Facile preparation of poly(melamine formaldehyde) sponge/bacterial nanocellulose composite for oil-water separation

The treatment of a large amount of oil-water mixture generated by oil leakage accidents is an environmental issue that cannot be ignored. Achieving fast and efficient oil-water separation is a goal pursued by both academia and industry. In this paper, poly(melamine formaldehyde) (PMF) sponges were modified by the hydrophilic bacterial nanocellulose (BNC) through mild and facile conditions. With increasing the concentration of BNC dispersion used, the pore size of the composite decreases. The water contact angle of the obtained PMF/BNC composite surface is 0° or so, and the underwater oil contact angle is greater than 150°. Based on its hierarchical pore structure, hydrophilicity and oil repellency, the PMF/BNC composite exhibits excellent oil-water separation efficiency and water treatment flux for layered oil-water mixture (>1.0×107L/(m2·h·bar)). In addition, the PMF/BNC composite also exhibits high water treatment flux and oil-water separation efficiency (> 95%) for oil-water emulsion with and without surfactant. Stacking several composite slices could achieve higher oil-water separation efficiency.


Introduction
The rapidly growing global energy demand is increasing for crude oil extraction, while frequent oil spills and accidents have become increasingly important environmental issues that cannot be ignored.The oil spill accident at sea has led to a surge in the content of toxic compounds in the ocean, threatening the survival of species on the marine food chain, from low-level algae to high-level mammals, and thus affecting the health and safety of humans.In addition, small scope oil spills in the chemical industry (textile, leather, metallurgy, etc.) cannot be neglected [1,2].Thus, the problem of oil spill treatment (oilwater separation) needs to be solved urgently.The conventional treatment methods for oil-water mixtures mainly include gravity sedimentation [3], liquid-liquid cyclone [4], chemical degradation [5], in-situ combustion [6], and membrane separation [7].However, these methods generally have drawbacks such as low efficiency, high cost, low processing flux, and even secondary pollution.For example, the super hydrophobic-superlipophilic polyvinylidene fluoride membrane can separate oil-water emulsion by gravity with a separation efficiency of 99.95 wt% [8].While these two-dimensional separation AMCE-2023 Journal of Physics: Conference Series 2713 (2024) 012018 IOP Publishing doi:10.1088/1742-6596/2713/1/012018 2 membranes have small pore sizes and short separation channels, they are prone to adsorb oil droplets and surfactants during the separation process, causing pore blockage and a significant decrease in treatment flux and efficiency [9].Polymer-modified stainless steel meshes are used for oil-water separation [10].The separation efficiency of some stainless steel meshes can reach 99%, but the number of cycles is limited.
Our group has introduced SiO 2 nanofibers into poly(melamine formaldehyde) (PMF) sponge and grew layered double hydroxide (LDH) nanoscrolls in situ to obtain a three-dimensional composite porous material with a micro-nano structure [11], which exhibits high water treatment flux and oil-water separation efficiency.However, the preparation of SiO 2 nanofibers requires harsh process conditions such as electrospinning and then calcination at 800°C [12].The in-situ growth of LDH nanoscrolls also was carried out at higher temperatures and special conditions [13].This paper develops a facile method to prepare the composite of PMF and hydrophilic bacterial nanocellulose (BNC) [14].Mild processes such as adsorption, oscillation, freeze-drying, and gas cross-linking at room temperature are used.Hydrophilic BNC fibre has a diameter of 20-50 nm and a length of 20-100 μm, which could enter into the PMF sponge porous with the diameter of 100 μm to adjust the pore size of PMF, so that the composite sponge exhibits hierarchical pore structure with micron sized and nanoscale pores inside.The strong hydrophilicity of BNC endows the surface and inner pores of PMF with strong hydrophilicity and oil repellency, which is expected to achieve high oil-water treatment flux and high separation efficiency simultaneously.

Preparation of PMF/BNC composite porous materials
The schematic diagram of the PMF sponge modified by BNC is shown in Figure 1.A quantitative amount of BNC dispersion was added to a bottle with deionized water.A high-speed mixer (T25 D S25, IKA company in Germany) is used to disperse BNC fibres to obtain homogeneous BNC dispersion with different concentrations (C BNC ).The PMF sponge was cut into a slice (4 cm  4 cm  5 mm), and then immersed in BNC dispersion with various C BNC to allow BNC fibres to enter the PMF pores by squeeze and diffusion.After that, the PMF slice was fast frozen using liquid nitrogen, and then dried at -46°C and 200 Pa for 12 h in a freeze-drying machine (FD-1A-50, Beijing Boyekang Experimental Instrument Co., Ltd.).20 mL of 50% glutaraldehyde aqueous solution was poured into a 1000 mL volumetric beaker with a stainless steel bracket.The freeze-dried modified sponge was placed on the bracket, and then sealed the beaker.Place the beaker at 25C for 12 h to obtain PMF/BNC composite.

Characterization
The viscosity of BNC dispersion was tested using a DHR rheometer (TA Instruments, USA).The contact angles of water and underwater oil (dichloroethane) were measured via a contact angle system (Harke-SPCA, Peking Harke Experimental Instrument Factory, China), and the final results were taken as the average of three tests.Scanning electron microscopy (SEM, Hitachi S-4800 Japan) was used to observe the morphology of PMF/BNC composite.The size of oil droplets in the emulsion was characterized by a fluorescence microscope (NIB900, Nexcope, USA) in optical mode.The concentration of oils remaining was analyzed by a Lambda 32 UV-vis absorption spectrometer (PerkinElmer Company, USA) at room temperature.

Separation of oil-water mixture
To test the separation efficiency of the modified PMF sponges, layered oil/water mixture, surfactantfree and stabilized emulsions were selected.n-hexane was mixed with water (v:v = 2:3) to form a layered oil-water mixture (the oil was dyed with red O). 1 mL of n-hexane was introduced into 99 mL distilled water, and then emulsified using a high-speed mixer (5000 rpm for 10 min) to obtain a surfactant-free emulsion.The emulsion with Tween 80 as the surfactant was also fabricated.A self-made filtration unit was constructed to evaluate the feasibility of oil/water separation, in which the modified PMF sponge was placed between two silica washers.The oil-water mixtures can simply pass through the membrane by gravity.During the oil/water separation process, the real-time flux was measured every 30 s, where the height of the mixture column was maintained at 3 cm.Through testing the oil content in feed and corresponding filtrate using the UV-vis absorption spectrometer, the oil rejection was defined.The water treatment flux according to the equation: , p gh   .Among them, F is the water treatment flux, which is the liquid volume (L), S is the cross-sectional area of the liquid column (m 2 ), d is the diameter of the liquid column (m), t is the time when the liquid completely passes through (h), p is the pressure (bar),  is the liquid density (kg/m 3 ), g is the gravitational acceleration (taken as 9.80 N/kg), and h is the height of the liquid column (m).

Viscosity of BNC dispersion with different concentration
Table 1 shows the viscosity of BNC aqueous dispersion at different concentrations (C BNC ).When C BNC = 0.3 mg/mL, its viscosity is approximately 0.12 Pas.As the concentration increases, the viscosity gradually increases.The viscosity is as high as 100 Pas at C BNC = 2.0 mg/mL.When the PMF sponge was immersed in the suspension, BNC nanofibers entered the PMF pores and overlapped on the skeleton, thus changing the pore size of the PMF sponge.In the case of low C BNC , fewer BNC nanofibers could enter into PMF pores.In BNC dispersion with high C BNC , more BNC nanofibers enter the pores and some of them even form BNC membranes.Therefore, different C BNC s give rise to the modified PMF sponges with different pore sizes, regulating their treatment flux and oil-water separation efficiency.Table 1.Viscosity of BNC suspension with different concentration C BNC (mg/mL) 0.1 0.2 0.3 0.5 1.0 2.0 Viscosity (Pas) 0.004 0.018 0.12 0.35 14.0 98.0

Effect of C BNC on the morphology of PMF/BNC composite
The effect of C BNC on the morphology of PMF/BNC composite porous material was studied as shown in Figure 2. It can be seen that BNC nanofibers are uniformly dispersed inside the PMF sponge pores in the case of lower C BNC .The modified sponge exhibits a hierarchical porous structure with a pore size of micron and nanometer.When C BNC increases to 2.0 mg/mL, more BNC nanofibers enter the sponge pores, some of them form membranes, and a few pores are blocked by the membrane.

Surface hydrophilicity and underwater hydrophobicity of PMF/BNC composite
There are numerous inherent OH groups on the surface of BNC fibres, ensuring that the modified sponges exhibit superhydrophilic and underwater superoleophobic properties.The permeation process of a water droplet was recorded by a high-speed camera (Figure 3).When the water droplet with a volume of 6 L touched the PMF/BNC (C BNC =1.0 mg/mL) composite surface, it quickly permeated into the composite sponge and the entire process was accomplished within 0.5 s, indicating that the composite has prominent water-wetting property.The water contact angles (WCA) of several PMF/BNC composites are all close to 0° (Figure 5), implying their good hydrophilicity.According to Young's equation and the Cassie equation, the superhydrophilic surface usually exhibited excellent underwater superoleophobic properties [15].Owing to the high density (1.257 g/cm 3 ), 1, 2-dichloroethane was selected as model oil.The underwater oil contact angle (OCA) of the PMF/BNC composite (C BNC =1.0 mg/mL) was 152.8°.During the testing, obvious deformation was hardly observed (Figure 4), demonstrating the pretty low oil adhesion of the PMF/BNC composite surfaces.All the PMF/BNC composites exhibit high OCA values as shown in Figure 5, implying their outstanding underwater oil repellency.An n-hexane jet sprayed onto the surface of the PMF/BNC composite could bounce off from the surface underwater without leaving any traces, confirming the feasibility of the PMF/BNC composite for practical application in a water environment.

Oil-water separation performance of PMF/BNC composites
To evaluate the separation efficiency of PMF/BNC composites, layered oil/water mixture, surfactantfree and stabilized emulsions were selected.The composite sponge slice was placed into the self-made separation device and fixed with an iron clip.The mixture of 40 mL n-hexane (dyed with oil red O) and 60 mL deionized water was poured into the upper mouth as shown in Figure 6.The superhydrophilic and superhydrophobic properties of PMF/BNC composite combining their large pore size make water quickly pass through the composite solely by gravity, and the dyed n-hexane is left on the upper part of the composite, achieving oil-water separation.PMF sponges without BNC suspension treatment cannot separate layered oil-water mixtures completely.While all the PMF/BNC composites could effectively segregate oil and water in layered mixtures, and their treatment fluxes are shown in Figure 7.When C BNC is 0.5~1.5 mg/mL, the treatment flux is above 1.0 × 10 7 L/(m 2 ꞏhꞏbar).When C BNC is 2.0 mg/mL, the treatment flux of the composite sponge decreases rapidly to about 5.0 × 10 6 L/(m 2 ꞏhꞏbar), but is still larger than that of the separation membrane (up to the order of 10 5 L/(m 2 ꞏhꞏbar)) [16].Separation for oil-water emulsion was further studied.Especially, emulsion-containing surfactants usually tend to form highly stable and complex droplets, which makes it difficult to achieve effective oil-water separation through conventional materials.The emulsion was prepared using Tween 80 as the surfactant.The droplet size of oil in emulsion without and with Tween 80 is shown in Figure 8.The diameter of oil droplets in emulsion without surfactant is about 12.3 μm, while it is about 9.6 μm in the case of Tween 80.The above two emulsions were separated with single-layer PMF/BNC composites and the separation efficiency and treatment flux were shown in Figure 9(a).The treatment fluxes of several PMF/BNC composites for the two emulsions are similar, so only the treatment fluxes for the emulsion with Tween 80 are given.Similar to Figure 7, with the increase of C BNC , the treatment flux of the PMF/BNC composite sponge for emulsion decreases.The separation efficiency of PMF modified by 0.5 mg/mL BNC dispersion for Tween 80 free emulsion is about 80%.As C BNC increases, the oilwater separation efficiency increases to over 90%.The separating efficiency of PMF/BNC composite for emulsion including Tween 80 is just about 60%.With increasing C BNC , the oil-water separation efficiency can increase to 90%, but it is still lower than that of the emulsion without Tween 80.
Considering that the aggregation of oil droplets in the internal pores of the sponge is an important step during the oil-water separation process, the infiltration path of the emulsion can be extended by increasing the number of PMF/BNC composite slices to achieve high oil droplet rejection.When two PMF/BNC composite slices (C BNC =1.0 mg/mL) are stacked, the water treatment flux decreases significantly (Figure 9(b)), but the oil-water separation efficiency is significantly improved.When C BNC > 1.0 mg/mL, the separation efficiency of the modified PMF sponge for both emulsions can reach about 95%.

Reusable PMF/BNC composites
To further evaluate the separation ability of PMF/BNC composite for oil/water mixtures in practical application, the water treatment flux of single-layer PMF/BNC composite sponge (C BNC = 1.0 mg/mL) over time was studied as shown in Figure 10.The continuous treatment of oil-water emulsion (without Tween 80) sustains about 60 min, and the flux decreases slowly due to the accumulation of oil droplets on the PMF/BNC composite surface.Nevertheless, the flux is still much larger than that of a twodimensional membrane, pure BNC membrane and aerogel.After washing with alcohol, the treatment flux can recover to its initial value, demonstrating that the PMF/BNC composite has excellent stability and anti-fouling performance.For simple layered oil-water mixtures, they can be continuously processed for more than 5 hours without blockage.

Conclusion
In order to achieve excellent oil-water separation efficiency and high water flux simultaneously, BNC nanofibers were introduced into the PMF sponge skeleton to regulate the pore diameter inside.The pore size of the PMF/BNC composite decreases with increasing BNC dispersion concentration and exhibits a hierarchical pore structure with micron-sized and nanoscale pores.PMF/BNC composite shows a water contact angle of 0° and an underwater oil contact angle of >150°.The modified sponge has treatment flux for a simple layered oil-water mixture greater than 1.0 × 10 7 L/(m 2 ꞏhꞏbar).The PMF/BNC composite also exhibits high oil-water separation efficiency for oil-water emulsion without and with the surfactant, i.e. up to 95%, and high water treatment flux.Stacking several composite slices can achieve higher oil-water separation efficiency.

Figure 1 .
Figure 1.Schematic diagram of PMF sponge modified by BNC nanofibers.

Figure 2 .
Figure 2. The upper surface of PMF sponge modified by BNC dispersion with different C BNC .(the number is C BNC ).

Figure 4 .
Figure 4. Underwater OCA of PMF/ BNC composite (C BNC =1.0 mg/mL).According to Young's equation and the Cassie equation, the superhydrophilic surface usually exhibited excellent underwater superoleophobic properties[15].Owing to the high density (1.257 g/cm 3 ), 1, 2-dichloroethane was selected as model oil.The underwater oil contact angle (OCA) of the PMF/BNC composite (C BNC =1.0 mg/mL) was 152.8°.During the testing, obvious deformation was hardly observed (Figure4), demonstrating the pretty low oil adhesion of the PMF/BNC composite surfaces.All the PMF/BNC composites exhibit high OCA values as shown in Figure5, implying their outstanding underwater oil repellency.An n-hexane jet sprayed onto the surface of the PMF/BNC composite could bounce off from the surface underwater without leaving any traces, confirming the feasibility of the PMF/BNC composite for practical application in a water environment.

Figure 6 .
Figure 6.Separation process of the layered nhexane/water mixture using a self-made filtration unit.

Figure 7 .
Figure 7. Treatment flux of PMF/BNC composite modified by different C BNC for layered oil/water mixture.

Figure 8 .
Figure 8. Optical micrograph of n-hexane droplets in the emulsions (a: without surfactant; b: with surfactant).

Figure 9 .
Figure 9. Water permeation flux and oil rejection of PMF/BNC composite (a: single layer; b: double layer) for n-hexane/water emulsion with and without surfactant-free.

Figure 10
Figure 10 Water treatment flux evolution with time and recovery of PMF/BNC composite sponge (C BNC = 1.0 mg/mL) for oil-water emulsion.