Preparation of Nano Lignin–Based Activated Carbon Depending on Ratio Impregnation of K2CO3, Activation Time and Activation Temperature on Yield for Congo Red Dye Adsorption

INTRODUCTION   
Water pollution is one of the serious environmental problems that threaten both human health and economic development [1]. Dyeing, pulp and paper, plastic, cosmetic, leather and food industries make use of dyes for staining products. The photolytic and chemical stability of dyes make them resistant towards degradation that ultimately led to water pollution [2, 3]. Increased human and industrial activities have led to various types of pollutants, such as organic, inorganic dyes and heavy metal ions [4]. Numerous technologies have been employed for mitigating water pollution, such as ozonation, sonocatalysis, photocatalysis, membrane filtration, ion exchange, electrochemical processes, and adsorption [5-16]. Among these techniques, adsorption has gained popularity due to its low cost, biodegradability, and eco-friendliness [17-19]. Important performance indicators for adsorption systems include adsorption capacity and adsorption rate. Different types of adsorbent materials have been investigated for the effective removal of dyes such as congo red (CR) dye [20]. Congo Red (CR) is a synthetic anionic azo dye derived mainly from benzidine. Continuous exposure to CR can lead to allergic reactions and its metabolism can produce benzidine, a known carcinogen. Its molecular structure in water can vary depending on pH that thereby affects its removal efficiency [21-23]. 
Activated carbon (AC) has proven to be a highly effective and widely used adsorbent for treating dye wastewater and removing various pollutants dissolved in water. Derived from agricultural waste and natural materials including rice husk, coconut shell, bituminous coal, and lignite, AC shows exceptional surface area and total pore volume, making it well-suited for adsorption applications [24, 25]. Lignin, a polyphenolic agro-waste having an amorphous structure and high surface area, is a promising precursor for the development of high-value products. Its complex structure, characterized by the presence of different ionic functional groups (including hydroxyl, phenolic, and carboxyl), makes it a versatile starting material. Lignin-derived activated materials have gained significant attention in recent years. The preparation of AC from lignin involves thermal processes including carbonization and activation. Activation process can be achieved through either physical or chemical ways. During carbonization, the temperature is increased to high levels i.e., 400-900 °C while chemical activation often involves the use of various agents such as H3PO4, ZnCl2, KOH, K2CO3  [26-28]. 
Keeping in view the above discussion, this study focuses on the synthesis of nano form of activated carbon from lignin, a natural source obtained from straw pulping, using both physical and chemical activation methods. Chemical activation was performed using potassium carbonate (K2CO3). The optimal synthesis parameters, such as mass ratio of potassium compound to lignin, activation temperature, and time, were investigated. Characterization of the activated carbon synthesized under optimal conditions with K2CO3 was conducted using Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller/ Barrett-Joyner-Halenda (BET/ BJH) analysis, and energy-dispersive X-ray spectroscopy (EDX). The adsorption behaviour of the synthesized activated carbon towards CR dye removal from water was also investigated. The effects of contact time, adsorbent dosage, initial pH, and temperature were studied, along with kinetic and thermodynamic studies to assess the dye adsorption efficiency.  

MATERIALS AND METHODS
Materials 
Lignin, a precursor obtained from straw pulping, was sourced from a local farm (shown in Fig. 1). Analytical-grade potassium carbonate (K2CO3) and CR dye were procured from Scharlau. Double-distilled water was used to prepare aqueous solutions for the experiments. 

Preparation of activated carbon (AC) 
Preparation of raw materials
Lignin, a precursor obtained from straw pulping, was collected from a local farm in Al-Diwaniyah city, southern Iraq. The samples were washed thoroughly with double-distilled water (DDW) to remove any kind of impurities present and then oven-dried at 105°C for 4 hours. 

Preparation of nano AC based lignin
Activated carbon was produced from lignin using a combination of both physical and chemical activation methods. Physical activation method involved pyrolysis to obtain char, followed by chemical activation with potassium carbonate (K2CO3). An accurately weighed 2 g of oven-dried lignin powder was impregnated with various mass ratios of K2CO3 (including 0.5:1, 1:1, 1.5:1, 2:1, and 2.5:1) in 20% aqueous solutions and placed in a crucible for the time period of 16 hours. The mixtures were then dried and dehydrated in an electric furnace. Carbonization process of the impregnated samples was conducted at temperatures varying from 450°C to 850°C for 20-60 minutes under a nitrogen atmosphere, with the heating rate of 200°C/ min. After activation with K2CO3, the chemical activating agent was removed by washing the samples multiple times with hot and cold distilled water until the washing water reached a neutral pH as measured by the pH meter. The liquid phase was then removed by centrifugation at 6000 rpm. Finally, the activated carbon nanoparticles were dried in an oven at 105 °C for the time period of 6 hours [29, 30]. The yield of activated carbon was calculated using the following equation [3, 31]:  

                               
here, and  refers to the mass of AC and the initial mass of oven-dried lignin. 

Characterization 
The prepared adsorbent was characterized in terms of yield and many other properties. The surface morphologies of lignin and AC were analyzed with the help of scanning electron microscopy (SEM) (Zeiss Libra 200 FE, Germany). The crystalline structures of lignin and activated carbon were studied by using X-ray diffractometry (XRD-6000, Shimadzu-Japan, 2θ range of 10° to 80°, λ=1.5405 Å). Transmission electron microscopy (TEM) images were recorded with Philips model FEL, Quanta 400. Additionally, Fourier transform infrared spectroscopy (FTIR) using a Shimadzu-8400S spectrophotometer was employed for characterization of different functional groups present in prepared adsorbent (32). The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area based on nitrogen adsorption isotherms at variable relative pressures (P/P0). Furthermore, pore size distribution was determined using Barrett-Joyner-Halenda (BJH) method. Elemental composition and chemical analysis were identified using energy-dispersive X-ray spectroscopy (EDX) [31, 33, 34]. 

Adsorption studies 
The adsorption efficiency of prepared adsorbent was evaluated under various experimental conditions, including contact time (0-45 min), adsorbent mass (0.001-0.01 g/ 25 ml), pH (4-10), initial dye concentration (50 mg/l for CR), and temperature (293-308 K). Batch adsorption experiments were conducted in conical flasks, where a known mass of activated carbon was added to 25 ml of CR dye solution. The pH of solutions was adjusted using 1 M HCl or 1 M NaOH. The mixtures were then stirred in an orbital shaker at the speed of 110 rpm for a specified time, followed by centrifugation at 6000 rpm for 10 minutes that helps in removing the adsorbent particles. The absorbance of the solutions was then measured using a UV-Visible spectrophotometer (PC 1650, Shimadzu, Japan) at the maximum wavelength (λmax) of 498 nm for CR dye. This general procedure was repeated with different experimental parameters to determine the removal efficiency (% removal) and adsorption capacity (Qe) of AC using Eqs. 2 and 3 respectively [35, 36]. 

where, Ci and Ce represent the initial and final concentrations of CR dye (mg/ l) respectively, V is the volume (L) of dye solution and m is the weight (g) of adsorbent. 

RESULTS AND DISCUSSION
Effect of activating agent ratio to lignin on yield 
Product yield is a crucial metric for evaluating the impact of K2CO3/ lignin mass ratio on AC synthesis. AC was prepared from lignin derived from straw pulping, and the optimal conditions were determined based on adsorption properties and yield. The effects of varying the K2CO3/ lignin ratio between 0.5 and 2.5 were studied, resulting in yields varying from 45.8% to 45%. As highlighted in Fig. 2, increasing the K2CO3/ lignin ratio (with a constant lignin mass) led to a corresponding increase in yield up to a ratio of 2:1. This ratio resulted in an increase in highest product yield of 53%, demonstrating superior effectiveness and removal efficiencies. The higher yield is attributed to the formation of more pores, which increases adsorption capacity. However, adding excess K2CO3 at a ratio of 2.5 resulted in a decrease in yield due to additional reactions between K2CO3 and microporous carbon. This leading to the destruction of formed structure and increased carbon consumption [37]. 

Effect of activation time on yield
The effect of activation time on the yield of activated carbon (with a K2CO3/ lignin ratio of 2:1) was investigated under experimental conditions. As revealed in Fig. 3, the yield of activated carbon increased with increasing activation time within the range of 10 to 50 minutes, reaching a maximum of 62.22%. However, after 50 minutes, further increases in activation time resulted in wider pores and carbon burn-off thereby leading to a decrease in both yield (40.11%) and adsorption capacity. 

Effect of activated temperature on yield
The K2CO3/ lignin ratio (2:1) and activation time (50 min) were maintained constant for evaluating the yield of AC at different temperatures ranging from 450 to 850 °C. Fig. 4 illustrates that increasing the activation temperature from 450 to 850 °C resulted in a significant decrease in carbon yield, from 65.3 % to 40 %. Within the temperature range of 450 to 750 °C, the yield decreased from 65.3 % to 52 %, accompanied by the formation of more pores [38]. However, further increases in temperature from 750 to 850 °C led to a decline in yield to 40 % due to pore widening and burn-off. The optimal temperature for synthesizing AC with a high number of pores was found to be 750 °C using K2CO3 as the activating agent. This activation process enhanced the adsorption capacity of AC. At higher temperatures of 850 °C, pore widening and burn-off occurred, leading to the formation of a white material (ash) and a decrease in adsorption ability [29]. 

Activation mechanism with chemical agent (K2CO3)
After alkaline pre-treatment of lignin with K2CO3, the sample was carbonized and activated. During activation with K2CO3, carbon is consumed, producing potassium metal (K) and carbon monoxide (CO). After carbonization process at 450-750 °C, gasification occurs via following reaction involving the alkali metal [39]: 

The carbon is removed as CO due to the reduction of K2CO3 by carbon, resulting in the products as elaborated in equation above. The surface area and pore volume increase at the optimal temperature of 750 °C. Elemental potassium decreases the shrinkage of raw material by occupying a significant volume when temperature increases. Washing with hot distilled water results in removing potassium, leaving behind micropores [40]. 

Characterization of activated carbon as adsorbent
FT-IR analysis 
Fourier-transform infrared spectroscopy (FTIR) was used to analyze the surface functional groups of raw straw pulping and prepared AC. FTIR spectra were obtained in the mid-infrared range between 500 and 4000 cm⁻¹. Fig. 5 shows a prominent peak at 3410 cm⁻¹, indicating the stretching vibration of hydroxyl, phenolic, and carboxylic groups in lignin. A sharp peak at 2854 cm⁻¹ is attributed to symmetric stretching of CH3 group in the methoxyl group. Furthermore, a distinct peak at 1714 cm⁻¹ corresponds to carbonyl stretching, associated with unconjugated carboxyl and ketone groups. The band obtained at 1033 cm⁻¹ corresponds to the C-O stretching vibrations in C-OH for primary alcohols and aromatic CH [5, 41-48]. 

X-ray diffraction 
X-ray diffraction (XRD) analysis was performed using monochromatic nickel-filtered CuKα radiation (λ = 1.5416 Å) for investigating the crystallinity of lignin and activated carbon. Fig. 6 shows the XRD patterns of raw lignin and carbon precursors impregnated with K2CO3. Results revealed more intense peaks in lignin when compared with AC. In activated carbon, the main peak is less intense due to presence of KHCO3 and weak peaks of K4H2(CO3)3. This suggests the formation of C-O-K complexes, which participate in the activation reaction. The absence of sharp peaks and appearance of a broad diffraction peak in AC with K2CO3 indicate the presence of an amorphous structure in studied material [49, 50]. 

SEM-EDX analysis
The surface texture of lignin and activated carbon was examined using SEM (Fig. 7). Results revealed that the lignin exhibited a heterogeneous structure with no visible pores. Activated carbon prepared at 750 °C displayed a porous honeycomb structure exhibiting irregular shapes and sizes. The activation process, involving the formation and widening of small pores due to vaporized moisture and precursor contents, resulted in introducing the porosity in its structure [51]. Additionally, K2CO3 related contaminants occupied the formed cavities during pyrolysis process [52-54]. Furthermore, EDX analysis revealed the elemental composition of AC and lignin. Activated carbon contained a high percentage of carbon (50.65%), followed by oxygen (45.10%), nitrogen (3.99%), and potassium (0.26%). Lignin contained a higher carbon content (74.37%) compared to oxygen (19.66%) and nitrogen (5.98%). These results revealed that carbon is the dominant element on surface of AC [55-57]. 

TEM analysis
TEM images were used to analyze the surface morphology and size properties of lignin and AC (results are elaborated in Fig. 8). Study revealed that the lignin appeared as microfibrils, while AC showed the intercalation of K species within the carbon matrix. This intercalation is mainly attributed to quasi-chemical bonds, such as –O-K bond [58, 59]. 

BET-BJH analysis 
The surface area of a solid is defined as the sum of its external surface and accessible internal pore surface. In this study, BET surface area and micropore volume were determined using N2 adsorption experiments. The adsorption capacity of AC is directly proportional to its surface area. Fig. 9 illustrates the N2 adsorption-desorption isotherms and pore size distribution curves for lignin and AC. Table 1 summarizes the BET surface area and pore size values. Results revealed that BET surface area increased significantly from 31.632 m²/ g for lignin to 270.42 m²/ g for AC prepared at 750 °C. Similarly, the pore volume increased from 22.125 cm³/ g to 48.841 cm³/ g. The porosity of lignin-based AC produced by chemical activation with K2CO3 increased with higher impregnation ratios and activation temperatures. The resulting AC exhibited a higher micropore volume. Additionally, increasing the impregnation ratio with incremental amounts of K2CO3 promoted the formation of micropores, leading to products with larger surface area and pore volume. However, study revealed that the excessive impregnation ratios and activation temperatures can cause pore widening and burn-off [52, 60]. 
 
Adsorption of CR dye onto AC 
Effect of contact time
A rapid uptake of pollutants and establishment of equilibrium within a short timeframe is an indication of adsorbent’s efficiency for water treatment. The adsorption of CR dye onto AC was evaluated under controlled experimental conditions, including a time range of 0-45 minutes, an adsorbent dose of 0.01 g/ 25 ml, at the pH of 7 and temperature of 25 °C. The initial CR dye concentration was 50 mg/l. The contact time was maintained at 45 minutes, with a constant stirring velocity of 110 rpm. The absorbance of the mixtures was analyzed using a UV-Vis spectrophotometer (Shimadzu, Japan) at a maximum wavelength (λmax) of 498 nm for CR dye. Fig. 10 showed that adsorption of CR dye onto AC increased with time until it gradually reached equilibrium. The adsorption process can be divided into three distinct stages. In the first stage, a rapid initial adsorption was observed due to the concentration gradient of CR dye between solution and surface of AC. Simultaneously, AC possesses a sufficient number of active sites, enabling a rapid increase in amount of adsorbed CR dye. In the second stage, however, the adsorption rate decreases as active sites on activated carbon surface become progressively occupied by dye molecules. The concentration gradient between the solution and adsorbent also diminishes due to repulsive interactions, making it more difficult for additional CR dye molecules to bind to active sites. In third stage of adsorption, all active sites are completely occupied, rendering further adsorption challenging. As a result, equilibrium is reached, and optimum contact time for this process is determined to be 20 min, corresponding to a removal efficiency of 70.21% [61-63].  

Effect of adsorbent dosage and dye concentration 
The adsorbent dose significantly affects the adsorption process by determining the adsorbent-adsorbate equilibrium within the system. The quantity of adsorbent directly relates to the number of available adsorption sites, which, in turn, affect removal efficiency. To investigate the effect of adsorbent dose on adsorption capacity, experiments were conducted using varying amounts of adsorbent (0.001-0.009 g/l) under optimal conditions (for 20 min at pH 7 and temperature of 25 °C). As depicted in Fig. 11 (left), increasing the AC dosage from 0.001 g/l to 0.007 g/l led to a corresponding increase in removal efficiency from 43.427 % to 68.553 %. This enhancement is attributed to the increased availability of active sites on adsorbent surface. However, further increasing the adsorbent dose beyond 0.007 g/l did not result in a significant increase in adsorption capacity. This saturation point suggests that the active sites on adsorbent surface become saturated, limiting additional adsorption [55, 64]. The results of the dye concentration on CR dye adsorption shown in Fig. 11 (right) highlight the continuous increase in adsorption capacity with an increase of dye concentration. With the increase in initial dye concentration from 10 mg/l to 50 mg/l, there was a continuous increase in adsorption capacity from 35.04 mg/g to 160.60 mg/g respectively. This behavior can be attributed to the increase in interaction between adsorbent surface and the dye molecules with continuous increase in dye concentration. These thereby results in increasing the adsorption of CR dye molecules on the surface of AC [65].  

Effect of pH 
The pH of aqueous solutions significantly affects the adsorption process by influencing the surface charge of adsorbent and degree of ionization of adsorbate, thereby impacting the adsorption mechanism [66-68]. To investigate the effect of initial pH on adsorption efficiency, experiments were conducted at pH values of 4, 6, 8, and 10 under optimal conditions (i.e., using adsorbent dose of 0.007 g/l for the contact time of 20 min using dye concentration of 50 mg/l at 25 °C). As shown in Fig. 12 (left), the maximum removal of CR dye (78.616%) was achieved at pH 4. At lower pH values, the uptake of CR dye, an anionic dye, is attributed to the interaction between acidic medium and dye. Conversely, at higher pH values, the increasing concentration of hydroxyl groups in basic medium leads to an increase in negative charges on both dye and adsorbent surface. This electrostatic repulsion between similarly charged species results in a decrease in removal efficiency, with only 25.157% removal observed at pH 10. The point of zero charge (pHpzc) is a critical parameter that determines the pH sensitivity range and indicates the type of active sites on adsorbent surface. Anionic dye adsorption is favoured at pH values below pHpzc when the surface is positively charged, while cationic dye adsorption is favoured at pH values above pHpzc due to presence of negative functional groups such as OH-. As illustrated in Fig. 12 (right), the pHpzc for AC was found to be 7.8. Below this value, the increased concentration of H+ ions in dye solution leads to a more acidic environment, resulting in a positively charged adsorbent surface and enhanced electrostatic attraction with anionic dye. Conversely, pH values above pHpzc lead to an increase in OH- ion concentration, resulting in a negatively charged adsorbent surface and reduced adsorption capacity due to electrostatic repulsion between similar charges.         

Effect of temperature and adsorption thermodynamic
Temperature is a critical parameter influencing adsorption capacity and overall adsorption process. To investigate the effect of temperature on uptake of CR dye by AC, experiments were conducted at temperatures ranging from 293 K to 308 K for 20 min. As shown in Fig. 13 (left), increasing temperature favoured the transport of adsorbate into pores of AC, leading to an increase in the amount of CR dye adsorbed by AC and removal efficiency from 77.98 % to 89.93 % as temperature increased from 20 °C to 35 °C. The enhanced adsorption capacity at higher temperatures can be attributed to both the decreased solubility of CR dye and increased molecular movement, enabling the dye molecules to interact more effectively with active sites on adsorbent surface. A thermodynamic study conducted over temperature range of 20-35°C provided insights into nature of adsorption process and the degree of freedom of dye molecules. Thermodynamic parameters such as enthalpy change (ΔH), entropy change (ΔS), and Gibbs free energy change (ΔG) were calculated using Eqs. 5-8 [69] and results are presented in Table 2. 

Where keq, is thermodynamic equilibrium constant, Qe is amount of dye adsorbed in solution (mg/g), Ce is equilibrium concentration of dye solution (mg/l), R is universal gas constant (8.314 J/ mol.K) and T is temperature in Kelvin. The values of ΔH and ΔS were determined from intercept and slope of plot shown in Fig. 13 (right), respectively. The results indicate that adsorption capacity increases with increasing temperature, suggesting an endothermic nature of adsorption process. Furthermore, the positive value of ΔH supports these results. Additionally, the positive value of ΔS suggests a higher degree of freedom for molecules in adsorbed phase compared to liquid phase, indicating increased randomness at solid-liquid interface. In contrast, negative values of ΔG at all temperatures clearly highlights that adsorption of CR dye onto adsorbent is feasible and spontaneous. The obtained thermodynamic values are summarized in Table 2. 

Adsorption isotherm studies
To assess the applicability of adsorption for removing CR dye from aqueous solutions using AC, several commonly used isotherm models, including Langmuir, Freundlich, and Temkin, were employed. 

Langmuir isotherm 
The Langmuir isotherm is applicable to solid-liquid systems and postulates that adsorption occurs at specific, homogeneous monolayer sites on adsorbent. It assumes that all active sites on surface have identical adsorption energies and adsorbate is distributed equally among these sites. The nonlinear and linearized forms of the Langmuir isotherm equation are as follows: 

here, qe is amount of dye adsorbed on unit mass of adsorbent at equilibrium (mg/g), qmax is maximum quantity of adsorbate to form monolayer adsorption capacity (mg/g), Ce is equilibrium dye concentration in solution (mg/l), b is Langmuir isotherm constant attributed to affinity of binding sites (l/mg). The parameter qm can be calculated from slope of linear plot of Ce/ qm versus Ce. The most important feature of Langmuir isotherm can be made on basis of a dimensionless constant RL, which is known as separation factor or equilibrium parameter and expressed as:

here, C₀ represents initial concentration of CR dye (mg/l). RL values indicate the shape of isotherm. If its value ranges between 0 and 1, the adsorption process is considered as favourable (0 < RL ≤ 1). When RL equates unity, the process is linear and irreversible. If RL is greater than 1 (RL > 1), the adsorption is unfavourable [70-72]. Fig. 14a and Table 3 present the results of Langmuir isotherm for CR dye. The experimental RL value of 0.0180 indicates a good fit to the Langmuir isotherm model, consistent with the requirements for favourable adsorption. 

Freundlich isotherm
The Freundlich isotherm model is applicable for non-ideal adsorption and is an empirical equation used to describe heterogeneous surfaces of adsorbents and multilayer adsorption. This model accounts for distribution of active sites on adsorbent surface and interactions between adsorbed molecules. The nonlinear form of the Freundlich equation can be expressed as below: 

In Freundlich isotherm model,  is a constant representing the adsorption capacity, while 1/n refers to the adsorption intensity. The value of 1/n varies with the heterogeneity of material, and n reflects the mutual interaction between adsorbed species. Qm represents the equilibrium adsorption capacity (mg/g), and Ce is equilibrium dye concentration (mg/l). The linear form of the Freundlich isotherm is represented by following equation [73]:

 
The values of kF and 1/n were obtained from intercept and slope of a straight-line plot of log Qe versus log Ce. The applicability of the isotherm equations was compared using correlation coefficient (R²). Fig. 14b and Table 3 show the results of Freundlich model revealing the better fit to Langmuir model compared to Freundlich model. These results suggest a homogeneous nature of the AC surface and formation of a monolayer of CR dye molecules on its exterior surface. Furthermore, the value of 1/n indicates the favourability of adsorption process. A value of n close to zero suggests a more heterogeneous system. The experimental values of n are greater than 1 (n > 1), indicating repulsive forces between adsorbed molecules and favourable adsorption conditions.  

Temkin isotherm
The Temkin model is used for heterogeneous surface systems with non-uniform adsorption heat distributions. This model assumes a linear decrease in heat of adsorption for all molecules in layer with low interactions taking place between studied adsorbent and adsorbate. The linear form of the Temkin isotherm is represented by the following equation [73, 74]: 

here, B refers to Temkin constant that is related to heat of adsorption (J/ mol), R = universal gas constant (8.314J/ mol.K), T = absolute temperature (K), b = Temkin isotherm constant, kT = Temkin isotherm equilibrium binding constant (l/mg) corresponding to maximum binding energy. Values B and kT can be calculated from slope and intercept of graph (qe versus log Ce) respectively. Fig. 14c shows the Temkin isotherm model and parameters calculated from this model are listed in Table 3. Results revealed that the best fit for CR dye was obtained with Langmuir revealing the R2 of 0.9620 followed by Temkin showing R2 of 0.9433 and Freundlich with R2 of 0.9233.  
In adsorption, molecules within the bulk material experience cohesive forces of equal magnetic magnitude in all directions. However, molecules on surface are pulled downwards and sideways. When the opportunity of adsorption increases, surface atoms or molecules will attempt to satisfy their unsaturated valence by bonding with other molecules or atoms. This bonding reduces surface energy. If the bonding occurs primarily through Van der Waals forces, the adsorption is known as physisorption. If the surface is unsaturated, chemical forces take place, leading to bond formation and chemisorption. This phenomenon is in good agreement with Langmuir isotherm. 

Adsorption kinetics
Kinetic studies elucidate the rate and mechanism of adsorption process. Solid-liquid adsorption processes on porous surfaces often involve multiple steps. Initially, the external mass of adsorbate transfers from liquid phase (solution) to solid surface through boundary layer. Subsequently, internal diffusion occurs within porous particles of adsorbent, followed by adsorption onto adsorbent surface. Based on adsorptive behaviours and potential rate-controlling mass transport and chemical reaction processes, adsorption kinetics were investigated using two kinetic models namely the Lagergren pseudo-first-order and pseudo-second-order models. The pseudo-first-order model is concerned with the physical forces between adsorbent and adsorbate and assumes that adsorption rate is proportional to unoccupied sites on adsorbent surface [75-79]. The pseudo-first order equation, is shown in Eq. 15 as below:  

The constant reaction rate k₁ (min⁻¹) can be determined from the slope of the (qe-qt) versus t plot. Here, qe represents equilibrium adsorption capacity (mg/g), qt represents amount of dye adsorbed at time t (mg/g), and k₁ refers to adsorption rate constant (min⁻¹). 
The pseudo–second order kinetic equation, is represented by Eq. 16: 

                     
                                                   
The constant reaction rate k₂ (g/mg·min) and qe are calculated from slope and intercept of straight-line plot of t/qt versus t, respectively. The kinetic study of CR dye removal on AC at equilibrium contact time revealed the characteristics of adsorption behaviour. Both pseudo-first-order and pseudo-second-order models were applied to experimental data obtained, as shown in Fig. 15a and b. The results, reported in Table 4, indicate correlation values of R² = 0.834 for pseudo-first-order model and R² = 0.997 for pseudo-second-order model. The higher correlation coefficient for pseudo-second-order model suggests a better fit to adsorption data than pseudo-first-order model. Additionally, the equilibrium adsorption capacity for pseudo-first-order model was found to be 0.861, which is significantly lower than qe value of 121.951 for pseudo-second-order model. This suggests that pseudo-second-order model is in close agreement with experimental data and implies that rate-limiting step is chemisorption-type, indicating an adsorption mechanism related to adsorbent and adsorbate. 
 
CONCLUSION
This study demonstrates the preparation of activated carbon (AC) with high porosity and specific surface area from agricultural waste lignin derived from straw pulping. Activation using K₂CO₃ as an activating agent and varying impregnation ratios affects yield of nano activated carbon. Activation time and carbonization temperature also play substantial roles in product quantity. The raw material and AC were characterized using various techniques. The efficiency of AC as an adsorbent was evaluated for removal of congo red (CR) dye from aqueous solutions. The adsorption process was influenced by experimental conditions such as contact time, adsorbent dose, pH, and temperature. The highest percentage removal of CR dye by AC under optimal conditions was 89.9375%. Thermodynamic studies revealed that adsorption of CR dye is endothermic and spontaneous. The Langmuir isotherm model provided best fit for CR adsorption, and kinetic study results indicated that adsorption of CR dye follows pseudo-second-order model. The findings of this work suggest that nano lignin-based carbon materials can be utilized as eco-friendly and low-cost adsorbent for adsorption of dyes. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.