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hollow hydroxyapatite microsphere photocatalyst for fast removal of antibiotic pollutants
Journal of Physics and Chemistry of Solids 139 (2020) 109353
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs
Novel and efficient red phosphorus/hollow hydroxyapatite microsphere photocatalyst for fast removal of antibiotic pollutants Rongjiang Zou a, Tianhong Xu b, Xiaofang Lei b, Qiang Wu b, *, Song Xue a, ** a
Department of Cardiovascular Surgery, Renji Hospital, School of Medicine, Shanghai Jiao Tong University, 200127, Shanghai, People’s Republic of China Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, College of Environmental and Chemical Engineering, Shanghai University of Electric Power, 200090, Shanghai, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Red phosphorus Hydroxyapatite Rifampicin Tetracycline Levofloxacin Photocatalysis
Photocatalysis is a well-established and green technique in environmental pollutant remediation. In the present study, a novel and green red phosphorus (5.0 wt%)/hollow hydroxyapatite microsphere photocatalyst was fabricated and applied forthe degradation of rifampicin, tetracycline, and levofloxacin. The as-prepared material exhibits outstanding photocatalytic activity and stability, and has universal application for degradation of each antibiotic. Notably, tetracycline, rifampicin, andlevofloxacin can be completely degraded within 10 min, 20 min, and 50 min, respectively, under a 300 W Xe lamp with full spectrum illumination, where the optimal conditions were an initial pH of 6.0 and a photocatalyst dosage of 1.0 g/L. In addition, a plausible photocatalytic reaction mechanism was proposed. The low-cost, highly efficient, and green red phosphorus (5.0 wt%)/hollow hy droxyapatite microsphere photocatalyst is a promising candidate for the removal of many hazardous pollutants.
1. Introduction Water contamination is one of the key challenges for our modern industrial societyandrequires effective methods and techniques to solve this problem [1–9]. Antibiotics have been widely used for treatment and cure of human and animal diseases. However, the presence of antibiotic pollutants in water is a great threat to human health and ecosystem equilibrium [10,11]. Thus, the removal of antibiotic pollutants from wastewater has become a hot issue. Many methods have been used to degrade and remove antibiotic pollutants, including physical adsorp tion, biodegradation, electrochemical oxidation, and photocatalysis [12–23]. Conventional techniques usually fail to achieve complete degradation of antibiotic pollutants and lead tothe formation of some toxic organic intermediates. In contrast, photocatalysisis regarded as a promising and green approach to fully remove antibiotic pollutants from wastewater [24–29]. Hydroxyapatite (Ca10(PO4)6(OH)2;HAp)has excellent biocompati bility and bioactivity, is nontoxic, environmentally benign, and of low cost, and has many applications in biomaterials [30,31]. In recent years, HAp has been explored as a cheap and green photocatalyst for degra dation of some organic pollutants [32–34]. However, its chemical
stability and photocatalytic efficiency are still far from satisfactory. Thus, it is important to develop a feasible and low-cost strategy to synthesize a novel structured HAp-based composite photocatalyst with high efficiency and stability. Very recently, red phosphorus (RP) has sparked a great deal of in terest in photocatalysis because of its Earth abundance, low cost, high stability, lack of metal components, and suitable band position [35–37]. It has been confirmed that adding RP to some photocatalysts could take full advantage of their abilities for organic pollutant degradation. In addition to their use as photocatalysts, hollow HAp microspheres tend to have high specific surface areas and enhanced active sites, and thus have potential as candidate carriers fora composite photocatalyst. As a result, hollow HApmicrospheres decorated with small amounts of RPwould hold promise as highly functionalized materials for potential application in wastewater pollutant treatment. Here, a newRP (5.0 wt%)/hollow HAp microsphere photocatalyst was fabricated and its applications in antibiotic pollutant removal were explored for rifampicin (RIF), tetracycline (TC), and levofloxacin (OFLX). As expected, the as-prepared RP (5.0 wt%)/hollow HAp microsphere composite possesses high photocatalytic efficiency in the degradation of the three a forementioned antibiotics in a short time. This
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Wu), [email protected] (S. Xue). https://doi.org/10.1016/j.jpcs.2020.109353 Received 6 October 2019; Received in revised form 6 January 2020; Accepted 7 January 2020 Available online 10 January 2020 0022-3697/© 2020 Elsevier Ltd. All rights reserved.
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Journal of Physics and Chemistry of Solids 139 (2020) 109353
fundamental research will provide a promising strategy for developing highly efficient and compatible photocatalysts with wide applications. 2. Experimental 2.1. Preparation of hollow HAp microspheres Hollow HAp microspheres were obtained with CaCO3 as a hard template through a hydrothermal route. First, CaCO3 template was prepared on the basis of a polystyrene sodiumsulfonate (PSS) polymertemplate method. Briefly, CaCl2 solution (30 mL, 0.2 M) was added to sodium polystyrene sulfonate solution (300 mL, 10 g/L) under magnetic stirring, followed by the dropwise addition of Na2CO3 (30 mL, 0.2 M) to the prepared mixture. After stirring for 1 h, the resulting solution was centrifuged and washed with deionized water and ethanol, respectively. Target CaCO3 template was collected after vacuum drying at 333 K overnight. Next, 0.2 g CaCO3 template was added to Na2HPO4solution (100 mL, 0.2 M) under stirring. The pH was adjusted to 11.0 with NaOH solution. Later the suspension was transferred to a 200 mL Teflon-lined autoclave. After hydrothermal treatment at 393 K for 1 h and natural cooling, the corresponding sample was collected by centrifugation and washed with deionized water and ethanol, respectively, several times, Finally, the resulting sample was dried under a vacuum at 333 K over night to obtain hollow HAp microspheres. 2.2. Controllable synthesis of RP/hollow HAp microsphere composites The typical synthetic procedure was as follows. A certain amount of the as-prepared hollow HAp microspheres was first placed in a suction filtration unit. Then a certain amount of purified commercial RP was ultrasonically dispersed in ethanol and slowly dropped onto the hollow HAp microspheres. Excess solution was removed by vacuum filtration. Afterwards, the resulting product was dried in air at 353 K for 12 h to obtain the desired composite. In the present study, composites with a 5.0 wt% ratio of RP to hollow HAp microspheres (abbreviated asRP (5.0 wt%)/HAp) were obtained and optimized for further use. 2.3. Characterization
Fig. 1. (a) X-ray diffraction patterns of red phosphorus (RP), hydroxyapatite (HAp), and RP (5.0 wt%)/HAp composite. (b) UV–visible absorption spectra of RP, HAp, and RP (5.0 wt%)/HAp composite.
X-ray diffraction (XRD) measurements were performed with a Bru kerD8 diffractometer with a CuKα radiation. Field-emission scanning electron microscopy (FE-SEM) measurements were performed with a Hitachi SU-1500 instrument. Transmission electron microscopy (TEM) images were recorded with a JEOLJEM-2010 electron microscope. Ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy was con ducted with a Shimadzu UV-2550 spectrophotometer. BrunauerEmmett-Teller (BET) specific surface area was measured with a Gemini VII 2390 instrument. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific ESCALAB250XiX-ray photo electron spectrometer. Electron spin resonance (ESR) spectroscopy was performed with a Bruker EPRA300-10/12 spectrometer.
¼ 335 nm for RIF,λmax ¼ 357 nm for TC, andλmax ¼ 294 nm for OFLX. The photocatalytic degradation efficiency (D) was calculated as follows: Dð%Þ ¼
C0
Ct C0
� 100% ;
(1)
whereC0 and Ct represent the initial antibioticconcentration and the antibioticconcentration after a certain irradiation time t. A recycling experiment was conducted for RIFto test the stability and reusability of the as-prepared photocatalyst. Trapping experiments were performed to elucidate the plausible photocatalytic degradation mechanism. 2-Propa nol, disodium ethylenediaminetetraacetic acid, and 1,4-benzoquinone were used as different scavengers to detect the main active species during the photocatalytic process. In addition, ESRspectroscopywas further performed to identify the active species generated in the pho tocatalytic process.
2.4. Photocatalytic experiments for antibiotics The photocatalytic performance of as-prepared RP (5.0 wt%)/HAp composite was evaluated by degradation of RIF, TC, and OFLX in a homemade glass reactor at room temperature. A 300 W xenon arc lamp (PLS-SXE 300, Perfectlight Co.) with full spectrum irradiation was used as the light source. In a typical photocatalytic experiment, 100 mg ofasprepared RP (5.0 wt%)/HApwas dispersed in 100 mL antibioticaqueous solution (10 mg/L) at pH6.0. Before irradiation, the solution was magnetically stirred in darkness for 30 min to ensure an adsorptiondesorption equilibrium. Then the lamp was turned on. At given irradi ation intervals, 4 mL of suspension was withdrawn and centrifuged, and then the supernatant was analyzed with a UV–vis spectrometer to determine its residual concentration according to the absorbance at λmax
3. Results and discussion The phase composition and crystallinity of the as-prepared RP (5.0 wt%)/HAp composite were characterized by the XRDtechnique (Fig. 1). For comparison, the XRD patterns ofcommerciallyavailableRP and asprepared HAp were also measured. For HAp (Fig. 1a), all the diffrac tion peaks match very well with the typical hexagonal structured HAp (JCPDScard no. 09–0432). As illustrated in Fig. 1b, one typical peak at 2
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Journal of Physics and Chemistry of Solids 139 (2020) 109353
stronger absorption both in the UV region and in the visible region. As expected, the light-harvesting ability of the as-prepared RP (5.0 wt %)/HAp composite was greatly enhanced in the whole region compared with that of the as-prepared HAp. This result demonstrates that the asprepared RP (5.0 wt%)/HAp composite can act as a promising photo catalyst for wastewater pollutant removal. FE-SEM and TEM characterization were performed to confirm the morphology of the as-prepared RP (5.0 wt%)/HAp composite. From the FE-SEM image in Fig. 2a, it can be seen that HAp microspheres are a good substrate to anchor and stabilize RP particles on their surface. As shown inthe TEM image (Fig. 2b), the as-prepared RP (5.0 wt%)/HAp compositehas a translucent center and a black edge, demonstrating that the composite has a hollow structure. The TEM image also indicates that RP particles were immobilized on the surface of hollow HAp micro spheres, which further confirms that hollow HAp microspheres can act as a good substrate for anchoring of RP particles. XPS analysis was used to analyze the chemical state and elemental composition of the as-prepared RP (5.0 wt%)/HAp composite. As shown in Fig. 3a, the survey spectrum of RP (5.0 wt%)/HApdemonstrates the coexistence of the elements P, Ca, and O. The high-resolution spectrum for O 1s (Fig. 3b)exhibits a peak at 532.1 eV, which corresponds to O2 . The P 2p spectrum (Fig. 3c) exhibits three characteristic peaks; the peak at 133.9 eV is ascribed to P 2p, andthe peaks at 130.2 eV and 129.4 eV are assigned to P2p1/2 and P2p3/2, respectively [38]. Furthermore, the high-resolution spectrum for Ca 2p (Fig. 3d) exhibits peaks at 347.4 eV and 350.9 eV, which are ascribed to Ca2þ. To verify the photocatalytic performance of the RP (5.0 wt%)/HAp composite, TC, RIF, and OFLX were used as model antibiotics for
Fig. 2. (a) Field-emission scanning electron microscopy image and (b) trans mission electron microscopy image of red phosphorus (5.0 wt%)/hydroxyapa tite composite.
15.3� , corresponding to (102) planes, was observed for commercially available RP. In addition, the RP (5.0 wt%)/HAp composite (Fig. 1c) exhibits an XRD pattern similar to that ofas-prepared HAp. The disap pearance of the RP characteristic peak in the composite was due to weak crystallinity and a trace amount of RP not being measured. No peaks of any other phases or impurities are present in the composite, indicating high purity of the final product. The results also indicate the successful synthesis of RP (5.0 wt%)/HAp composite in the present study. Fig. 1b shows the UV–vis diffuse reflectance spectra of pureRP, asprepared HAp, and RP (5.0 wt%)/HAp composite in the wavelength range from 200 to 800 nm. Obviously, the as-prepared HAp exhibits very weak absorption in the UV region. In contrast, RP has much broader and
Fig. 3. Binging energy spectra obtained by X-ray photoelectron spectroscopy of red phosphorus (5.0 wt%)/hydroxyapatite: (a) survey spectrum, (b) O 1s spectrum, (c) P 2p spectrum, and (d) Ca 2p spectrum. 3
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Journal of Physics and Chemistry of Solids 139 (2020) 109353
photocatalytic degradation. Fig. 4 demonstrates the photocatalytic degradation of the aforementioned antibiotics over RP (5.0 wt%)/HAp composite under 300W xenon lamp irradiation. Before irradiation, a dark adsorption experiment was conducted to ensure the establishment of adsorption-desorption equilibrium. As observed, the as-prepared RP (5.0 wt%)/HAp composite can adsorb certain amounts of TC and OFLX, and adsorption removal rates of approximately 23% and 8% were ob tained in the adsorption process for TC and OFLX, respectively. No obvious removal of RIF was observed after 30 min adsorption overRP (5.0 wt%)/HAp composite. The different adsorption abilities for different antibiotics over the as-prepared RP (5.0 wt%)/HAp composite might be ascribed tothe different chemical structures of the antibiotics and interactions between the RP (5.0 wt%)/HAp composite and the antibiotics, as verified in other literature [39]. As observed in Fig. 4, TC, RIF, and OFLX can be completely degraded over the as-prepared RP (5.0 wt%)/HAp composite within 10 min, 20 min, and 50 min, respectively, under xenon lamp irradiation and with a photocatalytic efficiency of approximately 77%, 100%, and 92%, respectively. Thus, the total removal efficiency for the three model antibiotics was approximately 100% over the as-prepared RP (5.0 wt%)/HAp composite. It can be inferred that the RP (5.0 wt%)/HAp composite has wide applications for the degradation of various antibiotic pollutants in wastewater. The BET specific surface area of RP (5.0 wt%)/HApwas calculated to be 60.3 m2/g, which is much higher than that of hollow HAp micro spheres (44.2 m2/g). It can be deduced that the enhanced BET specific surface area of RP (5.0 wt%)/HAp compared with hollow HAp micro spheres may favor an increase in the number of active sites and thus promote its photocatalytic activity toward antibiotic degradation. The reusability and stability of the as-prepared RP (5.0 wt%)/HAp composite was investigated by three consecutive photocatalytic degra dation runs for RIF to assess the possible use of the photocatalyst in practical applications. As shown in Fig. 5a, there was no obvious change in the photocatalytic activity of RP (5.0 wt%)/HAp composite even after three successive experimental runs under the same conditions, sug gesting its excellent recyclability in the RIF degradation reaction. In addition, the XRD patterns of the as-prepared RP (5.0 wt%)/HAp com posite before and after the photocatalytic degradation (Fig. 5b) provide good proof that the photocatalyst is very stable during the reaction. Therefore, the as-prepared RP (5.0 wt%)/HAp composite can be used as a stable and high-performance photocatalyst for antibiotic pollutant removal. As shown in Fig. 6a, when2-propanol,1,4-benzoquinone, or disodium ethylenediaminetetraacetic acid scavenger was added, the photo catalytic degradation efficiency was significantly reduced. Thus, it can be inferred that ⋅O2 , ⋅OH, and hþare the main active species responsible for the photocatalytic degradation ofthe antibiotics. Furthermore, the ESRanalysis (Fig. 6b–d) confirms that⋅O2 , ⋅OH, and hþ are the pre dominant active species in the photocatalytic process, which is in good agreement with trapping experiment results. On the basis of the above experimental results, a plausible degra dation pathway is proposed in Fig. 7. The possible photocatalytic equations are expressed as follows:
Fig. 4. Photocatalytic degradation of tetracycline (TC), rifampicin (RIF), and levofloxacin (OFLX) over red phosphorus (5.0 wt%)/hydroxyapatite composite under 300 W xenon lamp full spectrum irradiation.
Fig. 5. (a) Photocatalytic recycling runs for rifampicin over red phosphorus (5.0 wt%)/hydroxyapatite composite and (b) X-ray diffraction patterns of red phosphorus (5.0 wt%)/hydroxyapatite composite photocatalyst before and after rifampicin degradation. BQ, 1,4-benzoquinone; IPA, 2-propanol, EDTA disodium ethylenediaminetetraacetic acid.
HAp = RP þ hν→e þ hþ
(2)
e ðHApÞ→e ðRPÞ
(3)
O2 þ e ðRPÞ→⋅O2
(4)
H2 O þ hþ ðHApÞ → ⋅ OH þ Hþ
(5)
� � ⋅O2 ⋅OH hþ þ antibiotics→degraded products
(6)
As demonstrated in Fig. 7, hollow HAp microspheres and RP can act as an electron donor and an electron acceptor, respectively. Under a 300 W xenon lamp with full spectrum irradiation, the photogenerated elec trons (e ) in the valence band of the hollow HAp microspheres can be 4
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Journal of Physics and Chemistry of Solids 139 (2020) 109353
Fig. 6. (a) Results of trapping experiments for red phosphorus (5.0 wt%)/hydroxyapatite composite, (b)electron spin resonance (ESR) signals of (DMPO)-⋅O2 , (c)ESR signals of DMPO-⋅OH, and (d)ESR signals of (TEMPO)-hþ for red phosphorus (5.0 wt%)/hydroxyapatite.
fabricated and applied for the degradation of RIF, TC, and OFLX. It was shown that the RP (5.0 wt%)/hollow HAp microsphere composite has universal application for complete degradation of each antibiotic. Moreover, the RP (5.0 wt%)/hollow HAp microsphere composite pos sesses good stability during the photocatalytic process, confirming its promising potential as an efficient photocatalyst. As expected, the asprepared RP (5.0 wt%)/hollow HAp microsphere compositehas wide applications for environmental remediation. Declaration of competing interest We declare that we do not have any commercial or associative in terest that represents a conflict of interest in connection with the work submitted.
Fig. 7. Mechanism for photocatalytic degradation of antibiotics over red phosphorus (RP) (5.0 wt%)/hydroxyapatite (HAp)composite.
CRediT authorship contribution statement
excited to the conduction band, leaving behind positive holes (hþ). The excited electrons in the conduction band of the hollow HAp micro spheres can be transferred tothe RP surface. The photogenerated elec trons accumulating at the RP surface can react with O2 to form ⋅O2 , resulting in partial degradation of antibiotic molecules. Meanwhile, the separated positive holes in the valence band of RP can directly partici pate in the oxidation of antibiotic molecules. Besides, the separated positive holes in the valence band of the hollow HApmicrospheres can react with H2O to form ⋅OH, which also can lead to partial degradation of antibiotic molecules.
Rongjiang Zou: Conceptualization, Methodology, Investigation. Tianhong Xu: Data curation, Investigation. Xiaofang Lei: Resources, Visualization. Qiang Wu: Supervision, Writing - original draft, Writing review & editing. Song Xue: Writing - review & editing. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21107069), and the Science and Technology Commission of Shanghai Municipality (14DZ2261000).
4. Conclusion
Appendix A. Supplementary data
To sum up, a new RP (5.0 wt%)/hollow HAp microsphere photo catalyst with excellent photocatalytic performance was successfully
Supplementary data to this article can be found online at https://doi. 5
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Journal of Physics and Chemistry of Solids 139 (2020) 109353
org/10.1016/j.jpcs.2020.109353.
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6
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs
Novel and efficient red phosphorus/hollow hydroxyapatite microsphere photocatalyst for fast removal of antibiotic pollutants Rongjiang Zou a, Tianhong Xu b, Xiaofang Lei b, Qiang Wu b, *, Song Xue a, ** a
Department of Cardiovascular Surgery, Renji Hospital, School of Medicine, Shanghai Jiao Tong University, 200127, Shanghai, People’s Republic of China Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, College of Environmental and Chemical Engineering, Shanghai University of Electric Power, 200090, Shanghai, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Red phosphorus Hydroxyapatite Rifampicin Tetracycline Levofloxacin Photocatalysis
Photocatalysis is a well-established and green technique in environmental pollutant remediation. In the present study, a novel and green red phosphorus (5.0 wt%)/hollow hydroxyapatite microsphere photocatalyst was fabricated and applied forthe degradation of rifampicin, tetracycline, and levofloxacin. The as-prepared material exhibits outstanding photocatalytic activity and stability, and has universal application for degradation of each antibiotic. Notably, tetracycline, rifampicin, andlevofloxacin can be completely degraded within 10 min, 20 min, and 50 min, respectively, under a 300 W Xe lamp with full spectrum illumination, where the optimal conditions were an initial pH of 6.0 and a photocatalyst dosage of 1.0 g/L. In addition, a plausible photocatalytic reaction mechanism was proposed. The low-cost, highly efficient, and green red phosphorus (5.0 wt%)/hollow hy droxyapatite microsphere photocatalyst is a promising candidate for the removal of many hazardous pollutants.
1. Introduction Water contamination is one of the key challenges for our modern industrial societyandrequires effective methods and techniques to solve this problem [1–9]. Antibiotics have been widely used for treatment and cure of human and animal diseases. However, the presence of antibiotic pollutants in water is a great threat to human health and ecosystem equilibrium [10,11]. Thus, the removal of antibiotic pollutants from wastewater has become a hot issue. Many methods have been used to degrade and remove antibiotic pollutants, including physical adsorp tion, biodegradation, electrochemical oxidation, and photocatalysis [12–23]. Conventional techniques usually fail to achieve complete degradation of antibiotic pollutants and lead tothe formation of some toxic organic intermediates. In contrast, photocatalysisis regarded as a promising and green approach to fully remove antibiotic pollutants from wastewater [24–29]. Hydroxyapatite (Ca10(PO4)6(OH)2;HAp)has excellent biocompati bility and bioactivity, is nontoxic, environmentally benign, and of low cost, and has many applications in biomaterials [30,31]. In recent years, HAp has been explored as a cheap and green photocatalyst for degra dation of some organic pollutants [32–34]. However, its chemical
stability and photocatalytic efficiency are still far from satisfactory. Thus, it is important to develop a feasible and low-cost strategy to synthesize a novel structured HAp-based composite photocatalyst with high efficiency and stability. Very recently, red phosphorus (RP) has sparked a great deal of in terest in photocatalysis because of its Earth abundance, low cost, high stability, lack of metal components, and suitable band position [35–37]. It has been confirmed that adding RP to some photocatalysts could take full advantage of their abilities for organic pollutant degradation. In addition to their use as photocatalysts, hollow HAp microspheres tend to have high specific surface areas and enhanced active sites, and thus have potential as candidate carriers fora composite photocatalyst. As a result, hollow HApmicrospheres decorated with small amounts of RPwould hold promise as highly functionalized materials for potential application in wastewater pollutant treatment. Here, a newRP (5.0 wt%)/hollow HAp microsphere photocatalyst was fabricated and its applications in antibiotic pollutant removal were explored for rifampicin (RIF), tetracycline (TC), and levofloxacin (OFLX). As expected, the as-prepared RP (5.0 wt%)/hollow HAp microsphere composite possesses high photocatalytic efficiency in the degradation of the three a forementioned antibiotics in a short time. This
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Wu), [email protected] (S. Xue). https://doi.org/10.1016/j.jpcs.2020.109353 Received 6 October 2019; Received in revised form 6 January 2020; Accepted 7 January 2020 Available online 10 January 2020 0022-3697/© 2020 Elsevier Ltd. All rights reserved.
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Journal of Physics and Chemistry of Solids 139 (2020) 109353
fundamental research will provide a promising strategy for developing highly efficient and compatible photocatalysts with wide applications. 2. Experimental 2.1. Preparation of hollow HAp microspheres Hollow HAp microspheres were obtained with CaCO3 as a hard template through a hydrothermal route. First, CaCO3 template was prepared on the basis of a polystyrene sodiumsulfonate (PSS) polymertemplate method. Briefly, CaCl2 solution (30 mL, 0.2 M) was added to sodium polystyrene sulfonate solution (300 mL, 10 g/L) under magnetic stirring, followed by the dropwise addition of Na2CO3 (30 mL, 0.2 M) to the prepared mixture. After stirring for 1 h, the resulting solution was centrifuged and washed with deionized water and ethanol, respectively. Target CaCO3 template was collected after vacuum drying at 333 K overnight. Next, 0.2 g CaCO3 template was added to Na2HPO4solution (100 mL, 0.2 M) under stirring. The pH was adjusted to 11.0 with NaOH solution. Later the suspension was transferred to a 200 mL Teflon-lined autoclave. After hydrothermal treatment at 393 K for 1 h and natural cooling, the corresponding sample was collected by centrifugation and washed with deionized water and ethanol, respectively, several times, Finally, the resulting sample was dried under a vacuum at 333 K over night to obtain hollow HAp microspheres. 2.2. Controllable synthesis of RP/hollow HAp microsphere composites The typical synthetic procedure was as follows. A certain amount of the as-prepared hollow HAp microspheres was first placed in a suction filtration unit. Then a certain amount of purified commercial RP was ultrasonically dispersed in ethanol and slowly dropped onto the hollow HAp microspheres. Excess solution was removed by vacuum filtration. Afterwards, the resulting product was dried in air at 353 K for 12 h to obtain the desired composite. In the present study, composites with a 5.0 wt% ratio of RP to hollow HAp microspheres (abbreviated asRP (5.0 wt%)/HAp) were obtained and optimized for further use. 2.3. Characterization
Fig. 1. (a) X-ray diffraction patterns of red phosphorus (RP), hydroxyapatite (HAp), and RP (5.0 wt%)/HAp composite. (b) UV–visible absorption spectra of RP, HAp, and RP (5.0 wt%)/HAp composite.
X-ray diffraction (XRD) measurements were performed with a Bru kerD8 diffractometer with a CuKα radiation. Field-emission scanning electron microscopy (FE-SEM) measurements were performed with a Hitachi SU-1500 instrument. Transmission electron microscopy (TEM) images were recorded with a JEOLJEM-2010 electron microscope. Ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy was con ducted with a Shimadzu UV-2550 spectrophotometer. BrunauerEmmett-Teller (BET) specific surface area was measured with a Gemini VII 2390 instrument. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific ESCALAB250XiX-ray photo electron spectrometer. Electron spin resonance (ESR) spectroscopy was performed with a Bruker EPRA300-10/12 spectrometer.
¼ 335 nm for RIF,λmax ¼ 357 nm for TC, andλmax ¼ 294 nm for OFLX. The photocatalytic degradation efficiency (D) was calculated as follows: Dð%Þ ¼
C0
Ct C0
� 100% ;
(1)
whereC0 and Ct represent the initial antibioticconcentration and the antibioticconcentration after a certain irradiation time t. A recycling experiment was conducted for RIFto test the stability and reusability of the as-prepared photocatalyst. Trapping experiments were performed to elucidate the plausible photocatalytic degradation mechanism. 2-Propa nol, disodium ethylenediaminetetraacetic acid, and 1,4-benzoquinone were used as different scavengers to detect the main active species during the photocatalytic process. In addition, ESRspectroscopywas further performed to identify the active species generated in the pho tocatalytic process.
2.4. Photocatalytic experiments for antibiotics The photocatalytic performance of as-prepared RP (5.0 wt%)/HAp composite was evaluated by degradation of RIF, TC, and OFLX in a homemade glass reactor at room temperature. A 300 W xenon arc lamp (PLS-SXE 300, Perfectlight Co.) with full spectrum irradiation was used as the light source. In a typical photocatalytic experiment, 100 mg ofasprepared RP (5.0 wt%)/HApwas dispersed in 100 mL antibioticaqueous solution (10 mg/L) at pH6.0. Before irradiation, the solution was magnetically stirred in darkness for 30 min to ensure an adsorptiondesorption equilibrium. Then the lamp was turned on. At given irradi ation intervals, 4 mL of suspension was withdrawn and centrifuged, and then the supernatant was analyzed with a UV–vis spectrometer to determine its residual concentration according to the absorbance at λmax
3. Results and discussion The phase composition and crystallinity of the as-prepared RP (5.0 wt%)/HAp composite were characterized by the XRDtechnique (Fig. 1). For comparison, the XRD patterns ofcommerciallyavailableRP and asprepared HAp were also measured. For HAp (Fig. 1a), all the diffrac tion peaks match very well with the typical hexagonal structured HAp (JCPDScard no. 09–0432). As illustrated in Fig. 1b, one typical peak at 2
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stronger absorption both in the UV region and in the visible region. As expected, the light-harvesting ability of the as-prepared RP (5.0 wt %)/HAp composite was greatly enhanced in the whole region compared with that of the as-prepared HAp. This result demonstrates that the asprepared RP (5.0 wt%)/HAp composite can act as a promising photo catalyst for wastewater pollutant removal. FE-SEM and TEM characterization were performed to confirm the morphology of the as-prepared RP (5.0 wt%)/HAp composite. From the FE-SEM image in Fig. 2a, it can be seen that HAp microspheres are a good substrate to anchor and stabilize RP particles on their surface. As shown inthe TEM image (Fig. 2b), the as-prepared RP (5.0 wt%)/HAp compositehas a translucent center and a black edge, demonstrating that the composite has a hollow structure. The TEM image also indicates that RP particles were immobilized on the surface of hollow HAp micro spheres, which further confirms that hollow HAp microspheres can act as a good substrate for anchoring of RP particles. XPS analysis was used to analyze the chemical state and elemental composition of the as-prepared RP (5.0 wt%)/HAp composite. As shown in Fig. 3a, the survey spectrum of RP (5.0 wt%)/HApdemonstrates the coexistence of the elements P, Ca, and O. The high-resolution spectrum for O 1s (Fig. 3b)exhibits a peak at 532.1 eV, which corresponds to O2 . The P 2p spectrum (Fig. 3c) exhibits three characteristic peaks; the peak at 133.9 eV is ascribed to P 2p, andthe peaks at 130.2 eV and 129.4 eV are assigned to P2p1/2 and P2p3/2, respectively [38]. Furthermore, the high-resolution spectrum for Ca 2p (Fig. 3d) exhibits peaks at 347.4 eV and 350.9 eV, which are ascribed to Ca2þ. To verify the photocatalytic performance of the RP (5.0 wt%)/HAp composite, TC, RIF, and OFLX were used as model antibiotics for
Fig. 2. (a) Field-emission scanning electron microscopy image and (b) trans mission electron microscopy image of red phosphorus (5.0 wt%)/hydroxyapa tite composite.
15.3� , corresponding to (102) planes, was observed for commercially available RP. In addition, the RP (5.0 wt%)/HAp composite (Fig. 1c) exhibits an XRD pattern similar to that ofas-prepared HAp. The disap pearance of the RP characteristic peak in the composite was due to weak crystallinity and a trace amount of RP not being measured. No peaks of any other phases or impurities are present in the composite, indicating high purity of the final product. The results also indicate the successful synthesis of RP (5.0 wt%)/HAp composite in the present study. Fig. 1b shows the UV–vis diffuse reflectance spectra of pureRP, asprepared HAp, and RP (5.0 wt%)/HAp composite in the wavelength range from 200 to 800 nm. Obviously, the as-prepared HAp exhibits very weak absorption in the UV region. In contrast, RP has much broader and
Fig. 3. Binging energy spectra obtained by X-ray photoelectron spectroscopy of red phosphorus (5.0 wt%)/hydroxyapatite: (a) survey spectrum, (b) O 1s spectrum, (c) P 2p spectrum, and (d) Ca 2p spectrum. 3
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photocatalytic degradation. Fig. 4 demonstrates the photocatalytic degradation of the aforementioned antibiotics over RP (5.0 wt%)/HAp composite under 300W xenon lamp irradiation. Before irradiation, a dark adsorption experiment was conducted to ensure the establishment of adsorption-desorption equilibrium. As observed, the as-prepared RP (5.0 wt%)/HAp composite can adsorb certain amounts of TC and OFLX, and adsorption removal rates of approximately 23% and 8% were ob tained in the adsorption process for TC and OFLX, respectively. No obvious removal of RIF was observed after 30 min adsorption overRP (5.0 wt%)/HAp composite. The different adsorption abilities for different antibiotics over the as-prepared RP (5.0 wt%)/HAp composite might be ascribed tothe different chemical structures of the antibiotics and interactions between the RP (5.0 wt%)/HAp composite and the antibiotics, as verified in other literature [39]. As observed in Fig. 4, TC, RIF, and OFLX can be completely degraded over the as-prepared RP (5.0 wt%)/HAp composite within 10 min, 20 min, and 50 min, respectively, under xenon lamp irradiation and with a photocatalytic efficiency of approximately 77%, 100%, and 92%, respectively. Thus, the total removal efficiency for the three model antibiotics was approximately 100% over the as-prepared RP (5.0 wt%)/HAp composite. It can be inferred that the RP (5.0 wt%)/HAp composite has wide applications for the degradation of various antibiotic pollutants in wastewater. The BET specific surface area of RP (5.0 wt%)/HApwas calculated to be 60.3 m2/g, which is much higher than that of hollow HAp micro spheres (44.2 m2/g). It can be deduced that the enhanced BET specific surface area of RP (5.0 wt%)/HAp compared with hollow HAp micro spheres may favor an increase in the number of active sites and thus promote its photocatalytic activity toward antibiotic degradation. The reusability and stability of the as-prepared RP (5.0 wt%)/HAp composite was investigated by three consecutive photocatalytic degra dation runs for RIF to assess the possible use of the photocatalyst in practical applications. As shown in Fig. 5a, there was no obvious change in the photocatalytic activity of RP (5.0 wt%)/HAp composite even after three successive experimental runs under the same conditions, sug gesting its excellent recyclability in the RIF degradation reaction. In addition, the XRD patterns of the as-prepared RP (5.0 wt%)/HAp com posite before and after the photocatalytic degradation (Fig. 5b) provide good proof that the photocatalyst is very stable during the reaction. Therefore, the as-prepared RP (5.0 wt%)/HAp composite can be used as a stable and high-performance photocatalyst for antibiotic pollutant removal. As shown in Fig. 6a, when2-propanol,1,4-benzoquinone, or disodium ethylenediaminetetraacetic acid scavenger was added, the photo catalytic degradation efficiency was significantly reduced. Thus, it can be inferred that ⋅O2 , ⋅OH, and hþare the main active species responsible for the photocatalytic degradation ofthe antibiotics. Furthermore, the ESRanalysis (Fig. 6b–d) confirms that⋅O2 , ⋅OH, and hþ are the pre dominant active species in the photocatalytic process, which is in good agreement with trapping experiment results. On the basis of the above experimental results, a plausible degra dation pathway is proposed in Fig. 7. The possible photocatalytic equations are expressed as follows:
Fig. 4. Photocatalytic degradation of tetracycline (TC), rifampicin (RIF), and levofloxacin (OFLX) over red phosphorus (5.0 wt%)/hydroxyapatite composite under 300 W xenon lamp full spectrum irradiation.
Fig. 5. (a) Photocatalytic recycling runs for rifampicin over red phosphorus (5.0 wt%)/hydroxyapatite composite and (b) X-ray diffraction patterns of red phosphorus (5.0 wt%)/hydroxyapatite composite photocatalyst before and after rifampicin degradation. BQ, 1,4-benzoquinone; IPA, 2-propanol, EDTA disodium ethylenediaminetetraacetic acid.
HAp = RP þ hν→e þ hþ
(2)
e ðHApÞ→e ðRPÞ
(3)
O2 þ e ðRPÞ→⋅O2
(4)
H2 O þ hþ ðHApÞ → ⋅ OH þ Hþ
(5)
� � ⋅O2 ⋅OH hþ þ antibiotics→degraded products
(6)
As demonstrated in Fig. 7, hollow HAp microspheres and RP can act as an electron donor and an electron acceptor, respectively. Under a 300 W xenon lamp with full spectrum irradiation, the photogenerated elec trons (e ) in the valence band of the hollow HAp microspheres can be 4
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Journal of Physics and Chemistry of Solids 139 (2020) 109353
Fig. 6. (a) Results of trapping experiments for red phosphorus (5.0 wt%)/hydroxyapatite composite, (b)electron spin resonance (ESR) signals of (DMPO)-⋅O2 , (c)ESR signals of DMPO-⋅OH, and (d)ESR signals of (TEMPO)-hþ for red phosphorus (5.0 wt%)/hydroxyapatite.
fabricated and applied for the degradation of RIF, TC, and OFLX. It was shown that the RP (5.0 wt%)/hollow HAp microsphere composite has universal application for complete degradation of each antibiotic. Moreover, the RP (5.0 wt%)/hollow HAp microsphere composite pos sesses good stability during the photocatalytic process, confirming its promising potential as an efficient photocatalyst. As expected, the asprepared RP (5.0 wt%)/hollow HAp microsphere compositehas wide applications for environmental remediation. Declaration of competing interest We declare that we do not have any commercial or associative in terest that represents a conflict of interest in connection with the work submitted.
Fig. 7. Mechanism for photocatalytic degradation of antibiotics over red phosphorus (RP) (5.0 wt%)/hydroxyapatite (HAp)composite.
CRediT authorship contribution statement
excited to the conduction band, leaving behind positive holes (hþ). The excited electrons in the conduction band of the hollow HAp micro spheres can be transferred tothe RP surface. The photogenerated elec trons accumulating at the RP surface can react with O2 to form ⋅O2 , resulting in partial degradation of antibiotic molecules. Meanwhile, the separated positive holes in the valence band of RP can directly partici pate in the oxidation of antibiotic molecules. Besides, the separated positive holes in the valence band of the hollow HApmicrospheres can react with H2O to form ⋅OH, which also can lead to partial degradation of antibiotic molecules.
Rongjiang Zou: Conceptualization, Methodology, Investigation. Tianhong Xu: Data curation, Investigation. Xiaofang Lei: Resources, Visualization. Qiang Wu: Supervision, Writing - original draft, Writing review & editing. Song Xue: Writing - review & editing. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21107069), and the Science and Technology Commission of Shanghai Municipality (14DZ2261000).
4. Conclusion
Appendix A. Supplementary data
To sum up, a new RP (5.0 wt%)/hollow HAp microsphere photo catalyst with excellent photocatalytic performance was successfully
Supplementary data to this article can be found online at https://doi. 5
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org/10.1016/j.jpcs.2020.109353.
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