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.

1. Introduction

Lead ion (Pb2+) is one of the most common toxic pollutants in industrial wastewater [1]. Due to the non-degradability of Pb2+, 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 Pb2+ 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, it is 3,4-dihydroxy phenylethylamine, which belongs to the catecholamine group as it has a catechol ring and an amine side chain [11]. Such a chemical structure provides the possibility for it to be used as a cross-linking agent.

Sodium alginate (SA) is widely used in the preparation of adsorbents. SA is a biopolymer extracted from brown algae. It has wound-healing properties, good moisture absorption, and high viscosity in water, and it is also a non-toxic, naturally biodegradable green material [12,13]. Researchers found that the SA molecular chain contained many carboxyl and hydroxyl residues, which led to its ability to combine with additional ions to form gels, and the common ion was calcium ion [14]. By cross-linking calcium ions, a microsphere was formed, but the gel spheres had disadvantages, such as large pores, poor strength, and poor stability, so researchers often use blending and chemical modification to improve these properties [13].

As a natural polymer material, lignin not only has a three-dimensional amorphous network structure, but also is the only extendable aromatic polymer [15,16]. According to statistics, 700,000 tons of lignin was annually produced from the pulp and paper industry [17]. Unfortunately, there are still great restrictions on the value-added utilization of lignin [18,19], and more than 95% of lignin is not effectively utilized, resulting in environmental pollution [20]. Alkali lignin (AL), as a by-product of alkali pulping, can cause serious pollution of the water environment and pose a threat to human health if humans cannot properly handle it. Therefore, it is necessary to treat alkali lignin effectively. In addition, AL contains a large number of active functional groups, such as phenolic hydroxyl group and methoxy group, on its surface, so it can remove heavy metal ions in wastewater by chemical reaction. This effectively realizes the value-added utilization of lignin and avoids the waste of biomass energy. However, it was found that the adsorption efficiency of alkali lignin to heavy metal ions was not significant, so it was indispensable to carry out the chemical treatment of AL [21]. The researchers obtained modified lignin-based adsorbents by grafting the copolymerization, oxidation, esterification, sulfonation, and amination of lignin, and prepared lignin-based composite adsorbents by chemical recombination with other materials (such as chitosan, chitin, etc.). Moreover, using lignin, chitin, sodium alginate, and other green polymers to prepare heavy metal adsorbents can not only effectively avoid the pollution caused by these substances to the natural environment, but also purify the polluted wastewater [22]. For example, the maximum adsorption capacity of the lignin-based magnetic adsorbent prepared by Zhou et al., using melamine chloride as a cross-linking agent for Pb2+, was 111.23 mg/g [15]. Yan et al. obtained lignin (LGN) and GO (graphene oxide) composite nanospheres using the self-assembly method, and the maximum adsorption capacity was 368.78 mg/g for Cr6+ [23]. Popovic et al. prepared an amino-functionalized lignin-based microsphere, which showed a favorable adsorption performance for Cd2+ (74.84 mg/g), Cr6+ (54.20 mg/g), As5+ (53.12 mg/g), and Ni2+ (49.42 mg/g) [24]. Zhang et al. prepared functionalized lignin-based hybrid magnetic nanoparticles to adsorb various heavy metal ions; it showed the adsorption capacity of 150.33 mg/g and 70.69 mg/g for Pb2+ and Cu2+, respectively [25]. The lignosulfonate biosorbent (CLLS) prepared by Zhang et al. also had a favorable adsorption effect on lead ions, with a maximum adsorption capacity of 64.9 mg/g [26]. Chen et al. used MoS2 to react with lignin to obtain MoS2-lignin-derived carbon nanocomposites, which exhibited an outstanding adsorption performance for Cr6+ (198.70 mg/g) [27]. These treatment methods can effectively improve the adsorption effect of heavy metal ions. However, it was found that the lignin-based spherical structure demonstrated excellent adsorption efficiency for Cr6+, and there are few studies on the adsorption of lead ions. In recent years, spherical particles have attracted extensive attention because of their regular shape, uniform size, and positive fluidity. Moreover, they also have the advantages of sustainability and easy preparation [28]. According to the uniqueness of the spherical structure, we used a lignin-based spherical adsorbent to study the adsorption of Pb2+. In addition, compared with other biomaterials, lignin is abundant in nature, biodegradable, rich in content, environmentally friendly, and biocompatible [29]. As the spherical lignin particles also have the 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 Pb2+ 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 Pb2+.

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.

2. Materials and Methods

2.1. Materials

The alkali lignin (AL) in the present study was obtained from the Shanghai Trading, Co., Ltd. (Shanghai, China). and the ClCH2COOH, 98%H2SO4, and HCHO were derived from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). CS2, 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.

2.2. 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.

Figure 1

The preparation of the LMM.

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% H2SO4 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) Ca2+ solution and left for 12 h. Finally, these spheres were freeze-dried to obtain the LMM.

2.3. 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 Pb2+ concentrations were acquired using a GGX-600 AAS flame atomic absorption spectrometer (Beijing Haiguang Instrument Co., LTD., Beijing, China).

2.4. Adsorption Experiments

In order to research the effect of dosage, pH, temperature, duration, and concentration for the adsorption performance, the 30 mg/L Pb2+ solution was poured into a 150 mL conical flask with different conditions to study the adsorption capacity. The residual Pb2+ 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 Pb2+ 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.

2.5. 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 Pb2+ 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 Pb2+ again.

3. Results and Discussion

3.1. 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.

Figure 2

(a) Carboxyl content of lignin-based multilayer microspheres (LMM); (b) FT-IR spectrogram of alkali lignin (AL), sodium alginate (SA), and LMM; (c) N2 adsorption-desorption isotherms of LMM; (d) XRD spectra of AL, SA, and LMM.

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.

3.2. 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 1176 cm−1 indicates C-O-C stretching vibration in the ester group, and the weak peak at 1360 cm−1 indicates the presence of C-N in dopamine, which also indicates the chemical reaction between dopamine and DML.

Furthermore, Figure 2c displays the N2 adsorption-desorption curve, which shows that the specific surface area of the LMM was 2.14 m2/g. We found that the specific surface area of the LMM was higher than the AL (1.66 m2/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 Pb2+ 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 Pb2+ by the LMM.

Figure 3

SEM images of (a) alkali lignin (AL); (b) lignin-based multilayer microspheres (LMM); (c) adsorbing Pb2+; (d) size of LMM.

3.3. Adsorption Efficiency of the LMM for Pb2+

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.

Figure 4

Influence of (a) solution pH, (b) adsorbent dosage, (c) time, and (d) temperature on adsorption capacity of lignin-based multilayer microspheres.

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 Pb2+. As it can be seen from Figure 4b, the increased dosage of the LMM would decrease the adsorption capacity for Pb2+, 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 Pb2+ 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.

3.4. Adsorption Kinetics

In order to explore the adsorption mechanism of the LMM adsorbing Pb2+, 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 C0 is the initial concentration of Pb2+ (mg/L), Ct is the concentration of Pb2+ 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:

The pseudo-second-order is calculated as follows:

Meanwhile, the intraparticle diffusion model is expressed by the equation [35]: where qt and qe (mg/g) are the adsorption capacities at time t and equilibrium, respectively; k1 (1/min) and k2 (g/mg min) are the pseudo-first-order and pseudo-second-order model rate constants, respectively; kp is the intraparticle diffusion rate constant [mg/(g min1/2)]; 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 R2 of the pseudo-first-order and pseudo-second-order were 0.9696 and 0.9992, respectively. In addition, the R2 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 Pb2+ was mainly chemical adsorption and the interaction between the LMM and Pb2+ was not a simple diffusion [36].

Table 1

The adsorption kinetics parameters of the LMM for Pb2+.

Model	Pseudo-First-Order
C0 (mg/L)	qe (mg/g)	k1 (min−1)	R2
30	22.08	0.02349	0.9696
	Pseudo-second-order
C0 (mg/L)	qe (mg/g)	k2 (g/mg min)	R2
30	38.91	0.001929	0.9999
	intraparticle diffusion model
	C	Kp	R2
Step 1	10.29	2.739	0.9053
Step 2	20.65	1.252	0.9274
Step 3	36.04	0.05840	0.8944

Figure 5

Fitting adsorption kinetics of (a) pseudo-first-order model, (b) pseudo-second-order model, and (c) intraparticle diffusion model.

3.5. Adsorption Isotherm and Thermodynamics

The relation between LMM and Pb2+ 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 Pb2+ 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 Pb2+ is demonstrated in Figure 6 and the parameters are exhibited in Table 2. It can be seen that the R2 of Freundlich model was higher than the Langmuir model. Therefore, the multi-layer chemical adsorption could be used to display the adsorption process of Pb2+ 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 Pb2+ could easily react with the LMM [38].

Figure 6

Fitting adsorption equilibrium isotherms of (a) Freundlich model and (b) Langmuir model.

Table 2

The equilibrium isotherm models parameters for Pb2+ by the LMM.

Model	Langmuir Model
T (K)	qmax (mg/g)	KL (L/mg)	R2
303	185.19	0.07649	0.7248
313	204.81	0.1012	0.7721
323	250.0	0.2353	0.9422
	Freundlich model
T (K)	KF (L/g)	1/n	R2
303	40.85	0.3011	0.8130
313	46.53	0.3266	0.8783
323	58.56	0.4088	0.9900

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 Pb2+. It can be seen that the adsorption capacity of LMM for Pb2+ 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.

Table 3

The comparisons of the adsorption capacity of Pb2+ by other adsorbents.

Adsorbent	Adsorption Capacity (mg/g)	Reference
chitosan/lignosulfonate hydrogel	525	[18]
Bio-sourced phenolic foams	100.9	[39]
Cross-linked lignosulfonate bio-adsorbent	64.9	[26]
Modified alkaline lignin	73.7	[40]
Functionalized lignin-based adsorbent	91.4	[41]
Lignin-based hybrid magnetic nanoparticles	150.33	[25]
Magnetic polyethyleneimine lignin	96.60	[42]
LMM	250.0	This work

3.6. Thermodynamics of the LMM Adsorption

In order to investigate the thermodynamics of the LMM adsorption for Pb2+, the three thermodynamics parameters, including ΔG (kJ/mol), ΔH (kJ/mol), and ΔS (kJ/mol/K), were obtained using the following equations [43]: where Kd (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 Pb2+. ΔG < 0 showed that the adsorption process was a spontaneous reaction and the rates of ΔG 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.

Table 4

The Thermodynamics paraments of Pb2+ adsorption by the LMM.

T (K)	ΔG (KJ/mol)	ΔH (KJ/mol)	ΔS (KJ/mol/K)
303	−28.89	16.26	0.149
313	−30.38
323	−31.87

In addition, the positive value of ΔH 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 ΔS 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 Pb2+ by the LMM fitted well with this analysis.

3.7. Recyclability of the LMM

The recyclability of Pb2+ using the LMM was analyzed using an adsorption-desorption experiment. As shown in Figure 7a, the adsorption efficiency of the LMM for Pb2+ 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 Pb2+. 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 Pb2+. Therefore, in the follow-up study, we will conduct experiments and solve problems based on these reasons.

Figure 7

(a) Adsorption and desorption cycles of lignin-based multilayer microspheres (LMM) for Pb2+ and (b) selectivity of LMM for Pb2+, Cu2+, and Mn2+.

3.8. 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 Pb2+, Cu2+, and Mn2+ follow the order Pb2+ > Cu2+ > Mn2+. It was caused by the differences in their corresponding electronegativities (Pb2+ = 2.33, Cu2+ = 2.0, and Mn2+ = 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, Pb2+ was considered to a softer acid than Cu2+ and Mn2+, 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 Pb2+, Cu2+, and Mn2+ 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 Pb2+ was the best among the three heavy metal ions.

3.9. Adsorption Mechanism

The adsorption mechanism of the LMM for Pb2+ could be explored by XPS analysis. Figure 8 suggests the changes of the LMM and the LMM involving Pb2+. The peaks of N, S, and Pb explain the successful preparation of the LMM and the Pb2+ adsorption. The 4f5/2 and 4f7/2 signals appearing in Figure 8b illustrate the adsorption of the LMM for Pb2+. 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 Pb2+, and the peaks of COOH shifted from 288.1 eV to 287.6 eV after adsorption Pb2+, which would be caused by the appearance of COO-Pb (Figure 8c,g) [30]. Additionally, the -C-O and O-H peaks were 531.6 eV and 532.7 eV, respectively; after adsorbing Pb2+, the -C-O and O-H peaks were 531.2 eV and 532.0 eV, respectively. Moreover, the 535.2 eV signal explains the existence of -O-Pb [46]. The peaks of the N 1s spectra at 399.3 eV and 401.2 eV were caused by the -NH2-, -NH-, and C-N bonds of the LMM. Then, the peaks of -NH2-, -NH-, and C-N changed to 399.28 eV 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 Pb2+ [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 Pb2+, 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 Pb2+ and the LMM.

Figure 8

(a) XPS spectra of lignin-based multilayer microspheres (LMM) and LMM-Pb; (b) characteristic peaks of Pb and fitting peaks of (c,g) C 1s, (d,h) O 1s, (e,i) N 1s, and (f,j) S 2p.

In conclusion, according to the analysis of the XPS spectra, the main adsorption process of the LMM for Pb2+ 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 Pb2+ could better enter the interior of LMM, which was more conducive to the physical adsorption and internal chemical reaction of Pb2+ on the LMM. Therefore, the LMM adsorbed the Pb2+ 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).

Figure 9

Adsorption mechanism of lignin-based multilayer microspheres (LMM) for Pb2+.

4. 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 m2/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 Pb2+ by LMM was 250.0 mg/g, determined by Langmuir model analysis and the adsorption of Pb2+ 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 Pb2+ 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 Pb2+, Cu2+, and Mn2+, and the adsorption capacity (at 303 K, initial concentration = 30 mg/L, pH = 5, time = 180 min) followed the order of Pb2+(187.4 mg/g) > Cu2+(168.0 mg/g) > Mn2+(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.