Effect of different initial concentrations of metal ions on adsorption
The adsorption capacity and removal rates of lignite and MML for different initial concentrations of metal ions are shown in Fig. 1. The adsorption of heavy metal ions by lignite and MML increases with the increase of initial concentration (Fig. 1). This is due to the fact that the higher the initial concentration of metal ions, the higher the chance of collision with the adsorption sites on the surface of the adsorbent material and the better mass transfer driving force, which is conducive to reducing the mass transfer resistance and increasing the adsorption amount20. The removal rate of lignite and MML for metal elements decreases with increasing initial concentration, and the slope of the removal rate curve increases significantly near the initial concentration of Cu2+ at 30 mg/L, Zn2+ at 30 mg/L, and Pb2+ at 50 mg/L. This is because for a fixed amount of adsorbent material, the number of adsorption sites on its surface is limited and the adsorption effect will reach the best adsorption at a certain metal ion concentration.
Comparing the adsorption effects of lignite and MML on heavy metal ions, it can be seen that the adsorption capacity and removal rates of Cu2+, Zn2+ and Pb2+ by MML were higher than those by lignite at the same concentrated metal ion degree. In the initial concentration range of 10–90 mg/L, the removal rates of Cu2+ and Zn2+ by lignite and MML showed similar trends, and the difference in removal rates tended to increase. In contrast, the removal rate of Pb2+ by MML in the range of 10–50 mg/L was almost constant with increasing initial concentration, indicating that Pb2+ in AMD was well removed by MML in this range. In addition, the difference of adsorption of Cu2+, Zn2+ and Pb2+ between MML and lignite in the range of initial concentration 10–90 mg/L increased with the increase of initial concentration. This phenomenon indicates that MML has greater adsorption potential than lignite when the concentration of heavy metal ions in AMD solution is higher.
Adsorption isotherm
To clarify the mechanism of Cu2+, Zn2+ and Pb2+ adsorption by lignite and MML, the Langmuir model and Freundlich model were used to fit the experimental data based on the adsorption isotherm principle.
The Langmuir model assumes that monolayer adsorption occurs on the uniform adsorbent surface, with no interaction between adsorbates. The Langmuir model is expressed in the following form:
|
\(\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{e}}}=\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{m}}}+\frac{1}{\left({\text{K}}_{\text{L}}{\text{q}}_{\text{m}}\right)}\)
|
(1)
|
Where qm and qe are the maximum adsorption capacity and the adsorption capacity at equilibrium (mg/g), respectively, Ce is the adsorbate concentration in solution at equilibrium(mg/L), and KL is the Langmuir adsorption constant (L/mg). The values of qm and KL can be calculated by a linear relationship. In addition, the equilibrium constant RL of the Langmuir model can be used to describe the adsorption effect of the adsorption process.The RL equation is of the following form:
|
\({\text{R}}_{\text{L}}=1/\left(1+{\text{K}}_{\text{L}}{\text{C}}_{0}\right)\)
|
(2)
|
where C0 is the initial concentration of metal ions. The value RL<1 indicates good adsorption performance.
Based on multilayer adsorption on non-homogeneous surfaces, the empirical Freundlich equation without assumptions is expressed in the following form:
|
\(\text{ln}{\text{q}}_{\text{e}}=\text{ln}{\text{K}}_{\text{F}}+\frac{1}{\text{n}}\text{ln}{\text{C}}_{\text{e}}\)
|
(3)
|
The adsorption isotherms and corresponding parameters of Cu2+, Zn2+ and Pb2+ adsorption by lignite and MML are shown in Fig. 2 and Table 1, respectively. The correlation coefficient (R2 > 0.99) of the Langmuir model is higher, indicating that the processes of Cu2+, Zn2+ and Pb2+ adsorption by lignite and MML are more consistent with the Langmuir model (Table 1). On the basis of this result, it can be inferred that the processes of Cu2+, Zn2+ and Pb2+ adsorption by lignite and MML belong to Langmuir monolayer adsorption, where there is no interaction between the heavy metal ions adsorbed on the surface20. Also, RL was found to be less than 1 by analysis, indicating good adsorption of heavy metal ions21.The 1/n less than 1 in the Freundlich model also confirms good adsorption conditions22. Comparing the parameters of the adsorption isotherms of lignite and MML shows that the correlation coefficient of the Langmuir model for MML is larger,
Table 1
Adsorption isotherm constants for the adsorption of heavy metal ion onto different samples: Lignite; MML.
Metal ion
|
Adsorption material
|
Langmuir
|
Freundlich
|
KL (L/mg)
|
qm (mg/g)
|
R2
|
RL
|
KF (L/mg)
|
1/n
|
R2
|
Cu2+
|
Lignite
|
0.16684
|
13.36180
|
0.99576
|
0.16652
|
2.26204
|
0.48950
|
0.88550
|
MML
|
0.25769
|
16.21270
|
0.99926
|
0.11458
|
3.47001
|
0.46680
|
0.91549
|
Zn2+
|
Lignite
|
0.10161
|
14.79290
|
0.99571
|
0.24702
|
1.82106
|
0.54410
|
0.93473
|
MML
|
0.20573
|
15.80530
|
0.99772
|
0.13943
|
3.03035
|
0.47940
|
0.92316
|
Pb2+
|
Lignite
|
0.09121
|
17.52740
|
0.98246
|
0.17984
|
1.90594
|
0.63730
|
0.92816
|
MML
|
0.25673
|
18.38700
|
0.99755
|
0.07227
|
8.85303
|
0.24820
|
0.85616
|
which may be due to the homogeneous specific adsorption sites generated during the magnetization process23. The maximum adsorption capacities of MML were 16.2127 mg/g, 15.8053 mg/g, and 18.3870 mg/g for Cu2+, Zn2+ and Pb2+, respectively, while the maximum adsorption capacities of lignite were 13.3618 mg/g, 14.7929 mg/g, and 17.5274 mg/g for Cu2+, Zn2+ and Pb2+, respectively. It indicates that the adsorption capacity of MML is large. In addition, it has been reported that the larger the adsorption capacity of the adsorbent the larger the value of KF23. The value of KF of MML in this study were larger than those of the original lignite, further confirming that MML is more likely to adsorb heavy metal ions.
Adsorption kinetics
To clarify the adsorption mechanism of lignite and MML, the adsorption kinetics of Cu2+, Zn2+ and Pb2+ by lignite and MML were analyzed in this paper using quasi first-order kinetic model, quasi second-order kinetic model and intraparticle diffusion model.
Quasi first-order model
Lagergren proposed an adsorption analysis method based on solid adsorption capacity20, which is the quasi first-order kinetic equation in the following form:
|
\(\text{ln}\left({\text{q}}_{\text{e}}-{\text{q}}_{\text{t}}\right)=\text{ln}{\text{q}}_{\text{e}}-{\text{k}}_{1}\text{t}\)
|
(4)
|
where qe and qt are the amounts of adsorbed metal ions at equilibrium and at time t (mg/g), respectively, and k1 is the quasi first-order rate constant (min− 1).
Quasi second-order model
The quasi second-order kinetic model is based on the assumption that the adsorption rate is controlled by chemisorption24. The quasi second-order kinetic model is expressed in the following form:
where qe and qt are the amounts of adsorbed metal ions at equilibrium and at time t (mg/g), respectively, and k2 is the quasi second-order rate constant (min− 1).
The results of quasi first-order model and quasi-second-order kinetic fits for the adsorption of Cu2+, Zn2+ and Pb2+ by lignite and MML are shown in Fig. 3 and Table 2.
From Fig. 3(a) and (b), it can be seen that in the initial stage of adsorption, the slope of the tangent line of the curve is larger, indicating that the adsorption rate of MML and lignite is faster. As the reaction proceeds, the slope gradually decreases. This is due to the fact that there are enough effective adsorption sites on the surface of the adsorbent material in the initial stage, but the adsorption efficiency decreases after the adsorption sites are gradually occupied. As can be seen from Table 2, the quasi first-order kinetic parameters R2 for the adsorption of Cu2+, Zn2+ and Pb2+ by lignite were high, indicating that the adsorption process followed the quasi first-order kinetic model and was dominated by physical adsorption. The fitted equations of quasi first-order l kinetics of lignite for Cu2+, Zn2+ and Pb2+ were: y = 9.2602*(1 - e− 0.00607x), y = 10.2839*(1 - e− 0.00468x), and y = 11.8456*(1 - e− 0.01265x), respectively. However, the quasi second-order kinetic parameter R2 for the adsorption of heavy metal ions by MML is high, indicating that the adsorption process follows a quasi second-order kinetic model. The adsorption mechanism is dominated by chemisorption24–25, and the adsorption rate is influenced by the coordination of active sites on the material surface with metal ions26. The fitted equations for the quasi second-order kinetics of MML for Cu2+, Zn2+ and Pb2+ were: y = 0.09599x + 8.81861, y = 0.09333x + 10.01582, and y = 0.05103x + 5.02836, respectively.
Intra-particle diffusion model
The adsorption process usually involves two main mechanisms: film diffusion and particle diffusion. In order to determine the way of metal ions entering the adsorbent material from the solution, the intra-particle diffusion model (Eq. 6) was used to determine the adsorption rate control steps and the results are shown in Fig. 3 and Table 2.
|
\({\text{q}}_{\text{t}}={\text{k}}_{3}{\text{t}}^{1/2}+\text{C}\)
|
(6)
|
Where qt is the amount of metal ions adsorbed at any moment t (mg/g), k3 is the diffusion rate constant within the particle (min− 1), and C is the constant involving thickness and boundary layer. The larger the value of C, the greater the contribution
Table 2
Kinetic parameters of heavy metal ion adsorption on different samples: Lignite; MML.
Metal ion
|
Adsorption material
|
Quas first-order model
|
Quasi second-order model
|
Intra-particle diffusion model
|
K1
|
qe (mg/g)
|
R2
|
K2
|
qe (mg/g)
|
R2
|
K3
|
C
|
R2
|
Cu2+
|
Lignite
|
0.00607
|
9.26020
|
0.98854
|
0.00072
|
10.1926
|
0.85585
|
0.73173
|
-1.65400
|
0.87331
|
MML
|
0.01311
|
7.53811
|
0.99724
|
0.00104
|
10.4178
|
0.99906
|
0.87488
|
-1.72723
|
0.95551
|
Zn2+
|
Lignite
|
0.00468
|
10.28394
|
0.98526
|
0.00070
|
9.7447
|
0.78164
|
0.68009
|
-1.56120
|
0.84320
|
MML
|
0.01191
|
7.52736
|
0.99828
|
0.00087
|
10.7147
|
0.99925
|
0.85380
|
-1.78576
|
0.95279
|
Pb2+
|
Lignite
|
0.01251
|
11.85742
|
0.96693
|
0.00038
|
19.0330
|
0.79657
|
1.47048
|
-3.53485
|
0.98318
|
MML
|
0.01327
|
14.01561
|
0.98903
|
0.00052
|
19.5963
|
0.99463
|
1.63444
|
-3.37912
|
0.97145
|
of the boundary layer.
Figure 3 shows the linear relationship between qt and t1/2. Among them, the parameters of the intra-particle diffusion model for Cu2+, Zn2+ and Pb2+ adsorption by lignite and MML are shown in Table 2. According to reports, if the plots are linear and pass through the origin, indicating that intra-particle diffusion is the only rate control step; if the linear plot of the fitted results does not pass through the origin, indicating that the adsorption rate is also controlled by other adsorption stages27. As can be seen in Fig. 5, the fitted results for the adsorption of Cu2+, Zn2+ and Pb2+ by lignite and MML are linear and do not pass the origin, indicating that the rates of Cu2+, Zn2+ and Pb2+ adsorption by lignite and MML are jointly controlled by multiple adsorption stages.
Analysis of the removal effects of lignite and MML on Cu2+, Zn2+ and Pb2+
The dynamic removal effects of lignite and MML on Cu2+, Zn2+ and Pb2+ with time are shown in Fig. 4. The dynamic removal effects of both lignite and MML on Cu2+, Zn2+ and Pb2+ showed a similar trend (Fig. 4). Metal ions were removed rapidly in the first 13 days, with removal rates of Cu2+, Zn2+ and Pb2+ exceeding 95%, 92% and 97%, respectively, and then the removal rate gradually decreased from day 13 to day 22, and the removal rate was only about 10% at day 22. This phenomenon was attributed to the fact that there were enough binding sites on the surfaces of adsorbent lignite and MML for metal ions to occupy at the beginning of the reaction, which made the adsorption process easier. However, the number of effective adsorption sites on the surfaces of lignite and MML gradually decreased with time, which led to a decrease in the removal rate. During the whole dynamic removal cycle, the average removal rates of lignite for Cu2+, Zn2+ and Pb2+ were 78.00%, 76.97% and 78.65%, respectively, and the average removal rates of MML for Cu2+, Zn2+ and Pb2+ were 82.83%, 81.57% and 83.50%, respectively. Apparently, the adsorption capacity of MML for heavy metal ions increased due to the increase in specific surface area and pore volume18.
Characterization Analysis
SEM analysis
The lignite and MML before and after the dynamic test were taken for SEM inspection, and the results are shown in Fig. 5. From Fig. 5(a) and (b), it can be seen that the surface of lignite is smooth, while the surface of MML is slightly rough, which is mainly due to the successful loading of Fe3O4 on the surface of lignite leading to the rough surface of MML. A large number of Fe3O4 particles scattered on the surface of lignite increased the specific surface area of the adsorbent material, which was beneficial to the removal of Cu2+, Zn2+ and Pb2+ by the MML. As shown in Fig. 5(c) and (d), the surfaces of MML and lignite after adsorption of Cu2+, Zn2+ and Pb2+ were very rough, and the granularity of the MML surface increased significantly, indicating that more heavy metal ions were adsorbed on the surface of MML.
XRD analysis
The XRD test results of lignite and MML before the dynamic test are shown in Fig. 5(e). Compared with lignite, the number of broad peaks in the XRD pattern of MML was reduced, indicating that the lignite changed from amorphous phase to crystalline phase during the magnetic modification process. The peaks at 2θ = 30.09°, 35.42°, 43.05°, 56.93° and 62.52° show (220), (311), (400), (511) and (440) diffraction planes, respectively, which are consistent with the standard XRD data of the cubic phase Fe3O4, and it can be inferred that Fe3O4 is successfully loaded onto the surface of lignite. The significant surface phase changes are consistent with SEM, which further verifies that the surface of MML becomes rougher due to the presence of Fe3O4. The rough surface of MML leads to an increase in specific surface area, which facilitates the adsorption of metal ions. The XRD results of lignite and MML after dynamic tests are shown in Fig. 5(f). Comparing the XRD plots of lignite and MML before and after the dynamic tests, it can be seen that no other crystal phases appeared in lignite and magnetically modified lignite after adsorption of Cu2+, Zn2+ and Pb2+, indicating that the adsorption process basically did not affect the crystal structures of lignite and MML.
FTIR analysis
The lignite and MML before and after the dynamic test were taken for FTIR detection, and the results are shown in Fig. 5(g) and (h). From Fig. 5(g) and (h), it can be seen that the peaks at 3400 cm− 1 for lignite and MML are caused by O-H stretching vibrations of carboxylic acid groups, the peak at 2920 cm− 1 is attributed to the presence of -CH2 in the stretching of aliphatic compounds, and the peak at 1600 cm− 1 is related to the stretching vibrations of carboxylic acid functional groups 19,29. The disappearance of some peaks after magnetic modification of lignite, especially the generation of new peaks at 584 cm-1, is attributed to Fe-O stretching vibrations of Fe3O4 particles30. It shows that Fe3O4 was successfully loaded onto the lignite surface, which is consistent with the XRD results. After the adsorption of Cu2+, Zn2+ and Pb2+ by dynamic tests, the band at 3384 cm− 1 shifted to 3392 cm− 1 for lignite, and the same was observed in the FTIR pattern of MML. The band at 3420 cm− 1 shifted to 3426 cm− 1 after the adsorption of metal ions by MML, which indicates the involvement of surface carboxylic acid ions in the adsorption of metal ions. The shape of the peak changed after the absorption of metal ions because the hydroxyl group of the carboxylic acid group stretched the vibration to metal ions. In addition, the band of MML at 584 cm− 1 shifted to 580 cm− 1, indicating the interaction between Fe3O4 and metal ions during adsorption.