Bionanocomposite MIL-100(Fe)/Cellulose as a high-performance adsorbent for adsorption of methylene blue

Water refining treatment consisted to eliminate organic matter and improve the organoleptic qualities of the water (flavor, smell, clarity). There are different methods for removal of pollutants which among them, the adsorption process has been expanded due to its easiness and high efficiency. The development of innovative porous materials is therefore of primary importance for the treatment of effluents. Dyes are important source of pollution once discharged into the environment. World production of dyes is estimated at more than 800,000 t•yr -1 . The purpose of this research falls within the scope of the choice of an effective, local and inexpensive adsorbent to remove dyes from waste water. Adsorptive elimination of dyes by commonly accessible adsorbents is inefficient. The metal organic frameworks (MOFs) are important class of porous materials offering exceptional properties as adsorbents by improving separation efficiency compared to existing commercial adsorbents. However, its powder form limits its applications. One way to overcome this problem is to trap them in a flexible matrix to form a hierarchical porous composite. Therefore, in this work, we prepared MIL-100 (Fe) embedded in cellulose matrix named MIL-100(Fe)/Cell, and used it as an adsorbent of methylene blue (MB) dye. Adsorption was studied as a function of contact time, adsorbent mass and pollutant load (concentration), and pH, and the effect of each of them on absorption efficiency was optimized. MIL-100(Fe)/Cell was capable to remove 98.94% of MB dye with an initial concentration of 150 mg/L within 10 minutes. The obtained maximum adsorption capacity was 384.615 mg/g. The adsorption isotherm and kinetic were consistent with the Langmuir and pseudo-second order models. The mechanism of MB adsorption proceeds th rough п - п and electrostatic interactions.


Introduction
Today, more than 6.5 billion human beings have to make do with the same amount of water, which is why the raw material water, for a long time freely available in many parts of the earth, is still seriously threatened.Industrial effluents and pollutants resulting from the intensive use of fertilizers, pesticides, sanitary, agricultural and pharmaceutical products are the major causes of environmental pollution.Pollution generated by human activities represents an increasingly worrying threat to humans and ecosystems, which requires effective techniques to separate and remove them from the environment [1][2][3][4] .Among the industries that consume large quantities of water, textiles and dyeing are at the top of the list, the sectors of dyeing, printing or finishing of textiles occupy the following positions.
These effluents are heavily loaded with acidic or basic dyes, salts and additives [5][6][7][8][9] .Several complementary treatments are possible such as adsorption or degradation by oxidation 10,11 .There are different types of materials which can be used as sorbent.These materials include graphene oxide (GO), zeolites, MOFs, magnetic nanoparticles, natural polymers, etc.One of the important challenges that researchers are dealing with is to make a sorbent that is both easy to synthesize and effective for adsorbing pollutants [12][13][14][15][16][17] .
Metal-organic frameworks (MOFs) are a unique class of porous crystalline materials, formed by the interconnection of aggregates metal ions with organic molecules bricks.The characteristics of these structures include high specific surface area and porosity, adjustable functional groups, controlled pore size, and simple preparation process.These features cause MOFs materials have many applications such photocatalytic degradation, gas storage, drug delivery, sensors, energy conversion and storage, etc. 18 .The use of MOFs to adsorb dyes has been developed in recent years 19,20 .MOFs can be prepared with various dimensionality in addition to diverse sizes and shapes, reliant on the kind of preparation procedure and materials which used.There are different organic ligands which used for synthesis of MOFs and these ligands play significant rolls in structure of MOFs as well as in their properties like adsorption performance, thermal and aquatic stability.In addition, the MOFs can be synthesized with different types of natural polymers like cellulose, chitosan, starch, alginate, etc.This causes various properties to be added to the MOFs and increases their performance by adsorbing pollutants [21][22][23][24] .
Cellulose is polysaccharide mainly produced by plants.The important properties of cellulose are its abundance in nature, cheapness, non-toxicity, biocompatibility and biodegradability.Due to these features, cellulose is a suitable option for preparing natural supports in the manufacture of heterogeneous catalysts 25,26 .Cellulose was composed of many glucoses unites which bind together with 1,4-glycosidic bonds.Owing to its units, cellulose has many -OH functional groups which can be simplify functionalized with different materials 27 .In this research, we synthesized MIL-100(Fe) on cellulose support (MIL-100/Cell) by simple method at ambient temperature by means of water as a green solvent and used for MB adsorption.The MB was used as the target pollutants because it is broadly applied in industries like pharmaceuticals, food, textile and printing, and there are large amounts of its in wastewater.In addition, we optimized different factors for adsorption reaction like initial Con of the dye, Ads doses, pH and the time.

Characterization of MIL-100/Cell
The prepared MIL-100/Cell was analyzed by diverse analytical techniques like FTIR, XRD, SEM, TGA/DTA, BET, EDS and elemental mapping.
Figure 1 displayed the FTIR spectra of cellulose, MIL-100 and MIL-100/Cell.According to the cellulose spectrum, the absorption peaks at 901 and 1063 cm -1 correspond to β-glycosidic bond and -C-O-Cpyranose ring, respectively.The absorption bands at 1374 and 2900 cm -1 are correlated to the stretching and bending vibrations of -CH of glucose units of cellulose.Also, the absorption peak at 3400 cm -1 is related to the stretching vibrations of -OH in the structure of cellulose 28 .In the MIL-100 spectrum, the absorption peak related to the C=O bond at 1620 cm -1 , the C=C bond of the benzene ring at 1379 cm -1 and 1446, the -CH of benzene at 712 cm -1 and 763, and Fe-O bond at 615 cm -1 are detected.After placing MIL-100 on the cellulose substrate, some peaks undergo changes.The nitrogen adsorption/desorption isotherm of MIL-100/Cell is according to Figure 5.The results of BET analysis can be seen in Table 1.The BET isotherm corresponds to the type п isotherm, which corresponds to porous materials.

Adsorption studies
Factors such as the dye Con, amount of Ads, pH and reaction time are effective in the adsorption process.

Effect of MB concentration
One of the most significant and influential factors on the AdCa is the initial dye Con.To examine the effect of initial dye Con in the adsorption procedure, we choose different Con of MB including 50, 100, 150, 200 and 250 (mg/L) and their AdCa was measured in the presence of 10 mg of MIL-100/Cell (Figure 7 (a)).As the dye Con increases from 50 to 150 (mg/g), the AdCa increases and with further increase remain constant.

Effect of pH
Figure 7 (c) was displayed the AdCa of MIL-100/Cell through changes pH from 5 to 11.Experiments exhibited that with increasing the pH from 5 to 6.5 the AdCa increased.As the pH increases, the Ads surface becomes negatively charged and facilitates electrostatic interaction with cationic dye molecules.In the range of 6.5 to 11 the pH remains constant and does not change much, which indicates that electrostatic interaction is not the only mechanism of dye adsorption 30 .

Adsorption isotherm studies
Equilibrium data, commonly recognized as adsorption isotherms (AdIs), are applied to define the adsorption mechanism.The AdIs describe how the adsorbed molecules or ions interact with the Ads surface.In this work, Langmuir and Freundlich AdIs were applied to investigate the adsorption of MB dye by MIL-100/Cell using Eqs (3)   and ( 4): where qm (mg/g) is the maximum AdCa, qe(mg/g) is equilibrium AdCa, KL is Langmuir adsorption constant, KF is Freundlich constant and n indicates the adsorption intensity.The results of the data can be seen in Figure 8 and Table 2.According to the R 2 values obtained from the isotherm models, the MB adsorption process by MIL-100/Cell is more consistent with the Langmuir model (R 2 =0.9999) and adsorption occurs monolayer.

Kinetics of dye adsorption
The pseudo-first-order (Eq.5) and pseudo-second-order (Eq.6) kinetic models were applied to the experimental results to define the order of the adsorption reaction in the case of the adsorbent/adsorbate system studied.
Where K1 (min -1 ) indicates the rate constant of the pseudo first order kinetic model and K2 (min -1 ) represents the rate constant of pseudo second order kinetic model.The obtained results can be seen in Table 3.The obtained R 2 values display that the kinetics of MB by MIL-100/Cell is consistent with pseudo second order model (R 2 =0.9815) and adsorption occurs by chemisorption.

Thermodynamics of dye adsorption
In order to examine the effect of temperature on the MB adsorption by MIL-100/Cell ̦ free energy changes (ΔG°), enthalpy changes (ΔH°) and entropy changes (ΔS°) were investigated by the following Eqs: ΔG°= -RT ln Kd ( 8) In these Eqs, R is the global gas constant (8.314J mol -1 K -1 ), T is temperature (K) and Kd is the equilibrium constant.
The results of thermodynamic calculations can be seen in Table 4.
Table 4. Results of thermodynamic calculations.

T(K)
ΔG°(J mol -1 ) ΔH°(J mol -1 ) ΔS°(J mol -1 K - The obtained results show that the amount of adsorption decreases with increasing temperature ̦ so the adsorption of MB is physically by MIL-100/Cell.The negativity of ΔG° at all temperatures indicates spontaneity of the procedure, the negativity of ΔH° indicates the exothermic nature of the process and the positivity of ΔS° shows increase in disorder of the system in through the adsorption process.

Mechanism of MB adsorption
According to Figure 9 electrostatic and ᴫ-ᴫ stacking MB and MIL-100/Cell are involved in dye adsorption.The electrostatic interaction occurs among the positive charge of MB and the negative charge of MIL-100/Cell, while the ᴫ-ᴫ interaction is done between the aromatic rings of the dye and Ads.

Comparison of MIL-100/Cell with other adsorbents
The comparison of the efficiency and adsorption performance of MIL-100/Cell with previously reported Ads for the adsorption of MB is tabulated in Table 5.As can be seen, MIL-100/Cell adsorb Ads exhibitions higher adsorption performance in the removal of MB compared to the other Ads.In fact, in this work we used high concentrated of the MB and at least amount if the Ads.Compared to other works, according to the amount of MB Con, pH, and catalyst amount, the time used for the reduction reaction has been very appropriate.

Recycling of the MIL-100/Cell
When the adsorption of the MB was finished the MIL-100/Cell Ads was washed with ethanol after collected by centrifugation and dried at 70 C° until reused in next cycle.The results show that the dye RE is almost constant up to 3 cycles and after that it decreases at a slow rate Figure 10.In fact, the Ads can be recycled for 5 times without noteworthy loss of in performance.

Conclusion
Dyes are among the most released industrial effluents into the environment.The removal of organic dyes is a global challenge due to the occurrence of various diseases in humans.This study demonstrated the effectiveness of MIL-100/Cell in removing MB dye from aqueous medium.The MIL-100/Cell was synthesized by a simple and green method at room temperature using water as a green solvent.The influence of parameters related to operating conditions such as contact time, amount of adsorbent, pH, initial dye concentration and temperature was examined.The plot of the adsorption isotherms shows that the Langmuir model perfectly represents the adsorption of MB on MIL-100/Cell following a pseudo-second order models, and a maximum adsorption capacity of 384.6 mg/g after 10 min.The thermodynamic parameters obtained indicate that the adsorption of methylene blue dye on MIL-100/Cell is a spontaneous and exothermic process.The mechanism of MB adsorption proceeds through п-п and electrostatic interactions.According to the BET analysis the specific surface area of the synthesized MOF is 294 m 2 /g which related to the presence of the cellulose as efficient and green support.The MIL-100/Cell has many advantages such as simple and green synthesis method, use of inexpensive and non-toxic cellulose in the structure, high AdCa and short reaction time.The abundance of cellulose can offer a low-cost adsorption material that can potentially contribute to the treatment of textile effluents.

Experimental section 7.1 Materials and method
All materials applied in this study were bought from Merck and Aldrich Chemical Co and applied without extra purification.The prepared Ads characterized with different analytical methods like Fourier transform infrared (FTIR) ̦ X-ray powder diffraction (XRD)̦ Scanning electron microscope (SEM) ̦ Thermal gravimetric analysis (TGA/DTA) ̦ Ehlers-Danlos syndromes (EDS) and Brunauer-Emmett-Teller (BET).

Synthesis of MIL-100/Cell
MIL-100(Fe) on cellulose was prepared according to a method from the literature 37 .First 1.676 g of benzene-1,3,5tricarboxylic acid (BTC) was dissolved in NaOH (1 M, 30 mL) (Solution A).Then, FeCl2.4H2O(2.26 g) was dissolved in deionized water (97.2 mL) and added dropwise to cellulose (300 mg) and stirred for 30 min at room temperature (Solution B).Solution A was added dropwise to solution B and stirred at ambient temperature for 24 h.The obtained brown product was filtered and washed with EtOH and dried at 70 0 C for 24 h (MIL-100/Cell).The MIL-100/Cell was activated under vacuum at 120 0 C for 6h (Figure 11).

Figure 2 .
Figure 2. a) XRD and b) expand XRD pattern of the MIL-100/Cell.

Figure 3
Figure3shows the SEM images of the MIL-100 and MIL-100/Cell.According to the obtained images, the approximately hexagonal structure of MIL-100 is proved.
The existence of C, O and Fe elements in the MIL-100/Cell structure was confirmed by EDS and elemental mapping according to Figure6.These analyses displayed that the MIL-100/Cell was synthesized.

Figure 6 .
Figure 6.The EDS and elemental mapping analysis of the MIL-100/Cell.

Figure 7 (
Figure 7 (b) displays the AdCa for different amount of MIL-100/Cell.According to the achieved results, the highest amount of AdCa occurs in the presence of 0.01 g of MIL-100/Cell.

Figure 7 (
Figure 7 (d) displays the effect of reaction time on AdCa.By increasing the reaction time up to 10 min, the AdCa increases and reaches its maximum value in 10 min.The quick increase in AdCa at the start of the procedure is owing to the accessibility of most of the active sites for the adsorption of dye molecules.Active sites gradually fill up over time, which slows down adsorption 30 .

Table 1 .
The results of BET analysis.

Table 2 .
The AdIs factors from Langmuir and Freundlich model.

Table 5 .
Comparison of adsorption performance of the MIL-100/Cell with other Ads.