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Abstract

In this study, a new synthesized magnetic nanocomposite was used as an adsorbent for the removal of toxic cationic dye methylene blue (MB). The adsorbent incorporated three components to produce a nanocomposite named Fe₃O₄/GO/natural clay. The characterization techniques were used to study the physical and chemical properties of nanocomposite and approve the successful synthesis. XRD of the Fe₃O₄/GO/natural clay nanocomposite indicated the crystalline structure with the appearance of characteristic peaks of each component. Also, the FT-IR of Fe₃O₄/GO/clay nanocomposite showed a significant change in the peak intensity of natural clay after the addition of GO and magnetic NPs which attributed to the successful interaction between GO and magnetic NPs with the functional groups on the surface of clay. The TEM and SEM images of the nanocomposite showed that the magnetic NPs, clay, and GO excellently interacted in one appropriate structure with the deposition of clay and magnetic NPs on the GO surface which serves as a matrix holding them. The MB adsorption system was optimized by varying several parameters, including pH, adsorbent dosage, contact time, starting concentration, ionic strength, and competing ions. Furthermore, the adsorption isotherm showed that the highest adsorption capacities of 999.34 and 1108.54 mg/g for the Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay nanocomposites, respectively, followed the Langmuir isotherm. Additionally, the kinetic analysis showed that a pseudo-second-order model better explained the adsorption on both nanocomposites. The nanocomposite can be reused for multiple cycles with a little drop in its initial performance, according to reusability research conducted up to six times using ethyl alcohol as the eluent. As a result, this reasonably priced adsorbent might be employed as a beneficial and effective dye adsorbent for the removal of MB from water while maintaining the safety of the water quality.

Keywords

adsorption clay graphene oxide magnetic nanoparticles nanocomposites water treatment

Introduction

Environmental contamination is one of the most challenging issues facing the modern world. The most dangerous environmental contamination for humans is water pollution (Alsaiari et al., 2023). Water pollution results from the release of dyes, heavy metals, and pharmaceuticals to water sources. Dyes are the most worrisome of all the pollutants due to its toxic, carcinogenic, teratogenic or mutagenic nature (Deb et al., 2021; Saha et al., 2022). The rapid industrialization over the past few years has led to an increase in dye-containing industrial effluents. The plastics, pulp, paper, food processing, cosmetics, leather, and textile industries are just a few of the areas where organic dyes can be found in water streams (Hanafi and Sapawe, 2020; Samsami et al., 2020). The most popular dye used in these applications is methylene blue (MB). Many health issues, including vomiting, nausea, stupor, tissue necrosis, jaundice, retching, and eye burns have been linked to it (Thakur et al., 2016). In addition to health risks, its water deposit may prevent sunlight from penetrating, preventing photosynthesis from occurring, disquieting re-oxygenation capability, and creates an anaerobic condition that will stop the growth of plants (Deb et al., 2021; Surip et al., 2020). Treatment of dye wastewater is therefore quite important today. Due to the planar and complicated structure of this dye, it is difficult to remove it from water (Liu et al., 2018). Many methods, such as coagulation (Hadadi et al., 2023), adsorption (Kim et al., 2023), advanced oxidation (Mohod et al., 2023), photocatalytic remediation (Ma et al., 2024), and membrane separation (Li et al., 2023), are commonly used to remove pigments from wastewater. One of the most effective contemporary techniques for treating wastewater is adsorption, which is employed by the industry to lessen hazardous organic and inorganic components in wastewater. This is because of its various benefits, which include high removal efficiency, low running costs, high regeneration, and ease of usage (saad Algarni and Al-Mohaimeed, 2022). Several materials as adsorbents, such as clays (Amari et al., 2021; El Haouti et al., 2019), metal oxides (Noreen et al., 2020), activated carbons (Jawad et al., 2021), polymers (Stejskal, 2020), and their composites, have been used to adsorb dyes from water. The materials indicated above are used to remove colorants from wastewater; however, they have significant drawbacks, including a low adsorption capacity, non-biodegradability, high operational cost, poor recyclability, and a lack of specificity. The creation of optimal adsorbent materials that are efficient, affordable, accessible, and ecologically benign has therefore received a lot of attention.

Recent reports from several researchers suggest that multicomponent systems have unique opportunities for a variety of applications. Much effort has been paid to the creation of hybrid systems that incorporate nanoparticles (NPs). For example, Koochakzadeh et al. fabricated a hybrid for the improved adsorption of cationic dyes crystal violet, malachite green, and methylene blue (Koochakzadeh et al., 2023). This was attributed to the exceptional properties of NPs (Alsaiari et al., 2021; Singh et al., 2020). The metal oxide nanoparticles, which also act as fillers in the composite matrix, enhance the mechanical, swelling, thermal characteristics, and adsorption capacities of the nanocomposites (Makhado et al., 2019). The magnetic iron oxide (Fe₃O₄) NPs are among the metal oxide nanoparticles that have garnered the most study interest in a variety of applications. Fe₃O₄ NPs are regarded as good adsorbents in this context because of their exceptionally high surface-to-volume ratio, paramagnetic, and low toxicity features (Shahidi, 2021). Also, an external magnetic field could quickly separate them from the solution and give them an unparalleled advantage. For instance, green-synthesized Fe₃O₄ nanoparticles using the extract of Prosopis farcta were efficiently used for the removal of methylene blue dye (Mohammadpour et al., 2023). In a similar study, Cordia myxa leaf extracts were used to synthesize Fe₃O₄ nanoparticles for the removal of methylene blue as a model of organic pollutant (Ghoohestani et al., 2024).

However, they displayed low stability in acidic environments and had mediocre selectivity and adsorption capacities. As a result, scientists tried to increase the stability and adsorption capability of Fe₃O₄ NPs by functionalizing them with carbonaceous substances like GO. Compared to Fe₃O₄ NPs, these functionalized magnetic nanoparticles were found to be more affordable, chemically stable, and environmentally benign. Graphite is an allotropic form of carbon in which Van der Waals forces bind different layers of carbons in a systematic hexagonal shape. Graphene is the result of separating a single hexagonal layer from graphite. Graphite can be transformed into graphene oxide, which is graphene's functionalized state. The ability of graphene and graphene oxide to effectively adsorb dyes, arsenic, fluoride, and heavy metals (Ahmad et al., 2020; Liu et al., 2019; Sadeghi et al., 2020; Shang et al., 2019; Velusamy et al., 2021) from water has recently come to light. This is attributed to its high adsorption capacity and specific chemical structure. As graphene oxide has a wide variety of oxygen groups (hydroxyl and epoxy) at its surface and carbonyl and carboxyl groups at its edges (Nebol'Sin et al., 2020), where oxygen groups contribute hydrophilic characteristics and negative charge densities at the surface, it is particularly effective at capturing and eliminating contaminants (Peng et al., 2016). For instance, Fe₃O₄/GO was synthesized and investigated for effective removal of methyl orange and methylene blue with both adsorption and photocatalytic performance (Silva et al., 2023). This enhanced behavior was attributed to the functional groups on the GO surface as well as the porous structure and high surface area. Also, natural materials such as natural clay have been applied for water treatment from metals and dyes as it does not require any pre-treatment processes, and do not cause secondary pollution, they are abundantly available, and more economical and eco-friendly (Uddin, 2017). Due to their significant ion exchange capacity and propensity to form strong complexes with metal ions and dyes that prevent their release into the ecosystem, clay minerals are increasingly being used to remove water contaminants by adsorption. So, clay minerals have been used for the adsorption of many pollutants such as Ni(II), Cu(II), Cd(II), and Pb(II) (Anna et al., 2015; Kostenko et al., 2019; Li et al., 2020; Taha et al., 2016). Also, natural clay minerals were used for the removal of dyes. For instance, Natural Muscovite Clay was used to adsorb Methylene Blue (Amrhar et al., 2023), natural bentonite clay for the removal of Methylene Blue (Jawad et al., 2023), modified and unmodified montmorillonite for the removal of Methylene Blue (Zhang et al., 2023), natural and cationic surfactant modified pumice for Congo red removal (Shayesteh et al., 2016), natural and cationic surfactant modified clinoptilolite for the removal of Congo red, and kaolinite clay Basic Yellow 28 and Malachite Green (Nodehi et al., 2020).

Keeping in mind the above fact, the present research is formulated in such a way that magnetic Fe₃O₄ NPs, GO, and natural clay can be combined in one nanocomposite and investigated as an adsorbent for capturing the organic dye MB. Based on the adsorption behavior of each component of the ternary composite, the three components were combined to benefit from their properties to achieve enhanced adsorption capacity. Similar adsorbents were used for effective removal of pollutants from water. For example, Fe₃O₄/GO/clay composite was used for the removal of Cr (VI) ions from aqueous media (Esmaeili and Tamjidi, 2020). This work also intends to promote the use of an inexpensive Fe₃O₄/GO/natural clay nanocomposite for MB removal from water. FT-IR spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Raman spectra were among the methods used to characterize the produced nanocomposites. A thorough investigation was conducted into the suitability of the as-prepared nanoparticles as adsorbents for the removal of MB from water. Additionally, the kinetics and adsorption isotherms of the process were examined. Additionally, the reusability and regeneration study was used. The novelty of this clay nanocomposite adsorbent includes innovative methods for utilizing natural clay to synergistically synthesize, modify, and reinforce conventional adsorbent matrices, as well as potential advantages for environmental applications.

Experimental

Chemicals

Hydrogen peroxide (H₂O₂, 30%), sulfuric acid (H₂SO₄, 98%), sodium nitrate, potassium permanganate (KMnO₄, 99%), graphite, and methylene blue (MB, purity of 98%) dye were obtained from Merck Co. (Germany). Sodium hydroxide, hydrochloric acid, and phosphoric acid were purchased from El-Gomhouria Co., Egypt. Natural clay was collected from the environment, Mansoura city, Egypt. All reagents were of analytical grade and used as received without any purifications. The water used in the experimental part was deionized water. A definite amount of MB dye was dissolved in water to prepare a stock solution of concentration 1000 mg/L for further dilutions to be used in adsorption experiments.

Synthesis of graphene oxide

The first step to achieve the nanocomposite Fe₃O₄/GO/natural clay is the synthesis of GO. We weighed 18.0 g of potassium permanganate and 3.0 g of graphite and dissolved in a mixture of H₃PO₄ and H₂SO₄ with volumes of 40 mL and 360 mL, respectively. The mixture was stirred for 15 h at 55°C. At room temperature, the mixture was left to cool down. After that, hydrogen peroxide (H₂O₂, 30%) was added to the mixture slowly until the color of the solution was changed to light brown from black. Then, the solution was centrifuged and the precipitate was filtered and collected. The collected precipitate was washed several times with deionized water until the pH value reached neutral. Finally, the neutralized GO was dried for 1 day at 75°C in a vacuum oven (BOV-20, Being Instrument, USA).

Synthesis of nanocomposite

To achieve the nanocomposite Fe₃O₄/GO/natural clay, 100 mL of water was used to dissolve 0.50 g of GO and sonicated using an ultrasonic homogenizer (VCX 750 W; Sonics & Materials, Inc., USA) for half an hour. After that, the GO suspension was provided with 1.0 g of natural clay, and the sonication was continued for an additional 20.0 min. Then, the clay/GO mixture was provided with a previously prepared aqueous mixture of Fe(III) and Fe(II) (2:1 ratio) under stirring at 90°C for 20 min. The NaOH solution (4.0 M) was added to the previous solution drop by drop and the stirring was continued for up to 1 h under reflux conditions. The manufactured nanocomposite was removed from the experimental solution using an external magnet, and it was repeatedly rinsed with water until the supernatant's pH reached neutral. Ultimately, the produced nanocomposite was dried in an oven set to 100°C for one day. The nanocomposite's synthesis processes are depicted in Scheme 1.

Scheme 1.

The synthesis procedures of the Fe₃O₄/GO/natural clay nanocomposite.

The characterization techniques

The surface and characteristics properties of the as-prepared materials have been characterized using different techniques. Using Ni filtered CuK_α radiation at a wavelength of (λ = 1.5406 Å), XRD (PAnalytical Xpert PRO-diffractometer) was used to examine the structure of synthesized GO, Fe₃O₄ NPs, and Fe₃O₄/GO/natural clay. Morphology was analyzed using SEM (JSM7500F, JEOL microscope, Tokyo, Japan) to investigate the surface and its changes. The functional groups present in the synthesized materials were investigated using FT-IR (Broker Victor 22) in the wavenumber range 400 to 4500 cm⁻¹. Using a vibrating sample magnetometer (VSM), the magnetic characteristics of the produced Fe₃O₄ NPs and Fe₃O₄/GO/natural clay were ascertained. To study the internal morphology of the as-prepared materials, TEM (LEO 906) analysis was used at 80 kV. Raman spectra have been used to investigate of the successful synthesis of as-described materials better.

Adsorption studies

The as-prepared materials as adsorbents were investigated for the removal of MB dye in batch adsorption experiments. Briefly, MB dye aqueous solution (30.0 mL) was added to the optimum dosage of adsorbent material in an Erlenmeyer conical flask with a Teflon-lined screw cap. Adsorbent dosages ranging from 0.33 to 1.66 g/L were used to test the adsorption capacities to examine the effects of adsorbent dosage. The adsorption capacities were examined at various pH values ranging from 3 to 10 to investigate the impact of pH. The pH values of the solution were adjusted using 0.1 M of HCl and NaOH solutions. To study the effect of dye initial concentration, the adsorption capacities were investigated at different dye concentrations varied from 200 mg/L to 350 mg/L. The adsorption capacities at various durations ranging from 0 to 85 min were examined to determine the impact of contact time. In all adsorption experiments, the flasks were agitated at 160 rpm in a thermostatic shaker until they reached the equilibrium. After each adsorption experiment, the adsorbent was collected using an external magnet, and the solution was investigated for the existence of MB dye using a UV-vis spectrophotometer (λ_max= 661 nm). The adsorption capacity (q_e) and adsorption percentage were investigated using equations (1) and (2), respectively (Batubara et al., 2023). $q_{e} = (C_{o} - C_{e} / m) V$ (1) $E f f i c i e n c y (%) = (C_{o} - C_{e}) / C_{o} \times 100$ (2)Where C_e is the concentration at equilibrium (mg/L), C_o is the starting concentration of dye (mg/L), q_e is the adsorption capacity of dye at equilibrium (mg/g), and V is the volume of dye solution (L). The reusability study was performed by collecting the adsorbent after the adsorption study using an external magnet. Then, the adsorbent was immersed in ethyl alcohol and agitated for 2 h to desorb the dye from the surface of the adsorbent. The adsorbent was then washed using water and dried in an oven to be used in the next cycle. This adsorption-desorption process was done up to four cycles. All adsorption experiments were performed in duplicate. One of the most crucial surface characteristics for materials that can predict the surface charge of the material at various pH levels is the point of zero charge (pH_zpc). In this study, the pH_zpc of the synthesized Fe₃O₄/GO/natural clay was determined using the pH drift method as described in the literature (Zhou et al., 2019).

Results and discussion

Materials characterization

Different characterization techniques were used to characterize and approve the successful synthesis of materials Fe₃O₄/GO/natural clay, Fe₃O₄ NPs, and GO. To study the crystallinity of synthesized materials, XRD analysis was done to determine the structures of materials in scan range from 5° to 80° as shown in Figure 1(a). According to Figure 1(a), the structure of Fe₃O₄/GO/natural clay, Fe₃O₄ NPs, and GO is crystalline as characteristic peaks observed with different intensities in the XRD patterns of materials indicating its crystalline phases. XRD of GO showed the appearance of its characteristic peak at 2θ=10° which corresponds to the plane (001) (Yuan et al., 2011) indicating the successful synthesis of GO from graphite. Also, the XRD of synthesized Fe₃O₄ NPs showed the appearance of characteristic peaks at 2θ = 18.30°, 30.15°, 35.44°, 43.12°, 53.48°, 57°, and 63° which are corresponding to the planes (111), (220), (311), (400), (442), (422), and (511), respectively indicating the successful synthesis of magnetic NPs with cubic reverse spinel structure (Esmaeili and Farrahi, 2016; Petit and Bandosz, 2011). XRD of natural clay showed the appearance of characteristic peaks with different intensities at 2θ = 23.25°, 25.66°, 31.25°, 37.29°, 45.68°, 50.27°, 61.44°, and 64.75 indicating the presence of montmorillonite, calcite, quartz, and muscovite in the natural clay (Foroutan et al., 2019). When the three components are merged in one nanocomposite (Fe₃O₄/GO/natural clay), XRD of the nanocomposite showed the appearance of characteristic peaks of each component. The peak at 2θ = 11.79° is corresponding to GO, the peaks at 2θ = 63.55°, 54.34°, and 35.98° are corresponding to magnetic NPs, and the other peaks are corresponding to the clay. XRD of the nanocomposite indicates the successful fabrication of the ternary composite in which there was an excellent combination between its components.

Figure 1.

The XRD (a), FT-IR (b), Raman spectra (c), magnetization curves (d), nitrogen adsorption-desorption isotherms (e), and the pore size distribution curves (f) of the synthesized Fe₃O₄/GO/natural clay, Fe₃O₄ NPs, and GO.

To determine the functional groups present in the synthesized materials, FT-IR analysis was performed as shown in Figure 1(b). According to Figure 1(b), IR spectrum of GO displayed the attendance of absorption peaks at wavenumber of 1045, 1119, 1389, 1028, 2927 cm⁻¹ which attributed to C-O, C = C, C–C, and C–H (in carboxyl and carbonyl groups) tensile vibrations (Chang et al., 2014; Tayyebi et al., 2015). Additionally, the FT-IR of natural clay showed the appearance of absorption peaks at 1441, 1018, 535, and 474 cm⁻¹ which was attributed to the vibrations of carbonates, Si–O, Si-O-Al, and Si-O-Mg (Eloussaief et al., 2011). All synthesized materials Fe₃O₄/GO/clay, Fe₃O₄/clay, GO, and natural clay have OH groups in their structure as H₂O, Si-OH, and Al-OH which vibrate and showed a broad absorption peak in the wavenumber range of 3390 to 3438 cm⁻¹ (Pavia et al., 2008; Taha et al., 2016). The FT-IR of Fe₃O₄/GO/clay nanocomposite showed a significant change in the peak intensity of natural clay after the addition of GO and magnetic NPs which attributed to the successful interaction between GO and magnetic NPs with the functional groups on the surface of clay. Moreover, the FT-IR of Fe₃O₄/GO/clay nanocomposite showed the appearance of an absorption peak at a wavenumber of 571 cm⁻¹ which attributed to the vibrations of the Fe–O bond indicating the successful incorporation of magnetic NPs in the nanocomposite (Shafiee et al., 2019). Also, the band surface measurement and band intensity of the as-prepared GO and Fe₃O₄/GO/clay nanocomposite were analyzed using Raman spectroscopy as shown in Figure 1(c). According to Figure 1(c), the D band and G band which are characteristic of GO appeared at 1600 and 1350 cm⁻¹ due to sp³ structural disorder of C-atom and sp² vibrations of C-atom in the hexagonal graphite arrangement, respectively (Lin et al., 2013). The Raman spectrum of the Fe₃O₄/GO/clay nanocomposite has the same D and G bands with a slight shift indicating the existence of GO in the ternary nanocomposite. It appears from the Raman spectra of the nanocomposite that the incorporation of clay, iron, and oxygen changes the peak intensity ratio (I_D/I_G) to 0.947 from 0.915 indicating the good assembly of three components in one composite. The magnetic behavior is the main advantage of the synthesized materials. So, the magnetic properties of the synthesized Fe₃O₄ NPs, Fe₃O₄/clay, and Fe₃O₄/GO/clay were analyzed using a vibrating sample magnetometer (VSM) as shown in Figure 1(d). According to Figure 1(d), Fe₃O₄ NPs have the highest magnetization saturation value equal to 84.5 emu g⁻¹. The addition of clay to the magnetic NPs caused a decrease in the magnetization saturation value to 56.345 emu g⁻¹. Also, the addition of clay and GO to the magnetic NPs caused a decrease in the magnetization saturation value to 39.761 emu g⁻¹. This decrease is attributed to many reasons including the incorporation of non-magnetic materials, oxygen content, and particle size. However the decrease in magnetization saturation value, the synthesized materials still have paramagnetic properties and this is clear from the magnetization saturation values and the S-shaped diagrams. This gives the advantage to the synthesized materials to be easily separated using an external magnet from aqueous solutions. N₂ adsorption-desorption was used to measure the produced materials’ surface area and pore characteristics. Mesopores were present in all the materials, as demonstrated by the characteristic IUPAC type-IV adsorption isotherms with a hysteresis loop that were displayed in Figure 1(e). Fe₃O₄/clay and Fe₃O₄/GO/clay have specific surface areas of 55.50 and 127.15 m²/g, respectively. Therefore, the dispersion of Fe₃O₄ nanoparticles was improved by including GO in the composite of Fe₃O₄/GO/clay. The enhanced surface area of Fe₃O₄/GO/clay could be because the functional groups on the GO sheets bonded the clay and magnetic nanoparticles, supporting the pores and increasing the surface area as a result. Figure 1(f) shows the pore structure and surface morphology of the synthetic Fe₃O₄/GO/clay, demonstrating the high porosity of the Fe₃O₄/GO/clay. Furthermore, the mesoporous nature of both adsorbents was shown by the measurements of the pore volume of the Fe₃O₄/clay and Fe₃O₄/GO/clay composite, which were 0.24 and 0.34 cm³/g, respectively, with an average pore size of 3.1–10.8 and 3.1–10.5 nm, respectively. The results also indicated that the synthesized composite had a higher surface area than the Fe₃O₄/clay combination, which was advantageous for the trapping of the target ions. The surface properties and morphology of the synthesized materials were investigated using TEM analysis as shown in Figure 2(a)–(c). TEM images of natural clay and GO (Figure 2(a) and (b)) showed that their layer morphology and surface area are excellent. Also, Figure 2(c) displayed the TEM picture of the composite (Fe₃O₄/GO/clay) in which Fe₃O₄ NPs are less than 50 nm and spherical. Compared with classical adsorbents, the spherical particles have additional advantages in transport and mass diffusion. Moreover, the TEM image of the nanocomposite showed that the magnetic NPs, clay, and GO excellently interacted in one appropriate structure. The findings also show that clay and magnetic NPs are deposited on the GO surface which serves as a matrix holding them in one nanocomposite.

Figure 2.

TEM images of GO (a), clay (b), and Fe₃O₄/GO/clay (c) and SEM images of GO (d), clay (e), and Fe₃O₄/GO/clay (f).

Also, SEM analysis was used to determine the particle size, surface properties, and morphology of the synthesized materials as shown in Figure 2(d)–(f). Figure 2(d) shows an SEM image of GO that appears to have a layered structure allowing it to act as a matrix holding magnetic NPs and clay as approved also from TEM results. The outcomes of the clay SEM image (Figure 2(e)) also demonstrated that natural clay has a porous surface with various holes and degrees of roughness, which can be useful in MB adsorption. Also, an SEM image of Fe₃O₄/GO/clay nanocomposite (Figure 2(f)) showed that the clay, GO, and magnetic NPs present in the composite structure contain particles and surfaces of various sizes. It may be said that the interaction of natural clay, GO, and magnetic NPs is excellent as shown from the SEM of nanocomposite.

Batch adsorption experiments

Effect of adsorbent dosage

Many factors affect the adsorption process. An important factor is the adsorbent dose. So, the effect of adsorbent dose on the adsorption of MB dye over the surface of Fe₃O₄/ natural clay and Fe₃O₄/GO/natural clay composites was investigated as shown in Figure 3(a). According to Figure 3(a), the effect of adsorbent dose was investigated using different amounts of adsorbents in the range of 0.33 to 1.66 g/L. For two adsorbents Fe₃O₄/ natural clay and Fe₃O₄/GO/natural clay, as the adsorbent dose increased, the adsorption capacities decreased. Because there are many active sites accessible at high adsorbent doses, the osmotic pressure causes equilibrium to be achieved and produces an agglomeration on the adsorbent's surface, which lengthens the diffusion channel and lowers the adsorption capacities.

Figure 3.

The effect of adsorbent dose (a), the effect of pH (b), and pH_zpc values (c) for the adsorption of MB dye on the surface of synthesized materials.

However, at a low adsorbent dose, a high adsorption capacity resulted from the active sites participating fully in the MB adsorption process. But, more adsorbents mean more binding sites and more sites for dye adsorption meaning a higher removal rate of dye from aqueous solution. Numerous earlier studies have noted this tendency of the adsorbent dosage impact toward the removal of MB from aqueous solution (Eltaweil et al., 2020; Zou et al., 2019). Also, it was observed from Figure 3(a) that the adsorption capacity of Fe₃O₄/GO/natural clay was higher than that of Fe₃O₄/ natural clay due to the presence of GO layers.

Effect of pH value

The pH level is a significant additional item influencing the adsorption process. During any adsorption process, the pH value manages the combination between the surface of the adsorbent and the adsorbate particles. So, the effect of pH on the removal of MB dye over the surface of Fe₃O₄/ natural clay and Fe₃O₄/GO/natural clay composites was investigated as shown in Figure 3(b). According to Figure 3(b), the effect of pH value was investigated using different values of initial pH in the range of 3 to 10 at the temperature of 25.0°C. According to Figure 3(b), as the pH value increased, the adsorption capacities were decreased for two adsorbents. The adsorption capabilities of Fe₃O₄/ natural clay and Fe₃O₄/GO/natural clay nanocomposite rose from 310 to 590 mg/g and 530 to 595 mg/g, respectively, when the pH of the solution was raised from 3 to 10. This behavior can be interpreted as at low pH values, the functional groups on the surface of adsorbents are positively charged (protonated) causing the repulsion between cationic dye (MB) and positively charged adsorbents. While the cationic MB dye molecules and the adsorbent are more likely to interact electrostatically when the pH is raised because the active sites become negatively charged. As a result, both nanocomposites had higher adsorption capacities at high pH values. Additionally, the adsorption equilibrium was obtained at a pH of 7.1 for Fe₃O₄/GO/natural clay nanocomposite and at 6.0 for Fe₃O₄/ natural clay nanocomposite. Subsequently, there was no increase in the adsorption of dye higher than these pH values. From the study of the pH effect, it is clear that the adsorbent surface charge at different pH values is important in the adsorption process. Thus, as seen in Figure 3(c), the adsorbent surface charge was ascertained by computing the pH_ZPC parameter by the charting of ΔpH as a function of initial pH. According to Figure 3(c), the pH_ZPC was 6.0 and 7.1 for Fe₃O₄/ natural clay and Fe₃O₄/GO/natural clay composites, respectively. At pH higher than these values, the surface charge of the adsorbent is positive. While at pH lower than these values, the surface charge of the adsorbent is negative. Across the investigated pH range, both adsorbents displayed a similar tendency. The pH_pzc of the Fe₃O₄/ natural clay nanocomposite was slightly lower than that of the Fe₃O₄/GO/natural clay nanocomposite, though. The pH_pzc values demonstrate how the surface change was modified by the addition of GO to the nanocomposite network. So, modifications to the adsorption pathways occur due to the change in pH throughout the adsorption process as implied from these pH_pzc results. The adsorption process was shown to be influenced by the strong electrostatic interaction between MB dye and synthetic adsorbents, which was taken into consideration.

Isotherm study

Initially, as indicated in Figure 4(a), the impact of the initial dye concentration on the adsorption process on the surface of materials was measured in the range of 200 to 350 mg/L. Figure 4(a) shows that for Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay nanocomposites, respectively, an increase in the starting dye concentration from 200 to 350 mg/L resulted in an increase in the adsorption capacity from 500 to 989 mg/g and 500 to 1034 mg/g. This behavior of initial concentration with the adsorption capacity indicates that the adsorption process is derived by mass diffusion (Mahdavinia et al., 2017). The experimental adsorption data was fitted using adsorption isotherm models including Freundlich, Langmuir, and Temkin models as shown in Figure 4(b)–(d), respectively.

Figure 4.

The effect of initial dye concentration (a), fitting to Freundlich model (b), fitting to Langmuir model (c), and fitting to Temkin model (d) for the adsorption of MB dye on the surface of synthesized materials.

Freundlich, Langmuir, and Temkin models can be represented according to equations (3)–(5) respectively (Abdel-Raouf et al., 2023; Batubara et al., 2023). $q_{e} = K_{f} C_{e}^{1 / n}$ (3) $q_{e} = \frac{q_{m} K_{L} C_{e}}{1 + K_{L} C_{e}}$ (4) $q_{e} = B_{t} \ln k_{t} + B_{t} \ln C_{e}$ (5)Where C_e, q_e, K_f, n, q_m, and K_L represent the concentration of dye at equilibrium (mg/L), amount of dye adsorbed per unit mass of adsorbent (mg/g), Freundlich constant, a measure of adsorption intensity, maximum adsorption capacity, and Langmuir constant, respectively. k_t (L/g) is the binding constant of the Temkin isotherm, and B_t (J mol^–1) is the temperature-dependent Temkin constant. Plotting q_e against ln C_e yielded the slope and intercept, which were used to determine the values of the B_t and k_t constants. Adsorption isotherm models must be used to estimate adsorbate-adsorbent interactions and the distribution of adsorbate molecules between adsorbent surface and aqueous phases in the adsorption process. Table 1 displays the parameters of the Freundlich, Langmuir, and Temkin models. The findings shown in Table 1 indicate that the Langmuir model provides a more accurate description of the MB dye adsorption on the surface of Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay nanocomposites. This is supported by the values of R² (correlation coefficient). The R² value is higher for the Langmuir model than that of the Freundlich model. Freundlich isotherm model suggests multi-layer adsorption while the Langmuir isotherm model suggests monolayer adsorption. The Temkin isotherm model explains the relationship between the surface coverage and the heat of adsorption. The Temkin parameters, B_t and k_t, stand for the Temkin isotherm equilibrium binding constant and the adsorption heat, respectively. It is postulated that the reduced interaction between the adsorbent and adsorbate caused the heat of adsorption to gradually decrease with covering. The Temkin model's lower R² values in comparison to the R² values of the Langmuir model indicate that the Temkin model is not appropriate for explaining the adsorption of MB dye onto Fe₃O₄/GO/natural clay adsorbent.

Table 1.

The adsorption isotherm and kinetic parameters for the adsorption of MB dye on the surface of the synthesized materials.

Model	Parameter	Fe₃O₄/natural clay	Fe₃O₄/GO/natural clay
Langmuir	q_m (mg/g)	999.34	1108.54
	K_L (L/mg)	0.285	0.511
	R ²	0.999	0.977
Freundlich	N	3.219	7.230
	K_F (mg/g)(L/mg)^1/n	358.52	613.51
	R ²	0.974	0.637
Temkin	B_t (J mol⁻¹)	416.7	120.5
	K_t (L/g)	3.028	461.5
	R ²	0.999	0.560
Pseudo 1^st order	K₁ (min^{− 1})	0.018	0.008
	q_e	3.065	8.650
	R ²	0.956	0.985
Pseudo 2^nd order	K₂ (g/mg.h)	5.433	0.335
	q_e	599.0	546.5
	R ²	0.999	0.998
intra-particle diffusion	K_p (mg g^–1 min^–1/2)	5.571	14.555
	R ²	0.539	0.763

So, the present results suggest the monolayer adsorption of MB dye on the surface of Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay nanocomposites as the experimental data best fit to Langmuir model. According to the Langmuir model, the maximum adsorption capacity toward the removal of MB dye is 999.34 and 1108.54 mg/g for Fe₃O₄/natural clay and Fe₃O₄/GO/natural clay nanocomposites, respectively. The fact that the MB dye adsorbs in a monolayer on the surfaces of both adsorbents suggests that the adsorption took place over energetically comparable adsorption active sites in a homogeneous monolayer. The adsorption capacity of MB is greater for Fe₃O₄/GO/natural clay than that of Fe₃O₄/GO/natural clay due to the presence of GO layers which give the adsorbent additional adsorption efficiency. As shown in Table 2, the maximum adsorption capacities of both adsorbents in the current investigation for the removal of MB dye were contrasted with adsorbents that had previously been reported. Table 2 shows that compared to previously published adsorbents, the Fe₃O₄/natural clay and Fe₃O₄/GO/natural clay adsorbents had better adsorption capacities for the removal of MB dye from water. This indicates that the prepared materials as adsorbents are promising to be applied for water treatment. Additionally, the existence of magnetic NPs in the synthesized materials allows their magnetic separation from the adsorption medium, and this ease of separation provides these adsorbents with additional advantages. The high adsorption capacity of Fe₃O₄/GO/natural clay could be attributed to the different types of functional groups on its surface providing a variety of interactions between dye and adsorbent. The suggested adsorption mechanism of MB dye on the surface of Fe₃O₄/GO/natural clay is shown in Scheme 2. This complex adsorption mechanism can be attributed to clay and GO which hold abundant functional groups on its surfaces. According to Scheme 2, different types of interactions can exist between MB dye and adsorbent which include electrostatic attraction, ion exchange, surface complexation, π-π interaction, and hydrogen bonding. The crucial adsorption process may be the electrostatic interactions between the negative charge on the oxygen group and the positive charge on the nitrogen group of MB. Additionally, through the π-π interaction, the localized π electrons in the conjugated aromatic rings of the composite can interact with the C–C double bond of MB. A key factor in the adsorption of MB on the surface of the adsorbent is the hydrogen bonding interactions between the hydrogen-containing groups of MB and the amine or oxygen-containing groups of the composite. Also, the cations on the surface of clay like Na⁺ and K⁺ can be exchanged with MB dye providing an ion-exchange mechanism. Surface complexation exists between dye and surface functional groups (-COOH, -OH, and -NH₂) on adsorbents surface.

Scheme 2.

Different adsorption mechanisms of Fe₃O₄/GO/natural clay for MB dye.

Table 2.

Comparison of Fe₃O₄/GO/natural clay nanocomposite with previously reported adsorbents for the adsorption of MB dye.

Adsorbent	pH value	Contact time (minutes)	Removal capacity (mg/g)	Ref.
Fe₃O₄/GO/natural clay	7.1	60	1108.54	This study
Single-walled carbon nanotubes-amine	9.0	90	136	(Maazinejad et al., 2020)
Zeolitic imidazolate framework	-	-	10	(Wu et al., 2021)
Hydrolyzed polyacrylamide modified diatomite waste	9.0	120	37.0	(Ma et al., 2020)
Iron-based metal–organic framework	9.0	24 h	8.7	(Arora et al., 2019)
Reed biochar supported hydroxyapatite	8.0	10 h	21.1	(Li et al., 2018)
Nano-Fe₃O₄/baker's yeast composite	6.0	2 h	141.75	(Du et al., 2017)
APTES-Fe₃O₄/bentonite	8.0	120	91.83	(Lou et al., 2017)
EDTA-modified bentonite	5.5	45	160	(De Castro et al., 2018)
Graphene oxide	6.0	1 h	714	(Yang et al., 2011)

Adsorption kinetics

Firstly, the effect of contact time as an important factor on the adsorption process was determined as shown in Figure 5(a). For both adsorbents Fe₃O₄/natural clay and Fe₃O₄/GO/natural clay nanocomposites, the increased contact time caused an increase of the adsorption capacity. This behavior could be interpreted as the positively charged dye having more time to interact with the negatively charged surface of the adsorbent. According to Figure 5(a), the equilibrium was reached after 60 and 25 min for Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay, respectively.

Figure 5.

The effect of contact time (a), fitting experimental data to pseudo-first-order model (b), pseudo-second-order model (c), and intra-particle diffusion (d) for the adsorption of MB dye on Fe₃O₄/clay and Fe₃O₄/GO/clay nanocomposites.

This indicates the fast adsorption process of the dye on the surface of adsorbents. Furthermore, no discernible increase in adsorption capacity was observed with increasing contact time and after reaching the equilibrium, which could be due to the saturation of active sites with molecules of adsorbates (Alsaiari et al., 2022). Kinetic models pseudo-first-order and pseudo-second-order were used to fit these experimental data. The pseudo-first-order, pseudo-second-order, and intra-particle diffusion models can be expressed according to equations (6)–(8) respectively (Abdel-Raouf et al., 2023). $\log (q_{e} - q_{t}) = l n q_{e} - K_{1} t$ (6) $\frac{t}{q_{t}} = \frac{1}{K_{2} q_{e}^{2}} + \frac{1}{q_{e}} t$ (7) $q_{t} = K_{p} t^{0.5} + C_{i}$ (8)where K₁ and K₂ are the rate constants of the two kinetic models, and q_t (mg/g) and q_e (mg/g) are the adsorption capacity at time t (min) and equilibrium, respectively. Plotting log (q_e−q_t) versus time (t) is how the experimental data is fitted to the pseudo-first-order model, as illustrated in Figure 5(b). Additionally, as illustrated in Figure 5(c), fitting the experimental data to the pseudo-second-order model is accomplished by graphing t/q_t against t. The thickness of the boundary layer is denoted by C_i, and the intra-particle diffusion constant is K_p (mg g^–1 min^–1/2). Calculating the slope and intercept of a linear relationship between q_t and t^1/2 as shown in Figure 5(d) allows one to determine the values of K_p and C_i, respectively. Table 1 displays the pseudo-first-order and pseudo-second-order models’ kinetic parameters. Noticeably, dye adsorption by Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay fitted well with two kinetic models achieved, although the highest correlation coefficients (R²) were achieved by the pseudo-second-order model (0.999). Thus, surface adsorption was the rate-limiting stage in the process by which both adsorbents absorbed the organic dye through chemical and physical interactions. This means that the dye adsorption was chemically affected by active sites and functional groups on the surface and physically affected by the porous structure. The intra-particle diffusion model was proposed because it can provide crucial information about the rate-limiting steps in the adsorption process, as first-order and pseudo-second-order models may not always be sufficient to identify the mechanism of dye diffusion and may not be able to indicate all the steps that may be involved in the adsorption process (i.e., external film diffusion, adsorption, and intra-particle diffusion). The fact that there are multiple slopes as shown in Figure 5(d) suggests that there are numerous phases involved in the dye adsorption process. The transit of the dyes in the bulk solution, their film diffusion at the adsorbent's boundary layer, their diffusion of dye molecules from the bulk solution to the adsorbent material's exterior, and their diffusion via the adsorbent's tiny pores are all included in these processes (Osman et al., 2024). The chemical binding of dyes with the adsorption sites occurs after these stages. At last, the maximum amount of adsorption was acquired and the adsorption equilibrium was reached. The adsorption of dyes from the polluted water onto the manufactured adsorbent was then shown to be regulated by surface chemisorptions that occurred at the generated material's boundary layers, as demonstrated by the results of the kinetic modeling.

Effect of ionic strength and competing ions

Industrial effluents usually contain salts which may be affecting the removal process. So, different concentrations of NaCl were used to investigate the effect of ionic strength on the removal of MB dye using the synthesized adsorbents as shown in Figure 6(a). As the ionic strength increased, the removal effectiveness of MB dye dropped steadily, as seen in Figure 6(a). This may be attributed to the influence of ionic strength on the activity coefficient of dye that hinders their movement to the surface of adsorbent. Also, the presence of sodium cations may compete with cationic dye on the active sites and reduce their removal. Moreover, the shielding effect resulted from the presence of salt that damaged the electrostatic interactions between the surface of the adsorbent and dye reducing the removal efficiency. So, the MB adsorption on the surface of Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay nanocomposites was found to be affected by the ionic strength. However, this effect on Fe₃O₄/natural clay is lower than Fe₃O₄/GO/natural clay due to the lower active sites.

Figure 6.

The effect of ionic strength (a) and the effect of competing cations (b) on the removal of MB dye on the surface of Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay nanocomposites.

Moreover, the wastewater holds different cations with the organic molecules. So, the selectivity of adsorbents toward MB dye in the presence of competing cations must be assessed. Different cations including Mn(II), Zn(II), Ba(II), Al(III), Mg(II), and Hg(II) were used to understand the selectivity of synthesized adsorbents toward the cationic MB dye. The cations have a concentration 2-fold higher than the MB dye. The selectivity of adsorbents toward MB dye was investigated in the presence of different cations as shown in Figure 6(b). According to Figure 6(b), Fe₃O₄/GO/natural clay nanocomposite showed high removal efficiency toward MB dye even in the presence of higher concentration metallic ions. The Fe₃O₄/GO/natural clay showed high selectivity toward MB dye which is indicated by high removal efficiency (over 90%) in the presence of different cations. So, the Fe₃O₄/GO/natural clay nanocomposite has shown specific selectivity to the MB dye molecules based on the stable complexation ability.

Reusability study

The reusability and stability of any material as an adsorbent play a significant role in its commercial applicability in water treatment (Katubi et al., 2021). So, the adsorption-desorption study was applied to investigate the reusability of synthesized materials for the adsorption of MB dye up to six successive cycles. Figure 7(a) showed the reusability of Fe₃O₄/GO/natural clay and Fe₃O₄ /natural clay for the removal of MB dye. According to Figure 7(a), both adsorbents showed high adsorption efficiency even after six successive cycles. However, there was a decrease in the adsorption efficiency with increasing the cycles. Fe₃O₄/GO/natural clay showed a drop in adsorption efficiency from 99.4% in the 1^st cycle to 97% in the last cycle. At the same time, Fe₃O₄ /natural clay dropped from 97% in the 1^st cycle to 93.7% in the last cycle. This decrease may result from the damage of some active sites due to the adsorbed MB molecules. Fe₃O₄/GO/natural clay showed higher adsorption efficiency after six cycles than Fe₃O₄/ natural clay indicating its higher stability. The reusability study indicates that Fe₃O₄/GO/natural clay and Fe₃O₄ /natural clay nanocomposites can be used commercially for the adsorption of MB dye from an aqueous solution. Moreover, the magnetic separation of synthesized nanocomposites increases its advantages to be used as adsorbents for commercial purposes. Additionally, the stability of the adsorbent after the adsorption process is very important and significant in its commercial use. Consequently, to ascertain the stability of the Fe₃O₄/GO/natural clay nanocomposite after water treatment, XRD was carried out, as indicated in Figure 7(b). There is no change in the XRD patterns after and before adsorption. The XRD results before and after adsorption when compared indicated the excellent stability of the synthesized adsorbent.

Figure 7.

Reusability study (a), the XRD of Fe₃O₄/GO/natural clay nanocomposite (b), and UV-vis spectra of water containing dye after and before treatment on the surface of Fe₃O₄/GO/natural clay and Fe₃O₄ /natural clay nanocomposites (c).

To confirm the efficiency of the synthesized adsorbents for the removal of MB dye from an aqueous solution, UV-vis spectra of water-containing dye after and before treatment on the surface of Fe₃O₄/GO/natural clay and Fe₃O₄ /natural clay nanocomposites were investigated as shown in Figure 7(c) after six cycles. According to Figure 7(c), MB dye showed the appearance of characteristic peaks at 663, 614, 293, and 247 nm (Malatji et al., 2020). After adsorption, these characteristic peaks were significantly decreased which indicated the successful removal of dye from water on the surface of Fe₃O₄/GO/natural clay and Fe₃O₄ /natural clay nanocomposites. Furthermore, the adsorption process is remarkable on the surfaces of both adsorbents, as evidenced by the absence of peak shift following adsorption. Additionally, the UV-vis spectra confirmed the stability of adsorbents for dye removal even after reusability for six cycles.

Adsorption of MB from real water samples by Fe₃O₄/Go/natural clay nanocomposite

The suggested adsorbent was further tested on different water samples from the Sea, tap water, and spring water (Mansoura, Egypt), and laundry wastewater in order to determine its practical application for dye removal of real samples. Following optimal conditions and after treatment by an adsorbent, specific quantities of MB (200, 250, and 300 mg L⁻¹) dyes were added to all samples. The amount of dye that remained was determined using UV-Vis spectrophotometry. Good adsorption efficiency for MB dye is demonstrated by the data shown in Table 3, which supports the viability of the suggested technique for removing these dyes from actual samples.

Table 3.

Adsorptive removal of MB dye from real water samples using Fe₃O₄/GO/natural clay.

Sample	MB Conc. (mg/L)	Removal efficiency (%)
Laundry wastewater	200	90.5
	250	87.35
	300	81.16
Spring water	200	98.15
	250	92.65
	300	90.78
Tap water	200	98.65
	250	97.70
	300	94.80
Sea water	200	99.21
	250	98.15
	300	96.25

Conclusion

In the present study, the magnetic nanocomposite Fe₃O₄/GO/natural clay was investigated for the adsorption of organic dye MB. The as-prepared nanocomposite was investigated for MB adsorption. According to the adsorption results, the higher adsorption efficiency of Fe₃O₄/GO/natural clay was achieved at a pH of 7.1, contact time of 60 min, adsorbent dose of 0.33 g, and initial concentration of 200 mg/L. The adsorption on Fe₃O₄/GO/natural clay and Fe₃O₄/natural clay nanocomposites followed the Langmuir isotherm, with maximum adsorption capacities of 1108.54 and 999.34 mg/g, respectively, according to the adsorption isotherm assessment of the experimental data. The adsorption on both nanocomposites was best described by the pseudo-second-order model. So, both adsorbents adsorbed the organic dye through chemical and physical interactions. Moreover, the Fe₃O₄/GO/natural clay as adsorbent showed high adsorption efficiency toward MB dye even in the presence of competing cations which attributed to complexation ability. The reusability of adsorbents for MB removal was investigated using ethyl alcohol as eluent indicating that the synthesized adsorbents were reversible and can be used for several cycles. The decolorization of MB in different real water samples using Fe₃O₄/GO/natural clay demonstrated that the presence of ions did not affect MB adsorption in actual water samples. Thus, the results indicated that the synthesized adsorbents were cost-effective and showed higher capacity compared with previously prepared adsorbents and can be used with high efficiency and selectivity for water treatment.

Footnotes

Acknowledgments

The authors extend their appreciation to the Deanship of Research and Graduated studies at King Khalid University for funding this work through Large Research Project under grant number RGP.2/521/45. This research was also funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R19),Princess Nourah bint Abdulrahman University,Riyadh,Saudi Arabia. Authors would like to thank Dr. Zaina Algarni for her support of this work.

Author contributions

“N.S.A. and M.A.I. for conceptualization;N.B.A. for methodology;M.A.I. for software;N.B.A. for validation;N.B.A. and M.A.I. for formal analysis;M.A.I. for investigation;N.B.A. for resources;N.B.A. for data curation;M.A.I. for writing—original draft preparation;M.A.I. for writing—review and editing;A.A. for visualization;M.A.T. for supervision;M.A.T. for project administration;N.S.A. for funding acquisition. Each author has reviewed the published version of the manuscript and given their approval.”

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This research was funded by the Deanship of Research and Graduated studies at King Khalid University under grant number RGP.2/521/45. Additionally,this research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R19),Princess Nourah bint Abdulrahman University,Riyadh,Saudi Arabia.

ORCID iD

Mohamed A Tahoon

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Synthesis,characterization,and application of novel reusable clay-based magnetic nanocomposite for the removal of organic dyes from water

Abstract

Keywords

Introduction

Experimental

Chemicals

Synthesis of graphene oxide

Synthesis of nanocomposite

The characterization techniques

Adsorption studies

Results and discussion

Materials characterization

Batch adsorption experiments

Effect of adsorbent dosage

Effect of pH value

Isotherm study

Adsorption kinetics

Effect of ionic strength and competing ions

Reusability study

Adsorption of MB from real water samples by Fe3O4/Go/natural clay nanocomposite

Conclusion

Footnotes

Acknowledgments

Author contributions

Declaration of conflicting interests

Funding

ORCID iD

References

Adsorption of MB from real water samples by Fe₃O₄/Go/natural clay nanocomposite