Adsorption characteristics and mechanisms of Cd2+ from aqueous solution by biochar derived from corn stover

Thermogravimetric/differential thermogravimetry analyses of corn stover

Thermogravimetric/Differential Thermogravimetry (TG/DTG) curves are shown in Fig. 2. The pyrolysis process of corn stover could be divided into three stages. The first stage was the dehydration stage, which occurred at approximately 55–125 °C, and the weight loss was mainly accounted for by water¹⁹. The second stage was the pyrolysis stage, which occurred at approximately 200–400 °C and mainly involved the decomposition of cellulose, hemicellulose and a small amount of lignin. This process involved the generation of CO and CO₂ and the breaking of carbonaceous polymer bonds²⁰. In addition, a shoulder peak in the range of 265 to 300 °C in the DTG diagram could be caused by side chain decomposition and glycosidic bond cleavage of xylan during the pyrolysis of corn stover²¹. The third stage was the carbonization stage, which occurred above 400 °C; this stage mainly involved the decomposition of lignin^22,23. The carbonization process was relatively slow after 600 °C; this process was called the passive pyrolysis stage²⁴. In general, the TG loss in the pyrolysis process of corn stover was mainly from the moisture in the biomass sample in the first stage. Hemicellulose and cellulose decomposition occurred in the second stage, and lignin decomposition occurred in the third stage²⁵. In this experiment, the minimum pyrolysis temperature for the preparation of biochar was 400 °C. Therefore, the pyrolysis of biochar was relatively complete.

Characterization of biochar

Yield and specific surface area analyses

The yield and S_BET are presented in Table 2. BC, BC-H and BC-OH represent the origin, acid-modified, and base-modified biochar, respectively. The yield of corn stover biochar exhibited a negative correlation with the temperature and decreased from 39.65 to 28.26% when the pyrolysis temperature increased from 400 to 700 °C. This phenomenon could have occurred due to the loss of more volatile substances and the thermal degradation of lignocellulose with increasing temperature, thus reducing the yield of biochar^26,27. The S_BET of the original biochar showed little difference below 700 °C but increased significantly at 700 °C. Combined with the SEM analysis (Fig. 3), at low temperatures, more ashes on the surface of biochar could block its pores so that the change in S_BET was not obvious. At 700 °C, because the ash content significantly reduced and the pyrolysis was more sufficient, the pores of the biochar were more developed, and the S_EBT significantly increased. The S_BET of the acid/base-modified biochar increased with increasing temperature. The S_BET of biochar was larger than that of the original biochar after acid and base modification at 400–600 °C. This phenomenon occurred because the porous structure of biochar was enhanced by acid and base modification²⁸. Moreover, pickling removed most of the inorganic substances in biochar and reduced ash content, while alkali washing removed the tar on the surface of biochar to a certain extent²⁹. However, at 700 °C, the S_BET of biochar after acid/base modification was lower than that of the original biochar. Combined with the SEM (Fig. 4), the acid/base modification caused the nanopores of biochar to collapse into mesopores or macropores³⁰. Therefore, the well-developed pore structure of the biochar prepared at 700 °C was destroyed by acid/base modification, resulting in a significant decrease in S_BET.

Scanning electron microscopy analysis

The C1, C8, C12 and C16 biochars had the highest Cd²⁺ removal rates at 400, 500, 600 and 700 °C, respectively. Therefore, these BCs were selected for SEM analysis. Figure 3 clearly showed that as the pyrolysis temperature increased from 400 to 700 °C, the pore structure of biochar became more developed, with a smaller pore size and more pores. Although there were numerous pores at 500 °C, the pores were not fully developed and were blocked inside. At 700 °C, the skeleton structure appeared, and the particle size of ash blocked in the pores decreased.

By taking C16 biochar with the highest removal rate of Cd²⁺ as the research object, the changes in the biochar surface before and after modification were compared. C16-H and C16-OH represent acid-modified and base-modified biochar, respectively. After acid/base modification, the ash content on the surface of the biochar decreased, and the pore size increased (Fig. 4). Therefore, some skeleton structures could collapse after corrosion, which was consistent with the previous S_BET results. Sun et al. discovered that citric acid-modified biochar would lead to micropore wall collapse and micropore loss, resulting in a reduction in S_BET³¹. This finding was in agreement with the results of our study.

Fourier transform infrared spectroscopy analysis

The FTIR spectra of biochar at different pyrolysis temperatures are presented in Fig. 5a.

As the pyrolysis temperature increased from 300 to 700 °C, the absorption peak intensity showed a downwards trend. There was a remarkable decrease in features associated with stretch O–H (3400 cm⁻¹)³². The vibration peaks of C–H (2924 cm⁻¹) and C=O (1610 cm⁻¹) decreased with increasing temperature, which could be due to the reduction in –CH₂ and –CH₃ groups of small molecules and the pyrolysis of C=O into gas or liquid byproducts at high temperatures³³. In addition, the peak at 1435 cm⁻¹ was identified as the vibration of C=C bonds belonging to the aromatic skeleton of biochar. A decrease in the absorbance peaks was found at 1115 cm⁻¹, which corresponded to C–O–C bonds. The ratio of intensities for C=C/C=O (1550–1650 cm⁻¹) and C–O–C (1115 cm⁻¹) to the shoulder (1100–1200 cm⁻¹) gradually decreased, and the loss of –OH at 3444 cm⁻¹ indicated that the oxygen content in biochar reduced. The cellulose and wood components were dehydrated, and the degree of biochar condensation increased at higher temperatures. The bending vibration peaks of Ar–H at 856 and 877 cm⁻¹ changed little at different temperatures, which showed that the aromatic rings were relatively stable below 700 °C³⁴. Combined with the above analysis the condensation degree of biochar increased gradually above 400 °C^35,36. In summary, as the pyrolysis temperature increased, the degree of aromatization of biochar improved, and the numbers of oxygen-containing functional groups decreased continuously.

Figure 5b showed that after acid/base modification, the absorbance peaks at 3444 cm⁻¹, 1610 cm⁻¹ and 1115 cm⁻¹ increased, indicating that the number of oxygen-containing functional groups increased. However, the stretching vibration peak of aromatic ring skeleton C=C (1435 cm⁻¹) and the bending vibration peaks of Ar–H (856–877 cm⁻¹) changed little. The number of functional groups of acid-modified biochar increased more than that of alkali-modified biochar. Mahdi et al. found that acid modification increased the number of functional groups in a study of biochar modification³⁷. After acid/base modification, the number of oxygen-containing functional groups, such as hydroxyl and carboxyl groups, increased.

Optimization of biochar

Figure 6 illustrates that the removal rates of Cd²⁺ by corn stover biochar (original, acid-modified, and base-modified biochars) consistently increased with increasing pyrolysis temperature. The highest removal rate reached 95.79% at 700 °C. The removal rate decreased after modification, especially after pickling. The results showed that C16 biochar had the best removal effect on Cd²⁺.

Intuitive and variance analyses were employed to explore the influences of biochar preparation conditions on the removal rate of Cd²⁺.

1. 1.

   Intuitive analysis

   The intuitive analysis of the orthogonal experiment is shown in Table 3 and Fig. 7. The pyrolysis temperature had the most significant influence on the removal of Cd²⁺, followed by the retention time and finally the heating rate. Therefore, the optimal conditions for biochar preparation were a pyrolysis temperature of 700 °C, a retention time of 2.5 h, and a heating rate of 5 °C/min.

2. 2.

   Variance analysis

   Variance analysis showed that the effect of pyrolysis temperature on the removal rate of Cd²⁺ was very significant (Table 4). The effects of retention time and heating rate were not significant. This phenomenon was consistent with the conclusions obtained in the intuitive analysis.

Analysis of adsorption mechanism

The S_BET of the unmodified biochar did not change significantly with temperature, which indicated that S_BET could potentially not be a critical factor for Cd²⁺ adsorption. Qi et al. obtained a similar conclusion when studying the adsorption of Cd²⁺ in water by chicken litter biochar³⁸. In addition to S_BET, the four primary mechanisms involved in the removal of heavy metal ions by biochar were as follows: (1) Ion exchange: the alkali or alkaline earth metals in biochar (K⁺, Ca^{2 +}, Na⁺, and Mg²⁺) were the dominant cations in ion exchange³⁹. (2) The complexation of oxygen-containing functional groups mainly included hydroxyl and carboxyl groups⁴⁰. (3) Mineral precipitation: Cd²⁺ was precipitated by minerals on the surface of biochar to form Cd₃(PO₄)₂ and CdCO₃⁴¹. Soluble cadmium precipitated with some anions released by biochar, such as CO₃²⁻, PO₄³⁻ and OH^−42,43. (4) π electron interaction: Cd²⁺ coordinated with the π electrons of C=C or C=O at low pyrolysis temperatures^43,44. Biochar contains more aromatic structures at high pyrolysis temperatures, which could provide more π electrons. Therefore, the π electron interaction in adsorption of Cd²⁺ was effectively enhanced⁴⁵.

C1, C8, C12 and C16 were selected to study the adsorption mechanism. Related physicochemical properties are given in Table 5.

The CEC of biochar gradually increased as the pyrolysis temperature increased, reaching a maximum at 600 °C and slightly decreasing at 700 °C. This phenomenon could have occurred because the crystalline minerals under high pyrolysis temperatures inhibited the exchange of cations on the surface of biochar with Cd²⁺ in aqueous solution⁴⁶. Nevertheless, CEC did not change significantly with temperature; thus, CEC was not the main adsorption mechanism. With increasing pyrolysis temperature, the number of acidic functional groups decreased gradually, while the number of alkaline functional groups increased. The main functional groups used to remove Cd²⁺ were generally considered acidic oxygen-containing functional groups. However, the number of these functional groups decreased with increasing pyrolysis temperature, which weakened the complexation on the surface of the biochar. However, this result was contradictory to the results of Cd²⁺ adsorption. Therefore, the functional groups were not the main adsorption mechanism.

To further explore the adsorption mechanism of Cd²⁺, the biochar before and after the adsorption of Cd²⁺ was characterized by XRD. As shown in Fig. 7a, C16-100Cd and C16-200Cd represented the biochar after Cd²⁺ adsorption when the concentrations of cadmium solution were 100 mg/l and 200 mg/l, respectively. The results showed that new peaks appeared at 30.275° and 36.546° after adsorption, corresponding to CdCO₃. The spike at 29.454° was due to Cd(OH)₂. Additionally, the intensity of the CdCO₃ peak increased significantly from C16-100Cd to C16-200Cd, indicating that mineral precipitation occurred in adsorption. Liu et al. found similar results in a study on removing Cd²⁺ from water by blue algae biochar¹². However, as the concentration of Cd²⁺ increased from 0 to 200 mg/L, the diffraction peak at 2θ = 29.454° first increased and then decreased. This because the peak position of CaCO₃ at 2θ = 29.369° was very close to Cd(OH)₂ at 2θ = 29.454°. At low concentrations, the production of Cd(OH)₂ was greater than that of CdCO₃. When the initial concentration of Cd²⁺ increased, more CO₃²⁻ released by CaCO₃ combined with Cd²⁺ to form CdCO₃, resulting in a reduction in the diffraction peak.

As presented in Fig. 8b, the peak intensities of CdCO₃ and Cd(OH)₂ gradually increase with increasing pyrolysis temperature. On the one hand, this phenomenon could be ascribed to the increase in the mineral content of biochar with increasing pyrolysis temperature. On the other hand, the pH value of biochar increased with increasing pyrolysis temperature. In this way, more OH⁻ was released, thus forming more Cd(OH)₂. Wang et al. obtained similar results⁴². Moreover, the peak intensity of KCl at 2θ = 28.347° decreased after adsorption, as shown in Fig. 8a, which indicated that ion exchange took part in adsorption.

In addition, the FTIR spectra showed that the number of functional groups, such as C=C and C=O, in biochar decreased with increasing pyrolysis temperature, leading to the weakening of cation–π interactions between Cd²⁺ and C=C and C=O. In contrast, due to the enhanced aromatization of functional groups on the surface of biochar, many lone pair electrons existed in the electron-rich domains of the graphene-like structure, which in turn enhanced the cation–π interactions. Harvey et al., based on the study of Cd²⁺ adsorption by plant biochar, concluded that the electron-rich domain bonding mechanism between Cd²⁺ and the graphene-like structure on the surface of biochar played a more significant role in biochar with a high degree of carbonization⁴⁵. Therefore, π-electron interactions could play a dominant role in Cd²⁺ adsorption on high-temperature pyrolysis biochar. Moreover, the results showed that the number of alkaline functional groups increased while acidic functional groups decreased with the increase in pyrolysis temperature. It is generally believed that acidic functional groups could withdraw electrons, and basic functional groups could donate electrons^47,48. The biochar with higher pyrolysis temperature contained more alkaline functional groups, which improved the electron donating ability of biochar and enhanced the cation–π electron effect.

In summary, mineral precipitation and π electron coordination were the main mechanisms of removing Cd²⁺ from water by corn stover biochar. This phenomenon explained why the Cd²⁺ removal rate of acid/base–modified biochar decreased. After modification, the functional groups on the surface of biochar increased, but the inorganic minerals were removed. Pickling resulted in the loss of soluble minerals and alkaline functional groups on the surface of biochar, which was not conducive to adsorption⁴⁹. After alkaline washing, more PO₄³⁻, CO₃²⁻ and HCO₃⁻ were released, thereby reducing the mineral precipitation^50,51. Since NaOH had a weaker destructive effect than HCl and introduced some OH⁻, alkaline washing had little effect on the removal rate of Cd²⁺.

Adsorption isotherm and adsorption kinetics

Adsorption isotherm

The adsorption isotherms were fitted with Langmuir (Eq. 3) and Freundlich (Eq. 4) models, as shown in Fig. 9, and the fitting parameters are listed in Table 6.

The Langmuir model (R² > 0.963) was more suitable than the Freundlich model (R² > 0.919), indicating that the adsorption sites of biochar were evenly distributed, and adsorption was mainly monolayer. Parameter K_L reflected the difficulty of adsorption and was generally divided into four types: unfavourable (K_L > 1), favourable (0 < K_L < 1), linear (K_L = 1), or irreversible (K_L = 0)^52,53. The K_L values obtained by fitting were all between 0 and 1, suggesting that it was easy to adsorb. According to the fitting parameters of the Langmuir model, it could be inferred that the maximum adsorption capacity of corn stover biochar for Cd²⁺ was 13.4 mg/g. This result was higher than the maximum adsorption capacity of Cd²⁺ by biochar derived from oil seed rape, miscanthus and wheat in other studies (6.77, 11.33 and 12.35 mg/g, respectively)⁵⁴. The maximum adsorption capacity of Hickory wood biochar before and after sodium hydroxide modification for Cd²⁺ was 0.2 mg/g and 0.98 mg/g, respectively⁵⁵, which was lower than the biochar derived from corn stover in this study.

Adsorption kinetics

Pseudo-first-order (Eq. 5), pseudo-second-order (Eq. 6) and Elovich (Eq. 7) models were employed to fit the adsorption kinetics process, and the results are presented in Fig. 10a and Table 7.

According to the fitting parameters of the kinetic model, the fitting effect of the pseudo-second-order model was better than that of the pseudo-first-order model. This phenomenon indicated that adsorption could be controlled by chemisorption, which could be roughly divided into two stages: rapid adsorption within 4 h and slow adsorption after 4 h. The adsorption capacity reached 6.98 mg/g at 4 h, accounting for 72.9% of the total adsorption capacity. In the fast adsorption stage, due to the existence of numerous active adsorption sites on the sample, the adsorption capacity increased significantly with time. With the decrease in the number of adsorption sites, the samples entered the slow stage, and the adsorption rate slowed and gradually approached equilibrium⁵⁶. In comparison, the Elovich model had the best fitting effect on adsorption (R² > 0.944), indicating that the adsorption of Cd²⁺ by corn stover biochar occurred by heterogeneous chemisorption⁵⁷. The results were consistent with the previous adsorption mechanism.

The Webber–Morris intraparticle diffusion model (Eq. 8) is often used to predict the possibility of intraparticle diffusion⁵⁸. The adsorption process could be divided into different stages according to the adsorption characteristics⁵⁹. The Webber–Morris intraparticle diffusion model showed that adsorption consisted of two stages, as reflected in Fig. 10b. The first stage was the diffusion of Cd²⁺ to the surface of the biochar. The second stage was the adsorption of Cd²⁺ on biochar. Since K_1d was greater than K_2d (Table 8), the second stage was the control step of adsorption. Neither of the two fitting lines passed through the origin, indicating that intraparticle diffusion was not the only rate-determining step in adsorption⁶⁰. The adsorption process could be affected by liquid film diffusion and the physicochemical interaction between Cd²⁺ and biochar. Similar results were obtained by Pholosi et al. using magnetite-coated biomass to adsorb Cr(VI)⁶⁰.

Optimal conditions for Cd²⁺ adsorption by biochar

Electrolyte concentration

Figure 11a showed that the electrolyte concentration was negatively correlated with the removal rate and adsorption capacity. The removal rate of Cd²⁺ decreased from 74.465 to 36.02% as the CaCl₂ concentration increased from 0.01 to 0.3 mol/L, which could be caused by the competitive adsorption between Ca²⁺ and Cd²⁺ and the formation of a water-soluble metal–anion complex (CdCl⁺)^61,62. Therefore, the removal rate of Cd²⁺ was the highest when the concentration of CaCl₂ was 0.01 mol/L.

pH

As shown in Fig. 11b, the removal rate of Cd²⁺ improved significantly with the initial pH increase, and the upwards trend gradually slowed after the pH value reached 4, which could be explained by the competitive adsorption of Cd²⁺ and H⁺, the electrostatic repulsion between Cd²⁺ and the positive charge on the adsorbent surface. With the increase in pH and the decrease in protons, more binding sites were exposed, promoting the adsorption of Cd^2+12,63. In addition, mineral precipitates dissolved at low pH values, affecting adsorption. The optimal pH value was 7. Considering the economic benefits, combined with the Cd²⁺ removal rate, the pH value should be selected in a neutral range.

Biochar dosage

The biochar dosage had an important effect on the adsorption of Cd²⁺ (Fig. 11c). As the biochar dosage increased, the removal rate of Cd²⁺ gradually increased to 97.96% and then stabilized. This phenomenon could have occurred because with the increase in biochar dosage from 0.1 to 0.4 g, the surface adsorption sites increased rapidly, thus promoting the adsorption of cadmium and increasing the removal rate of cadmium. When the biochar dosage was higher than 0.4 g, the removal rate did not increase, indicating that the Cd²⁺ in the solution reached adsorption equilibrium⁶⁴. However, the adsorption capacity decreased from 11.42 to 4.14 mg/g with increasing biochar dosage. This phenomenon could have occurred because although the adsorption sites increased with the addition of biochar dosage, the amount of adsorbate remained constant; thus, the mass of Cd²⁺ adsorbed per unit mass of biochar decreased⁶⁵. Additionally, due to the aggregation and overlap of adsorption sites, which could be caused by the increase in biochar dosage, the effective adsorption area decreased, and the diffusion path length increased, reducing the adsorption capacity⁶⁶. Therefore, under the experimental concentration, the optimal addition amount of biochar was 0.4 g.

Source: Ecology - nature.com