Simple Preparation of Lignin-Based Phenolic Resin Carbon and Its Efficient Adsorption of Congo Red

Abstract

Biomass porous carbon is a low-cost, environmentally friendly material with no secondary pollution and has great potential in the field of dye pollutant adsorption. In this work, we used lignin, a renewable resource abundant in nature, to completely replace phenol and develop a lignin-based phenolic resin carbon (LPFC) adsorbent with high dye removal capacity, high recyclability, and low production cost. The samples were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. Then the effects of adsorbent dosage (1 g/L, 2 g/L, 3 g/L, 4 g/L, and 5 g/L), temperature (30 °C, 45 °C, and 60 °C), initial dye concentration (100, 200, 300, 400, 500, 600, 700, and 800 mg/L), and pH (3, 4, 6, 8, 10, and 12) on the adsorption capacity were investigated during the adsorption process. The experimental results showed that the pore structure of LPFC was richer and more graphitized than that of phenolic resin carbon (PFC). The adsorption performance of LPFC on CR was better than that of PFC. The adsorption characteristics of LPFC were investigated from the adsorption isotherm and kinetic perspectives. The Langmuir isothermal adsorption model and the proposed second-order kinetic model were able to fit the adsorption data better. The adsorption process preferred monolayer adsorption, and the proposed second-order model predicted a maximum adsorption capacity of 425.53 mg/g. After five cycles, the removal of CR by LPFC only decreased from 92.1 to 79.2%. It can be seen that LPFC adsorbents have great potential in the field of wastewater treatment and can effectively realize the high-value application of lignin.

1. Introduction

At present, with the rapid development of industrialization and urbanization, water pollution problems are becoming more and more serious [1]. There has been an exponential increase in the release of harmful pollutants into the water from industries such as paper [2], printing [3], plastics [4], and dyes [5], particularly from the textile industry [6]. About 10–25% of water-soluble dyestuffs are lost during the dyeing process in the textile industry, generating large amounts of wastewater, and 2–20% of the wastewater is discharged into the water, causing serious water pollution [7]. Dyes are organic molecules with complex structures that are chemically stable and toxic [8,9]. Dye wastewater also contains large amounts of sulfur, chloride, bromine, metal ions, inorganic salts, and organic salts and has a high chemical oxygen demand, which makes subsequent degradation and treatment extremely difficult [10]. Many dyes can cause allergies, cancer, and mutations in living organisms, posing a serious threat to biological health [11]. Dyes have complex molecular structures that are difficult to degrade in the natural environment and can pose a serious risk to the environment [12]. Congo red (CR), one of the common dyes, is a structurally complex anionic dye containing a biphenyl group and two naphthalene units that are difficult to degrade by adsorption [13]. It is more representative of organic dyes. It is extremely hazardous, preventing water from re-oxidizing, and contact with eyes and skin can cause irritation, stomach pain, nausea, vomiting, and diarrhea [14]. Therefore, there is an urgent need to purify CR dye wastewater in order to protect water resources. The common purification methods for dye wastewater include coagulation [15], membrane separation [16], catalytic degradation [17], flocculation [18], adsorption [19,20], and photocatalytic degradation [21,22]. The adsorption method uses the adsorbent’s porous structure or special functional groups to remove the dye from the wastewater by adsorbing it onto the adsorbent [23]. The adsorption method is considered a very promising technology for water purification because of its low expense, high practicality, and good recovery performance. The exploration of efficient adsorbent materials has been the focus of research in recent years. The literature has reported the application of carbon nanotubes, biochar, clay, agricultural wastes, composite nanomaterials, and metal-organic skeletons in the treatment of polluted wastewater [24,25,26].

Porous carbon materials have a highly developed pore structure [27], a large specific surface area [28], strong mechanical properties, high chemical stability, and many excellent physical properties [29,30], such as electrical conductivity, thermal conductivity, and thermal stability. Porous carbon materials can adsorb dyes efficiently as well as play a catalytic function in the degradation of dyes [31], which is a kind of adsorbent material that can be widely used in dye wastewater purification and has become popular among researchers in recent years [32,33]. The adsorption effect of porous carbon materials on dyes is mainly influenced by their pore structure and surface heteroatoms. Under the premise of selecting suitable precursors, the pore structure can be effectively regulated by adjusting the temperature, time, and activator ratio during the activation process [34]. Biomass-derived carbon materials use biomass as a precursor to value, utilize low-cost, renewable biomass resources, and have a unique porous structure [35]. Lignin is a naturally abundant biomass resource [36] and is considered an ideal carbon source for building porous carbon. It is inexpensive, widely available, contains up to 60% carbon, has high aromaticity, a high calorific value, and abundant active sites [37,38]. Nowadays, lignin-based porous carbon adsorbents have achieved remarkable results in the field of wastewater purification [39,40], but there are still some problems, such as the fact that the pore structure is not prominent and the production process is complex and requires a large amount of activator, which is not good for low-cost, sustainable, and large-scale industrial production [41].

In recent years, the application of phenolic resin carbon materials for dye adsorption in industrial wastewater has also received much attention [42,43]. Wanassi et al. [44] successfully synthesized a composite carbon material using phenolic resin and waste cotton fibers as precursors and applied the material to remove Alizarin red from wastewater. Li et al. [45] used phenolic resin and triisopropyl borate as precursors and pluronic F127 as a soft templating agent to synthesize a porous adsorbent with a significant adsorption effect on crystalline violet and CR. In order to examine the impact of pore structure on dye adsorption and the impact of various environments on the adsorption performance of porous carbon, Ariyanto et al. [46] employed phenolic resin as the starting point and frequently used a pyrolysis process to synthesize porous carbon. Wu et al. [47] synthesized graded porous carbon materials containing metal ions using phenolic resin with FeCl₂ as a raw material. The adsorption capacity of the material for methylene orange can reach 175.91 mg/g, which has good adsorption performance. Liu et al. [48] combined the excellent properties of chitosan and phenolic resin to prepare a composite porous carbon adsorbent by hydrothermal carbonization and chemical activation. The adsorbent has good regenerative properties and can effectively adsorb phenolic pollutants. From these studies, it is clear that phenolic resin carbon materials have great potential for development in the field of dye wastewater purification. However, the phenolic resins used in most of the studies still need to be prepared from the petroleum product phenol. This does not make the product environmentally friendly. It cannot alleviate the problem of energy shortages. Additionally, the adsorption effect of most adsorbents is not significant. The adsorption of anionic dyes such as CR by phenolic resin porous carbon is not abundantly studied. Zong et al. [49] generated high-activity and petal-like Zn-Al layered double hydroxide/multiwalled carbon nanotubes supported by sodium dodecyl sulfonate (Zn-Al LDH/SDS/MWCNTs) with a maximum adsorption of CR of 1.42 × 10⁻³ mol/g, which is greatly superior to other composites reported so far. It is shown that the adsorption performance of Zn-Al-LDH/SDS/MWCNTs composites for CR is to some extent related to surface complexation, ionic changes, π-π interactions, electrostatic interactions, and hydrogen bonding. Although the method is more complicated, it provides a strong basis for our subsequent study on the interaction mechanisms of other adsorbents with CR so as to improve its adsorption capacity.

Based on this, in this study, lignin-based phenolic resin (LPF) was synthesized by using alkali lignin as a raw material instead of phenol, a petroleum product, and reacted with formaldehyde to prepare lignin-based phenolic resin carbon (LPFC) composites by chemical activation and pyrolysis methods to explore a green, efficient, and easy-to-produce dye adsorbent. The LPFC adsorbent was then used in simulated CR wastewater to examine how the adsorption capacity of LPFC on CR changed under various adsorbent mass, pH, and temperature conditions and to optimize the adsorption conditions to get the best adsorption effect. The obtained experimental data were analyzed by thermodynamic and kinetic models to further explore the mechanism of this adsorption process. In this study, phenol in phenolic resins is completely replaced by lignin, which can reduce the use of the toxic chemical phenol and alleviate petroleum energy while making the production process safer. The phenolic resin prepared from lignin was chosen as the carbon source, which improved the adsorption capacity and recycling performance compared to the activated carbon adsorbent prepared from conventional phenolic resin.

2. Materials and Chemicals

2.1. Materials

Alkali lignin (AR) was purchased from UPM (UPM-Kymmene Corporation, Helsinki, Finland). Hydrochloric acid (AR), formaldehyde (AR), methanol (AR), and potassium hydroxide (AR) were from Nanjing Maclean’s Reagent Company, Nanjing, China, and the laboratory water was ultrapure water.

2.2. Methods

Take 12.5 g of alkali lignin, add to a 250 mL reactor, place in an oil bath at 50 °C for heating and refluxing, and keep stirring. Then add 38.8 mL of a 37% formaldehyde aqueous solution to the reactor, and add 2 mL of hydrochloric acid as a catalyst. After the temperature is raised to 85 °C, add 2 mL of hydrochloric acid and react for about 6–7 h. When the endpoint of the reaction is reached, it is obvious that the reactant has changed from a liquid to a solid. After the reaction reaches its endpoint, take them out. First, pre-dry them in a 100 °C oven for 12 h, and then dry them in a 170 °C vacuum oven for 6 h to prepare lignin-based phenolic resin. The dried LPFC is then heated in a tube furnace with nitrogen gas at a rate of 5 °C/min while remaining at 800 °C for 2 h. Finally, the carbon material obtained after pre-carbonization is mixed with KOH in a 1:3 mass ratio. Then carry out secondary carbonization, activate the mixture in a tubular furnace from room temperature to 850 °C at a rate of 10 °C/min under nitrogen flow, and conduct pyrolysis activation at 850 °C for 2 h. The obtained material was washed with 10% HCl, distilled to neutrality, and finally dried at 60 °C for 8 h to prepare LPFC. Under the same preparation conditions, pure PFC was prepared by replacing alkali lignin with phenol. See Supplementary Materials for the experimenta sectionl part of characterization and adsorption.

3. Results

3.1. Characterization of LPFC

Scanning electron microscopy (SEM) was used to analyze the morphology and microstructure of PFC, LPFC, and LPFC materials with adsorbed CR, as can be seen in Figure 1. From Figure 2a,b, it is visible that the surface structure of PFC consists of stacked fragments of different sizes. Figure 2c–g shows that the surface of LPFC then appears to have more obvious pores and voids; pores with a larger pore size appear concentrated; many small pores are located on both sides and bottom of these large pores in a scattered arrangement; and pores with smaller pores also appear densely arranged, which may be due to the addition of lignin. For porous carbon adsorbents, the size of the specific surface area and the structure and size of the pores are the key factors determining their adsorption capacity. Porous carbon materials with a larger pore size and a larger specific surface area tend to have a more significant adsorption effect [50]. Therefore, the structure of LPFC may be more favorable for the adsorption of dyes. Figure 2h,i shows clearly that CR covers the pores of LPFC materials, and the presence of CR makes the surface of LPFC more rough, sparse, and dispersed. The elemental composition of LPFC was quantitatively analyzed using Energy Dispersive Spectroscopy (EDS). The EDS element images of LPFC (Figure 2j,k) clearly show that C and O elements occupy the surface of LPFC, and the C element occupies a very large area. From the EDS analysis data, it can be seen that the LPFC consists of 83.61% C and 16.39% O elements.

The degree of graphitization of the LPEC material was characterized by Raman tests, and the results are shown in Figure 3. The Raman spectra have two clearly distinguishable peaks at 1300 cm⁻¹ and 1600 cm⁻¹, called D-peak and G-peak, which are indicated as the disordered state and graphitized structure of LPFC and PFC, respectively [51]. The ID/IG values of LPRC and LPR were 1.4 and 1.62, respectively, by fitting the Raman data to the analysis. It shows that the addition of lignin increases the graphitization of carbon materials. Making the graphitization of the carbon material adsorbent increase in a certain range is beneficial to enhance its chemical stability and thus its performance in recycling during adsorption. In fact, increasing the pores or defects of the material can provide more adsorption and active sites during the adsorption of dyes, thus improving the adsorption capacity of the adsorbent.

The FT-IR spectra of LPFC, PFC, CR, and LPFC with adsorbed CR (LPFC-CR) were characterized as shown in Figure 3 to compare the differences in the structures of LPFC and PFC as well as the modifications of LPFC both before and after adsorption of CR. The results indicated that the FT-IR spectra of LPFC and PFC were approximately the same. For example, the peak near 3435 cm⁻¹ corresponds to the characteristic phenol-OH- or alcohol-OH stretching vibration, the peak at 2901 cm⁻¹ corresponds to the characteristic aliphatic-CH₂ vibration, and the C-O group vibration is between 500 and 800 cm⁻¹ [52,53]. The peak at 1629 cm⁻¹ corresponds to the aromatic ring C=C stretching vibration, and the peak at 1077 cm⁻¹ is related to the bicyclic aromatic ring bending vibration or C-C bond bending vibration, which indicates that the sample is aromatic [54]. In contrast to the FT-IR spectrogram of PFC, the peaks of phenolic -OH and alcohol -OH, and the peak of aliphatic -CH₂ are attenuated and shifted to higher wavelengths in LPFC, which may be influenced by the cross-linking between the alkylmethoxyphenol structure in lignin and its derivatives. As shown in the IR spectra of CR and LPFC before and after adsorption in Figure 4, several distinct absorption characteristic peaks were still observed after CR adsorption by LPFC, similar to those before adsorption. However, the presence of CR resulted in a weakening of the aromatic ring’s C=C vibration at 1579 cm⁻¹, a slight reduction of the peak in the frequency band of 900–1200 cm⁻¹, a broadening of the peak’s width, a shift of the peak’s position to a longer wavelength, and a smaller absorption peak appearing at 1045 cm⁻¹, which could confirm the adsorption of CR by LPFC.

3.2. Removal of Organic Dyes

The effects of adsorption time, temperature, dye solution pH, and initial concentration of the solution on the adsorption of CR by LPFC were investigated in this experiment using the single variable method to compare the adsorption performance of two adsorbents, LPFC and PFC, and to identify the best adsorption conditions for LPFC.

Firstly, we investigated the effect of different adsorbent dosages on the adsorption performance of LPFC and RFC. LPFC and PFC adsorbents (10 mg, 20 mg, 30 mg, 40 mg, 50 mg, and 60 mg) were accurately weighed and placed in 100 mL of a 100 mg/L concentration of CR solution and left to adsorb for 2 h at 25 °C. The adsorption results are shown in Figure 5. In the adsorption mass range of 5–10 mg, the adsorption capacity shows an increasing trend, which is due to the increase in surface area and number of active sites with the increase in adsorbent dosage. The maximum adsorption capacities of LPFC and PFC for CR were 306.06 and 159.37 mg/g, respectively, at which time the masses of the adsorbents added were both 10 mg. Subsequently, the capacity of the adsorbents decreased as the masses of the adsorbents increased, and the adsorption of CR leveled off when the amount of adsorbent input was greater than 30 mg. This is because more adsorbent can be used to provide more adsorption sites during the adsorption of the dye, increasing the removal rate. However, because the initial concentration of the dye is constant, the dye concentration decreases as the removal rate increases, lowering the dye’s adsorption capacity per mass of adsorbent. This is consistent with the conclusion presented in the report by Wang et al. [45] that the relative adsorption rate decreases when the number of adsorbents increases, leading to a decrease in the adsorption capacity. It can be determined that the adsorption capacity of LPFC is better than that of PFC. This may be mainly attributed to the fact that the LPFC surface has a large number of pore structures (Figure 2), which can provide more adsorption sites for CR molecules [55]. The pore size of LPFC may be more suitable for the size of CR molecules. Whether the pore structure of the surface in porous carbon materials can provide enough adsorption sites for the adsorbent is the key to the strength of the adsorption capacity. It is worth noting that the reactive groups in lignin, such as phenolic hydroxyl and carboxyl groups, can act as active centers for dye adsorption, thus improving the adsorption capacity of the samples, which may also be the reason for the superior adsorption capacity of LPFC.

In order to optimize the adsorption conditions and lead to a better adsorption effect, the effect of pH on the adsorption performance continued to be investigated. A total of 10 mg of adsorbent was placed in a solution containing 100 mL of CR at a concentration of 100 mg/L, so that the pH of the dye solution was 3, 4, 6, 8, 10, and 12, respectively, and the adsorption temperature was 25 °C and the adsorption time was 2 h. The adsorption results are shown in Figure 6, and the adsorption amount of CR reached its maximum at a pH of six when the pH was increased from 3 to 12. The maximum adsorption capacity of LPFC for CR was 335.68 mg/g, and that of PFC for CR was 142.75 mg/g. The adsorption capacity increased gradually when the pH value varied from 3 to 6, and the increasing trend of the adsorption capacity was more significant the closer the pH value was to six. In the molecular structure of the LPFC, elements like ionizable groups and charge density may have an impact on how well the adsorption performs in response to pH. The sulfonic acid group of CR undergoes protonation at pH values below six, which reduces the negative charge of the anionic group in these anionic dyes. This has an impact on the electrostatic adsorption of the dyes by LPFC. Additionally, the anionic groups of dyes are hindered from adhering to LPFC by the electrostatic adsorption of OH⁻ under alkaline circumstances [56]. This suggests that the initial pH of the dye can determine the electrostatic or molecular interaction between the adsorbent and the adsorbate due to the charge distribution and has a crucial influence on the final adsorption effect.

One of the key elements influencing the adsorption effect is the dye’s initial concentration. We also investigated the impact of various initial dye concentrations on the adsorption effect to determine the ideal adsorption circumstances. Different initial concentrations of CR dye solutions (100, 200, 300, 400, 500, 600, 700, and 800 mg/L) were prepared, and 10 mg of LPFC and PFC adsorbent were added to them for 2 h each. Other influencing factors, such as time, temperature, and pH, were kept consistent. The outcomes are displayed in Figure 7. Other influencing factors, such as time, temperature, and pH, were kept consistent. The results are shown in Figure 7. The adsorption capacities of LPFC and PFC for CR showed an increasing trend with the increase of the initial concentration at the initial concentration of 50–100 mg/L [57]. At lower solution concentrations, the adsorption capacity of CR increased significantly, indicating a high affinity between the CR dye molecules and the adsorbent surface. After the initial concentration of CR solution reached 100 mg/L, the adsorption capacities of the two adsorbents showed a rapidly decreasing trend. The higher the initial concentration of dye, the more significant the adsorption effect, but when the dye concentration is greater than the equilibrium concentration, the total amount of dye is still increasing while the input amount of adsorbent is constant. At this time, the adsorbent has no ability to adsorb the remaining dye, so the unit adsorption of CR by LPFC and PFC will decrease with the initial concentration.

This experiment further investigated the effect of different adsorption times on the adsorption effect and determined the optimal adsorption time. It is obviously seen from Figure 8 that the absorption of CR by LPFC and PFC increases with time, especially at the beginning of 10 min, when the adsorption amount of the adsorbent on CR increased rapidly and the adsorption rate of LPFC was slightly larger than that of PFC. Then the increasing trend tended to level off, and the change curve showed a slow decreasing trend after 60 min. The optimum adsorption time was determined to be 60 min, at which time the adsorption amount of LPFC to CR was 406.35 mg/g, which was still higher than that of PFC.

Based on the aforementioned experimental findings, it is clear that LPFC has a higher adsorption capability than PFC. We further investigated the effect of different temperatures on the adsorption effect of the LPFC adsorbent on the dyestuff and determined its optimal adsorption temperature conditions. The temperature gradients of 30 °C, 45 °C, and 60 °C (i.e., 303 K, 318 K, and 333 K) were set, and 10 mg LPFC was added to 100 mL of CR solution with an initial concentration of 100 mg/L and left to adsorb for 2 h. Figure 9 shows the changes in the adsorption amount of CR by the LPFC adsorbent at different temperatures from 0 to 120 min. At the beginning of the adsorption process, the adsorption amount increased rapidly at different temperatures, and the increase was approximately the same; after 20 min, the effect of different temperatures on the adsorption reaction gradually appeared. The adsorption performance of LPFC was the best at 45 °C, and the adsorption effect of LPFC was poor at 30 °C. The adsorption capacity increased in the range of 30 °C to 45 °C, which may be due to the fact that the process is a heat-absorbing reaction. During this process, the adsorption sites of CR increased, and the molecular mobility of CR increased. [56] The adsorption capacity of the adsorbent for CR decreased with a further increase in temperature, which may be due to the decrease in intermolecular interactions with an increase in temperature.

Thermodynamic parameters, such as a change in the Gibbs free energy ΔG⁰, enthalpy ΔH⁰, and entropy ΔS⁰, were used to investigate the adsorption process of LPFC on CR at different temperatures. The equations are as follows [58]:

$$K^0=\frac{Q_e}{C_e},$$

(1)

$$\Delta G^0=-RT\times ln{K}^0,$$

(2)

$$ln{K}^0=-\frac{\Delta H^0}{RT}+\frac{\Delta S^0}{R},$$

(3)

R—the universal gas constant, 8.314 J/mol·K;

T—the absolute temperature, K;

K⁰—equilibrium constant.

The thermodynamic parameters are shown in

Table 1. Thermodynamic parameters of the adsorption of CR by LPFC.

Samples	$\Delta {\mathit{G}}^0/\left(\mathbf{KJ}·{\mathbf{mol}}^{-1}\right)$			$\Delta {\mathit{H}}^0/\left(\mathbf{KJ}·{\mathbf{mol}}^{-1}\right)$	$\Delta {\mathit{S}}^0/\left(\mathbf{KJ}·{\mathbf{mol}}^{-1}\right)$
	30 °C	45 °C	60 °C
CR	−6.37	−6.67	−7.00	3.00	21.62

. From the table, it can be observed that ΔG⁰ < 0 decreases with increasing temperature, which indicates that the adsorption process proceeds spontaneously and that higher temperatures are favorable to increasing the equilibrium adsorption capacity. This may be due to the fact that at higher temperatures, the heat provided is converted into kinetic energy, which promotes the adsorption of CR molecules with LPFC. ΔG⁰ > 0 indicates an increase in the degree of freedom of the adsorbate and an increase in the disorder at the solid-liquid two-phase interface during the adsorption process. ΔH⁰ > 0 indicates that the adsorption process is an irreversible heat absorption reaction.

3.3. Adsorption Kinetics and Adsorption Isotherms

In adsorption processes involving mass transfer rates and diffusivity of substances, adsorption kinetics is commonly used to investigate the relationship between adsorption capacity and adsorption time. The commonly used models are the proposed first-order kinetic model, the proposed second-order kinetic model, and the intra-particle diffusion model [59,60]. In general, the adsorption process can be divided into three main stages: the film diffusion stage, the intra-particle diffusion order, and the adsorption process of the adsorbed material at the adsorption site of the adsorbent. The adsorption equilibrium will be rapidly established at the adsorption site, and the film diffusion stage and the intra-particle diffusion stage have higher mass transfer resistance and slower rates. The total rate of the adsorption process is determined by the largest and slowest steps. Thus, membrane diffusion, or intra-particle diffusion, mostly determines the control stage of the adsorption process [55]. To better understand the pace of the procedure of adsorption, the time it takes to reach equilibrium, and to investigate the adsorption mechanism of the LPFC adsorbent, the kinetics of the adsorption process were examined in this experiment.

Equations (4) and (5) represent the quasi-primary and quasi-secondary kinetic models, respectively, as follows [59]:

$$\mathit{ln}(q_e-q_t)=\mathit{ln}{q}_e-k_{1}t,$$

(4)

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e},$$

(5)

where q_e—equilibrium adsorption amount, mg/g;

q_t—the adsorption amount per unit mass of adsorbent at any adsorption time t, mg/g;

k₁—adsorption rate constant, min⁻¹;

k₂—adsorption rate constant in this model, g/(mg·min).

The intraparticle diffusion model is mainly used for describing how particulate matter diffuses within a porous medium, and its equation is given in Equation (6) [60]:

$$q_t=k_p{t}^{\frac{1}{2}}+C$$

(6)

where k_p—particle internal diffusion rate constant, g/(mg·min).

The results of the linear fit using the fitted first-order model, the fitted second-order model, and the intragranular diffusion model are shown in Figure 10. The parameters associated with these three kinetic models are listed in

Table 2. Parameters related to the quasi-primary kinetic model and the quasi-secondary kinetic model; and the intra-particle diffusion model for adsorption of CR.

Samples	Quasi-Primary Adsorption Kinetic Model			Quasi-Secondary Adsorption Kinetic Model
	${\mathit{Q}}_1(\mathbf{mg}/\mathbf{g})$	${\mathit{k}}_1\left(\mathbf{min}{-}^1\right)$	R2	${\mathit{Q}}_2(\mathbf{mg}/\mathbf{g})$	${\mathit{k}}_2(\mathbf{g}/(\mathbf{mg}·\mathbf{min}\left)\right)$	R2
CR	185.30	0.049	0.8079	425.53	0.00030	0.9994
Samples	The intra-particle diffusion model
	${\mathit{K}}_{\mathit{p}\mathbf{1}}$ (mg·g−1·min−0.5)		$R_1^2$	${\mathit{K}}_{\mathit{p}\mathbf{2}}$ (mg·g−1·min−0.5)		$R_2^2$
CR	61.52		0.9176	13.59		0.6717

. The R² value obtained in the fitted first-order model was 0.8079, and the fitted second-order R² was 0.9994, and the fitted second-order value was much larger than the fitted first-order. The equilibrium adsorption capacity of CR estimated in the fitted second-order model was 425.53 mg/g, which was closer to the actual experimental results, indicating that the fitted second-order model can better describe the adsorption process. Adsorption is more inclined toward monolayer adsorption, controlled by chemisorption. From the linear fit plot of the intra-particle diffusion model with relevant parameters, it can be seen that the adsorption process is a two-order adsorption process, which is divided into two stages of the adsorption process, i.e., external surface adsorption and intra-particle diffusion. Additionally, the first stage rate constant (K_P1) is greater than the second stage rate constant (K_p2), indicating that the fast adsorption in the first stage is controlled by boundary layer diffusion, while the slow adsorption in the later stage is controlled by intra-particle diffusion [55].

Isothermal adsorption models are often used to study the relationship between adsorbate uptake and the surface of adsorbent materials [61,62]. To investigate the adsorption mechanism of LPFC materials in depth, these four models (Langmuir isothermal adsorption model, Freundlich isothermal adsorption model, Temkin isothermal adsorption model, and Redlich–Peterson isothermal adsorption model) were used in the current study to investigate the thermodynamic parameters of two dyes.

Langmuir isothermal adsorption model equation, as shown in Equation (7) [61]:

$$\frac{C_e}{Q_e}=\frac{1}{K_L{Q}_m}+\frac{C_e}{Q_m},$$

(7)

where C_e—Equilibrium concentration, mg/L;

K_L—Langmuir constant, L/mg;

Q_e—Equilibrium adsorption capacity, mg/g;

Q—Maximum adsorption capacity per unit mass of adsorbent, mg/g.

Freundlich isothermal adsorption model equation, as shown in Equation (8):

$$\ln Q_e=\mathit{ln}{K}_F+\frac{1}{n}\mathit{ln}{C}_e,$$

(8)

where K_F—adsorption equilibrium constant;

n—intensity factor.

The Temkin isothermal adsorption model is a two-parameter adsorption model proposed by Temkin and Pyzhev that considers the effect of adsorbent–adsorbent interaction on the adsorption energy. The model assumes that the adsorption energy decreases linearly, rather than logarithmically, with increasing surface coverage in a moderate concentration range, with the following Equation (9) [63]:

$$q_e=B_t\mathit{ln}\left(K_T\right)+B_t\mathit{ln}\left(C_e\right),$$

(9)

B_t—is the Temkin constant associated with the heat of adsorption.

The Redlich–Peterson isothermal adsorption model, which uses the following Equation (10), assumes a specific adsorption space on the adsorbent surface and a temperature-independent adsorption potential at each location of the adsorption space [62].

$$ln\frac{Ce}{qe}=\beta lnCe-lnA,$$

(10)

β—Redlich–Peterson isotherm index;

A—Redlich–Peterson isotherm constant.

The results of linear fits of Langmuir (Figure 11a), Freundlich (Figure 11b), Temkin (Figure 11c), and Redlich–Peterson models (Figure 11d) for CR adsorption isotherms are shown in Figure 11.

Table 3. Adsorption isotherm model parameters.

Langmuir	CR	Freundlich	CR	Temkin	CR	Redlich–Peterson	CR
Qm (mg/g)	416.67	KF	284.60	Bt	41.84	β	26.18
KL (L/mg)	1.64	1/n	0.13	KT	927.97	A (mg/L)	0.0026
R2	0.9978	R2	0.9202	R2	0.9002	R2	0.9860

displays the relevant parameters of the four adsorption isotherm models for CR dyes. The linear fitting results show that the Langmuir adsorption model of LPFC has a correlation coefficient R² of 0.9978, which is the best fit among these four models; the Redlich–Peterson isotherm adsorption model has a relatively low fit; the Freundlich isotherm adsorption model is the second best fit; and the Temkin isotherm adsorption model is not applicable to this adsorption process. This suggests that the Langmuir model can describe adsorption isotherms with larger R² values and that the adsorption of CR by LPFC occurs as a monolayer on the material surface, which is consistent with the above conclusions. According to the Langmuir model, the maximal adsorption capacity of the LPFC pair of CR was calculated to be 416.6 mg/g, which is roughly comparable to the actual experimental results. In comparison with the previously reported biomass adsorption, the LPFC adsorbent showed excellent adsorption performance, and its maximum adsorption capacity was much larger than that of other biomass adsorbents (

Table 4. Maximum adsorption capacity of different adsorbents for CR.

Adsorbent	Maximum Capacity (mg/g)	Removal Rate (%)	Reference
Magnetically separable mesoporous TiO2 modified with gamma-Fe2O3.	125.00	97	[64]
Coconut shell FexCo3-xO4 nanoparticles.	128.60	86.12	[65]
MgZnCr-TiO2 layered double hydroxide.	526.32	99.68	[66]
NiAl magnetite humic acid.	178.571	97.29	[67]
Chitosan-modified hydrogen titanate nanowires membrane.	374.4	99.5	[68]
Photogenic MnO2/Ag metal nanocomposites.	97.10	98	[69]
Fibrous xonotlite.	574.71	95.5	[70]
Polypyrrole magnetic nanocomposite.	119.76	94	[71]
Ground nut shells charcoal.	117.60	83	[72]
ZnO/chitosan nanocomposite.	227.30	94.8	[73]
Reduced graphene oxide composite.	333.32	89	[74]
Porous molybdenum disulfide and reduced graphene oxide nanocomposite.	440.9	97	[75]
LPFC activated carbon.	406.35	92.1	This work

). Moreover, the preparation process of LPFC adsorbent is relatively simple and feasible, which can effectively realize the high-value application of biomass.

3.4. Reuse of Adsorbents

In order to avoid the solid waste accumulation of adsorbent after the adsorption of dyes and pollution of the environment, the reusability of adsorbent suction is considered a key indicator for industrial applications [60]. In order to evaluate the recyclability of LPFC, we conducted a reuse test. The used LPFC (10 mg) was placed in 20 mL of methanol and sonicated for 60 min. After filtration, the adsorbent was washed three times with distilled water and dried overnight at 80 °C. The adsorbent was then reused for the next process of dye adsorption. The regenerated LPFC was reused in the next process of dye adsorption with the same adsorption conditions. The results are shown in Figure 12. The adsorption capacity was essentially the same at the 1st and 2nd reuses, and the adsorption rate decreased from 92.1% to 79.2% after five reuses. The decrease in adsorbent rate may be due to some strong adsorption sites that are currently being prepared. LPFC adsorbent is not yet able to regenerate, but it also shows better recyclability. We will also follow up with a more in-depth study on the reuse of the adsorbent.

3.5. Adsorption Mechanism

As shown in Figure 13, LPFC is positively charged in a neutral environment, and CR is an anionic dye. Therefore, LPFC can adsorb the negatively charged CR on its surface by electrostatic adsorption. In addition, the high adsorption capacity of LPFC on CR may be attributed to the very strong π-π interaction between the benzene ring structure of LPFC (π-rich electron providing region) and the C=C of the benzene ring (π-electron acceptor) of the CR molecule [49]. Another possible mechanism is attributed to the hydrogen bonding interaction between the hydroxyl group contained in LPFC and the amino group contained in CR [76].

4. Conclusions

In summary, we prepared LPFC adsorbent by a simple KOH activation method using lignin, a renewable resource, as a complete replacement for phenol. The adsorbent demonstrated great potential in the field of anionic dye adsorption with a maximum adsorption capacity of 406.35 mg/g, and we also determined its optimal adsorption conditions (initial dye concentration of 100 mg/L, initial pH = 6 in solution, temperature of 30 °C, reaction time of 1 h, and adsorbent dosage of 1 g/L). The experimental data better fit the Langmuir isothermal adsorption model and the kinetic proposed second-order model, indicating that the adsorption process prefers monolayer adsorption, and the equilibrium adsorption capacity of LPFC on CR estimated in the proposed second-order model is 425.53 mg/g. The experimental results also indicate that the replacement of phenol by lignin results in a carbon material with a richer pore structure and a larger surface ratio. This greatly improved the adsorption capacity of the carbon material adsorbent, which demonstrated the feasibility and superiority of lignin to replace the traditional petroleum product phenol. These findings provide a feasible and green pathway for the high-value utilization of lignin, which is important for the rational design, working mechanism, and application of biomass materials in wastewater treatment.