Preparation and Characterization of New CrFeO 3 -Carbon Composite Using Environmentally Friendly Methods to Remove Organic Dye Pollutants from Aqueous Solutions

: Globally, environmental pollution is an important issue. Various pollutants present in water resources, such as bacteria, heavy-metal ions, and organic pollutants, cause serious problems to the environment, animals, plants, and human health. Among the water resources, pollutants, dyestuff, which is discharged from dyeing, textile, and other industrial processes, is an important class of pollutants. Removing these dye pollutants from water resources and wastewater is vital and important due to their toxicity. In this work, a CrFeO 3 -carbon nanotube (CNT) adsorbent was synthesized using environmentally friendly methods. The synthesized CrFeO3-CNT adsorbent was characterized stoichiometrically, spectroscopically, and morphologically. The synthesized CrFeO 3 CNT adsorbent was tested for the removal of two dyes: Methyl violet 2B (MV) and Azocarmine G2 (AC) from an aqueous solution. Crushing CrFeO 3 composite with multi-walled fullerene CNT to prepare CrFeO 3 -CNT adsorbent improved the adsorption performance of free multi-walled fullerene CNT towards MV dye by 30% and towards AC dye by 33.3%.


Introduction
Water, which covers about 71% of the earth's total surface, is a valuable resource for sustaining life. Among this percentage, only 1% of total water is found as freshwater, which is used for various purposes (i.e., domestic use, agriculture, and drinking) [1]. The global population is increasing; human society is fast growing, and industrial technology is rapidly progressing and developing. This has led to a significant amount of environmental water pollution, and reusing sustainable resources such as water has become a serious issue of concern worldwide. Year after year, the demand for freshwater is exceeding supply in parts of the world, though freshwater resources remain limited. Obtaining freshwater in the future is of utmost importance and will depend on preserving the quality of water through treatment and recycling techniques.
Dyestuff is widely used in many industries, such as food processing, plastic, rubber, drug, printing, cosmetic, leather tanning, paper and paperboard, and textile [2]. Among the main industries that consume and produce a significant amount of wastewater that pollutes water resources are the dye and textile industries. The tremendous discharge of MO, USA). The multi-walled fullerene carbon nanotube (CNT) was purchased from Alfa Aesar, Thermo Fisher Scientific (76870 Kandel, GmbH, Germany). Specifications of the CNT are: multi-walled, 3-20 nm OD, 1-3 nm ID, 0.1-10 microns long, 95% nanotubes. The dyes' aqueous solutions throughout the experiments were prepared using deionized (DI) water. All materials were used as received without further purification. The instruments used to characterize the synthesized composite and adsorbent materials include a scanning electron microscope (SEM model JSM-6390LA JEOL) coupled with an energydispersive X-ray spectrometer (EDXRF model JED-2300) (Tokyo, Japan) for SEM-EDX data, a Bruker compact Fourier-transform infrared (FT-IR) spectrophotometer (model Alpha) (Bruker Optik GmbH, Ettlingen, Germany) for the IR spectra, and a Perkin-Elmer CHN Microanalyzer (model PE 2400 series II) (Perkin-Elmer Inc, Waltham, MA, USA) for the C, H, and N (in %) elemental analyses. A Perkin-Elmer UV/vis spectrophotometer (model Lambda 25) (Perkin-Elmer Inc, Waltham, MA, USA) was used for the ultraviolet-visible measurements. An X'Pert Philips X-ray Powder Diffractometer was used for the XRD measurements from a diffraction angle (2θ) of 5 • to 70 • .

CrFeO 3 Composite
FeCl 3 (1 mmol), CrCl 3 (1 mmol), and urea (6 mmol) were dissolved in a 100 mL binary solvent mixture (H 2 O: MeOH) (1:1). The mixture was stirred for 24 h at 80 • C. The resultant precipitate was separated by filtration and washed several times with hot water to remove all unreacted compounds, and then thermally decomposed at 800 • C for 3 h in an air atmosphere to generate the nanostructured dark-red CrFeO 3 composite. This composite was also obtained by using another chromium salt [Cr(NO 3 ) 3 ·9H 2 O] instead of CrCl 3 . The composite generated from CrCl 3 was labeled as Composite A, and that generated from Cr(NO 3 ) 3 ·9H 2 O was labeled as Composite B. The composites were grounded into powder with a particle size of 2-4 mm and characterized by IR spectroscopy and an SEM-EDX instrument.

Adsorbent
A 100 mg amount of the CrFeO 3 composite was added to 1.0 g of the multi-walled fullerene CNT on a dry, clean, porcelain mortar. A porcelain pestle was used to mix the two materials thoroughly; after that, a few drops of methanol was added to the mixture, then all components were thoroughly mixed and ground together for 20 min. The black CrFeO 3 -CNT adsorbent was collected from the mortar and vacuum dried in a desiccator with anhydrous CaCl 2 .

Adsorption Experiments
In a typical run, 100 mL of aqueous dye solution (1 g/L; MV or 10 g/L; AC) was added into a 250 mL conical flask. The conical flask was put on a mechanical shaker. A 40 mg amount of the synthesized adsorbent amount was added to the dye solution. The mixture was shaken at room temperature. Aliquots (5 mL) were taken after pre-defined time intervals (2, 4, 6, 8, 10, 12 min) and centrifuged for 10 min to remove the adsorbent, then the absorbance was measured in a Perkin-Elmer UV/vis spectrophotometer at the wavelength of 588 nm in the case of MV and at 516 nm in the case of AC. All the dye adsorption experiments were repeated three times. The degree of decolorization of the dye was determined using the following equation [22,23]: where A o is the initial absorptance of the dye solution, and A t is the absorbance of the dye solution at time t.

Preparation and Characterization of Composites A and B
Composite A was generated by two stages: Stage one: Reacting 1 mmol of FeCl 3 and 1 mmol of CrCl 3 with 6 mmol of urea in a binary solvent mixture (H 2 O: MeOH) (1:1) at 80 • C. This produced a solid precipitate with a brown color ( Figure 1) containing [Fe(NH 2 CONH 2 ) 6 ]Cl 3 and [Cr(NH 2 CONH 2 ) 6 ]Cl 3 solid mixtures [24].

Preparation and Characterization of Composites A and B
Composite A was generated by two stages: Stage one: Reacting 1 mmol of FeCl3 and 1 mmol of CrCl3 with 6 mmol of urea in a binary solvent mixture (H2O: MeOH) (1:1) at 80 °C. This produced a solid precipitate with a brown color ( Figure 1) containing [Fe(NH2CONH2)6]Cl3 and [Cr(NH2CONH2)6]Cl3 solid mixtures [24].
Stage two: Thermal decomposition of the resultant solid mixture was performed at 800 °C for 3 h in an air oxygen atmosphere. This combustion generated the nanostructured dark-red CrFeO3 composite (Composite A) (Figure 1) [24].
Composite B (Figure 1) was obtained by using another chromium salt; Cr(NO3)3·9H2O was generated by the same two stages. Stage two: Thermal decomposition of the resultant solid mixture was performed at 800 • C for 3 h in an air oxygen atmosphere. This combustion generated the nanostructured dark-red CrFeO 3 composite (Composite A) ( Figure 1) [24].
Composite B (Figure 1) was obtained by using another chromium salt; Cr(NO 3 ) 3 ·9H 2 O was generated by the same two stages. Figure 2 depicts the IR spectra of the solid mixtures generated from stage one. These solids are Solid Mixture A, which contains [Fe(NH 2 CONH 2 ) 6 ]Cl 3 and [Cr(NH 2 CONH 2 ) 6 ]Cl 3 , and Solid Mixture B, which contains [Fe(NH 2 CONH 2 ) 6 ]Cl 3 and [Cr(NH 2 CONH 2 ) 6 ](NO 3 ) 3 . Generally, both mixtures displayed similar IR features, including the following absorption bands: (i) a broad medium-strong intensity band accumulated at 3340 cm −1 , which was assigned to the ν(NH 2 ) vibrations; (ii) a band with a medium intensity observed at 1632 cm −1 in Solid Mixture A and at 1641 cm −1 observed in Solid Mixture B. These are due to the ν(C=O) vibrations; (iii) a band appeared at 1440 cm −1 in Solid Mixture A and at 1380 cm −1 in Solid Mixture B. These could have resulted from the δ def (NH 2 ) vibrations; (iv) a band resonated, respectively, at 830 and 811 cm −1 Solid Mixture A and Solid Mixture B, attributed to the δ twist (NH 2 ) modes; and (v) a band resonating at 640 cm −1 for both mixtures, attributed to the δ rock (NH 2 ) modes.     , δ def (NH 2 ), δ twist (NH 2 )) were no longer observed in their corresponding composites. Instead of these bands, only two bands were observed in the IR spectra of Composites A and B. These bands resonated at 547 and 450 cm −1 for both composites and resulted from the ν(Fe-O) and ν(Cr-O) vibrations [25,26]. Figure 4a,b shows SEM micrographs of Solid Mixtures A and B, re-spectively. EDX analysis indicated the presence of Fe, Cr, C, O, N, and Cl elements in Solid Mixture A and elements of Fe, Cr, C, O, and N in Solid Mixture B. Morphologically, there are no differences in the surface topology and shape of the microstructure between the two solid mixtures, as evidenced by their SEM pictures. Both mixtures consisted of small particles, and most of these particles had similar sizes. Solid Mixtures A and B had stone-like-shaped particles. This indicated that using chromium chloride salt, CrCl 3 , or chromium nitrate salt, Cr(NO 3 ) 3 ·9H 2 O, shows no differences in the morphology of the solid mixture. Burning Solid Mixtures A and B at 800 • C resulted in the formation of highly homogenized and uniform materials (Composites A and B), as evidenced by their SEM micrographs shown in Figure 5a,b. EDX analysis evidenced the presence of Fe, Cr, and O elements in both composites. The elemental percentages of these elements are listed in Table 1. These elemental percentages agree well with the proposed chemical structure for Composites A and B (CrFeO 3 ). EDX analysis showed the presence of 4% C elements in Composite A and 7% C elements in Composite B, which indicated that some carbons remain as residual from the combustion of Solid Mixtures A and B at 800 • C. Figure 6 depicts XRD spectra of Composites A and B. Both composites exhibited characteristic peaks related to chromium ferrite.

Preparation and Characterization of the CrFeO3-CNT Adsorbent
A multi-walled fullerene carbon nanotube (CNT) was used for the preparation of the adsorbent. Its SEM micrographs are presented in Figure 7. The SEM micrographs visualize the nanotubes, and EDX analysis showed that this material is 100% pure carbon. Figure 8 presents the IR spectra of this CNT material along with the synthesized CrFeO3-CNT adsorbent material. No clear bands were observed in any area of the IR spectrum of the fullerene CNT. The IR spectrum of the synthesized CrFeO3-CNT adsorbent material showed only one clear, broad absorption band with medium intensity resonating at 505 cm −1 , which can be attributed to the ν(M-O) vibrations (M: Fe or Cr). The CrFeO3-CNT adsorbent was generated by grinding the CrFeO3 composite with the fullerene CNT material at a 1:10 molar ratio in the presence of a few drops of methanol solvent as described in Figure  9. This process produced a homogenate, black, solid adsorbent material that has a magnetic property ( Figure 10). Information on the size, shape, surface morphology, topology, and elemental composition of the synthesized CrFeO3-CNT adsorbent was collected from its SEM micrographs shown in Figure 11. The SEM micrographs captured between 1000× and 2500× magnification revealed that the adsorbent material consisted of stone-likeshaped particles. These stones have different irregular shapes, sizes, and features. The surface of the particles is rough, and some of the particles are clumped into large agglomerates. EDX analysis evidenced the presence of iron, chromium, oxygen, and carbon ele-

Preparation and Characterization of the CrFeO 3 -CNT Adsorbent
A multi-walled fullerene carbon nanotube (CNT) was used for the preparation of the adsorbent. Its SEM micrographs are presented in Figure 7. The SEM micrographs visualize the nanotubes, and EDX analysis showed that this material is 100% pure carbon. Figure 8 presents the IR spectra of this CNT material along with the synthesized CrFeO 3 -CNT adsorbent material. No clear bands were observed in any area of the IR spectrum of the fullerene CNT. The IR spectrum of the synthesized CrFeO 3 -CNT adsorbent material showed only one clear, broad absorption band with medium intensity resonating at 505 cm −1 , which can be attributed to the ν(M-O) vibrations (M: Fe or Cr). The CrFeO 3 -CNT adsorbent was generated by grinding the CrFeO 3 composite with the fullerene CNT material at a 1:10 molar ratio in the presence of a few drops of methanol solvent as described in Figure 9. This process produced a homogenate, black, solid adsorbent material that has a magnetic property ( Figure 10). Information on the size, shape, surface morphology, topology, and elemental composition of the synthesized CrFeO 3 -CNT adsorbent was collected from its SEM micrographs shown in Figure 11. The SEM micrographs captured between 1000× and 2500× magnification revealed that the adsorbent material consisted of stone-like-shaped particles. These stones have different irregular shapes, sizes, and features. The surface of the particles is rough, and some of the particles are clumped into large agglomerates. EDX analysis evidenced the presence of iron, chromium, oxygen, and carbon elements in the adsorbent material.          Figure 11. SEM micrographs of the synthesized CrFeO3-CNT adsorbent material.

Investigated Dyes
The adsorption performance of the synthesized CrFeO3-CNT adsorbent was inves gated with two model dyes, Methyl violet 2B (MV) and Azocarmine G2 (AC). An aqueo solution of MV at pH = 7 has a violet color, whereas an aqueous solution of AC at pH has a pink color. Figure 12 shows the UV-visible spectra of MV (1 mg in 100 mL of deio ized water) and AC (10 mg in 100 mL of deionized water). Both dyes absorb across a wi range from 200 nm to 650 nm. The MV dye displayed two absorption bands: (i) a narro weak band at 302 nm and (ii) a strong and broadband from 460 nm to 635 nm (~175 nm width). This wideband has a λmax at 588 nm. The UV-visible spectrum of AC dye w characterized by two absorption bands: (i) a medium-intensity band that had two hea at 293 and 334 nm, where the two heads had approximately the same intensity, and (ii strong, broad absorption band that appeared at a much wider wavelength region. Th wideband ranged from 460 nm to 588 nm and had two heads at 516 and 550 nm. T intensity of the head that appeared at 516 nm was a little higher than that at 550 nm.

Investigated Dyes
The adsorption performance of the synthesized CrFeO 3 -CNT adsorbent was investigated with two model dyes, Methyl violet 2B (MV) and Azocarmine G2 (AC). An aqueous solution of MV at pH = 7 has a violet color, whereas an aqueous solution of AC at pH = 7 has a pink color. Figure 12 shows the UV-visible spectra of MV (1 mg in 100 mL of deionized water) and AC (10 mg in 100 mL of deionized water). Both dyes absorb across a wide range from 200 nm to 650 nm. The MV dye displayed two absorption bands: (i) a narrow, weak band at 302 nm and (ii) a strong and broadband from 460 nm to 635 nm (~175 nm of width). This wideband has a λ max at 588 nm. The UV-visible spectrum of AC dye was characterized by two absorption bands: (i) a medium-intensity band that had two heads at 293 and 334 nm, where the two heads had approximately the same intensity, and (ii) a strong, broad absorption band that appeared at a much wider wavelength region. This wideband ranged from 460 nm to 588 nm and had two heads at 516 and 550 nm. The intensity of the head that appeared at 516 nm was a little higher than that at 550 nm.
Crystals 2021, 11, x FOR PEER REVIEW 12 of Figure 11. SEM micrographs of the synthesized CrFeO3-CNT adsorbent material.

Investigated Dyes
The adsorption performance of the synthesized CrFeO3-CNT adsorbent was inves gated with two model dyes, Methyl violet 2B (MV) and Azocarmine G2 (AC). An aqueo solution of MV at pH = 7 has a violet color, whereas an aqueous solution of AC at pH = has a pink color. Figure 12 shows the UV-visible spectra of MV (1 mg in 100 mL of deio ized water) and AC (10 mg in 100 mL of deionized water). Both dyes absorb across a wid range from 200 nm to 650 nm. The MV dye displayed two absorption bands: (i) a narrow weak band at 302 nm and (ii) a strong and broadband from 460 nm to 635 nm (~175 nm width). This wideband has a λmax at 588 nm. The UV-visible spectrum of AC dye w characterized by two absorption bands: (i) a medium-intensity band that had two hea at 293 and 334 nm, where the two heads had approximately the same intensity, and (ii strong, broad absorption band that appeared at a much wider wavelength region. Th wideband ranged from 460 nm to 588 nm and had two heads at 516 and 550 nm. T intensity of the head that appeared at 516 nm was a little higher than that at 550 nm.

Adsorption of MV
An aqueous solution of MV dye was prepared (1 mg in 100 mL of deionized water), then 40 mg of the fullerene CNT was added. The mixture was mechanically shaken at room temperature. Aliquots (5 mL) were taken every 2 min (2, 4, 6, 8, 10 min) and centrifuged for 10 min to remove the CNT material. The absorbance of the pure aliquot was measured spectrophotometrically, and the decrease of the intensity of the band at 588 nm was used as a measure of decolorization degree. Figure 13 illustrates the removal efficiency (%) of MV using 40 mg of the fullerene CNT at different contact times. This figure indicates that the fullerene CNT adsorbed around 96% of the dye content in 10 min. The test was run using 40 mg of the synthesized CrFeO 3 -CNT adsorbent, which reached a decolorization degree of 98% after just 7 min, as indicated in Figure 14. This outcome suggests that crushing CrFeO 3 composite with fullerene CNT improves the adsorption performance of the free fullerene CNT towards MV dye by 30%.

Adsorption of MV
An aqueous solution of MV dye was prepared (1 mg in 100 mL of deionized water), then 40 mg of the fullerene CNT was added. The mixture was mechanically shaken at room temperature. Aliquots (5 mL) were taken every 2 min (2, 4, 6, 8, 10 min) and centrifuged for 10 min to remove the CNT material. The absorbance of the pure aliquot was measured spectrophotometrically, and the decrease of the intensity of the band at 588 nm was used as a measure of decolorization degree. Figure 13 illustrates the removal efficiency (%) of MV using 40 mg of the fullerene CNT at different contact times. This figure indicates that the fullerene CNT adsorbed around 96% of the dye content in 10 min. The test was run using 40 mg of the synthesized CrFeO3-CNT adsorbent, which reached a decolorization degree of 98% after just 7 min, as indicated in Figure 14. This outcome suggests that crushing CrFeO3 composite with fullerene CNT improves the adsorption performance of the free fullerene CNT towards MV dye by 30%.

Adsorption of AC
An aqueous solution of AC dye was prepared (10 mg in 100 mL of deionized water), then 40 mg of the fullerene CNT was added. The mixture was mechanically shaken at room temperature. Aliquots (5 mL) were taken every 2 min and centrifuged for 10 min to remove the CNT material. The absorbance of the pure aliquot was measured spectrophotometrically, and the decrease in the intensity of the band at 516 nm was used as a measure of decolorization degree. The removal efficiency (%) of AC using 40 mg of the fullerene CNT at different contact times is shown in Figure 15 and using 40 mg of the synthesized CrFeO 3 -CNT adsorbent is shown in Figure 16. These figures indicate that the fullerene CNT alone adsorbed around 96% of the AC dye content in 12 min, whereas the CrFeO 3 -CNT adsorbent adsorbed around 95.5% of the AC content in just 8 min. This suggested that grinding CrFeO 3 composite with the fullerene CNT increases the adsorption performance of the free fullerene CNT towards AC dye by 33.3%.

Adsorption Mechanisms, Reusability, and Regeneration
Fullerenes have surface defects, a high surface-to-volume ratio, and high electron affinity. Fullerene CNT can adsorb organic dyes into the spaces/defects between the carbon nanoclusters via surface complexation formation, film diffusion, intra-particle diffusion, and pore diffusion [29][30][31]. During the adsorption process, organic dyes can enter the

Adsorption Mechanisms, Reusability, and Regeneration
Fullerenes have surface defects, a high surface-to-volume ratio, and high electron affinity. Fullerene CNT can adsorb organic dyes into the spaces/defects between the carbon

Adsorption Mechanisms, Reusability, and Regeneration
Fullerenes have surface defects, a high surface-to-volume ratio, and high electron affinity. Fullerene CNT can adsorb organic dyes into the spaces/defects between the carbon nanoclusters via surface complexation formation, film diffusion, intra-particle diffusion, and pore diffusion [29][30][31]. During the adsorption process, organic dyes can enter the mesoporous spaces of carbon nanoclusters (physisorption). The adsorption performance of the free fullerene CNT towards MV and AC dyes improved by 30% and 33.3%, respectively, after grinding the fullerene CNT with the CrFeO 3 composite, probably due to the physicochemical bond between dyes and the CrFeO 3 composite. The reusability of the synthesized CrFeO 3 -CNT adsorbent was investigated by determining its desorption efficiency. After the adsorption experiment, the CrFeO 3 -CNT adsorbent was filtrated off and washed with deionized water. Deionized water, 0.1 N H 2 SO 4 , 0.1 N HCl, 0.1 N HNO 3 , 0.1 N KOH, and 0.1 N EDTA were used as eluting agents to elute adsorbed dyes from the CrFeO 3 -CNT adsorbent. The most efficient desorbing solution to recover MV and AC dyes from the CrFeO 3 -CNT adsorbent was HNO 3 (~93%). Reusability of the CrFeO 3 -CNT adsorbent was determined by running and adsorption-desorption experiment for several cycles. We found that the CrFeO 3 -CNT adsorbent can be reused at least 12 times in adsorption-desorption cycles.

Conclusions
With the fast development of our society, a large amount of wastewater polluted by heavy-metal ions and organic pollutants, such as dyes and others, are released from papermaking, textiles, leather, and paints, and other industries, causing serious environmental problems to animals, plants, and human health. Dealing with these environmental problems, not by methods that release more pollutants but by the use of environmentally friendly methods, is urgent and has attracted increasing attention worldwide. In this work, we aimed to improve the adsorption performance of a fullerene carbon nanotube (CNT) towards organic dyes by combining it with a CrFeO 3 composite by the grinding process. The synthesized CrFeO 3 -CNT adsorbent was physicochemically (elemental analysis, SEM/EDX analysis, and FT-IR spectroscopy) characterized to describe its composition, texture, and surface morphology. Next, the batch technique was conducted to assess the adsorption performance of the synthesized CrFeO 3 -CNT adsorbent in comparison with the multi-walled fullerene CNT material alone. Outcomes indicated that grinding a CrFeO 3 composite with a multi-walled fullerene CNT to prepare a CrFeO 3 -CNT adsorbent increases the adsorption properties of the free fullerene CNT towards MV dye by 30% and towards AC dye by 33.3%. Combining a CrFeO 3 composite with a fullerene CNT not only improves the adsorption properties of the free fullerene CNT but also gives the fullerene CNT material magnetic properties, making it easily separable from the solution via an external magnetic field.