Facile fabrication of organic superhydrophobic corn silk-derived cellulose acetate nanofiber for the effective sequestration of oil from oil–water mixture

The oil sector is a key supporter of economic and social advancement in modern civilization (Bai et al. 2023). The rapid development of industry has led to an increase in both the regularity of oil spills and the vast release of wastewater containing oil, both of which have become global environmental hazards that put both humans and aquatic organisms in danger (Ye et al. 2023). Separation methods for oil–water mixtures include centrifugation, adsorption, and flocculation, all of which have been shown to be effective in the past (Zhou et al. 2022). Because the water-in-oil emulsions with droplet sizes below 20 μm are more stabilized, these technologies are best for separating immiscible water-in-oil but challenging for separating water-in-oil emulsions (Kou et al. 2021). Furthermore, these techniques are plagued by secondary contamination, high costs, and poor separation efficiency (Ge et al. 2018). Separation methodology employing superhydrophobic membranes could be developed to successfully extract oil from water (Li et al. 2019). As a result, the development of novel materials as a means of achieving oil–water separation in a manner that is kind to the environment is of the utmost importance (Ye et al. 2023). In order to separate oil–water emulsions, researchers were devoted to creating superhydrophobic membranes, which had been motivated in part by the hydrophobic nature of leaves, butterfly wings, and spider silk (Zhang et al. 2019). Oil from an emulsified oil–water aqueous system can be isolated using materials like manufactured inorganic mineral substances, fiber-based materials, and polymers that are organic in nature. When choosing the optimal sorbent material for oil removal, it is essential to prioritize high absorption capacity, hydrophobicity, and oleophilicity, in addition to strong recoverability (Wang et al. 2016). There is a substantial threat to the environment, the recyclability, and the long-term viability posed by treatments composed of inorganic particles and synthetic compounds. Inorganic particles and synthetic compounds are neither renewable nor biodegradable, posing a significant threat to the environment and social welfare. According to the findings of studies, there are a number of different materials that are capable of achieving hyperhydrophobicity. These materials include biodegradable polymers, cellulose, chitosan, and plant waxes. Recent years have witnessed a substantial quantity of research on natural organic materials, including the use of Mj-fiber (Wang et al. 2020c), duck down fiber (Fang et al. 2022), cellulose nanofiber (Shu et al. 2020), flax fibers (Liu 2020), jute fibers (Kovačević et al. 2023), kapok and waste cotton (Singh et al. 2023), and luffa sponge (Alvarado-Gómez et al. 2021). Although most of the natural and organic fiber materials are biodegradable, few of those can absorb water as well as oil, which reduces the effectiveness of separation and some others, and can sink during separation in rough sea conditions (Elmaghraby et al. 2022). Nanofibers are prepared by electrospinning, which is a simple and efficient technique that produces large specific surfaces (Xue et al. 2019). The resulting electrospun nanofiber mats are noteworthy because they exhibit highly interconnected porous nanostructures, exceptionally large specific surface areas, and a modifiable nature, and hence, this approach provides a fantastic way to make distinctive wettable surfaces (Mahmoud 2020). When compared to other fabrication methods, electrospinning stands out due to its flexibility in producing fibres with a wide variety of arrangements (such as aligned fibres, random orientations, or their combinations) and morphological structures (such as tubular scaffolds, flat surfaces, and asymmetrical configurations). The technique of making solid threads from solution does not necessitate the use of coagulation chemicals or high temperatures. Because of this, the method excels in producing fibres from huge and complicated compounds (Islam et al. 2019).

One of the most abundant and durable biopolymers in the earth is cellulose, which can be feasibly converted into the lucrative derivatives of cellulose acetate (CA) that has a variety of industrial applications (Nemr et al. 2017). Cellulose has a wide range of desirable characteristics, including low cost, biocompatibility, biodegradability, and chemical modifiability (Ma et al. 2016). The field of nanofibers has seen growing attention in CA (which is an acetate ester of cellulose) over the past decade. CA is frequently employed as an alternative to cellulose substitution due to cellulose's poor solubility in common solvents (Eleryan et al. 2021). Wheat straw, corn husk, rice hulls, sugar cane bagasse, rice husk, bamboo pulp, bamboo bark, coconut fiber, cotton tree, sugarcane bagasse, Pinus sp. sawdust, and many other agricultural wastes are some of the biomasses that have been utilized as raw material to create CA (Biswas et al. 2006; Israel et al. 2008; Embong et al. 2021; Trejo et al. 2022).

In this investigation, we sought to develop a novel, environmentally friendly CA nanofiber synthesized from corn silk (CS) for the purpose of recovering oil from oil–water emulsion. Since CS was not extensively researched for its environmental applications, it was decided to examine using CS to create extremely hydrophobic organic fiber. In the current study, CA has been synthesized into nanofibers (membranes) from CS to boost its ability to dissolve and hydrophobicity, as well as to facilitate electrospinning into nanofiber membranes for oil–water sequestration. The as-prepared CS-derived CA (CSCA) membrane was characterized using scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) for analyzing their morphological features. Wettability characteristics in response to water and oil were analyzed using water contact angle (WCA) measurement. The viscosity and the conductivity of PAN solutions of varying concentrations were measured and analyzed. The ability of the as-prepared CSCA membrane was evaluated for its capacity to effectively isolate oils from oil–water mixtures as well as oil–water emulsions. Concurrently, the oil-sorbed sorbent, which could be reconstituted for several life cycles after being cleaned with 100% ethanol, was assessed.

The CS residue that was used came from the agricultural cooperatives in the Warangal region, India. All chemicals and reagents employed in this study were analytical grade.

Employing an integrated green pretreatment that has been documented in the literature, cellulose has been extracted from dried CS (Araujo et al. 2019). Treatment with hot water was used in the first stage of a two-step procedure, which was then followed by treatment with a diluted NaOH solution. The treatment with hot water was conducted at a temperature of 190 °C for 30–40 min while undergoing constant mechanical stirring in a cylindrical reactor made of stainless steel with a rough capacity of 2.5 L with a 10% w/v material feed loading rate. The substances that were insoluble were collected shortly after this treatment, completely rinsed in deionized water, and then desiccated at 60 °C. After being treated with hot water, the CS was then steeped in NaOH (2 wt%) at 80 °C for 12 h while being stirred magnetically. After being treated with alkali, the CS was scrubbed and repeatedly rinsed with deionized water until it became colorless. The final product was stored after having been dried at 60 °C.

Despite the fact that cellulose and CA are thermoplastic polymers, cellulose's tight ordering renders it insoluble in organic solvents and prevents it from flowing when heated. However, CA may be fluidized by heat and dissolves in certain organic solvents. CA was produced from cellulose obtained from CS using acid treatment with slight modification that has been reported (Asriza et al. 2021). The extracted cellulose was mixed with 25–30 mL of glacial acetic acid and agitated continuously for 1 h at room temperature (RT) to form activated cellulose. About 15–20 mL of anhydride acetic acid and 3–5 drops of concentrated H₂SO₄ were combined in a separate container at 0 °C. The preceding solution combination had this solution mixture added to it, and then it was agitated mechanically for 3 h at a temperature of 45 °C. After 24 h of stirring at RT, we added 25 mL of 60% glacial acetic acid to the solution mixture in a drop-by-drop fashion. All of the solution was filtered through a Hoover. The resulting product was dripped with demineralized water several times until the pH reads neutral and also to obtain a white polymer precipitate. Gel permeation chromatography (GPC) determined that the molecular weight (M_n) of the final product, CA, was between 30,000 and 50,000. Furthermore, the produced CA was dried at 40 °C in a vacuum oven.

An electrospinning solution was prepared using CSCA and polyacrylonitrile (PAN, mol. wt. 150,000) at 12% (w/v) using a solvent of dimethylformamide (DMF). To prepare a 12% CA electrospinning solution, an appropriate amount of CSCA and PAN in the ratio of 3 : 1 was added to the DMF solvent. To create a uniform CA electrospinning solution combination, the mixture was swirled vigorously for 24 h under magnetic stirring at 45 °C. The electrospinning solution had been loaded into a 15 mL syringe (with a 22 gauge blunt needle as the spinneret). During the electrospinning process, the voltage was set at 20 kV, the distance across the tip of the needle and the collector was 15 cm, the injection rate remained set at 1 mL/h, the electrospinning period was set at 10 h, and the humidity was set at 40%. The temperature was set at ambient. The accumulation of viscous fluid during the electrospinning procedure causes the tip of the needle to become clogged, which undermines the jet, and also prevents fiber creation. In spite of the fact that there was no airflow throughout this part of the experiment, the needle tip had to be meticulously cleaned during each and every run of the spinning procedure so as to keep the fiber creation intact. After that, the fibrous mat that had accumulated on the aluminum foil was collected and preserved for later investigation.

At RT, a Brookfield digital viscometer and a conductivity meter were used to measure the viscosity and conductivity of the CSCA and CA/PAN solutions, respectively. Electrospun CSCA nanofiber shape and diameter were investigated on a scanning electron microscope (TESCAN VEGA3 LMU). The contact measurements of the electrospun CSCA nanofiber were investigated using a contact angle goniometer (GBX Digidrop Goniometer). X-ray diffraction determined the CSCA nanofiber crystalline structure. To explore the interatomic characteristics of bonding and to examine the functional group existing in the produced CSCA nanofiber, FTIR (BRUKER ALPHA-II) analysis was conducted out in the wave number range of 400–4,000 cm⁻¹. In order to investigate the thermal degrading behavior of the CSCA nanofiber, thermogravimetric analysis techniques (Perkin Elmer TGA-7) were employed. An energy-dispersive X-ray analyzer (Shimadzu EDX-7000/8000) was also used to identify the individual elements comprising the CSCA nanofibers.

During the process of separating oil from water emulsion, V (L) indicates the volume of filtered oil in the duration of t (h) and A (m²) denotes the area of the membrane through which the filtrate travels.

When selecting a product, the extent to which it may be reused is one of the most crucial factors to take into account from a financial point of view. The aforementioned oil–water separation cross-flow filtration system was utilized to conduct 25 consecutive 5-min tests on the CSCA membrane to determine its recycling capability. After every cycle, the oil-saturated CSCA nanofiber membrane was collected, squeezed mechanically to extract as much of the oil as possible, and then washed many times in 100% ethanol to get rid of any lingering traces of oil.

In Figure 1(b), we see the effect of varying PAN concentrations (0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 4) in a CA solution of 12 wt% on the solution's conductivity. At PAN concentrations >2.5 wt%, the conductivity rose rapidly from a rather stable range of 0.5–2.5 wt%. The addition of PAN at lower concentrations increased the mobility, but the charge of excess PAN induced an increase in repulsion in the CA/PAN solution when the PAN concentration was more than 2.5 wt%. Because there was no miscibility at higher concentrations >3 wt%, certain PAN molecules were not mixed with CA molecules and were found independently in the solution. Furthermore, the increased conductivity owing to excess PAN seems to induce the production of finer nanofibers.

By using traditional techniques, it is significantly more challenging to separate an oil–water emulsion, particularly when surfactants are present. Oil–water emulsions may be effectively separated via membrane technology, especially when the particles of oil are substantially smaller than 10 μm. As a result, similar research on the CSCA membrane's capacity to cleanse oil–water emulsion was conducted utilizing a cross-flow filtration apparatus. With the exception of the apparatus being upright, the experiment was conducted similarly to the oil–water separation. The emulsion's own gravitational pull is mostly responsible for driving the separation process. Oil–water emulsions of various types (D : W, P : W, E : W, T : W, and M : W) were then separated using the CSCA nanofiber membrane. Figure 7(b) depicts the oil–water emulsion separation using various oils. The CSCA membrane showed the best rejection to M : W emulsion but a relatively lower rejection to diesel and petrol emulsion, because diesel and petrol were viscous and quickly closed the pores on nanofibers during the filtration process. The reason for this difference is that diesel and petrol were more difficult to reject. The separation fluxes of the CSCA membrane for oil–water (D : W, P : W, E : W, T : W, and M : W) emulsion are 434, 442, 418, 412, and 402 L/m²/h, respectively. These results indicate that the more the viscosity of the fluid, the lesser the flux. Similarly, M : W emulsion was shown to have the highest separation efficiency, whereas D : W emulsion had the lowest. In addition, mineral oil is able to diffuse through the hydrophobic membrane under the influence of gravity, whereas water is rejected by the hydrophobic surface, resulting in a clear emulsion. The separation efficiency exhibited no apparent deviations and it is >95% for every type of oil–water emulsion. The foregoing findings demonstrate that the CSCA nanofiber membrane has high separation capability for oil–water emulsions stabilized by surfactants. Table 1 presents a comparative analysis of the performance of CA nanofiber membranes in relation to previously reported nanofiber membranes.

Cellulose was extracted from dried CS by using an integrated green pretreatment. Furthermore, through the acetylation of obtained cellulose, CA has been produced. Electrospinning was used to make CSCA nanofibers with a porous structure via blending with PAN. Because of the porous, hydrophobic, and oleophilic properties of the fibers, they are an excellent contender for the role of a sustainable oil sorbent. It was found that mineral oil had the highest oil sorption capacity at 908%, while toluene had the lowest at 664%. This result is four times greater than the oil sorption capacities of commercially available oil sorbents. For all of the prioritized oil–water emulsions, the separation fluxes were higher than 400 L/m²/h. In addition, the separation efficiency was >90% for all of the selected oil–water mixtures and >95% for oil–water emulsions. The separation efficiency was still higher than 96% after 25 cycles of separation experiments, and the resultant oily phase was still 99.6% pure, demonstrating exceptional potential for reuse. Because of the high surface porosity, the surface area of the fibers was significantly enhanced, which, in practice, implies that there was more contact area for the oil on the sorbent, which ultimately led to a high oil sorption capacity. The solution's viscosity is inversely proportional to its CA content. The bonding information in CA fiber may be understood by structural analysis. These membranes provide a low-priced, high-efficiency method for cleaning oil out of water. Still, challenges in managing nanofiber shapes continue. More work is needed, as well as novel methodologies, to efficiently design the surface morphology of nanofibers; this will pave the way for further exploration of fibrous materials and spur more study. As a consequence, the results confirmed that the CSCA membranes, due to their simple construction and enhanced properties, may be used by industries as an ecologically friendly and cost-competitive technology to separate oil from oil-contaminated industrial effluent.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.