The Synthesis of MnFe 2 O 4-Activated Carbon Composite for Removal of Methyl Red From Aqueous Solution

In this study, MnFe2O4-activated carbon composite was synthesized by co-precipitation method and applied to adsorb methyl red dye. MnFe2O4-activated carbon composite was made with the mass ratio of activated carbon and MnFe2O4 of 1:2. The composite characterization by using X-Ray Diffraction, Fourier Transform Infrared Spectroscopy, Scanning Electron Micrograph, and Energy-Dispersive X-Ray Spectroscopy, the surface area using the Brunauer, Emmett and Teller and magnetic properties by using Vibrating Sample Magnetometer. The adsorption parameters include contact time, and adsorbent weight. The spectra of FTIR MnFe2O4-activated carbon composite analysis show the presence of Fe-O and Mn-O which is not present in the spectra of activated carbon. SEM analysis shows that the composite has pores and MnFe2O4 oxides are spread on its surface. The composition of the composite consists of C, O, Fe, and Mn. The composite has a surface area of 143.992 smaller than that of the activated carbon of 217.697 m 2 /g. However, the composite has magnetic properties with the saturatization magnetization of 17.91 emu/g. The optimum condition of the composite for adsorption of methyl red was obtained at a weight of 0.15 g, and contact time of 100 minutes. The adsorption of the composite was in accordance by pseudo-second-order kinetic and Langmuir isotherms with adsorption capacity of 81.97 mg/g.


INTRODUCTION
Activated carbon is an adsorbent that is often used in waste treatment.The adsorption process by using activated carbon has an advantage because the active carbon has a large porosity and surface area.Also, the adsorption process occurs rapidly, and it can be used to adsorb various compounds with a simple and regenerable design (Tan, Ahmad, & Hameed, 2008;Djilani et al., 2015).To increase the adsorption capacity, the activated carbon can be combined with other materials so that it has new properties as we desire.
Magnetic ferrite nanoparticle is a material of concern to be developed in recent years because it has unique magnetic and electrical properties.This material has different properties in its bulk (Deraz, & Alarifi, 2012;Aslibeiki et al., 2016).Magnetic ferrite has many uses.In the field of physics, it functions as magnetic sensors, broadband transformer, ferrofluids, etc.In the biomedical field, it functions as drug delivery, biosensor, magnetic resonance imaging, etc. and it has some functions also in the environmental field.One of the magnetic ferrites is MnFe 2 O 4 .The magnetic suspension of MnFe 2 O 4 is higher than of the other ferrites such as MgFe 2 O 4 , CoFe 2 O 4 , ZnFe 2 O 4 and NiFe 2 O 4 with the magnetic spin of 5μb (Zhong, Yang, Chen, Qiu, & Zhang, 2015;Tawainella et al., 2014).Also, MnFe 2 O 4 is superior mechanical and luminescent (Kanagesan et al., 2016).
The use of MnFe 2 O 4 as an adsorbent has the advantage of a nano-size material so that it has a large surface area.After the adsorption process, the adsorbent can be pulled easily and quickly from the solution by using magnets because MnFe 2 O 4 has magnetic properties.Several studies have used MnFe 2 O 4 for the adsorption of both organic and inorganic pollutants, for example to removal of Pb (II) and Cu (II) (Ren et al., 2012), congo red (Zhong et al., 2015), phosphate (Xia et al., 2016), chromium (Sezgin, Yalcin, & Koseoglu, 2016), and rhodamin B (Riyanti, Hariani, & Purwaningrum, 2018).
In this study, a composite synthesis from activated carbon and MnFe 2 O 4 was done by using the co-precipitation method.The formation of nanomagnetic composites can increase the adsorption capacity and the efficiency of the adsorption process.The MnFe 2 O 4 -activated carbon composite produced was used to adsorb methyl red dyes.Yamaguchi, Bergamasco, & Hamoudi (2016) reported that the synthesized MnFe 2 O 4graphene composite using a one-pot facile method has a larger adsorption capacity than that of MnFe 2 O 4 and graphene.
The methyl red is an anionic dye having a molecular formula C 15 H 15 N 3 O 2 with a molecular weight of 269.30 g/mol (Khan, Shahjahan, & Khan, 2018).Methyl red has an azo group (-N = N-).This dye is widely used in industry in the coloring process because it has a bright and stable color.Synthetic dye is reported to cause carcinogenesis, respiratory toxicity, mutagenic in living organisms, skin irritation, chromosomal fractures, and teratogenicity (Santhi, Manonmani, & Smitha, 2010;Saxena, & Sharma, 2016).The presence of dyes in waters with low concentrations (<1 mg/L) may prevent sunlight from entering the waters, thus interfering with the process of photosynthetic plankton (Khan et al., 2018).Therefore, the treatment of wastewater containing synthetic dyes is very necessary before it is discharged into the environment.

Chemical
The activated carbon used in this study is a local product having a surface area of 217.697 m 2 /g, moisture content and ash content of 14.179 and 6.518%.
Into this mixture, 2.307 g of activated carbon was added.This mixture was stirred, and NaOH 5 M was slowly added until its pH reached ± 10.The stirring was continued for 30 minutes.The composite is separated from the solution using a magnet.Then washed with distilled water until neutral pH and heated in the oven at 100 °C for 2 hours.The composites were then determined by pHpzc and characterized by using FTIR, SEM-EDS, and SAA.

Determination of pH Point Zero Charge
A total of 50 mL of a 0.01M NaNO 3 solution was put respectively into 11 Erlenmeyer.The initial pH of the solution was adjusted from 2˗12 by adding 0.1M HCl or 0.1M NaOH solution.Then into each Erlenmeyer, 0.2 g of the composite was added, it was shaken by a shaker for 2 hours.The mixture was allowed to stand for two days and measured its final pH using pH meters of each solution.

Adsorption Process
The adsorption process was carried out by the batch method.An amount of 100 mL of 100 mg/L methyl red solution (pH 4.4) was placed in a glass bottle, 0.15 g of MnFe 2 O 4activated carbon composite was added.The experiment was performed at room temperature.The glass bottle was placed on the shaker, stirring was done in the range of 10-140 minutes with a speed of 150 rpm.The adsorbent was separated from the solution using a permanent magnet.The concentration of the remaining dye was measured using a UV-Vis spectrophotometer at λ 490 nm.The effect of adsorbent weight was measured by 0.05-0.3g adsorbent weight variation, whereas adsorption isotherm was measured using concentration of methyl red from 50-100 mg/L, the adsorbent weight of 0.15 g and contact time of 100 minutes.
The analysis with FTIR was performed on activated carbon and MnFe 2 O 4 -activated carbon composite.Both spectra have a major difference in the wave number of 515.0 cm -1 indicating the presence of manganese ferrites vibrations which were not found in the activated carbon spectra.Hidarian, & Hishemian (2014) reported the characteristics of the presence of MnFe 2 O 4 on the composite appearing at wave numbers of 415-691 cm -1 .The similar result reported Riyanti et al. ( 2018) obtained at wave number 561.10 cm -1 as the vibration of Mn-O.The FTIR spectra of the MnFe 2 O 4 -activated carbon composite and the activated carbon are shown in Figure 2.
The comparison of the wave number of activated carbon and MnFe 2 O 4 -activated carbon composite is shown in Table 1.The wave number of 3382.9 cm -1 in the composite shows the presence of stretching and bending O-H groups.The peak that appears at the wave number of 2920.0 cm -1 shows the vibration of C-H.The wave number at 1562.2 cm -1 shows the presence of the C=C group.The presence of C-O was identified at wave numbers 1267.1 cm -1 .

Activated carbon Composite
The morphology of the activated carbon and MnFe 2 O 4 -activated carbon composite identified using SEM with 5000 x magnification is shown in Figure 3.In the Figure, it can be observed that the activated carbon has a surface morphology with wide, nonhomogeneous porous.The MnFe 2 O 4activated carbon composite has more pores and tends to be uniform with smaller pore sizes than those of the activated carbon.This occurs because of the addition of MnFe 2 O 4 on the activated carbon which causes the oxide to enter the pores and is also dispersed randomly on the composite surface.Table 2 shows the results of EDS analysis.
Activated carbon contains a major element of carbon at 98.92 %, and oxygen in a small percentage of 1.08 %.The addition of MnFe 2 O 4 to the activated carbon causes a decrease in carbon content and an increase of O, Fe, and Mn.This proves that the composite synthesis process has been successful.The presence of Cl on the composite shows that there is still the remain of Cl from the synthesis process that has not been lost during the washing process.This is also reinforced by the data of the measurement of surface area in which the activated carbon used has a surface area of 217.697 m 2 /g, with the addition of MnFe 2 O 4 the surface area decreases to 143.992 m 2 /g.However, with the addition of MnFe 2 O 4, the activated carbon may have magnetic properties.The result of the measurement of magnetic properties using VSM is shown in Figure 4.    Do et al. (2011) showed that the greater the nano ferrites content in the composite so the stronger the magnetic properties and the smaller the surface area.The active carbon pore space is blocked by nano-sized magnetic ferrites.The magnetic properties of this composite have the advantage, the separation of adsorbents from the solution can be done quickly using magnets without filtering, more effective and economical than non-magnetic adsorbents.
pH pzc (point zero charges) is a state in which the positive and the negative charges on the surface of a substance are equal so that the surface is neutrally charged.Figure 5 shows the pH pzc of the MnFe 2 O 4 -activated carbon composite and activated carbon.The composite has a pH pzc of 5 indicating that at that pH the composite surface is neutral.At pH <5 the composite group is positively charged so that it attractive the anion more easily, whereas at pH > 5 the composite surface is negatively charged so that it is easier to adsorb the cation.The activated carbon has a pH pzc 6 that is bigger than composite.

Adsorption Study
The adsorption process of methyl red using activated carbon and MnFe 2 O 4 -activated carbon composite was performed at pH of methyl red solution of ± 4.4.Methyl red dye is an anionic dye having a pH range of 4.2-6.2.In that pH range, the dye is negatively charged, while the adsorbent is positively charged (pH solution <pH pzc), so that the tensile pull between the dye and the adsorbent is more effective.Other studies have reported that the methyl red adsorption process is effective at pH 4 using activated carbon from Annona squmosa seed (Santhi et al., 2010) and activated carbon from coconut husk (Tan et al., 2008).
Figure 6 shows the effect of contact time on methyl red adsorption process using activated carbon and MnFe 2 O 4 -activated carbon composite.The concentration of methyl red was 100 mg/L, the composite's weight was 0.15 g and the pH was 4.4.The adsorption rate increased with the increase of contact time.At the contact time of 10-60 minutes, the adsorption process fast.At the contact time of 60 minutes, more than 50% of methyl red was adsorbed.After that, the adsorption process slowly and reached the optimum at the contact time of 100 minutes.
The amount of adsorbent is a parameter related to the cost of an effective amount of adsorbent to adsorb an adsorbate.Figure 7 shows the effect of the weight of adsorbent that is the weight of 0.05-0.3g, the concentration of methyl red dye of 100 mg/L, the contact time of 100 minutes and the pH of the solution of 4.4.The adsorption rate increases with the addition of adsorbent from 0.05 -0.15 g.The increase in the amount of adsorbent also increases the number of active sites that can bind to the dye.However, with the addition of more adsorbent (> 0.15 g), the % of removal is relatively constant.reported that an increase in the amount of adsorbent could decrease the adsorption capacity since the increase in the amount of adsorbent is not proportional to the increase in the amount of the dye adsorbed.Similarly, the study of Khan et al. (2018) showed the same pattern on methyl red adsorption using activated carbon.

Adsorption Isotherm
The adsorption capacity of activated carbon and MnFe 2 O 4 -activated carbon composite in removal methyl red can be illustrated using an adsorption isotherm model.Langmuir's isotherm theory is based on the assumption that the surface of the adsorbent is homogeneous, whereas Freundlich's isotherm illustrates that the adsorbent has a heterogeneous surface so that the adsorption process has an energy difference.The equation of Langmuir's and Freundlich's isotherm is expressed in the following equation: in which q m and b represent the adsorption capacity (mg/g) and the adsorption equilibrium (mg/L), whiles k f and n are the Freundlich's constants expressing the adsorption capacity (mg/L) and the adsorption intensity (Hayeeye et al., 2017).

Kinetic Adsorption
Kinetic parameters and correlation coefficients of the adsorption process are shown in Table 4. From the table, it can be seen that the correlation coefficient value of the pseudo-second-order is greater than the pseudo-first-order for activated carbon and MnFe 2 O 4 -activated carbon composite.The equation of pseudo-first-order and pseudosecond-order is shown in the following equation: -) = (4) + (5) and are the number of adsorbates which are absorbed per unit of mass at equilibrium and every time, and are equilibrium constants for pseudo-first-order and pseudosecond-order.From the table, it can be seen that the correlation coefficient value of the pseudo-second-order is greater than the pseudo-first-order for activated carbon and MnFe 2 O 4 -activated carbon composite.The methyl red adsorption process is more compatible with pseudo-second-order.The adsorption mechanism depends on the interaction of the adsorbate and the adsorbent (Sarma, SenGupta, & Bhattacharyya, 2018).

CONCLUSIONS
In this study, the synthesis of MnFe 2 O 4activated carbon composite has been successfully performed by the co-precipitation method and it was applied to the removal of methyl red dye.The composite has magnetic properties with a saturation magnetization of 17.91 emu/g.The Langmuir isotherm is more suitable for describing methyl red dye adsorption in MnFe 2 O 4 -activated carbon composite.The adsorption capacity of MnFe 2 O 4 -activated carbon composite is greater than that of the activated carbon, namely 81.97 and 60.24 mg/g, respectively.Thus, MnFe 2 O 4activated carbon composite is potentially used as a commercial adsorbent for waste treatment.

Figure 6 .Figure 7 .
Figure 6.The effect of contact time on the adsorption of methyl red using activated carbon and MnFe 2 O 4 -activated carbon composite s and Freundlich's isotherm graphs are shown in Figure8 and 9.The correlation coefficient (R 2 ) and other constants listed in

Table 1 .
The Comparison of the wave number of activated carbon and MnFe 2 O 4 -activated carbon composite

Table 2 .
The Elements of activated carbon and MnFe 2 O 4 -activated carbon composite

Table 3
Co is the initial concentration (mg/L).The data calculation reveals that the value of R L of the activated carbon is 0.188 while of the composite is 0.244.This indicates that Langmuir isotherm is favorable.

Table 3 .
Parameters in the Langmuir and Freundlich models for methyl red adsorption

Table 4 .
Kinetic parameters for adsorption methyl red by activated carbon and MnFe 2 O 4 -activated carbon composite