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

: These days, environmental pollution, notably water pollution, has increasingly caused severe human health problems. The major water pollutants are heavy metals. MnFe 2 O 4 /GO nanocomposite was prepared in the current work via in situ method and tested to remove lead ion Pb 2+ 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 Pb 2+ 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 Pb 2+ 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 Pb 2+ and NR dye, respectively. The optimum pH for the adsorption of both pollutants was found to be 6.0. The reusability of MnFe 2 O 4 /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.


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

Nanocomposite Synthesis
In situ method was used for the synthesis of MnFe 2 O 4 /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 MnFe 2 O 4 NPs were required for characterization and comparison; so synthesized by the Tour method [42] and in situ method [43], respectively.

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 K α = 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.

Batch Adsorption Experiments
In a preliminary test, 8 mg of the synthesized adsorbent was mixed with 50 mL of working solutions of Pb 2+ and NR in a 100 mL Erlenmeyer flask to determine the optimum conditions of the adsorption. For lead ions (Pb 2+ ) 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 Pb 2+ ions over the nanocomposite surface, Pb 2+ 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 HNO 3 . Also, 50 mg/L of Pb 2+ 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 Pb 2+ 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 Pb 2+ 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: where C t and C o 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 q e denotes the adsorption capacity (mg/g).

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 MnFe 2 O 4 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 MnFe 2 O 4 /GO nanocomposite that clearly appears the well-distribution of MnFe 2 O 4 NPs over the GO sheets with nanocomposite size ranging from 11 to 26 nm. where Ct and Co represent the final and initial pollutant concentration, respectively. m notes the mass of adsorbent; V denotes the volume of solution. E denotes the remo efficiency and qe denotes the adsorption capacity (mg/g).

Nanocomposite Characterization
The synthesized materials morphology was characterized using a TEM image shown in Figure 1. According to Figure 1, the TEM image of MnFe2O4 nanoparticles Figure 1a showed an agglomeration of the nanoparticles, while Figure 1b showed stack of GO sheets that are appeared to be folded and wrinkles. Figure 1c showed the T image of MnFe2O4/GO nanocomposite that clearly appears the well-distribution MnFe2O4 NPs over the GO sheets with nanocomposite size ranging from 11 to 26 nm.   Figure 1d shows the homogenous distribution of spherical MnFe2O4 NPs over sheets that confirms the TEM image results. MnFe2O4/GO nanocomposite XRD was sho in Figure 2a to investigate the crystalline structure and phase purity.    , 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. and MnFe 2 O 4 /graphene oxide nanocomposite. Graphene oxide peak was observed at 12.99 • that represents (001) plane. The graphene oxide plane decreased in the XRD of MnFe 2 O 4 /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 MnFe 2 O 4 nanoparticles alone and MnFe 2 O 4 in nanocomposite was calculated using the Debye-Scherrer equation, which was found equal to 12 nm and 9 nm, respectively. The decreased size of MnFe 2 O 4 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  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 MnFe 2 O 4 /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 MnFe 2 O 4 /graphene oxide were showed in Figure 3c. The structural defects are responsible for the D peak that resulted from out of the plane of sp 2 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 MnFe 2 O 4 /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.  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 sp 2 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.

Effect of Initial Concentration
The ratio between accessible active sites of MnFe 2 O 4 /graphene oxide nanocomposite adsorbent and the concentration of adsorbate (Pb 2+ 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 Pb 2+ and NR over MnFe 2 O 4 /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 Pb 2+ 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 Pb 2+ 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 Pb 2+ 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.
increase of concentration led to the increase of adsorption capacity from 63 to 625 mg/g for Pb 2+ 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 Pb 2+ 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.

Dose Effect
Adsorption of contaminants dramatically depends on the dose of MnFe2O4/GO nanocomposite that extremely affects the removal efficiency of Pb 2+ ions and NR dye due to the variations in available functional groups responsible for contaminants capturing. The adsorbent dose effect on the adsorption of Pb 2+ 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 Pb 2+ 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 Pb 2+ 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%.

Dose Effect
Adsorption of contaminants dramatically depends on the dose of MnFe 2 O 4 /GO nanocomposite that extremely affects the removal efficiency of Pb 2+ ions and NR dye due to the variations in available functional groups responsible for contaminants capturing. The adsorbent dose effect on the adsorption of Pb 2+ 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 Pb 2+ 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 Pb 2+ 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 Pb 2+ 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. Pb 2+ 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.

Effect of pH
The adsorption of Pb 2+ 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 Pb 2+ and NR dye, respectively. According to Figure 6, the removal efficiency 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 Pb 2+ 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. Pb 2+ 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.

Effect of pH
The adsorption of Pb 2+ and NR dye over MnFe 2 O 4 /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 Pb 2+ and NR dye, respectively. According to Figure 6, the removal efficiency for Pb 2+ ions and NR dye was decreased by the decrease in pH value as predicted. This minimum removal efficiency of Pb 2+ ions at a low pH value was attributed to the existence of hydronium ions H 3 O + 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 Pb 2+ adsorption capacities over MnFe 2 O 4 /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 Pb 2+ and NR dye adsorption over MnFe2O4/GO nanocomposite was 6.0.

Effect of Contact Time
One of the most significant factors affecting the adsorption equilibrium is the contact time between adsorbent and pollutant. Pb 2+ 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 Pb 2+ ions and NR dye over MnFe2O4/GO nanocomposite at different time intervals were shown in Figure 7. 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 MnFe 2 O 4 /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 Pb 2+ and NR dye adsorption over MnFe 2 O 4 /GO nanocomposite was 6.0.

Effect of Contact Time
One of the most significant factors affecting the adsorption equilibrium is the contact time between adsorbent and pollutant. Pb 2+ and NR dye's adsorption capacities over MnFe 2 O 4 /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 Pb 2+ ions and NR dye over MnFe 2 O 4 /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 Pb 2+ 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 Pb 2+ ions and NR dye increased quickly at the first stage then gradually became slower. The first stage was observed for Pb 2+ ion and NR dye adsorption at the first 15 and 20 min, respectively. The tremendous available numbers of adsorption sites over MnFe 2 O 4 /GO nanocomposite are responsible for this rapid increase of adsorption capacity at the first Processes 2021, 9, 589 9 of 15 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 Pb 2+ ions and NR dye maximum removal were 98% and 94%, respectively. Subsequently, the optimum contact times for Pb 2+ ions and NR dye were 120 and 30 min, respectively at which the equilibrium reached.
tional 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 Pb 2+ and NR dye adsorption over MnFe2O4/GO nanocomposite was 6.0.

Effect of Contact Time
One of the most significant factors affecting the adsorption equilibrium is the contact time between adsorbent and pollutant. Pb 2+ 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 Pb 2+ 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 Pb 2+ 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 Pb 2+ ions and NR dye increased quickly at the first stage then gradually became slower. The first stage was observed for Pb 2+ 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 Pb 2+ ions and NR dye maximum removal were 98% and 94%, respectively. Subsequently, the optimum contact times for Pb 2+ ions and NR dye were 120 and 30 min, respectively at which the equilibrium reached.

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

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 q e = ln K F + (1/n ln C e ), where n and K F denotes Freundlich constants. C e and q e denote equilibrium concentration and adsorption capacity, respectively. The linear fit of ln C e against ln q e 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: where K L , C e , q m , and q e denote Langmuir constant, equilibrium concentration, maximum adsorption capability, and equilibrium adsorption capacity, respectively. The linear fit between C e and C e /q e used for these parameters' calculations. Freundlich and Langmuir adsorption isotherm data are shown in Figure 8 and different parameters were tabulated in Table 1.
tween 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 Pb 2+ ions and NR dye adsorption than Freundlich isotherm. This is indicated from the R 2 value equal to 0.998 and 0.996 (Langmuir fit), 0.986, and 0.859 (Freundlich fit) for Pb 2+ 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 Pb 2+ 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.   According to the results in Table 1, Langmuir isotherm is better fitted to the experimental results for Pb 2+ ions and NR dye adsorption than Freundlich isotherm. This is indicated from the R 2 value equal to 0.998 and 0.996 (Langmuir fit), 0.986, and 0.859 (Freundlich fit) for Pb 2+ 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 Pb 2+ 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.

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 MnFe 2 O 4 /GO was studied for reusability toward removing Pb 2+ ions and NR dye from the water for up to five cycles. The reusability study of MnFe 2 O 4 /GO nanocomposite toward capturing Pb 2+ ions and NR dye was shown in Figure 9. The results showed that the first cycle has the highest removal efficiency for Pb 2+ 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 Pb 2+ ions and NR removals from water are efficient with low cost over the synthesized MnFe 2 O 4 /GO nanocomposite. study of MnFe2O4/GO nanocomposite toward capturing Pb 2+ ions and NR dye was shown in Figure 9. The results showed that the first cycle has the highest removal efficiency for Pb 2+ 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 Pb 2+ ions and NR removals from water are efficient with low cost over the synthesized MnFe2O4/GO nanocomposite.

Comparative Study
The adsorption capacity of MnFe2O4/GO nanocomposite toward removing Pb 2+ 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 Pb 2+ ions and NR dye from water. Also, many reviews reported the application of different materials for the removal of Pb 2+ 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 Pb 2+ 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 Pb 2+ ions. However, the most effective adsorbent for Pb2+ removal is few-layered graphene oxide synthesized by Zhao et al.

Comparative Study
The adsorption capacity of MnFe 2 O 4 /GO nanocomposite toward removing Pb 2+ 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 Pb 2+ ions and NR dye from water. Also, many reviews reported the application of different materials for the removal of Pb 2+ 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 Pb 2+ ions than that synthesized in the present study. This indicates that MnFe 2 O 4 /GO nanocomposite synthesized in the literature has a higher effective surface area and larger functional groups for the chelation of Pb 2+ 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.

Conclusions
In this study, MnFe 2 O 4 /GO nanocomposite was synthesized and characterized through the in-situ method for applying water treatment from Pb 2+ 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 Pb 2+ 2 and NR dye adsorption over MnFe 2 O 4 /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 Pb 2+ 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 Pb 2+ 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 Pb 2+ ions and NR dye's adsorption capacity increased quickly at the first stage and then gradually became slower. The first stage was observed for Pb 2+ ion and NR dye adsorption at the first 15 and 20 min, respectively. The tremendous available numbers of adsorption sites over MnFe 2 O 4 /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 Pb 2+ ions and NR dye were 120 and 30 min, respectively. The monolayer adsorption was concluded for Pb 2+ 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 Pb 2+ 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 MnFe 2 O 4 /GO nanocomposite is a promising adsorbent for removing Pb 2+ ions and NR dye from water.