Novel porous oil-water separation material with super-hydrophobicity and super-oleophilicity prepared from beeswax, lignin, and cotton

Journal Pre-proof Novel porous oil-water separation material with superhydrophobicity and super-oleophilicity prepared from beeswax, lignin, and cotton

Yuqing Zhang, Yiwen Zhang, Qiping Cao, Chunyu Wang, Chao Yang, Yao Li, Jinghui Zhou PII:

S0048-9697(19)35802-4

DOI:

https://doi.org/10.1016/j.scitotenv.2019.135807

Reference:

STOTEN 135807

To appear in:

Science of the Total Environment

Received date:

11 September 2019

Revised date:

17 November 2019

Accepted date:

26 November 2019

Please cite this article as: Y. Zhang, Y. Zhang, Q. Cao, et al., Novel porous oil-water separation material with super-hydrophobicity and super-oleophilicity prepared from beeswax, lignin, and cotton, Science of the Total Environment (2019), https://doi.org/ 10.1016/j.scitotenv.2019.135807

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© 2019 Published by Elsevier.

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Novel Porous Oil-water Separation Material with Super-hydrophobicity and Super-oleophilicity Prepared from Beeswax, Lignin, and Cotton

Yuqing Zhang, Yiwen Zhang, Qiping Cao, Chunyu Wang, Chao Yang, Yao Li*, Jinghui Zhou

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Liaoning Province Key Laboratory of Pulp and Papermaking Engineering, Dalian Polytechnic University, Qinggongyuan NO.1, Ganjingzi District, Dalian, Liaoning

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Province, l16034, China.

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* To whom correspondence should be addressed.

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E-mail: [email protected]

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Abstract: The traditional fluorinated porous material with super-hydrophobicity and super-oleophilicity is an effective strategy for oil-water separation. However, in recent years, fluorinated materials have been classified as “Emerging Environmental Pollutants” by U. S. Environmental Protection Agency because of difficult

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degradation and bio-accumulation. It is unacceptable to introduce new pollutants while solving environmental disasters. Therefore, it is great requirement to explore a

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low-cost, environmentally friendly, and renewable technique for the fabrication of

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novel porous materials with super-hydrophobicity and super-oleophilicity to separate

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oil-water mixtures. In this work, renewable beeswax, lignin, and cotton have been

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chosen to prepare the biomass-based porous materials with super-hydrophobicity and

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super-oleophilicity for oil-water separation. The mixture of beeswax and lignin is modified on the surface of cotton to obtain the biomass-based porous materials with

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super-hydrophobicity and super-oleophilicity. The beeswax and lignin provide low surface energy and micro/nanoscale structures, respectively. The introduction of lignin effectively improves the thermal stability of the porous materials. The apparent contact angle still remains to be above 150° after a long-time heating. The porous materials effectively separate oil-water mixtures and have good absorption effect for heavy oil (density greater than water). Moreover, the porous materials are easily recyclable after reactivation. This strategy of preparing oil-water separation materials from renewable natural polymers not only helps to clean the environment, but also helps to recover valuable oil.

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Keywords: Biomass-based; Super-hydrophobic; Oil-water separation material

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1. Introduction Oil spill accidents in the Gulf of Mexico (2010) and New Zealand (2011) caused billions of gallons of oil spill(Prathap and Sureshan, 2017). These oil spills are difficult to clean up and recover(Mao et al., 2018), and thus result in colossal economic damage and severe long-lasting environmental disasters(Barry et al., 2018).

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Therefore, for environmental protection and economic development, many strategies have been developed to prepare oil-water separation systems to eliminate the

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environmental pollution of oil quickly(Darmanin and Guittard, 2014; Ge et al., 2017;

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Gupta et al., 2017; Li et al., 2019). Among these, the biomimetic porous material with

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super-hydrophobicity and super-oleophilicity is an effective and simple strategy to

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quickly separate oil-water mixture(Chen and Chen, 2019; Tursi et al., 2018), control

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environmental pollution and recover crude oil(Cao et al., 2016; Tursi et al., 2019). The ideal porous material for oil-water separation should be super-hydrophobic

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and super-oleophilic to ensure that it selectively absorbs oil rather than water(Chen and Zheng, 2014). The super-hydrophobic phenomena widely exist in the plant leaves, fish skins, and insect wings(Darmanin and Guittard, 2014). Lotus leaves (Nelumbo nucifera) have evolved super-hydrophobicity to resist the invasion of external liquid and sludge. Liquid drops are almost spherical, rolling freely and cleaning the sludge on the lotus leaf surface. This special super-hydrophobic (high water contact angle and small sliding angle) is attributed to two main factors cause: biological wax and micro/nanoscale structures on the lotus leaf surface(Li et al., 2018b; Shi et al., 2013). The biological wax provides low surface energy(Zhang et al., 2019), which is

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conducive to reducing liquid wettability. The micro/nanoscale structures could trap vapor pockets(Fang et al., 2010; Gentile et al., 2011; Piao and Park, 2015), increasing gas/liquid contact area, so that the wetting behavior of liquid drops as a Cassie physical model(Chen et al., 2014; Wu et al., 2017). Recently, many techniques have been chosen to prepare the oil-water separation

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material with super-hydrophobicity and super-oleophilicity (Boinovich et al., 2014; Darmanin and Guittard, 2014; Emelyanenko et al., 2017; Manna and Lynn, 2013; Pan

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et al., 2018; Wang et al., 2014; Zhao et al., 2012). A main approach is to modify the

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surface of porous materials with small fluorine-containing molecules(Liao et al., 2014;

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Wang et al., 2013). However, these fluorinated porous materials are fabricated by

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sophisticated processes(Wang et al., 2013), which are prohibitively expensive and

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uncontrollable for large-areas. Moreover, U. S. Environmental Protection Agency has classified the fluorinated materials as “Emerging Environmental Pollutants” because

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of difficult degradation and bio-accumulation. It is unacceptable to introduce new pollutants while solving environmental disasters. Therefore, it is a great challenge to explore a low-cost, easy-to-operate, environmentally friendly(Li et al., 2018a), and renewable

technique

for

the

fabrication

of

the

porous

materials

with

super-hydrophobicity and super-oleophilicity to separate oil-water mixtures(Greca et al., 2018; Limongi et al., 2013). Beeswax is a kind of biological wax secreted by bees to build honeycombs, and it is also a main by-product in the process of honey preparation. Lignin is the second most abundant biomacromolecule with three-dimensional structures(Liu et al., 2018;

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Pu et al., 2019b). In the pulp and paper industry, only a small fraction (about 2%) of the 500 million tons of lignin produced annually is effectively utilized(Pu et al., 2019a), and most of the lignin is burned or discharged into rivers. The utilization of these precious renewable biological resources is of great significance. Being inspired by nature, in this work, renewable beeswax, lignin, and cotton have been chosen to

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prepare the biomass-based porous materials with super-hydrophobicity and super-oleophilicity for oil-water separation. The beeswax and lignin provide low

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surface energy and micro/nanoscale structures, respectively. The mixture of beeswax

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and lignin is modified on the surface of cotton to obtain the biomass-based porous

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materials with super-hydrophobicity and super-oleophilicity. The introduction of

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lignin effectively improves the thermal stability of the porous materials (the

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super-hydrophobicity and super-oleophilicity is well maintained at high temperature for a long time). The porous materials effectively separate oil-water mixtures and

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have good absorption effect for heavy oil (density greater than water). 2. Experimental section 2.1. Materials

Beeswax was purchased from Aldrich Chemical Co. (Shanghai, China). Lignin is poplar lignin purchased from Dongsheng New Materials Co., Ltd. N-hexane, chloroform, and other chemicals were purchased from commercial suppliers and used without further purification. 2.2. Preparation of the super-hydrophobic coating

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Beeswax (0.2 g) and lignin (0.1 g) were mixed and heated at 100 °C for 20 min to melt the beeswax completely in a 50 ml beaker. The mixture was stirred to make the beeswax completely wrapped around the lignin surface. Then, n-hexane (30 ml) was added to obtain a uniform suspension. The suspension was sprayed onto the surface of the cotton which was held at a distance of 12 cm from the spray coater

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nozzle. The pressure during spraying was kept constant at 30 psi. Finally, the super-hydrophobic cotton (SC-2) was obtained after leaving in the room temperature

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for 24 h. SC-1 (containing 0 g of lignin and 0.3 g of beeswax), SC-3 (containing 0.15

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g of lignin and 0.15 g of beeswax) and SC-4 (containing 0.2 g of lignin and 0.1 g of

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beeswax) were obtained according to the above similar process with different

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beeswax and lignin content. The preparation schematic diagram of the biomass-based

Figure 1a.

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2.3. Characterization

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porous materials with super-hydrophobicity and super-oleophilicity is shown in

Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer Spectrum 10 FTIR infrared spectrometer (PerkinElmer, Norwalk, CT, United States) in the wavelength range of 4000−1000 cm−1. Scanning electron microscopy (SEM) was conducted on a JSM 7800F electron microscope (JEOL, Tokyo, Japan) with the primary electron energy of 10 kV. The decomposition temperature thermogravimetric analysis (TGA) was performed on a Q50 type thermogravimetric analyzer (TA, United States). The test conditions were: nitrogen flow rate of 40 ml/min; heating rate of 20 °C/min; temperature range of 30 to 600 °C. Differential scanning calorimetry

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(DSC) was performed with a Q200 differential scanning calorimeter (TA, United States) at a heating rate of 10 °C/min under a nitrogen atmosphere. The measurements of water contact angle were performed using a Data Physics OCA 35 goniometer (Biolin Scientific, Sweden/Finland) equipped with SCA 20 software. The type of the spray coater is HD-130, and the size for nozzle is 0.3mm. For static water contact

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angles measurements, 10μL of water were dropped onto the sample surface and the water contact angle was measured at a relative humidity of 50 ± 5% and a temperature

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of 23 ± 2 °C. Each surface was measured at least five times so that the average static

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water contact angle could be determined. For the oil-water separation efficiency test,

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the samples were cut into 4 × 4 cm2 pieces and measured at 23 ± 2 °C and 50 ± 5%

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relative humidity. After recording the thickness and initial mass, the sample was

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completely immersed in a layered solution of chloroform and water. Subsequently, the sample was taken out of the water and weighed again, and the amount of oil

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absorbed was calculated. Each sample was measured at least three times so that the average oil-water separation efficiency could be determined. 3. Result and discussion

3.1. Introduction of super-hydrophobic and super-oleophilic surface

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Fig. 1 (a) Preparation schematic diagram and (b) oil-water separation mechanism diagram of the biomass-based porous materials with super-hydrophobicity and super-oleophilicity

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According to the theory of surface thermodynamics, the wetting behavior of

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liquids is attributable to the reduced interfacial free energy, which is mainly determined by surface chemical composition, microscopic geometry and macroscopic geometry. The super-hydrophobicity of the surface refers to the process by which the gas

on

the

solid

surface

cannot

be

replaced

by

the

liquid.

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hydrophilic-hydrophobic properties of the surface are generally determined by the wettability of pure water. The wettability is determined by the Young’s equation: 𝛾𝑠𝑔 = 𝛾𝑠𝑙 + 𝛾𝑙𝑔 cos 𝜃 𝛾𝑠𝑔 , 𝛾𝑠𝑙 and 𝛾𝑙𝑔 represent the interfacial tension at the solid/gas, solid/liquid and liquid/gas interfaces, respectively(Feng and Jiang, 2006). The equation applies to a smooth surface, and resulting contact angle is called the intrinsic contact angle.

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However, few solid surfaces are truly smooth. The influence of roughness should be considered on the wettability of the surface(Tian and Jiang, 2013). The actual wettability is briefly categorized by Wenzel physical model or Cassie physical model. Wenzel physical model refers to the infiltration of liquid at the surface containing microstructures,

and

the

wettability

increases

with

the

increase

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microstructures(Zhang et al., 2015). Cassie physical model means that the hydrophobic region is not easily invaded into the surface microstructure, thereby

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trapping air to produce a gas film. The liquid drops appear to "sit" above the tip of the

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surface microstructure, appearing as a rounded sphere with a small rolling angle(Zhu

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et al., 2016). The oil-water separation mechanism diagram of the biomass-based

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porous materials with super-hydrophobicity and super-oleophilicity is shown in

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Figure 1b. The lignin particles coated with beeswax adhere to cotton surface with air flow. The beeswax and lignin provide low surface energy and micro/nanoscale

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structures, respectively. On this surface, water drops are almost spherical, rolling freely on this surface. This wetting behavior of water drops is Cassie physical model. On the contrary, the oil drops show Wenzel physical model, which are instantly absorbed and form a flat oil film once touched. This special wettability (super-hydrophobic and super-oleophilic) is necessary for the oil-water separation.

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Fig. 2 (a) Image of water and oil drop on the cotton surface; (b) image of water and oil drop on the

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biomass-based porous material with super-hydrophobicity and super-oleophilicity (SC-2) surface;

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(c) flow scour image of the biomass-based porous material with super-hydrophobicity and super-oleophilicity (SC-2)

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Figure 2a shows the images of water and oil drop on the cotton surface. The

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cotton not only absorbs oil drop, but also water drop. It's obvious that the cotton cannot selectively absorb oil drop instead of water drop. Compared with the cotton, the biomass-based porous material (SC-2) exhibits super-hydrophobic property obviously (Figure 2b). The water drop is almost spherical, rolling freely on the SC-2 surface. On the contrary, the oil drop is instantly absorbed and forms a flat oil film once touched with the SC-2 surface. This selective absorption (absorbing only oil does not absorb water) can effectively separate oil drops from water. Figure 2c shows the flow scour image of the biomass-based porous material with super-hydrophobicity and super-oleophilicity (SC-2). Water flow is bounced off the SC-2 surface, leaving

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no water drops on the SC-2 surface. It is noteworthy that no substance is taken away from the SC-2 surface during the process of water drops bouncing off.

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3.2. Surface morphologies

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Fig. 3 (a) SEM image of the SC-1 surface; (b) SEM image of the SC-2 surface; (c) SEM image of the SC-3 surface; (d) SEM image of the SC-4 surface

This special super-wettability (super-hydrophobic and super-oleophilic) is attributed to two main factors cause: biological wax and micro/nanoscale structures on the biomass-based porous material surface. The surface morphologies of the biomass-based porous materials with super-hydrophobicity and super-oleophilicity are shown in Figures 3. It is obvious that the original fibrous morphologies of the cotton are still maintained after the cotton was processed into the biomass-based porous materials with super-hydrophobicity and super-oleophilicity. Furthermore, many micron-sized papillary structures (size of 0–5μm) were found on the

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biomass-based porous materials surface. With the increase of the lignin content, these micron-sized papillary structures increase obviously. Figures 3 (inset) shows a high magnification

SEM

image

of

the

biomass-based

porous

materials

with

super-hydrophobicity and super-oleophilicity. Many nano-scale wrinkle structures (size of 50-80 nm) are found on the micron-sized papillary structure surface. These

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micro/nanoscale structures are similar to the lotus leaf surface, which is necessary to obtain the super-hydrophobicity and super-oleophilicity simultaneously.

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3.3. Stability

Fig. 4 (a) Contact angle of aqueous solution with different pH values on the SC-1 surface; (b) contact angle of aqueous solution with different pH values on the SC-2 surface; (c) contact angle of aqueous solution with different pH values on the SC-3 surface; (d) contact angle of aqueous solution with different pH values on the SC-4 surface

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In the practical application for oil-water separation, the biomass-based porous oil-water separation material often need to be exposed to the strong acid solution and strong alkali solution, especially in the treatment of wastewater in factories. Therefore, the biomass-based porous oil-water separation material is necessary for the stability of acidic and alkaline solutions. Fig. 4 is contact angle of aqueous solution with different

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pH values on the surface of the biomass-based porous oil-water separation materials. It is obvious that the contact angle of the coating surface is maintained at about 150°

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for the strong acid solution and strong alkali solution such as pH=2 and 13. All

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biomass-based porous oil-water separation materials exhibit super-hydrophobicity for

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the strong acid solution and strong alkali solution. These results imply that the

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biomass-based porous oil-water separation materials can effectively prevent the

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infiltration of the strong acid solution and strong alkali solution and be applied for oil-water separation in the oil-water mixing system with extreme pH. This

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phenomenon is mainly attributed to the micro/nanoscale structures of the biomass-based porous oil-water separation materials effectively capture a large amount of gas, and make it form a layer of air film. However, super-hydrophobicity of the biomass-based porous oil-water separation materials decreased significantly after long time immersion in strong acid solution and strong alkali solution. The wetting state of the super-hydrophobic interface is the Cassie model. The Cassie physical model means that the hydrophobic region is not easily invaded into the surface microstructure, thereby trapping air to produce a gas film. The liquid drops appear to "sit" above the tip of the surface microstructure, appearing as a rounded

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sphere with a small rolling angle. In fact, the contact area of the acid or alkali solution with the surface of the biomass-based porous material with super-hydrophobicity and super-oleophilicity is very limited, so it can withstand the attack of acid and alkali in a

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short time.

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Fig. 6 (a) Water contact angle of the biomass-based porous materials with super-hydrophobicity and super-oleophilicity after being heated at 120 °C for 60 min; (b) temperature resistance

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mechanism of the biomass-based porous materials with super-hydrophobicity and super-oleophilicity; (c) SEM image of SC-1 surface after being heated at 120 °C for 60 min; (d) SEM image of SC-2 surface after being heated at 120 °C for 60 min; (e) SEM image of SC-3 surface after being heated at 120 °C for 60 min; (f) SEM image of SC-4 surface after being heated at 120 °C for 60 min

High temperature environments are often experienced in the application and storage of the biomass-based porous oil-water separation materials. Therefore, the temperature stability of the biomass-based porous oil-water separation materials is very necessary. As shown in Figure 6a, the SC-1 has lost its super-hydrophobicity

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(water contact angle from 163° to 80°) after being heated at 120 °C for 15 min. In contrast, the super-hydrophobicity of the biomass-based porous materials containing the lignin is maintained after being heated at 120 °C for 60 min. The water contact angle of SC-2, SC-3, and SC-4 remains to be above 135°. This phenomenon is mainly attributed to the beeswax melting at high temperature, resulting in the collapse of

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micro/nanoscale structure and then decreasing super-hydrophobicity (Figure 6b). In contrast, with the introduction of lignin, the micro/nanoscale structures are maintained.

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The introduction of lignin with three-dimensional structures effectively supports the

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micro/nanoscale structures, thus maintaining the micro/nanoscale structures of the

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biomass-based porous materials in high temperature environment. In order to prove

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this mechanism, the surface morphologies of the biomass-based porous materials after

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being heated at 120 °C for 60 min are shown in Figure 6c-f. As shown in Figure 6c, the micron-sized papillary structures on the surface of SC-1 disappear obviously, only

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a few nano-scale wrinkle structures are preserved. This result suggests that 120 °C has exceeded the melting point of beeswax, and melting of beeswax results in the collapse of the original micron-sized papillary structures on the SC-1 (without the lignin), then decreasing the super-hydrophobicity. With the introduction of lignin, the original micron-sized papillary structures and nano-scale wrinkle structures for the SC-2 are maintained. It is noteworthy that the papillary structures increase significantly with the increase of lignin content (Figure 6f). This trend is consistent with the surface morphologies of the biomass-based porous materials before heat treatment (Figure 3), suggesting that these micron-sized papillary structures that do not melt are lignin. The

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introduction of lignin effectively prevents the micro/nanoscale structural damage caused by the melting of beeswax. That’s the reason why the contact angles of SC-2, SC-3, and SC-4 have not changed significantly after being heated at high temperature.

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3.4. Surface density

Fig. 5 (a) Water contact angle of SC-1 surface with a different surface density; (b) water contact angle of SC-2 surface with a different surface density; (c) water contact angle of SC-3 surface with a different surface density; (d) water contact angle of SC-4 surface with a different surface density

Because

the

particular

super-wettability

(super-hydrophobicity

and

super-oleophilicity) is caused by two main factors: bio-wax and surface micro/nanostructures, the surface density (ρs, the weight per unit area of the coating) has a strong influence on the super-hydrophobicity of the biomass-based porous

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materials. As shown in Figure 5, with an insufficient surface density, the water drops on the biomass-based porous materials surface exhibits a low contact angle (about 80°), which is not suitable for application of the super-hydrophobic materials. With the increase of surface density, the hydrophobicity of the biomass-based porous materials y increases significantly. The SC-1, SC-2, SC-3, and SC-4 achieve super-hydrophobicity at the surface densities of 2.67 mg/cm 2, 3.56 mg/cm 2, 3.56

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mg/cm 2, and 3.56 mg/cm 2, respectively. It is noteworthy that the porous materials

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with the large lignin content need more surface coverage density to obtain

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super-hydrophobic properties. This phenomenon is mainly attributed to the lignin

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with three-dimensional network structure reduces the continuity of the coating. At the

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same time, the hydroxyl groups in the chemical structure of lignin increase the surface

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energy of the whole film. Furthermore, continuing to increase surface density (SC-1: ρs ≥ 2.67 mg/cm2, SC-2: ρs ≥ 3.56 mg/cm2, SC-3: ρs ≥ 3.56 mg/cm2, SC-4: ρs ≥ 3.56

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mg/cm2) has no effect on the apparent contact angle. This result implies that the increase of the surface energy only changes the thickness of the coating without changing the surface morphologies of the biomass-based porous materials. 3.5. Oil-water separation performance

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Fig. 7 (a) Oil/water separation process of the biomass-based porous materials with

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super-hydrophobicity and super-oleophilicity for the low density oil; (b) oil/water separation

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process of the biomass-based porous materials with super-hydrophobicity and super-oleophilicity

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for the high density oil; (c) oil/water separation process of the cotton for the high density oil

As mentioned above, the biomass-based super-wettability porous materials

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exhibit super-hydrophobicity and super-oleophilicity simultaneously, and therefore are suitable for exploiting in oil/water separation. Figure 7 shows a mixture of oil and water, in which the colorless transparent part is water and the purple part is oil dyed with crystal violet dyes. As shown in Figure 7a, the biomass-based porous oil-water separation material (SC-4) floats on the surface of water and cannot absorb any water drops on the air/water interface, showing extreme super-hydrophobicity. Once contacted, the floating oil drop (ethyl acetate) is quickly and selectively collected inside the biomass-based porous material. The oil drops in the treated oil/water mixture have completely disappeared. In the practical application of oil-water

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separation materials, it is often necessary to separate oil drop whose density is greater than water. Figure 7b shows the oil/water separation process of the biomass-based porous materials for the high density oil. The high density oil drops (trichloromethane) precipitate at the bottom of beaker. By applying an external force, the biomass-based porous material (SC-4) sinks into the beaker, and no water drop is absorbed. It is

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noteworthy that a layer of air film appears on the SC-4 surface. It is because of the existence of air layer that the biomass-based porous material prevents water from

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penetrating into the interior. Once contacted, the floating oil drop (trichloromethane)

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is quickly and selectively collected inside the biomass-based porous material. By

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contrast, the cotton is completely wetted by water before coming into contact with the

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oil drop (Figure 7c), and therefore the uncoated cotton is completely ineffective in

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removing the sedimentary the high density oil under water.

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Fig. 8 (a) SEM image of recycled SC-1 surface; (b) SEM image of recycled SC-2 surface; (c) SEM image of recycled SC-3 surface; (d) SEM image of recycled SC-4 surface

As a porous material for environmental pollution control, its environmental friendliness and reusability are very important. A similar spraying process is used to reactivate the biomass-based porous materials to regain their super-hydrophobicity.

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The recycled surface morphologies of the biomass-based porous oil-water separation materials with are shown in Figure 8. After recycling, the original micron-sized

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papillary structures and nano-scale wrinkle structures are reappeared. With the

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increase of the lignin content, these micron-sized papillary structures increase

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obviously. The results are similar to those of the original biomass-based porous

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materials with super-hydrophobicity and super-oleophilicity. There is no difference

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between the structure after recycling and the original super-hydrophobicity and

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super-oleophilicity porous materials.

Fig. 9 (a) The absorption ratio of SC-3 for different kinds of model oil; (b) the absorption ratio of the biomass-based porous materials with super-hydrophobicity and super-oleophilicity for

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trichloromethane/water separation; (c) the absorption ratio of the biomass-based porous materials with super-hydrophobicity and super-oleophilicity after being heated for trichloromethane/water separation; (d) the absorption ratio of the biomass-based porous materials with super-hydrophobicity and super-oleophilicity after recycling for trichloromethane/water separation; (e) the recovery rate of the biomass-based porous materials with super-hydrophobicity and

The

biomass-based

super-wettability

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super-oleophilicity for trichloromethane/water separation

porous

materials

exhibit

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super-hydrophobicity and super-oleophilicity simultaneously, and therefore are

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suitable for exploiting in oil/water separation. Figure 9a show the oil-water separation

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efficiency of the biomass-based porous material (SC-3) for the different kinds of

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model oil. The results show that the biomass-based porous materials can absorb

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model oil (ethyl acetate, soybean oil, methylbenzene, and trichloromethane) more than 17 times their own weight in oil/water separation. The absorption ratio for

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trichloromethane/water separation is more than 20 times its own weight. Therefore, the trichloromethane is used as a model oil to further evaluate the oil/water separation efficiency of the biomass-based porous materials. As shown in Figure 9b, the absorption ratio of biomass-based porous materials for trichloromethane/water separation is more than 20 times their own weight. With the increase of the lignin content, the oil/water separation efficiency of the biomass-based porous materials for trichloromethane

increases

slightly.

The

absorption

ratio

of

SC-4

for

trichloromethane/water separation is more than 21 times its own weight. High temperature environments are often experienced in the application of the

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biomass-based porous materials with super-hydrophobicity and super-oleophilicity. So it is necessary to evaluate their oil/water separation efficiency after high temperature treatment. As shown in Figure 9c, the absorption ratio of SC-1 for trichloromethane/water separation decreases to 17 times of their own weight after being heated at 120 °C for 60 min. With the increase of lignin content, the absorption

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ratio of the biomass-based porous materials for trichloromethane/water separation increase gradually, and the absorption ratio of SC-4 for trichloromethane/water

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separation still maintains 20 times its own weight. This phenomenon is attributed to

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the introduction of lignin. Lignin effectively supports the micro/nanoscale structure,

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thus maintaining the micro/nanoscale structure of the biomass-based porous materials

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in high temperature environment. So that the absorption ratio of SC-4 for

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trichloromethane/water separation is not significantly lower than before. As a porous material for environmental pollution control, the reusability of the biomass-based

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porous materials with super-hydrophobicity and super-oleophilicity is very important. A similar spraying process is used to reactivate the biomass-based porous materials to regain their super-hydrophobicity. As shown in Figure 9d, the absorption ratio of the biomass-based porous materials for trichloromethane/water separation still maintains more than 18.4 times its own weight. A condensation reflux device is designed to recover separated model oil (trichloromethane). As shown in Figure 9e, the recovery rate of the biomass-based porous materials for trichloromethane/water separation reaches at least 88.92% and the highest is 93.78%. The recovered oil can still be

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reused, which has good utilization value in practical applications. In our future research, we will develop materials that can be reused without re-spraying. 4. Conclusion This work shows a simple, low-cost, and environmentally friendly preparation method

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biomass-based

porous

oil-water

separation

material

with

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super-hydrophobicity and super-oleophilicity. We have chosen cotton as the matrix and beeswax/lignin compound as the super-hydrophobicity and super-oleophilicity

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functional structure. The beeswax/lignin compound adsorb to the matrix through

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hydrogen bonds making the cotton simultaneously super-hydrophobicity and

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super-oleophilicity. Due to the super-oleophilicity and high porosity of the porous

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oil-water separation material, it absorbs the oil into the voids instantaneously. The

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biomass-based porous materials show very high oil up-taking capacity for different kinds of model oils. Furthermore, the introduction of lignin effectively improves the

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thermal stability of the biomass-based porous oil-water separation materials, thus broadening the application environment. The apparent contact angle of the biomass-based porous materials can remain to be above 150° after a long-time heating. This biomass-based porous oil-water separation material is superior to all the known oil-water separation systems as it overcomes all the usual limitations such as pure green raw materials and reusability making this material attractive towards the implementation. This strategy for preparing oil-water separation materials from renewable natural polymers helps not only cleans the environment but also to recover

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the valuable oil. In our future research, we will develop materials that can be reused without re-spraying. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 31800498, No. 31770635 and No. 31470604) References

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical abstract

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Highlights 1.

All raw materials are green and renewable.

2.

The introduction of lignin effectively improves the thermal stability of the porous materials, which will greatly expand the application range of materials. The recovery rate of the biomass-based porous materials for heavy oil /water

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lP

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separation reaches up to 93.78%.

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3.