Investigation on the oil sorption capability of silkworm cocoon waste

Abstract

In this study, degummed and undegummed silkworm cocoon wastes were used to investigate their oil sorption capabilities for three kinds of oils, namely motor oil, vegetable oil, and crude oil. The laboratory-scale oil sorption results reveal that degummed silkworm cocoon waste has sorption capacities of 16.7, 15.8, and 14.2 g/g for motor oil, vegetable oil, and crude oil, respectively. It exhibits higher sorption capacities for these three oils compared with raw cocoons and cocoon scrap. These sorbents also exhibited excellent oil/water selectivity, retention capacity, and reusability. According to the large-scale experiment, the crude oil can be removed completely by degummed silkworm cocoon waste in 20 min, which shows that degummed silkworm cocoon waste could be considered a good alternative to synthetic sorbent materials applied to oil removal and recycling.

Keywords

Silkworm cocoon waste oil sorbent sorption capacity reusability oil removal

The environmental pollution caused by oil leakage and oil-bearing wastewater from industrial production is threatening the global ecosystem.^1
–4 In addition to petroleum spills, other types of oils, such as fats, lubricants, cutting liquids, and heavy and light hydrocarbons, have also caused water contamination worldwide.⁵ This oily wastewater has negative impacts on aquatic organisms, economy, tourism activities, and human health if not treated properly.^6
–8 Furthermore, oils like petroleum are considered valuable commodities as they are crucial sources of energy and raw materials for industries. Therefore, it is important to clear and recycle oil in water promptly to ensure sustainable and stable environmental development, as well as to harness the benefits that oil provides.⁹ In light of this, there is an urgent need to address water pollution using high-efficiency, energy-saving, and recyclable oil–water separation materials.

Generally, oil sorbents are considered the most effective tools for cleaning up and collecting spilled oil and they can be classified as inorganic mineral materials, synthetic organic polymers, and natural materials.^10
–13 In general, the oil sorption performance of synthetic composite sorbents prepared from synthetic non-biodegradable polymers in different forms, such as sponge,¹⁴ aerogel,¹⁵ and membrane,¹⁶ is superior to that of natural materials. However, in the future, a decrease in the use of synthetic polymeric absorbent materials is expected owing to their very high cost and non-biodegradability. The utilization of bio-sorbents derived from natural materials with environmentally friendly, low-cost, reusability, abundant, and biodegradability properties can be an ideal alternative for conventional synthetic sorbents, which will have a positive effect on sustainability and the circular economy. Natural sorbents include rice husks,^13,17 coconut husks,¹⁸ wood chips,¹⁸ walnut shells¹⁹, kapok fiber,^20,21 cotton grass,²² wood ships,²³ etc. However, it is important to note that many natural materials have a limited sorption capacity, and need pretreatment to enhance their effectiveness as oil sorbents. Moreover, the industrialization of these materials poses significant challenges due to the multiple preparation stages involved, as well as limited resources and high costs.¹² Therefore, exploring alternative abundant natural resources (especially using waste natural materials)^24
–31 offers a promising solution to overcome the aforementioned difficulties.

The domesticated silkworm, known as Bombyx mori, has been a valuable source of silk fibers for clothing materials for a long time. These fibers have not only played a significant role in the fashion industry but also have been utilized as medical sutures for many decades. Silk is a protein-based material known for its outstanding mechanical properties and low cytotoxicity, making it a promising contender for the next generation of structural materials, potentially replacing petroleum-based alternatives. However, despite the increasing focus on silk fibers, a significant amount of silk is still wasted globally. In the industrial process, dirty and damaged cocoons, as well as those created jointly by multiple silkworms, are typically removed. Furthermore, it is estimated that tons of used silk garments are discarded annually. The recycling and full utilization of those abundant waste resources can certainly make sericulture more prosperous.^32,33 Currently, waste silk materials are usually used as blended fibers and for the reinforcement of composites, biological tissue materials, electrodes, and filter media for air and water.³⁴ Silkworm cocoons consist of two proteins (sericin and fibroin) and show good biodegradability and amphiphilic properties, and have been used for dye sorption as well as oil sorption.^35,36 Moreover, only a few reports have been published on the use of animal protein materials as sorbents for oil removal,^37
–39 as listed in Table 1.

Table 1.

Reported natural protein fibrous oil sorbents

Sorbent	Sorption capacity (g/g)	Oil types	References
Recycled wool-based nonwoven material	10.60	Diesel fuel	[37]
	12.46	Crude oil	[37]
	14.48	Vegetable oil	[37]
	14.78	Base oil	[37]
	15.73	Motor oil	[37]
	5.48	Oily wastewater	[38]
	15–19	Oily wastewater	[39]
Loose natural wool	5.56	Oily wastewater	[38]
Loose natural wool	33–43	Oily wastewater	[39]

Therefore, in this study, a comprehensive investigation of the oil sorption capacity research of silkworm cocoon wastes on a laboratory scale was carried out, and a large-scale oil sorption simulation using degummed silkworm cocoon (DSC) wastes was also conducted. The findings from this research will contribute to the understanding of the sorption capacity of silkworm cocoon wastes, both with and without sericin, and will also help in the development of their new applications.

Materials and methods

Experimental materials

Defective silkworm cocoons (also called silkworm cocoon wastes) are used for the preparation of sorbent materials (Chinese strain “Liang guang er hao” from farmers in Guangxi Province). According to the normal procedure, silkworm cocoon wastes were dried for 1 h at 110°C and 3 h at 80°C in a vacuum drying oven before use. The dried silkworm cocoon wastes were further treated and divided into three groups (as shown in Figure 1): (1) untreated silkworm cocoons (SCs), cut into two halves; (2) silkworm cocoon scrap (SCS), cut into small pieces (about 5 mm × 5 mm); (3) DSCs, which were obtained by cutting dried silkworm cocoon wastes into pieces and treating them three times with 0.5 wt % aqueous sodium carbonate solution at 100°C for 30 min, followed by washing them with deionized water and natural drying.

Three types of oils, namely motor oil, vegetable oil, and crude oil, were employed to investigate the oil sorption characteristics of silkworm cocoon waste. Oils were obtained from the market and the main characteristics of the investigated oils are presented in Table 2. All the oils were used without any modification.

Table 2.

Characteristics of the investigated oils

Oil sample	Viscosity (cP)	Density (g/cm³)
Motor oil	151	0.88
Vegetable oil	60	0.92
Crude oil	23	0.86

Characterization

Micrographs of the samples were examined using a scanning electron microscope (SEM, Quanta FEG 450). Before SEM observation, all samples were fixed on aluminum stubs and coated with gold. The surface areas of the samples were evaluated by the Langmuir model and the Brunauer–Emmett–Teller (BET) model. The surface water or oil contact angle of the samples was characterized by a contact angle meter (OCA15EC, Dataphysics, Germany).

Laboratory-scale oil sorption experiments

To evaluate the oil sorption capacity of sorbent materials accurately, oil spill conditions were simulated in a 250 mL glass beaker. The sorption experiments were conducted in a static system at 20 ± 4°C. A volume of 100 mL of oil was poured into a glass beaker. Samples of 1 g were immersed into the liquid and left to soak until they reached sorption saturation for the determination of sorption capacity. The experiments were performed at different time intervals from 1 to 30 min. The samples were then taken out of the oil and the wetted adsorbents were obtained after being drained for 1 min in a sustainer, to separate any loosely attached liquid. The sorption capacities (Q) of the samples were weighed before and after the sorption, and calculated according to

Q = \frac{M_{s} - M_{n s}}{M_{n s}}

(1)

where Q (g g⁻¹) is the sorption capacity of the adsorbent and M_s and M_ns are the weight of samples after and before oil sorption, respectively. All oil sorption capacities were measured three times and the average values were used.

The additional sorption tests were carried out for a mixture of oil and water to compare the removal capacity of the samples in the oil/water mixture system. Oil was poured into a 250 mL glass beaker, containing 200 mL of tap water, to obtain oil films of 4 mm in thickness on the water surface. The mixture of oil and water was constantly agitated (600 rpm) by a magnetic stirrer. Then the samples were weighed and placed in the glass beaker. After 30 min of sorption, the wetted sorbents were obtained after being drained for 1 min. Water will directly evaporate at a relatively high temperature while oil will not. So, the samples were dried in an oven (temperature of 50°C) until no more water existed, for about 50 min. Crude oil is not used in the experiment due to its high volatility. So, motor oil and vegetable oil were carried out to evaluate the selectivity of the sorbents in the oil/water mixture system. The oil/water sorption ratio (η) was determined by the weight before and after drying, and calculated according to

η = \frac{M_{2}}{M_{1} - M_{2}}

(2)

where η is the oil/water sorption ratio of the sample and M₁ and M₂ are the weight of the sample before and after drying, respectively. All the sorption coefficients were measured three times and the average value was used.

The procedure mentioned above was utilized to examine the oil retention and reusability of the materials. Three cycles of the sorption process were conducted for every sample. After each cycle, the saturated sorbents were squeezed and the resulting liquids were centrifuged for 5 min at 1000 rpm to separate the oil from the samples. The samples were weighed before and after centrifugation. Subsequently, the samples that had undergone centrifugation were employed in the subsequent cycle experiment. The sorption capacities of each cycle were calculated using Equation (1), while the weight ratios before and after centrifugation determined the oil retention capacity.

Large-scale oil sorption experiments

To simulate an oil spill in nature and reflect the sorption capacity of the silkworm cocoon waste directly, a 1 m × 1.5 m water tank and 500 g crude oil were used for a large-scale oil sorption experiment. Crude oil was poured into the tank, and the thickness of the oil was about 0.4 mm. Then DSC samples (50 g) with the best sorption capacity were dropped over the oil surface and pictures were taken of the sorption process every 5 min.

Results and discussion

Characterization of adsorbents

SEM photographs of SC and DSC fibers are shown in Figure 2. The photos reveal the surface texture of the samples. The silkworm cocoon is a macroporous network composed in an interwoven manner, whilst the surface of the silkworm cocoon is rough owing to the coverage of sericin, as shown in Figure 2(a). This coarser and network surface can not only increase the surface area of the fiber but also improve the adhesion property of oil on the sample surface, which is beneficial for the retention of oil in silkworm cocoons. From Figure 2(b), the degummed process makes the silk fiber surface smooth, implying that the sericin coating has been removed from the fiber surface. Silk fiber without sericin (also called single fibroin) is composed of thousands of microfibers, which contribute to the sorption capacity.⁴⁰

It is also known that an adsorbent with a higher surface area has been found to facilitate the sorption of oil.³⁸ According to the BET test result presented in Table 3, DSCs have a higher BET surface area (0.54 m²/g) than that of SCS (0.35 m²/g), which indicates that DSCs will have a higher sorption capacity than that of SCS.

Table 3.

Surface areas of silkworm cocoon waste

Samples	Surface area (m²/g)
Silkworm cocoon scrap	0.35
Degummed silkworm cocoons	0.54

Sorption capacity of silkworm cocoon waste

To assess the maximum oil sorption capacity of silkworm cocoon waste accurately, a series of oil sorption tests were conducted using three different types of pure oils, excluding any water content. In addition, water sorption experiments were performed to compare the sorption capacity of water with that of oil. The sorption capacities of SCs, SCS, and DSCs are represented in Figure 3(a). The sorption capacities of SCs for motor, vegetable, and crude oil and water are about 4, 3, 2.67, and 1.1 g g⁻¹, respectively. The sorption capacities of SCS are higher than those of SCs, and the sorption capacities of SCs for the four liquids mentioned above reached about 12.0, 8.4, 4.4, and 4.5 g g⁻¹, respectively. DSCs show the best oil sorption capacities, with sorption capacities of 16.7, 15.8, 14.2, and 18.3 g g⁻¹ for the liquids, respectively. The results imply that the removal of sericin significantly enhances the oil sorption capability of the material. This improvement can primarily be attributed to the large BET surface area of DSCs. In addition, it is noteworthy that only DSCs exhibited a higher water saturation sorption capacity compared to oil. This discrepancy can be explained by the removal of sericin and the lower viscosity of water, which likely play significant roles in this performance. Interestingly, samples with higher viscosity oils demonstrated a greater sorption capacity, indicating a positive correlation between viscosity and sorption efficiency. The high viscosity of oil can lead to two opposite effects: a decrease of sorption as the penetration through the interior of the fiber is inhibited, and improved sorption since the oil is better adhered to the material surface.²⁹ This is particularly noticeable in the sorption test of crude oil, which has almost seven times lower viscosity than motor oil. Initially, a great amount of crude oil was absorbed into the material, but subsequently, it was rapidly drained during 1 min in the sustained period. In contrast, motor oil was easily adhered to a sorbent's surface.³⁷

The relationship between contact time and oil sorption capacity is shown in Figures 3(b)–(d), representing motor oil, vegetable oil, and crude oil, respectively. Oil sorption capacity curves for the three types of oil show similar trends. Along with the extension of treatment time, all the oil sorption capacities show a sharp decrease for all the sorbents within 1 min and reach the maximum values. Afterward, the sorbents tend to begin a slight variation toward a steady state for both motor oil and vegetable oil. However, a slight increase can be observed in Figure 3(d) in oil sorption capacity when extending the contact time, 5 min for SCS and 3 min for DSCs. It is noteworthy that the average oil sorption capacities of the three sorbents align with the observed results. This expeditious sorption rate can be predominantly attributed to the effective interstice structure between interconnected single fibers.^20,41 The presence of structured fibrous assemblies, such as needle-punched nonwovens, offers the advantage of small pores that facilitate the oil's entry into the sorbent materials and the subsequent retention of oil particles. In addition, these structured fibrous assemblies can be easily collected from the affected area after sorption, exhibiting superior reusability and oil retention performance compared to loose fibrous assemblies.⁴² The high oil sorption capacity and fast sorption rate of DSCs make them a potential protein material for oil removal.

Removal capacity in an oil/water mixture system

In practical oil spill cleanup operations, the treatment of discharged or spilled oil is usually performed in a water environment; it is therefore very important to evaluate the oil removal capacity of sorbents in oil/water mixture systems. The contact angles and oil sorption capacities of three sorbents in an oil/water mixture system are depicted in Figure 4. As illustrated in Figure 4(a), SCs and DSCs exhibit water contact angle values of 96.9° and 117.1°, respectively. However, the vegetable oil and motor oil contact angles for these two samples are relatively lower compared to that of water, with vegetable oil contact angles of 67.3° and 62.5° and motor oil contact angles of 77.4° and 74.4°, respectively. The oil/water sorption ratio (η) was used to compare the oil and water sorption capacity in the mixture system. The values of the oil/water sorption ratio are greater than 1, indicating that sorbents prefer oil to water in the oil/water mixture system, as shown in Figure 4(b). It is observed that the oil/water sorption ratio values of the materials in motor oil and vegetable oil are greater than 10, showing their good oil/water selectivity. Meanwhile, vegetable oil is easier to be selected in the mixture system than motor oil by SCs and SCS, while a similar selectivity appeared for DSCs. Although DSCs have a superior sorption capacity for water compared with that for pure oil, it turns out to be entirely alien in the oil/water mixture system. The results indicate that both SCS and DSCs have a high sorption capacity as well as good oil/water selectivity, which means the water pickup is negligible in comparison to the oil sorption. Good oil/water selectivity means very high efficiency of a single sorption cycle when the sorbents take up large amounts of oil without further efforts required for oil/water separation.

Reusability of silkworm cocoon waste

One important criterion for assessing the reusability of sorbents is their ability to endure multiple cycles without experiencing problems such as squeezing, crushing, tearing, or overall deterioration. To evaluate the reusability of silkworm cocoon waste, two factors that need to be considered are the variations in oil sorption capacity and the percentage of oils that can be effectively removed using reasonable effort and equipment. As a result, we conducted multiple experiments to determine the sorption and retention capabilities of the sorbents. Figure 5 presents the results of these experiments, displaying the variation in oil sorption and retention capacity across three sorption cycles for motor, vegetable, and crude oil. The results demonstrate that oil sorption capacities decrease with the number of sorption cycles. The motor oil and vegetable oil sorption capacities decrease at the same tendency and reach a level of around 40% for SCs, 30% for SCS, and 50% for DSCs in cycle 2 followed by a steady state. However, crude oil sorption capacities decrease continuously after three sorption cycles. From Figures 5(d)–(f), approximately 70–90% of oil is removed from three samples after the first cycle of centrifugation. However, the removal percentage increases after the second cycle (approximately 95%) and did not significantly change in the following sorption cycle. The retention capacity for motor oil and vegetable of SCs is much better than those of SCS and DSCs, as shown in Figures 5(d) and (e). Nevertheless, no significant difference can be found in the crude oil sorption test. The differences between crude oil sorption and motor or vegetable oil sorption indicate that crude oil is difficult to remove from the sorbents. One possible reason is that crude oil, with low viscosity, easily penetrates the interior of the sorbents. Strong mechanical pressure, such as unloading oil by putting pressure directly on the sorbent or squeezing an oil-loaded sorbent with two rollers can cause the severe loss of the intrinsic sorption capability of porous sorbents due to the occurrence of contraction and irreversible deformation.^37,43 As a result, the sorbents can only be utilized for a limited time. In this study, oil is removed by centrifugation without severe disruption of the fiber hollow lumen, which increases the service life of the sorbents.

Large-scale oil sorption experiment

From the results above, the sorption capacity of DSCs is better than that of the other two sorbents, and crude oil is relatively more difficult to remove than motor oil and vegetable oil. Therefore, DSCs and crude oil are chosen for a large-scale oil sorption experiment to evaluate the sorption capacity visually. Figure 6 displays pictures of the cleanup of pure crude oil by DSCs. Some 500 g pure crude oil is placed in the tank to form a 0.4 mm oil layer. After adding the DSCs on the surface of the oil system, most of the oil is quickly sucked into the DCS sorbents in 5 min. Amazingly, crude oil is removed completely by the DSC sorbents 20 min later. It can be speculated that the faster oil removal capacity of DSCs would also occur for motor oil and vegetable oil. For natural sorbents like silkworm silk, several factors including the amount of surface sericin, the molecular arrangement of the polymer, and the physical characteristics of the fiber, such as surface roughness, twist, crimp, and fineness, play a significant role in determining the rate and extent of oil sorption.⁴⁴ However, the main mechanism behind silk fiber's ability to hold oil is through surface sorption facilitated by capillary bridging among the fiber bundle.^45
–47 The removal of sericin through degumming treatment enhances the oil sorption capacity of the silkworm silk. In conclusion, the DSC waste has excellent oil sorption properties, high reusability, and is biodegradable, making it a promising alternative to the synthetic sorbents available in the market.

Conclusions

Silkworm cocoon wastes are low-cost recycled resources that were used to investigate their sorption capacities for motor oil, vegetable oil, and crude oil in this study. The laboratory-scale oil sorption experiment results reveal that DSCs with sorption capacities of 16.7, 15.8, and 14.2 g/g for motor oil, vegetable oil, and crude oil, respectively, show higher sorption capacities than the cocoon scrap and raw cocoons for these three oils. Motor oil, with higher viscosity, is easier to remove by sorbents than vegetable oil and crude oil. Along with the extension of treatment time, all the oil sorption capacities reach the maximum values within 1 min. In the oil/water mixture system, sorbents have good oil/water selectivity. Approximately 95% of oil can be removed from three samples after three cycles of centrifugation, which indicates excellent retention capacity and reusability of the sorbents. Due to the special structure and high BET surface area, DSCs have excellent oil sorption capacity and reusability. According to the large-scale experiment, the crude oil can be removed completely by DSC sorbents in 20 min. Silkworm cocoon waste is relatively economical compared to synthetic sorbents, which can be recovered from the discarded pierced or contaminated cocoons for reuse as oil sorbents and to reduce industrial wastes. Therefore, DSC waste can be considered a good alternative to widely used synthetic sorbent materials for oil removal and recovery.

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 following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China under Grant Nos. 32102613, Yibin Academy of Southwest University (XNDX20221302), State Key Laboratory of New Textile Materials and Advanced Processing Technologies, No. FZ20230015.

ORCID iD

Lan Cheng

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