Synthesis of α-MnO 2 Nanomaterial from a Precursor γ-MnO 2 : Characterization and Comparative Adsorption of Pb ( II ) and Fe ( III )

α-MnO2 nanostructure was successfully synthesized via hydrothermal treatment of a precursor γ-MnO2. Structure, morphology, and BET surface area were characterized using X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), and Brunauer-Emmett-Teller nitrogen adsorption (BET-N2 adsorption). Thermal analysis result showed that α-MnO2 nanorods were formed from γ-MnO2 at 600C. In addition, Pb(II) and Fe(III) adsorptive properties were investigated in an optimal condition. Results showed that equilibrium adsorption was obtained after 60 minutes for Pb(II) at pH = 4.0 and 80 minutes for Fe(III) at pH = 3.5 with 240 rpm of shaking speed overall. Experimental data was analyzed using three models: Langmuir, Freundlich, and Sips. Adsorption capacities (qm) from the Langmuir isotherm models are 124.87mg/g for Pb(II) and 30.83mg/g for Fe(III). Along with the highest corelation coefficients, it is clear that the adsorption of Pb(II) and Fe(III) ions on α-MnO2 surface followed Sips model. Kinetic studies indicated that the uptake of Pb(II) and Fe(III) occurred in the pseudo-second-order model with two stages for Pb(II) and three stages for Fe(III).


Introduction
Nowadays, the human health and the environment are in danger owing to the increase in industrial and mining activities [1].Lead and iron ions seriously affect human life even in trace concentrations.According to the Environmental Quality Act, the limit for lead and iron in water is less than 0.1 mg/L and 1.0 mg/L.Therefore, there are many physicochemical treatments used to remove them, such as chemical precipitation, adsorption and ionic exchange, membrane technology, and solvent extraction.One of the most efficient and promising methods for treatment of lead and iron ions from an aqueous solution is the adsorption technology because of its high enrichment efficiency and ease of phase separation [2].
In this study, -MnO 2 was synthesized from heating precursor -MnO 2 and used as an adsorbent to remove Pb(II) and Zn(II) from an aqueous solution in an optimal condition.Comparative adsorption of two ions on -MnO 2 was also investigated using three isotherm models: Langmuir, Freundlich, and Sips.The uptake kinetics were analyzed using

Materials and Methods
2.1.Preparation of -MnO 2 from -MnO 2 .-MnO 2 which was successfully synthesized by Le and Van Phuc [2] from ethanol and potassium permanganate was heated at 600 ∘ C. The product after being investigated by XRD, SEM, and BET was used as an absorbent to remove lead and iron ions from the aqueous solution.
2.2.Adsorbent.Lead(II) and iron(II) ions were used as adsorbates.1000 mg/L standard stock solution of each metal ion was prepared by dissolving Pb(NO 3 ) 2 and Fe(NO 3 ) 3 in distilled water.

2.3.
Instruments.X-ray diffractometer D5000 made in Germany by Siemens with X-ray radiation CuK,  = 1,54056 Å, was used to examine the phase of the crystalline structure.
The morphology of the material was investigated by Ultra High Resolution Scanning Electron Microscopy S-4800, a transmission electron microscope.
Atomic Absorption Spectrophotometer (Atomic Absorption Spectrometer AA-7000 made in Japan by Shimadzu) was used to determine the BET surface area and pore site.
The pH measurements were done with a pH meter (Martini Instruments Mi-150, Romania); the pH meter was standardized using Hanna Instruments buffer solutions with pH values of 4.01 ± 0.01, 7.01 ± 0.01, and 10.01 ± 0.01.
A temperature-controlled shaker (Model IKA R5) was used for equilibrium studies.

2.4.
Diagram for These Studies.See Figure 1.

Adsorption Study. 50 mL solution of heavy metal (Pb 2+
and Zn 2+ ) ions was placed into a 100 mL conical flask containing 0.1 g -MnO 2 .During the uptake, influence of pH of the initial solution was adjusted within 2-5 range using HNO 3 0.1 M or NaOH 0.1 M solutions.Adsorption studies were also conducted in batch experiments with various adsorption times (20-240 minutes) and concentrations of metal ion (100-500 mg/L).Concentrations of heavy metal ions in the filtrate before and after uptake were determined using AAS.
Adsorption capacity was calculated by using the mass balance equation for the adsorption [1]: And the adsorption efficiency (%) was calculated from the formula where  is the adsorption capacity (mg/g) at equilibrium,  0 and   are the initial and the equilibrium concentrations (mg/L), respectively,  is the volume (L) of the solution, and  is the mass (g) of the adsorbent used.

Characterization of 𝛼-MnO 2 Nanostructures
3.1.1.Thermal Analysis Result.Thermal analysis (Figure 2) showed that there were two specific peaks corresponding to the decrease in volume of a product.From 100 ∘ C to 300 ∘ C, the endothermic process occurred which combined with a significant reduction of volume (approximately 16.40%) showed chemical dehydration as well as melting of material at 186.41 ∘ C. From 300 ∘ C to 800 ∘ C, there was a slight decrease in volume of about 5.40% which was the continued melting.In addition, there was an endothermic peak at 535.93 ∘ C corresponding to allotropic transformation.It was predicted that -MnO 2 crystalline structure can be formed at above 536 ∘ C.

Scanning Electron Microscopy Analysis Results
. Figure 4 showed that although -MnO 2 particles were flocculated at 400 ∘ C, the morphology of material was not changed.It can be concluded that -MnO 2 nanostructures were not formed.At 600 ∘ C and 800 ∘ C, the morphology of the material was changed from nanospheres to nanorods which coincided with -MnO 2 crystalline nanostructure corresponding to XRD results.However, these nanorods were broken and smashed at 800 ∘ C.

BET Surface Area and BJH Analysis.
The BET surface area as well as BJH adsorption-desorption pore size of -MnO 2 and -MnO 2 was provided in Table 1.Whereas precursor -MnO 2 had a BET surface area of about 65 m 2 ⋅g −1 , the synthesized -MnO 2 had smaller surface, approximately 9.4 m 2 ⋅g −1 .This can be explained by the fact that -MnO 2 was composed of nanospheres, had a porous surface, and was small in size at room temperature.When heating -MnO 2 to form -MnO 2 at 600 ∘ C, nanospheres were melted, flocculated, and showed an increase in size.As a result, -MnO 2 had a smaller surface area than precursor -MnO 2 .Both -MnO 2 and -MnO 2 had BJH adsorption average pore width from 20 Å to 50 Å which were mesopores materials.However, with the fact that BJH adsorption pore size of -MnO 2 is smaller than that of -MnO 2 , it was revealed that adsorptive properties of -MnO 2 were better than those of -MnO 2 .

Effective Factors. pH plays an important role in absorb-
ing Pb(II) and Fe(III) ions onto -MnO 2 material surface.
At higher pH value, hydroxo ion MOH + or/and insoluble hydroxide M(OH)  was easily formed which inhibits the adsorption of these ions.In contrast, at low pH value, adsorption sites may be decreased because part of the MnO 2 nanomolecules can be dissolved in an acid solution as in the following reaction: As a result, adsorption reached a maximum at pH = 4.0 for Pb(II) and pH = 3.5 for Fe(III) (Figure 5(a)).
Adsorption time is one of the important factors which helps us to predict kinetics as well as the mechanism of the uptake of heavy metals on material surface.The influence of contact time on the adsorption process of Pb(II) and Fe(III) onto -MnO 2 was shown in Figure 5(b).As a result, the adsorption equilibrium was obtained after 60 minutes for Pb(II) and 80 minutes for Fe(III).

Isotherm Equation Studies.
To understand the nature of the adsorption of Pb(II) and Fe(III) on material surface, some isotherm equations, such as Langmuir, Freundlich, and Sips, were investigated.While Langmuir model can help us to calculate the maximum adsorption capacity on a monolayer, Freundlich model shows the interaction between the absorbent and a multilayer.Plots of these nonlinear equations and equilibrium isotherm parameters were shown in Figure 6 and Table 2. Results showed that the maximum adsorption capacities calculated from Langmuir model were 124.87 mg/g for Pb(II) and 30.83 mg/g for Fe(III).This can be explained by the fact that lead ion is more satisfied with adsorption pore size because it has ion radius larger than iron.In addition, the separation factor,   , which is a dimensionless constant equilibrium parameter, was given by Based on the   value, it can be revealed that the smaller the value of   is, the more favorable the adsorption is.The calculated   values which were smaller than 1 and higher than 0 in both the lowest and the highest concentrations showed that -MnO 2 was an appropriate adsorbent for Fe(III) and Pb(II).However,   value of Pb(II) > Fe(III) indicated that Fe(III) will compete for binding sites faster than Pb(II) in a mixed metal ions system.
Moreover, the 1/ values calculated from Freundlich model for Fe(III) higher than Pb(II) indicated that the interaction between -MnO 2 and Fe(III) is more favorable than -MnO 2 and Pb(II).
However, the material surface shows heterogeneity.Hence, the uptake of heavy metal ions onto MnO 2 can occur on different mechanisms.Sips model, which is Langmuir and Freundlich models combined, generally described exactly the nature of adsorption.Sips isotherm equation was given by formula [1] where   is Sips isotherm model constant (L/mg);   is Sips isotherm model exponent;   is Sips constant;   (mg/L) is the equilibrium concentration; and   (mg/g) is adsorption capacity.Sips equation is a three-parameter model, and thus it can be solved by Solver Add-In in Microsoft Excel.
In comparison with the correlation coefficients ( 2 ), the root mean square error (RMSE), and the nonlinear chi-square test ( 2 ) values of the three models, it was clear that the adsorption of Pb(II) and Fe(III) onto -MnO 2 followed Sips model due to the highest  2 value as well as the lowest RMSE and  2 ones.

Kinetic Studies.
Pseudo-first-order and pseudosecond-order models are often used to describe the uptake of heavy metal ions onto material surface for adsorption time.Kinetic parameters give essential information for designing and modeling the adsorption processes.However, the two modes cannot determine clearly the nature of the adsorption of Pb(II) and Fe(III) onto -MnO 2 nanomaterial.Therefore, intraparticle diffusion model which was developed by Weber and Morris was applied for ascertaining a mechanism of the uptake of Pb(II) and Fe(III) onto -MnO 2 .Plots of these kinetic models were shown in Figure 7 and kinetic parameters were given in Table 3.
Results showed that the theoretical   values which were calculated from pseudo-second-order kinetic models were more accordant with the experimental values,  (exp) , than pseudo-first-order kinetic ones.In addition, correlation coefficients in these pseudo-second-order kinetic models for both Pb(II) and Fe(III) were higher than 0.9950.It was revealed that pseudo-second-order models were satisfied with describing kinetics of the uptake of Pb(II) and Fe(III) on -MnO 2 .
Furthermore, intraparticle diffusion models showed that the uptake of Pb(II) followed two stages: firstly, Pb(II) ions were quickly adsorbed on -MnO 2 surface; secondly, there was gradual diffusion of absorbents from surface sites into the inner pores.And it was observed that there are three stages in the sorption of Fe(III) ion on the material surface.In the first one, Fe(III) ion was rapidly transferred from the solution to -MnO 2 surface.In the next one, intraparticle diffusion which is the controlling factor gradually occurred.Lastly, the adsorption equilibrium was obtained to correspond with the saturation of absorbent sites.The intraparticle diffusion constants for all these stages were given in Table 3.

Conclusion
In this report, -MnO 2 nanostructure synthesized from a precursor -MnO 2 was used as an adsorbent to remove Pb(II) and Fe(III) from an aqueous solution at an optimal Time (minutes) 250 300 200 150 100 50 0 t/q t t/q t = 0.0091t + 0.0075 R 2 = 1.0000 condition.Results showed that the maximum capacities were 124.87 mg/g for Pb(II) and 30.83 mg/g for Fe(III) at pH = 4.0 for Pb(II) and pH = 3.5 for Fe(III).Experiment data which was analyzed using three isotherm models, that is, Langmuir, Freundlich, and Sips, indicated that Sips model was fitted better than the other two.Kinetics studies showed that both Pb(II) and Fe(III) corresponded with pseudo-second-order model with corelation coefficient constants ( 2 ) higher than 0.9950.Intraparticle diffusion kinetics confirmed that the uptake of Pb(II) on -MnO 2 includes two stages while it includes three stages for Fe(III).

Figure 5 :
Figure 5: Influence of pH (a) and contact time (b) on adsorption of Pb(II) and Fe(III) at room temperature with 240 rpm of shaking speed.

Figure 7 :
Figure 7: Pseudo-first-order kinetic plots for the adsorption of Pb(II) (a) and Fe(III) (c).Pseudo-second-order kinetic plots for the adsorption of Pb(II) (b) and Fe(III) (d).Intraparticle diffusion kinetic plots for the adsorption of Pb(II) (e) and Fe(III) (f).