ZHAO Yifan, PENG Changxin, CUI Sheng＊, WU Xiaodong, JIANG Shengjun

(1. College of Materials Science and Engineering, Nanjing Tech University, Nanjing 211800, China; 2. Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing 211800, China)

Abstract: Superhydrophobic cotton cellulose (CC) aerogel was prepared from budget cotton with huge output. CC aerogels were synthesized by using commercial cotton extract as a precursor through the nano-self-assembly process, CO2 supercritical drying process, and vapor deposition. Typical,uniform hydrophobic CC aerogel structure can be confirmed by several characterization techniques.Maximum specific surface areas before and after modification are 213.80 m2·g-1 and 184.33 m2·g-1 with average pore sizes at 14.01 nm and 32.56 nm. The water contact angle is up to 153°, showing its superhydrophobic property. The maximum adsorption efficiency can be up to 16.0 g·g-1, which exhibits excellent cycling property after 5 adsorption tests. The oil absorption mechanism of the superhydrophobic CC aerogels is investigated in detail as well. This superhydrophobic CC aerogel has excellent adsorption performance and simple, rapid, effective oil contamination treatment which has promising applications in the field of oil adsorption and water remediation in oil adsorption.

Key words: super-hydrophobic; oil adsorption; cotton cellulose; aerogel

1 Introduction

In recent years, frequent oil spill accidents have caused increasingly serious oil pollution, which has caused widespread concern in the international community[1,2]. Some statistics show that more than 100 million gallons of crude oil spill into the ocean each year[3]. To date, various technologies have been developed for oil absorption and separation, including methods such as chemical treatment, bioremediation,and physical extraction[4,5]. Among these methods,the use of specific adsorbents to remove oil directly from contaminated areas is the most effective and cost-effective[6,7]. Besides, if the absorbed crude oil can be recycled through simple extrusion, distillation,or extraction, it will bring great convenience to environmental restoration and avoid waste of resources[8,9].

Nowadays, oil sorbents with various characteristics have been produced by using natural[10,11]and synthetic[12,13]materials. In general, synthetic oil sorbents have shown better practical properties,including higher absorption and oil retention, better mechanical properties, and reusability, because these properties can be achieved through the use of appropriate fabrication techniques[14,15]. On the other hand, natural adsorbents have been considered unique advantages. They have less impact on the environment due to their biodegradability, low cost, and rich source.However, they often exhibit relatively poor absorption and retention properties. Therefore, it will be a great breakthrough in the field of environmentally friendly adsorption to produce natural oil adsorbents, which have adsorption performance comparable to synthetic adsorbents and maintains their advantages, such as biodegradability.

As one of the most promising materials in the 21st century, aerogel was born with inorganic silicaas the main raw quality[16]. With the deepening of the research on aerogel materials, biomass-based aerogels,as a new type of multifunctional materials, which have a wide range of potential applications, have attracted more and more attention from researchers due to their rich sources, low cost, and environmental friendliness. Yuanet al[17]successfully produced a compressible, super-hydrophobic biomass carbon＠SiO2＠MnO2aerogel (HBCSM) using sisal cellulose as the main raw material. They first prepared cellulose＠SiO2aerogel (CS) by a sol-gel method and then carbonized to form biomass carbon＠SiO2aerogel(BCS). Subsequently, HBCSM was prepared by assembling MnO2nanosheets in situ on the surface of the BCS aerogel under hydrothermal conditions.HBCSM has an excellent super-hydrophobicity and the water contact angle is 155°. The prepared modified HBCSM aerogel has a very large absorption capacity(60-120 g·g-1) for different oils and organic solvents.The absorption capacity keeps well after 9 cycles of repeated absorption and release of several oils and solvents slight decline. Fenget al[18]used paper waste and polyamide 3,3-epoxybutyronitrile resin (Kymene)crosslinking agent to successfully develop a simple and low-cost biocompatible cellulose aerogel. After modification, it has excellent oil absorption capacity up to 95 g·g-1whose oil absorption kinetics fits well with the pseudo-secondary model. Fanet al[19]prepared a porous, hydrophobic cellulose-based aerogel using newspaper as the raw material and glutaraldehyde as a cross-linking agent via thermochemical vaporphase synthesis. The aerogel has a low density (17.4-28.7 mg·cm-3) and a mesoporous internal structure.All these properties give the new aerogel great adsorption capacity for oils and organic solvents(including kerosene, nitrobenzene, and chloroform),as well as excellent soot filtration capabilities. Cuiet al[20]prepared a MnFe2O4-Cellulose magnetic composite aerogel (MnCA) with high specific surface areas of 236-288 m2·g-1and total pore volume of 0.55-0.88 cm3·g-1. The hybrid aerogel showed superparamagnetism with maximum saturation magnetization reaching up to 18.53 emu·g-1. This magnetic recycling method can be assisted for further cycling usage of the adsorbent. Coincidentally, one year later, Wanget al[21]uniformly fixed ultrafine MnFe2O4(8-13 nm) on the cellulose backbone through rapid hydrothermal and freeze-drying processes. The prepared super-hydrophilic MnFe2O4/cellulose aerogel(MCA) has a three-dimensional network structure and also has interconnected and bifurcated fibrils, a porous structure, and a high specific surface area. Combining the adsorption-aggregation of cellulose with the high surface activity of low agglomerated ultrafine MnFe2O4,the aerogel’s adsorption efficiency has been greatly improved, thereby achieving a higher utilization of the material. In order to be able to further treat severe oil pollution in harsh environments, (heptadecafluoro-1,1,2,2-tetradecyl) Trimethoxysilane (FAS-17) was used to improve MnFe2O4/cellulose aerogel (F-MCA)to make full use of its inherent structural characteristics.The lipophilic F-MCA has a large carrying capacity for water and fast adsorption performance for oil / organic solvents.

Cotton is a common plant seed fiber that is abundant in tropical and subtropical regions. For example,China, the United States, and India all have huge annual cotton production. According to the National Bureau of Statistics of China, the total national cotton output in 2019 was 5.889 million tons, which indicates that cotton is obtainable easily and cheap. Coincidentally,we also encountered cotton when we were looking for such natural biological material. Therefore, in order to further reduce the cost of aerogel preparation and improve material properties, this work used cotton fibers, which are cheaper and more readily available,as the cellulose source. It is popular today that uses natural organic matter as a source of biomass materials.

2 Experimental

2.1 Materials

Fig.1 Schematic illustration for the preparation of superhydrophobic CC aerogel

Lithium hydroxide (LiOH), Urea, and Methanol(CH3OH) was purchased from Sinopharm Chemical Reagent Co., Ltd, China. Kerosene was provided by Shanghai Macklin Biochemical Co., Ltd, China.Absolute ethanol (EtOH) was received from Wuxi City Yasheng Chemical Co., Ltd, China. Deionized water (H2O) was supplied by Nanjing WANQING chemical glassware & instrument Co., Ltd, China.The cotton fiber was produced by Jiangsu Mishawa Medical Supplies Co., Ltd. Machine oil was sponsored by Tianjin Total Industries Ltd, China.Methyltrimethoxysilane (MTMS) was obtained by Shanghai Aladdin Bio-Chem Technology Co., Ltd,China. Gasoline was procured from China National Petroleum Corporation, China. Pump oil was bought from Wonfull Petrochemical Co., Ltd. China. Cooking oil was purchased from Yihai Grain and Oil Industry Co., Ltd. China. All chemicals and solvents were used as received without further purification. Deionized water was used in all experiments.

2.2 Preparation process of super-hydrophobic cotton cellulose aerogel

2.2.1 Preparation of cotton cellulose (CC) aerogel

Prepared and stirred a mixed solution with a mass ratio ofm(Urea):m(LiOH):m(H2O) =12:7:81. A certain amount of cotton fiber was dispersed uniformly in the mixed solution using ultrasonic cell disruption for 30 min, the power was 500 W. Then it was then frozen at a low temperature of -30 °C for 24 hours.After taking out, a cloudy cotton cellulose sol was obtained with high-speed mechanical stirring. Then the sol was centrifuged at 8000 rpm for 5 min to remove air bubbles, which was changed from a turbid state to a clear state. The clear sol was poured into a mold prepared in advance and left to stand in a methanol atmosphere to form a gel. The regenerated CC hydrogel was washed multiple times with absolute ethanol to remove chemical residues. Wash 3-4 times with an interval of 12 hours. The structure of the wet gel is strengthened during this replacement process, from a soft state to a hard one. Drying with supercritical CO2was performed using a supercritical CO2fluid extraction HELIX 1.1, Applied Separations Inc., by exchanging ethanol to liquid CO2at 10 MPa for 10 h at 40 ℃, and finally by slow release CO2. The CC aerogels with cotton cellulose contents of 1 g, 1.5 g, and 2 g were numbered C1, C2, and C3, respectively.

2.2.2 Hydrophobic modification of CC aerogel

Place four 10 mL beakers in a regular large glass container, and put the aerogel made above in the center of the container. Add 2 mL of MTMS to each beaker.Seal the glass container, placed it in a preheated oven,and silanized it at 70 ℃ for 12 hours. Then put the aerogel into a dry beaker, and dried in an oven at 50℃ for 24 h to obtain a super-hydrophobic cellulose aerogel coded C1-M, C2-M, and C3-M, respectively.

2.3 Characterization

Apparent density is obtained by ρ=m/V, here ρ is the apparent density of the material(g/cm3),mthe mass of the material (g), andVthe volume of the material(cm3). X-ray diffraction (XRD) patterns were obtained using CuKα radiation (λ=0.15406 nm) with a Rigaku Smart Lab-3000. Scanning electron microscopy (SEM)was carried out on an Ultra-55 scanning electron microscope. Surface areas, pore-volume, and pore distribution were measured by using a V-sorb 2800P surface area after the samples. Infrared spectrum analysis was investigated by using a Nicolet Avatar Fourier transform infrared spectrometer. Contact angle test was carried out on a JY-82B contact angle tester.

3 Results and discussion

3.1 XRD spectroscopy analysis

Fig.2 is an XRD patterns of CC aerogel before and after modification, which is mainly for analyzing the crystal structure change of CC aerogel before and after hydrophobic modification. It can be seen from the figure that the crystal forms of cotton cellulose aerogels before and after modification are not much different.The diffraction peaks are mainly concentrated around 12°, 20°, and 21°. The crystal planes corresponding to the diffraction angles are(110), and (200). This crystalline form distribution corresponds to the peak shape of cellulose type II.

Fig.2 XRD patterns of cotton cellulose aerogels before and after modification

3.2 SEM microanalysis

Fig.3 is the SEM images of CC aerogels. The prepared CC aerogels generally exhibit a connected three-dimensional network structure, and this network structure has the characteristics of uniform distribution and loose porosity. Intuitively, cotton cellulose fibers are entangled and nodded with each other like a“rope”. The existence of a large number of micropores in the gap between the ropes ensures the oil storage performance of the CC aerogel material. It can be seen from the high-magnification pictures that the microporous structure has an irregular shape and size.This rough microstructure is essential for aerogels to achieve super-hydrophobia. With the increase of cotton cellulose content, the cross-linked fibers become thicker and denser, and the network structure gradually becomes denser.

Fig.3 SEM images of cotton cellulose aerogels: (a-c) C1, C2, and C3 with low magnification; (d-f) C1, C2, and C3 with high magnification

The SEM image of the superhydrophobic cellulose cotton aerogel is shown in Fig.4. The modified CC aerogel still presents a connected porous three-dimensional network structure. However, there are some changes in the cross-linked fibers after modification. The fibers of unmodified CC aerogel appear to be clearer and smoother, and the overall network pathway is also clear. After the modification, a small substance is generated on the silanized fiber, just like a germinated branch. These “buds” do not block the microporous structure itself, so the hydrophobic CC aerogel still has the ability to make oil flow through the entire network.

Fig.4 SEM images of superhydrophobic cellulose cotton aerogels: (a-c) C1-M, C2-M, and C3-M with low magnification; (d-f) C1-M, C2-M and C3-M with high magnification

3.3 EDS of hydrophobically modified CC aerogel

In order to understand the results of hydrophobic modification more clearly and intuitively, EDS spectrum analysis was performed on the prepared hydrophobic aerogel. The CC-M aerogel has a dense and uniform distribution of Si, C, and O elements,which proves the success of the hydrophobic modification of MTMS.

3.4 N2 adsorption-desorption test and aperture analysis

Fig.6 shows the N2adsorption-desorption curve and BJH pore size distribution curve of cotton cellulose aerogel. Table 2 records the specific surface area and pore structure physical properties of CC aerogels obtained from the N2adsorption-desorption test analysis. Characteristics of the adsorption curves in the analysis chart is listed. When the relative pressureP/P0is between 0.0-0.05, the sample undergoes a single molecular layer adsorption process. After reaching a characteristic point (inflection point), the sample starts from the single-molecule adsorption process Turning to the multi-layer adsorption process,the adsorption process continued, and the relative pressureP/P0also gradually increased. The nitrogen adsorption in the pores produced a cohesive effect,which directly caused the nitrogen adsorption amount to increase sharply. The amount eventually reaches saturation. The desorption curve and adsorption curve of the sample form a closed hysteresis loop, which indicates that the prepared material conforms to the type IV adsorption isotherm, which corresponds to a mesoporous material. The specific surface areasSBETof the prepared cotton cellulose aerogel samples C1,C2, and C3 are 150.19 m2·g-1, 121.91 m2·g-1, and 213.80 m2·g-1; the average pore diametersDporeof the samples C1, C2, and C3 are 16.28 nm, 32.56 nm,16.71 nm; the pore volumesVporeof samples C1, C2,and C3 were 0.72 cm3·g-1, 1.26 cm3·g-1, and 0.77 cm3·g-1, respectively. The N2adsorption-desorption curve of the MTMS hydrophobically modified cotton cellulose aerogel sample is approximately the same as that of the unmodified cotton cellulose aerogel. The specific surface areasSBETof the samples C1-M, C2-M, and C3-M are 184.33 m2·g-1, 180.76 m2·g-1, and 139.99 m2·g-1, respectively; the average pore diametersDporeare 12.50 nm, 14.01 nm, and 15.83 nm; the pore volumesVporeare 0.51 cm3·g-1, 0.58 cm3·g-1, and 0.55 cm3·g-1, respectively. The average pore diameter ofthe samples before and after modification is distributed between 12 nm and 33 nm, which is consistent with the results of the above curve analysis and belongs to mesoporous materials. Some subtle changes are found, which is the average pore size and pore volume of the hydrophobically modified material decreased.Combining SEM and EDS analysis, hydrophobic modification is mainly achieved by surface modification of materials. After modification of CC aerogel, the addition of surface silane groups caused the reduction of pore size and pore volume to some extent.

Table 1 Pros and cons of different oil/water treatment approaches

Table 2 Physical parameters of cotton cellulose aerogels

Table 3 Density and viscosity of the oils used in the experiment

Fig.5 (a) EDS layered images of hydrophobic cotton cellulose aerogels; (b) Elemental energy spectra of hydrophobic cotton cellulose aerogels

Fig.6 N2 adsorption-desorption isotherms and BJH pore size distributions of cotton cellulose aerogels

3.5 FTIR spectroscopy and water contact angle analysis

Fig.7(a) is a water contact angle test chart and a water contact angle histogram of a hydrophobic cotton cellulose aerogel. It can be seen from the figure that the water contact angle of all samples is greater than 145°, which shows the excellent hydrophobic properties of the material. Combined with the infrared spectrum analysis chart of cotton cellulose aerogel before and after hydrophobic modification in Fig.7(b),the changes of aerogel surface groups before and after hydrophobic modification are analyzed. The infrared absorption peaks between 3 500-3 450 cm-1wavenumbers are mainly the stretching vibration peaks of -OH in the hydrogen bonds between cellulose and intramolecular[22,23]. The infrared absorption peaks between 3000-2750 cm-1wavenumbers are caused by the stretching vibration of -CH, of which 2 960 cm-1and 2 874 cm-1are anti-symmetric, symmetrical stretching vibration peaks of -CH3; 2 920 cm-1and 2 850 cm-1are the anti-symmetric, symmetrically stretching vibration peaks of -CH2-, which all indicate the infrared spectral characteristics of cotton cellulose fibers[24,25]. The strong absorption band between wave number 1 100-1 000 cm-1is caused by the antisymmetrical stretching vibration of Si-O-Si, and the strong absorption peak near 800 cm-1is caused by Si-O-Si bending vibration[26]. The above analysis shows that MTMS is successfully grafted into the cellulose backbone, which greatly improves the adsorption capacity of cellulose aerogel. The hydrophobic angles of C1-M, C2-M, and C3-M are 145.3°, 151.0°, and 153.0°, respectively. They have approached or even reached the characteristics of superhydrophobic, and they are increasing. The content of cotton cellulose increases from 1 g to 2 g, the skeleton structure of cotton cellulose aerogel becomes more compact,and more fibers receive hydrophobic modification treatment, resulting in a larger and larger hydrophobic angle. The analysis of the water contact angle also verifies the completion of hydrophobic modification of the material.

Fig.7 (a)Contact angles and bar charts of contact angles of hydrophobic cotton cellulose aerogels; (b)FTIR spectra of cotton cellulose aerogels before and after modification

3.6 Oil absorption performance analysis

An oil adsorption test was performed on hydrophobic cotton cellulose aerogel to study the adsorption performance of different samples. Fig.8(a) is the oil absorption process diagram of the hydrophobic cotton cellulose aerogel. The dyed kerosene was selected as the adsorption object. It can be seen from the figure that the sample can absorb the oil more quickly, showing the potential of this material to quickly treat oil. Fig.8 (b) is the adsorption efficiency graph of hydrophobic cotton cellulose aerogel.The adsorption capacity of different samples has reached several times its own weight. The adsorption efficiencies of samples C1-M, C2-M, and C3-M are 16.0 g·g-1, 12.8 g·g-1, and 7.8 g·g-1, respectively. With the increase of cotton cellulose content, the adsorption efficiency shows a downward trend. The adsorption efficiency is inversely proportional to the density of the aerogel. This is mainly due to the increase in the specific gravity of cellulose, which directly leads to the aerogel’s skeleton being heavier. Under the condition that the accommodation space is not much different, the adsorption efficiency of the aerogel will be relatively reduced.

Fig.8 (a)Oil absorption progress of the hydrophobic cotton cellulose aerogel; (b)Absorption capacity of hydrophobic cotton cellulose aerogels

In order to further measure the adsorption performance of the hydrophobic CC aerogel, the samples are tested for the adsorption of different oils. The selected adsorption objects were hexane,machine oil, cooking oil, pump oil, gasoline, and kerosene. Fig.9(a) is the adsorption efficiency graph of hydrophobic cotton cellulose aerogel for various oils.The prepared hydrophobic cotton cellulose aerogel has good adsorption performance for various oils,and the max adsorption efficiency can reach 17 g·g-1.Oil absorption is generally considered to be related to the density and viscosity of the oily substance.However, in this experiment, the adsorption capacity of hydrophobically modified CC aerogels for different oils is not significantly related to the above two physical and chemical properties, and a certain model can not be established.

There are generally two types of adsorption for oils, physical adsorption and chemical adsorption.Chemically adsorbed materials are more difficult to desorbed. This is mainly because the traditional desorption operation can not break the chemical bond between the material and the oily substance. This is mainly because traditional desorption operations, such as squeezing, rinsing, and supercritical operations,cannot break the chemical bond between the material and the oily substance. This is why many oil-absorbing materials can not be reused. Fig.9(b) is a cycle efficiency graph of a hydrophobic cellulose aerogel.After five cycles, the adsorption efficiency of the sample decreased, which was due to the destruction of the internal pore structure of the material by multiple adsorptions. As analyzed above, this oil absorption process has both physical and chemical adsorption. Chemical adsorption is used to improve adsorption efficiency, and physical adsorption is used to achieve the reusability of materials. In summary,the hydrophobic cotton cellulose aerogel has excellent adsorption performance and recyclability and can perform simple, fast, and effective adsorption treatment on oil stains.

Fig.9 (a) Absorption capacity of hydrophobic cotton cellulose aerogels for various oils; (b) Recyclability of hydrophobic cotton cellulose aerogels

4 Conclusions

The superhydrophobic cotton cellulose aerogel has certain adsorption properties, which can perform simple, fast, and effective adsorption treatment on oil stains. In this work, superhydrophobic CC aerogels are synthesized by using commercial cotton extract as a precursor through the nano-self-assembly process, CO2supercritical drying process, and vapor deposition. The max specific surface areas before and after modification are 213.80 m2·g-1and 184.33 m2·g-1with average pore sizes at 14.01 nm and 32.56 nm. All samples achieve excellent hydrophobicity, with a maximum hydrophobic angle of 153°. The maximum adsorption efficiency can be up to 16.0 g·g-1, which exhibits excellent cycling property after 5 adsorption tests. According to the analysis of water contact angle, the increase in the content of cotton cellulose makes the skeleton structure of aerogel more compact. At the same time,more fibers are received hydrophobic modification treatment, resulting in a larger hydrophobic angle. The oil absorption process has both physical and chemical adsorption. Chemical adsorption is used to improve adsorption efficiency, and physical adsorption is used to achieve the reusability of materials. In general, this superhydrophobic cotton cellulose aerogel has good application prospects in the field of oil stain treatment.