect and pH effect for reducing arsenic in water.

TiO₂ has been used to degrade ROX in recent years. In 2010, Huo et al. (2010) reported the degradation of ROX by poly-o-phenylenediamine/TiO₂/fly ash hollow microbeads. In 2014, Kwiecien et al. (2014) proposed the UV-induced TiO₂ photocatalytic degradation of ROX. It is obvious that TiO₂ possesses enough ability to reduce ROX. However, the facet effect and the pH effect, which are the important factors during the reducing process, have not been discussed yet. Therefore, in the present paper, three kinds of anatase TiO₂ crystals with the main facets of {101}, {001} and {100} are used to show the ROX photocatalytic degradation kinetics and the pH effect.

ROX was obtained from JK Chemical Co., Ltd, China. Degussa titanium dioxide P25 (80% anatase and 20% rutile) was obtained from Chengdu Cloning Chemical Reagent Factory, China. Sulfuric acid (H₂SO₄), hydrochloric acid (HCl), potassium hydroxide (KOH) and sodium hydroxide (NaOH) were purchased from Guangzhou Hualisen Trading Co., Ltd, China. Ammonium nitrate (NH₄NO₃), hexamethylenetetramine (C₆H₁₂N₄) and ammonium carbonate ((NH₄)₂CO₃) were purchased from Shanghai Chemical Reagent Factory, China. The different TiO₂ crystals have been synthesized as in our previous paper (Han et al. 2009; Li & Xu 2010; Pan et al. 2011; Wei et al. 2019), and the experimental details are in the appendix.

During the experiments, all reagents were analytically pure and the solutions were prepared in distilled water.

Powder X-ray diffraction (XRD, Rigaku-Ultima III) analyses were carried out to determine the crystal structures of the obtained anatase TiO₂. Scanning electron microscope (SEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEM-2100F from JEOL. Ltd) analyses were carried out to characterize the morphologies of TiO₂ crystals. The Brunauer-Emmett-Teller (BET, Quantachrome Instruments, USA) analyses gave the specific surface area of three types of TiO₂. The visible spectrophotometer analyses were used to detect the concentrations of ROX, and the detail methods and the standard curves are in the appendix, Figure S2. Liquid chromatography-mass spectrometry (LC/MS, LC/MS-2020 from Daojin, Japan) analyses were used to identify the degradation products of ROX.

The photooxidation process of ROX was carried out with a 0.5 L glass flask containing 0.3 g TiO₂ (2,000 mg/L) and 0.25 L of ROX (100 mg/L). 0.1020 g of ROX was dissolved in a volumetric flask of 100 mL with 10 mL 0.01 M HCl, then diluted to the volume with distilled water to obtain 1,000 mg/L solution. 10 mL of 1,000 mg/L ROX was added to a 100 mL volumetric flask, and diluted to the volume with distilled water to obtain the preparation solution. Then the solution with 100 mg/L ROX was adjusted to the desired pH values, such as pH 3 to pH 10, by adding HCl or NaOH. The adsorption equilibrium of ROX was obtained by magnetic stirring for 30 minutes in the dark.

Hydroquinone, ammonium oxalate and isopropanol were added to ROX in order to capture ▪O²⁻, ▪OH and h⁺, respectively (Andrei et al. 2019). At the same time, the experimental conditions with C_TiO2 = 2,000 mg/L, C_ROX = 100 mg/L, C_hydroquinone = 0.1 mmol/L (or C_{ammonium oxalate} = 0.1 mmol/L or C_isopropanol = 0.1 mmol/L), and the best pH environments for the oxidation were chosen.

Then, the photooxidation reaction was carried out by two Xenon lamps (285–750 nm, 92.3 mW/cm²) with an oxygen flux of 1.5 L/min. An aliquot sample of 10 mL was taken from the suspension through a syringe filter of 0.22 μm and sampled at 0, 30, 60, 90, 120, 150, 180, and 210 min (The first 30 minutes were in the dark). All experimental points were tested three times and the average values were taken.

In order to study the adsorption processes, the concentration of ROX was decreased to 10 mg/L and the concentration of TiO₂ was reduced to 500 mg/L. Then, the best and the worst pH environments for the oxidation were chosen. An aliquot sample of 10 mL was taken from the suspension through a syringe filter of 0.22 μm and sampled at 0, 30, 60, 90, 120, 150, 180, and 210 min. The experiments were conducted in a dark environment.

In order to characterize the photooxidation processes of ROX (Li-Ming et al. 2004; Zhang et al. 2005), the data were based on the relationship with f (C_t) = f (t), f (⁠⁠) = f (t) and f (1/C_t) = f (t), where C_t was the concentration of ROX at t min and t stood for time, and the order of photooxidation reaction was determined, then the rate constants of k and R² about zero-order model, first-order model and second-order model were calculated.

The different TiO₂ crystals with the main facets of {101}, {001} and {100} have been synthesized with the methods in our previous paper (Han et al. 2009; Li & Xu 2010; Pan et al. 2011; Wei et al. 2019). The experimental details, as well as the XRD, the SEM and the TEM images, were published in our previous work (Wei et al. 2019). The BET of the three kinds of TiO₂ were also measured in the order of {101} (21.21 m²/g) > {001} (19.22 m²/g) > {100} (17.08 m²/g). The exposed percent of {101}, {001} and {100} respectively were 91.4, 67.1 and 66.7% (Yu et al. 2014). The details are in the appendix (Figure S3).

Variations of ROX concentrations before and during Xenon lamp for three types of TiO₂ at pH 7 are shown in Figure 1. It can be seen from Figure 1 that there is no obvious effect for in the dark adsorption of 30 min, which is consistent with Kwiecien's results (Kwiecien et al. 2014). And the TiO₂ with {101} facet maintains the best degradation effect in neutral environment, while the TiO₂ with {001} surface is the second and the TiO₂ with {100} facet has the worst effect. A set of low concentration experiments were also carried out, where the concentration of ROX was 5 mg/L and the concentration of TiO₂ was 500 mg/L, such as in Figure S4. It is shown that the photocatalytic oxidation abilities also follow the order of {101} facet > {001} facet > {100} facet, which agree with the results from the high ROX concentration such as 100 mg/L.

The photodegradation efficiencies of ROX are characterized by the zero-order kinetic model, first-order kinetic model and the second-order kinetic model.

The catalytic kinetic data k and the fitness R² of the three types of TiO₂ to ROX from pH 3 to pH 10 are presented in Table 1. From Table 1, for {101} facet TiO₂ from pH 5 to pH 10, the photooxidation kinetics conform to the first-order kinetics with the well linear relationship, which is consistent with the results of Kwiecien et al. (2014). However, at the low pH values such as pH 3 and pH 4, it is more consistent with the zero-order kinetics.

For {001} facet TiO₂, most of the photooxidation kinetics conform to the zero-order kinetics such as from pH 3 to pH 5 and from pH 8 to pH 10. At pH 6, the first-order kinetics is more suitable for the reaction; at pH 7, the second-order kinetics is the best one.

For {100} facet TiO₂, the results can fit with zero, first and second orders, while the zero-order kinetics are the best for pH values 4, 6, 9 and the second-order kinetics are the best for pH values 3, 5, 7, 8, 10. The speed of reaction rate may be the result of many factors, such as pH value, reactant concentration, temperature, etc. We will focus on the pH effects in the next section.

Figure 2 presents the degradation effects of ROX by Xenon lamp and TiO₂ crystals at pH values from pH 3 to pH 10 after 30 minutes adsorption.

From Figure 2(a), it can be seen that the best degradation effects by the {101} facet TiO₂ are obtained at pH 4 and pH 7–8, and the lowest point is at pH 8. The kinetics for {101} facet TiO₂ at pH values 4, 7 and 8 are shown in Figure 3, where Figure 3(a) is for zero-order kinetics, Figure 3(b) is for first-order kinetics, and Figure 3(c) is for second-order kinetics. At pH 4, from Figure 3 the zero-order kinetics are more suitable than the first-order and the second-order kinetics with the equation of and the fitness R² of 0.9868. At pH 7 and 8, the first-order kinetics are best with the equations of (R² = 0.9972) and (R² = 0.9933), respectively. It seems that the reaction mechanisms may be different at different pH values.

From Figure 2(b), it is obvious that the best degradation effects by the {001} facet TiO₂ are obtained in weak acidity environments such as at pH 5 and pH 6. The details for {001} facet TiO₂ at pH values 5 and 6 are also shown in Figure 4, where Figure 4(a) is for zero-order kinetics, Figure 4(b) is for first-order kinetics, and Figure 4(c) is for second-order kinetics. At pH 5, from Figure 4 both the zero-order kinetics and the first-order kinetics can fit well with the reaction, and the zero-order kinetics is the best one with the equation of (R² = 0.9958). At pH 6, the first-order kinetics is the best fitting for the reaction with the equation of (R² = 0.9993).

From Figure 2(c), pH values 4 and 9 possess the best degradation potential for ROX on the TiO₂ with {100} facet. However, the {100} facet is so weak for ROX degradation, even at pH 4 the concentration of after 180 min is only 0.7364, which is much lower than 0.4501 for {001} facet at pH 6 in Figure 2(b) and 0.1964 at pH 8 for {101} facet in Figure 2(a). The details for {100} facet TiO₂ at pH values 4 and 9 are also shown in Figure 5, where the zero-order kinetics in Figure 5(a) are the best for the reaction with the equations of (R² = 0.9869) and (R² = 0.9940) for the pH values 4 and 9, respectively.

In summary, the photocatalytic abilities of the TiO₂ facets follow the order of {101} > {001} > {100}, and the best pH value for the {101} facet in our results is 8. Then the MS diagram of the ESI⁺ scan of the LC/MS was carried out at pH 8, which is shown in Figure 6. It should be noted that the reaction rate at pH 8 in our experiment, such as in Figure 5(b), (R² = 0.9933), is a little faster than that from Kwiecien et al.'s result (Kwiecien et al. 2014) judging from the reaction rate constant of 0.00768 min⁻¹. Therefore, in Figure 6(b) we chose the same reaction time as Kwiecien et al. (2014), such as after 120 min reaction for the LC/MS test. Figure 6(a) shows a scan of ROX and background before the reaction and Figure 6(b) shows the results after 120 min reaction. By comparing Figure 6(a) and 6(b), the new peak m/z 544.50 appears, which stands for the product (c). The structure of product (c) is shown in Figure 6(c), and it had been analyzed by MS/MS technique in some articles (Kwiecien et al. 2014; Xu et al. 2017). The detailed comparison of Figure 6(a) and 6(b) is in the appendix. It seems that product (c) can be proposed as a product of ROX after 120 min reaction. However, the catalytic process is complicated and the product (c) is only a deduction from the LC/MS results of Figure 6(a) and 6(b). Further research is still needed to know the reaction paths and the whole reaction mechanism.

Hydroquinone, ammonium oxalate and isopropanol were used to capture ▪O²⁻, ▪OH and h⁺, respectively (Andrei et al. 2019). Then the experimental conditions were C_TiO2 = 2,000 mg/L, C_ROX = 100 mg/L, C_hydroquinone = 0.1 mmol/L (or C_{ammonium oxalate} = 0.1 mmol/L or C_isopropanol = 0.1 mmol/L), pH = 8 for {101} facet TiO₂, pH = 6 for {001} facet TiO₂, and pH = 4 for {100} facet TiO₂, and the reaction time was 3 hours, as shown in Figure 2. The results have been collected and shown in Figure 7. In Figure 7, the removal rate is calculated from (1-C_t/C₀) % and the free capture results are also shown which is the same as that in Figure 2. It is clear that in Figure 7 isopropanol gives the greatest impact on the reaction. Therefore h⁺ might be the main oxidant.

In our previous research, TiO₂ was used to oxidize arsenite, and the results showed that the catalytic ability of TiO₂ was related to its adsorption ability (Wei et al. 2019). Therefore, some experiments were carried out to test whether there were some relations between the adsorption and the oxidation for ROX. It has been reported that in the condition such as C_ROX = 100 mg/L and C_TiO2 = 2,000 mg/L, the adsorption effects were not obvious. In order to study the adsorption processes, the concentration of ROX was decreased to 10 mg/L and the concentration of TiO₂ was reduced to 500 mg/L. Then, the best and the worst pH environments for the oxidation were chosen such as pH 8 and pH 5 for {101} facet TiO₂, pH 6 and pH 3 for {001} facet TiO₂, and pH 4 and pH 3 for {100} facet TiO₂. The results are shown in Figure 8.

By comparing with Figure 2, the better oxidation is corresponding to the stronger adsorption for each kind of TiO₂, such as pH 8> pH 5 for {101} facet TiO₂, pH 6 > pH 3 for {001} facet TiO₂, and pH 4 > pH 3 for {100} facet TiO₂. It should be noted that in Figure 2, the catalytic efficiencies follow the order of pH 8 ({101} facet) (C_t/C₀ = 0.1964) > pH 6 ({001} facet) (C_t/C₀ = 0.4501) > pH 5 ({101} facet) (C_t/C₀ = 0.5055) > pH 4 ({100} facet) (C_t/C₀ = 0.7364) > pH 3 ({001} facet) (C_t/C₀ = 0.7802) > pH 3 ({100} facet) (C_t/C₀ = 0.9080). It is clear that the adsorption ability of TiO₂ generally agrees with its catalytic ability such as pH 8 ({101} facet) (15.22%) > pH 6 ({001} facet) (12.77%) > pH 5 ({101} facet) (11.29%) > pH 4 ({100} facet) (10.20%) > pH 3 ({001} facet) (9.80%) > pH 3 ({100} facet) (4.59%). In addition, the kinetic models such as the pseudo-first order kinetic model, the pseudo-second order kinetic model and the Weber-Morris kinetic model have been used to fit with the data in Figure 8 and the details are listed in the appendix (Figures S5–S7). And the results suggest that the pseudo-second order kinetic is the best one to simulate the adsorption process.

In this paper, the photooxidation of ROX on three types of TiO₂ has been studied. It shows that the catalytic abilities of the three types of TiO₂ follow the order of {101} > {001} > {100}. From pH 5 to pH 10, the photooxidation of ROX on the {101} facet follows the first-order kinetics, whereas the {001} and {100} facets obey the zero-order and second-order kinetics, respectively, at most of the pH values. The {101} facet possesses the highest photooxidation rate for ROX at pH 8; the {001} facet works best at pH 5 and pH 6; and the {100} facet has a relatively obvious effect at pH 4. Based on these results, the photooxidation reaction of ROX possesses both a facet effect and pH effect, then the {101} facet at the pH value of 8 is the most favorite condition. A product has also been suggested based on the LC/MS results. However, further work about the reaction mechanism is still needed. Furthermore, the results with traps and adsorption show that h⁺ might be the main oxidant, and the catalytic ability of TiO₂ is related with its adsorption ability. Comparing with pseudo-first order kinetic model and Weber-Morris kinetic model, the pseudo-second order kinetic is the best one to simulate the ROX adsorption process on TiO₂.

The financial supports of this work are the National Natural Science Foundation of China (21373104, 21173022, 20803014), the National Natural Science Foundation of Guangdong Province (2016A030313704), and the Guangdong University funding program (201911845135, 201911845184).

All relevant data are included in the paper or its Supplementary Information.