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Article

Synthesis of Manganese Ferrite/Graphene Oxide Magnetic Nanocomposite for Pollutants Removal from Water

by
Khadijah Mohammedsaleh M Katubi
1,
Norah Salem Alsaiari
1,*,
Fatimah Mohammed Alzahrani
1,*,
Saifeldin M. Siddeeg
2,3 and
Mohamed A. Tahoon
2,4
1
Chemistry Department, College of Science, Princess Nourah bint Abdulrahman University, Riyadh 11671, Saudi Arabia
2
Department of Chemistry, College of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
3
Chemistry and Nuclear Physics Institute, Atomic Energy Commission, P.O. Box 3001, Khartoum 11111, Sudan
4
Chemistry Department, Faculty of Science, Mansoura University, 35516 Mansoura, Egypt
*
Authors to whom correspondence should be addressed.
Processes 2021, 9(4), 589; https://0-doi-org.brum.beds.ac.uk/10.3390/pr9040589
Submission received: 8 March 2021 / Revised: 24 March 2021 / Accepted: 25 March 2021 / Published: 28 March 2021
(This article belongs to the Special Issue Application of Carbon-Based Nanocomposites in Bioengineering and Biomedicine)
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Abstract

:
These days, environmental pollution, notably water pollution, has increasingly caused severe human health problems. The major water pollutants are heavy metals. MnFe2O4/GO nanocomposite was prepared in the current work via in situ method and tested to remove lead ion Pb2+ and neutral red (NR) dye from water. The prepared nanocomposite was characterized using different techniques, including X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectra, and vibrating sample magnetometer. The prepared nanocomposite showed high adsorption capacity toward Pb2+ and NR dye removal according to Langmuir fitting indicating the monolayer homogeneous adsorption of pollutants over the adsorbent surface and can be separated easily with an external magnet. The effect of different factors, including contact time, pH, initial concentration, and adsorbent dose on the adsorption, were also studied. The increased concentration of pollutants led to increased adsorption capacity from 63 to 625 mg/g for Pb2+ ions and from 20 to 90 mg/g for NR dye. The increased adsorbent dose led to increased removal efficiency from 39% to 98.8% and from 63% to 94% for Pb2+ and NR dye, respectively. The optimum pH for the adsorption of both pollutants was found to be 6.0. The reusability of MnFe2O4/GO nanocomposite was studied for up to five cycles. The nanocomposite can keep its efficiency even after the studied cycles. So, the prepared magnetic nanocomposite is a promising material for water treatment.

1. Introduction

The environment and living organisms face serious health risks resulting from the continuous discharging of industrial effluents directly to the environment, such as cosmetics effluents, plastic, paper, textiles, leather, fertilizers, pesticides, fuel, and energy production, metallurgy, mining, and the discharged wastes that contain heavy metal ions [1,2,3]. The direct discarding of such harmful industrial pollutants without treatment is a significant concern [4]. Thus, many techniques are developed for the treatment of water/wastewater from toxic species, such as adsorption [5], co-precipitation [6], electrocoagulation [7], biochemical degradation [8], photocatalytic degradation [9], photo-degradation [10], oxidative degradation [11], and electrochemical degradations [12]. However, several reasons make the adsorption technique widely used for water treatment over other techniques, such as eliminating several types of toxins, high efficiency, less environmental effect, the ability to reuse adsorbents, and the ease of processing [13,14,15]. The examples of the most dangerous substances are heavy metal ions and organic dyes that have harmful environmental impacts. Heavy metals can accumulate inside human and living organisms over a long-timescale as these metals are not metabolized inside organisms and humans [16]. In the current study, we focus on lead ions and neutral red dye. Lead ions are thrown away directly to the environment with automobiles industrial parts, paints, and batteries [17]. It is a very toxic metal and has a harmful effect on the central and peripheral nervous system [18]. Many dangerous effects include antisocial behaviors, nausea, loss of appetite, anemia, muscle paralysis, and abdominal pains. According to the World Health Organization (WHO), the lead ions limited concentration in blood should not exceed 0.1 gm−3 [19] that makes its removal from water/wastewater mandatory. Many efforts have been made for the removal of Pb2+ ions from water using different carbon-based materials such as the poly(acrylamide-co-itaconic acid)/multi-walled carbon nanotubes (MWCNTs) [20], mesoporous NiO/ZnCl2-Rosa Canina-L seeds activated carbon nanocomposite [21], and magnetic Fe3O4/H2SO4-activated Myrtus Communis leaves carbon nanocomposite [22]. Dyes can consume soluble oxygen due to their deep color [23]. The degradation products of many dyes are poisonous and carcinogenic that can destroy marine organisms [24]. Cosmetic industries, food additives, textiles, paper industries, and leather are well-known dye sources [25]. Neutral red is a mono-cationic dye widely used in nuclear counterstaining in biological studies [26]. The direct release of these colored compounds leads to cancer, mutations, and carcinogenesis [27]. Hence, elimination of these pollutants from water/wastewater is essential to control the contamination of the environment. Carbon-based materials have been utilized to remove dyes from the water with high efficiency [28].
Nanomaterials [29,30,31,32,33] as adsorbents, especially magnetic ones such as (MnFe2O4, NiFe2O4, Fe3O4) are promising and widely used for water treatment in the last decade. However, these nanomaterials have many deficiencies, like agglomeration and instability [34]. Especially, nanoparticles of MnFe2O4 have many uses disadvantages such as ease of oxidation at low pH values, weak mechanical properties, agglomeration, and instability that makes its attachment to the substrate is essential to enhance their properties. The excellent dispersion and hydrophilic properties of graphene oxide resulted from various oxygen-containing functional groups, and high negative charge density on its surface and high specific surface area [35] enable it to be the ideal substrate to enhance magnetic MnFe2O4 NPs for water treatment applications. The attachment of graphene oxide and MnFe2O4 to form MnFe2O4/graphene oxide nanocomposite has enhanced properties over its counterparts such as high removal efficiency, magnetic separation, and reusability that can be perfect adsorbent for the elimination of neutral red dye and lead ions from wastewater. Various studies have been reported the combination of carbon-based materials and magnetic nanoparticles [36,37,38] for water treatment. Moreover, hierarchically porous reduced graphene oxide decorated with manganese ferrite (MrGO) was used to uptake methylene blue and malachite green dyes with high efficiency [39].
This study aimed to synthesize and characterize a novel nanocomposite of MnFe2O4/graphene oxide to absorb neutral red dye and lead ions from wastewater. The effect of different factors like contact time, adsorbent dosage, and pH on the adsorption process was investigated and discussed. The separation of the selected nanocomposite for reusability was determined. In a similar previous study, MnFe2O4/graphene oxide was synthesized for the removal of Pb2+, As3+, and As5+ from water [40]. As the synthesis method, conditions, and phase ratios in the composite affect the surface area of the adsorbent and, consequently, affect the nanocomposite’s adsorption capacity. Therefore, the current study clarifies the suitable method and conditions of MnFe2O4/graphene oxide synthesis for more effective pollutants removal and determining the applicability of this nanocomposite for the removal of additional pollutants.

2. Materials and Methods

2.1. Chemicals

All used reagents are analytical grade and used without any further purifications. Nitric acid (HNO3, 64%), sodium hydroxide (NaOH), ferric chloride hexahydrate (FeCl3·6H2O, 98%), and manganese chloride tetrahydrate (MnCl2·4H2O, 98%) were supplied from Shanghai Chemical Reagent Co. while graphite powder and neutral red (NR), and Lead nitrate Pb(NO3)2 (99%) were supplied from Sigma-Aldrich, Darmstadt, Germany. We used in all experiments double distilled water.

2.2. Nanocomposite Synthesis

In situ method was used for the synthesis of MnFe2O4/graphene oxide nanocomposite [41] in which graphene oxide suspension (prepared by dissolving 0.4 g in 200 mL of water) was mixed with manganese chloride tetrahydrate (0.94 g) and ferric chloride hexahydrate (2.54 g), so that the molar ratio of Mn:Fe in the solution is 1:2.7. After that, the mixture’s sonication was continued up to half-hour followed by adjustment of mixture pH to 10. Then, a vigorous stirring of the mixture was continued up to one hour at 85 °C. The produced precipitate was finally washed several times and dried. Graphene oxide and MnFe2O4 NPs were required for characterization and comparison; so synthesized by the Tour method [42] and in situ method [43], respectively.

2.3. Nanocomposite Characterization

The synthesized materials were characterized using different techniques. The materials morphology and size were determined using Transmission Electron Microscopy (TEM, JEOL JEM-1010, JEOL USA, Inc., Massachusetts, USA) as well as Scanning Electron Microscopy (SEM, Zeiss Ultra plus 55, Oberkochen, Germany). The X-ray diffraction (XRD, Bruker D5005, Bruker Co., Massachusetts, USA) was used to determine the crystalline nature and phase purity using monochromatic Cu radiation Kα = 0.15418 nm. Raman spectroscopy was used to study the material structural defects. Fourier transform infrared (FTIR, a Nicolet 6700, Thermo Fisher Scientific, Massachusetts, USA) spectroscopy was used to study the chemical groups in the range of 400 cm−1 to 4000 cm−1. X-ray photoelectron spectroscopy (XPS) was used to study the chemical state of the samples. A vibrating sample magnetometer (MicroSense EV9, MicroSense, LLC, Massachusetts, USA) was used to study the sample magnetic properties.

2.4. Batch Adsorption Experiments

In a preliminary test, 8 mg of the synthesized adsorbent was mixed with 50 mL of working solutions of Pb2+ and NR in a 100 mL Erlenmeyer flask to determine the optimum conditions of the adsorption. For lead ions (Pb2+) removal, a stock solution was prepared by dissolving (1.6 g) of lead nitrate in distilled water (1000 mL) for all batch experiments at room temperature. The dilution method is used to obtain the desired concentration. To study the pH effect on the adsorption of Pb2+ ions over the nanocomposite surface, Pb2+ ions solution (50 mg/L, 50 mL) was used to study the adsorption in the pH range (1.0 to 6.0) adjusted using 1 M of NaOH and HNO3. Also, 50 mg/L of Pb2+ ions solution was used to study the contact time effect in the range of 5 to 120 min using 8 mg (adsorbent dosage). 50 mL of 50 mg/L Pb2+ ions solutions in the range of 2 to 30 mg were used to study the dosage effect. For neutral red dye, a concentration of (20 mg/L, 6 mL) was used. The pH range was altered as 4 to 9, contact time effect as 30 to 480 min, and dosage effect from 2 to 12 mg. Water bath shaker was used to shake all experiments flasks at 300 rpm. For the analysis of pollutants residues in the solutions after each experiment, the adsorbent was separated using a magnet and the solution was analyzed for the presence of Pb2+ ions and NR using atomic adsorption spectroscopy and UV-Vis spectrophotometer at 526 nm, respectively. The synthesized adsorbent adsorption capacity and removal efficiency were calculated according to the following equations:
E = [(Co − Ct)/Co] × 100,
qe (mg/g) = [(Co − Ct)/m] × V,
where Ct and Co represent the final and initial pollutant concentration, respectively. m denotes the mass of adsorbent; V denotes the volume of solution. E denotes the removal efficiency and qe denotes the adsorption capacity (mg/g).

3. Results and Discussions

3.1. Nanocomposite Characterization

The synthesized materials morphology was characterized using a TEM image, as shown in Figure 1. According to Figure 1, the TEM image of MnFe2O4 nanoparticles in Figure 1a showed an agglomeration of the nanoparticles, while Figure 1b showed stacking of GO sheets that are appeared to be folded and wrinkles. Figure 1c showed the TEM image of MnFe2O4/GO nanocomposite that clearly appears the well-distribution of MnFe2O4 NPs over the GO sheets with nanocomposite size ranging from 11 to 26 nm.
Figure 1d showed the SEM image of MnFe2O4/GO nanocomposite at high magnification. Figure 1d shows the homogenous distribution of spherical MnFe2O4 NPs over GO sheets that confirms the TEM image results. MnFe2O4/GO nanocomposite XRD was shown in Figure 2a to investigate the crystalline structure and phase purity.
According to Figure 2a, XRD shows the peaks corresponding to (731), (533), (440), (511), (422), (400), (222), (311), and (220) crystal plans that appeared in both XRD of MnFe2O4 and MnFe2O4/graphene oxide nanocomposite. Graphene oxide peak was observed at 12.99° that represents (001) plane. The graphene oxide plane decreased in the XRD of MnFe2O4/graphene oxide nanocomposite, indicating the reduction of graphene oxide during nanocomposite synthesis. The purity of the synthesized sample was indicated from the absence of XRD peaks related to the formation of metal oxides, either manganese or iron, and indicated the formation of the nanocomposite. As nanoparticles grown on graphene oxide surface preclude it from restacking, the peak of graphene oxide is significantly reduced. The average particle size of MnFe2O4 nanoparticles alone and MnFe2O4 in nanocomposite was calculated using the Debye-Scherrer equation, which was found equal to 12 nm and 9 nm, respectively. The decreased size of MnFe2O4 in the nanocomposite was attributed to the blocked growth of nanoparticles from one side during the growth over graphene oxide sheets.
For more details about the nanocomposite composition, FT-IR spectra were shown in Figure 2b to determine the functional groups. According to the FT-IR results, the vibrations of functional groups O-C, C=C, C=O, and OH graphene oxide were represented by 750.20, 1383.48, 1628.34, 1628.34, 1722.43, and 3450 cm−1, respectively [44]. Additionally, MnFe2O4 (Mn-O and Fe-O) bonds are represented by 481.25 and 663.29 cm−1, respectively. The absorption peak at 540 cm−1 is a characteristic peak corresponding to the stretching vibrations of the Mn-Fe-O linkage. FT-IR results showed an excellent linkage between graphene oxide and MnFe2O4 NPs [45]. Furthermore, the composition more details were obtained via wide scan XPS, as shown in Figure 3a for MnFe2O4/GO nanocomposite. According to Figure 3a, Fe 2p, Mn 2p, O 1s of adsorbed oxygen, and C 1s in sp2 carbon were represented by peaks at 710.33, 640.14, 528.59, and 284.34, respectively.
The high purity of the synthesized nanocomposite was also indicated by the absence of any other pollutants’ peaks in the wide scan XPS.
As one of the advantages of this nanocomposite is the magnetic property that allowing the simple separation using a magnet, we study the magnetization curve of the currently synthesized nanocomposite as shown in Figure 3b. According to Figure 3b, the saturation magnetism (Ms) was 28.8 and 67 emu/g for MnFe2O4/GO and manganese ferrite alone, respectively, indicating the reduced size nanocomposite after the loading of manganese ferrite over graphene oxide. Both of them exhibit superparamagnetic behavior. The nanocomposite’s magnetic properties are attributed to the load of magnetic ferrite over the graphene oxide [46,47].
Raman spectra of graphene oxide and MnFe2O4/graphene oxide were showed in Figure 3c. The structural defects are responsible for the D peak that resulted from out of the plane of sp2 bonded carbon atoms, while the G peak is attributed to in-plane vibrations [48]. The defect of the material was represented by the (ID/IG) intensity ratio. According to Figure 3c, ID/IG ratio for MnFe2O4/graphene oxide nanocomposite is 1.13 that is higher than that of graphene oxide (which is equal to 0.94), indicating the interaction between graphene oxide’s oxygen groups and manganese ferrite nanoparticles [49]. This interaction is responsible for the increased degree of the defect and causes this difference in peak ratio.

3.2. Effect of Initial Concentration

The ratio between accessible active sites of MnFe2O4/graphene oxide nanocomposite adsorbent and the concentration of adsorbate (Pb2+ and NR dye) is the adsorption efficiency (%) that also associates adsorbent surface coverage. The concentrations range from 10 to 300 and 20 to 200 mg/L were used to study the adsorption of Pb2+ and NR over MnFe2O4/graphene oxide nanocomposite, respectively as shown in Figure 4. According to Figure 4, the increased concentration of the adsorbate led to a decrease of removal efficiency from 99% to 33% for Pb2+ ions and from 92.8 to 89% for NR dye. Nevertheless, the increase of concentration led to the increase of adsorption capacity from 63 to 625 mg/g for Pb2+ ions and from 20 to 90 mg/g for NR dye due to the accelerated mass transfer of pollutants toward the surface of nanocomposite that attributed to high pollutants concentrations [50]. The removal efficiency reached its maximum value of Pb2+ ions and NR adsorption at lower adsorbate concentrations due to high available adsorption active sites. This inverse relationship between adsorption efficiency and adsorption capacity during initial concentration increase was observed in many similar reports.

3.3. Dose Effect

Adsorption of contaminants dramatically depends on the dose of MnFe2O4/GO nanocomposite that extremely affects the removal efficiency of Pb2+ ions and NR dye due to the variations in available functional groups responsible for contaminants capturing. The adsorbent dose effect on the adsorption of Pb2+ and NR was investigated in dosage range from 2 to 30 mg and 2 to 12 mg, respectively as shown in Figure 5. The volume and concentration of Pb2+ ions and NR dye were fixed to be (50 mL, 50 mg/L) and (6 mL, 50 mg/L), respectively. As the amount of nanocomposite dose increased from 2 mg to 8 mg, the removal efficiency of Pb2+ ions were increased from 39% to 98.8%. In comparison, the increased dose from 2 mg to 12 mg led to an increase in removal efficiency from 63% to 94%.
The increased overall surface area and the number of active sites accompanying the increased adsorbent dosage are responsible for the nanocomposite’s increased adsorption efficiency toward Pb2+ and NR dye that easily penetrate these vacant adsorption sites. As observed in Figure 5, the adsorption capacity was decreased by increasing the adsorbent dosage. Pb2+ ions and NR dye’s absorption capacity were decreased from 960.0 mg/g to 166.0 mg/g and from 80 mg/g to 25 mg/g, respectively with the increase of adsorbent dose. This behavior is due to the high existed number of active adsorption sites compared to the lower number of contaminants molecules that led to lesser adsorbates per unit mass of adsorbent. Thus, the adsorption capacity decreased.

3.4. Effect of pH

The adsorption of Pb2+ and NR dye over MnFe2O4/GO nanocomposite at different pH values were studied, as shown in Figure 6. The studied range was from 2.0 to 6.0 and from 1.5 to 13.0 for Pb2+ and NR dye, respectively. According to Figure 6, the removal efficiency for Pb2+ ions and NR dye was decreased by the decrease in pH value as predicted. This minimum removal efficiency of Pb2+ ions at a low pH value was attributed to the existence of hydronium ions H3O+ that compete with the positively charged metal ions on the adsorption sites over adsorbent surfaces [51]. While the increased value of pH enhanced the ionization of carboxylic and hydroxyl groups over the nanocomposite surfaces, increasing the adsorption of metal ions by ionized functional groups [52]. Furthermore, the hydroxides and oxides of lead ions precipitate at a pH of more than 6. The pH increased from 2.0 to 6.0, leading to an increase of Pb2+ adsorption capacities over MnFe2O4/GO nanocomposite from 155.70 to 463.0 mg/g.
Also, for cationic NR dye, at low pH values, the protonation of nanocomposite functional groups and the existing hydronium ions negatively affected the dye adsorption. So, the NR adsorption capacities over MnFe2O4/GO nanocomposite are increased from 17.0 to 24.0 mg/g by pH increase from 1.5 to 3.5, and the adsorption capacity reached its maximum by pH increase to 6.0 then reached the equilibrium and not affected by the further pH increase. Thus, the optimum pH value for Pb2+ and NR dye adsorption over MnFe2O4/GO nanocomposite was 6.0.

3.5. Effect of Contact Time

One of the most significant factors affecting the adsorption equilibrium is the contact time between adsorbent and pollutant. Pb2+ and NR dye’s adsorption capacities over MnFe2O4/GO nanocomposite were investigated at contact time ranging from 0 to 120 min and 0 to 30 min, respectively. The adsorption capacity and removal efficiencies of Pb2+ ions and NR dye over MnFe2O4/GO nanocomposite at different time intervals were shown in Figure 7.
During the contact time effect study, we hold other parameters constant including adsorbent dose, volume, and concentration. The dose, volume, and concentration of Pb2+ ions and NR dye were (8 mg, 50 mL, and 50 mg/L) and (12 mg, 6 mL, and 50 mg/L), respectively. According to Figure 7, the removal efficiency and adsorption capacity of Pb2+ ions and NR dye increased quickly at the first stage then gradually became slower. The first stage was observed for Pb2+ ion and NR dye adsorption at the first 15 and 20 min, respectively. The tremendous available numbers of adsorption sites over MnFe2O4/GO nanocomposite are responsible for this rapid increase of adsorption capacity at the first stage. After this stage, the surface saturation was reached, and hence the equilibrium reached with no further significant increase in the adsorption capacity. As indicated from the results, the adsorption equilibrium was not significantly dependent on the adsorbate concentration. The Pb2+ ions and NR dye maximum removal were 98% and 94%, respectively. Subsequently, the optimum contact times for Pb2+ ions and NR dye were 120 and 30 min, respectively at which the equilibrium reached.

3.6. Adsorption Isotherm

The distribution of the molecule at the equilibrium time between liquid and solid phases is indicated by the adsorption isotherms. The affinities of adsorbent material and insights about surface properties and the understanding of adsorption mechanisms were achieved using both adsorption isotherms. The relation between the adsorption ability of the adsorbent and adsorbate concentration can be completely investigated through adsorption isotherms. The first type of adsorption isotherm is Freundlich isotherm [53] that supposes heterogeneous adsorption and the second is Langmuir isotherm [54] supposes monolayer adsorption with energetically similar active sites. Freundlich isotherm introduces the opinion of heterogeneous surface and multilayer adsorption depending on the fact that non-ideal sorption can be modeled using an exponentially decaying adsorption site energy distribution. The linearized form of the Freundlich model can be written according to the following equation:
ln qe = ln KF + (1/n ln Ce),
where n and KF denotes Freundlich constants. Ce and qe denote equilibrium concentration and adsorption capacity, respectively. The linear fit of ln Ce against ln qe used for these parameters′ calculation. More system heterogeneity is indicated by a higher n value. Monolayer chemisorption is represented by the Langmuir isotherm that can be applied according to linearized form according to the following equation:
Ce/qe = (1/ KL qm)+ Ce/qm,
where KL, Ce, qm, and qe denote Langmuir constant, equilibrium concentration, maximum adsorption capability, and equilibrium adsorption capacity, respectively. The linear fit between Ce and Ce/qe used for these parameters’ calculations. Freundlich and Langmuir adsorption isotherm data are shown in Figure 8 and different parameters were tabulated in Table 1.
According to the results in Table 1, Langmuir isotherm is better fitted to the experimental results for Pb2+ ions and NR dye adsorption than Freundlich isotherm. This is indicated from the R2 value equal to 0.998 and 0.996 (Langmuir fit), 0.986, and 0.859 (Freundlich fit) for Pb2+ and NR dye adsorption, respectively.
These results indicate the homogenous energy distribution for active adsorption sites with a monolayer the adsorption process over the adsorbent surface. Langmuir isotherm indicated that Pb2+ and NR dye’s maximum adsorption capacity are 636.94 and 46.08 mg/g, respectively. Future studies must be including the thermodynamic parameters [55,56,57] of the adsorption process to get a full image of the removal process.

3.7. Nanocomposite Regeneration

The excellent adsorbent not enough to have excellent adsorption capacity but also must be regenerated according to the industrial, practical, and economic point of view. The adsorbent’s reusability ability led to a decrease in the total cost of materials and adsorption processing. For the adsorption/desorption study, the nanocomposite was separated after each adsorption study using a magnet and regenerated again by immersing in 0.2 M HCl to be used again. The nanocomposite MnFe2O4/GO was studied for reusability toward removing Pb2+ ions and NR dye from the water for up to five cycles. The reusability study of MnFe2O4/GO nanocomposite toward capturing Pb2+ ions and NR dye was shown in Figure 9. The results showed that the first cycle has the highest removal efficiency for Pb2+ ions and NR dye removal due to the intact pores and active sites. After that, the removal efficiency was slightly decreased due to the damaged sites after the adsorption/desorption cycle. The reusability study indicates that Pb2+ ions and NR removals from water are efficient with low cost over the synthesized MnFe2O4/GO nanocomposite.

3.8. Comparative Study

The adsorption capacity of MnFe2O4/GO nanocomposite toward removing Pb2+ ions and NR dye from the water was compared with earlier synthesized adsorbents even the same adsorbent synthesized by other methods as shown in Table 2. The information in the table indicates that the nanocomposite synthesized in this study is suitable for removing Pb2+ ions and NR dye from water. Also, many reviews reported the application of different materials for the removal of Pb2+ ions and dyes [58,59,60]. According to Table 2, the similar nanocomposite synthesized by Kumar et al. [40] via a different method, conditions, and different percentages of composite phases showed a higher adsorption capacity toward Pb2+ ions than that synthesized in the present study. This indicates that MnFe2O4/GO nanocomposite synthesized in the literature has a higher effective surface area and larger functional groups for the chelation of Pb2+ ions. However, the most effective adsorbent for Pb2+ removal is few-layered graphene oxide synthesized by Zhao et al. [61]. This high adsorption capacity is attributed to the existence of a large number of oxygen-containing functional groups over its surfaces due to the higher surface site density of few-layered graphene oxide that is about two times more than other graphene materials.

4. Conclusions

In this study, MnFe2O4/GO nanocomposite was synthesized and characterized through the in-situ method for applying water treatment from Pb2+ ions and NR dye. Different techniques were used for the nanocomposite characterization, including SEM, TEM, FT-IR, XRD, Raman, and XPS to determine its size, composition, and morphology. The optimal conditions for Pb2+ 2 and NR dye adsorption over MnFe2O4/GO nanocomposite were determined by studying different factors, including initial concentration, dose, pH, and contact time. The initial concentration-effect study indicated that the increased concentration of pollutants led to the increase of adsorption capacity from 63 to 625 mg/g and from 20 to 90 mg/g for Pb2+ ions and NR dye removal, respectively. The dose-effect study indicated that the increased adsorbent dose led to increased removal efficiency from 39% to 98.8% and from 63% to 94% for Pb2+ and NR dye, respectively, due to the increased number of active adsorbent sites. The pH effect study indicated that the optimum pH for both pollutants’ adsorption was found to be 6.0. The contact time effect study indicated that Pb2+ ions and NR dye’s adsorption capacity increased quickly at the first stage and then gradually became slower. The first stage was observed for Pb2+ ion and NR dye adsorption at the first 15 and 20 min, respectively. The tremendous available numbers of adsorption sites over MnFe2O4/GO nanocomposite are responsible for this rapid increase of adsorption capacity at the first stage. After this stage, the surface saturation was reached, and hence the equilibrium reached with no further significant increase in the adsorption capacity. The optimum contact times for Pb2+ ions and NR dye were 120 and 30 min, respectively. The monolayer adsorption was concluded for Pb2+ and NR dye adsorption over the synthesized nanocomposite that indicated from experimental fitting to Langmuir isotherm. The adsorption capacities are 636.94 and 46.08 mg/g for the adsorption of Pb2+ and NR dye, respectively according to Langmuir isotherm. The reusability study of this adsorbent for up to five cycles indicated that this nanocomposite is reusable and stable, reducing the overall cost of treatment. We can conclude that MnFe2O4/GO nanocomposite is a promising adsorbent for removing Pb2+ ions and NR dye from water.

Author Contributions

Conceptualization, N.S.A.; K.M.MK., F.M.A., and M.A.T.; investigation, M.A.T.; data curation, M.A.T., and S.M.S.; writing—original draft preparation, M.A.T. and S.M.S.; writing—review and editing, N.S.A.; K.M.MK., and F.M.A.; supervision, M.A.T. and S.M.S.; project administration, S.M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Deanship of Scientific Research at King Khalid University.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through research groups program under grant number R.G.P.2/111/42. Also, this research was funded by the Deanship of Scientific Research at Princess Nourah bint Abdulrahman University through the Fast-track Research Funding Program.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. TEM image of MnFe2O4 nanoparticles (a), graphene oxide (b), and MnFe2O4/graphene oxide nanocomposite (c); SEM image of MnFe2O4/graphene oxide nanocomposite (d).
Figure 1. TEM image of MnFe2O4 nanoparticles (a), graphene oxide (b), and MnFe2O4/graphene oxide nanocomposite (c); SEM image of MnFe2O4/graphene oxide nanocomposite (d).
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Figure 2. XRD (a) and FT-IR (b) of MnFe2O4/graphene oxide nanocomposite and its components.
Figure 2. XRD (a) and FT-IR (b) of MnFe2O4/graphene oxide nanocomposite and its components.
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Figure 3. Wide scan XPS (a), room temperature magnetization curve (b), and Raman spectra (c) of MnFe2O4/GO nanocomposite.
Figure 3. Wide scan XPS (a), room temperature magnetization curve (b), and Raman spectra (c) of MnFe2O4/GO nanocomposite.
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Figure 4. The effect of initial concentration of Pb2+ ions (a) and neutral red (NR) dye (b) for the adsorption over MnFe2O4/GO nanocomposite.
Figure 4. The effect of initial concentration of Pb2+ ions (a) and neutral red (NR) dye (b) for the adsorption over MnFe2O4/GO nanocomposite.
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Figure 5. The effect of adsorbent dose on Pb2+ ions (a) and NR dye (b) adsorption over MnFe2O4/GO nanocomposite.
Figure 5. The effect of adsorbent dose on Pb2+ ions (a) and NR dye (b) adsorption over MnFe2O4/GO nanocomposite.
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Figure 6. The effect of pH on Pb2+ ions (a) and NR dye (b) adsorption over MnFe2O4/GO nanocomposite.
Figure 6. The effect of pH on Pb2+ ions (a) and NR dye (b) adsorption over MnFe2O4/GO nanocomposite.
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Figure 7. The effect of contact time on Pb2+ ions (a) and NR dye (b) adsorption over MnFe2O4/GO nanocomposite.
Figure 7. The effect of contact time on Pb2+ ions (a) and NR dye (b) adsorption over MnFe2O4/GO nanocomposite.
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Figure 8. Freundlich adsorption isotherm (a); Langmuir adsorption isotherm (b) for Pb2+ and NR dye adsorption over MnFe2O4/GO nanocomposite.
Figure 8. Freundlich adsorption isotherm (a); Langmuir adsorption isotherm (b) for Pb2+ and NR dye adsorption over MnFe2O4/GO nanocomposite.
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Figure 9. Regeneration of MnFe2O4/GO nanocomposite for Pb2+ and NR dye removal up to five cycles.
Figure 9. Regeneration of MnFe2O4/GO nanocomposite for Pb2+ and NR dye removal up to five cycles.
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Table
Table 1. Freundlich and Langmuir adsorption isotherms different parameters for Pb2+ ions and NR dye adsorption over MnFe2O4/GO nanocomposite.
Table 1. Freundlich and Langmuir adsorption isotherms different parameters for Pb2+ ions and NR dye adsorption over MnFe2O4/GO nanocomposite.
PollutantFreundlichLangmuir
KFNR2qmKLR2
Pb2+300.06.5230.986636.940.1680.998
NR dye11.110.1600.85946.080.0650.996
Table
Table 2. Reported adsorbents for adsorption of Pb2+ ions and NR dye.
Table 2. Reported adsorbents for adsorption of Pb2+ ions and NR dye.
AdsorbentPollutantIsothermqm (mg/g)Ref.
Banana and orange peels NR dyeLangmuir18.0 and 14.0[62]
Halloysite nanotubesNR dyeLangmuir54.85[63]
Peanut huskNR dyeLangmuir37.5[64]
GO-Fe3O4NR dye-171.3[47]
GOPb2+Langmuir488.0[65]
Carboxylated chitosan magnetic submicron spheresPb2+Langmuir142.0[66]
Tetraethylenepentamine modified CS/CoFe2O4Pb2+Langmuir228.0[67]
GO-EDTAPb2+Langmuir455.0[68]
Pinecone activated carbonPb2+Langmuir27.6[69]
EDTA-GOPb2+Freundlich509.0[70]
GO-MnFe2O4 nanohybridsPb2+Langmuir673[40]
Few-layered graphene oxidePb2+Langmuir842[61]
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Katubi, K.M.M.; Alsaiari, N.S.; Alzahrani, F.M.; M. Siddeeg, S.; A. Tahoon, M. Synthesis of Manganese Ferrite/Graphene Oxide Magnetic Nanocomposite for Pollutants Removal from Water. Processes 2021, 9, 589. https://0-doi-org.brum.beds.ac.uk/10.3390/pr9040589

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Katubi KMM, Alsaiari NS, Alzahrani FM, M. Siddeeg S, A. Tahoon M. Synthesis of Manganese Ferrite/Graphene Oxide Magnetic Nanocomposite for Pollutants Removal from Water. Processes. 2021; 9(4):589. https://0-doi-org.brum.beds.ac.uk/10.3390/pr9040589

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Katubi, Khadijah Mohammedsaleh M, Norah Salem Alsaiari, Fatimah Mohammed Alzahrani, Saifeldin M. Siddeeg, and Mohamed A. Tahoon. 2021. "Synthesis of Manganese Ferrite/Graphene Oxide Magnetic Nanocomposite for Pollutants Removal from Water" Processes 9, no. 4: 589. https://0-doi-org.brum.beds.ac.uk/10.3390/pr9040589

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