Enhanced Photocatalytic Degradation of Environmental Pollutants

Apr 5, 2017 - Although nanotechnology has offered effective and efficient solutions for environmental remediation, the full utilization of sustainable...
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Enhanced Photocatalytic Degradation of Environmental Pollutants under Visible Irradiation by a Composite Coating Shuqin Liu,† Qingkun Hu,† Junlang Qiu,† Fuxin Wang,† Wei Lin,† Fang Zhu,† Chaohai Wei,‡ Ningbo Zhou,*,§ and Gangfeng Ouyang*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, No. 135, Xingang Xi Road, Guangzhou, Guangdong 510275, China ‡ College of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China § College of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, PR China S Supporting Information *

ABSTRACT: Although nanotechnology has offered effective and efficient solutions for environmental remediation, the full utilization of sustainable energy and the avoidance of secondary pollution are still challenges. Herein, we report a two-step modification strategy for TiO2 nanoparticles by first forming a thin, surface-adherent polydopamine (PDA) shell onto the nanoparticles and then assembling core−shell nanoparticles as a photodegradation coating. The composite coating modified from TiO2 could not only realize the highly efficient utilization of photons from the visible region but also avoid the secondary pollution of nanoparticles during application. Additionally, improvements in the adsorption ability after modification greatly facilitated the photocatalytic process of the modified materials. A preliminary in vivo study on Daphnia magna and a wastewater treatment experiment suggest that treatment with the composite coating can effectively eliminate fluorene and significantly reduce its lethality. We believe the two-step modification scheme can open new avenues for the facile modification of nanomaterials for designed purposes, especially in the field of environmental remediation. gap17,18 by promoting electron transfer from the valence band to the conduction band and facilitating the formation of oxidative species. However, the existence of some transition metals has been proven to facilitate the probability of electron− hole recombination, which would lead to a decline in the photocatalytic performance.6 Recently, some studies have shown that the band gap narrowing of TiO2 could be betterrealized by adding nonmetal elements (N, C, S, F, etc.) into the material.9,19,20 Such modified TiO2 materials showed larger responses to irradiation in the visible region and increased the degradation performance of hazardous pollutants under visible light irradiation, especially under natural solar light irradiation.6,20 Currently, polydopamine (PDA) has opened a new route to the functionalization of a variety of substrates because dopamine can self-polymerize and deposit PDA on almost all kinds of inorganic or organic surfaces.21 In addition, PDA is expected to offer a large number of active sites for binding organic pollutants via electrostatic interactions, hydrogen bonding, or π−π stacking interactions, owing to the existence

1. INTRODUCTION Environmental pollution has long been a global problem due to the rapid pace of urbanization and heavy industrialization.1,2 Although much effort has been spent on the removal and degradation of hazardous environmental contaminants,3−5 environment remediation is still a topic of global concern. Over the past decades, the emergence of nanotechnology has given tremendous possibilities and scope to remedy polluted water by utilizing nanomaterials designed with specific properties.6−8 As a well-known and the most investigated functional material in semiconductor photocatalysis, TiO2 nanoparticles have been extensively used in the degradation of a variety of toxic pollutants in air and water.9−11 Despite possessing numerous advantages, such as strong oxidizing power, high stability, nontoxicity, and superior photoelectric effect, the large band gap energy (3.2 eV) of TiO2 facilitates the necessity of utilizing ultraviolet (UV) excitation to realize specific applications.12−14 However, it should be noted that only a small amount of solar radiation comes from the UV region,6 which leads to requirements for enhancing the utilization of photons from the visible region by TiO2 nanoparticles. A viable strategy for narrowing the band gap of TiO2 can be realized by doping foreign elements into the materials.15,16 The modification of TiO2 with metal elements has been reported to successfully decrease the TiO2 band © 2017 American Chemical Society

Received: Revised: Accepted: Published: 5137

February 6, 2017 March 26, 2017 April 5, 2017 April 5, 2017 DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145

Article

Environmental Science & Technology

columns with the viscous mixture following the heating steps. Before each use, the TiO2/PDA/GA coating and fibers were conditioned at 90 °C overnight under a vacuum. 2.4. Physical Characterizations. The detection of pollutants was carried out on an Agilent 7890 gas chromatograph-flame ionization detector (GC-FID, Palo Alto, CA) and a gas chromatograph−mass spectrometer (GC-MS, 6890−5975 B) equipped with an HP-5 MS capillary column (30 m × 0.32 mm i.d. × 0.25 μm film thickness) from Agilent (Palo Alto, CA). Scanning electron microscopy (SEM) images and elemental mappings were obtained from a Quanta 400 Thermal FE environmental scanning electron microscope (FEI, Netherlands). Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu kα). Transmission electron microscopy (TEM) images were obtained via a JEM-2010HR transmission electron microscope (JEOL). Fourier transform infrared (FTIR) spectra were recorded on a Bruker EQUINOX 55 Fourier transform infrared spectrometer. BET surface areas and N2 adsorption isotherms (77.3 K) were obtained by a Micromeritics ASAP 2020 V3.04 H surface and porosity analyzer. X-ray photoelectron spectra (XPS) patterns were recorded on an ESCALab250 X-ray photoelectron spectrometer (Thermo Fisher Scientific). Element analysis (EA) was carried out by an Elementar Vario EL element analyzer, and UV−vis diffuse reflectance spectra were collected on a UV−visNIR spectrophotometer (Shimadzu UV-3600). Commercial polydimethylsiloxane (PDMS, 100 μm) and polyacrylate (PA, 85 μm) solid-phase microextraction (SPME) fibers were purchased from Supelco (Bellefonte, PA). 2.5. Adsorption of Geosmin and Fluorene. The TiO2/ PDA/GA coating was submersed in 10 mL of 1000 ppb geosmin or fluorene aqueous solutions in the dark. Agitation was carried out using a magnetic stir bar at room temperature. The real-time concentrations of the residual pollutants in the solutions were detected by headspace (HS) SPME coupled with GC-FID methods (see details in the Supporting Information). Similarly, the adsorption of geosmin or fluorene by the TiO2 or TiO2/PDA core−shell nanoparticles was conducted by replacing the TiO2/PDA/GA coating with the nanoparticles (the same amount as the coating materials). 2.6. Photodegradation Studies. Photocatalytic experiments were carried out in a homemade photoreactor (Figure 4a). In experiments with the coatings, a coated quartz glass tube was submersed in 10 mL of aqueous solution of 1000 ppb geosmin or fluorene under agitation after reaching extraction equilibrium. The mixture was irradiated with a 350 W xenon lamp under different wavelengths. Tinfoil was attached to the internal surface of the light blocking box. Photodegradation under UV was conducted by equipping the lamp with a 300− 400 nm filter, while photodegradation under visible light was realized by equipping the lamp with a 420 nm cutoff filter. In addition, irradiation from UV to visible light was provided with no filter equipped. After irradiation for a given time interval, the coating was removed from solution. The residual concentrations of the pollutant were detected by the headspace solidphase microextraction (HS-SPME) and gas chromatography− flame ionization detection (GC-FID) methods. Similar to the degradation of geosmin or fluorene by TiO2 nanoparticles, the TiO2/PDA core−shell nanoparticles or brush-like photodegradation device was constructed by replacing the TiO2/ PDA/GA coating with the nanoparticles or the assembled fiber array.

of abundant functional groups, such as catechol groups, amine groups, and aromatic moieties, which are beneficial to secondary modification as well.21−25 Moreover, as a major component of naturally occurring melanin, which is widely distributed in the human body, the excellent biocompatibility of PDA22 decreases the occurrence of adverse effects during biological and environmental applications. Therefore, in this contribution, PDA was used as a nonmetal element dopant to realize the band gap narrowing of rutile TiO2 by the simple immobilization of a thin, surface-adherent, and biocompatible PDA shell onto the TiO2 nanoparticles. However, separation is another major bottleneck that limits the application of TiO2 nanoparticles26−28 because it is hard to achieve photodegradation and separation simultaneously in powder form and difficult to recycle TiO2 due to its suspended dispersive properties in water.28,29 Moreover, the incomplete separation of nanoparticles may subsequently give rise to the secondary pollution of water.30 In this context, glutaraldehyde (GA) was used to assemble the prepared TiO2/PDA core−shell nanoparticles into a homogeneous photodegradation coating by connecting with the abundant functional groups in PDA shells. After two-step modification of rutile TiO2 nanoparticles, the resulting TiO2/PDA/GA coating possessed satisfactory visible light photodegradation capacity toward organic pollutants. As expected, the contaminated water treated by the composite coating was safe, as proven by preliminary in vivo studies.

2. EXPERIMENTAL SECTION 2.1. Synthesis of TiO2 Nanoparticles. First, 18 mL of ammonium hydroxide (30%), 32 mL of ultrapure water, and 44.5 mL of TEOS were added into 750 mL of ethanol with stirring for 24 h. The translucent SiO2 was collected by centrifugation and sintered at 500 °C. Then, 66 mL of aqueous TiCl4 (20 mM) containing 10 g of the sintered product was heated at 70 °C for 1 h. SiO2 was obtained after washing, drying, and resintering (500 °C). Afterward, 2.6 g of SiO2 and 6.68 g of 1-methylimidazolium tetrafluoroborate were mixed into 200 mL of the 120 mM TiF4 aqueous solution (pH = 2.1), which was transferred to a Teflon-lined autoclave and heated at 130 °C for 12 h. The product collected by filtration was then added into a 2 M NaOH aqueous solution at 80 °C for 1 h to remove SiO2. The remaining TiO2 nanoparticles were finally obtained by filtration, washing, and drying. 2.2. Synthesis of TiO2/PDA Core−Shell Nanoparticles. TiO2 nanoparticles (100 mg) and dopamine (40 mg) were dispersed in 320 mL of ultrapure water by ultrasonication for 30 min to form a suspension. Subsequently, 80 mL of Tris buffer (10 mM, pH: ∼8.5) was added to the suspension, and the mixture was stirred at room temperature for 24 h. Afterward, the product was obtained by centrifugation, washed three times with ultrapure water and ethanol, and then dried at 80 °C. 2.3. Preparation of TiO2/PDA/GA Coating and Fibers. First, a cleaned quartz glass tube with an external diameter of 5 mm was immersed into a solution of dopamine (2 mg/mL) with 10 mM Tris buffer (pH: ∼8.5) overnight at room temperature. The PDA-coated tube was rinsed with distilled water to remove residual dopamine. Afterward, 1 g of the TiO2/PDA core−shell nanoparticles were dispersed in 1 mL of GA (50% in water) to form a viscous mixture. The mixture was then spin-coated on the treated quartz glass tube and held at 60 °C for 4 h. Similarly, the TiO2/PDA/GA fibers were prepared by spin-coating cleaned gas chromatograph (GC) capillary 5138

DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145

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Environmental Science & Technology

Figure 1. Synthetic scheme of the TiO2/PDA core−shell nanoparticles and assembly of the TiO2/PDA/GA coating.

Figure 2. (a,b) TEM images of the TiO2/PDA nanoparticles; inset is the FFT-generated diffraction pattern. (c) SEM images of the TiO2/PDA nanoparticles. (d,e) TEM images of the TiO2/PDA/GA coating; inset is the FFT-generated diffraction pattern. (f) SEM images of the TiO2/PDA/ GA coating; inset is the photograph of the coating.

2.7. Lethality Studies in Daphnia magna. Daphnia magna were cultured in the artificial freshwater (AFW) under the conditions described in the guidelines from the Organization for Economic Cooperation and Development for the testing of chemicals (OECD, 2008). The AFW was prepared according to the method described previously.31 In brief, a total of 294.0 mg of CaCl2·2H2O, 123.2 mg of MgSO4· 7H2O, 5.8 mg of KCl, and 64.8 mg of NaHCO3 were mixed into 1 L of ultrapure water. The mixture was aerated vigorously for 24 h to dissolve the chemicals and stabilize the medium. The culture temperature was set at 20 ± 0.1 °C, and the photoperiod was 16 h:8 h light/dark. D. magna were fed twice a week with a suspension of Scenedesmus subspicatus. In the lethality experiments, D. magna were cultured under untreated AFW, AFW contaminated by fluorene (1000 ppb in concentration), and contaminated AFW that was treated by a

TiO2/PDA/GA coating under visible light irradiation for 120 min. Each group contained 100 D. magna. The lethality (%) of D. magna was monitored every 12 h. 2.8. Coking Wastewater Treatment. Coking wastewater was collected from a coke plant in Shanghai, China. Before treatment and analysis, the wastewater was filtered through a 0.45 μm filter membrane to remove insoluble substances. For each treatment experiment, 100 mL of coking wastewater was used.

3. RESULTS AND DISCUSSION 3.1. Synthesis of TiO2/PDA Core−Shell Nanoparticles and Assembly of TiO2/PDA/GA Coating. Dopamine has been known to self-polymerize and generate thin, surfaceadherent PDA membranes onto virtually all types of inorganic or organic substances.22 The simple immersion of TiO2 5139

DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145

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Figure 3. Photocatalytic degradation of geosmin and fluorene. (a) Schematic illustration of the removal and analysis of the pollutants. (b,d,f) Removal of geosmin by the TiO2 nanoparticles, the TiO2/PDA core−shell nanoparticles, and the TiO2/PDA/GA coating. (c,e,g) Removal of fluorene by the TiO2 nanoparticles, the TiO2/PDA core/shell nanoparticles, and the TiO2/PDA/GA coating.

2a, the TEM image of the scraped coating (Figure 2d) showed that the TiO2/PDA nanoparticles were almost aggregated after GA treatment, which corresponds to the self-assembly of the TiO2/PDA/GA coating. High-resolution transmission electron microscopy (HRTEM) images (Figure 2b,e) and fast Fourier transform (FFT) generated diffraction patterns (inset images in Figure 2a,d) confirmed that the nanoparticles consisted of a crystalline TiO2 core and an amorphous polymer shell. In the HRTEM images, a diagnostic lattice fringe was found in the core, while no lattice fringe appeared in the shell. The desired crystalline structure of the TiO2 cores in the TiO2/PDA core− shell nanoparticles and the TiO2/PDA/GA coating were also validated by PXRD patterns (Figure S2). The surface morphologies of the TiO2/PDA core−shell nanoparticles and the TiO2/PDA/GA coating exhibited distinct

nanoparticles into a diluent of dopamine (2 mg/mL), buffered to a pH typical of marine environments (10 mM Tris, pH 8.5), gave rise to the spontaneous formation of a homogeneous adherent polymer shell on the surface of the nanoparticles (Figure 1). Rutile TiO2 nanoparticles with sizes of 80−100 nm (Figure S1) were synthesized via a seeded nucleation scheme.32 TEM images (Figure 2a,b) of the resulting TiO2/PDA core− shell nanoparticles revealed that the thickness of the outer PDA shell was almost 12 nm. Multifunctional groups, including catechols, amines, and imines, present in the PDA shell were further modified with GA. Owing to the cross-linking effect of GA, the TiO2/PDA core−shell nanoparticles were assembled onto the PDA-modified quartz glass tube, leading to the formation of the TiO2/PDA/GA coating (Figure 1). At variance with the separated core−shell nanoparticles in Figure 5140

DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145

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Figure S6b presents the XPS valence band spectra of the prepared materials, showing that the edges of the valence band maximum energy for the TiO2 nanoparticles, the TiO2/PDA core−shell nanoparticles and the TiO2/PDA/GA coating were 2.28, 2.97, and 2.46 eV, respectively. Once the energy gap and the valence band maximum energy positions are confirmed, the energy band structures of the prepared materials can also be determined, as shown in Figure S6c. According to the results discussed above, the as-prepared TiO2/PDA core−shell nanoparticles and the TiO2/PDA/GA coating were expected to possess favorable visible light photocatalysis enhancement compared with the unmodified TiO2 nanoparticles. Herein, geosmin and fluorene were chosen as target pollutants to investigate the photocatalytic performance of the modified materials. Geosmin, a bicyclic tertiary alcohol formed in aqueous environments by cyanobacteria or actinomycetes,28,33 is of great importance to the water purification and aquaculture industries. Although geosmin is nontoxic, its presence in drinking water causes an unpleasant taste. In contrast, as one of the polycyclic aromatic hydrocarbons (PAHs) that contains three fused rings, fluorene is a ubiquitous pollutant with potently mutagenic and carcinogenic toxicities, which is labeled a priority pollutant by the U.S. Environment Protection Agency.34 Given the above reasons, geosmin and fluorene, which were commonly found in the environment, were selected as the testing pollutants. HS-SPME coupled with GC-FID was used in this work to determine the real-time concentration of the pollutants (Figure 3a). SPME is a solvent-free and green equilibrium-based sample pretreatment technique that integrates sampling, isolation, and enrichment into one step.35−37 It involves the utilization of a fused silica or metal fiber coated with specific materials, which is introduced into or over the sample matrixes. After the target analytes are extracted onto the fiber, the SPME fiber is transferred to the injection port of the GC to complete the desorption and analysis procedures. The as-proposed SPME− GC methods could analyze a variety of compounds with low detection limits. Herein, geosmin and fluorene analyzed by HSSPME−GC−FID using a PDMS fiber was validated from 1 to 1000 ppb and 5 to 1000 ppb, respectively. The calibration curves of the two pollutants showed a linear response with correlation coefficients (R2) of 0.9990 and 0.9996 (Figure S7). Herein, the photocatalytic decomposition of geosmin and fluorene was carried out using the TiO2 nanoparticles, the TiO2/PDA core−shell nanoparticles, and the TiO2/PDA/GA coating under different lighting conditions. The time-dependent concentrations of geosmin and fluorene are shown in Figure 3. The schematic illustration in Figure 3a demonstrates the removal and analysis processes of pollutants using the TiO2/PDA/GA coating. The photodegradation of pollutants using the nanoparticles was conducted by replacing the coating with the dispersed nanoparticles. As Figure 3b shows, the TiO2 nanoparticles can remove only 28.1% of the geosmin after visible light irradiation for 120 min, which indicates the poor photocatalytic ability of the TiO2 nanoparticles under visible light irradiation. However, 67% and 70.2% of the geosmin was degraded with catalysis from the TiO2 nanoparticles under UV irradiation and UV−visible light irradiation, respectively. In contrast, the TiO2/PDA core−shell nanoparticles exhibited much better photocatalytic ability toward geosmin not only under visible light irradiation but also under UV irradiation (Figure 3d). In detail, 90.0%, 90.9%, and 92.2% of geosmin was degraded after visible, UV, and UV−visible light irradiation, for

differences, as shown in Figure 2c,f. After treatment by GA, numerous macropores (Figure 2f) appeared on the surface of the TiO2/PDA/GA coating (inset image in Figure 2f), and almost no separated particles were observed, which was greatly different from the SEM image of the TiO2/PDA core/shell nanoparticles (Figure 2c). Elemental mappings (Figure S3) based on energy-dispersive X-ray spectroscopy (EDS) and EA confirmed the existence of Ti, O, C, and H elements in the TiO2/PDA core−shell nanoparticles and the TiO2/PDA/GA coating (Table S1). In addition, the nitrogen-to-carbon (N/C) atomic ratio, determined to be 0.127, of the TiO2/PDA core− shell nanoparticles was close to the theoretical N/C value (0.125) of dopamine, indicating that the polymer shell of the nanoparticles was derived from the polymerization of dopamine. After treatment by GA, the hydrogen-to-carbon (H/C) ratio increased, while the N-to-C ratio greatly decreased, which demonstrated the involvement of GA during the assembly of the TiO2/PDA/GA coating. XPS and FTIR analyses were conducted to further verify the functional groups in the TiO2/PDA core−shell nanoparticles and the TiO2/PDA/GA coating. In the FTIR spectra (Figure S4), bands at 450−700 cm−1 indicated the presence of Ti−O− Ti bonds, and peaks from 1450 to 1600 cm−1 resulted from benzene skeletal vibrations. In addition, both spectra showed a broad band from 3000 to 3600 cm−1, which synergistically resulted from O−H bonds, N−H bonds, and C−H bonds from the benzene ring framework. However, the core−shell nanoparticles exhibited a distinct absorption peak at approximately 1100 cm−1 due to C−N stretching vibrations, while the absorption peak of the coating at the same wavenumber was much weaker, which proved that amine groups in the PDA shell participated in the cross-linking reaction with GA. Additionally, in the FTIR spectra of the TiO2/PDA/GA coating, peaks at 2870 and 2934 cm−1 were generated by C−H stretching vibrations of the methylene bonds, the peaks at 1680 and 1720 cm−1 were caused by the CO bonds of the unreacted aldehyde group, and CN bonds formed from the aldehydeammonia condensation reaction. XPS analysis (Figure S5) and EA results (Table S1) also agree with the above discussion. 3.2. Photocatalytic Performance. By the introduction of nonmetal elements, including C and N, onto the surface of the TiO2 nanoparticles, the PDA shell was expected to narrow the band gap energy and enhance visible light absorption for the modified TiO2, as shown in Figure S6a, which displays the UV−vis diffuse reflectance spectra (DRS) of the proposed materials. As expected, the rutile TiO2 nanoparticles showed their typical absorption edge at approximately 400 nm. In comparison with pure TiO2, the TiO2/PDA core−shell nanoparticles and the TiO2/PDA/GA coating exhibited significantly enhanced absorption not only in the visible region but also in the UV region. In addition, it should be noted that the absorption edges of the core−shell nanoparticles and the coating showed an obvious red shift relative to that of TiO2, which implies that the narrowing of the band gap energy occurred after modification. The band gap energies can be determined through extrapolation of the adsorption edges according to the equation Eg = 1240/λ. Consequently, the band gap energies of the prepared materials were estimated to be approximately 3.10 eV for the TiO2 nanoparticles, 2.28 eV for the TiO2/PDA core−shell nanoparticles, and 2.25 eV for the TiO2/PDA/GA coating, indicating that the modified materials are able to utilize visible light to decompose specific environmental pollutants. 5141

DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145

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Environmental Science & Technology

coating, respectively. Obviously, the adsorption ability of the TiO2 nanoparticles was also improved after modification. Thus, the increased efficiency in the adsorption of pollutants contributed to the enhanced photocatalytic ability of the composites. In fact, modification by PDA and GA immensely increased the BET surface area from 24.0 m2/g for the TiO2 nanoparticles to 87.3 m2/g for the TiO2/PDA core−shell nanoparticles and 99.0 m2/g for the TiO2/PDA/GA coating, which resulted in the improvement in the adsorption efficiency mentioned above. After the modification process, the modified materials exhibited significantly enhanced adsorption of photons in visible region and UV region (Figure S6a), which contributed to the photocatalytic process. Moreover, the adsorption improvement further facilitated the photocatalytic process of the modified materials not only under visible light irradiation but also under UV irradiation. Adsorption kinetics were investigated to further explore the adsorption behavior of the materials toward geosmin and fluorene (Figure S11 and Table S3). According to the results, the adsorption processes were more accordant with the pseudofirst-order kinetics. That is to say, the adsorption of the materials toward geosmin and fluorene was mainly influenced by the diffusion process of the compounds from the sample to the adsorbent. During the photodegradation process, as the adsorbed pollutant was decomposed, the adsorption equilibrium was broken, and the pollutant would quickly move from the solution to the core−shell nanoparticles and the coating. The efficient adsorption process significantly accelerated the enrichment of the pollutant to the catalyst surface, which subsequently improved the photodegradation performance. 3.4. In Vivo Study on D. magna and Wastewater Treatment. D. magna is among the most-sensitive researched organism to toxic compounds, which is diffusely distributed in aquatic freshwater habits and has been broadly utilized in aquatic risk assessment.40 Accordingly, it is always used to evaluate the toxicity response of various aquatic systems.41,42 To evaluate the impact of the photodegradation of toxic fluorene using the TiO2/PDA/GA coating under visible light irradiation, a D. magna lethality experiment was carried out. D. magna were cultured under untreated AFW, AFW contaminated by fluorene, and the contaminated AFW that was treated by the TiO2/PDA/GA coating under visible light irradiation. In contaminated AFW, all D. magna died after exposure to fluorene for 36 h, while 96% of the D. magna survived without exposure to fluorene (Figure 4a). The results in Figure 4a and Movie S1 revealed that treatment of contaminated AFW by the TiO2/PDA/GA coating significantly reduced the lethality of the contaminated AFW, such that 92% of the D. magna survived. The in vivo experiment suggests that treatment of contaminated water with the as-prepared TiO2/PDA/GA coating might prove advantageous to decreasing the toxic effects of some chemicals. Coking wastewater generated from high-temperature carbonation, coal gas purification, and chemical refining in coke plants always contains a certain amount of PAHs.43 To further broaden the application of the TiO2/PDA/GA coating to eliminate fluorene, a coking wastewater treatment experiment was carried out. The calibration curve of fluorene in a low concentration range was obtained via the HS-SPME−GC−MS method using PDMS fibers (Figure S12). The wastewater treatment device was setup as shown in Figure S13. It is believed to be difficult for light to enter the dark-brown wastewater, which would greatly lower the amount of light

120 min, respectively. As expected, the TiO2/PDA/GA coating showed favorable photocatalytic ability to decompose geosmin as well and that 87.9%, 88.7%, and 91.5% of the geosmin was removed after treatment under visible, UV, and UV−visible light irradiation for 120 min using the TiO2/PDA/GA coating (Figure 3f). Similarly, in the fluorene decomposition experiments, only 25.6% of the pollutant was removed by the TiO2 nanoparticles after visible light irradiation for 120 min, while 73.0% and 74.2% of the fluorene was degraded under UV and UV−visible light irradiation (Figure 3c). The TiO2/PDA core− shell nanoparticles and the TiO2/PDA/GA coating also exhibited much better photocatalytic ability toward fluorene, as shown in Figure 3e,g. As a result, 98.0%, 98.2%, and 99.0% of the fluorene was removed by the TiO2/PDA core−shell nanoparticles and 97.3%, 98.4% and 99.0% of the fluorene was degraded by the TiO2/PDA/GA coating under different lighting conditions. In summary, after modification by PDA and GA, the promotion of the photocatalytic ability of the prepared rutile TiO2 under visible light irradiation was successfully achieved. As a matter of fact, separation is one of the major bottlenecks that limits the application of nanoparticles. Apart from the involvement of separation and filtration, circulating processes are difficult to achieve, considering the dispersed nanoparticles form particulate suspensions. Therefore, in this context, taking advantage of the cross-linking effect of GA and PDA, a TiO2/ PDA/GA coating was self-assembled to overcome disadvantages of the direct utilization of nanoparticles. It is noted that the coating exhibited photocatalytic ability toward the pollutants comparable to that of the modified TiO2/PDA core−shell nanoparticles. Although the coating needed 80 min to remove the pollutants, while the core−shell nanoparticles needed only 40 min, the avoidance of filtration and separation processes could compensate for the extra time consumption. The photodegradation of geosmin and fluorene using the TiO2/PDA/GA coating under visible light irradiation was also monitored for five consecutive cycles of 80 min each (Figure S8). No significant reduction in the photodegradation performance was observed over the five sequential cycles, demonstrating the good stability of the proposed photodegradation coating under visible light irradiation. Compounds that were only detected after the photocatalytic treatment of the contaminated solutions, instead of the untreated ones, were determined to be degradation intermediates or products of the pollutants. Degradation intermediates of geosmin and fluorene in the presence of the TiO2/ PDA/GA coating under visible- light irradiation were detected by direct immersion (DI) SPME coupled with GC−MS using a PA fiber. A total of four peaks were found to be related to geosmin, and two peaks were found to be produced by the photodegradation of fluorene, as shown in Table S2. Some of the degradation intermediates listed in the table correspond to those in previous reports.38,39 The possible reaction pathways were proposed accordingly for the decomposition of fluorene and geosmin, as shown in Figure S9. 3.3. Adsorption Ability. The adsorption performances of the prepared materials toward geosmin and fluorene were examined in the dark. As displayed in Figure S10, all the materials reached extraction equilibrium toward the two pollutants after 60 min. Under adsorption equilibrium, 22.2%, 62.2%, and 59.7% of the geosmin and 28.2%, 62.2%, and 61.7% of the fluorene were adsorbed by the TiO2 nanoparticles, the TiO2/PDA core−shell nanoparticles and the TiO2/PDA/GA 5142

DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145

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Environmental Science & Technology

received by the coating. To solve the mentioned limitation, the coating was adhered to the glass wall of a vial, and a water membrane was formed between the coating and the glass wall. The water membrane was thin enough to be penetrated by light so that photodegradation could successfully occur. The results in Figure 4b show that the initial concentration of fluorene existed in the wastewater itself was 158 ng/L and that no fluorene was detected in the wastewater after 520 min treatment with the TiO2/PDA/GA coating under visible light irradiation. Although agitation can accelerate the flow of the water membrane, the elimination rate of fluorene in coking wastewater was still much lower than that in the pure water system. Moreover, the proposed TiO2/PDA/GA coating was proven to be capable of removing fluorene in wastewater, which provides potential for future application of the coating in industrial wastewater treatment and environmental remediation. 3.5. Evaluation of the Modified Degradation Device. Effective utilization of the TiO2/PDA/GA coating avoids separation processes, but it takes two times longer for the TiO2/PDA core/shell nanoparticles to remove pollutants (Figure 3) due to the much-smaller contact area between the coating and the pollutants. With the purpose of improving the photocatalytic efficiency of the coating, GC capillary columns with external diameters of nearly 350 μm were used to prepare photodegradation fibers. The fibers were subsequently assembled as a brush-like photodegradation device. Under visible light irradiation, the photodegradation performance of the TiO2/PDA/GA fiber array toward geosmin and fluorene was compared with those of the TiO2/PDA nanoparticles and the TiO2/PDA/GA coating (Figure 4a,b). The modified degradation device is shown in Figure 5c. According to the results in Figure 4a,b, the fiber array exhibited an elimination rate of the pollutants between that of the core−shell

Figure 4. (a) Treatment of fluorene-contaminated AFW by the TiO2/ PDA/GA coating that significantly reduced the lethality of Daphnia magna. (b) Fluorene in coking wastewater was removed by the TiO2/ PDA/GA coating under visible-light irradiation.

Figure 5. Photodegradation performance of the TiO2/PDA/GA fiber array under visible light irradiation. (a,b) Removal of geosmin and fluorene by the TiO2/PDA/GA fiber array. (c) Schematic illustration of pollutant removal by the TiO2/PDA/GA fiber array. (d) Comparison of fluorene removal in coking wastewater by the TiO2/PDA/GA coating and the fiber array. 5143

DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145

Article

Environmental Science & Technology nanoparticles and the coating. Moreover, the fiber array needed only 40 min to remove the pollutants, which is the same time required for removal by the TiO2/PDA nanoparticles. Table S4 illustrated the comparison of the treating time and visible light degradation ratio for the pollutants of the reported nonmetal modified TiO2 materials and the TiO2/PDA/GA composite proposed in this work. Generally speaking, compared with other nonmetal modified TiO2 materials, the TiO2/PDA/GA composite exhibited rapid pollutant degradation ability, especially for the TiO2/PDA/GA fiber array, and satisfactory pollutant elimination ratios. For the purpose of further proving the effectiveness of the modified degradation device, the removal of fluorene from coking wastewater was conducted (Figure S14). According to Figure 5d, no fluorene was detected in wastewater after a 280 min treatment of the TiO2/PDA/GA fiber array under visible light irradiation, which was 240 min faster than when the TiO2/ PDA/GA coating was used. The above results proved that the adoption of the miniature TiO2/PDA/GA fiber array could greatly improve the photocatalytic efficiency of the proposed material. Actually, more time was required for fiber preparation and device assembly processes. However, this will not be a problem if batch production is adopted. In this work, we proposed a two-step modification scheme to improve the photocatalytic performance of rutile TiO 2 nanoparticles under visible light. First, the immobilization of a PDA shell onto the TiO2 core narrowed the band gap energy of the TiO2 nanoparticles, which consequently improved the visible light response compared to unmodified TiO2. Secondary modification by GA, which resulted in the formation of the TiO2/PDA/GA coating, avoided separation processes for continuous application of the modified materials while maintaining the photoresponse ability to visible light irradiation of the TiO2/PDA core−shell nanoparticles. In other words, the self-assembled nanostructured materials can be used without environmental exposure to nanomaterials. Additionally, the adsorption ability of the unmodified TiO2 nanoparticles was also improved after modification. The increased adsorption efficiency of pollutants contributed to the photocatalytic performance enhancement of the composites. The safety and effectiveness of the coating, proven by the in vivo study with D. magna and the coking wastewater treatment experiment, suggests that it may provide an innocuous alternative to the utilization of nanotechnology in environmental remediation. Because dopamine has been proven to undergo a selfpolymerization reaction and form a PDA membrane on almost all kinds of inorganic and organic surfaces,21 the two-step modification scheme proposed in this work is expected to provide a versatile strategy for the modification of nanomaterials and other functional substrates. We believe the findings in this work could provide new insights in various fields, such as environmental science, catalytic science, materials science, and life science.

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reflectance spectra and band gap states, relative response of geosmin and fluorene vs their concentrations, recycled photodegradation performance, reaction pathways, adsorption performance, adsorption kinetics, and coking wastewater treatment devices. Tables showing element analysis, intermediate products of photodegradation, adsorption kinetics parameters, and performance comparisons. (PDF) A movie showing the Daphnia magna lethality experiment. (AVI)

AUTHOR INFORMATION

Corresponding Authors

*Phone and Fax: +86-2084110845; e-mail: [email protected]. edu.cn. *E-mail: [email protected]. ORCID

Gangfeng Ouyang: 0000-0002-0797-6036 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the projects of NNSFC (nos. 21377172, 21477166, 21527813, and 21677182) and the NSF of Guangdong Province (no. S2013030013474).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b00350. Details of analysis of experimental conpounds and their degradation intermediates. Figures showing TEM images, XRD patterns, SEM images and elemental mappings, FTIR spectra, XPS spectra, UV−visible diffuse 5144

DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145

Article

Environmental Science & Technology

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DOI: 10.1021/acs.est.7b00350 Environ. Sci. Technol. 2017, 51, 5137−5145