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

Abstract

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.

1. 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.

Therefore, several methods have been developed and employed for the removal and/or degradation of hazardous organic dyes from contaminated wastewater, such as photodegradation, extraction, adsorption, membrane separation, coagulation, flocculation, chemical oxidation, ion exchange and biological treatment [3,4,5,6,7,8,9]. Among all the techniques, adsorption by natural and synthetic adsorbents has been widely used because of its simplicity and efficiency for toxic dye removal in wastewater [10,11,12,13,14,15].

Several natural adsorbents are effective in removing dyes from aqueous effluents [10,11,16,17], of which activated carbon is the most commonly used, mainly due to its superior adsorption efficiency [18,19]. Nevertheless, commercially available activated carbon is very expensive [20].

There is therefore a great need to develop efficient adsorbents showing superior performance in their ability to remove pollutants and having suitable properties, such as ease of separation from the solution, regeneration and efficiency, even after several cycles of use, in order to enable the recovery of valuable compounds.

In recent decades, metal oxides as potential materials with different capabilities have been studied [21]. The family of metal molybdates is one of the most promising examples of the mixed metal oxides, which have been extensively studied in recent years [22,23,24,25].

Metal molybdate compounds with the formula MMoO₄ or M₂(MoO₄)₃ are important inorganic materials that have attracted great research interest, because they have important industrial applications. These include photocatalytic materials, humidity sensors, scintillator materials, photoluminescent compounds and optical fibers [26,27]; microwave applications and electrochemical and magnetic properties are also key features of these compounds. Metal molybdate compounds synthesized in the nanoscale were also used for environmental applications such as the removal of dyes from water by adsorption [23,28], oxidation of methylene blue dye [29] and photocatalytic oxidation of dyes [30].

Among all molybdenum-containing mixed oxides, the most studied is perhaps iron molybdate Fe₂(MoO₄)₃. It has garnered an exponentially increasing degree of interest, it has been synthesized at the nanoscale by different methods and it has been involved in many environmental and industrial applications [31]. In fact, iron molybdate has important applications in solid oxide fuel cells, sodium and lithium ion batteries, catalysis and sensors [32], propylene oxide production via the oxidation of propylene [33,34], methanol oxidation to formaldehyde [35] and photo-combined heterogeneous activation of persulfate for the removal of micropollutants [36]. In addition, the coprecipitation preparation of Fe₂(MoO₄)₃ nanopowder was applied for the photocatalytic degradation of rhodamine B with an efficiency of ~97% [37].

In the present work, iron molybdate nanoparticles prepared at a rather low temperature via a relatively cost-effective and very simple procedure, as previously described in the literature [22], were investigated as an adsorbent for MB dye removal. The effects of different parameters such as solution pH, temperature, contact time, adsorbent dosage and initial dye concentration on the removal of MB by the synthesized Fe₂(MoO₄)₃ nanosorbents were studied. In addition, the adsorption isotherms and kinetics were evaluated. Furthermore, the removal efficiency after regeneration of the used nanosorbent by calcination at high temperature was evaluated.

2. Results and Discussion

2.1. MB Removal

2.1.1. pH Effect

An essential parameter in terms of controlling the dye removal is pH [38]. While it does not modify the adsorbent site separation, it nevertheless changes the chemistry and the structure of the dye [39]. Therefore, the effect of pH on MB removal using iron molybdate Fe₂(MoO₄)₃ nanosorbent was investigated by varying its values between 3 and 11 at room temperature (i.e., 20 °C) with an initial concentration of 80 ppm. As shown in Figure 1, there is a pronounced effect of pH on MB removal. For instance, an increasing removal percentage from 69% to 88% was observed with increasing pH values from 3 to 11. The same trend was also observed for the dye removal amount per adsorbent 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 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.

2.1.2. 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].

2.1.3. 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 Fe₂(MoO₄)₃ surface, which, as a consequence of the sorption process, gradually decreased by filling up these sites with increasing contact times [44].

2.1.4. 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:

$$\Delta {\mathrm{G}}^{\mathrm{o}}=-{\mathrm{RTLnK}}_{\mathrm{d}}$$

(1)

$${\mathrm{K}}_{\mathrm{d}}=\frac{{\mathrm{C}}_{\mathrm{a}}}{{\mathrm{C}}_{\mathrm{e}}}$$

(2)

$${\mathrm{LnK}}_{\mathrm{d}}=\frac{\Delta {\mathrm{S}}^{\mathrm{o}}}{\mathrm{R}}-\frac{\Delta {\mathrm{H}}^{\mathrm{o}}}{\mathrm{RT}}$$

(3)

where K_d 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, C_a is the amount of dye adsorbed at equilibrium, C_e 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 K_d versus 1/T (Figure 5), and ∆G° values were calculated from Equation (1). All data are shown in

Table 1. Methylene blue (MB) removal by Fe₂(MoO₄)₃ thermodynamic parameters.

Adsorbent	Adsorbate	∆H° (KJ·mol−1)	∆S° (KJ·mol−1·K)	∆G° (KJ·mol−1)
Fe2(MoO4)3	MB	83.79	0.291	296K	323K	343K
				−2.904	−8.934	−16.402

.

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 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].

2.2. 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. Kinetic model equations.

Model	Equation	Parameters
Pseudo-first-order (PFD) [50]	$\ln \left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)={\ln \mathrm{q}}_{\mathrm{e}}+{\mathrm{K}}_1\mathrm{t}$ (4)	qt: the removal capacity at time t (mg/g) qe: the removal capacity at equilibrium (mg/g) K1: the rate constant of pseudo-first-order adsorption (1/min)
Pseudo-second-order (PSD) [50]	$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{K}}_2{\mathrm{q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$ (5)	qt: the removal capacity at time t (mg/g) qe: the removal capacity at equilibrium (mg/g) K2: the pseudo-second-order rate constant (g. mg−1.min−1)
Intraparticle diffusion (IPD) [51]	${\mathrm{q}}_{\mathrm{t}}={\mathrm{K}}_{\mathrm{I}}{\mathrm{t}}^{0.5}+\mathrm{l}$ (6)	I (mg/g) and KI (mg/(g.min0.5)) are the intraparticle diffusion constants qt: the removal capacity (mg/g) at time t t: the contact time (min)

.

The three model parameters, namely intraparticle diffusion, pseudo-first-order and pseudo-second-order, are arranged in

Table 3. MB removal by Fe₂(MoO₄)₃ kinetic parameters.

Dye (Ci mg/L)	Pseudo-First-Order				Pseudo-Second-Order			Intraparticle Diffusion Model
	qexp (mg/g)	qe (mg/g)	k1 (1/min)	R12	qe (mg/g)	k2 (g/mg min)	R22	I (mg/g)	ki (mg/g min0.5)	R32
50	4999	378	0.020	0.956	4981	0.00021	1.000	4596	36	0.904
60	5967	496	0.014	0.814	5871	0.00016	0.999	5444	37	0.782
65	5979	565	0.019	0.752	5922	0.00016	1.000	5135	85	0.748
70	6179	57	0.021	0.993	6176	0.00154	1.000	6117	6	0.993

and shown in Figure 6, Figure 7 and Figure 8, respectively. These models varied in the values of correlation coefficients (R²) 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 R² value was equal to or near 1 for the pseudo-second-order model, the latter fit very well to the experimental data.

2.3. 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. Adsorption isotherm models for MB dye removal using Fe₂(MoO₄)₃.

Model	Equation	Parameters
Freundlich [52]	${\ln \mathrm{q}}_{\mathrm{e}}={\ln \mathrm{q}}_{\mathrm{F}}+\frac{1}{\mathrm{n}}{\ln \mathrm{C}}_{\mathrm{e}}$ (7)	qF: Freundlich constant (mg(1−1/n)L1/ng−1) n: heterogeneity factor (g/L) qe: amount of MB dye adsorbed by α-Fe2(MoO4)3 at equilibrium (mg/g) Ce: MB concentration at equilibrium (ppm)
Langmuir [52]	$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}}}$ (8)	qe: amount of MB dye adsorbed by α-Fe2(MoO4)3 at equilibrium (mg/g) Ce: MB concentration at equilibrium (ppm) qm: maximum amount of MB dye removed by Fe2(MoO4)3 (mg/g) KL: Langmuir adsorption constant (L/mg)
	${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{i}}}$ (9)	Ci: initial concentration of MB KL: Langmuir constant RL: values specify that the removal of MB dye could be linear (RL = 1), irreversible (RL = 0), favorable (0 < RL < 1) or unfavorable (RL > 1)
Dubinin-Radushkevich (D-R) [53]	${\ln \mathrm{q}}_{\mathrm{e}}={\ln \mathrm{q}}_{\mathrm{m}}-K{\epsilon }^2$ (10) $\mathsf{\epsilon }=RT\ln (1+\frac{1}{{\mathrm{C}}_{\mathrm{e}}})$ (11)	K: sorption energy constant (mol2/kJ2) ε: Polanyi potential T: temperature (K) R: universal gas constant (8.314 J.mol−1 K−1) qm: theoretical saturation capacity Ce: MB concentration at equilibrium (ppm)
Temkin [54]	${\mathrm{q}}_{\mathrm{e}}={\mathrm{B}}_{\mathrm{T}}{\ln \mathrm{A}}_{\mathrm{T}}+{\mathrm{B}}_{\mathrm{T}}{\ln \mathrm{C}}_{\mathrm{e}}$ (12)	bT: Temkin constant related to heat of sorption (J/mol), BT = RT/bT R: gas constant (8.314 J/mol K) AT: Temkin isotherm constant (L/g) T: absolute temperature (K)

.

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 (R²) are given in

Table 5. MB removal by Fe₂(MoO₄)₃ isotherm parameters.

Langmuir				Freundlich			Temkin			Dubinin-Radushkevich
qm (mg/g)	KL (L/mg)	R2	Range RL	qF (mg(1−1/n)L1/ng−1)	1/n	R2	AT (L/g)	BT	R2	qm (mg/g)	R2	E (Kj/mol)
6173	5	0.999	0.0024–0.0038	5825	0.02	0.866	5E18	135	0.870	6063	0.971	944

, as extracted from Figure 9. The highest value for R² (0.999) was obtained from the Langmuir model, while the fittings of the Freundlich and Temkin models showed the lowest values of R² (0.866 and 0.870, respectively), whereas an intermediary value was achieved for the D-R model (R² = 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 Fe₂(MoO₄)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 Fe₂(MoO₄)₃. Therefore, the investigated nanosorbent had excellent removal efficiency when compared to other materials (

Table 6. The maximum removed amount (q_m) of MB dye reported in the literature.

Nanosorbent	qm (mg/g)	Reference
Magnetic β-cyclodextrin-chitosan nanoparticles	2783.30	[55]
Fe2O3	1124.70	[56]
Zinc molybdate nanoparticles	217.86	[23]
CoO	5501.93	[56]
Molybdenum trioxide nanorods and stacked nanoplates	152.00	[45]
Iron molybdate (Fe2(MoO4)3)	6173.00	This work

).

2.4. Regeneration and Characterization of the Fe₂(MoO₄)₃ Nanosorbent

2.4.1. 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 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.

2.4.2. 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.

2.5. MB Removal Mechanism

As discussed earlier, MB removal by Fe₂(MoO₄)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 Fe₂(MoO₄)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 (Fe₂(MoO₅²⁻)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(MoO₅²⁻)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.

3. Experimental

3.1. 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, Spain.

Iron molybdate nanosorbent (Fe₂(MoO₄)₃) was produced by iron molybdenum complex thermal breakdown, in the solid state, by reacting iron nitrate (Fe(NO₃)₃·9H₂O), oxalic acid (H₂C₂O₄·2H₂O) and ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O), as reported previously in the literature [22]. Iron nitrate (Fe(NO₃)₃·9H₂O), oxalic acid (H₂C₂O₄·2H₂O) and ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O) were mixed together in a molar ratio of 2:10:0.43. The obtained homogeneously powdered mixture was heated at 160 °C on a hot plate. The iron molybdenum complex obtained was then decomposed in a tubular furnace (with both ends open) at 500 °C under static air for two hours.

3.2. 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 Fe₂(MoO₄)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:

$$\mathrm{Removal}\%=\frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_0}\times 100$$

(13)

$${\mathrm{q}}_{\mathrm{e}}=\frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})}{\mathrm{M}}\times \mathrm{V}$$

(14)

where C₀ and C_e (ppm) are the initial and equilibrium concentrations of MB, respectively, M (g) is the added mass of Fe₂(MoO₄)₃ and V (L) is the volume of solution used. The results were reported in triplicate.

3.3. 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 Fe₂(MoO₄)₃ used was calcined for 1 h at 400 °C under atmospheric air after being dried at 100 °C upon filtration. The calcined Fe₂(MoO₄)₃ was tested for recycling purposes with the same conditions as that of freshly used Fe₂(MoO₄)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).

3.4. 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 Fe₂(MoO₄)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 m²/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).

4. 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.