Equilibrium and kinetic studies on methylene blue adsorption by simple polyol assisted wet hydroxyl route of NiFe₂O₄nanoparticles

Abstract

A novel approach has been adopted in the synthesis of nickel ferrite nanoparticles and their adsorption capacity was studied in the effective removal of MB dye from aqueous solution. Nanoparticles have a main advantage of treating large amount of wastewater within a short time and producing less contamination. The synthesized Spinel ferrites show high adsorption capacity, magnetic performance, and an eco-friendly material which effectively removes dyes. In the current work Nickel ferrite nanoparticles have been synthesized by wet hydroxyl chemical route using ethylene glycol as a chelating agent. XRD analysis indicates cubic spinel phase nickel ferrite and the average crystallite size is found to be 56.11 nm. An FTIR spectrum illustrates two intense absorption bands in the range between 1000 and 400 cm⁻¹ corresponding to the presence of nickel ferrite. The shape and morphology of Nickel ferrite are examined by SEM analysis. The constituent elements and chemical composition analyzed using EDX spectrum showed that the estimated atomic percentages of O, Fe, and Ni are in good agreement with the theoretical value. VSM analysis clarifies soft ferromagnetic nature at room temperature. The equilibrium time for the removal of MB dye was found to be 180 mins. The capacity of nickel ferrite nanoparticles to adsorb the MB dye was proved from its maximum adsorption capacity of 72 mg g⁻¹ from Langmuir model. The Equilibrium parameter (R_L) and % error was calculated and found that Langmuir isotherm and Second-order kinetic model gave a good fit to the experimental data.

Introduction

Textile, leather, rubber, plastic, and paper printing industries have been used more than ten thousand organic dyes to enrich the appealing values of their products. They generate a huge volume of effluent annually. These untreated colored effluents are highly toxic. Direct disposal of effluent spoils the quality of water continuously. Therefore effective removal of pollutants from wastewater remains a challenge [1].

Methylene blue (MB) is phenothiazine dye, which is widely applied in chemistry, biology, medical science, and dyeing industries. It is a non-biodegradable when discharged into the water because of its stable heterocyclic aromatic structure. The discharge of MB is hazardous for both toxicological and aesthetical reasons. They considerably reduce oxygen concentration and light penetration which in turn harmfully affect the aquatic ecosystems. Therefore, the removal of Methylene blue from wastewater is crucial for the preservation of the human health [2].

Various techniques such as coagulation, membrane filtration, solvent extraction, biological treatment, and adsorption have been used for the removal of dye from wastewater. Due to high removal efficiency, simplicity and low cost, the adsorption is believed to be a fruitful method for the confiscation of colorants from aqueous solution [3]. Owing to their high surface area, activated carbon mesoporous silica has been used for the removal of dyes from aqueous solution. Though the adsorption technique can be carried out with low cost, the materials used in this method are costly. Thus, it becomes a challenging method because of the cost escalation, recovery, and possible reuse.

The choice of the adsorbents needs to be considered for the following factors: the high adsorption capacity; the excellent magnetic performance, and the nontoxic and eco-friendly material. Nanoparticles are broadly applied in the fields of medicine, bioinorganic chemistry and molecular biology, but they are also applied in environmental science. The main advantage of this technology consists in its capacity of treating large amount of wastewater within a short time and producing less contamination. Recent studies show that Spinel ferrites have received tremendous attention worldwide for its effective removal of dyes, mainly for its effortless separation by a simple magnetic process and intrinsic magnetic properties. Nickel ferrite is an inverse spinel structure and it comes under soft magnetic material [4]. These magnetic properties, coupled with the surface area of the nanoparticles of magnetite, are the main cause for the increased removal of dyes. It can be an important prospect in practical applications such as high-density magnetic recording media, photocatalysts, and adsorbents in the process of water purification.

Nickel ferrite can be prepared by various methods such as citrate precursor method [5], glyoxylate precursor method [6], sucrose method [7], hydrothermal processing [8], combustion technique [9], etc.

A considerable amount of work has also been reported in the journals for removal of synthetic dyes from wastewater using Nickel Ferrite [2, 10–18].

This study proposes the feasibility and efficiency of using nickel ferrite for the removal of methylene blue dye from its aqueous solutions. The reason for choosing MB is, for its strong adsorption onto solids. Even though the investigations on the structural and magnetic properties of Nickel ferrite have been reported earlier, the simultaneous detailed study of these properties of Nickel ferrite synthesized using polyol assisted Wet Hydroxyl Route, along with adsorption of methylene blue has not been reported so far. The experimental data for adsorption have been analyzed using the Langmuir and Freundlich isotherm pseudo-first-order kinetic model and also with the pseudo-second-order model.

Experimental section

Synthesis

In a typical synthesis, 0.2 M ferric nitrate (Fe (NO₃)₃·9H₂O) and 0.1 M of nickel nitrate (Ni (NO₃)₂·6H₂O) (99·9% pure MERK) were dissolved in 50 ml of ethylene glycol solution separately. Further, both precursors were mixed with each other and magnetically stirred for 6 h. NaOH solution was added drop wise till its pH reached 8.The precipitate was washed several times with distilled water. Then the product was dried on a hot plate at 120 °C to transform it into nickel ferrite. The resultant dried product was powdered using an agate mortar. The obtained ferrite powder was annealed for 2 h at 800 °C. Ethylene glycol was added to prevent from agglomeration and with this process; we got the ultrafine Nickel ferrite [4].

Batch adsorption study

The structure and properties of methylene blue used for the present work are given in Table 1.The dye solution is prepared by dissolving 1 g of methylene blue in 1000 mL of distilled water to obtain 1000 mgL⁻¹ of the synthetic dye solution.

Table 1.

Structure and properties of methylene blue

For equilibrium studies, the batch technique was used because of its simplicity. For batch adsorption study, a known concentration of dye solution is added to 0.5 g (100 mL)⁻¹ adsorbent and is kept in 250 mL conical flask. Then these flasks are placed in the rotatory shaker at room temperature. The inspiration of various factors like contact time and the effect of the concentration of dye on adsorption of methylene blue are marked. Finally, the mixture is separated using the centrifuge and the final concentration of the dye is analyzed using the UV spectrophotometer at a wavelength of 665 nm (λ_max). Isotherm and kinetics studies are monitored after the experiments to find out the best fit model.

The adsorption capacity of methylene blue onto the NiFe₂O₄ nanoparticles could be calculated by the following Eqs. (1, 2).

$$\mathrm{Q}=\frac{\left(C_0-C_e\right)V}{m}$$ 1

The dye percent removal (%) was calculated using the following equation:

$$\text{Removal}\%=\frac{\left(C_0-C_e\right)}{C_0}\times 100$$ 2

Where V is the solution volume (L), C₀ is the initial methylene blue concentration (mg/L), Ce is the methylene blue concentration at an equilibrium time (mg/L), and m is the adsorbent mass (g).

Characterization of the adsorbent

The crystallinity and phase of the sample are observed by X-ray diffraction (XRD)(PanalyticalX’Pert Powder X’CeleratorDiffractometer) using Cu Kα radiations (λ=1.5406 Å) in 2θ ranging from 20^o to 80^o. The surface morphology of all the samples is gathered with the support of the Scanning electron microscopy (SEM) (Carl Zeiss SUPRA-55). Elemental analysis is done by Energy Dispersive X-Ray analysis (EDX) (Quantax 200 with X-Flash – Bruker). The Fourier transform infrared spectrum (FTIR-Shimadzu) of the sample is recorded in the range of 400 cm⁻¹ – 4000 cm⁻¹. The coercivity and saturation magnetization are studied by using Vibrating Sample Magnetometer (VSM) (Lakeshore 7410) up to 1.5 T at room temperature. The absorbance is observed by using the UV-Vis Spectrophotometer (Hitachi U-2000).

Desorption studies

To investigate the possible repeatable use of the adsorbent, desorption and regeneration study has been carried out. In general, 0.1 g (100 mL)⁻¹ of the adsorbent was stirred along with 20 mg L⁻¹ of the MB dye concentration for 180 min. The final concentration of the analyte was measured. After the adsorption cycle, the MB loaded nanoparticle was eluted using 0.1 M H₂SO₄ as desorbing agent for the same operating conditions as mentioned above [19]. The supernatant was analysed for the MB dye and the desorbed nanoparticles was further used for the next adsorption cycle as the regenerated adsorbent.

Results and discussion

Structural analysis

Figure 1 shows the XRD pattern of Nickel ferrite nanoparticles. The reflection from the XRD patterns depicts the characteristic peaks (220), (311), (222), (400), (422), (511), and (440) of spinel nickel ferrite and it agrees well with the JCPDS # (82–2267) [20].

Fig. 1.

XRD pattern of nickel ferrite nanoparticles

The average crystallite size of the samples was calculated using the Debye–Scherrer formula

$$D=\frac{k\lambda }{\beta \mathit{\text{cosθ}}}$$ 3

Where D is the average crystallite size,k is the dimensionless constant and value used is 0.9, λ is the x-ray wavelength, β is the Full width half maximum, θ is the Bragg angle. The average crystallite size of Nickel ferrite is found to be 56.11 nm.

Also, the lattice parameter and X-Ray density were calculated using the following formula

$$\mathrm{a}=\mathrm{d}\sqrt{{\mathrm{h}}^2+{\mathrm{k}}^2+{\mathrm{l}}^2}$$ 4

$$d_x=\left(8M\right)/\left(N{a}^3\right)$$ 5

ɑ is the Lattice constant, d is the inter-planar distance, (h,k,l) is the Miller indices, M is the molecular weight of the samples, N is the Avogadro’s number. The acquired lattice parameter and X-ray density are 8.347 Å and 5.38 g cm⁻³ respectively.

FTIR studies

FTIR spectrum of nickel ferrite nanoparticles is shown in Fig. 2 in the region of 4000 to 400 cm⁻¹. It indicates the formation of the single phase of spinel ferrites which are having two sublattices. The two main absorptions are tetrahedral site and octahedral site. The occurrence of the first band at a higher wave number of 598 cm⁻¹ (ʋ₁) has been assigned to the intrinsic vibrations of the tetrahedral complexes and the second band at the lower wave number of 401 cm⁻¹ (ʋ₂) is attributed to the intrinsic vibrations of the octahedral complexes [21].

Fig. 2.

FTIR spectra of nickel ferrite nanoparticles

Morphological analysis

Figure 3 shows the SEM images which reveal the microscopic structure and morphology of ferrite nanoparticles. The enlarged image clearly shows that the nanoparticles are spherical in shape and it seems to be condensed and well-connected grain to each other [22, 23].

Fig. 3.

SEM image of ferrite nanoparticles

Elemental analysis

Figure 4 shows the elemental composition of nickel ferrite nanoparticles. It is clear from the figure that it has a proper elemental composition such as Ni-13.03%, Fe-28.47%, O-58.50% and no other additional compounds were present indicating the compound to be free from impurities. The estimated atomic percentages of O, Fe, and Ni are in good agreement with the theoretical value.

Fig. 4.

Elemental composition of nickel ferrite nanoparticles

Magnetic properties

Magnetic hysteresis loop was used to determine magnetic parameters such as the saturation magnetization (Ms), and coercivity (Hc) of the sample (Fig. 5). The saturation magnetization is an intrinsic factor that is only influenced by the chemical composition and the crystalline structure. The saturation magnetization and coercivity values were observed to be 30.74 emu g⁻¹ and 445.75 Oe, respectively for Nickel ferrite which is a typical ferromagnetic behavior at room temperature [24]. A small increase in the magnetization at a high field of 5 Oe was observed in the hysteresis curves. This tendency of saturation magnetizations is analyzed by the extrapolation to the intercepts of the magnetization axes in magnetization vs. reciprocal of magnetic field. The saturation magnetization and remanence magnetization values are directly taken from the magnetization vs.field graph.

Fig. 5.

VSM analysis of nickel ferrite nanoparticles

Adsorption of methylene blue dye

Adsorptive removal activity of the prepared sample was evaluated by measuring the adsorption rate of MB in the presence of nickel ferrite nanoparticles.

Effect of contact time and concentration of dye on batch adsorption

The equilibrium conditions are used to standardize the adsorption isotherms. A series of contact time experiments have been carried out for different initial MB dye concentrations. Figure 6 illustrates the effects of contact time for different initial MB dye concentrations (5–40 mg L⁻¹) and it is noted that the adsorption capacity increases from 42 mgg⁻¹ to 62 mgg⁻¹ with an increase in initial dye concentration from 5 mgL⁻¹ to 40 mgL⁻¹ and the equilibrium time is conquered at 180 mins. The experiments were carried out at pH 6 as the obtained pH_ZPC was 5.8.

Fig. 6.

Effects of contact time for different initial MB dye concentrations (Initial MB concentration = 5–40 mg L⁻¹, adsorbent dosage = 0.1 g (100 mL)⁻¹, pH = 6, Temperature = 30 °C)

The amount of the adsorbed dye onto the nanoparticle increases with the increase in time. At some particular time, there will be no further adsorption of dye from the solution, and this is calledas the equilibrium time [25, 26]. The maximum adsorption capacity of the adsorbent, from the above-declared process, exposes that the amount of dye adsorbed at the equilibrium time of 180 mins.

Effect of pH

Figure 7 gives the adsorption capacity within pH range of 2.0–12.0. The uptake of MB was found to increase with increase in pH and attained maximum adsorption capacity at pH 6. A lower adsorption capacity was observed at a lower pH and this is due to the H⁺ ion competing for the binding sites on the nanoparticle [27]. The nanoparticle surface was negatively charged at an increased pH. Also from the pH_ZPC, a value of pH > 5.8 becomes more negatively charged and this increases the adsorption capacity on the adsorbate surface.

Fig. 7.

Effects of pH on MB adsorption (Initial MB concentration = 20 mg L⁻¹, adsorbent dosage = 0.1 g (100 mL)⁻¹, Temperature = 30 °C)

Effect of adsorbent dosage

The adsorption capacity of the adsorbent during the adsorption process is strongly influenced by the dosage of the adsorbent. The effect of adsorption capacity by varying the adsorbent dosage on the adsorption of MB dye was investigated and is shown in Fig. 8. The adsorption capacity decreased from 62.51 mg g⁻¹ to 19.2 mg g⁻¹ as the adsorbent dose was varied from 0.01 g to 0.1 g. The decrease in the adsorption capacity of the nanoparticle for the MB dye adsorption could be due to the increase in the adsorbent surface area resulting in increased numbers of adsorption sites available for adsorption [26].

Fig. 8.

Effects of adsorbent dosage on MB adsorption (Initial MB concentration = 20 mg L⁻¹, pH = 6, Temperature = 30 °C)

Equilibrium studies

Adsorption isotherms are used to analyze the experimental data of MB molecules interact with the adsorbent surface. The most widely used isotherms are Langmuir and Freundlich equations [28–31].

Langmuir isotherm

The Langmuir isotherm explains the adsorption of dye into adsorbent. A basic assumption of the Langmuir theory is that adsorption takes place at specific sites within the adsorbent [32].

Langmuir isotherm is calculated using the following equation,

$${\mathrm{C}}_{\mathrm{e}}/{\mathrm{q}}_{\mathrm{e}}=1/\left({\mathrm{Q}}_{\mathrm{o}}\mathrm{b}\right)+\left(1/{\mathrm{Q}}_{\mathrm{o}}\right){\mathrm{C}}_{\mathrm{e}}$$ 6

Where C_e is the equilibrium concentration of the MB (mg L⁻¹), q_eis the amount of adsorbate adsorbed per unit mass of the adsorbent (mg g⁻¹), Q_oand b show the maximum monolayer adsorption capacity and the affinity of binding sites, respectively.

Figure 9 shows the linear plot of C_e/q_e versus C_e from which the values of Q₀ and b were calculated by the slope and intercept. The necessary descriptions of the Langmuir isotherm can be expressed in terms of a dimensionless equilibrium parameter (R_L), which is defined by the following equation:

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+b{C}_0}$$ 7

where C₀ is the highest initial concentration of the adsorbate. The value of R_L specifies the type of the isotherm to be, unfavorable (R_L is greater than 1), linear (R_L is equal to 1), favorable (R_L is between 0 and 1), or irreversible (R_L = 0).

Fig. 9.

Comparative isotherm graph for Langmuir, Freundlich and experimental values (Temperature = 30 °C, pH = 6, carbon dosage = 0.1 g (100 mL)⁻¹, time = 180 min)

Freundlich isotherm

The Freundlich isotherm was developed mainly to allow for an empirical account of the variation in adsorption heat with the concentration of an adsorbate (vapor or solute) on an energetically heterogeneous surface.

Freundlich isotherm is calculated using the following equation:

$${\text{logq}}_{\mathrm{e}}=log{\mathrm{k}}_{\mathrm{F}}+\left(1/\mathrm{n}\right)log{\mathrm{q}}_{\mathrm{e}}$$ 8

Here K_F and n are Freundlich constants, which signify adsorption capacity and adsorption strength correspondingly.

Langmuir and Freundlich isotherm constants and the value of R_Lis favorable for Langmuir isotherm and the values are given in Table 2. Conformation of the experimental data into Langmuir isotherm model indicates the homogeneous nature of nanoparticle surface, i.e. each dye molecule/nanoparticle adsorption has equal adsorption activation energy. The results also demonstrate the formation of monolayer coverage of dye molecule at the outer surface of the nanoparticles. From the R² values, it is confirmed that data fit well with the Langmuir isotherm and this model describe the equilibrium data best because the maximum adsorption capacity obtained from the model is close to the experimental data.

Table 2.

Isotherm constants and model fit test

Langmuir isotherm constants	Values	Freundlich isotherm constants	Values
Qo (mgg−1)	72	1/n	0.153
b (mg−1)	1.212	kF [(mgg−1) (mg−1)1/n]	177.8
R2	0.893	R2	0.789
RL	0.0252
χ2	1.3047	χ2	23.2996

Chi-square test (χ²)

Chi-square test is used as the model fit test to find out whether the experimental data is close to the theoretical data.

$${\chi }^2=\sum _i{\left(q_i^{\mathit{\text{calc}}}-q_i^{\mathit{obs}}\right)}^2/q_i^{\mathit{\text{calc}}}$$ 9

where q^(obs) is the observed adsorption capacity (mg g⁻¹); q^(calc) - calculated adsorption capacity (mg g⁻¹); and i - integer index of summation indicating the successiveness of the experimental data. As the χ² value tends towards 0, the experimental data is behaving more close to the calculated isotherm model. Table 2 gives the values calculated for the model fit test of Langmuir and Freundlich isotherms. From the table, it can be concluded that the data fits well with the Langmuir isotherm [33].

Adsorption kinetics

The pseudo-first and pseudo-second-order models are used to review the kinetics of the adsorption process of MB by the Nickel ferrite nanoparticles. The pseudo-first-order kinetic model is expressed by.

$$ln\left(q_e-q_t\right)=ln{q}_e-k_{1}t$$ 10

where q_e and q_t are the adsorption capacity (mg g⁻¹) at equilibrium and time t(min), respectively; kis the rate constant.

The linear form of the pseudo-second-order rate equation

$$\frac{t}{q_t}=\frac{1}{{K_2}^{\ast }{q}_e^2}+\frac{t}{q_e}$$ 11

Where k₂ denotes the rate constant in g mg⁻¹ h⁻¹ for pseudo-second-order kinetic model.

The determination coefficient, R², was used to estimate the suitability of different models [34]. The higher the R² value, the better the model fits the experimental data. Pseudo-first and pseudo-second-order rate constants and experimental and calculated q_e values for different initial MB concentrations are given in Table 3. It has been found that the adsorption of MB onto the nanoparticle can be described effectively using the second-order kinetic model [35]. Figure 10 shows the Comparative kinetics graph for Pseudo first order, Pseudo second order and experimental values.

Table 3.

The comparative table for the pseudo-first and pseudo-second order rate constants and experimental and calculated q_e values for different initial MB concentrations

Initial concentration (mg L−1)	qe, exp. (mgg−1)	First-order kinetic model			Second-order kinetic model
		K1 (h−1)	qe, cal (mgg−1)	R2	K2 [g(mg h)−1]	qe, cal (mgg−1)	R2
5	42.50	0.3476	10.74	0.47	0.0354	45.98	0.98
10	54.34	0.2963	22.56	0.52	0.00274	60.02	0.98
20	62.12	0.2186	37.01	0.49	0.00253	63.33	0.98
30	65.91	0.0139	51.57	0.74	0.00168	68.14	0.98
40	68.78	0.1545	53.77	0.65	0.00142	70.04	0.98

Fig. 10.

Comparative kinetics graph for Pseudo first order, Pseudo second order and experimental values (Initial MB concentration = 20 mg L⁻¹, Temperature = 30 °C, pH = 6, carbon dosage = 0.1 g (100 mL)⁻¹)

Desorption and regeneration study of the adsorbent

To find out the commercial applicability of the adsorbent, desorption and regeneration study is of crucial importance. The desorption percentage of MB was found to be 85% for the first two cycles. After the second cycle a decrease in the dye loading capacity was observed. Hence, the synthesized nanoparticles can be used for 4 complete cycles of adsorption and desorption.

Comparison of adsorption parameters of various dyes with nickel ferrite

The adsorption parameters of anionic and cationic dyes onto NiFe₂O₄ are reported in Table 4.

Table 4.

Comparison of adsorption parameters of dyes with NiFe₂O₄ sorbents

S. no	Sample	Dye	Adsorption parameter	Reference
1	PANI-NiFe2O4	Methylene blue	Removal % = 88.13	[10]
2	NiFe2O4	Sunset yellow	Removal % = 93.97	[9]
3	NiFe2O4	RB5	Adsorption capacity = 31.25 mg/g	[12]
4	Ethanol assisted NiFe2O4	Congo red	Adsorption capacity = 92.5 mg/g	[16]
5	Polyol assisted NiFe2O4	Methylene blue	Removal % = 52	Present work

Conclusion

Nano crystallinepolyol assisted nickel ferrite is thus synthesized by a wet hydroxyl chemical route. XRD, SEM, EDX pattern affirm the crystallinity, grain size, and chemical composition separately. The FT-IR spectra indicate two primary absorption bands ʋ₁ and ʋ₂ comparing to the stretching vibration of the tetrahedral and octahedral sites around 600 and 400 cm⁻¹ individually. From VSM analysis, it is found that specific magnetizations have soft and ferromagnetic behavior. Methylene blue adsorption is found to be more effective by using the nickel ferrite nanoparticles. The isotherm can be expounded well using the Langmuir model. The experimental data fit well with the second-order kinetic model. The magnetism confirms a super magnetic separability of the adsorbent, which makes the application of nickel ferrite in sewage treatment field possible. The parameters obtained can be further used for modeling of the adsorption column for methylene blue adsorption using nanoparticles for the pilot scale purpose.

Compliance with ethical standards

Conflicts of interest

There is no conflict of interest in the submission.