Multifunctional Chitosan@TiO₂/PPy nanocomposite for high-performance adsorption of malachite green: Kinetics, isotherms, and thermodynamics

Ali Q. Alorabi

doi:10.25259/JKSUS_960_2025

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Research Article

2025

:37;

9602025

doi:

10.25259/JKSUS_960_2025

Multifunctional Chitosan@TiO₂/PPy nanocomposite for high-performance adsorption of malachite green: Kinetics, isotherms, and thermodynamics

Ali Q. Alorabi^a,*

aDepartment of Chemistry, Faculty of Science, Al-Baha University 4781, Al-Baha 65779, Saudi Arabia

* Corresponding author E-mail address: aalorabi@bu.edu.sa (A. Q. Alorabi)

Received: 2025-05-31, Accepted: 2025-08-01, Epub ahead of print: 2025-09-29, Published: 2025-10-06

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Abstract

Industrial discharge of synthetic dyes poses serious threats to aquatic ecosystems and human health. In this study, a novel ternary nanocomposite, Chitosan@TiO₂/Polypyrrole (PPy) nanocomposite was fabricated through a green in situ oxidative polymerization approach for Malachite Green (MG) removal, utilizing the synergistic adsorption capabilities of TiO₂, PPy, and chitosan via electrostatic, hydrogen bonding, and π–π interactions. Characterization confirmed the formation of a thermally stable, mesoporous structure with a surface area of 52.11 m²/g and pHₚzc of 6.56. The synthesized composite exhibited excellent adsorption capacity, reaching 304.45 mg/g under optimal conditions (25°C, 50 ppm MG concentration). The kinetic behavior followed a pseudo-second-order model (R² = 99.2), indicating that the adsorption mechanism likely involved chemical interactions. Equilibrium analysis showed strong agreement with the Langmuir isotherm (R² = 98.6–99.2), reflecting monolayer coverage of MG molecules on a homogeneous surface. Thermodynamic parameters confirmed the process as energetically favorable and exothermic in nature (ΔH° = –44.15 kJ/mol; ΔG° = –25.73 kJ/mol at 298 K), underscoring the material’s efficiency and suitability for sustainable dye remediation applications.

Keywords

Adsorption

Chitosan

Isotherm study

Malachite green

Polypyrrole

TiO2

Wastewater treatment

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1. Introduction

The widespread discharge of synthetic dyes into aquatic environments, originating from industries such as textiles, leather processing, paper manufacturing, and printing, has become a major environmental concern, mainly due to the dyes’ vivid coloration, high chemical persistence, and potential toxicity (Abdi et al., 2024; Ali, 2010; Sharma et al., 2021). Malachite Green (MG), a widely used cationic dye, finds extensive application in areas including textile processing, aquaculture treatments, and biological staining protocols. (Shanker et al., 2017; Sharma et al., 2023). Despite its wide applications, MG is associated with adverse health effects, including mutagenicity, cytotoxicity, and carcinogenicity, as well as ecological impacts such as inhibition of aquatic photosynthesis and bioaccumulation in aquatic species (Okereafor et al., 2020; Shahid et al., 2020). Therefore, developing efficient strategies for the removal of MG from wastewater is vital for safeguarding environmental and public health (Anderson et al., 2022; Hanjra et al., 2012).

Conventional methods for treating dye-contaminated wastewater, including coagulation, chemical oxidation, and membrane-based processes, are commonly employed but often exhibit limitations in terms of removal efficiency and the potential formation of secondary pollutants (Saravanan et al., 2021). As an alternative, adsorption has gained considerable attention due to its simplicity, cost-effectiveness, high removal capacity, and operational flexibility (Qiu et al., 2022; Rashid et al., 2021). The effectiveness of adsorption processes is strongly influenced by the physicochemical characteristics of the adsorbent. Recent studies have emphasized the development of advanced adsorbent materials, particularly hybrid composites composed of polymers and metal oxides, for enhanced dye uptake (Ahmaruzzaman et al., 2025; Bassie Gelaw et al., 2021; Iqbal et al., 2022).

Natural biopolymers like chitosan, cellulose, and alginate have attracted increasing interest as sustainable adsorbents owing to their biodegradability, low toxicity, and rich content of functional groups (e.g., –OH, –NH₂) that facilitate interactions with dye pollutants (Das et al., 2024; Kanmani et al., 2017). Chitosan, obtained by deacetylating chitin, is particularly notable for its high binding affinity toward both anionic and cationic dyes via mechanisms such as electrostatic attraction, hydrogen bonding, and chelation (Mohamed, 2021; Tarique et al., 2023). However, pure chitosan suffers from low surface area, poor mechanical stability, and limited regeneration potential (Liu et al., 2022). To overcome these drawbacks, various chitosan-based composites have been developed by incorporating inorganic fillers to improve adsorption kinetics, surface roughness, and reusability (Ahmed and Mohamed, 2023). For example, Fahad Abdulaziz et al. prepared a chitosan–Fe₂O₃ hybrid that demonstrated improved adsorption performance for Reactive Blue 19 dye, with a maximum uptake of 140 mg/g under favorable pH conditions (Abdulaziz and Alanazi, 2024).

Titanium dioxide (TiO₂) is extensively researched as a metal oxide owing to its exceptional chemical resilience, expansive surface area, strong oxidative capabilities, and notable photocatalytic performance under both ultraviolet and visible light exposure (Ijaz and Zafar, 2021). To improve the performance of biopolymer-based adsorbents, TiO₂ has been embedded within polymer frameworks, contributing to greater adsorption capacity and material stability. For instance, TiO₂-loaded chitosan films (Ahamad et al., 2025), TiO₂-modified alginate beads (Dallabona et al., 2021), and TiO₂/GO composites (Ramos et al., 2020) have shown enhanced removal of various dyes attributed to the introduction of additional surface hydroxyl groups, electrostatic forces, and π–π electron donor–acceptor interactions.

Despite these advancements, TiO₂-based systems alone often exhibit limitations, such as light dependency and particle agglomeration, which reduce adsorption efficiency in dark conditions. To address such limitations, conducting polymers like polypyrrole (PPy) have been explored as functional components in hybrid adsorbents. PPy is a π-conjugated nitrogen-containing polymer known for its electrical conductivity, chemical resistance, and the existence of positively charged nitrogen sites that facilitate interaction with anionic and aromatic dye molecules (Zarean Mousaabadi et al., 2024; Zhou and Xu, 2017). Its redox-active surface also enhances dye degradation when combined with photocatalysts. Several PPy-based composites, such as PPy/Fe₃O₄ (Joshi et al., 2021), PPy/GO (Noreen et al., 2021), and PPy/TiO₂ (Sultan et al., 2016), have demonstrated notable adsorption and photocatalytic capabilities due to synergistic polymeric networks and the embedded inorganic constituents. The standalone application of TiO₂ or PPy in dye removal is limited by low adsorption capacity in the absence of light, particle agglomeration (TiO₂), and poor structural stability with low surface area (PPy) (Jamali Alyani et al., 2024). Multifunctional ternary nanocomposites integrating biopolymers, metal oxides, and conductive polymers have been explored in other domains such as flame-retardant and anti-corrosive coatings (Xavier, 2024) (Priyadharshini and Xavier, 2025), demonstrating superior structural synergy, stability, and performance. However, their translation to aqueous-phase environmental applications such as dye adsorption remains underexplored. However, a key research gap persists: most existing systems are binary composites that do not fully exploit the potential synergistic interplay between biopolymers, metal oxides, and conductive polymers. These binary systems often suffer from trade-offs between surface area, adsorption selectivity, and structural integrity. Furthermore, the mechanisms governing adsorption in such hybrids remain insufficiently understood or optimized for practical applications.

In response to the limitations of conventional and binary composite adsorbents, this work introduces a novel multifunctional ternary nanocomposite, Chitosan@TiO₂/PPy, designed to synergize the physicochemical properties of TiO₂, the conductive features of PPy, and the bioaffinity of chitosan. The material was synthesized through an eco-friendly in situ oxidative polymerization technique, aiming to achieve improved structural integrity, adsorption efficiency, and diverse interaction pathways with dye molecules. This hybrid aims to overcome the performance limitations of binary systems by enabling stronger and more varied interaction mechanisms such as electrostatic forces, hydrogen bonding, π–π stacking, and surface complexation. Comprehensive characterization and systematic evaluation of its performance toward MG were conducted, including kinetic, isotherm, and thermodynamic analyses. The findings confirm that the synthesized composite exhibits excellent dye uptake performance, structural integrity, and preferential interaction with MG, underscoring its applicability in large-scale wastewater purification. This work introduces an innovative approach for developing multifunctional and eco-friendly adsorbent materials tailored for advanced water treatment systems.

2. Materials and Methods

2.1 Materials

The pyrrole monomer (PPy, C₄H₅N, 98%), ferric chloride hexahydrate (FeCl₃·6H₂O, ≥98%), and titanium (IV) oxysulfate (TiOSO₄, 99.99%) were sourced from Sigma-Aldrich. Additional reagents, including hydrochloric acid (HCl, 36%), nitric acid (HNO₃, 68.0–70.0%), and sodium hydroxide (NaOH, ≥97%) were supplied by BDH, UK. Various dyes including Crystal Violet (CV, C₂₅H₃₀ClN₃. 97%), methylene blue (MB, C₁₆H₁₈ClN₃S, 95%), MG (C₁₆H₁₈N₃SCl, ≥90%), methyl green (MtG, C₂₆H₃₃Cl₂N₃, 85%), methylene blue (MtB, C₃₇H₂₇N₃Na₂O₉S₃, ≥95%), Congo Red (CR, C₃₂H₂₂N₆Na₂O₆S₂, ≥85%), methyl orange (MO, C₁₄H₁₄N₃NaO₃S, 85%), bromophenol blue (BPB, C₁₉H₁₀Br₄O₅S, 90%), reactive black (RB, C₂₆H₂₁N₅Na₄O₁₉S₆, 50%), and chitosan ((C₆H₁₁O₄N)_n, ∼85%) were also acquired from Sigma-Aldrich, USA.

2.2 Preparation of TiO₂ NPs

TiO₂ NPs were synthesized using a hydrolysis-precipitation approach. Specifically, 10 mL of TiOSO₄ was diluted in 120 mL of deionized water (DW) and magnetically stirred for 30 min to ensure complete dissolution. Subsequently, a 0.2 M NaOH solution was gradually added dropwise to the mixture to induce hydrolysis and control the pH within the alkaline range of 9-10, which promotes the formation of titanium hydroxide precursors under basic conditions. This alkaline range was chosen to ensure complete precursor hydrolysis and form crystalline, low-aggregation TiO₂ NPs with high surface area for improved adsorption (Wang et al., 2022). The prepared mixture was placed into a Teflon-lined autoclave and subjected to hydrothermal treatment at 230°C for 7 h. After the reaction, the system was allowed to cool naturally to room temperature. The resulting precipitate was then collected by centrifugation, thoroughly washed with deionized water and ethanol to eliminate residual impurities, and subsequently dried at 85°C.

2.3 Synthesis of Chitosan@TiO₂/PPy composite

The Chitosan@TiO₂/PPy composite was fabricated via an in situ oxidative polymerization approach. Initially, 1 g of TiO₂ NPs and 1 g of chitosan were separately dispersed in 50 mL of DW and subjected to ultrasonic treatment for 20 minutes. These dispersions were then combined and further sonicated for 40 minutes to achieve homogeneous mixing. In parallel, a 1.1 M solution of pyrrole was prepared by dissolving 0.5 g of pyrrole monomer in 70 mL of DW, while the oxidizing agent (2 M) was obtained by dissolving 18.3 g of FeCl₃·6H₂O in 60 mL of DW. The pyrrole solution was gradually introduced into the FeCl₃ solution under constant stirring in an ice bath maintained at 0-6°C to control the polymerization process. Subsequently, the previously prepared chitosan-TiO₂ mixture was added and stirred at a low temperature for 6 h, a duration chosen to ensure complete polymerization and uniform incorporation of all components based on prior optimization (Spoială et al., 2022). The resulting black solid was isolated by centrifugation, followed by multiple washings with DW and ethanol to remove any residual reactants, and subsequently dried in an oven at 95°C.

2.4 Characterization of Chitosan@TiO₂/PPy composites

A range of analytical techniques was employed to evaluate the structural integrity, surface morphology, elemental composition, and surface characteristics of the Chitosan@TiO₂/PPy nanocomposites. Fourier-transform infrared (FTIR) spectroscopy (Nicolet™ iS50, Thermo Scientific, USA) was utilized to identify functional groups and confirm chemical interactions among the composite constituents. Morphological features and elemental mapping were examined using scanning electron microscopy (SEM) (JSM-7001F, JEOL Ltd., Japan) equipped with energy-dispersive spectroscopy (EDS) (Oxford X-MaxN, UK) operating at 15 kV. Crystalline phases were determined through X-ray diffraction (XRD) analysis using a Shimadzu XRD-6100 diffractometer. Specific surface area, pore size distribution, and porosity were evaluated via nitrogen adsorption–desorption isotherms using the Brunauer-Emmett-Teller (BET) method (Micromeritics ASAP 2020, USA). Thermal stability was investigated through thermogravimetric analysis (TGA) (Q50, TA Instruments, USA) under nitrogen atmosphere up to 800°C. Additionally, the surface charge behavior and pH at the point of zero charge (pHₚzc) were determined using a Zetasizer Nano ZS (Malvern Panalytical, UK) (Salmi et al., 2024a).

2.5 Adsorption tests

The adsorption performance of the synthesized Chitosan@TiO₂/PPy composite toward MG dye was systematically evaluated under varying operational parameters. The effects of solution pH (3-10), contact time (15, 30, 60, 120, 180, 240, 300, and 360 min), adsorbent dosage (10, 30, 50, 70, 100, and 130 mg), and initial dye concentration (50-300 ppm) were investigated in batch mode. For each experiment, 0.07 g of the Chitosan@TiO₂/PPy composite was dispersed in 50 mL of an aqueous MG solution (C₀: 50 ppm) and agitated for 6 h to facilitate equilibrium adsorption. The pH of the solution was carefully controlled using 0.1 M HCl or NaOH, with continuous monitoring via a pH meter.

Following the adsorption process, the remaining concentration of dye in solution was quantified using UV–visible spectrophotometry at the characteristic absorption wavelength of MG (λₘₐₓ = 617 nm). Equilibrium adsorption capacity (qₑ, mg/g) and removal efficiency (Re, %) were computed using standard adsorption equations (Nayak et al., 2024):

(1)

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

(2)

R e . (%) = \frac{C_{o} - C_{e}}{C_{o}} \times 100

Here, C₀ and Cₑ denote the initial and equilibrium concentrations of MG in mg/L, respectively. The volume of the dye solution is represented by V (L), and m refers to the mass of the adsorbent in g.

3. Results and Discussion

3.1 XRD analysis

The XRD patterns provide clear evidence of the structural features and phase composition of the synthesized materials (Fig. 1). The TiO₂ NPs exhibit sharp and intense diffraction peaks at 2θ values of 25.31°, 38.57°, 48.04°, 53.89°, 55.06°, 62.69°, 68.76°, 70.29°, 75.05°, and 82.68°, which correspond to the (101), (112), (200), (105), (211), (204), (116), (220), (215), and (224) crystallographic planes, respectively. These reflections are characteristic of the tetragonal anatase phase of TiO₂ and are in excellent agreement with the standard JCPDS card No. 01-084-1285, confirming the phase purity and successful hydrothermal synthesis. In contrast, the XRD pattern of PPy is characterized by a broad hump around 2θ = 24°, which is typically associated with its amorphous or partially ordered structure (Shariq et al., 2021). In the case of the Chitosan@TiO₂/PPy composite, the characteristic diffraction peaks of TiO₂ remain visible, though with diminished intensity and slight broadening. This change implies potential interaction between TiO₂ and the polymeric components, along with a reduction in crystallite size likely caused by surface encapsulation. Crystallite size calculations using the Debye–Scherrer equation (Salmi et al., 2024b) yielded an average of 29.7 nm for pure TiO₂ and approximately 25.9 nm for the composite, supporting the observed broadening and reflecting the impact of polymer incorporation on crystal growth. These findings validate the formation of a structurally integrated hybrid composite with preserved TiO₂ crystallinity and modified nanoscale features.

XRD profiles of TiO₂ NPs, PPy, and the fabricated Chitosan@TiO₂/PPy composite.

3.2 FTIR analysis

The FTIR spectra of TiO₂ NPs, PPy, and the Chitosan@TiO₂/PPy composite reveal distinct vibrational bands confirming the successful synthesis and functional integration of the composite constituents (Fig. 2). In all spectra, a broad band around 3418 cm⁻¹ is observed, corresponding to O–H stretching vibrations from hydroxyl groups or adsorbed water (Liu et al., 2015). The TiO₂ NPs spectrum shows a characteristic Ti–O–Ti stretching vibration at approximately 488 cm⁻¹, indicative of the metal-oxygen framework in the anatase phase (Wandre et al., 2016). For pure PPy, notable peaks appear at 1550 and 1186 cm⁻¹, which can be assigned to C=C ring stretching and C–O stretching vibrations (Ma et al., 2020; Ţucureanu et al., 2016), respectively, while the band near 920 cm⁻¹ corresponds to C–H in-plane deformation (Agatonovic-Kustrin et al., 2020). The spectrum of Chitosan@TiO₂/PPy exhibits characteristic bands at 1618, 1188, 920, and 782 cm⁻¹, indicating the integration of Ppy and chitosan within the TiO₂ framework. Notably, the band at 1618 cm⁻¹, associated with N–H bending vibrations (Asyana et al., 2016), affirms the retention of chitosan’s functional groups in the composite. The retention and slight shifts of these functional groups indicate strong intermolecular interactions, possibly hydrogen bonding or coordination between the polymer matrices and the TiO₂ surface (Tian et al., 2022), which supports the formation of a stable hybrid structure with enhanced physicochemical characteristics.

3.3 TGA analysis

The TGA profiles of TiO₂, PPy, and the Chitosan@TiO₂/PPy composite reveal distinct thermal stability behaviors indicative of their compositional characteristics (Fig. 3). TiO₂ shows excellent thermal resistance, with a total weight loss of only 8.3%, attributed to surface-bound water evaporation below 107°C and minor decomposition of adsorbed species up to 800°C (Caramazana-Gonzalez et al., 2017). PPy, in contrast, exhibits significant thermal degradation, with an initial 8% weight loss between 41-145°C due to moisture loss, followed by a major 37% reduction from 145-802°C resulting from the breakdown of the polymer backbone and oxidative degradation. The Chitosan@TiO₂/PPy composite demonstrates improved stability compared to PPy alone, undergoing a 7% weight loss between 28-135°C corresponding to moisture evaporation, followed by a 4.5% loss from 156-355°C attributed to the decomposition of chitosan and partial degradation of PPy (Zhang et al., 2023). A further 15.5% mass loss is observed between 355-799°C, associated with the progressive thermal decomposition of the remaining organic matrix. The composite’s enhanced thermal resistance reflects the synergistic effect of TiO₂ and chitosan in stabilizing the PPy network, confirming the formation of a thermally robust hybrid structure.

TGA analysis of TiO₂ NPs, PPy, and the Chitosan@TiO₂/PPy composite.

3.4 BET analysis

The nitrogen adsorption–desorption analysis of the Chitosan@TiO₂/PPy composite (Fig. 4a) displayed a characteristic Type IV isotherm, along with a distinct H3-type hysteresis loop. This profile reflects the presence of mesopores and is commonly linked to the formation of slit-like pores or aggregates of plate-like particles, confirming the mesoporous architecture of the synthesized material (Barrett et al., 1951). This profile indicates capillary condensation occurring within mesopores, confirming the composite’s porous architecture. The BJH pore size distribution curve (Fig. 4b) shows a sharp peak centered around 9.9 nm, further corroborating the mesoporous nature of the material. According to the BET analysis, the specific surface area is 52.1060 m²/g, suggesting a moderately high surface area that facilitates efficient adsorptive and catalytic interactions. The BJH adsorption and desorption surface areas are slightly higher, recorded at 56.987 m²/g and 62.7538 m²/g, respectively, indicating good porosity with accessible internal surfaces. The corresponding pore volumes are 0.141091 cm³/g (adsorption) and 0.147211 cm³/g (desorption), which support the material’s capacity to accommodate guest molecules. The consistent average pore diameters from both BJH and D-H methods, ranging from approximately 9.38 to 9.91 nm, reflect uniform mesopore distribution (Sharifigaliuk et al., 2022). Compared to similar ternary composites reported in previous studies, such as ZnFe₂O₄/Fe₂O₃/chitosan (29.053 m²/g) (Ehsanizadeh et al., 2025) and chitosan–TiO₂–ZnO (39.48 m²/g) (Asadzadeh Patehkhor et al., 2021), the Chitosan@TiO₂/PPy composite exhibits a notably higher surface area and well-defined mesoporosity. This improvement is likely due to the integrated design of biopolymer, metal oxide, and conductive polymer, which effectively prevents agglomeration and facilitates the formation of a structurally stable and highly porous network (Ganesan and Xavier, 2024). These features make the composite particularly promising for use in adsorption-driven and catalytic environmental applications. These characteristics confirm that the Chitosan@TiO₂/PPy composite possesses a well-developed mesoporous network with ample surface area and pore volume, making it suitable for applications such as adsorption, catalysis, and environmental remediation.

(a) N2 adsorption–desorption isotherms of Chitosan@TiO₂/PPy composite; (b) BJH pore size distribution curve.

3.5 SEM analysis

The SEM image of PPy (Fig. 5a) displays a rough, disordered morphology with flake-like features and high porosity, typical of chemically polymerized PPy. TiO₂ NPs (Fig. 5b) exhibit uniform, spherical particles with smooth surfaces and relatively narrow size distribution, indicating successful crystallization via hydrothermal synthesis. The composite structure (Fig. 5c) shows spherical TiO₂ particles embedded within an amorphous polymer matrix of chitosan and PPy, forming a heterogeneous yet integrated hybrid surface. The polymeric coating surrounding the TiO₂ particles appears continuous and non-agglomerated, ensuring high accessibility of the surface for dye molecules. Additionally, the visible surface roughness and microscale porosity in the composite suggest enhanced adsorption capacity by providing increased surface area and diffusion pathways (Ganesan and Xavier, 2025; Xavier, 2025). The rougher texture and dispersed morphology confirm the effective incorporation of the individual components.

SEM micrographs of (a) PPy, (b) TiO₂ NPs, and (c) Chitosan@TiO₂/PPy composite; along with EDX spectra of (d) TiO₂ NPs, (e) PPy, and (f) Chitosan@TiO₂/PPy composite.

The corresponding EDX spectra further confirms the elemental composition of each sample. PPy (Fig. 5d) exhibits a nearly balanced distribution of carbon (51.21 wt%) and nitrogen (48.79 wt%), indicating the nitrogen-rich conjugated polymer structure. TiO₂ NPs (Fig. 5e) show dominant titanium (77.51 wt%) and oxygen (22.49 wt%) peaks, confirming high purity and stoichiometry. In the composite (Fig. 5f), the coexistence of carbon (45.73 wt%), nitrogen (20.59 wt%), oxygen (26.68 wt%), and titanium (7.00 wt%) evidences successful integration of the three constituents. The lower titanium content in the composite, compared to pure TiO₂, suggests uniform dispersion within the organic matrix. Together, these analyses confirm the morphological and elemental synergy achieved in the Chitosan@TiO₂/PPy composite, supporting its potential in multifunctional applications, such as adsorption, catalysis, and environmental remediation.

3.6 Zeta potential analysis

The zeta potential profile of the Chitosan@TiO₂/PPy composite (Fig. 6) indicates a well-defined pH-dependent surface charge behavior across the pH range of 3 to 9.5, with the isoelectric point (pHₚzc) occurring at pH 6.56. Below this threshold, the surface exhibits a net positive charge because of the protonation of amine and hydroxyl groups, while above this pH, deprotonation leads to a progressive increase in negative surface potential. This charge inversion directly influences electrostatic interactions during adsorption, particularly favoring the removal of cationic species such as MG at alkaline pH values (Raval et al., 2017). The continuous decline in zeta potential with increasing pH reflects the dynamic surface chemistry of the composite, which facilitates tunable interactions with adsorbates (Raval et al., 2017). Such responsive behavior highlights the material’s versatility in treating effluents across a broad pH spectrum, enhancing its suitability for practical wastewater remediation applications.

Fig. 6.

Zeta potential Profile of Chitosan@TiO₂/PPy composite as a function of pH (range 3-9.5).

3.7 Study of adsorption properties

3.7.1 Selective adsorption behavior

The adsorptive selectivity of the Chitosan@TiO₂/PPy composite was evaluated using various anionic and cationic dyes under standardized conditions (initial dye concentration: 20 ppm; adsorbent dose: 0.02 g; solution volume: 50 mL; contact time: 6 h; ambient temperature) (Fig. 7). The composite exhibited notably high removal efficiencies for MG (87.7%), CR (81.2%), and RB (70.5%), while significantly lower adsorption was recorded for CV (25.9%), MB (10.63%), and MO (3.4%). This pronounced selectivity can be attributed to the structural compatibility between the dye molecules and the active sites on the composite surface. The enhanced uptake of MG, CR, and RB likely stems from the presence of amino and hydroxyl groups in chitosan and the π-conjugated backbone of PPy, which together enable electrostatic attraction, π–π interactions, and hydrogen bonding with the dye structures (Gao et al., 2023). These findings highlight the effectiveness of Chitosan@TiO₂/PPy as a selective and efficient adsorbent for the removal of specific dyes from wastewater, demonstrating strong potential in environmental remediation applications.

Comparative dye adsorption efficiency of Chitosan@TiO₂/PPy composite for selected anionic (CR, RB, MO) and cationic (M.G, CV, MB) dyes.

3.7.2 Influence of pH

The adsorption behavior of the Chitosan@TiO₂/PPy composite was examined under varying initial pH conditions ranging from 3 to 10 (Fig. 8). The adsorption rate exhibited a significant dependence on pH, with a marked increase in performance as the pH shifted from acidic to neutral conditions (Reza et al., 2017). At pH 3, the removal efficiency was limited (∼21%), which can be attributed to repulsive electrostatic forces between the protonated adsorbent surface and the positively charged MG dye molecules. A sharp enhancement was observed between pH 4 and 5, with adsorption exceeding 90%, indicating improved interaction between the composite and MG. Beyond pH 5, the removal efficiency plateaued, reaching a maximum of approximately 99% around pH 9-10. This high efficiency in the neutral to mildly alkaline range suggests favorable electrostatic and π–π interactions (Kansal et al., 2009). The stability of adsorption across a wide pH window reflects the robustness of the composite and its adaptability for practical wastewater treatment applications (Alkaim et al., 2014).

Effect of initial solution pH on the removal of MG by the Chitosan@TiO₂/PPy composite.

3.7.3 Impact of experimental conditions on dye adsorption efficiency

The time-dependent adsorption profile of MG onto the Chitosan@TiO₂/PPy composite reveals a distinct kinetic progression. Initial adsorption within the first 60 min was relatively limited (17.7%), likely due to restricted diffusion and gradual access to active sites (Figs. 9a and b). A marked enhancement in uptake occurred between 60 and 240 min, with removal efficiency increasing to 85.7%. This phase reflects efficient dye interaction with the composite surface, facilitated by functional groups such as –NH₂ and –OH from chitosan and π-electron systems from PPy, promoting electrostatic and π–π interactions (Lin et al., 2024). Beyond 240 minutes, the adsorption rate diminished, approaching equilibrium at 360 minutes with a maximum removal of 92.1%, suggesting saturation of available binding sites. The consistent decline in absorbance at λ_max = 617 nm further confirms effective dye removal. This kinetic trend underscores the composite’s capacity for rapid and high-efficiency MG uptake, demonstrating its applicability in wastewater treatment where both speed and effectiveness are critical.

(a) UV–Vis spectra of MG solution at various contact times with Chitosan@TiO₂/PPy composite; (b) Time-dependent removal efficiency of MG; (c) Effect of adsorbent dosage on MG uptake; (d) Influence of initial dye concentration and temperature on adsorption capacity.

The influence of adsorbent dosage on MG removal was examined to identify the optimal quantity for efficient adsorption (Fig. 9c). As the dosage increased from 10 mg to 70 mg, a notable enhancement in removal efficiency was achieved, reaching 96.06%. This enhancement is attributed to the increased number of available active sites, facilitating more effective interaction between dye molecules and the composite surface. At lower dosages, the limited surface area restricts the number of accessible binding sites, resulting in lower uptake. As the dosage increases, the adsorption surface becomes more available, improving the contact between the adsorbent and dye molecules. Beyond 70 mg, further increases to 100 mg and 130 mg led to only slight improvements, reaching 97.18% and 97.56%, respectively. This marginal difference suggests that most active sites were already saturated, and the system was approaching equilibrium. At higher dosages, the adsorption efficiency showed minimal improvement, likely due to the onset of particle agglomeration, which limits the accessible surface area and reduces the number of effective binding sites. Therefore, 70 mg is identified as the most efficient dosage for achieving near-complete removal, balancing performance and material economy.

The adsorption performance of the composite was examined under varying dye concentrations (50-600 ppm) and temperatures (25, 35, and 45°C). The adsorption capacity (Q, mg/g) increased with concentration due to enhanced dye diffusion toward active sites (Fig. 9d). At 600 ppm, maximum capacities reached 278.35, 269.40, and 262.22 mg/g at 25, 35, and 45°C, respectively. The inverse relationship between temperature and Q suggests an exothermic process, likely governed by physisorption, where weak interactions such as hydrogen bonding and electrostatic forces are disrupted at elevated temperatures (Peng et al., 2023). The results confirm that lower temperatures and higher dye concentrations enhance adsorption efficiency, emphasizing the material’s suitability for applications involving high pollutant loads under ambient conditions.

The comparative analysis demonstrates that the Chitosan@TiO₂/PPy composite outperforms previously reported adsorbents for MG removal (Table 1). Its superior performance stems from the synergistic effects of its components: PPy facilitates π–π interactions, chitosan offers hydrogen bonding through amine and hydroxyl groups, and TiO₂ contributes surface complexation via hydroxyl sites (Salahuddin et al., 2020). This multifunctional integration, combined with a porous structure that enhances site accessibility, results in stronger dye affinity and higher removal efficiency.

Table 1. Comparison of the MG dye removal by Chitosan@TiO₂/PPy composite with other adsorbents.

Adsorbent	Conditions					q_max (mg/g)	Refs
Adsorbent	C_o (mg/L)	pH	m (g)	T (K)	t (h)	q_max (mg/g)	Refs
MWCNT/TiO₂/CS coposite	25	7	0.4	298	3	269.98	(Ahamad et al., 2025)
Nanotitania/calcium alginate composite	20–500	5	0.2–2.0	298	24	210.08	(Hassan et al., 2023)
TiO₂ modified fly ash	30	7	0.04	308	1	179.95	(Singh et al., 2023)
Zeo−PPY∕TiO₂ nanoparticles	5–40	4	1–15	298	70 min	21.92	(Motamedi et al., 2022)
Fe₃O₄ decorated chitosan	1	7	2	303	3.5	55.8659	(Mashkoor et al., 2024)
Chitosan/TiO₂	30-300	-	50	298	-	32	(Habiba et al., 2019)
TiO₂ NPs	3.6 - 22	-	0.1 - 0.4 g/100 mL	303	30 min	6.3	(Abou-Gamra and Ahmed, 2015)
Chitosan@TiO₂/PPy composite	50	7	0.07	298	6	304.45	This study

3.7.4 Kinetic analysis

The adsorption kinetics of MG onto the Chitosan@TiO₂/PPy composite were evaluated using multiple models—namely, pseudo 1^st order, pseudo 2^nd order, Elovich, and the Weber–Morris intra-particle diffusion approach—to elucidate the governing mechanisms and identify potential rate-limiting steps (Fig. 10). These models were applied to gain a deeper understanding of the adsorption dynamics and the interaction pathways involved.

Fitting of kinetic models for MG adsorption onto Chitosan@TiO₂/PPy composite: (a) pseudo 1st order, (b) pseudo 2nd order, (c) Elovich model, and (d) Weber–Morris model.

The pseudo 1^st order model, described by the linear equation (Lagergren, 1898):

(3)

\ln (q_{e} - q_{t}) = l n q_{e} - k t

assumes that adsorption is primarily governed by the rate of physical adsorption. The model yielded a calculated equilibrium capacity q_e1,cal=21.98 mg/g and rate constant k₁=0.0112 min⁻¹, with a correlation coefficient R²=98.3% (Table 2). However, the predicted qe deviates notably from the experimental value (34.24 mg/g), indicating poor model fit.

Table 2. Estimated kinetic parameters for MG adsorption onto Chitosan@TiO₂/PPy composite using different kinetic models. (T = 298 K, Chitosan@TiO₂/PPy dosage = 0.07 g, C₀= 50 ppm, t = 360 min, pH = 7.0).

Model	Parameters	Value
	C_o: 50 ppm; q _e,exp:34.24 mg/g	Value
Pseudo-first-order	q_{e1, cal.} (mg/g)	21.98
	K₁ (1/min)	0.0112
	R²(%)	98.3
Pseudo-second-order	q_{e2, cal.}(mg/g)	36.23
	K₂ (g/mg-min)	1.05× 10⁻³
	R²(%)	99.2
Elovich	α (mg/g min)	698.82
	Β (g/mg)	0.202
	R²(%)	88.1
Weber–Morris		First stage	Second stage
	k_id(mg/g·min^0.5)	0.811	0.658
	C	13.47	22.07
	R²(%)	99	96

In contrast, the pseudo 2^nd order model, given by:

(4)

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

provided a much better fit. The close match between the calculated capacity (q_e,cal = 36.23 mg/g) and experimental data, along with a high R² value (99.2%), confirms the model’s suitability and indicates chemisorption as the dominant mechanism, likely involving electron exchange with active surface sites.

The Elovich model, expressed as (Ho and McKay, 1999):

(5)

q_{t} = \frac{1}{β} \ln (α β) + \frac{1}{β} \ln (t)

is typically applied to systems with heterogeneous adsorption surfaces. B represents the desorption constant, while α refers to the initial adsorption rate. This model produced α = 698.82 mg/g·min and B = 0.202 g/mg, indicating an initially rapid uptake. However, its lower R² value (88.1%) reflects poor alignment with experimental data. In contrast, the pseudo 2^nd order model provided a superior fit, confirming chemisorption as the dominant pathway, driven by π–π interactions, hydrogen bonding, and electrostatic attraction.

To gain deeper insight into the adsorption mechanism of MG onto the Chitosan@TiO₂/PPy composite, the Weber–Morris intra-particle diffusion model was utilized, as defined by equation (6) (Banerjee et al., 2024):

(6)

q_{t} = k_{i d} • t^{0.5} + C

where k_id donates the intra-particle diffusion rate constant and C represents the boundary layer resistance. The resulting plot (Fig. 10d) displayed two sequential linear regions, suggesting a multi-stage adsorption pathway. The initial phase is attributed to external (film) diffusion, while the second corresponds to intra-particle transport within the porous matrix of the composite (Obayomi et al., 2022). As presented in Table 2, the first stage is characterized by a steeper slope and lower intercept, indicative of rapid adsorption at the outer surface influenced by boundary-layer resistance (López-Luna et al., 2019). The subsequent stage showed a reduced diffusion rate and higher intercept, reflecting a shift to slower dye migration into internal pores. The non-zero intercepts indicate that intra-particle diffusion is involved, but not the sole rate-controlling mechanism, with surface interactions also contributing significantly (Prajapati and Mondal, 2020). This suggests that both external surface adsorption and internal pore diffusion contribute to the overall uptake mechanism.

3.7.5 Isotherm study

The equilibrium adsorption of MG onto the Chitosan@TiO₂/PPy composite was investigated at 25°C, 35°C, and 45°C to elucidate the adsorption mechanism and thermodynamic characteristics (Fig. 11). The data were well-fitted by the Langmuir isotherm, which describes adsorption occurring on a homogenous surface with finite, identical sites (Craik and Leibovich, 1976):

(7)

q_{e} = \frac{q_{m a x} K_{L} C_{e}}{1 + K_{L} C_{e}}

Isotherm analysis of MG adsorption onto Chitosan@TiO₂/PPy composite at 25°C, 35°C, and 45°C: (a) equilibrium adsorption capacity (qₑ) versus equilibrium concentration (Cₑ); (b–d) nonlinear fitting results based on the Langmuir and Freundlich.

This model provided an excellent fit across all temperatures, with R² values ranging from 98.59 to 99.24 and high adsorption capacities (q_max) of 304.45, 313.65, and 307.49 mg/g, respectively (Table 3). The gradual decrease in the Langmuir constant (K_L) from 0.104 to 0.0342 L/mg suggests a reduction in surface affinity as temperature increases, indicating an exothermic nature of adsorption. The highest uptake was observed at 35°C, suggesting a slightly thermally assisted interaction, beyond which desorption effects become more significant.

Table 3. Isotherm adsorption parameters calculated from equilibrium modeling of MG dye uptake onto the Chitosan@TiO₂/PPy composite at different temperatures.

Model	Parameters	25°C	35°C	45°C
Langmuir	q_max (mg/g)	304.45	313.65	307.49
	K_L (L/mg)	0.104	0.046	0.034
	R² (%)	98.59	96.84	99.24
Freundlich	K_F (mg⁽¹⁻ⁿ⁾·Lⁿ/g)	75.31	52.1	40.37
	n	3.71	3.04	2.73
	R² (%)	82.74	85.65	92.15

In contrast, the Freundlich model, which describes multilayer adsorption on heterogeneous surfaces, is defined by (Proctor and Toro-Vazquez, 1996):

(8)

q_{e} = K_{F} . C_{e}^{\frac{1}{n}}

The fitting accuracy of this model was lower (R² = 82.74–92.15), and both K_F and n declined with increasing temperature, indicating diminished adsorption intensity and surface heterogeneity under thermal influence. These findings imply that while multilayer adsorption may occur to a limited extent, the surface is predominantly uniform and energetically consistent corroborated by BET and SEM analyses. Altogether with kinetic studies, the adsorption of MG onto Chitosan@TiO₂/PPy is governed by a spontaneous, exothermic chemisorption mechanism, characterized by high surface specificity, thermal stability, and selective interaction. This behavior underscores the material’s suitability for advanced wastewater treatment applications requiring efficient and robust adsorbent systems.

3.7.6 Thermodynamic study

The thermodynamics of MG adsorption onto the Chitosan@TiO₂/PPy composite were investigated to understand the nature of energy exchange, molecular ordering, and spontaneity (Fig. 12). The change in standard Gibbs free energy (ΔG°) was determined using the following expression:

(9)

Δ G ° = - R T l n K c

Van’t Hoff plot of ln(Kc) vs. 1/T for MG adsorption onto Chitosan@TiO₂/PPy composite.

The equilibrium constant K_c was obtained from the Langmuir constant K_L using two thermodynamically consistent expressions (Shu et al., 2015):

(10)

K_{c} = K_{L} \times C^{°}

where C°=1 mol/L, and

(11)

K_{c} = M . wt \times 1000 \times K_{L}

with M_wt as the molar mass of MG and the factor 1000 ensuring proper unit conversion from mg to g.

To further explore the enthalpic and entropic contributions, the Van’t Hoff expression was applied (Lima et al., 2020):

(12)

\ln K_{c} = - \frac{Δ H^{\circ}}{R T} + \frac{Δ S^{\circ}}{R}

Thermodynamic analysis based on the Van’t Hoff plot (ln Kc vs. 1/T) revealed an enthalpy change (ΔH°) of –44.15 kJ/mol and an entropy change (ΔS°) of –61.79 J/mol·K (Table 4). The corresponding ΔG° ranged from –25.73 to –24.49 kJ/mol within 298–318 K, confirming that the adsorption process is spontaneous and energetically favorable across the examined temperature range. The decrease in ΔG° with rising temperature indicates a thermodynamically favorable process that becomes slightly less driven as thermal agitation increases. The negative enthalpy confirms the exothermic character of the interaction, while the entropy reduction suggests increased structural ordering at the solid-liquid interface as dye molecules align with active sites on the composite (Darwish et al., 2019). The use of dimensionless and molar-normalized constants in calculating K_c enhances the reliability and comparability of the derived thermodynamic parameters, addressing the limitations of the simplified q_e/C_e approach.

Table 4. Results of thermodynamic study.

ΔH° (kJ/mol)	ΔS° (J/mol·K)	ΔG° (kJ·mol⁻¹)
−44.15	−61.79	298 K	308 K	318 K
−44.15	−61.79	−25.73	−25.11	−24.49

3.7.7 Adsorption mechanism

The adsorption of MG onto the Chitosan@TiO₂/PPy composite is primarily driven by electrostatic forces, hydrogen bonding, and π–π stacking (Fig. 13) (Hassan et al., 2019). Below the pHₚzc (≈6.56), surface protonation of –NH₂ and –OH groups in chitosan and PPy imparts a positive charge, promoting strong interactions with the electron-rich regions of MG molecules (Ghosh and Das, 2025), enhancing electrostatic attraction with the anionic components or electron-rich regions of MG molecules. In addition, the aromatic structure of PPy contributes to π–π stacking interactions with the conjugated rings of MG, facilitating stable molecular association (Mosch et al., 2015). Hydrogen bonding also plays a critical role, where donor–acceptor interactions occur between the functional groups of MG (e.g., –N(CH₃)₂, –C=C–) and surface –OH or –NH₂ groups (Salahshoori et al., 2023). Furthermore, the mesoporous structure and high surface area of the composite ensure effective diffusion and accessibility of active sites. These synergistic interactions collectively drive the high affinity and selectivity of the composite toward MG, supporting a predominantly chemisorptive mechanism with contributions from physisorption under optimized conditions.

Proposed adsorption mechanism of MG dye onto Chitosan@TiO₂/PPy composite.

4. Conclusions

A novel Chitosan@TiO₂/PPy composite was successfully synthesized via a green, in situ oxidative polymerization technique and evaluated for its efficacy in removing MG dye from aqueous solutions. The composite demonstrated excellent structural integrity, thermal stability, and mesoporous surface characteristics (BET surface area: 52.11 m²/g; pore size: ∼9.9 nm), which contributed to its high adsorption performance. At 25°C, the composite exhibited a high monolayer adsorption capacity of 304.45 mg/g. The adsorption behavior was best described by the pseudo 2^nd order kinetic model (R² = 99.2%) and showed a strong fit with the Langmuir isotherm, suggesting a chemisorptive mechanism involving uniform surface interactions. Thermodynamic evaluation indicated that the adsorption process was spontaneous and exothermic (ΔH° = –44.15 kJ/mol; ΔG° < 0 across all temperatures). The combined presence of chitosan, TiO₂, and PPy enabled diverse interaction pathways, including electrostatic forces, hydrogen bonding, and π–π interactions, enhancing the composite’s adsorption performance. These findings position the Chitosan@TiO₂/PPy composite as a robust, eco-friendly material with strong potential for application in advanced wastewater treatment technologies. Although this study demonstrates high adsorption performance and material stability, future work should include adsorption–desorption cycle analysis to confirm the composite’s regeneration capacity and operational sustainability in repeated use and evaluate the feasibility of scaling up for industrial applications.

Acknowledgments

The authors would like to thank the Department of Chemistry, Faculty of Sciences, Al-Baha University, for providing the required laboratory facilities.

CRediT authorship contribution statement

Ali Q. Alorabi: Investigation, data acquisition, Conceptualization; Formal analysis; methodology; Writing—original draft; review and editing; Data curation; Data analysis; Resources.

Declaration of competing interest

The authors declare that they have no competing financial interests or personal relationships that could have influenced the work presented in this paper.

Data availability

All data generated or analyzed during this study are included in this published article.

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.

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Multifunctional Chitosan@TiO₂/PPy nanocomposite for high-performance adsorption of malachite green: Kinetics, isotherms, and thermodynamics

Abstract

Keywords

Adsorption

Chitosan

Isotherm study

Malachite green

Polypyrrole

TiO2

Wastewater treatment

1. Introduction

2. Materials and Methods

2.1 Materials

2.2 Preparation of TiO₂ NPs

2.3 Synthesis of Chitosan@TiO₂/PPy composite

2.4 Characterization of Chitosan@TiO₂/PPy composites

2.5 Adsorption tests

3. Results and Discussion

3.1 XRD analysis

3.2 FTIR analysis

3.3 TGA analysis

3.4 BET analysis

3.5 SEM analysis

3.6 Zeta potential analysis

3.7 Study of adsorption properties

3.7.1 Selective adsorption behavior

3.7.2 Influence of pH

3.7.3 Impact of experimental conditions on dye adsorption efficiency

3.7.4 Kinetic analysis

3.7.5 Isotherm study

3.7.6 Thermodynamic study

3.7.7 Adsorption mechanism

4. Conclusions

Acknowledgments

CRediT authorship contribution statement

Declaration of competing interest

Data availability

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

References

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