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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Expanded Graphite-Polyurethane Foams for Water−Oil Filtration Lía Vásquez,§,‡ Laura Campagnolo,§,‡ Athanassia Athanassiou,§ and Despina Fragouli*,§ §

Smart Materials, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy Dipartimento di Chimica e Chimica Industriale (DCCI), Università degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Italy

‡

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S Supporting Information *

ABSTRACT: Herein, expanded graphite is successfully combined with waterborne polyurethane to develop porous foams with underwater oleophobic properties for the separation of surfactant-free, oil-in-water mixtures and emulsions. To obtain foams with different pore sizes and therefore with different performances in the oil−water filtration process, two solvent-free fabrication processes are adopted. In the first one, the expanded graphite granules are mixed with the waterborne polyurethane (PUEGr), and in the second method, calcium carbonate is introduced to the two-component mixture (PUEGr_t). In both cases, the obtained foams exhibit hydrophilicity and oleophilicity in air and oleophobicity underwater, and they have porous interconnected networks, while their pore size distribution differs significantly. The foams can be used as 3D filters, able to separate, through gravity, surfactant-free, oil-in-water mixtures (10% w/w oil in water) with high oil rejection efficiencies and flow rates that depend on the type of foam. In particular, in the gravitydriven filtration process using 100 mL of the feed liquid, the PUEGr foams have an oil rejection efficiency of 96.85% and flow rate of 9988 L m−2 h−1, while for the PUEGr_t foams the efficiency is higher (99.99%) and the flow rate is lower (8547 L m−2 h−1) due to their smaller pore size. Although the PUEGr_t foams have slower separation performance, they are more efficient for the separation of surfactant-free emulsions (1% w/w oil in water) reaching an oil rejection efficiency of 98.28%, higher than the 95.66% of the PUEGr foams of the same thickness. The foams can be used for several filtration cycles, as well as in harsh conditions without deteriorating their performance. The nature of raw materials, the simple solvent-free preparation method, the effective gravity-driven filtration even in harsh conditions, and their reusability suggest that the herein engineered foams have great potential for practical applications in oil−water separation through highly energy-efficient filtration. KEYWORDS: underwater oleophobicity, porous materials, waterborne polyurethane, water remediation, oil rejection efficiency

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limited applicability in the filtration of oil−water mixtures and emulsions. In fact, such materials are unsuitable for continuous gravity-driven filtration processes, as a water barrier layer usually naturally settles below oil due to its higher density, preventing the oil permeation and therefore blocking the filtration. Furthermore, during demulsification, such porous materials can be easily fouled by oil. On the contrary, because of the superhydrophilic and superoleophobic, or underwater superoleophobic, character of the water-removing materials the water permeates selectively the porous materials, protecting them from the oil fouling.2 Such types of materials inspired by the underwater oil repellency of fish scales can trap abundant water on their rough surface. The trapped water greatly weakens the direct contact between oily substances and the superhydrophilic surface of the material, resulting in low oil-adhesion superoleophobic surfaces. Many research works are focused on the fabrication of water-removing porous materials through the combination of their surface chemistry and porosity. In particular, appropriate inorganic and organic compounds have been used, such as

INTRODUCTION With the fast expansion of the earth’s population and the industrialization, the sources of oily wastewater have been significantly increased, creating serious pollution problems and a great impact on the environment. As a result, the necessity to clean oil-polluted wastewaters is an emerging issue that has led to the development of a wide range of remediation technologies based on biological, chemical, and physical methods.1,2 Among them, chemical oxidation, in situ burning, electrochemical treatment, and bioremediation exhibit some disadvantages, such as secondary pollutant generation,3−5 limitations in space, and high energy consumption.6 On the other hand, physical separation technologies, like absorption or filtration, are more effective, cheaper, scalable, and have a minimal environmental impact.7 Owing to these promising characteristics, numerous studies have been performed on the development of low-cost porous materials able to separate effectively water from oil with filtration or absorption processes.7−10 Based on their preferred interactions toward water or oil, such porous materials can be classified as either “oil-removing” or “water-removing” systems.11 An oil-removing material is a superhydrophobic and superoleophilic material suitable for the effective selective separation of dispersed oil on the surface of water but with © XXXX American Chemical Society

Received: May 6, 2019 Accepted: July 25, 2019

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DOI: 10.1021/acsami.9b07907 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces nanomaterials12,13 and polymers,10,14−16 in order to post-treat commercially available meshes, textiles, membranes, and foams17−24 or to directly form porous systems with the appropriate properties.25−28 Among them, carbon-based materials such as graphene,15 graphene oxide,29 and carbon nanotubes30 have attracted great research interest for the fabrication of engineered filtration systems able to carry out effective oil−water separation due to their porous structure, high surface area, and chemical stability.31,32 Usually for such a scope, thin 2D membranes of carbon-based materials have been formed following assembling methods, used as is or as components of filtration cells.33 Despite the impressive performance of some of these materials, most of the fabrication processes followed often require multistep, sophisticated, and time- and energy-consuming procedures, restricting, thus, their scalability and their further wide application.27,34−36 Hence, it is of great importance to develop functional materials with stable underwater superoleophobicity for gravity-driven oil− water filtration in a simple, economical, and scalable approach. On the top, the performance of these 2D membranes in gravity-driven filtration processes is often declining due to the membranes’ small pore sizes and short permeation channels, which are easily contaminated during long-term operation due to surfactant adsorption and pore blocking. A valuable alternative to achieve efficient separation performance is the development of 3D porous structures with controlled pore sizes and interconnected permeation paths.36 However, nowadays, 3D functional materials are mainly used as absorbents for oil−water separation,9,11,37,38 while their application in filtration processes is still explored. The combination of carbon-based materials with polymers in order to form flexible 3D filters that efficiently separate oil− water mixtures/emulsions through gravity could be a valuable alternative to the current filtration processes. Such filters could have extended use in water remediation since the carbon-based materials are proven to work efficiently not only in oil−water separation but also in the removal of other pollutants from water through adsorption processes.39−41 Herein, we report an approach to fabricate hydrophilic and underwater oleophobic foams with highly interconnected pores of controlled size for the separation of oil−water mixtures and surfactant-free emulsions through gravity-driven filtration, following a solvent-free straightforward fabrication process. One of the two components of the foams is the expanded graphite (EGr) in the form of granules, a lightweight highly porous cost-effective carbon-based material42−44 previously used as a sorbent for oil−water separation32,45,46 but still not explored for filtration processes. The EGr granules are combined with waterborne polyurethane (WPU) through a simple mixing process, and after the water evaporation, foams with interconnected pores and highly accessible surface area are formed. The designed foams have hydrophilic and underwater oleophobic properties, key factors in the effective gravity-driven oil−water mixture and emulsion separation.36 The premise of our design is to exploit the porosity and high surface area of the EGr to generate pore sites, with surface wettability modified by the WPU, which partially coats and binds together the EGr granules. With the aim to study how the morphology, pore size, and porosity of the EGr-based foams influence the wetting behavior and the oil−water separation process, two types of foams were fabricated. The first one was fabricated by mixing WPU and EGr, while in order to tune the pore size and further increase the porosity,

the calcium carbonate salt was introduced in the formulation of the second foam as a pore size regulator. The developed foams present hydrophilic properties and tunable pore size, which promote the rapid gravity-driven filtration of surfactant-free oil−water mixtures and emulsions while maintaining high separation efficiency above 98%. The herein presented methodology for the fabrication of foams, using low-cost materials and water-based processes, which separate efficiently oil−water mixtures and emulsions through gravity-driven filtration, is a highly energy-efficient green alternative to the so far reported systems, easily scalable, and widely applicable.

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EXPERIMENTAL SECTION

Materials. The expanded graphite (EGr) was supplied by Directa Plus. Waterborne polyurethane (Ico-thane BF 10) was purchased from I-Coats. Calcium carbonate (CaCO3), analytical-grade hydrochloric acid (HCl, 37%), sea salts, and sodium hydroxide (NaOH) were purchased from Sigma Aldrich. Diesel oil (SAE 15 W-40, dynamic viscosity = 287.23 mPa s, surface tension γoil = 31 mN m−2, and ρoil = 0.8787 g cm−3 at room temperature (RT))11 was purchased from a local market and was employed for the wettability characterization and filtration experiments. Silicone oil and toluene were purchased from Sigma Aldrich. Soybean oil (Bunge) was purchased from a local market, and vacuum pump oil (Elmo Rietschle) was purchased from Gardner Denver. All materials were used as received without further purification. Preparation of the Foams. Two types of EGr-based foams were fabricated by manually mixing the EGr granules and the WPU. The WPU dispersion was composed of 35% w/w solid and 65% w/w water, and all further weight ratios were calculated according to the weight of the solid component. PUEGr foams were prepared with a WPU/EGr w/w ratio of 2:1. The homogeneous mixture was poured into a Teflon Petri dish and subsequently placed in a convection oven at 80 °C for 2 h in order to accelerate the water evaporation. PUEGr_t foams were prepared by mixing WPU, EGr, and CaCO3 in a w/w ratio of 3.5:1:24, respectively. The mixture was placed in a Teflon Petri dish and subsequently in a convection oven at 80 °C for 4 h. Because of the presence of CaCO3 that decelerates the drying process, extra drying steps were necessary, and therefore, the samples were subsequently placed in a vacuum oven at 70 °C for 4 h in order to ensure the total evaporation of water. Then they were dipped in a 1 M HCl solution overnight to remove the salt content. Finally, the PUEGr_t foams were rinsed with Milli-Q water until the water reached neutral pH. The cleaned foams were then dried in a vacuum chamber at RT overnight. Characterization. Morphological and Structural Characterization. The morphology of the foams was analyzed using scanning electron microscopy (SEM) performed with a JEOL JSM-6490LA microscope. The images were acquired in high vacuum and backscattered electron imaging mode with an acceleration voltage of 10 kV. Before imaging, samples were coated with a 10 nm gold layer using a high-resolution sputter coater (Cressington 208 HR). To measure the droplet size of the oil-in-water mixtures and emulsions, few drops of the dispersions were cast on glass slides, and images were acquired using a Nikon Eclipse 80i optical microscope equipped with NIS Elements F software. Chemical Characterization. In order to investigate the possible interactions between the WPU and the EGr, Fourier transform infrared attenuated total reflectance (FTIR-ATR) measurements were carried out using a Vertex 80 Bruker FTIR instrument equipped with an ATR unit (zinc selenide (ZnSe) crystal). 64 scans were collected for each spectrum ranging from 4000 to 600 cm−1 with a scan resolution of 4 cm−1. X-ray diffraction analysis (XRD) was performed using a PANalytical Empyrean X-ray diffractometer with a 1.8 kW Cu Kα ceramic X-ray tube (λ = 1.5418 Å) and PIXcel3D area detector (2 × 2 mm2) operating at 45 kV and 40 mA to evaluate the crystallinity of the prepared foams. The diffractograms were recorded in the 2θ range of 10 to 60°, with a step time of 185.4 s and step size of 0.026°. B

DOI: 10.1021/acsami.9b07907 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces In order to investigate the interaction between the WPU and the EGr, X-ray photoelectron spectroscopy (XPS) was performed using an electron spectrometer (Lab2, Specs) equipped with a monochromatic X-ray source (set at 1253 eV) and with a hemispherical energy analyzer (Phoibos, HSA3500, Specs). CasaXPS software was used for the experimental data analysis. Structural Characterization. The pore size distribution of the foams and their specific surface area were determined by both mercury intrusion porosimetry (MIP) and nitrogen adsorption− desorption isotherms. Nitrogen adsorption−desorption isotherms were performed by an Autosorb-iQ gas sorption analyzer (Quantachrome Instruments). The specific surface area and the pore size distribution were determined by employing the Brunauer− Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) techniques. MIP was performed by Pascal 140 Evo and Pascal 240 Evo mercury porosimeters (Thermo Fisher Scientific). The measurements were conducted in two stages. The first stage was the low-pressure stage with a pressure running from 0 to 0.14 MPa. In the second stage, the high-pressure stage, the pressure range was set from 0.14 to 200 MPa. The pore diameter at each pressure step was analyzed using the SOL.I.D Evo Software by the Washburn equation. It was set at a mercury contact angle of 140°, a mercury surface tension of 0.48 N m−1, and a cylindrical and plate surface area model. For this analysis, the presence of open pores in the foams was considered, not the pores opened by the mercury intrusion process, in a pore size range of 0.01−110.00 μm. Water/Oil Permeation. The maximum absorption capacity was determined by immersing the foams into water or oil until their complete saturation. In particular, the foams were weighed (Wi) and placed in a beaker with 10 mL of Milli-Q water or oil at RT. Subsequently, the weight of the saturated foams (Wf) was measured after their removal from the liquid and draining on tissue paper in order to remove the excess liquid. The absorption capacity in each case was calculated according to eq 1. Wwater/oil Wfoam

ij W − Wi yz zz = jjj f j Wi zz k {

J = (V /A)t

(2)

Here, V is the volume of the permeate liquid at time t and A is the effective filtration area of the foam.47,48 A was estimated by multiplying the surface area of the foams with their respective porosity. The porosity in each case was calculated as described in the Supporting Information. After filtration, the content of oil in the permeate liquid was determined using a Varian CARY 300 Scan UV− visible spectrometer. Detailed information of the analysis is presented in the Supporting Information and in Figure S1. The oil rejection efficiency was calculated using the following equation % oil rejection = (Cf − C p)/Cf × 100

(3)

where Cf and Cp are the contents of oil in the feed and permeate solution, respectively.49−51 The chemical oxygen demand (COD) of the permeate liquids was determined using a COD analyzer (QuickCOD-02E1717, LAR Process Analysers). The permeate liquids were placed in a vortex mixer (500 rpm for 1 min), and a 150 μL portion in each case was injected in the analyzer. At least three measurements were performed for each sample. Stability Tests. The abrasion tests were performed using a discpolishing machine (Labpol Duo 8, EXTEC). First, the samples were placed at a radius of 3.2 cm from the center of the sandpaper disc (grade 240), and on their surface was applied a weight of 150 g. The disc was allowed to rotate while the sample was held fixed. Every disc rotation represents 20 cm of linear distance and was defined as one abrasion cycle. After every five cycles, the sample was blown with N2, and filtration tests were performed. Additionally, SEM and oleophobicity underwater analysis of the samples after 25 abrasion cycles were performed. For the test of the stability in corrosive environments, the saline solution was prepared using 35% w/w sea salts dissolved in Milli-Q water. It should be mentioned that, unless it is specified, all experiments of permeation, filtration, and stability evaluation were performed with diesel oil (named oil).

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RESULTS AND DISCUSSION Development of the Porous Structures and Physicochemical Characterization. As shown in Figure 1a,b, the

(1)

In order to study the wettability of the foams and their permeation performance, filtrations of pure water or oil using dry or wet PUEGr and PUEGr_t foams were carried out. The dry foams (∼7 mm thick) were fixed between two rubber o-rings with a diameter of 26 mm inside the filtration setup suspended over a graduated tube placed on a scale and a weight balance. 10 mL of Milli-Q water or diesel oil was poured on top of the foams, and the volume and weight of the permeate liquids over time were registered. The underwater oil contact angle was determined using a DataPhysics OCA 20 contact angle goniometer. A glass cuvette was filled with 5 mL of Milli-Q water, the samples were placed slightly below the surface of the water, after being fixed on a glass support. 10 to 50 μL of oil droplets was then dispensed inside the water and deposited on the surface of the sample using an upward needle. Underwater roll-off angles were determined by tilting the stage at a rate of 1° s−1. Filtration Process. The oil rejection efficiency of surfactant-free mixtures and emulsions was studied. The oil-in-water mixture was prepared by mixing oil and water in a 10% w/w using a vortex mixer (Heidolph Multi Reax, Germany) at 500 rpm for 5 min. The oil-inwater emulsion was prepared by mixing oil and water and in 1% w/w and sonicating with an ultrasonic probe at 40% amplitude for 30 s on/ off cycles for 20 min, until a white milky liquid was obtained. The emulsion was cooled down at RT and was stable for 15 min, and after this period, a yellowish thin layer could be noticed on the top of the white solution. The foams were previously wetted with water before placing them into the filtration system. Different volumes of the prepared oil-in-water mixtures were poured onto the foams for the gravity-driven filtration, and the volume and the weight of the permeate liquid over time were registered. The flow rate (J) during filtration and permeation tests was derived from the slopes of the linear fit of the permeate volume versus filtration time (V(t) vs t) graphs, according to eq 2.

Figure 1. Photographs of the (a) PUEGr and (b) PUEGr_t foams.

formed PUEGr and PUEGr_t structures are compact solid porous materials. Their size and shape depend on the mold in which the WPU/EGr mixture is cast, revealing the great number of choices for the macroscopic morphological features of the foams. The SEM imaging of the PUEGr foam (Figure 2a) reveals the presence of macroscopic pores characteristic of the wrapping spaces52 formed by the permanent stacking of the EGr granules on each other due to the presence of the WPU, with an average size of 218 ± 77 μm (pore size distribution analysis, inset of Figure 2a). Indeed, as shown in Figure S2, the EGr granules have morphology similar to the PUEGr foam reinforcing the concept that the WPU is used as “glue” in order to stack the granules to each other. A closer look to the highermagnification SEM image (Figure 2b) reveals a second type of pore formed by the packed layers of the EGr granules (around 6 μm), with plate-like morphology similar to the pure EGr, proving once again that the WPU coating does not affect significantly the original structure of the EGr and, therefore, C

DOI: 10.1021/acsami.9b07907 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Complementary information obtained by combining nitrogen physisorption and MIP measurements on the two types of foams is essential for the quantification of the specific surface area and the more precise pore size distribution study in each case.54,55 With the MIP technique, the macropore structures can be analyzed (under 110 μm), while this method is not suitable for the characterization of pore sizes below 0.01 μm. Therefore, for smaller pores, the nitrogen physisorption technique is adopted in order to investigate the structural characteristics of the pores in the microscopic (50 nm), the MIP method was adopted. The obtained mercury intrusion and extrusion curves for both foams (Figure S4, Supporting Information) confirm the presence of an interconnected porous network since a large volume of mercury is trapped in the foams after the extrusion.57 Figure 3c,d depicts the cumulative pore volume of the mercury

Figure 2. (a,b) SEM images of the PUEGr foam. (c,d) SEM images of the PUEGr_t foam. The green cycles in panels (a) and (c) indicate some of the wrapping spaces. Insets: size distribution of the wrapping spaces and magnified morphologies of the corresponding foams. The red arrows in panel (b) indicate the packed layers of the EGr granules.

also its characteristics, like the high specific surface area.53 With the aim to affect the macroscopic pore size without reducing the porosity of the foams, the CaCO3 salt was introduced to the WPU and EGr mixture, while a higher amount of WPU was used. After the solidification of the foams and the salt removal, the PUEGr_t foam has an architecture of interconnected open pores of two types (Figure 2c,d). The first pore type is formed by the stacking of the EGr granules with each other due to the presence of the WPU modified by the salt template. As evidenced in Figure 2c, the pores of the PUEGr_t foam have a mean size of 69 ± 26 μm (pore size distribution analysis, inset of Figure 2c), smaller than the one of the PUEGr foams due to the presence of a higher amount of polymer ligand. The second type of pore is attributed to the presence of the salt-template-modified polymer layer with smaller pores of size 99.48%) (Figure 7b). Moreover, when an oil−saline water mixture was used as feed solution, both foams can effectively separate the mixture with oil rejection efficiencies of 99.94% for the PUEGr and 99.97% for the PUEGr_t foam. To further prove the stability of the foams in corrosive environments, the underwater oleophobicity is investigated after immersing the foams in aqueous solutions of pH 1, pH 9, and in saline water for 1 h. As shown in Figure 7c,d, both foams maintain their high underwater oil contact angles (over 134° for the PUEGr foam and over 141° for the PUEGr_t foam) and low sliding angles, demonstrating that the underwater oleophobic properties of the foams are not altered during their exposure to corrosive or saline conditions. Emulsion Separation. In order to study the performance of the filtration process for oil−water mixtures with smaller oil droplets size, a 10 mL surfactant-free emulsion of 1% w/w oil in water was prepared by ultrasonication. The size of the oil droplets has a mean value of 1.1 ± 0.7 μm (Figure S13, Supporting Information), and the emulsion is stable for 15 min. As can be seen in Figure 8a,b, the filtration process using the PUEGr water-wet foam is faster than that using the waterwet PUEGr_t, reaching flow rates of 17141 and 595 L m−2 h−1, respectively. Nonetheless, as shown in Figure 8c, the oil rejection efficiency is 70.84% when the PUEGr foam is used, while it increases to 93.80% for the PUEGr_t foam. The higher oil rejection efficiency and the smaller flow rate of the PUEGr_t can be attributed to its smaller pores compared to the PUEGr foam, which block and repel the oil droplets of similar size when in contact with the surface of the foam, but also, possibly, to the different tortuous paths that the feed liquid follows in the two types of foams. When the thickness of the foams is increased from 7 to 21 mm, the flow rates decrease (13409 L m−2 h−1 for PUEGr; 57 L m−2 h−1 for PUEGr_t), but the one of the foams with the bigger pores (PUEGr) is always higher than the PUEGr_t foams. Most importantly, the increase of the thickness of the foams improves significantly the oil rejection efficiency in both cases, reaching 95.66 and 98.28% for the PUEGr and PUEGr_t foams, respectively. Indeed, the use of a thicker foam increases the rough surface area (introduced by the small pores) and the

has smaller pores than the PUEGr, with a high amount in the range of 50−100 μm proving that these types of foams can efficiently separate by gravity-driven filtration several volumes of oil−water mixtures. The developed foams can be reused several times after cleaning as demonstrated in Figure 6e,d. For each cycle, 10 mL of 10% w/w oil-in-water mixture was filtered. After each cycle, the foams were washed with water before the next filtration. As shown, even after 20 cycles of filtration, for both types of foams, the oil rejection efficiencies were between 99.82 and 99.92% for the PUEGr foam and higher than 99.97% for the PUEGr_t foam, while the COD concentrations are around 200 mg L−1 in both cases. The obtained results prove that the foams can be reused effectively for several cycles without affecting their oil/water separation efficiency as well as their high flow rates (Figure S10, Supporting Information). Material Stability. In order to test the mechanical durability and chemical stability of the foams, the filtration of oil−water mixtures in harsh conditions was investigated, including abrasion and corrosion tests. The scheme of the abrasion test is shown in the inset of Figure 7a, and the

Figure 7. (a) Oil rejection efficiency variation during abrasion cycles of the PUEGr and PUEGr_t foams, determined upon filtration of 10 mL of an oil water mixture (10% w/w). Insets: scheme of the system used to perform the abrasion cycles. Video S3 is provided in the Supporting Information. (b) Oil rejection efficiency of both types of foams under corrosive and saline conditions (feed liquid 10 mL of 10% w/w oil-in-water mixture (depending on the case, water: pH 1, pH 9, and simulated seawater). Underwater oil contact angles and sliding angles after immersion of the (c) PUEGr and (d) PUEGr_t foams in corrosive and saline liquid environments.

experimental details of the abrasion cycles are described in the Experimental Section. Figure 7a shows that, for 25 abrasion cycles, the oil rejection efficiency of the PUEGr_t foam remains almost constant and higher than 99.80%, while for the PUEGr foam, the oil rejection efficiency gradually decreases, reaching 94.63% at the 25th abrasion cycle. Although after 25 cycles of abrasion some parts of the surface of the foams are flattened, their internal configuration did not change as clearly proven by the SEM analysis presented in Figure S11.68 Compared to the PUEGr_t foam, the PUEGr’s surface is less rough after the abrasion, and this affects the underwater oleophobicity and thus the oil rejection efficiency of the foam.69 The underwater oil contact angle of the PUEGr foam H

DOI: 10.1021/acsami.9b07907 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

h−1),71 while the separation efficiency is in the same range71 or even higher than other72 reported materials. This in combination with the fact that our foams can be used in harsh and saline conditions and for diverse filtration cycles and that they are prepared following a simple and solvent-free method using cost-effective materials, while the separation principle is based solely on gravity, makes them valuable candidates for water−oil separation processes through filtration using 3D porous materials.

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CONCLUSIONS In summary, we demonstrate that, by simply mixing WPU with EGr granules, foams with hierarchical architectures and underwater oleophobic pore sites can be formed. The prepared foams are two versatile filters that can be used for different filtration processes, even under harsh conditions, and can effectively separate oil−water mixtures with high flow rates driven only by gravity, showing comparable performance with other filtration materials. Such foams combine an alternative practical and simple solvent-free method of preparation, with an energy free filtration process, which make them promising candidates for oil−water filtration applications.

Figure 8. (a,b) Permeate liquid after filtration of 10 mL of surfactantfree 1% w/w oil-in-water emulsion using PUEGr and PUEGr_t foams of 7 and 21 mm thickness. (c) Oil rejection efficiency of the filtration of 1% w/w oil-in-water emulsion for the PUEGr and PUEGr foams of 7 and 21 mm thickness. (d) Oil rejection efficiency of the PUEGr_t foam 21 mm thick of the permeate liquid after the filtration of 10 mL of surfactant-free 1% w/w oil in water for several filtration cycles. After each filtration cycle, the foam was cleaned with water in order to continue with the next filtration.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07907.

path that the liquid should pass in order to permeate the system (tortuosity), increasing thus the possibility for the smaller oil droplets to coalesce forming bigger droplets, which are subsequently repelled by the water−solid interface of the foams. Also, in this case, the PUEGr_t foam is more efficient but slower for the separation of surfactant-free emulsions compared to the PUEGr, revealing once again the possibility to choose the fabrication process and therefore the appropriate foam structure according to the needs of the filtration and the characteristics of the feed liquid. Since the thicker PUEGr_t foam (21 mm) can separate efficiently the surfactant-free emulsion, the reusability is explored after performing succeeding filtrations using 10 mL of 1% w/w oil-in-water emulsions. As shown in Figure 8d, in the first three filtration cycles, the foam has oil rejection efficiency of values between 97.60 and 96.92%. In the next two filtration cycles, the efficiency decreases, reaching 92.24% after the 5th cycle. These results demonstrate that the PUEGr_t foam can be reused at least three times in order to filter efficiently nonstabilized oil-in-water emulsions. Therefore, the functionality of the foam 3D structure can be tuned and used according to the needs of the filtration. In particular, the PUEGr foam can be used for fast filtrations with an oil rejection efficiency above 97%, while the PUEGr_t foam can be used for more efficient but slower filtration processes. The gravity-driven filtration performance, using oil/water surfactant-free mixtures as feed liquids, of the herein presented foams is comparable or even better than other previously developed materials as shown in Table S1 in the Supporting Information. In particular, although in some cases the flow rates of our EGr-based foams are slower than those of other developed 2D membrane filters that can reach even 25200 L m−2 h−1,70 in the case of 3D filters, our developed materials have a superior performance. In particular, the herein presented foams have the highest flow rates (>500 L m−2

Determination of porosity; oil content with UV−vis spectroscopy; SEM images of the EGr granules; nitrogen adsorption−desorption isotherms; mercury intrusion and extrusion curves; XPS survey spectrum of the WPU; wettability characterization; digital images of the oil permeation test; underwater oil droplet deposition; oil−water interfacial tension; digital images of the mixture filtration; flow rate vs hydrostatic pressure curve; hydrostatic pressure definition; flow rate dependence on the filtration cycles; SEM images of the foams after 25 abrasion cycles and underwater oleophobicity; stability test of the WPU under aqueous solutions of different pH values; microscope image and oil droplet size distribution in the surfactant free emulsion; sequence of the filtration of the emulsion using the PUEGr and the PUEGr_t foam (PDF) Separation process of an oil−water mixture using the PUEGr foam (AVI) Separation process of an oil−water mixture using PUEGr_t foam (AVI) Abrasion test of the foams (AVI) Hydrophilicity of a dry PUEGr_t foam (AVI)

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Hydrophilicity of a wet PUEG_t foam (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Despina Fragouli: 0000-0002-2492-5134 Notes

The authors declare no competing financial interest. I

DOI: 10.1021/acsami.9b07907 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS The authors acknowledge Directa Plus (Lomazzo, Italy) for the donation of the EGr, Dr. Alexander Davis for his help with the abrasion test, Riccardo Carzino for his support to the XPS analysis, Giorgio Mancini for the mercury intrusion measurements, and Gabriele La Rosa for the nitrogen physisorption measurements.

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ABBREVIATIONS EGr, expanded graphite WPU, waterborne polyurethane J, flow rate Phydr, hydrostatic pressure Pbreakthrough, breakthrough pressure

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DOI: 10.1021/acsami.9b07907 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX