A versatile and cost-effective reduced graphene oxide-crosslinked polyurethane sponge for highly effective wastewater treatment

Abstract

Gradually deteriorating water quality caused by oil or chemical leakage and heavy metal wastewater discharge is becoming a global problem. To address this issue, a versatile absorbent material was designed and fabricated by coating reduced graphene oxide (rGO) on porous polyurethane (PU) sponges. Firstly, calcium carbonate nanoparticles (nano-CaCO₃) and graphene oxide (GO) were added during the preparation of the PU sponge. A PU sponge with nanosized porous framework was obtained after etching the nano-CaCO₃. After the reducing process, rGO was precisely coated on the prepared porous PU via crosslinking to obtain porous rGO-based PU sponge (porous PU@rGO). The physicochemical properties of the as-fabricated porous PU@rGO indicated that it was recyclable, hydrophobic and conductive. Batch experiments were carried out, and results suggested that porous PU@rGO has a widespread potential for applications in oil–water separation, the break-up of oil-in-water emulsions, as well as hazardous ion adsorption.

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Introduction

Water is the most important resource and an essential component for living beings on earth. However, our water resources are deteriorating continuously because of exponential population growth, industrialization, agricultural activities and so on. Thousands of organic and inorganic contaminants have been reported with adverse effects on human health and the environment.^1–4 Therefore, the fabrication of appropriate materials for treating wastewaters is necessary to reduce the harmful factors.^5–7

As a two-dimensional sheet of covalently bonded carbon atoms, graphene has attracted tremendous attention in oil–water separation and hazardous ion removal from wastewater owing to its excellent mechanical flexibility, extremely high electrical conductivity and large specific surface area.^8–15 Recently, graphene foam has been successfully fabricated by different methods for water remediation. However, relatively low mechanical performance is usually shown by these graphene foams, which largely restrict its further application.^8,16 Therefore, a new type of foam with robust physical properties is urgently required.

PU sponge, a commercially available three-dimensional (3D) material, has become a conventional adsorbent aimed at wastewater treatment due to its porous structure, low density and good elasticity, as well as its low price.^17–22 However, most commercial PU sponges exhibit hydrophilicity or poor hydrophobicity,²¹ which limits their practical application for wastewater treatment, especially for the removal of oils and organic solvents from water. Hence, surface modification is required to change the wettability of PU sponges from hydrophilic or poorly hydrophobic to superhydrophobic. Recently, Kuo and coworkers employed a commercially available melamine sponge for graphene coating.¹⁸ Jiang and coworkers fabricated PU-based graphene foams for oil–water separation.¹¹ These methods greatly enhanced the mechanical properties and cycle performance of the foams. However, the graphene sheets were attached around the skeletons in the foams via π–π stacking, and the weak force of the graphene sheets might lead to unstable coating. Thus, some other effective methods should be developed to enhance stability.

Herein, porous PU sponge crosslinked with rGO was fabricated via a unique fabrication procedure. Firstly, nano-CaCO₃ and GO were mixed well with the raw materials before the PU foaming process. After that, nano-CaCO₃ was etched in a mildly acidic solution, and a PU framework with porous nanostructure was obtained. The sponge was then coated with a thin rGO layer via a crosslinking method to obtain rGO-coated porous PU sponge (Fig. 1). The crosslinking of rGO not only makes the porous PU@rGO hydrophobic but also increases its electrical conductivity. Therefore, the porous PU@rGO was applied to a combination of oil–water separation and electrolytic decomposition. Compared with other previously reported materials, our PU@rGO materials exhibited multifunctional properties in wastewater treatment, including oil–water separation, breaking up oil-in-water emulsion via electrochemical method, as well as hazardous ion absorption. Moreover, the porous structure and the coating of rGO endow the sponge a large surface area and rich surface functional groups, which could enhance the adsorption capacity, suggesting that the porous PU@rGO could be a potential material for hazardous ion adsorption from water.

Fig. 1 Schematic depiction of the fabrication of porous PU@rGO.

Experimental section

Materials

Polyether polyols (NJ-330) and diphenyl-methane-diisocyanate (MDI, suprasec 5005) were purchased from Huntsman. 1,4-diazabicyclo[2.2.2]octane, stannous octoate, polydimethylsiloxane, nano-calcium carbonate, and graphite flakes were obtained from Sinopharm Chemical Reagent Co., Ltd. Potassium persulfate, phosphorus pentoxide and potassium permanganate were purchased from Sinopharm Chemical Reagent Co., Ltd. The commercial lubricant oil and soluble oil were provided by Royal Dutch/Shell Group of Companies.

Fabrication of porous PU@rGO

The overall preparation procedures of the porous PU@rGO are illustrated in Fig. 1. First, the raw materials of commercial PU foam were mixed with the other two components (nano-CaCO₃ and GO). Then, the mixture was immersed in the acid solution to remove nano-CaCO₃; thus a skeleton with macropores was obtained. Finally, the porous PU was immersed in GO solution so that the GO sheets would crosslink with GO in the skeletons during the reducing procedure, leading to the assembly of rGO around the skeletons of the PU sponge.

Preparation of GO

GO was synthesized from purified natural graphite according to published procedures.^13,23 Briefly, graphite (3 g) was stirred with concentrated H₂SO₄ (12 mL), then potassium persulfate (2.5 g) and phosphorus pentoxide (2.5 g) were added. The solution was kept at 80 °C and stirred for 4.5 h. The mixture was cooled to room temperature, and then 0.5 L water was added to dilute the solution. The solution was filtered, and the residue was dried in vacuum. The dry residue was put into a flask with concentrated H₂SO₄ (120 mL), and the mixture was stirred continuously at a temperature below 20 °C. Then, potassium permanganate (15 g) was added. Next, the temperature was increased to 30 °C, and 0.7 L water was added to the mixture. Finally, 30% H₂O₂ was added until the color of the mixture changed to brilliant yellow. The mixture was filtered and washed with 5% aqueous HCl to remove metal ions and then washed with distilled water to remove the acid. The resulting filter cake was dried in air and then dispersed into water. Suspended GO sheets were obtained after ultrasonic treatment. The GO was freeze-dried for 48 h subsequently for mixing with PU foams.

Preparation of porous PU

10 g diphenyl-methane-diisocyanate, 10 g nano-CaCO₃, 1 g GO and 5 g polyether polyols were stirred for a while to obtain the pre-polymer. 0.55 g of 1,4-diazabicyclo[2.2.2]octane was dissolved in 6 g water, and the solution was added into a bottle with 50 g polyether polyols, 0.2 g stannous octoate and 1 g polydimethylsiloxane. Then, 5 g of this mixture was poured into the pre-polymer and stirred continuously for 10 s. After 1 h, the foaming process was finished, and the PU sponge was immersed into 1 M HCl solution (water : alcohol = 1 : 1, v/v) with continuous extrusion until no bubble was produced. Then, the acidifying PU sponge was washed with water and dried, and porous PU sponge was obtained.

Preparation of porous PU@rGO

Porous PU foam was immersed in GO solution (4 mg mL⁻¹) with 5% ethylene diamine (EDA) by repeatedly squeezing the foam. After squeezing, the vessel was closed and placed in an oven and heated to 90 °C for 8 h. After the black foam was washed with flowing water, it was dried, and porous PU@rGO was obtained.

Characterization of the porous PU@rGO

The contact angle was obtained using an optical contact angle measuring device (OCA 20110524, Dataphysics Instruments, Germany). The elemental composition was determined using X-ray photoelectron spectroscopy (XPS) (Axis Ultra HAS, KRATOS, Japan). The porous structure was observed by scanning electron microscopy (SEM) (S-4700, HITACHI, Japan). Stress–strain curves were performed by an electromechanical tester (CMT6502, SANS, MTS, USA) with a crosshead speed of 10 mm min⁻¹ at room temperature. The concentration of Cu²⁺ was determined by an atomic absorption spectrometer (AA-Duo, Varian, USA). The chemical oxygen demand (COD) was measured with a COD reactor (HI839800, HANNA Instruments, USA) and a wastewater treatment photometer (HI83224, HANNA Instruments, USA). Electrochemical measurements were performed in an electrochemical workstation (CorrTest, China).

Results and discussion

Characterization of the porous PU@rGO

The morphology of porous PU and porous PU@rGO sponge was observed by scanning electron microscopy, and the images are shown in Fig. 2. For comparison, PU without porous structure (a recipe without nano-CaCO₃) is also shown in Fig. 2. After being etched, a porous framework was formed (Fig. 2c–e) that was obviously different from the smooth surface of the normal PU (Fig. 2a and b). After crosslinking with rGO, we could see that wrinkles covered the porous framework of PU@rGO (Fig. 2f and g). These wrinkles were probably caused by the thin coating of rGO.²¹ Moreover, because rGO coated the porous PU by crosslinking with the GO inside, the rGO was strongly attached to the PU sponge skeleton. These results strongly confirm the presence of micropores in the skeletons of PU and the rGO coatings, which is crucial for further applications of this porous PU@rGO.

Fig. 2 SEM photos of normal PU (a and b), porous PU (c–e) and porous PU@rGO (f and g).

The variation of elemental composition and functional groups of porous PU@rGO were measured by XPS, and the results are shown in Fig. 3. In C 1s spectra, the peaks at 284.6 eV and 286.1 eV were ascribed to C–C and C–N bond, respectively (Fig. 3b and d). The intensity change of O 1s in the survey spectrum and the C 1s peak at 285.1 eV in Fig. 3d confirms the presence of amine groups after EDA-mediated crosslinking.

Fig. 3 XPS spectra of porous PU (a) and porous PU@rGO (c), C 1s spectrum of porous PU (b) and porous PU@rGO (d).

Cyclic stress–strain measurements were done to evaluate the mechanical properties of porous PU@rGO. As shown in Fig. 4, the stress rose above and then returned to zero, suggesting that the sponge can rapidly and completely recover to its original state even after 20 cycles, indicating that the porous PU@rGO has good elasticity even after the acid etching and rGO crosslinking. Meanwhile, the morphology of porous PU@rGO was observed by SEM. As shown in Fig. S1,† the porous PU@rGO maintained its original porous structure, and the skeleton of the as-fabricated sponge is still uniformly covered with rGO sheets, indicating the strong adhesion force between rGO and the PU sponge skeleton.

Fig. 4 The stress–strain curves of porous PU@rGO at the maximum strain of 55%.

The above measurements prove the porous structure and successful rGO crosslinking of the porous PU@rGO, and that the porous PU@rGO exhibits excellent elasticity. The contact angle was also measured to determine the hydrophobicity or oleophilicity of the as-fabricated sponge. As shown in Fig. 5a, the water contact angle of the as-fabricated sponges was 154.99°, whereas the water contact angle of the normal sponges was 110.99°, indicating the as-fabricated sponge's hydrophobicity was obviously enhanced owing to hydrophobic and wrinkled rGO sheets coated on its skeleton. Meanwhile, water droplets could stand on the surface of as-fabricated porous sponge PU@rGO with a “near-spherical” shape, whereas the lubricating oil droplets completely spread out and were infused into the sponge. The PU@rGO exhibited excellent hydrophobicity even after treatment in harsh conditions such as by immersion into acid–base solutions and hexane (Fig. S2†). These results indicate that the rGO coating made the porous PU@rGO superhydrophobic and superoleophilic.

Fig. 5 Water droplet standing on the normal PU (a) and porous PU@rGO (b), water droplets and lubricating oil droplets placed on the porous PU@rGO (c).

Application of the porous PU@rGO in wastewater treatment

The observable 3D porous structures of the as-fabricated sponge and its verified superhydrophobicity and superoleophilicity permit its application in sewage treatment. To test this, a porous PU@rGO sponge was placed on a toluene–water mixture (Fig. 6a); the sponge quickly and selectively absorbed the toluene from the mixture and left a clear ring around the sponge (Fig. 6b). The toluene-filled sponge then floated on the water (Fig. 6c); Movie S1† also shows this quick oil absorption. Furthermore, the sponge also absorbed a high-density organic solvent (e.g. chloroform) from water effectively (Fig. 6d–f and Movie S2†). When a piece of sponge was pressed into water and contacted with chloroform, the chloroform was quickly sucked into the sponge within a few seconds. The oil/water separation test was carried out with a glass tube filled with porous PU@rGO, and no other force was applied. As shown in Fig. 6g–i and Movie S3,† porous PU@rGO had an effective separation performance for oil/water mixtures. As known to all, the reusability of absorbent materials is a key criterion for oil clean-up application. The porous PU@rGO maintained a high adsorption capacity and excellent oil/water separation performance even after 20 test cycles (Fig. S3a and b†). Importantly, the 3D porous structures of PU@rGO provide a large space to absorb and collect the oils or organic solvents, resulting in a higher absorption capacity than other sorbents, such as PDMS sponge²⁴ and spongy graphene.⁸ Meanwhile, SEM images show that the rGO sheets were coated on the skeletons of PU foam after 20 cycles in reusability tests (Fig. S3c and d†), which is important for using PU@rGO material in practical applications. All these tests indicate that the porous PU@rGO could be a promising candidate for oil–water mixture separation, including low- and high-density organic solvents.

Fig. 6 Selective absorption of toluene (dyed by Sudan I) on water (a–c), chloroform (dyed by Sudan I) in water (d–f), and chloroform (dyed by Sudan I)/water (dyed by methylene blue) in a mixture separator (g–i) by using the porous PU@rGO.

Electrochemically assisted technologies have been successfully applied to the treatment of oil-in-water emulsions. In this work, the porous PU@rGO was used to study the break-up of emulsions because of the rGO coating on the surface, which has excellent electrical conductivity.^25–28 To investigate the conductivity performance of the porous PU@rGO, electrochemical measurements were performed using a three-electrode cell configuration in an electrochemical analyzer. The prepared sponge was cut to a strip (5 cm × 0.5 cm × 0.2 cm) for use as the working electrode. The working electrode and counter (platinum) electrode were assembled in a test cell with saturated calomel electrode as reference electrode. The electrolyte was 0.5 M NaCl solution. As shown in Fig. 7a, the porous PU@rGO showed good conductivity, whereas the normal PU without rGO coating was nearly non-conducting. This property could be used to break up oil-in-water emulsions.

Fig. 7 Cycle voltammetry curves of porous PU@rGO and normal PU in 0.5 M NaCl solution (a), COD curve of the electrolytic process and photograph of the emulsion before (left) and after (right) de-emulsification (b).

The oily phase of the emulsion is composed of commercial lubricant oil and soluble oil. To prepare the emulsion, the same amounts of both lubricant and soluble oils (50 : 50, w/w) were mixed and stirred until a homogenous liquid was obtained. Then, brine was added slowly while the mixture was being stirred to finally obtain the oil-in-water emulsion. The emulsion was placed into an electrolytic cell with the clubbed sponge as negative pole and aluminum bar as positive pole. Electric current was applied using a DC Power Supply PS-305DM, and the voltage was maintained at 35 V. The aluminum was necessary to destabilize the emulsion due to the formation of aluminum hydroxide in the positive pole. After decomposition, an oil layer was assembled by sponge, which accelerates the break-up speed of the emulsions. As shown in Fig. 7b, chemical oxygen demand (COD) easily decreased to half or less in 10 min, and the white emulsion also turned to clear water after 60 min.

Furthermore, the porous PU@rGO could also be used to absorb hazardous ions (e.g. Cu²⁺).^29,30 Cu²⁺ adsorption tests were conducted, and the results are shown in Fig. 8. From Fig. 8a, we could see that the as-prepared porous PU@rGO has a high Cu²⁺ adsorption capacity, and the PU nearly could not adsorb Cu²⁺, in contrast. We also studied the performance of porous PU@rGO at a low initial Cu²⁺concentration, and the result is shown in Fig. 8b; the concentration of Cu²⁺ decreased to almost zero at 2 h, and this result strongly demonstrates the strong interaction between rGO and Cu²⁺. The good performance of porous PU@rGO in hazardous ion adsorption also deserves future research.

Fig. 8 The adsorption curves of Cu²⁺ on PU and porous PU@rGO (a). The adsorption curve of Cu²⁺ on porous PU@rG for 2 h (b).

Conclusions

In this study, a versatile PU@rGO sponge was fabricated by coating the rGO on the porous skeletons of PU sponge. The as-fabricated porous PU@rGO sponge with unique binary structure combines the advantage of graphene and polyurethane foams, leading to high physical strength, hydrophobic performance, and remarkable electrical conductivity. Moreover, because rGO was coated on the porous PU by crosslinking with the GO inside the skeleton, the rGO was strongly attached to the PU surface. Thus, the multifunctional sponge could be used to absorb oil in wastewater and break up oil-in-water emulsions by electrochemical methods. Besides, the porous PU@rGO sponge also showed great hazardous ion adsorption efficiency owing to the rGO crosslinking and the porous structure. We believe that the multifunctional porous PU@rGO, produced by our simple and effective strategy, should be a promising potential material for wastewater treatment.

Acknowledgements

We gratefully acknowledge the financial support provided by National Natural Science Foundation of China (21336005, 21301125, 51573122), Industry-Academia Research Project of Jiangsu (BY2014059-05), and Suzhou Nano-project (ZXG2013001, ZXG201420).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05450a

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