Eco-friendly approach for the construction of biochar-based superhydrophobic membrane for effective oil/water separation

Researchers are looking at improved oil/water separation techniques due to the industry’s struggles with the separation of oily wastewater. One promising approach is to use superhydrophobic, SHP, membranes to separate oil from oily wastewater. In this study, we developed SHP textile fabric based on biochar, BC. The BC was synthesized from banana leaves by pyrolysis and then modified with nickel metal to produce Ni@BC. The textile fabric, TF, was submerged in an ethanolic solution of Ni@BC, and stearic acid, SA, to produce the SHP coating. The Ni@BC was utilized to improve the roughness of the surface of the pristine TF, and SA was utilized to reduce its surface energy. Scanning electron microscope, SEM, was used to investigate the surface morphology of the pristine and modified TF. The SEM results show that the modified TF shows a micro-nano structure. Atomic force microscopy, AFM, was utilized to study the surface roughness. The results show that the modified TF has a surface roughness greater than the pristine TF. The energy-dispersive x-ray spectroscopy techniques and Fourier transform infrared spectroscopy confirmed the structure of Ni@BC and the grafting of both SA and Ni@BC to the TF. The wettability finding demonstrated that the modified TF had a high degree of superhydrophobicity, with a high-water contact angle of 161° and a low water sliding angle of 1°. The modified TF showed excellent oil/water separation performance, with a separation efficiency of 99.9%. The oil absorption capacity of the TF was also high, with a capacity of 303 g g−1 for silicone oil, and it also has a high flux rate. The modified TF was also found to be mechanically and chemically stable, with no significant degradation after 10 cycles of use. The results of this study demonstrate that the biochar-based SHP TF is a promising material for oil/water separation.


Introduction
Oil pollution is a major environmental problem that can have a significant impact on human health and the environment. Oil pollution can occur from a variety of sources, including accidents during oil production and transportation, in addition to the intentional discharge of oily wastewater [1][2][3][4][5][6]. Oil pollution can harm wildlife, contaminate drinking water, and damage ecosystems. To separate oil from oily wastewater, numerous technologies were been developed, such as functional material absorption [7], vacuum dehydration [8], gravity separation [9], microbiological degradation [10], demulsifiers [11], and centrifugation separation [12].
To overcome the drawbacks of current technologies, such as bulky machinery, high energy consumption, the ease with which secondary pollutants can be created, and low separation efficiency, researchers have been working to develop more efficient separation approaches for oil/water, O/W, separation [13][14][15]. The most effective methods for separating O/W mixtures are membrane separation techniques. The membrane separation techniques like paper [16,17], sponge [17,18], fabric [19,20], and metal mesh [13,21] are popular because they require less maintenance and have a low energy requirement.
A promising new method for O/W separation is superhydrophobic, SHP, membranes. SHP surfaces are very water-repellent surfaces with a water contact angle larger than 150 degrees [22]. There is a lot of curiosity about these surfaces because of their potential uses in a variety of industries, including O/W separation, microfluidic devices, biomedicine, solar cells, sensors, and antifouling technologies [22][23][24]. SHP surfaces can be produced using several different methods, such as spraying, electrodeposition, immersion, sol-gel, electrospinning, anodization, chemical vapour deposition, chemical etching, and 3D printing [25][26][27]. By designing a surface with a high degree of roughness and a low level of surface energy, SHP surfaces can be produced [28]. The creation of this novel technique for O/W separation has the potential to significantly advance the effort to combat oil pollution. The coating is cheap to create and simple to apply to many different surfaces. This makes it an attractive option for use in many applications, such as water treatment and the cleanup of oil spills.
Biochar, BC, is produced by the pyrolysis of biomass, such as banana leaves, residual seeds, and rice straw in the presence of a little amount of oxygen [29]. In recent years, BC has grown in popularity. Given that it is less expensive than graphene, biochar has a great potential to replace it in a variety of applications [30]. BC is used as an effective adsorbent to remove a variety of impurities from water all over the world [31]. BC's surface area is expanded via treatment with cobalt and nickel metallic nanoparticles [32]. Having desired properties including hardness, and magnetism, corrosion resistance, nickel is a key industrial metal [33]. According to our knowledge, this is the first report of a SHP textile fabric, TF, made of BC doped with Ni, Ni@BC, that is used for O/W separation. In this study, we use Ni@BC as an additive to improve the surface roughness, this is the primary condition for achieving superhydrophobicity. We have established a green fabrication technique for producing BC from banana leaves with sustainable biomass resources.
In terms of gathered metric tonnes, banana leaves are one of the most valuable agricultural products, with an average length of two meters, a width of half a meter, and eight to twelve leaves per tree. Banana leaves are available year-round, and a single plant may yield up to 40 leaves every cycle. So that the waste has been converted into more valuable materials such as BC [34].
The goal of this research is to create a SHP BC-based TF for O/W separation. BC is synthesized from the banana leaves by pyrolysis, then modified by nickel metal to produce Ni@BC. The TF is submersed in the ethanolic solution of Ni@BC and stearic acid, SA, to construct the SHP coating. The oil absorption capacity, O/ W separation performance, flux and chemical stability, morphology, wettability, and surface composition of the created SHP TF were investigated.

Preparation method 2.2.1. Biochar preparation
To produce the biochar (BC), it was necessary to thoroughly clean the banana leaves by rinsing them with distilled water several times to eliminate any contaminants. Once cleaned, the leaves were air-dried and then subjected to overnight drying in an oven at 60°C. Afterward, the dried banana leaves were ground into a fine powder using a mixer. A total of 10 grams of the resulting powder was then pyrolyzed in a muffle furnace at a temperature of 600°C for two hours, resulting in the synthesis of BC.

Nickel-modified biochar preparation
Nickel-modified biochar material, Ni@BC, was obtained by adding 0.45 g of NiSO 4 .6H 2 O to a solution containing 1.0 g of BC and 100 ml of deionized water. After 30 min of sonication, the mixture was stirred for an hour and then dried overnight at 60°C. The resulting mixture was pyrolyzed at 600°C for two hours in a muffle furnace, resulting in the production of Ni@BC.

Fabrication of superhydrophobic textile fabric
A solution containing SA (8.0 g l −1 ) and Ni@BC (0.8 g l −1 ) was prepared by dissolving them in 250 ml of ethanol and agitating the mixture for an hour. The solution was further sonicated for an hour. Afterward, a circular TF with a diameter of 10.5 mm was submerged in the solution for 30 min. Then, the modified TF was cleaned using ethanol and dried at 60°C for 2.0 h.

Characterization
A scanning electron microscope, SEM, (model JSM-200 IT, JEOL) was utilized to investigate the morphology of the TF before and after grafting with SHP coating. An energy-dispersive x-ray spectrometer (EDX JEM-2100 Japan) was used to study the elemental composition of the produced materials and surfaces. The chemical components at the surface were assessed using Fourier-transform infrared spectroscopy, FTIR, (model: Bruker Tensor 37 FTIR). The 4000-400 cm −1 range was used to get the FTIR spectra. Using the Rame-hart WCA instrument (model 190-F2), the water contact angle (CA) and water slide angle (SA) were determined using water droplets of 5 μl. For measuring SA, a water droplet of 5 μl was carefully placed on the membrane. The inclined stage of the instrument was gradually tilted till the water droplet began to move down the surface. At this point, the angle at which the water droplet began to slide was recorded as the SA. The process of measuring SA was repeated multiple times on different regions of the membrane to ensure the reliability and reproducibility of the measurements. The presented CA and SA values are the averages of four tests made at different points on the created SHP TF surface. With the help of sulfuric acid and sodium hydroxide, the pH of the water droplets was altered. A Scanning Probe Microscope (SPM9600-Shimadzu Japan) was used to perform the atomic force microscopy, AFM.
The prepared SHP TF's mechanical stability was evaluated using an abrasion test. The prepared SHP fabric was laid out on 800 mesh sandpaper, dragged with a weight of 100 g, and the CA was measured every 5 cm. By examining the impact of soaking several manufactured modified TF in solutions of different pH (1 to 13) for time ranging from 1 to 5 h on the value of CA, the chemical stability of the created SHP TF was investigated [35]. The presented mechanical and chemical stability findings are the averages of the three experiments.

Absorption capacity tests
The ability to absorb liquids was tested by submerging a sample of the manufactured SHP TF for one minute in n-hex, PE, or SO draining for a short period of time, and then wiping with filter paper to eliminate any remaining organic solvents. The absorption capacity (k) of n-hex, PE, and SO was estimated using weights measured using the equation below [36]: M 0 denotes the sample weight of the modified TF, while M 1 denotes the sample weight after absorption of the organic solvent.
2.6. Separation performance of O/W mixture An oily effluent was produced by mixing oil and water in a 1:1 volume ratio in a 50 ml container. Distilled water was colorized by the addition of Methyl red. Different organic solvents, such as SO, PE, and n-hex, were utilized to create the oil phases. The SHP TF was used as a filter membrane for the separation of oil and water during the separation process. Upon introduction of the oily wastewater into the separation system, the modified TF facilitated the passage of oil, which then accumulated in the lower container, while the water remained in the upper container. The separation efficiency (W) was calculated using the equation shown below [36]: The weight of the oil at the start of the experiment is denoted as M 3 , while the weight of the oil that was collected at the end of the experiment is referred to as M 2 . To calculate the Flux of the redesigned TF, the following equation was utilized [36]: Where V is the liquid volume that passed through the membrane during the time t, A is the effective area of the membrane, and Flux is the rate of flow of the liquid through the membrane per unit area. Figure 1 shows a schematic representation of the fabrication of modified textile fabric and the separation process. Figure 2 illustrates the FTIR spectra of the BC, Ni@BC, pristine TF, and modified TF. The FTIR graph of BC displays several peaks that may be attributed to the presence of diverse functional groups. The broad peak observed at 3240 cm −1 is indicative of O-H stretching vibrations, while the two small peaks at 2856 cm −1 and 2922 cm −1 correspond to C-H stretching vibrations [37]. The sharp peak at 1590 cm −1 can be attributed to   The existence of these functional groups provides evidence that nickel has been effectively incorporated into the biochar.

FTIR results
The FTIR spectrum of a pristine TF shows several peaks indicative of various functional groups. The peak at 3422 cm −1 is due to the stretching of N-H groups in secondary amides. The peaks at 2904 cm −1 and 2955 cm −1 are caused by the asymmetric and symmetric stretching of -CH 2 − groups, respectively [38]. The peak at 1717 cm −1 is due to the stretching of C=O bonds, while the peak at 1406 cm −1 is due to the of C-H bonds bending. The peaks at 1092 cm −1 and 1231 cm −1 are caused by the stretching of C-N bonds, while the peak at 870 cm −1 is due to the out-of-plane bending of N-H groups [39]. Finally, the peak at 721 cm −1 is attributed to the bending of C-H bonds [39]. The spectrum of modified TF shows the same peaks as Ni@BC and pristine TF with a shift in the stretching peaks of C=O and N-H 2 stretch peak which was detected at 1717 cm −1 and 3244 cm −1 , representing the modification of TF with SA and Ni@BC.

EDX results
The chemical composition of the BC, Ni@BC, pristine TF, and modified TF were examined using the EDX, as shown in figure 3. The BC micrograph displays peaks of oxygen and carbon indicating the successful synthesis of BC. The Ni@BC micrograph displays peaks of oxygen, carbon, and nickel indicating the successful doping of BC with nickel. The pure TF micrograph displays peaks for oxygen, carbon, and nitrogen. The micrograph of the modified TF displays the same peaks of pristine TF with an extra peak for nickel, proving that the TF was grafted with Ni@BC.

SEM and wettability results
The SHP property is determined by surface morphology and chemical composition. The morphology of the pristine and altered TF was examined using the SEM, figure 4. The surface of the pristine TF is smooth, as shown in the image. The altered TF surface roughness was greatly enhanced due to the extensive micro/ nanostructures.
The measurement of CA was used to examine the wettability of pristine and altered TFs, figure 5. The pristine TF is hydrophilic, CA near 0°, and the water droplets don't move down even if the TF is upset. The altered textile material demonstrated exceptional SHP qualities with CA values of 161°± 0.9°and a SA value of 1°± 0.1°.
Atomic force microscopy (AFM) was employed to investigate the surface topography and roughness of both the pristine and modified TF, as illustrated in figure 6. The 3D AFM image in figure 6(a) reveals that the pristine TF had an arithmetic average surface roughness (Ra) of 0.7 μm, indicating a relatively smooth surface. After coating the TF with Ni@BC, the Ra value increased to 2.1 μm, figure 6(b), demonstrating that the deposited Ni@BC coating significantly increased the TF surface roughness. The increased surface roughness provided by the Ni@BC layer is beneficial as it is a critical parameter to attain superhydrophobicity.

Absorption capacity measurements
The created SHP TF's ability to absorb n-hex, PE, and SO was evaluated. Although the altered SHP TF selectively absorbed only oil, the pristine TF absorbed both oil and water. The created modified TF's SHP properties allow for quick absorption of organic solvents.  Figure 7 shows the absorption capacity of the prepared SHP TF for n-hex, PE, and SO. The absorption capacity of the synthesized SHP TF was tested repeatedly for 10 cycles. Following each cycle, the oil absorption capacity was measured, and the resulting SHP TF was squeezed to complete the desorption process of the organic solvent. The figure shows that n-hex has the lowest absorption capacity, whereas SO has the highest. Numerous earlier studies have revealed that as viscosity and oil density rise, its absorption capacity improves [40][41][42][43][44][45][46]. The extended release of oil from the SHP membrane is attributed to its slower desorption rate, particularly for oils with higher viscosities and densities. This characteristic enables the retention of a larger amount of oil within the  porous structure, thereby enhancing the absorption capacity of the membrane [43]. The oils of higher density will weigh heavier in the same number of pores of the SHP membrane than the light oil, giving it a greater absorption capacity.
The developed SHP TF for oil separation is recyclable because the absorption capacity basically stays constant even after 10 cycles. After ten cycles, the prepared SHP/oleophilic TF's CA was 157°± 1.1°, indicating a high degree of mechanical stability. These absorbents have better absorption capacities than previously known absorbents [47,48].

O/W separation efficiency
The created SHP TF's separation efficiency after 10 cycles of use with various O/W mixtures is shown in figure 8. For the different O/W mixtures under examination, the modified TF has an excellent separation efficiency. The SHP TF consequently demonstrates exceptional oil selectivity. The figure demonstrates that the initial cycle  exhibits the highest separation efficiency, while the efficiency gradually decreases with an increase in the number of cycles. During the separation process, oil adhesion to the funnel and sample is the main cause of the small loss of the collected oil. Additionally, the volatility of the oil should result in a decline in separation effectiveness [36]. The modified TF has a high oil flux because of the porosity that allows the oil to permeate it. Additionally, the rough features make it easy to create oil channels. The separation efficiency of the modified TF is higher than that of formerly reported absorbents [49,50].

Mechanical stability
It has been demonstrated that improving the abrasion resistance of SHP coatings is crucial for their industrial applications. Figure 9 shows the variation of both CAs and SAs as a function of abrasion length to demonstrate the susceptibility of the prepared SHP coat to abrasion resistance. The figure shows that as the abrasion length increases, the CAs reduced and the SAs increased. The superhydrophobicity of the prepared SHP TF is maintained for 750 cm abrasion length where the CA is larger than 150°and the SA is lower than 10°. The created SHP TF exhibits more mechanical stability than numerous values that have been previously reported [48,51].
The surface topography and roughness of the modified TF after the abrasion test was investigated using AFM. The 3D AFM image in figure 10 reveals that the modified TF had an arithmetic average surface roughness (Ra) of 1.46 μm, indicating that the nano-sized circular-like particles were destroyed. Since achieving a specific surface roughness is crucial for creating superhydrophobic coatings, the destruction of the nano-sized circularlike particle roughness leads to a loss of the superhydrophobic properties in the manufactured coatings.

Chemical stability and Flux rate
The SHP coating stability is a crucial concern for practical applications. The impact of alkali and acid solutions on the CA was examined to determine the chemical stability of the developed SHP TF. The produced fabric was submerged in solutions with pH ranges of 1 to 13 for varying immersion times (1-5 h), and the CA was calculated every hour. Figure 11 depicts the relationship between the pH and CA of the SHP TF for various immersion times. The CA values are higher than 150 o at pH 5-11, demonstrating that the modified TF is still SHP. The modified SHP TF maintains superhydrophobicity till 4 h at pH 3, till 3 h at pH 13, and till 2 h at pH 1. The resulting TF's chemical stability is higher than that of previously reported values [49,52].
The prepared modified TF's porosity makes it easy for oil channels to form, which contributes to the high oil flux. Equation (3) was utilized to calculate the modified TF Flux. The Flux values for SO, PE, and n-hex equal 18100, 17800, and 17200 l m −2 h −1 , respectively. The modified fabric's oil flux rate was high. After 10 cycles of Figure 11. Effect of pH values on the CA of the modified TF at different immersion times. separation procedures, the flux stability of the modified TF in the O/W separation process was measured. After ten separation cycles, the Flux values for n-hex, PE, and SO are 17900, 17400, and 16900 l m −2 h1, respectively. The Flux has somewhat lowered, demonstrating the improved TF's good Flux stability.
The differences in flux levels are caused by the viscosities of different organic solvents; flux is inversely proportional to liquid viscosity [53]. The modified TF can be used to quickly and efficiently separate oil from water. The Flux rates of the modified TF are higher than that of many reported absorbents [51,54].
The surface topography and roughness of the modified TF were examined using AFM after a chemical stability test that involved immersing the modified TF in a pH 5 solution for 5 h. The 3D AFM image in figure 12 reveals that the modified TF had an arithmetic average surface roughness (Ra) of 1.25 μm, indicating that the nano-sized circular-like particles were destroyed. Since surface roughness is a critical requirement for the fabrication of SHP surfaces, the destroying of the surface roughness causes the SHP coatings to lose their SHP properties.

Conclusion
This study has successfully developed a biochar-based SHP TF for effective O/W separation. The fabric was fabricated by modifying the surface of the TF with Ni@BC and SA. The modified TF exhibited excellent O/W separation performance, high oil absorption capacity, good mechanical and chemical stability, and good flux rates. The SHP fabric exhibited excellent O/W separation performance, with a separation efficiency of 99.9%. The fabric was also found to be mechanically and chemically stable, with no significant changes in its properties after 10 cycles of use. The prepared SHP TF is also recyclable, making it an environmentally friendly solution for O/W separation. This research provides a new perspective on developing sustainable SHP membranes for practical applications in O/W separation.

Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is sufficiently accessible or reusable by other researchers. The data that support the findings of this study are available upon reasonable request from the authors.