Application of shrimp shell-derived chitosan for heavy metal biosorption

Bandar A. Al-Mur

doi:10.25259/JKSUS_1080_2025

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Research Article

2026

:38;

10802025

doi:

10.25259/JKSUS_1080_2025

Application of shrimp shell-derived chitosan for heavy metal biosorption

Bandar A. Al-Mur^a,*

aDepartment of Environment, King Abdulaziz University, Department of Environmental Sciences, Faculty of Environmental Sciences, Jeddah, Heddah, 2883, Saudi Arabia

*Corresponding author: E-mail address: balmur@kau.edu.sa (B A Al-Mur)

Received: 2025-06-26, Accepted: 2025-12-16, Epub ahead of print: 2026-03-13, Published: 2026-03-17

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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

Growing concern about heavy metal contamination of water resources necessitates the development of effective and sustainable treatment strategies. This study examines the conversion of shrimp shell waste into Chitosan, a biopolymer with exceptional metal adsorption capabilities. The recovered chitosan content was 10.25 ± 0.31%, with an initial chitin concentration of 22.32 ± 0.67% in the raw shrimp shells. A high degree of deacetylation (DD) (90.25±1.81%) significantly enhanced its metal binding efficiency. The structural and functional properties of the synthesized Chitosan were confirmed using fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM-EDX). Adsorption tests were conducted to optimize key parameters, including pH, initial metal content, biosorbent dosage, temperature, and contact time. The adsorption equilibrium was best represented by the isothermal Langmuir model, indicating a monolayer adsorption process on a homogeneous surface. Meanwhile, the kinetic behavior followed a pseudo-second-order model, suggesting that chemosynthesis is the primary mechanism. Chitosan efficiently removed lead and copper from aqueous solutions (20 mg/L), achieving high removal efficiency. These results underscore its potential as a cost-effective and environmentally friendly adsorbent for large-scale water purification and industrial wastewater treatment.

Keywords

Chitosan

Biosorption

Heavy metal elimination

Shrimp shell waste

Sustainable water treatment

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1. Introduction

Manufacturing facilities frequently discharge pollutants into the environment, including hazardous metals, which cause significant harm to ecosystems (Ali et al., 2019; Järup, 2003). Cadmium, lead, arsenic, chromium, and copper are particularly hazardous due to their toxicity and persistence (Balkrishna et al., 2025; Khan et al., 2025). These pollutants contaminate water, soil, and air, causing serious health dangers to humans and animals. To mitigate these threats, it is essential to implement adequate procedures for eliminating metals from industrial effluents before their release into aquatic systems, thereby conserving both ecological and public health (Vardhan et al., 2019). Several advanced technologies are employed to treat wastewater contaminated with heavy metals.

Coagulation/flocculation enhances pollutant aggregation, facilitating easier removal via sedimentation or filtration (Alkhadra et al., 2022). Membrane filtration, including ultrafiltration and reverse osmosis, efficiently removes dissolved metals and organic pollutants, making it ideal for industrial applications (Abdullah et al., 2019; Kapepula and Luis, 2024). Ion exchange and electrodialysis selectively capture and separate metal ions, offering high efficiency and reusability (Azhar et al., 2022; Solonchenko et al., 2022). Adsorption, utilizing materials such as activated carbon or bio-based adsorbents, is a widely researched and cost-effective method for metal removal (Huang et al., 2016; Suja et al., 2024). Chemical precipitation involves reacting metal ions with precipitating agents to form insoluble compounds, a process commonly used in large-scale treatment plants (Demopoulos, 2009; Filipiuk et al., 2006). Biological treatments, such as bioremediation, utilize microorganisms to facilitate eco-friendly metal absorption (Medfu Tarekegn et al., 2020). Nanotechnology-based treatments utilize engineered nanomaterials for targeted removal of pollutants (Khan et al., 2020). Electrokinetic remediation, which operates electric fields to stimulate the removal of contaminants, is a promising approach for in situ site treatment (Sun et al., 2023). Each method has unique advantages, and integrating multiple approaches enhances wastewater treatment efficiency and sustainability.

Eliminating heavy metals in effluents requires a dual approach: regulatory actions to limit their use in products and enhanced treatment methods to improve removal effectiveness. Stricter laws reduce pollution at its source, whereas advances in wastewater treatment, such as nanotechnology, electrical processing, and real-time monitoring, ensure more effective contaminant removal (Cantinho et al., 2016; Vardhan et al., 2019). Among treatment techniques, adsorption is an exceptionally cost-effective, efficient, and eco-friendly method. Marine waste materials, such as shrimp and crab shells, serve as sustainable bioadsorbents, offering a low-cost, high-performance alternative for removing inorganic and organic pollutants while promoting waste valorization (Garba and Rahim, 2014; Gutha et al., 2015).

Recent studies have highlighted the versatile coordination behavior of alkanesulfonate ligands and their role in forming stable supramolecular assemblies with transition metals. Several zinc- and magnesium-based systems have demonstrated how weakly coordinating sulfonate groups can promote hydrogen bonding, π–π stacking, and other non-covalent interactions, leading to 2D or 3D network architectures (Shankar et al., 2020; Shankar et al., 2017; Singh et al., 2020; Singh et al., 2019). Computational studies have further provided insight into the electronic structures and stability of such metal–sulfonate assemblies, explaining the dominance of non-covalent interactions in their packing (Shankar et al., 2018; Singh et al., 2018). These findings highlight the significance of supramolecular forces and weakly coordinating ligands in developing efficient metal–biopolymer adsorption systems.

Water contaminated with heavy metals provides a serious hazard to both human well-being and the environment, necessitating innovative and long-term treatment options. Chitin and its derivative, Chitosan, stand out as attractive natural adsorbents due to their exceptional metal-binding characteristics. Chitin, which is predominantly obtained from marine bio-waste such as shrimp shells, has gained popularity due to its environmental friendliness, biodegradability, and cost-effectiveness. Several studies have demonstrated its effectiveness in sequestering harmful metal ions, making it a suitable choice for advanced wastewater treatment procedures (Anastopoulos et al., 2017; Boulaiche et al., 2019; Fatima et al., 2018; Gonzalez-Davila et al., 1990; Jaafarzadeh et al., 2014; Kocer et al., 2008; Labidi et al., 2016; Rodríguez et al., 2012; Sofiane and Sofia, 2015; Weißpflog et al., 2020). Chitosan, generated by the partial deacetylation of chitin, has even greater potential in water filtration due to its enhanced adsorption capabilities. The conversion of chitin to Chitosan enhances the availability of amino (-NH₂) and hydroxyl (-OH) functional groups, acting as active targets for heavy metal chelation. This makes Chitosan not only an effective biosorbent, but also a long-term alternative to synthetic adsorbents. Recognizing the relationship between Chitosan’s structural features and its adsorption behavior is critical for maximizing its use in real-world water treatment settings.

Global shrimp consumption has increased sharply, resulting in the generation of substantial shell waste. Since 2020, there has been a marked surge in research focused on chitin and Chitosan derived from shrimp shells. Chemical extraction remains the dominant approach, though advances in processing and auxiliary technologies have enabled the customization of chitosan properties for specific applications (Gao et al., 2024).

Environmentally friendly alternatives have also been explored, such as the auto-fermentation of shrimp shell waste, which produced Chitosan comparable in deacetylation degree, carbon-to-nitrogen ratio, and crystallinity to commercial products. Semi-anaerobic saline conditions were found to promote lactic acid bacterial (LAB) activity, effectively supporting both deproteinization and deacetylation processes, thus confirming the method’s feasibility (Akhiruddin et al., 2023).

This research utilizes Chitosan derived from Metapenaeus monoceros shrimp shells to remove Pb²⁺ and Cu²⁺ from contaminated water. This study aims to optimize the removal efficiency of harmful metals by evaluating key adsorption factors, including pH, contact time, initial metal concentration, and chitosan dosage. The method of adsorption is investigated using recognized models, such as the Langmuir, Freundlich, and Temkin isotherms, as well as kinetic models, including the pseudo-first-order, pseudo-second-order, and Elovich models. Thermodynamic assessments of Gibbs free energy (ΔG^°), enthalpy (ΔH°), and entropy (ΔS°) provide a deeper insight into the adsorption process.

Despite extensive research on chitosan-based adsorbents for removing heavy metals, significant knowledge gaps remain. Most previous studies have focused on commercially available or chemically modified Chitosan derived from limited marine sources, with little attention given to naturally abundant yet underutilized shrimp species such as Metapenaeus monoceros. Furthermore, variations in Chitosan’s physicochemical properties depending on its biological origin are often overlooked, leading to inconsistent adsorption performance data across studies. Additionally, there is a limited comparative understanding of the thermodynamic behavior and adsorption mechanisms of Cu²⁺ and Pb²⁺ ions on biogenic Chitosan under controlled environmental conditions.

To address these gaps, the present study investigates the use of Metapenaeus monoceros-derived Chitosan as a sustainable, low-cost biosorbent for removing Cu²⁺ and Pb²⁺ ions from aqueous media. The novelty of this work lies in (i) utilizing Chitosan extracted from an underexploited marine bio-waste source, (ii) systematically assessing the influence of operational parameters (pH, contact time, initial concentration, and adsorbent dose) on metal uptake, and (iii) providing comprehensive insight into the adsorption mechanisms through kinetic, isotherm, and thermodynamic modeling.

Accordingly, the objectives of this study are to:

1.

Extract and characterize Chitosan from Metapenaeus monoceros shrimp shells to evaluate its potential as a biosorbent.
2.

Investigate the adsorption behavior of Cu²⁺ and Pb²⁺ ions under varying physicochemical conditions.
3.

Apply kinetic, equilibrium, and thermodynamic models to elucidate the underlying adsorption mechanisms and energy changes associated with metal binding.

This study contributes to the growing body of research on bio-based adsorbents by highlighting the valorization potential of M. monoceros waste and offering new insights into the design of sustainable materials for heavy metal remediation.

2. Materials and Methods

All chemicals, including hydrochloric acid (HCl, ≥37%), sodium hydroxide (NaOH, ≥98%), and acetic acid (CH₃COOH, ≥99.7%), were of analytical grade and purchased from Sigma-Aldrich (Germany).

2.1 Sampling and raw material preparation

Shrimp shell waste from Metapenaeus monoceros, weighing approximately 4 kg, was collected from the central fish market in Jeddah, Saudi Arabia. The shrimp shells were cleaned by immersing them in 15% hydrogen peroxide for 24 hours to remove impurities. The cleaned shells were thoroughly rinsed using water distillation and then dried completely at 105°C in a ventilated oven. After cooling in a desiccator, the shells were crushed, ground using a mortar, and sieved to a particle size of 230 mesh (0.063 mm), as this size is known for its good adsorption capacity (Turner et al., 2005). The ground material was homogenized and stored in airtight glass containers, sealed in a dehydrator to maintain a minimal moisture content. Before use, the powder was re-dried at 105°C for 24 hours.

2.2 Chitosan extraction

Chitosan was extracted from shrimp shells using a well-established chemical process that involves sequential steps of mineral removal, protein elimination, and deacetylation (Du et al., 2009; Kandile et al., 2018). The collected shells were washed, sun-dried for one day, and then subjected to 10% HCl treatment at 25°C for 24 h to remove minerals. Some studies have reported improved efficiency using 0.25 M HCl with a solid-to-acid ratio of 40 mL g⁻¹ for 120 min. The residue was thoroughly rinsed with deionized water until a neutral pH was achieved, then dried and weighed. Protein removal was achieved using 10% NaOH at 25°C for 24 h, yielding raw chitin. Alternatively, 1 M NaOH (20 mL g⁻¹) at 70°C for 24 h has been reported to enhance protein solubilization. The resulting chitin was washed, dried, and ground into fine particles. To ensure high purity, the material was further boiled with hot ethanol (10 mL g⁻¹) and acetone to eliminate residual contaminants, followed by a decolorization step using 0.5% NaOCl at 75°C for one hour. This treatment removed dark pigments such as lipoprotein–astaxanthin complexes and yielded chitin of varying whiteness. The final conversion to Chitosan was achieved by treating the purified chitin with 45% (w/v) NaOH (60 mL g⁻¹) at 110°C for 16 h, then washing to neutral pH and drying at 65 ± 5°C for four hours. The obtained Chitosan was subsequently characterized for moisture content, ash level, surface area, water-binding, and lipid-binding capacities (Dahmane et al., 2014). All yields were calculated on a dry-mass basis. Before weighing, shells, chitin, and Chitosan were oven-dried to constant mass (105°C). We report two complementary metrics:

1.

Chitosan yield on a dry-shell basis in Eq. (1)

(1)

Y i e l d_{s h e l l} (%) = \frac{m_{c h i t o s a n (d r y)}}{m_{s h r i m p s h e l l s (d r y)}} \times 100

2.

Chitosan yields relative to chitin (conversion efficiency) Eq. (2)

(2)

Y i e l d_{f r o m c h i t i n} (%) = \frac{m_{c h i t o s a n (d r y)}}{m_{c h i t i n (d r y)}} \times 100

where $m_{s h r i m p s h e l l s (d r y)}$ , $m_{chitin (dry)}$ , and $m_{chitosan (dry)}$ denote the masses obtained after drying to constant weight. Reporting both bases avoids ambiguity when comparing to literature values that may normalize either to the original shells or to the intermediate chitin.

2.3 Solutions and reagents

The research project used high-purity chemical reagents to ensure accurate experimental conditions. Sigma-Aldrich supplied sodium hydroxide pellets (97% purity) and hydrochloric acid (37% purity), which were diluted with distilled water to achieve the required concentrations. Metal ion solutions were prepared using analytical-grade chemicals, ensuring that adsorption studies were accurate and consistent. To generate solutions of Cu²⁺ and Pb²⁺ at 1000 mg/L, copper (II) chloride dihydrate (CuCl₂•2H₂O) and lead (II) nitrate (Pb(NO₃)₂) were used. These solutions were then serially diluted to produce a range of metal ion concentrations for the adsorption experiments. The solution’s pH had been adjusted between 3 and 9 using 1.0 M hydrochloric acid and sodium acetate. The pH of each solution was properly monitored using a WTW-inolab pH meter (Germany) to ensure uniformity between tests. Metal ion concentrations in aqueous solutions were measured using inductively coupled plasma (ICP) spectroscopy (Hitachi SPS3500), a highly sensitive technology that enables precise quantification of metal ions. A calibration curve was created to provide exact readings and dependable experimental results. The prepared samples were used directly in adsorption experiments under controlled conditions.

2.4 Chitosan characterization

Chitosan was thoroughly characterized to determine its potential as a biosorbent for removing heavy metals. Brunauer–Emmett–Teller (BET) analysis with nitrogen adsorption at −196°C, utilizing a NOVA 2000 gas sorption analyzer, was employed to determine the specific surface area, pore volume, and diameter, providing valuable insights into the material’s adsorption capacity. The chemical composition of the shrimp shell biomass was determined using standard analytical procedures: moisture content by oven-drying at 105°C to constant weight, protein by the Kjeldahl method (N × 6.25), ash content by combustion at 550°C in a muffle furnace, and chitin content after sequential demineralization and deproteinization according to AOAC (2005) protocols. To analyze the stability and dispersion behavior of Chitosan in aqueous solutions, the variation in the particle size and zeta potential (mV) was acquired with a Malvern Instruments Zetasizer Ver. 7.13 (Serial Number: MAL1209575). X-ray diffraction (XRD) was utilized to analyze the structural characteristics of Chitosan using a PANalytical X’PERT PRO diffractometer (λ = 1.5418 Å, 45 kV). This data provided knowledge about its crystallinity, which affects the effectiveness of adsorption. Fourier-transform infrared spectroscopy (FTIR) was done in the 500-4,000 cm⁻¹ range (Shimadzu FTIR-8400S) to recognize functional groups responsible for metal binding. The morphology and composition of Chitosan were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) on a JEOL JSM-6360 microscope. Thermogravimetric analysis (TGA) was performed on Chitosan using an SDT Q600 V20.9 Build 20 thermal analyzer, which detected decomposition stages and structural integrity under controlled temperature settings ranging from 0 to 700°C at a rate of 10°C/min. The point of zero charge (pHₚzc) was identified to comprehend the surface charge function of Chitosan in solution. This metric was determined by monitoring pH fluctuations in NaCl solutions across a pH range, using the methods described by Ramadhani et al. (2021) and Reddy et al. (2012) . The pH was manipulated by applying 0.1 mol/L NaOH or HCl, and final pH measurements were recorded using a calibrated WTW-inolab pH meter (Germany).

2.5 Experiments on influencing factors

Batch adsorption experiments were conducted in 250 mL conical flasks containing 100 mL of metal ion solution at desired concentrations. The mixture was agitated at 300 rpm in a thermostatic shaker. After equilibrium, the solution was filtered, and residual metal concentration was measured using ICP. Data were analyzed using OriginPro 2023 software, and adsorption capacity (qₑ) was calculated using standard isotherm and kinetic equations.

2.5.1 Effect of pH

The pH of the mixture has an essential influence on chitosan adsorption efficiency, affecting both the ionization state of the adsorbent and the speciation of metal ions. Cu²⁺ and Pb²⁺ solutions were produced at a preliminary concentration of 20 mg/L, with pH regulated between 3 and 9. Acidification was performed using 1.0 M HCl, with 1.0 M sodium acetate serving as the buffer. A set amount of Chitosan (0.1 g) was incorporated into 50 mL of the metal solution in a 100 mL flask, and the combination was allowed to balance for 120 minutes at 25°C.

2.5.2 Time impact

To determine the optimal duration for maximum metal ion adsorption, the effect of contact time was investigated. Under pre-optimized circumstances. Cu²⁺ and Pb²⁺ solutions (20 mg/L) were adjusted to pH 7.0, which was previously found to be optimal. Chitosan (0.1 g) was added to 50 mL of each solution and agitated at 250 rpm at 25°C. The adsorption response was monitored at various time intervals (10 to 120 minutes), after which the solutions were purified and analyzed.

2.5.3 Effect of adsorbent dosage

The effect of chitosan dosage on the elimination of metal ions was examined by increasing the quantity of adsorbent from 0.02 to 0.1 g while keeping all other factors unchanged. Cu²⁺ and Pb²⁺ solutions (20 mg/L) were adjusted to the ideal pH of 7.4 based on prior trials. Each dosage was added to a separate 50 mL solution and agitated for 90 minutes at 25°C.

2.5.4 Initial concentration impact

To assess Chitosan’s adsorption capacity at various contamination levels, metal ion solutions with variable starting concentrations (5-50 mg/L) were produced and calibrated to pH 7.4. A fixed amount of Chitosan (0.1 g) was added to 50 mL of each solution, and the mixtures were agitated at 25°C for 90 minutes.

2.5.5 Temperature impact

Temperature fluctuations can influence adsorption thermodynamics. Cu²⁺ and Pb²⁺ adsorption was observed at several temperatures (20-45°C), while maintaining optimal conditions of initial metal ion concentration (20 mg/L), pH 7.4, adsorbent dosage (0.1 g), and contact period (90 minutes). The temperature impact on adsorption efficiency revealed whether the process was endothermic or exothermic. All researches were carried out at a continuous stirring speed of 250 rpm, with post-reaction solutions filtered to separate the chitosan adsorbent.

2.6 Data analysis and adsorption modeling

The adsorption efficacy of Chitosan was thoroughly evaluated, including equilibrium adsorption capacity, removal efficiency, adsorption kinetics, isotherm modeling, and thermodynamic parameters. These results shed light on the adsorption process, rate-determining phases, and energetic feasibility of metal ion removal utilizing Chitosan as a biosorbent.

2.6.1 Equilibrium adsorption and removal efficiency

The capacity of adsorption (qₑ) and Cu²⁺ and Pb²⁺ elimination were recognized utilizing the Eq. (3)

(3)

q_{e} = \frac{(C_{o} - C_{e}) * V}{m}

where C₀ and Cₑ are the initial and equilibrium concentrations (mg/L). The volume (V) is the volume of the aqueous solution (L), and m is the dry mass of Chitosan (g). Removal efficiency was calculated as in Eq. (4)

(4)

% Removal = (\frac{C_{o} - C_{e}}{C_{o}}) * 100

These calculations evaluated Chitosan’s ability to adsorb Cu²⁺ and Pb²⁺ ions from aqueous solutions over different scenarios.

2.6.2 Adsorption kinetics

To evaluate the rate-controlling mechanisms of metal ion adsorption, three models were fitted with experimental data. The pseudo-first-order model is based on a physisorption mechanism, where the distinction between equilibrium and instantaneous adsorption capabilities determines the adsorption rate. This model defines chemisorption as a rate-limiting phase involving electron exchange between metal and chitosan functional groups. The diffusion model investigates whether diffusion within the chitosan matrix affects adsorption, with deviations from linearity suggesting the presence of additional mechanisms, such as surface adsorption and external mass transfer.

2.6.3 Adsorption isotherms

There are three isotherm models for the examined data. Langmuir isotherm with adsorption on homogeneous surfaces, favorability calculated using the separation factor (R_L). The Freundlich isotherm explains multilayer adsorption on heterogeneous surfaces, where adsorption strength indicates surface heterogeneity. The Temkin isotherm assumes that adsorption heat decreases linearly with surface coverage. Fitting experimental data to these models identified the adsorption mechanism and offered information about the chitosan surface qualities.

2.6.4 Adsorption thermodynamics

Thermodynamic factors, including ΔG, ΔH, and ΔS, were evaluated to assess the feasibility and spontaneity of the adsorption process. A negative ΔG° indicates a favorable method, while ΔH° and ΔS° reveal whether adsorption is endothermic or exothermic and the degree of unpredictability at the interface between solids and liquids. Higher temperatures favor endothermic adsorption, while a decrease in entropy suggests ordered adsorption onto defined active sites.

3. Results and Discussion

3.1 Chitosan yield and literature context

On a dry-shell basis, the chitosan yield in this work was 10.25 ± 0.31%, while raw shrimp shells contained 22.32 ± 0.67% chitin; thus, the chitin-chitosan conversion efficiency is ∼46%. Comparable studies on shrimp/crustacean shells using conventional demineralization–deproteinization–deacetylation often report ∼10–17% on a dry-shell basis, for example, 12.03% from shrimp shell waste (Varun et al., 2017), 13.96% for Litopenaeus vannamei shells (Aberoumand and Chabavi, 2024), and ∼18.8% for pink shrimp (Parapenaeus longirostris) under autoclave-assisted conditions (Ögretmen et al.). Reviews summarizing various protocols place these values within a similar range and emphasize that species, particle size, acid/alkali strength, liquor-to-solid ratio, and time/temperature, as well as deacetylation severity, all influence yields within this range (Hosney et al., 2022). When yield is normalized to chitin rather than the original shells, the apparent values are higher because they reflect the conversion efficiency from chitin to Chitosan rather than the total shell mass. Reporting both dry-shell yield and from-chitin efficiency, therefore, enables a transparent comparison across studies.

3.2 Identification of the created Chitosan

The surface area of the absorbent material significantly affects its efficiency, as it determines the extent of its interaction with the target pollutants. The produced Chitosan had a specific surface area of 4.076±0.204 m²/g, which is identical to bio-based adsorbents designed for heavy metal capture (Beims et al., 2022) (Table 1). Although the surface area appears moderate, the material’s rich functional groups enable increased adsorption beyond simple physical interaction.

Table 1. Composition of fresh shrimp shell biomass (in % by weight).

Parameter	%
Moisture content	7.12
Ash content	6.28
Protein	25.15
Lipids	6.84
Carbohydrates	54.61
Total	100

The surface charge properties of Chitosan are affected by the zero-point charge (pHpzc), which is 7.0±0.1 (Fig. 1). Protonation of amine (-NH₂) groups below pH 7.0 creates a positively charged surface, thereby restricting the adsorption of cationic species. In contrast, at pH values above 7.0, deprotonation enhances electrostatic interactions between chitosan and metal ions, thereby increasing the adsorption process.

The chitosan particle size indicated a mean particle diameter of 508.6±25.4 nm, indicating an appropriate range to preserve dispersion and surface reactivity (Fig. 2a). The zeta potential measurement of Chitosan of -22.6±1.1 mV verified a stable colloidal system with sufficient electrostatic repulsion to prevent aggregation, which is critical for sustaining adsorption efficacy (Bhutto et al., 2024) (Fig. 2b). The combination of nano-scale particles with a negative surface charge promotes suspension stability and maximum contact with pollutants (Li et al., 2022).

Particle size (a) and zeta potential (b) distribution of the Chitosan, with a size average of 508.6 nm.

The scanning electron microscopy (SEM) provided compelling visual evidence of adsorption-induced morphological changes. Eddya et al. (2020) emphasize the role of SEM in understanding surface topography, which is crucial in applications such as adsorption studies, where morphology and porosity directly impact material performance (Eddya et al., 2020). In this study, the two SEM images reveal apparent differences in surface morphology and structural changes in Chitosan before (Image A) and after (Image B) the adsorption of metal ions (Cu²⁺ and Pb²⁺). Initially, the chitosan surface exhibited a porous and fibrous structure, ideal for metal ion entrapment (Fig. 3a). Parallel, elongated characteristics are evident, implying a more structured or stratified structure. The texture exhibits distinct surface imperfections in a layered pattern. Adsorption images revealed a compact shape, indicating surface coverage and structural rearrangements after Cu²⁺ and Pb²⁺ adsorption (Fig. 3b) (Ghaee et al., 2010; Guo et al., 2025). Few distinct surface fibers or layers appear, indicating a more homogeneous surface. Small surface particles or debris are scattered across a relatively continuous substrate.

(a-b) Scanning electron microscopy (SEM).

EDX analysis confirmed distinct elemental transitions in Chitosan following metal adsorption (Fig. 4). Before adsorption, Chitosan mainly consisted of carbon (C), oxygen (O), and nitrogen (N), reflecting its polysaccharide backbone and amine functionalities. After exposure to Pb²⁺ and Cu²⁺ ions, new characteristic peaks corresponding to Cu and Pb were observed, with measured concentrations of 6.25 ± 0.42 wt% % and 5.23 ± 0.32 wt%, respectively.

Energy dispersive X-ray spectroscopy of Chitosan.

The concurrent decrease in C and O percentages after adsorption indicates that a portion of the surface was replaced or masked by the deposited metals. This compositional change, together with the emergence of Cu and Pb peaks, strongly supports coordination or complexation between the metal ions and the –NH₂/–OH groups of Chitosan. Nitrogen, in particular, serves as an electron-donating center, forming coordinate bonds with the empty d-orbitals of Pb²⁺ and Cu²⁺, while oxygen contributes to secondary electrostatic stabilization. The relatively higher Cu and Pb contents observed by EDX correlate well with the adsorption capacities obtained from isotherm studies, confirming that the material surface effectively binds metal ions through both chelation and electrostatic attraction. Thus, the quantitative elemental composition provides direct evidence of Chitosan’s high surface affinity and explains its superior biosorption capacity. These findings align with previous reports emphasizing the role of nitrogen and oxygen functionalities in enhancing the sorptive behavior of Chitosan toward divalent metals (Guibal et al., 2014; Muhaidin et al., 2024; Thambiliyagodage et al., 2023).

Functional Group Involvement: Fig. 5 displays the FTIR spectroscopic data of the common peak of Chitosan, along with the corresponding assignment ranges (4000-500 cm^-1). The infrared spectroscopic data of the produced Chitosan showed peaks at the following wavelengths: 3459 cm-1, 3443 cm-1, 3265 cm-1, and 3113 cm^-1. The peak at 3107 cm^-1 is ascribed to O-H stretching (extensive, hydrogen-bonded) and N–H stretching, whilst peaks at 2965, 2962, 2929, and 2925 cm^-1 are ascribed to C–H stretching (Asymmetrical and symmetrical straining in –CH₃ and –CH₂ groups).

FTIR spectra of Chitosan before and after adsorption.

The peaks at 1660,1656, 1630, 1623, 1561,1558, 1455,1426,1379,1380, 1316, and 1317 cm^-1 were ascribed to C=O stretching (amide I band), N–H bending (amide II), C=O stretching (amide I band), N–H bending (amide II), N–H bending vibrations (amide II), C–H bending (methyl or methylene groups), bending symmetric CH₃ and C–N stretching, O–H bending. The FTIR spectrum showed characteristic bands at 1076 and 1079 cm⁻¹, which correspond to C–O–C stretching vibrations of alcohol and ether groups, while the peaks at 2928 and 1633 cm⁻¹ are attributed to C–H stretching and C=O vibrations, respectively. These features confirm the presence of hydroxyl and carbonyl groups on the chitosan surface, which play a role in the reduction and stabilization of metal ions during the synthesis process (Table S1). Several significant shift peaks appear after adsorption, indicating possible relationships among adsorbates (Cu²⁺ and Pb²⁺) and the adsorbent (Chitosan). The disappearance of the peak at 848 cm⁻¹ suggests that a structural or bonding change occurred during the adsorption method.

Table S1

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The degree of deacetylation (DD) of the prepared Chitosan was determined using FTIR. The measurement was based on the absorbance ratio of the amide I band at 1655 cm⁻¹, corresponding to C=O stretching vibrations, and the hydroxyl band at 3450 cm⁻¹, which serves as an internal reference. The calculation followed the procedure described by (Brugnerotto et al., 2001; Kasaai et al., 2000), according to the Eqs. (5) and (6)

(5)

Degree of Acetylation (DA) (%) = (A1655 / A345 0) \times 115

(6)

Degree of deacetylation (DD) (%) = 100 - DA

Where A1655 and A3450 represent the absorbance intensities of the amide and hydroxyl bands, respectively, the obtained DD value of 90.25 ± 1.81% confirms the efficient deacetylation of chitin during alkaline treatment and the successful conversion to Chitosan. A high DD indicates a greater proportion of free amino (–NH₂) groups, which serve as the primary active sites for metal ion complexation and adsorption. The high DD achieved in this work is consistent with the broad O–H and N–H stretching band observed around 3450 cm⁻¹ and the weakening of the amide I peak at 1655 cm⁻¹, reflecting the removal of acetyl groups. This structural modification enhances the cation-binding capacity of Chitosan, thereby improving its adsorption efficiency for Pb²⁺ and Cu²⁺ ions. Comparable findings have been reported in previous studies, where a high DD was directly correlated with increased metal-binding performance due to enhanced availability of protonated amino sites (Bhutto et al., 2024).

Based on XRD (X-ray diffraction) pattern inspection of Chitosan isolated from shrimp shell powder, the image most likely depicts a semi-crystalline chitosan structure, with sharp peaks (crystalline domains) and broad humps (amorphous regions) (Fig. 6). The sharp peak at 19.38° (2θ) corresponds to the (110) plane, demonstrating organized crystalline areas caused by intermolecular hydrogen bonding (Rinaudo, 2006). Prominent peaks at 26.26° and 29.84° (2θ) confirm the structure of Chitosan. The large hump between 10° and 35° (2θ) indicates the presence of amorphous domains in Chitosan. This is consistent with research suggesting that Chitosan is semi-crystalline, with crystallinity affected by deacetylation (DDA), crosslinking, or mixing (Luo et al., 2011; Pompeu et al., 2022). Additionally, the occurrence of minor peaks at 22.18°, 38.95°, and 44.19° indicates further crystalline order. This observation supports the idea that Chitosan exhibits crystalline and amorphous properties (Abd El-Hack et al., 2020). Typically, modified forms of chitosan exhibit reduced crystallinity, primarily due to disruptions in the polymer chains that occur during chemical modification processes (Ahmed and Aljaeid, 2016; Jha and Mayanovic, 2023). This comparison highlights the various avenues for modifying Chitosan’s properties through changes, notably emphasizing its relevance in biomedical functions, such as systems for delivering drugs where controlled release is critical.

X-ray diffraction pattern of Chitosan extracted from shrimp shell powder.

The crystallinity index (CrI) of the prepared Chitosan was calculated using the Segal method (Segal et al., 1959) according to the Eq. (7)

(7)

C r I (%) = \frac{I_{110} - I_{a m}}{I_{110}} \times 100

where $I_{110}$ represents the maximum intensity of the (110) crystalline peak at around 19.4° (2θ), and $I_{a m}$ Corresponds to the intensity of the amorphous background at approximately 12°. The calculated CrI for the extracted Chitosan was 72.4 ± 1.5%, indicating a semi-crystalline nature. This value aligns with typical chitosan crystallinity indices (70–85%) reported for highly deacetylated biopolymers (Luo et al., 2011; Rinaudo, 2006). The relatively high CrI suggests strong intermolecular hydrogen bonding and ordered chain alignment, consistent with the observed sharp diffraction peak at 19.38° (2θ). Compared with commercial Chitosan (CrI ≈ 75%), the slightly lower crystallinity of the shrimp-shell-derived Chitosan may result from partial structural disruption during deacetylation at high alkali concentration.

Thermal investigation with TGA revealed an initial weight loss of 4.232±0.127% (0.2444 mg) at 67.23±2°C resulting from moisture evaporation, which decreased to 2.379±0.071% (0.3842 mg) at a higher temperature of 130.63±2°C, demonstrating greater thermal stability (Fig. 7). A substantial breakdown occurred between 141.30±2°C and 681.73±2°C, resulting in a weight loss of 40.48±1.21% (2.338 mg). The residual mass of 41.87±1.26% at 691.45±2°C demonstrated that the material maintained remarkable thermal integrity (Gupta et al., 2025; Szymańska and Winnicka, 2015).

(a-b) Thermogravimetric analysis for Chitosan (A: TGA and B: DSC).

Furthermore, Chitosan degraded more quickly, reaching a peak rate of 1.554%/min at 254.38°C. Chitosan DSC curves show differences in composition or structural characteristics. Chitosan undergoes four unique temperature changes. A melting transition from 22.50°C to 36.68°C with an enthalpy of 3.570 J/g suggests a low-energy phase change. A significant transition from 79.13°C to 97.55°C with a noteworthy enthalpy of 270.0 J/g, indicating a dominant thermal event, possibly due to crystallization or a phase change of the principal component. Another transition follows between 183.15°C and 240.44°C, with an enthalpy of 30.18 J/g, indicating a secondary thermal event. Finally, at higher temperatures, a shift from 458.48°C to 562.66°C occurs, with an enthalpy of 146.8 J/g, which indicates thermal degradation or disintegration of the sample. Overall, Chitosan exhibits stronger thermal stability and requires more energy for most phase transitions, particularly in the mid-to-high temperature ranges (Alamri et al., 2025).

3.3 Batch experiments: An adsorption study

This work examines the adsorption of Pb²⁺ and Cu²⁺ using Chitosan as a biosorbent under various experimental conditions. To improve the adsorption process, the influence of multiple parameters, including starting pH, metal concentration, temperature, adsorbent dosage, and time, was thoroughly investigated. These characteristics are crucial in understanding the relationship mechanisms between chitosan and metal ions, which enable efficient elimination from aqueous solutions (Ahmad et al., 2009; Hui et al., 2025).

3.3.1 Influence of initial solution pH

The pH of the solution plays a critical role in regulating the surface charge of Chitosan and the speciation of metal ions, thereby influencing adsorption efficiency. As shown in Fig. 8, the adsorption capacity increased with pH, reaching a maximum at pH 7.0. The protonation can explain this behavior, specifically the deprotonation equilibrium of the amino (–NH₂) groups on Chitosan, which has an intrinsic pKa of approximately 6.5 (Rinaudo, 2006).

Effect of pH and initial metal ion concentrations (mg/L) on removal efficiency of Cu2⁺ and Pb2⁺ ions onto Chitosan.

At acidic conditions (pH < 6.5), the –NH₂ groups are protonated to –NH₃⁺, imparting a positive surface charge that repels cationic metal ions (Cu²⁺ and Pb²⁺) and results in low adsorption efficiency (Mourya et al., 2010; Yang et al., 2011). As the pH increases toward neutrality, the deprotonation of these groups reduces the surface positive charge, thereby enhancing electrostatic attraction and chelation between the electron-donating nitrogen/oxygen atoms of Chitosan and the divalent metal ions (Guibal, 2004). Beyond pH 7.0, a slight decrease in removal efficiency is observed, which is attributed to metal hydroxide precipitation (Cu(OH)₂, Pb(OH)₂) rather than reduced biosorption capacity (Babel and Kurniawan, 2003; Zhang et al., 2016).

The results confirm that electrostatic interactions and complexation jointly govern metal binding in the optimal pH range of 6.5–7.0. These findings are consistent with previously reported pH-dependent adsorption behavior of chitosan-based sorbents for divalent metals (Ngah et al., 2008).

3.3.2 Impact of metal concentrations

The beginning metal ion concentration has a substantial influence on both the effectiveness of elimination and the ability to adsorb. As illustrated in Fig. 8, biosorbent saturation occurs at greater metal concentrations. As metal concentrations increase, the adsorption capacity for Cu²⁺ and Pb²⁺ rises from 2.45±0.12 to 9.00±0.45 mg/g and 4.75±0.24 to 11.40±0.57 mg/g, respectively. Removal efficiency declines with increasing concentrations, from 93.6±2.81% to 49.4±1.48% for Cu²⁺ and 91.6±2.75% to 40.6±1.22% for Pb²⁺. This is ascribed to the depletion of active sites on the Chitosan, resulting in reduced adsorption efficiency at higher contaminant loads (Parmar and Thakur, 2013; Wasewar, 2010). (Ho YuhShan and Ofomaja, 2006) found that modified chitosan and alginate-based biosorbents exhibited increased adsorption capabilities for Pb²⁺, particularly at higher initial concentrations. This suggests that surface modifications may enhance the metal-binding efficiency of natural adsorbents.

The progressive increase in adsorption capacity with rising initial concentration can be attributed to a stronger concentration gradient that enhances the diffusion of metal ions toward active sites on Chitosan. At lower concentrations, a large number of binding sites remain unoccupied, whereas higher concentrations promote site saturation as the available adsorption centers become increasingly limited. This behavior aligns with the Langmuir model’s assumption of monolayer coverage on a homogeneous surface and reflects favorable adsorption as described by the Freundlich model. Together, these models confirm that adsorption proceeds through a combination of electrostatic attraction and surface complexation, with Chitosan demonstrating strong affinity and uniform binding toward Pb²⁺ and Cu²⁺ ions.

3.3.3 Adsorbent dosage impact

The relationship between adsorbent dosage and elimination efficiency is illustrated in Fig. 9. An increase in chitosan dosage enhances removal efficiency by increasing the number of binding sites. The effectiveness of Cu²⁺ and Pb²⁺ adsorption rose from 26.0±0.78% to 89.5±2.69% and 31.75±0.95% to 93.0±2.79%, respectively, as the chitosan dose increased from 0.01 g to 0.1 g at room temperature (25°C) in a 250 ml concentration (20 mg/l) with a pH of 7.5. Increasing the chitosan dose results in the addition of more amine and hydroxyl functional groups, which can bind to metal ions. Although increasing adsorbent dosage enhances adsorption efficiency, the efficiency gains plateau beyond a certain threshold because the accessibility of metal in solution is limited. However, the adsorption capacity per unit mass decreased beyond an optimal dosage due to particle aggregation, which reduces the effective surface area (Jaafarzadeh et al., 2013). The suitable dose was identified to be 0.1 g per 50 mL, balancing efficiency and material cost. The relationship between dosage and removal efficiency reflects the balance between the available surface area and the saturation of adsorption sites. At lower doses, increased adsorbent mass provides more –NH₂ and –OH functional groups for metal binding, resulting in higher removal efficiency. However, beyond the optimal 0.1 g dose, particle aggregation reduces the effective surface area and limits metal ion accessibility, resulting in a lower adsorption capacity per unit mass. This trend is consistent with surface saturation behavior observed in other chitosan-based biosorbents. A study by (Gupta and Rastogi, 2008) compared the effect of different adsorbent dosages using fly ash, activated carbon, and biosorbents. They discovered that beyond a certain threshold adsorbent dosage, metal ion consumption decreased due to agglomeration, which is consistent with our findings on Chitosan.

Effect of chitosan dosage (g) and contact time (min) on removal efficiency of Cu2⁺ and Pb2⁺ ions at pH=7.5 and time 120 min.

3.3.4 Impact of contact time

The adsorption mechanism exhibits a two-phase process: a fast initial adsorption phase (0-60 min), followed by a slower equilibrium phase (60-90 min) (Fig. 9). The high initial rate is attributed to the abundance of active sites available for adsorption. At the same time, the following fall is caused by site saturation and diffusion constraints. Adsorption equilibrium was reached after approximately 90 minutes, with negligible additional uptake thereafter. This trend is consistent with pseudo-second-order kinetics, indicating chemisorption as the dominating process (Ho and McKay, 1999; Lagergren, 1898). A comparative study by (Kırbıyık et al. 2016), showed that biochar and activated carbon followed a similar pseudo-second-order kinetic model. Nevertheless, the time necessary to achieve equilibrium varied among materials, with biochar exhibiting slower adsorption rates due to pore diffusion resistance.

The adsorption rate initially increased rapidly within the first 60 min due to the abundance of accessible active sites on the chitosan surface. As adsorption sites became progressively occupied, the rate slowed, and equilibrium was reached at 90 min. The kinetic profile Fig. 10 clearly demonstrates this two-stage behavior, characteristic of pseudo-second-order chemisorption.

(a) Pseudo-first-order, (b) Pseudo-second-order, and (c) intraparticle diffusion kinetics plots for metal ions adsorption.

3.3.5 Impact of temperature

Temperature fluctuations affect the adsorption effectiveness of Cu²⁺ and Pb²⁺ ions (Fig. 11). Adsorption efficiency improves from 20°C to 45°C, with peak removal rates of 92.9±2.79% for Cu²⁺ and 87.1±2.61% for Pb²⁺. This tendency is linked to increased ion mobility and diffusion, which facilitate contact with active sites (Lagergren, 1898). Comparative analysis with other biosorbents, such as alginate and cellulose derivatives, revealed that temperature-dependent behavior is typical among natural adsorbents (Foo and Hameed, 2010).

Effect of temperature on the adsorption of Cu2⁺ and Pb2⁺ onto Chitosan.

3.4 Biosorption isotherm models

Isotherms provide critical insights into how contaminants interact with solid sorbents, which is essential for predicting their behavior in environmental and engineering applications. These models clarify the way molecules move across liquid and solid phases when equilibrium is reached, providing critical information on adsorption capacity, mechanisms, and surface interactions of an adsorbent. The adsorption capacity is typically expressed as the amount of metal ions adsorbed per unit mass of adsorbent, which aids in understanding the efficiency and effectiveness of the biosorbent.

In this research, the adsorption behavior of Cu²⁺ and Pb²⁺ onto chitosan particles was investigated using three well-established adsorption isotherm models: the Freundlich, Langmuir, and Tempkin. Each model presents a distinct perspective on the adsorption mechanism. The Freundlich isotherm is suitable for describing adsorption on heterogeneous surfaces with different energy distributions. In contrast, the Langmuir isotherm implies adsorption on a homogeneous surface with a limited number of adsorption sites. The Temkin isotherm, on the other hand, considers the linear reduction in adsorption energy resulting from interactions between the adsorbate and the adsorbent.

3.4.1 Adsorption models

The Freundlich model is an analytical formula commonly used to characterize adsorption on diverse surfaces. It accounts for multilayer adsorption and indicates that the sorption potential falls dramatically as the adsorption process progresses. The linearized Freundlich formula is calculated as in Eq. (8) (Freundlich, 1906):

(8)

\ln q_{e} = {lnK}_{f} + \frac{1}{n_{f}} {LnC}_{e}

where K_f (mg/g) represents the adsorption of the Freundlich model, and n is the adsorption intensity indicator, indicating the surface heterogeneity and binding strength. A value of 1/n < 1 suggests favorable adsorption, while 1/n > 1 implies cooperative adsorption.

In the present study, the Freundlich equilibrium constants were determined from the plot of log q_e versus log C_e (Fig. 12). Critical parameters can be inferred from the Freundlich isotherm, which helps characterize the adsorption mechanism. The n and 1/n_f the quantity given represents the level of nonlinearity between metal ions concentration and Chitosan as biosorption, which can help to characterize the isotherm adsorption mechanism: If n_f equals one, adsorption is linear; If n_f < 1, adsorption is a chemical process; otherwise, adsorption is physical (Awwad and Salem, 2012; Yusuf et al., 2024). The calculated values of n_f were 5.8173 ± 0.183 and 4.6189 ± 0.231 for Cu²⁺ and Pb²⁺, respectively, confirming that the adsorption process is predominantly physical (Table 2). Cu²⁺ and Pb²⁺ are physically absorbed onto Chitosan, as shown by n_f values between 1 and 10. The regression rates for this model were 0.8093 for Cu²⁺ and 0.6791 for Pb²⁺, indicating a moderate fit of the experimental findings to the Freundlich model.

(a) Freundlich, (b) Langmuir, and (c) Temkin isotherm plots for biosorption of Cu2+and Pb2+ ions onto Chitosan.

Table 2. Langmuir and the Freundlich isotherm parameters for Pb²⁺ and Cu²⁺ adsorption onto Chitosan. qₘₐₓ = maximum adsorption capacity (mg g⁻¹); K_ᴸ = Langmuir constant (L mg⁻¹); R_ᴸ = dimensionless separation factor; K_f = Freundlich constant [(mg g⁻¹)(L mg⁻¹)¹/ⁿ]; n = Freundlich intensity parameter; R² = coefficient of determination.

Isotherm models	Parameters	Cu²⁺	Pb²⁺
Langmuir	q_max (mg/g)	8.20±0.41	10.49±0.52
	K_L (L/mg)	71.76±7.5	11.48±1.15
	R_LR²	0.0007±7*10^-50.9923	0.0043±43*10^-50.9951
Freundlich	K_f (L/mg)	3.72±0.19	10.01±0.50
	n_f (g/L)	3.67±0.18	4.62±0.23
	1/n_f (L/g)	0.27	0.2165
	R²	0.8093	0.6791
Tempkin	A_t(L/mg)	6.36±0.23	2.05±0.103
	B_T (kg/mol)	1.57±0.079	2.42±0.121
	R²	0.7592	0.9210

The Langmuir isotherm implies that adsorption occurs at specified and homogeneous sites on the adsorbent surface, forming a monolayer coverage (Langmuir, 1918). Once an adsorption region is established, no further adsorption may occur at that spot, leading to saturation at the maximum adsorption capacity. The linearized Langmuir is in Eq. (9) below:

(9)

1 / qe = 1 / qmax + (1 / (qmax KL)) (1 / Ce)

qₑ represents the equilibrium capability for adsorption (mg/g), qₘₐₓ is the monolayer) max. (adsorption capability (mg/g), K_L is the Langmuir constant (L/mg), and Cₑ is the equilibrium metal ion content (mg/L). From the linear plot of 1/q_e versus 1/c_e, the Langmuir parameters (q_max and K_L) were determined (Fig. 12, Table 2). The results show that the sorption capacities for Cu²⁺ and Pb²⁺ are 8.1967 ± 0.41 and 10.4932 ± 0.52 mg/g, respectively. The high regression coefficients (R^² = 0.9923 for Cu²⁺ and R^² = 0.9951 for Pb²⁺) suggest that the Langmuir isotherm provides an excellent fit for the experimental findings, indicating a monolayer adsorption process on a uniform surface. This behavior signifies chemisorption, where adsorption occurs through ionic or covalent bonding between Chitosan and metals.

Additionally, the fundamental features of the Langmuir model can be described by the undefined separating coefficient (R_L), which is calculated as Eq. (10) (Ayawei et al., 2017; Özer et al., 2004)

(10)

R_{L} = 1 / 1 + K_{L} C_{o}

0 < RL < 1 suggests a good adsorption, R_L > 1 suggests unfavorable adsorption, R_L = 1 reflects linear adsorption, and RL = 0 means permanent adsorption. The Langmuir factor is denoted by KL (L/mg), whereas Co represents the initial adsorbate concentration of 20 mg/L. In this situation, K_L values were 71.76 and 11.48 for Cu²⁺ and Pb²⁺, respectively. R_L values were 0.0007 and 0.0043 for Cu²⁺ and Pb²⁺, respectively. Lower RL values indicate that adsorption becomes more efficient. These results were in close agreement with those previously studied (Awwad and Salem, 2012; Yusuf et al., 2024).

The Tempkin isotherm model accounts for indirect connections between the adsorbate and adsorbent by assuming that the thermal energy of adsorption reduces linearly, not logarithmically, with the surface coverage. The linear Temkin isotherm Eq. (11) is expressed as:

(11)

q_{e} = B_{T} {lnA}_{T} + B_{T} {LnC}_{e}

where A_T is the Tempkin equilibrium interaction factor (L/g), b is the adsorption heat constant (J/mol), R represents the universal gas constant (8.314 J/mol·K), and T represents the actual temperature (K). In this study, the Temkin parameters were detected from the plot of qe versus ce (Fig. 12). The obtained regression coefficients were 0.7592 for Cu²⁺ and 0.9210 for Pb²⁺, implying that the Temkin model provided a moderate to good fit for the adsorption process.

3.4.2 Adsorption kinetics

Kinetic modeling provides insight into the systems that govern adsorption rates, aiding in process design. The adsorption of Pb²⁺ and Cu²⁺ onto Chitosan was studied using three kinetic approaches: pseudo-first-order, pseudo-second-order, and intraparticle diffusion. The appropriateness of each model was assessed by comparing experimental findings with anticipated values; the results are presented in Table 3.

Table 3. Kinetic model parameters for the adsorption of Pb²⁺ and Cu²⁺ onto Chitosan. qₑ = equilibrium adsorption capacity (mg g⁻¹); k₁ = pseudo-first-order rate constant (min⁻¹); k₂ = pseudo-second-order rate constant (g mg⁻¹ min⁻¹); kᵢd = intraparticle diffusion rate constant (mg g⁻¹ min^⁻½); C = intercept; α = initial adsorption rate (mg g⁻¹ min⁻¹); β = desorption constant (g mg⁻¹); R² = coefficient of determination.

Kinetic	Parameter	Cu²⁺	Pb²⁺
qe (experimental) (mg/g)		18.60	19.02
Pseudo-first order	K₁(min^-1)q_e (mg/g)R²	-0.0293±0.0015 14.98±0.75 0.5665	-0.0357±0.0018 15.15±0.76 0.6200
Pseudo-second order	K₂ (g/mg/min) q_e (mg/g) R²	0.00123±1.23*10^-3 19.96±1 0.9539	0.00127±1.27*10^-3 20.75±1.04 0.9634
Intraparticle diffusion model	C (mg/L) k_dif(mg/g min^1/2) R²	0.97±0.048 1.90±0.095 0.9539	1.58±0.079 2.19±0.109 0.9634

The pseudo-first-order model, proposed by (Lagergren, 1898), assumes that adsorption occurs through physisorption and that Adsorption frequency depends on the distinction across equilibrium capacity for adsorption (qₑ) and adsorption potential at time t (qₜ) (Lagergren, 1898) Eq. (12)

(12)

\ln (q_{e} - q_{t}) = \ln q_{e} - k_{1} t

k₁ (min⁻¹) is the rate constant of the pseudo-first-order reaction. The linear plot of Ln(q_e−q_t) versus t was used to determine k₁ and q_e. As shown in Table 3, the pseudo-first-order yielded lower correlation coefficients (R² = 0.5665 for Cu²⁺ and R² = 0.6200 for Pb²⁺), indicating an insufficient match with the experimental results. Moreover, the calculated q_e values (14.97±0.75 mg/g for Cu²⁺ and 15.14±0.76 mg/g for Pb²⁺) deviated significantly from the experimentally determined values (18.60 mg/g and 19.02 mg/g, respectively). These discrepancies indicate that metal ion adsorption onto Chitosan does not follow a simple physisorption process. Instead, stronger interactions such as chemical bonding or ion exchange may be involved (Hubbe et al., 2019). Similar deviations have been reported in previous studies, confirming that the pseudo-first model is often insufficient for biosorption onto polymeric materials (Ngah and Hanafiah, 2008).

The pseudo-second model assumes that the adsorption frequency is maintained by chemisorption, suggesting valency forces and electron exchange between metal ions and active sites on the adsorbent surface Eq. (13) (Ho and McKay, 1999)

(13)

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}

where k₂ (g/mg·min) is the frequency factor of the pseudo-second-order reaction. This model is widely applied to biosorption processes due to its accuracy in describing adsorption onto polymeric materials, such as Chitosan. The linear fit of t/q_t versus t yielded higher coefficients (R² = 0.9539 for Cu²⁺ and R² = 0.9634 for Pb²⁺) compared to the pseudo-first-order model. Additionally, the obtained q_e values (19.96±1 mg/g for Cu²⁺ and 20.75±1.04 mg/g for Pb²⁺) closely matched the experimental values (Table 3, Fig. 10). The strong consistency between experimental and calculated q_e values confirms that chemisorption governs the adsorption process. The higher k² values for Pb²⁺ suggest a slightly faster adsorption rate compared to Cu²⁺, which may indicate good interactions between Pb²⁺ ions and chitosan functional groups (Ho, 2006; Tran et al., 2017). This model’s superiority in describing metal ion adsorption onto Chitosan has been widely reported, supporting its applicability to polymer-based adsorbents (Abourehab et al., 2022).

To evaluate whether intraparticle diffusion has an essential role in the process of adsorption, the Weber and Morris (1963) model was applied Eq. (14) (Weber Jr and Morris, 1963)

(14)

q_{t} = k_{id} t^{1 / 2} + C

where kₑ (mg/g·min½) is the intraparticle diffusion factor and C is a boundary parameter. A linear correlation between qₜ and t¹/² indicates that intraparticle diffusion is the dominant step. In contrast, deviations from linearity suggest the involvement of additional mechanisms, such as surface adsorption and external mass transfer. The intraparticle diffusion model produced a good fit (R² > 0.95) for both Cu²⁺ and Pb²⁺ (Table 3). However, the nonzero C values (0.97±0.048 mg/L for Cu²⁺ and 1.58±0.079 mg/L for Pb²⁺) indicate that intraparticle diffusion was not the sole rate-limiting step. While intraparticle diffusion contributes significantly to the overall process, it does not fully control the adsorption kinetics. Instead, a hybrid mechanism is likely, where metal ions first bind to active sites on Chitosan (chemisorption), followed by gradual diffusion into the polymer matrix (Ngah and Hanafiah, 2008; Tran et al., 2017). This aligns with studies reporting that biosorption onto chitosan-based materials involves intraparticle diffusion and surface adsorption (Bhattacharyya and Gupta, 2008).

3.4.3 Thermodynamic studies

The Pb²⁺ and Cu²⁺ adsorption behavior onto Chitosan was evaluated through thermodynamic factors, including ∆G^o, ∆H^o, and ∆S^o. These factors were determined using equilibrium distribution coefficients (K_d) and analyzed to comprehend the adsorption technique. The thermodynamic parameters provide insight into improvement, heat effects, and disorder associated with the adsorption technique. The distribution factor (K_d) was first estimated based on equilibrium adsorption results using the Eq. (15)

(15)

K_{d} = q_{e} / C_{e}

where q_e represents the equilibrium level of capacity for adsorption (mg/g), and C_e is the equilibrium metal ion concentration in solution (mg/L). The spontaneity of adsorption was assessed by computing ΔG° utilizing the Van’t Hoff Eq. (16)

(16)

Η G ° = - R T L n K_{d}

where R is the global gas factor (8.314 J/mol·K), and T is the ultimate temperature (K). Negative ΔG° values indicate a thermodynamically favorable and spontaneous adsorption process, with more negative values indicating a stronger adsorption affinity.

To further investigate the adsorption mechanism, the ΔH° and ΔS° were calculated from the linearized Van’t Hoff Eq. (17)

(17)

L n K_{d} = \frac{Δ S °}{R} - \frac{Δ H °}{R T}

Both the intercept and the slope from the ln Kd vs. 1/T figure were used to estimate ΔH° and ΔS° (Fig. 13). The signs and intensities of these factors provide essential information on the adsorption technique. A positive ΔH° suggests an endothermic adsorption mechanism, indicating that adsorption is more favorable at higher temperatures. Conversely, a negative ΔH° implies an exothermic process, where adsorption decreases with increasing temperature. Similarly, the ΔS° represents the level of unpredictability at the solid-liquid junction throughout adsorption. A positive ΔS° indicates a rise of instability at the interface, suggesting greater affinity between the metal ions and the chitosan surface, possibly due to structural rearrangements or dehydration effects. A negative ΔS°, on the other hand, signifies a decrease in system randomness, often associated with the ordered adsorption of molecules onto well-defined active sites.

Van’t Hoff plot for the adsorption of Cu2+ and Pb2+ onto Chitosan.

The computed thermodynamic parameters for the adsorption of Cu2+ and Pb2+ onto Chitosan at various temperatures are presented in Table 4. The negative ΔG° values for both Cu²⁺ and Pb²⁺ indicate spontaneous adsorption. The values become increasingly negative with rising temperature, implying improved spontaneity at higher temperatures. The outcomes are consistent with earlier investigations, such as that of Chand et al. (2009), who reported comparable thermodynamic trends for heavy metal adsorption onto modified biopolymer adsorbents.

Table 4. Thermodynamic factors of Pb²+ and Cu²+ adsorption onto Chitosan at variable temperatures, including distribution coefficient (K_d), ∆G^o, ∆H^o, and ∆S^o.

Metals	T (⁰C)	T (K)	K_d	∆G⁰(kJ/mol)	∆H⁰(kJ/mol)	∆S⁰(J/mol K)	R²
Cu²⁺	20	293	1.02	-1.6885	40.39±2	143.83±6	0.9953
	25	298	1.45	-2.5803
	30	303	1.96	-3.1559
	35	308	2.55	-3.8515
	40	313	5.66	-4.734
	45	318	8.27	-4.9659
Pb²⁺	20	293	1.44	-0.524	52.02±2.6	179.82±7.5	0.9871
	25	298	1.93	-1.6803
	30	303	3.08	-2.5069
	35	308	5.93	-2.7687
	40	313	9.16	-4.5357
	45	318	9.60	-4.6608

The positive ΔH° values confirm that the adsorption of Cu²⁺ and Pb²⁺ onto Chitosan is an endothermic process, implying that heat input enhances ion diffusion and interaction with available active sites. The relatively low magnitudes of ΔH° (below 40 kJ mol⁻¹) indicate that the process is mainly physisorption, involving weak electrostatic and van der Waals interactions rather than chemical bonding. Similar endothermic behavior and stability trends have been reported in supramolecular metal–sulfonate frameworks, where hydrogen bonding and secondary interactions significantly influence the thermodynamic parameters (Shankar et al., 2020; Singh et al., 2019). Theoretical DFT analyses of such systems confirmed that weakly coordinated sulfonate groups favor entropy-driven adsorption processes and contribute to overall spontaneity (Shankar et al., 2018; Singh et al., 2018).

The positive ΔS° values further reveal an increase in randomness at the solid–liquid interface, which can be attributed to the desolation of hydrated ions and minor structural rearrangements of the chitosan matrix during adsorption. Similar entropy-driven behavior has been observed for other biopolymeric sorbents (Foo and Hameed, 2010), supporting the notion that metal ion binding onto Chitosan involves both diffusion enhancement and structural adaptation of the sorbent surface.

4. Desorption Studies and Reusability of Chitosan for Heavy metal Removal

The efficiency of an adsorbent is not only dependent on its metal removal capacity but also on its ability to be regenerated and utilized for multiple cycles. Desorption studies evaluate whether metal-laden Chitosan can be effectively regenerated using suitable desorbing agents while maintaining its structural integrity. This work systematically examines the desorption of Pb²⁺ and Cu²⁺ from Chitosan, focusing on the effect of desorbing agent type and concentration, desorption kinetics (interaction time), and sorption-desorption cycles to assess reusability. To determine the most effective desorbing agent, two commonly used eluents, nitric acid (HNO₃, 0.1–2.0 M) and hydrochloric acid (HCl, 0.1–2.0 M), were tested. The desorption efficiency (%D) of Pb²⁺ and Cu²⁺ at different concentrations of HNO₃ and HCl is noted in (Fig. 14).

Desorption efficiency of Pb2⁺ and Cu2⁺ at different concentrations of HNO₃ and HCl.

The findings show that desorption efficiency increased with increasing acid concentration, reaching its highest values (>90%) at 0.5 M HNO₃ and 1.0 M HCl. However, at lower concentrations (0.01 M HNO₃ or HCl), desorption efficiency remained below 20%, suggesting insufficient elution strength for metal ion removal. Notably, 0.5 M HNO₃ emerged as the optimal desorbing agent, achieving over 98% Pb²⁺ and Cu²⁺ desorption, making it the preferred choice for subsequent kinetic and reusability tests. These findings align with previous studies that employed HNO₃ as a highly effective desorbing agent. Zhao et al. (2020) confirmed the effectiveness of nitric acid in regenerating modified chitosan composites (Zhao et al., 2020). To evaluate desorption kinetics, Pb²⁺ and Cu²⁺-loaded chitosan samples were subjected to 0.5 M HNO₃ for time intervals ranging from 1 to 360 minutes. A rapid desorption phase was observed within the first 60 minutes, with over 80% of Pb²⁺ and Cu²⁺ desorbed. The equilibrium desorption point was attained at 360 minutes, with maximum recovery rates of 98.5±1% for Pb²⁺ and 99.1±1% for Cu²⁺ (Fig. 15).

Effect of desorption time on Pb2⁺ and Cu2⁺ recovery using 0.5 M HNO₃.

The fast initial desorption phase may be related to the rapid release of surface-bound metal. In contrast, the slower phase suggests the involvement of stronger electrostatic interactions or metal ion chelation. (Costa et al., 2021) Observed similar equilibrium times for metal desorption from chitosan-based adsorbents. Regarding large-scale use, an adsorbent must have a high adsorption capacity after numerous reuse cycles. To test the reusability of Chitosan, 10 adsorption-desorption cycles with 0.5 M HNO₃ were performed, with the adsorption capacity measured after each cycle. After 10 cycles, Chitosan preserved approximately 90% of its initial adsorption ability. Pb²⁺ adsorption decreased by 9.4% and Cu²⁺ by 8.7% (Table 5). Despite this gradual decrease, the biosorbent remained highly effective in metal removal, demonstrating excellent regeneration potential. These results align with earlier research. Zhao et al. (2020) reported a 10.92% decrease in La(III) adsorption capacity after 10 cycles, and Costa et al. (2021) found that modified Chitosan retained approximately 90% of its original adsorption performance after repeated use.

Table 5. Changes in adsorption capacity after multiple desorption cycles.

Cycle number	Pb²⁺ adsorption (mg/g)	Cu²⁺ adsorption (mg/g)	% Decrease in adsorption
1	10.49±0.52	8.19±8.19	0.00%
3	10.21±0.51	7.94±0.40	2.50%
5	9.93±0.50	7.72±0.39	5.00%
7	9.72±0.49	7.55±0.38	7.30%
10	9.49±0.47	7.48±0.37	9.4% (Pb ²⁺), 8.7%(Cu ²⁺)

Desorption investigations indicate that Chitosan can be regenerated and reused for Pb²⁺ and Cu²⁺ adsorption with minimal loss of efficiency. The optimal desorption conditions were established using 0.5 M HNO₃, with equilibrium achieved after 360 minutes, resulting in a high desorption efficiency (>98%). Over 10 adsorption-desorption cycles, Chitosan preserved more than 90% of its original adsorption capability, indicating its promise for long-term wastewater treatment applications. These findings demonstrate the economic viability, environmental sustainability, and efficiency of Chitosan as a regenerable biosorbent, making it an attractive alternative for large-scale elimination of heavy metals and wastewater treatment.

The adsorption of Pb²⁺ and Cu²⁺ ions onto Chitosan occurs primarily through ion exchange and chelation mechanisms involving the amino (–NH₂) and hydroxyl (–OH) functional groups. In aqueous media, the deprotonation of these groups facilitates coordination with metal cations, where the lone pair electrons of nitrogen and oxygen atoms interact with the vacant orbitals of Pb²⁺ and Cu²⁺, forming stable coordination complexes. The simultaneous participation of –NH₂ and –OH groups enhances the overall metal-binding strength through synergistic electrostatic and covalent interactions. The FTIR spectral shifts observed for the N–H and O–H stretching bands after metal adsorption confirm the participation of these functional groups in metal ion binding. Additionally, EDX analysis, which shows the presence of Pb and Cu peaks, further validates the successful surface complexation. Fig. 16 illustrates the proposed adsorption mechanism of Pb²⁺ on Chitosan, highlighting ion exchange between protonated amino groups and Pb²⁺ ions, followed by chelation involving nitrogen and oxygen donor sites. This dual mechanism agrees with previous reports describing Chitosan–metal interactions as a combination of electrostatic attraction and coordination complex formation (Babel and Kurniawan, 2003; Guibal, 2004; Muhaidin et al., 2024; Ngah and Fatinathan, 2008; Thambiliyagodage et al., 2023).

(a) illustrates the chelation mechanism and (b) illustrates the ion-exchange mechanism.

5. Summary of Adsorption Performance and Practical Relevance

Under the optimized adsorption conditions (pH 7.5, contact time = 90 min, adsorbent dose = 0.1 g per 50 mL, and temperature = 25°C), the Chitosan derived from shrimp shells demonstrated outstanding removal capacities, achieving 93.0 ± 2.8% removal of Pb²⁺ and 89.5 ± 2.7% removal of Cu²⁺ from aqueous solutions. These high efficiencies confirm the strong affinity of the amino (–NH₂) and hydroxyl (–OH) functional groups toward divalent metal ions, consistent with the chemisorption mechanism established by kinetic and isotherm analyses.

The combination of high removal efficiency, chemical stability, and reusability (retaining over 90% of the adsorption capacity after ten regeneration cycles) highlights Chitosan’s potential as a sustainable biosorbent for heavy-metal remediation. Its low cost, biodegradability, and availability from seafood waste further enhance its environmental and economic appeal.

Nonetheless, practical challenges remain for large-scale implementation. In real wastewater containing multiple metal ions and competing solutes, the adsorption efficiency may decline due to competitive binding effects. Moreover, regeneration using acidic eluents must be carefully optimized to avoid polymer degradation. Therefore, further studies should focus on pilot-scale testing, assessing long-term stability, and developing modified chitosan composites with improved selectivity and mechanical performance.

6. Conclusions

This study demonstrated that shrimp-shell-derived Chitosan is an efficient and sustainable biosorbent for removing Pb²⁺ and Cu²⁺ ions from aqueous solutions. The adsorption process was strongly influenced by pH, contact time, and dosage, achieving maximum removal efficiencies of 93.0 ± 2.8% for Pb²⁺ and 89.5 ± 2.7% for Cu²⁺ under optimized conditions (pH 7.5, 90 min, 0.1 g/L per 50 mL). Kinetic analysis confirmed that the adsorption followed a pseudo-second-order model, while the equilibrium data fitted well to the Langmuir isotherm, indicating monolayer chemisorption on homogeneous sites. The mechanistic investigation supported a combined ion-exchange and chelation process involving amino and hydroxyl groups on the chitosan surface. The material demonstrated excellent reusability, retaining more than 90% of its adsorption capacity after ten regeneration cycles, confirming its potential for repeated use. The findings highlight the environmental value of converting marine waste into effective biosorbents. However, extending these results to real wastewater systems requires further investigation, particularly in the presence of multiple competing ions and varying ionic strengths. Future studies should focus on optimizing regeneration methods, assessing performance in complex effluents, and developing modified chitosan composites to enhance selectivity, mechanical stability, and long-term usability. Overall, this work advances the application of chitosan-based biosorbents by introducing a regionally sourced, highly deacetylated material with proven regeneration potential, offering a sustainable pathway for large-scale wastewater treatment.

Acknowledgement

This Project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, Saudi Arabia, under grant no. (IPP: 49-155-2025). The authors, therefore, acknowledge with thanks DSR for technical and financial support.

CRediT authorship contribution statement

The author contributed to the study conception and design, material preparation, data collection and analysis, investigation, and writing of the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Results will be provided upon an adequate inquiry.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Supplementary data

Supplementary material to this article can be found online at https://dx.doi.org/10.25259/JKSUS_1080_2025.

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Application of shrimp shell-derived chitosan for heavy metal biosorption

Abstract

Keywords

Chitosan

Biosorption

Heavy metal elimination

Shrimp shell waste

Sustainable water treatment

1. Introduction

2. Materials and Methods

2.1 Sampling and raw material preparation

2.2 Chitosan extraction

2.3 Solutions and reagents

2.4 Chitosan characterization

2.5 Experiments on influencing factors

2.5.1 Effect of pH

2.5.2 Time impact

2.5.3 Effect of adsorbent dosage

2.5.4 Initial concentration impact

2.5.5 Temperature impact

2.6 Data analysis and adsorption modeling

2.6.1 Equilibrium adsorption and removal efficiency

2.6.2 Adsorption kinetics

2.6.3 Adsorption isotherms

2.6.4 Adsorption thermodynamics

3. Results and Discussion

3.1 Chitosan yield and literature context

3.2 Identification of the created Chitosan

3.3 Batch experiments: An adsorption study

3.3.1 Influence of initial solution pH

3.3.2 Impact of metal concentrations

3.3.3 Adsorbent dosage impact

3.3.4 Impact of contact time

3.3.5 Impact of temperature

3.4 Biosorption isotherm models

3.4.1 Adsorption models

3.4.2 Adsorption kinetics

3.4.3 Thermodynamic studies

4. Desorption Studies and Reusability of Chitosan for Heavy metal Removal

5. Summary of Adsorption Performance and Practical Relevance

6. Conclusions

Acknowledgement

CRediT authorship contribution statement

Declaration of competing interest

Data availability

Declaration of generative AI and AI-assisted technologies in the writing process

Supplementary data

References

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