3.1 Structural characterizations of the RCF
The morphological and structural evolution of the CCF and RCF were comparatively studied by SEM images with different magnification (Figure 1b-1g). As presented in Figure 1b, the original CCF possesses a woven structure composed of twisted microscale fibers with a diameter of about 12 μm. The individual fibers have some intrinsic wrinkles on their surfaces and are covered with an amorphous material (Figure 1c), consisting primarily of non-cellulose compounds such as pectin, protein, and wax. When the CCF underwent dissolution and freeze drying process, the surface morphology of the fabric changed significantly (Figure 1d-1f). The surface of the RCF is seen to be fully covered by a relatively dense skin-layer (Figure 1d). For individual fiber of the RCF, a large number of symmetrical deep concave valleys morphology structures were constructed and thus generated high surface roughness (Figure 1e), resulting from the dissociated cellulose. The magnified image of the top view (Figure 1f) shows that the surface of the RCF has numerous interconnected pores with a diameter of 300-500 nm. Interestingly, the relatively flat part of the RCF surface in low resolution SEM image was further magnified (Figure 1g), and a large number of nanopores with pore size ranging from 10 nm to 50 nm were randomly distributed on the RCF surface.
The porous feature of the RCF was further determined by nitrogen adsorption/desorption isotherms (Figure 1h). The RCF has a large surface area of ∼120 m2/g with type-IV isotherms measured by the Brunauer-Emmett-Teller (BET) method. The Barrett-Joyner-Halenda (BJH) method verified that the average pore size of the mesopore structure was 17 nm and the pore volume was 0.31 cm3/g (inset of Figure 1h). According to the above SEM observation and pore structure measurement, these results demonstrate that the RCF display hierarchical porosity at the nano-, micro-, and macroscales. In addition, all these pores are connected to each other to form an unimpeded transport channel. The connection of regeneration cellulose with partially dissolved fabric results in an interconnected network with a particularly stable pore structure.
The XRD patterns of the CCF and RCF were displayed in Figure 1i. It could be seen that the main diffraction peak of CCF appears near 2θ at 15.4° (101), 17.1° (101), 23.2° (002), and 35.2° (040), which belonged to the feature diffraction peaks of cellulose I.[23] In comparison, the characteristic diffraction peaks of cellulose I was also observed in the RCF. At the same time, the XRD profiles of the RCF also presented the typical peaks of cellulose II crystal at 11.5° and 20.5°, which corresponded well to the lattice planes of (110) and (110).[24] The results indicated that the RCF was a mixture of cellulose I and cellulose II. The structural transformation of cellulose I to cellulose II is caused by the partial destruction and rearrangement of the cellulose crystal structure in the dissolving-regenerating process of the CCF.
The thermal stability of the CCF and RCF were investigated by their respective TGA curves in ambient condition (Figure 1j). According to the TGA curves, the two samples had a small initial mass loss around 100 °C due to the evaporation of absorbed moisture. As the temperature further increased, the major decomposition occurred in the temperature range of 260-350 °C with ∼70 wt % weight loss for the materials. When the temperature exceeded 510 °C, the residual mass of both materials was almost 0%, indicating that the materials completely decompose in the air. Compared with the CCF, the lower thermal stability of the RCF may be due to the fact that it contains cellulose II, which has been reported to degrade at significantly lower temperature.[25]
In order to further clarify the changes of surface chemical structures of the CCF and RCF, FTIR was used to characterize. The FTIR spectra of the CCF and RCF were illustrated in Figure 2a. For the CCF, the band located at 3319 cm-1 corresponds to the stretching vibration of hydroxyl groups, which were mainly derived from the cellulose and the “wax-type” material on the surface of cotton fibers. The absorption peak at 2852 and 1458 cm-1 belonged to the C-H stretching vibration and C-H bending vibration of -CH2- in long alkyl chain, respectively. After self-dissolving and regenerating treatment, cellulose characteristic peaks were observed at 3350, 2900, 1063 and 894 cm-1 in the RCF, characteristic of O-H, C-H and C-O stretching vibration and -C1H distortion in glucose, respectively.[26] The bands at 1107, 1054 and 709 cm-1 were observed to decrease or even disappear, which were planar stretching vibration of ring skeleton, CO stretching vibration at C3 and C-C stretching and C-O-H out-of-plane bending of cellulose Iβ, respectively.[24] These results further support the conversion of cellulose I to cellulose II and are consistent with the XRD results.
The surface chemical structures were further analyzed by XPS. For the wide-scan XPS spectra and the respective elemental ratios of the CCF and RCF (Figure 2b), only carbon and oxygen species were detected. It is worthwhile to mention that the O1s signal of the RCF increased sharply compared with the CCF. After dissolution and regeneration treatment, the oxygen content in the materials increased from 18.1% to 37.2%. The O/C elemental ratio increased from 0.22 for the CCF to 0.59 for the RCF, implying that more oxygen-containing groups were exposed on the surface. Moreover, as shown in the high-resolution C1s XPS spectra (Figure 2c, 2d), the oxygen species of C-O (hydroxyl, 286.3 eV), O-C-O or C=O (carboxyl, 287.8 eV) for the RCF increased significantly after self-dissolving-regenerating process. This was evident and could be seen clearly by comparing the peak intensities of oxygen-containing groups in the CCF and the RCF (Figure 2e and 2f). The oxygen content from the CCF to the RCF increased after dissolution and regeneration, which may be attributed to the exposure of more glucose groups and hydrogen bonds in the cellulose surface.
3.2. Wettability of the RCF
The wettability behaviors of the RCF and CCF were examined by measuring the WCA and OCA in air, as shown in Figure 3. The original CCF presented a hydrophobicity (WCA, 143°) (Figure 3a) and a superoleophilicity (OCA, 0°) (Figure 3b). This result is easy to understand because of the presence of the naocellulosic compound, such as proteins, pectin and waxes on the surface of cotton fibers. On the contrary, it could be seen from Figure 3c that the RCF surface had excellent superhydrophilicity in air (WCA, 0°), and the water droplet spread and permeated into the surface at a very fast speed when they contacted the membrane. At the same time, the surface of RCF still maintained the superoleophilicity in air (Figure 3d), which indicates that the RCF had a unique superamphipathicity. This result was due to the synergistic effect of hydrophilic cellulose surface modification and hierarchical structure morphology of the RCF.
Figure 3e was the underwater oil CA of the RCF to different kinds of oil. Underwater superoleophobicity is a key factor in the application of the membrane for efficient oil/water separation.[27] When a few oil droplets (1,2-dichloroethane, n-hexane, toluene, hexadecane, soybean oil, pump oil and crude oil) were placed on the surface of the submerged RCF, the oil droplets remained spherical rather than spreading out (Figure 3e). The underwater oil CA of these different oil droplets was around 160°, which indicated that the RCF had excellent underwater superoleophobic property. The RCF surface with the developed hydrophilic hierarchical architecture could increase hydration ability and formed a stable hydration layer to realize underwater superoleophobic feature. Therefore, without the need for any chemical modification or coating, the superhydrophilic RCF was superoleophobic in water.
The underwater superoleophobic stability of the RCF in 1 M HCl solution, 1 M NaOH solution and 10 wt% NaCl solution was also evaluated. As indicated in Figure 3f, the underwater CA of the oil (1M HCl solution, 1M NaOH solution and 10wt% NaCl solution) under the corrosive condition were all greater than 150° (approximately spherical), which confirmed the stable underwater superoleophobicity of the RCF in these harsh environment.
With respect to oil-in-water emulsion separation, the ideal separation material should have very low (negligible) underwater-oil adhesion on the surface to resist oil contamination, while also showing high separation selectivity. To better explore the low-oil-adhesion performance of the RCF, a dynamic underwater oil adhesion experiment was conducted (Figure 4a, 4b). Figure 4a presents the adhesion property when an oil droplet was extruded on the surface of the RCF and then lifted up underwater. During compression relaxtion, the shape of the oil droplet changed from spherical to elliptical and eventually returned to spherical shape (Figure 4a). When the oil droplet left the RCF surface, there was little distortion in the corresponding photographs. Figure 4b displayed the forward and backward behavior of underwater-oil adhesion on the RCF surface. No force was observed during the forward and backward processes, and images of the corresponding oil droplet observed throughout the process showed no deformation. These results confirmed that the RCF possessed anti-oil-adhesion property.
Due to its excellent underwater anti-oil-adhesion property, the RCF had excellent underwater self-cleaning performance for both light and viscous oil. Under water, a trickle of toluene (dyed red) easily bounced off the RCF surface without leaving any oil droplets (Figure 4c, Movie S1), which meant that its adhesion to the oil was extremely low. Furthermore, when the pre-wetted RCF adhered to heavy crude oil in air, the preloaded crude oil would automatically escaped from the RCF surface when the RCF was immersed into water (Figure 4d). In addition, when the RCF was lifted and repeatedly immersed in the crude oil/water mixture, the RCF was not contaminated with oil and remained clean underwater (Movie S2). The result showed the RCF had excellent anti-oil-adhesion and self-cleaning properties in an aqueous environment.
In order to further accurately evaluate the underwater-oil-adhesion force of the RCF surface, dynamic measurements of underwater-oil-adhesion force were further performed, and the results were given in Figure 4e and 4f. A load was applied to the oil droplets to ensure adequate contact with the membrane surface and to lift them up from the membrane under water. As indicated in Figure 4e and 4f, a smooth adhesion force curves were observed during the whole process of advance and retreat. The adhesion forces of the RCF to toluene and n-hexane in water was extremely low, which were 3 μN and 1 μN, respectively. Besides, no droplet deformation was observed when the oil droplet was detached from the surface of membrane (inset in Figure 4e and 4f). The RCF showed remarkable underwater anti-oil-adhesion property was mainly due to its superhydrophilicity.
3.3. Oil-in-water Emulsion Separation Performance
The RCF had excellent characteristics, such as selective wettability, hierarchical porous structure, and anti-oil-adhesion property, which made it had a promising capability to separate different kinds of oil-in-water emulsions. Therefore, the separation performance of the RCF for diverse oil-in-water emulsions was comprehensively evaluated. As preliminarily studies, SDS-stabilized toluene/water and n-hexane/water emulsions were used to conduct separation tests on the RCF. The digital photos and liquid droplet distribution of the corresponding emulsion/filtrate were shown in Figure 5a-b. Intuitively, the image of the bottle containing the emulsion and filtrate indicated that the feed emulsion was milky white and the filtrate was wholly transparent. In addition, the optical microscope pictures showed that the emulsions feed contained a large number of random oil droplets, while the filtrate did not contain any visible liquid droplets. The result confirmed that the RCF membrane could effectively retained oil droplets in the emulsion.
The permeability as an important parameter of the RCF separation performance was also comprehensive evaluated. As displayed in Figure 5c, as for the three SDS-stabilized emulsions, the fluxes of toluene-in-water, hexadecane-in-water, and n-hexane-in-water emulsions are 1600, 1460 and 1680 L/m2h, respectively. The flux difference between different emulsions, which was mainly ascribed to their different viscosity of hexadecane, toluene and n-hexane, which is 3.03 mPa/s, 0.59 mPa/s and 0.31 mPa/s, respectively. Basically, oil-in-water emulsions with lower oil viscosity will generally exhibit higher flux. The separation fluxes of hexadecane-based emulsions were relatively smaller compared to other emulsions, possibly due to their higher viscosity. This result was consistent with the Hagen-Poiseuille equation, in which the flux of a liquid was just inversely proportional to its viscosity.[28]
Besides, the oil rejection ratios (hexadecane, toluene, and n-hexane) of the RCF were further quantitatively measured. As indicated in Figure 5c, the separation efficiency of all the measured emulsions is up to 99.5%, indicating an outstanding separation property of the RCF for SDS-stabilized oil-in-water emulsions. Impressively, compared to some of the previously reported representative oil-water emulsions separation materials as listed in Table 1, the RCF in this work demonstrated superior separation property for SDS-stabilized oil-in-water emulsions.
Table1 Comparison of preparation, flux and separation efficiency of the RCF with that in other emulsions separation works.
Substrate
|
Coating material
|
Fabrication method
|
Separation
type of oil
|
Water flux
(L/m2h)
|
Separation efficiency
(%)
|
Ref.
|
Cotton fabric
|
Mg(OH)2
|
1.Desizing
2.Dip coating
|
n-Dodecane-in-
water
|
957
(200 mL/min)
|
99.5
|
[22]
|
Cotton fabric
|
Cellulose
|
1.Desizing
2.Dip coating
|
Toluene-in-
water
|
4200
(-0.01 MPa)
|
93.2
|
[29]
|
Stainless mesh
|
TiO2
|
1.Electrostatic self-assembly
2.Annealing
|
Decane-in-
water
|
2329
(gravity)
|
94.8
|
[30]
|
Nylon mesh
|
Tunicate cellulose nanocrystal
|
Vacuum filtration
|
n-Hexane-in-
water
|
1549
(0.5 bar)
|
99.9
|
[31]
|
Steel mesh
|
ZnO-Co3O4
|
Hydrothermal synthesis
|
Toluene-in-
water
|
66.9
(gravity)
|
-
|
[32]
|
Carbon cloth
|
I-doped (BiO)2CO3
|
Hydrothermal synthesis
|
Toluene-in-
water
|
195
(gravity)
|
99.9
|
[33]
|
Filter paper
|
TEMPO-
CNF
|
Dip-coating
|
n-Hexane-in-
water
|
89.6
(gravity)
|
99.1
|
[34]
|
PP membrane
|
PDA/PEI
|
Co-deposition
|
Dichloroethane-in-
water
|
100
(1 bar)
|
98
|
[35]
|
GO
|
Palygorskite
|
Vacuum filtration
|
Hexadecane-in-
water
|
1867
(0.5 bar)
|
99.9
|
[36]
|
PVDF
membrane
|
Chitosan silica
|
Dip-coating
|
Gasoline-in-
water
|
-
|
99
|
[37]
|
Cotton fabric
|
Cellulose
|
Self-dissolving-regenerating process
|
Toluene-in-
water
|
1600
(0.5 bar)
|
99.5
|
This
work
|
In addition, the reusability of the RCF was further tested through cyclic separation of SDS/toluene/H2O emulsion. The flux and the separation efficiency were monitored over five cycles. At the end of each cycle, the RCF was washed with water and ethanol to remove deposited oil-foulants. As indicated in Figure 5d, the emulsion permeation flux of the RCF could maintain without significant flux decline during the whole test. Moreover, there was no remarkable change in the separation efficiency during the cycle test. The result presented that the RCF possessed the outstanding anti-oil-fouling capacity and recyclability. The RCF had excellent reusability and antifouling property, mainly due to its outstanding underwater superoleophobicity and hydration ability, which was conducive to the formation of a hydration layer on the membrane surface during filtration, avoiding oil adhesion and accelerating water permeation.
Therefore, based on the above discussions, the excellent performance of the RCF was attributed to the hierarchical porous structure and hydrophilic surface of the RCF. The numerous hydrophilic functional groups on the cellulose surface could provide an excellent hydrophilic interface, which was the key to achieve oil/water separation of the RCF membrane. The hierarchical porous structure provided multifarious pathways as size-sieving for water molecules rapid permeation while blocking larger oil molecules.[38] The water molecules are capable of establishing hydrogen bonds with cellulose surface and passing through the hierarchical pores rapidly due to its small size.[39]