3.1 Analysis of adsorbability results
The prepared 100 mg/L Methylene blue was diluted to 2 mg/L, 4 mg/L, 6 mg/L, 8 mg/L, 10 mg/L, and 12 mg/L. After decantation and filtration, the equilibrium concentrations of dye in the solution were measured at 665 nm using UV-visible spectrophotometer, \(y = 0.07727 x + 0.05234\). According to the equation, the adsorption capacity of activated carbon at different Methylene blue concentration was calculated. The fitted equation is shown in Fig. 2, \({R }^{2}\)= 0.9941.
The adsorption capacity and removal rate of activated carbon for methylene blue are carried out according to formulas (2) and (3), thus measured data is shown in the Table.1.
$${q}_{e}=\frac{\left({C}_{0}-{C}_{e}\right)V}{W}$$
2
$$R=\frac{{C}_{0}-{C}_{e}}{{C}_{0}}\times 100\%$$
3
Table.1 The influence of carbonization temperature and NaOH-to-C ratio on the adsorption capacity and removal rate of activated carbon
Temperature
|
\(\text{NaOH/C}\)
|
\({\text{q}}_{\text{e}}\text{ (mg/g)}\)
|
\(\text{R}\)(%)
|
300℃
|
2: 1
|
482.23
|
96.45
|
3: 1
|
395.22
|
88.64
|
4: 1
|
468.78
|
93.76
|
5: 1
|
457.95
|
92.59
|
350℃
|
2: 1
|
492.92
|
98.58
|
3: 1
|
403.82
|
94
|
4: 1
|
475.5
|
97.76
|
5: 1
|
470
|
95.1
|
400℃
|
2: 1
|
497.23
|
99.45
|
3: 1
|
406.62
|
93.14
|
4: 1
|
495.92
|
99.32
|
5: 1
|
465.7
|
99.18
|
450℃
|
2: 1
|
496.43
|
99.29
|
3: 1
|
405.65
|
91.13
|
4: 1
|
489.91
|
97.98
|
5: 1
|
486.28
|
97.26
|
*\({\left[MB\right]}_{initial}\)=300mg/L |
3.1.1 The effect of carbonization temperature on adsorption
According to Fig. 3, the amount of methylene blue adsorbed by activated carbon increases with the increase of carbonization temperature, reaches a peak when the temperature reaches 400°C, and then begins to decrease.
The main function of the carbonization process of MIP is to enrich the fixed carbon in the raw materials, reduce the volatile components and moisture in the raw materials, thereby improve the strength of the carbonized materials, and at the same time generate initial porosity, which helps the activation process. If the carbonization temperature is too low, the components will not be completely volatilized, so that sufficient initial porosity will not be generated, which will affect the adsorption. If the temperature is too high, the graphite crystallites in the carbonized product will change in an orderly manner, reduce the gaps between the crystallites, and affect the activation process.
3.1.2 The effect of NaOH-to-C ratio on adsorption
According to Fig. 4.The ratio of NaOH-to-C ratio has a significant effect on the adsorption capacity of activated carbon. When the ratio of NaOH-to-C ratio is 2:1, the adsorption capacity reaches 497.23 mg/g.
With different NaOH-to-C ratio, the properties of the prepared materials are different, which is mainly reflected in the difference in specific surface area.
The chemical activation method is used to prepare more pore structures. With the increase in the proportion of the activating reagent NaOH, the precursor of the material is brittle and cannot withstand severe chemical changes, resulting in larger pores or through-holes. Because of surface area is reduced, so that the adsorption capacity of activated carbon is not well.
3.2 TG results and analysis
According to Fig. 5. From room temperature to about 250 ℃ is the initial stage of pyrolysis temperature rise. MIP raw material sample absorbs heat and evaporates water, thereby reducing the weight. The temperature is in the range of 250 ~ 400 ℃, which is the pyrolysis stage of MIP, mainly hemicellulose and cellulose pyrolysis, so the thermal weight loss is the most, and the weight loss rate is the fastest. When temperature rises to 400 ℃, the main ingredients of the melamine facing paper enter the end of pyrolysis, the structure gradually stabilizes and the formation of a carbon layer, and finally, the carbon residue rate of MIP is 25.06%, which is similar to that of ordinary agricultural waste.
3.3 SEM – morphological studies
A SEM study was explored to reveal microstructural morphological features of activated carbon. Figure 6a-d as shown in prepared carbonization/pyrolysis processes revealed morphological features of activated carbon. Small and large particles microstructures view with different magnifications explored at 20 − 10µm. The major part of the samples NaOH is chemically treated and thermally activated like pyrolysis/carbonization effects of 3D–porous carbon. The carbonization temperature is 400℃, when the ratio of NaOH-to-C ratio is 2:1, the surface of activated carbon is relatively dense, and the pore structure is rich, and the pore size ranges from 0.1nm to 4nm; when NaOH-to-C ratio to 3:1, there are fewer micropores on the surface of activated carbon. when the ratio of NaOH-to-C ratio is 4:1 and 5:1, the pore structure of activated carbon is not uniform, and the minimum pore size is about 0.2 nm. Clearly, shown that Fig. 6a lower magnification micro-structural images of 3D-porous fully inter-connected network combine of Cellular Structure forming an interconnected porous network.
3.4 BET analysis
To investigate the textural features (surface area and pore size distribution) of the adsorbent, the nitrogen adsorption-desorption isotherm was determined by using surface area analyzer. BET(Fig. 7) survey curve showed major contribution of activated carbon which is clearly observed about N2 adsorption-desorption isotherm and pore size distribution. The curve is mainly s-shaped, which is a type II isotherm. There is an inflection point in the low-pressure zone, indicating that the single-layer adsorption is saturated at this time, and the way to continue the adsorption is multi-layer adsorption. As seen in Fig. 6b when the NaOH-to-C ratio is 3:1 that the adsorption capacity is the lowest.
As shown in Table.2, shows the pore structure changes in the activated carbon caused by temperature variation during the preparation process. The maximum specific surface area of activated carbon is 608.55 m2/g, the average pore diameter is mainly between 1.93 nm to 3.04 nm, the micropore volume is small, and the pore diameter distribution is mainly concentrated between micropores and mesopores, and minimum pore volume is 0.131 cm3/g.
Table.2 Specific surface area and pore structure parameters of activated carbon under different conditions
Sample
|
BET Surface Area(m²/g)
|
Total pore volume (cm3/g)
|
Micropore volume(cm3/g)
|
Average pore size(nm)
|
400℃, 2: 1
|
608.548
|
4.4309
|
0.221061
|
1.9295
|
400℃, 3: 1
|
364.329
|
8.4167
|
0.13134
|
2.345
|
400℃, 4: 1
|
602.828
|
3.7402
|
0.205702
|
2.3277
|
400℃, 5: 1
|
442.619
|
4.9573
|
0.144457
|
3.0432
|
Therefore, based on the understanding and control strategy of activated carbon pore structure formation process, creating more micropores is conducive to the improvement of MB adsorption performance.
3.2 Isotherm study
Interpreting the isotherm information is significant for originating an equation that can be utilized. The Langmuir (4) and Freundlich (5) models were used to analyze the adsorption thermodynamics of MIP activated carbon. (see Fig. 8)
$${q}_{e}=\frac{{q}_{m}{K}_{L}{C}_{e}}{1+{K}_{L}{C}_{e}}$$
4
In the formula, Ce(mg/L) is the concentration of adsorbate in the solution at equilibrium, qe(mg/g) is the adsorption capacity per unit mass of adsorbent at equilibrium, qm(mg/g) is the maximum single-layer adsorption per unit mass of adsorbent. KL(L/mg) is the Langmuir isotherm adsorption energy constant. When RL>1.0, it means that it is not a single-layer adsorption; when RL=1.0, it means that the relationship is linear; when RL<1.0, it means that it is a single-layer adsorption; when RL=0, it means that it is irreversible adsorption. Kf is the Freundlich isotherm adsorption empirical constant, which is related to the adsorption capacity and the adsorption strength.
The correlation coefficients obtained by the nonlinear fitting of Langmuir and Freundlich isotherm models are 0.96233 and 0.94119, respectively, indicating that the adsorption of methylene blue by activated carbon is more consistent with the Langmuir model, and the adsorbent surface is uniform and is a single-layer adsorption. Calculated parameters of Isotherms for MB removal are given in Table.3.
Table.3 Langmuir and Freundlich equation parameters
3.3 Kinetic study
In order to further explore the adsorption state of activated carbon in the Kinetic models, the typical pore size models were combined.
The adsorption kinetics of methylene blue on activated carbon was investigated under the conditions of 30 ℃, not pH value adjustment, and 0.03 g of activated carbon. The nonlinear fitting of the quasi-first-order kinetics model (6) and the quasi-second-order kinetics model (7) was performed respectively. (Fig. 9)
$${q}_{t}={q}_{e}\left(1-{exp}^{-{k}_{1}t}\right)$$
6
$${q}_{t}=\frac{{{q}_{e}}^{2}{k}_{2}t}{1+{q}_{e}{k}_{2}t}$$
7
In the formula, t (h or min) is the adsorption time, qt, qe (mg/g) are the adsorption capacity at time t and when equilibrium is reached, k1 (h− 1 or min− 1) is the quasi-first order kinetic rate constant, k2(mg·g− 1·min− 1) is the quasi-second-order kinetic rate constant.
Table.4, the correlation coefficient of the experimental data obtained by the non-linear fitting of the quasi-second-order kinetic model is 0.9961, and the correlation coefficient of the quasi-first-order is 0.70495. To a certain extent, the adsorption amount qe,cal value calculated by the quasi-second-order kinetic equation is relatively close to the experimental data. It showed that the adsorption kinetics of methylene blue on melamine facing paper-based activated carbon conforms to the quasi-second-order kinetic model, and chemical adsorption is dominant.
Table.4 Equation parameters