Application of three types of aminated lignins for efficient removal of Cd(II) and Pb(II) ions in aqueous solution

Abstract

Lignin is a renewable natural aromatic polymer that is generated as a co-product during lignocellulosic biorefinery processes, and it has been applied widely as a functional biomaterial. In this study, the adsorption behavior of Cd(II) and Pb(II) metal ions was investigated via ion chelation using aminated lignins (ALs) with primary, secondary, and tertiary amine groups. ALs exhibited optimal Cd(II) and Pb(II) adsorption capacities in solution under neutral conditions due to their chelating, electrostatic, and cationic–π interactions with metal ions. The AL with the primary amine group showed the highest adsorption capacities for both Cd(II) and Pb(II), reaching 83.2 and 159.7 mg·g-1, respectively, followed by the ALs with secondary and tertiary amine groups. The adsorption kinetics and isotherm analysis demonstrated that all adsorption behaviors followed the Langmuir isotherm and pseudo-second-order kinetics. Thermodynamic studies revealed that the adsorption processes of Cd(II) and Pb(II) using the ALs were spontaneous and endothermic. These results demonstrate that ALs are promising adsorbents for the removal of Cd(II) and Pb(II) metal ions.

GRAPHICAL ABSTRACT

INTRODUCTION

Heavy metal ions pose potential risks to human health and environment because of their toxic, non-biodegradable, and bioaccumulative nature (Boamahet al. 2015; Duanet al. 2018; Liuet al. 2018). Heavy metals released to the environment are detected in river, ground, and surface waters, where the sediments accumulate to form hazardous materials (Gochtet al. 2001; Gauret al. 2005). Cd(II), one of the most common heavy metals, is metabolized slowly in the human body (Boparaiet al. 2013; Bianet al. 2015). Therefore, it accumulates in organs, such as the kidneys, and has a long half-life (Bianet al. 2015; Genchiet al. 2020). Cadmium is released into the environment during the combustion of fossil fuels, mineralization, cement production, electroplating, and manufacturing of batteries and pigments (Liet al. 2003b; Bianet al. 2015). Pb(II) is a poisonous trace metal, which is hazardous to public health, particularly to humans, even at a low concentration (10 μg·L^-1) in drinking water (Wanget al. 2020b; Zhouet al. 2021). Lead is a potent water pollutant that is widely distributed in environmental water bodies owing to its extensive applications in various industries, including chemical manufacturing of paint, batteries, and cosmetics, as well as in coal mining, extractive metallurgy, nuclear power plants, and military usage (Sekaret al. 2004; Shahabuddinet al. 2018). Because of the hazardous nature of Cd(II) and Pb(II) ions, they must be removed from industrial wastewater before its release to the environment (Awaleh and Soubaneh 2014; Shahabuddinet al. 2018). Various techniques have been proposed to address water pollution, including adsorption, solvent extraction, ion exchange, membrane filtration, biological treatment, and chemical oxidation (Duttaet al. 2001; Kadirveluet al. 2003; Heet al. 2008; Liet al. 2020). Among these techniques, adsorption remains a priority for environmental scientists because it is economical, fast, and environmentally friendly (Liet al. 2003a; Stafiej and Pyrzynska 2007; Yuet al. 2021).

Natural materials from industry and agricultural operations have been used as prospective “green” adsorbents (Kyzas and Kostoglou 2014; Pyrzynska 2019). For example, lignin is the most abundant natural aromatic polymer and is composed of various phenyl propane units (C₆–C₃) (Zonget al. 2018). It is a sustainable, cost-effective, and biodegradable biopolymer (Santanderet al. 2021). Lignin-based adsorbents can be used to treat different water pollutants, including organic dyes and heavy metal ions, because of their remarkable three-dimensional network with a cross-linked structure of macromolecules and various functional groups (Wanget al. 2019; Santanderet al. 2021). Despite their potential, many unmodified lignins have insufficient ability to adsorb metal ions (Lora and Glasser 2002). To overcome this limitation, lignin surface modification introduces functional groups with high affinity for metal ion chelation, which increase the adsorption capacity of adsorbents. A previous study showed that amine-functionalized lignins can selectively adsorb harmful dyes, such as methylene blue and Congo red (Heoet al. 2022). Chen et al.(2021) have reported that amine groups were grafted onto the lignin by 2-chloroethylamine hydrochloride, which offers the lignin a higher capacity to capture silver ions. In addition, lignins can be functionalized with amine groups to significantly enhance heavy metal removal from wastewater (Zhanget al. 2019b; Zhouet al. 2021). The amine group is a suitable functional group for the development of adsorbents. Since the nitrogen atom has a free electron doublet that can react with the metal cation, the amine group may be responsible for the uptake of the metal cation by the chelation and electrostatic interaction (Wambaet al. 2018; Gdulaet al. 2019). However, since the amine group is easily protonated in an acidic solution, electrostatic repulsion with the metal cation can act. Therefore, it can be expected to exhibit excellent metal cation uptake under non-acidic conditions (Guibal 2004).

In this study, the adsorption behavior of harmful Cd(II) and Pb(II) heavy metal ions was investigated using aminated lignins (ALs) with primary, secondary, and tertiary amine groups. In the heavy metal ion adsorption experiment, the effects of the adsorbent dosage, setting time, pH, initial concentration of metal ions, and temperature were examined. The relationship between adsorption capacity and adsorption mechanisms was investigated using adsorption isotherms, adsorption kinetics, and thermodynamic studies. Furthermore, the change in the chemical structure of the adsorbent after heavy metal ion adsorption was analyzed using X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FT-IR) spectroscopy, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) to examine the adsorption mechanism in detail.

EXPERIMENTAL

Materials

Methanol lignin (ML) was isolated from kraft lignin (Moorim Pulp and Paper Co., Ltd., South Korea) (Anet al. 2020; Heoet al. 2022). (3-Aminopropyl)trimethoxysilane (APTMS; 97%), (N-methylaminopropyl)trimethoxysilane (MAPTMS; 95%), (N,N-dimethyl-3-amino)propyltrimethoxysilane (DMAPTMS; 96%), cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O; 99.997 %), and lead(II) nitrate (Pb(NO₃)₂; 99.999%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Anhydrous methanol (99.9%), anhydrous toluene (99.8%), pyridine (99.5%), acetic anhydride (98%), 0.1 N sodium hydroxide solution, and 0.1 N hydrochloric acid solution were procured from Daejung Chemicals and Metals Co., Ltd. (Seoul, South Korea). All purchased chemicals were used without further purification.

Synthesis of Adsorbents (ALs)

ALs were prepared using the method described previously (Fig. 1) (Heoet al. 2022). In summary, ML (2 g) was dispersed in 100 mL of anhydrous toluene under agitation (350 rpm for 30 min in a 250 mL round-bottom flask). Amino-silane (APTMS, DAPTM, or DMAPTMS; 18 mmol) was added dropwise under N₂ atmosphere and heated at 70 °C for 24 h. After the reaction, the suspension was filtered and washed several times with toluene. The solid obtained was dried in a vacuum oven at 40 °C for 72 h. The ALs were named PAL (–NH₂), SAL (–NHCH₃), and TAL (–N(CH₃)₂).

Fig. 1. Synthesis of three types of ALs

Adsorption Experiments

Heavy metal ion solutions with concentrations of 40 and 80 mg/L were prepared from Cd(NO₃)₂ and Pb(NO₃)₂, respectively. Next, 30 mL of the heavy metal ion solution was mixed with each AL (0.005 to 0.1 g) at different pH values (2 to 7) in an Erlenmeyer flask, and adsorption experiments were performed in a shaking incubator (IST-4075, JEIO TECH, Daejeon, South Korea) at a fixed speed (200 rpm) and different temperatures (298.15, 313.15 and 333.15 K) for 0 to 1440 min. The pH was adjusted using NaOH (0.1 N) and HCl (0.1 N) solutions. After the adsorption of heavy metal ions, the suspension was immediately filtered through a nylon membrane (NY020047A, HYUNDAI MICRO, Seoul, South Korea) with 0.20 µm pore size, and the concentrations of residual metal ions in the solution were determined using inductively coupled plasma optical emission spectroscopy (OPTIMA 7300 DV, PerkinElmer Korea, Seoul, South Korea). The emission lines of Cd(II) and Pb(II) were 214.439 and 220.353 nm, respectively. For satisfactory precision, data with relative standard deviation (RSD) values less than 5% were used. The adsorption experiments were repeated at least 3 times, and the average values were used. The adsorbed amount (Q) and the adsorption efficiency (E) of Cd(II) and Pb(II) ions were calculated according to the following equations,

(1)

Characterization of ALs

FT-IR spectroscopy was employed to evaluate the chemical structure of ALs. As shown in Fig. S1, peaks in the spectra of the ALs confirmed the presence of amine groups and silicone-containing groups. These results were similar to those presented previously (Heoet al. 2022). In addition, acetylated ALs were used to evaluate the presence of amine groups. After the acetylation of the amine groups in PAL and SAL, a new peak appeared at 1645 cm^-1, which was assigned to the amide group (Fig. S1) (Chenet al. 2021). In the TAL spectrum, a peak at 1645 cm^-1 was not observed, suggesting that the tertiary amine groups did not yield amide groups because the tertiary amine group did not react easily with acetic anhydride. This result confirmed that the chemical structures of the ALs contained three types of amine groups. In addition, elemental contents of the ALs were detected using elemental analysis, and the results are listed in Table S1. The N contents were in the following order: PAL (2.40%) > SAL (1.98%) > TAL (1.60%). Moreover, the N/C (%) of PAL was higher than those of the other ALs, suggesting that APTMS has higher reactivity toward hydroxyl groups and endows more amine groups into lignins. In this study, three types of aminated lignins were successfully synthesized.

The particle size of the adsorbent is a decisive parameter determining the efficiency of the adsorption process, and the particle sizes of the ALs are summarized in Table S1. The particle sizes of PAL (12.5 μm), SAL (12.0 μm), and TAL (9.3 μm) were higher than that of ML (8.7 μm). According to the previous study, the specific surface areas of the ALs were also higher than that of ML (5.2 m²·g^-1) and were in the following order: TAL (14.7 m²·g^-1) > SAL (11.2 m²·g^-1) > PAL (9.4 m²·g^-1) (Heoet al. 2022). The particle sizes and the specific surface areas of ALs were higher than that of ML, which may explain that the grafting of amino-silane improves the specific surface area owing to the polycondensation of amino-silane (Lu 2013; Budnyaket al. 2018).

Operational Parameters of the Metal Ion Adsorption

To evaluate the adsorption ability of the ALs for Cd(II) and Pb(II), the effects of the AL dosage, pH, and setting time were investigated. The detailed experimental method is provided in the Appendix (Supplementary Information, S1. Adsorption experiments).

The dosage is a vital parameter for determining the adsorption capacity of ALs (Yuet al. 2021). The amounts of Cd(II) and Pb(II) ions removed at different adsorbent dosages are presented in Figs. 2(a) and (b), respectively. The removal efficiency of metal ions increased with the increasing adsorbent dosage, suggesting that increasing the adsorbent dosage resulted in a better removal efficiency owing to the interaction between the adsorbent particles and adsorbates with more active sites or surface area (Liet al. 2003a). Notably, the removal efficiency of metal ions onto PAL was higher than that of SAL and TAL. Moreover, the ALs exhibited better adsorption abilities for Pb(II) ions than for Cd(II) ions. To compare the adsorption abilities of the ALs, the amount of adsorbent that resulted in a heavy metal removal efficiency of approximately 98% was regarded as the optimal dosage for further adsorption experiments.

The role of pH in the adsorption ability of an adsorbent is one of the noteworthy in the adsorption experiments. In addition, its impacts on the ionization degree and surface charge of the functional groups on the active sites of adsorbent and adsorbate are very important (Shahabuddinet al. 2018; Murthyet al. 2020). The adsorption capacities of Cd(II) and Pb(II) ions at different pH values are shown in Figs. 2(c) and (d), respectively. The adsorption capacities of both Cd(II) and Pb(II) ions by ALs increased with an increase in pH. The adsorption capacities of ALs were poor for both heavy metals at pH 2.0 to 3.0. The protonation of ALs in an acidic environment caused the formation of cationic ammonium groups, resulting in the strong repulsion between ALs and cationic heavy metal ions (Liet al. 2019a). In contrast, under neutral conditions, ALs have a relatively good adsorption capacity of cationic heavy metal ions, which are expected to chelate with heavy metal cations because of the presence of lone pair electrons in ALs (Xuet al. 2016). Moreover, PAL exhibits a relatively high density of amine groups in the form of multilayered primary amine groups (Heoet al. 2022), suggesting that PAL has the highest ability to adsorb heavy metals. This can be explained by the relatively high number of active sites that can chelate with heavy metal ions. In addition, metal ions tend to form precipitates at high pH values, as shown in Fig. S2. When the pH exceeds 8, cadmium hydroxide is formed and cadmium is precipitated, and when the pH exceeds 7, lead precipitates due to the formation of lead hydroxide. Therefore, to simulate the optimization conditions, the adsorption experiments of Cd(II) and Pb(II) were conducted at pH 6.0 and 5.0, respectively.

The setting time provides an evaluation of the adsorbent properties between the adsorbent and adsorbate in the adsorption process. The Cd(II) and Pb(II) ion adsorption capacities for ALs at various setting times in the range of 0 to 1440 min are presented in Figs. 2(e) and (f), respectively. With an increase in the contact time from 0 to 160 min, the adsorption capacities of the ALs increased rapidly at first, followed by a gradual increase, and they reached equilibrium after 1080 min. This trend shows that the adsorption capacities of metal ions onto the ALs increased with increasing setting time until the adsorption sites were fully occupied and equilibrium was attained (Qinet al. 2017). For the adsorption of Cd(II) ions, the equilibrium adsorption capacities of PAL, SAL, and TAL were 28.92, 19.3, and 11.8 mg·g^-1, respectively. For the adsorption of Pb(II) ions, the equilibrium adsorption capacities of PAL, SAL, and TAL were found to be 112.6, 39.8, and 38.9 mg·g^-1, respectively. These results indicate that the ALs have a higher binding affinity for Pb(II) than for Cd(II).

Fig. 2. Effect of the adsorbent dosage on the removal efficiency of (a) Cd(II) and (b) Pb(II) ions; effect of pH on the removal efficiency of (c) Cd(II) and (d) Pb(II) ions; effect of the setting time on the removal efficiency of (e) Cd(II) and (f) Pb(II) ions

Adsorption Kinetics

Adsorption kinetics indicate the rate of the adsorption process, which is important for interpreting the kinetic parameters of the adsorbents (Xuet al. 2016; Zhanget al. 2020a). The kinetic parameters were interpreted using pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic, as shown in Eqs. 3 and 4, respectively,

(3)

(4)

where q_e is the equilibrium adsorption capacity (mg·g^-1), q_tis the adsorption capacity (mg·g^-1) at time t (min), and k₁ and k₂ are the kinetic rate constants of the PFO and PSO kinetic models, respectively.

The error percentage was interpreted using Eq. 5.

(5)

Based on the experimental data (Figs. 2(e) and (f)) obtained from the analysis of effects of the setting time on the removal rate, the PFO and PSO kinetic models were used to fit the data, and the results are presented in Fig. 3. The characteristic kinetic equation parameters are listed in Table 1.

Fig. 3. Linear fitting curves of the PFO and PSO adsorption kinetic models for the adsorption of Cd(II) and Pb(II) ions onto ALs

Table 1. Characteristic Parameters of PFO and PSO Kinetics for the Adsorption of Cd(II) and Pb(II) Ions onto Als

For the adsorption of both Cd(II) and Pb(II) ions, the q_e values calculated using the PSO kinetic model (q_e(cal)) were in good agreement with the experimental values (q_e(exp)). The error percentage of PSO kinetics was smaller than that of PFO kinetics. For all heavy metal ion adsorption processes, the coefficients of determination R² for the PSO kinetic models (R²≥ 0.995) were higher and closer to 1 than those for the PFO kinetic models (R²≤ 0.981). Therefore, the adsorption of Cd(II) and Pb(II) ions onto the ALs followed the PSO kinetic models, suggesting that the adsorption process involves diffusion of ions into narrow pores as the rate-limiting step (Hubbe et al. 2019)

Adsorption Equilibrium Isotherms

Adsorption isotherm studies explore the adsorption properties of materials and provide important data to determine whether the ALs can be used as adsorbents (Zhanget al. 2019a). The isotherm uptake parameters were analyzed using the Langmuir, Temkin, and Freundlich isotherms, which are given by Eqs. 6, 7, and 8, respectively,

(6)

(7)

(8)

where q_e is the equilibrium adsorption capacity (mg·g^-1), C_e is the adsorbate concentration at equilibrium (mg·L^-1), q_max is the maximum adsorption capacity (mg·g^-1), k_L is the Langmuir constant, and B = RT·b_t^-1 (kJ·mol^-1), in which b_t (kJ·mol^-1) and k_T (L·mg^-1) are the Temkin constants related to the heat of adsorption and equilibrium binding constant, respectively. R (8.314 J·mol^-1·K^-1) and T (K) are the universal gas constant and absolute solution temperature, respectively, whereas k_F and n are the Freundlich constants.

Figure S3 shows the effects of the initial adsorbate (Cd(II) and Pb(II)) concentrations on the adsorption capacity of PAL, SAL, and TAL. Figure 4 shows the experimental data fitted to the Langmuir, Temkin, and Freundlich isotherm equations, and the relevant linear equation parameters are summarized in Table 2. Based on the R² values, the experimental data for the Cd(II) and Pb(II) ions were well-fitted to the models in the following order: Langmuir > Temkin > Freundlich.

For the Langmuir isotherms, the dimensionless separation coefficient (R_L), which is an important characteristic of the Langmuir isotherm, was analyzed with Eq. 9,

(9)

where C_i (mg·L^-1) is the initial dye concentration, and k_L (L·mg^-1) is the Langmuir constant. Based on the R_L value, the isotherms can be classified as irreversible (R_L = 0), favorable (0 < R_L < 1), linear (R_L = 1), or unfavorable (R_L > 1) (Vargaset al. 2012; Wanget al. 2017). Therefore, the R_L values for all adsorption processes of metal ions are between 0 and 1, as listed in Table 2, which clearly indicate that metal ion adsorption on ALs via the monolayer coverage of metal ions was a favorable process.

The Temkin isotherm contains a factor that explicitly considers adsorbent–adsorbate interactions (Araújoet al. 2018). For Temkin isotherms, the b_t constant is related to the variation in the adsorption energy. The b_t values for the adsorption of Cd(II) ions onto PAL, SAL, and TAL were 19.6, 22.5, and 31.4 kJ·mol^-1, respectively, whereas those for Pb(II) were 41.6, 49.0, and 54.3 kJ·mol^-1, respectively. These results imply that all the adsorption processes were chemisorption because b_t > 0.020 kJ·mol^-1 (Araújoet al. 2018).

Fig. 4. Linear fitting curves of the adsorption isotherm models (Langmuir, Temkin, and Freundlich models) for the adsorption of Cd(II) and Pb(II) onto ALs

Table 2. Characteristic Parameters of Langmuir, Temkin, and Freundlich Models for the Adsorption of Cd(II) and Pb(II) Ions onto ALs

Adsorption Thermodynamics

Temperature is an important parameter that affects the mechanism and adsorption behavior of adsorbents (Shahabuddinet al. 2018; Zhanget al. 2019a). The thermodynamic studies were performed within the temperature range of 298.15 to 333.15 K. The data from the adsorption measured at various temperatures can be used to estimate thermodynamic parameters, such as changes in the standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) according to Eqs. 10, 11, and 12, respectively,

(10)

(11)

(12)

where K_d is the adsorption distribution coefficient, which was obtained by plotting ln(q_e/C_e) versus C_eat different temperatures and extrapolating to zero C_e, ΔH° is the standard enthalpy change (kJ·mol^-1), ΔS° is the standard entropy change (J·mol^-1·K^-1), ΔG° is the standard free energy change (kJ·mol^-1), Ris the gas constant (8.314 J·mol^-1·K^-1), and Tis the absolute temperature (K).

Figure S4 shows the effects of temperature (298.2 to 333.2 K) on the adsorption capacity of the ALs for Cd(II) and Pb(II) ions. To evaluate the effects of temperature and sorption mechanism, a thermodynamic model based on Eqs. 10 to 12 was investigated. The thermodynamic parameters can be determined from the slope and intercept of the plot of ln(K_d) versus 1/T (Fig. 5). The details of the thermodynamic parameters are summarized in Table 3.

All ∆G° values for the metal ion adsorption were negative (Table 3), which demonstrates the feasibility and spontaneity of the process (Zhanget al. 2019a). The magnitude of the ∆G° values increased with increasing temperature, indicating that a better adsorption ability of ALs for metal ions was obtained at higher temperatures. The positive values of ΔH° and ΔS° of the ALs reflect an endothermic process and increased randomness in the sorption process (Table 3), respectively, which imply an increased affinity of the ALs for metal ions at higher temperatures (Popovicet al. 2020; Zhanget al. 2020b). The overall ΔS° values were positive because as the metal ions were adsorbed onto the surface of ALs, water molecules located around the metal ions in the solid–liquid interface were released into the solution (Shahabuddinet al. 2018).

Fig. 5. Plots of ln(Kd) of ALs versus inverse temperature (1/T) for the adsorption of (a) Cd(II) and (b) Pb(II)

Table 3. Characteristic Parameters of Thermodynamic Studies for the Adsorption of Cd(II) and Pb(II) Ions onto ALs

Characterization of the Interaction between ALs and Heavy Metal Ions

To analyze the interactions with metal ions, the ALs were characterized before and after the adsorption of Cd(II) and Pb(II) via XPS. As shown in Fig. S5, the peaks of C 1s, O 1s, N 1s, and Si 2p were observed initially in the ALs. After the adsorption of metal ions, new clear peaks that correspond to Cd(II) and Pb(II) appeared in the XPS spectra of ALs-Cd(II) and ALs-Pb(II), respectively. These results indicate that Cd(II) and Pb(II) ions were successfully adsorbed onto the ALs. In addition, the core-level Cd 3d and Pb 4f spectra of PAL, SAL, and TAL after adsorption are illustrated in Fig. S6. After the Cd(II) adsorption, the core-level spectra showed two distinct peaks at approximately 404 and 411 eV, which were assigned to Cd 3d_5/2 and Cd 3d_3/2, respectively (Chenet al. 2017; Penget al. 2021). After the Pb(II) adsorption, the core-level spectra showed two clear peaks at approximately 137 and 142 eV, which were assigned to Pb 4f_7/2 and Pb 4f_5/2, respectively (Liet al. 2019b). Figure 6 presents the N core-level spectra before and after metal ion adsorptions onto PAL, SAL, and TAL. After the adsorption of Cd(II) and Pb(II), the amine group (–NR₂) peaks shifted to 397.9 eV and 380 to 398.8 eV, respectively, whereas the protonated amine group (–NR₃⁺) peaks shifted to 399.7 to 400.6 eV and 400.0 to 400.6 eV, respectively, demonstrating the participation of the N-containing group in the metal ion adsorption processes (Liet al. 2019b; Heoet al. 2021). The lone pair of electrons in the N atoms was shared with Pb(II), leading to an increase in the binding energies (Xuet al. 2016). These results indicate that chemical interactions induce the adsorption of Cd(II) and Pb(II) onto ALs.

This adsorption mechanism can be explained using Pearson’s hard/soft acid/base (HSAB) theory, where Cd(II) and Pb(II) are considered Lewis soft acids that tend to react with Lewis soft bases, such as amine groups of ALs (Xuet al. 2016; Jinet al. 2017). Electrostatic interaction and chelation occurred between the metal ions and ALs, which led to changes in the N electronic environment (Zhanget al. 2020b). This is further evidence that the binding mechanism mainly proceeds through chelation mechanisms owing to the preference of Cd(II) and Pb(II) for amine groups.

Fig. 6. N 1s core-level spectra of (a) PAL, (b) SAL, and (c) TAL before the adsorption; N 1s core-level spectra after the adsorption of Cd(II) onto (d) PAL, (e) SAL, and (f) TAL; N 1s core-level spectra after the adsorption of Pb(II) onto (g) PAL, (h) SAL, and (i) TAL

The FT-IR spectra (Fig. 7) of the ALs show the difference before and after metal ion adsorptions. The broad peaks at approximately 3400 and 3170 cm^-1 were attributed to hydroxyl (–OH) and amine (–NR₂) groups, respectively (Anet al. 2020; Popovicet al. 2020; Chenet al. 2021). In addition, the peak at 1100 cm^-1was assigned to Si–O, C–N, and C–O–C bonding, while the peak at 1030 cm^-1 was assigned to Si–O–Si and C–O bonding (Liet al. 2019a; Heoet al. 2022). The peaks at 737 and 693 cm^-1were assigned to Si–C and Si–O–Si, respectively (Heoet al. 2022). However, after metal ion adsorption, slightly different absorption peaks were observed.

After metal ion adsorption, peaks were observed at 3400, 1662, 1265, 1215, 1150, 737, and 693 cm^-1. The peaks at approximately 3400 cm^-1 (hydroxyl group) were stronger than those of the ALs before adsorption. In addition, the peak at 3170 cm^-1after metal ion adsorption indicates the presence of amine groups, whereas the peaks at 1265, 1215, and 1150 cm^-1 were broader and stronger. Significantly, a new peak at 1662 cm^-1 was observed, corresponding to amine group bending (Liet al. 2019b). This can be explained by the interaction between the amine groups of the ALs and the metal ions (Xuet al. 2016). In contrast, the peaks at 737 and 693 cm^-1disappeared, indicating that the strength of the hydroxyl group increased, and the peaks disappeared because the structure of ALs changed because of the interconversion of siloxane to silanol via hydration (Warringet al. 2016). It could be suggested that an increase in silanol may participate in the interaction with metal ions (Liet al. 2020). Therefore, owing to the shift and intensity changes of the amine and hydroxyl groups, these two groups were established as the main functional groups that interact with the metal ions (Leiet al. 2019).

The morphologies of the ALs before and after the adsorption are shown in Fig. S7. After adsorption of Cd(II) and Pb(II), many metal ion particles were adsorbed on the surface of ALs. According to Heo et al. (2022), the Brunauer–Emmett–Teller surface analysis confirmed that ALs have a mesoporous structure, implying that the metal ion adsorption capacity of ALs was enhanced owing to the presence of exposed specific surface area and pore. In addition, Table S2 shows the surface atomic content of the ALs after Cd(II) and Pb(II) adsorptions. After the adsorption of metal ions, Cd(II) and Pb(II) appeared on the surface of the ALs, suggesting the successful adsorption of the metal ions. In this study, PAL exhibited the highest adsorption ability for metal ions among the ALs.

Fig. 7. FT-IR spectra of (a) PAL, (b) SAL, and (c) TAL before and after metal ion adsorptions

Adsorption Mechanism of Cd(II) and Pb(II) Ions

The heavy metal ion adsorption behaviors of the AL adsorbents were compared with those of other reported modified lignin-based adsorbents, as summarized in Table 4 (Peterneleet al. 1999; Geet al. 2014; Klapiszewskiet al. 2015; Jinet al. 2017; Klapiszewskiet al. 2017; Zhouet al. 2021). The maximum Cd(II) and Pb(II) adsorption capacity values of PAL were 83.19 and 159.7 mg·g^-1, respectively, which are higher than those of the recently reported modified lignin-based adsorbents. This superior adsorption behavior of PAL is attributed to the presence of many hydroxyl and amine groups (active sites) provided by the amino-silane grafted on the lignin surface. The presence of lone pair electrons in the O- and N-containing groups, which enable chelation with metal ions, explains the superior behavior of PAL for the adsorption of Cd(II) and Pb(II).

Table 4. Comparison of q_e of the ALs for Cd(II) and Pb(II) Metal Ions with Other Modified Lignin-Based Adsorbents

Notably, the ALs showed higher adsorption capacity for Pb(II) ions than for Cd(II) ions. In particular, some studies have reported that Pb(II) ions have a greater chelating capacity with amine groups than Cd(II) ions (Xuet al. 2016; Heoet al. 2021; Zhouet al. 2021). This suggests that in the acid/base classification, the metal can be ranked as borderline (Pb(II)) and soft acid (Cd(II)), according to the HSAB theory (Zhouet al. 2021). Therefore, the chelating ability between the ALs and Pb(II) is superior to that between the ALs and Cd(II). Moreover, the adsorption mechanism involves the cation–π interaction between the positively charged metal ion and π-electron cloud of the negatively charged aromatic ring (Wanget al. 2020a; Zhao and Zhu 2020). Therefore, the synergistic effects of various interactions between metal ions and ALs, including the chelating, electrostatic, and cation–π interactions, contribute to the adsorption process (Fig. 8) (Zhao and Zhu 2020). By introducing amino-silane into lignin, the ability of ALs to adsorb metal ions was improved by the inclusion of aromatic rings, amines, and hydroxyl groups in the structure of ALs.

Fig. 8. Hypothetical adsorption mechanism of metal ions onto aminated lignin

CONCLUSIONS

1. ALs with primary, secondary, and tertiary amine groups were synthesized through one-step modification using three types of amino-silane reagents and applied to adsorb heavy metal ions.

2. The ALs exhibited optimal Cd(II) and Pb(II) adsorption capacities in a neutral environment, where the attraction between the lone pair electrons of ALs and the cationic Cd(II) and Pb(II) species was maximized.

3. The adsorption process followed Langmuir isotherms, suggesting that metal ions were adsorbed onto the ALs by forming a homogeneous monolayer. The maximum adsorption capacities for Cd(II) and Pb(II) ions were 83.2 and 159.7 mg·g^-1, respectively.

4. The attraction between the N- and O-containing groups in the ALs and cationic metal ions also plays a critical role in the adsorption.

5. This study presents bio-adsorbents for high-performance lignin-based adsorption for the removal of hazardous water pollutants.

APPENDIX

Supplementary Information

Fig. S1. (a) FTIR spectra of ML and ALs; (b) FTIR spectra of acetylated ML and acetylated ALs

Table S1. Elemental Contents and Surface Elemental Contents of ML, PAL, SAL, and TAL

S1 Adsorption Experiments

To evaluate effect of adsorbent dosage, heavy metal ion solutions with a concentration of 40 and 80 mg·L^-1 were prepared from Cd(NO₃)₂ and Pb(NO₃)₂, respectively. Next, 30 mL of the heavy metal ion solution was mixed with each AL (0.005–0.1 g) in an Erlenmeyer flask, and then adsorption experiments were performed without pH adjustment at a speed of 200 rpm, and 298.15 K for 1440 min. This result of adsorption experiment was listed in Fig. 1(a) and (b).

To investigate effect of pH, heavy metal ion solutions with a concentration of 40 and 80 mg·L-1 were prepared from Cd(NO₃)₂ and Pb(NO₃)₂, respectively. The optimum dosage of PAL, SAL, and TAL for Cd(II) ions were 0.04 g, 0.06 g, and 0.1 g, respectively. Furthermore, the optimum dosage of PAL, SAL, and TAL for Pb(II) ions were 0.02 g, 0.06 g, and 0.06 g, respectively. Next, 30 mL of the heavy metal ion solution was mixed with each optimum dosage of ALs in an Erlenmeyer flask, and then adsorption experiments were performed with a pH of 3 to 7 at a speed of 200 rpm, and 298.15 K for 1440 min. This result of adsorption experiment was listed in Fig. 1(c) and (d).

To evaluate effect of setting time, heavy metal ion solutions with a concentration of 40 and 80 mg·L^-1 were prepared from Cd(NO₃)₂ and Pb(NO₃)₂, respectively. Next, 30 mL of the heavy metal ion solution was mixed with each optimum dosage of ALs in an Erlenmeyer flask, and then adsorption experiments were performed without pH adjustment at a speed of 200 rpm, and 298.15 K for 0 to 1440 min. This result of adsorption experiment was listed in Fig. 1(e) and (f).

To investigate adsorption isotherms, optimum dosage of ALs was added into 30 mL of Cd(II) ion solution (concentration gradient: 10-300 mg·L^-1) with a pH of 5.5 at 298.15 K for 1440 min. For Pb(Ⅱ) ions adsorption, the optimum dosage of ALs, and the concentration of Pb(II) ion solution is 10-300 mg/L with a pH of 5.0 at 298.15 K for 1440 min. This result of adsorption experiment is listed in Fig. S3.

To investigate effect of temperature for metal ion adsorption, about optimum dosage of ALs was added into 30 mL of metal ion solution (concentration gradient: 40 mg·L^-1for Cd(II) solution and 80 mg·L^-1 for Pb(II) solution) with a pH of 5.0 at 298.15 313.15, and 333.15 K for 1440 min. This result is shown in Fig. S4.

Fig. S2. Cadmium and lead species in the aqueous system as a function of pH by Visual MINTEQ (V3.0)

Fig. S3. Effect of initial concentration on Cd(II) ions adsorption capacity of PAL (a), SAL (b), and TAL (c) and Pb(II) ions adsorption capacity of PAL (d), SAL (e), and TAL (f)

Fig. S4. (a) Cd(II) adsorption capacity of PAL, SAL, and TAL according to temperature (295.15, 313.15, and 333.15 K); (b) Pb(II) adsorption capacity of PAL, SAL, and TAL according to temperature (295.15, 313.15, and 333.15 K)

Fig. S5. (a) XPS survey spectra of ALs before adsorption; (b) XPS survey spectra of ALs after Cd(II) adsorption; (c) XPS survey spectra of ALs after Pb(II) adsorption

Fig. S6. Cd 3d spectrum after Cd(II) adsorption onto (a) PAL, (b) SAL, and (c) TAL; Pb 4f spectrum after Pb(II) adsorption onto (d) PAL, (e) SAL, and (f) TAL

Fig. S7. Surface morphology of (a) PAL, (b) SAL, (c) and TAL before metal ion adsorption; surface morphology of (d) PAL, (e) SAL, and (f) TAL after Cd(II) adsorption; surface morphology of (g) PAL, (h) SAL, and (i) TAL after Pb(II) adsorption

Table S2. Surface element contents of ALs before and after Cd(II), and Pb(II) Adsorption from SEM-EDS

ACKNOWLEDGMENTS

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2021R1A2C2008178), and the Ministry of Education (2018R1A6A1A03025582).

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