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The existence of metals emitted by microwave radiation is controversial because metals ignite easily. But what is interesting is that the researchers found that the arc discharge phenomenon offers a promising route for the synthesis of nanomaterials by splitting molecules. This study is developing a one-step yet affordable synthetic method that combines microwave heating and an electric arc to convert crude palm oil into magnetic nanocarbon (MNC), which can be considered as a new alternative for palm oil production. It involves the synthesis of a medium with permanently wound stainless steel wire (dielectric medium) and ferrocene (catalyst) under partially inert conditions. This method has been successfully demonstrated for heating in the temperature range from 190.9 to 472.0°C with various synthesis times (10-20 min). Freshly prepared MNCs showed spheres with an average size of 20.38–31.04 nm, a mesoporous structure (SBET: 14.83–151.95 m2/g) and a high content of fixed carbon (52.79–71.24 wt.%), as well as D and G bands (ID/g) 0.98–0.99. The formation of new peaks in the FTIR spectrum (522.29–588.48 cm–1) testifies in favor of the presence of FeO compounds in ferrocene. Magnetometers show high magnetization saturation (22.32–26.84 emu/g) in ferromagnetic materials. The use of MNCs in wastewater treatment has been demonstrated by evaluating their adsorption capacity using a methylene blue (MB) adsorption test at various concentrations from 5 to 20 ppm. MNCs obtained at the synthesis time (20 min) showed the highest adsorption efficiency (10.36 mg/g) compared to others, and the MB dye removal rate was 87.79%. Therefore, Langmuir values are not optimistic compared to Freundlich values, with R2 being about 0.80, 0.98 and 0.99 for MNCs synthesized at 10 min (MNC10), 15 min (MNC15) and 20 min (MNC20 ) respectively. Consequently, the adsorption system is in a heterogeneous state. Therefore, microwave arcing offers a promising method for converting CPO to MNC, which can remove harmful dyes.
Microwave radiation can heat the innermost parts of materials through the molecular interaction of electromagnetic fields. This microwave response is unique in that it promotes a fast and uniform thermal response. Thus, it is possible to speed up the heating process and enhance chemical reactions2. At the same time, due to the shorter reaction time, the microwave reaction can ultimately produce products of high purity and high yield3,4. Due to its amazing properties, microwave radiation facilitates interesting microwave syntheses that are used in many studies, including chemical reactions and the synthesis of nanomaterials5,6. During the heating process, the dielectric properties of the acceptor inside the medium play a decisive role, since it creates a hot spot in the medium, which leads to the formation of nanocarbons with different morphologies and properties. A study by Omoriyekomwan et al. Production of hollow carbon nanofibers from palm kernels using activated carbon and nitrogen8. In addition, Fu and Hamid determined the use of a catalyst for the production of oil palm fiber activated carbon in a 350 W9 microwave oven. Therefore, a similar approach can be used to convert crude palm oil to MNCs by introducing suitable scavengers.
An interesting phenomenon has been observed between microwave radiation and metals with sharp edges, dots or submicroscopic irregularities10. The presence of these two objects will be affected by an electrical arc or spark (commonly referred to as an arc discharge)11,12. The arc will promote the formation of more localized hot spots and influence the reaction, thereby improving the chemical composition of the environment13. This particular and interesting phenomenon has attracted various studies such as contaminant removal14,15, biomass tar cracking16, microwave assisted pyrolysis17,18 and material synthesis19,20,21.
Recently, nanocarbons such as carbon nanotubes, carbon nanospheres, and modified reduced graphene oxide have attracted attention due to their properties. These nanocarbons hold great potential for applications ranging from power generation to water purification or decontamination23. In addition, excellent carbon properties are required, but at the same time, good magnetic properties are required. This is very useful for multifunctional applications including high adsorption of metal ions and dyes in wastewater treatment, magnetic modifiers in biofuels and even high efficiency microwave absorbers24,25,26,27,28. At the same time, these carbons have another advantage, including an increase in the surface area of the sample’s active site.
In recent years, research into magnetic nanocarbon materials has been on the rise. Typically, these magnetic nanocarbons are multifunctional materials containing nanosized magnetic materials that can cause external catalysts to react, such as external electrostatic or alternating magnetic fields29. Due to their magnetic properties, magnetic nanocarbons can be combined with a wide range of active ingredients and complex structures for immobilization30. Meanwhile, magnetic nanocarbons (MNCs) show excellent efficiency in adsorbing pollutants from aqueous solutions. In addition, the high specific surface area and pores formed in MNCs can increase adsorption capacity31. Magnetic separators can separate MNCs from highly reactive solutions, turning them into a viable and manageable sorbent32.
Several researchers have demonstrated that high quality nanocarbons can be produced using raw palm oil33,34. Palm oil, scientifically known as Elais Guneensis, is considered to be one of the important edible oils with a production of around 76.55 million tons in 202135. Crude palm oil or CPO contains a balanced ratio of unsaturated fatty acids (EFAs) and saturated fatty acids (Singapore Monetary Authority). Most of the hydrocarbons in CPO are triglycerides, a glyceride composed of three triglyceride acetate components and one glycerol component36. These hydrocarbons can be generalized due to their huge carbon content, making them potential green precursors for nanocarbon production37. According to the literature, CNT37,38,39,40, carbon nanospheres33,41 and graphene34,42,43 are usually synthesized using crude palm oil or edible oil. These nanocarbons have great potential in applications ranging from power generation to water purification or decontamination.
Thermal synthesis such as CVD38 or pyrolysis33 has become a favorable method for the decomposition of palm oil. Unfortunately, the high temperatures in the process increase the cost of production. Producing the preferred material 44 requires lengthy, tedious procedures and cleaning methods. However, the need for physical separation and cracking is undeniable due to the good stability of crude palm oil at high temperatures45. Therefore, higher temperatures are still required to convert crude palm oil into carbonaceous materials. The liquid arc can be considered as the best potential and new method for the synthesis of magnetic nanocarbon 46 . This approach provides direct energy for precursors and solutions in highly excited states. An arc discharge can cause the carbon bonds in crude palm oil to break. However, the electrode spacing used may need to meet stringent requirements, which will limit the industrial scale, so an efficient method still needs to be developed.
To the best of our knowledge, research on arc discharge using microwaves as a method for synthesizing nanocarbons is limited. At the same time, the use of crude palm oil as a precursor has not been fully explored. Therefore, this study aims to explore the possibility of producing magnetic nanocarbons from raw palm oil precursors using an electric arc using a microwave oven. The abundance of palm oil should be reflected in new products and applications. This new approach to palm oil refining could help boost the economic sector and be another source of income for palm oil producers, especially affected small farmers’ palm oil plantations. According to a study of African smallholders by Ayompe et al., smallholders only earn more money if they process fresh fruit clusters themselves and sell raw palm oil rather than selling it to middlemen, which is a costly and tedious job47. At the same time, an increase in factory closures due to COVID-19 has affected palm oil-based application products. Interestingly, since most households have access to microwave ovens and the method proposed in this study can be considered feasible and affordable, MNC production can be considered as an alternative to small-scale palm oil plantations. Meanwhile, on a larger scale, companies can invest in large reactors to produce large TNCs.
This study mainly covers the synthesis process using stainless steel as the dielectric medium for various durations. Most general studies using microwaves and nanocarbons suggest an acceptable synthesis time of 30 minutes or more33,34. In order to support an accessible and feasible practical idea, this study aimed to obtain MNCs with below average synthesis times. At the same time, the study paints a picture of technology readiness level 3 as the theory is proven and implemented on a laboratory scale. Later, the resulting MNCs were characterized by their physical, chemical, and magnetic properties. Methylene blue was then used to demonstrate the adsorption capacity of the resulting MNCs.
Crude palm oil was obtained from Apas Balung Mill, Sawit Kinabalu Sdn. Bhd., Tawau, and is used as a carbon precursor for synthesis. In this case, a stainless steel wire with a diameter of 0.90 mm was used as a dielectric medium. Ferrocene (purity 99%), obtained from Sigma-Aldrich, USA, was chosen as a catalyst in this work. Methylene blue (Bendosen, 100 g) was further used for adsorption experiments.
In this study, a household microwave oven (Panasonic: SAM-MG23K3513GK) was converted into a microwave reactor. Three holes were made in the upper part of the microwave oven for the inlet and outlet of gas and a thermocouple. The thermocouple probes were insulated with ceramic tubes and placed under the same conditions for each experiment to prevent accidents. Meanwhile, a borosilicate glass reactor with a three-hole lid was used to accommodate the samples and the trachea. A schematic diagram of a microwave reactor can be referred to in Supplementary Figure 1.
Using crude palm oil as a carbon precursor and ferrocene as a catalyst, magnetic nanocarbons were synthesized. About 5% by weight of the ferrocene catalyst was prepared by the slurry catalyst method. Ferrocene was mixed with 20 ml of crude palm oil at 60 rpm for 30 minutes. The mixture was then transferred to an alumina crucible, and a 30 cm long stainless steel wire was coiled and placed vertically inside the crucible. Place the alumina crucible into the glass reactor and secure it securely inside the microwave oven with a sealed glass lid. Nitrogen was blown into the chamber 5 minutes prior to the start of the reaction to remove unwanted air from the chamber. The microwave power has been increased to 800W because this is the maximum microwave power that can maintain a good arc start. Therefore, this may contribute to the creation of favorable conditions for synthetic reactions. At the same time, this is also a widely used power range in watts for microwave fusion reactions48,49. The mixture was heated for 10, 15 or 20 minutes during the reaction. After completion of the reaction, the reactor and microwave were naturally cooled to room temperature. The final product in the alumina crucible was a black precipitate with helical wires.
The black precipitate was collected and washed several times alternately with ethanol, isopropanol (70%) and distilled water. After washing and cleaning, the product is dried overnight at 80°C in a conventional oven to evaporate unwanted impurities. The product was then collected for characterization. Samples labeled MNC10, MNC15, and MNC20 were used to synthesize magnetic nanocarbons for 10 min, 15 min, and 20 min.
Observe MNC morphology with a field emission scanning electron microscope or FESEM (Zeiss Auriga model) at 100 to 150 kX magnification. At the same time, the elemental composition was analyzed by energy-dispersive X-ray spectroscopy (EDS). The EMF analysis was carried out at a working distance of 2.8 mm and an accelerating voltage of 1 kV. Specific surface area and MNC pore values were measured by the Brunauer-Emmett-Teller (BET) method, including the adsorption-desorption isotherm of N2 at 77 K. The analysis was performed using a model surface area meter (MICROMERITIC ASAP 2020).
The crystallinity and phase of the magnetic nanocarbons were determined by X-ray powder diffraction or XRD (Burker D8 Advance) at λ = 0.154 nm. Diffractograms were recorded between 2θ = 5 and 85° at a scan rate of 2° min-1. In addition, the chemical structure of MNCs was investigated using Fourier transform infrared spectroscopy (FTIR). The analysis was performed using a Perkin Elmer FTIR-Spectrum 400 with scan speeds ranging from 4000 to 400 cm-1. In studying the structural features of magnetic nanocarbons, Raman spectroscopy was performed using a neodymium-doped laser (532 nm) in U-RAMAN spectroscopy with a 100X objective.
A vibrating magnetometer or VSM (Lake Shore 7400 series) was used to measure the magnetic saturation of iron oxide in MNCs. A magnetic field of about 8 kOe was used and 200 points were obtained.
When studying the potential of MNCs as adsorbents in adsorption experiments, the cationic dye methylene blue (MB) was used. MNCs (20 mg) were added to 20 ml of an aqueous solution of methylene blue with standard concentrations in the range of 5–20 mg/L50. The pH of the solution was set at a neutral pH of 7 throughout the study. The solution was mechanically stirred at 150 rpm and 303.15 K on a rotary shaker (Lab Companion: SI-300R). The MNCs are then separated using a magnet. Use a UV-visible spectrophotometer (Varian Cary 50 UV-Vis Spectrophotometer) to observe the concentration of the MB solution before and after the adsorption experiment, and refer to the methylene blue standard curve at a maximum wavelength of 664 nm. The experiment was repeated three times and the average value was given. The removal of MG from the solution was calculated using the general equation for the amount of MC adsorbed at equilibrium qe and the percentage of removal %.
Experiments on the adsorption isotherm were also carried out with stirring of various concentrations (5–20 mg/l) of MG solutions and 20 mg of the adsorbent at a constant temperature of 293.15 K. mg for all MNCs.
Iron and magnetic carbon have been extensively studied over the past few decades. These carbon-based magnetic materials are attracting increasing attention due to their excellent electromagnetic properties, leading to various potential technological applications, mainly in electrical appliances and water treatment. In this study, nanocarbons were synthesized by cracking hydrocarbons in crude palm oil using a microwave discharge. The synthesis was carried out at different times, from 10 to 20 min, at a fixed ratio (5:1) of the precursor and catalyst, using a metal current collector (twisted SS) and partially inert (undesirable air purged with nitrogen at the beginning of the experiment). The resulting carbonaceous deposits are in the form of a black solid powder, as shown in Supplementary Fig. 2a. The precipitated carbon yields were approximately 5.57%, 8.21%, and 11.67% at synthesis times of 10 minutes, 15 minutes, and 20 minutes, respectively. This scenario suggests that longer synthesis times contribute to higher yields51—low yields, most likely due to short reaction times and low catalyst activity.
Meanwhile, a plot of synthesis temperature versus time for the obtained nanocarbons can be referred to in Supplementary Figure 2b. The highest temperatures obtained for MNC10, MNC15 and MNC20 were 190.9°C, 434.5°C and 472°C, respectively. For each curve, a steep slope can be seen, indicating a constant rise in temperature inside the reactor due to the heat generated during the metal arc. This can be seen at 0–2 min, 0–5 min, and 0–8 min for MNC10, MNC15, and MNC20, respectively. After reaching a certain point, the slope continues to hover to the highest temperature, and the slope becomes moderate.
Field emission scanning electron microscopy (FESEM) was used to observe the surface topography of the MNC samples. As shown in fig. 1, magnetic nanocarbons have a slightly different morphological structure at a different time of synthesis. Images of FESEM MNC10 in fig. 1a,b show that the formation of carbon spheres consists of entangled and attached micro- and nanospheres due to high surface tension. At the same time, the presence of van der Waals forces leads to the aggregation of carbon spheres52. The increase in synthesis time resulted in smaller sizes and an increase in the number of spheres due to longer cracking reactions. On fig. 1c shows that MNC15 has an almost perfect spherical shape. However, the aggregated spheres can still form mesopores, which can later become good sites for methylene blue adsorption. At a high magnification of 15,000 times in Fig. 1d more carbon spheres can be seen agglomerated with an average size of 20.38 nm.
FESEM images of synthesized nanocarbons after 10 min (a, b), 15 min (c, d) and 20 min (e–g) at 7000 and 15000 times magnification.
On fig. 1e–g MNC20 depicts the development of pores with small spheres on the surface of magnetic carbon and reassembles the morphology of magnetic activated carbon53. Pores of different diameters and widths are randomly located on the surface of magnetic carbon. Therefore, this may explain why MNC20 showed a greater surface area and pore volume as shown by BET analysis, as more pores formed on its surface than at other synthetic times. Micrographs taken at a high magnification of 15,000 times showed inhomogeneous particle sizes and irregular shapes, as shown in Fig. 1g. When the growth time was increased to 20 minutes, more agglomerated spheres were formed.
Interestingly, twisted carbon flakes were also found in the same area. The diameter of the spheres varied from 5.18 to 96.36 nm. This formation may be due to the occurrence of differential nucleation, which is facilitated by high temperature and microwaves. The calculated sphere size of the prepared MNCs averaged 20.38 nm for MNC10, 24.80 nm for MNC15, and 31.04 nm for MNC20. The size distribution of spheres is shown in the supplementary fig. 3.
Supplementary Figure 4 shows the EDS spectra and elemental composition summaries of MNC10, MNC15, and MNC20, respectively. According to the spectra, it was noted that each nanocarbon contains a different amount of C, O, and Fe. This is due to the various oxidation and cracking reactions occurring during the additional synthesis time. A large amount of C is believed to come from the carbon precursor, crude palm oil. Meanwhile, the low percentage of O is due to the oxidation process during synthesis. At the same time, Fe is attributed to iron oxide deposited on the nanocarbon surface after ferrocene decomposition. In addition, Supplementary Figure 5a–c shows the mapping of MNC10, MNC15, and MNC20 elements. Based on fundamental mapping, it was observed that Fe is well distributed over the MNC surface.
Nitrogen adsorption-desorption analysis provides information about the adsorption mechanism and the porous structure of the material. N2 adsorption isotherms and graphs of the MNC BET surface are shown in Figs. 2. Based on the FESEM images, the adsorption behavior is expected to exhibit a combination of microporous and mesoporous structures due to aggregation. However, the graph in Fig. 2 shows that the adsorbent resembles the type IV isotherm and the type H2 hysteresis loop of IUPAC55. This type of isotherm is often similar to that of mesoporous materials. The adsorption behavior of mesopores is usually determined by the interaction of adsorption-adsorption reactions with the molecules of the condensed matter. S-shaped or S-shaped adsorption isotherms are usually caused by single-layer-multilayer adsorption followed by a phenomenon in which gas condenses into a liquid phase in pores at pressures below the saturation pressure of the bulk liquid, known as pore condensation 56. Capillary condensation in pores occurs at relative pressures (p/po) above 0.50. Meanwhile, the complex pore structure exhibits H2-type hysteresis, which is attributed to pore plugging or leakage in a narrow range of pores.
The physical parameters of the surface obtained from the BET tests are shown in Table 1. The BET surface area and total pore volume increased significantly with increasing synthesis time. The average pore sizes of MNC10, MNC15, and MNC20 are 7.2779 nm, 7.6275 nm, and 7.8223 nm, respectively. According to the IUPAC recommendations, these intermediate pores can be classified as mesoporous materials. The mesoporous structure can make methylene blue more easily permeable and adsorbable by MNC57. Maximum Synthesis Time (MNC20) showed the highest surface area, followed by MNC15 and MNC10. Higher BET surface area can improve adsorption performance as more surfactant sites are available.
X-ray diffraction patterns of the synthesized MNCs are shown in Fig. 3. At high temperatures, ferrocene also cracks and forms iron oxide. On fig. 3a shows the XRD pattern of MNC10. It shows two peaks at 2θ, 43.0° and 62.32°, which are assigned to ɣ-Fe2O3 (JCPDS #39–1346). At the same time, Fe3O4 has a strained peak at 2θ: 35.27°. On the other hand, in the MHC15 diffraction pattern in Fig. 3b shows new peaks, which are most likely associated with an increase in temperature and synthesis time. Although the 2θ: 26.202° peak is less intense, the diffraction pattern is consistent with the graphite JCPDS file (JCPDS #75–1621), indicating the presence of graphite crystals within the nanocarbon. This peak is absent in MNC10, possibly due to the low arc temperature during synthesis. At 2θ there are three time peaks: 30.082°, 35.502°, 57.422° attributed to Fe3O4. It also shows two peaks indicating the presence of ɣ-Fe2O3 at 2θ: 43.102° and 62.632°. For MNC synthesized for 20 min (MNC20), as shown in Fig. 3c, a similar diffraction pattern can be observed in MNK15. The graphical peak at 26.382° can also be seen in the MNC20. The three sharp peaks shown at 2θ: 30.102°, 35.612°, 57.402° are for Fe3O4. In addition, the presence of ε-Fe2O3 is shown at 2θ: 42.972° and 62.61. The presence of iron oxide compounds in the resulting MNCs can have a positive effect on the ability to adsorb methylene blue in the future.
The chemical bond characteristics in the MNC and CPO samples were determined from the FTIR reflectance spectra in Supplementary Figure 6. Initially, the six important peaks of crude palm oil represented four different chemical components as described in Supplementary Table 1. The fundamental peaks identified in CPO are 2913.81 cm-1, 2840 cm-1 and 1463.34 cm-1, which refer to the CH stretching vibrations of alkanes and other aliphatic CH2 or CH3 groups. The identified peak foresters are 1740.85 cm-1 and 1160.83 cm-1. The peak at 1740.85 cm-1 is a C=O bond extended by the ester carbonyl of the triglyceride functional group. Meanwhile, the peak at 1160.83 cm-1 is the imprint of the extended CO58.59 ester group. Meanwhile, the peak at 813.54 cm-1 is the imprint of the alkane group.
Therefore, some absorption peaks in crude palm oil disappeared as the synthesis time increased. Peaks at 2913.81 cm-1 and 2840 cm-1 can still be observed in MNC10, but it is interesting that in MNC15 and MNC20 the peaks tend to disappear due to oxidation. Meanwhile, FTIR analysis of magnetic nanocarbons revealed newly formed absorption peaks representing five different functional groups of MNC10-20. These peaks are also listed in Supplementary Table 1. The peak at 2325.91 cm-1 is the asymmetric CH stretch of the CH360 aliphatic group. The peak at 1463.34-1443.47 cm-1 shows CH2 and CH bending of aliphatic groups such as palm oil, but the peak begins to decrease with time. The peak at 813.54–875.35 cm–1 is an imprint of the aromatic CH-alkane group.
Meanwhile, the peaks at 2101.74 cm-1 and 1589.18 cm-1 represent CC 61 bonds forming C=C alkyne and aromatic rings, respectively. A small peak at 1695.15 cm-1 shows the C=O bond of the free fatty acid from the carbonyl group. It is obtained from CPO carbonyl and ferrocene during synthesis. The newly formed peaks in the range from 539.04 to 588.48 cm-1 belong to the Fe-O vibrational bond of ferrocene. Based on the peaks shown in Supplementary Figure 4, it can be seen that synthesis time can reduce several peaks and re-bonding in magnetic nanocarbons.
Spectroscopic analysis of Raman scattering of magnetic nanocarbons obtained at different times of synthesis using an incident laser with a wavelength of 514 nm is shown in Figure 4. All spectra of MNC10, MNC15 and MNC20 consist of two intense bands associated with low sp3 carbon, commonly found in nanographite crystallites with defects in vibrational modes of carbon species sp262. The first peak, located in the region of 1333–1354 cm–1, represents the D band, which is unfavorable for ideal graphite and corresponds to structural disorder and other impurities63,64. The second most important peak around 1537–1595 cm-1 arises from in-plane bond stretching or crystalline and ordered graphite forms. However, the peak shifted by about 10 cm-1 compared to the graphite G band, indicating that the MNCs have a low sheet stacking order and a defective structure. The relative intensities of the D and G bands (ID/IG) are used to evaluate the purity of crystallites and graphite samples. According to Raman spectroscopic analysis, all MNCs had ID/IG values in the range of 0.98–0.99, indicating structural defects due to Sp3 hybridization. This situation can explain the presence of less intense 2θ peaks in the XPA spectra: 26.20° for MNK15 and 26.28° for MNK20, as shown in Fig. 4, which is assigned to the graphite peak in the JCPDS file. The ID/IG MNC ratios obtained in this work are in the range of other magnetic nanocarbons, for example, 0.85–1.03 for the hydrothermal method and 0.78–0.9665.66 for the pyrolytic method. Therefore, this ratio indicates that the present synthetic method can be widely used.
The magnetic characteristics of the MNCs were analyzed using a vibrating magnetometer. The resulting hysteresis is shown in Fig.5. As a rule, MNCs acquire their magnetism from ferrocene during synthesis. These additional magnetic properties may increase the adsorption capacity of nanocarbons in the future. As shown in Figure 5, the samples can be identified as superparamagnetic materials. According to Wahajuddin & Arora67, the superparamagnetic state is that the sample is magnetized to saturation magnetization (MS) when an external magnetic field is applied. Later, residual magnetic interactions no longer appear in the samples67. It is noteworthy that the saturation magnetization increases with the synthesis time. Interestingly, MNC15 has the highest magnetic saturation because strong magnetic formation (magnetization) can be caused by optimal synthesis time in the presence of an external magnet. This may be due to the presence of Fe3O4, which has better magnetic properties compared to other iron oxides such as ɣ-Fe2O. The order of the adsorption moment of saturation per unit mass of MNCs is MNC15>MNC10>MNC20. The obtained magnetic parameters are given in table. 2.
The minimum value of magnetic saturation when using conventional magnets in magnetic separation is about 16.3 emu g-1. The ability of MNCs to remove contaminants such as dyes in the aquatic environment and the ease of removal of MNCs have become additional factors for the obtained nanocarbons. Studies have shown that the magnetic saturation of the LSM is considered to be high. Thus, all samples reached magnetic saturation values more than sufficient for the magnetic separation procedure.
Recently, metal strips or wires have attracted attention as catalysts or dielectrics in microwave fusion processes. Microwave reactions of metals cause high temperatures or reactions within the reactor. This study claims that the tip and conditioned (coiled) stainless steel wire facilitate microwave discharge and metal heating. Stainless steel has pronounced roughness at the tip, which leads to high values of surface charge density and external electric field. When the charge has gained sufficient kinetic energy, the charged particles will jump out of the stainless steel, causing the environment to ionize, producing a discharge or spark 68 . Metal discharge makes a significant contribution to solution cracking reactions accompanied by high temperature hot spots. According to the temperature map in Supplementary Fig. 2b, the temperature rises rapidly, indicating the presence of high-temperature hot spots in addition to the strong discharge phenomenon.
In this case, a thermal effect is observed, since weakly bound electrons can move and concentrate on the surface and on the tip69. When stainless steel is wound, the large surface area of the metal in solution helps induce eddy currents on the surface of the material and maintains the heating effect. This condition effectively helps to cleave the long carbon chains of CPO and ferrocene and ferrocene. As shown in Supplementary Fig. 2b, a constant temperature rate indicates that a uniform heating effect is observed in the solution.
A proposed mechanism for the formation of MNCs is shown in Supplementary Figure 7. The long carbon chains of CPO and ferrocene begin to crack at high temperature. The oil breaks down to form split hydrocarbons that become carbon precursors known as globules in the FESEM MNC1070 image. Due to the energy of the environment and pressure 71 in atmospheric conditions. At the same time, ferrocene also cracks, forming a catalyst from carbon atoms deposited on Fe. Rapid nucleation then occurs and the carbon core oxidizes to form an amorphous and graphitic carbon layer on top of the core. As time increases, the size of the sphere becomes more precise and uniform. At the same time, the existing van der Waals forces also lead to the agglomeration of spheres52. During the reduction of Fe ions to Fe3O4 and ɣ-Fe2O3 (according to X-ray phase analysis), various types of iron oxides are formed on the surface of nanocarbons, which leads to the formation of magnetic nanocarbons. EDS mapping showed that the Fe atoms were strongly distributed over the MNC surface, as shown in Supplementary Figures 5a-c.
The difference is that at a synthesis time of 20 minutes, carbon aggregation occurs. It forms larger pores on the surface of MNCs, suggesting that MNCs can be considered as activated carbon, as shown in the FESEM images in Fig. 1e–g. This difference in pore sizes may be related to the contribution of iron oxide from ferrocene. At the same time, due to the reached high temperature, there are deformed scales. Magnetic nanocarbons exhibit different morphologies at different synthesis times. Nanocarbons are more likely to form spherical shapes with shorter synthesis times. At the same time, pores and scales are achievable, although the difference in synthesis time is only within 5 minutes.
Magnetic nanocarbons can remove pollutants from the aquatic environment. Their ability to be easily removed after use is an additional factor for using the nanocarbons obtained in this work as adsorbents. In studying the adsorption properties of magnetic nanocarbons, we investigated the ability of MNCs to decolorize methylene blue (MB) solutions at 30°C without any pH adjustment. Several studies have concluded that the performance of carbon absorbents in the temperature range of 25–40 °C does not play an important role in determining MC removal. Although extreme pH values play an important role, charges can form on the surface functional groups, which leads to disruption of the adsorbate-adsorbent interaction and affects adsorption. Therefore, the above conditions were chosen in this study considering these situations and the need for typical wastewater treatment.
In this work, a batch adsorption experiment was carried out by adding 20 mg of MNCs to 20 ml of an aqueous solution of methylene blue with various standard initial concentrations (5–20 ppm) at a fixed contact time60. Supplementary Figure 8 shows the status of various concentrations (5–20 ppm) of methylene blue solutions before and after treatment with MNC10, MNC15, and MNC20. When using various MNCs, the color level of MB solutions decreased. Interestingly, it was found that MNC20 easily discolored MB solutions at a concentration of 5 ppm. Meanwhile, the MNC20 also lowered the color level of the MB solution compared to other MNCs. The UV visible spectrum of MNC10-20 is shown in Supplementary Figure 9. Meanwhile, the removal rate and adsorption information are shown in Figure 9. 6 and in table 3, respectively.
Strong methylene blue peaks can be found at 664 nm and 600 nm. As a rule, the intensity of the peak gradually decreases with decreasing initial concentration of the MG solution. In the additional Fig. 9a shows the UV-visible spectra of MB solutions of various concentrations after treatment with MNC10, which only slightly changed the intensity of the peaks. On the other hand, the absorption peaks of MB solutions decreased significantly after treatment with MNC15 and MNC20, as shown in Supplementary Figures 9b and c, respectively. These changes are clearly seen as the concentration of the MG solution decreases. However, the spectral changes achieved by all three magnetic carbons were sufficient to remove the methylene blue dye.
Based on Table 3, the results for the amount of MC adsorbed and the percentage of MC adsorbed are shown in Fig. 3. 6. The adsorption of MG increased with the use of higher initial concentrations for all MNCs. Meanwhile, the adsorption percentage or MB removal rate (MBR) showed an opposite trend when the initial concentration increased. At lower initial MC concentrations, unoccupied active sites remained on the adsorbent surface. As the dye concentration increases, the number of unoccupied active sites available for the adsorption of dye molecules will decrease. Others have concluded that under these conditions saturation of the active sites of biosorption will be achieved72.
Unfortunately for MNC10, MBR increased and decreased after 10 ppm of MB solution. At the same time, only a very small part of MG is adsorbed. This indicates that 10 ppm is the optimum concentration for MNC10 adsorption. For all MNCs studied in this work, the order of adsorption capacities was as follows: MNC20 > MNC15 > MNC10, the average values were 10.36 mg/g, 6.85 mg/g and 0.71 mg/g, the average removal of MG rates was 87, 79%, 62.26% and 5.75%. Thus, MNC20 demonstrated the best adsorption characteristics among the synthesized magnetic nanocarbons, taking into account the adsorption capacity and the UV-visible spectrum. Although the adsorption capacity is lower compared to other magnetic nanocarbons such as MWCNT magnetic composite (11.86 mg/g) and halloysite nanotube-magnetic Fe3O4 nanoparticles (18.44 mg/g), this study does not require the additional use of a stimulant. Chemicals act as catalysts. providing clean and feasible synthetic methods73,74.
As shown by the SBET values of the MNCs, a high specific surface provides more active sites for the adsorption of the MB solution. This is becoming one of the fundamental features of synthetic nanocarbons. At the same time, due to the small size of MNCs, the synthesis time is short and acceptable, which corresponds to the main qualities of promising adsorbents75. Compared to conventional natural adsorbents, the synthesized MNCs are magnetically saturated and can be easily removed from solution under the action of an external magnetic field76. Thus, the time required for the entire treatment process is reduced.
Adsorption isotherms are essential to understand the adsorption process and then to demonstrate how the adsorbate partitions between the liquid and solid phases when equilibrium is reached. The Langmuir and Freundlich equations are used as standard isotherm equations, which explain the mechanism of adsorption, as shown in Figure 7. The Langmuir model well shows the formation of a single adsorbate layer on the outer surface of the adsorbent. Isotherms are best described as homogeneous adsorption surfaces. At the same time, the Freundlich isotherm best states the participation of several adsorbent regions and the adsorption energy in pressing the adsorbate to an inhomogeneous surface.
Model isotherm for Langmuir isotherm (a–c) and Freundlich isotherm (d–f) for MNC10, MNC15 and MNC20.
Adsorption isotherms at low solute concentrations are usually linear77. The linear representation of the Langmuir isotherm model can be expressed in an equation. 1 Determine adsorption parameters.
KL (l/mg) is a Langmuir constant representing the binding affinity of MB to MNC. Meanwhile, qmax is the maximum adsorption capacity (mg/g), qe is the adsorbed concentration of MC (mg/g), and Ce is the equilibrium concentration of the MC solution. The linear expression of the Freundlich isotherm model can be described as follows:
Post time: Feb-16-2023