Xu Mengabc,
Yanyan Donga,
Yajun Zhaoa and
Liping Liang*a
aCollege of Textile and Garment, College of Life Science, Shaoxing University, Shaoxing, China. E-mail: liangliping0702@163.com; Fax: +86-575-88341506; Tel: +86-575-88341506
bKey Laboratory of Clean Dyeing and Finishing Technology of Zhejiang Province, Shaoxing University, Shaoxing, 312000, China
cZhejiang Sub-center of National Carbon Fiber Engineering Technology Research Center, Shaoxing, 312000, China
First published on 16th November 2020
This work presents a facile preparation and modification of cellulose sponge with hydrophobic/oleophilic surface wetting properties. The modification method included several steps: sodium hydroxide (NaOH) and urea were introduced to dissolve cellulose, after which calcium carbonate (CaCO3) and epichlorohydrin were added into the solution for formation of a hydrogel. Finally, the cellulose sponge was obtained through hydrochloric acid (HCl) etching of CaCO3, and octadecyl trichlorosilane (OTS) self-assembly modification. The prepared cellulose sponge exhibited hydrophobicity with a water contact angle of 153.5°, and oleophilicity with an oil contact angle of 0°. The prepared cellulose sponge demonstrated a separation efficiency as high as 92% for various types of oil/water mixtures. The prepared cellulose sponge could achieve continuous separation with the assistance of a peristaltic pump. The material will be a promising candidate to be used in oil/water separation.
So far, there have been various kinds of methods for oil/water separation, such as gravity separation, degradation separation, absorption methods, etc.5–7 Adsorption is one of the simple and convenient methods for oil/water separation. However, most of the traditional oil/water separation materials have weak absorption capacity, poor selectivity, low separation efficiency, and the disadvantage of not being easy to recycle which is not in line with the theme of green development. Therefore, it is extremely urgent to develop an environmentally friendly material with good performances. Sponges which can be manufactured with the materials such as melamine, polyurethane, carbon, polyvinyl alcohol, and polydimethylsiloxane have attracted attention due to their high absorbency.8–12 Among them, cellulose sponge is considered as a green and environmentally friendly material due to its abundant resources, renewability, high porosity, biodegradability and low cost. In addition, it is characterized by low density, which reduces the barrier layer formed by the direct contact of water (greater density than oil) and the surface of the high-density material to promote oil penetration.13–15 In order to make the materials have unique wetting properties, various substances with different properties have been introduced on the sponge matrix to change their surface properties and surface structures.16–19
In recent years, it has become a hot spot to prepare oil/water separation materials with environmental-friendly substances as the matrix.20,21 Thus, to date, a lot of researches on super hydrophobic and super lipophilic cellulose sponges have been conducted. In this respect, Halim et al.22 prepared a super lipophilic and underwater super oleophobic cellulose sponge by modifying with bamboo-based counter collision cellulose nanofiber and wood-free fiber-based 2,2,6,6-tetramethylpiperidine-1-oxyl oxidized cellulose nanofiber, respectively, whose separation efficiency could both come up to 99% with gravity alone. Chen et al.23 fabricated a porous cellulose sponge with steady super hydrophilicity and oleophobicity even under harsh environment by grafting amphiphilic molecular brushes of polyethylene glycol with short perfluorinated end caps on the surface of isocyanate-functionalized cellulose sponge, which had excellent separation efficiency and antifouling property. However, the complex preparation processes and high cost limited their applications in practice. Hence, it is significant to search after a convenient and low-cost method for preparing oil/water separation cellulose sponge.
In this work, the cellulose sponge was fabricated with a cost-efficient and simple operation and ended with vacuum freeze-drying technology. And then, the obtained sponge was grafted with OTS, which endowed the sponge with a certain hydrophobic/oleophilic property. Fig. 1 showed the design scheme of the modification of the cellulose sponge and the oil/water separation process of the as-prepared sponge. The modified cellulose sponge with the above properties could be an environmentally friendly and efficient oil/water separation material in practical application.
Fig. 1 (a) Schematic diagram of the modification of cellulose sponge, (b) the oil/water separation process of prepared cellulose sponge. |
The morphological structures of the original and OTS-grafted sponge were investigated by SEM. From Fig. 3a and a′, it could be clearly seen that the untreated sponge exhibited a highly porous structure and a smooth skeleton surface. As shown in Fig. 3b and b′, the modification with OTS led to the significant change on the sponge surface. The grafted sponge covered uniformly by OTS exhibited its evident rough surface, which might be the reason of the reduction of pore structure. And this difference also proved that grafting reaction had taken place on the surface of the sponge. It is worth noting that one of the prerequisites for hydrophobic/oleophilic property is the rough surface mentioned above.28
Fig. 3 SEM images of prepared cellulose sponge: (a) control, (a′) a control one with high magnification, (b) cellulose sponge after modifying with OTS, (b′) a modifying one with high magnification. |
The maximum operating temperature is determined by the thermal performance of the sponge. TG was conducted for the original sponge and the modified one, and the results were shown in Fig. 4. From the curve, it could be found that the mass loss of each sponge was recorded three stages. For the blank sponge, the maximum decomposition temperature of the first stage was between about 40 °C and 170 °C, which was the result of the loss of water and cellulose. Compared to the blank one, the modified sponge's maximum decomposition temperature was increased to about 180 °C and the weightlessness rate was lower, which indicated its better thermal stability due to the addition of OTS and less water adsorbed from the air. In the second stage, both the blank and modified sponges lost their weight rapidly, which could be owing to the degradation of the cellulose sponge backbone.
Fig. 4 The TG curves of cellulose sponge before and after modifying: (a) control, (b) the modified one. |
The contact angle measurements were implemented to evaluate the surface wettability of the original and grafted sponge, and the consequences were presented in Fig. 5. The pristine sponge was superhydrophilicity and superoleophilicity with very low WCA and OCA, which indicated that both water and oil could easily wet this sponge. Whereas, from the figure, it could be clearly seen that the water droplet exhibited spherical shape on the surface and the water contact angle increased rapidly to 153.5° with OTS grafted on the surface of the sponge. In the meantime, when an oil droplet dropped to the surface of OTS-grafted sponge, it spread and completely wetted the sample with the oil contact angle of about 0°. These consequences demonstrated the super-hydrophobicity/oleophilicity of the modified sponge.
As mentioned above, the modified sponge showed superhydrophobic and superoleophilic. The phenomenon of water droplets and chloroform droplets on the sponge, and the sponge in the water and vegetable oil was looked into to further prove the unique wetting properties of the treated sponge. As shown in Fig. 6a, when a dye-dyed water droplet and a dye-dyed chloroform were placed on the surface, the blank sponge could be penetrated and wetted rapidly by both water and chloroform due to its superhydrophilicity and superoleophilicity. It could be seen from Fig. 6b that the OTS-grafted sponge still possessed the superlipophilicity and were only permeated by dye-dyed chloroform. Whereas, it was worth noting that the treated sponge showed a different wettability than before which might have something to do with their low surface energy and repelled the dye-dyed water which took on an almost perfect sphere on the surface.29 Fig. 6c showed the phenomena of the original sponge and modified sponge being placed on the surface of water and vegetable oil. It could be found that the blank sponge swelled rapidly caused by the water adsorption and suspended under the surface of water. In contrast, the OTS-grafted sponge floated on the surface of water without any visible changes, indicating the superhydrophobicity and certain structural strength of it. When immersed in vegetable oil, nevertheless, the treated sponge was soaked rapidly and suspended beneath the oil without swelling because of its super-oleophilicity.
Fig. 7 illustrated the adsorption capacities of the OTS-grafted sponge which could be estimated by several kinds of oils/organics, including vegetable oil, hexane, cyclohexane and chloroform. The results showed that the adsorption capacities of the as-prepared sponge for oils/organics were above 200% to around 700% of its weight, exhibiting a great oil adsorption performance. The multi-aperture structure and low surface energy of it might be the reason of this great property.30 Therefore, this modified sponge had a broad prospect as a sort of oil adsorbing material with great oil adsorption performance. However, there was such a huge difference in the adsorption capacities of vegetable oil and chloroform. The reason of this might be that the molecules of chloroform were smaller and more likely to enter the pores of the sponge.
Fig. 7 The absorption capacities of modified cellulose sponge for various types of oil or organic solvents. |
As shown in Fig. 8, the OTS-grafted sponge could be reused to absorb oil on the surface of oil/water mixture with high absorption capacity and great adsorption selectivity. In addition, the absorbed oil could be restored. These characteristics mentioned above were very crucial in practical applications. The separation process of collecting toluene from the surface of oil/water mixture by grafted sponge was demonstrated in Fig. 8a. The toluene in the mixture was rapidly absorbed by the OTS-grafted sponge when they came into contact with each other, and the sponge still remained the water-repellent at the same time. Obviously, it could be seen that the modified sponge still floated on the surface of water even after absorbing the toluene due to its light weight and superhydrophobicity. What's more, the as-prepared sponge could absorb toluene completely without absorbing water and the absorbed toluene could achieve recycling with mechanical extrusion. Fig. 8b showed the durability of the modified sponge after 5 adsorption–desorption cycles, taking vegetable oil as the sample. The results demonstrated that the absorption capacity of the sponge only reduced a little bit after 5 times of cyclic extrusion and still sustained about 200% of its weight, which indicated a great recyclability.
Fig. 8 (a) The separation process of collecting toluene (dyed by dyestuff) from water surface, (b) the changes of absorption capacity for vegetable oil after 5 adsorption–desorption cycles. |
Fig. 9 showed the investigation results on the separation efficiency of the OTS-grafted sponge which was an important indicator for evaluating the separation property. It was worth noting that the modified sponge exhibited outstanding separation efficiencies of all above 92% towards these experimental mixtures (vegetable oil, hexane, cyclohexane, chloroform). Among them, the as-prepared sponge had the most excellent separation ability to chloroform, which could come up to about 98%. These consequences demonstrated that the modified sponge was a material worth choosing for oil/water separation.
Fig. 10 showed the continuous separation ability of the grafted sponge for toluene/water mixture, which was investigated through a constant flow pump. When the pump was taken on, toluene was drawn at a constant rate through a rubber tube to the beaker on the other side (Video S1 in ESI†). After a while, toluene was removed completely from the mixture, and no water was pumped away to the toluene collected beaker, which indicated that the modified sponge still maintained its superhydrophobicity and superoleophilicity even in the presence of pressure difference. These consequences indicated that the grafted sponge had a good application prospect in oil/water separation.
Fig. 10 The continuous oil/water separation process of modified cellulose sponge under being assisted by a peristaltic pump. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra07910c |
This journal is © The Royal Society of Chemistry 2020 |