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A Novel All-Nature Material for Oil/Water Separation Fajun Wang, Sheng Lei, Junfei Ou, Changquan Li, and Wen Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05535 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Industrial & Engineering Chemistry Research

A Novel All-Nature Material for Oil/Water Separation

Fajun Wang*, Sheng Lei, Junfei Ou, Changquan Li, Wen Li School of Materials and Engineering, Jiangsu University of Technology, Changzhou 213001, P. R. China

* To

whom correspondence should be addressed. Email: [email protected] (Fajun Wang)

Abstract: Traditional oil/water separation materials were prepared with partial or all non-renewable raw materials, which would not only increase resource consumption, but also cause new environmental pollution after abandoned. In this work, a superhydrophobic and superoleophilic oil/water separation all-natural material were prepared using a simple and fast emulsion immersing method. The raw materials used including natural sponges and natural vegetable waxes, all of which are renewable. The as-prepared all-natural material can not only absorb various of oils from water with high oil absorption capacity, fast oil absorption rate and good recyclability, but also achieve the reuse of the absorbed oil by squeezing. In addition, the as-prepared all-natural material exhibits strong stability against cyclic compression, excellent resistance to corrosive aqueous solutions contacting and organic solvents immersing. Furthermore, the surface wax coating can be easily recycled. This work provides a new method to prepare oil/water separation materials by using all-natural and renewable raw materials.

Keywords: oil/water separation; superhydrophobicity; superoleophilicity; all-natural material; natural sponge; natural wax.

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Introduction Recently, porous materials with surface superhydrophobicity and superoleophilicy have received great attention in the field of oil/water separation due to water pollution problems caused by oil spill accidents and leakages of organic chemicals.1-3 The famous natural superhydrophobic surface is lotus leaf.4 The surface of lotus leaf exhibits superhydrophobicity can be attributed to the combination of rough surface micro-structures and a low-surface-energy wax layer on the surface.4, 5 Inspired by nature, a larger amount of artificial superhydrophobic surfaces have been prepared through two typical routes. The first route was constructing appropriate microstructures on the surfaces of intrinsic hydrophobic materials, such as Teflon and polydimethylsiloxane (PDMS).6, 7 The second route was creating rough structures on a solid surface first and then modifying the rough surface with low-surface-energy material, such as silane, fluorosilane and stearic acid, and so on.8−10 For oil/water separation, porous substrates are preferable because the porous structures could provided pathways for passing through of oils, while repels water from penetrating. Various synthesized and natural porous substrates, such as metallic meshes and foams,11,

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filter paper,13 sponges,14 aerogel,15

glass fibre cloth,16 fibres and fabrics have been used for fabricating superhydrophobic and superoleophilic absorbent materials for oil/water separation.17, 18 Among these oil/water separation materials, sponges have received special attention due to their excellent elasticity. After oil separation, the absorbed oil in the pores could be easily collected by mechanical squeezing, thus achieving the reuse of the separated oil.19 Synthesized sponges, including PDMS sponge,20 melamine sponge,21 polyurethane sponge and Carbon Nanotube Sponge with surface superhydrophobicity and superoleophilicity have been extensively investigated.

19, 22

However, these synthesized sponges are non-renewable materials. The heavy use of synthetic materials will result in a great consumption of non-renewable resources. 23 In addition, after oil/water separation cycles, the synthesized sponges can not be degraded in nature, which can cause environmental pollution again after throwing away. Therefore, an ideal oil/water separation material should be prepared using renewable and environmentally friendly materials, including sponge substrates, materials for constructing roughening structures, as well as low-surface-energy materials.24, 25 Marine sponges (Porifera) are an ancient group of multicellular animals.26−28 Most of marine 2


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sponges live in oceans and about 8000 species exist today.27 Natural sponges have been widely used as sanitary ware since ancient times, and the murals of ancient Greek island of Crete were described as natural sponges.26 Despite being challenged by cheap synthetic sponges, natural sponge are more widely used in bath care, art decoration, painting, laboratory, hospital, high-grade leather care due to its outstanding characteristics, such as pure natural, soft, durable, antistatic and rich in seabed minerals. In today's global-warming world, the least use of oil and petroleum derivatives is responsible for the earth. However, the natural sponge exhibits strong water absorption.26 Hence, the unmodified natural sponge can not be used in the application filed of oil/water separation. Natural waxes, such as carnauba wax and beeswax, possess the characteristics of pure natural, renewable, non-toxic, easy to surface coating processing and low surface energy.29−31 Wax based superhydrophobic coatings have been fabricated for reducing residual liquid food,28, 29 wood surface protection and hydrophobic treatment of paper due to their highly hydrophobic nature.31−33 Compared with other preparation methods, natural wax can simplify the preparation process, because the formation of coating, the constructing of surface rough structures and the modification of low-surface-energy materials can be completed in one step without post treatment. To the best of our knowledge, the oil/water separation properties of a superhydrophobic and superoleophilic natural sponge modified with natural waxes have not been reported. This work is focused on developing all natural oil/water separation materials which are completely harmless to the environment. For this purpose, natural sponge (NS) is selected as porous substrate. Two types of natural waxes, that is, rice bran wax (RBW) and candelilla wax (CW), were selected as coating material in order to endow the surface of NS with superhydrophobicity and superoleophilicity. Studies on the preparation of superhydrophobic surface from these two vegetable waxes have not been reported. Therefore, developing RBW and CW will help to not only reduce the consumption of carnauba wax and beeswax, but also expand the application fields of other natural waxes, both of which are very beneficial to the rational and effective utilization of various natural wax resources. The superhydrophobic and superoleophilic natural sponge was prepared using a simple emulsion immersing method. The modified natural sponge is stable against repeatedly compressing. In addition, the modified sponge shows highly resistance to various corrosive aqueous solutions and organic solvents. The modified natural sponge can absorb both light oils and heavy oils from water. Importantly, the absorbed oil can be collected for reuse by squeezing. Furthermore, after 3



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reaching the service life, the waxes can be recycled by a simple immersing method and the recycled wax emulsion can be used for preparing superhydrophobic coating again. The method adopted in this work maximizes the use of renewable resources while recycling the waste, which is conducive to the sustainable development of resources and environment.

Fig.1 (a) Ethanol was heated to reflux; (b) wax pellets were added successively into hot ethanol; (c) the suspension was stirred continuously and a wax-in-ethanol emulsion was formed; (d) the emulsion was transferred to a water bath to maintain a constant temperature of 60 °C；(e) a piece of SS was immersed into the suspension; (f) the sample was compressed using two glass rod; (g) the SS sample was taken out; (h) the wax modified SS sample was obtained after naturally cooled.

2. Experimental 2.1. Materials Natural sponge (NS) was purchased from the local market. A large NS was cut into pieces with a typical size in the range of 2 cm to 4 cm. Rice bran wax (RBW) and candelilla wax (CW) were purchased from Jiangsu Tianliao new material co. LTD, see Figure S1 in Supplementary Materials (SM). Analytically pure ethyl alcohol and chloroform was obtained from Shanghai chemical reagent co. LTD. Deionized water was prepared in the lab using an ultra-pure water system. Hexadecane was purchase form Aladdin chemical reagent co. LTD. Kerosene was obtained from Sinopec. Rapeseed oil was purchased from supermarket. All of the materials were used without any purification. 2.2 Preparation 4


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The wax modified NS samples were prepared using a simple solution immersing method. The preparation procedure was illustrated in Fig.1. Briefly, 200 ML of ethyl alcohol was heated to reflux under stirring [Fig. 1(a)]. Then, 0.2 g CW and 0.2 g RBW were added successively into the hot ethyl alcohol [Fig.1(b)]. An emulsion with a concentration of 2.0 mg/ML was formed after 30 min of stirring [Fig.1(c)]. Afterwards, the wax-in-ethanol emulsion was transfer to a water bath and the temperature of the emulsion was maintained at about 60 C° [Fig.1(d)]. A piece of NS was immersed into the hot wax-in-ethanol emulsion using two glass stirring rods [Fig.1(e) and (f)]. The NS sample was squeezed in the emulsion and then taken out [Fig.1(e) and (f)]. After naturally cooled to room temperature, the wax modified NS (MNS) samples were obtained [Fig.1(g) and (h)]. Different modified NS samples were obtained by repeating the above process of immersing, squeezing and drying.

2.3 Characterization Contact angles and rolling angles for various liquids (water, hexadecane, Kerosene, rapeseed oil, chloroform) were measured using a droplet shape analyzer (KRÜSS DSA30, Germany). The surface morphologies of the NS samples before and after wax modification were observed using a Field Emission Scanning Electron Microscope (FESEM, FEI, QUANTA F250). The composition of NS were measured using Energy Disperse Spectroscopy (EDS, GENESIS, EDAX company). The chemical bonds and functional groups of the samples were measured using an FT-IR Spectrometer (NICOLET 5700, America) under reflective mode. The bouncing behaviour of a water droplet on the superhydrophobic surface of the sponge was captured by a high-speed camera (5KF10C, Fuhuangjunda, China).

2.4 Oil/water separation property measurement Four kinds of representative oils, i.e., hexadecane, chloroform, kerosene and rapeseed oil, were chosen for the measurement of oil/water separating properties of the modified NS. Hexadecane was represented as a water insoluble organic compound which has a lower density than water, while chloroform was insoluble in water and heavier than water. Kerosene was used as the representative of fuel oil. Rapeseed oil was used as the representative of edible oil. The oil absorption capacity (κ) was calculated using the following formula:20, 33 5



κ

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m1  m0 m0

Where m0 is the weight of MNS before oil adsorption and m1 is the weight of MNS after it is saturated with oil (i.e., maximum oil absorption). The oil absorption capacity was measured at room temperature. The oil separation efficiency (η) was calculated using the following formula:20, 33

  V VV 0

1

0

Where V0 is the volume of oil mixed with water and V1 is the volume of the collected oil. Both the oil absorption capacity and separation efficiency were measured at room temperature. The oil adsorption kinetics of MNS was determined by measuring the weight of MNS at different adsorption time by making contact with oil. Then, the kinetics of four types of oils adsorption by the MNS were analyzed by pseudo first-order kinetic model according to literature:33, 34

Ln (qs  qt )  Ln(qs )  K1t Where qs is the amount of oil absorbed by MNS (g/g) at saturation, qt is the amount of oil adsorbed by MNS (g/g) at time, K1 is the rate constant (s-1)

3. Results and Discussion 3.1. Characterization of the pristine natural sponge

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Fig.2 (a) Photo of the initial NS; (b) and (c) SEM images of the pristine NS surface observed at different magnification; (d) XPS survey spectrum of the pristine NS; (e) EDS analysis of the pristine NS. Fig. 2(a) shows the appearance of a complete NS. The yellow NS displays a porous structure. The NS is made up of three dimensional interconnected fibrous framework and exhibits pore sizes ranging from 20 μm to 200 μm [see Fig.2(b)]. The diameter of the NS fibre is about 20 μm and some protrusions can be observed on fibre surface [see Fig.2(c)]. XPS spectrum indicates that only four elements exist on the fibre surface, i.e., elemental carbon, nitrogen, oxygen and calcium [Fig.2(d)]. The result of EDS analysis is completely consistent with that of XPS test [Fig.2(e)]. The total element content (At%) of C, N and O is more than 97%. Small amounts of elemental calcium (

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