1 Introduction

The frequently occurring oil spills and discharge of chemical organic solvents have caused significant damage to marine ecosystems and human health and safety [1,2,3]. For example, the oil spill in the Gulf of Mexico released 4.9 million barrels of crude oil, covering ~ 149,000 km2 of surface area according to the satellite images. The pollution affected human health in the near-shore water. Moreover, the perniciousness of the oils lasts for several years [4]. Therefore, many traditional oil spill remediation methods were developed, including membrane separation, [5,6,7,8] in situ combustion [9], biodegradation [10], chemical treatment [11], and mechanical removal [12]. Many drawbacks exist in traditional oil spill remediation, such as complicated fabrication processes, air pollution produced, and limited removal capacities. Absorption based on porous materials is a good alternative for oil recovery because of its low cost, fast and effective oil absorption, and high adsorption. Recently, several absorption materials have been developed, such as sponges [13,14,15,16,17,18], aerogels [19,20,21], biochar [22], electrospun nanofibers [23], cotton fabrics [24,25,26], carbon nanotubes [27], and foams [28].

Given the excellent properties of high adsorption capacity and easy oil recovery, sponges have been developed as an alternative absorbent. However, the pristine sponge is amphiphilic, restricting the separation of oil from oily water. Hence, surface modification of sponges, transferring the hydrophilic surface to hydrophobic surface, attracted much attention. The hydrophobic surface with a water-repellent property hinders water to permeate the sponge.

Many superhydrophobic materials exist in nature, such as lotus [29], water striders [30], mussels, rice leaves [31], kales [32], butterfly wings [33], and gecko setaes [34] The surface of lotus leaves is randomly distributed with convex micro- and nanostructures. The structure makes the water drop from an air barrier layer when it contacts the lotus leaf’s surface. Nature-inspired hydrophobic surfaces with macro or nanostructures have been the subject of interest in the past decade. The geometrical structure and surface-free energy govern surface wettability characteristics. Many studies have reported on constructing superhydrophobic surfaces by combining a rough structure with a low energy surface. Li et al. [35] fabricated a superhydrophobic/superoleophilic sponge using a core-shell fluorinated-polyacrylate latex and hydrophobic SiO2 NPs. The as-prepared sponge has a high water-contact angle (WCA) of 150.5° and is maintained above 140° after abrading using sandpaper for 10 cycles. The coated sponge had a faster oil absorption rate than the pristine sponge. Mi et al. [36] synthesized a superhydrophobic fluorinated fibrous silica sponge by crosslinking and fluorination using glutaraldehyde and perliuorodecyltriethoxysilane. The obtained sponge had a high WCA of 151°, high oil absorption capacities of ~ 122 times its weight, and excellent separation efficiency of 98% after five cycles of selective absorption. Guselnikova et al. [37] reported on a superhydrophobic/superoleophilic polyurethane sponge with 3,5-bis(trifluoromethyl)benzenediazonium tosylate. The as-prepared sponge had an extremely high WCA of 160 ± 1°, with an absorption capacity of 35–77 times its weight, and excellent separation efficiency (over 99.9% for emulsion). The fluorinated compounds were used in many hydrophobic materials because of their low surface energy. However, the fluorinated compounds are non-biodegradable and could bioaccumulate in the environment, which is concerning to the environment and human life when absorbents are used in oil spill remediation. Therefore, it is challenging to fabricate environmentally-friendly oil absorbent materials.

Up to now, various methods have been reported for preparing superhydrophobic sponges, such as layer-by-layer assembly, dip-coating, chemical vapor deposition, spray coating, electrospinning, and etching. Among these methods, the dip-coating method was a facile preparation. To prepare superhydrophobic surfaces, rough structures were the most common by incorporating nanosized particles, such as TiO2 [5, 38], SiO2 [14, 39], ZnO [40], and Fe3O4 [41, 42], on the skeletons of the materials. SiO2 was a common modification material for constructing hydrophobic surfaces. Yao et al. [43] proposed a superhydrophobicity bio-based polybenzoxazine/SiO2-modified fabric for oil/water separation. The obtained fabrics had high oil fluxes of 6500–9500 L/(m2·h) and strong durability in harsh environments. Ge et al. [44] developed a superhydrophobic polyurethane (PU) sponge for oil/water separation by coating with SiO2 nanoparticles (NPs). The sponge exhibited high elasticity, high oil absorption capacity, and excellent recyclability.

We construct a rough surface of porous melamine sponge by coating the surface with hydrophobic SiO2 NPs to obtain a superhydrophobic/superoleophilic oil absorbent material. A problem in the sponge modification process is that the hydrophobic SiO2 NPs is weakly adhered to the surface of the sponge and easily peeled off. We present silk fibroin (SF) as a binder, which originated from Bombyx mori silk. Bombyx mori silk is composed of SF and silk sericin (SS). Sericin could be easily removed from Bombyx mori silk by boiling it in Na2CO3 water [45]. SF predominantly consists of glycine, alanine, serine, tyrosine, and valine, with ratios of 45.9%, 30.3%, 12.1%, 5.3%, and 1.8%, respectively [46]. SF has a light chain (~ 390 kDa) and a heavy chain (~ 26 kDa). A single disulfide bridge linked the light and heavy chains [47,48,49]. The heavy chain contains a hydrophobic region, consisting of a highly duplicated hexapeptide amino acid sequence (Gly-Ala-Gly-Ala-Gly-Ser) and a highly duplicated dipeptide amino acid sequence (Gly-Ala/Ser/Tyr). Gly and Ala amino acid sequences were hydrophobic. Silk secondary structures include metastable silk I and stable silk II. Silk II has an antiparallel β-sheet structure and confers insolubility in water. Silk I can form silk II by high temperature, sonication, and chemical solvents, such as methyl alcohol or potassium phosphate [50, 51].

SF is a multifunctional biomaterial because of its high mechanical strength, outstanding biocompatibility, nontoxicity, and biodegradability [48]. Because of these performances, SF has been widely used in wound healing [52, 53], hydrogel [54], aerogel [55], membrane [56], sponge [57], and scaffold [58]. Chen et al. [59] fabricated a porous scaffold to promote osteochondral repair using biocompatible and biodegradable SF. Wang et al. [60] developed a silk film for information security. The obtained film had excellent mechanical strength, long-term structural stability, and a unique response mechanism. The film shows excellent potential in carrying optical information. Zhou et al. [57] fabricated a hydrophobic silk fibroin-graphene oxide functionalized melamine sponge for oil/water separation. SF is a natural polymer to combine graphene oxide with a melamine sponge.

In this work, we propose a facile route to fabricate a superhydrophobic functionalized melamine sponge using a simple dip-coating method. SF and hydrophobic SiO2 NPs were introduced to form a rough nanoscopic morphology on the skeletons of melamine sponge. To prepare the superhydrophobic melamine sponge, SF was used as a binder to adhere the hydrophobic SiO2 to the skeletons of melamine sponge. A combination of the hydrophobic region in SF with the roughness of the NPs reinforced the hydrophobicity. The functionalization process was performed using a one-step coating of hydrophobic SiO2 NPs coated to the surface of the melamine sponge, and SF as a binder combined the melamine sponge with hydrophobic SiO2 NPs. The process was reacted at room temperature without additional equipment, directly achieving a superhydrophobic surface without further organic hydrophobic modifications. For the hydrophobization of melamine sponge, this approach is more cost-effective and simpler than in previous reports. The obtained sponge exhibited superhydrophobicity, with a WCA exceeding 150°, separating the oil and water mixtures quickly and effectively, exhibiting excellent mechanical properties, and excellent recyclability. PDMS solution.

2 Experimental methods

2.1 Materials

MF sponge without oil/water selectivity (water contact value of 0°, n-hexane contact value of 0°) was used as raw material, and was purchased from a local store. Bombyx mori cocoons were obtained from Hangzhou Jiuyuan Silk Culture Co. Ltd, sodium carbonate anhydrous (99.8%) was bought from Fuchen (Tianjin) Chemical Reagent Co. Ltd, dodecane (98%) and LiBr (99%) was supplied by Shanghai Macklin Biochemical Co. Ltd, and hydrophobic SiO2 NPs (7–40 nm of diameter, 260 m2/g, hydrophobicity originated from alkylation treatment), 1,1,2,2-tetrachloroethane (TTCE, 98%), and N,N-dimethylformamide (DMF, ≥ 99.5%) were supplied by Shanghai Aladdin Biochemical Technology Co. Ltd. n-Hexane (≥ 97.0%) was obtained from Guangdong Guanghua Sci-Tech Co. Ltd, pump oil was supplied by Zhejiang Feiyue Electromechanical Co. Ltd, corn oil was obtained from Shenzhen Lam Soon Edible Oil Co. Ltd, and isopropyl alcohol (99.7%) was bought from Guangzhou Chemical Reagent Factory.

2.2 Preparation of SF extraction

Bombyx mori cocoons were boiled in a 0.02 M sodium carbonate aqueous solution for 30 min under constant stirring. The boiled cocoons were washed with deionized (DI) water repeatedly to remove sericin, followed by vacuum drying at 60 °C for 12 h. For preparing SF solution, 2 g degummed silk was dissolved in 100 mL 9.3 M LiBr at 80 °C for 2 h under constant stirring. After cooling to 25±2 °C, the solution was dialyzed to remove impurity ions and diluted to 0.4 mg/mL.

2.3 Preparation of SiO2/SF/MS

A series of SF/SiO2 solutions with different SiO2 concentrations (0, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, and 0.6 wt%) were prepared. 2 mL SF solution was mixture with 1 mL CaCl2 mixture solution(CaCl2:C2H5OH:H2O molar ratio 1:2:8), SiO2/SF/MS were prepared by dispersing the hydrophobic SiO2 NPs of different weights into 40 mL SF/CaCl2 aqueous solution (0.4 mg/mL) under sonication at 25 ± 2 °C for 30 min. The raw melamine sponge was cut into 1 cm3 (1 cm × 1 cm × 1 cm) pieces, followed by washing with DI and ethanol water, respectively. The washed sponge was spin-dried at 1500 rpm for 3 min and vacuum dried at 60 °C for 6 h. The sponge pieces were immersed in the SF/SiO2 solution for 24 h under continuous shaking. The sponge pieces were spin-dried at 1500 rpm for 3 min to remove the residual solution, then washed with ethanol three times. Lastly, the sponge pieces were vacuum dried at 60 °C for 6 h. SiO2/SF/MS was obtained. Unless otherwise specified, the composite SiO2/SF/MS used for characterization and oil adsorption measurement was SiO2/SF/MS with the SiO2 content of 0.3 wt%.

2.4 Characterization

The surface morphology of the sponge was analyzed using a high-resolution field emission (FE) scanning electron microscope (SEM) (FE-SEM, Merlin, Zeiss, Germany). Fourier transform infrared (FTIR) spectra were characterized using FTIR spectroscopy (Nicolet IS50 - Nicolet Continuum). For further structural analysis, thermogravimetric analysis (TGA) was observed using a simultaneous thermal analyzer (STA 449 C, Netzsch, Germany) at 25–1000 °C at a ramp rate of 10 °C/min under nitrogen atmosphere.

2.5 Surface wettability measurements

To evaluate the surface wettability of the materials, the WCA of all sponge sample surfaces was measured using a contact angle measuring system (OCA15, KRUSS, Germany). Five tests were measured at random locations on each sample using 2 µL water droplets. The final WCA results were averaged within each sponge.

2.6 Mechanical properties

The mechanical properties of the sponge were conducted using a dynamic mechanical analyzer (DMA Q800, TA, USA) to obtain the compressive stress–strain curves. The samples were on a cyclic compression-desorption process for 100 cycles, with a fixed strain of 70% to evaluate its long-term recoverability.

2.7 Measurement of oil absorption capacity

To evaluate the oil-capturing ability, the SiO2/SF/MS (taking 0.3 wt% as an example) was immersed in n-hexane, dodecane, isopropyl alcohol, pump oil, corn oil, DMF, and TTCE. In detail, a weighed small piece of SiO2/SF/MS was placed into the oils or organic solvents for 30 s until saturated. The residual oil was removed by holding the sponge in the air for 30 s. Then, the saturated mass of the sponge was instantaneously measured. The mass absorption capacity (Q) was evaluated using the equation

$$\text{Q}=\frac{{m}_{t}-{m}_{i}}{{m}_{i}},$$

where Q represents the oil absorption capacity (g/g) of SiO2/SF/MS, mt (g) represents the weight of the sponge after absorption, and mi (g) represents the weight of the sponge before adsorption.

2.8 Measurement of reusability

The reusability of the modified sponge for oils and organic solvents were evaluated using a repeated absorption-desorption test for 10 cycles. The sponge was weighed immediately after being immersed in oils or organic solvents for 30 s and holding for 30 s. The saturated sponges were spin-dried at 1500 rpm for 3 min to remove the absorbed oils or organic solvents. The absorption-desorption procedures were repeated 10 times. For each adsorption-desorption cycle, the weight of the as-prepared sponge before and after absorption was recorded to measure its absorption capacity.

3 Results and discussion

3.1 The effect of SiO2 concentrations on SiO2/SF/MS

The hydrophobicity of the sponge surface and absorption capacity were used to evaluate the optimum concentration of SiO2 NPs. To obtain the optimal superhydrophobic sponge, the effect of SiO2 concentrations on the wettability of SiO2/SF/MS was investigated. The wettability of the materials was evaluated using the WCA, and the high WCA corresponds to high hydrophobicity [61]. Figure 1a demonstrates that the WCA of the sponge increased with the increment of consistence of SiO2, and the WCA reached 151.9° at 0.3 wt% SiO2. The superhydrophobic surface is defined by WCA (θ > 150°); therefore, the SiO2/SF/MS exhibits superhydrophobicity at a SiO2 content of 0.3 wt%. The absorption capacity of each concentration was evaluated and recorded. As shown in Fig. 1b, the absorption capacity of n-hexane and TTCE increased first and decreased with the increment of the SiO2 concentration and reached a maximum at the SiO2 concentration of 0.3 wt%. Considering both aspects, the optimal concentration of SiO2 would be at 0.3 wt%.

Fig. 1
figure 1

The water contact angle (a) and absorption capacity for n-hexane and TTCE (b) of SiO2/SF/MS with different SiO2 content (Color figure online)

Full size image

3.2 Characterization of SiO2/SF/MS

3.2.1 Morphology of SiO2/SF/MS

The surface morphology of the pristine melamine sponge and SiO2/SF/MS with different concentrations of SiO2 were characterized using FE-SEM (Fig. 2). Note that the pristine melamine sponge displays a three-dimensional open-cell structure with pore sizes ranging from 100 to 180 μm, vital for possessing a high oil absorption capacity for absorbent materials. The higher magnification images of the pristine sponge show that the skeletons were smooth and bead-free (Fig. 2a1–a3). After immersion in the SF solution, the original open-cell structure was retained, SF flocs were generated on the surface of the sponge skeletons (Fig. 2b1–b3), leading to rough skeletons. Furthermore, the nanoscale dimension particles were adhered to the backbones of the sponges after adding hydrophobic SiO2 NPs. From Fig. 2c–h, more particles loaded on the junction points, attributed to the increasing SiO2 content. At the high SiO2 concentration, numerous NPs aggregated into different size balls, enhancing the surface roughness as necessary. These images confirm that SF and hydrophobic SiO2 NPs were attached to the surface and interior of the sponge successfully. Therefore, the melamine sponge was transformed from a hydrophilic to superhydrophobic sponge.

Fig. 2
figure 2

The photographs of a MS, b SF/MS, c 0.1% SiO2/SF/MS, d 0.2% SiO2/SF/MS, e 0.3% SiO2/SF/MS, f 0.4% SiO2/SF/MS, g 0.5% SiO2/SF/MS, and h 0.6%SiO2/SF/MS

Full size image

3.2.2 FTIR analysis of SiO2/SF/MS

To verify the presence of hydrophobic SiO2 NPs, FTIR spectra of MS, SF/MS SiO2/SF/MS, SF, and hydrophobic SiO2 NPs were analyzed and compared (Fig. 3). Strong absorption peaks occurred at 3330, 1540, 1159, and 810 cm−1 related to secondary amine (N–H), C=N stretching, C–O stretching vibration, and triazine ring bending vibration, all observed in MS, SF/MS, and SiO2/SF/MS. Furthermore, absorption peaks at 1333 and 980 cm−1 linked to C–H bending also appeared in pure and modified sponges. SF predominantly consists of three conformation states: random coils, α-helix (silk I), and β-sheets (silk II). The SF secondary structures could be confirmed by the characteristic bands corresponding to amide I and amide II [62]. From the FTIR spectra of SF, SF/MS, and SiO2/SF/MS, the strong absorption peaks at 1625 cm−1 and 1521 cm−1 were ascribed to the stretching vibrations of amide I and amide II, corresponding to the Silk II conformation (β-sheet). Furthermore, the absorption peak at 2964 cm−1 in the spectrum of the SiO2 NPs was ascribed to –CH3 antisymmetric stretching vibration, which provides the SiO2 NPs hydrophobicity. Besides, strong absorption peaks at 1100 cm−1 and 810 cm−1 shown in the hydrophobic SiO2 NPs and SiO2/SF/MS spectra were ascribed to Si–O–Si antisymmetric and symmetric stretching vibrations, indicating that the melamine sponge was successfully modified using hydrophobic SiO2 NPs.

Fig. 3
figure 3

The FTIR spectra of MS, SF/MS, SiO2/SF/MS, SF, and superhydrophobic SiO2 NPs

Full size image

3.2.3 Wettability property

The specific wettability is a critical consideration for the absorbent in applying the treatment of oil spill accidents. The pristine and modified sponges, with mass SiO2 concentrations of 0.3 wt%, exhibited different wettability. To better visualize the wetting performance of pristine melamine and modified sponges, we dyed water with methylene blue and n-hexane with sudan III. Figure 4a shows that water and n-hexane droplets could penetrate the pristine melamine sponge quickly and entirely, leaving a blue and red circle contact area. However, the n-hexane droplet permeated into SiO2/SF/MS immediately, whereas the water droplet maintained a spherical shape, indicating that the air in the sponge was replaced by n-hexane. Figure 1a shows that the WCA of SiO2/SF/MS (0.3 wt%) is 151.9°. Furthermore, the water droplet on the interior of the modified sponge also maintained a spherical shape (Fig. 4b), verifying superhydrophobicity and superoleophobicity of the surface and interior of the SiO2/SF/MS.

Fig. 4
figure 4

a Water (dyed blue) and n-hexane droplets (dyed red) dropped onto the surface of MF, and SiO2/SF/MS. b The water droplet wetting behaviors on the the surface and section of the SiO2/SF/MS. c The images of the MS and SiO2/SF/MS after placing it on the water surface. d Water immersion images of SiO2/SF/MS by external force. The images of the absorption process of e1-e3 light oil (n-hexane), (f1-f3) heavy oil (TTCE) (Color figure online)

Full size image

When placed into water, the pure melamine sponge imbibed water immediately and sank to the bottom of the beaker (Fig. 4c), whereas the SiO2/SF/MS could continuously float on the water surface and without sinking. When an external force was applied to the SiO2/SF/MS (Fig. 4d), the sponge was immersed in the water, with a silver mirror-like surface, attributing to the air bubble layer trapped between water and superhydrophobic melamine sponge, preventing the water from entering the interior.

The three-dimensional porous structure and superhydrophobicity of the sponge make it a promising candidate absorbent for efficient oil/water separation. The porous structure creates a high absorbance for oils, and the superhydrophobicity endows excellent water repellency. To confirm the selective absorption performance of SiO2/SF/MS, n-hexane and TTCE were chosen as light oil and heavy oil samples, respectively. For better visualization of the separation process, n-hexane and TTCE were dyed with sudan III. As shown in Fig. 4e1–e3 and f1–f3, when the SiO2/SF/MS is placed on the water with n-hexane dispersing on the water surface, the sponge absorbed the oil immediately and floated on the water surface because of its water repellency. No oil droplets are left on the remaining liquid surface after removing the sponge. Similarly, when the SiO2/SF/MS was placed into water to absorb TTCE at the bottom of the beaker, the sponge quickly absorbed TTCE, and no residual oil droplets were observed after removing the sponge from the water.

3.2.4 Absorption capacity of SiO2/SF/MS

To assess the absorption capacity of SiO2/SF/MS, absorption experiments were conducted on various oils and organic solvents, including hydrocarbon solvent (n-hexane and dodecane), halogenated hydrocarbon (TTCE), alcohol solvents (isopropyl alcohol ), oils (pump oil and corn oil), DMF, and water.

From Fig. 5, the absorption capacities of the original sponge and SiO2/SF/MS for different oils and organic solvents were almost the same, but the water absorption capacities showed significant difference. However, the water absorption capacity of original sponge was 108.7 g/g, while that of the SiO2/SF/MS was only 0.12 g/g. The hydrophilic modification didn’t affect the absorption capacities of sponge for the oils and organic solvents, but made the sponge own oil/water selectivity. The absorption capacities of the SiO2/SF/MS, for n-hexane, dodecane, isopropyl alcohol, pump oil, corn oil, DMF, and TTCE were 70.9, 80.9, 86.8, 97.7, 102.0, 103.3, and 160.8 g/g, respectively. Especially, the absorption capacity for TTCE reached 160.8 times its weight. The differences in the results because of the density, viscosity, and surface tension values of the oils and solvents [63, 64], show that the absorption capacity is proportionate to the oil or organic solvents’ density. For instance, TTCE has the highest density among these liquids, and the absorption capacity is higher than others by absorbing the same volume. The density of n-hexane is the lowest, and the absorption capacity is relatively less. The n-hexane is the one of most widely used chemical to evaluate the adsorption capability. So the n-hexane absorption capacity was used to compare the performance with the other researches, the results were shown in Table 1. The n-hexane absorption capacity of SiO2/SF/MS in this work are better than most of the reported sponge materials. Moreover, the as-prepared SiO2/SF/MS sponge possessed unique merits, including low density, high buoyancy, and low fabrication cost, making it a promising oil absorbent for oil/water mixture remediation.

Fig. 5
figure 5

Absorption capacity of or original sponge and SiO2/SF/MS with SiO2 concentration of 0.3 wt% for different oils and organic solvents

Full size image
Table 1 Comparsion of modification methods, absorption capacity and WCA of other sponge absorbents reported in literature
Full size table

3.2.5 Reusability and stability

For oil absorbents, two vital parameters should be considered. One is the absorption capacity, and another is reusability because of economic and ecological demands. The prepared melamine sponge was used to absorb n-hexane for 30 cycles. Saturated organic solvents were removed using a centrifugal dehydrator. Figure 6 shows the variation of absorption capacity of SiO2/SF/MS for n-hexane during 30 repetitive absorption-desorption cycles. The results showed that the modified sponge performed stable oil absorption capacity and contact value. In details, the decrease of oil absorption capacity was less than 4%, and the contact angle value was still above 150° after 30 cycles. Hence, the modified sponge achieved admirable stability and durability.

Fig. 6
figure 6

Recyclability of SiO2/SF/MS over 30 absorption-desorption cycles for adsorption and contact angle value

Full size image

3.2.6 Simultaneous thermal analysis (STA)

STA was used to confirm the attachment of silica and SF on the sponge skeleton surface, revealing the composition thermal stability of the neat MF and SiO2/SF/MS. From Fig. 7, all samples exhibited similar thermogravimetric curves, demonstrating that the addition of the SF and hydrophobic SiO2 NPs did not change the thermal degradation mechanism of the melamine sponge. This was ascribed to the low SF and hydrophobic SiO2 NPs concentrations that did not alter the principal compositions of the melamine sponge. The first weight loss (~10 wt%) under 120 °C was caused by water evaporation. The second weight loss, in the range of 120–360 °C, was attributed to eliminate formaldehyde from the ether bridge to form methylene bridges. The primary weight loss between 380 and 400 °C is linked to the breakdown of methylene bridges of melamine sponge. The final weight loss between 400 and 1000 °C of all samples was ascribed to the thermal decomposition of the triazine ring. Although all sample curves exhibited similar thermal behavior, the residual weight of uncoated MF (12.28%) was lower than that of SiO2/SF/MS at the SiO2 content between 0.4 wt% and 0.6 wt% after being heated to 1000 °C. The residual mass of SiO2/SF/MS (at 0–0.1 wt% SiO2) was lower than the pure melamine sponge and decreased with increasing SiO2 concentration. The reason could be that at the low SiO2 content, the SF adhered to the sponge. The results demonstrated that the thermal stability of the sponge was enhanced after modification using enough hydrophobic SiO2 NPs.

Fig. 7
figure 7

TGA curves of MF and SiO2/SF/MS

Full size image

3.2.7 Mechanical properties

Mechanical performance is a significant parameter for superwetting materials in oil/water mixture separation, which is vital for sustainable reuse and operation flexibility. The stress–strain curves of the modified sponge were obtained from a repeated 100-times compression-releasing process with a maximum strain of 70%. Even after 100 cycles, the pure melamine sponge with a 3D porous structure exhibits excellent elasticity and flexibility, making it an excellent template for preparing oil absorbent. The method we adopted did not damage the structure of the pristine melamine sponge. To investigate the mechanical behaviors of pristine melamine sponge and SiO2/SF/MS (taking 3 wt% as an example), compressive stress versus strain curves in a strain-range from 0 to 70% were obtained from consecutive cyclic compression and release tests (Fig. 8). From the compressive stress–strain curves, the compressive stress at 70% strain of SF/MS was 0.0195 MPa, which is higher than 0.011 MPa of pure melamine sponge, indicating that SF addition reinforced its compressive stress, which could be attributed to its unique mechanical strength on SF. Furthermore, with increasing hydrophobic SiO2 NPs, the stresses declined first and increased at hydrophobic SiO2 NPs concentrations of 0.3 wt%, indicating that adding high SiO2 concentrations reinforced its compressive stress. Furthermore, the sponge regained its original shape and is highly elastic with no obvious fractures even after 100 cycles when the stress was released. This could be observed from the curves returning to pristine points after releasing the force, which demonstrated its excellent elasticity and flexibility. The outstanding mechanical property of SiO2/SF/MS makes it a reusable oil absorbent in oily water remediation.

Fig. 8
figure 8

The compressive stress–strain curves of a MF and SiO2/SF/MS with different hydrophobic SiO2 NPs concentration and b cyclic stress–strain curves of SiO2/SF/MS at 70% strain for 100 cycles

Full size image

4 Conclusions

We have fabricated a novel and environmentally-friendly superhydrophobic/superoleophilic SiO2/SF/MS as an absorbing material using a simple dip-coating approach with SF and hydrophobic SiO2 NPs. The eco-friendly SF functioned as a binder, which coated the melamine sponge backbone with hydrophobic SiO2 NPs and transformed the smooth backbones into roughness, and consequently, in superhydrophobicity and superoleophobicity. The fabricated superhydrophobic and superoleophilic melamine sponge was founded on the principles of constructing roughness on the surface of the skeletons to enhance its wettability property. This was realized by introducing hydrophobic SiO2 NPs onto the surface of the skeletons by SF solution coating. The hydrophobic SiO2 NPs enhanced the roughness of the skeletons, endowed the melamine sponge hydrophobicity, and increased its affinity with oil. The SiO2/SF/MS exhibited a high WCA of 151.9°. The modified melamine sponge showed excellent oil-absorbing ability and excellent heat-resistance ability. Our findings demonstrated that the commercial melamine sponge could be successfully transferred into high-efficient high-viscosity oil absorbents using simple modification, which is beneficial to alleviate the increasing oil spillage problems and provide a feasible route to reuse the waste melamine sponge.