Biobased Chitosan–Carbon Composite Beads from Azadirachta excelsa for Dual Dye Adsorption

Biobased Chitosan–Carbon Composite Beads from Azadirachta excelsa for Dual Dye Adsorption | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Biobased Chitosan–Carbon Composite Beads from Azadirachta excelsa for Dual Dye Adsorption Ria Nurwidiyani, Deni Agus Triawan, Siska Mawarti This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7678806/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Investigation of adsorption of Congo Red (CR) and Methylene Blue (MB) dyes by chitosan beads (CB) and chitosan carbon based of Azadirachta excelsa composite beads (CAECB) has been carried out. The adsorbent was synthesized with varying mass ratio of chitosan and carbon. The composite was synthesized with PVA as a cross linker agent. This research was carried out in several stages for adsorbent preparation, degree of swelling, the stability of the adsorbent, and the effect of contact time (adsorption kinetics), and isotherm adsorption. The degree of swelling of the adsorbent range from 380–610% and the adsorbent was stabled when soaked using distilled water until the 5th day (0% deformation). The best adsorbent for adsorption of MB and CR are CAECB 0,5 and chitosan beads (CB), respectively. The optimum adsorption time of MB was obtained at 90 minutes with the adsorption percentage of 99.10% by CAECB 0.5. The optimum adsorption time of CR was obtained at 150 minutes with the adsorption percentage of 96.01% by CB. The kinetics result follow pseudo second order rate equation for CAECB 0.5-MB, CB-MB, and CB-KR. The kinetics result for CAECB 0.5-MB was followed pseudo first order rate equation. adsorption beads composites methylene blue congo red Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Globally, pollution remains a pressing issue that attracts significant attention. As human civilization advances and the demand for technological development increases, the resulting waste from various production processes also proliferates. Pollution of air, water, and soil has become inevitable. Water, a vital resource for all living organisms on Earth, is increasingly difficult to access in clean and usable forms. Every year, more than 3.575 million lives are lost due to water-related diseases, while over two billion people still struggle to access safe drinking water due to polluted water sources [ 1 ]. Industrial wastewater pollution presents a significant challenge to industry stakeholders and researchers, requiring urgent attention due to its substantial environmental and public health impacts [ 2 ]. Textile dyeing industries are one of the primary contributors to environmental pollution, discharging more than 300,000 tons of wastewater containing dyes annually [ 3 ]. Wastewater from dyeing processes is typically treated using various techniques, photocatalysis or photolysis driven by UV or visible light, adsorption on to activation carbon, advanced oxidation process, ozonation and electro precipitation represent strategies for waste water treatment, however this methods are often limited by hight operational cost and significant energy demand [ 4 ]. Among these methods, adsorption is the most commonly used, although careful selection of adsorbents is crucial. Adsorption offers several advantages over other methods, including high efficiency in dye removal, applicability to a wide range of dyes, and high regeneration capability. In contrast, other methods often have limitations, such as the generation of difficult-to-handle sludge in precipitation, filter clogging in filtration methods, or the failure of biological methods when interfering substances disrupt microbial activity [ 5 ]. Chitosan, a derivative of chitin, is predominantly sourced from the exoskeletons of arthropods, such as crabs, shrimp, and lobsters. As a biomaterial with cationic properties, chitosan can adsorb heavy metals and textile dyes from wastewater [ 6 ]. Several studies have explored the use of chitosan and it’s modification for dye adsorption, such as its application in Congo Red adsorption [ 7 ], Direct Blue-218 adsorption [ 8 ], and Red BF-4B adsorption [ 9 ]. However, chitosan's use as an adsorbent has limitations due to its solubility in acidic conditions. When combined with other materials, chitosan forms composites with improved dye adsorption capabilities. For instance, chitosan/alumina composites have been employed for Methyl Orange adsorption [ 10 ], chitosan microspheres for Methyl Orange adsorption [ 11 ], chitosan-modified activated carbon beads for Congo Red adsorption [ 12 ], and chitosan-activated carbon-metal oxide hydrogel beads for adsorbing anionic and cationic dyes [ 13 ]. To enhance surface area and stability, chitosan can be combined with activated carbon, a porous material that increases the adsorbent's surface area, thereby improving its adsorption capacity. The addition of fillers to composites can further enhance the mechanical properties of the composite matrix. Activated carbon can be produced during the pyrolysis process of wood containing lignin, cellulose, and lignocellulose [ 14 ]. Neem wood (commonly known as Bawang wood in Indonesian regions), scientifically known as Azadirachta excelsa (Jack) M. Jacobs, is native to Bengkulu and is widely used in the furniture industry due to its strength, durability, and resistance to termite attack. However, the extensive use of this wood is not matched by optimal processing of sawdust waste. Previous research [ 15 ] has shown that A. excelsa wood contains 46.42% cellulose and 33.16% lignin, which makes it a promising raw material for activated carbon production, adding economic value to the sawdust waste. Chitosan-activated carbon composites derived from neem wood are synthesized as beads. They are expected to be more easily applied as adsorbents for dye wastewater due to their ease of separation from the filtrate. 2. Methodology 2.1 Equipment/Tools/Materials The equipment used in this study included a hot plate stirrer, an analytical balance, a mortar and pestle, a pyrolysis set, an oven, a 100-mesh sieve, a Buchner funnel, and various glassware. The analytical equipment comprised an infrared spectrophotometer (Bruker, Germany), a UV-Visible spectrophotometer (SPECTROstar Nano, BMG Lab), and a Phenom Desktop Scanning Electron Microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX) (Thermo Fisher Scientific). The materials used included sawdust from A. excelsa wood (obtained from furniture manufacturers in Bengkulu), sodium hydroxide (Merck), hydrochloric acid (Merck), polyvinyl alcohol (PVA) (BM = 13,000–23,000) (Sigma Aldrich), chitosan with a degree of deacetylation > 75% (Himedia), acetic acid (Merck), Congo Red (Merck), Methylene Blue (Merck), demineralized water (Brataco Chem, Indonesia), and Whatman 42 filter paper. 2.2 Preparation of Azadirachta excelsa (AE) Carbon The carbon used in this study was produced from sawdust of A. excelsa , a byproduct of the furniture industry. It was prepared by pyrolysis at 300°C for 6 hours. Subsequently, the resulting charcoal was activated with 0.1 M HCl for 24 hours, then separated and washed with deionized water until reaching a neutral pH. The activated carbon was dried at 105°C for 1 hour and sieved using a 100-mesh sieve. 2.3 Synthesis of Chitosan Beads and Chitosan-Activated Carbon Composite Beads Chitosan beads (CB) and chitosan-activated carbon composite beads (CAECB) were synthesized by dissolving 0.5 g of PVA powder in 25 mL of deionized water and stirring using a magnetic stirrer until dissolved at 70°C. Then, 0.1 g of activated carbon was added to 25 mL of 3% acetic acid, followed by the addition of 0.9 g of chitosan. The mixture was stirred until a gel was formed. Both solutions were then mixed and stirred for 10 minutes at 70°C [ 16 ]. Bead formation was done by transferring the chitosan-carbon mixture into a 1 mL syringe and slowly dripping it to form uniform beads into a beaker containing 2.5 M NaOH solution. The beads were stirred for 1 hour to complete the formation, then filtered and washed with deionized water until neutral pH. The neutralized adsorbents were dried in an oven at 100°C until a constant weight was achieved. The same procedure was followed for carbon compositions of 0.3 g, 0.5 g, and 0.7 g of A. excelsa . 2.4 Effect of Carbon Composition of Azadirachta excelsa on Chitosan Beads in Methylene Blue and Congo Red Adsorption A total of 10 mL of Methylene Blue (MB) and 10 mL of Congo Red (CR) solutions were adsorbed using 0.1 g of various adsorbent compositions (CAE 0; CAECB 0.1; CAECB 0.3; CAECB 0.5; and CAECB 0.7). The mixtures were shaken at 200 rpm for 30 minutes. The adsorbent was then separated, and the filtrate was analyzed using a UV-Vis spectrophotometer at its maximum wavelength. The best composition was then characterized and further studied for adsorption. 2.5 Characterization of Adsorbents Functional groups were determined using the Fourier Transform Infrared Spectroscopy (FTIR), and surface morphology was observed using the Scanning Electron Microscopy (SEM). The stability and deformation of the adsorbents in solvents were analyzed by swelling studies, where the adsorbents were immersed in deionized water for 1, 2, 3, 4, and 24 hours. 2.6 Effect of Contact Time (Adsorption Kinetics Study) The effect of contact time was studied from 15 to 150 minutes. The adsorbent with the best composition of A. excelsa carbon (AE) for each dye (0.1 g) was added to 10 mL of Methylene Blue and Congo Red solutions and shaken at 200 rpm for 15, 30, 45, 60, 90, 120, and 150 minutes. The adsorbent and filtrate were separated by decantation, and the filtrate absorbance was measured using a UV-Vis spectrophotometer at its maximum wavelength. Adsorption kinetics were analyzed using the pseudo-first-order and pseudo-second-order kinetic models as follows. Pseudo-first-order kinetics ln (q e – q t ) = log (q e ) – k 1 t (1) Where q t is the amount of adsorbate adsorbed by the adsorbent at time t (mg/g), q e is the amount adsorbed at equilibrium (mg/g), and k 1 is the rate constant of the pseudo-first-order reaction (1/min). Pseudo-second-order kinetics $$\:\frac{t}{{q}_{\left(t\right)}}=\:\frac{1}{{k}_{2}{q}_{e}^{2}}+\:\frac{1}{{q}_{e}}\:t$$ 2 By plotting t/q t against t , q e and k 2 could be determined from the slope and intercept [ 17 ]. 2.7 Effect of Initial Concentration (Adsorption Isotherm Study) The investigation into the effect of initial concentration was conducted with concentrations of 10, 25, 50, 100, 150, and 200 ppm. A total of 0.1 g of the adsorbent was added to each solution and shaken for the optimum contact time determined in previous experiments. Subsequently, the adsorbent and filtrate were separated by decantation, and the absorbance of the filtrate was measured using a UV-Vis spectrophotometer at the maximum wavelength. The adsorption isotherms for Methylene Blue and Congo Red were studied using two models: the Langmuir and the Freundlich isotherms. The Langmuir Isotherm : \(\:\frac{Ce}{Qe}\) = \(\:\frac{1}{{K}_{L}\:{q}_{m}}\) + \(\:\frac{Ce}{{q}_{m}}\) (3) Where q e is the amount of dye adsorbed per unit of adsorbent (mg/g), C e is the equilibrium concentration (the concentration that is not adsorbed by the adsorbent) (mg/L), q m is the Langmuir maximum adsorption capacity (mg/g), and KL is the Langmuir constant, representing the adsorption energy (L/mg). The Freundlich Isotherm : ln q e = ln K F + \(\:\frac{1}{n}\) ln C e (4) Where q e is the amount of dye adsorbed per unit of adsorbent (mg/g), C e is the equilibrium concentration (mg/L), and K F is the Freundlich constant . 3. Results and Discussion 3.1. Characterization of Chitosan Beads and Chitosan-Activated Carbon Composite Beads Using Fourier Transform Infrared Spectroscopy and Scanning Electron Microscopy The adsorbents were characterized using FTIR to analyze the functional groups and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX) to examine the surface morphology of the adsorbents. Functional group characterization was conducted on chitosan, chitosan beads, activated carbon, and chitosan-activated carbon composite beads. The FTIR spectra of chitosan, chitosan beads, activated carbon, and chitosan-activated carbon composite beads are presented in Fig. 1 . The CAECB analyzed had a chitosan-to-carbon ratio of 0.5 (AE 0.5). As shown in Fig. 1 , the broad absorption between 3000–3500 cm − 1 corresponds to the stretching vibration of -OH, which overlaps with NH, indicating the presence of hydroxyl and amine groups [ 18 ]. Absorption at 1650 cm − 1 and 1559 cm − 1 confirms the presence of the C = O stretching vibration of Amide I and NH bending of Amide II, confirming the amine structure in chitosan, chitosan beads, and chitosan-activated carbon composite beads. The C-O-C stretching and β-glycosidic ring vibrations observed at 1018–1083 cm − 1 and 886–894 cm − 1 confirm the characteristic glycosidic bonds of chitosan [ 2 ]. The absorption peaks at 1026 cm − 1 and 1060 cm − 1 in the composite beads confirm the C-O stretching and C-N bending bonds, indicating successful formation of CAECB [ 12 ]. This finding is further supported by the decrease in absorption intensity at 3259 cm − 1 , indicating interactions between OH and NH groups. The CH stretching peak still appeared in the composite beads, indicating that the aliphatic groups remained intact after the material was formed into composite beads. The morphology of CB and CAECB was analyzed using SEM-EDX. Figure 2 shows that CB without added carbon had a more homogeneous and uniform surface, while the CB with A. excelsa wood activated carbon exhibited a more textured surface morphology. The addition of activated carbon to the composite results in the formation of pores on the composite surface. The presence of pores in the A. excelsa wood carbon chitosan beads could enhance the adsorbent's surface area, optimizing the interaction between the adsorbent and adsorbate. Based on the EDX spectra (see Fig. 3 ) of the observed areas, the main components of CB and CAECB consisted of carbon, oxygen, and nitrogen. In CB, the atomic composition was 61.6% carbon, 2.1% nitrogen, and 36.3% oxygen. In CAECB, the atomic composition consisted of 51.2% carbon, 1.3% nitrogen, 45.2% oxygen, and 2.3% sodium. The presence of sodium in CAECB was likely due to NaOH used in the CAECB preparation process, which was retained in the carbon pores on the adsorbent surface. The decrease in nitrogen content in CAECB was attributed to the reduced chitosan content in the beads due to the increased carbon content. 3.2. Effect of A. excelsa Carbon Composition on Chitosan Beads for Adsorption of Methylene Blue and Congo Red The difference in the composition of activated carbon to chitosan had a significant effect on its adsorption capacity for Congo Red and Methylene Blue dyes. Chitosan contains active groups such as -NH 2 and -OH, which can be used to remove pollutants from water, such as heavy metal ions and dyes [ 19 ]. Based on Fig. 4 , it is evident that as the carbon composition in the composite increased, the adsorption capacity for Methylene Blue increased. However, the opposite occurred for Congo Red, where increasing the carbon composition in the composite resulted in a decreased adsorption capacity for Congo Red. In the case of Methylene Blue adsorption, the higher the composition of activated carbon in the composite, the higher the adsorption capacity for Methylene Blue. On the other hand, for Congo Red adsorption, the higher the carbon composition in the composite, the lower the adsorption capacity for Congo Red. CB are more effective at adsorbing anionic dyes due to the amino groups in chitosan. Chitosan has -NH 2 groups that, in solution, can become protonated and thus interact electrostatically with Congo Red, an anionic dye [ 19 ]. The higher the chitosan ratio in the composite, the greater its adsorption capacity for Congo Red. In contrast, Methylene Blue forms positive ions in solution, so the higher the chitosan composition in the composite, the lower its adsorption capacity for Methylene Blue. In addition to surface area and porosity, activated carbon also has functional groups that play an active role in adsorbing Methylene Blue, such as -OH, -C = O, -NH 2 , -C = S, -C-O-C, -S = O, dan -C = N [ 20 ]. Methylene Blue is a cationic dye that exists as a positive ion in solution and will easily accumulate on negatively charged surfaces through electrostatic interaction [ 21 ]. Thus, based on this research, the best adsorbent for Methylene Blue was Chitosan-Activated Carbon Composite Beads (CAECB) 0.5, while the best adsorbent for Congo Red was chitosan beads (CB). 3.3. Swelling and Deformation Studies The swelling degree refers to a biopolymer's ability to absorb water. The addition of activated carbon, a porous material, increases water absorption. Therefore, when compared to CB without activated carbon, the swelling degree of CAECB was higher. However, the swelling degree decreased as the amount of activated carbon added increased. This was due to the reduced amount of polymer in the composite. Swelling occurs due to the expansion of the polymeric network from water absorption; thus, the drastic reduction in polymer content in the composite decreases its swelling degree [ 22 ]. Based on Fig. 5 , the swelling degree increased over time as the soaking duration increased. However, the beads did not undergo any damage during soaking for 1, 2, 3, 4, and 5 days. Therefore, it could be concluded that the synthesized adsorbent was stable when used in water solvents. 3.4. Effect of Contact Time (Kinetic Study) This study used a batch system at room temperature and neutral pH to investigate the adsorption of Methylene Blue and Congo Red at a concentration of 25 mg/L. The contact time was varied from 0 to 150 minutes. As shown in Fig. 6 , the results indicate that during the first 15 minutes, Methylene Blue and Congo Red adsorbed rapidly onto the surface of the adsorbent, while the subsequent adsorption process occurred at a slower rate. This initial difference in adsorption speed is related to the availability of active sites on the adsorbent that have not yet bonded with the adsorbate. Over time, the number of available active sites decreases as they become occupied by the adsorbate [ 23 ]. The adsorption of Methylene Blue on both adsorbents reached equilibrium in 60 minutes, while the adsorption of Congo Red reached equilibrium in 150 minutes. The difference in equilibrium times for the two dyes is attributed to the varying particle sizes of each compound, with Congo Red having larger particles compared to Methylene Blue. Larger molecules require more time to diffuse into the pores of the adsorbent. The correlation between contact time and adsorption capacity ( q t ) is shown in Fig. 6 . Two kinetic models (pseudo-first-order and pseudo-second-order) were employed to study the adsorption kinetics of CB and CAECB on Methylene Blue and Congo Red. The experimental conditions involved an initial concentration of 25 ppm, 0.1 g adsorbent, and varying contact times between 10 and 150 minutes. According to Table 1 , the pseudo-second-order kinetic model exhibited a higher R² value than the pseudo-first-order model for Methylene Blue and Congo Red, using both CB and CAECB adsorbents. Therefore, it can be concluded that the pseudo-second-order kinetic model better described the adsorption kinetics in this study. The pseudo-first-order model (Lagergren) is generally associated with physisorption mechanisms, which are affected by physical interactions such as van der Waals forces and external mass transfer. In contrast, the pseudo-second-order model (Ho & McKay) better represents chemisorption, which involves strong chemical interactions such as covalent bond formation or ion exchange [ 24 , 25 ]. Theoretically, the pseudo-second-order kinetic model suggests that adsorption involves more than one active site [ 26 ]. Based on the values of the rate constant for the pseudo-second-order model, the order of adsorption performance was as follows: CAECB-MB > CB-MB > CB-CR > CAECB-CR, indicating that, kinetically, CAECB was more effective for Methylene Blue adsorption, while CB was better suited for Congo Red. Table 1 Adsorption Kinetic Parameters Adsorben Kinetic Models Parameters Methylene blue Congo Red CB CAECB Pseudo first order (PFO) Qe k 1 R 2 qe k 1 R 2 0,1988 0,0068 0,0423 0,2172 0,0296 0,8695 1,0279 0,0257 0,9859 1,1590 0,0131 0,8514 CB CAECB Pseudo second order (PSO) Qe k 2 R 2 qe k 2 R 2 1,4209 0,2168 0,9796 2,4987 0,2724 0,9999 2,4248 0,0429 0,9995 1,8754 0,0086 0,9138 3.5 Effect of Initial Concentrations (Adsorption Isotherm) The initial concentration of the adsorbate significantly affects the maximum adsorption capacity at equilibrium ( q e ) [ 13 ]. In this regard, the effect of initial concentration was investigated by varying the adsorbate concentration between 1–200 mg/L. The results indicate that as the concentration of the dye increased, the adsorption capacity also increased. This is related to the greater driving force for mass transfer that occurs at higher adsorbate concentrations [ 19 ]. As the adsorbate concentration increases, the number of interactions between the adsorbent and adsorbate increases, filling the active sites on the adsorbent until a saturated condition is reached, resulting in equilibrium [ 13 ]. Based on Fig. 7 , the adsorption capacity of CAECB 0.5 for Methylene Blue was significantly higher compared to CB. In neutral solutions and at higher pH values (pH > isoelectric point, pH ≈ 3), Congo Red was negatively charged, while Methylene Blue was positively charged [ 27 ]. The difference in charges between the adsorbates leads to varying interactions with the adsorbents. Hence, CAECB demonstrated better adsorption capacity for Methylene Blue, while CB was more effective for Congo Red. Adsorption isotherm analysis is a crucial step in understanding the interactions between the adsorbate and the adsorbent surface, as well as estimating the maximum adsorption capacity. In this study, two commonly used isotherm models, the Langmuir and the Freundlich, were applied to evaluate the adsorption capabilities of two adsorbents (CB and CAECB) for two types of dyes, Methylene Blue and Congo Red. The Langmuir model postulates that adsorption takes place as a single molecular layer on a finite number of specific sites distributed over the adsorbent surface [ 28 ]. The fitting results show that the CAECB adsorbent had a much higher maximum adsorption capacity (a) for Methylene Blue (27.32 mg/g) compared to CB (8.49 mg/g), indicating that modification of CB significantly enhanced its ability to adsorb Methylene Blue. On the other hand, the highest correlation coefficient (R² = 0.9962) was obtained for the CB–MB system, indicating that the Langmuir model most accurately described this system. For Congo Red adsorption, the highest Langmuir capacity was found with the CB adsorbent (a = 11.53 mg/g), compared to CAECB (a = 4.47 mg/g), suggesting that Congo Red adsorption was more effective on the CB surface. The Langmuir affinity constant (b) for Methylene Blue adsorption on CAECB was also relatively high (0.646), reflecting strong interactions between the cationic dye and the modified adsorbent surface. The Freundlich model, which reflects multilayer adsorption on a heterogeneous surface, also provided good results, especially for the CAECB–MB system, with the highest coefficient of determination (R² = 0.9978). The adsorption intensity parameter ( n ) for this system was 1.705, indicating a favorable adsorption process ( n > 1). The highest K value was also observed in the CAECB–MB system ( K = 9.247 mg/g), demonstrating that the CAECB surface had a strong affinity for Methylene Blue. In the CB–CR system, the value of n was very high (5.21), indicating that while the maximum adsorption capacity (a) was not as large, the surface interactions were highly favorable.. Overall, the results indicate that the adsorption of Methylene Blue (a cationic dye) was more effective on the modified CAECB adsorbent, which likely had a greater negative surface charge due to the increased functional groups after modification. In contrast, the adsorption of Congo Red (an anionic dye) was more effective on CB, possibly due to differences in surface charge characteristics and pore structure. Table 2 Adsorption Isotherm Parameters Adsorbent Adsorbate Langmuir (Ce/(x/m) = 1/ab + 1/a Ce Freundlich Log(x/m) = log K + 1/n Log Ce a (mg/gram) b R 2 n K (mg/gram) R 2 CB MB 8,49 0,026 0,9962 1,616 2,832 0,9804 CAECB MB 27,32 0,646 0,965 1,705 9,247 0,9978 CB CR 11,53 0,17 0,9408 5,21 4,617 0,9224 CAECB CR 4,47 0,262 0,9899 3,61 1,268 0,9323 Based on the previous discussion, the most dominant interaction was chemisorption, with potential interactions including hydrogen bonding, van der Waals forces, pore filling by the adsorbate, and π-π interactions between the aromatic rings of the dye and the carbon surface [ 9 ] (see Fig. 8 ). At neutral pH, methylene blue (MB) predominantly exists in a cationic form, whereas congo red (CR) is present in an anionic form. Accordingly, MB is electrostatically attracted to the negatively charged sites of the adsorbent, while CR interacts with the positively charged sites. In addition, the hydroxyl (–OH) groups on the adsorbent surface facilitate hydrogen bonding with the amine functionalities of both CR and MB. Furthermore, π–π interactions are likely to occur between the aromatic rings of the dye molecules and the aromatic domains of the carbonaceous components incorporated within the adsorbent. 4. Conclusion Chitosan-activated carbon composite beads (CAECB) exhibited superior adsorption capacity compared to chitosan beads (CB) for Methylene Blue, with an optimum capacity of 27.32 mg/g. However, their adsorption capacity for Congo Red decreased to 8,39 mg/g. The adsorption kinetics for both adsorbents concerning Methylene Blue and Congo Red followed the pseudo-second-order kinetic model. The adsorption isotherm studies revealed that the adsorption of Methylene Blue using CB and CAECB followed the Langmuir isotherm with an R² value of 0.9962 and the Freundlich isotherm with an R² value of 0.9978, respectively. Declarations Acknowledgment The authors express their gratitude to Universitas Bengkulu for funding this research through the Budget Implementation List ( DIPA ) of Universitas Bengkulu for the fiscal year 2024, Contract Number: 2916/UN30.15/PT/2024. Funding This research was supported by funding from the University of Bengkulu Data Availability The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request. Conflict of Interest The researchers consent to the submission of this manuscript for review and declare that they have no competing interests. Ethical Approval Not Applicable Informed Consent Not Applicable References Kumar P, Uppaluri RVS. (2025) Environmental sustainability through adsorption : A review of chitosan’s potential in dye pollution remediation. Sustain Chem Pharm 46:102096. https://doi.org/10.1016/j.scp.2025.102096. El S, El A, Raza A, Moussaoui F, Rakcho Y, El F, et al. 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Mater Today Chem 16:100233. https://doi.org/10.1016/j.mtchem.2019.100233. Abdulhameed AS, Heider AL Omari R, Althahban S, Jazaa Y, Abualhaija M, Algburi S. (2025) Green vegetable waste composited with chitosan as a bioadsorbent for effective removal of methylene blue dye from water : Insight into physicochemical and adsorption characteristics. Biomass and Bioenergy 193:1–13. https://doi.org/10.1016/j.biombioe.2024.107528. Jung S, Jung M, Yoon J, Kim J, Jin H, Won H. (2024) Chitosan-derived activated carbon / chitosan composite beads for adsorptive removal of methylene blue and acid orange 7 dyes. React Funct Polym 204. https://doi.org/10.1016/j.reactfunctpolym.2024.106028. Idohou EA, Fatombi JK, Osseni SA, Agani I, Neumeyer D, Verelst M, et al. (2020) Preparation of activated carbon/chitosan/Carica papaya seeds composite for efficient adsorption of cationic dye from aqueous solution. Surfaces and Interfaces 21. https://doi.org/10.1016/j.surfin.2020.100741. Hubbe MA, Azizian S, Douven S. (2019) Implications of Apparent Pseudo Second Order Adsorption Kinetics onto Cellulosic Materia. BioResources 14:7582–626. Qiu H, Lv L, Pan B, Zhang Q, Zhang W, Zhang Q. (2009) Critical review in adsorption kinetic models *. J Zhejiang Univ Sci A 10:716–24. https://doi.org/10.1631/jzus.A0820524. Purnaningtyas MAK, Sudiono S, Siswanta D. (2020) Synthesis of activated carbon/chitosan/alginate beads powder as an adsorbent for methylene blue and methyl violet 2b dyes. Indones J Chem 20:1119–30. https://doi.org/10.22146/ijc.49026. Litefti K, Freire MS, Stitou M, González-álvarez J. (2019) Adsorption of an anionic dye ( Congo red ) from aqueous solutions by pine bark. Sci Rep 9:1–11. https://doi.org/10.1038/s41598-019-53046-z. Abid LH, Mussa ZH, Farhan I, Al-ameer LR, Falih H, Al-saedi S, et al. (2025) Walnut Shell as a bio-activated carbon for elimination of malachite green from its aqueous solution : Adsorption isotherms , kinetics and thermodynamic studies. Results Chem 14:1–10. https://doi.org/10.1016/j.rechem.2025.102124. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7678806","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":529608659,"identity":"8453d736-fcee-428d-aa61-921c25635173","order_by":0,"name":"Ria Nurwidiyani","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+ElEQVRIiWNgGAWjYDACZgjFw3AYRBnYAEV4oCIHiNOSRoQWOIAoOAxWjCyCAczbmZ895t1zWIbvOI/h54KC84nb2XkPMPyoYZDhw6FF5jCbuTHPs8M8kod5jKVnGNxO3NnMl8DYc4yBRxKHFglmBjNpngO3eQwO8xhI8wC1bAAyGHgbGHgMcGph/wbTYvybx+AcWAvjX7xaeOC2ABkGB8BamPHbwlMmOefAf6Bf2MqseQySjTcc5ks4LHNMArdf+I9vk3hzIM2e7/zhzbd5/tjJbjh/9uDDNzU29rhCDAlwGMCZQMUSBNUDAfsDYlSNglEwCkbBCAQA2phTFEniIjkAAAAASUVORK5CYII=","orcid":"","institution":"University of Bengkulu","correspondingAuthor":true,"prefix":"","firstName":"Ria","middleName":"","lastName":"Nurwidiyani","suffix":""},{"id":529608660,"identity":"23346974-e24d-422c-815a-7125c6d1b8c7","order_by":1,"name":"Deni Agus Triawan","email":"","orcid":"","institution":"University of 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15:42:56","extension":"html","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":105715,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/19ea9e075aa3d58946080572.html"},{"id":94472438,"identity":"988ac22d-a7dc-4f13-97fe-71473716b245","added_by":"auto","created_at":"2025-10-27 15:41:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":744057,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of chitosan, chitosan beads, \u003cem\u003eA. excelsa\u003c/em\u003e wood carbon, and chitosan-activated carbon composite beads.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/13ee72846c0d056818674629.png"},{"id":94473230,"identity":"74921286-d020-488c-8a36-076207270b45","added_by":"auto","created_at":"2025-10-27 15:43:29","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1293025,"visible":true,"origin":"","legend":"\u003cp\u003eSurface morphology and EDX spectra: (a) CB (1000x magnification); (b) CB (10,000x magnification); (c) CAECB 0.5 (1000x magnification); (d) CAECB 0.5 (10,000x magnification).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/0439f63443687144b1cc928b.png"},{"id":94473055,"identity":"df502e81-e358-42f6-85d7-ee75154a8580","added_by":"auto","created_at":"2025-10-27 15:42:28","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":378152,"visible":true,"origin":"","legend":"\u003cp\u003eEDX spectra: (a) CB; (b) CAECB\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/0ddf1481f8b702a6b6b14bb8.png"},{"id":94472270,"identity":"3b2458e9-ccb7-499d-8b38-9e520bef9a51","added_by":"auto","created_at":"2025-10-27 15:41:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":59190,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of \u003cem\u003eA. excelsa\u003c/em\u003e carbon content (grams) in chitosan beads on Methylene Blue and Congo Red adsorption.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/470fa7eb93f3258c029d7ee3.png"},{"id":94472559,"identity":"18412e39-e829-4bc2-ab03-99a128819bd0","added_by":"auto","created_at":"2025-10-27 15:41:58","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":58584,"visible":true,"origin":"","legend":"\u003cp\u003eSwelling of CB and CAECB.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/34137b5fffd292f81e8f4f24.png"},{"id":94472554,"identity":"2eeb4980-a0e7-4cec-a9b0-faf5045338e5","added_by":"auto","created_at":"2025-10-27 15:41:57","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":99396,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of contact time of CB and CAECB adsorbents on Methylene Blue and Congo Red adsorption\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/a7c020d475e9677db8126b91.png"},{"id":94472901,"identity":"06ace8aa-d861-474b-bd26-599317c48e9c","added_by":"auto","created_at":"2025-10-27 15:42:03","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":83813,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of initial concentration of Methylene Blue and Congo Red.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/913aa3d4211126431eb48442.png"},{"id":94472440,"identity":"ff47ec98-9abe-4b84-a45f-15c020d49343","added_by":"auto","created_at":"2025-10-27 15:41:44","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":286360,"visible":true,"origin":"","legend":"\u003cp\u003eEstimated interaction of CAECB with methylene blue and congo red\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/b53b8b72a3d66960f2cad6b0.png"},{"id":95222210,"identity":"bd105626-16c1-4a06-8538-93c114ca65da","added_by":"auto","created_at":"2025-11-05 16:20:19","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4507185,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7678806/v1/9e51ab4c-d395-4d9e-a6ef-7d451230e722.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Biobased Chitosan–Carbon Composite Beads from Azadirachta excelsa for Dual Dye Adsorption","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eGlobally, pollution remains a pressing issue that attracts significant attention. As human civilization advances and the demand for technological development increases, the resulting waste from various production processes also proliferates. Pollution of air, water, and soil has become inevitable. Water, a vital resource for all living organisms on Earth, is increasingly difficult to access in clean and usable forms. Every year, more than 3.575\u0026nbsp;million lives are lost due to water-related diseases, while over two billion people still struggle to access safe drinking water due to polluted water sources [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Industrial wastewater pollution presents a significant challenge to industry stakeholders and researchers, requiring urgent attention due to its substantial environmental and public health impacts [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Textile dyeing industries are one of the primary contributors to environmental pollution, discharging more than 300,000 tons of wastewater containing dyes annually [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eWastewater from dyeing processes is typically treated using various techniques, photocatalysis or photolysis driven by UV or visible light, adsorption on to activation carbon, advanced oxidation process, ozonation and electro precipitation represent strategies for waste water treatment, however this methods are often limited by hight operational cost and significant energy demand [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Among these methods, adsorption is the most commonly used, although careful selection of adsorbents is crucial. Adsorption offers several advantages over other methods, including high efficiency in dye removal, applicability to a wide range of dyes, and high regeneration capability. In contrast, other methods often have limitations, such as the generation of difficult-to-handle sludge in precipitation, filter clogging in filtration methods, or the failure of biological methods when interfering substances disrupt microbial activity [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eChitosan, a derivative of chitin, is predominantly sourced from the exoskeletons of arthropods, such as crabs, shrimp, and lobsters. As a biomaterial with cationic properties, chitosan can adsorb heavy metals and textile dyes from wastewater [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Several studies have explored the use of chitosan and it\u0026rsquo;s modification for dye adsorption, such as its application in Congo Red adsorption [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], Direct Blue-218 adsorption [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], and Red BF-4B adsorption [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. However, chitosan's use as an adsorbent has limitations due to its solubility in acidic conditions.\u003c/p\u003e\u003cp\u003eWhen combined with other materials, chitosan forms composites with improved dye adsorption capabilities. For instance, chitosan/alumina composites have been employed for Methyl Orange adsorption [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], chitosan microspheres for Methyl Orange adsorption [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], chitosan-modified activated carbon beads for Congo Red adsorption [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], and chitosan-activated carbon-metal oxide hydrogel beads for adsorbing anionic and cationic dyes [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eTo enhance surface area and stability, chitosan can be combined with activated carbon, a porous material that increases the adsorbent's surface area, thereby improving its adsorption capacity. The addition of fillers to composites can further enhance the mechanical properties of the composite matrix. Activated carbon can be produced during the pyrolysis process of wood containing lignin, cellulose, and lignocellulose [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Neem wood (commonly known as \u003cem\u003eBawang\u003c/em\u003e wood in Indonesian regions), scientifically known as \u003cem\u003eAzadirachta excelsa\u003c/em\u003e (Jack) M. Jacobs, is native to Bengkulu and is widely used in the furniture industry due to its strength, durability, and resistance to termite attack. However, the extensive use of this wood is not matched by optimal processing of sawdust waste. Previous research [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] has shown that \u003cem\u003eA. excelsa\u003c/em\u003e wood contains 46.42% cellulose and 33.16% lignin, which makes it a promising raw material for activated carbon production, adding economic value to the sawdust waste. Chitosan-activated carbon composites derived from neem wood are synthesized as beads. They are expected to be more easily applied as adsorbents for dye wastewater due to their ease of separation from the filtrate.\u003c/p\u003e"},{"header":"2. Methodology","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003cp\u003e2.1 Equipment/Tools/Materials\u003c/p\u003e\n \u003cp\u003eThe equipment used in this study included a hot plate stirrer, an analytical balance, a mortar and pestle, a pyrolysis set, an oven, a 100-mesh sieve, a Buchner funnel, and various glassware. The analytical equipment comprised an infrared spectrophotometer (Bruker, Germany), a UV-Visible spectrophotometer (SPECTROstar Nano, BMG Lab), and a Phenom Desktop Scanning Electron Microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX) (Thermo Fisher Scientific). The materials used included sawdust from \u003cem\u003eA. excelsa\u003c/em\u003e wood (obtained from furniture manufacturers in Bengkulu), sodium hydroxide (Merck), hydrochloric acid (Merck), polyvinyl alcohol (PVA) (BM\u0026thinsp;=\u0026thinsp;13,000\u0026ndash;23,000) (Sigma Aldrich), chitosan with a degree of deacetylation\u0026thinsp;\u0026gt;\u0026thinsp;75% (Himedia), acetic acid (Merck), Congo Red (Merck), Methylene Blue (Merck), demineralized water (Brataco Chem, Indonesia), and Whatman 42 filter paper.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003cp\u003e2.2 Preparation of \u003cem\u003eAzadirachta excelsa\u003c/em\u003e (AE) Carbon\u003c/p\u003e\n \u003cp\u003eThe carbon used in this study was produced from sawdust of \u003cem\u003eA. excelsa\u003c/em\u003e, a byproduct of the furniture industry. It was prepared by pyrolysis at 300\u0026deg;C for 6 hours. Subsequently, the resulting charcoal was activated with 0.1 M HCl for 24 hours, then separated and washed with deionized water until reaching a neutral pH. The activated carbon was dried at 105\u0026deg;C for 1 hour and sieved using a 100-mesh sieve.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003cp\u003e2.3 \u003cstrong\u003eSynthesis of Chitosan Beads and Chitosan-Activated Carbon Composite Beads\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003eChitosan beads (CB) and chitosan-activated carbon composite beads (CAECB) were synthesized by dissolving 0.5 g of PVA powder in 25 mL of deionized water and stirring using a magnetic stirrer until dissolved at 70\u0026deg;C. Then, 0.1 g of activated carbon was added to 25 mL of 3% acetic acid, followed by the addition of 0.9 g of chitosan. The mixture was stirred until a gel was formed. Both solutions were then mixed and stirred for 10 minutes at 70\u0026deg;C [\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eBead formation was done by transferring the chitosan-carbon mixture into a 1 mL syringe and slowly dripping it to form uniform beads into a beaker containing 2.5 M NaOH solution. The beads were stirred for 1 hour to complete the formation, then filtered and washed with deionized water until neutral pH. The neutralized adsorbents were dried in an oven at 100\u0026deg;C until a constant weight was achieved. The same procedure was followed for carbon compositions of 0.3 g, 0.5 g, and 0.7 g of \u003cem\u003eA. excelsa\u003c/em\u003e.\u003c/p\u003e\u003cspan\u003e\u003cstrong\u003e2.4 Effect of Carbon Composition of\u003c/strong\u003e \u003cstrong\u003eAzadirachta excelsa\u003c/strong\u003e \u003cstrong\u003eon Chitosan Beads in Methylene Blue and Congo Red Adsorption\u003c/strong\u003e\u003c/span\u003e\u003cbr\u003e\n \u003cp\u003eA total of 10 mL of Methylene Blue (MB) and 10 mL of Congo Red (CR) solutions were adsorbed using 0.1 g of various adsorbent compositions (CAE 0; CAECB 0.1; CAECB 0.3; CAECB 0.5; and CAECB 0.7). The mixtures were shaken at 200 rpm for 30 minutes. The adsorbent was then separated, and the filtrate was analyzed using a UV-Vis spectrophotometer at its maximum wavelength. The best composition was then characterized and further studied for adsorption.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003cp\u003e2.5 Characterization of Adsorbents\u003c/p\u003e\n \u003cp\u003eFunctional groups were determined using the Fourier Transform Infrared Spectroscopy (FTIR), and surface morphology was observed using the Scanning Electron Microscopy (SEM). The stability and deformation of the adsorbents in solvents were analyzed by swelling studies, where the adsorbents were immersed in deionized water for 1, 2, 3, 4, and 24 hours.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003cp\u003e2.6 Effect of Contact Time (Adsorption Kinetics Study)\u003c/p\u003e\n \u003cp\u003eThe effect of contact time was studied from 15 to 150 minutes. The adsorbent with the best composition of \u003cem\u003eA. excelsa\u003c/em\u003e carbon (AE) for each dye (0.1 g) was added to 10 mL of Methylene Blue and Congo Red solutions and shaken at 200 rpm for 15, 30, 45, 60, 90, 120, and 150 minutes. The adsorbent and filtrate were separated by decantation, and the filtrate absorbance was measured using a UV-Vis spectrophotometer at its maximum wavelength. Adsorption kinetics were analyzed using the pseudo-first-order and pseudo-second-order kinetic models as follows.\u003c/p\u003e\n \u003cp\u003e\u003cem\u003ePseudo-first-order kinetics\u003c/em\u003e\u003c/p\u003e\n \u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003e\u003cem\u003eln (q\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e\u0026ndash; q\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e)\u0026thinsp;=\u0026thinsp;log (q\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e) \u0026ndash; k\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003et\u003c/em\u003e (1)\u003c/p\u003e\n \u003c/div\u003e\n \u003cp\u003eWhere \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e is the amount of adsorbate adsorbed by the adsorbent at time \u003cem\u003et\u003c/em\u003e (mg/g), \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e is the amount adsorbed at equilibrium (mg/g), and \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e is the rate constant of the pseudo-first-order reaction (1/min).\u003c/p\u003e\n \u003cp\u003e\u003cem\u003ePseudo-second-order kinetics\u003c/em\u003e\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cp class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\:\\frac{t}{{q}_{\\left(t\\right)}}=\\:\\frac{1}{{k}_{2}{q}_{e}^{2}}+\\:\\frac{1}{{q}_{e}}\\:t$$\u003c/p\u003e\n \u003cp class=\"EquationNumber\"\u003e2\u003c/p\u003e\n \u003c/div\u003e\n \u003cp\u003eBy plotting \u003cem\u003et/q\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e against \u003cem\u003et\u003c/em\u003e, \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e could be determined from the slope and intercept [\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003cp\u003e2.7 Effect of Initial Concentration (Adsorption Isotherm Study)\u003c/p\u003e\n \u003cp\u003eThe investigation into the effect of initial concentration was conducted with concentrations of 10, 25, 50, 100, 150, and 200 ppm. A total of 0.1 g of the adsorbent was added to each solution and shaken for the optimum contact time determined in previous experiments. Subsequently, the adsorbent and filtrate were separated by decantation, and the absorbance of the filtrate was measured using a UV-Vis spectrophotometer at the maximum wavelength.\u003c/p\u003e\n \u003cp\u003eThe adsorption isotherms for Methylene Blue and Congo Red were studied using two models: the Langmuir and the Freundlich isotherms.\u003c/p\u003e\n \u003cp\u003e\u003cem\u003eThe Langmuir Isotherm\u003c/em\u003e:\u003c/p\u003e\n \u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{Ce}{Qe}\\)\u003c/span\u003e\u003c/span\u003e = \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{{K}_{L}\\:{q}_{m}}\\)\u003c/span\u003e\u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{Ce}{{q}_{m}}\\)\u003c/span\u003e\u003c/span\u003e (3)\u003c/p\u003e\n \u003c/div\u003e\n \u003cp\u003eWhere \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e is the amount of dye adsorbed per unit of adsorbent (mg/g), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e is the equilibrium concentration (the concentration that is not adsorbed by the adsorbent) (mg/L), \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003em\u003c/em\u003e\u003c/sub\u003e is the Langmuir maximum adsorption capacity (mg/g), and \u003cem\u003eKL\u003c/em\u003e is the Langmuir constant, representing the adsorption energy (L/mg).\u003c/p\u003e\n \u003cp\u003e\u003cem\u003eThe Freundlich Isotherm\u003c/em\u003e:\u003c/p\u003e\n \u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003e\u003cem\u003eln q\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e= ln K\u003c/em\u003e\u003csub\u003e\u003cem\u003eF\u003c/em\u003e\u003c/sub\u003e \u003cem\u003e+\u003c/em\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{n}\\)\u003c/span\u003e\u003c/span\u003e \u003cem\u003eln C\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e (4)\u003c/p\u003e\n \u003c/div\u003e\n \u003cp\u003eWhere \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e is the amount of dye adsorbed per unit of adsorbent (mg/g), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e is the equilibrium concentration (mg/L), and \u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003eF\u003c/em\u003e\u003c/sub\u003e is the Freundlich constant .\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003e\u003cb\u003e3.1. Characterization of Chitosan Beads and Chitosan-Activated Carbon Composite Beads Using Fourier Transform Infrared Spectroscopy and Scanning Electron Microscopy\u003c/b\u003e\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe adsorbents were characterized using FTIR to analyze the functional groups and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX) to examine the surface morphology of the adsorbents. Functional group characterization was conducted on chitosan, chitosan beads, activated carbon, and chitosan-activated carbon composite beads. The FTIR spectra of chitosan, chitosan beads, activated carbon, and chitosan-activated carbon composite beads are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe CAECB analyzed had a chitosan-to-carbon ratio of 0.5 (AE 0.5). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the broad absorption between 3000\u0026ndash;3500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the stretching vibration of -OH, which overlaps with NH, indicating the presence of hydroxyl and amine groups [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Absorption at 1650 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1559 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e confirms the presence of the C\u0026thinsp;=\u0026thinsp;O stretching vibration of Amide I and NH bending of Amide II, confirming the amine structure in chitosan, chitosan beads, and chitosan-activated carbon composite beads. The C-O-C stretching and β-glycosidic ring vibrations observed at 1018\u0026ndash;1083 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 886\u0026ndash;894 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e confirm the characteristic glycosidic bonds of chitosan [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The absorption peaks at 1026 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1060 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the composite beads confirm the C-O stretching and C-N bending bonds, indicating successful formation of CAECB [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. This finding is further supported by the decrease in absorption intensity at 3259 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, indicating interactions between OH and NH groups. The CH stretching peak still appeared in the composite beads, indicating that the aliphatic groups remained intact after the material was formed into composite beads.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe morphology of CB and CAECB was analyzed using SEM-EDX. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows that CB without added carbon had a more homogeneous and uniform surface, while the CB with \u003cem\u003eA. excelsa\u003c/em\u003e wood activated carbon exhibited a more textured surface morphology. The addition of activated carbon to the composite results in the formation of pores on the composite surface. The presence of pores in the \u003cem\u003eA. excelsa\u003c/em\u003e wood carbon chitosan beads could enhance the adsorbent's surface area, optimizing the interaction between the adsorbent and adsorbate.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBased on the EDX spectra (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) of the observed areas, the main components of CB and CAECB consisted of carbon, oxygen, and nitrogen. In CB, the atomic composition was 61.6% carbon, 2.1% nitrogen, and 36.3% oxygen. In CAECB, the atomic composition consisted of 51.2% carbon, 1.3% nitrogen, 45.2% oxygen, and 2.3% sodium. The presence of sodium in CAECB was likely due to NaOH used in the CAECB preparation process, which was retained in the carbon pores on the adsorbent surface. The decrease in nitrogen content in CAECB was attributed to the reduced chitosan content in the beads due to the increased carbon content.\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003e\u003cb\u003e3.2. Effect of\u003c/b\u003e \u003cb\u003eA. excelsa\u003c/b\u003e \u003cb\u003eCarbon Composition on Chitosan Beads for Adsorption of Methylene Blue and Congo Red\u003c/b\u003e\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe difference in the composition of activated carbon to chitosan had a significant effect on its adsorption capacity for Congo Red and Methylene Blue dyes. Chitosan contains active groups such as -NH\u003csub\u003e2\u003c/sub\u003e and -OH, which can be used to remove pollutants from water, such as heavy metal ions and dyes [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Based on Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, it is evident that as the carbon composition in the composite increased, the adsorption capacity for Methylene Blue increased. However, the opposite occurred for Congo Red, where increasing the carbon composition in the composite resulted in a decreased adsorption capacity for Congo Red. In the case of Methylene Blue adsorption, the higher the composition of activated carbon in the composite, the higher the adsorption capacity for Methylene Blue. On the other hand, for Congo Red adsorption, the higher the carbon composition in the composite, the lower the adsorption capacity for Congo Red. CB are more effective at adsorbing anionic dyes due to the amino groups in chitosan. Chitosan has -NH\u003csub\u003e2\u003c/sub\u003e groups that, in solution, can become protonated and thus interact electrostatically with Congo Red, an anionic dye [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The higher the chitosan ratio in the composite, the greater its adsorption capacity for Congo Red. In contrast, Methylene Blue forms positive ions in solution, so the higher the chitosan composition in the composite, the lower its adsorption capacity for Methylene Blue. In addition to surface area and porosity, activated carbon also has functional groups that play an active role in adsorbing Methylene Blue, such as -OH, -C\u0026thinsp;=\u0026thinsp;O, -NH\u003csub\u003e2\u003c/sub\u003e, -C\u0026thinsp;=\u0026thinsp;S, -C-O-C, -S\u0026thinsp;=\u0026thinsp;O, dan -C\u0026thinsp;=\u0026thinsp;N [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Methylene Blue is a cationic dye that exists as a positive ion in solution and will easily accumulate on negatively charged surfaces through electrostatic interaction [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Thus, based on this research, the best adsorbent for Methylene Blue was Chitosan-Activated Carbon Composite Beads (CAECB) 0.5, while the best adsorbent for Congo Red was chitosan beads (CB).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Swelling and Deformation Studies\u003c/h2\u003e\u003cp\u003eThe swelling degree refers to a biopolymer's ability to absorb water. The addition of activated carbon, a porous material, increases water absorption. Therefore, when compared to CB without activated carbon, the swelling degree of CAECB was higher. However, the swelling degree decreased as the amount of activated carbon added increased. This was due to the reduced amount of polymer in the composite. Swelling occurs due to the expansion of the polymeric network from water absorption; thus, the drastic reduction in polymer content in the composite decreases its swelling degree [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Based on Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the swelling degree increased over time as the soaking duration increased. However, the beads did not undergo any damage during soaking for 1, 2, 3, 4, and 5 days. Therefore, it could be concluded that the synthesized adsorbent was stable when used in water solvents.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.4. Effect of Contact Time (Kinetic Study)\u003c/h2\u003e\u003cp\u003eThis study used a batch system at room temperature and neutral pH to investigate the adsorption of Methylene Blue and Congo Red at a concentration of 25 mg/L. The contact time was varied from 0 to 150 minutes. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the results indicate that during the first 15 minutes, Methylene Blue and Congo Red adsorbed rapidly onto the surface of the adsorbent, while the subsequent adsorption process occurred at a slower rate. This initial difference in adsorption speed is related to the availability of active sites on the adsorbent that have not yet bonded with the adsorbate. Over time, the number of available active sites decreases as they become occupied by the adsorbate [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The adsorption of Methylene Blue on both adsorbents reached equilibrium in 60 minutes, while the adsorption of Congo Red reached equilibrium in 150 minutes. The difference in equilibrium times for the two dyes is attributed to the varying particle sizes of each compound, with Congo Red having larger particles compared to Methylene Blue. Larger molecules require more time to diffuse into the pores of the adsorbent. The correlation between contact time and adsorption capacity (\u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e) is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTwo kinetic models (pseudo-first-order and pseudo-second-order) were employed to study the adsorption kinetics of CB and CAECB on Methylene Blue and Congo Red. The experimental conditions involved an initial concentration of 25 ppm, 0.1 g adsorbent, and varying contact times between 10 and 150 minutes. According to Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the pseudo-second-order kinetic model exhibited a higher R\u0026sup2; value than the pseudo-first-order model for Methylene Blue and Congo Red, using both CB and CAECB adsorbents. Therefore, it can be concluded that the pseudo-second-order kinetic model better described the adsorption kinetics in this study. The pseudo-first-order model (Lagergren) is generally associated with physisorption mechanisms, which are affected by physical interactions such as van der Waals forces and external mass transfer. In contrast, the pseudo-second-order model (Ho \u0026amp; McKay) better represents chemisorption, which involves strong chemical interactions such as covalent bond formation or ion exchange [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Theoretically, the pseudo-second-order kinetic model suggests that adsorption involves more than one active site [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Based on the values of the rate constant for the pseudo-second-order model, the order of adsorption performance was as follows: CAECB-MB\u0026thinsp;\u0026gt;\u0026thinsp;CB-MB\u0026thinsp;\u0026gt;\u0026thinsp;CB-CR\u0026thinsp;\u0026gt;\u0026thinsp;CAECB-CR, indicating that, kinetically, CAECB was more effective for Methylene Blue adsorption, while CB was better suited for Congo Red.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eAdsorption Kinetic Parameters\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAdsorben\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eKinetic Models\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eParameters\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMethylene blue\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCongo Red\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCB\u003c/p\u003e\u003cp\u003eCAECB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003ePseudo first order (PFO)\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eQe\u003c/p\u003e\u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eqe\u003c/p\u003e\u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0,1988\u003c/p\u003e\u003cp\u003e0,0068\u003c/p\u003e\u003cp\u003e0,0423\u003c/p\u003e\u003cp\u003e0,2172\u003c/p\u003e\u003cp\u003e0,0296\u003c/p\u003e\u003cp\u003e0,8695\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1,0279\u003c/p\u003e\u003cp\u003e0,0257\u003c/p\u003e\u003cp\u003e0,9859\u003c/p\u003e\u003cp\u003e1,1590\u003c/p\u003e\u003cp\u003e0,0131\u003c/p\u003e\u003cp\u003e0,8514\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCB\u003c/p\u003e\u003cp\u003eCAECB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003ePseudo second order (PSO)\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eQe\u003c/p\u003e\u003cp\u003ek\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003cp\u003e\u003cem\u003eqe\u003c/em\u003e\u003c/p\u003e\u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e1,4209\u003c/p\u003e\u003cp\u003e0,2168\u003c/p\u003e\u003cp\u003e0,9796\u003c/p\u003e\u003cp\u003e2,4987\u003c/p\u003e\u003cp\u003e0,2724\u003c/p\u003e\u003cp\u003e0,9999\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2,4248\u003c/p\u003e\u003cp\u003e0,0429\u003c/p\u003e\u003cp\u003e0,9995\u003c/p\u003e\u003cp\u003e1,8754\u003c/p\u003e\u003cp\u003e0,0086\u003c/p\u003e\u003cp\u003e0,9138\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Effect of Initial Concentrations (Adsorption Isotherm)\u003c/h2\u003e\u003cp\u003eThe initial concentration of the adsorbate significantly affects the maximum adsorption capacity at equilibrium (\u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. In this regard, the effect of initial concentration was investigated by varying the adsorbate concentration between 1\u0026ndash;200 mg/L. The results indicate that as the concentration of the dye increased, the adsorption capacity also increased. This is related to the greater driving force for mass transfer that occurs at higher adsorbate concentrations [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. As the adsorbate concentration increases, the number of interactions between the adsorbent and adsorbate increases, filling the active sites on the adsorbent until a saturated condition is reached, resulting in equilibrium [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Based on Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the adsorption capacity of CAECB 0.5 for Methylene Blue was significantly higher compared to CB. In neutral solutions and at higher pH values (pH\u0026thinsp;\u0026gt;\u0026thinsp;isoelectric point, pH\u0026thinsp;\u0026asymp;\u0026thinsp;3), Congo Red was negatively charged, while Methylene Blue was positively charged [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The difference in charges between the adsorbates leads to varying interactions with the adsorbents. Hence, CAECB demonstrated better adsorption capacity for Methylene Blue, while CB was more effective for Congo Red.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAdsorption isotherm analysis is a crucial step in understanding the interactions between the adsorbate and the adsorbent surface, as well as estimating the maximum adsorption capacity. In this study, two commonly used isotherm models, the Langmuir and the Freundlich, were applied to evaluate the adsorption capabilities of two adsorbents (CB and CAECB) for two types of dyes, Methylene Blue and Congo Red. The Langmuir model postulates that adsorption takes place as a single molecular layer on a finite number of specific sites distributed over the adsorbent surface [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The fitting results show that the CAECB adsorbent had a much higher maximum adsorption capacity (a) for Methylene Blue (27.32 mg/g) compared to CB (8.49 mg/g), indicating that modification of CB significantly enhanced its ability to adsorb Methylene Blue. On the other hand, the highest correlation coefficient (R\u0026sup2; = 0.9962) was obtained for the CB\u0026ndash;MB system, indicating that the Langmuir model most accurately described this system.\u003c/p\u003e\u003cp\u003eFor Congo Red adsorption, the highest Langmuir capacity was found with the CB adsorbent (a\u0026thinsp;=\u0026thinsp;11.53 mg/g), compared to CAECB (a\u0026thinsp;=\u0026thinsp;4.47 mg/g), suggesting that Congo Red adsorption was more effective on the CB surface. The Langmuir affinity constant (b) for Methylene Blue adsorption on CAECB was also relatively high (0.646), reflecting strong interactions between the cationic dye and the modified adsorbent surface. The Freundlich model, which reflects multilayer adsorption on a heterogeneous surface, also provided good results, especially for the CAECB\u0026ndash;MB system, with the highest coefficient of determination (R\u0026sup2; = 0.9978). The adsorption intensity parameter (\u003cem\u003en\u003c/em\u003e) for this system was 1.705, indicating a favorable adsorption process (\u003cem\u003en\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;1). The highest \u003cem\u003eK\u003c/em\u003e value was also observed in the CAECB\u0026ndash;MB system (\u003cem\u003eK\u003c/em\u003e\u0026thinsp;=\u0026thinsp;9.247 mg/g), demonstrating that the CAECB surface had a strong affinity for Methylene Blue.\u003c/p\u003e\u003cp\u003eIn the CB\u0026ndash;CR system, the value of \u003cem\u003en\u003c/em\u003e was very high (5.21), indicating that while the maximum adsorption capacity (a) was not as large, the surface interactions were highly favorable.. Overall, the results indicate that the adsorption of Methylene Blue (a cationic dye) was more effective on the modified CAECB adsorbent, which likely had a greater negative surface charge due to the increased functional groups after modification. In contrast, the adsorption of Congo Red (an anionic dye) was more effective on CB, possibly due to differences in surface charge characteristics and pore structure.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eAdsorption Isotherm Parameters\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eAdsorbent\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eAdsorbate\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c5\" namest=\"c3\"\u003e\u003cp\u003eLangmuir\u003c/p\u003e\u003cp\u003e(Ce/(x/m)\u0026thinsp;=\u0026thinsp;1/ab\u0026thinsp;+\u0026thinsp;1/a Ce\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e\u003cp\u003eFreundlich\u003c/p\u003e\u003cp\u003eLog(x/m)\u0026thinsp;=\u0026thinsp;log K\u0026thinsp;+\u0026thinsp;1/n Log Ce\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003ea\u003c/b\u003e (mg/gram)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003eb\u003c/b\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003eR\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003en\u003c/b\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003eK\u003c/b\u003e\u003c/p\u003e\u003cp\u003e(mg/gram)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003e\u003cb\u003eR\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8,49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0,026\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0,9962\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1,616\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e2,832\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0,9804\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCAECB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e27,32\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0,646\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0,965\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1,705\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e9,247\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0,9978\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e11,53\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0,17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0,9408\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e5,21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e4,617\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0,9224\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCAECB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e4,47\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0,262\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0,9899\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e3,61\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e1,268\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0,9323\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eBased on the previous discussion, the most dominant interaction was chemisorption, with potential interactions including hydrogen bonding, van der Waals forces, pore filling by the adsorbate, and π-π interactions between the aromatic rings of the dye and the carbon surface [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] (see Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). At neutral pH, methylene blue (MB) predominantly exists in a cationic form, whereas congo red (CR) is present in an anionic form. Accordingly, MB is electrostatically attracted to the negatively charged sites of the adsorbent, while CR interacts with the positively charged sites. In addition, the hydroxyl (\u0026ndash;OH) groups on the adsorbent surface facilitate hydrogen bonding with the amine functionalities of both CR and MB. Furthermore, π\u0026ndash;π interactions are likely to occur between the aromatic rings of the dye molecules and the aromatic domains of the carbonaceous components incorporated within the adsorbent.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eChitosan-activated carbon composite beads (CAECB) exhibited superior adsorption capacity compared to chitosan beads (CB) for Methylene Blue, with an optimum capacity of 27.32 mg/g. However, their adsorption capacity for Congo Red decreased to 8,39 mg/g. The adsorption kinetics for both adsorbents concerning Methylene Blue and Congo Red followed the pseudo-second-order kinetic model. The adsorption isotherm studies revealed that the adsorption of Methylene Blue using CB and CAECB followed the Langmuir isotherm with an R\u0026sup2; value of 0.9962 and the Freundlich isotherm with an R\u0026sup2; value of 0.9978, respectively.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgment\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors express their gratitude to Universitas Bengkulu for funding this research through the Budget Implementation List (\u003cem\u003eDIPA\u003c/em\u003e) of Universitas Bengkulu for the fiscal year 2024, Contract Number: 2916/UN30.15/PT/2024.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by funding from the University\u0026nbsp;of\u0026nbsp;Bengkulu\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe researchers consent to the submission of this manuscript for review and declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot Applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInformed Consent\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot Applicable\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eKumar P, Uppaluri RVS. 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Results Chem 14:1\u0026ndash;10. https://doi.org/10.1016/j.rechem.2025.102124.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"adsorption, beads composites, methylene blue, congo red","lastPublishedDoi":"10.21203/rs.3.rs-7678806/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7678806/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eInvestigation of adsorption of Congo Red (CR) and Methylene Blue (MB) dyes by chitosan beads (CB) and chitosan carbon based of Azadirachta excelsa composite beads (CAECB) has been carried out. The adsorbent was synthesized with varying mass ratio of chitosan and carbon. The composite was synthesized with PVA as a cross linker agent. This research was carried out in several stages for adsorbent preparation, degree of swelling, the stability of the adsorbent, and the effect of contact time (adsorption kinetics), and isotherm adsorption. The degree of swelling of the adsorbent range from 380\u0026ndash;610% and the adsorbent was stabled when soaked using distilled water until the 5th day (0% deformation). The best adsorbent for adsorption of MB and CR are CAECB 0,5 and chitosan beads (CB), respectively. The optimum adsorption time of MB was obtained at 90 minutes with the adsorption percentage of 99.10% by CAECB 0.5. The optimum adsorption time of CR was obtained at 150 minutes with the adsorption percentage of 96.01% by CB. The kinetics result follow pseudo second order rate equation for CAECB 0.5-MB, CB-MB, and CB-KR. The kinetics result for CAECB 0.5-MB was followed pseudo first order rate equation.\u003c/p\u003e","manuscriptTitle":"Biobased Chitosan–Carbon Composite Beads from Azadirachta excelsa for Dual Dye Adsorption","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-27 14:23:12","doi":"10.21203/rs.3.rs-7678806/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"92a9a559-e839-47f3-8bd4-3301c7578f07","owner":[],"postedDate":"October 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-03T12:38:53+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-27 14:23:12","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7678806","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7678806","identity":"rs-7678806","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}