Dual-Modified Lignin-Assembled Multilayer Microsphere with Excellent Pb2+ Capture

With the continuous research on lignin-based sorbents, there are still limitations in the research of spherical sorbents with a high adsorption capacity for Pb2+. In order to solve the problem of low adsorption effect, alkali lignin (AL) was modified and assembled to increase the adsorption active sites. In this work, we used dual-modified lignin (DML) as a raw material to assemble a singular lignin-based multilayer microsphere (LMM) with sodium alginate (SA) and dopamine. The prepared adsorbent had various active functional groups and spherical structures; the specific surface area was 2.14 m2/g and the average pore size was 8.32 nm. The adsorption process followed the Freundlich isotherm and the second-order kinetic model. Therefore, the LMM adsorbed Pb2+ ascribed by the electrostatic attraction and surface complexation; the adsorption capacity was 250 mg/g. The LMM showed a selective adsorption performance for Pb2+ and the adsorption capacity followed the order Pb2+ (187.4 mg/g) > Cu2+(168.0 mg/g) > Mn2+(166.5 mg/g). After three cycles, the removal efficiency of Pb2+ by the LMM was 69.34%, indicating the reproducibility of LMM.


Introduction
Lead ion (Pb 2+ ) is one of the most common toxic pollutants in industrial wastewater [1]. Due to the non-degradability of Pb 2+ , it can accumulate in the human body through the food chain and accordingly become hazardous for human health. The World Health Organization (WHO) stipulates that the level of lead ions in drinking water should be less than 0.01 mg/L. Moreover, when the daily intake of lead exceeds 0.3 mg, it is considered harmful to human health [2,3]. When the contents of lead ions in the body exceed a certain amount, it makes the human body have kidney failure, high blood pressure, and psychiatric disorders; at the same time, when excess lead ions accumulate in bones, brain, and muscles, they can cause serious developmental disorders, injury, disease, and even death [4]. As a consequence, effectively removing the Pb 2+ from wastewater has drawn the attention of researchers. In the process of continuous research, coagulation, ion exchange, reverse osmosis, membrane separation, and adsorption are the common wastewater treatment methods [5]. Meanwhile, the adsorption method exhibits broad application prospects, which are due to high efficiency, low cost, recyclability, easy operation, etc. [6].
In previous studies, few researchers used dopamine as a cross-linking agent to assemble adsorbents in the field of lignin-based adsorbent materials. As an important neurotransmitter, dopamine plays an important role in many physiological processes [7], For example, it plays an important role in the modulation of neuroplasticity [8], and Matt et al. found that dopamine also has an effect on immune cell dysfunction in diseases [9]. Landgraf et al. also found a role of dopamine in the circadian rhythm [10]. In terms of chemical structure, advantage of biodegradability, it has broad application prospects in the field of biological materials [28]. Therefore, on the basis of previous studies, in order to achieve an effective removal of Pb 2+ in wastewater and improve the adsorption capacity of adsorbent, we tried to combine dual-modified lignin (DML), dopamine, and sodium alginate to obtain a singular lignin-based multilayer microsphere (LMM). In addition, in view of current studies, spherical adsorption materials based on lignin are mainly monolayer and there is still a lack of studies on double-layer microspheres. This study also provides a new idea for the assembly of lignin-based double-layer heavy metal adsorption materials. At the same time, compared with other lignin-based spherical adsorption materials, it also effectively improves the adsorption performance of Pb 2+ .
In the present study, we assembled lignin multilayer microspheres (LMM) using dopamine and sodium alginate for the first time. In order to successful obtained the LMM, we used dual-modified lignin with different carboxyl content values (1.15 and 1.57 mmol/g) to react with dopamine, then assembled with SA. Finally, the LMM not only had multilayer microspheres structures, but had mounts of active groups after modification. Consequently, this work researched the influence of time, pH, temperature, and initial concentration for the adsorption capacity. Meanwhile, we also evaluated the ion selectivity and recyclability of the LMM. This kind of lignin-based adsorbent with high absorbability, multi-absorbability, and simple preparation not only further realized the value-added utilization of lignin, but also demonstrates broad application prospects in wastewater treatment.

Materials
The alkali lignin (AL) in the present study was obtained from the Shanghai Trading, Co., Ltd. (Shanghai, China). and the ClCH 2 COOH, 98%H 2 SO 4 , and HCHO were derived from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). CS 2 , NaOH, sodium alginate, and NaCl were purchased from the Tianjin Damao Chemical Reagent Factory (Tianjin, China). The dopamine hydrochloride was collected from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). All reagents were analytically pure.

Methods
The preparation of lignin multilayer microspheres (LMM) is shown in Figure 1 and the preparation of dual-modified lignin (DML) followed previous methods [30]. After that, the different carboxyl contents of DML were obtained by adjusting the ratio of chloroacetic acid and NaOH.
LMM was prepared using a two-step method including esterification and cross linking: (i) Amounts of 0.1 g DML (carboxyl content = 1.15 mmol/g), 10 mL 10% (w/v) dopamine hydrochloride, and 20 mL 72% H 2 SO 4 were poured into a three-necked flask, and the mixture reacted for 5 h at 60 • C. (ii) The lignin prepared above and DML (carboxyl content = 1.57 mmol/g) was combined with sodium alginate in a beaker and was stirred for 3 h until the solution was fully mixed. Then, using an injector, this solution was added the 2% (w/v) Ca 2+ solution and left for 12 h. Finally, these spheres were freeze-dried to obtain the LMM.

Characterization of the LMM
Fourier transform infrared (FT-IR) spectroscopy was used to analysis the structure of the LMM (ALPHA Infrared spectrometer Shimazu Corporation, Japan) and the microstructure of the AL and LMM were recorded on an SEM (HitachiRegulus8220 Hitachi, Japan). The S BET data was obtained using a Mike ASAP 3020 analyzer (McMurtic Instruments Co., LTD., USA). The XRD data was received using a multifunctional powder XRD instrument (D8-ADVANCE, Bruker, Germany) and the chemical structure variation of the LMM before and after adsorption was tested using an XPS instrument (ESCALABXi+ American Brooker Company, USA). A Shanghai Ray Magnetic Multi-Parameter Analyzer (DZS-706) exhibited the carboxyl content of the LMM. The Pb 2+ concentrations were acquired using a

Characterization of the LMM
Fourier transform infrared (FT-IR) spectroscopy was used to analysis the structure of the LMM (ALPHA Infrared spectrometer Shimazu Corporation, Japan) and the microstructure of the AL and LMM were recorded on an SEM (HitachiRegulus8220 Hitachi, Japan). The SBET data was obtained using a Mike ASAP 3020 analyzer (McMurtic Instruments Co., LTD., USA). The XRD data was received using a multifunctional powder XRD instrument (D8-ADVANCE, Bruker, Germany) and the chemical structure variation of the LMM before and after adsorption was tested using an XPS instrument (ESCALABXi+ American Brooker Company, USA). A Shanghai Ray Magnetic Multi-Parameter Analyzer (DZS-706) exhibited the carboxyl content of the LMM. The Pb 2+ concentrations were acquired using a GGX-600 AAS flame atomic absorption spectrometer (Beijing Haiguang Instrument Co., LTD., Beijing, China).

Adsorption Experiments
In order to research the effect of dosage, pH, temperature, duration, and concentration for the adsorption performance, the 30 mg/L Pb 2+ solution was poured into a 150 mL conical flask with different conditions to study the adsorption capacity. The residual Pb 2+ concentration was recorded using the atomic adsorption method. The adsorption capacity (qe) of the LMM followed the expression: Meanwhile, the removal efficiency (E) of the LMM followed the expression: where C0 (mg/L) and Ce (mg/L) represent the Pb 2+ solution concentration before and after adsorption, respectively; V (L) is the volume of solution; M (g) is the mass of the adsorbent. The experiments were repeated three times.

Adsorption Experiments
In order to research the effect of dosage, pH, temperature, duration, and concentration for the adsorption performance, the 30 mg/L Pb 2+ solution was poured into a 150 mL conical flask with different conditions to study the adsorption capacity. The residual Pb 2+ concentration was recorded using the atomic adsorption method. The adsorption capacity (q e ) of the LMM followed the expression: Meanwhile, the removal efficiency (E) of the LMM followed the expression: where C 0 (mg/L) and C e (mg/L) represent the Pb 2+ solution concentration before and after adsorption, respectively; V (L) is the volume of solution; M (g) is the mass of the adsorbent. The experiments were repeated three times.

The Reproducibility of the LMM
The recoverability of the LMM was measured by adsorption-desorption experiments. In this process, 100 mg LMM was added to the 50 mL Pb 2+ solution (30 mg/L), then left to react for 4 h at pH 5. After finishing adsorption experiments, the lead-loaded LMM was obtained by filtration, soaking it in 50 mL 1M HCl for 60 min, and repeating twice. After that, the sample was washed in deionized water, underwent centrifugation, and was dried at 80 • C for 12 h. Finally, this adsorbent adsorbed Pb 2+ again.

The Determination of Carboxyl Content in DML
In this work, we used conductivity titration to measure the carboxyl content of the DML. When the ratio of NaOH and chloroacetic acid changed, the content of carboxyl groups in the DML also changed. When the conditions of NaOH/chloroacetic acid content ratio were 1:1 and 1.5:1, the NaOH consumption-conductivity curve was constructed (as shown in Figure 2a), and the carboxyl content of different DMLs was calculated using a carboxyl content calculation formula (Equation (3)). It was found that the carboxyl content of the DML were 1.57 and 1.15 mmol/g.

The Determination of Carboxyl Content in DML
In this work, we used conductivity titration to measure the carboxyl content of the DML. When the ratio of NaOH and chloroacetic acid changed, the content of carboxyl groups in the DML also changed. When the conditions of NaOH/chloroacetic acid content ratio were 1:1 and 1.5:1, the NaOH consumption-conductivity curve was constructed (as shown in Figure 2a), and the carboxyl content of different DMLs was calculated using a carboxyl content calculation formula (Equation (3)). It was found that the carboxyl content of the DML were 1.57 and 1.15 mmol/g.
The carboxyl content calculation formula is as follows: where VB is the consumption of NaOH at the end of conductivity equilibrium and VA is the consumption at the beginning of conductivity stabilization.  The carboxyl content calculation formula is as follows: where V B is the consumption of NaOH at the end of conductivity equilibrium and V A is the consumption at the beginning of conductivity stabilization.

The Characterization of the LMM
As shown in Figure 2b, the spectra of the DML, SA, and the LMM are introduced. As can be seen from the spectra, the structure of adsorption changed significantly and the peak of sodium alginate at 1407 cm −1 shifted towards 1429 cm −1 due to the symmetric deformation of -COO-, indicating the cross-linking of carboxylic acid base group in the system [31]. At the same time, the changes at 1282, 1220, 1087, and 1031 cm −1 also explain the successful cross linking between sodium alginate and DML. In addition, the peak at Furthermore, Figure 2c displays the N 2 adsorption-desorption curve, which shows that the specific surface area of the LMM was 2.14 m 2 /g. We found that the specific surface area of the LMM was higher than the AL (1.66 m 2 /g), while the average pore size of the LMM (8.32 nm) was lower than the AL (10.5 nm). These parameters indicate that the microstructure and average pore size of specific surface area of AL were changed by modification.
Similarly, the XRD data showed the crystal structure of the AL, sodium alginate, and the LMM; as can be seen from Figure 2d, with the addition of sodium alginate, the peak value of LMM was weakened but still maintained non-crystal structure, showing that the crystal structure of LMM did not change significantly after modification and curing, and that it had a non-crystal structure [32].
In order to research the microscopic surface morphology of the AL and LMM, this work used SEM to discuss the surface structure. It is shown in Figure 3 that the micromorphology evidently changed and the LMM had a spheroidal structure, which differed from the blocky structure of the AL. From Figure 3b, it can be seen that the LMM did not have rough morphology; however, pore structures existed on the surface of the LMM and it also had aggregation of microspheres in the ores. This shows that the multilayer microspheres were successfully prepared using sodium alginate and dopamine. Figure 3c shows the microscopic surface morphology of Pb 2+ adsorbed by LMM. In addition, Figure 3d displays the diameter changes of the LMM, the LMM with deionized water, and the LMM with lead ions. These results show the successful adsorption of Pb 2+ by the LMM.

Adsorption Efficiency of the LMM for Pb 2+
To discuss the influence of pH on the adsorption capacity of the LMM, the adsorption experiments was carried out with different pH values, and the mass of the LMM was 0.8 g/L at 30 °C for 4 h. It can be seen from Figure 4a that the LMM had an adsorption effect at pH 2, and with the increase of the pH values, the removal efficiency and capacity evidently increased. In addition, when the pH was 5, the removal efficiency and capacity

Adsorption Efficiency of the LMM for Pb 2+
To discuss the influence of pH on the adsorption capacity of the LMM, the adsorption experiments was carried out with different pH values, and the mass of the LMM was 0.8 g/L at 30 • C for 4 h. It can be seen from Figure 4a that the LMM had an adsorption effect at pH 2, and with the increase of the pH values, the removal efficiency and capacity evidently increased. In addition, when the pH was 5, the removal efficiency and capacity were 98.66% and 36.99 mg/g, respectively. When the pH was higher than 6, the removal efficiency and capacity reached 99.67% and 37.38 mg/g, respectively. These results were caused by the production of Pb(OH) 2 [33]. In conclusion, this adsorption test allowed the selection of pH 5 as the best condition in next step.  The dosage of the LMM was also an important role in adsorption capacity. Therefore, the test with varying dosages (0.2, 0.4, 0.6, 0.8, and 1.0 g/L) was conducted to research the adsorption capacity for Pb 2+ . As it can be seen from Figure 4b, the increased dosage of the LMM would decrease the adsorption capacity for Pb 2+ , while the adsorption efficiency would increase, and the maximum adsorption efficiency and capacity were 99% and 142.3 mg/g, respectively. Therefore, 0.8 g/L LMM was selected as the optimal condition in the following experiments. Figure 4c exhibits the effect of time on the adsorption capacity. The adsorption capacity of the LMM rose quickly when the adsorption time ranged from 0 to 30 min, and then the adsorption capacity increased slowly until 180 min. This phenomenon may be caused by the large number of active sites on the surface of the LMM. Meanwhile, the adsorption process reached sorption equilibrium as the adsorption time reached 180 min and the adsorption capacity was 37.1 mg/g.
The temperature was a crucial effect for the adsorption performance. Figure 4d showed the temperature on the removal of Pb 2+ by the LMM. The adsorption capacity increased when the temperature increased, indicating that the adsorption process was an endothermic process; high temperature was more conducive to adsorption. The dosage of the LMM was also an important role in adsorption capacity. Therefore, the test with varying dosages (0.2, 0.4, 0.6, 0.8, and 1.0 g/L) was conducted to research the adsorption capacity for Pb 2+ . As it can be seen from Figure 4b, the increased dosage of the LMM would decrease the adsorption capacity for Pb 2+ , while the adsorption efficiency would increase, and the maximum adsorption efficiency and capacity were 99% and 142.3 mg/g, respectively. Therefore, 0.8 g/L LMM was selected as the optimal condition in the following experiments. The temperature was a crucial effect for the adsorption performance. Figure 4d showed the temperature on the removal of Pb 2+ by the LMM. The adsorption capacity increased when the temperature increased, indicating that the adsorption process was an endothermic process; high temperature was more conducive to adsorption.

Adsorption Kinetics
In order to explore the adsorption mechanism of the LMM adsorbing Pb 2+ , the adsorption kinetics were analyzed by determining the adsorption capacity and time ranging from 30 to 300 min at pH 5 and 30 • C. Then, the experiment data were fitted with the pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetics models [34].
The adsorption capacity of the LMM at time t was obtained by this equation: where C 0 is the initial concentration of Pb 2+ (mg/L), Ct is the concentration of Pb 2+ at time t (mg/L), V is the volume of the solution (L), and the M is the mass of the LMM (g).
The pseudo-first-order is expressed using the following equation: log q e − q t = logq e − k 1 2.303 t The pseudo-second-order is calculated as follows: Meanwhile, the intraparticle diffusion model is expressed by the equation [35]: where q t and q e (mg/g) are the adsorption capacities at time t and equilibrium, respectively; k 1 (1/min) and k 2 (g/mg min) are the pseudo-first-order and pseudo-second-order model rate constants, respectively; k p is the intraparticle diffusion rate constant [mg/(g min 1/2 )]; and C represents the thickness of the boundary layer. The relation parameters of the pseudo-first-order, pseudo-second-order, and the intraparticle diffusion model are listed in Table 1. The fitting curves ( Figure 5) exhibit that the adsorption process followed pseudo-second-order, and the R 2 of the pseudo-first-order and pseudo-second-order were 0.9696 and 0.9992, respectively. In addition, the R 2 of the intraparticle diffusion model were 0.9053, 0.9274, and 0.8944, and the C was different from zero. According these results, it could be concluded that the adsorption process of the LMM for Pb 2+ was mainly chemical adsorption and the interaction between the LMM and Pb 2+ was not a simple diffusion [36].  Figure 5) exhibit that the adsorption process followed pseudo-second-order, and the R 2 of the pseudo-first-order and pseudo-second-order were 0.9696 and 0.9992, respectively. In addition, the R 2 of the intraparticle diffusion model were 0.9053, 0.9274, and 0.8944, and the C was different from zero. According these results, it could be concluded that the adsorption process of the LMM for Pb 2+ was mainly chemical adsorption and the interaction between the LMM and Pb 2+ was not a simple diffusion [36].

Adsorption Isotherm and Thermodynamics
The relation between LMM and Pb 2+ was investigated using two equilibrium isotherm models with the Langmuir model and Freundlich model [37].
The Langmuir model can be expressed as follows: where C e is the concentration of Pb 2+ at equilibrium (mg/L), q e is the equilibrium adsorption capacity (mg/g), and K L (L/mg) (L/g) is the constant of the Langmuir model. The Freundlich model is obtained as the equation: where K F [(mg/g) (L/mg) 1/n ] and n are the Freundlich constants. The adsorption isotherm curves of the LMM for Pb 2+ is demonstrated in Figure 6 and the parameters are exhibited in Table 2. It can be seen that the R 2 of Freundlich model was higher than the Langmuir model. Therefore, the multi-layer chemical adsorption could be used to display the adsorption process of Pb 2+ by the LMM. According to the Langmuir model parameters, it could be obtained that the maximum adsorption capacity was 250 mg/g. In addition, because of the 1/n was between 0 and 1, it illustrated that the Pb 2+ could easily react with the LMM [38].

Adsorption Isotherm and Thermodynamics
The relation between LMM and Pb 2+ was investigated using two equilibrium isotherm models with the Langmuir model and Freundlich model [37].
The Langmuir model can be expressed as follows: where Ce is the concentration of Pb 2+ at equilibrium (mg/L), qe is the equilibrium adsorption capacity (mg/g), and KL (L/mg) (L/g) is the constant of the Langmuir model. The Freundlich model is obtained as the equation: where KF [(mg / g) (L / mg) 1/n ] and n are the Freundlich constants. The adsorption isotherm curves of the LMM for Pb 2+ is demonstrated in Figure 6 and the parameters are exhibited in Table 2. It can be seen that the R 2 of Freundlich model was higher than the Langmuir model. Therefore, the multi-layer chemical adsorption could be used to display the adsorption process of Pb 2+ by the LMM. According to the Langmuir model parameters, it could be obtained that the maximum adsorption capacity was 250 mg/g. In addition, because of the 1/n was between 0 and 1, it illustrated that the Pb 2+ could easily react with the LMM [38].    Furthermore, this material could be comparable to the other adsorbent materials reported in the past. Table 3 shows the adsorption capacity of the different adsorbent materials that adsorbed Pb 2+ . It can be seen that the adsorption capacity of LMM for Pb 2+ was at a medium level, and the adsorption performance was better than that of other spherical adsorbents. This may be because the double spherical structure provides more adsorption sites for lead ion adsorption. Moreover, the LMM has the advantage of low cost, easy preparation, and high hydrophobicity, which makes the LMM a wide application prospect.

Thermodynamics of the LMM Adsorption
In order to investigate the thermodynamics of the LMM adsorption for Pb 2+ , the three thermodynamics parameters, including ∆G (kJ/mol), ∆H (kJ/mol), and ∆S (kJ/mol/K), were obtained using the following equations [43]: where K d (mL/g) is the equilibrium constant, T (K) is the temperature, and R is the gas constant (J/mol/K). Table 4 exhibits the thermodynamics parameters about the adsorption of the LMM for Pb 2+ . ∆G < 0 showed that the adsorption process was a spontaneous reaction and the rates of ∆G were −28.89, −30.38, and −31.87 KJ/mol at 303, 313, and 323 K, respectively. These results indicate that the adsorption process spontaneously enhanced when the temperature increased.
In addition, the positive value of ∆H explained that the adsorption process was an endothermic process and this result describes the reason that the adsorption capacity increased as the temperature raised. Similarly, the positive ∆S suggested the adsorption reaction led to the disordered movement of molecules and it was an entropy-driven process. Meanwhile, the experiment results of the effect of temperature with adsorbing Pb 2+ by the LMM fitted well with this analysis.

Recyclability of the LMM
The recyclability of Pb 2+ using the LMM was analyzed using an adsorption-desorption experiment. As shown in Figure 7a, the adsorption efficiency of the LMM for Pb 2+ decreased slowly with cycles increased. It can be seen that the removal efficiency was 98.82% in the initial adsorption; after the three cycles, the removal efficiency decreased to 69.34%. These results show that the LMM could be reused to adsorb Pb 2+ . Although the adsorption efficiency certainly declined, the LMM showed regeneration properties. At the same time, we analyzed the reasons for the decrease of LMM adsorption performance, which may be caused by the destruction of LMM's chemical structure and the incomplete desorption of Pb 2+ . Therefore, in the follow-up study, we will conduct experiments and solve problems based on these reasons. 69.34%. These results show that the LMM could be reused to adsorb Pb 2+ . Although the adsorption efficiency certainly declined, the LMM showed regeneration properties. At the same time, we analyzed the reasons for the decrease of LMM adsorption performance, which may be caused by the destruction of LMM's chemical structure and the incomplete desorption of Pb 2+ . Therefore, in the follow-up study, we will conduct experiments and solve problems based on these reasons.

Selectivity of the LMM Adsorption
The selectivity of the LMM was accessed using the adsorption capacity of other heavy metal ions by the LMM. The adsorption experiment result is shown in Figure 7b; it indicates that the adsorption capacity of the LMM for Pb 2+ , Cu 2+ , and Mn 2+ follow the order Pb 2+ > Cu 2+ > Mn 2+ . It was caused by the differences in their corresponding electronegativities (Pb 2+ = 2.33, Cu 2+ = 2.0, and Mn 2+ = 1.55) [44]. Moreover, the appearance of this phenomenon could also be explained by the Pearson's hard/soft acid/base (HSAB) theory. According to acid base classification, Pb 2+ was considered to a softer acid than Cu 2+ and Mn 2+ , and COO-, CSS¯, C/N, and NH2 contained in LMM can be used as soft bases, thus producing a better affinity for lead ions [15,45]. Meanwhile, it was also found that the LMM exhibited good adsorption capacity for three heavy metal ions and the maximum adsorption capacity of the LMM for Pb 2+ , Cu 2+ , and Mn 2+ were 187.4, 168.0, and 166.5 mg/g, respectively. Thus, it could be concluded that the LMM had versatility and the adsorption performance of Pb 2+ was the best among the three heavy metal ions.

Adsorption Mechanism
The adsorption mechanism of the LMM for Pb 2+ could be explored by XPS analysis. Figure 8 suggests the changes of the LMM and the LMM involving Pb 2+ . The peaks of N, S, and Pb explain the successful preparation of the LMM and the Pb 2+ adsorption. The 4f 5/2 and 4f 7/2 signals appearing in Figure 8b illustrate the adsorption of the LMM for Pb 2+ . Meanwhile, the peaks of the C-H and C-O shifted from 284.5 eV and 286.2 eV to 284.5 eV

Selectivity of the LMM Adsorption
The selectivity of the LMM was accessed using the adsorption capacity of other heavy metal ions by the LMM. The adsorption experiment result is shown in Figure 7b; it indicates that the adsorption capacity of the LMM for Pb 2+ , Cu 2+ , and Mn 2+ follow the order Pb 2+ > Cu 2+ > Mn 2+ . It was caused by the differences in their corresponding electronegativities (Pb 2+ = 2.33, Cu 2+ = 2.0, and Mn 2+ = 1.55) [44]. Moreover, the appearance of this phenomenon could also be explained by the Pearson's hard/soft acid/base (HSAB) theory. According to acid base classification, Pb 2+ was considered to a softer acid than Cu 2+ and Mn 2+ , and COO-, CSS , C/N, and NH 2 contained in LMM can be used as soft bases, thus producing a better affinity for lead ions [15,45]. Meanwhile, it was also found that the LMM exhibited good adsorption capacity for three heavy metal ions and the maximum adsorption capacity of the LMM for Pb 2+ , Cu 2+ , and Mn 2+ were 187.4, 168.0, and 166.5 mg/g, respectively. Thus, it could be concluded that the LMM had versatility and the adsorption performance of Pb 2+ was the best among the three heavy metal ions.

Adsorption Mechanism
The adsorption mechanism of the LMM for Pb 2+ could be explored by XPS analysis. Figure 8 suggests the changes of the LMM and the LMM involving Pb 2+ . The peaks of N, S, and Pb explain the successful preparation of the LMM and the Pb 2+ adsorption. The 4f 5/2 and 4f 7/2 signals appearing in Figure 8b illustrate the adsorption of the LMM for Pb 2+ . Meanwhile, the peaks of the C-H and C-O shifted from 284.5 eV and 286.2 eV to 284.5 eV and 285.4 eV, respectively, after adsorption Pb 2+ , and the peaks of COOH shifted from 288.1 eV to 287.6 eV after adsorption Pb 2+ , which would be caused by the appearance of COO-Pb (Figure 8c,g) [30].  [2,15,18]. In addition, the S 2p XPS spectra could be fitted into two peaks at 163.1 eV and 164.2 eV, which were C-S and C=S bonds. After the LMM reacted with Pb 2+ , the peaks of C-S and C=S were 162.7 eV and 163.6 eV, respectively. The S 2p spectra produced two new peaks at 165.7 eV and 168.1 eV, which were appointed to C-S-Pb and C=S-Pb [47], respectively. In conclusion, the emergence of new peaks of Pb and chemical bonds reacting with Pb can be seen from Figure 8. Meanwhile, the other chemical bonds had significant changes after adsorbing lead ions. These consequences illustrated the formation of coordination complexes between Pb 2+ and the LMM. and 399.7 eV and the new peaks appeared at 406.6 eV (Pb-N); these results demonstrate that the nitrogenous functional groups participated in the adsorption process of the LMM for Pb 2+ [2,15,18]. In addition, the S 2p XPS spectra could be fitted into two peaks at 163.1 eV and 164.2 eV, which were C-S and C=S bonds. After the LMM reacted with Pb 2+ , the peaks of C-S and C=S were 162.7 eV and 163.6 eV, respectively. The S 2p spectra produced two new peaks at 165.7 eV and 168.1 eV, which were appointed to C-S-Pb and C=S-Pb [47], respectively. In conclusion, the emergence of new peaks of Pb and chemical bonds reacting with Pb can be seen from Figure 8. Meanwhile, the other chemical bonds had significant changes after adsorbing lead ions. These consequences illustrated the formation of coordination complexes between Pb 2+ and the LMM. In conclusion, according to the analysis of the XPS spectra, the main adsorption process of the LMM for Pb 2+ was chemical adsorption, including complexation and electrostatic attraction. On the other hand, the SEM images and specific surface area parameters also showed that the adsorption process involved physical adsorption. Meanwhile, the pore size of LMM was 8.32 nm while the diameter of the lead ion was about 0.15 nm. This In conclusion, according to the analysis of the XPS spectra, the main adsorption process of the LMM for Pb 2+ was chemical adsorption, including complexation and electrostatic attraction. On the other hand, the SEM images and specific surface area parameters also showed that the adsorption process involved physical adsorption. Meanwhile, the pore size of LMM was 8.32 nm while the diameter of the lead ion was about 0.15 nm. This indicated that Pb 2+ could better enter the interior of LMM, which was more conducive to the physical adsorption and internal chemical reaction of Pb 2+ on the LMM. Therefore, the LMM adsorbed the Pb 2+ mechanism mainly using chemical adsorption because of the existence of active groups and there was some physical adsorption in the adsorption process because of the microstructure (as shown in Figure 9).

Conclusions and Future Perspectives
Lignin-based bilayer microspheres prepared by the green method have the advantages of easy preparation, renewability, high adsorption, and versatility. In the present study, dual-modified lignin was assembled by dopamine and sodium alginate to obtain a double-layer lignin microsphere. The diameter of the microsphere obtained was about 800 µm, the specific surface area was 2.14 m 2 /g, and the average pore size was 8.2 nm. The adsorption process followed the pseudo-second-order kinetic equation and Freundlich model, indicating that the adsorption was multi-layer adsorption dominated by chemical adsorption. The maximum adsorption capacity of Pb 2+ by LMM was 250.0 mg/g, determined by Langmuir model analysis and the adsorption of Pb 2+ by LMM was mainly composed of electrostatic attraction and surface complexation. In addition, it was found that after three cycles, the adsorption performance of LMM for Pb 2+ decreased from 98.82% to 69.34%, indicating that LMM also had a certain reproducibility. The LMM showed high adsorption performance for various heavy metal ions, including Pb 2+ , Cu 2+ , and Mn 2+ , and the adsorption capacity (at 303 K, initial concentration = 30 mg/L, pH = 5, time = 180 min) followed the order of Pb 2+ (187.4 mg/g) > Cu 2+ (168.0 mg/g) > Mn 2+ (166.5 mg/g). The lignin bilayer microspheres prepared from low-cost and readily available lignin have great potential in the removal of heavy metal ions. In the next research process, the assembly and adsorption properties of lignin-based adsorption materials will be further studied to expand their application in water treatment materials. At the same time, the multi-functional use of lignin-based adsorbent materials further realizes the value-added utilization of lignin.

Conclusions and Future Perspectives
Lignin-based bilayer microspheres prepared by the green method have the advantages of easy preparation, renewability, high adsorption, and versatility. In the present study, dual-modified lignin was assembled by dopamine and sodium alginate to obtain a doublelayer lignin microsphere. The diameter of the microsphere obtained was about 800 µm, the specific surface area was 2.14 m 2 /g, and the average pore size was 8.2 nm. The adsorption process followed the pseudo-second-order kinetic equation and Freundlich model, indicating that the adsorption was multi-layer adsorption dominated by chemical adsorption. The maximum adsorption capacity of Pb 2+ by LMM was 250.0 mg/g, determined by Langmuir model analysis and the adsorption of Pb 2+ by LMM was mainly composed of electrostatic attraction and surface complexation. In addition, it was found that after three cycles, the adsorption performance of LMM for Pb 2+ decreased from 98.82% to 69.34%, indicating that LMM also had a certain reproducibility. The LMM showed high adsorption performance for various heavy metal ions, including Pb 2+ , Cu 2+ , and Mn 2+ , and the adsorption capacity (at 303 K, initial concentration = 30 mg/L, pH = 5, time = 180 min) followed the order of Pb 2+ (187.4 mg/g) > Cu 2+ (168.0 mg/g) > Mn 2+ (166.5 mg/g). The lignin bilayer microspheres prepared from low-cost and readily available lignin have great potential in the removal of heavy metal ions. In the next research process, the assembly and adsorption properties of lignin-based adsorption materials will be further studied to expand their application in water treatment materials. At the same time, the multi-functional use of lignin-based adsorbent materials further realizes the value-added utilization of lignin.