Pharmaceuticals Removal by Novel Nanoscale Photocatalyst Bi4VO8Cl

Article pubs.acs.org/IECR

Pharmaceuticals Removal by Novel Nanoscale Photocatalyst Bi4VO8Cl: Influencing Factors, Kinetics, and Mechanism Xingyun Hu,† Jing Fan,*,† Kelei Zhang,‡ Ning Yu,§ and Jianji Wang† †

School of Chemical and Environmental Sciences, Henan Key Laboratory for Environmental Pollution Control, Key Laboratory for Yellow River and Huai River Water Environmental and Pollution Control, Ministry of Education and ‡School of Physics and Information Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China § Environmental Protection and Monitoring Station of Xinxiang, Xinxiang, Henan 453003, P. R. China ABSTRACT: A novel nanoscale photocatalyst Bi4VO8Cl was prepared by hydrothermal synthesis and characterized. This catalyst was used for the degradation of six pharmaceuticals including metronidazole, aciclovir, levofloxacin hydrochloride, sulfonamide, adrenaline hydrochloride, and ribavirin in aqueous solutions by visible light irradiation. It was shown that all the drugs except for metronidazole were mineralized completely during 10 h visible light irradiation, while metronidazole could be degraded easily into biodegradable small molecules. Then metronidazole was selected as a representative drug to evaluate removal efficiency of the photocatalytic system under visible light, ultraviolet light, and solar light, to investigate the degradation kinetics, and to study the possible degradation pathway from degradation products measurements. Results indicated that under the optimal conditions, metronidazole could also be completely degraded within 10 min ultraviolet light irradiation or 6 h solar light irradiation. Therefore, Bi4VO8Cl would be a promising photocatalyst for the removal of pharmaceuticals from waste waters under solar irradiation. Bi4NbO8Cl,40 Bi4TaO8I41 and Bi4NbxTa(1‑x)O8I,42 have good visible-light-response property and have been extensively investigated as a new class of photocatalysts independently of TiO2 and its modified forms.24−26,43 Considering the fact that nanoscale photocatalysts are favorable to the enhancement of photocatalytic efficiency due to its high surface-to-volume ratio and high separation efficiency of the photogenerated electrons or holes,43,44 nanoscale photocatalyst Bi4VO8Cl was prepared by using hydrothermal synthesis and characterized with XPS, UV−vis DRS, SEM, and TEM in the present work. Then this nanoscale photocatalyst was used for the degradation of six typical drugs under visible light irradiation. It was found that unlike aciclovir, levofloxacin hydrochloride, sulfonamide, adrenaline hydrochloride, and ribavirin, metronidazole (MTZ) could not be mineralized completely under visible light irradiation. Thus, MTZ was used to evaluate removal efficiency of the photocatalyst under ultraviolet and solar light. Degradation reaction kinetics of MTZ was also investigated, and the pseudo-first-order kinetics model was successfully used to describe the degradation kinetic behavior. In addition, the change of total organic carbon (TOC) for all the drugs before and after the degradation and some intermediates in the degradation process of MTZ were determined. It was shown that although MTZ could not be mineralized completely, this pharmaceutical could be degraded easily into biodegradable small molecules. Based on these results, a possible mechanism was suggested for the degradation of MTZ in aqueous solutions.

1. INTRODUCTION The presence of pharmaceuticals and personal care products (PPCPs) in surface water is an emerging environmental issue and has received increasing concern in recent years. Pharmaceuticals are extensively used in medical and veterinary practice with an amount estimated in several hundreds of tons per year, if measures are not taken.1 More than 80 pharmaceutical compounds belonging to different therapeutic families have been detected up to the ppm level in sewage, surface, and ground waters.2 The pollution of these drugs includes the development of antibiotic resistance in aquatic bacteria, direct toxicity to microflora and microfauna, and possible risks to human health by the consumption of contaminated nontarget fauna.3 Therefore, PPCPs must be either degraded or removed from water supplies to ensure that they will not contaminate aquatic environments. Pharmaceutical compounds can be poorly removed by conventional wastewater treatment technologies.4,5 Several alternatives to eliminate pharmaceutical compounds in water have been reported in the literature.6,7 These include reverse osmosis,8 adsorption onto activated carbons,9 ozonation,10,11 nanoscale zerovalent iron particles,12,13 advanced oxidation processes (AOPs) such as Fenton or photo-Fenton system,14 ultrasound,15,16 peroxidation combined with UV light,17 photocatalysis using TiO2,18−21 and advanced oxidation hybrid processes.22 Among these AOPs, heterogeneous photocatalysis appears to be one of the most attractive technologies.23−26 However, to efficiently utilize the largest proportion of the solar spectrum and artificial visible light, development of high activity photocatalysts under a wide range of visible light irradiation is indispensable. A literature survey reveals that Bi-containing compounds, such as BiMO4 (M = V, Nb, and Ta),27−29 Bi3NbO7,30 Bi4Ti3O12,31 Bi2MoO6,32 Bi2WO6,33 Bi2Mo3O12,34 Bi2W2O9,35 Bi2Mo2O9,36 and the layered Bi-based oxyhalides BiOX (X = Cl, Br, I),37 BiOBr1−xIx,38 BiOIxCl1−x,39 © 2014 American Chemical Society

Received: Revised: Accepted: Published: 14623

May 6, August August August

2014 24, 2014 30, 2014 30, 2014

dx.doi.org/10.1021/ie501855r | Ind. Eng. Chem. Res. 2014, 53, 14623−14632

Industrial & Engineering Chemistry Research

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Table 1. Chemical Information of the Pharmaceuticals Investigated in the Present Work

2. MATERIALS AND METHODS 2.1. Chemicals. Bi2O3, NH4VO3, and Bi(NO3)3·5H2O were purchased from Shanghai Sinopharm Chemical Reagent Corporation, and HCl and NH3·OH used to adjust solution pH were from Zhengzhou Paini Reagent Corporation. These chemicals were analytical grade (≥99.0% purity) and used as received. Six pharmaceuticals were selected for this investigation based on consumption and environmental relevance. The names and usages of these compounds are shown in Table 1. These pharmaceuticals include pharmaceuticals of anti-infection, antiviral, anti-inflammatory, and antibiosis and treatment agents of the cardiovascular system. They were injections purchased from Beijing Shuguang Pharmaceutical Co., Ltd. These pharmaceuticals were used without further purification. 2.2. Preparation of the Photocatalysts. BiOCl powder was prepared by a hydrolysis method based on the procedure described in the literature.45 For the synthesis of Bi4VO8Cl by a hydrothermal method, 7.5 mmol of Bi(NO3)3·5H2O and 2.5 mmol of BiOCl were dissolved in 40 mL of aqueous nitric acid solution (4 mol/L), and then the mixture was slowly added into 20 mL of diluted ammonia containing 2.5 mmol of NH4VO3. By addition of aqueous NH3·OH, the aqueous solution of precursor suspensions was adjusted to pH 3, and then the system was treated hydrothermally for 20 h in a 100 mL capacity Teflon-lined stainless steel autoclave at 160 ◦C. Subsequently, the autoclave was cooled to room temperature naturally, and the obtained samples were filtered, washed with deionized water, and dried at 80 ◦C in air. For comparison purposes, a sample of Bi4VO8Cl was also synthesized by conventional solid-state reaction of Bi(NO3)3·5H2O and BiOCl with NH4VO3. The blended reagents with stoichiometric proportion were calcined for 24 h in air at 700 ◦C. The product named as SS-Bi4VO8Cl was ground for further characterization. 2.3. Characterization of the Photocatalysts. Qualitative analysis of the photocatalysts was carried out by a X-ray

photoelectron spectroscopy (XPS) using a Kratos Axis Ultra DLD high performance electron spectrometer with a monochromatic Al K source and a charge neutralizer. All binding energies were referred to the C 1s peak at 284.63 eV of the surface adventitious carbon and revised using BaSO4 as reference. UV−vis diffuse reflection spectra (UV−vis DRS) were recorded on a Shimadzu UV-3100 spectrophotometer. Morphology of the samples was detected by using a JSM-63901 field emission scanning electron microscope (SEM) with an accelerating voltage of 30 kV and a JEOL JEM-2100 transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. The specific surface area was measured by a 004A N2-adsorption surface area measuring apparatus (China). The potential of Bi4VO8Cl was determined by a nano-ZS90 zetasizer (England). 2.4. Measurements of Photocatalytic Activity. Photocatalytic activity of Bi4VO8Cl was evaluated for the degradation reaction of six drugs in aqueous solutions. To simulate the visible light spectrum, a 300 W Xe lamp with a cutoff filter (λ > 420 nm) was used as a light source which was positioned inside a quartz cell with a circulating water jacket to keep the system at room temperature. In the ultraviolet light irradiation experiment, a 300 W high pressure mercury-vapor lamp (λ < 380 nm) was applied as the light source. A direct solar irradiation experiment was conducted on a sunny day in Xinxiang (geographical location is N35.18°and E113.52°). In all the experiments, 250 mL of suspensions was magnetically stirred in the dark for 60 min to reach the adsorption−desorption equilibrium between the photocatalyst and the drugs. During the reaction, about 3 mL of aliquots was taken from the reactor at a given time interval for subsequent concentration analysis after filtering. Their concentrations were determined by a Persee T6 General UV−vis spectrophotometer (China). The photocatalytic degradation efficiency (η) of the drugs can be calculated by the following equation η = (1 − C /C0) × 100% 14624

(1)



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Figure 1. XPS fitted spectra of Bi4VO8Cl: (a), Bi4f spectrum; (b), V2p spectrum; (c), O1s spectrum; and (d), Cl2p spectrum.

where C0 (mg/L) and C (mg/L) are the initial concentration and the concentration at reaction time t for each drug, respectively. 2.5. Determination of Degradation Products of the Drugs. The possible degradation products of the drugs were analyzed through an ACQUITY TQD HPLC-MS system (Waters Corporation) equipped with a BEH-C18 column (100 mm × 2.1 mm, 5 μm). Methanol−water (45/55, v/v) and acetonitrile−water (20/80, v/v) solutions were used as the eluents, at a flow rate of 0.3 mL/min, for the degradation study of MTZ and the other five drugs, respectively. A 3 μL sample was injected by using an autosampling device, and the eluent from the chromatographic column enters a UV−vis detector, an electrospray ionization (ESI) interface, and then the mass analyzer. Mass spectra (MS) analysis in the negative mode was performed on ion trap mass spectrometer equipped with an atmospheric pressure ionization (API) interface and an ESI ion source. The flow rate of high purity nitrogen (heater temperature, 350 °C) was maintained at 650 L/h. The spectrometer was scanned at m/z = 100−600 during the recording of mass spectra. The TOC was determined by a Shimadzu 5000A-TOC analyzer (Japan).

The peaks with binding energy of 514.8 and 521.9 eV were assigned to V2p3/2 and V2p1/2 of V5+.47 The binding energy at 528.1 and 530.5 eV for O1s can be assigned to the O2− state48 and those at 196.1 and 197.7 eV for Cl2p3/2 and Cl2p could be assigned to the Cl− state.49 All these are accorded with the molecular formula of Bi4VO8Cl. UV−vis DRS (Figure 2) showed that Bi4VO8Cl would have photoabsorption from UV light to visible light, and the absorption edge was at around 525 nm. The band gap energy is 2.36 eV as estimated by the equation:50λg = 1239.8/Eg, which

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Photocatalyst. The qualitative XPS analysis result of the photocatalyst is shown in Figure 1, and the peaks of Bi4f, V2p, Cl2p, and O1s were observed. The two strong peaks located at 162.7 and 157.4 eV in the Bi region were assigned to Bi4f5/2 and Bi4f7/2 of Bi3+.46

Figure 2. UV−vis absorption spectrum of Bi4VO8Cl. 14625



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Figure 5. Pseudo-first-order linear plots for the degradation kinetics of MTZ (10 mg/L) under visible-light irradiation and different pH conditions at the catalyst dosage of 1 g/L.

Table 2. Degradation Kinetic Data of Metronidazole by Photocatalyst Bi4VO8Cl under Visible Light Irradiation and Different Conditions

Figure 3. SEM and TEM images of Bi4VO8Cl: (a), SEM and (b), TEM.

dose (g/L)

C0(mg/L)

initial pH

K1 (h−1)

R

2.0 2.0 2.0 2.0 0.6 1.0 1.5 2.0 2.5 1.0 1.0 1.0 1.0 1.0 1.0

5 10 15 20 10 10 10 10 10 10 10 10 10 10 10

6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 3.0 5.0 7.0 9.0 10.0 11.0

0.0714 0.0633 0.0493 0.0457 0.0566 0.0628 0.0680 0.0711 0.0505 0.2908 0.0751 0.0676 0.0762 0.0534 0.3226

0.998 0.995 0.993 0.997 0.996 0.999 0.999 0.998 0.997 0.996 0.992 0.994 0.996 0.994 0.999

Figure 4. Comparison of photocatalytic degradation efficiency of nano-Bi4VO8Cl and SS-Bi4VO8Cl for MTZ (10 mg/L) under visible light irradiation at pH 6.7 and 1 g/L catalyst.

Figure 6. Infrared spectra of the original and the recovered Bi4VO8Cl.

suggested the potential photocatalytic activity under visible light. Morphology of the photocatalyst was characterized by SEM and TEM, as shown in Figure 3. An obvious layer structure has

been observed, and the lattice spacing of Bi4VO8Cl is about 3 nm according to the TEM graph. The layer structure is considered to be convenient for electronic transmission between the layers. This favors charge separation so that excited 14626



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degradation of MTZ by Bi4VO8Cl under different conditions. A blank experiment showed that after 12 h adsorption in the dark, the concentration of MTZ was only decreased by 10.3% and the photolysis efficiency of MTZ was about 8%. However, with Bi4VO8Cl as the photocatalyst, 55% of MTZ was degradated after 12 h irradiation by visible light. This suggested that Bi4VO8Cl has good photocatalytic activity under visible light irradiation, and the removal of MTZ is mainly caused by photocatalysis rather than adsorption and photolysis. For comparison, the photocatalytic activity of SS-Bi4VO8Cl (synthesized by conventional solid-state reaction) was also evaluated under the same conditions. It was found that degradation efficiency of MTZ by Bi4VO8Cl was about 1.5 times greater than that of SS-Bi4VO8Cl after 12 h visible light irradiation. This may be attributed to the smaller specific surface area of SS-Bi4VO8Cl (0.47 m2/g) than that of nanoscale Bi4VO8Cl (6.81 m2/g), which results in less adsorption efficiency of MTZ by SSBi4VO8Cl (5.0%) than that by Bi4VO8Cl (10.3%). The internal electric fields in plated material Bi4VO8Cl are considered to favor charge separation, which can subsequently induce redox reactions on the semiconductor surface and enhance photocatalytic activity.51,52 3.2.1. Initial pH Effect and Photocatalytic Degradation Kinetics of MTZ in Aqueous Solution. To investigate the effect of the pH value on the degradation of MTZ by the Bi4VO8Cl photocatalyst, aqueous MTZ solution was adjusted to different initial pH values (3, 5, 7, 9, and 11) by diluted nitric acid or ammonia. Figure 5 shows the pH dependence of degradation efficiency of MTZ after 9 h visible light irradiation. Clearly, degradation efficiency of MTZ at pH 3 and pH 11 were greater than that at other pH values. In addition, it was found that at a given pH value, the photocatalytic degradation of MTZ obeys pseudo-first-order kinetics53 with respect to the MTZ concentration

Figure 7. Effect of initial MTZ concentrations on the pseudo-firstorder linear plots of MTZ under visible-light irradiation at the catalyst dosage of 1 g/L and pH 6.7.

ln(C0/C) = k1t Figure 8. Effect of photocatalyst dosage on the pseudo-first-order linear plots of MTZ (10 mg/L) under visible-light irradiation at pH 6.7.

(2)

where the definition of C and C0 is the same as in eq 1, and k1 is the observed rate constant (h−1) and t is reaction time (h). The fitted kinetic data and correlation coefficients were summarized in Table 2. It is clear that the observed degradation rate constants at pH 11 and pH 3 were about 5 times greater than that at other pH values, which was related to the adsorption of MTZ on the photocatalyst at different pH values. The adsorption efficiency after 30 min in the dark was found to be 14.3, 9.9, 9.1, 9.6, and 13.3% at pH 3, 5, 7, 9, and 11, respectively. This indicated that adsorption of MTZ on the photocatalyst was stronger at pH 3 and pH 11 than that at other pH values. This may be ascribed to the different charges available on the catalyst surface and MTZ molecules at different pH values. It was known that zeta potentials of Bi4VO8Cl at pH 3, 5, 7, 9, and 11 were −2.5, −12, −20, −12.44, and −26.8 mV, respectively, and the more negative zeta potential means more negative charges on the surface of photocatalyst. Considering the fact that MTZ (pKa = 2.6) is negatively charged at pH above 2.6, it is clear that the pH increase enhances the electrostatic repulsion between the catalyst surface and the MTZ molecules, resulting in less adsorption of MTZ on the catalyst surface. More adsorption at pH 3 leads to more charges exchange between MTZ and Bi4VO8Cl, which facilitates photocatalytic reaction. However, the maximum catalysis efficiency was observed at pH 11. Presumably, intermediate degradation products were dissolved in strong alkaline pH, and more reactive sites were released. Infrared spectra of Bi4VO8Cl before and after reaction at pH 11 (concentration of

Figure 9. Cycling runs for the Bi4VO8Cl catalyst in photocatalytic degradation of MTZ under visible light irradiation.

e− and h+ can reduce or oxidize chemical species, respectively, leading to enhanced photocatalytic activity of the photocatalyst.40 3.2. Photocatalytic Activity under Visible Light Irradiation. As an example, Figure 4 displays the photocatalysis 14627



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Table 3. Removal Efficiency and Mineralization Efficiency of the Pharmaceuticals concentration (mg/L)

TOC (mg/L)

pharmaceutical

original

after degradation

removal efficiency

original

after degradation

mineralization efficiency

metronidazole aciclovir levofloxacin hydrochloride sulfonamide adrenaline hydrochloride ribavirin

10 10 10 10 10 10

0 0 0 0 0 0

100% 100% 100% 100% 100% 100%

16.4 11.5 24.9 16.9 15.9 11.0

9.8 0 0 0 0 0

40% 100% 100% 100% 100% 100%

to the MTZ concentration. The kinetic data summarized in Table 2 showed the linear relationship described by eq 2 with the pseudo-first-order degradation rate constants changed from 0.0714 to 0.0457 h−1 as the initial MTZ concentrations ranged from 5 to 20 mg/L. It is evident that the k1 values decrease with an increase of the initial concentration of MTZ. The possible reason is that the quantum efficiency of Bi4VO8Cl is constant under a constant photon flux. However, the content of MTZ is much higher than that of Bi4VO8Cl in aqueous solution, which prevents light absorption of the photocatalyst. Another possible reason is that more intermediate products were formed at higher MTZ concentrations during the photocatalytic degradation of MTZ. This may compete with the MTZ molecules for the limited adsorption and catalytic sites on the surface of catalyst particles, thus inhibiting the degradation of MTZ to a certain extent. Generally speaking, the initial MTZ concentrations have small influence on the photocatalytic reaction process. 3.2.3. Effect of Catalyst Dosage. Catalyst dosage ranging from 0.6 to 2.5 g/L was used to study the effect of catalyst dosage on the degradation efficiency of MTZ, and the results are shown in Figure 8. The kinetic data under the selected catalyst dosages were also included in Table 2. It is shown that the degradation reaction rate increases with the catalyst dosage up to 2.0 g/L. This is due to an increase of available adsorption and catalytic sites on the surface of the Bi4VO8Cl catalyst. A further increase in catalyst dosage (2.5 g/L), however, may cause light scattering and screening effects, thus reducing the specific activity of the catalyst. Considering the catalyst dosage and the degradation efficiency together, 1 g/L was adopted as the optimal catalyst dosage in the present work. 3.2.4. Reusability of the Catalyst. The stability of the Bi4VO8Cl catalyst was also evaluated by recycling experiments. For this purpose, after every 9 h of photodegradation, the separated photocatalyst was washed with deionized water and dried at 80 °C in a drying oven. The variation of degradation efficiency in each run is shown in Figure 9. It was clearly noted that after three recycling experiments, the photocatalytic activity of the Bi4VO8Cl catalyst was scarcely decreased. Besides, the stability of Bi4VO8Cl before and after reaction at pH 11 was confirmed by infrared spectra of the catalyst. These results indicated that the photocatalyst was quite stable and its recycling use was feasible. 3.3. Removal of the Other Pharmaceuticals by Bi4VO8Cl under Visible Light Irradiation and the Degradation Mechanism. In order to reveal the photocatalytic properties of Bi4VO8Cl for the other five kinds of pharmaceuticals, the degradation of aciclovir, levofloxacin hydrochloride, sulfonamide, adrenaline hydrochloride, and ribavirin was also investigated under visible light irradiation. Complete mineralization of organic compounds is our ideal goal in the treatment of organic pollutants in water. To evaluate the degree of mineralization of the above drugs by Bi4VO8Cl, TOC was monitored before and

Figure 10. (a) UV−vis spectra evolution of MTZ during the photocatalytic reaction under visible light irradiation at the initial MTZ concentration of 10 mg/L, catalyst dosage of 1 g/L and initial pH 11. (b) The liquid chromatography of MTZ systems (from up to down): aqueous MTZ solution, aqueous MTZ solution after 16 h visible light irradiation, and the blank (aqueous catalyst).

MTZ, 10 mg/L; dosage of catalyst, 1 g/L) is shown in Figure 6. It can be seen that the position of absorption peaks was unchanged after reaction, illustrating that MTZ or intermediate products were dissolved from the surface of the photocatalyst. 3.2.2. Effect of Initial Concentrations of MTZ. The effect of initial MTZ concentrations (5, 10, 15, and 20 mg/L) in aqueous solutions on the degradation efficiency was evaluated. As shown in Figure 7, the degradation efficiency decreases with increasing concentration of MTZ. It was found that the photocatalytic degradation of MTZ obeys pseudo-first-order kinetics with respect 14628



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Figure 11. Suggested degradation pathway of MTZ byBi4VO8Cl in aqueous solutions.

Figure 12. Photocatalytic efficiency of MTZ under ultraviolet light irradiation at the initial MTZ concentration of 10 mg/L, catalyst dosage of 1 g/L, and initial pH 6.7.

Figure 13. Photocatalytic efficiency of MTZ under solar irradiation at the initial MTZ concentration of 10 mg/L, catalyst dosage of 1 g/L, and initial pH 6.7.

after the degradation reactions at the initial contaminant concentration of 10 mg/L and natural pH. The data in Table 3 clearly indicates that aciclovir, levofloxacin hydrochloride, sulfonamide, adrenaline hydrochloride, and ribavirin were totally degraded and mineralized under 10 h visible light irradiation. This has been verified by liquid chromatography analysis: the liquid chromatography graphs for the five drugs after a 10 h reaction were the same as that of the blank (aqueous catalyst). However, only about 40% of TOC was removed from the reaction system of MTZ after a 10 h reaction under visible light irradiation. This result indicated that MTZ was not completely

mineralized, which can also be demonstrated by a UV−vis absorption spectrum (Figure 10a) and liquid chromatography (Figure 10b) of MTZ in aqueous solution. It can be seen that the maximum absorption appears at 318 nm, which is associated with characteristic absorption peak of MTZ. The intensity of this peak decreases gradually during the irradiation and finally disappears in the degradation processes. However, a new peak appears at 220 nm, indicating that MTZ was destroyed, and a new product was generated during the photocatalytic reaction. This is consistent with the results obtained from liquid chromatography analysis of MTZ. It was also found from 14629


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Figure 10b that after a 10 h reaction, the peak at a retention time of 1.58 min for MTZ disappeared, but a new peak at the retention time of 2.65 min emerged, suggesting that MTZ was degraded and a new substance was generated. Furthermore, the intermediates produced in the photocatalytic degradation process of MTZ were observed from HPLC-MS measurements, and seven main values of m/z =157, 128, 113, 89, 74, 69, and 61 were detected. According to the photocatalytic reaction principle,54 both hydroxyl radicals and photogenerated holes were responsible for the heterogeneous MTZ degradation. They may attack different chemical bonds of MTZ or oxidize MTZ to form different intermediates. From the m/z values mentioned above, a possible degradation pathway of MTZ by Bi4VO8Cl is shown in Figure 11. It was found that although MTZ could not be mineralized completely, this pharmaceutical has been completely degraded into biodegradable small molecules. The removal of the antibiotic metronidazole from aquatic solutions was also investigated by nanoscale zerovalent iron particals. It was shown that metronidazole (80 mg/L) was almost completely removed, but less than 5% of mineralization efficiency was observed within 90 min, indicating that MTZ was barely mineralized by nanoscale zerovalent iron particals.12 3.4. Photocatalytic Activity under Ultraviolet and Solar Irradiation. In order to extend the photocatalytic application of the catalytic materials, we investigated the photocatalytic activity of Bi4VO8Cl under ultraviolet and solar light irradiation, and the results are shown in Figure 12. It can be seen that the adsorption efficiency of MTZ by Bi4VO8Cl is about 11% during 60 min adsorption in the dark, and the photolysis efficiency is about 33% under 10 min ultraviolet irradiation without the catalyst. Interestingly, degradation efficiency in the presence of the Bi4VO8Cl catalyst was found to be about 91, 98, and 99% after 5, 8, and 10 min ultraviolet irradiation, respectively. This indicates that the photocatalytic activity of Bi4VO8Cl is high enough to completely degrade MTZ in 10 min under ultraviolet light irradiation. With solar irradiation, the photocatalytic activity of Bi4VO8Cl for the degradation of MTZ was studied in a sealed conical borosilicate glass reactor with a lid to avoid water evaporation and concentration change. The solution was stirred magnetically, and the experimental conditions were as follows: solar irradiation from 9:30 a.m. to 15:30 p.m., 10 mg/L of MTZ, 1 g/L of catalyst dosage, and pH 6.7. Figure 13 illustrated the photocatalytic activity of Bi4VO8Cl by solar irradiation. It should be mentioned that under solar irradiation, the reaction temperature was changeable, and the maximum temperature could be up to about 60 °C during the reaction process. For the purpose of comparison, the degradation in the absence of the catalyst and the adsorption in the dark were also investigated under the same conditions. It was shown that the degradation efficiency under the above-mentioned conditions was negligible compared to that in the presence of the catalyst, where the removal efficiency of MTZ reached 96% during 6 h solar irradiation. These results indicated that when ultraviolet light and solar light were used, Bi4VO8Cl also possessed excellent photocatalytic activity for the degradation of MTZ in aqueous solutions.

300 W long-arc xenon lamp with a UV cutoff glass filter (λ > 420 nm), 300 W high-pressure mercury lamp (UV) and solar light

300 W long-arc xenon lamp with a UV cutoff glass filter (λ > 420 nm)

500 W high-pressure mercury lamp (UV) 300 W long-arc xenon lamp with UV cutoff glass filter (λ > 420 nm)

2.36 eV

Bi4Nb0.1Ta0.9O8I42

4. CONCLUSIONS Novel nanoscale photocatalyst Bi4VO8Cl was prepared and used for the degradation of metronidazole, aciclovir, levofloxacin hydrochloride, sulfonamide, adrenaline hydrochloride, and ribavirin in aqueous solutions under visible light irradiation.

Bi4VO8Cl

2.48 eV

Bi4NbO8Cl40 Bi4TaO8I41

Bi4Ti3O1231 Bi2W2O935 BiOX37

3.08 eV 2.87−2.96 eV BiOCl, 3.22 eV BiOBr, 2.64 eV BiOI, 1.77 eV 2.38 eV 2.43 eV

methyl orange (10 mg/L) methyl orange (10 mg/L), pentachlorophenol (10 mg/L) methyl orange (10 mg/L) Bisphenol A (20 mg/L) metronidazole (MTZ), aciclovir, levofloxacin hydrochloride, sulfonamide, adrenaline hydrochloride, and ribavirin (10 mg/L)

20 W UV lamp (360 nm) xenon lamp of 10 000 K with a luminous flux of 2100 lm 500 W halogen-tungsten lamp with a 420 nm cutoff filter

100% removal of OG/60 min; 100% removal of AG/ 45 min; 40% removal of MV/60 min 90% removal/4 h 90% removal/300 min BiOCl, 15% removal BiOBr, 21% removal BiOI, 80% removal/3 h 100% removal and 60% mineralization/100 min 80% removal of methyl orange/25 h 100% removal of pentachlorophenol/10 h 92% removal of methyl orange/12 h 99% mineralization of bisphenol A/12 h 100% mineralization of aciclovir/10 h 100% removal of levofloxacin hydrochloride sulfonamide, adrenaline hydrochloride and ribavirin/10 h 40% mineralization of MTZ/10 h visible light 100% mineralization of MTZ/10 min UV 95% mineralization of MTZ/6 h solar light 125 W high-pressure mercury lamp (UV)

light source target pollutant

orange G (OG) and alizarin green (AG) (25 mg/L) and methyl violet (MV, 10 mg/L) methyl orange (10 mg/L) rhodamine B (5 mg/L) methyl orange (10 mg/L) 3.40 eV

band gap Bi-based photocatalyst

Table 4. Comparison of Bi4VO8Cl with Other Bi-Based Photocatalysts

BiTaO429

removal or mineralized percentage/reaction time


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The degradation of metronidazole was also investigated under ultraviolet and solar light irradiation. It was found that the Bi4VO8Cl catalyst appeared to have high photocatalytic activity toward the removal of these pharmaceuticals. In addition, a comparison of Bi4VO8Cl with other Bi-based photocatalysts was made in Table 4. Although these photocatalysts possess different optical properties and were used to degrade different pollutants under different light sources, we still found that Bi4VO8Cl has superior photocatalytic degradation ability for pharmaceuticals. In aqueous solutions, acyclovir, levofloxacin hydrochloride, sulfonamide, adrenaline hydrochloride, and ribavirin could be degraded and mineralized completely under visible light irradiation. However, although MTZ molecules could be completely degradated into biodegradable small molecules by Bi4VO8Cl under the irradiation of 9 h visible light, 10 min ultraviolet light, or 6 h solar light, only about 40% of the TOC was eliminated during 9 h visible light irradiation. This suggests that the pharmaceuticals with benzene ring and nonimidazole ring can be mineralized completely by our nanoscale photocatalyst Bi4VO8Cl, while the pharmaceutical with the imidazole ring was resistant against visible light irradiation. In addition, it was found that the photocatalytic degradation of MTZ could be described by pseudo-first-order kinetics. The photocatalyst was highly stable in the photocatalysis and could be easily recovered. Consequently, the hydrothermal technique presented here is an economical and easy way to enhance photocatalytic activity of Bi4VO8Cl, and Bi4VO8Cl is expected to be a potentially effective photocatalyst for the degradation of pharmaceuticals in wastewaters under solar irradiation.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86 373 3325971. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21377036) and the Science Foundation of Henan Province (Grant No. 102102210061).

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Pharmaceuticals Removal by Novel Nanoscale Photocatalyst Bi4VO8Cl

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