Facile fabrication of superhydrophobic, superoleophilic photocatalytic membrane for efficient oil-water separation and removal of hazardous organic pollutants

Accepted Manuscript Facile fabrication of superhydrophobic, superoleophilic photocatalytic membrane for efficient oil-water separation and removal of hazardous organic pollutants Umair Baig, Asif Matin, M.A. Gondal, S.M. Zubair PII:

S0959-6526(18)33093-2

DOI:

10.1016/j.jclepro.2018.10.079

Reference:

JCLP 14481

To appear in:

Journal of Cleaner Production

Received Date: 26 May 2018 Revised Date:

26 September 2018

Accepted Date: 9 October 2018

Please cite this article as: Baig U, Matin A, Gondal MA, Zubair SM, Facile fabrication of superhydrophobic, superoleophilic photocatalytic membrane for efficient oil-water separation and removal of hazardous organic pollutants, Journal of Cleaner Production (2018), doi: https:// doi.org/10.1016/j.jclepro.2018.10.079. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract Facile fabrication of superhydrophobic, superoleophilic photocatalytic membrane for efficient oil-water separation and removal of hazardous organic pollutants Umair Baig1, Asif Matin1, M.A. Gondal2*, S. M. Zubair1,3 1

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Center for Research Excellence in Desalination and Water Treatment, King Fahd University of Petroleum and Minerals, Saudi Arabia 2 Department of Physics and Center for Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Saudi Arabia 3 Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Saudi Arabia *Corresponding author: M.A. Gondal; Email address: [email protected]

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Telephone: +96638602351 and +96638603274

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Resultant wordcount: 7080

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Facile fabrication of superhydrophobic/superoleophilic photocatalytic membrane for efficient oil-water separation and removal of hazardous organic pollutants

Umair Baig1, Asif Matin1, M.A. Gondal2*, S.M. Zubair1,3 1

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Center for Research Excellence in Desalination and Water Treatment, King Fahd University of Petroleum and Minerals, Saudi Arabia 2 Department of Physics and Center for Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Saudi Arabia 3 Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Saudi Arabia

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*Corresponding author: M.A. Gondal; Email address: [email protected] Telephone: +96638602351 and +96638603274 Abstract

In this work, the fabrication and characterization of a dual-purpose membrane is reported: (i) oilwater separation with high efficiency, and (ii) effective and rapid photocatalytic degradation of

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hazardous environmental pollutants. Nanoparticles of a rare-earth oxide, cerium (IV) oxide, synthesized by a simple co-precipitation technique, were spray-coated onto stainless steel membranes to obtain a uniform coating with superwetting characteristics. The as-synthesized cerium (IV) oxide nanoparticles and its spray-coated stainless steel membrane were

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characterized by advanced analytical techniques. X-ray diffraction, selected area electron diffraction, and electron microscopy results of the cerium oxide particles showed a high degree of crystallinity and uniform particle size of ~ 50 nm. Elemental surface analysis using X-ray

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photoelectron spectroscopy showed the characteristic peaks for Ce4+, thereby confirming the presence of cerium oxide nanoparticles. Contact angle measurements showed the coated membranes to be superhydrophobic (~ 150°) and superoleophobic (~ 0°). When used in a simple laboratory setup, the membranes were able to separate oil and water with a high efficiency (~ 99%). Furthermore, the coated membrane showed enhanced photocatalytic activity under UV light irradiation resulting in the rapid degradation of organic persistent pollutants like MB dye in water. To summarize, this work presents an innovative approach in the application of multifunctional membranes for effective water treatment. 1

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Keywords: Cerium oxide; Membrane; Wettability; Photo-catalytic degradation.

1. Introduction

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Nowadays, the generation of large volumes of oily wastewater from a wide variety of industries presents a major environmental hazard as well as wastage of water resources. In addition, the frequent occurrence of accidents such as oil spillages (Dubansky et al., 2013) and leakage of oil

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and organic solvents have resulted in potential hazards to the environment and living organisms (Shannon et al., 2008). Moreover, in oil-producing regions such as Saudi Arabia and other Gulf

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countries, produced water is contaminated with significant amounts of oil. Also, many waste streams from chemical plants contain organic dyes that are toxic to the flora and fauna. With ever increasing stringent regulations concerning the environment, it has become very essential to purify and decontaminate the wastewater coming out of different sources.

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The removal of oil and organic contaminants from water is a highly challenging task (Joye et al., 2015 and Pettinato et al., 2015). Currently, the techniques used in industry and commercial sector focus on either the collection of oil in its pure form or dispersion using

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specialty chemicals (Kleindienst et al., 2015). These include but are not limited to filtration, electrochemical, separation by gravity and floatation, and centrifugation methods (Nordvik et al.,

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1996 and Aloulou et al., 2018). However, most of these technologies have major disadvantages in that they are either expensive and energy-exhaustive or unable to provide high levels of separation efficiency.

In recent decades, membranes have been increasingly used for different kinds of water

purification processes e.g. seawater desalination, wastewater reclamation, etc (Sarkar et al., 2015a and Shannon et al., 2008). There are several inherent advantages associated with

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membrane-based techniques that include (Fane et al., 2015), ease of synthesis, surface modification by thin film deposition, a high degree of selectivity and simplicity of operation. Membrane science and technology has seen rapid advancements in several aspects such as

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fouling resistance, higher permeation rates, and increased selectivity against solutes. Chakraborty et al., 2012, developed a two separate phase submerged biocatalytic membrane reactor for the production of fatty acids and glycerol from residual vegetable oil streams.

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Recently, many researchers have investigated the effectiveness and feasibility of using superhydrophobic fabrics (Mohsin et al., 2016) microporous membranes (Elanchezhiyan et al.,

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2016) and adsorbents (Doshi et al., 2018) for oil/water separation. Most of the efforts have focused on a surface modification to impart contrasting wetting characteristics to the membrane (Yue et al., 2018). The basic idea is that one of the fluids completely wets the surface and passes through rapidly, while the other is repelled by the membrane surface. In a detailed review on bio-

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inspired surfaces with superwettability, Gondal et al. 2014 have discussed the 4 different extreme wetting states with regards to oil and water possible for any surface: superhydrophilic, superhydrophobic, superoleophilic and superoleophobic. For a membrane to achieve separation

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of oil and water, it should have one of the following combinations: (i) superhydrophobic and superoleophilic, (ii) superhydrophilic and superoleophobic. Multiple research efforts have

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focused on both configurations with each having its own merits and demerits. Rare-earth oxides are an interesting class of materials in that they are intrinsically

hydrophobic as opposed to their regular metal counterparts (e.g. Alumina, titania, etc.) that are generally hydrophilic (Carchini et al., 2016). Their different wetting behavior is explained by their unique electronic structure (unfilled 4f orbitals shielded from interaction with the surrounding environment by the full octet of electrons in the 5s2p6 outer shell) that discourages

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the formation of hydrogen bonds with water molecules on the surface. In addition, their mechanical stability and robustness give them a clear edge over their polymeric and organic counterparts as the latter are usually fragile (Azimi et al. 2013).

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Researchers have investigated the feasibility of some rare-earth oxides for versatile applications (Matin et al., 2018). Cerium (IV) oxide (CeO2) nanoparticles have been synthesized using different techniques (Charbgoo et al., 2017) such as co-precipitation, electrochemistry and

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studied for biomedical, nanomedicine and catalysis applications. Similarly, yttrium oxide (Y2O3), another member of the same family, has been explored for cutting tool applications due to its

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excellent mechanical properties (Kumar et al., 2004). Lanthanum oxide (La2O3), the first compound of the lanthanide series, is used for water treatment (Xie et al., 2015), catalysis (Vishnyakov et al. 2010) and piezoelectric and thermoelectric applications (Ghiasi et al. 2015). In this work, we report the fabrication of a membrane by facile spray-deposition of

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synthesized ceria nanoparticles for two major water treatment applications: oil-water separation and photocatalytic degradation of hazardous environmental pollutants. For this purpose, two different characteristics of the oxide are utilized; wetting properties and photocatalytic abilities.

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We believe that this is the first instance of reporting a rare-earth oxide for a dual application. The fabricated membranes show promising results in both aspects; high efficiency of oil-water

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separation and rapid photocatalytic degradation of an organic dye in the presence of UV light.

2. Materials & Methods 2.1. Materials

Cerium(IV) ammonium sulfate, Sodium hydroxide (NaOH), Tetrahydrofuran (THF), acetone (maximum water 0.03%), methanol (HPLC grade), Octane, Isopropanol, Ethanol, Hydrochloric

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acid were procured from Sigma Aldrich, USA. Stainless steel meshes (Pore size: ~ 50 µm) were purchased from TWP Inc. USA. Glass slides were obtained from Paul Marienfeld GmbH & Co.

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Germany.

2.1. Facile synthesis of CeO2 nanoparticles

CeO2 nanoparticles was prepared by facile, one-pot precipitation reaction of the aqueous

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solutions of cerium(IV) ammonium sulphate and alkaline solution of sodium hydroxide as shown in Fig. 1. A 4M sodium hydroxide solution in DI water was added dropwise to a 1M solution of

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the sulfate. This mixture was carefully stirred at 25 oC on a magnetic stirrer to get a precipitate of CeO2 nanoparticles. The precipitate was stirred for about 3 h. It was then left to stand for another 24 h at room temperature to form an absolute precipitate, which was filtered by the suction pump, washed with demineralized water and then dried at 100 oC for 24 h. The dried powder was

(Kumar et al., 2013).

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calcined for 6 h at 800 ºC in a muffle furnace to obtain the crystalline CeO2 nanoparticles

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2.2. Fabrication of CeO2 nanoparticles coated membranes CeO2 nanoparticles coated membranes were fabricated using facile spray deposition of

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nanoparticle dispersion. The homogeneous dispersion of CeO2 nanoparticles in solution was prepared by adding 100 mg nanoparticles to 10 mL of Tetrahydrofuran and subjecting the dispersion to ultrasonic vibrations (BRANSON-3510, Branson Ultrasonic Bath, USA; Power: 100 W and Frequency: 42 kHz) at 25 oC for 1 h. Overnight stirring was done to obtain welldispersed nanoparticles. The dispersed solution was then used to coat glass slide substrates and stainless steel mesh substrates. A spray gun with a nozzle diameter of ~ 1mm was utilized for the

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deposition of particles inside a fume hood with a nitrogen pressure of ~ 300 kPa. The substrate was placed at a distance of 10 cm from the nozzle. After spray coating, annealing of the samples

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was carried out at 200 oC for 4 hours.

2.3. Characterization of CeO2 nanoparticles and its dispersion coated membranes

The synthesized powders were characterized by powder X-ray diffraction system (Bruker AXS; Advance

X-ray

diffractometer),

UV–Visible

spectroscopy

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(JASCO;

V-670

spectrophotometer). TEM and SAED images were taken by JEOL (JEM-2100F) transmission

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electron microscope. FESEM images of the synthesized powders and its coated stainless steel membranes were taken by Field emission scanning electron microscope (FESEM-Tescan Lyra-3) operated at 20 KV. Contact angle measurements were performed by a KRUSS (DSA20X, GmbH Germany) standard drop shape analysis system. To ensure precise contact angle test, contact

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angles were measured thrice and averaged. A high-resolution X-ray photoelectron spectroscopy system (Thermo Scientific ESCALAB 250Xi) with monochromatic aluminum X-ray source was used to confirm the chemical composition of our sample. Thermogravimetric analyses of the

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synthesized ceria nanoparticles was also carried out to determine the thermal stability of Cerium (IV) oxide esp. at high temperatures. For this analysis, a bench-top analyzer (EXSTAR TG/DTA

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6300) with an alumina pan and N2 at a rate of 200 mL/min was used. A small quantity (10.00 mg) of the crystalline ceria nanoparticles was subjected to a temperature range of 35 – 1000 °C with Al2O3 powder as the reference material.

2.4. Oil-Water Separation

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The oil-water separation efficiency of the membranes modified with crystalline ceria nanoparticles was investigated using a simple laboratory-made setup. The membrane was placed between two circular Teflon discs with a circular cavity (~ 1 in diameter) in the center and O-

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rings. Pyrex glass tubes of outer diameter ~ 1 in were connected on either side of the discs to allow for controlled fluid flow. The oil-water mixture was poured from the top and the permeate collected at the bottom in a 100 mL beaker. For easy distinguishing of the fluids, water was dyed

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blue (using methylene blue dye) while hexadecane (model oil) was dyed red (using Sudan dye). 2.5. Photocatalytic activity

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After the oil-water separation, CeO2 nanoparticles-coated membrane could be used for photocatalytic removal of hazardous environmental pollutants. An OmniCure UV lamp (30 mW/cm2 intensity) was utilized as a light source for the photo-catalytic removal of aqueous methylene blue (MB) dye as an organic pollutant. The CeO2 nanoparticles-coated membrane was

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utilized as a photocatalyst for the removal of MB dye under UV light irradiation. The percentage degradation was calculated using the formula [(Co-C)/Co] x100), where Co is the initial dye

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concentration (10 ppm) and C is the instantaneous dye concentration (Das et al., 2014).

3. Results and discussion

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3.1. X-ray diffraction analysis and surface morphology of CeO2 nanoparticles The complete XRD spectrum (20° < 2θ < 80°) for the synthesized CeO2 nanoparticles is shown in Fig. 2a. The exhibited XRD peaks correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) of a cubic fluorite structure of CeO2 and are identified using the standard data (Yang et al. 2005). The peaks are in perfect agreement with those observed for bulk cerium (IV) oxide in other studies (Kumar et al. 2013). No additional peaks were observed that would

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correspond to any secondary phase in the sample. Also, the sharp and narrow peaks are a clear proof of the high degree of crystallinity associated with the ceria particles annealed at 800 °C as compared to broader peaks of samples processed at lower temperatures (Zhang et al, 2006).

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Similar results were obtained for CdSe synthesized using a wet chemical technique and at different temperatures (Nag et al., 2007)

FESEM images of the synthesized and thermally processes ceria nanoparticles show

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extensive agglomeration to form clusters (Fig. 2b). The morphology of these particles becomes clearer in high-resolution TEM images (Fig. 2c) that show most of them to be spherical in shape.

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Furthermore, these images provide a good estimate of the average particle size that appears to be around 50 nm. However, some variation in size can be spotted with particles as small as 20 nm and as large as 100 nm. The highly crystalline nature of the synthesized nanoparticles is further confirmed by the brightness and good size of spots visible in the SAED pattern (Fig. 2d)

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Similar findings on crystal structure and particle morphology have been reported in several other studies. Farahmandjou and co-researchers (Farahmandjou et al., 2016) also prepared oxide nanoparticle using a co-precipitation method but with different precursors. The

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XRD results and FESEM images of the synthesized particles were pretty much similar in that the peaks corresponded to the fluorite structure and the clusters of agglomerated particles were

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clearly visible. Yet another group of researchers, prepared the oxide nanoparticles using Cerium nitrate and NaOH. The only major difference in their findings was the particle size ~ 20 nm and a polycrystalline structure.

Figure S1 shows the complete TGA curve of the CeO2 nanoparticles for the entire

temperature range. The curve shows a very small total weight loss (~ 5 %) that is consistent with other findings (Caputo et. al. 2017) and confirms that the oxide does not chemically decompose

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within this temperature range. Most of the weight loss occurs until 300 °C and is due to the evaporation of the residual solvent and moisture and carbonate species adsorbed on the surface. The TGA findings are a good indication of the thermal stability of the synthesized ceria

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nanoparticles as no decomposition is visible up to a temperature of 1000 °C.

3.2. X-ray photoelectron surface analysis

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XPS of a ceria nanoparticle coated glass was carried out to determine the elements present at the surface and their approximate ratios. Fig. 3 shows the complete as well as the individual spectra

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for the main elements, Ce and O. The elements present in CeO2 nanoparticles coated surface are clearly identified. In addition to the two elements, the spectra shows trace amounts of Carbon from atmospheric contamination. In order to confirm the composition of the CeO2 nanoparticles, high resolution scans were recorded for Ce 3d, O 1s and C 1s core level spectra.

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Fig. 3b represents the fitted spectra of Ce 3d spin-orbit doublet peaks of the sample. The peaks are fitted with the contributions from nanocrystalline CeO2. The Ce 3d peaks from CeO2 have binding energies of 916.98, 908.08, 904.28, 899.28, 886.08 eV and 882.18 eV; the first

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three > 900 eV corresponding to contributions from Ce 3d3/2 spin orbital, and the last three < 900 to Ce 3d5/2 orbital. The full width at half maximum (FWHM) are 3.06, 2.19, 4.47, 3.61,

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4.36 and 3.5 eV for binding energies of 916.98, 908.08, 904.28, 899.28, 886.08 eV and 882.18 eV respectively. The high resolution peaks are in good agreement with those reported in literatures and majority belong to an oxidation state of 4+ (Beche et al., 2008). Fig. 3c shows the spectra of O 1s peaks of CeO2 nanoparticles. The peak is fitted with

three peaks located around at 529.0, 531.48 and 532.38 eV with FWHM of 3.42, 1.19 and 1.08 eV respectively. The presence of the main peak ~ 529 eV conforms that most of the oxygen is

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present as lattice oxygen and the content of non-lattice oxygen is negligible. The atomic percent estimated from the fittings of Ce 3d and O 1s spectra for Ce and O were calculated to be 28.46% and 60.07%, respectively. This is very close to the stoichiometric composition of CeO2

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compound (1:2.2) indicating that oxygen from atmospheric contaminants such as water vapor and CO2 is minimal. This composition also explains the wetting behavior of the oxide (Khan et

3.3. Optical properties of CeO2 nanoparticles

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al., 2015) that is studied in detail in the next sections.

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Figure 4a shows the absorption spectra of the nanoparticles where the CeO2 nanoparticles have the highest absorption in the UV region. CeO2 is a direct bandgap semiconductor material. The band gap of CeO2 nanoparticles is depicted in Fig. 4b by using Tauc plot (Baig et al., 2017; Yang et al., 2017), which is basically (Fhv) versus photon energy (hv) for direct band gap

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material, where F is the Kubelka–Munk function (Dasgupta et al., 2017), which is the equivalent of absorption coefficient deduced from reflectance as in equation 1 and hv is the incident photon energy. Generally the absorption coefficient (α) is related to the band gap energy (Eg) as

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shown in equation 2, where A is a constant known as band tailing parameter and n is the power factor of the transition mode which depends on the nature of the material. The value of n in

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equation 2 is taken as ½ for direct band gap materials and taken as 2 for indirect band gap materials, where the transitions are assisted by phonons to conserve momentum. Transforming equation 2 into linear form leads to equation 3 and extrapolating the linear part of the Tauc plot and its intercept on the x axis directly yields the band gap energy of the material (Dasgupta et al., 2017). α = F(R) =

( ) 

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(1)

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(2)



(αhv) = A(hv − E )

(3)

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α=



(  )

The band gap energy of CeO2 nanoparticles was found to be 3.30 from Touc plot, which is in agreement with the literature (Kumar et al., 2013).

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3.4. Surface morphology of nanoparticles coated membranes

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Superhydrophobic/superoleophilic filtering surfaces were fabricated by simple spray-coating of facile-synthesized CeO2 nanoparticles on stainless steel membranes. The major advantages of this method are: less time consumption, straightforwardness, versatility and economics. A schematic diagram of the fabrication of CeO2 nanoparticles-coated membrane using pressured spray gun technique is shown in Fig. 5.

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The surface morphology of both the uncoated stainless steel membrane, and the one coated with CeO2 nanoparticles was investigated by field emission electron microscope. Scanning electron micrographs of the membrane in both conditions are shown in Fig. 6. This

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figure gives a good comparison and brings out the contrast between the virgin and surface-

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modified membranes at different magnifications. The top row shows representative FE-SEM images of the uncoated membranes (Fig. 6a – 6c), while the bottom row is representative of the coated ones. The relatively smooth surface of the virgin membrane fibers is prominent and a good estimate of the mean pore size (~ 50 µm) is obtained. The oxide particles and their agglomerates are clearly visible on the fiber surface after spray coating. The apparent surface roughness of the fibers shows an increase and this factor is crucial for obtaining superwetting properties as will be explained later. CeO2 nanoparticles 11

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deposits can be clearly visualized as granular structures at microporous stainless steel substrate. CeO2 nanoparticles-coated membranes possess nanoscale and microscale surface roughness, which is important for controlling surface wettability. A closeup view at high magnification (Fig.

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6f) shows good coverage of the membrane surface with the spray-coated particles and absence of voids/gaps between the particles.

FE-SEM was used to study both the surface morphology and the cross-section of the

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substrate coated with crystalline ceria nanoparticles. Fig. S2 represents a collection of crosssectional and surface images taken at different magnifications. The image of the cross-section at

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relatively lower magnification (Fig. S2a) shows the coating to be uniform and continuous with a thickness of ~ 3 µm, which is typical of films obtained by spray-deposition. The surface images show the conformal nature of the coatings as the substrate is completely covered by the ceria particles (Fig. S2b). The high magnification image of the surface reveals extensive

3.5. Surface wettability

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agglomeration of the nanoparticles to form clusters that are of different sizes (Fig. S2c).

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Goniometric measurements were performed to assess the wettability of CeO2 nanoparticlescoated membranes. Fig. 7 displays the outcomes of contact angle measurements for both the

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uncoated and coated membranes. Droplets of both fluids, water and oil, completely wet the virgin membrane, giving an angle of ~ 0° (Fig. 7a & b). From this observation it can be safely concluded that the membranes by default are superhydrophilic and superoleophilic in air. On the other hand, after the surface modification, membrane surface becomes superhydrophobic (~ 150°) but maintains its affinity for oil (0°) (Fig. 7 c & d). The contrasting behavior of the two

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fluids on the surface-modified membrane indicates its strong potential for effecting efficient separation of oil and water.

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In addition to images from the goniometer, digital pictures of oil and water droplets on the mesh surface were also taken. These reiterate the contact angle findings given above: the water droplet is almost spherical (Fig. 7 f), whereas, the model oil, hexadecane, spreads almost instantaneously

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and completely wets the coated membrane surface (Fig. 7 g). The non-wetting behavior is explained by the inherent hydrophobicity of the oxide that shows a contact angle > 100° on a

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smooth surface. When the oxide is deposited on a surface with nanoscale roughness, the low surface energy together with the surface topology, results in a Cassie state of wetting to give an angle ~ 150° (Murakami et al., 2014).

3.6. Oil-Water separation performance

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Several oil-water separation experiments were carried out to examine the performance of CeO2 nanoparticles coated membranes for separating water from an oil-water mixture. In this experiment, hexadecane was used for the oil phase. An oil-water mixture (oil – hexadecane –

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was colored red and water was colored blue with methylene dye) was poured onto the nanoparticles-coated membrane that was altered between two glass tubes and fixed with Teflon

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flange oil-water separation set-up (Fig. 8). Owing to the exceptional superhydrophobic behavior of CeO2 nanoparticles coated

meshes, water was blocked by membrane while oil with greater thickness than water (Fig. 8d) penetrated through the coated mesh rapidly. No traces of water were visually observed in the permeated oil (Fig. 8d), a clear testimony of the rigorous and thorough nature of the oil-water separation process. The entire purification procedure occurred rapidly, in a few minutes. For

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control purposes, an uncoated mesh yielded almost negligible oil-water separation (Fig. 8b), since the entire mixture (of both oil and water) flowed past the mesh to reach the collecting

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beaker below. The separation efficiency for oil and water was quantified with the aid of the following equation (Gondal et al., 2017; Shi et al., 2015): "#

"$

% × 100 (4)

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Efficiency (%) = 1 −

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Where Co and Cp represent the percentage volume (vol/vtotal) of oil in the mixture of oil and water above the membrane and the filtered permeate respectively. These parameters were quantified by measuring the volume proportion of oil present in the mixture by utilizing a graduated analytical cylinder. The CeO2 nanoparticles coated membrane exhibited excellent performance in the overall purification process, effecting an oil removal rate in excess of 99.95%

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starting from an oil-water mixture.

To additionally contemplate the separation capacity of the CeO2 nanoparticles coated

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membrane, the intrusion pressure of water flowing through the modified membrane was estimated, which showed the maximum height of water that could be supported. The intrusion

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pressure for water was estimated by pouring water onto a CeO2 nanoparticles coated membrane to accomplish the maximum height and it is given by the weight of water, subsequently, test esteems can be obtained by the equation below (Gondal et al., 2014): P = ρgh,-.

(5)

Here, ρ is the water density, g the acceleration due to gravity (9.8 m/s2), and hmax is the height of the water column at which the water just begins to permeate through the coated membrane. This

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height was determined to be ~ 12 cm. Inserting the values for the different variables, the intrusion pressure was calculated to be ~ 1.176 KPa. This infers for intrusion pressure equivalent to or not as much as this, the water cannot course through the CeO2 nanoparticles coated

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membrane and exhibiting that our CeO2 nanoparticles coated membrane has a decent stability. The water flux (F) was likewise estimated under a settled section of water and the esteem

J=

0

1

(6)

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were computed utilizing condition (Gondal et al., 2014):

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Here, V represents the water volume that was passed through the membrane (0.1 L), A represents the actual area of the membrane through which the passage occurs, and t is the time measured for the water to permeate. Using these values, the flux was calculated to be ~ 17 L m-2s-1. The following equation is used for the calculation of the intrusion pressure for the fluid flow through the membrane (Gondal et al., 2014):

3$4 "567$4

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∆P =

(7)

Where γOW is the interfacial tension between oil and water, θow is the contact angle that the

water meniscus.

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water droplet makes on the coated membrane, and R represents the radius of curvature for the

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The above observations regarding oil-water separation and intrusion pressure can be explained in the following manner. By virtue of having a lower density than water, the oil comes into contact with the membrane surface first and completely wets it (Fig. 9). The incoming water contacts the oil instead of the membrane as the latter has infiltrated the surface. The mutual contact of the oil and water in this manner results in a vertical component of the interfacial tension acting upwards, Fγ.

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Depending upon the contact angle that the incoming fluid makes with the surface, there are two distinct, yet entirely different scenarios. In the first case (θ > 90º), which is the situation for the membrane coated with crystalline ceria nanoparticles, the two major forces acting in the

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vertical direction, surface tension and fluid pressure, are acting in exactly opposite directions. Therefore, the interfacial tension will resist the passage of water through the membrane unless and until the column of water on the other side becomes high enough that FP > Fy. However, in

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the reverse case (θ < 90º), which is the situation for uncoated membranes, the force associated with surface tension acts in a direction complimentary to the intrusion force that results in a

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negative (< 0) intrusion pressure, meaning spontaneous penetration of the water through the membrane.

3.7. Photocatalytic performance of CeO2 nanoparticles coated membrane

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In order to understand the photocatalytic behavior of CeO2 nanoparticles coated membrane, we studied the photo-catalytic degradation of MB dye in water under UV illumination. The absorbance spectrum of degradation of MB dye without photocatalytic membrane and CeO2

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nanoparticles coated membrane under UV illumination are shown in Fig. 10a and 10b

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respectively. In order to make sure that the degradation of MB dye is not due to photo-induced (non photo-catalytic) effect, the decay curve in the absence of photo-catalytic membrane (only light) under the same irradiation condition was also studied. Decay curves for degradation of MB dye in the presence of uncoated membrane and CeO2 nanoparticles coated membrane without UV light irradiation (under dark condition) are shown in Fig. S3. From Fig. S3, it is clear that in the presence of uncoated membrane and CeO2 nanoparticles coated membrane, the adsorption of MB dye is 0 and 2.0 % respectively in 80 min under dark condition. It is also clear from Fig. S3

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that the photocatalytic degradation efficiency of MB dye in water in the presence of CeO2 nanoparticles coated membrane is approximately 99.96% as compare to 23.47% uncoated membrane after 80 minutes of UV irradiation. In the presence of light and CeO2 nanoparticles

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coated membrane, the photo-catalytic process is triggered and this leads to the increased degradation efficiency of MB dye.

Figure 11a shows the photo-catalytic removal of MB dye (ln(C/Co) versus time). It is very

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obvious that in the presence of the coated membrane, the photo-catalytic degradation of MB dye

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is very rapid in comparison to the uncoated membrane (only UV illumination). The rate of degradation (k) of MB and the k values for the coated and uncoated membranes were estimated by curve linearization to be 0.002 min-1 and 0.041 min-1 respectively (Sarkar et al., 2015b and Chakraborty et al. 2017). The bar diagram in Fig. 11b demonstrates the percentage photocatalytic degradation efficiency of MB in water at different irradiation time, where it is evident

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that the photocatalytic degradation efficiency of CeO2 nanoparticles coated photo-catalytic membrane is approximately 99.96% as compare to 23.47% without photo-catalytic membrane

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after 80 minutes of UV irradiation.

3.8. Mechanism for the photocatalytic degradation process

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Schematic diagram for the photocatalytic degradation process of methylene blue (MB) dye in water using CeO2 nanoparticles coated membrane is depicted in Fig. 12. During the photocatalysis, photons incident on a photocatalytic CeO2 nanoparticles trigger the production of electron-hole pairs. The electrons are concentrated in the conduction band (CB) whilst the holes are produced in the valence band (VB). The CB electrons reduce the dissolved O2 molecule whilst the VB holes oxidize the surface-adsorbed hydroxide ions to generate OH* radicals and superoxide respectively. The OH* radicals and superoxides are responsible for the oxidation of 17

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MB dye molecules. The following reactions describes the photo-excitation (equation 1), oxidation (equation 2 and 3) and reduction (equation 4 and 5) process during photo-catalysis using CeO2 nanoparticles.

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CeO + hv → CeO (h= < ; e?< ) → Photoexcitation (8)

H 2 O + hvb+ → H 2 O + → OH • + H + (9)

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OH • + OH • → H 2 O + 1 2 O2 (10)

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H + + ecb− → H • + H • → H 2 (11) O 2 +e- cb → O 2 −• (12)

4. Conclusion

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To summarize, cerium (IV) oxide nanoparticles of high purity were synthesized by a simple chemical method. The nanoparticles were then deposited on microporous membranes by a facile spray coating technique. The ceria particles displayed a high degree of crystallinity and a

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uniform particle size. Contact angle measurements on the coated membrane showed contrasting wetting characteristics for two different fluids, oil and water. The superwetting properties of the

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prepared surface resulted in efficient oil/water separation; the oil passing through rapidly, whereas, a fairly high column of water was blocked. The photocatalytic properties of CeO2 coated membrane were also utilized for the efficient degradation of a dye. In the presence of UV light, the membrane coated with nanoparticles showed rapid and near complete degradation of the organic dye. The main findings of this study showed the utility of a membrane coated with particles of a rare-earth oxide in effectively dealing with two different yet major aspects of water treatment. The work presented in this paper is highly beneficial for future studies on scaling up 18

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of this technique for commercial applications for oil water separation and degradation of organic

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pollutants using solar radiations.

Acknowledgements

The financial support by KFUPM under DSR project# DISC1505 to complete this research work

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is appreciatively acknowledged.

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Figure Captions:

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Fig. 1. Schematic sketch and illustration of the experimental procedure undertaken for the synthesis of crystalline CeO2 nanoparticles. NaOH and Ce(NH4)SO4 are mixed thoroughly that results in the precipitation of ceria particles. The resulting mixture is filtered, washed with DI water, dried and the nanoparticles annealed at 800ºC

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Fig. 2. Crystal structure and morphology of the synthesized nanoparticles (a) XRD pattern showing all the peaks associated with the Ce(IV) oxide cubic fluorite crystal structure. Note the sharpness of all the peaks indicating a high degree of crystallinity (b). FESEM image showing clusters of agglomerated nanoparticles. (c) TEM image at high magnification confirming the agglomeration at a smaller scale and giving a good idea of average particle size ~ 50 nm. (d) Selected Area Electron Diffraction (SAED) pattern of the nanoparticles. Note the brightness and good size of the spots that again is an indication of crystalline structure.

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Fig. 3. XPS spectra of a glass substrate coated with the CeO2 nanoparticles. (a) A survey scan showing the presence of major elements, Cerium, and Oxygen. (b) high-resolution scan for the Ce peaks with fitting showing all the peaks associated with an oxidation state of 4+. The peaks with binding energy below 900 eV are attributed to a spin-orbital of 5/2 while the ones above 900 eV emanate from the 3/2 orbital. (c) High-resolution spectra for Oxygen with curve fitting showing the major peak at 529 eV that corresponds to lattice oxygen. Fig. 4. The UV-Vis absorbance characteristics of the synthesized nanoparticles (a) UV-DRS spectra of the crystalline CeO2 nanoparticles (b) Tauc plot for the particles showing bandgap of CeO2 around 3.3 eV. Fig. 5. Schematic sketch and illustration of the spray deposition of the synthesized CeO2 nanoparticles on the stainless steel membrane. A stable suspension of the particles obtained by stirring in an organic solvent, THF, is sprayed on the membrane with pressurized gas to obtain a conformal coating. Fig. 6. SEM images of the membranes at different magnifications showing the surface morphology (a-c) virgin membrane prior to deposition (d-f) the surface-modified after deposition 23

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of ceria nanoparticles. Note the increase in apparent surface roughness after the deposition that contributes to superhydrophobic behavior. Also, the absence of voids between the deposited particles on one of the membrane fibers in the magnified image (f).

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Fig. 7. Contact angle images of oil droplet in air (a) and water droplet in air (b) on uncoated membrane surface. Contact angle images of oil droplet in air (c) and water droplet in air (d) on CeO2 nanoparticles coated membrane surface. Digital photographs of uncoated membrane surface showing water is spreading (e) and oil is spreading (f). Digital photographs of CeO2 nanoparticles coated membrane surface showing oil is spreading (g) and Water droplet is not spreading, and is superhydrophobic (f).

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Fig. 8. Laboratory-made set-up for oil-water separation test. (a & b) Digital photograph of separation of oil and water mixture using an uncoated membrane and (c & d) photograph of separation of oil and water mixture using CeO2 nanoparticles coated membrane. Water is dyed blue (methylene blue dye) while hexadecane is dyed red. In Fig. b, oil and water mixture totally go through the uncoated membrane giving no separation at all. In Fig. d, Oil permeates through the CeO2 nanoparticles coated membrane while water is retained.

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Fig. 9. Schematic sketch illustration of the analytical model used to explain the observed superwetting behavior and oil-water separation efficiency (a) The coated membrane is completely wetted by oil and lets it permeate rapidly; however, water does not wet the surface at all and is blocked from passing through (b) the wetting behavior of water (θ > 90º) and the subsequent intrusion pressure that acts against the pressure due to column of water above the membrane surface (c) the wetting behavior of oil (θ < 90º) and the subsequent intrusion pressure that acts in the same direction and allows free passage.

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Fig. 10. Absorption spectra taken at a regular time interval in the process of photo-catalytic degradation of MB dye (a) without photocatalytic membrane and (b) CeO2 nanoparticles coated membrane under UV radiation.

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Fig. 11. (a) Plots of ln(C/Co) vs. irradiation time for the photo-catalytic degradation of MB dye without photocatalytic membrane and CeO2 nanoparticles coated membrane under UV radiation and inset of Fig. a is showing decay curves for photocatalytic degradation of MB dye without photocatalytic membrane and CeO2 nanoparticles coated membrane under UV radiation. (b) Percentage of photocatalytic degradation of MB dye without photocatalytic membrane and CeO2 nanoparticles coated membrane under UV light irradiation. Fig. 12. Schematic sketch for the photocatalytic degradation process of Methylene Blue (MB) dye using CeO2 nanoparticles coated membrane.

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Research Highlights – Facile fabrication of superhydrophobic/superoleophilic and photocatalytic membranes

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Applied for oil-water separation and photocatalytic degradation of dyes

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High efficiency of oil-water separation achieved by nanoparticles coated membrane

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Excellent photocatalytic activity of membrane for removal of toxic organic dyes

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