Nowadays, the human health and the environment are in danger owing to the increase in industrial and mining activities [
Being well known as a low-cost and environmentally friendly material, manganese oxides have been studied and applied to different areas since they have a great number of crystalline structures (
In this study,
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(NO3)2 and Fe(NO3)3 in distilled water.
X-ray diffractometer D5000 made in Germany by Siemens with X-ray radiation CuK
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
A temperature-controlled shaker (Model IKA R5) was used for equilibrium studies.
See Figure
50 mL solution of heavy metal (Pb2+ and Zn2+) ions was placed into a 100 mL conical flask containing 0.1 g
Adsorption capacity was calculated by using the mass balance equation for the adsorption [
And the adsorption efficiency (%) was calculated from the formula
Thermal analysis (Figure
TG-DTA analysis.
XRD analysis results showed that
XRD image of MnO2 at different temperatures: gamma-MnO2 (a);
Figure
SEM images of material at 100°C (a), 400° (b), 600°C ((c), (d)), and 800°C ((e), (f)).
The BET surface area as well as BJH adsorption-desorption pore size of
BET and BJH analysis results of
Material | BET surface area | BJH adsorption pore size | BJH desorption pore size |
---|---|---|---|
| 65.00 m2 | 417.83 Å | 340.23 Å |
| 9.37 m2 | 162.95 Å | 734.37 Å |
pH plays an important role in absorbing Pb(II) and Fe(III) ions onto
As a result, adsorption reached a maximum at pH = 4.0 for Pb(II) and pH = 3.5 for Fe(III) (Figure
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.
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
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
Equilibrium isotherm parameters.
Isotherm | Nonlinear forms | Isotherm parameters | ||
---|---|---|---|---|
Pb(II) | Fe(III) | |||
Langmuir | | | 0.3340 | 0.135 |
| 124.9 | 30.83 | ||
RMSE | 3.529 | 1.504 | ||
| 0.9109 | 0.9042 | ||
| 0.6618 | 0.7603 | ||
| 0.0291 | 0.0689 | ||
| 5.95 | 0.0146 | ||
| ||||
Freundlich | | | 0.1115 | 0.2033 |
| 74.76 | 11.52 | ||
RMSE | 1.493 | 0.695 | ||
| 0.9840 | 0.9795 | ||
| 0.1148 | 0.1296 | ||
| ||||
Sips | | | 6.75 | 11.84 |
| −0.9142 | 0.1986 | ||
| 0.0070 | 0.3442 | ||
RMSE | 1.4689 | 0.6272 | ||
| 0.9846 | 0.9833 | ||
| 0.1133 | 0.1122 |
Plots of adsorption isotherm models: Langmuir, Freundlich, and Sips models.
Based on the
Moreover, the
However, the material surface shows heterogeneity. Hence, the uptake of heavy metal ions onto MnO2 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 [
In comparison with the correlation coefficients (
Pseudo-first-order and pseudo-second-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
Kinetic parameters.
Kinetic models | Linear forms | Kinetic parameters | ||
---|---|---|---|---|
Pb(II) | Fe(III) | |||
| 109.80 | 16.80 | ||
| ||||
Pseudo-first-order model | | | 0.01428 | 1.38 |
| 0.5998 | 0.6873 | ||
| 5.433 | 18.12 | ||
| ||||
Pseudo-second-order model | | | 0.01108 | 4.5 |
| 1.000 | 0.9958 | ||
| 109.89 | 17.92 | ||
| ||||
Intraparticle diffusion model | | | 0.3722 | 0.0309 |
| 0.0038 | 0.1759 | ||
| 0.0017 |
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).
Results showed that the theoretical
Furthermore, intraparticle diffusion models showed that the uptake of Pb(II) followed two stages: firstly, Pb(II) ions were quickly adsorbed on
In this report,
The authors declare that there are no competing interests regarding the publication of this paper.