Insights into kinetics, thermodynamics, and mechanisms of chemically activated sunflower stem biochar for removal of phenol and bisphenol-A from wastewater

This study synthesized a highly efficient KOH-treated sunflower stem activated carbon (KOH-SSAC) using a two-step pyrolysis process and chemical activation using KOH. The resulting material exhibited exceptional properties, such as a high specific surface area (452 m2/g) and excellent adsorption capacities for phenol (333.03 mg/g) and bisphenol A (BPA) (365.81 mg/g). The adsorption process was spontaneous and exothermic, benefiting from the synergistic effects of hydrogen bonding, electrostatic attraction, and stacking interactions. Comparative analysis also showed that KOH-SSAC performed approximately twice as well as sunflower stem biochar (SSB), indicating its potential for water treatment and pollutant removal applications. The study suggests the exploration of optimization strategies to further enhance the efficiency of KOH-SSAC in large-scale scenarios. These findings contribute to the development of improved materials for efficient water treatment and pollution control.


Material characterization
The X-ray powder diffraction patterns of the synthesized materials were acquired using a Rigaku D/Max-2500 diffractometer.Surface elemental composition was analyzed using the ESCALAB-210 X-ray photoelectron spectroscopy instrument (Spain).The SEM analysis of the sample was conducted using the Hitachi S-4300 and EDX-350 instruments from Japan.The Brunauer, Emmett, and Teller (BET) surface area and pore diameters of the materials were calculated based on data obtained from N 2 -physisorption measurements using an Autosorb-1 instrument from Quantachrome (USA).pH readings were obtained using a pH meter (DKK-TOA, HM-42X, Japan), and the pH of the solution was adjusted using 0.1 mol/L HCl and NaOH solutions.The residual radionuclide concentration was determined using an autosampler-fitted Optima 2100 DV inductive coupled plasmaoptical emission spectroscopy (ICP-OES) instrument from PerkinElmer (USA).

Adsorption experiments
The experiment aimed to investigate the adsorption behavior of KOH-SSAC on BPA and phenol in aqueous solutions.The range of the pollutants' concentration in the solutions, which was used to determine the adsorption capacity, was 10 to 500 mg/L.Batch adsorption studies were conducted using 20 mg of dried KOH-SSAC mixed with 50 mL of pollutant solution.Various adsorption periods were employed to collect the contact time and kinetic data.A phenol and BPA content was determined using HPLC equipment after the liquid samples were collected by filtering.A mobile phase containing methanol and water in a 50:50 ratio was utilized for phenol, whereas for BPA, a 40:60 mixture of water and ACN was used.A 20 µL sample was injected for phenol and BPA, and 0.4 ml and 0.60 mL of fluid, were flowed at different rates.The experiment was conducted three times, using three different pH values for the liquids.It was constantly agitated at 150 rpm while maintaining a steady temperature of 298 K.The adsorption capacity and kinetics of KOH-SSAC towards BPA and phenol from aqueous solutions were evaluated using these data.Optimizing the circumstances for the sorption of these pollutants from water resources is another application for the data.The comprehensive tests were carried out in triplicate, and the average outcomes were considered for additional computations.The equation used to obtain the adsorption capacity (q e ) at equilibrium is shown in Eq. ( 1).
where q e (mg/g) represents the pollutant's adsorption capacity at equilibrium, C 0 (mg/L) is the starting concentration of the pollutant, Ce (mg/L) stands for the pollutant concentration at equilibrium, V (L) is the volume of the pollutant, and m (g) is the adsorbent weight.

Characterization of activated carbon derived from biochar
Figure 1a illustrates the significant differences between the XRD analyses of the KOH-SSAC and SSB samples, highlighting the significance of the KOH activation process.Compared with SSB and KOH-SSAC, the match of two wide, distinctive peaks with centers at 23° and 44° is visible, as shown in Fig. 1a.These peaks correspond to graphite-aromatic and amorphous carbon structures' (0 0 2) and (1 0 1) diffractions, respectively 35 .In addition, KOH-SSAC exhibited several unique and sharp diffraction peaks.The loss of water and the cracking process cause the salting-out effect, which is responsible for the sharp diffraction peaks.This indicates that KOH-generated potassium salts were removed, leaving no trace impurities.However, they had a significant effect on the morphology of the SSB, causing it to hold on to more distinct crystalline forms, as evidenced by the peaks at nearly 61° and 77.5°, which correspond to crystalline carbon and graphite-like carbon nanofibers, respectively 36 .The BET analysis performed with the N 2 desorption/adsorption isotherms of SSB and KOH-SSAC are depicted in Fig. 1b.The adsorption isotherms for both SSB and KOH-SSAC fall under the type IV isotherms as per the IUPAC ( 1) classification, of both micro-and mesopore existence were confirmed.The BET surface areas were found to be 369.29 m 2 /g for SSB and 413.04 m 2 /g for KOH-SSAC.Evidently, after KOH activation, the specific surface areas (SBET) were increased, which is assumed to be responsible for the enhanced adsorption efficiency.Furthermore, pore volumes also decreased from 0.226 to 0.1853 cm 3 /g upon activation.
FESEM microscopy and TEM are used to investigate surface morphology, including the textural aspects of prepared materials (Fig. 2). Figure 2a and b depict the surface of SSB, which exhibits a heterogeneous structure with irregular pores, lacking any definite shape.This results from the decomposition of hemicellulose and cellulose macromolecules 37 .However, after KOH activation, Fig. 2c and d demonstrate increased homogeneity of porosity with decreasing pore volume and a layered surface appearance in KOH-SSAC.SEM-EDX analysis of KOH-SSAC revealed elemental composition with an atomic percentage in the order of C > O > K > Ca > Cl > P. Notably, C and O exhibited higher concentrations compared to other elements, particularly with negligible concentrations observed for Cl and P, as illustrated in Fig. S1 and Table S1.The results of the TEM analysis provided valuable insights into the structural characteristics of the adsorbent made from sunflower stem biochar (SSB) after activation with KOH, as shown in Fig. 2e and f.The TEM analysis was conducted to investigate the origin of the mesoporous structure observed in KOH-SSAC.The sheet-like morphology observed in the TEM images suggests the formation of graphene-like layers within the KOH-SSAC material, which conforms to the previous www.nature.com/scientificreports/results of FE-SEM and XRD.These layers contribute to developing a highly porous structure, providing ample surface area for adsorption observed in subsequent BET studies.The mesopores formed through the activation process play a crucial role in enhancing the adsorption capacity of the KOH-SSAC adsorbent.These results were found to be consistent with previous studies on KOH activation of biomass-derived materials and the resulting mesoporous structures 38,39 .The presence of sheet-like structures confirms the successful activation of SSB using KOH, leading to the development of a desirable mesoporous morphology.

Effect of contact time and adsorption isotherms
Investigations were carried out on the optimized adsorbent KOH-SSAC to comprehend the influence of contact period on the adsorption of BPA and phenol, with varying initial concentrations (10, 20, 40, 60, 80, and 100 mg/L).This is depicted in Fig. 3a and b.Notably, a significant portion of Phenol and BPA was adsorbed within 30 min, and then an incremental adsorption procedure was performed until equilibrium was reached.
The swift initial adsorption rate results from the abundant adsorption sites on the KOH-SSAC surface, stemming from Van der Waals and electrostatic forces, as well as the quick diffusion of the pollutant toward these adsorbent surfaces.Conversely, the subsequent slower adsorption rate is attributed to the overabundance of active sites on the sorbent surface by adsorbates.
The study of adsorption kinetics is vital as it provides insightful information on the potential application of composites and a thorough knowledge of the adsorption process.Various models were introduced to elucidate the sorption mechanism.The obtained experimental data were fitted to the pseudo-first-order (PFO) and pseudosecond-order (PSO) models.These fitting outcomes were organized in Table 1, while the visual representation of Phenol and BPA removal on KOH-SSAC was demonstrated in Fig. 3a and b.This study critically evaluated the applicability of the PFO and PSO models in the context of Phenol and BPA adsorption onto KOH-SSAC adsorbent.These kinetic models facilitate the understanding of sorption mechanisms, shedding light on ratecontrolling steps like transfer of mass and chemical reactions.Two kinetic models were employed to investigate these processes.The nonlinear kinetic models depicted the effect of time on the adsorption of both Phenol and BPA onto the developed KOH-SSAC.The calculated adsorption capacity (q e , cal ) aligned well with the theoretical adsorption capacity (q e,th ) derived from the PSO model.Notably, The PSO model's R 2 value was much higher than the PFO model's, underscoring that the chemisorption of Phenol and BPA onto KOH-SSAC governs the rate-controlling stage of the removal process 40 .This indicates that chemical adsorption is the dominant factor, influenced by various factors such as electron-donor-acceptor interactions and hydrogen bonding.To gain insights into the interactions between phenol, BPA, and the KOH-SSAC surface, various adsorption models, such as Langmuir, Freundlich, Temkin, and D-R, were employed to fit the experimental data.The corresponding model parameters and determination coefficients (R 2 ) can be found in Table 2.The Freundlich model exhibited a significantly higher R 2 value than the Langmuir, Temkin, and D-R models.Further, the statistical error and chi-square calculations (Table S2) found that Freundlich's model fitting shows a low chi-square value.These results suggest that each adsorption site on the KOH-SSAC surface possesses distinct adsorption energies and affinities for phenol and BPA 41 .
Consequently, this leads to a non-uniform distribution of phenol and BPA on the surface, confirming the efficacy of multilayer adsorption for the adsorption of phenol and BPA by KOH-SSAC.From Langmuir isotherm, the maximum sorption capacity (q m ) for phenol and BPA was determined to be 333.03mg/g and 365.81 mg/g, respectively.KOH-SSAC shows high adsorption capacity for both pollutants when compared with SSB (258.15 mg/g for phenol and 289.25 mg/g for BPA) due to the KOH-SSAC having a high surface area.SSB isotherms are shown in Fig. S2 and Table S3 for comparison of results before and after activation of SSB (KOH-SSAC).

Adsorption thermodynamic studies
The thermodynamics of phenol and BPA adsorption onto KOH-SSAC were analyzed under optimized conditions (pH 6, adsorbent dose = 0.4 g/L, contact time = 20 min, initial concentration = 100 mg/L) and across varying temperatures (298, 308, and 323 K) to assess the efficacy of the removal process.Thermodynamic parameters related to the adsorption, including ΔH (enthalpy, kJ/mol), ΔG (Gibbs free energy, kJ/mol), and ΔS (entropy, kJ/ mol/K) as calculated using Eq. ( 2), played a pivotal role in predicting and characterizing the adsorption mechanism.Figure 4 illustrates a linear plot (lnK d vs. 1/T) corresponding to the temperature-dependent equilibrium constant.The point of intersection on the Van't Hoff plots was employed to determine ΔH and ΔS.K d represents the thermodynamic equilibrium constant, C q (mg/L) is adsorbed concentration of pollutant, T denotes the absolute temperature measured in Kelvin (K), and R is the ideal gas constant, with a value of 8.314 J/mol/K.
(2) Table 2. Phenol and BPA adsorption isotherms onto KOH-SSAC (0.4 g/L) at pH 6.0 and 298 K ± 2. q max and q D : the maximum adsorption capacity; n: heterogeneity factor, K L , K F , K T and K DR : constants of the Langmuir, Freundlich, Temkin, and D-R isotherms, respectively; B T : the heat of adsorption of in the Temkin.

Pollutants
Langmuir q e = (q max K L Ce)/(1 + K L Ce) Freundlich q e = K F + C e

1/n
Temkin q e = RT BT LnK T C e D-R q e = q D − exp(βε 2 ) q max , mg/g K L , L /mg R 2 K F , mg/g (L /mg) A diminution of the q e for phenol and BPA with increasing temperature suggests an exothermic adsorption process.This phenomenon can be attributed to reducing the attractive forces involving the pollutant molecules and the active binding sites on the adsorbent surface 42 .Table 3 presents the results, indicating a negative sign for ΔG° at the studied temperatures.This implies that the sorption of phenol and BPA onto the adsorbent is thermodynamically favourable and occurs spontaneously.Moreover, these results underscore the effectiveness of KOH-SSAC in adsorbing phenol and BPA 43 .ΔG values falling between − 20 to 0 kJ/mol generally indicate physisorption processes 44 .The values of ΔG, along with their decreasing trend as temperature rises, further confirm that the adsorption of these compounds occurs primarily through physisorption.Similarly, the negative sign of the enthalpy change, ΔH°, signifies an exothermic adsorption behaviour.Additionally, the significant value of ΔS° indicates an increase in randomness at the thermodynamically stable interface throughout the adsorption process 43 .

Comparative study
Table 4 comprehensively compares the q max of phenol and BPA across various adsorbents.In this study, we employed sunflower stem biochar treated with KOH as the adsorbent, and our findings revealed a commendable efficacy in the removal of both phenol and BPA.Compared to other adsorbents, KOH-SSAC consistently exhibited favourable results, showcasing its effectiveness as a potent adsorbent for the sorption of BPA and phenol from aqueous solutions.

Influence of external environmental conditions
Among the various water quality parameters, ionic strength, pH, and organic matter significantly influenced sorption As depicted in Fig. 5a, pH had a notable impact on removing BPA and phenol.The effective sorption of these contaminants remained relatively stable across a pH range of 2 to 10.However, when the solution pH  exceeded this range, the removal efficiency for phenol and BPA began to decline.This phenomenon can be ascribed to changes in the charge on the surface of KOH-SSAC and the nature of the pollutants.At higher pH values, the disassociation equilibrium of phenol led to the formation of its anionic form (C 6 H 5 O − ) at pH > pKa (phenol, pKa = 9.92).This anionic form rendered the adsorption onto KOH-SSAC more challenging.Further reasons for the decline in adsorption efficiency include electrostatic repulsion between phenol anions and the KOH-SSAC surface and repulsion between phenolate-phenolate anions in the solution.Also, the stronger interactions between water and phenol anions caused by the higher solubility of phenol anions in aqueous solution made it harder for them to be displaced before adsorption 45 .When the pH was less than 9.0 (BPA, pKa = 9.6), the dominancy of BPA in the solution was the almost neutral molecule (BPA 0 ).Hydrophobic interactions or hydrogen bonding between the contaminant and KOH-SSAC probably significantly eliminated BPA 46 .However, BPA molecules steadily lost protons as the pH of the solution rose over 9.5, transitioning in their ionic forms (BPA − or BPA 2− ), leading to a rapid decrease in their hydrophobic interactions or hydrogen bonding with KOH-SSAC due to the increased hydrophilicity of BPA species 47 .Additionally, the functional groups of KOH-SSAC underwent partial deprotonation (pHzpc = 7.8), increasing the negative charge on KOH-SSAC.Consequently, electrostatic repulsion between KOH-SSAC and the protonated BPA species reduced the adsorption capacity under strong alkaline conditions.Future investigated the impact of natural organic matter, specifically humic acid (HA), on removing phenol and BPA.As the concentration of HA increased, the removal efficiency of phenol and BPA by KOH-SSAC gradually declined, as illustrated in Fig. 6b.Notably, KOH-SSAC exhibited a robust resistance to HA interference at concentrations below 25 mg/L compared to the control sample.However, once the HA concentration reached 25 mg/L, its influence on the removal efficiency of both pollutants became negligible.This behaviour can be attributed to several factors.First, the competitive adsorption of HA led to a reduction in the available pore  volume of KOH-SSAC through hydrophobic interactions or van der Waals forces.This reduction consequently resulted in a decline in phenol and BPA adsorption capacity 48 .Secondly, as HA was absorbed, it introduced oxygen-bearing surface functionality of the activated carbon surface.This addition had the effect of reducing the surface hydrophobicity of the activated carbon.As a result, the hydrophobic interactions between KOH-SSAC, phenol, and BPA were weakened, leading to decreased adsorption removal of these contaminants 49 .

Desorption and regeneration studies
Adsorbents are sometimes discarded due to their inability to be regenerated, leading to their classification as secondary pollutants 50 .A regeneration process involving solvent treatment and heating was employed for the absorbent to address this issue.Subsequently, this regenerated adsorbent was used for the adsorption of BPA and phenol, with each adsorption experiment involving 20 mg of the regenerated adsorbent and 10 mg/L of phenol and BPA in a 50 mL solution.Like the adsorption tests, a 60-min desorption procedure was performed using a shaker.The process of separating the adsorbent from the liquid phase during adsorption and desorption was accomplished by centrifugation.The mixture underwent centrifugation at a speed of 4000 rpm for 10 min.
After the desorption step, the adsorbent was meticulously cleansed at specific intervals using distilled water before oven drying.After the fifth cycle of heating and washing with 0.1 M NaOH, the regenerated adsorbent's maximum adsorption efficiency was measured, and there was a decrease in efficiency.Phenol decreased from 97.8 to 62%, while BPA dropped from 99 to 64%, as shown in Fig. 6.This decrease in effectiveness is caused by the interaction between phenol and NaOH, converting it into sodium phenoxide; it is water-soluble and aids in desorbing phenol from the adsorbent.The high regeneration ability of NaOH can be attributed to three simultaneous mechanisms: (1) The hydrolysis of oxygen groups on the adsorbent's surface and the interaction with the hydroxyl group (-OH) of phenol, (2) The significant solubility of phenol in water in alkaline conditions, (3)  The repulsion between deprotonated acidic groups on the adsorbent's surface and the adsorbed phenol, induced by NaOH.A series of consecutive adsorption-desorption cycles were performed to evaluate the adsorbent's regeneration capability, totaling five cycles.

Possible mechanism
Analysis of kinetics and isotherms revealed that the adsorption of phenol and BPA onto KOH-SSAC involved a mechanism combining physical and chemical properties.The PSO model suggested the presence of chemical interactions influencing the sorption process.Additionally, isotherm model fitting data revealed the formation of multilayer adsorption on KOH-SSAC.Interestingly, as the temperature increased, the adsorption capacity for phenol and BPA on KOH-SSAC decreased, indicative of an exothermic adsorption process.Moreover, higher initial concentrations of these contaminants facilitated their migration from the solution to the KOH-outer SSAC's surface.Notably, a larger pore volume within the adsorbent played a beneficial role in enhancing the removal process.The role of pore-filling, which is closely associated with the porous structure, was pivotal in the adsorption of contaminants on activated carbon.This resulted in elevated initial removal rates 51 .Furthermore, π-π interactions represent a significant Mechanism of intermolecular interaction between the pollutants and KOH-SSAC.To gain insights into this adsorption mechanism, we conducted X-ray photoelectron spectroscopy (XPS) characterization on KOH-SSAC before and after the adsorption process, as portrayed in Fig. 7.In the C 1s spectra, the peaks at 284.72, 285.31, and 287.31 eV typically correspond to C-C/C=C, C-O, and C=O bonds, respectively 52 .Notably, after adsorption, these carbon functional groups shifted towards lower energy levels, which can be attributed to the adsorption of phenol and BPA.Particularly, certain groups, such as C-O, C-C, and C=O, are known to enhance their π-electron-donor potential towards BPA and phenol 53 .This suggests that π-π interactions are pivotal in facilitating the adsorption process.
According to the hydrogen bonding process, surface functional groups on KOH-SSAC that include oxygen can establish hydrogen bonds with other functional groups, including the phenolic hydroxyl group found in organic compounds.This interaction promotes the sorption of pollutants onto the adsorbent.Simultaneously, water molecules can be adsorbed onto the activated carbon surface by forming hydrogen bonds.Subsequently, these water molecules act as secondary adsorption sites, forming clusters that prevent the penetration of organic substances into the activated carbon's micropores 54 .Hydrogen bonds are the primary driving force behind the mobility of oxygen functional groups on the adsorbent's surface.This indicates that the oxygen-containing functionalities of phenol and BPA also participate in these interactions.In summary, Electrostatic interaction, π-π interactions, and hydrogen bonding substantially contribute to the adsorption capacity, and their mechanisms are illustrated in Fig. 8.

Conclusions
In this research, KOH-activated carbon derived from sunflower stems (KOH-SSAC) exhibited remarkable efficacy as an adsorbent for phenol and BPA removal from aqueous solutions.The KOH activation process facilitated the generation of active surface sites and increased material porosity, significantly enhancing adsorption performance compared to SSB alone.Adsorption isotherm studies revealed impressive maximum adsorption capacities of 333.03 mg/g for phenol and 365.81 mg/g for BPA, surpassing other materials in similar studies.Thermodynamic analyses confirmed the spontaneous and exothermic nature of the adsorption process, with KOH-SSAC exhibiting robust regeneration capabilities and prolonged usage life.Mechanistic insights highlighted electrostatic attraction, π-π interactions, and hydrogen bonding as key contributors to its exceptional adsorption performance.Future directions include optimizing pore size distribution and surface area, exploring continuous column conditions, assessing synergistic treatment approaches, and conducting pilot-scale studies for practical water treatment applications.These efforts are essential for validating the effectiveness and feasibility of deploying KOH-SSAC on a larger scale, ensuring safe and efficient water treatment.

Figure 3 .
Figure 3. Kinetic effect (a, b) and Isotherms (c, d) of phenol and BPA at 0.4 g/L and pH of 6.

Figure 7 .
Figure 7. (a) C 1s and (b) O 1s XPS spectra of the region before and after the reaction.

Figure 8 .
Figure 8. Mechanism of phenol and BPA adsorption on KOH-SSAC.

Table 4 .
Comparison of phenol and BPA adsorption capacity on KOH-SSAC with reported adsorbents.