Application of Lignin-Derived Carbon Materials in Adsorption and Separation

Abstract

In the context of sustainable human development and the depletion of petroleum resources, lignin has received widespread attention as a carbon-rich, low-cost, and renewable resource. Owing to their distinctive physical and chemical properties, carbon materials are extensively applied in the fields of adsorption and separation. The conversion of lignin into diverse multifunctional carbon materials, such as porous carbon, activated carbon, carbon fibers, carbon foams, and carbon aerogels, has emerged as a pivotal strategy for the high-value utilization of lignin. In this paper, representative examples of various lignin-based carbon materials utilized in the field of adsorption and separation over the past decade are reviewed and categorized according to the type of carbon materials, and their preparation methods and adsorption effects are described.

1. Introduction

With the continuous development of human society, the issue of environmental pollution has become increasingly severe [1,2,3], while petrochemical resources are dwindling [4,5,6,7,8]. People are gradually realizing that environmental protection and the effective use of renewable resources are the two main challenges for the persistence of human civilization [9,10,11,12]. In pursuit of the sustainable development of human society, the search for green and renewable resources has become urgent [13,14,15]. Lignin has received widespread attention due to its ubiquitous distribution, abundant reserves [16], low cost [17], high carbon content [18], renewability, and other advantages [19]. The high-value utilization of lignin can mitigate the reliance on non-renewable petroleum resources and decrease greenhouse gas emissions [20,21], which is of great significance for environmental protection and sustainable development [22,23]. Lignin is an aromatic polymer inherently present in terrestrial plants, alongside cellulose and hemicellulose, typically comprising 15–40% of dry biomass [24,25]. Therefore, to ensure efficient utilization, it is usually necessary to pre-separate lignin from the raw material prior to use. Currently, there are three primary methods for lignin extraction: physical, chemical, and biological. The physical method, which primarily includes the ball milling method and steam explosion techniques, has a minimal environmental impact but suffers from low separation efficiency. The chemical method, encompassing acid treatment, alkali treatment, hydrothermal pretreatment, and organic solvent extraction, is simple to operate and boasts high extraction efficiency. However, the extensive use of chemical reagents generates significant sewage, posing substantial environmental harm if not properly treated. Biological methods, which primarily consist of enzymatic hydrolysis and microbiological methods, offer low energy consumption for lignin extraction. However, the cost of enzymes remains high. In the practical production process, the appropriate method is usually selected based on the specific composition of different precursor types and the impact of the extraction method on the final product [26,27,28]. It has been reported that up to 50 million tons of lignin is extracted from the pulp and paper industry each year [29]. Unfortunately, only a small portion of this valuable resource is commercialized as a by-product, while the majority is burned directly for heat and power generation [30]. However, lignin possesses a relatively low calorific value, meaning that direct combustion not only constitutes a significant underutilization of this resource but also results in the emission of greenhouse gases. Consequently, the challenge of effectively utilizing lignin for high-value conversion demands urgent attention.

The chemical structure of lignin is relatively intricate and varies according to its source and purification method, rendering its precise structure elusive [31]. Faix et al. proposed an approximate empirical chemical formula for lignin [32], indicating an elemental composition of approximately 63.4% carbon, 30% oxygen, and 5.9% hydrogen [33]. Lignin is commonly envisioned as a three-dimensional highly branched amorphous phenolic polymer with a disordered reticular structure [34,35]. It can be rationalized into three primary phenylpropane structural units, namely p-coumarol alcohol, coniferyl alcohol, and sinapyl alcohol [36], corresponding to p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) structural units [37], respectively (Figure 1). These units are interconnected through various C-C and C-O bonds, predominantly β-O-4, β-β, β-5, 4-O-5, and 5-5. Among these, β-O-4 bonds usually occupy the major portion of the lignin structure [38]. In addition, a range of oxygen-containing groups such as methoxy, hydroxyl, and carbonyl groups exist in the C-α and C-β bonds of lignin, contributing to its complex structure [39]. The proportion of H, G, and S units varies based on the tree species [40,41]. Depending on the source, lignin can be broadly classified into hardwood lignin (HL), softwood lignin (SL), and grass lignin (GL). Among them, HL primarily consists of S and G units, SL comprises G units and some S and H units, and GL includes p-coumaric and ferulic acid in addition to S, G, and H units [31]. Due to its intricate structure, the high-value utilization of lignin poses a significant challenge.

Carbon is one of the elements in nature that is most closely related to human beings [42]. It is superior to other materials in terms of hardness, radiation resistance, optical properties, electrical conductivity, and insulating properties, making carbon materials versatile and widely applicable. As researchers continue to uncover more advantages of carbon materials, such as their high specific surface area, abundant functional groups, and high electrical conductivity, bio-based carbon materials, in particular, are gaining popularity due to their environmental friendliness and cost-effectiveness [43,44,45]. Lignin, with its abundance of benzene rings and over 60% carbon content, emerges as an exceptional precursor for carbon materials [46]. By converting lignin into a variety of useful carbon materials, including porous carbon, activated carbon, carbon fiber, carbon foam, and carbon aerogel, we can not only enhance the value of lignin but also avoid the environmental pollution associated with direct combustion and greenhouse gas emissions.

The primary methods for preparing carbon materials from lignin are pyrolysis-activation and the template method. Pyrolysis is a common method for the preparation of carbon materials, involving the thermal decomposition of lignin at high temperatures. However, direct pyrolysis often results in carbon materials with poor pore distribution and a small specific surface area, making them difficult to utilize. To improve these properties, activation methods, such as physical and chemical activation, are employed [47]. Physical activation involves carbonizing lignin under an inert gas and then activating it using air and water vapor, while chemical activation performs carbonization and activation simultaneously, often using reagents like KOH and ZnCl₂. In contrast, the template method utilizes templates to effectively control the pore structure of the synthesized carbon materials [48,49].

Over the past few decades, the preparation of carbon materials from lignin has become an important approach for the high-value utilization of lignin. To date, carbon materials derived from lignin have found widespread applications in the fields of adsorption and separation [50,51], catalysis [52,53,54], energy storage [55,56,57], environmental remediation [58,59], and medicine [60]. Remarkably, carbon materials derived from lignin are particularly well-suited for applications in adsorption and separation, attributed to their abundant pore structure and exceptionally high specific surface area. In recent years, many reviews have emerged regarding adsorbents derived from lignin. For example, Wang et al. [61] focused on the utilization of lignin-based adsorbents in wastewater treatment. In another comprehensive review, Supanchaiyamat et al. [62] categorized the advancements in the application of lignin-derived adsorbents in the domain of adsorption and separation, based on their adsorption targets. However, there is still a lack of relevant reviews focusing exclusively on the application of lignin-derived carbon materials in the field of adsorption and separation. Therefore, we deem it imperative to summarize the recent advancements in this field. In this paper, we have compiled a selection of important papers from the past decade in this field, categorizing them based on the type of carbon material and presenting them in chronological order.

2. Type of Carbon Materials

2.1. Porous Carbon

Lignin-derived porous carbon materials have received widespread attention in recent years. When converted into a porous carbon material, lignin exhibits unparalleled advantages over other biomass resources such as cellulose, polysaccharides, and proteins. Lignin-based porous carbon has the advantages of high porosity, large specific surface area, low density, good chemical stability, and regenerability, which have made it the focus of extensive research in the field of adsorption in recent years [63,64,65].

In 2019, Wang et al. prepared a novel lignin-based porous carbon material (LPC) embedded with layered graphene through an innovative chemical solution combustion method. They used urea as a nitrogen source for the doped carbon-based materials and ammonia as both an activator and pore expander. The process involved mixing lignin with urea, dissolving the mixture in ammonia, and subsequently calcining the materials at 350 °C for 1 h, followed by heating at 500 °C under an argon atmosphere for 1 h (Figure 2) [66]. The resulting LPC material has an abundant and highly interconnected porous structure, primarily attributed to the effects of urea and ammonia. The adsorption of Pb(II) and Cd(II) by the LPC material increased rapidly within 1 h. Beyond this point, the adsorption sites were close to saturation, with optimum adsorption capacities of 250.5 mg/g and 126.4 mg/g, respectively, surpassing previously reported adsorption levels for similar adsorbents (

Table 1. The adsorption capacities of Pd(II) and Cd(II) for previously reported adsorbents.

Adsorbents	Heavy Metals	Adsorption Capacity	Ref.
Activated tea waste	Pd(II)	81 mg/g	[67]
Pomelo peel biochar	Pd(II)	88.74 mg/g	[68]
Mango peel waste	Cd(II)	68.9 mg/g	[69]

) [67,68,69]. Kinetic analysis showed that the adsorption kinetics conformed to a pseudo-second-order model, indicating chemisorption, and the Freundlich model provided a better description of the adsorption process. Zeta potential measurements showed that the surface potential of the material shifted from positive to negative as the pH increased from 3 to 9, and the adsorption properties enhanced with pH due to electrostatic interactions between the material and heavy metal ions. Notably, the LPC material exhibited excellent recyclability, maintaining 96% removal of Pb(II) and 92% removal of Cd(II) even after five consecutive cycles.

In 2021, Tan et al. successfully prepared lignin-based porous carbon (LPC) for the adsorption of methylene blue (MB) using a combination of hydrothermal methods and KOH activation (Figure 3) [70]. They initially dispersed the lignin in sulfuric acid (5 wt%) using ultrasound, followed by heating it to 180 °C in a high-pressure reactor for 18 h. The resulting brown solid from the hydrothermal treatment was neutralized with deionized water and dried to obtain a lignin precursor (CL), which exhibited a surface covered with loosely distributed pores of varying sizes. Subsequently, the CL material was thoroughly mixed with KOH and deionized water, dried, and then carbonized in a tube furnace at 750 °C for 2 h under a N₂ flow rate of 60 mL/min, with a temperature increase rate of 3 °C/min. After cooling naturally, the material was rinsed with a hydrochloric acid solution (10 wt%) to remove KOH, washed with deionized water, and dried. The obtained LPC material possessed a highly abundant porous structure, with a large number of micropores and mesopores that facilitated efficient MB adsorption. The synthesized LPC material showed a rapid increase in MB adsorption within 25 min, followed by a gradual increase in adsorption rate, reaching an optimum adsorption capacity of 1119.18 mg/g. This high adsorption performance was maintained even after four cycles. The adsorption kinetics conformed to the pseudo-second-order kinetic model, while the Langmuir isotherm model provided a more accurate description of the adsorption process, with a calculated maximum adsorption capacity of 1119.65 mg/g, closely matching the experimental results. In addition, the authors revealed the adsorption mechanism and tested the material’s performance with anionic and cationic dyes. The adsorption capacities for cationic malachite and rhodamine B were 2467.30 mg/g and 1657.82 mg/g, respectively, whereas the adsorption capacities for anionic methyl orange and amaranth were significantly lower at 526.58 mg/g and 206.12 mg/g, respectively. This difference is attributed to the electrostatic interactions caused by the negative charges on the surface of the LPC material, which are the primary drivers of adsorption, along with hydrogen bonding and π-π stacking interactions that also contribute to the adsorption process.

In the same year, Zhu et al. extracted lignin from a corn stover as the raw material and incorporated Fe₃O₄ nanoparticles into a lignin suspension (2 wt%, 100 mL). The suspension was stirred at 7000 rpm for 3 min, sonicated at 30% amperage for 5 min, filtered utilizing a 0.22 μm membrane, and subsequently dried. Carbonization was carried out for 2 h at 400 °C under a N₂ flow rate of 200 mL/min, with a heating rate of 5 °C/min. The solid–liquid ratio was maintained at 1:15 (g/mL), and the material was mixed with sulfuric acid (98%), followed by etching and sulfonating for 12 h at 150 °C under a nitrogen atmosphere. After cooling, the material was filtered, washed with hot deionized water to remove sulfate ions, and dried under vacuum at 70 °C overnight to synthesize a novel lignin-based sulfonated porous carbon (LSPC) (Figure 4) [71]. After etching, LSPC exhibited an abundance of loose and disordered porous structures, which are very favorable for dye adsorption. The authors conducted binary dye adsorption performance tests and observed a gradual shift toward anionic dye colors within 60 min, highlighting the excellent selectivity of the material for cations. Due to the introduction of a large number of -SO₃H negatively charged groups during the etching sulfonation process, LSPC materials have strong electrostatic interactions with cationic dyes, resulting in excellent cation adsorption properties. Further adsorption tests revealed that when the initial concentration of MB was 500 mg/L (pH = 5), adsorption increased rapidly during the first 15 min and reached equilibrium at 36 h, with an optimum adsorption capacity of 420.40 mg/g. The adsorption process conforms to the second-order kinetic model, and the Langmuir isotherm model is more suitable to describe the adsorption process, with a maximum adsorption capacity of 423.73 mg/g. Interestingly, the adsorption of MB increased dramatically from 234.19 mg/g to 621.52 mg/g when the solution pH was increased from 2 to 11. In addition, LSPC showed excellent regeneration performance for desorption in an acidic methanol solution.

In 2023, Liang et al. used industrial alkali lignin as a raw material, blending it with urea stirred in water and stirring for 10 min. The mixture was subsequently heated in a reactor at 200 °C for 200 min, filtered, and then dried at 100 °C to produce lignin-based carbon spheres (L-CSs). Although the resultant L-CS materials have very small pores and are not suitable for adsorption applications, the formed skeleton laid the groundwork for future activation. The authors then mixed the L-CSs with KOH in water and stirred the solution for 48 h. The solution was dried to remove water and heated in a box atmosphere furnace under an argon atmosphere at 800 °C with a heating rate of 8 °C/min for 2 h. At the end of the reaction, the material was washed with deionized water and dried at 100 °C to obtain lignin-based layered porous carbon (L-HPC) (Figure 5) [72]. The KOH activation process significantly enhanced the specific surface area of the material and generated numerous pore structures, which were favorable for adsorption. The adsorption of Cr(Ⅳ) by the synthesized L-HPC material increased dramatically within the initial 30 min and reached equilibrium after 12 h, with an optimal adsorption capacity of 887.8 mg/g. The adsorption performance was maintained above 83% after 12 consecutive uses, demonstrating excellent regeneration performance. Kinetic analysis confirmed that the adsorption process was chemisorption-driven, and the abundance of N and O functional groups on the surface of L-HPC plays a key role. Additionally, hydrogen bonding was observed between -OH and Cr(Ⅳ). XPS analysis indicated the presence of Cr(IV) and Cr(III) after adsorption, probably due to the N donation of electrons to reduce Cr(IV) to Cr(III). The zeta potential shows that the surface of the material carries a positive charge when pH < 2.5, probably due to the electrostatic interaction of N with HCrO⁴⁻ and Cr₂O₇²⁻ in the form of positive ions.

In the same year, Zhao et al. prepared a series of porous carbon materials from pulping waste liquid (black liquor lignin BLL) using both chemical activation and template methods [73]. The chemical activation method was performed by mixing BLL with ZnCl₂ or KOH and stirring overnight at room temperature, drying at 105 °C for 24 h, and then heating in a tube furnace for 2 h under a nitrogen atmosphere at 800 °C with a ramp rate of 5 °C/min. Finally, the material was washed with hydrochloric acid, rinsed to neutrality with distilled water, dried at 105 °C for 24 h, and ground to yield C-BLL-ZnCl₂ or C-BLL-KOH. In contrast, the template method involved mixing BLL with basic magnesium carbonate (BMC) or magnesium oxide (MgO), dispersing the mixture in distilled water using ultrasound, stirring at 80 °C, grinding after drying, and then heating in a tube furnace under nitrogen atmosphere at 600 °C for 2 h, with a heating rate of 5 °C/min. The template was removed using hydrochloric acid, and the material was washed with distilled water and then dried for 24 h at 105 °C to obtain C-BLL-BMC or C-BLL-MgO. The porous carbon materials prepared by the chemical activation method have a large number of pore structures, compared to those prepared using the template method, highlighting the significant influence of the synthesis technique on the structure of the carbon materials. Among these, C-BLL-KOH, prepared by KOH activation, demonstrated the highest degree of carbon structural disorder and a maximum specific surface area of 1336.5 m²/g. Microporosity accounts for 93.2% of its total pore volume, which is very favorable for CO₂ capture. Subsequently, the authors investigated the CO₂ adsorption performance of four materials, C-BLL-ZnCl₂, C-BLL-KOH, C-BLL-BMC, and C-BLL-MgO, at different temperatures (0 °C, 25 °C, 50 °C). Among them, C-BLL-KOH showed excellent adsorption performance, with adsorption capacities of 5.20, 3.6, and 2.23 mmol/g at 0 °C, 25 °C, and 50 °C, respectively, under a pressure of 100 kPa. The C-BLL-ZnCl₂ material followed closely in terms of adsorption performance, while the template-derived materials, C-BLL-BMC and C-BLL-MgO, showed poor adsorption performance. This indicates that lignin extracted from BLL is more suitable for the preparation of carbon materials for CO₂ capture using the chemical activation method.

In 2025, Xu et al. dissolved lignin in ethylene glycol through sonication. Subsequently, 4-acetoxystyrene, diethylene glycol terephthalate, and azobisisobutyronitrile (AIBN) were added and heated for 8 h at 80 °C. At the end of the reaction, the product was filtered and washed with methanol and deionized water, followed by drying at 80 °C to obtain the lignin derivative (ADL). Then, the ADL was milled with ZnCl₂ and heated in a tube furnace for 1 h under a nitrogen atmosphere at 500 °C with a heating rate of 5 °C/min. After cooling to room temperature, it was immersed in deionized water for 12 h, washed again with methanol and deionized water, and dried at 105 °C to obtain lignin-derived porous carbon (ADLC) (Figure 6) [74]. The surface of the obtained ADL material is very smooth, with a surface area of only 17 m²/g. After chemical activation, the surface of the obtained ADLC material has a rich honeycomb porous structure, with a surface area of 1101 m²/g, which led to a significant enhancement of the adsorption performance. Immediately after that, the authors further assessed the adsorption of ADLC material in both iodine vapor and iodine/cyclohexane solution, which reached the maximum value at 9 h. The optimum adsorption capacities were 2340 mg/g and 354 mg/g, respectively, and the material demonstrated excellent recyclability, maintaining high adsorption performance even after five cycles. Subsequently, the authors proposed an adsorption mechanism wherein the presence of abundant oxygen-containing functional groups on the surface of ADLC facilitated electron transfer with iodine molecules to form polyiodide anions, which could be adsorbed onto ADLC through electrostatic interactions. In addition, the benzene ring established a π-π conjugated bond with iodine, while hydrogen bonding interactions and van der Waals forces between the hydroxyl group and the iodine molecule are also involved in the adsorption process.

The porous structure of lignin-based porous carbon provides a large specific surface area and abundant reaction sites, which is very favorable for adsorption and separation [75]. Currently, this innovative material has proven its mettle in diverse applications, including the adsorption of CO₂ [72], the removal of heavy metal ions [66,73], the purification of organic wastewater [70,71], and the sequestration of nuclear waste [74]. Despite its successful employment in the realm of adsorption and separation, lignin-based porous carbon faces several challenges and shortcomings. Notably, the preparation process of the adsorbent is relatively intricate, necessitating further refinement to enhance its adsorption performance. Additionally, there is a pressing need to elevate the number of adsorption cycles and expand its range of applications.

2.2. Activated Carbon

Activated carbon typically exists as a black powder or granule, characterized by numerous internal pore structures, a substantial specific surface area, and excellent adsorption capacity. The preparation of activated carbon involves three primary stages: raw material selection, carbonization, and activation. Currently, the majority of commercially available activated carbon is primarily produced from fossil resources, which have the potential to pollute the environment. In contrast, lignin stands out as an appealing precursor for activated carbon due to its high carbon content, low cost, renewability, and environmental friendliness [76].

In 2019, Wang et al. used corn kernels as the raw material to extract hemicellulose using dilute sulfuric acid [77]. Enzymatic digestion was then carried out using cellulase at a pH of 4.8, a temperature of 50 °C, and a stirring speed of 180 rpm for 72 h, targeting the enzymatic degradation of the cellulose. At the end of the reaction, the residue was collected and dried. The authors used different impregnation ratios and activators (CaCl₂, H₃PO₄, NaOH, and NaCO₃) to prepare the materials, which were oven-dried at 105 °C for 12 h, heated in a tube furnace at an N₂ flow rate of 80 mL/min, and ramped up at 10 °C/min to 500–750 °C for 30–90 min. After activation, the materials were cooled to room temperature and washed with HCl and deionized water until the pH was neutral. Finally, they were dried at 105 °C for 12 h. The highest yield of activated carbon was obtained when CaCl₂ was used as the activator at an impregnation ratio of 2:1 (mass ratio), a temperature of 600 °C, and a reaction time of 1 h. The activated carbon produced with CaCl₂ exhibited well-developed and uniformly distributed pores, resulting in a significant increase in Cu(II) adsorption within 10 min, reaching equilibrium after 60 min, with an optimal adsorption capacity of 49.21 mg/g. The Langmuir isothermal adsorption model provided a better description of the adsorption process. FTIR analysis revealed that the activated carbon contains a large number of carboxyl groups, and Cu(II) ions initially diffuse into the activated carbon before being adsorbed through these carboxyl groups.

In 2021, Zhu et al. homogeneously mixed black liquor lignin (BLL) with Fenton sludge and stirred the mixture for 1 h. Subsequently, KOH was added for activation, and the mixture was stirred for an additional 2 h before being dried at 105 °C for 12 h. The dried mixture was then heated to 800 °C in a tube furnace at an N₂ flow rate of 100 mL/min and a heating rate of 10 °C/min and maintained at this temperature for 1 h. After cooling to room temperature, the pH was adjusted to neutral. Finally, the lignin-based magnetic activated carbons (MACs) were obtained by vacuum drying at 60 °C for 24 h [78]. Following the same procedure, the authors also prepared control activated carbons (ACs) without the addition of Fenton sludge. The incorporation of sludge significantly increased the mesopore volume and porosity of the material compared to the control sample. The adsorption of methylene blue (MB) was investigated by adjusting the ratio of sludge, lignin, and KOH. The adsorption of MAC_0.5–2 (Fenton sludge:KOH = 0.5:2, mass ratio) increased rapidly within 20 min and reached equilibrium after 60 min, with an optimal adsorption capacity of 307.2 mg/g. The adsorption capacity of MAC_1–2 (Fenton sludge: KOH = 1:2, mass ratio) increased rapidly within 90 min and reached equilibrium after 180 min, with an optimal adsorption capacity of 184.1 mg/g. After five cycles of adsorption, the recovery rates were 84% and 92.1%, respectively. The adsorption kinetics conformed to the proposed second-order kinetic model, and the Sips isothermal model provided a better description of the adsorption process. Subsequently, the authors analyzed the possible mechanism of the adsorption process. After MB adsorption, the MACs showed a significant decrease in pore space and specific surface area, and carboxylate anions were detected, suggesting the presence of pore filling and electrostatic interactions during adsorption. The MAC material possesses a rich benzene ring structure, and FTIR spectroscopy revealed shifts in the peaks corresponding to aromatic C-C bonds before and after adsorption, indicating that the π-π bonding interactions may have contributed to the adsorption process. In addition, shifts in the peaks of Fe-O bonds were observed, suggesting that the magnetic particles may have improved the adsorption properties of the MAC materials.

In 2023, Li et al. washed corn stover lignin (CSLAC), lignin sulfonate lignin (LSAC), and sugarcane bagasse lignin (BLAC) with distilled water to remove impurities. Subsequently, these lignins were soaked at 80 °C for 8 h, filtered, and dried at 105 °C for 24 h. Then, they were mixed with KOH, soaked in distilled water for 3 h, filtered, and dried at 120 °C for 24 h. The sample was then extracted and dried at 120 °C. Then, the sample was calcinated with a heating rate of 3 °C/min under a nitrogen atmosphere, and the temperature was raised to 250 °C in a tube furnace for 2 h. Subsequently, the temperature was raised to an activation temperature of 600–900 °C, yielding a series of lignin-based activated carbons [79]. The authors observed that as the temperature increased from 600 °C to 800 °C, the specific surface area and pore volume of the material gradually increased but decreased when the temperature reached 900 °C. The results showed that CSLAC-800 (CSLAC with a carbonization temperature of 800 °C) possessed the most abundant mesopores and micropores and demonstrated the best adsorption performance for phenol. It achieved significant optimal adsorption within 15 min at 30 °C, reaching adsorption equilibrium within 60 min, with an optimal adsorption capacity of 612 mg/g. The adsorption kinetics conformed to the proposed secondary kinetic model, while the Freundlich isothermal adsorption model provided a better description of the adsorption process. The process occurred through a combined mode of physical and chemical adsorption, where the abundant benzene ring structure and oxygen-containing functional groups in the activated carbon material interacted with phenol through hydrogen bonding and π-π bonding interactions.

In 2024, Cao et al. prepared a series of lignin-based activated carbons using both physical and chemical activation techniques. For physical activation, steam was used to heat the water, with the temperature controlled between 700 and 900 °C. The sample was placed inside the reactor and subjected to a reaction time of 30–120 min. For chemical activation, coir or lignin was immersed in either K₂CO₃ or KOH solution and subsequently dried at 105 °C for 24 h. The heating process was conducted at a rate of 10 °C/min, reaching 800 °C under a nitrogen atmosphere for 40 min in a tube furnace. After cooling to room temperature, the sample was washed with distilled water until a neutral pH was achieved. Finally, the lignin-based activated carbon material was obtained by drying at 105 °C for 24 h (Figure 7) [80]. During the physical activation process, the specific surface area exhibited an increase followed by a decrease with the gradual increase in the temperature, reaching a maximum of 1065 m²/g at 800 °C. Similarly, in the chemical activation process, both chemical activators reached a maximal specific surface area at 800 °C. Although an increase in temperature led to a gradual decrease in the yield of lignin-based activated carbon, it significantly enhanced the specific surface area, promoting the formation of micropores. The FTIR and adsorption results showed that the lignin-based activated carbon activated by K₂CO₃ exhibited the best performance. During the process of K₂CO₃ activation, abundant micropores and oxygen-containing functional groups were formed, providing more adsorption sites and improving the adsorption performance. The adsorption efficiency of the samples increased significantly within 10 min, basically reaching equilibrium at 40 min, with a maximum adsorption efficiency of 98.22% at 60 min. After three cycles, the adsorption rate still remained above 93%, and the adsorption kinetics aligned with the proposed one-stage kinetic model.

In the same year, Yun et al. utilized a microwave-assisted heat treatment method by mixing sulfated lignin (KL) with KOH, placing them in a reactor, and heating them using microwaves (MW) under a nitrogen atmosphere (Figure 8) [81]. The mixture was heated for 10 min at 3% of its mass, with a power setting of 1000 W. Subsequently, it was heated at 450 W for 10 min, with stirring intervals every 2 min. Then, the mixture was washed with deionized water until it reached a neutral pH and then dried at 105 °C for 24 h. In the conventional method of preparation of lignin-based activated carbon, approximately 8.16 KWh of electricity is consumed. However, in this study, only 0.075 KWh of electricity was consumed at 450 W, demonstrating a significant improvement in energy efficiency. In the process of MW-assisted heating of KL, the cleavage of functional groups like hydroxyl and methoxyl led to a reduction in oxygen content and a substantial increase in carbon content. This process facilitated the generation of more activated carbon, and a pore structure with a larger specific surface area of 1342.79 m²/g can be obtained by chemical activation with KOH. The adsorption capacities of methylene blue and acid orange 7 by the sulfated lignin-based activated carbon were 548.54 mg/g and 543.82 mg/g, respectively. The adsorption rate of the sulfated lignin-based activated carbon remained above 97% even after five cycles.

Over the past decade, activated carbons derived from lignin have attracted considerable attention from researchers due to their unique properties [82]. Their highly developed internal pore structure and high specific surface area, coupled with their exceptionally stable chemical properties and abundant chemical functional groups on their surface, create favorable conditions for adsorption processes. Currently, researchers have successfully utilized activated carbon prepared from different lignin sources such as corn stover, alkali lignin, and sulfate lignin, for the effective adsorption of heavy metal ions [77], organic dyes [78,81], and volatile organic compounds (VOCs) [79,80].

2.3. Carbon Aerogel

Carbon aerogel is an ultra-lightweight three-dimensional mesh structure with many micropores and mesopores, rendering it a versatile material in applications such as adsorption and separation due to its unique physicochemical properties [83,84]. However, the majority of current methods for synthesizing carbon aerogels are cumbersome and expensive, thereby limiting their widespread adoption. Currently, lignin has emerged as a promising precursor for the production of eco-friendly and biodegradable lignin-based carbon aerogels that are extensively utilized in the realm of adsorption and separation [85].

In 2020, Geng et al. prepared suspensions of sulfated lignin and TEMPO-oxidized cellulose nanofibers (TOCNFs) with different concentrations by putting them into distilled water. Then, they employed ice templating and freeze-drying techniques to synthesize lignin/TOCNF precursors, which were carbonized at 1000 °C to obtain different lignin-based carbon aerogel LTCAs (Figure 9) [86]. When the TOCNF content was 12 wt%, the material exhibited a specific surface area of 806 m²/g and a porosity of 93.4%. The authors found that the specific surface area of the material was further increased to 1101 m²/g after washing the LTCAs with water. SEM-EDX analysis revealed a reduction in Na content before and after washing, likely attributable to the removal of sodium salts by water washing and exposure of more micropores, which improves the specific surface area and adsorption performance. The material demonstrated a CO₂ adsorption capacity of 5.23 mmol/g at 273 K and 100 kPa.

In the same year, Lv et al. added κ-carrageenan to distilled water and stirred the mixture at 80 °C for 30 min. Then, sodium lignosulfonate was added and the stirring continued for 30 min. The solution was then allowed to cool to room temperature and sealed with parafilm for 24 h. Finally, it was placed in the refrigerator for 24 h and freeze-dried for 72 h. The resulting NaLS-kA was heated in a tube furnace, with a N₂ flow rate of 100 mL/min at 550 °C and a heating rate of 10 °C/min for 30 min. Then, it was immersed in a KOH solution for 12 h and dried at 105 °C. After drying, it was placed in a tube furnace and heated for 30 min, washed with distilled water until a neutral pH was achieved, and then dried at 80 °C to obtain the lignin-based carbon aerogel NaLS-kACA [87]. After etching with KOH, the carbon aerogel exhibited an abundance of meso- and microporous structures with a specific surface area of 594.6 m²/g. EDS and XPS analyses showed that the carbon aerogel was rich in hydroxyl and carboxyl functional groups. Upon deprotonation, the carbon aerogel showed a negative charge along with an increase in pH, which was very favorable for the adsorption of cationic dyes. The adsorption rate of methylene blue (MB) by the carbon aerogel increased significantly within 90 min and then plateaued, with an optimum adsorption capacity of 421.94 mg/g. The adsorption kinetics adhered to the proposed second-order model, while the adsorption process was accurately described by the Freundlich isothermal model. Thermodynamic analysis indicated that the interaction between the carbon aerogel and MB was spontaneous and endothermic.

In the same year, Meng et al. used diethylenetriamine (DETA) to modify alkali lignin (AL), resulting in the formation of aminated lignin. This aminated lignin, along with N, N′-methylenebisacrylamide (MBA), and acrylic acid (AA), was dissolved in a graphene oxide (GO) solution. Ammonium persulfate (APS) was then added, and the mixture was transferred to a reaction flask, which was sealed with nitrogen and maintained at 50 °C for 12 h. This process led to the formation of graphene oxide-reinforced lignin-based hydrogel (LHGO). The hydrogel was subsequently immersed in deionized water for 72 h and freeze-dried. Lignin-based carbon aerogel (LCAGO) was obtained after annealing in a tube furnace under a nitrogen atmosphere, with a heating rate of 5 °C/min at 350 °C for 2 h (Figure 10) [88]. FTIR analysis revealed that, after annealing, the hydrophilic groups underwent rearrangement into aromatic and aliphatic carbon chains, transforming from hydrophilic to hydrophobic. The rough surface of the annealed LCAGO is suitable for hydrophobic applications with water contact angles up to 150°. The authors then performed water–oil separation tests, and the data showed that LCAGO exhibited moderate adsorption capacities of 32.5 g/g and 34 g/g for light and heavy oils, respectively, when compared to previous reports.

Carbon aerogel has a high specific surface area due to its unique porous structure, significantly enhancing its effectiveness in adsorption processes [89]. At present, the carbon aerogel prepared by researchers using lignin as raw material has been successfully applied to the adsorption of CO₂ [86], organic dyes [87], and oil–water separation [88]. However, the existing carbon aerogels are primarily composites, and the creation of single-lignin-based carbon aerogels with a uniform pore size distribution and interconnected pores remains challenging. In addition, the preparation of lignin-based carbon aerogels mainly consists of three steps: preparation of precursors, drying, and carbonization. Initially, the molecules are polymerized to form a three-dimensional mesh structure by the addition of a cross-linking agent. Then, the precursor undergoes drying to remove the solvent, a critical step that significantly impacts the final performance of the carbon aerogel, necessitating a drying process that preserves the pore structure of the material. Finally, carbonization is carried out at high temperatures under inert gas to improve the mechanical properties of the carbon aerogels. However, the current process for preparing carbon aerogels is more expensive and not suitable for large-scale production, so further efforts by researchers are needed to reduce the cost [90].

2.4. Carbon Foam

Carbon foam is a three-dimensional porous material with excellent physical and chemical properties, including high porosity, ultra-lightweight nature, and good thermal stability. These qualities render carbon foam a high-performance material with pivotal applications in the field of adsorption and separation [91,92]. Carbon foams can be prepared from a variety of sources, including graphite, organic polymers, intermediate-phase bitumen, biomass, etc. Among these, lignin has received much attention as a precursor for carbon foam preparation because of its low cost and environmental friendliness. The main methods for the preparation of lignin-based carbon foams include an ultrasonic method, template method, and radiation method.

In 2017, Qu et al. added lignin and phenol into a NaOH solution, stirring the mixture for 20 min. Subsequently, they added a formalin solution and allowed it to react at 71 °C for 1 h to synthesize phenolic resin (LPF). The templates were obtained by placing polyurethane (PU) foam in NaOH solution at 50 °C, allowing it to react for 1 h. The templates were then used in the subsequent reaction. The LPF was applied to the PU foam, with frequent squeezing applied to remove air bubbles, followed by curing at 180 °C for 80 min. The resulting lignin-based carbon foam (PU/LPF) was then calcined at 950 °C for 2 h under a nitrogen atmosphere, with a heating rate of 2 °C/min, to remove the template [93]. A large amount of phenol is present in conventional phenolic resins. However, the authors ingeniously substituted a portion of the phenol with grass lignin, thereby reducing the potential threat to human health. The synthesized carbon foams exhibited exceptional hydrophobicity, with a water contact angle of 149°. Subsequently, the authors tested the oil–water separation performance of the material, revealing rapid adsorption rates with complete adsorption achieved in about 20 s. The adsorbed oil or solvent could be removed directly by combustion without damaging its structure. Even after 10 cycles of testing, the adsorption rate was maintained above 83%.

In 2021, Vannarath et al. mixed lignin extracted from betel seed shells with ZnNO₃·6H₂O and starch, which transformed into a gelatinous liquid upon heating at 180 °C, followed by the emergence of a yellowish-brown foam, which ultimately solidified into black foamy material as lignin-based carbon foam (LCF) (Figure 11) [94]. The LCF exhibited a rough surface and a highly porous structure with a maximum water contact angle of 132°, resulting in excellent hydrophobicity. The LCF demonstrated remarkable thermal stability, proving resistant to ignition even at 1100 °C. These excellent properties indicate that LCF is very suitable for the application of oil–water separation. In the adsorption tests on various oils, the LCF material showed excellent adsorption capacity. The adsorption rate increased rapidly within 30 min, reaching the adsorption equilibrium in 180 min. Notably, the optimal adsorption amount was 7842.71 mg/g for diesel oil and 1127.58 mg/g for sunflower seed oil. The adsorbates were removed by direct combustion and the adsorption rate still remained 93.7% after five cycles. The adsorption process conforms to the quasi-secondary kinetics as a chemisorption process. There are abundant carboxyl and hydroxyl functional groups on the surface of the LCF material, while the oil consists of a mixture of hydrocarbons. Adsorption occurs due to the interaction of hydrogen bonds between these components.

In 2022, Xu et al. prepared a foaming precursor by mixing phenolic resin, surfactant, and ZnCl₂ in deionized water and subjecting it to strong extrusion for 1 h at room temperature. Then, the precursor was heated in a muffle furnace to 140–180 °C at a heating rate of 5 °C/min for 1 h to induce foaming, followed by further heating to 600 °C at a rate of 2 °C/min for 2 h. After cooling down to room temperature, the material was washed with hydrochloric acid and deionized water until a neutral pH was achieved, resulting in lignin-based carbon foams [95]. The authors first examined various types of surfactants and found Tween-80 to be the most effective, with the compressive strength of the material being only 0.4 MPa without the addition of any surfactant and increasing to 2.23 MPa with the addition of Tween-80. In addition, surfactants greatly enhanced the porosity and reduced the density of the material. Upon heat treatment, the material remained stable even at temperatures up to 1500 °C, exhibiting a water contact angle of 115° and an oil contact angle of zero. It demonstrated excellent oil–water separation capabilities, rapidly and effectively separating n-hexane and water within seconds and retaining 91% of the adsorption performance after 20 cycles. This performance is comparable to carbon foams derived from fossil resources.

In 2023, Bai et al. immersed wood blocks in a deep eutectic solvent (DES) consisting of ZnCl₂ and lactic acid, which was heated up to 90 °C for 3 h. Then, under an argon atmosphere, the temperature was elevated to 500–900 °C and the material was heated in a tube furnace for 2 h to produce lignin-based carbon foams (DW) (Figure 12) [96]. ZnCl₂ was not removed from the DES, with ZnO-deposited foam carbon formed directly in situ. The deposition of ZnO greatly increased the specific surface area of the material, reaching a maximum of 446 m²/g when the carbonization temperature was 800 °C (DW-800). The deposition of ZnO also improved the roughness of the material, resulting in a water contact angle of 133°. Subsequently, the authors tested the adsorption performance of the material, finding that the adsorption capacity of CO₂ was 3.03 mmol/g at 25 °C and 1 bar. The material could be regenerated by heating it to 80 °C for 2 h, and the adsorption performance of the material remained relatively stable even after eight cycles of adsorption. In addition, the authors examined the oil–water separation properties of the material, reporting adsorption capacities of 95 g/g and 157 g/g for soybean oil and chloroform, respectively.

In the same year, Li et al. added polyvinylpyrrolidone (PVP) to degraded lignin, and the mixture was stirred for 10 min and dried under vacuum at 80 °C for 12 h. Subsequently, the material was heated in a tube furnace for 2 h at 800 °C under a nitrogen atmosphere, with a heating rate of 5 °C/min. After cooling to room temperature, the material was soaked in 3 M hydrochloric acid solution overnight, washed with deionized water, and dried under vacuum at 80 °C. (Figure 13) [97]. Using undegraded lignin as a control, the authors discovered that the material prepared using degraded lignin possessed a larger specific surface area and a hierarchical porous structure, which provided more active sites for adsorption. The prepared material exhibited excellent adsorption performance (30.2 mg/g) and retained 94.4% of the adsorption capacity after 30 electrosorption cycles. The incorporation of PVP played a crucial role in altering the structure of the material, acting as a network template that facilitated the distribution of depolymerized lignin. The reintegration of depolymerized lignin fragments with different structures during the carbonization process readily formed a pore-rich carbon material.

Lignin-based prepared carbon foams have significant pores, which enhance their specific surface area and the adsorption properties [98]. Currently, these materials have been successfully applied in various fields, including CO₂ adsorption [96] and oil–water separation [93,94,95,96]. In oil–water separation, lignin-based carbon foams exhibit excellent oil adsorption properties, and the sorbent can be recovered and reused through direct combustion. In addition, carbon foam is easily obtainable and highly favorable for large-scale oil spill applications.

2.5. Carbon Fiber

Carbon fiber has the advantages of a high specific surface area, low density, lightweight, high strength, corrosion resistance, excellent electrical conductivity, etc. The current preparation of carbon fiber primarily relies on expensive raw materials such as polyacrylonitrile, asphalt, and viscose, which limits its widespread application. Therefore, researchers are focused on finding inexpensive and renewable raw materials [99,100]. Lignin-based carbon fibers can be produced by electrostatic spinning under certain conditions, offering advantages such as high plasma carbon content, low cost, and renewability. Lignin is considered a viable feedstock for carbon fiber production and shows great potential in the field of adsorption and separation.

In 2017, Song et al. prepared a polyvinyl alcohol (PVA) solution by mixing lignin and acetic acid to obtain a lignin solution. This solution was stirred at 60 °C for 15 min and then allowed to stir at room temperature for 2 h [101]. The PVA solution and lignin solution were combined and subjected to sonication for 1 h. After cooling to room temperature, lignin fibers (LFs) were produced by electrostatic spinning. As a control, lignin fibers (LF-Fe) with a Fe₃O₄ catalyst were synthesized by incorporating the catalyst during the mixing of the PVA and lignin solutions. The resulting material was then placed in a tube furnace and heated to 200 °C for 36 h at a heating rate of 0.5 °C/min, followed by heating to 600 °C for 90 min under a nitrogen atmosphere, with a heating rate of 10 °C/min. LCFs and LCFs-Fe were obtained by cooling to room temperature. The surface area of the micropores was determined using the Brunauer-Emmett-Teller (BET) analysis, revealing that LCF-Fe had a significantly higher specific surface area and microporous volume, making it more favorable for adsorption compared to LCF without the added Fe₃O₄ catalyst. The LCFs-Fe exhibited an optimal adsorption capacity of 439 mg/g for toluene at room temperature (20 °C), which is comparable to commercially available activated carbon. The isothermal data were better described by the D-R equation and the effect of humidity change on adsorption was investigated, showing a significant reduction when the humidity reached 80%.

In 2019, Song et al. added alkali lignin to polyvinyl alcohol (PVA) and acetic acid solution and stirred the mixture at 80 °C for 30 min. Subsequently, they utilized electrostatic spinning to produce lignin-based fibers (LFs). These fibers were heat-stabilized in a tube furnace at an airflow rate of 150 mL/min with heating to 200 °C for 3 h. The authors then used three types of activation methods. The physical activation method involves heating the LF to 600–1000 °C and carbonizing them for 1 h under a nitrogen flow rate of 150 mL/min, which was then activated at 800 °C for 30 min to obtain ACFS. the chemical activation method involved uniformly mixing LF and KOH and then performing the same carbonization step to obtain ACFK. The metal activation method involves the addition of different metal nitrates to the spinning solution after the same carbonization step (iron nitrate ACFF, zinc nitrate ACFZ, nickel nitrate ACFN) (Figure 14) [102]. During the carbonization process, the specific surface area of each material reaches its optimum at 800 °C. Beyond this temperature, the internal pore structure of the material will be destroyed. Consequently, the authors chose 800 °C as the carbonization temperature. Among the three activation methods, ACFK, prepared by the chemical activation method, exhibited an excellent specific surface area of 1147.16 m²/g and an optimal toluene adsorption capacity of 463 m²/g. In addition, an investigation into the dynamic adsorption properties of ACFK revealed that the lignin-based carbon fibers prepared by the chemical activation method performed exceptionally well in both static and dynamic adsorption of toluene.

In 2020, Nordin et al. prepared a PAN solution by dissolving polyacrylonitrile (PAN) in DMF and produced PAN fibers by electrostatic spinning. PAN and sago lignins (SLs) were mixed and uniformly dissolved in DMF, and PAN/SL fibers were obtained by electrostatic spinning. The fibers were heat stabilized by exposing them to an air atmosphere at 250 °C for 1 h. Following this, the samples were carbonized in a tube furnace at 1000 °C for 1 h under a nitrogen atmosphere, resulting in the formation of PANCNF and PAN/SLCNF. The PAN/SLCNF was then added to nitric acid and allowed to react at 120 °C for 48 h. After cooling to room temperature, the material was washed with deionized water until it reached neutrality and subsequently dried at 100 °C overnight to obtain PAN/SLCNFs [103]. Following nitric acid activation, the surface area of the material appeared to be significantly reduced and the pore size broadened. However, the surface of the material was enriched with more oxygen-containing functional groups. Consequently, the adsorption capacity for Pb(II) ions was three times higher than that before activation. The rate of Pb(II) ion adsorption by the material increased significantly within 2 h and reached equilibrium at 4 h, with an optimal adsorption amount of 524 mg/g. The adsorption kinetics conformed to the proposed secondary kinetic model, and the Langmuir model provided a better description of the adsorption process, with a maximum adsorption capacity of 588.24 mg/g, which was classified as chemisorption.

In 2021, Chen et al. prepared a solution by adding polyethylene oxide (PEO) to DMF and stirring it thoroughly at 80 °C. Subsequently, bio-cleaned lignin was added and stirred for 12 h. After cooling to room temperature, bio-clean lignin fibers were produced by electrostatic spinning. The samples were thermally stabilized by heating them to 250 °C in a tube furnace for 1 h under an air atmosphere with a temperature increase rate of 0.5 °C/min. Following this, the samples were then heated for 1 h in a tube furnace. After cooling to room temperature, they were heated again to 250 °C under an argon atmosphere at the same heating rate and continued to be heated to 1000 °C for 1 h at a heating rate of 5 °C/min. After carbonization, the samples were cooled to room temperature to obtain bio-clean lignin-based carbon fibers (Bio-KLB-C) (Figure 14) [104]. After carbonization, the fiber diameter decreased and the porosity increased, enhancing the adsorption surface area and providing sufficient reaction sites for methylene blue (MB) adsorption. The dynamic adsorption of the material reached equilibrium after 3 h. The adsorption kinetics were more in line with the proposed one-level kinetic model, and physical adsorption dominated the adsorption process, which was better described by the Langmuir model with a maximum adsorption capacity of 548.65 mg/g. The authors then performed adsorption cycling tests, which revealed a reduction in adsorption performance to 75% after five cycles.

Figure 14. Flow chart of the process for the preparation of lignin carbon fibers by electrostatic spinning and a schematic diagram of the adsorption of methylene blue [104].

Figure 14. Flow chart of the process for the preparation of lignin carbon fibers by electrostatic spinning and a schematic diagram of the adsorption of methylene blue [104].

Carbon fibers, distinguished by their unique fiber morphology and higher specific surface area compared to activated carbon, have received much attention in the field of adsorption [105]. Currently, lignin-based carbon fibers have demonstrated excellent adsorption performance in applications such as VOCs adsorption [101,102], heavy metal ion adsorption [103], and organic dye adsorption [104]. However, the preparation of lignin-based carbon fibers typically involves spinning technology, and the complexity and variability of lignin’s structure, along with the significant impact of carbon fiber diameter on specific surface area and adsorption performance, underscore the importance of selecting suitable lignin precursors and meticulously controlling spinning parameters. We have summarized the material covered in this article and plotted the tables in the order of the articles (

Table 2. Summary of the carbon materials.

Raw Material	Preparation Method	Carbon Material	Adsorption Target	Adsorption Capacity	Cycle Times	Ref.
Native lignin	Chemical solution combustion method	Porous carbon	Pb(II), Cd(II)	250.5 mg/g, 126.4 mg/g	5	[66]
Native lignin	Hydrothermal, chemical activation	Porous carbon	Methylene blue, Malachite, Rhodamine B	1119.18 mg/g, 2467.30 mg/g, 1657.82 mg/g	4	[70]
Corn stover lignin	Template method	Porous carbon	Methylene blue	420.40 mg/g	3	[71]
Industrial alkali lignin	Chemical activation	Porous carbon	Cr(VI)	887.8 mg/g	12	[72]
Black liquid lignin	Chemical activation	Porous carbon	CO2	5.20 mmol/g	5	[73]
Graft copolymerization lignin	Chemical activation	Porous carbon	I2	2340 mg/g, 354 mg/g	5	[74]
Corncob residue lignin	Chemical activation	Activated carbon	Cu(II)	49.21 mg/g	-	[77]
Black liquid lignin	Chemical activation	Activated carbon	Methylene blue	307.2 mg/g	5	[78]
Corn stover lignin	Chemical activation	Activated carbon	Phenol	612 mg/g	-	[79]
Coconut shell, Alkali lignin	Physical activation, Chemical activation	Activated carbon	Phenol	removal rate 98.22%	3	[80]
Sulfated lignin	Microwave-assisted heat treatment, chemical activation	Activated carbon	Methylene blue, lime 7	543.82 mg/g, 548.54 mg/g	5	[81]
Sulfated lignin	Ice template method	Carbon aerogels	CO2	5.23 mmol/g	-	[86]
Sodium lignosulfonate	Chemical activation	Carbon aerogels	Methylene blue	421.94 mg/g	-	[87]
Alkali lignin	Physical activation	Carbon aerogels	Light oil, heavy oil	32.5 g/g, 34 g/g	-	[88]
Sedge grass lignin	Template method	Carbon foam	oil–water separation	about 20 s	10	[93]
Betel nut shell lignin	Template method	Carbon foam	Diesel oil, sunflower oil	7842.71 mg/g, 1127.58 mg/g	5	[94]
hardwood lignin	Self-bubbling method	Carbon foam	Hexane/water	seconds	20	[95]
Balsa lignin	Deep eutectic solvent assisted method	Carbon foam	CO2, light oil, heavy oil	3.03 mmol/g, 95 g/g, 157 g/g	8	[96]
Degradation of lignin	Depolymerization and reorganization	Carbon foam	NaCl	30.2 mg/g	30	[97]
Alkali lignin	Electrostatic spinning	Carbon fiber	Toluene	439 mg/g	-	[101]
Alkali lignin	Electrostatic spinning, chemical activation	Carbon fiber	Toluene	463 mg/g	-	[102]
Sago lignin	Electrostatic spinning	Carbon fiber	Pb(II)	588.24 mg/g	-	[103]
Bioclean Lignin	Electrostatic spinning	Carbon fiber	Methylene blue	548.65 mg/g	5	[104]

).

3. Conclusions and Prospects

This paper reviews the application of lignin-derived carbon materials in the field of adsorption and separation. Lignin-based carbon materials, characterized by their exceptional specific surface area and pore structure, have garnered considerable attention due to their environmental friendliness, renewability, and cost-effectiveness, aligning with the current focus on sustainable human development. In this paper, we categorized the carbon materials based on their types, selected representative examples applied in the adsorption and separation since the last decade, and described the preparation methods and adsorption effects of lignin-based carbon materials. While lignin-based carbon materials have achieved promising results in adsorption and separation, there are still several challenges that remain. (1) The diversity of lignin structures necessitates tailored preparation methods for converting them into carbon materials, posing the question of whether a universal method can be devised. (2) The high-temperature and sometimes alkaline conditions used in the preparation of lignin-based carbon materials are environmentally unfriendly and pose safety risks at large scales, necessitating the urgent development of gentler methods. (3) Adsorption experiments conducted under controlled laboratory conditions may not fully reflect real-world scenarios, where wastewater contains multiple contaminants. Therefore, there is a need to develop materials that can effectively adsorb a wide range of pollutants under practical conditions. As a result, this crucial field remains in its early stages of development, and there is an urgent necessity for additional research to address the aforementioned challenges.