Evaluation of surface structure and specific surface area of biochar
SEM images of untreated biochars of P. vittata and those chemically modified by FeCl3 and NaOH that were calcined at 600 °C, 800 °C, and 1200 °C are shown in Figs. 1 to 3, respectively.
The leaf biochars at each calcination temperature (Fig. 1) showed an increase in the number of particles on the surface of the grain as the firing temperature increased. On the other hand, there was no significant difference in the surface structure of the biomass as the calcination temperature was changed. Furthermore, in the case of chemical modifications of leaf biochars, the FeCl3-treated sample (Fig. 2) was found to have crystalline particles attached to the surface. In the NaOH-treated sample (Fig. 3), some of the layered structures on the biochar surface were detached. In addition, cracks on the biochar surface were observed in all the FeCl3- and NaOH-treated samples.
Granular materials were identified on the surface of the non-treated biochar calcined at a temperature of 1200 °C, and they indicated that the functional groups on the surface of P. vittata leaves were decomposed and basic surface oxides were formed. Sasaki et al. reported that surface functional groups decompose and basic oxides are formed when carbon is heated in a vacuum or inert air stream at temperatures greater than 1000 °C23. Leaves contain more chloroplasts than stems, and thus have a large quantity of sugars, which explains the presence of granular material in the biochars of leaves calcined at 1200 °C.
The adhesion of crystalline particles was observed on the chemically modified biochar surfaces of samples in the case of FeCl3 modification probably caused by the precipitation of Fe in the FeCl3 on the surface, either directly or as complexes. Fe3+ ions generated when FeCl3 is dissolved in water are known to be reduced to Fe in the presence of activated carbon in the aqueous phase24, which explains the modification of Fe on the biochar surface. A porous structure caused by exfoliation of the surface structure was observed on the biochar surface treated with NaOH. Li et al.20 reported that the specific surface area of biochar was increased as a result of NaOH treatment. Therefore, it was considered that the surface of the biochar was eroded by the corrosive effect of NaOH resulting in the exfoliation of the surface structure and the generation of pores. In addition, the granular material identified in the leaf biochar calcined at 1200 °C disappeared after the NaOH treatment. This suggests that the granular material was composed of alkali-soluble substances. In addition, the cracks observed on the chemically modified biochar surface could be due to physical crushing by the agitator during the modification procedure.
The specific surface areas of biochars at each calcination temperature are listed in Table 1. The specific surface areas of the leaf biochar were 6.57 m2/g and 4.31 m2/g at 600 °C and 800 °C, respectively, with no significant difference. However, at 1200 °C, it was 34.54 m2/g, nearly 5 to 7 times higher. The results showed that the specific surface area of the biochars increased with high-temperature calcination. In addition, the specific surface areas of the Fe-modified leaf biochar were 160 m2/g, 114 m2/g, and 129 m2/g for calcination temperatures of 600 °C, 800 °C, and 1200 °C, respectively, displaying a maximum increase of almost five times compared to that before modification. The specific surface areas of the NaOH-modified biochar were 73.79 m2/g, 31.92 m2/g, and 65.38 m2/g at 600 °C, 800 °C and 1200 °C, respectively. In the case of the NaOH-modified biochar, the highest specific surface area was observed at 600 °C. Therefore, the increase in the specific surface area of the biochar was higher with FeCl3 treatment than that of NaOH treatment. Considering the observation of the surface structure by SEM, it can be inferred that the specific surface area of the FeCl3-treated sample increased because of the adhesion of Fe to the surface and the formation of pores by physical breaking of the biochar surface. In the case of the NaOH-modified biochars, exfoliation of the surface structure was also observed, which may be the reason for the increase in the specific surface area. Surface cracks were observed in both FeCl3- and NaOH-treated biochars, suggesting that the adhesion of Fe to the surface is a more significant factor in increasing the specific surface area.
Table 1
Specific surface area (m2/g) of biochars at each calcination temperature and after chemical modification.
|
Calcination temperature
|
Treatment
|
600 ℃
|
800 ℃
|
1200 ℃
|
No treatment
|
6.57
|
4.31
|
34.54
|
FeCl3-treated
|
160.83
|
114.78
|
129.35
|
NaOH-treated
|
72.23
|
31.92
|
65.38
|
Reported examples of biochar from other biological resources are rapeseed straw biochar calcined at 600 °C (19.13 m2/g) by Li et al.20, cotton stalk biochar calcined at 600 °C (224 m2/g) by Zhang et al.25, and rice straw biochar calcined at 800 °C (36.4 m2/g) by Basta et al.26. In comparison to these studies, the surface areas of the P. vittata biochars in this study were similar under similar calcination conditions. The specific surface area of FeCl3-modified biochar was reported by Cope et al. for rice husk at 550 °C (77.3 m2/g)27 and Li et al. for rapeseed straw at 600 °C (6.82 m2/g)20. NaOH-modified sawdust at 700 °C (423.28 m2/g) was reported by Chen et al.28, and rapeseed straw at 600 °C (43.18 m2/g) was reported by Li et al.20. The surface areas of the FeCl3-modified biochars of this study using P. vittata exceeded the values reported in previous studies. In addition, the NaOH-modified biochars of this study had surface areas similar to those of the rapeseed straw biochar. Therefore, FeCl3 modification of biochar has a greater effect on the increase in specific surface area compared to previous studies.
Evaluation of As and Cd adsorption capacities of P. vittata biochar
Figure 4 shows the adsorption isotherms of As(V) and Cd for the biochars calcined at 600 °C, 800 °C, and 1200 °C. Biochars tended to adsorb more Cd than As. The adsorption of Cd was particularly pronounced in the initial concentration range of 1–150 mg/L. At low initial concentrations (10 mg/L), calcination temperature of biochar did not have an effect on the adsorption of As. However, Cd adsorption increased in biochars that were calcined at high temperatures (800 °C and 1200 °C). At initial concentrations of above 100 mg/L, the adsorption capacities of the samples calcined at 600 °C and 800 °C were higher. The quantity of toxic elements adsorbed per unit weight of biochar at an initial concentration of 1000 mg/L was approximately 450 mg/g-biochar at any calcination temperature. The best adsorption values were 451.69 mg/g-biochar at 800 °C for As and 454.65 mg/g-biochar at 600 °C for Cd. Furthermore, Figs. 5 and 6 show the adsorption isotherms of As and Cd for the FeCl3- and NaOH-modified biochars, respectively. It was confirmed that Cd tended to be adsorbed more readily than As, even in chemically modified biochars. The best adsorption by biochar at the initial concentration of 1000 mg/L were 458.40 mg/g-biochar at 800 °C for FeCl3 modification and 455.38 mg/g-biochar at 1200 °C for NaOH modification. Adsorption capacity increased by approximately 2% in the case of FeCl3 modification, while it was not significant in the case of NaOH modification.
The adsorption rates of As and Cd on the biochars with and without chemical modification at each initial concentration are shown in Fig. 7. While the specific surface areas of unmodified biochars were much larger at 1200 °C than at 600 °C and 800 °C, the adsorption rate of As(V) was only about 90% at any initial concentration. However, the adsorption rate of the FeCl3-modified biochars significantly improved at all initial concentrations at all calcination temperatures. In particular, it was shown that the adsorption rate in the low concentration range of 1 to 10 mg/L of initial concentration, which was not observed for unmodified biochars, was improved when biochars calcined at 800 °C and 1200 °C were used as feedstock. Although some improvement or decrease in adsorption performance was observed for biochars modified with NaOH, no significant change in the overall Cd adsorption properties was observed for biochars prepared at any calcination temperature as feedstock. The unmodified biochars showed a very high adsorption rate of about 97–98% under an initial Cd concentration of 1 to 10 mg/L, from which the adsorption rate decreased as the initial concentration increased, and the adsorption rate remained at approximately 90% for biochars at any calcination temperature at an initial concentration of 1000 mg/L. In the case of chemical modification with NaOH, the adsorption rate of the biochars calcined at 600 °C improved in the initial Cd concentration range of 1 to 10 mg/L, but the adsorption rate slightly decreased at higher concentrations compared to untreated biochars. In the case of biochars calcined at 800 °C and 1200 °C, the adsorption rate at low concentrations was lower than that of the unmodified samples, but it was slightly higher than that of unmodified samples for initial Cd concentrations of 100 to 250 mg/L.
Because the specific surface area was evaluated using the monolayer adsorption model with the Brunauer–Emmett–Teller (BET) equation by nitrogen adsorption, it was not affected by the functional groups and the chemical state of the biochar surface. In contrast, the adsorptions of As(V) and Cd were not affected by the specific surface area suggesting some sites on the biochar surface specifically bind arsenate regardless of the calcination temperature. The adsorption of As(V) was not affected by the specific surface area. On the other hand, Cd showed an extremely high adsorption capacity at low initial concentrations of 1 to 10 mg/L, while the adsorption rate decreased by 90% at an initial concentration of 1000 mg/L. This is because oxygen-containing functional groups, such as hydroxyl (-OH) and carbonyl (-COOH) groups, have high affinity and bind preferentially to cations such as Cd, but as the number of oxygen-containing functional groups decreases with increasing concentration, the ratio of binding by surface π-electrons increases. Therefore, this is a two-step adsorption mechanism.
The specific surface area of the FeCl3-modified P. vittata biochar was larger than that of the unmodified biochar, and the As adsorption capacity improved. The specific surface area of the biochar calcined at 600 °C was the largest among the three types, but the increase in As adsorption capacity by FeCl3 modification was the highest for the biochar calcined at 1200 °C. However, the increase in the amount of As adsorbed by Fe-modification was the highest when the biochar calcined at 1200 °C was used as the raw material probably because the Fe3+ ions were attracted to the π-electrons or oxygen-containing functional groups on the surface of this biochar than in the other two types29. The leaf biochar calcined at 1200 °C before modification has a larger specific surface area than those calcined at 600 °C and 800 °C, and in this case, the adsorption by π-electrons is stronger because the oxygen-containing functional groups on the surface are less due to the high-temperature calcination. Therefore, it is suggested that the π-electrons of the biochar calcined at 1200 °C attracted more Fe3+ ions present in the aqueous solution, which increased the attachment of the reduced Fe particles to the biochar surface or they caused the formation of complexes with oxygen-containing functional groups resulting in an increase in As adsorption and specific surface area. In addition, the reason for the improved arsenate adsorption at low initial concentrations of 1 to 10 mg/L in the case of FeCl3 modification is that the biochars calcined at 800 °C and 1200 °C had a sufficient amount of Fe that was adsorbed on to their surfaces, which may have resulted in the formation of a high-affinity complex between Fe and arsenate. In the biochar calcined at 600 °C, Fe was less modified than in the other two conditions, which may have caused the difference in the adsorption behavior after modification. On the other hand, it was confirmed that the adsorption performance tended to decrease at initial concentrations of above 100 mg/L compared to that at lower concentrations because the complex formation by Fe has high affinity, but the number of adsorption sites is limited, and hence the adsorption rate may have decreased due to the increase in arsenate present in the surrounding area.
Although the specific surface area of P. vittata biochar modified with NaOH was larger than that before modification, the Cd adsorption capacity improved only at the initial Cd concentration of 100–250 mg/L probably because of the decrease in carboxyl groups (-COOH) associated with NaOH modification. It has been reported that the modification of biochar with NaOH results in the formation of lactones by dehydration condensation of the surface functional group COOH with the hydroxyl group (-OH) of NaOH and an increase in OH20. The P. vittata biochar calcined at 600 °C before modification had the highest amount of -COOH, and that calcined at 1200 °C was considered to have the strongest binding by π-electrons on the biochar surface. However, NaOH modification caused a decrease in -COOH, which is a surface functional group, and the presence of surface functional groups seems to have changed significantly.