Abstract

To obtain a kind of superhydrophobic sponge with high oil and water selectivity, the MS/TiO2/PDMS sponge was prepared via a two-step hydrophobic fabrication based on the melamine sponge (MS), tetrabutyl titanate (TBOT), and polydimethylsiloxane (PDMS). The effects of modification time, the concentrations of TBOT and PDMS on the properties of the MS/TiO2/PDMS sponge were studied, and the separation mechanism was also discussed based on the interaction between the oil and the surface of the MS/TiO2/PDMS sponge. The results suggest that under optimal conditions, the MS/TiO2/PDMS sponge show superhydrophobicity. The contact angle and adsorption capacity for oil of the MS/TiO2/PDMS sponge are 149.2° and 98.5 g·g−1, respectively, and they can be recycled for about 25 cycles after oil–water separation test. This study prepares a new composite material with high oil–water selectivity, which is a good foundation for the development and research of new oil adsorbents.

HIGHLIGHTS

  • The MS/TiO2/PDMS sponge was prepared via a two-step hydrophobic fabrication.

  • The contact angle of the MS/TiO2/PDMS sponge was 149.2°.

  • The adsorption capacity for oil of the MS/TiO2/PDMS sponge are 98.5 gg−1.

  • The MS/TiO2/PDMS sponge can be recycled about 25 cycles after oil-water separation test.

INTRODUCTION

With the growth of industry and marine transportation in recent few decades, rivers and oceans have become increasingly polluted, which is mainly due to the leakage of crude oil, the uncontrolled disposal of waste water containing heavy metal ions, and the leakage of chemical reagents (Varela et al. 2013; Fan et al. 2015; Zou et al. 2015). Pollutants in oil-bearing wastewater have significant impacts on air, surface, and underground water, so their removal is essential because of their harmful nature and serious environmental risks. Therefore, it is necessary to treat the oil pollution to achieve water purification. The conventional oil spill methods mainly include mechanical collection, physical adsorption, in situ burning, chemical dispersion, bioremediation, and oil–water separation techniques (Yu et al. 2015). Although these methods are simple and convenient, they often cause secondary pollution during the clean-up process, and thus it is necessary to seek new methods to realize the goal of environmental protection. Compared with those commonly used methods mentioned above, the physical adsorption method has many advantages, such as low cost, simple operation, low secondary pollution, and good recyclability (Raju et al. 2017). The method is helpful for extensive research and application. However, the synthesis of adsorbent materials used in physical adsorption is still a challenge in that the adsorbent materials need to have the properties of low cost, superior adsorption capacity, good selectivity, and good reusability.

Familiar sorbents and dispersants such as sponge, mineral products (Adebajo et al. 2003), chemical dispersants (Kleindienst et al. 2015), polymers (Ge et al. 2016), textiles/fibers (Seddighi & Hejazi 2015; Hu et al. 2016), metallic meshes (Wang et al. 2015), carbon materials (Gupta & Tai 2016), and membranes (Sadiq et al. 2020) have been used widely to separate oil from water. Among them, the three-dimensional porous structure materials play an important role in the field of oil removal from water due to their low cost, they are easily mass-produced, they have high mechanical strength, excellent adsorption capacity, and are suitable for recycling (Guan et al. 2019). Melamine sponges (MS) are frequently used in oil–water separation field because of their advantages of low density, large porosity, large specific surface area, high thermal stability, corrosion resistance, and flame retardancy (Qiu et al. 2015). However, the MS without modification are naturally hydrophilic and cannot selectively adsorb oils and organic solvents from water, which greatly affects their oil–water removal efficiency and restricts its application to oil–water treatment (Lei et al. 2017a, 2017b). A summary for the adsorption capacities of oils solvents by different MS-based adsorbents is shown in Table 1. To satisfy the demand of application in water oils separation, it is necessary to exploit an efficient and facile method to tune the opposite surface of sponge. Recent reports have focused on the superhydrophobic modification of the sponges by various methods, including dip coating (Qiang et al. 2017), chemical vapor deposition (Khosravi & Azizian 2015; Zhang & Seeger 2015), in situ chemical reaction (Chung et al. 2018), carbonization (Yao et al. 2017), and other methods (Lei et al. 2017a, 2017b).

Table 1

Summary for the adsorption capacities of oils solvents by different MS-based adsorbents

Sponge typeOil typeAdsorption capacity (g/g)References
MS/ODTS Mineral oil
Motor oil 
100.7
94 
Pham & Dickerson (2014)  
PODS-MS Light petroleum ∼65 Ke et al. (2014)  
PDMS-MS(M8) Silicone oil
Motor oil
Toluene 
61.4
46.3
71.5 
Chen et al. (2016)  
Polybenzoxazine-modified MS Sunflower oil
Motor oil 
90
80 
Ejeta et al. (2021)  
HDTMS/rGO-MF Pump oil
Chlorotom 
11
22 
Zhang et al. (2020b)  
MS-DA-PEI Crude oil
Pump oil 
98
110 
Liu et al. (2021)  
Fluorizated kaoline modified MS (K-MS) Soybean oil
Motor oil
Diesel 
123
102
101 
Wang et al. (2019)  
Graphene/MS Chloroform 165
Soybean oil 95 
165
95 
Nguyen et al. (2012)  
CMS/rGO/PFDT Olive oil
Vacuum pump oil
Diesel oil
Gasoline 
52
52
45
36 
Duman et al. (2021)  
MS/TiO2 Chloroform 88.1 Cho et al. (2016)  
Fe3O4/MS Gasoline 26 Wang & Deng (2019)  
MS@SiO2@VTMS sponge Soybean oil 71 Gao et al. (2018)  
SiO2-DTMS-MS Peanut oil
Motor oil
Gasoline 
46
36
24 
Liu et al. (2019) 
MS@TiO2@PPy CCl4 110.7 Yan et al. (2020)  
Ag/PDA/MS Soybean oil
Pump oil 
90
76 
Xu et al. (2015)  
S-PDA-Fe3O4-Ag-ODA Gasoline
Diesel
Lubricating oil
Edible oil 
56
74
85
91 
Chen et al. (2021)  
CW-coated Fe3O4@MS Bean oil
Diesel 
78
70 
Yin et al. (2020)  
AC−TiO2 − PDMS@PDMS /PU Diesel oil
Motor oil
Vegetable oil
Rapeseed oil 
∼100
∼100
∼130
∼140 
Shi et al. (2020)  
TA/Fe3+/MS Beeswax-in-water emulsion 99.8 ± 3.6 Zeng & Taylor (2020)  
MS/TiO2/PDMS Edible blend oil 98.5 Current work 
Sponge typeOil typeAdsorption capacity (g/g)References
MS/ODTS Mineral oil
Motor oil 
100.7
94 
Pham & Dickerson (2014)  
PODS-MS Light petroleum ∼65 Ke et al. (2014)  
PDMS-MS(M8) Silicone oil
Motor oil
Toluene 
61.4
46.3
71.5 
Chen et al. (2016)  
Polybenzoxazine-modified MS Sunflower oil
Motor oil 
90
80 
Ejeta et al. (2021)  
HDTMS/rGO-MF Pump oil
Chlorotom 
11
22 
Zhang et al. (2020b)  
MS-DA-PEI Crude oil
Pump oil 
98
110 
Liu et al. (2021)  
Fluorizated kaoline modified MS (K-MS) Soybean oil
Motor oil
Diesel 
123
102
101 
Wang et al. (2019)  
Graphene/MS Chloroform 165
Soybean oil 95 
165
95 
Nguyen et al. (2012)  
CMS/rGO/PFDT Olive oil
Vacuum pump oil
Diesel oil
Gasoline 
52
52
45
36 
Duman et al. (2021)  
MS/TiO2 Chloroform 88.1 Cho et al. (2016)  
Fe3O4/MS Gasoline 26 Wang & Deng (2019)  
MS@SiO2@VTMS sponge Soybean oil 71 Gao et al. (2018)  
SiO2-DTMS-MS Peanut oil
Motor oil
Gasoline 
46
36
24 
Liu et al. (2019) 
MS@TiO2@PPy CCl4 110.7 Yan et al. (2020)  
Ag/PDA/MS Soybean oil
Pump oil 
90
76 
Xu et al. (2015)  
S-PDA-Fe3O4-Ag-ODA Gasoline
Diesel
Lubricating oil
Edible oil 
56
74
85
91 
Chen et al. (2021)  
CW-coated Fe3O4@MS Bean oil
Diesel 
78
70 
Yin et al. (2020)  
AC−TiO2 − PDMS@PDMS /PU Diesel oil
Motor oil
Vegetable oil
Rapeseed oil 
∼100
∼100
∼130
∼140 
Shi et al. (2020)  
TA/Fe3+/MS Beeswax-in-water emulsion 99.8 ± 3.6 Zeng & Taylor (2020)  
MS/TiO2/PDMS Edible blend oil 98.5 Current work 

Studies reported that hydrophilic surfaces can be converted to hydrophobic surfaces by silanization (Xu et al. 2015; Yujing et al. 2019; Yan et al. 2020; Shi et al. 2020; Yin et al. 2020; Chen et al. 2021), carbonization (Feng & Yao 2018; Peng et al. 2019), and fluorination (Ruan et al. 2014; Gurav 2015; Li & Guo 2017). The silanization treatment has more application prospects because fluorination has certain toxicity and will cause secondary pollution (Chen et al. 2016). Polydimethylsiloxane (PDMS) is considered to be one of the most commonly used hydrophobic reagents for the modification of oily material adsorbents, not only because of their hydrophobicity, but also because of their stability and mechanical flexibility (Ge et al. 2016). It can firmly bond onto the sponge skeleton surface and does not fall off easily (Chen et al. 2016). Meanwhile, hydrophobic nanoparticles, such as TiO2 (Chen et al. 2016), SiO2 (Yeom & Kim 2016), Ag (Qin et al. 2018), ZnO (Tran & Lee 2017), Fe3O4 (Li et al. 2018), and attapulgite particles have been used to modify the surface of sponges. In particular oleophilic TiO2 nanoparticles, which are widely used as a functional material with good chemical stability, were adsorbed onto the surface of the MS' skeleton to enhance the surface roughness of the sponge and the adsorption performance for oil spill cleanup (Padervand et al. 2011; Ge et al. 2014; Li et al. 2020). The nanoscale protrusion of hydrophobic TiO2 forms a rough surface similar to lotus leaf, which enhances their superhydrophobicity. The incorporated TiO2 nanoparticles produce rough structures, and PDMS is available for design of hydrophobic surface. Although good progress has been made in improving the lipophilicity and hydrophobicity of the MS, these materials still have drawbacks such as poor oil–water selectivity and poor practical application.

Inspired by these achievements (Shi et al. 2014; Li et al. 2015; Wei et al. 2018), this study developed a two-step hydrophobic fabrication of MS/TiO2/PDMS sponge with low cost, simple operation, high adsorption capacity, and high oil and water selectivity. The MS/TiO2 sponge was first prepared using the sol-gel method with the MS and tetrabutyl titanate (TBOT) as the matrix and precursor, and the MS/TiO2/PDMS sponge was finally synthesized by using dipping method with PDMS as the matrix. During this process, the TiO2 nanoparticles can be uniformly grown in situ on the MS sponge framework under the water atmosphere, and no other operation is required. This is followed by surface modification with PDMS to take advantage of its hydrophobic siloxane in an effort to enhance the hydrophobicity of the sponge.

EXPERIMENTAL SECTIONS

Materials

The MS was purchased from Zhengzhou Fengtai Nanomaterials Co. Ltd. TBOT (AR, ≥ 99.0%) was purchased from Tianjin Kemiou Chemical Reagent Co. Ltd. PDMS (40cst, HO-(C2H6OSi)n-H) was purchased from Shanghai Maclean Biochemical Technology Co. Ltd. The oil used in this study is an edible blend oil and sesame oil for golden arowana. All chemicals used in this study were of commercially available analytical grade.

Preparation of MS/TiO2/PDMS sponge

At room temperature, the pretreated melamine sponge was first immersed in 5%TBOT/absolute ethanol solution for 3.5 h, and then hydrolytic polycondensation occurred using the sol-gel method in a moist closed system to obtain MS/TiO2 sponge. The prepared materials were immersed in 0.5% PDMS/n-hexane solution for 3.5 h, followed by washing and drying. A new functional composite material of MS/TiO2/PDMS sponge was then obtained.

During the experiment process, the volume concentrations of TBOT absolute ethanol solution selected were 10%, 15%, 20%, and 25%, and the volume concentration of PDMS/n-hexane solution selected were 1%, 2%, and 4%.

Oil-water separation tests

The adsorption capacity of MS, MS/TiO2, and MS/TiO2/PDMS sponges for oils was determined with a weighing method. At room temperature, the sponge was placed in a beaker containing 50 mL of oil for 30 min and weighed before and after adsorption. The preparation process is shown in Figure 1. When the sponge stopped dripping water (oil), it was put it in a clean beaker and weighed it to calculate the water (oil) adsorption capacity. The adsorption capacity can be calculated according to the following formula:
formula
where Q refers to the water (oil) adsorption capacity of the sponge, g·g−1; m1 the mass after the sponge adsorbs water (oil) saturation, g; m0 the mass when the sponge does not adsorb water (oil), g. During the above oil adsorption test, oil was measured three times, and the final water (oil) adsorption capacity was averaged.

Characterization

The surface functional groups of MS, and MS/TiO2, MS/TiO2/PDMS sponges were analyzed using an attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR, PerkinElmer, Spectrum Two) (Mohammed et al. 2014; Khalaf et al. 2021). The surface morphology was observed using scanning electron microscopy (SEM, Hitachi Model S-3400N, with gold spray). The contact angle was measured using a video optical contact angle tester (OCA20, Dataphysics). A 4 μL water sample was dropped as an indicator onto the surface of the sponges by sessile drop, and the measurements were performed three times on each sample and the mean contact angle values were obtained.

Reuse test of MS/TiO2/PDMS sponge in oil–water separation

The modified MS/TiO2/PDMS sponge was subjected to an oil (water) adsorption test, and the oil (water) adsorption capacity was calculated. After the test, the oil (water) was removed by squeezing and then the MS/TiO2/PDMS sponge was washed with anhydrous ethanol and dried. The sponge that had been washed was subjected to repeated oil (water) adsorption experiments, and reusability capacity was finally measured.

RESULTS AND DISCUSSION

Morphology of materials

To study the effect of modification methods on the surface morphology of MS, the surface morphology of the MS before and after modification were monitored by SEM (as shown in Figure 2). It can be seen that the unmodified MS presents a 3D porous network structure which allows the sponge to have a large specific surface area, while the skeleton is particularly smooth (Figure 2(e)). In comparison, the surface of the MS/TiO2 composite has adherents, and the surface roughness increases in Figure 2(b)–2(d). With an enlarged surface of MS/TiO2 sponge, it is clearly shown that there are small nanoparticles on the MS/TiO2 when the concentration of TBOT solution was 5% (Figure 2(f)). TiO2 nanoparticles on the skeleton of MS/TiO2 sponge increase with the concentration of TBOT solution (Figure 2(g)); however, the laminated TiO2 adhered on the skeleton of the MS/TiO2 sponge when TBOT concentration was 25%. When the MS is treated by TBOT solution and then placed in a moist atmosphere, the reaction of hydrolytic polycondensation of the TBOT happened on the skeleton of MS, and TiO2 nanoparticles were generated (Wang et al. 2007). The surface roughness of MS/TiO2 increased because the TiO2 nanoparticles were loaded on the skeleton of MS.

Figure 1

Procedure for preparation of MS/TiO2/PDMS sponge.

Figure 1

Procedure for preparation of MS/TiO2/PDMS sponge.

Figure 2

SEM images of MS (a, e) and MS/TiO2 sponge with different TBOT solutions: (b, f) 5%; (c, g) 20%; (d, h) 25%.

Figure 2

SEM images of MS (a, e) and MS/TiO2 sponge with different TBOT solutions: (b, f) 5%; (c, g) 20%; (d, h) 25%.

Figure 3 shows SEM images of different concentrations of PDMS-modified MS/TiO2 under the following conditions: 20% TBOT concentration and modification time of 3.5 h. From the enlarged surface of the MS/TiO2/PDMS sponge (Figure 3(d)–3(f), it can be seen that the MS/TiO2/PDMS sponge retained a 3D porous network compared to the unmodified MS (Figure 2(c)) and MS/TiO2 (Figure 2(d)). The MS/TiO2 sponge presents a coarser skeleton structure and it can be clearly observed that the surface of MS/TiO2 sponge is covered with a layer of adherent material. However, it is also observed from Figure 3(d)–3(f) that the dispersion of PDMS is heterogeneous, and is mainly distributed at the junction of the sponge skeleton, and the amount of sediment on the surface increases with the increase of PDMS concentration. Furthermore, deposit sediment (PDMS: 4%) shows little change when the concentrations of PDMS exceed 2%. In addition, by comparing to Figures 2 and 3, it can be clearly seen that the pore size remains the same.

Figure 3

SEM images of the MS/TiO2/PDMS sponge prepared in different PDMS concentrations (PDMS concentrations; 1% (a, d), 2% (b, e), and 4% (c, f).

Figure 3

SEM images of the MS/TiO2/PDMS sponge prepared in different PDMS concentrations (PDMS concentrations; 1% (a, d), 2% (b, e), and 4% (c, f).

Chemical composition of materials

The chemical composition of unmodified MS, MS/TiO2 sponge, and MS/TiO2/PDMS sponge were investigated by infrared (IR)-ATR analysis, as shown in Figures 4 and 5. Figure 4 shows the IR-ATR image of unmodified MS (Figure 4(a)) and modified MS with different concentrations of TBOT (Figure 4(b) and 4(c)) at 3.5 h. The spectrum of the MS shows absorption bands at 807 cm−1, 1,540 cm−1, and 3,309 cm−1, which are assigned to triazine ring bending, C–N stretching, and N–H stretching, respectively. Furthermore, bands around 1,325 cm−1 and 1,467 cm−1 are indicative of –CH– bending. Moreover, two small peaks at 2,800–2,900 cm−1 are attributed to –CH2 and –CH3 stretching. These groups of absorption bands verify the chemical composition of the MS (Figure 4(a)). When compared to unmodified MS, the weak stretching vibration absorption peak of Ti–O bond appears at 679 cm−1 (see Figure 4(b) and 4(c)), which confirms that the TiO2 particles were successfully loaded on the surface of MS. At the same time, the intensity of the stretching vibration peak of the hydrophilic group −CO at 1,140 cm−1 and 1,100 cm−1, and the absorption peak intensity of the triazine ring bending vibration at 1,325 cm−1 and 807 cm−1 gradually decrease or even disappear (Figure 4(b) and 4(c)) with the increasing concentration of TBOT. According to Ge's report (Ge et al. 2016), the surface of TiO2 has a certain interaction force with the hydrophilic group. The hydrophilic groups (−NH, −C = O and −CO) of the surface of MS and that of the TiO2 interact via a secondary bond. The force makes the TiO2 nanoparticles tightly attach to the surface of the MS skeleton. The functional group of the MS surface has changed, and the surface of the sponge changes from a hydrophilic surface to a hydrophobic surface. The analysis results showed that the surface composition of the modified MS was changed and the hydrophilic group was relatively weakened. In other words, the MS/TiO2 sponge gradually changed from hydrophilic to hydrophobic after treatment with the TBOT solution.

Figure 4

IR-ATR image of MS (a) and MS/TiO2 prepared in different TBOT concentrations (TBOT concentrations: 15% (b), 20% (c) respectively).

Figure 4

IR-ATR image of MS (a) and MS/TiO2 prepared in different TBOT concentrations (TBOT concentrations: 15% (b), 20% (c) respectively).

Figure 5

IR-ATR image of MS/TiO2/PDMS sponge (TBOT concentration was 20%) prepared in different PDMS concentrations (PDMS concentrations was 0.5% (a); 1% (b); 4% (c) and 2% (d) respectively).

Figure 5

IR-ATR image of MS/TiO2/PDMS sponge (TBOT concentration was 20%) prepared in different PDMS concentrations (PDMS concentrations was 0.5% (a); 1% (b); 4% (c) and 2% (d) respectively).

The IR-ATR images of the MS/TiO2/PDMS sponge with different concentrations of PDMS-modified MS/TiO2 (TBOT: 20%) are shown in Figure 5. It can be seen that the stretching vibration peaks of the Si–O bond are observed at 1,011 cm−1 and 1,084 cm−1, the stretching vibration peak of the Si–C bond is observed at 866 cm−1 and 795 cm−1, and −CH3 groups are shown at the absorption peak at 2,962 cm−1. These peaks belong to the characteristic absorption peaks of PDMS. Meanwhile, the intensity of the absorption peak at 2,962 cm−1 of −CH3 groups, the stretching vibrational absorption peak of Si–O bond and the stretching vibration absorption peak of Si–C increases gradually as the concentration of PDMS increases. The peak intensity is the maximum when the PDMS concentration is 2%, while the peak intensity decreases as the concentration of PDMS is 4%. The above results indicate that the concentration of PDMS has a certain effect on the hydrophobicity of the MS. This result is consistent with the SEM image.

Surface wettability of materials

Effect of TBOT concentration on the contact angles of MS/TiO2 sponge

Figure 6 shows the contact angles of unmodified MS and MS/TiO2 with different concentrations of TBOT at 3.5 h. From Figure 6(a), it can be observed that the unmodified MS has particularly good wettability, water droplets are adsorbed in less than 1 s, and the contact angle is 0 °. To make the MS have the selectivity of oil and water, the surface of MS was modified with differents concentrations of TBOT solution. Compared to the unmodified MS, the MS/TiO2 manifests a certain hydrophobicity (see Figure 6). The results of the measurement of the contact angle are depicted in Figure 6. From Figure 6, it can be seen that the contact angle of the MS/TiO2 gradually increases as TBOT concentration increases. When the TBOT concentration is 20%, the contact angle reaches a maximum value of 142.2°. Nevertheless, the contact angle decreases to 138.2° when the TBOT concentration increases 25%. The contact angle increases first and then decreases with the increase of TBOT concentration. The nanoscale protrusion of the hydrophobic TiO2 combined with microporous structure of the original sponge forms a double rough surface, which is similar to the structure of lotus leaf, resulting in an enhanced superhydrophobicity (Barthlott & Neinhuis 1997; Cho et al. 2016). The surface roughness caused by TiO2 nanoparticles from TBOT hydrolysis increases with the increase of the concentration of TBOT solution, and the contact angle increase with surface roughness, leading to the gradual increase of hydrophobicity. When excess TBOT was hydrolyzed, the large amount of TiO2 particles obtained aggregated on the MS framework and had a laminaceous structure (see Figure 2(h)), which resulted in macro-scale roughness, but a decrease in the micro-scale roughness. As a result, hydrophobicity is reduced. Furthermore, the sponge modified with solid particles showed a decrease in oil adsorption capacity due to the increase of the sponge density to some extent, which indicates the contact angle decrease (Zhang et al. 2020a, 2020b). The result was consistent with the SEM. Therefore, the optimal concentration of TBOT modified MS is 20%.

Figure 6

Contact angle for unmodified MS (a) and MS/TiO2 sponge prepared in different TBOT concentrations (TBOT concentrations: 5%, 10%, 15%, 20%, and 25% respectively).

Figure 6

Contact angle for unmodified MS (a) and MS/TiO2 sponge prepared in different TBOT concentrations (TBOT concentrations: 5%, 10%, 15%, 20%, and 25% respectively).

Effect of modification times on the contact angles of MS/TiO2/PDMS sponge

Figure 7 examines the effects of different modification times on the properties of modified MS when TBOT concentration was 20% and PDMS concentration was 2%. Figure 7 shows that the immersion time has a great effect on the contact angle of the MS/TiO2/PDMS sponge. The contact angle of the MS/TiO2/PDMS sponge gradually increases from 136.7° to 149.2° as the immersion time increases from 2 h to 3.5 h. However, the contact angle decreases to 147.2° when the immersion time increased to 4 h. During the experiment, white powder appeared on the surface of the MS/TiO2/PDMS sponge when the immersion time reached 4 h. The reason may be that the MS/TiO2/PDMS sponge pores were filled with a large amount of white hydrolyzed TiO2 powders after drying, and this white powder easily falls off because there was no force between TiO2 particles. With the increase in modification time, the contact angle of MS/TiO2/PDMS sponge increased first and then decreased mainly due to the long immersion time and the TiO2 nanoparticles blocking the pores of the sponge. The TiO2 nanoparticles and the siloxane cannot continue to adhere to the surface of the sponge skeleton, resulting in the decrease of contact angle of MS/TiO2/PDMS sponge. Therefore, the optimal modification time of MS/TiO2/PDMS sponge is 3.5 h.

Figure 7

Contact angle of MS/TiO2/PDMS sponge prepared with different modification times (TBOT concentration was 20% and PDMS concentration was 2%).

Figure 7

Contact angle of MS/TiO2/PDMS sponge prepared with different modification times (TBOT concentration was 20% and PDMS concentration was 2%).

Effect of PDMS concentrations on the contact angle of MS/TiO2/PDMS sponge

Figure 8 shows the effect of PDMS concentrations on the contact angle of the MS/TiO2/PDMS sponge with different concentrations of PDMS solution when the modification time of a given concentration (20%) of TBOT was 3.5 h. Compared to the unmodified sponge (see Figure 6(a)), it can be observed that the MS/TiO2/PDMS sponge (see Figure 8) have superior hydrophobicity. Figure 8 shows that the concentration of PDMS solution has a significant effect on the contact angle of MS/TiO2/PDMS sponge. The contact angle of the MS/TiO2/PDMS sponge increases rapidly as the concentration of PDMS solution increases. When the concentration of PDMS is 2%, the contact angle reaches a maximum value of 149.2°. The contact angle is reduced as the concentration of PDMS is over 2%. From the analysis of the experimental process, the roughness and the contact angle of the MS/TiO2/PDMS sponge decreases because the high concentration of 4% PDMS solution were used, and the porous structure of the MS/TiO2/PDMS sponge was clogged during the impregnation process, which hindered the further adhesion of the PDMS to the surface of the MS/TiO2 composite. The optimum concentration of PDMS is 2%.

Figure 8

Contact angle for MS/TiO2/PDMS sponge prepared with different PDMS concentrations (0.5%, 1%, 2% and 4%) when the modification time of a given concentration (20%) of TBOT was 3.5 h.

Figure 8

Contact angle for MS/TiO2/PDMS sponge prepared with different PDMS concentrations (0.5%, 1%, 2% and 4%) when the modification time of a given concentration (20%) of TBOT was 3.5 h.

Oil adsorption capacity of the sponge

Oil-water adsorption capacity of the sponge before and after modification

To test oil (or water) adsorption capacity of the sponge after modification, the MS/TiO2/PDMS sponge was prepared using a 2% PDMS concentration, and 20% of modified TBOT was used for 3.5 h. Figure 9 demonstrates the MS and MS/TiO2/PDMS sponge immersed in water and oil. Compared to the unmodified MS, the MS/TiO2/PDMS sponge seems to be plated with a bright silver film on the surface (see Figure 9(a)) after being immersed in water, due to the superhydrophobicity of the MS/TiO2/PDMS sponge. When the MS/TiO2/PDMS sponge is squeezed and immersed in water under external force, a dense layer of air bubbles adheres to the surface of the modified sponge, and this layer of bubbles reflects incidental light and produces a silver film, which demonstrates the hydrophobicity of the modified sponge. When the pressure stops, the MS/TiO2/PDMS sponge will immediately emerge from the water, and will not adsorb water for several hours. (Figure 9(b) shows the sponge on the water surface. The quality of the sponge will also not change significantly before and after immersing in the water. The Cassie–Baxter nonwetting behavior leads to this phenomenon (Cassie & Baxter 1944; Pham & Dickerson 2014). Figure 9(c) and 9(d) are the unmodified MS and the modified MS/TiO2/PDMS sponge immersed in oil, respectively. It can be observed that when they are immersed in oil, they both rapidly adsorb oil and sink. The experimental results show that the modified sponge has lipophilic and hydrophobic properties, it is not wetted in the water, and it floats on the water surface; however, it quickly adsorbs oil and sinks in the oil. It has a selectivity for oil and water. The unmodified sponge has lipophilic and hydrophilic properties. It quickly adsorbs water, and sinks in the water, and it also adsorbs oil and sinks in the oil. This means it does not have oil–water selectivity. Therefore, MS/TiO2/PDMS sponge can be used as oil–water adsorptive separation materials.

Figure 9

MS and MS/TiO2/PDMS immersed in water (a, b) and sesame oil (c, d).

Figure 9

MS and MS/TiO2/PDMS immersed in water (a, b) and sesame oil (c, d).

Figure 10 shows the process of the MS/TiO2/PDMS sponge selective adsorption of oil in water. It can be observed from the image that the modified MS/TiO2/PDMS sponge can quickly and selectively adsorb oil in water and achieve the effect of oil–water separation and purification of water, which shows a potential use for the separation of oil–water mixtures in real life.

Figure 10

Photographs of MS/TiO2/PDMS sponge selective adsorption of edible blend oil from water.

Figure 10

Photographs of MS/TiO2/PDMS sponge selective adsorption of edible blend oil from water.

Effect of TBOT concentrations on the oil–water adsorption capacity of MS/TiO2/PDMS sponge

Figure 11 shows the oil–water adsorption capacity of MS/TiO2/PDMS sponge prepared with different concentrations of TBOT solution when the modification time of a given concentration (2%) of PDMS was 3.5 h. It can be observed from the image that the adsorption capacity of the unmodified sponge to water and oil were 136 g·g−1 and 123 g·g−1, respectively, which are much higher than that of MS/TiO2/PDMS sponge. In the unmodified MS, the condensation product structure of melamine and formaldehyde contains both hydrophilic polar group (imine) and hydrophobic group (methylene). MS therefore has a certain hydrophilic and hydrophobic property (see Figure 11). However, the water adsorption capacity of the MS/TiO2/PDMS sponge gradually decreases as the concentration of the TBOT solution gradually increases, i.e. the hydrophobicity of MS/TiO2/PDMS sponge increases. When the TBOT concentration is 20%, the adsorption capacity of MS/TiO2/PDMS sponge reaches a maximum of 98.5 g·g−1; however, the hydrophobicity of MS/TiO2/PDMS sponge decreases when the TBOT concentration is over 20%. This is consistent with the measurement of the contact angle of MS/TiO2/PDMS sponge. There is no significant change in oil adsorption capacity before and after MS modification. That is because although the MS/TiO2/PDMS sponge has an increased roughness compared to the unmodified sponge, the porosity of the modified MS/TiO2/PDMS sponge decreases, resulting in no increase in adsorption capacity. Compared with the adsorption capacity of MS@SiO2@VTMS sponge to soybean oil (71 g·g−1) (Gao et al. 2018), SiO2-DTMS-MS to peanut oil (46 g·g−1) (Yujing et al. 2019), Ag/PDA/MS to soybean oil (90 g·g−1) (Xu et al. 2015), S-PDA-Fe3O4-Ag-ODA to edible oil (91 g·g−1) (Chen et al. 2021), CW-coated Fe3O4@MS to bean oil (78 g·g−1) (Yin et al. 2020) (see Table 1), the adsorption capacity of MS/TiO2/PDMS sponge far exceeds the adsorption capacity reported in the literature, and the preparation process is simpler. The results show that the MS/TiO2/PDMS sponge has higher adsorption capacity and can be used in practice.

Figure 11

Edible blend oil and water adsorption capacity of MS/TiO2/PDMS sponge with different TBOT concentrations when the modification time of a given concentration (2%) of PDMS was 3.5 h.

Figure 11

Edible blend oil and water adsorption capacity of MS/TiO2/PDMS sponge with different TBOT concentrations when the modification time of a given concentration (2%) of PDMS was 3.5 h.

Effect of PDMS concentrations on the oil–water adsorption capacity of the MS/TiO2/PDMS sponge

Figure 12 shows the oil–water adsorption capacity of the MS/TiO2/PDMS sponge prepared with different concentrations of PDMS solution when the modification time of a given concentration (20%) of TBOT was 3.5 h. It can be observed from the image that the adsorption capacity of the modified MS/TiO2/PDMS sponge gradually increases as the concentration of the PDMS solution gradually increases, i.e. the hydrophobicity of MS/TiO2/PDMS sponge increases. When the PDMS concentration is 2%, the adsorption capacity of MS/TiO2/PDMS sponge reaches a maximum of 98.5 g·g−1; however, the adsorption capacity of the MS/TiO2/PDMS sponge decreases when the PDMS concentration is over 2%. This is consistent with the measurement of the contact angle of MS/TiO2/PDMS sponge (see Figure 8). The possible reason is that when an appropriate amount of PDMS is added, PDMS completely interacts with TiO2 and remains multistage structural pore of the sponge. When excessive PDMS is added, the excess PDMS clogs the porous structure of the MS/TiO2/PDMS sponge, resulting in declining surface hydrophobicity, and the oil absorption ability is reduced.

Figure 12

Edible blend oil and water adsorption capacity of MS/TiO2/PDMS sponge with different PDMS concentrations when the modification time of a given concentration (20%) of TBOT was 3.5 h.

Figure 12

Edible blend oil and water adsorption capacity of MS/TiO2/PDMS sponge with different PDMS concentrations when the modification time of a given concentration (20%) of TBOT was 3.5 h.

Effect of pH on the oil–water adsorption capacity of MS/TiO2/PDMS sponge

To investigate the effects of solution pH on adsorption capacity, MS/TiO2/PDMS was immersed in oily wastewater (prepared using HCl, distilled water, and NaOH) with pH values of 4–10. Figure 13 shows that the adsorption capacity of the MS/TiO2/PDMS sponge at different pH when the modification time of a given concentration (2%) of PDMS and (20%) of TBOT was 3.5 h. Under acidic conditions, the adsorption capacity remains constant with increasing pH. When the pH was 7, the adsorption capacity of MS/TiO2/PDMS sponge reached a maximum of 98.5 g·g−1. When the pH value continued to rise and the solution became alkaline, the adsorption capacity decreased. The above results indicate that for MS/TiO2/PDMS sponges, both alkaline and acidic environments are not suitable for the adsorption, and that the optimum pH for adsorbing oil by MS/TiO2/PDMS sponge was 7. The possible reasons are, on the one hand, under alkaline condition, due to partial saponification of vegetable oil, the charge changed from ester group to carboxylate, which enhanced the hydrophilicity of modified sponge; on the other hand, partial hydrolysis of PDMS occurs under strong alkaline conditions, resulting in the decrease of the lipophilicity of the adsorbent. The results were consistent with the reported in the literature (Cai et al. 2019; Wang et al. 2019; Wang et al. 2021), which the contact angle of modified MS firstly increases and then decreases with the increase of solution pH, and reaches the maximum value when pH = 7.

Figure 13

Edible blend oil and water adsorption capacity of MS/TiO2/PDMS sponge with different pH when the modification time of a given concentration (2%) of PDMS and (20%) of TBOT was 3.5 h.

Figure 13

Edible blend oil and water adsorption capacity of MS/TiO2/PDMS sponge with different pH when the modification time of a given concentration (2%) of PDMS and (20%) of TBOT was 3.5 h.

Reusability of MS/TiO2/PDMS sponge

Figure 14 shows the results of repeated use of the unmodified and modified sponges prepared was chosen, when the concentration of TBOT solution was 20%, the concentration of PDMS solution was 2%, and modification time was 3.5 h. It can be seen that MS has high adsorption capacity of not only water (123 g·g−1) but also of oil (117 g·g−1) (see Figure 14) due to MS having a hydrophilic polar group (imine) and hydrophobic group (methylene). However, the water adsorption capacity of the original MS decreases continuously with an increase in cycle times, and the oil adsorption capacity of MS is basically stable after reusing 10 times. For the MS/TiO2/PDMS sponge, with the increase of the number of cycles, the oil adsorption capacity is stable, while the water adsorption capacity increases slowly. The increase of water adsorption capacity of the MS/TiO2/PDMS sponge is because the TiO2 nanoparticles and PDMS molecule on the surface of the modified sponge skeleton is removed when continuously squeezed. From the experiments of reusing, the modified MS/TiO2/PDMS sponge can be recycled more than 10 times. This result shows that the MS/TiO2/PDMS sponge has better oil–water selectivity and reusability.

Figure 14

Reuse of MS sponge (left) and MS/TiO2/PDMS sponge (right) (the concentration of TBOT solution was 20%, the concentration of PDMS solution was 2%, and modification time was 3.5 h).

Figure 14

Reuse of MS sponge (left) and MS/TiO2/PDMS sponge (right) (the concentration of TBOT solution was 20%, the concentration of PDMS solution was 2%, and modification time was 3.5 h).

The oil–water separation mechanism with MS/TiO2/PDMS sponge

According to the wettability theory, high selective adsorption of oil or organic solvent can be achieved on the sponge by constructing the rough surface morphology or modifying the material with low surface energy (Zhou et al. 2019). The MS/TiO2/PDMS sponge indicated an excellent superhydrophilic and porous structure, which prompted a further study of the mechanism of oil–water separation. Figure 15 shows the schematic of the oil–water separation mechanism with the MS/TiO2/PDMS sponge. Before modification, the condensation product structure of melamine and formaldehyde contains both hydrophilic polar group (imine) and hydrophobic group (methylene); therefore, MS has a certain hydrophilic and hydrophobic (see Figure 11). When MS was immersed in TBOT/absolute ethanol solution, the TiO2 nanoparticles were uniformly grown in situ on the MS sponge framework by the sol-gel method. The nanoscale protrusion of the hydrophobic TiO2 combined with microporous structure of the original sponge forms a double rough surface, which is similar to the structure of lotus leaf (Barthlott & Neinhuis 1997; Cho et al. 2016). Furthermore, TiO2 nanoparticles with certain hydrophobicity and high surface-to-volume ratio were supported on the porous wall of the MS, and the rough structure strengthened its hydrophobicity. After further modification by hydrophobic PDMS, the hydrophobicity of the sponge wall was further strengthened, so that the sponge formed rich hydrophobic pores with multistage structure. The hydrophobicity of MS/TiO2/PDMS sponge was significantly enhanced, which enhanced its oil–water separation efficiency. Furthermore, under the action of capillary force, the oil is easily adsorbed into the sponge, while water is completely excluded from the hydrophobic surface due to the high surface tension, indicating that MS/TiO2/PDMS sponge has the ability of oil–water separation. Due to oleophilic silanization of PDMS on the sponge's interconnected skeleton, the adsorbed oil rapidly diffuses into the interior space of the sponge. The oil is then stored inside the porous MS/TiO2/PDMS sponge, leading to its high oil adsorption capacity (Gao et al. 2018). In a word, the oil adsorption capacity of the MS/TiO2/PDMS sponge after hydrophobicity modification is determined by the double factors of multistage structural pore and hydrophobic pore wall.

Figure 15

Schematic of the oil–water separation mechanism with MS/TiO2/PDMS sponge.

Figure 15

Schematic of the oil–water separation mechanism with MS/TiO2/PDMS sponge.

CONCLUSION

In this paper, the MS was chosen as the raw material to prepare the MS/TiO2 sponge via the sol-gel process. The superhydrophobic MS/TiO2/PDMS sponges were then prepared by dipping MS/TiO2 sponge in PDMS solution. These fabricated MS/TiO2/PDMS sponges displayed excellent hydrophobilicity, and also higher oil–water selectivity and adsorption capacity for different kinds of oils. The results showed that the optimal hydrophobicity of the prepared MS/TiO2/PDMS sponge was achieved when the TBOT concentration was 20%, the PDMS concentration was 2%, and the modification time was 3.5 h. The contact angle of the MS/TiO2/PDMS sponge reached149.2°, while the adsorption capacity was 98.5 g·g−1. After the oil–water separation test, the MS/TiO2/PDMS sponge was recycled more than 25 times. The super oil adsorption capacity of the MS/TiO2/PDMS sponge after hydrophobicity modification is determined by the double factors of multistage structural pore and hydrophobic pore wall. These conclusions suggest that by changing the chemical composition of the surface of the 3D porous material, it is possible to prepare oil-adsorbing materials that have super oil–water selectivity properties, and are convenient and environmentally friendly to use. These materials have potential to be widely used in oil–water separation processes.

ACKNOWLEDGEMENTS

This paper was financially supported by the project of the National Science Foundation of China (21303135), the Natural Science Foundation of Shaanxi Province (2021JM-509, 2021JQ-798), the Youth Innovation Team of Shaanxi Universities (Environmental Pollution Monitoring and Control Innovation Team, 51), and Research Team of Xi'an University (XAWLKYTD018).

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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