1 Introduction

Nature has always inspired the man with its outstanding creations. Among these, natural architectures possessing unusual properties capture special attention. We are inspired by nature: for example, geckos for having extraordinary adhesive capabilities [1], moth eyes for anti-reflective coatings [2] and lotus leaf for self-cleaning surfaces [3]. In particular, the inspiration from natural superhydrophobic surfaces like lotus leaf have received unusual reflection to prepare and modify synthetic surfaces for self-cleaning and anti-icing purposes [4]. After an inclusive research in this field, it appears to be a well-established fact that superhydrophobic phenomenon can be mimicked by combining the surface roughness of a material along with low energy characteristic. Application based research is now being targeted to exploit superhydrophobic properties for the development of cutting edge technologies for oil–water separation and self-cleaning surfaces. The development of a commercially feasible and scalable method for the separation of industrial oily wastewater and spills can indeed address a mighty threat to our environment [5]. After the demonstration of superhydrophobic metallic meshes, various attempts have been made to fabricate superhydrophobic materials for the separation of oil–water mixtures [6]. Some of the recent attempts include the filter papers coated with ZnO followed by modification with octyltrimethoxysilane [7], superhydrophobic metallic meshes prepared by immersion method [8] or coated with TiO2 nanoparticles [9], SiO2 based coatings on cotton fabrics [10, 11], superhydrophobic magnetic foams prepared by the deposition of Fe3O4 [12, 13], polyurethane (PU) foams modified with chitosan and titanate nanotubes followed by functionalization with dodecylmercaptan [14], ZnO coated meshes [15–17], electrospinned polyvinylidene fluoride membranes [18], magnetic sponges [19], and copper oxide coated foams [20].

Inspired by our previous work on silica nanoparticles [21, 22] we have now directed our efforts to develop a cost-effective and convenient method for the synthesis of hydrophobic silica nanoparticles (H-SiO2 NPs) from sodium silicate solution (SSS) and explored its application in oil–water separation. The synthesis of H-SiO2 is generally carried out by using tetraethylorthosilicate as a precursor. Sodium silicate is a cost-effective source of silica, which can be utilized for the synthesis of hydrophobic silica nanoparticles by selecting appropriate reaction conditions. The H-SiO2 NPs synthesized from sodium silicate can be utilized for a variety of applications including self-cleaning surfaces and oil–water separation. We have utilized the H-SiO2 NPs prepared from low-cost sodium silicate for deposition on commercially available filter paper, polyurethane (PU) foam and glass slide. After optimizing the processing parameters, the superhydrophobic/superoleophilic materials were employed to separate oil–water mixtures via filtration, absorption, and vacuum. The superhydrophobic filter paper proved to be an efficient medium for lab scale separation of oil–water mixtures and PU foam is an efficient material for absorption of oil from the surface of the water. Moreover, the continuous separation of oil from the oil–water mixture was performed by employing the vacuum system.

2 Experimental

2.1 Materials

SSS (Na2O = 7.5–8.5 %, SiO2 = 25.5–28.5 %) with a pH of 11.0–11.5 and trimethylchlorosilane (TMCS) were purchased from Merck Millipore. Filter paper Whatman®, polyurethane foam and glass slides acquired from the local market and used in as received form. Ethanol was purchased from Labscan Ltd.

2.2 Synthesis of H-SiO2 NPs

H-SiO2 NPs were prepared by diluting 2.5 mL of SSS with 100 mL of deionized water under constant stirring, which was followed by the addition of 40 mL of ethanol. The obtained sol was aged for 1 h. Later 9 mL of TMCS was added and left to stir for 1 h. Afterward, H-SiO2 NPs were collected, washed, dried and dispersed in ethanol with 0.25 wt% loading followed by sonication for 2 h for uniform dispersion.

2.3 Deposition of H-SiO2 NPs on filter paper and polyurethane foam

The suspension of H-SiO2 NPs in ethanol was taken in petri dish and Whatman® filter paper (7 cm) was dipped to coat H-SiO2 NPs, which was subsequently dried at 50 °C. The dip coating of the filter paper was assisted with sonication using probe sonicator so as to disperse H-SiO2 NPs in the porous structure of filter paper. A range of specimens with different dip coating cycles of 3, 6 and 9 was prepared to explore the optimum conditions. Similarly, PU foam was dipped in the suspension and left to dry at 50 oC. However, in the case of PU foam, only one step was enough for deposition of H-SiO2 NPs because PU foam absorbs the large volume of suspension in a single run. The PU foam was left to dry under same conditions in the presence of the absorbed suspension of nanoparticles.

Additionally, the pristine filter paper was functionalized with TMCS to compare its properties with H-SiO2 NPs coated filter paper. For functionalization, the filter paper was treated with three solutions prepared by adding 3, 6 and 9 mL TMCS in 50 mL ethanol for 2 h.

2.4 Characterization

The chemical nature and information about functional groups of H-SiO2 NPs were obtained using fourier-transformed infrared spectroscopy (FTIR). The size distribution of H-SiO2 NPs and their distribution on filter paper and PU foam were investigated by field emission gun scanning electron microscope. The hydrophobicity was measured by contact angle measurement using goniometer. The performance of filter papers for oil–water separation was evaluated by using water mixtures with four different oils including n-hexane, gasoline, diesel and kerosene. For this purpose, 50:50(v/v) oil–water mixture was prepared to contain 5 mL of each liquid. The efficiency (e) of these filter papers was measured by using the following expression:

$$e = \left( {V - V^\circ /V} \right) \times 100$$

V is the volume of liquid before separation, V o the volume of liquid lost during separation.

The absorption capacities of PU foam were measured by immersing superhydrophobic foam in the oil–water mixture. The following expression was used for calculation of absorption capacity:

$$a = \left( {W^\circ - W/W} \right) \times 100$$

W is the initial weight of PU foam, W o the final weight after absorption.

The continuous separation of oil was performed by attaching a vacuum system with superhydrophobic PU foam.

3 Results and Discussion

Spherical H-SiO2 NPs of size 35 ± 8 nm were produced and successfully deposited on filter paper as can be seen in their scanning electron microscopy (SEM) images (Figs. 1 and 3). The functionalization of H-SiO2 NPs with methyl groups along with siloxane moieties was confirmed by FTIR studies (Fig. 2), as also reported elsewhere [23, 24]. These H-SiO2 NPs were formed by hydrolysis of sodium silicate followed by condensation by the addition of ethanol [21, 25]. It can be inferred from the results that when TMCS was introduced in the sol containing SiO2 NPs, it resulted in the replacement of OH groups with O–Si–(CH3)3 functionalities on the surface of particles; these groups on the surface of SiO2 NPs are responsible for their hydrophobic behavior. The FTIR analysis of filter paper modified with TMCS indicates the presence of Si-CH3 and CH3 groups (Supplementary Fig. 1) but there is no significant difference in wetting behavior of modified filter paper as compared to the pristine filter paper (Supplementary Fig. 3). The SEM images of pristine and functionalized filter paper are presented in Supplementary Fig. 2. The porous network of fibers is visible in pristine filter paper. The SEM images of functionalized filter paper indicate the presence of tiny holes which may be due to the formation of HCL during the functionalization with TMCS. The presence of HCL may have caused the degradation of cellulose fibers [26–28]. It was observed that the filter paper became more flimsy with an increase in the concentration of TMCS.

Fig. 1
figure 1

SEM image of hydrophobic silica nanoparticles at two different magnifications

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Fig. 2
figure 2

FTIR spectrum of hydrophobic silica nanoparticles

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To produce a superhydrophobic filter paper for oil–water separation, the H-SiO2 NPs were coated on filter paper via dip-coating and coating cycles were optimized. H-SiO2 NPs are present on the filter paper with three dip-coats but the surface of filter paper was not completely coated with NPs (Fig. 3a, b and c). The incomplete coating produced partial repellency and low contact angle towards the water, as the water droplets were absorbed on filter paper after a while (Supplementary Fig. 3). The improved distribution of H-SiO2 NPs was obtained after six coats (Fig. 3d, e and f), which also improved the water contact angle to 144°. Further improvement in the distribution of H-SiO2 NPs and water contact angle (155°) was observed in the filter paper with nine coats (Fig. 3g, h and i). The coated H-SiO2 NPs prevented the wetting and induced superhydrophobic character to the filter paper. It is due to synergistic effect of surface roughness and the presence of methyl groups on the surface of H-SiO2 NPs. The inherent rough structure of the filter paper combined with H-SiO2 NPs provides an ideal condition for superhydrophobic and superoleophilic properties, as shown in Fig. 5a, b and f. A few drops of the suspension of H-SiO2 NPs were dropped on glass slide followed by drying to form semi-transparent superhydrophobic surface. It was observed that a semi-transparent superhydrophobic surface was formed on glass slide due to the deposition of small sized H-SiO2 NPs, as shown in Fig. 5d. Transparency towards visible light can be achieved by engineering roughness below the visible range i.e., 400 nm.

Fig. 3
figure 3

SEM images of filter papers after three (a, b and c), six (d, e, and f) and nine (g, h and i) dip coating cycles of hydrophobic silica nanoparticles

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Similarly, the H-SiO2 NPs were deposited on PU foam as shown in Fig. 4. It is evident from the SEM image that one coat was enough for the deposition of H-SiO2 NPs, which may be attributed to the fact that the PU foam absorbs a large volume of suspension in a single run as compared to the filter paper. The H-SiO2 NPs have covered the surface of the foam thus making a porous superhydrophobic and superoleophilic structure. The deposition of H-SiO2 NPs on the structure of PU foam helped to retain its superhydrophobic properties. Originally, the pristine PU foam shows repellency towards the water, which moderates with time and upon the application of force, it completely absorbs water as shown in Fig. 5e. That’s why pristine foam was not used for oil–water separation. In the case of modified PU foam, the H-SiO2 NPs are present on the external structure, which help to retain its resistance towards the water. The modified PU foam remained superhydrophobic even after 200 g of weight was applied to it as shown in Fig. 5g. The PU foam was gone underwater under the influence of weight but it was exhibiting silver shade, which reflects its resistance towards the water.

Fig. 4
figure 4

SEM images of (a and b) pristine polyurethane foam (c and d) after deposition of hydrophobic silica nanoparticles

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Fig. 5
figure 5

Photographic image of a superhydrophobic, a filter paper, b wetting behavior of filter paper against water c wetting behavior of coated PU foam against water, d glass slide coated with hydrophobic silica nanoparticles, e pristine PU foam immersed in water, f wetting behavior of filter paper against water (blue) and oil, g coated PU foam immersed in water under 200 g weight displaying silver shade (Color figure online)

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The application of superhydrophobic filter paper for lab-scale separation of oil-water mixtures was demonstrated by using mixtures of water with four types of oils including n-hexane, diesel, gasoline and kerosene. Here, the oils were separated under the influence of gravity and no external force was applied. The efficiencies of filter papers were calculated by observing the volume of collected oil and water left on filter paper (Fig. 6a). Oil separation efficiency of filter paper was found to be 84 ± 1 and 82 ± 1 % for n-hexane and gasoline, respectively, while diesel and kerosene oil showed the efficiency of 88 ± 1 % each (Fig. 7). The water which stayed on the surface of filter paper during separation process was collected and its efficiency turned out to be 98 ± 1 % for all type of oil-water mixtures. The variation in efficiencies may be attributed to different volatilities of these liquids. The lost volume of oil may be due to the evaporation during filtration or absorbance of oil in filter paper. On the other hand, the minimal loss in volume of water may be attributed to the losses during handling as there was no indication of water in the separated oil and the filter papers were able to separate the mixture very effectively.

Fig. 6
figure 6

a Photographic image of oil and water separation via filter paper, b and c absorption of n-Hexane, d and e absorption of engine oil

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Fig. 7
figure 7

Oil and water separation efficiency of filter paper

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In absorption based separation, oils were separated by immersing H-SiO2 NPs coated PU foam in water mixtures with five types of oils (Fig. 8). The absorbance capacities in terms of percentage weight gain for n-hexane, gasoline, diesel, kerosene and engine oil are 2858, 3625, 3851, 3775 and 3948 %, respectively. PU foam contains a large amount of pores, which if made hydrophobic can selectively absorb large volumes of oil from the surface of the water. This method can be used for the collection of oils from oil spills. In this investigation, the coated PU foam was able to collect highly viscous oil such as engine oil proving its potential industrial application. It is evident from Fig. 6b, c, d and e that the oil readily absorbed in coated PU foam, which was later collected in a separated beaker. The separated oil was used for the calculation of absorption capacity.

Fig. 8
figure 8

Absorption capacity of coated PU foam for different types of oils

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Finally, the continuous separation of the oil–water mixture was performed by using a vacuum assisted system (Fig. 9). For this purpose, a piece of coated PU foam was inserted into a pipe, which was connected to a vacuum system. The coated PU foam was placed at the oil–water interface in such a way that approximately half portion was dipped in water. When the pressure was maintained at 20 KPa, the vacuum assisted system was able to separate oil from water continuously. The vacuum assisted system separated oil very effectively with a flow rate of 2 mL/s without any sign of water in the separated oil.

Fig. 9
figure 9

a Vacuum assisted system for continuous separation of oil–water mixture b separated oil and water

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4 Conclusion

Superhydrophobic filter paper and PU foam were prepared by using silica nanoparticles synthesized from sodium silicate. The filter paper fabricated under optimized conditions exhibited the water contact angle of 155° and separated n-hexane, gasoline, diesel, and kerosene efficiently. Furthermore, the coated PU foam was used for the absorption based separation of oil and it was able to absorb a variety of oils even with high viscosities. The PU foam has the capacity to separate large volumes of oil from the surface of the water and can perform continuous separation upon connection with a vacuum system.