Iron Molybdate Fe2(MoO4)3 Nanoparticles: Efficient Sorbent for Methylene Blue Dye Removal from Aqueous Solutions

The present study investigated iron molybdate (Fe2(MoO4)3), synthesized via a simple method, as a nanosorbent for methylene blue (MB) dye removal from aqueous solutions. Investigations of the effects of several parameters like contact time, adsorbent dose, initial dye concentration, temperature and pH were carried out. The results showed that MB removal was affected, significantly, by adsorbent dose and pH. Interestingly, lower values of adsorbent dose resulted in the removal of higher amounts of MB. At the optimum pH, the removal efficiency of 99% was gained with an initial MB concentration of ≤60 ppm. The kinetic study specified an excellent correlation of the experimental results with the pseudo-second-order kinetics model. Thermodynamic studies proved a spontaneous, favorable and endothermic removal. The maximum amount of removal capacity of MB dye was 6173 mg/g, which was determined from the Langmuir model. The removal efficiency was shown to be retained after three cycles of reuse, as proven by thermal regeneration tests. The presence and adsorption of the dye onto the Fe2(MoO4)3 nanoparticle surface, as well as the regeneration of the latter, was ascertained by scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). These findings are indicative that the investigated nanosorbent is an excellent candidate for the removal of MB in wastewater.


Introduction
The textile and dyeing industries are major problem sources of water pollution [1,2]. Dye effluent released into clean water causes a variety of health hazards in marine organisms, freshwater sources and humans [2]. Thus, dyes are considered to be highly toxic and carcinogenic, and their removal is of great interest. unit mass at equilibrium (q e ), which increased from 110 to 137 mg/g. There was a strong electrostatic interaction of the charges between the MB dyes and the Fe 2 (MoO 4 ) 3 adsorbent, and this was shown by increasing the pH values, which resulted in a higher percentage obtained. In fact, the hydroxyl group (OH − ) in the solution at pH 11 favors the positive charge of the MB, which has a pKa equal to 3.8 [40]. However, at acidic values, the lower removal efficiency could be linked to the excess of proton ions in the solution competing with the basic dye cations on the removal sites of Fe 2 (MoO 4 ) 3 . Similar findings were reported by Kooli et al. [41] in a study of waste bricks applied as a promising removal agent for basic blue 41 from aqueous solutions. Thus, the best value for MB removal using Fe 2 (MoO 4 ) 3 nanosorbent was shown at pH 11.
Molecules 2020, 25, x FOR PEER REVIEW 3 of 16 increasing pH values from 3 to 11. The same trend was also observed for the dye removal amount per adsorbent unit mass at equilibrium (qe), which increased from 110 to 137 mg/g. There was a strong electrostatic interaction of the charges between the MB dyes and the Fe2(MoO4)3 adsorbent, and this was shown by increasing the pH values, which resulted in a higher percentage obtained. In fact, the hydroxyl group (OH -) in the solution at pH 11 favors the positive charge of the MB, which has a pKa equal to 3.8 [40]. However, at acidic values, the lower removal efficiency could be linked to the excess of proton ions in the solution competing with the basic dye cations on the removal sites of Fe2(MoO4)3. Similar findings were reported by Kooli et al. [41] in a study of waste bricks applied as a promising removal agent for basic blue 41 from aqueous solutions. Thus, the best value for MB removal using Fe2(MoO4)3 nanosorbent was shown at pH 11.

Adsorbent Dose Effect
The adsorbent dose is regarded as one of the important parameters in the adsorption processes [42]. MB dye removal using Fe2(MoO4)3, with an initial dye concentration of 70 ppm, was explored by varying the dose of the adsorbent between 0.001 and 0.05 g/L. As can be depicted from Figure 2, the percentage (%) of removed MB increased, and its concentration (mg/g) decreased when the adsorbent dose increased from 0.001 to 0.05 g/L. This was an expected tendency, since the active sites of the adsorbent's surface area increase with the increasing adsorbent dose, which therefore leads to an increasing amount of removed MB [43].

Initial Dye Concentration and Contact Time Effect
Removal studies, in light of the effect of initial MB dye concentration and contact time, were conducted at pH 11, and the results are shown in Figure 3. A percentage of 95% removal was achieved within 30 min for both Ci of 50 and 60 ppm, which increased to 99% after a 120-min contact time. As

Adsorbent Dose Effect
The adsorbent dose is regarded as one of the important parameters in the adsorption processes [42]. MB dye removal using Fe 2 (MoO 4 ) 3 , with an initial dye concentration of 70 ppm, was explored by varying the dose of the adsorbent between 0.001 and 0.05 g/L. As can be depicted from Figure 2, the percentage (%) of removed MB increased, and its concentration (mg/g) decreased when the adsorbent dose increased from 0.001 to 0.05 g/L. This was an expected tendency, since the active sites of the adsorbent's surface area increase with the increasing adsorbent dose, which therefore leads to an increasing amount of removed MB [43]. increasing pH values from 3 to 11. The same trend was also observed for the dye removal amount per adsorbent unit mass at equilibrium (qe), which increased from 110 to 137 mg/g. There was a strong electrostatic interaction of the charges between the MB dyes and the Fe2(MoO4)3 adsorbent, and this was shown by increasing the pH values, which resulted in a higher percentage obtained. In fact, the hydroxyl group (OH -) in the solution at pH 11 favors the positive charge of the MB, which has a pKa equal to 3.8 [40]. However, at acidic values, the lower removal efficiency could be linked to the excess of proton ions in the solution competing with the basic dye cations on the removal sites of Fe2(MoO4)3. Similar findings were reported by Kooli et al. [41] in a study of waste bricks applied as a promising removal agent for basic blue 41 from aqueous solutions. Thus, the best value for MB removal using Fe2(MoO4)3 nanosorbent was shown at pH 11.

Adsorbent Dose Effect
The adsorbent dose is regarded as one of the important parameters in the adsorption processes [42]. MB dye removal using Fe2(MoO4)3, with an initial dye concentration of 70 ppm, was explored by varying the dose of the adsorbent between 0.001 and 0.05 g/L. As can be depicted from Figure 2, the percentage (%) of removed MB increased, and its concentration (mg/g) decreased when the adsorbent dose increased from 0.001 to 0.05 g/L. This was an expected tendency, since the active sites of the adsorbent's surface area increase with the increasing adsorbent dose, which therefore leads to an increasing amount of removed MB [43].

Initial Dye Concentration and Contact Time Effect
Removal studies, in light of the effect of initial MB dye concentration and contact time, were conducted at pH 11, and the results are shown in Figure 3. A percentage of 95% removal was achieved within 30 min for both Ci of 50 and 60 ppm, which increased to 99% after a 120-min contact time. As

Initial Dye Concentration and Contact Time Effect
Removal studies, in light of the effect of initial MB dye concentration and contact time, were conducted at pH 11, and the results are shown in Figure 3. A percentage of 95% removal was achieved within 30 min for both C i of 50 and 60 ppm, which increased to 99% after a 120-min contact time. As for a C i value of 65 ppm, the removal percentage maximum (92%) was reached with a contact time of 120 min. However, removal maxima of 88% and 76% were obtained with C i values of 70 and 80 ppm, respectively, after a 120-min contact time. Thus, the removal capacity increased notably from 4999 mg/g to 6179 mg/g with increasing initial concentrations of dye from 50 to 80 ppm. Such a trend could be attributed to the initially abundant empty sites onto the Fe 2 (MoO 4 ) 3 surface, which, as a consequence of the sorption process, gradually decreased by filling up these sites with increasing contact times [44].
Molecules 2020, 25, x FOR PEER REVIEW 4 of 16 for a Ci value of 65 ppm, the removal percentage maximum (92%) was reached with a contact time of 120 min. However, removal maxima of 88% and 76% were obtained with Ci values of 70 and 80 ppm, respectively, after a 120-min contact time. Thus, the removal capacity increased notably from 4999 mg/g to 6179 mg/g with increasing initial concentrations of dye from 50 to 80 ppm. Such a trend could be attributed to the initially abundant empty sites onto the Fe2(MoO4)3 surface, which, as a consequence of the sorption process, gradually decreased by filling up these sites with increasing contact times [44].

Temperature Effect
Temperature is another parameter of prime importance that has a great impact on dye removal [45]. The process of removing the MB dye was investigated from 20 to 70 °C, as can be seen in Figure  4. The effect of temperature studies showed that the percentage removal increased from 76% to 99% at an initial dye concentration of 80 ppm, with an increased removal capacity from 6109 mg/g to 7999 mg/g. In fact, the removal motion of the adsorbent sites improved with increasing temperature, which, in turn, caused the motion of the dye molecules to increase [46]. Thermodynamic factors are important parameters in the adsorption processes [47]. The probability and the adsorption mechanism are predictable in the light of thermodynamic parameters [47]. These can be evaluated by means of the following equations:

Temperature Effect
Temperature is another parameter of prime importance that has a great impact on dye removal [45]. The process of removing the MB dye was investigated from 20 to 70 • C, as can be seen in Figure 4. The effect of temperature studies showed that the percentage removal increased from 76% to 99% at an initial dye concentration of 80 ppm, with an increased removal capacity from 6109 mg/g to 7999 mg/g. In fact, the removal motion of the adsorbent sites improved with increasing temperature, which, in turn, caused the motion of the dye molecules to increase [46]. for a Ci value of 65 ppm, the removal percentage maximum (92%) was reached with a contact time of 120 min. However, removal maxima of 88% and 76% were obtained with Ci values of 70 and 80 ppm, respectively, after a 120-min contact time. Thus, the removal capacity increased notably from 4999 mg/g to 6179 mg/g with increasing initial concentrations of dye from 50 to 80 ppm. Such a trend could be attributed to the initially abundant empty sites onto the Fe2(MoO4)3 surface, which, as a consequence of the sorption process, gradually decreased by filling up these sites with increasing contact times [44].

Temperature Effect
Temperature is another parameter of prime importance that has a great impact on dye removal [45]. The process of removing the MB dye was investigated from 20 to 70 °C, as can be seen in Figure  4. The effect of temperature studies showed that the percentage removal increased from 76% to 99% at an initial dye concentration of 80 ppm, with an increased removal capacity from 6109 mg/g to 7999 mg/g. In fact, the removal motion of the adsorbent sites improved with increasing temperature, which, in turn, caused the motion of the dye molecules to increase [46]. Thermodynamic factors are important parameters in the adsorption processes [47]. The probability and the adsorption mechanism are predictable in the light of thermodynamic parameters [47]. These can be evaluated by means of the following equations: Thermodynamic factors are important parameters in the adsorption processes [47]. The probability and the adsorption mechanism are predictable in the light of thermodynamic parameters [47]. These can be evaluated by means of the following equations: Molecules 2020, 25, 5100 where K d is the distribution constant, T is the absolute temperature (K), R is the gas constant (J.mol −1 .K −1 ), ∆G • is the free energy, C a is the amount of dye adsorbed at equilibrium, C e is the equilibrium concentration (mol/L) and ∆H • and ∆S • are the standard enthalpy and standard entropy, respectively. The values of ∆S • and ∆H • were determined from the intercept and slope of the plot ln K d versus 1/T ( Figure 5), and ∆G • values were calculated from Equation (1). All data are shown in Table 1.
The negative sign of ∆G • indicated a favorable and spontaneous adsorption. The removal of MB dye, as indicated by the positive value of ∆H • (83.79 KJ.mol −1 ), was proven to occur via a physisorption process [48]. The positive values of ∆S • were proof of the increased randomness and disorder at the solid-solution interface of Fe 2 (MoO 4 ) 3 and MB. The adsorbate molecules caused the adsorbed water molecules to move, and consequently, an additional translational energy was gained, resulting in a random system taking place [49].
Molecules 2020, 25, x FOR PEER REVIEW 5 of 16 where Kd is the distribution constant, T is the absolute temperature (K), R is the gas constant (J.mol −1 .K −1 ), ΔG° is the free energy, Ca is the amount of dye adsorbed at equilibrium, Ce is the equilibrium concentration (mol/L) and ΔH° and ΔS° are the standard enthalpy and standard entropy, respectively. The values of ∆S° and ∆H° were determined from the intercept and slope of the plot ln Kd versus 1/T ( Figure 5), and ∆G° values were calculated from Equation (1). All data are shown in Table 1.
The negative sign of ∆G° indicated a favorable and spontaneous adsorption. The removal of MB dye, as indicated by the positive value of ∆H° (83.79 KJ.mole -1 ), was proven to occur via a physisorption process [48]. The positive values of ∆S° were proof of the increased randomness and disorder at the solid-solution interface of Fe2(MoO4)3 and MB. The adsorbate molecules caused the adsorbed water molecules to move, and consequently, an additional translational energy was gained, resulting in a random system taking place [49].

Kinetic Study
A kinetic test for the removal of MB was investigated in order to provide an indication regarding the adsorption system [38].
The kinetics of MB dye elimination using Fe2(MoO4)3 nanosorbent were evaluated using intraparticle diffusion and pseudo-first-order and pseudo-second-order kinetic models. The equations of the considered models are given in Table 2. Table 2. Kinetic model equations.

Kinetic Study
A kinetic test for the removal of MB was investigated in order to provide an indication regarding the adsorption system [38].
The kinetics of MB dye elimination using Fe 2 (MoO 4 ) 3 nanosorbent were evaluated using intraparticle diffusion and pseudo-first-order and pseudo-second-order kinetic models. The equations of the considered models are given in Table 2.
The three model parameters, namely intraparticle diffusion, pseudo-first-order and pseudo-second-order, are arranged in Table 3 and shown in Figures 6-8, respectively. These models varied in the values of correlation coefficients (R 2 ) of the linear regressions. These values were estimated as follows: 0.748 to 0.993 for intraparticle diffusion, 0.752 to 0.993 for pseudo-first-order model and 0.999 to 1.000 for pseudo-second-order model for the studied concentrations. Since the R 2 value was equal to or near 1 for the pseudo-second-order model, the latter fit very well to the experimental data.

Adsorption Isotherms
It is essential to examine adsorption isotherms because of the information they provide when planning to use the adsorption method [50]. In the present work, we studied the four major adsorption models, namely Freundlich, Langmuir, Dubinin-Radushkevich and Temkin. These are governed by the equations presented in Table 4.

Model Equation Parameters
Pseudo-first-order (PFD) [50] ln q e − q t = ln q e + K 1 t (4) q t : the removal capacity at time t (mg/g) q e : the removal capacity at equilibrium (mg/g) K 1 : the rate constant of pseudo-first-order adsorption (1/min) Pseudo-second-order (PSD) [50] q t : the removal capacity at time t (mg/g) q e : the removal capacity at equilibrium (mg/g) K 2 : the pseudo-second-order rate constant (g. mg −1 .min −1 ) Intraparticle diffusion (IPD) [51] q t = K I t 0.5 + l (6) I (mg/g) and K I (mg/(g.min 0.5 )) are the intraparticle diffusion constants q t : the removal capacity (mg/g) at time t t: the contact time (min)

Adsorption Isotherms
It is essential to examine adsorption isotherms because of the information they provide when planning to use the adsorption method [50]. In the present work, we studied the four major adsorption models, namely Freundlich, Langmuir, Dubinin-Radushkevich and Temkin. These are governed by the equations presented in Table 4.
The Dubinin-Radushkevich, Temkin, Freundlich and Langmuir models were studied and attempted to fit the experimental data. The model parameters and the regression correlation coefficients (R 2 ) are given in Table 5, as extracted from Figure 9. The highest value for R 2 (0.999) was obtained from the Langmuir model, while the fittings of the Freundlich and Temkin models showed the lowest values of R 2 (0.866 and 0.870, respectively), whereas an intermediary value was achieved for the D-R model (R 2 = 0.971). Accordingly, the Langmuir isotherm had the best fit with the experimental results, suggesting that the dye removal proceeded via the formation of an MB monolayer onto the Fe 2 (MoO 4 ) 3 adsorbent surface, with a high adsorption capacity of 6173 mg/g, leading to a homogenous surface. On the other hand, the separation factor R L , ranging from 0.0024 to 0.0038, indicated a favorable dye removal by Fe 2 (MoO 4 ) 3 . Therefore, the investigated nanosorbent had excellent removal efficiency when compared to other materials (Table 6). Table 4. Adsorption isotherm models for MB dye removal using Fe 2 (MoO 4 ) 3 .

Regeneration Efficiency
The repeatability and regeneration of the nanosorbent are very important parameters for its eventual practical applications. In the literature, several regeneration procedures were suggested, such as microwave irradiation, bio-regeneration, supercritical regeneration, chemical extraction, thermal treatment, etc. [17,45,[57][58][59]. The thermal regeneration used in the present work was similar to that discussed in our previous work [45]. In this research, this thermal treatment method was tested for the regeneration process, as the structure of the Fe2(MoO4)3 removal agent was stable.
The results showed that Fe2(MoO4)3 was prone to regeneration by thermal treatment. Figure 10 shows the recycled efficiency of Fe2(MoO4)3 for the removal of methylene blue for three

Regeneration Efficiency
The repeatability and regeneration of the nanosorbent are very important parameters for its eventual practical applications. In the literature, several regeneration procedures were suggested, such as microwave irradiation, bio-regeneration, supercritical regeneration, chemical extraction, thermal treatment, etc. [17,45,[57][58][59]. The thermal regeneration used in the present work was similar to that discussed in our previous work [45]. In this research, this thermal treatment method was tested for the regeneration process, as the structure of the Fe 2 (MoO 4 ) 3 removal agent was stable.
The results showed that Fe 2 (MoO 4 ) 3 was prone to regeneration by thermal treatment. Figure 10 shows the recycled efficiency of Fe 2 (MoO 4 ) 3 for the removal of methylene blue for three cycles. In fact, the results showed a decrease of dye removal from 99% to 96%, with a decreasing removal capacity from 5932 to 5777 mg/g. The adsorbent regeneration through calcination at 400 • C under air atmosphere was shown to be extremely efficient, in addition to its excellent reusability, as suggested by the observed high removal efficiencies.

Fourier-Transform Infrared Spectroscopy
With the aim of fully elucidating the removal process of MB dye by Fe2(MoO4)3 nanosorbent, an FTIR spectroscopic study was carried out on the material prior to and after exposure to MB dye. Figure 11 displays the spectra of the Fe2(MoO4)3 nanosorbent in both cases. As can be noticed, clear flexing and stretching vibrations characteristic of the metal-oxygen bonds were situated at frequencies between 700 and 1000 cm −1 , corresponding to the vibrations of the Mo-O bond of the MoO4 tetrahedra in the Fe2(MoO4)3 [60]. The pure MB spectrum exhibited bands between 1700 and 1000 cm −1 [61], while after MB adsorption, additional bands located at 1600 cm −1 were shown in the FTIR spectrum of (Fe2(MoO4)3-MB). These were attributed to MB C=C bond stretching, inferring the presence of MB as a result of its attachment to Fe2(MoO4)3-active sites [62]. The regenerated Fe2(MoO4)3 FTIR spectrum, hereby denoted as (Fe2(MoO4)3-MB-Reg), obtained upon thermal treatment was very much comparable with that of fresh Fe2(MoO4)3, indicating the complete combustion of the attached MB on the surface. In addition, the obtained spectrum confirmed the purity of the regenerated material and the efficiency of the reused adsorbent. In the same sense, the X-ray diffraction (XRD) pattern of iron molybdate was taken before and after the regeneration, showing the same results.

Fourier-Transform Infrared Spectroscopy
With the aim of fully elucidating the removal process of MB dye by Fe 2 (MoO 4 ) 3 nanosorbent, an FTIR spectroscopic study was carried out on the material prior to and after exposure to MB dye. Figure 11 displays the spectra of the Fe 2 (MoO 4 ) 3 nanosorbent in both cases. As can be noticed, clear flexing and stretching vibrations characteristic of the metal-oxygen bonds were situated at frequencies between 700 and 1000 cm −1 , corresponding to the vibrations of the Mo-O bond of the MoO 4 tetrahedra in the Fe 2 (MoO 4 ) 3 [60]. The pure MB spectrum exhibited bands between 1700 and 1000 cm −1 [61], while after MB adsorption, additional bands located at 1600 cm −1 were shown in the FTIR spectrum of (Fe 2 (MoO 4 ) 3 -MB). These were attributed to MB C=C bond stretching, inferring the presence of MB as a result of its attachment to Fe 2 (MoO 4 ) 3 -active sites [62]. The regenerated Fe 2 (MoO 4 ) 3 FTIR spectrum, hereby denoted as (Fe 2 (MoO 4 ) 3 -MB-Reg), obtained upon thermal treatment was very much comparable with that of fresh Fe 2 (MoO 4 ) 3 , indicating the complete combustion of the attached MB on the surface. In addition, the obtained spectrum confirmed the purity of the regenerated material and the efficiency of the reused adsorbent. In the same sense, the X-ray diffraction (XRD) pattern of iron molybdate was taken before and after the regeneration, showing the same results. Fe2(MoO4)3 FTIR spectrum, hereby denoted as (Fe2(MoO4)3-MB-Reg), obtained upon thermal treatment was very much comparable with that of fresh Fe2(MoO4)3, indicating the complete combustion of the attached MB on the surface. In addition, the obtained spectrum confirmed the purity of the regenerated material and the efficiency of the reused adsorbent. In the same sense, the X-ray diffraction (XRD) pattern of iron molybdate was taken before and after the regeneration, showing the same results.

MB Removal Mechanism
As discussed earlier, MB removal by Fe 2 (MoO 4 ) 3 nanoparticles was found to proceed via an adsorption mechanism. In this respect, FTIR spectroscopic data depicted that no chemical decomposition of MB took place during its removal upon adsorption of the dye's cations, and there was no evidence of any intermediate compounds. Moreover, the removal effectiveness of MB using Fe 2 (MoO 4 ) 3 nanoparticles increased with increasing pH values up to 11, which were attributed to the alkaline media. From these findings, we proposed the removal mechanism given in Figure 12. During the first step, MB (pKa = 3.8) maintained its positive charge at pH 11 [40]. In the same conditions, the iron molybdate (Fe 2 (MoO 5 2− ) 3 ) ion was produced, without intermediate compounds, by the reaction of Fe 2 (MoO 4 ) 3 with the hydroxyl groups (OH − ) present in solution [63]. Hence, the electrostatic interactions were governing the adsorptive process. These strong interactions were notable between the negatively charged iron molybdate (Fe 2 (MoO 5 2− ) 3 ) surface and the positively charged MB cations [45]. To gain insights, at each adsorption step, into the morphological evolution of the Fe 2 (MoO 4 ) 3 nanosorbent, SEM micrograph images were taken, as shown in Figure 13, which provided some indication of how the starting pure iron molybdate (Fe 2 (MoO 4 ) 3 ) particles formed aggregates and showed good porosity, which could permit better adsorption of the dye ( Figure 13A). However, the micrographs in Figure 13B,D,F,H indicated a less porous powder after the adsorption tests, i.e., the MB molecules filled the pores existing in the starting samples. Figure 13C,E,G,I showed that the sample morphology was not changed after regeneration and the first, second and third reuse. For all three cases, these less-agglomerated particles manifested as extremely porous powder. Generally, the morphology of Fe 2 (MoO 4 ) 3 was not considerably altered, even after the second or third reuse, as shown in Figure 13G,I. the first step, MB (pKa = 3.8) maintained its positive charge at pH 11 [40]. In the same conditions, the iron molybdate (Fe2(MoO5 2− )3) ion was produced, without intermediate compounds, by the reaction of Fe2(MoO4)3 with the hydroxyl groups (OH − ) present in solution [63]. Hence, the electrostatic interactions were governing the adsorptive process. These strong interactions were notable between the negatively charged iron molybdate (Fe2(MoO5 2− )3) surface and the positively charged MB cations [45]. To gain insights, at each adsorption step, into the morphological evolution of the Fe2(MoO4)3 nanosorbent, SEM micrograph images were taken, as shown in Figure 13, which provided some indication of how the starting pure iron molybdate (Fe2(MoO4)3) particles formed aggregates and showed good porosity , which could permit better adsorption of the dye ( Figure 13A). However, the micrographs in Figure 13B,D,F,H indicated a less porous powder after the adsorption tests, i.e., the MB molecules filled the pores existing in the starting samples. Figure 13C,E,G,I showed that the sample morphology was not changed after regeneration and the first, second and third reuse. For all three cases, these less-agglomerated particles manifested as extremely porous powder. Generally, the morphology of Fe2(MoO4)3 was not considerably altered, even after the second or third reuse, as shown in Figure 13G,I.

Iron Molybdate Nanosorbent Preparation
All chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received without any changes, except for the methylene blue (MB) dye, which was supplied by Panreac, Barcelona

Adsorption Investigations
Methylene blue (MB) dye removal was investigated in batch equilibrium experiments [38]. The MB solution pH was controlled by the addition of either 0.01-N HCl or 0.01-N NaOH solutions. The removal of MB by Fe 2 (MoO 4 ) 3 was conducted with continual stirring of a specific quantity of the nanosorbent in MB solution (V = 100 mL) with known concentrations at various temperatures (T = 25 • C, 50 • C and 70 • C) and for different contact times (10 min, 30 min, 60 min, 90 min and 120 min). Next, a 0.22-µm (Whatman) syringe filter was employed to filter the solution, which was then investigated by UV-visible spectrometry at λ max = 665 nm (Thermo Fisher Scientific, Madison, WI, USA). The percentage removed (%) and the amount of MB removed at equilibrium (q e (mg/g)) were determined using the following equations: where C 0 and C e (ppm) are the initial and equilibrium concentrations of MB, respectively, M (g) is the added mass of Fe 2 (MoO 4 ) 3 and V (L) is the volume of solution used. The results were reported in triplicate.

Method for Adsorbent Regeneration
For the experiments of adsorbent regeneration, an extended equilibrium time of 1 h was allocated for the removal with a 60-ppm solution that was used. The fresh Fe 2 (MoO 4 ) 3 used was calcined for 1 h at 400 • C under atmospheric air after being dried at 100 • C upon filtration. The calcined Fe 2 (MoO 4 ) 3 was tested for recycling purposes with the same conditions as that of freshly used Fe 2 (MoO 4 ) 3 . The whole regeneration cycle was repeated thrice under the same conditions. The removed percentage (%) and the MB dye amount removed at equilibrium (q e (mg/g)) were determined using Equations (13) and (14).

Characterization
Analysis of XRD (λ Cu-Kα = 1.5406 Å and Ni filter on a Shimadzu X-ray diffractometer 6000, Tokyo, Japan) was carried out for the identification of the synthetized Fe 2 (MoO 4 ) 3 nanosorbent material before and after its use for the removal of MB dye, as presented in Figure 14. The Scherrer equation was used to estimate the particle size from the XRD pattern of the as-prepared nanoparticles as follows: D XRD = 0.9λ/(βcos θ), where D XRD is the average particle diameter, λ is the Cu kα wavelength, β is the full-width at half-maximum (FWHM) of the diffraction peak and θ is the diffraction angle. The first two peaks were used to calculate the crystallite size D XRD , and the crystallite size was found to be 45 nm in both cases.  The nitrogen adsorption isotherm was employed for the determination of the specific surface area [22] with a value close 8.03 m 2 /g.
The presence of MB dye molecules on the Fe2(MoO4)3 nanoparticles was confirmed by FTIR spectroscopy, using the KBr pellet technique in the range of 400 to 4000 cm −1 , on a Shimadzu apparatus (IR Affinity-1S, Shimadzu, Tokyo, Japan).
A Quanta FEG 250 scanning electron microscope (SEM; Thermo Fisher Scientific, Hillsboro, OR, USA) was used to study the surface morphology and the particle sizes of the synthesized materials.
The concentration at equilibrium of the MB dye was determined using a UV-visible spectrophotometer (Thermo Scientific Genesys 10S, Madison, WI, USA).

Conclusions
Fe2(MoO4)3 nanosorbent was synthesized and utilized as an MB removal agent in aqueous solutions. A strongly pH-dependent removal was observed, with an achieved removal efficiency of 99% after only 120 min of contact time at pH 11, using an initial dye concentration of 50 to 60 ppm. Further kinetic investigations revealed that MB removal followed a pseudo-second-order model, while the thermodynamic study showed that the Langmuir isotherm was the best fitted model to the experimental adsorption data. Interestingly, Langmuir model-based calculations showed that the removal capacity attained a maximum of 6173 mg/g. Efficient regeneration was possible upon calcination at 400 °C, and afterwards, the nanosorbent was ready for further reuse. The Fe2(MoO4)3 removal efficiency for MB was higher even after three cycles of reuse. The data showed that Fe2(MoO4)3 was indeed as an effective nanosorbent, endowed with excellent removal performance for the studied MB dye, and that it was not altered after several recycling tests.  The nitrogen adsorption isotherm was employed for the determination of the specific surface area [22] with a value close 8.03 m 2 /g.
The presence of MB dye molecules on the Fe 2 (MoO 4 ) 3 nanoparticles was confirmed by FTIR spectroscopy, using the KBr pellet technique in the range of 400 to 4000 cm −1 , on a Shimadzu apparatus (IR Affinity-1S, Shimadzu, Tokyo, Japan).
A Quanta FEG 250 scanning electron microscope (SEM; Thermo Fisher Scientific, Hillsboro, OR, USA) was used to study the surface morphology and the particle sizes of the synthesized materials.
The concentration at equilibrium of the MB dye was determined using a UV-visible spectrophotometer (Thermo Scientific Genesys 10S, Madison, WI, USA).

Conclusions
Fe 2 (MoO 4 ) 3 nanosorbent was synthesized and utilized as an MB removal agent in aqueous solutions. A strongly pH-dependent removal was observed, with an achieved removal efficiency of 99% after only 120 min of contact time at pH 11, using an initial dye concentration of 50 to 60 ppm. Further kinetic investigations revealed that MB removal followed a pseudo-second-order model, while the thermodynamic study showed that the Langmuir isotherm was the best fitted model to the experimental adsorption data. Interestingly, Langmuir model-based calculations showed that the removal capacity attained a maximum of 6173 mg/g. Efficient regeneration was possible upon calcination at 400 • C, and afterwards, the nanosorbent was ready for further reuse. The Fe 2 (MoO 4 ) 3 removal efficiency for MB was higher even after three cycles of reuse. The data showed that Fe 2 (MoO 4 ) 3 was indeed as an effective nanosorbent, endowed with excellent removal performance for the studied MB dye, and that it was not altered after several recycling tests.