Immobilized TiO2-reduced graphene oxide nanocomposites on optical fibers as high performance photocatalysts for degradation of pharmaceuticals
Graphical abstract
Introduction
In recent decades, pharmaceuticals have been detected in wastewater, surface water, and even in drinking water due to their extensive uses [1]. One of the major pathways of pharmaceuticals in surface water is the discharge of urban wastewater effluent because conventional wastewater treatment is not capable of removing pharmaceuticals effectively. Although pharmaceuticals present in surface water are usually at low concentrations, their adverse effects on terrestrial and aquatic organisms have been a prevailing environmental concern [2]. Recently, advanced oxidation processes (AOPs) using O3, H2O2, and ultraviolet (UV) irradiation have been employed for removal of pharmaceuticals in water and wastewater [3], [4]. However, AOPs have the shortcomings of high chemical usage, intense energy consumption, and considerable cost.
Photocatalysis is an attractive technology because it can use solar energy to degrade organics and inactivate pathogens. In comparison to traditional oxidation processes, photocatalytic oxidation has the advantages of energy-neutral, chemical-free, and operation-simple. Many refractory organic contaminants could be destroyed by photocatalytic reactions [5]. However, low solar energy utilization efficiency and slow photocatalytic degradation rate must be improved before practical applications [6].
Among various photocatalysts, TiO2 is the most widely studied in water treatment due to its strong oxidizing ability, excellent chemical stability, long durability, water insolubility, superhydrophilicity, and low cost [6], [7], [8]. TiO2 has three main crystalline structures: anatase, brookite, and rutile. Both anatase and rutile phases are commonly used in photocatalysis, with anatase generally demonstrating greater photocatalytic performance [9]. However, anatase TiO2 is not an ideal sunlight-driven photocatalyst due to its large band gap (band-gap energy 3.2 eV) and low quantum yield, because anatase TiO2 absorbs UV light with a wavelength less than 387 nm (only 5% of solar light) and an energy higher than 3.2 eV [9].
Substantial efforts have been devoted to improve the light utilization efficiency of TiO2, such as doping with metal ions, nonmetal ions, and creation of heterojunctions with other semiconductors [10], [11], [12], [13]. Due to the unique electron-transferring property, incorporation of the emerging graphene and TiO2 is considered a promising nanocomposite to expand the light absorption region [14], [15], [16], [17]. Graphene could transform wide-band-gap semiconductors (including TiO2) into visible light photocatalysts [18]. Considerably higher photoactivity was attained than the commonly used Degussa P25 TiO2 powder [19]. As a result, TiO2–graphene particles can absorb wider light region for both UV and visible light, as well as have faster photocatalytic kinetics [20], [21], [22], [23], [24], [25], [26], [27]. Additionally, graphene can work as an electron acceptor/transporter for TiO2 particles; graphene is therefore anticipated to significantly enhance the lifetime of electron-hole pairs [28]. Higher activity of the coupled adsorption and photocatalytic oxidation can be achieved due to the large specific surface area of graphene along with its high adsorption capacity [29]. Because of the high production costs of graphene, one of the most popular approaches to graphene-based nanomaterials is to reduce graphene oxide (GO). GO can be produced at low cost by chemical oxidation of graphite [30].
Most researches used TiO2-GO or reduced GO (rGO) as suspended photocatalysts in the solution of traditional heterogeneous slurry photoreactors to remove contaminants in water [19], [22], [23], [31], [25], [32]. The model contaminants studied were predominantly dyes (e.g., rhodamine B, methylene blue, and methyl orange) although other compounds such as 2,4-dichlorophenoxyacetic acid [33], butane [34], diphenhydramine [35], [36], and 4-nitrophenol [37] were also studied to evaluate the photocatalytic performance of TiO2-GO/rGO composites. Such suspended particles contact well with contaminants in water, thereby achieving the highest possible catalytic efficiency. These reactors however, are mostly limited to laboratory study due to low light utilization, loss of photocatalysts, and difficulty and high cost for separation of suspended photocatalyst particles from aqueous solutions. Hence an ideal photoreactor should be able to recover catalysts from treated water easily, and reduce the light loss from liquid absorption and catalyst particles scattering.
So far the use of immobilized TiO2–graphene/GO/rGO for water treatment is still in its inception. There are only a limited number of studies on this new area. For example, coating TiO2–graphene nanocomposite on glass surface degraded butane in a gas phase under UV and visible light [38], [39]. Incorporation of TiO2-GO in filtration membranes improved water flux and achieved higher removal of methylene blue, organic dyes, and diphenhydramine in water [36], [40], [41].
An immobilized photoreactor with catalyst-coated side-glowing optical fibers (SOFs) was developed during our previous study to treat organic contaminants in water [42] and desalination concentrate [43]. SOF is an innovative fiber with nude quartz glass fiber as the core and coated with silicone rubber, light irradiance distribution for SOFs is more uniform along the fiber length as compared to conventional optical fibers (i.e., end-emitting optical fibers) [42]. Bundles of SOFs were incorporated in batch or continuous-flow photoreactors, which provide both light transmission and catalyst support. In comparison to a conventional photoreactor, the SOFs allow the light to transmit directly through the inner fiber cores to reach the photocatalysts coated on the surface, as well as on the exterior surface of the photocatalysts, thus significantly improving the light utilization efficiency. This is an economical way to deliver photons efficiently and uniformly in a large-scale reactor while avoiding the separation step of photocatalysts from water.
In this study, TiO2-rGO nanocomposite thin films were synthesized on SOFs using polymer assisted hydrothermal deposition (PAHD) method. PAHD is a relatively simple and inexpensive process that enables the formation of a range of high quality materials by precise control of the stoichiometric ratio of precursor solutions, polymers, and dopants, for multi-phase materials. Polymers used in the PAHD can enhance the durability and stability of SOFs coated with photocatalysts in air and water. In addition, hydrothermal method requires lower deposition temperature (e.g., 180–200 °C), which allows the deposition of catalysts on SOF because the silicone rubber coating of SOFs can only endure 250 °C.
The designed photoreactor with immobilized TiO2-rGO SOFs was investigated to degrade pharmaceuticals including ibuprofen, carbamazepine, and sulfamethoxazole using different light sources. These pharmaceuticals are commonly used anti-inflammatory drug, anticonvulsant and mood stabilizer, and antibiotic used for bacterial infections (Fig. 1). The objective of this study was to characterize and optimize the photocatalytic efficiency of TiO2-rGO nanocomposites, and to investigate the photodegradation and mineralization of pharmaceutical compounds under different light irradiations in immobilized photoreactors. The durability of the synthesized TiO2-rGO nanocomposites as photocatalyst was evaluated during multiple treatment cycles. The overall goal of the present work was to develop highly efficient SOFs photocatalytic reactors with immobilized catalysts to degrade contaminants of emerging concern (e.g., pharmaceuticals) in water and wastewater using natural sunlight.
Section snippets
Synthesis of graphene oxide (GO) nanosheets
GO nanosheets were synthesized from graphite using modified Hummer’s method [30]. Three grams of graphite (ACS reagent grade, Sigma–Aldrich, St Louis, MO) and 1.5 gram of sodium nitrate (ACS reagent grade, Fisher Scientific, Fair Lawn, NY) were added into 75 mL of 98% sulfuric acid (ACS reagent grade, Sigma–Aldrich, St Louis, MO). The mixture was cooled down to 4 °C in an ice-bath and stirred for 2 h. Nine grams of potassium permanganate (ACS reagent grade, Sigma–Aldrich, St Louis, MO) were added
Characterization of TiO2-rGO nanocomposites
Fig. 3a shows FTIR spectra of the synthesized GO and TiO2-2.7% rGO nanocomposite. GO presented different types of functionalities as confirmed at 3400 cm−1 (O–H stretching vibration), 2900 cm−1 (C–H stretching vibration), 1720 cm−1 (CO stretching vibration), 1600 cm−1 (skeletal vibration from unoxidized graphite), 1355 cm−1 (C–N stretching vibration), 1220 cm−1 (C–OH stretching vibration), and 1040 cm−1 (C–O stretching vibration). The FTIR spectrum of TiO2-2.7% rGO presented significant peak reduction
Conclusions
This study investigated the photocatalytic efficiency of treating pharmaceuticals using an immobilized optical fiber photoreactor with a series of TiO2-rGO nanocomposites under UV and visible light irradiation. TiO2 was present in the mixture phases in the TiO2-rGO nanocomposite. The photocatalytic activity increased with increasing concentrations of rGO (0–2.7%) in composites, but the degradation was inhibited when the rGO concentration was larger than 2.7%. The highest catalytic activity was
Acknowledgments
Support for this study was provided by the United States National Science Foundation (NSF) Engineering Research Center Program under Cooperative Agreement EEC-1028968 (ReNUWIt), New Mexico State University (NMSU), and NMSU College of Engineering Research Center.
References (58)
- et al.
Drugs degrading photocatalytically: kinetics and mechanisms of ofloxacin and atenolol removal on titania suspensions
Water Res.
(2010) - et al.
Photocatalytic degradation of antibiotics: the case of sulfamethoxazole and trimethoprim
Catal. Today
(2009) - et al.
Heterogeneous photocatalysis of moxifloxacin in hospital effluent: effect of selected matrix constituents
Chem. Eng. J.
(2015) - et al.
Continuous-flow photocatalytic treatment of pharmaceutical micropollutants: activity, inhibition, and deactivation of TiO2 photocatalysts in wastewater effluent
Appl. Catal. B
(2013) - et al.
A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions
Adv. Environ. Res.
(2004) - et al.
TiO2 photocatalysis: design and applications
J. Photochem. Photobiol. C
(2012) - et al.
Titanium dioxide photocatalysis
J. Photochem. Photobiol. C
(2000) - et al.
Preparation of Cu-doped ZnS QDs/TiO2 nanocomposites with high photocatalytic activity
Appl. Catal. B
(2014) - et al.
Design of H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 film-coated optical fiber photoreactor for the degradation of aqueous rhodamine B and 4-nitrophenol under simulated sunlight irradiation
Chem. Eng. J.
(2012) - et al.
Carbonaceous nanomaterials for the enhancement of TiO2 photocatalysis
Carbon
(2011)
Semiconductor/reduced graphene oxide nanocomposites derived from photocatalytic reactions
Catal. Today
Enhanced photocatalytic degradation of methylene blue under visible irradiation on graphene@TiO2 dyade structure
Appl. Catal. B
Preparation of graphene–TiO2 composite by hydrothermal method from peroxotitanium acid and its photocatalytic properties
Colloids Surf. A
Enhanced chemical interaction between TiO2 and graphene oxide for photocatalytic decolorization of methylene blue
Chem. Eng. J.
TiO2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants
Carbon
Enhanced photocatalytic activity of graphene oxide decorated on TiO2 films under UV and visible irradiation
Curr. Appl. Phys.
Advanced nanostructured photocatalysts based on reduced graphene oxide–TiO2 composites for degradation of diphenhydramine pharmaceutical and methyl orange dye
Appl. Catal. B
Graphene oxide based ultrafiltration membranes for photocatalytic degradation of organic pollutants in salty water
Water Res.
Graphene-based materials for the catalytic wet peroxide oxidation of highly concentrated 4-nitrophenol solutions
Catal. Today
Membrane surface modification with TiO2–graphene oxide for enhanced photocatalytic performance
J. Membr. Sci.
Multifunctional graphene oxide–TiO2 microsphere hierarchical membrane for clean water production
Appl. Catal. B
Enhanced photocatalysis using side-glowing optical fibers coated with Fe-doped TiO2 nanocomposite thin films
J. Photochem. Photobiol. A
Effect of ionic strength and hydrogen peroxide on the photocatalytic degradation of 4-chlorobenzoic acid in water
Appl. Catal. B
Combined advanced oxidation processes for the synergistic degradation of ibuprofen in aqueous environments
J. Hazard. Mater.
Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack
J. Catal.
Photocatalytic degradation of 17-β-oestradiol on immobilised TiO2
Appl. Catal. B
Photocatalytic degradation pathway of methylene blue in water
Appl. Catal. B
Photocatalytic degradation of commercial azo dyes
Water Res.
Photocatalytic degradation of methyl orange by TiO2-coated activated carbon and kinetic study
Water Res.
Cited by (163)
Floating ZnO nanoparticles-coated micro glass bubbles for the efficient photodegradation of micropollutants in water
2024, Separation and Purification TechnologyHybrid membrane processes in advanced wastewater treatment
2024, Current Trends and Future Developments on (Bio-) Membranes: Advances on Membrane EngineeringMultifunctional photocatalytic membrane distillation for treatment of hypersaline produced water using hydrophobically modified tubular ceramic membranes
2023, Journal of Environmental Chemical EngineeringFabrication of photocatalytic PAN nanofiber membrane loading with TiO<inf>2</inf>@RGO by electro-spinning & electro-spraying
2023, Composites Part B: Engineering