Open Access 
Abstract
This study fabricated chitosan beads obtained from pineapple peels as sustainable biosorbents for hexavalent chromium (Cr (VI)) extraction from water, thus supporting circular economy initiatives. Heavy metal pollution in water systems is a crucial environmental issue that has serious consequences for human health, aquatic ecosystems, and the overall environment. Heavy metals such as lead (Pb), cadmium (Cd), chromium (Cr), zinc (Zn), and copper (Cu) are particularly problematic since they are non-biodegradable, tend to bioaccumulate in organisms, and have the ability to impair cellular function, posing long-term ecological and public health problems. Two biosorbents, PPA (glutaraldehyde crosslinked) and PPB (citric acid crosslinked), were synthesized and investigated using SEM and FTIR to investigate the structural and functional changes caused by the crosslinking agents. While PPB included more carboxyl groups due to citric acid crosslinking, FTIR analysis confirmed the presence of functional groups required for Cr (VI) binding. With optimal Cr (VI) removal at pH 3.0, testing results revealed maximal adsorption capacities of 18.87 mg/g for PPA and 21.01 mg/g for PPB. Increased availability of functional groups and adsorption surface stability improve PPB performance. Adsorption isotherm analysis revealed that both biosorbents followed the Freundlich model, indicating a heterogeneous adsorption mechanism. Kinetic investigations identified pseudo-first-order chemisorption as the major mechanism. Thermodynamic investigation revealed negative Gibbs free energy values (∆G), confirming the spontaneous nature of Cr (VI) adsorption. With PPB showing improved performance, this study demonstrates the efficacy of chitosan beads generated from pineapple peels as a sustainable and low cost biosorbent for heavy metal cleanup. The findings emphasize the importance of crosslinking agents in improving biosorbent performance, giving valuable information for the development of efficient and cost-effective wastewater treatment methods aligned with sustainability and circular economy concepts.
Introduction
Heavy metal pollution in water systems is a crucial environmental issue that has serious consequences for human health, aquatic ecosystems, and the overall environment 1 . Heavy metals such as lead (Pb), cadmium (Cd), chromium (Cr), zinc (Zn), and copper (Cu) are particularly problematic since they are non-biodegradable, tend to bioaccumulate in organisms, and have the ability to impair cellular function, posing long-term ecological and public health problems 2 . Prolonged exposure to heavy metals can have a variety of negative health consequences, including neurotoxicity, carcinogenesis, and kidney impairment, emphasizing the critical need for efficient and sustainable technologies to remove these contaminants from water sources 3 . Chemical precipitation, ion exchange, membrane filtration, and electrochemical procedures are some of the traditional heavy metal removal methods 4 . While these methods are efficient, they are frequently associated with considerable disadvantages. Chemical precipitation, for example, produces a substantial volume of sludge that requires additional treatment, whereas ion exchange and membrane filtering are expensive and require continual material maintenance and regeneration 5 , 6 , 7 . Furthermore, the energy-intensive nature of these processes, as well as their potential to emit secondary pollutants, has prompted research into more sustainable alternatives 8 .
In recent years, biosorbents—biological materials capable of adsorbing contaminants—have emerged as a promising alternative due to their eco-friendliness, cost-effectiveness, and natural abundance. Plants, fungus, and agricultural waste can all be used to produce biosorbents 9 . Agricultural waste products, such as fruit peels, sawdust, and rice husks, have received a lot of interest because they are high in cellulose, hemicellulose, lignin, and other components that help heavy metal ions bond together 10 , 11 . The use of agricultural waste as a feedstock for biosorbents is consistent with circular economy ideas, which involve transforming waste materials into valuable resources for environmental purposes 12 . Pineapple peel is a common agricultural waste that has great potential as a biosorbent. Pineapple peel includes considerable amounts of cellulose, hemicellulose, lignin, and pectin, all of which have natural adsorption properties for metal ions 13 . Chitosan, a biopolymer formed from the deacetylation of chitin, has been extensively researched for its ability to adsorb heavy metals due to the presence of functional groups such as amino (-NH 2 ) and hydroxyl (-OH) groups that readily bind with metal ions 14 . Chitosan has a strong affinity for metals such as Pb, Cd, and Cu, as evidenced by electrostatic interaction between its amine groups and positively charged metal ions 15 . However, pure chitosan has some drawbacks, such as solubility in acidic solutions and susceptibility to breakdown at high pH levels, limiting its practical use as an adsorbent 16 . This has prompted researchers to investigate crosslinking chitosan with other materials to improve its stability, mechanical strength, and overall adsorption capability.
Crosslinking chitosan with pineapple peel combines the characteristics of both materials, increasing the biosorbent’s ability to operate over a wide pH range while maintaining structural integrity and adsorption capacity 17 . Crosslinked chitosan-based biosorbents have been demonstrated to be more resistant to chemical degradation and have better mechanical properties, making them more appropriate for practical water treatment applications 18 . Furthermore, the generation of crosslinked chitosan-pineapple peel biosorbents provides further environmental advantages. By recycling waste materials, the strategy decreases the environmental impact of agricultural waste disposal while also contributing to a closed-loop system that promotes sustainable waste management practices. This is consistent with the broader goals of green chemistry and environmental sustainability, making crosslinked biosorbents a feasible alternative for tackling the global problem of heavy metal pollution in water bodies 19 .
In the biosorbents, glutaraldehyde and citric acid work together as crosslinking agents to enhance structural stability and adsorption performance. Glutaraldehyde reacts with hydroxyl (-OH) groups in the pineapple peel and amine (-NH 2 ) groups in chitosan, forming strong covalent bonds that create a durable, rigid framework with improved mechanical strength and resistance to degradation 20 , 21 . On the other hand, citric acid, as a natural and environmentally friendly crosslinker, forms ester bonds with hydroxyl and carboxyl (-COOH) groups, adding more carboxyl groups that serve as active sites for heavy metal ion binding 22 . The combination of glutaraldehyde for durability and citric acid for increasing functional groups results in a robust, highly effective, and sustainable biosorbent capable of removing heavy metals like Cr(VI), Pb(II), and Cu(II) in wastewater treatment 23 .
This work aims to create an effective biosorbent by crosslinking agents with pineapple peel -chitosan beads and testing its efficacy in heavy metal adsorption from aqueous solutions. The findings of this study will add to the expanding body of knowledge on sustainable water treatment technologies and provide a scalable solution for the removal of heavy metals from industrial and municipal wastewater.
Materials and methods
Chemicals and Materials
In this study, isopropyl alcohol (C 3 H 8 O), succinic acid (C 4 H 6 O 4 ), chitosan, methanol (CH 3 OH), citric acid (C 6 H 8 O 7 ), sulfuric acid (H 2 SO 4 ), sodium hydroxide (NaOH), Glutaraldehyde 2.5 %, and 1,5 Diphenylcarbazide were obtained from Alpha Chemical Reagent Co., Ltd. (Tianjin, China) and qualified as an analytical grade. All working solutions used during all experiments were prepared by diluting chemicals in deionized water.
Preparation of absorbent
Pineapple peel waste was collected and thoroughly washed with water several times to remove dirt. The cleaned material was blended using a 400W blender. As the blended material still retained moisture, it was subsequently dried in an oven at 80 °C for 12 hours to ensure complete dehydration. To prepare biosorbent beads, pineapple powder, chitosan, and 7% acetic acid were mixed in a ratio of 1 g : 10 g : 240 mL in a beaker. The mixture was stirred continuously using a magnetic stirrer for 12 hours. The resulting slurry was degassed under vacuum and then dropwise added into 200 mL of an alkaline coagulating solution composed of distilled water, methanol, and NaOH in a weight ratio of 4:5:1. This process yielded spherical beads with an average diameter of 3.5 mm. The formed beads were collected and thoroughly rinsed with distilled water to remove residual chemicals and achieve neutral pH.
For the preparation of the glutaraldehyde-crosslinked biosorbent (PPA), the beads were soaked in a 2% glutaraldehyde solution for 12 hours, followed by repeated washing with distilled water until a neutral pH was reached. Alternatively, for the citric acid-crosslinked biosorbent (PPB), the beads were soaked in 3% citric acid for 1 hour. After treatment, the beads were washed several times with distilled water until neutral pH was achieved. The final biosorbent beads (PPA and PPB) were stored in a cool, dry environment to prevent moisture absorption before being used in metal ion adsorption experiments.
Batch experiment
Biosorption experiments were conducted using a batch experiment. All experiments were performed in duplicate by mixing 2 g/L of the biosorbents (PPA or PPB) with 100 mL of Cr(VI) solution at an initial concentration of 25 mg/L. The mixtures were agitated in an orbital shaker incubator at 200 rpm and 303 K. After equilibration, the solutions were filtered, and the residual Cr(VI) concentrations were determined using the diphenylcarbazide method.
The effect of agitation time on Cr(VI) removal was evaluated at 303 K and 200 rpm using 2 g/L of biosorbent. The influence of adsorbent dosage was also investigated, ranging from 0.5 to 5 g/L, to determine the optimal amount for maximum Cr(VI) uptake. The effect of pH on biosorption was examined across a pH range of 2.0–7.0, adjusted using appropriate buffer solutions. Additionally, the impact of temperature was studied over the range of 303–318 K under optimal agitation time and pH conditions.
where: C e (mg/L) is the Cr(VI) concentration at equilibrium; C 0 (mg/L) is the initial concentration of Cr(VI); q e (mg/g) is the adsorption capacity at equilibrium; V (L) is the volume of Cr(VI) solution; m (g) is the mass of the adsorbent.
Absorbent characterization
Their surface morphology was displayed using scanning electron microscope (SEM) images (JSM-6510 LV; Japan). Fourier-transform infrared spectroscopy (FTIR; FT/IR-4600; Japan) determined the main functional groups on their surface.
Absorbent characterization
Their surface morphology was displayed using scanning electron microscope (SEM) images (JSM-6510 LV; Japan). Fourier-transform infrared spectroscopy (FTIR; FT/IR-4600; Japan) determined the main functional groups on their surface.
Results and discussion
Absorbent characterization
Figure 1 . SEM images of PPA (a, b) and PPB (c,d) materials; Before and After Cr(VI) adsorption; (e) FITR spectra of PPA material before and after Cr(VI) adsorption; (f) FITR spectra of PPB material before and after Cr(VI) adsorption.
The SEM images provide clear evidence of morphological changes in the biosorbents PPA and PPB before and after Cr (VI) adsorption. The PPA surface prior to adsorption ( Figure 1 , a) appears rough, porous, and heterogeneous with layered and cracked structures, indicating a high surface area favorable for adsorption. After Cr (VI) adsorption ( Figure 1 , b), the PPA surface becomes noticeably smoother, with visible pore blockage and flake-like deposits, suggesting that Cr(VI) ions were successfully adsorbed onto and partially filled the surface cavities. In contrast, the PPB biosorbent exhibits a relatively smooth and compact surface before adsorption ( Figure 1 , c), consistent with the effect of citric acid crosslinking, which may have contributed to a more uniform surface structure. The following adsorption ( Figure 1 , d), significant surface deformation, cracks, and particle deposition are observed on PPB, indicating a strong interaction between Cr(VI) and the carboxyl-rich surface. These morphological changes confirm that both biosorbents effectively adsorbed Cr(VI), with PPB showing more pronounced surface modifications, supporting its higher adsorption performance.
Figure 1 (e) and (f) showed the FTIR analysis of both PPA and PPB biosorbents before and after Cr(VI) adsorption and revealed significant spectral changes, confirming the involvement of functional groups in metal binding. A broad band around 3400 cm⁻¹ corresponding to –OH and –NH stretching was observed to shift and intensify after adsorption, suggesting hydrogen bonding and possible coordination with Cr(VI). In both spectra, peaks around 1600–1700 cm⁻¹ (attributed to C=O or N–H bending) also showed noticeable shifts, indicating the participation of carbonyl or amine groups in the adsorption process. Additionally, alterations in the 1000–1300 cm⁻¹ region, associated with C–O and C–N stretching, further support the chemical interaction between Cr(VI) and the biosorbent surface. These changes collectively demonstrate that hydroxyl, amino, and carboxyl functional groups play key roles in Cr(VI) biosorption for both glutaraldehyde-crosslinked PPA and citric acid-crosslinked PPB materials.
Effect of pH on adsorbents
pH is crucial in all adsorption studies, and even minor changes in solution pH can greatly improve adsorbent efficiency ( Figure 2 ). The current investigation focused on pH levels ranging from 2 to 7. pH changes were conducted with 0.1 M H 2 SO 4 and 0.1 M NaOH. Figure 2 shows the observed variations in removal effectiveness as a function of pH. As the pH increased from 2 to 7, Cr (VI) removal effectiveness decreased from 39.5% to 1.0% for PPA material and 67.4% to 41.4% for PPB material. The results demonstrate that eliminating Cr (VI) in an aqueous solution works better at lower pH. The increased removal efficacy of Cr (VI) under acidic conditions could be attributed to charge density. At lower pH, Cr (VI) exhibits a large negative charge density due to the existence of ions like HCrO 4 − , Cr 2 O 7 2− , and CrO 4 2− in the solution 29 . Because of the large concentration of H + ions on the adsorbent surface, these ionic forms preferentially adhere there. Adsorbent positively charged surfaces exhibit high electrostatic attraction with Cr (VI) ions 30 . As the pH increases, the interaction between the anionic species and the adsorbent surface diminishes for two primary reasons: (a) the adsorbent surface acquires a negative charge at elevated pH levels, and (b) there is an increase in hydroxyl ions in the aqueous solution 31 . The electrostatic interaction between oppositely charged metal ions and the adsorbent’s surface diminishes, resulting in reduced removal of Cr (VI) ions.
Figure 2 . Effect of pH on Adsorption Capacity (Qe) and Removal Efficiency (Re) of PPA and PPB Adsorbents
Effect of contacting time and adsorbents ‘dosage on Cr(VI) removal capacity
In general, removal efficiency improves with increased contact duration. Figure 3 shows the percentage removal of Cr (VI) as a function of time. It was discovered that when the contact time rose from 0 to 480 minutes, more Cr (VI) ions were eliminated. After 480 minutes, there was no obvious increase in Cr (VI) removal. Due to the instantaneous sorption of metal ions on active binding sites that are more numerous on the adsorbent’s outer surface, the initial rate of Cr (VI) adsorption was observed to be higher. At the end of the process, there is no visible change in the rate of adsorption and removal because metal ions gradually enter the adsorbent’s internal pore structures as the contact length increases, further blocking the active binding sites 32 .
Figure 3 . Effect of contact time on Adsorption Capacity (Qe) of PPA and PPB Adsorbents
Figure 4 . Effect of adsorbent dose on Adsorption Capacity (Qe) and Removal Efficiency (Re) of PPA and PPB Adsorbents
Figure 4 shows the results of effect of adsorbent dose on adsorption capacity (Qe) and removal efficiency (Re) of PPA and PPB adsorbents. In metal removal investigations, the adsorbent dosage is crucial. In batch studies, the adsorbent dosage ranged from 0.5 to 5 g/L. The results indicated that the removal efficiency of Cr (VI) increased with the adsorbent dosage; for example, 0.5 g/L of Cr (VI) removed 27.9% of the material for PPA and 31.9% of the material for PPB within 480 minutes, while 5 g/L removed all of the Cr (VI) within 480 minutes, indicating 100% removal ( Figure 4 ). This could be because a higher adsorbent dosage will have more surface area, which will bind more metal ions 33 . After all, the adsorption process will have many new binding sites accessible, preferring a high rate of adsorption.
Adsorption kinetics, isotherms and thermodynamics
The kinetic analysis of heavy metal adsorption onto biosorbents (PPA and PPB) provides valuable insights into the adsorption mechanisms and efficiency of these materials. The study utilized three widely accepted kinetic models—pseudo-first order (PFO), pseudo-second-order (PSO), and intra-particle diffusion models—to interpret the adsorption behavior observed experimentally.
The parameters and correlation coefficients (R 2 ) for the kinetic models are presented in Table 1 . For the PPA biosorbent, R 2 values ranging from 0.91 to 0.99 indicate that the pseudo-first order (PFO) model is applicable compared to other kinetic models. This implies that the adsorption process is predominantly governed by physical adsorption mechanisms, such as weak Van der Waals forces. However, the possibility of chemical adsorption cannot be excluded, as suggested by the high correlation coefficient (R 2 = 0.98), which points to interactions between heavy metal ions and functional groups on the chitosan and pineapple peel matrix via electron sharing or exchange. On the other hand, the pseudo-second order (PSO) model demonstrated the best fit for the PPB biosorbent, with an adjusted R 2 value of 0.98. This indicates that chemisorption is the dominant mechanism, involving strong chemical interactions such as ion exchange or complexation between heavy metals and the active sites on the biosorbent. Additionally, as the concentration of Cr(VI) ions increased, the adsorption capacity rose while the rate constant decreased. This behavior can be explained by the reduced competition for active sites at lower concentrations, whereas at higher concentrations, the competition for these sites became significantly more intense. Identifying the rate-limiting steps for the adsorption of Cr(VI) ions on PPA and PPB required a thorough understanding of the adsorption mechanism, which was not adequately explained by pseudo-first order and pseudo-second-order kinetic models. The intraparticle diffusion model was employed to elucidate the diffusion mechanism. The results from the intraparticle diffusion model for PPA and PPB reveal significant differences in their adsorption mechanisms. For PPA, the model showed a high R 2 value of 0.91 indicating that, while pore diffusion is important, surface adsorption or external mass transfer also contributes, especially in the initial stages. This suggests that PPA’s relatively uniform structure, due to crosslinking, facilitates the effective diffusion of Cr(VI) ions into its pores. In contrast, PPB exhibited a lower R 2 value of 0.76, indicating that intraparticle diffusion is less significant in its adsorption process. While the Ki value of 0.24 indicates faster pore diffusion to a more dominant surface adsorption mechanism in the early stages. This can be attributed to the more heterogeneous surface of PPB, which likely limits the contribution of intraparticle diffusion. Overall, PPA relies more on both surface interactions and pore diffusion due to its homogeneous structure, while PPB’s adsorption is largely driven by surface interactions, reflecting its more varied surface characteristics.
The adsorption isotherm analysis of pineapple peel crosslinked chitosan biosorbents, PPA and PPB, provides insight into their adsorption mechanisms and capacity for heavy metal removal. Both biosorbents were evaluated using the Langmuir, Freundlich, Temkin, and Redlich-Peterson models, which allowed us to assess their adsorption performance and determine the nature of the adsorption interactions. Table 2 shows the R 2 value and extra coefficients from the nonlinear regression study. The isotherm models were sorted based on R² values, with the best-fitting model discovered first. The models are arranged as follows: Freundlich, Langmuir, Redlich-Peterson, and Temkin. The Freundlich value "n" ranges from 0 to 10, indicating that the adsorption of Cr(VI) ions onto PPA and PPB is physical in nature. The results showed that the Freundlich model fit the adsorption equilibrium data better than the other isotherm models. This suggests that Cr(VI) ions are adsorbed onto biosorbents in a heterogeneous, multilayered way. The Langmuir maximum monolayer adsorption capacities of the PPA and PPB adsorbents were determined to be 18.87 mg/g and 20.37 mg/g, respectively. The Temkin model predicts that the heat of adsorption values for the current adsorption system are less than 8 kJ/mol, indicating a weak interaction between the adsorbent and Cr(VI) ions. This suggests that the current adsorption method relies on physical adsorption. The Redlich-Peterson isotherm shows that PPA and PPB have βRP values of 0.64 and 0.72, respectively. "βRP" refers to an exponent between 0 and 1. This section explains the relevance of βRP. When βRP = 0, the Redlich-Peterson equation simplifies to Henry's law, while βRP = 1 corresponds to the Langmuir equation. The Redlich-Peterson equation can be understood as being consistent with Henry's law for low Cr(VI) ion concentrations and with the Freundlich isotherm model at higher values.The isotherm data indicate that the Freundlich isotherm model best describes the current adsorption system, as demonstrated by a greater correlation coefficient and lower error values.
Thermodynamic parameters for the adsorption of Cr(VI) by PPA and PPB at varying temperatures are shown in Table 3 . At every temperature, both materials show negative Gibbs free energy (ΔG°) values, suggesting that the adsorption process is spontaneous 34 . PPB has a similar tendency, with ΔG° falling from -73.76 kJ/mol to -79.04 kJ/mol over the same temperature range, whereas PPA’s declines from -72.38 kJ/mol at 303 K to -78.18 kJ/mol at 318 K. This implies that both materials benefit from the adsorption process at higher temperatures. The endothermic process is indicated by the enthalpy change (ΔH°) of 44.41 kJ/mol for PPA and 33.09 kJ/mol for PPB, which is also endothermic but to a lesser extent 35 . With PPA at 387.96 J/mol·K and PPB at 352.71 J/mol·K, the entropy change (ΔS°) is positive for both materials, indicating greater randomness at the solid-liquid interface during adsorption 36 . This can be explained by the fact that water molecules that are displaced by adsorbents gain more entropy than those that are lost due to HCrO 4 - anions dissociating. These findings demonstrate how well both materials work for Cr(VI) adsorption, with PPB performing somewhat better at higher temperatures.
Conclusions
According to the study's findings, PPA and PPB materials show great promise as biosorbents, with both being particularly effective at removing Cr(VI) from aqueous solutions. It was observed that the optimal pH for Cr(VI) biosorption on these materials was 3.0, which corresponds to the conditions that allow for the highest adsorption efficiency. PPB was the most effective adsorbent of the materials tested, with an even higher capacity of 21.01 mg/g at 303 K than PPA compounds, particularly PPA, which had an adsorption capacity of 18.87 mg/g. The Freundlich isotherm model fit the experimental data well, demonstrating the diversity of adsorption sites on different biosorbents. Physical sorption may be the rate-limiting step, as kinetic experiments revealed that the adsorption process followed the pseudo-first-order model. The biosorption process was endothermic and spontaneous, as evidenced by negative ∆G values for Cr(VI) uptake and positive ∆H values for heat absorption during adsorption (thermodynamic analysis). Thus, PPA and PPB materials have considerable promise for real-world applications in removing heavy metals from contaminated water sources, especially when crosslinked with citric acid.
Acknowledgements
The author would like to greatly acknowledge the support of time and facilities from Ho Chi Minh City University of Technology and Education, Vietnam for this study.
Competing Interests
The author declares that they have no conflict of interest.
Authors' Contributions
Nguyen My Linh: Supervision, Writing - original draft, reviewing and Editing; Data curation, Formal analysis; Methodology, Conceptualization.
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