Enhanced Photocatalytic Degradation of Environmental Pollutants

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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, Chao-Hai Wei, Ningbo Zhou, and Gangfeng Ouyang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00350 • Publication Date (Web): 05 Apr 2017 Downloaded from http://pubs.acs.org on April 6, 2017

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Environmental Science & Technology

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Enhanced Photocatalytic Degradation of Environmental

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Pollutants under Visible Irradiation by a Composite

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Coating

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Shuqin Liua, Qingkun Hua, Junlang Qiua, Fuxin Wanga, Wei Lina, Fang Zhua,

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Chaohai Weib, Ningbo Zhou*c, Gangfeng Ouyang*a

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a

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Chemistry, Sun Yat-Sen University, No.135, Xingang Xi Road, Guangzhou,

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of

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Guangdong 510275, China

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b

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Guangzhou 510006, P. R. China.

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c

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Technology, Yueyang 414006, P. R. China.

College of Environment and Energy, South China University of Technology,

College of Chemistry and Chemical Engineering, Hunan Institute of Science and

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* Corresponding author. Tel. & Fax: +86-2084110845

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E-mail: [email protected]; [email protected]

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ACS Paragon Plus Environment


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ABSTRACT

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Although nanotechnology has offered effective and efficient solutions for

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environmental remediation, the full utilization of sustainable energy and the

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avoidance of secondary pollution are still challenges. Herein, we report a two-step

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modification strategy for TiO2 nanoparticles by first forming a thin, surface-adherent

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polydopamine (PDA) shell onto the nanoparticles and then assembling core/shell

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nanoparticles as a photodegradation coating. The composite coating modified from

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TiO2 could not only realize the highly efficient utilization of photons from the visible

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region but could also avoid the secondary pollution of nanoparticles during

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application. Additionally, improvements in the adsorption ability after modification

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greatly facilitated the photocatalytic process of the modified materials. A preliminary

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in vivo study on Daphnia magna and a wastewater treatment experiment suggest that

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treatment with the composite coating can effectively eliminate fluorene and

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significantly reduce its lethality. We believe the two-step modification scheme can

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open new avenues for the facile modification of nanomaterials for designed purposes,

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especially in the field of environmental remediation.

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1. INTRODUCTION

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Environmental pollution has long been a global problem due to the rapid pace of

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urbanization and heavy industrialization.1,2 Although much effort has been spent on

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the removal and degradation of hazardous environmental contaminants,3-5

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environment remediation is still a topic of global concern. Over the past decades, the

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emergence of nanotechnology has given tremendous possibilities and scope to

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remedy polluted water by utilizing nanomaterials designed with specific

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properties.6-8 As a well-known and the most investigated functional material in

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semiconductor photocatalysis, TiO2 nanoparticles have been extensively used in the

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degradation of a variety of toxic pollutants in air and water.9-11

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Despite possessing numerous advantages, such as strong oxidizing power, high

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stability, nontoxicity and superior photoelectric effect, the large band gap energy (3.2

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eV) of TiO2 facilitates the necessity of utilizing ultraviolet (UV) excitation to realize

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specific applications.12-14 However, it should be noted that only a small amount of

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solar radiation comes from the UV region,6 which leads to requirements for

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enhancing the utilization of photons from the visible region by TiO2 nanoparticles. A

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viable strategy for narrowing the band gap of TiO2 can be realized by doping foreign

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elements into the materials.15,16 The modification of TiO2 with metal elements has

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been reported to successfully decrease the TiO2 band gap17,18 by promoting electron

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transfer from the valence band to the conduction band and facilitating the formation

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of oxidative species. However, the existence of some transition metals has been

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proven to facilitate the probability of electron-hole recombination, which would lead 3



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to a decline in the photocatalytic performance.6 Recently, some studies have shown

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that the band gap narrowing of TiO2 could be better realized by adding nonmetal

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elements (N, C, S, F, etc.) into the material.9,19,20 Such modified TiO2 materials

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showed larger response to irradiation in the visible region and increased the

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degradation performance of hazardous pollutants under visible light irradiation,

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especially under natural solar light irradiation.6,20

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Currently, polydopamine (PDA) has opened a new route to the functionalization

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of a variety of substrates since dopamine can self-polymerize and deposit PDA on

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almost all kinds of inorganic or organic surfaces.21 In addition, PDA is expected to

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offer a large number of active sites for binding organic pollutants via electrostatic

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interactions, hydrogen bonding or π−π stacking interactions, owing to the existence

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of abundant functional groups, such as catechol groups, amine groups, and aromatic

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moieties, which are beneficial to secondary modification as well.21-24 Moreover, as a

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major component of naturally occurring melanin, which is widely distributed in the

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human body, the excellent biocompatibility of PDA22 decreases the occurrence of

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adverse effects during biological and environmental applications. Therefore, in this

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contribution, PDA was used as a nonmetal element dopant to realize the band gap

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narrowing of rutile TiO2 by the simple immobilization of a thin, surface-adherent

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and biocompatible PDA shell onto the TiO2 nanoparticles.

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However, separation is another major bottleneck that limits the application of TiO2

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nanoparticles,26-28 since it is hard to achieve photodegradation and separation

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simultaneously in powder form and difficult to recycle TiO2 due to its suspended 4


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dispersive properties in water.28,29 Moreover, the incomplete separation of

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nanoparticles may subsequently give rise to the secondary pollution of water.30 In

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this context, glutaraldehyde (GA) was used to assemble the prepared TiO2/PDA

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core/shell nanoparticles into a homogeneous photodegradation coating by connecting

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with the abundant functional groups in PDA shells. After two-step modification of

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rutile TiO2 nanoparticles, the resulting TiO2/PDA/GA coating possessed satisfactory

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visible light photodegradation capacity towards organic pollutants. As expected, the

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contaminated water treated by the composite coating was safe, as proven by

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preliminary in vivo studies.

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2. EXPERIMENTAL SECTION

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2.1. Synthesis of TiO2 Nanoparticles. First, 18 mL of ammonium hydroxide

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(30%), 32 mL of ultrapure water and 44.5 mL of TEOS were added into 750 mL

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ethanol with stirring for 24 h. The translucent SiO2 was collected by centrifugation

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and sintered at 500 °C. Then, 66 mL of aqueous TiCl4 (20 mM) containing 10 g of

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the sintered product was heated at 70 °C for 1 h. SiO2 was obtained after washing,

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drying and resintering (500 °C). Afterwards, 2.6 g of SiO2 and 6.68 g of

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1-methylimidazolium tetrafluoroborate were mixed into 200 mL of the 120 mM TiF4

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aqueous solution (pH=2.1), which was transferred to a Teflon-lined autoclave and

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heated at 130 °C for 12 h. The product collected by filtration was then added into a 2

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M NaOH aqueous solution at 80 °C for 1 h to remove SiO2. The remaining TiO2

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nanoparticles were finally obtained by filtration, washing and drying.

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2.2. Synthesis of TiO2/PDA Core/Shell Nanoparticles. TiO2 nanoparticles (100 5



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mg) and dopamine (40 mg) were dispersed in 320 mL of ultrapure water by

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ultrasonication for 30 min to form a suspension. Subsequently, 80 mL of tris-buffer

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(10 mM, pH: ~8.5) was added to the suspension, and the mixture was stirred at room

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temperature for 24 h. Afterwards, the product was obtained by centrifugation,

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washed three times with ultrapure water and ethanol, and then dried at 80 °C.

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2.3. Preparation of TiO2/PDA/GA Coating and Fibers. First, a cleaned quartz

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glass tube with an external diameter of 5 mm was immersed into a solution of

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dopamine (2 mg/mL) with 10 mM tris-buffer (pH: ~8.5) overnight at room

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temperature. The PDA-coated tube was rinsed with distilled water to remove

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residual dopamine. Afterwards, 1 g of the TiO2/PDA core/shell nanoparticles were

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dispersed in 1 mL of GA (50% in water) to form a viscous mixture. The mixture was

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then spin-coated on the treated quartz glass tube and held at 60 °C for 4 h. Similarly,

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the TiO2/PDA/GA fibers were prepared by spin-coating cleaned gas chromatograph

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(GC) capillary columns with the viscous mixture, following the heating steps. Before

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each use, the TiO2/PDA/GA coating and fibers were conditioned at 90 °C overnight

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under vacuum.

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2.4. Physical Characterizations. The detection of pollutants was carried out on

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an Agilent 7890 gas chromatograph-flame ionization detector (GC-FID, Palo Alto,

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CA, USA) and a gas chromatograph-mass spectrometer (GC-MS, 6890-5975 B)

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equipped with an HP-5 MS capillary column (30 m × 0.32 mm i.d. × 0.25 µm film

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thickness) from Agilent (Palo Alto, CA, USA). Scanning electron microscopy (SEM)

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images and elemental mappings were obtained from a Quanta 400 Thermal FE 6


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Environmental Scanning Electron Microscope (FEI, Netherlands). Powder X-ray

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diffraction (PXRD) patterns were recorded on a Bruker D8 ADVANCE X-ray

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powder diffractometer (Cu kα). Transmission electron microscopy (TEM) images

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were obtained via a JEM-2010HR transmission electron microscope (JEOL, Japan).

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Fourier transform infrared (FTIR) spectra were recorded on a Bruker EQUINOX 55

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Fourier transform infrared spectrometer. BET surface areas and N2 adsorption

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isotherms (77.3 K) were obtained by a Micromeritics ASAP 2020 V3.04 H surface

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and porosity analyzer. X-ray photoelectron spectra (XPS) patterns were recorded on

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an ESCALab250 X-ray photoelectron spectrometer (Thermo Fisher Scientific, USA).

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Element analysis (EA) was carried out by an Elementar Vario EL element analyzer,

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and UV-vis diffuse reflectance spectra were collected on a UV-vis-NIR

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spectrophotometer (Shimadzu UV-3600). Commercial polydimethylsiloxane (PDMS,

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100 µm) and polyacrylate (PA, 85 µm) solid-phase microextraction (SPME) fibers

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were purchased from Supelco (Bellefonte, PA, USA).

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2.5. Adsorption of Geosmin and Fluorene. The TiO2/PDA/GA coating was

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submersed in 10 mL of 1000 ppb geosmin or fluorene aqueous solutions in the dark.

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Agitation was carried out using a magnetic stir bar at room temperature. The

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real-time concentrations of the residual pollutants in the solutions were detected by

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headspace (HS) SPME coupled with GC-FID methods (see details in the Supporting

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Information). Similarly, the adsorption of geosmin or fluorene by the TiO2 or

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TiO2/PDA core/shell nanoparticles was conducted by replacing the TiO2/PDA/GA

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coating with the nanoparticles (the same amount as the coating materials). 7



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2.6. Photodegradation Studies. Photocatalytic experiments were carried out in a

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homemade photoreactor (Figure 4a). In experiments with the coatings, a coated

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quartz glass tube was submersed in 10 mL of aqueous solution of 1000 ppb geosmin

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or fluorene under agitation after reaching extraction equilibrium. The mixture was

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irradiated with a 350 W xenon lamp under different wavelengths. Tinfoil was

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attached to the internal surface of the light blocking box. Photodegradation under

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UV was conducted by equipping the lamp with a 300-400 nm filter, while

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photodegradation under visible light was realized by equipping the lamp with a 420

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nm cutoff filter. In addition, irradiation from UV to visible light was provided with

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no filter equipped. After irradiation for a given time interval, the coating was

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removed from solution. The residual concentrations of the pollutant were detected by

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the HS-SPME and GC-FID methods. Similar to the degradation of geosmin or

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fluorene by TiO2 nanoparticles, the TiO2/PDA core/shell nanoparticles or brush-like

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photodegradation device was constructed by replacing the TiO2/PDA/GA coating

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with the nanoparticles or the assembled fiber array.

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2.7. Lethality Studies in Daphnia Magna. Daphnia magna were cultured in the

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artificial freshwater (AFW) under the conditions described in the guidelines from the

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Organization for Economic Cooperation and Development for the testing of

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chemicals (OECD, 2008). The AFW was prepared according to the method

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described previously.31 In brief, a total of 294.0 mg CaCl2·2H2O, 123.2 mg

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MgSO4·7H2O, 5.8 mg KCl and 64.8 mg NaHCO3 were mixed into 1 L of ultrapure

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water. The mixture was aerated vigorously for 24 h to dissolve the chemicals and 8


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stabilize the medium. The culture temperature was set at 20±0.1 °C, and the

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photoperiod was 16:8 light:dark. Daphnia magna were fed twice a week with a

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suspension of Scenedesmus subspicatus. In the lethality experiments, Daphnia

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magna were cultured under untreated AFW, AFW contaminated by fluorene (1000

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ppb in concentration) and contaminated AFW that was treated by a TiO2/PDA/GA

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coating under visible light irradiation for 120 min. Each group contained 100

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Daphnia magna. The lethality (%) of Daphnia magna was monitored every 12 h.

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2.8. Coking Wastewater Treatment. Coking wastewater was collected from a

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coke plant in Shanghai, China. Before treatment and analysis, the wastewater was

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filtered through a 0.45 µm filter membrane to remove insoluble substances. For each

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treatment experiment, 100 mL of coking wastewater was used.

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3. RESULTS AND DISCUSSION

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3.1. Synthesis of TiO2/PDA Core/Shell Nanoparticles and Assembly of

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TiO2/PDA/GA Coating. Dopamine has been known to self-polymerize and generate

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thin, surface-adherent PDA membranes onto virtually all types of inorganic or

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organic substances.22 The simple immersion of TiO2 nanoparticles into a diluent of

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dopamine (2 mg/mL), buffered to a pH typical of marine environments (10 mM tris,

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pH 8.5), gave rise to the spontaneous formation of a homogeneous adherent polymer

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shell on the surface of the nanoparticles (Figure 1). Rutile TiO2 nanoparticles with

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sizes of 80-100 nm (Figure S1) were synthesized via a seeded nucleation scheme.32

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TEM images (Figures 2a,b) of the resulting TiO2/PDA core/shell nanoparticles

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revealed that the thickness of the outer PDA shell was almost 12 nm. Multifunctional 9



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groups, including catechols, amines and imines, present in the PDA shell were

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further modified with GA. Owing to the cross-linking effect of GA, the TiO2/PDA

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core/shell nanoparticles were assembled onto the PDA-modified quartz glass tube,

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leading to the formation of the TiO2/PDA/GA coating (Figure 1). At variance with

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the separated core/shell nanoparticles in Figure 2a, the TEM image of the scraped

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coating (Figure 2d) showed that the TiO2/PDA nanoparticles were almost aggregated

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after GA treatment, which corresponds to the self-assembly of the TiO2/PDA/GA

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coating. High-resolution transmission electron microscopy (HRTEM) images

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(Figures 2b and 2e) and fast Fourier transform (FFT) generated diffraction patterns

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(inset images in Figures 2a and 2d), confirmed that the nanoparticles consisted of a

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crystalline TiO2 core and an amorphous polymer shell. In the HRTEM images, a

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diagnostic lattice fringe was found in the core, while no lattice fringe appeared in the

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shell. The desired crystalline structure of the TiO2 cores in the TiO2/PDA core/shell

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nanoparticles and the TiO2/PDA/GA coating were also validated by PXRD patterns

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(Figure S2).

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The surface morphologies of the TiO2/PDA core/shell nanoparticles and the

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TiO2/PDA/GA coating exhibited distinct differences, as shown in Figures 2c and 2f.

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After treatment by GA, numerous macropores (Figure 2f) appeared on the surface of

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the TiO2/PDA/GA coating (inset image in Figure 2f), and almost no separated

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particles were observed, which was greatly different from the SEM image of the

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TiO2/PDA core/shell nanoparticles (Figure 2c). Elemental mappings (Figure S3)

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based on energy-dispersive X-ray spectroscopy (EDS) and EA confirmed the 10


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existence of Ti, O, C and H elements in the TiO2/PDA core/shell nanoparticles and

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the TiO2/PDA/GA coating (Table S1). In addition, the nitrogen-to-carbon (N/C)

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atomic ratio, determined to be 0.127, of the TiO2/PDA core/shell nanoparticles was

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close to the theoretical N/C value (0.125) of dopamine, indicating that the polymer

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shell of the nanoparticles was derived from the polymerization of dopamine. After

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treatment by GA, the hydrogen-to-carbon (H/C) ratio increased, while the N/C ratio

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greatly decreased, which demonstrated the involvement of GA during the assembly

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of the TiO2/PDA/GA coating.

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XPS and FTIR analyses were conducted to further verify the functional groups in

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the TiO2/PDA core/shell nanoparticles and the TiO2/PDA/GA coating. In the FTIR

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spectra (Figure S4), bands at 450-700 cm-1 indicated the presence of Ti-O-Ti bonds,

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and peaks from 1450 to 1600 cm-1 resulted from benzene skeletal vibrations. In

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addition, both spectra showed a broad band from 3000-3600 cm-1, which

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synergistically resulted from O-H bonds, N-H bonds and C-H bonds from the

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benzene ring framework. However, the core/shell nanoparticles exhibited a distinct

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absorption peak at approximately 1100 cm-1 due to C-N stretching vibrations, while

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the absorption peak of the coating at the same wavenumber was much weaker, which

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proved that amine groups in the PDA shell participated in the cross-linking reaction

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with GA. Additionally, in the FTIR spectra of the TiO2/PDA/GA coating, peaks at

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2870 and 2934 cm-1 were generated by C-H stretching vibrations of the methylene

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bonds, the peaks at 1680 and 1720 cm-1 were caused by the C=O bonds of the

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unreacted aldehyde group, and C=N bonds formed from the aldehyde-ammonia 11



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condensation reaction. XPS analysis (Figure S5) and EA results (Table S1) also

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agree with the above discussion.

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3.2. Photocatalytic Performance. By introducing nonmetal elements, including

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C and N, onto the surface of the TiO2 nanoparticles, the PDA shell was expected to

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narrow the band gap energy and enhance visible light absorption for the modified

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TiO2, as shown in Figure S6a, which displays the UV-vis diffuse reflectance spectra

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(DRS) of the proposed materials. As expected, the rutile TiO2 nanoparticles showed

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their typical absorption edge at approximately 400 nm. In comparison with pure

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TiO2, the TiO2/PDA core/shell nanoparticles and the TiO2/PDA/GA coating

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exhibited significantly enhanced absorption not only in the visible region but also in

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the UV region. In addition, it should be noted that the absorption edges of the

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core/shell nanoparticles and the coating showed an obvious red shift relative to that

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of TiO2, which implies that the narrowing of the band gap energy occurred after

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modification. The band gap energies can be determined through extrapolation of the

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adsorption edges according to the equation Eg=1240/λ. Consequently, the band gap

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energies of the prepared materials were estimated to be approximately 3.10 eV for

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the TiO2 nanoparticles, 2.28 eV for the TiO2/PDA core/shell nanoparticles and 2.25

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eV for the TiO2/PDA/GA coating, indicating that the modified materials are able to

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utilize visible light to decompose specific environmental pollutants.

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Figure S6b presents the XPS valence band spectra of the prepared materials,

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showing that the edges of the valence band maximum energy for the TiO2

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nanoparticles, the TiO2/PDA core/shell nanoparticles and the TiO2/PDA/GA coating 12


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were 2.28 eV, 2.97 eV and 2.46 eV, respectively. Once the energy gap and the

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valence band maximum energy positions are confirmed, the energy band structures

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of the prepared materials can also be determined, as shown in Figure S6c.

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According to the results discussed above, the as-prepared TiO2/PDA core/shell

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nanoparticles and the TiO2/PDA/GA coating were expected to possess favorable

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visible light photocatalysis enhancement compared with the unmodified TiO2

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nanoparticles. Herein, geosmin and fluorene were chosen as target pollutants to

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investigate the photocatalytic performance of the modified materials. Geosmin, a

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bicyclic tertiary alcohol formed in aqueous environments by cyanobacteria or

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actinomycetes,28,33 is of great importance to the water purification and aquaculture

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industries. Although geosmin is nontoxic, its presence in drinking water causes an

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unpleasant taste. In contrast, as one of the polycyclic aromatic hydrocarbons (PAHs)

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that contains three fused rings, fluorene is a ubiquitous pollutant with potently

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mutagenic and carcinogenic toxicities, which is labeled a priority pollutant by the

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U.S. Environment Protection Agency.34 Given the above reasons, geosmin and

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fluorene, which were commonly found in the environment, were selected as the

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testing pollutants.

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HS-SPME coupled with GC-FID was used in this work to determine the real-time

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concentration of the pollutants (Figure 3a). SPME is a solvent-free and green

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equilibrium-based sample pretreatment technique that integrates sampling, isolation

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and enrichment into one step.35-37 It involves the utilization of a fused silica or metal

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fiber coated with specific materials, which is introduced into or over the sample 13



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matrixes. After the target analytes are extracted onto the fiber, the SPME fiber is

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transferred to the injection port of the GC to complete the desorption and analysis

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procedures. The as-proposed SPME-GC methods could analyze a variety of

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compounds with low detection limits. Herein, geosmin and fluorene analyzed by

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HS-SPME-GC-FID using a PDMS fiber was validated from 1 ppb to 1000 ppb and 5

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ppb to 1000 ppb, respectively. The calibration curves of the two pollutants showed a

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linear response with correlation coefficients (R2) of 0.9990 and 0.9996 (Figure S7).

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Herein, the photocatalytic decomposition of geosmin and fluorene was carried out

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using the TiO2 nanoparticles, the TiO2/PDA core/shell nanoparticles and the

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TiO2/PDA/GA coating under different lighting conditions. The time-dependent

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concentrations of geosmin and fluorene are shown in Figure 3. The schematic

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illustration in Figure 3a demonstrates the removal and analysis processes of

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pollutants using the TiO2/PDA/GA coating. The photodegradation of pollutants using

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the nanoparticles was conducted by replacing the coating with the dispersed

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nanoparticles. As Figure 3b shows, the TiO2 nanoparticles can remove only 28.1% of

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the geosmin after visible light irradiation for 120 min, which indicates the poor

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photocatalytic ability of the TiO2 nanoparticles under visible light irradiation.

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However, 67% and 70.2% of the geosmin was degraded with catalysis from the TiO2

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nanoparticles under UV irradiation and UV-visible light irradiation, respectively. In

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contrast, the TiO2/PDA core/shell nanoparticles exhibited much better photocatalytic

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ability towards geosmin not only under visible light irradiation but also under UV

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irradiation (Figure 3d). In detail, 90.0%, 90.9% and 92.2% of geosmin was degraded 14


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after visible, UV and UV-visible light irradiation, for 120 min, respectively. As

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expected, the TiO2/PDA/GA coating showed favorable photocatalytic ability to

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decompose geosmin as well and that 87.9%, 88.7% and 91.5% of the geosmin was

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removed after treatment under visible, UV and UV-visible light irradiation for 120

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min using the TiO2/PDA/GA coating (Figure 3f). Similarly, in the fluorene

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decomposition experiments, only 25.6% of the pollutant was removed by the TiO2

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nanoparticles after visible light irradiation for 120 min, while 73.0% and 74.2% of

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the fluorene was degraded under UV and UV-visible light irradiation (Figure 3c).

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The TiO2/PDA core/shell nanoparticles and the TiO2/PDA/GA coating also exhibited

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much better photocatalytic ability towards fluorene, as shown in Figures 3e and 3g.

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As a result, 98.0%, 98.2% and 99.0% of the fluorene was removed by the TiO2/PDA

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core/shell nanoparticles and 97.3%, 98.4% and 99.0% of the fluorene was degraded

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by the TiO2/PDA/GA coating under different lighting conditions.

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In summary, after modification by PDA and GA, the promotion of the

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photocatalytic ability of the prepared rutile TiO2 under visible light irradiation was

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successfully achieved. As a matter of fact, separation is one of the major bottlenecks

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that limits the application of nanoparticles. Apart from the involvement of separation

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and filtration, circulating processes are difficult to achieve, considering the dispersed

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nanoparticles form particulate suspensions. Therefore, in this context, taking

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advantage of the cross-linking effect of GA and PDA, a TiO2/PDA/GA coating was

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self-assembled to overcome disadvantages of the direct utilization of nanoparticles.

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It is noted that the coating exhibited photocatalytic ability towards the pollutants 15



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comparable to that of the modified TiO2/PDA core/shell nanoparticles. Although the

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coating needed 80 min to remove the pollutants, while the core/shell nanoparticles

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needed only 40 min, the avoidance of filtration and separation processes could

329

compensate for the extra time consumption. The photodegradation of geosmin and

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fluorene using the TiO2/PDA/GA coating under visible light irradiation was also

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monitored for five consecutive cycles of 80 min each (Figure S8). No significant

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reduction in the photodegradation performance was observed over the five sequential

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cycles, demonstrating the good stability of the proposed photodegradation coating

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under visible light irradiation.

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Compounds that were only detected after the photocatalytic treatment of the

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contaminated solutions, instead of the untreated ones, were determined to be

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degradation intermediates or products of the pollutants. Degradation intermediates of

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geosmin and fluorene in the presence of the TiO2/PDA/GA coating under visible

339

light irradiation were detected by direct immersion (DI) SPME coupled with GC-MS

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using a PA fiber. Four peaks were found to be related to geosmin, and two peaks

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were found to be produced by the photodegradation of fluorene, as shown in Table

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S2. Some of the degradation intermediates listed in the table correspond to those in

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previous reports. 38,39 The possible reaction pathways were proposed accordingly for

344

the decomposition of fluorene and geosmin, as shown in Figure S9.

345

3.3. Adsorption Ability. The adsorption performances of the prepared materials

346

towards geosmin and fluorene were examined in the dark. As displayed in Figure

347

S10, all the materials reached extraction equilibrium towards the two pollutants after 16


Page 16 of 35

Page 17 of 35


348

60 min. Under adsorption equilibrium, 22.2%, 62.2%, and 59.7% of the geosmin and

349

28.2%, 62.2%, and 61.7% of the fluorene were adsorbed by the TiO2 nanoparticles,

350

the TiO2/PDA core/shell nanoparticles and the TiO2/PDA/GA coating, respectively.

351

Obviously, the adsorption ability of the TiO2 nanoparticles was also improved after

352

modification. Thus, the increased efficiency in the adsorption of pollutants

353

contributed to the enhanced photocatalytic ability of the composites. In fact,

354

modification by PDA and GA immensely increased the BET surface area from 24.0

355

m2/g for the TiO2 nanoparticles to 87.3 m2/g for the TiO2/PDA core/shell

356

nanoparticles and 99.0 m2/g for the TiO2/PDA/GA coating, which resulted in the

357

improvement in the adsorption efficiency mentioned above. After the modification

358

process, the modified materials exhibited significantly enhanced adsorption of

359

photons in visible region and UV region (Figure S6a), which contributed to the

360

photocatalytic process. Moreover, the adsorption improvement further facilitated the

361

photocatalytic process of the modified materials not only under visible light

362

irradiation but also under UV irradiation.

363

Adsorption kinetics were investigated to further explore the adsorption behavior

364

of the materials towards geosmin and fluorene (Figure S11 and Table S3). According

365

to the results, the adsorption processes were more accordant with the

366

pseudo-first-order kinetics. That is to say, the adsorption of the materials towards

367

geosmin and fluorene was mainly influenced by the diffusion process of the

368

compounds from the sample to the adsorbent. During the photodegradation process,

369

as the adsorbed pollutant was decomposed, the adsorption equilibrium was broken, 17



370

and the pollutant would quickly move from the solution to the core/shell

371

nanoparticles and the coating. The efficient adsorption process significantly

372

accelerated the enrichment of the pollutant to the catalyst surface, which

373

subsequently improved the photodegradation performance.

374

3.4. In vivo Study on Daphnia Magna and Wastewater Treatment. Daphnia

375

magna is among the most sensitive researched organism to toxic compounds, which

376

is diffusely distributed in aquatic freshwater habits and has been broadly utilized in

377

aquatic risk assessment.40 Accordingly, it is always used to evaluate the toxicity

378

response of various aquatic systems.41,42 To evaluate the impact of the

379

photodegradation of toxic fluorene using the TiO2/PDA/GA coating under visible

380

light irradiation, a Daphnia magna lethality experiment was carried out. Daphnia

381

magna were cultured under untreated AFW, AFW contaminated by fluorene and the

382

contaminated AFW that was treated by the TiO2/PDA/GA coating under visible light

383

irradiation. In contaminated AFW, all Daphnia magna died after exposure to

384

fluorene for 36 h, while 96% of the Daphnia magna survived without exposure to

385

fluorene (Figure 4a). The results in Figure 4a and Movie S1 revealed that treatment

386

of contaminated AFW by the TiO2/PDA/GA coating significantly reduced the

387

lethality of the contaminated AFW such that 92% of the Daphnia magna survived.

388

The in vivo experiment suggests that treatment of contaminated water with the

389

as-prepared TiO2/PDA/GA coating might prove advantageous to decreasing the toxic

390

effects of some chemicals.

18


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391

Coking wastewater generated from high-temperature carbonation, coal gas

392

purification, and chemical refining in coke plants always contains a certain amount

393

of PAHs.43 To further broaden the application of the TiO2/PDA/GA coating to

394

eliminate fluorene, a coking wastewater treatment experiment was carried out. The

395

calibration curve of fluorene in a low concentration range was obtained via the

396

HS-SPME-GC-MS method using PDMS fibers (Figure S12). The wastewater

397

treatment device was setup as shown in Figure S13. It is believed to be difficult for

398

light to enter the dark brown wastewater, which would greatly lower the amount of

399

light received by the coating. To solve the mentioned limitation, the coating was

400

adhered to the glass wall of a vial, and a water-membrane was formed between the

401

coating and the glass wall. The water-membrane was thin enough to be penetrated by

402

light so that photodegradation could successfully occur. The results in Figure 4b

403

show that the initial concentration of fluorene existed in the wastewater itself was

404

158 ng/L and that no fluorene was detected in the wastewater after 520 min

405

treatment with the TiO2/PDA/GA coating under visible light irradiation. Although

406

agitation can accelerate the flow of the water-membrane, the elimination rate of

407

fluorene in coking wastewater was still much lower than that in the pure water

408

system. Moreover, the proposed TiO2/PDA/GA coating was proven to be capable of

409

removing fluorene in wastewater, which provides potential for future application of

410

the coating in industrial wastewater treatment and environmental remediation.

411

3.5. Evaluation of the Modified Degradation Device. Effective utilization of the

412

TiO2/PDA/GA coating avoids separation processes, but it takes two times longer for 19



413

the TiO2/PDA core/shell nanoparticles to remove pollutants (Figure 3), due to the

414

much smaller contact area between the coating and the pollutants. With the purpose

415

of improving the photocatalytic efficiency of the coating, GC capillary columns with

416

external diameters of nearly 350 µm were used to prepare photodegradation fibers.

417

The fibers were subsequently assembled as a brush-like photodegradation device.

418

Under visible light irradiation, the photodegradation performance of the

419

TiO2/PDA/GA fiber array towards geosmin and fluorene was compared with those of

420

the TiO2/PDA nanoparticles and the TiO2/PDA/GA coating (Figures 4a and 4b). The

421

modified degradation device is shown in Figure 5c. According to the results in

422

Figures 4a and 4b, the fiber array exhibited an elimination rate of the pollutants

423

between that of the core/shell nanoparticles and the coating. Moreover, the fiber

424

array needed only 40 min to remove the pollutants, which is the same time required

425

for removal by the TiO2/PDA nanoparticles. Table S4 illustrated the comparison of

426

the treating time and visible light degradation ratio for the pollutants of the reported

427

non-metal modified TiO2 materials and the TiO2/PDA/GA composite proposed in

428

this work. Generally speaking, compared with other non-metal modified TiO2

429

materials, the TiO2/PDA/GA composite exhibited rapid pollutant degradation ability,

430

especially for the TiO2/PDA/GA fiber array, and satisfactory pollutant elimination

431

ratios.

432

For the purpose of further proving the effectiveness of the modified degradation

433

device, the removal of fluorene from coking wastewater was conducted (Figure S14).

434

According to Figure 5d, no fluorene was detected in wastewater after a 280 min 20


Page 20 of 35

Page 21 of 35


435

treatment of the TiO2/PDA/GA fiber array under visible light irradiation, which was

436

240 min faster than when using the TiO2/PDA/GA coating. The above results proved

437

that the adoption of the miniature TiO2/PDA/GA fiber array could greatly improve

438

the photocatalytic efficiency of the proposed material. Actually, more time was

439

required for fiber preparation and device assembly processes. However, this will not

440

be a problem if batch production is adopted.

441

In this work, we proposed a two-step modification scheme to improve the

442

photocatalytic performance of rutile TiO2 nanoparticles under visible light. First, the

443

immobilization of a PDA shell onto the TiO2 core narrowed the band gap energy of

444

the TiO2 nanoparticles, which consequently improved the visible light response

445

compared to unmodified TiO2. Secondary modification by GA, which resulted in the

446

formation of the TiO2/PDA/GA coating, avoided separation processes for continuous

447

application of the modified materials while maintaining the photoresponse ability to

448

visible light irradiation of the TiO2/PDA core/shell nanoparticles. In other words, the

449

self-assembled nanostructured materials can be used without environmental

450

exposure to nanomaterials. Additionally, the adsorption ability of the unmodified

451

TiO2 nanoparticles was also improved after modification. The increased adsorption

452

efficiency of pollutants contributed to the photocatalytic performance enhancement

453

of the composites. The safety and effectiveness of the coating, proven by the in vivo

454

study with Daphnia magna and the coking wastewater treatment experiment,

455

suggests that it may provide an innocuous alternative to the utilization of

456

nanotechnology in environmental remediation. Since dopamine has been proven to 21



Page 22 of 35

457

undergo a self-polymerization reaction and form a PDA membrane on almost all

458

kinds of inorganic and organic surfaces21, the two-step modification scheme

459

proposed in this work is expected to provide a versatile strategy for the modification

460

of nanomaterials and other functional substrates. We believe the findings in this

461

work could provide new insights in various fields, such as environmental science,

462

catalytic science, materials science and life science.

463

ASSOCIATED CONTENT

464

Supporting Information Available: Details of analysis geosmin, fluorene and their

465

degradation intermediates. Figures: TEM images, XRD patterns, SEM images and

466

elemental mappings, FTIR spectra, XPS spectra, UV-visible diffuse reflectance

467

spectra and band gap states, relative response of geosmin and fluorene versus their

468

concentrations,

469

adsorption performance, adsorption kinetics and coking wastewater treatment

470

devices. Tables: element analysis, intermediate products of photodegradation,

471

adsorption kinetics parameters and performance comparisons. Movie S1: (Daphnia

472

magna lethality experiment).

473

AUTHOR INFORMATION

474

Corresponding Author.

475

*Email: [email protected] (G. Ouyang); [email protected] (N. Zhou)

476

Notes.

477

The authors declare no competing financial interest.

recycled

photodegradation

performance, reaction

22


pathways,

Page 23 of 35


478

ACKNOWLEDGMENTS

479

We acknowledge financial support from the projects of NNSFC (21377172,

480

21477166, 21527813, 21677182) and the NSF of Guangdong Province

481

(S2013030013474).

482

REFERENCES

483

(1) Schwarzenbach, R. P.; Escher, B. I.; Fenner, K.; Hofstetter, T. B.; Johnson, C. A.;

484

von Gunten, U.; Wehrli, B. The challenge of micropollutants in aquatic systems.

485

Science 2006, 313 (5790), 1073-1077.

486

(2) Daughton, C. G.; Ternes, T. A. Pharmaceuticals and personal care products in the

487

environment: agents of subtle change? Environ. Health Perspect. 1999, 107 (Suppl

488

6), 907-938.

489

(3) Alsbaiee, A.; Smith, B. J.; Xiao, L.; Ling, Y.; Helbling, D. E.; Dichtel, W. R.

490

Rapid removal of organic micropollutants from water by a porous β-cyclodextrin

491

polymer. Nature 2016, 529, 190-194.

492

(4) Richardson, S. D.; Ternes, T. A. Water analysis: emerging contaminants and

493

current issues. Anal. Chem. 2014, 86 (6), 2813-2848.

494

(5) Murray, K. E.; Thomas, S. M.; Bodour, A. A. Prioritizing research for trace

495

pollutants and emerging contaminants in the freshwater environment. Environ.

496

Pollut. 2010, 158 (12), 3462-3471.

497

(6) Khin, M. M.; Nair, A. S.; Babu, V. J.; Murugan, R.; Ramakrishna, S. A review on

498

nanomaterials for environmental remediation. Energy Environ. Sci. 2012, 5 (8),

499

8075-8109. 23



Page 24 of 35

500

(7) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas B. J.;

501

Mayes, A. M. Science and technology for water purification in the coming decades.

502

Nature 2008, 452 (7185), 301-310.

503

(8) Brandl, F.; Bertrand, N.; Lima, E. M.; Langer, R. Nanoparticles with

504

photoinduced precipitation for the extraction of pollutants from water and soil. Nat.

505

Commun. 2015, 6, 7765.

506

(9) Li, X.; Liu, H.; Cheng, L.; Tong, H. Photocatalytic oxidation using a new catalyst

507

TiO2 microsphere for water and wastewater treatment. Environ. Sci. Technol. 2003,

508

37 (17), 3989-3994.

509

(10) Chalasani, R.; Vasudevan, S. Cyclodextrin-functionalized Fe3O4@TiO2:

510

reusable,

511

endocrine-disrupting chemicals in water supplies. ACS Nano 2013, 7 (5), 4093-4104.

512

(11) Park, C.; La, Y.; An, T. H.; Jeong, H. Y.; Kang, S.; Joo, S. H.; Ahn, H.; Shin, T. J.

513

Kim, K. T. Mesoporous monoliths of inverse bicontinuous cubic phases of block

514

copolymer bilayers. Nat. Commun. 2015, 6, 6392.

515

(12) Liu, H.; Ma, H.; Li, X.; Li, W.; Wu, M.; Bao, X. The enhancement of TiO 2

516

photocatalytic activity by hydrogen thermal treatment. Chemosphere 2003, 50 (1),

517

39-46.

518

(13) Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 surfaces:

519

principles, mechanisms, and selected results. Chem. Rev. 1995, 95 (3), 735-758.

520

(14) Lee, J.; Kim, J.; Choi, W. TiO2 photocatalysis for the redox conversion of

521

aquatic pollutants. ACS Symp. Ser. 2011, 1071, 199-222.

magnetic

nanoparticles

for

photocatalytic

24


degradation

of

Page 25 of 35


522

(15) Sato, S.; Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Photocatalysts

523

sensitive to visible light. Science 2002, 295 (5555), 626-627.

524

(16) Yamashita, H.; Harada, M.; Misaka, J.; Takeuchi, M.; Neppolian, B.; Anpo, M.

525

Photocatalytic degradation of organic compounds diluted in water using visible

526

light-responsive metal ion-implanted TiO2 catalysts: Fe ion-implanted TiO2. Catal.

527

Today 2003, 84 (3), 191-196.

528

(17) Sung-Suh, H. M.; Choi, J. R.; Hah, H. J.; Sang, M. K.; Bae, Y. C. Comparison

529

of Ag deposition effects on the photocatalytic activity of nanoparticulate TiO2 under

530

visible and UV light irradiation. J. Photochem. Photobiol. 2004, 163 (1), 37-44.

531

(18) Salthammer, T.; Fuhrmann, F. Photocatalytic surface reactions on indoor wall

532

paint. Environ. Sci. Technol. 2007, 41 (18), 6573-6578.

533

(19) Kosowska, B.; Mozia, S.; Morawski, A. W.; Grzmil, B.; Janus, M.; Kalucki, K.

534

The preparation of TiO2-nitrogen doped by calcination of TiO2·xH2O under

535

ammonia atmosphere for visible light photocatalysis. Sol. Energy Mater. Sol. Cells

536

2005, 88 (3), 269-280.

537

(20) Mao, W.; Lin, X.; Zhang, W.; Chi, Z.; Lyu, R.; Cao, A.; Wan, L. Core-shell

538

structured TIO2@polydopamine for highly active visible-light photocatalysis. Chem.

539

Commun. 2016, 52(44), 7122-7125.

540

(21) Rehman, S.; Ullah, R.; Butt, A. M.; Gohar, N. D. Strategies of making TiO2 and

541

ZnO visible light active. J. Hazard. Mater. 2009, 170 (2), 560-569.

542

(22) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-inspired

543

surface chemistry for multifunctional coatings. Science 2007, 318 (5849), 426-430. 25



544

(23) Liu, Y.; Ai, K.; Lu, L. Polydopamine and its derivative materials: synthesis and

545

promising applications in energy, environmental, and biomedical fields. Chem. Rev.

546

2014, 114 (9), 5057-5115.

547

(24) D’Ischia, M.; Napolitano, A.; Ball, V.; Chen, C. T.; Buehler, M. J.

548

Polydopamine and eumelanin: from structure-property relationships to a unified

549

tailoring strategy. Acc. Chem. Res. 2014, 47 (12), 3541-50.

550

(25) Sureshkumar, M.; Lee, C. K. Polydopamine coated magnetic-chitin (MCT)

551

particles as a new matrix for enzyme immobilization. Carbohydr. Polym. 2011, 84

552

(2), 775-780.

553

(26) Praetorius, A.; Scheringer, M.; Hungerbühler, K. Development of environmental

554

fate models for engineered nanoparticles: a case study of TiO2 nanoparticles in the

555

Rhine River. Environ. Sci. Technol. 2012, 46 (12), 6705-6713.

556

(27) Weir, A.; Westerhoff, P.; Fabricius, L.; Hristovski, K.; Von Goetz, N. Titanium

557

dioxide nanoparticles in food and personal care products. Environ. Sci. Technol.

558

2012, 46 (4), 2242-2250.

559

(28) Liu, H.; Li, X.; Leng, Y.; Li, W. An alternative approach to ascertain the

560

rate-determining steps of TiO2 photoelectrocatalytic reaction by electrochemical

561

impedance spectroscopy. J. Phys. Chem. B 2003, 107 (34), 8988-8996.

562

(29) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, B. W. Environmental

563

Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95 (1), 69-96.

564

(30) Chen, G.; Qiu, J.; Liu, Y.; Jiang, R.; Cai, S.; Liu, Y.; Zhu, F.; Zeng, F.; Luan, T.;

565

Ouyang, G. Carbon nanotubes act as contaminant carriers and translocate within 26


Page 26 of 35

Page 27 of 35


566

plants. Sci. Rep. 2015, 5, 15682.

567

(31) Petersen, E. J.; Akkanen, J.; Kukkonen, J. V.; Jr, W. W. Biological uptake and

568

depuration of carbon nanotubes by daphnia magna. Environ. Sci. Technol. 2009, 43

569

(8), 2969-2975.

570

(32) Crossland, E. J. W.; Noel, N.; Sivaram, V.; Leijtens, T.; Alexander-Webber, J. A.;

571

Snaith, H. J. Mesoporous TiO2 single crystals delivering enhanced mobility and

572

optoelectronic device performance. Nature 2013, 495, 215-219.

573

(33) Wert, E. C.; Korak, J. A.; Trenholm, R. A.; Rosario-Ortiz, F. L. Effect of oxidant

574

exposure on the release of intracellular microcystin, MIB, and geosmin from three

575

cyanobacteria species. Water Res. 2014, 52, 251-259.

576

(34) Luo, S.; Chen, B.; Lin, L.; Wang, X.; Tam, N. F. Y.; Luan, T. Pyrene degradation

577

accelerated by constructed consortium of bacterium and microalga: effects of

578

degradation products on the microalgal growth. Environ. Sci. Technol. 2014, 48 (23),

579

13917-13924.

580

(35) Ouyang, G.; Vuckovic, D.; Pawliszyn, J. Nondestructive sampling of living

581

systems using in vivo solid-phase microextraction. Chem. Rev. 2011, 111 (4),

582

2784-814.

583

(36) Liu, S.; Chen, D.; Zheng, J.; Zeng, L.; Jiang, J.; Jiang, R.; Zhu, F.; Shen, Y.; Wu,

584

D.; Ouyang, G. The sensitive and selective adsorption of aromatic compounds with

585

highly crosslinked polymer nanoparticles. Nanoscale 2015, 7 (40), 16943-16951.

586

(37) Vuckovic, D.; de Lannoy, I.; Gien, B.; Shirey, R.E.; Sidisky, L. M.; Dutta, S.;

587

Pawliszyn, J. In Vivo Solid-Phase Microextraction: Capturing the Elusive Portion of 27



588

Metabolome. Angew. Chem., Int. Ed. 2011, 50 (23), 5344-5460.

589

(38) Kinani, S.; Souissi, Y.; Kinani, A.; Vujovic, S.; Ait-Aissa, S.; Bouchonnet, S.

590

Photodegradation of fluorene in aqueous solution: Identification and biological

591

activity testing of degradation products. J. Chromatogr. A 2016, 1442, 118-128.

592

(39) Xue, Q.; Liu, Y.; Zhou, Q.; Utsumi, M.; Zhang, Z.; Sugiura, N. Photocatalytic

593

degradation of geosmin by Pd nanoparticle modified WO3, catalyst under simulated

594

solar light. Chem. Eng. J. 2016, 283, 614-621.

595

(40) Barata, C.; Varo, I.; Navarro, J. C.; Arun, S.; Porte, C. Antioxidant enzyme

596

activities and lipid peroxidation in the freshwater cladoceran Daphnia magna

597

exposed to redox cycling compounds. Comp. Biochem. Physiol. C: Pharmacol.

598

Toxicol. 2005, 140 (2), 175-186.

599

(41) Lovern, S. B.; Strickler, J. R.; Klaper, R. Behavioral and physiological changes

600

in Daphnia magna when exposed to nanoparticle suspensions (titanium dioxide,

601

nano-C60, and C60HxC70Hx). Environ. Sci. Technol. 2007, 41 (12), 4465-4470.

602

(42) Blewett, T. A.; Delompré, P. L.; He, Y.; Folkerts, E. J.; Flynn, S. L.; Alessi, D.

603

S.; Goss, G. G. The sub-lethal and reproductive effects of acute and chronic exposure

604

to flowback and produced water from hydraulic fracturing on the water flea Daphnia

605

magna. Environ. Sci. Technol. 2017, 51 (5), 3032-3039.

606

(43) Yang, W.; Li, X.; Pan, B.; Lv, L.; Zhang, W. Effective removal of effluent

607

organic matter (EfOM) from bio-treated coking wastewater by a recyclable aminated

608

hyper-cross-linked polymer. Water Res. 2013, 47 (13), 4730-4738.

609

28


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610

Figure Captions

611

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

612

of the TiO2/PDA/GA coating.

613

Figure 2. (a,b) TEM images of the TiO2/PDA nanoparticles; inset is the

614

FFT-generated diffraction pattern. (c) SEM images of the TiO2/PDA nanoparticles.

615

(d,e) TEM images of the TiO2/PDA/GA coating; inset is the FFT-generated

616

diffraction pattern. (f) SEM images of the TiO2/PDA/GA coating; inset is the

617

photograph of the coating.

618

Figure 3. Photocatalytic degradation of geosmin and fluorene. (a) Schematic

619

illustration of the removal and analysis of the pollutants. (b,d,f) Removal of geosmin

620

by the TiO2 nanoparticles, the TiO2/PDA core/shell nanoparticles and the

621

TiO2/PDA/GA coating. (c,e,g) Removal of fluorene by the TiO2 nanoparticles, the

622

TiO2/PDA core/shell nanoparticles and the TiO2/PDA/GA coating.

623

Figure 4. (a) Treatment of fluorene-contaminated AFW by the TiO2/PDA/GA

624

coating significantly reduced the lethality of Daphnia magna. (b) Fluorene in coking

625

wastewater was removed by the TiO2/PDA/GA coating under visible light

626

irradiation.

627

Figure 5. Photodegradation performance of the TiO2/PDA/GA fiber array under

628

visible light irradiation. (a,b) Removal of geosmin and fluorene by the

629

TiO2/PDA/GA fiber array. (c) Schematic illustration of pollutant removal by the

630

TiO2/PDA/GA fiber array. (d) Comparison of fluorene removal in coking wastewater

631

by the TiO2/PDA/GA coating and the fiber array.

632 29



633

Figure 1.

634

635 636 637

30


Page 30 of 35

Page 31 of 35


638

Figure 2.

639

640 641 642 643

31



644

Figure 3.

645

646 647 648 649

32


Page 32 of 35

Page 33 of 35


650

Figure 4.

651

652 653 654

33



655

Figure 5.

656

657 658 659 660

34


Page 34 of 35

Page 35 of 35


661

For TOC only

662

663 664 665 666

35


Enhanced Photocatalytic Degradation of Environmental Pollutants

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