3.1. Characterization of the Precursor and Biochars
The elemental composition of the precursor and its acid–base properties are described in
Table 1.
The content of mineral substance in the precursor was 3.2 wt.%, which means it was almost 3times lower than that in the residues of supercritical CO
2 extraction of marigold [
11]. The lower content of ash in the precursor used in this study was most probably a consequence of washing with hot distilled water. This procedure was not used for the precursor used in [
11]. The precursor used was characterized by a low degree of carbonization, 49.4 wt.%, and a very low content of sulfur, 0.1 wt.%. The contents of other elements were as follows: H
daf = 9.9, N
daf = 4.6 and O
daf = 36.0 wt.%. As the content of acidic groups on the precursor’s surface was much higher (over 4-fold) than that of basic groups, its surface had a strongly acidic character.
Elemental analysis of the biochars isdescribed in
Figure 2a and shows the dominant presence of elemental carbon that varies in the range 72.1–79.8 wt.%. The activated carbons and biochars obtained as a result of one-stage activation (with CO
2) or two-stage activation (pyrolysis and activation with CO
2) contain about 80 wt.% of elemental carbon [
30,
31]. According to the data presented in
Figure 2a, the increase in the activation temperature by 100 °C results in a significant decrease in the carbon content relative to that of sample A6, activated at 600 °C, which is interpreted as a consequence of enhanced aromatization of the carbon structure at higher temperatures [
32]. As forthe effect of increased activation temperature on the content of nitrogen, it is slightly reduced (by 0.5 wt.%) in sample A7.
An important procedure in the preparation of the carbon adsorbents was their washing with a 5% solution of HCl and then with hot distilled water to remove a significant part of the mineral substance from the biochar structure (
Figure 2b). As shown in
Figure 2b, this procedure resulted in a considerable reduction in ash content, which was nearly 8 times smaller in the washed samples A6 and A7 relative to that in the samples not subjected to washing. For sample A6, the content of ash decreased from 22.6 to 2.9 wt.%, while for sample A7 the analogous decrease was from 27.8 to 3.5 wt.%. That is why the elemental analysis and the determination of the textural parameters, acid–base properties and adsorption capacity were performed for the samples washed with diluted hydrochloric acid and hot distilled water.
The porous structure of the biochars ispresented in
Table 2. The SEM images are presented in
Figure 3. The brighter fragments observed for biochars may be due to the presence of ash.
Upon activation of the precursor, the specific surface area, area of micropores and total pore volume of the biochars increase with increasing activation temperature. Sample A7 showed a greater specific surface area than biochar A6. An increase in the activation temperature by 100 °C results in the specific surface area increasingby almost 200 m
2/g. The surface area of biochar A7 looks very good against a background of those of the other adsorbents described in the literature. S
BET of sample A7 is much larger than those of the carbon adsorbents obtained by activation of, e.g., hay [
19] or pistachio nutshells [
32], with CO
2. Moreover, the S
BET of sample A7 is greater than those of the adsorbents obtained by much more expensive chemical activation [
33,
34]. For instance, Neme et al. [
34] in their paper described the synthesis of an adsorbent from castor seed hulls by activation with H
3PO
4, using different amount ratios of the precursor to the activator. The greatest S
BET they obtained was 785 m
2/g, which was determined in [
33] for the activated carbon synthesized at the highest amount ratios heated at 700 °C for 60 min. The S
BET of the sample obtained from the castor seed hull was smaller than that of biochar A6 and sample A7. Moreover, we used a lower (600 °C) or the same (700 °C) temperature of activation and the process of physical activation, which is much cheaper than the chemical one. The specific surface area of our biochars is comparable to or greater than those of the commercial products [
35,
36] used for the removal of liquid and gas pollutants. The specific surface area of samples A6 and A7 was greater than that of the commercial carbon prepared from palm shell charcoal (838 m
2/g) produced by physical activation [
35]. In view of the above exemplary literature data, the method of activation of the residues of supercritical extraction of common nettle seeds with CO
2 may be successfully used for the production of materials of comparable or even better textural properties than those described in the literature. As shown in
Table 2, the mean diameters of the pores in the biochars we obtained are 4.1 nm and 3.5 nm for samples A6 and A7, respectively, which indicates the dominance of small mesopores.
The chemical character of the surface of the obtained biochars provides information on the type of reactions taking place upon adsorption of pollutants. Of key importance is the content of oxygen functional groups that may be basic or acidic. The amount of basic and acidic oxygen functional groups was estimated by Boehm titration, theresults of which are presented in
Figure 4. The activation of the precursor with CO
2 was found to generate acidic and basic groups, but the sample activated atthe higher temperature exhibiteda decrease in the number of acidic groups and an increase in the number of basic ones. Sample A7 was also found to show richer chemistry of the surface; the total amount of oxygen functional groups was 3.80 mmol/g, including 0.55 mol/g of acidic ones and 3.25 mmol/g of basic ones. Sample A6 contained 0.78 mmol/g of basic groups and 2.81 mmol/g of basic ones.
3.2. NO2Adsorption
The obtained biochars were tested as adsorbents of the gas pollutant NO
2. At present, it is important to have NO
2 adsorbents ready to use as in a time of economic crisis, people may resort to using all kinds of products for heat generation. Adsorption of NO
2 by samples A6 and A7 was tested in wet and dry conditions, and the results are presented in
Table 3.
The effectiveness of NO
2 removal was observed to depend on the activation temperature and conditions of adsorption. Irrespective of the conditions of the process, the sorption capacity of sample A6 was lower than that of A7. In the process with no access toair, the sorption capacity of A7 increased by 7.4 mg, while in wet conditions it increased by 16.3 mg. The greater sorption capacity of A7 is a consequence of its better-developed porous structure; moreover, biochar A7 has more surface oxygen functional groups that may interact with the pollutant [
37]. The sorption capacities of adsorbents A6 and A7 (irrespective of the conditions of adsorption) were greater than those of the activated carbons obtained in our earlier studies [
32]. Moreover, the samples prepared by activated carbons obtained by direct activation of pistachio nutshells [
32] needed a higher activation temperature and longer time of heating than the corresponding values used in this study for the residues of supercritical CO
2 extraction of common nettle seeds. Therefore, the activation method proposed may be considered effective.
The effectiveness of NO
2 removal of the biochars obtained wasdependent on the conditions of adsorption. The sorption capacities of samples A6 and A7 were over twice greater if the process was performed in the presence of steam. For example, a doubling in the sorption capacity towards NO
2 was not noted for carbonaceous adsorbents obtained by direct activation of hay (microwave method) [
19] and adsorbents prepared by direct activation from pistachio nutshells (conventional method) [
32]. Biochar A6 adsorbed 20.1 mg of NO
2 in dry conditions and 42.8 mg NO
2 in the presence of steam. The analogous values for sample A7 were 28.7 mg/g and 59.1 mg/g in dry and wet conditions, respectively. Analysis of the results suggests that sample A6 proved less susceptible to the impact of the conditions of NO
2adsorption than A7.
The isotherms of NO
2 adsorption/desorption are presented in
Figure 5a,b. Irrespective of the conditions of the process, their shapes are similar. The ideal adsorbent should be characterized by the breakthrough curve with zero concentration of NO
2 for a long time, followed by the rapid breakthrough of the carbon bed and a fast increase in the concentration of the gas studied [
38]. In wet conditions (
Figure 5b), the shape of the isotherms is close to such an ideal shape. The curve obtained for sample A7 is particularly close to the ideal shape: the concentration of gas was 0 ppm for nearly 90 min and then it rapidly increased to 20 ppm. The character of NO
2 curves recorded for sample A7 in wet conditions when compared to that obtained for bio-activated carbon presented in [
19] confirms the effectiveness of the applied method of activation of the starting material. For sample A7, the 0 ppm concentration of gas was maintained for much longer than that for the sample obtained from hay, and the time of adsorption was shorter. The time of zero concentration of NO
2 was longer for the adsorption carried out in the presence of steam, which consequently gave greater sorption capacities in wet conditions (
Table 3). On the basis of the shapes of the adsorption/desorption isotherms, it can be concluded that both in dry and wet conditions, after the NO
2 influx to the carbon bed is cut off, the concentration of NO
2decreasesto zero. This may indicate that NO
2 was strongly bound in the structure of samples A6 and A7 or that it was chemisorbed [
37]. It should be emphasized that the shape of NO2 adsorption/desorption curves recorded for samples A6 and A7 implies that the mechanism of NO
2 adsorption on their surfaces is the same.
Moreover, it can be inferred that the following reactions take placein dry conditions [
39]:
In the presence of steam, the adsorption of NO
2 may lead to the generation of a mixture of acids:
which would lead to greater sorption capacities in wet conditions [
39,
40].
In the process of NO
2 adsorption, changes in the NO concentration were also observed in dry (
Figure 5c) and wet (
Figure 5d) conditions. The shapes of the curves recorded imply that the biochars show a rather good ability to reduce NO
2 to NO, but for sample A7, this ability is stronger, irrespective of the conditions of adsorption. Additionally, in the process run in wet conditions, the process of reduction is more effective, which is well pronounced for sample A7, as the concentration of 200 ppm (NO) in the process of adsorption on this sample in wet conditions was achieved in the time twice shorter than that in dry ones.
Table 4 presents the sorption capacities of different selected adsorbents towards NO
2. Biochar A7 is less effective in the removal of NO
2 than the adsorbents prepared in our earlier studies [
11,
25]. The NO
2 adsorption by the biocarbon obtained from marigold [
11] was performed in mixed dry conditions; that is, prior to adsorption, the biocarbon surface was wetted with air of 70% humidity for 30 min. This procedure undoubtedly had a great impact on the sorption capacity of this adsorbent, which was twice greater than that of sample A7. The activated carbon prepared from hops [
25] was subjected to chemical activation of the precursor by Na
2CO
3. The synthesis of this activated carbon was performed using the activator to precursor amount ratio of 3:1. The sorption capacity of the activated carbon described in [
25] towards NO
2 was 155.3 mg/g. However, the synthesis of this adsorbent was much more time-consuming and expensive than the synthesis of biochars presented in this paper. The sorption capacity of the activated carbon obtained from sawdust pellets (54.7 mg/g) [
41] was similar to that of biochar A7 (59.1 mg/g). Much lower effectiveness in the removal of NO
2was shown bythe adsorbent obtained by chemical activation of waste tires with KOH [
42]; it was over 5times lower than that of sample A7.
3.3. Methylene BlueAdsorption
The effect of the initial concentration of MB on the sorption capacities of the samples studied was checked (MB concentrations were 5–110 mg/L). According to the character of the curves presented in
Figure 6, the efficiency of MB removal decreased with theincrease in theinitial MB concentration. Thisis a consequence of a decrease in the number of active centers available during the adsorption process. The higher effectiveness of MB removal at its low initial concentrations follows from a smaller ratio of the number of MB molecules to that of the active centers on the biochar surface [
43]. The temperature of activation also had a considerable impact on the sorption capacities of the biochar samples. Sample A6 was able to adsorb 150 mg of MB, while sample A7 (activated at 700°C) showed asorption capacity of 239 mg/g. Therefore, in addition to its greater effectiveness in NO
2 adsorption,biochar A7 proved to be a more effective adsorbent towards methylene blue in water solution.
The experimental data describing the adsorption process were fitted by the Langmuir and Freundlich models (
Table 5).
The Langmuir equation assumes that the adsorption takes place in a monolayer with homogeneously distributed active centers forming on the adsorbent surface and that the heat of adsorption does not depend on the area of the adsorbent covered. The maximum sorption capacity q
max and the value of constant K
L describing affinity between MB and the biochar can be read from the slope of the dependence plotted in
Figure 7a and the intercept of this plot with the y-axis. The Freundlich equation assumes that the adsorption takes place in a multilayer structure in which the adsorbate is adsorbed in a heterogeneous system [
43]. The strength of adsorption (parameter n) and sorption capacity (parameter K
F) are read from the plot presented in
Figure 7b. According to the R
2 values, included in
Table 5, the correlation of the experimental data to the Langmuir model predictions is stronger: for sample A6, the value of R
2 is 0.9950, while for A7, it is 0.9967. For comparison, the R
2 values obtained assuming the Freundlich model for samples A6 and A7 are 0.9215 and 0.9240, respectively. Thus,the MB adsorption on the biochars studied is the process of monolayer chemical adsorption. The process of adsorption on sample A7 was better fitted by the Langmuir model than that on sample A6. Moreover, a review of the literature shows that the adsorption of MB on porous adsorbents is much more often described by the Langmuir model [
17,
19,
25,
44]. The values of the maximum sorption capacity at equilibrium calculated assuming the Langmuir model (A6–153.85 mg/g, A7–243.90 mg/g) are close to the experimental values (
Figure 6). As the value of the K
L constant was higher for sample A6, the bonds between MB and the biochar surface arestronger for this sample. The last parameter calculated assuming the Langmuir model was the dimensionless coefficient R
L, whose values were in the range from 0 to 1,indicating favorable adsorption. The value of 1/n, characterizing the strength of adsorption, was in the range 0 < n < 1, indicating that the chemical bonds formed between the biochars and MB are strong [
45]. Taking into account the value of K
F, it can be inferred that biochar A7 was more selective towards MB than sample A6.
Moreover, we determined the parameters of the nonlinear forms of the Langmuir and Freundlich equations [
46]. The results are presented in
Table 6 and
Figure 8. As follows from these results, the Langmuir model better describes the experimental data than the Freundlich model, although the correlation coefficient (R
2) values are much smaller than those obtained for the linear form of the Langmuir equation. As follows from a comparison of the fit of results obtained for both biochars to the nonlinear Langmuir equation, the fit was better for sample A6, as indicated by a higher value of R
2calculated for the results of this sample.
In the next stage, the impact of contact time on the adsorption of methylene blue on A6 and A7 samples was evaluated. The experimental data are shown in
Figure 9. The course of the isotherms recorded for samplesA6 and A7 implies that in the first 60 min of the process, the sorption capacity of the biochars towards MB was rapidly increasing, which is related to a large number of free active centers. After this time, the effectiveness of the process started decreasing, and after 7 h, a state of adsorption equilibrium was reached. This decrease is a consequence of the fact that MB molecules gradually occupy the active centers on the biochar surface, slowing down the process of adsorption until the equilibrium is reached [
29]. At the stage of adsorption equilibrium, a higher sorption capacity was shown bysample A7. A saturation point of 480 min has also been noted for the activated carbons described by AlOthman et al. [
47]. However, the samples they studied showed lower sorption capacities towards methylene blue at the state of equilibrium (varied from 76 to 128.89 mg/g) than those obtained for samples A6 and A7.
Knowing the impact of the time of adsorbent/adsorbate contact on the adsorbent sorption capacity, we were able to calculate the kinetic parameters for the two models of kinetics. The calculated values are given in
Table 7, while the plots illustrating the fits with these two models are presented in
Figure 10.
According to the above-presented results, the values of q
e,expfor samples A6 and A7 are significantly different from q
e,cal predicted by the PFO model. In addition, the value of R
2 for this model is much lower than 0.999, which excludes the fit to experimental data. The sorption capacity predicted by the PSO model is much closer to the experimental results, which is also confirmed by the value of R
2. Therefore, it may be inferred that the adsorption of MB from water solution has the character of chemisorption [
48]. Data presented in
Table 7 show that biochar A7 shows a higher affinity to the PSO model, as the value of R
2for this sample is higher. However, a smaller difference between q
e,exp and q
e,cal was noted for sample A6.
Next, we checked the effect of the temperature of adsorption on the effectiveness of removal of MB from its water solution by the biochars obtained. Three temperature variants were followed (
Figure 11).
According to the experimental data presented in this figure, the temperature of adsorption has no significant impact on the effectiveness of MB adsorption on the biochars studied. However, with anincrease in thetemperature of adsorption, the sorption capacities of the biochars A7 and A6 increase; the increase was greater for A7. It should be noted that A6 and A7 increase their sorption capacity with increasing temperature of adsorption, while for the biochars described in [
49], the opposite tendency was observed; their sorption capacities decreased with increasing temperature of adsorption.
The measurements also permitted the determination of thermodynamic parameters: Gibbs free energy, enthalpy and entropy (
Table 8) [
50]. As the values of ΔG vary in the range of −20 to 0 kJ/mol, it was concluded that the process has a physical character. With increasing temperature, ΔG takes more negative values, which means that the process becomes increasingly spontaneous, and it is more spontaneous for sample A7. Positive values of ΔH confirm that the adsorption of MB from water solution is endothermic and needs energy input [
50]. A comparison of the ΔH values obtained for the two biochars shows that a greater energy input is needed for adsorption on sample A7.
The sorption capacity of biochar A7 towards methylene blue was compared with the results obtained for other materials (
Table 9). The effectiveness of methylene blue removal by adsorption on sample A7 was much lower than that of the biochar obtained in a one-step template method [
51] using heavy bio-oil produced from biomass pyrolysis as a precursor [
51]. Zang et al. have proved that this material was able to adsorb 411 mg/g of methylene blue. Similar to that for biochar A7, the adsorption of methylene blue on this material was described by the Langmuir and pseudo-second-order models. A similar value of sorption capacity to that of sample A7 was reported for the activated carbon obtained by chemical activation of
Dipterocarpus alatus fruit with ZnCl
2 [
52]. However, it should be noted that the synthesis of the latter adsorbent is more expensive than that of the synthesis of our samples, A7 and A6. Biochar A7 showed over twice greater effectiveness in the removal of methylene blue from its water solution than the coal gangue-based zeolite granules (108 mg/g) [
53]. The adsorbent described in [
54] showed a much lower sorption capacity than that of sample A7. Soury et al. [
54] have reported that the adsorption of methylene blue on this material was described by the Freundlich isotherm.