To accurately analyze the micro-morphology of the material and the distribution of MnOx, the original BC and modified BC were observed by scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS) (Figure 1a, 1b, and 1c). The surface of the CSBC400 was smoother than CSBC600 and MCSBC, with uneven pleats, no significant pores, many particles deposited on the surface, and a non-porous structure, thereby indicating that the biomass was not completely cleaved and the pores were not completely opened. The surface of CSBC600 was rough, with irregular pleats, large protrusions, and some pores due to the escape of small molecules during pyrolysis (Liu et al. 2021). There were developed pores on the MCSBC surface, which was mainly a complex network structure with pores possibly provided by the MnOx or embedded BC (Song et al. 2014; Tan et al. 2018). Therefore, providing more adsorption sites on MCSBC, thereby enhancing its adsorption capacity (Dai et al. 2020).
The surface of the adsorbent has a significant influence on its adsorption capacity for organic pollutants (Tong et al. 2019). Understanding the surface properties of adsorbents will help understand the adsorption kinetics and adsorption mechanism. The surface properties of BC largely depend on the feedstock and process used after pyrolysis (post-treatment) (Liu et al. 2018). Table 1 shows that the C content in the BC samples was in the order of CSBC600 (73.03 %) > CSBC400 (63.60 %) > MCSBC (60.40 %). MCSBC had a high O content, which was related to the introduction of MnOx. The modification process resulted in an increase in the O/C ratio, thereby indicating the increased hydrophobicity of the BC samples modified by KMnO4 (Jang et al. 2018). The BET method was used to analyze the structural characteristics of the BC samples, and the results showed that the BET specific surface area of MCSBC (242.394 m2/g) was larger than that of CSBC600 (140.130 m2 /g) and CSBC400 (8.619 m2/g). The change in the BET specific surface area might have been due to the removal of impurities (Hao et al. 2018), which was also confirmed by the subsequent XRD analysis results. In addition, the attachment of MnOx promoted the pyrolysis of CS, thereby resulting in the formation of many pores and roughening the surface of the modified BC, which led to an increase in the external specific surface area of the BC and an increase in the contact surface with TC, which was more conducive to the adsorption of TC (Song et al. 2014). Song et al. (2014) found that KMnO4 has strong oxidizing properties that are capable of destroying some nanopore structures by converting them into mesopores or macropores. The conversion could have caused an increase in the pore diameter, thereby increasing the specific surface area and pore volume. Furthermore, an increase in the pore volume contributed to the entry of TC into the internal pore system (Shen et al. 2020).
Adsorption effect of BC
The adsorption effect of BC prepared under different conditions was evaluated so as to optimize the adsorption material for water treatment. The adsorption performance of CSBC600 at high pyrolysis temperatures was significantly better than that of CSBC400 at lower pyrolysis temperatures (Figure 2). Thus, CSBC600 was selected as the modified precursor material in subsequent experiments, and the adsorption mechanism was explored. The best adsorption performance was achieved with MCSBC modified with 0.035 M KMnO4 solution, thereby indicating that the introduction of MnOx significantly improved the surface structure of BC, and then led to an increase in the TC adsorption amount, which was confirmed by SEM-EDS analysis. However, modification with the high Mn concentration decreased the adsorption performance of BC (Figure 2), which might have occurred because many MnOx particles agglomerated on the CSBC surface, thereby leading to partial blockage of the pore structure (Song et al. 2014).
Adsorption kinetics and effects of contact time
The kinetic adsorption of TC pollutants on porous materials is usually classified into rapid adsorption and slow adsorption (Li et al. 2021). In the initial stage of adsorption, the adsorption sites are quickly occupied by the adsorbent molecules. With time, all the surface adsorption sites become occupied and reach the adsorption equilibrium (Jiang et al. 2017). As shown in Figure 3, rapid adsorption of TC by CSBC400, CSBC600, and MCSBC occurred within 30 min, during which the accessible sites on the adsorbent surface were rapidly occupied. The hydroxyl group on TC and the O-containing functional group of BC combined with H bonds and π–π electron donors and acceptors (Song et al. 2014; Tan et al. 2018) resulting in a rapid adsorption capacity of more than 90 % of the equilibrium adsorption capacity. The slow adsorption stage lasted 360 min. The equilibrium adsorption capacity of CSBC400, CSBC600, and MCSBC was 9.834 mg/g, 10.875 mg/g, and 13.254 mg/g, respectively. The modified BC prepared by oxidizing sawdust and soaking FeCl2 and KMnO4 solution adsorbed TC, and the adsorption capacity after modification was approximately three times that before modification (Zhang et al. 2021). One study used the method of chemical co-precipitation to synthesize MnO2-supported BC for TC adsorption, and showed a good adsorption capacity compared with that of other adsorbents (Shen et al. 2020). In this study, the TC adsorption capacity of MCSBC was 1.7 times that of CSBC600. This might have been due to the modification of MnOx, which increased the number of adsorption sites on the surface of the material (Tan et al. 2018; Wan et al. 2020). There was a large concentration difference between the BC and the solution interface at the initial adsorption stage, thereby resulting in a large mass transfer driving force (Iqbal et al. 2021; Liu et al. 2021), which led to the rapid occupation of the BC adsorption sites by TC. As the reaction time increased, the concentration difference of the solution decreased, which weakened the driving force of adsorption, thereby resulting in a slow increase in the adsorption amount (Wang et al. 2018). The adsorption of TC by BC gradually stabilized after 360 min.
To better explain the adsorption mechanism, pseudo-first-order and pseudo-second-order adsorption models were used to fit the kinetic experimental data. The fitting results of the pseudo-first-order and pseudo-second-order adsorption models are listed in Table 2. The pseudo-second-order adsorption model accurately described the TC adsorption behavior of the original BC and modified BC (R2 > 0.93). The TC adsorption amount obtained by fitting the pseudo-second-order kinetics equation was closer to the experimentally measured value, thereby indicating that chemisorption was predominant throughout the adsorption process (Shepherd et al. 2017). The adsorption mechanism and rate-limiting steps of TC on all the BCs were further examined using the intra-particle diffusion model (Jiang et al. 2017). Figure 4 shows the fitting curve of intra-particle diffusion is composed of two linear segments: the first stage is straight but not at the origin, thereby indicating that TC adsorption is a multi-step process and that intra-particle diffusion is not the only rate-limiting step (Xiong et al. 2018). In different diffusion stages, the linear relationship between the adsorption amount of TC and t0.5 was mainly manifested by the difference in kdi. The large kd1 of the first stage indicated that external diffusion was the dominant process (Eltaweil et al. 2020; Ma et al. 2015), and the diffusion time at this stage was generally less than 90 min. TC adsorption may be controlled by molecular diffusion and membrane diffusion (Zeng et al. 2019), and the diffusion rate of MCSBC and CSBC600 was approximately three times that of CSBC400, which might have been due to the increased surface area of MCSBC and CSBC600, thereby providing more adsorption sites (Zhang et al. 2021). The slow slope of the second stage indicated that TC adsorption gradually transformed from the liquid film diffusion process to the particle internal diffusion stage, and TC was transferred through the liquid film to the pores of the BC matrix (Wang et al. 2018; Zhang et al. 2019a). The diffusion rate of MCSBC was approximately six times that of CSBC600 and CSBC400, thereby indicating that the diffusion rate in the particles decreased sharply with the complete adsorption and filling of the pores in MCSBC; however, TC molecules continued to migrate slowly on the surface until they entered the pores of the particles due to activation (Song et al. 2014). The intercept C represents the range of the boundary layer thickness, that is, the larger the intercept, the greater the boundary layer effect (Limousin et al. 2007). As shown in Table 3, the value of C2 was greater than that of C1, and the diffusion boundary effect was more significant in the second layer, thereby indicating that the total adsorption rate was controlled by both liquid film diffusion and intra-particle diffusion (Limousin et al. 2007; Ma et al. 2015).
Adsorption isotherms are used to describe the distribution of adsorbate on adsorbents, thereby revealing their adsorption characteristics (Eltaweil et al. 2020; Grisales-Cifuentes et al. 2021; Limousin et al. 2007). With an increase in the adsorption equilibrium concentration, the adsorption amount of TC on the three materials increased significantly. The higher the concentration of TC, the larger the concentration gradient formed at the solid–liquid interface between TC and BC, thereby resulting in a greater driving force for mass transfer and an increased adsorption amount of TC on the BC surface (Hao et al. 2021; Moussavi and Barikbin, 2010). The probability of TC capture by BC also increased as the TC concentration increased (Zhang et al. 2019). The Langmuir and Freundlich models were used to evaluate the isothermal adsorption characteristics of CSBC400, CSBC600, and MCSBC (Figure 5). The fitting results are listed in Table 4. The fitting coefficient of the Freundlich model (R2 > 0.98) was better than that of the Langmuir model, thereby indicating that adsorption is a heterogeneous, multi-molecular layer adsorption process (Wang et al. 2018). The critical parameter 1/n is related to the degree of non-uniformity of the adsorption site (Pezoti Junior et al. 2014; Zhang et al. 2019). The minimum 1/n value of the modified BC sample was 0.456, indicating that the adsorption process of MCSBC can occur easily (Eniola et al. 2020; Moussavi and Barikbin, 2010). The specific surface area and pore volume of the BC were increased by Mn modification, and the surface adsorption capacity of the pore filling was improved (Song et al. 2014). In addition, the role of MnOx and TC functional groups in the formation of complexes cannot be ignored (Shen et al. 2020; Song et al. 2014). kF reflects the adsorption capacity of the adsorbent. A larger kF value represents a stronger TC adsorption capacity of BC (Jang et al. 2018). The larger kF of MCSBC indicated that the TC adsorption force of MCSBC was stronger than that of CSBC400 and CSBC600. One study (Liu et al. 2019) showed that the Langmuir model could better describe the TC adsorption process of BC, and the adsorption was mainly single-layer adsorption. However, another study (Jang et al. 2018) showed that the Freundlich model could better describe the adsorption of TC by loblolly pine BC, and the adsorption process was heterogeneous. The equilibrium adsorption capacity of the Mn-modified BC prepared in this study was 42.076 mg/g, which was higher than that of CSBC400 (26.936 mg/g) and CSBC600 (28.038 mg/g). Therefore, MCSBC can potentially be applied as a water treatment material for the adsorption and removal of TC.
Effect of temperature on TC adsorption
As shown in Figure 6, as the temperature increased from 25 to 45 ℃, the TC adsorption capacity of the three BC materials gradually increased. The TC adsorption capacity of BC increased as the temperature and speed of molecular movement increased (Shen et al. 2020). In addition, the chemical processes in the adsorption process are intensified at high temperatures (Zhang et al. 2019). Chemisorption dominated the adsorption process of TC by modified BC, which led to a better adsorption performance of modified BC at high temperatures.
Effect of solution pH on TC adsorption
pH not only affects the solubility of the adsorbate, but also changes the surface charge characteristics of the adsorbent, thereby changing the interaction between the adsorbent and the adsorbate (Xiang et al. 2019). TC is an amphoteric molecule. TC species include H4TC+, H3TC, H2TC-, and HTC2-, respectively, with pH values of < 3.3, 3.4–7.7, 7.8–9.7, and > 9.7, respectively (Xiang et al. 2019).
The effect of solution pH on the removal of TC by different BCs is shown in Figure 7. As the pH of the solution increased from 3 to 9, the TC adsorption capacity of CSBC600 and MCSBC decreased gradually, whereas the adsorption capacity of CSBC400 increased gradually, thereby indicating that the TC adsorption capacity of BC depends on both the pH and BC properties. Under different pH values, TC showed different species distributions. As the pH of the solution increased from 3 to 9, the TC adsorption capacity of CSBC600 and MCSBC decreased gradually because of the strong electrostatic repulsion between the TC molecules (H4TC+ and H2TC-) and the positive and negative charges on the surface of the adsorbent (Zhu et al. 2014). When the pH of the solution was lower than 3.3, the main type of TC was protonated TC (TC+) and the BC surface was negatively charged. Therefore, the interaction between TC and BC is mainly electrostatic, and the adsorption effect is the best (Zhang et al. 2019a). When the pH was between 3.4 and 7.7, TC molecules (i.e., H3TC) had no net charges, the electrostatic attraction was weak, and the adsorption efficiency decreased (Liu et al. 2021). In contrast, the TC adsorption capacity of CSBC400 showed an upward trend, which also verified that pH can affect the adsorption capacity of BC, thereby indicating that adsorption mechanisms such as surface complexation and cation bridging might have existed (Zhang et al. 2019). The study showed that ash affected the specific surface area and porosity of BC (Li et al. 2017), and thus affected its adsorption effect. The difference in ash content is a key factor affecting pH (Li et al. 2017). The ash content of the CSBC differed and was the key factor affecting the pH and zeta potential. Therefore, the difference in the adsorption effect at different pH values might have been caused by ash (Zhang et al. 2019). In conclusion, changes in pH affect the surface charge and TC type distribution on BC, which further shows that the electrostatic interaction between BC and TC is not the only influencing factor and mechanism.
Influence of ionic strength on adsorption capacity
The effects of salt ions (Na+, K+, Ca2+, and NH4+) and the TC removal intensity using the various BCs are shown in Figure 8. As the concentration of monovalent cations in the solution increased, BC had a slight promoting effect on TC removal, which might have occurred because the low Na+ concentration can improve the ionization level of TC molecules (Xiang et al. 2020), thereby resulting in a strong electrostatic interaction between the charged TC and the surface of the BC adsorbent. Related studies (Ersan et al. 2017; Gao et al. 2012) have reported that Na+ and K+ have a slight promoting effect on TC adsorption by BC; moreover, these studies speculate that the mechanism was attributed to salting-out. The studies also indicated that the non-electrostatic force could offset part of the electrostatic repulsion force, thereby increasing the adsorption capacity. Compared with the above salt ions, the bivalent cation Ca2+ also had a promoting effect on TC adsorption, which might have been due to the electrostatic interaction between Ca2+ and the BC adsorbent (Liu et al. 2017). However, the low TC concentration in the solution did not allow the formation of a complex between Ca2+ and TC, thereby affecting the adsorption of TC (Liang et al. 2019).
FTIR comparison before and after adsorption
Surface functional groups are important factors that affect the adsorption of pollutants by BC materials (Hao et al. 2018; Toles et al. 1999). As shown in Figure 9, the peak values of 3410–3660 cm-1, 1585 cm-1, 1400 cm-1, and 1096–1131 cm-1 corresponded to the stretching vibrations of –OH, C=C, –CH2, and C–O–C groups, respectively, thereby indicating the existence of such functional groups on the surface of the BC. No organic functional groups (C–O–C) were found on the modified BC, thereby indicating that some O-containing functional groups were removed during BC modification. The peak value of the modified samples was 623 cm-1, which corresponded to Mn–O. The peak value of MCSBC at 623 cm-1 was wide, which might have been due to Mn–O vibrations in KMnO4. The Mn–O, C=C, and C=O peaks confirmed the successful combination of KMnO4 and BC (Dong et al. 2018). In addition, the total amount of functional groups on the surface of the modified BC decreased gradually and the vibration absorption peak in the scanning wavelength weakened gradually, thereby indicating that the functional groups were involved in the formation of MnOx and the structural properties of BC were more stable. No difference was found between the functional groups on the surface of the MCSBC before and after adsorption, thereby indicating that the types of functional groups did not change during adsorption. After TC adsorption, the –OH stretching of MCSBC at 3730 cm-1 decreased to 3740 cm-1, which also revealed that the carboxyl functional group on MCSBC participated in the adsorption process and that the –OH and Mn–OH groups on the surface of MCSBC might have a complexing effect on TC (Liu and Fan, 2018). In addition, the C=O functional group of MCSBC at 1605 cm-1 migrated after TC adsorption, which might have been related to the metal–π electron interaction between the part of BC not loaded with MnOx and the metal cation in the adsorption process (Tan et al. 2018). The wave number of Mn–O functional groups decreased after adsorption, which indicated that there was a chemical interaction between MnOx and metal ions in the adsorption process (Song et al. 2014) and that MnOx plays an important role in adsorption.
XRD comparison before and after adsorption
The crystal structure and phase composition characteristics of the BC samples were characterized using XRD, as shown in Figure 10. In the CSBC400, CSBC600, and MCSBC structures, the diffraction peaks at 28.0°, 41.0°, and 25.9° could be assigned to SiO2, which has also been reported in previous studies (Lin et al. 2017). After modification, sharp diffraction peaks were observed at 29.9°, 35.0°, and 43.0° in the MCSBC structure, thereby indicating the presence of Mn3O4. However, Mn3O4 peaks were not found in the CSBC400 and CSBC600 structures, thereby indicating that Mn3O4 was successfully loaded into the MCSBC. After modification, the intensity of the C diffraction peak of CSBC600 was weakened, thereby indicating that there was a chemical effect between CSBC600 and KMnO4 in the modification process. The sharp diffraction peak of Mn3O4 in the MCSBC structure was significantly weakened after adsorption, which proved that TC adsorption had an effect on the phase of the MCSBC material structure. However, MCSBC still had a diffraction peak similar to that of the original CSBC600 after adsorption, thereby indicating that MCSBC maintained a high degree of crystallinity after adsorption.
BC adsorption mechanisms include pore diffusion, Lewis acid-base action, electrostatic action, ion exchange, surface complexation, a cationic–π mechanism, H bonding, and π–π stacking (Hoslett et al. 2021; Peiris et al. 2017; Zhao et al. 2021). The specific mechanism depends on the physical and chemical properties of the BC and the properties of the solution.
The well-developed pore structure and large specific surface area of BC are conducive to the physical diffusion of TC molecules, which is also an important factor for the effective removal of TC by high-temperature BC (Liu and Fan, 2018). Electrostatic action can affect the removal of TC by BC, but it is clearer in strongly acidic or alkaline environments (Hao et al. 2018). The abundant carboxyl and hydroxyl functional groups on the surface of BC can form H bonds with the carboxyl, hydroxyl, and amino functional groups of TC (Shen et al. 2020; Tan et al. 2018). The protonated amino group of TC can also form a cationic–π mechanism with the aromatic structure of BC (He et al. 2018). BC rich in mineral ions can bind TC via an ion exchange mechanism (Ersan et al. 2017; Gao et al. 2012). In addition, the complete aromatic structure of high-temperature BC can form π–π stacking with the benzene ring structure of TC. In this study, the specific surface area of the CSBC was in the order of MCSBC > CSBC600 > CSBC400. The TC adsorption capacity of CSBC was in the order of MCSBC > CSBC600 > CSBC400. Therefore, pore action is an important mechanism for TC removal by BC.
The effect of solution pH on the adsorption of TC by BC indicates that the effect of electrostatic action is limited because pH changes the surface charge of BC and the presence of TC, thereby affecting the interaction between BC and TC (Liu et al. 2021).
FTIR analysis showed that –OH, C–O–C, the aromatic structure, and –CH2 were involved in TC adsorption, and the mechanisms involved included H bonding and π–π stacking. The reaction mechanism can be inferred from the changes in the XRD binding energies and relative contents of the functional groups. The O-containing functional groups of MCSBC were different from those of CSBC600 and CSBC400. XRD analysis showed that the changes in the O-containing functional groups in MCSBC after TC adsorption were related to π–π stacking and H bonding (Wang et al. 2018). FTIR analysis and XRD showed that the aromatic structure and O-containing functional groups of BC played an important role in TC adsorption.
In conclusion, the main mechanisms of TC adsorption by MCSBC are pore diffusion, H bonding, electrostatic interactions, and π–π stacking.