Cellulose-based aerogel from Eichhornia crassipes as an oil superabsorbent

Abstract

Cellulose-based aerogels (CBAs) were prepared based on Eichhornia crassipes as a raw material and a paper wet-strengthening agent as a cross-linker via a green and simple process. The hydrophobic CBAs had excellent properties, including low density (<0.0055 g cm⁻³), high porosity (>99.6%), excellent oil/solvent sorption capacities (58.06–101.14 g g⁻¹), superhydrophobicity (water contact angle as high as 154.8°), and superior elasticity. Moreover, the absorbed oil could be quickly recovered by squeezing, and the oil sorption capacity was still as high as 75% of the original sorption capacity after 16 cycles. This work provides a facile and cost-effective method to fabricate a novel superabsorbent from waste biomass materials, which is very promising in terms of using waste to deal with oil pollution.

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Introduction

Water pollution caused by oil spills has become one of the most serious disasters due to its severe threat to the marine ecosystem.^1,2 Thus, it is vital to choose appropriate strategies for collecting oil completely from water in time. Among the existing measures used for oil recovery, physical adsorption is generally considered as an effective and promising method, due to its low-cost and convenience of operation.^3,4 At present, various materials have been used for oil sorption, including inorganic mineral materials, organic synthetic products and organic natural materials.^5,6 Compared with the inorganic and synthetic organic adsorbents, natural organic adsorbents have received widespread attention because of their higher biodegradability, high biomass and cost effectiveness. High oil sorption capability is one of the most important criteria when choosing oil sorption materials, and mainly depends on the storage space of materials.⁷ However, the low storage space as well as the insufficient buoyancy of most natural organic materials limited their further application. Even so, there is a growing demand for synthesizing oil superabsorbents based on renewable and sustainable natural organic materials.

Owing to its low density and high porosity, aerogels have been extensively investigated. Various types of aerogels can be classified into two typical groups: inorganic aerogels (e.g., SiO₂,⁸ carbon nanotubes⁹ and graphene^10–12) and organic aerogels (e.g., polypropylene¹³ and cellulose^14–17). Besides high oil sorption capability, an ideal sorbent should be low-cost, renewable and easily fabricated. Among them, expensive raw materials or complicated synthetic procedures of inorganic aerogels and the slow degradation of aerogels that are based on synthetic polymers (e.g., polypropylene¹³) have limited their widespread application. Therefore, cellulose-based aerogels appear to be a promising sorbent considering that cellulose is a cheap and abundant source, and due to their high structural flexibility and good mechanical properties.^18–22 For achieving cellulose-based aerogels, several efforts have been made using some cellulose solvents, e.g., alkaline/urea,²³ NaOH/PEG aqueous solutions,²⁴ a hydrothermal process (KOH, 3 mol L⁻¹)²⁵ and ionic liquids (AmImCl).¹⁸ However, the above preparation processes use a large amount of chemicals and produce alkaline and acidic waste. Even though ionic liquids as green cellulose solvents were introduced to fabricate cellulose aerogels, the expenditure of ionic liquids is still too expensive to use in industrial manufacturing.

Herein, a cost-effective and environmentally friendly method is reported to prepare cellulose-based aerogels (CBAs) by using Eichhornia crassipes as a raw material and a paper wet-strengthening agent (the effective component is 12.5 wt% polyamide epichlorohydrin resin, PAE) as a cross-linker. Using Eichhornia crassipes as a starting material for the synthesis of aerogels provides the possibility of being able to recycle these aquatic plants, thereby reducing any heavy burden on the environment. PAE is commonly used in the papermaking industry as it can significantly enhance the wet-strength of the paper.²⁶ Thus, PAE can be directly used in cellulose-based aerogels without any pretreatment and can significantly reduce the use of solvent and the entire synthesis duration. Through a combination of freezing, freeze-drying and subsequent hydrophobic modification, cellulose-based aerogels (CBAs) that have good flexibility, high porosity, super-hydrophobicity and outstanding oil sorption capabilities were obtained.

Materials and methods

Materials

Eichhornia crassipes was harvested from the waterways of Shanghai, China. Methyltrimethoxysilane (98%, MTMS), and other chemicals including benzene, absolute ethyl alcohol, sodium chlorite, glacial acetic acid, sodium hydroxide and hydrochloric acid were all of analytical reagent grade and were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd, China. The paper wet-strengthening agent (effective component is 12.5 wt% polyamide epichlorohydrin resin, PAE) was supplied by Jiangsu Si Lida Chemical Co., Ltd, China. Deionized water was self-made in the laboratory. Crude oil (density at 20 °C = 0.6770 g cm⁻³, viscosity = 0.994 mm² s⁻¹) was supplied by the China National Offshore Oil Corporation. Diesel oil (density at 20 °C = 0.82 g cm⁻³, viscosity = 6.51 mm² s⁻¹), soybean oil (density at 20 °C = 0.93 g cm⁻³, viscosity = 7.26 mm² s⁻¹), lubricating oil (density at 20 °C = 0.79 g cm⁻³, viscosity = 47–57 mm² s⁻¹) and silicone oil (density at 20 °C = 0.96 g cm⁻³, viscosity = 95 mm² s⁻¹) were purchased from the local market, Shanghai, China.

Purification of cellulose from Eichhornia crassipes

The harvested plant samples were washed and sun-dried. The stalks of Eichhornia crassipes were ground and then screened. Powder with 40 mesh was used for the preparation of cellulose. The purification of cellulose was determined according to the procedures described by Shaoliang Xiao et al.²⁷ Briefly, 10 g of dried powder was extracted with benzene/ethanol (2 : 1, v/v) at 90 °C for 6 h in a Soxhlet extractor. Next, the treated sample was air-dried and treated with 600 mL 1 wt% NaClO₂ solution (pH = 4–5 adjusted by glacial acetic acid) at 75 °C for 5 h. The sample was washed with deionized water and then immediately treated with 250 mL 2 wt% NaOH at 90 °C for 2 h. Thereafter, the sample was collected and washed with deionized water again before being treated with 300 mL 1 wt% HCl at 80 °C for 2 h. Finally, the purified cellulose was collected and stored in a refrigerator at 4 °C before use. To avoid the generation of strong hydrogen bonding among the cellulose fibers, the sample was kept in a water-swollen state during the whole chemical process.

Preparation of the CBAs

The preparation process and a schematic of the CBAs are illustrated in Fig. 1. A mixture of an aqueous suspension of cellulose fibers (0.35, 0.50, 0.75 and 1.00 wt%) and PAE (20–60 wt% of dry cellulose fibers) (Fig. 1a and c) was violently stirred for 15 min. Next, the suspension was poured into a 150 mL glass beaker and sealed with a plastic wrap. The samples were then placed into a low temperature thermostat at −15 °C for 24 h to allow them to form solids. Afterwards, the frozen samples were freeze-dried at −35 °C for 48 h. Finally, the freeze-dried cellulose-based aerogels were kept at 120 °C for 3 h to strengthen the crosslinking among the cellulose fibers.

Fig. 1 The schematic and preparation process of CBAs. (a) Schematic of the fabrication process, (b) cellulose, (c) PAE, (d) frozen solid, (e) freeze-dried solid and (f) an ultralight CBA on a flower after cross-linking reaction at 120 °C.

Hydrophobic coating of CBAs

A tightly sealed glass bottle containing 1 mL MTMS, 1 mL H₂O and the samples was placed in an oven at 80 °C for 4 h. Then, the silanized samples were taken out and placed in a vacuum oven for 24 h at 60 °C to remove any excess MTMS. After that, hydrophobic CBAs were obtained. The CBA synthesized under different conditions was marked as _xCBAy, where x and y denote the fiber content and PAE concentration, respectively.

Characterizations

The surface morphology of the samples coated with Au was examined using Scanning Electron Microscopy (SEM, SU-1510, Hitachi Ltd, Japan). The FTIR spectra of the samples were recorded on a NICOLET-380 spectrometer (Thermo Fisher Scientific, USA). Contact angle measurements of the samples treated with MTMS were carried out using a drop of water (3 μL) or diesel oil (10 μL) (OCA30, Dataphysics, Germany).

Density and porosity of CBAs

The density (ρ, g cm⁻³) and porosity (%) of the unmodified CBAs were calculated according to the following formula (1) and (2):

ρ = m/V (1)

Porosity = (1 − ρ/ρ_c) × 100 (2)

where m and V represent the mass and volume of the CBAs and ρ_c is the density of crystalline cellulose (1.528 g cm⁻³).^20,23

Oil sorption capacity experiments

To determine the sorption capacity of the MTMS treated aerogels, this test was performed in various oils (crude oil, diesel oil, soybean oil, lubricating oil and silicone oil) and organic solvents (toluene, n-hexane, trichloromethane, acetone and ethanol). A weighed sample (about 0.03 g) was put into a stainless-steel mesh, and the sample and mesh were immersed together into the oil for 5 min. Then, the sample and mesh were taken out from the oil together and drained for 2 min before the weight measurement. The oil sorption capacity can be referred to as the weight gain defined by the mass of absorbed oil per unit mass of dry sample. This measurement was repeated three times and the average value was presented.

Recyclability of CBAs

After oil absorption by the CBAs, the absorbed oil was recovered by simple mechanical squeezing (a 200 g balance weight), and the remnant oil in the squeezed CBAs was weighed. Then the squeezed CBAs were allowed to absorb oil again and the oil in the CBAs (adsorbed oil) was measured again. This process was repeated several times to characterize the recyclability of the CBAs.

Results and discussion

Characterizations

Light-weight (Fig. 1f) and porous CBAs were successfully prepared via a green and facile method. Fig. 2 describes the density and porosity of the CBAs before modification as a function of the cellulose content and PAE concentration. Irrespective of the PAE concentration, the density of the CBAs can reach as low as 0.0055 g cm⁻³ (i.e., _0.35CBA10), and the porosity can be as high as 99.6% (i.e., _0.35CBA10) with decreasing cellulose concentration from 1.00 wt% to 0.35 wt%. At the same cellulose content, the density of the CBAs increased with an increase in the PAE concentration, while the porosity decreased. This shows that the density and porosity were influenced by the cellulose content and PAE concentration. In short, the prepared CBAs with high porosity (98.6–99.6%) and low density (0.0055–0.021 g cm⁻³) are lightweight materials, indicating that the CBAs would be able to float well on the water surface after absorbing oil (see below).

Fig. 2 Density and porosity of unmodified CBAs as a function of cellulose concentration.

The wettability of the CBAs is shown in Fig. 3. Before being modified by MTMS, the CBAs could not be directly used for oil sorption due to the hydrophilicity of the cellulose fibers. As shown in Fig. 3a, a water droplet was immediately absorbed by the unmodified CBAs within 0.2 s, and the water contact angle was nearly 0°. On the contrary, the water droplet exhibited a typical spherical shape on the outside and remained on the surface of the MTMS-modified CBAs after 60 s, while the diesel oil droplet could immediately penetrate the MTMS-modified CBAs in less than 0.2 s (Fig. 3b–d). Moreover, the water contact angle and oil contact angle were 154.8° and 0° (Fig. 3b and c), indicating the combined superhydrophobic and oleophilic features of the MTMS-modified CBAs. As shown in Fig. 3f, a piece of superhydrophobic CBA could easily absorb chloroform that was dyed with Sudan III from underwater without any water being absorbed, further confirming the good hydrophobicity of the MTMS-modified CBAs.

Fig. 3 The wettability of the CBAs. Water contact-angle measurements of (a) unmodified CBAs and (b) MTMS-modified CBAs. (c) Diesel oil contact-angle measurements of the MTMS-modified CBA at room temperature. (d) Water and diesel oil were dropped onto the surface of the MTMS-modified CBA. (e) Water was dropped onto the inner surface of the MTMS-modified CBA. (f) Chloroform was absorbed by the MTMS-modified CBA from underwater (water dyed with copper sulfate, diesel oil and chloroform dyed with Sudan III, for clear presentation).

The FTIR spectra of the unmodified CBAs and modified CBAs are presented in Fig. 4a and b. After modification with MTMS, the vibrations of the C–H of Si–CH₃ groups were identified at 2964 cm⁻¹.²⁸ The bands at 1270 and 854 cm⁻¹ were assigned to the Si–C bonds²⁹ and the Si–O–Si stretching vibration was also observed at 777 cm⁻¹.^20,29 The peak intensity of the O–H stretching at 3336 cm⁻¹ decreased, indicating a successful silanization reaction between the abundant hydroxyl groups of the fibers and MTMS. The successful silanization on the CBAs was also confirmed by the energy dispersive X-ray spectrum (EDS) (Fig. 4c and d). Compared with the unmodified CBAs, the EDS spectrum of the modified CBAs shows a sharp silicon peak and the relative silicon atomic weight percentage was 16.21%.

Fig. 4 FTIR spectra of the (a) unmodified CBA and (b) MTMS-modified CBA, and the EDS spectra of the (c) unmodified CBA and (d) MTMS-modified CBA.

Scanning electron microscope (SEM) images show that the CBAs are porous and have interconnected three-dimensional network structures with a pore size of around 100–200 μm (Fig. 5). To avoid the collapse of the structures due to moisture being absorbed by the cellulose, the freeze-dried CBAs were modified with MTMS to improve their hydrophobicity and retain the porous structure of the samples. The morphologies of the CBAs before and after hydrophobic modification are shown in Fig. 5, and there were no significant changes observed after hydrophobic modification. This indicated that the silanizing modification had little affect on the structures of the CBAs.

Fig. 5 SEM micrographs of (a) unmodified _0.35CBA40 (500×), (b) unmodified _0.35CBA40 (200×), (c) MTMS-modified _0.35CBA40 (500×) and (d) MTMS-modified _0.35CBA40 (200×).

Effects of the cellulose content and PAE concentration on the oil sorption of hydrophobic CBAs

The effect of cellulose content (0.35, 0.50, 0.75 and 1.00 wt%) on the oil sorption capacities of the CBAs with 40 wt% PAE for crude oil, diesel oil and lubricating oil is shown in Fig. 6a. Obviously, with an increase in the cellulose content from 0.35 to 1.00 wt%, the oil sorption capacities for crude oil, diesel oil and lubricating oil decreased significantly from 62.69 to 39.53 g g⁻¹, 78.03 to 46.33 g g⁻¹ and 90.04 to 54.93 g g⁻¹. It could be concluded that the CBAs with the lowest cellulose content possessed the highest oil absorption capacities, due to the lowest density (0.0068 g cm⁻³) and highest porosity (99.56%) being obtained at the lowest cellulose content level (i.e., _0.35CBA40).

Fig. 6 (a) Effect of cellulose content (0.35, 0.50, 0.75 and 1.00 wt%) on the oil sorption capacities of the CBAs with 40 wt% PAE. (b) Effect of PAE concentration (10, 20, 40 and 60 wt%) on the oil sorption capacities of the CBAs with 0.35 wt% cellulose.

The presence of PAE played an important role in facilitating the formation of the three-dimensional structures of the aerogels, thus impacted the oil sorption capacities. The effect of PAE concentration (10, 20, 40 and 60 wt%) on the oil sorption capacities of the CBAs with 0.35 wt% cellulose is depicted in Fig. 6b. Notably, the oil sorption capacities of _0.35CBA10, _0.35CBA20, _0.35CBA40 and _0.35CBA60 for oil decreased significantly with increasing PAE concentration from 10 to 60 wt%. For example, the oil sorption capacities of _0.35CBA10, _0.35CBA20, _0.35CBA40 and _0.35CBA60 for lubricating oil were 106.54, 106.49, 90.04 and 80.10 g g⁻¹, respectively. This showed that the CBAs with lower PAE concentrations possessed higher sorption capacities, which could be attributed to the higher porosity and lower density.

Recyclability

The choice of a suitable material for practical applications not only depends on the high oil sorption capacity but also the recyclability of the sorbents. Fig. 7 shows the recyclability of the CBAs by studying the weight change of saturated absorbed oil. In this work, a simple squeezing method was selected to recover the absorbed oil, mainly due to the fact that the oil absorbed by the CBAs could be easily recovered through squeezing, and the squeezed CBAs could be reused at once. The first compression/release process using the squeezing method is represented in Fig. 7a, with negligible changes being found on the morphology of _0.35CBA40, suggesting the excellent mechanical properties and elasticity of the aerogel. The use of PAE enhanced the mechanical strength and flexibility of the materials, and increased the reusability of the materials. The sorption capacities of _0.35CBA10, _0.35CBA20, _0.35CBA40 and _0.35CBA60 for diesel oil as a function of the cycle number are exhibited in Fig. 7b–e. After 16 compression/release cycles, there is still about 50%, 59%, 75% and 77% of the initial sorption capacity remaining for _0.35CBA10, _0.35CBA20, _0.35CBA40 and _0.35CBA60, and the corresponding sorption quantity capacities were 45.60, 51.88, 58.61 and 51.08 g g⁻¹, respectively. The changes in the sorption capacities of _0.35CBA10, _0.35CBA20, _0.35CBA40 and _0.35CBA60 after 16 cycles were due to the different mechanical properties of the CBAs caused by the different PAE concentrations. For a comprehensive consideration of the oil sorption capacities and mechanical properties of the CBAs, the CBAs prepared with 0.35 wt% cellulose and 40 wt% PAE (_0.35CBA40) were the best samples. For the convenience of the research, _0.35CBA40 was selected in the subsequent experiments.

Fig. 7 (a) Compressing–release processes of hydrophobic _0.35CBA40 in the oil system. The diesel oil sorption capacities of (b) _0.35CBA10, (c) _0.35CBA20, (d) _0.35CBA40 and (e) _0.35CBA60 as a function of cycle number.

Oil absorption performance

The lightweight properties, porous structures, superhydrophobicity and excellent recyclability of the CBAs make them an ideal candidate for the sorption of oils and other organic pollutants. Fig. 8a shows the sorption process of a piece of hydrophobic CBA with diesel oil that was floating on the water. It is noteworthy that the oil could be completely absorbed within 10 s and without any oil release or water absorption, and the CBAs saturated with oil can float well on the water surface due to their low density and hydrophobicity. Furthermore, the water contact angle of the CBAs dropped to 113.1–128.8° after 16 cycles, which means that the recycled CBAs still hardly adsorbed water in an oil–water mixture.

Fig. 8 (a) Absorption process of a piece of hydrophobic _0.35CBA40 with diesel oil. (b) Sorption capacity (g g⁻¹) of hydrophobic _0.35CBA40 for various oils and organic solvents. (c) Sorption capacity (mL g⁻¹) of hydrophobic _0.35CBA40 for various oils and organic solvents.

For practical applications, an ideal oil sorption material should be very good at dealing with all kinds of oil pollution. The sorption capacity of hydrophobic _0.35CBA40 for a series of commonly used oils and organic solvents was investigated (Fig. 8b). Obviously, _0.35CBA40 exhibited a rather high absorption capacity for the oils and organic solvents, ranging from 58.06 to 101.14 g g⁻¹. For example, the sorption capacities of hydrophobic _0.35CBA40 for crude oil, diesel oil, soybean oil, lubricating oil and silicone oil were 62.69, 77.03, 81.32, 88.04 and 94.00 g g⁻¹ respectively. The excellent sorption capacity of hydrophobic _0.35CBA40 could be attributed to its high porosity and low density as well as its uniform superoleophilic and superhydrophobic properties. However, the sorption capacity of _0.35CBA40 for oils and solvents with different densities was not the same.^14,30 The volume of trichloromethane absorbed was the lowest, the volume of ethanol, toluene, soybean oil and n-hexane was average, and the volume of crude oil, diesel oil, acetone, silicone oil and lubricating oil was the highest.

In general, the sorption capacities of the CBAs were much higher than those of other cellulose-based aerogels^5,18,23,31 and conventional sorbent materials including natural materials.^32,33 It was also observed that their sorption capacity was lower than that of some superabsorbents, such as MCF aerogel from microfibrillated cellulose fibers (88–228 g g⁻¹)²⁰ and graphene sponge (60–160 g g⁻¹),³⁴ but the method for the manufacturing process of the CBAs studied here was much simpler and lower in cost, and its raw material, i.e., Eichhornia crassipes was a sustainable waste biomass. Thereby, these CBAs could be promising sorbents used for spilled oil remediation.

Conclusions

Lightweight, porous and flexible cellulose-based aerogels were successfully prepared from Eichhornia crassipes via a facile process, without the need for special conditions. The superior CBA with both superior oil absorbency and good mechanical properties was obtained at 0.35 wt% cellulose and 40 wt% PAE (i.e., _0.35CBA40). The prepared CBA possessed excellent oil/solvent sorption capacities and super-hydrophobicity, as well as superior elasticity. Furthermore, the loss of sorption capacity of the hydrophobic _0.35CBA40 for diesel oil was only 25% after 16 cycles. Coupled with the fact that waste biomass is used as the raw material and the fabrication process is environmentally friendly, the low density and high porosity CBA is an ideal alternative for environmental protection.

Conflict of interest

The authors have declared no conflict of interest.

Acknowledgements

The work was funded by the National Natural Science Foundation of China (No. 41373097), and Program for Innovative Research Team in University (No. IRT13078).

References

1. F. Aguilera, J. Mendez, E. Pasaro and B. Laffon, J. Appl. Toxicol., 2010, 30, 291–301.
2. B. Wang, W. Liang, Z. Guo and W. Liu, Chem. Soc. Rev., 2015, 44, 336–361.
3. K. Zhu, Y. Y. Shang, P. Z. Sun, Z. Li, X. M. Li, J. Q. Wei, K. L. Wang, D. H. Wu, A. Y. Cao and H. W. Zhu, Front. Mater. Sci., 2013, 7, 170–176.
4. J. Feng, S. T. Nguyen, Z. Fan and H. M. Duong, Chem. Eng. J., 2015, 270, 168–175.
5. S. Han, Q. Sun, H. Zheng, J. Li and C. Jin, Carbohydr. Polym., 2016, 136, 95–100.
6. J. Li, M. Luo, C.-J. Zhao, C.-Y. Li, W. Wang, Y.-G. Zu and Y.-J. Fu, Bioresour. Technol., 2013, 128, 673–678.
7. R. Wahi, L. A. Chuah, T. S. Y. Choong, Z. Ngaini and M. M. Nourouzi, Sep. Purif. Technol., 2013, 113, 51–63.
8. J. M. Zhu, S. H. Guo and X. H. Li, RSC Adv., 2015, 5, 103656–103661.
9. L. Chen, R. Du, J. Zhang and T. Yi, J. Mater. Chem. A, 2015, 3, 20547–20553.
10. H. Bi, X. Xie, K. Yin, Y. Zhou, S. Wan, L. He, F. Xu, F. Banhart, L. Sun and R. S. Ruoff, Adv. Funct. Mater., 2012, 22, 4421–4425.
11. H. Bi, X. Xie, K. Yin, Y. Zhou, S. Wan, R. S. Ruoff and L. Sun, J. Mater. Chem. A, 2014, 2, 1652–1656.
12. J. Y. Hong, E. H. Sohn, S. Park and H. S. Park, Chem. Eng. J., 2015, 269, 229–235.
13. G. W. Wang and H. Uyama, Sci. Rep., 2016, 6, 21265.
14. F. Jiang and Y.-L. Hsieh, J. Mater. Chem. A, 2014, 2, 6337.
15. H. Bi, X. Huang, X. Wu, X. Cao, C. Tan, Z. Yin, X. Lu, L. Sun and H. Zhang, Small, 2014, 10, 3544–3550.
16. H. Bi, Z. Yin, X. Cao, X. Xie, C. Tan, X. Huang, B. Chen, F. Chen, Q. Yang, X. Bu, X. Lu, L. Sun and H. Zhang, Adv. Mater., 2013, 25, 5916–5921.
17. Z. Y. Wu, H. W. Liang, L. F. Chen, B. C. Hu and S. H. Yu, Acc. Chem. Res., 2016, 49, 96–105.
18. C. Jin, S. Han, J. Li and Q. Sun, Carbohydr. Polym., 2015, 123, 150–156.
19. Y.-Q. Li, Y. A. Samad, K. Polychronopoulou, S. M. Alhassan and K. Liao, ACS Sustainable Chem. Eng., 2014, 2, 1492–1497.
20. S. Wang, X. Peng, L. Zhong, J. Tan, S. Jing, X. Cao, W. Chen, C. Liu and R. Sun, J. Mater. Chem. A, 2015, 3, 8772–8781.
21. J. P. Zhang, B. C. Li, L. X. Li and A. Q. Wang, J. Mater. Chem. A, 2016, 4, 2069–2074.
22. G. Hayase, K. Kanamori, M. Fukuchi, H. Kaji and K. Nakanishi, Angew. Chem., Int. Ed., 2013, 52, 1986–1989.
23. R. Lin, A. Li, T. Zheng, L. Lu and Y. Cao, RSC Adv., 2015, 5, 82027–82033.
24. C. Wan, Y. Lu, J. Cao, Q. Sun and J. Li, Fibers Polym., 2015, 16, 302–307.
25. S. Yang, L. Chen, L. Mu, B. Hao and P.-C. Ma, RSC Adv., 2015, 5, 38470–38478.
26. E. J. Siqueira, M. C. B. Salon, M. N. Belgacem and E. Mauret, J. Appl. Polym. Sci., 2015, 132, 42144.
27. S. Xiao, R. Gao, Y. Lu, J. Li and Q. Sun, Carbohydr. Polym., 2015, 119, 202–209.
28. H. Z. Sai, R. Fu, L. Xing, J. H. Xiang, Z. Y. Li, F. Li and T. Zhang, ACS Appl. Mater. Interfaces, 2015, 7, 7373–7381.
29. A. Borba, M. Almangano, A. A. Portugal, R. Patricio and P. N. Simoes, J. Phys. Chem. A, 2016, 120, 4079–4088.
30. Q. F. Zheng, Z. Y. Cai and S. Q. Gong, J. Mater. Chem. A, 2014, 2, 3110–3118.
31. A. Mulyadi, Z. Zhang and Y. Deng, ACS Appl. Mater. Interfaces, 2016, 8, 2732–2740.
32. J. Wang, Y. Zheng and A. Wang, Chem. Eng. J., 2012, 213, 1–7.
33. W. Chai, X. Liu, J. Zou, X. Zhang, B. Li and T. Yin, Carbohydr. Polym., 2015, 132, 245–251.
34. J. P. Zhao, W. C. Ren and H. M. Cheng, J. Mater. Chem., 2012, 22, 20197–20202.

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Footnote

† Current address: College of Environment and Chemical Engineering, Shanghai University, No. 99, Shangda Road, Baoshan District, 200444, Shanghai, China.

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