Chinese Chemical Letters  2019, Vol. 30 Issue (6): 1214-1218   PDF    
Photocatalytic degradation of amoxicillin by carbon quantum dots modified K2Ti6O13 nanotubes: Effect of light wavelength
Qiankun Chena, Long Chena, Juanjuan Qia, Yingqian Tonga, Yitao Lva, Chaokai Xua, Jinren Nia,b, Wen Liua,b,*     
a The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China;
b The Beijing Innovation Center for Engineering Science and Advanced Technology(BIC-ESAT), Peking University, Beijing 100871, China
Abstract: A novel carbon quantum dots modified potassium titanate nanotubes (CQDs/K2Ti6O13) composite photocatalyst was synthesized by hydrothermal treatment combined with calcination. X-ray diffraction (XRD) pattern and transmission electron microscopy (TEM) indicated formation of potassium titanate nanotubes and successful deposition of CQDs onto K2Ti6O13. The photocatalytic performance of CQDs/K2Ti6O13 composite was evaluated by degradation of amoxicillin (AMX) under the irradiation of visible light and lights with the wavelengths of 365, 385, 420, 450, 485, 520, 595 and 630 nm. The results showed that the photocatalytic activity of CQDs/K2Ti6O13 hybrid material was greatly enhanced compared with the neat K2Ti6O13 calcined at 300 ℃. The narrowed band gap energy (Eg) and transfer of photo-excited electron by CQDs inhibited the immediate combination of electron-hole pairs, thus promoting photocatalytic activity. Moreover, CQDs/K2Ti6O13 exhibited a broad spectrum of photocatalytic ability and it was interesting that the photocatalytic activity decreased with the increase of the irradiation wavelength. Reactive oxygen species (ROS) quenching tests suggested the hole (h+) and hydroxyl radical (·OH) played the primary roles in photocatalytic degradation of AMX. Moreover, CQDs/K2Ti6O13 showed good reusability for AMX photocatalytic degradation after five successive runs. This study proposed an available method for titanate nanomaterials modification, and the developed novel CQDs/K2Ti6O13 hybrid material is promising for potential application on antibiotics removal from water and wastewater.
Keywords: Potassium titanate nanotubes     Carbon quantum dots     Photocatalysis     Antibiotics     Photo energy    

In the past few years, the antibiotics in aquatic environment have aroused increasing widespread concerns because of their potential risks to natural ecosystem and human health [1-3]. It was reported that about 100, 000–200, 000 tons of antibiotics were used all over the world every year [4]. Over 65% of the global antibiotic consumption belonged to β-lactam antibiotics [5]. Amoxicillin (AMX) is one of the most common β-lactam antibiotics, which is widely used throughout the world due to its remarkable antibacterial effect [6]. Because of its low biodegradability, AMX is frequently detected in the effluents of wastewater treatment plants and surface waters [7]. Although low concentration of AMX existed in surface water ranging from a few ng/L to μg/L, AMX may show various unexpected hazards to environment, eco-system and human exposure due to its adverse long-term effects [8-10]. Unfortunately, conventional wastewater treatment processes are not able to efficiently remove AMX [11]. Therefore, it is urgent and crucial to develop new techniques for AMX removal.

Heterogenous catalysis using functional materials has been widely applied for antibiotics removal [12-14]. Photocatalysis using nanomaterials, which can eliminate refractory organic pollutants including AMX from water thoroughly, has been believed to be the green and energy-saving strategy considering the potential solar energy utilization [15-18]. Titanate and its modified materials are confirmed to show great environmental remediation potentials [19], for example, removal of various contaminants through photocatalysis [20-22]. In recent years, potassium hexatitanate (K2Ti6O13) nanotubes which exhibit unique tunnel crystal structure and large specific surface area have attracted growing attention due to their chemical stability, intrinsic optical performance, electronic property, and photocatalytic activity for removing refractory organic pollutants [23-25]. However, K2Ti6O13 nanotubes can only be excited by UV light and cannot make full use of solar energy [23]. In addition, K2Ti6O13 nanotubes cannot achieve the expected excellent reactivity owing to the rapid recombination rate of photogenerated electron-hole pairs [26]. Therefore, modification of K2Ti6O13 nanomaterials to enhance the photocatalytic activity is necessary.

Carbon quantum dots (CQDs) are a new class of zerodimensional (0D) nanomaterials with sizes below 10 nm in diameter [27]. CQDs exhibit high aqueous solubility, good biocompatibility, low toxicity, tunable photoluminescence and have been applied in a series of fields, such as bioimaging, biosensing, photovoltaic devices [28]. Very recently, CQDs have been introduced to photocatalytic technology [29]. On the one hand, CQDs can display excellent up-converted photoluminescence (UCPL) behavior, leading to expanded utilization of solar energy from ultraviolet to visible range by the nanoscale photocatalysts [30]. On the other hand, CQDs possess superior ability for charge transport due to the conjugated structure, and thus can inhibit the recombination efficiency of photogenerated charges effectively [31]. Up to now, a large variety of CQDs modified photocatalysts have been reported, such as CQDs/TiO2 [32], CQDs/ g-C3N4 [33], CQDs/BiOX (X = Cl, Br or I) [17, 19], CQDs/Bi2WO6 [31]. Likewise, improved photocatalytic performance towards AMX removal was expected for CQDs modified K2Ti6O13 nanotubes.

In this study, we developed a hydrothermal treatment combined with calcination method for preparing CQDs/K2Ti6O13 composite photocatalyst. Compared with the neat K2Ti6O13 nanotubes, the obtained CQDs/K2Ti6O13 hybrid material exhibited greatly enhanced photocatalytic activity for AMX degradation under light irradiation with different wavelengths. The underlying mechanism on enhanced photocatalytic activity was discussed. The CQDs/K2Ti6O13 composite is a promising photocatalyst in the environmental remediation area on removal of antibiotics from water and wastewater.

All chemicals used in the experiments were of analytical grade, and deionized (DI) water was used to prepare all the solutions. Details related to the chemicals were provided in Supporting information.

The CQDs solution was prepared through hydrothermal treatment according to the previous study [34]. The K2Ti6O13 nanotubes were synthetized via a single alkali treatment [35]. Typically, 1.2 g P25 TiO2 was dispersed into 66.7 mL of 10 mol/L KOH solution. After magnetic stirring for 12 h, the mixture was dumped into a Teflon-lined reactor and then heated at 200 ℃ for 12 h. Afterwards, the white product was washed with 0.01 mol/L HCl solution till pH ~6 and oven-dried at 105 ℃ for 12 h.

The CQDs/K2Ti6O13 composite was fabricated by a simple calcination route. 0.2 g K2Ti6O13 was dispersed into 50 mL DI water under magnetic stirring, and then 2.0 mL CQDs solution was added. After being continuously stirred for 12 h at room temperature, the mixture was centrifuged at 8000 rpm for 5 min and the obtained substance was washed with DI water for five times. Afterwards, the product was dried at 105 ℃ for 12 h. Subsequently, the obtained powder was heated to 300 ℃ for 2 h. The pure K2Ti6O13 was also treated along the same process except addition of CQDs solution. Details on the characterizations of the prepared materials were provided in Supporting information.

In order to compare the photocatalytic activity of K2Ti6O13 and CQDs/K2Ti6O13 composite, a series of experiments on photocatalytic degradation of AMX were conducted under the irradiation of 365, 385, 420, 450, 485, 520, 595, 630 nm light and standard white light (visible light with wavelength ≥420 nm). A multichannel photochemical reaction system (PCX50C, PerfectLight, China) with nine light-emitting diode (LED) lights was used to supply all the lights, and the irradiation intensity was fixed at 10 mW/cm2. Batch experiments were carried out in quartz glass reactors at 25 ℃ with a circulating cool water system to maintain the reaction temperature. For each test, 50 mL of 1 mg/L AMX solution was mixed 0.2 g/L photocatalyst in the quartz glass reactor. The initial pH of solution was ~6.0. Before light irradiation, the mixture of photocatalyst and AMX solution was stirred for 30 min to reach adsorption-desorption equilibrium. During the photocatalytic degradation of AMX, 1 mL sample was taken at specified time intervals (0–90 min), and immediately filtered through 0.22 μm polytetrafluoroethylene (PTFE) membrane. Thus, the reaction was terminated due to separation of photocatalyst and no light supply. The concentration of AMX in the filtrate was detected by using a high performance liquid chromatography (HPLC, Agilent 1260 Infinity II, USA). A Zorbax SB-C18 column (4.6 mm × 150 mm, 3.5 μm) was equipped and the wavelength of UV detector (G1314 A VWD) was set as 220 nm. The mobile phase was composed of HPLC grade methanol and 0.1% formic acid in water at the ratio 20:80 (v/v) with a flow rate of 0.4 mL/min [36].

To investigate the primary reactive oxygen species (ROS) for AMX defecation during photocatalysis, scavenger quenching tests were carried out. Typically, 20 mmol/L of AO, IPA and TI were individually used for quenching hole (h+), hydroxyl radical (·OH), and superoxide radical (·O2-), respectively [33, 37, 38]. The method on reusability of CQDs/K2Ti6O13 was provided in Supporting information.

Fig. 1 presents the XRD patterns of the various titanium materials. P25 TiO2 nanoparticles exhibited mixed phases of anatase and rutile [20]. After hydrothermal reaction, all the TiO2 phase transformed into potassium titanate. The as-prepared potassium titanate showed distinct diffraction peaks of (200), (110), (310), (312), (602), (020) and (424) planes at 2θ = 11.5°, 24.1°, 29.3°, 33.8°, 43.5°, 47.9° and 66.3° respectively, which can be indexed to monoclinic phase of K2Ti6O13 (JCPDS card No. 40-0403) [35]. The heat treatment in air at 300 ℃ had little effect on its XRD pattern, which is in accordance with that the low-temperature calcination cannot change the crystal structure of titanate [39]. Moreover, there is no characteristic peak for CQDs in CQDs/ K2Ti6O13, which can be due to the relatively low CQDs content in the composite [19, 33].

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Fig. 1. XRD patterns of various titanium materials.

Fig. 2 presents the TEM images of different materials. TiO2 (P25) were spherical nanoparticles with a diameter of 30–50 nm (Fig. 2a) [20]. Figs. 2b and c show that CQDs/K2Ti6O13 composite photocatalyst exhibited multiwalled tubular structure, which was completely different from the morphology of TiO2. The length of the K2Ti6O13 nanotube was up to several hundreds of nanometers and diameter was ~10 nm (Figs. 2b and c). The introduction of CQDs onto K2Ti6O13 did not change the basic structure of K2Ti6O13 nanotubes. Some nanodots about 5 nm in diameter attached on the surface of K2Ti6O13 nanotubes were observed, indicating the CQDs and K2Ti6O13 were coupled successfully.

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Fig. 2. TEM images of different materials: (a) TiO2, (b) and (c) CQDs/K2Ti6O13.

The UV-DRS spectra of K2Ti6O13 and CQDs/K2Ti6O13 were displayed in Fig. S1 (Supporting information), and the energy gaps (Eg) of the two materials were got through Kubelka-Munk (K-M) transformation [40]. The Eg decreased to 3.42 eV of CQDs/K2Ti6O13 compared with 3.63 eV of unmodified K2Ti6O13, and the light adsorption edge shifted to 363 nm from 342 nm. In addition, light absorbance was greatly enhanced in a broad spectrum ranging from about 350 nm to 700 nm after CQDs modification. The narrowed Eg and enhanced light absorbance both facilitated the photocatalytic activity of K2Ti6O13.

Fig. 3 shows photocatalytic degradation of AMX by materials under various light sources. Lights with nine wavelengths were used as the irradiation sources, which were marked as UV365 (365 nm), UV385 (385 nm), white (visible light), purple (420 nm), blue (450), cyan (485 nm), green (520 nm), orange (595 nm) and red (630 nm) according to the light color, respectively. Before the photocatalytic degradation of AMX, blank experiments were performed without addition of any photocatalyst. Fig. S2 (Supporting information) shows that the photolysis of AMX was neglectable (< 1%) under the irradiation of UV 365 nm light and standard white light. In addition, UV365 had the highest photo energy, so direct degradation of AMX under other lights also can be ignored. As shown in Fig. 3, adsorption of AMX by the two materials could quick reach equilibrium within 10 min, while the removal of AMX by the two materials through adsorption was limited in dark (< 5%). At pKa1 (3.2) < pH (6.0) < pKa2 (11.7) (Table S1 in Supporting information), AMX existed in the form of zwitterionic AMX±, so the titanate cannot efficiently adsorb AMX through electrostatic attraction or cation ion exchange [41, 42]. Photocatalytic degradation of AMX by neat calcined K2Ti6O13 only found under UV lights (Fig. 3a), with a removal efficiency of 30.0% at 365 nm and 19.8% at 385 nm at 90 min, respectively. However, the photocatalytic activity of CQDs/K2Ti6O13 composite was remarkedly enhanced compared with the neat K2Ti6O13. Comparatively, after 90 min light irradiation at 365 nm and 385 nm, 100% of AMX removal was observed by using CQDs/K2Ti6O13 (Fig. 3b). More importantly, the CQDs/K2Ti6O13 hybrid material also displayed good photocatalytic ability under visible light irradiation as shown in Fig. 3b, which is consistent with the UV-DRS results (Fig. S1). For example, AMX removal efficiency by CQDs/K2Ti6O13 increased to 73.6% under standard white light irradiation at 90 min.

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Fig. 3. Photocatalytic degradation of AMX by (a) neat calcined K2Ti6O13 and (b) CQDs/K2Ti6O13 hybrid material; Pseudo-first order kinetic model fitting of the degradation of AMX by (c) neat calcined K2Ti6O13 and (d) CQDs/K2Ti6O13 hybrid material. Experimental conditions: initial AMX 1 mg/L, material dosage 0.2 g/L, pH 6.0 ± 0.1, temperature 25 ℃.

The AMX degradation kinetic data was further described by the pseudo-first order kinetic model (Eq. (1)) [20]:

(1)

where C0 and Ct (mg/L) are the AMX concentrations at initial and time t (min), respectively; and k1 (min-1) is the pseudo-first order rate constant.

The photocatalytic degradation kinetics of AMX can be well described by the pseudo-first order kinetic model (R2>0.99) (Table 1). The rate constant (k1) decreased with the increase of the light wavelength, indicating that the reactionwas strongly dependent on the irradiation wavelength. Zhan et al. also found higher H2 production efficiency via photocatalytic at lower wavelength of irradiation [43]. Chen et al. reported that the AgI/BiOIO3 heterostructure exhibited better photocatalytic performance for degradation of methyl orange under UV light than under visible light [44]. For the mechanism on different photocatalytic reactivity of AMX on the material, the photo energy (E) of various wavelengths was the key parameter for the nine irradiations. The photo energy (E, eV) at specific wavelength is obtained by Eq. (2) [45]:

Table 1
The best-fitted parameters of pseudo-first order kinetic model for AMX photocatalytic degradation by CQDs/K2Ti6O13 under irradiation with different wavelengths.

(2)

in which h is the Planck constant (6.626 × 10-34 J s), v is the photon frequency (s-1), C is the speed of light in vacuum (3 ×108 m/s), and λ is the wavelength of light (m).

Table 1 lists the calculated photo energy of lights with various wavelengths. Fig. S3 (Supporting information) displays the relationship between E and k1, which is in the terms of k1 = 0.0349E - 0.0732. The high correlation coefficient (R2 = 0.9271) indicated wavelength of light was a key parameter in the photocatalysis reaction. Specifically, photon with higher E can more efficiently excite the electron (e-) in the conduction band (CB) of photocatalyst, thus leading to easily separation of electronhole pairs. Therefore, more ROS (e.g., ·OH) will be produced and higher degradation efficiency of organics will be obtained.

To further explore the roles of ROS in the photocatalytic degradation of AMX, radicals quenching tests were carried out under UV365 and standard white lights (Fig. 4). AO, IPA and TI were individually added into the reaction system for quenching hole (h+), hydroxyl radical (·OH) and superoxide radical (·O2-), respectively. Addition of TI showed almost no effect on photocatalytic degradation of AMX (Fig. 4), indicating ·O2- was not responsible for AMX degradation. However, when the AO and IPA were added, the degradation efficiency of AMX was inhibited, especially for AO. The k1 value for AMX degradation under standard white light was decreased from 0.0144 to 0.0033 min-1 by AO and to 0.0094 min-1 by IPA, respectively. It is similar that the k1 value for AMX degradation under 365 nm UV light was decreased from 0.0495 to 0.0072 min-1 by AO and 0.0295 min-1 by IPA, respectively. The results revealed that h+ and ·OH were the primary ROS in the reaction systems and h+ played a more important role.

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Fig. 4. Photocatalytic degradation of AMX by CQDs/K2Ti6O13 in the presence of quenching agents under (a) UV365 and (b) standard white light irradiation. Experimental conditions: initial AM X 1 mg/L, material dosage 0.2 g/L, pH 6.0 ± 0.1, temperature 25 ℃, quenching agents dosage 20 mmol/L.

Fig. S4 (Supporting information) presents the reusability of CQDs/K2Ti6O13 composite for AMX photocatalytic degradation under the irradiation of 365 nm light and standard white light. CQDs/K2Ti6O13 exhibited good reusability, as the removal efficiency of AMX in the fifth cycle was slightly decreased from 100% to 91.7% under the irradiation of 365 nm light and from 73.5% to 66.8% under the irradiation of standard white light, respectively. In addition, the TOC in solution after each run was below detection limit, indicating good stability of the composite material. Therefore, CQDs/K2Ti6O13 is a promising material in practical application for organic contaminant removal.

Fig. S5 (Supporting information) depicts the schematic diagram on enhanced photocatalytic mechanism after CQDs deposition. Under light irradiation, photo-excited electron (e-) will escape from the semiconductor material, K2Ti6O13, thus resulting in formation of conduction band (CB) (Eq. (3)) [46]. The reminding part is valence band (VB, h+) with high oxidation ability (Eq. (3)), which can further oxidation H2O to ·OH (Eq. (6)). The excited e- can be captured by O2 molecule to form ·O2- (Eq. (5)). The CQDs in the CQDs/K2Ti6O13 composite can efficiently transfer electron due to its high quantum effect and electroconductivity (Eq. (4)) [19, 28, 33], thus inhibiting recombination of electron-hole pairs and promoting photocatalytic activity. AMX in the system is further oxidized by h+ and ·OH to organic products or achieve mineralization to CO2 and H2O (Eq. (7)).

(3)
(4)
(5)
(6)
(7)

In this work, a novel CQDs/K2Ti6O13 hybrid material was synthesized via an initial hydrothermal treatment and subsequent calcination. TEM and XRD confirmed the composite material maintain tubular structure of K2Ti6O13 while CQDs also successfully deposited. Compared with the neat K2Ti6O13 nanotubes, the photocatalytic activity of CQDs/K2Ti6O13 composite was greatly enhanced under both UV and visible light irradiations. CQDs/ K2Ti6O13 composite showed high degradation efficiency of AMX through photocatalysis. After 90 min light irradiation at 365 nm and 385 nm, AMX was completely removed by CQDs/K2Ti6O13, while application of the neat K2Ti6O13 only achieved 30.0% and 19.8% of AMX removal, respectively. After introduction of CQDs, the CQDs/K2Ti6O13 composite exhibited good photocatalytic performance at a broad spectrum of lights. More interesting, the photocatalytic activity decreased with the increase of the irradiation wavelength due to variation on photo energy. h+ and ·OH were the main ROS for photocatalytic degradation of AMX. The recycling experiments proved that the CQDs/K2Ti6O13 composite photocatalyst remained effective after five successive runs. The new CQDs/K2Ti6O13 hybrid material is promising for organics removal in the environmental remediation area.

Acknowledgments

Financial supports from Innovative Research Group of the National Natural Science Foundation of China (NFSC) (No. 51721006) and China Postdoctoral Science Foundation (No. 2017M620132) are much acknowledged.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.03.002.

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