Comparative analysis of the static adsorption of oil droplets and the dynamic absorption capacity of their aggregates by cotton,kapok,cattail and flax fibers

Abstract

Natural fiber has the advantages of being abundant, renewable and biodegradable, and it will be very promising to use it in the field of oil absorption. The adsorption and absorption process of engine oil, waste oil and salad oil were observed and quantitatively characterized by using four natural fibers of cotton, kapok, cattail and flax fiber. The main reason for the different forms of adsorbed oil droplets is that the static contact angles of the four fibers have little difference in the three oils, which further proves that flax is both lipophilic and hydrophilic. Judging from the shape of the oil droplets absorbed by the four types of fiber flakes, because the fibers changed from the local wetting state to the diffusion and expansion stage, and finally reached a stable state of saturation, the immersion time of the first half of the circle was much lower than that of the second half. The analysis of the influencing factors, such as the wax content on the fiber surface and the viscosity and tension of the oil, provides a theoretical reference for the application of natural fibers in the field of oil and sewage treatment.

Keywords

Natural fiber oil adsorption static contact angle oil dynamic absorption surface adsorption characteristics

Oily sewage that has not been properly treated is directly discharged into the ocean or rivers, which not only wastes resources but also causes ecological damage. One of the most ideal ways to treat oily sewage is by using lipophilic materials to absorb the oil in a large amount of oily sewage on its surface or into the pores for transfer, separation and recovery. The currently reported chemical synthetic materials, such as polypropylene fibers, polyurethane foams, polystyrene resins and polyester nonwovens, have the characteristics of higher stability and selectivity, easy large-scale production and low cost.^1

–6 These materials are widely used in the field of dealing with oil pollution. However, these materials also have some weaknesses, which are that they easily cause secondary pollution and are non-degradable and difficult to recycle.

In recent years, the use of natural fibers, such as wool, milkweed, poplar seed hair and radish seed hair, as oil-absorbing carrier materials has been given attention by researchers.^7

–10 These materials have the characteristics of low cost, convenient material acquisition, biodegradability and low environmental pollution during disposal. Some natural fiber materials even have a higher oil absorption capacity than industrial synthetic materials. Choi et al.^11
–13 showed that the oil absorption rate (30–40 g/g) of milkweed fiber, cotton fiber and wool fiber to crude oil was higher than that of polypropylene fiber. Annunciado et al.¹⁴ concluded that silk wool fibers have strong oil absorption and hydrophobic properties, and the oil absorption rate for petroleum reaches 85 g/g. Likon et al.¹⁵ reported that the adsorption rate of poplar seed wool fibers for high-density motor oil could be as high as 211 g/g. In addition, Khan et al.^16,17 found that compared with natural materials such as kapok fiber, sage and wood chips, cattail fiber has the best adsorption effect on polycyclic aromatic hydrocarbons (PAHs) in water. Of course, there is also literature showing that most natural adsorption materials have a low oil absorption capacity or no lipophilic or hydrophobic properties, and chemical modification is required to obtain hydrophobic and lipophilic properties. Wahi et al.¹⁸ reviewed the oil absorption properties of some natural fibers (rice husks, barley straw, wood chips, etc.), and concluded that most natural fiber materials have a small oil absorption rate (about 5 g/g), weak buoyancy performance and easily become damp and rotten.

The kinetics of adsorption and absorption of droplets by fibers and their aggregates involves a variety of fundamental physical phenomena, including wetting and adsorption on the fiber surface, permeation and absorption and the flow and diffusion of oil droplets inside the aggregates. For example, Carroll¹⁹ and McHale and Newton²⁰ reported that there are two possible equilibrium shapes (barrel-like and shell-like) for droplets on the fibers, and deduced the analytical equation for the droplet shape and gave out-of-transition conditions. The wicking performance of fiber aggregates generally refers to the process in which liquid water diffuses on the surface of fibers and is absorbed and transported by capillaries formed between fibers. The Washburn capillary model is more classically used to describe the wicking behavior.²¹ From a more microscopic point of view, lipophilic fibers utilize van der Waals forces, hydrogen bonding forces, the affinity of lipophilic groups and capillary forces to adsorb and absorb oils.

Cotton, kapok, cattail and flax are common cellulose fibers in nature. Generally speaking, there are few reports on the adsorption and absorption process of oil by these four fibers, and the description of the microscopic oil absorption process is still unclear. At present, much of the literature mainly focuses on the expression of the absorption characteristics of these fibers after modification. For example, Onuma et al.²² reported the characterization of the oil absorption performance of cotton fibers after chemical modification. Ik and Urakan²³ reported a test of the absorption properties of heavy metal ions and dyes after flax fiber was modified. Rouison et al.²⁴ reported a description of the water absorption properties of linen fabrics. A fiber can absorb both oil and water droplets, which is actually detrimental to the separation of oil and sewage. The research on oil absorption characteristics of kapok and cattail mainly focuses on fiber aggregates,^25
–27 both of which are in the initial stage of exploratory research.

In order to explore the adsorption and absorption process of the four natural fibers of cotton, kapok, cattail and flax on oil, we used engine oil (the most commonly used lubricating oil in engines and an important pollution source of industrial oil), waste oil (the source is collected and used salad oil, which is also an important pollution source of domestic sewage) and salad oil (the most commonly used edible oil in life) as a representative of the oil absorbed by the fiber material. By observing the adsorption and absorption state of oil, we quantitatively characterize and compare the static contact angle of oil on a single fiber and the relationship between the shape and time of oil on fiber aggregates, and analyze the surface adsorption and time of oil on the fiber. The internal absorption characteristics and influencing factors provide a reference for the application of natural fibers in the field of oil and sewage treatment.

Materials and methods

Samples

The cotton, kapok, cattail and flax fibers were all produced in China. Before the experiment, these four fibers were left standing at room temperature for 24 hours without any physical or chemical treatment. The appearance of their loose fibers is shown in Figure 1. The color and fiber length of the cotton in Figure 1(a) and the kapok in Figure 1(b) are not very obvious. The cattail fiber in Figure 1(c) has a tuft-shaped structure, and the small yellow particles in the figure are the seeds of the cattail fiber. Compared with the other three fibers, flax fibers are whiter in color and longer in fiber length, as shown in Figure 1(d). There are four kinds of experimental liquids used, namely distilled water, motor oil, waste oil and salad oil. Distilled water is used to compare the physical indexes of the three kinds of oil, and also to test the water absorption performance of flax fiber. The appearance colors of the three oils are shown in Figures 1(e)–(g): the waste oil in Figure 1(f) is the used salad oil collected from a kitchen range hood, and its color is dark yellow. The colors in order from light to dark are salad oil, motor oil and waste oil.

Figure 1.
Natural loose fibers and experimental oil. (Color online only.)

Index formula

The adsorption properties of natural fibers are related to the complexity of the arrangement of macromolecules on the surface of the fibers, and the complexity of this arrangement is usually expressed by the index of crystallinity. We use a D/max-2550PC X-ray diffractometer to measure the diffraction peak intensity and amorphous dispersion peak intensity, and further use the graphical sub-peak to calculate the crystallinity. The calculation formula is shown in Equation (1)
$X_{c w} = \frac{I_{c} - I_{a}}{I_{c}} \times 100 %$
(1)

In this formula, X_cw represents the crystallinity of the sample (%), I_c represents the diffraction peak intensity (a.u.) and I_a represents the amorphous dispersion peak intensity (a.u.).

We used the Soxhlet extraction method to determine the lipid and wax content on the fiber surface. The whole extraction process was carried out in a Soxhlet extractor, in which benzene and ethanol were used as organic solvents. The calculation formula is shown in Equation (2)
$M_{z} = \frac{W_{m} - W_{n}}{W_{m}} \times 100 %$
(2)

In the formula, M_z is the lipid wax content (%), W_m is fiber sample weight before extraction (g) and W_n is fiber sample weight after extraction (g).

The surface tension of the oil is measured by a DCAT11 surface tension measuring instrument. The principle is that the surface tension measuring instrument is used to partially immerse a standard platinum-iridium sheet in the oil, and the maximum pulling force W_f required when the platinum-iridium sheet is separated from the liquid surface is measured; its calculation formula is shown in Equation (3)
$σ = \frac{W_{t} - W_{f}}{2 (1 + d)}$
(3)

In this formula, d represents the thickness of the standard sheet (m), W_f represents the maximum pulling force (mN) required for the platinum-iridium sheet to separate from the liquid surface, W_t represents the force (mN) of the measured standard sheet in the liquid and σ represents surface tension (mN/m).

Test methods

We used a JSM-5600LV scanning electron microscope and Labomed optical microscope to observe the morphology of the cross-sections of the above four fibers and the state of a single fiber after oil absorption. We used an OCA15EC optical contact angle measuring instrument to test the static contact angle between the droplet and a single fiber and take pictures of the adsorption state of the droplet by the fiber flakes. In addition, we used an SNB-2 digital rotational viscometer and the DCAT11 surface tension meter to measure the physical properties of the experimental liquid, such as viscosity and surface tension.

Results and discussion

Observation of the microscopic morphology of fibers

We used the JSM-5600LV scanning electron microscope to observe the cross-sections of the four fibers without any treatment, and the microscopic cross-sections are shown in Figure 2. In Figure 2(a), the surface of the cotton fiber is concave and convex when viewed along the direction of the fiber axis, and there is a natural twist structure. Although the cotton fibers are hollow and flat in the transverse cross-section with rough incisions, the flat shape of the fibers may return to a tubular shape due to swelling after oil absorption. As shown in Figure 2(b), kapok is a natural cellulose fiber with a smooth surface and a tubular shape, with a thin-walled large hollow structure, in which the hollowness rate can be as high as 80%,²⁸ and its hollow interior provides storage space for oil.^29,30

Figure 2.
Micro-sections of the four kinds of fiber: (a) cotton; (b) kapok; (c) cattail and (d) flax.

The macroscopic cattail fibers shown in Figure 1 above are tuft-like fibers, and each tuft contains one seed and dozens of single fibers.³¹ We randomly picked one of the single fibers for scanning electron microscopy (SEM) observation. From Figure 2(c), it is found that its surface consists of four grooves with depressions in the middle, and the cross-sectional shape is similar to the “∏” type,^32,33 forming a special multi-faceted semi-open natural shaped fiber. Therefore, among the four single fibers of the same volume weight, the surface area of the cattail fiber is the largest. The flax fiber shown in Figure 2(d) is a non-solid layered structure with an irregular circular cross-section, and its surface is not continuous and smooth but rather has convex parts. Compared with the other three fibers, its mechanical strength is the largest due to the thicker layered structure, but its application in the field of oily sewage treatment is also limited because it is a hydrophilic and lipophilic fiber.

Morphology comparison of oil adsorption by a single fiber

We used the Labomed optical microscope to observe the morphology of the four single fibers after adsorbing oil, and the microscopic morphology is shown in Figure 3. We observed that the large oil droplets wrapped around a single fiber showed an oval shape, as expressed by Carroll¹⁹ and McHale and Newton,²⁰ and their volumes were in descending order for kapok, cattail, flax and cotton fibers. In addition to the adsorption of large droplets by the fibers, some oil is spread and adsorbed on the surface of the fibers, forming a surface adsorption oil layer, among which the most obvious is the flax fiber. There is also some oil that penetrates into the cavity of the fiber through the fiber, forming an inner cavity to absorb micro droplets, among which the kapok fiber is more obvious, and the oil is adsorbed at the “node” of the fiber, such as the position of the natural twist of cotton fibers and the baffle of cattail fibers.³¹

Figure 3.
Morphology of the four kinds of fibers adsorbing oil droplets: (a) cotton; (b) kapok; (c) cattail and (d) flax.

The different morphology of the surface and interior of the four kinds of fibers is the main reason for the difference in the properties of the adsorbed oil droplets. Specifically, cotton fibers have the characteristics of natural twist, grooves are formed at the twist and the oil is stably adsorbed at the groove. Therefore, in Figure 3(a), the oil content is relatively high at the twist of the fiber. The kapok fiber has high hollowness and a thinner wall layer; these characteristics promote oil droplets to enter the inner cavity through the micro–nano pores on its surface, and they form stable spherical droplets in the cavity, as shown in Figure 3(b). The oil droplets at the “nodes” of the cattail fibers we see in Figure 3(c), similar to the oil droplet storage at the inflection points of cotton fibers, all form concave–convex grooves, forming “retained” oil. However, the difference is that they are formed in a different way. Cotton fibers are formed by natural twisting, while cattail fibers are formed by naturally growing profiled surfaces. In Figure 3(d), the surface of flax fiber adsorbs thick and uniform oil, which is related to the smooth surface of flax fiber and the uniform distribution of lipophilic groups. At the same time, we found oil storage at the “nodes” of the flax fiber. In other words, due to the effect of surface tension and the existence of the wicking phenomenon, oil droplets form large oil droplets at the “nodes” of fibers, small oil droplets inside hollow fibers and oil-absorbing layers on smooth surfaces.

Static contact angle test

We use a syringe to inject the droplet onto the fiber, so that the droplet hangs on a single fiber, and then use the OCA15EC optical contact angle meter to find the baseline parallel to the fiber axis and the tangent of the contact between the droplet and the fiber, the intersection of these two lines. The formed angle is the static contact angle, and we usually call this method the pendant drop method. It can be seen from Figure 4 that the static contact angles of the four fibers have little difference with the three oils, and their contact angles are all lower than 65°. Generally speaking, the contact angle of fiber to oil is lower than 90°, indicating that the fiber has the ability to adsorb oil. The smaller the contact angle, the stronger the ability to absorb oil. We take the contact angle of the four kinds of fibers to the same oil, such as motor oil. We find that the contact angles from small to large are flax, cattail, cotton and kapok, so we can conclude that flax fiber has the best lipophilicity, while the lipophilicity of kapok fiber is the worst. We speculate that the reason is related to the content of lipophilic groups on the surface and inside of the fiber. Due to the thicker cortex structure of flax fiber and the thin hollow structure of kapok fiber and the lipophilic group contained in flax, the mass is larger than the amount contained on the kapok surface.

Figure 4.
Static contact angles of the three oils and four fibers.

From Figure 5, we found that although flax fiber has better lipophilicity, the static contact angle between flax and water is about 43° (less than 90°), indicating that flax fiber is both lipophilic and hydrophilic. Thus, the water absorption phenomenon of flax fabrics in the previous literature was confirmed,²³ and it was further speculated that flax fibers could not be used as a separation material in the field of oily sewage treatment.

Figure 5.
Static contact angles of the four kinds of fibers and water.

Quantification of the dynamic absorption process of oil droplets

We used the OCA15EC optical contact angle measuring instrument to photograph the spherical drop state obtained during the absorption process of oil droplets on the fiber flakes. Figure 6 shows the state in which the oil was absorbed, and six pictures were taken for each material. We found that the time it took for the droplets to be completely absorbed on the four fibers was different. The cotton fiber batting (5000 ms) was the fastest absorbed, and the kapok fiber batting (15,000 ms) was the slowest absorption, and the cattail and flax fibers were in the middle. Judging from the shape of the oil droplets absorbed, the immersion time of the first half of the circle took less than 200 ms, and the second half of the circle was completely immersed for much longer than the first half. If cotton needs more than 4800 ms, kapok is as high as 14,800 ms. This shape change law can also be represented by the relationship between the instantaneous contact angle and time. As shown in Figure 7, it is found that the instantaneous contact angle decreases rapidly with the prolongation of time before 1500 ms, and it tends to decrease gradually after 1500 ms. Figure 8 is the state of the process of testing the absorption of water droplets on the flax fiber flakes, once again verifying that the flax fibers are hydrophilic and lipophilic fibers. Judging from the shape of the water droplets absorbed, the immersion time for the first half of the circle took more than 500 ms, and the full immersion took 50,000 ms. Whether it was half immersion or full immersion, the time spent was much longer than that of the three oils.

Figure 6.
The process of oil droplets being absorbed on the four types of fiber flakes: (a) cotton; (b) kapok; (c) cattail and (d) flax, where ms is the time in milliseconds.

Figure 7.
Variation curve of the contact angle of fiber flakes during dynamic absorption of oil droplets.

Figure 8.
The process of water droplets being absorbed on the flax fiber flakes, where ms is the time in milliseconds.

The process of oil absorption by fiber flakes can be divided into four stages: wetting, diffusion, swelling and saturation. When the droplet starts to contact the fiber surface from 0 ms, the main factors are gravity, wicking and surface tension, which shows that the droplet spreads rapidly when it first contacts the fiber surface, that is, the contact angle decreases rapidly. When the time is 1500 ms, the droplets have penetrated into the fiber, and the fiber is partially wetted and enters the diffusion and expansion stage, and finally reaches a stable state of saturation; the interior of the oil reaches a fully spread state, resulting in a decrease in the surface tension, and the change of the contact angle between the fiber and the oil is not obvious with time. Therefore, after 1500 ms, as shown in Figure 7, the oil spreading speed becomes slower and tends to balance.

Effect of fiber crystallinity and wax content

Since the four kinds of fibers are all natural cellulose fibers and have the same four crystal planes, their corresponding 2θ angle values are 15°, 16.5°, 22.5° and 34.5°, as shown in Figure 9. The 2θ values of the diffraction peak intensity value position and the dispersion peak intensity value position are 22.5° and 18°, respectively. The characteristic shapes of the four natural fibers are similar, indicating that some characteristic groups are similar, and the different intensity values of the peaks also indicate that the content of the characteristic groups contained is different. From the calculation results, the crystallinity of cotton and flax fibers is greater than that of cattail and kapok fibers, and the macromolecular arrangement structure on the fiber surface is also closely related to the adsorption characteristics of the fiber surface. This is because the lateral attraction of macromolecules in the fiber crystalline region makes the arrangement of macromolecules relatively neat and dense, and there are fewer gaps and voids, so there is less space for storing microscopic oil and less adsorption capacity. In contrast, the smaller the crystallinity of the fibers, the larger the gaps and voids, which is more conducive to the adsorption of various liquids by the fibers. Therefore, in terms of crystallinity, compared with cotton and flax fibers, kapok and cattail fibers have lower crystallinity, and kapok and cattail fibers have better adsorption capacity.

Figure 9.
X-ray diffraction patterns of the four kinds of fibers.

It can be seen from Table 1 that the surface wax content of cattail fibers is the largest, followed by flax, and the surface wax content of cotton and kapok fibers is the smallest. The presence of wax on the fiber surface has two effects on the adsorption performance of the fiber surface: firstly, it provides a relatively hydrophobic surface that is conducive to oil absorption; secondly, it provides a lower surface energy environment for the capillary transport of oil. Therefore, the greater the surface wax content, the more favorable the lipophilic and hydrophobic properties of the fiber surface. It can be speculated that the waxy surface of cattail fiber is an important reason for its excellent oil absorption performance. In addition, compared with cotton, flax and kapok fibers, the moisture regain of cattail fibers is the lowest, which may be due to its high lipid wax content, which will make the fiber water repellent and reduce its hygroscopicity.

Table 1.
Physical test indicators for the four kinds of fiber

Parameter index Cotton Kapok Cattail Flax

Crystallinity (%) 68.00 32.70 45.41 66.12

Wax content (%) 0.60 0.80 10.64 2.26

Moisture regain (%) 11.10 10.11 8.98 12.00

Influence of the surface tension and viscosity of oil droplets

We used DCAT11 surface tension measuring instrument and SNB-2 digital rotational viscometer to test the surface tension and viscosity of water droplets and oil droplets, respectively. The specific measured values are shown in Table 2. The surface tension of a liquid refers to the intermolecular force when a fluid is applied to the interface of another substance. The lower the surface tension, the easier it is for the liquid to penetrate into another substance and be held inside the substance. From the measured viscosity value, we found that the surface tension of distilled water is about two times that of oil, but the viscosity value is much smaller than that of oil, and water is about 270 times that of oil. The greater the viscosity of the liquid, the greater the friction between the liquid molecules and the weaker the fluidity of the liquid. Therefore, the high viscosity oil is of great help to the adsorption of the fiber surface. However, it is disadvantageous for the complete absorption of the fibrous batt due to the poor flowability. When comparing the contact angles of the same fiber to different oils, we found that the contact angles of the fibers to engine oil, waste oil and salad oil increased sequentially, which indicated that the adsorption properties of the three oils decreased sequentially. As shown in Table 2, the corresponding viscosity values decreased sequentially, followed by oil at 270.70 mPs, waste oil at 131.20 mPs and salad oil at 72.34 mPs.

Table 2.
Physical parameters of water and oil

Parameter index Water Engine oil Waste oil Salad oil

Surface tension (mN/m) 71.79 31.17 32.93 32.95

Viscosity (mPs) 1.00 270.70 131.20 72.34

Density (g · cm^–3) 1.04 0.88 0.95 0.93

Figure 10 shows the relationship between the static contact angle of the four fibers and the three oils with the increase of the surface tension of the oil. For the same type of oil, the contact angle of the cattail fibers is the largest, and the contact angle of the cotton fiber is the smallest. For the same fiber, the smaller the surface tension of the oil, the smaller the contact angle. In addition, the difference in surface tension between waste oil and salad oil is small, but the contact angle between the two oils and the fibers is quite different, which may be due to other impurities in the waste oil. These phenomena indicate that the greater the surface tension of the droplet, the more difficult it is for the droplet to enter the surface and interior of the fiber and the weaker the fiber's ability to adsorb droplets.

Figure 10.
The relationship between surface tension and contact angle. 1: the contact angle between fiber and oil; 2: the contact angle between fiber and waste oil and 3: the contact angle between fiber and salad oil.

Conclusions

In this paper, four kinds of natural fibers, cotton, kapok, cattail and flax fiber, were used to observe and quantitatively characterize the adsorption and absorption process of three oils, namely motor oil, waste oil and salad oil. The contact angle and dynamic absorption process were compared and analyzed, and the influence of the physical parameters on the oil adsorption performance of the fiber was discussed from the perspectives of oil and fiber, and the following conclusions were obtained.

Firstly, the different morphology of the surface and interior of the four natural fibers is the main reason for the different morphology of the adsorbed oil droplets. The oil absorbed by the cotton fibers will be abundant in the natural twists of the fibers. The oil adsorbed by the kapok fiber passes through the micro–nano pores on the surface of the fiber and enters the cavity inside the fiber, forming stable spherical droplets. The concave and convex grooves on the surface of the cattail fibers promote the “retention” of oil. The adsorption layer on the surface of flax fiber is relatively thick.

Secondly, the static contact angles of the four kinds of fibers to the three kinds of oils have little difference, and their contact angles are all lower than 65°, indicating that the four kinds of fibers have the ability to adsorb oil. However, the contact angle of flax fiber with water is around 43°, which further proves that flax is both lipophilic and hydrophilic. As the fiber transitions from the local wetting state to the diffusion and expansion phase, and finally reaches a saturated steady state, the first half of the circular immersion time is much lower than the second half of the circular immersion time.

Finally, from the perspective of influencing factors, the greater the wax content on the fiber surface, the more favorable the lipophilic and hydrophobic properties of the fiber surface. The high viscosity of oil is very helpful for the adsorption of the fiber surface, but it is unfavorable for the complete absorption of the fiber flakes due to the weak fluidity. In addition, the greater the surface tension of the droplet, the more difficult it is for the droplet to enter the surface and interior of the fiber and the weaker the fiber's ability to adsorb droplets.

Parameter index	Cotton	Kapok	Cattail	Flax
Crystallinity (%)	68.00	32.70	45.41	66.12
Wax content (%)	0.60	0.80	10.64	2.26
Moisture regain (%)	11.10	10.11	8.98	12.00

Parameter index	Water	Engine oil	Waste oil	Salad oil
Surface tension (mN/m)	71.79	31.17	32.93	32.95
Viscosity (mPs)	1.00	270.70	131.20	72.34
Density (g · cm^–3)	1.04	0.88	0.95	0.93

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the research, authorship and/or publication of this article: This work was supported by the Open Project of Key Laboratory of Silk Engineering of Jiangsu Province (No. KJS2059) and the 2020 Shanghai Higher Education Teacher Training Plan of Shanghai Municipal Education Commission Teacher Professional Development Project.

ORCID iDs

Yuanyuan Li

Zhenjie Zhang

References

Haoyi

Weifeng

Bubakir

, et al . Polypropylene fibers fabricated via a needleless melt-electrospinning device for marine oil-spill cleanup. J Appl Polym Sci 2014; 131: 1–9.

Lin

Shang

Ding

, et al. Nanoporous polystyrene fibers for oil spill cleanup. Mar Pollut Bull 2012; 64: 347–352.

Wang

Zeng

, et al. Three-dimensional structured sponge with high oil wettability for the clean-up of oil contaminations and separation of oil-water mixtures. Polym Chem 2014; 5: 5942–5948.

Zhang

, et al. Mechanical-and oil-durable superhydrophobic polyester materials for selective oil absorption and oil/water separation. J Colloid Interface Sci 2014; 413: 112–117.

Brown

Bhushan

Durable superoleophobic polypropylene surfaces. Phil Trans R Soc 2016; A374: 1–9.

Zhu

Chu

Wang

, et al. Robust superhydrophobic polyurethane sponge as a highly reusable oil-absorption material. J Mater Chem A 2013; 1: 5386–5393.

Wenta

Jian

Ran

, et al . Developing superhydrophobic rock wool for high-viscosity oil/water separation. Chem Eng J 2019; 368: 837–846.

Panahi

Moghaddam

Moezzi

Assessment of milkweed floss as a natural hollow oleophilic fibrous sorbent for oil spill cleanup. J Environ Manage 2020; 268: 110688.

Liu

Lyu

Guo

, et al. Sound absorption performance of the poplar seed fiber/PCL composite material. Materials. 2020; 13: 1465.

10.

Wang

, et al. Metaplexis japonica seed hair fiber: a hydrophobic natural fiber with robust oil–water separation properties. Cellulose 2020; 27: 2427–2435.

11.

Choi

Moreau

Oil sorption behavior of various sorbents studied by sorption capacity measurement and environmental scanning electron microscopy. Microsc Res Tech 1993; 25: 447–455.

12.

Choi

Kwon

Moreau

Cotton nonwovens as oil spill cleanup sorbents. Text Res J 1993; 63: 211–218.

13.

Choi

Cloud

Natural sorbents in oil spill cleanup. Environ Sci Technol 2002; 26: 772–776.

14.

Annunciado

Sydenstricker

Amico

SC.

Experimental investigation of various vegetable fibers as sorbent materials for oil spills. Mar Pollut Bull 2005; 50: 1340–1346.

15.

Likon

Remškar

Ducman

, et al. Populus seed fibers as a natural source for production of oil super absorbents. J Environ Manage 2013; 114: 158–167.

16.

Khan

Virojnagud

Ratpukdi

Use of biomass sorbents for oil removal from gas station runoff. Chemosphere 2004; 57: 681–689.

17.

Khan

Khaodhir

Rotwiron

Polycyclic aromatic hydrocarbon removal from water by natural fiber sorption. Water Environ Res 2007; 79: 901–911.

18.

Wahi

Chuah

Choong

, et al. Oil removal from aqueous state by natural fibrous sorbent: an overview. Sep Purif Technol 2013; 113: 51–63.

19.

Carroll

The accurate measurement of contact angle, phase contact areas, drop volume, and Laplace excess pressure in drop-on-fiber systems. J Colloid Interface Sci 1976; 57: 488–495.

20.

McHale

Newton

Global geometry and the equilibrium shapes of liquid drops on fibers. Colloid Surface A 2002; 206: 79–86.

21.

Washburn and Edward W. The dynamics of capillary flow. Phys Rev 1921; 17: 273–283.

22.

Onuma

Ray

Yunfei

, et al . Surface characterisation of selected sorbent materials for common hydrocarbon fuels. Surf Sci 2007; 601: 2066–2076.

23.

Urakan

Adsorption of methylene blue on sodium alginate–flax seed ash beads: Isotherm, kinetic and thermodynamic studies. Int J Biol Macromol 2021; 167: 1156–1167.

24.

Rouison

Couturier

Sain

, et al. Water absorption of hemp fiber/unsaturated polyester composites. Polym Compos 2005; 26: 509–525.

25.

Dong

Cao

Highly efficient and recyclable depth filtrating system using structured kapok filters for oil removal and recovery from wastewater. J Hazard Mater 2017; 321: 859–867.

26.

Dong

Cao

Highly porous oil sorbent based on hollow fibers as the interceptor for oil on static and running water. J Hazard Mater 2016; 305: 1–7.

27.

Cao

Dong

, et al. Study on structure and wetting characteristic of cattail fibers as natural materials for oil sorption. Environ Technol 2016; 37: 3193–3199.

28.

Yukawa

Aoki

Kuroda

, et al . Kapok fiber paper sheets containing solid fluorescence materials. Electrophotography 2013; 52: 411–414.

29.

Cao

Dong

, et al. Oil spill cleanup by hydrophobic natural fibers. J Nat Fibers 2017; 14: 727–735.

30.

Dong

Wang

Theoretical and experimental study on the oil sorption behavior of kapok assemblies. Ind Crops Prod 2014; 61: 325–330.

31.

Cao

Dong

, et al. Cyclic filtration behavior of structured cattail fiber assembly for oils removal from wastewater. Environ Technol 2018; 39: 1833–1840.

32.

Cui

Liu

Oil sorption mechanism and capability of cattail fiber assembly. J Ind Text 2012; 43: 330–337.

33.

Dong

Wang

Oil spill cleanup by structured natural sorbents made from cattail fibers. Ind Crops Prod 2015; 76: 25–33.