Waste-derived nano-Al₂O₃-loaded pyranopyrazole composite for high-capacity cadmium and methylene blue removal with mechanistic and DFT validation

Abstract

Upcycling industrial aluminum waste into functional adsorbent materials offers a sustainable strategy for addressing complex wastewater contamination by coexisting organic dyes and toxic heavy metals. In this study, a waste-derived nano-Al₂O₃-loaded pyranopyrazole composite is rationally designed to enable high-capacity and simultaneous removal of methylene blue and Cd(II) ions from aqueous systems. The heteroatom-rich pyranopyrazole framework provides multiple organic binding domains, while nano-alumina incorporation introduces additional Al–O active sites and surface hydroxyl groups that collectively enhance interfacial adsorption efficiency. Comprehensive structural, spectroscopic, and microscopic characterization confirms homogeneous alumina anchoring and the formation of a stable organic–inorganic hybrid interface. The resulting composite exhibits markedly improved adsorption performance relative to the pristine pyranopyrazole, achieving maximum experimental uptake capacities of 189 mg g⁻¹ for methylene blue and 343 mg g⁻¹ for Cd(II) under optimized conditions. Adsorption kinetics follow a pseudo-second-order model, indicating surface-controlled chemisorption, while equilibrium data are best described by the Freundlich isotherm, reflecting adsorption on energetically heterogeneous surfaces. Thermodynamic analysis reveals that both adsorption processes are spontaneous and exothermic. Spectroscopic evidence combined with density functional theory calculations demonstrates that adsorption proceeds through synergistic coordination, electrostatic attraction, hydrogen bonding, and π-interactions involving nitrogen- and oxygen-donor sites of the organic framework together with alumina-derived Al–O centers. The composite maintains high removal efficiency over repeated adsorption–desorption cycles and exhibits effective performance in real wastewater matrices. Overall, this work establishes a clear structure–interaction–performance relationship for waste-derived alumina–organic hybrids and highlights their potential as sustainable, reusable, and multifunctional adsorbents for advanced water remediation targeting both organic dyes and heavy-metal contaminants.

Introduction

Global pressure to secure clean water resources has intensified as industrial expansion and urban growth continue to discharge chemically complex effluents into aquatic environments¹. Industrial wastewater frequently contains persistent organic dyes and toxic heavy metals whose coexistence presents a critical challenge for conventional treatment technologies². These contaminants are characterized by high chemical stability, limited biodegradability, and strong tendencies toward bioaccumulation, leading to long-term ecological imbalance and serious human health risks^3,4.

Cationic dyes constitute a particularly problematic class of organic pollutants due to their extensive industrial use and resistance to natural degradation pathways. Methylene blue is widely employed in textile processing, paper production, and pharmaceutical formulations; however, its uncontrolled release into aquatic systems is associated with acute and chronic toxicity, including oxidative stress and cellular damage, even at low exposure levels. Its aromatic structure and chemical persistence necessitate efficient removal prior to wastewater discharge^5,6,7,8.

Heavy metals further complicate wastewater remediation due to their non-degradable nature and cumulative toxicity⁹. Among them, cadmium is classified as a highly hazardous element owing to its carcinogenicity and its ability to induce renal, hepatic, and pulmonary damage¹⁰. Cadmium contamination commonly arises from electroplating, pigment manufacturing, battery production, and metal finishing industries, and its high solubility and mobility in aqueous media significantly limit the effectiveness of many conventional separation approaches^11,12.

A wide range of physical, chemical, and biological remediation strategies have been explored, including advanced oxidation, membrane separation, ion exchange, coagulation, and photocatalysis¹³. Despite their technical effectiveness, many of these methods suffer from inherent limitations such as high energy consumption, secondary waste generation, complex operational requirements, or poor long-term stability¹⁴. Adsorption has therefore emerged as a particularly attractive alternative owing to its operational simplicity, regeneration potential, and compatibility with low-cost materials, provided that adsorbents with sufficiently high affinity and stability can be developed¹⁵.

Activated carbon remains the most widely used adsorbent; however, its relatively high production and regeneration costs restrict large-scale application¹⁶. This limitation has stimulated growing interest in structurally engineered organic and hybrid materials capable of offering tunable surface chemistry and enhanced binding efficiency¹⁷. In this context, heterocyclic systems incorporating nitrogen and oxygen donor atoms are especially promising, as they provide versatile coordination environments for metal ions while simultaneously enabling electrostatic and π-interactions with aromatic dye molecules^{18,19,20,21,22,23}.

Pyrano[2,3-c]pyrazole derivatives feature a conjugated heterocyclic architecture enriched with multiple functional groups capable of participating in coordination, hydrogen bonding, and charge-transfer interactions. While these compounds have been extensively explored in medicinal and synthetic chemistry, their potential as adsorption platforms for environmental remediation remains largely underexplored. Importantly, the electronic structure of pyranopyrazole frameworks allows rational modulation of adsorption behavior through strategic hybridization with inorganic components^{24,25,26,27,28,29,30}.

Incorporation of metal oxides into organic frameworks represents an effective approach to enhance adsorption performance by introducing additional active sites, surface hydroxyl groups, and improved interfacial stability³¹. Aluminum oxide is particularly attractive due to its chemical robustness, amphoteric surface chemistry, and strong affinity toward cationic species. Moreover, the transformation of aluminum-containing waste into functional nano-alumina offers a sustainable pathway that simultaneously addresses waste valorization and water purification challenges^32,33.

Despite these advances, the rational integration of heterocyclic organic frameworks with waste-derived metal oxides for the simultaneous removal of organic dyes and toxic heavy metals remains insufficiently explored, particularly from a mechanistic and electronic-structure perspective.

In this work, a waste-derived nano-Al₂O₃-loaded pyranopyrazole composite is designed as a multifunctional adsorbent for the concurrent removal of methylene blue and cadmium ions from aqueous systems. The heteroatom-rich pyranopyrazole scaffold provides multiple organic coordination sites, while nano-alumina incorporation enhances surface reactivity and binding accessibility through Al–O functionalities, enabling cooperative organic–inorganic adsorption pathways within a single material architecture.

Comprehensive spectroscopic, microscopic, and surface analyses are employed to confirm composite formation and elucidate structure–property relationships. Adsorption behavior is systematically evaluated under variable physicochemical conditions and interpreted using kinetic, isotherm, and thermodynamic models. Density functional theory calculations are further applied to establish electronic structure–interaction correlations that rationalize the experimentally observed adsorption trends. The performance of the composite is additionally validated using real industrial wastewater to assess its practical applicability.

By integrating waste-derived material design, electronic structure analysis, and adsorption performance evaluation, this study provides mechanistic insight into hybrid adsorbent development and highlights the potential of alumina–heterocycle composites as sustainable platforms for simultaneous dye and heavy-metal remediation.

Materials and methods

Materials

All chemicals and solvents were obtained from commercial suppliers and used as received without further purification. Reaction progress and product purity were routinely monitored by thin-layer chromatography using Merck silica gel 60 F254 plates, and the developed spots were visualized under ultraviolet irradiation at 254 nm. Analytical-grade methylene blue dye and cadmium nitrate tetrahydrate (Cd(NO₃)₂·4 H₂O) were employed as model cationic organic and inorganic pollutants, respectively. Glacial acetic acid, hydrochloric acid, and sodium hydroxide were used during the preparation of aluminum oxide nanoparticles, with post-consumer aluminum beverage cans serving as the aluminum source.

Instrumentation

¹H and ¹³C NMR spectra were recorded on a Bruker spectrometer operating at 400 and 100 MHz, respectively, using DMSO-d₆ as the solvent and tetramethylsilane (TMS) as an internal reference. Chemical shifts are reported in parts per million (ppm), and coupling constants are expressed in hertz (Hz). Proton signal multiplicities are denoted as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), and br (broad). Mass spectra were obtained using a SHIMADZU GC/MS instrument operated at an ionization energy of 70 eV. Elemental (CHN) analyses were performed using a Vario EL III Elementar Analyzer (Germany).

Surface morphology was examined using a JSM-7500FA high-resolution cold field emission scanning electron microscope. Prior to imaging, samples were sputter-coated with a thin gold layer to enhance electrical conductivity and minimize charging effects. Elemental composition was analyzed using an energy-dispersive X-ray spectroscopy (EDX) detector coupled to the SEM. Fourier transform infrared spectra were recorded on a JASCO FT/IR-4100 spectrophotometer in the range of 4000–400 cm⁻¹ using KBr pellets. Residual dye concentrations in aqueous solutions were determined using a Shimadzu UV-2450 UV–Vis spectrophotometer. Cadmium ion concentrations were quantified using an atomic absorption spectrophotometer (AAS).

Preparation of Pyranopyrazole adsorbent (Pyrano PY)

The compound 6-amino-4-(4-hydroxy-3,5-dimethoxyphenyl)−3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile, abbreviated as Pyrano PY, was synthesized and employed as the organic adsorbent.

Method A (Conventional heating):

Phenylhydrazine (1 mmol) and ethyl acetoacetate (1 mmol) were mixed in an aqueous ethanol solution (1:1 v/v) and stirred at room temperature for 5 min to generate the corresponding hydrazone intermediate in situ. Syringaldehyde (1 mmol), malononitrile (1 mmol), and piperidine (5 mol%) were subsequently added under ambient conditions. The reaction mixture was stirred for approximately 20 min until a solid precipitate formed. Reaction completion was confirmed by TLC using ethyl acetate/n-hexane (1:2 v/v) as the eluent. The precipitated solid was filtered, washed with cold water followed by ethyl acetate/n-hexane (20:80 v/v), dried, and recrystallized from ethanol to afford Pyrano PY.

Method B (Microwave irradiation):

A mixture of ethyl acetoacetate (1 mmol), phenylhydrazine (1 mmol), syringaldehyde (1 mmol), and malononitrile (1 mmol) was dissolved in ethanol and transferred to a sealed microwave vial. The reaction was irradiated at 270 W (approximately 60 °C) for 5 min. After cooling to room temperature, the resulting solid was filtered, washed with ethanol, dried, and purified by recrystallization to obtain Pyrano PY.

Method C (Ultrasound-assisted synthesis):

Ethyl acetoacetate (1 mmol), phenylhydrazine (1 mmol), malononitrile (1 mmol), and syringaldehyde (1 mmol) were dissolved in 12 mL of an ethanol/water mixture (1:1 v/v) and subjected to ultrasonic irradiation at room temperature for 20 min. Reaction progress was monitored by TLC. The precipitated product was filtered, washed with ethanol, dried, and collected as Pyrano PY.

Preparation of waste-derived nano-Al₂O₃

Post-consumer aluminum beverage cans were used as the raw material for nano-alumina synthesis. The cans were cut into approximately 2 × 2 cm² pieces and treated with glacial acetic acid to remove surface coatings and polymer layers. The cleaned aluminum fragments were gradually dissolved in 6 M hydrochloric acid under controlled conditions to regulate hydrogen gas evolution. After complete dissolution, 6 M sodium hydroxide was added dropwise to precipitate aluminum hydroxide. The resulting precipitate was repeatedly washed with distilled water to eliminate residual sodium chloride formed during neutralization. The solid was filtered, dried at 90 °C for 24 h, and calcined at 1000 °C for 3 h to obtain nano-aluminum oxide³⁴.

Loading of nano-Al₂O₃ onto pyrano PY

Nano-alumina loading was achieved by dispersing 0.2 g of nano-Al₂O₃ and 1 g of dried Pyrano PY in 100 mL of distilled water. The suspension was magnetically stirred at 60 °C for 24 h to facilitate interfacial interaction and deposition. The solid composite (Al₂O₃@Pyrano PY) was then recovered by centrifugation, thoroughly washed, dried, and stored for subsequent adsorption experiments.

Preparation of adsorbate solutions

A methylene blue stock solution (1000 mg L⁻¹) was prepared by dissolving 1.0 g of analytical-grade dye in 1 L of distilled water and stored in an amber bottle to prevent photodegradation. Working solutions were freshly prepared by appropriate dilution. A cadmium stock solution (1000 mg L⁻¹, as Cd²⁺) was prepared by dissolving 2.74 g of cadmium nitrate tetrahydrate in distilled water and diluting to a final volume of 1 L. Solution pH was adjusted using 0.1 M HCl or 0.1 M NaOH and measured with a calibrated Mettler Toledo pH meter.

Batch adsorption experiments

Batch adsorption experiments were conducted to examine the effects of pH initial concentration contact time and temperature on the adsorption of methylene blue and cadmium ions using Pyrano PY and Al₂O₃@Pyrano PY. Experiments were performed in tightly sealed tubes agitated at 150 rpm in a temperature-controlled shaker maintained at 25 35 45 or 55 °C. A summary of the optimized operational conditions is provided in Table S1 in the Supporting Information.

Effect of pH

The initial pH was adjusted using 0.1 M HCl or 0.1 M NaOH. For methylene blue, pH values ranged from 2 to 12, while cadmium experiments were conducted at pH 4, 6, 7, and 8. In each experiment, 0.05 g of adsorbent was added to 20 mL of solution containing 25 mg L⁻¹ dye or 200 mg L⁻¹ Cd²⁺. Suspensions were shaken for 120 min at 25 °C, filtered, and analyzed.

Effect of contact time

Kinetic experiments were performed using initial concentrations of 25 mg L⁻¹ for methylene blue and 200 mg L⁻¹ for cadmium. Adsorbent dosage was fixed at 0.05 g per 20 mL solution. Samples were withdrawn at predetermined intervals, filtered, and analyzed to determine adsorption rates and equilibrium times.

Effect of initial concentrations

Initial pollutant concentrations ranging from 10 to 200 mg L⁻¹ were examined at a constant adsorbent dose of 0.05 g and contact time of 120 min. Residual concentrations were measured after filtration.

Effect of temperature

Temperature-dependent adsorption was evaluated at 25, 35, 45, and 55 °C using methylene blue (10 mg L⁻¹) and cadmium (15 mg L⁻¹) solutions at pH 8.

Adsorption capacity (qₑ) and removal efficiency were calculated using the following Eq. 

$$\:\text{A}\text{d}\text{s}\text{o}\text{r}\text{p}\text{t}\text{i}\text{o}\text{n}\:\text{C}\text{a}\text{p}\text{a}\text{c}\text{i}\text{t}\text{y}\:\left(\text{q}\text{e}\right)=\frac{(Co-Ce)\times\:V}{m}$$

(1)

$$\% \:{\rm adsorption \:removal }= \:\frac{Co-Ce}{Co}\times100$$

(2)

where m is the mass of adsorbent (g), V is the solution volume (L), and C₀ and Cₑ are the initial and equilibrium concentrations (mg L⁻¹), respectively³⁵.

Computational methodology

Density functional theory calculations

All quantum-chemical calculations were performed using density functional theory to investigate electronic structure and adsorption-relevant properties. Geometry optimization was carried out without symmetry constraints using a hybrid exchange–correlation functional³⁶. Frequency calculations confirmed true minimum with no imaginary frequencies (Fig. 1).

Fig. 1

Optimized structure of Pyrano PY.

A split-valence basis set augmented with polarization functions was employed for all non-metal atoms (C, H, N, and O), providing a balanced description of molecular geometry and electronic properties. Total energies and molecular orbitals were obtained from the optimized structures and used for subsequent analyses of frontier orbitals and reactivity descriptors.

Frontier molecular orbital analysis

HOMO and LUMO energies and spatial distributions were extracted to assess electron donation and acceptance behavior. The energy gap was calculated using:

$$\:{\varDelta\:E}_{g}=\:{E}_{L}-\:{E}_{H}$$

(3)

Where $\:{E}_{L}$ and $\:{E}_{H}$ represent the LOMO and HOMO energies³⁷.

Global reactivity descriptors

Ionization potential, electron affinity, chemical hardness, softness, electronegativity, chemical potential, and electrophilicity index were calculated using Koopmans’ theorem^38,39:

$$\eta=\:\:\frac{I-A}{2}$$

(4)

$$\sigma=\:\frac{1}{2\eta}$$

(5)

$$\:x=\:\frac{I+A}{2}$$

(6)

$$\upmu=-\:\frac{I+A}{2}$$

(7)

$$\omega=\:\frac{{{\upmu\:}}^{2}}{2{\upeta}}$$

(8)

Results and discussion

Synthesis and characterization of the Pyrano PY sorbent

Synthesis of the Pyrano PY sorbent

The Pyrano PY adsorbent was successfully synthesized through an environmentally benign and operationally straightforward one-pot four-component reaction that proceeds efficiently under conventional heating, microwave irradiation, or ultrasound-assisted conditions. In all synthetic routes, the target pyranopyrazole framework was obtained reproducibly in high isolated yield, reaching approximately 95%, highlighting the robustness and scalability of the synthetic protocol. The reaction combines phenylhydrazine, ethyl acetoacetate, malononitrile, and syringaldehyde, the latter being a naturally occurring phenolic aldehyde derived from lignin-rich biomass, thereby introducing a renewable component into the molecular architecture of the adsorbent (Scheme 1).

From a mechanistic perspective, the reaction sequence is initiated by the controlled in situ formation of the hydrazone intermediate through premixing phenylhydrazine with ethyl acetoacetate. This initial step is critical, as it directs the subsequent multicomponent assembly toward the desired heterocyclic scaffold while suppressing competing side reactions. The sequential addition of syringaldehyde and malononitrile triggers a rapid Knoevenagel condensation, followed by intramolecular cyclization and annulation processes, ultimately furnishing the 1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile core in high purity.

Scheme 1

Synthesis of pyrano[2,3-c]pyrazole adsorbent (Pyrano PY).

The efficiency of this synthetic strategy arises from the synergistic coupling of condensation and cyclization events within a single reaction vessel, minimizing intermediate isolation and solvent consumption. The use of catalytic piperidine under mild conditions further enhances reaction selectivity and accelerates product formation, resulting in short reaction times across all activation modes. Notably, comparable yields and product purity were achieved under conventional, microwave, and ultrasonic conditions, demonstrating that the reaction pathway is not strongly dependent on the mode of energy input and confirming its intrinsic chemical efficiency.

Beyond synthetic convenience, the molecular design of Pyrano PY is particularly relevant to its function as an adsorbent. The resulting heterocyclic framework incorporates multiple heteroatom-containing functional groups, including amino, cyano, phenolic hydroxyl, and pyrazole nitrogen moieties, all of which are strategically positioned to participate in coordination, hydrogen bonding, and electrostatic interactions with target pollutants. In addition, the conjugated aromatic system derived from syringaldehyde and phenylhydrazine provides π-electron density capable of engaging in π–π interactions with aromatic dye molecules. These structural features collectively establish Pyrano PY as a multifunctional adsorption platform with high chemical accessibility and interaction versatility.

Overall, the synthetic protocol combines green chemistry principles, high atom economy, and structural functionality, yielding a heterocyclic adsorbent that is not only synthetically accessible but also intrinsically suited for subsequent surface modification and adsorption applications. The clean reaction profile, high yield, and incorporation of renewable building blocks further support the suitability of Pyrano PY as a sustainable core material for advanced water remediation systems.

Spectroscopic characterization of Pyrano PY adsorbent

The molecular structure of the Pyrano PY adsorbent was unambiguously confirmed through a combination of infrared spectroscopy, proton and carbon nuclear magnetic resonance spectroscopy, electron-impact mass spectrometry, and elemental analysis. The collective spectroscopic data is fully consistent with the proposed pyrano[2,3-c]pyrazole architecture and confirms the successful formation of the targeted heterocyclic framework.

The ¹H NMR spectrum supports the structural assignment and reveals well-resolved signals corresponding to all proton environments within the molecule (Figure S1). The phenolic hydroxyl proton appears as a highly deshielded singlet at δ 8.91 ppm, reflecting its involvement in intramolecular hydrogen bonding. The amino protons resonate as a singlet at δ 7.13 ppm, consistent with an –NH₂ group attached to an electron-deficient heterocyclic system. Aromatic protons from the phenyl and substituted aromatic rings appear as multiplets in the range δ 7.78–6.82 ppm. The pyran C₄–H proton is observed as a sharp singlet at δ 4.58 ppm, serving as a diagnostic signal for pyran ring formation. Additional singlets at δ 3.75 ppm and δ 1.83 ppm are assigned to the methoxy substituents and the methyl group on the pyrazole ring, respectively, completing the proton assignment.

The ¹³C NMR spectrum further corroborates the proposed structure (Figure S2). A characteristic resonance at δ 120.82 ppm corresponds to the cyano carbon, while the signal at δ 166.43 ppm is attributed to the pyran C6 carbon bearing the amino group. Aromatic carbon resonances are distributed across the δ 112.31–166.34 ppm region, consistent with the conjugated aromatic framework. Well-defined signals at δ 56.05 ppm correspond to the methoxy carbons, whereas the pyran C₄ carbon and the pyrazole methyl carbon resonate at δ 29.97 ppm and δ 11.16 ppm, respectively.

Electron-impact mass spectrometry reveals a molecular ion peak at m/z 404.16 (Figure S3), which is in excellent agreement with the calculated molecular mass of 404.15, confirming the molecular integrity of the synthesized compound. Elemental analysis further supports the proposed molecular formula, as the experimentally determined carbon, hydrogen, and nitrogen contents closely match the theoretical values, indicating high purity and compositional accuracy.

Collectively, the spectroscopic and analytical results confirm that Pyrano PY possesses a heteroatom-rich architecture comprising phenolic hydroxyl, amino, pyrazole nitrogen, and cyano functionalities embedded within a conjugated aromatic system partially derived from naturally sourced syringaldehyde. This dense distribution of electron-donor sites provides multiple coordination, hydrogen-bonding, and π–π interaction pathways, which are essential for effective binding of both Cd²⁺ ions and aromatic dye molecules. These structural attributes establish Pyrano PY as a robust and multifunctional adsorption platform suitable for advanced wastewater remediation applications.

FTIR analysis of Pyrano PY and Al₂O₃@Pyrano PY

The FTIR spectrum of the pristine Pyrano PY adsorbent exhibits well-defined absorption bands corresponding to its functional groups, as illustrated in Fig. 2. The broad bands observed at 3437–3350 cm⁻¹ are assigned to the N–H stretching vibrations of the amino group, while the phenolic O–H stretching vibration appears at approximately 3240 cm⁻¹. Aromatic and aliphatic C–H stretching modes are clearly visible at 3077 and 2922 cm⁻¹, respectively. The sharp absorption band at 2185 cm⁻¹ confirms the presence of the cyano group, whereas the bands at 1648 and 1620 cm⁻¹ are attributed to C = N and C = C stretching vibrations within the heterocyclic and aromatic framework. In addition, the C–O–C stretching vibration associated with the pyran ring appears at 1226 cm⁻¹.

Upon loading Pyrano PY with nano-Al₂O₃, distinct spectral changes are observed, confirming successful composite formation. Two new absorption bands emerge in the low-frequency region at approximately 500 and 600 cm⁻¹, which are characteristic of Al–O and Al–O–Al stretching vibrations. The increased intensity and slight broadening of the O–H stretching band indicate the introduction of additional surface hydroxyl groups contributed by the alumina nanoparticles. These observations collectively confirm effective deposition of nano-alumina onto the organic pyranopyrazole framework.

Following Cd²⁺ adsorption, further changes in the FT-IR spectra are evident. Notable shifts in the positions and intensities of the N–H, O–H, C = N, and C–O–C bands are observed, reflecting direct interactions between cadmium ions and the nitrogen- and oxygen-containing functional groups of the composite. These spectral shifts indicate that Cd²⁺ adsorption proceeds through a combination of coordination interactions with heteroatom donor sites and electrostatic attraction at the alumina-modified surface. The involvement of Al–O functionalities further enhance metal binding, demonstrating the synergistic role of the organic and inorganic components within the hybrid adsorbent.

Fig. 2

FT-IR spectra of Pyrano PY and Al₂O₃@Pyrano PY before and after Cd²⁺ adsorption.

SEM-EDX analysis

The surface morphology of the prepared Pyrano PY adsorbent was examined by scanning electron microscopy to elucidate the textural features governing its adsorption behavior. As shown in Fig. 3a, the pristine Pyrano PY exhibits a highly porous and fibrous morphology characterized by irregularly shaped fibers and interconnected voids distributed throughout the surface. This open architecture generates a rough and heterogeneous texture that is advantageous for adsorption, as it provides a large accessible surface area and facilitates mass transfer of adsorbate species from the bulk solution to internal active sites. The interconnected pore network further promotes efficient diffusion of both metal ions and dye molecules into the interior of the adsorbent matrix.

Following the deposition of aluminum oxide nanoparticles, a pronounced change in surface morphology is observed, as illustrated in Fig. 3b. The originally fibrous surface becomes partially coated with granular features, indicating successful anchoring of Al₂O₃ onto the Pyrano PY framework. The alumina nanoparticles introduce nanoscale roughness and generate additional surface irregularities, which increase the density of accessible adsorption sites. Importantly, the underlying fibrous structure remains partially visible, suggesting that alumina loading occurs predominantly through surface decoration rather than pore blockage. This partial coverage preserves the porous architecture while simultaneously enhancing surface reactivity, thereby contributing to improved adsorption performance.

After Cd²⁺ adsorption, the surface morphology undergoes further transformation, as depicted in Fig. 3c. The adsorbent surface appears more compact and aggregated, with nanoparticles and adsorbed cadmium species visibly attached to the Pyrano PY matrix. The fibrous features become less distinct due to surface coverage by the adsorbate and the formation of cadmium-associated aggregates. These morphological changes provide direct visual evidence of effective interaction between Cd²⁺ ions and the active sites present on the Al₂O₃@Pyrano PY composite, confirming successful adsorption and occupation of previously available binding sites.

Fig. 3

SEM images of Pyrano PY adsorbents at 4000× magnification: (a) pristine Pyrano PY, (b) Al₂O₃@Pyrano PY, and (c) Al₂O₃@Pyrano PY after Cd²⁺ adsorption, illustrating progressive morphological evolution and surface coverage.

Energy-dispersive X-ray spectroscopy was employed to complement the SEM observations and to verify the elemental composition of the adsorbent at different modification stages. The EDX spectrum of the pristine Pyrano PY (Fig. 4a) reveals the presence of carbon nitrogen and oxygen as the only detectable elements, with atomic percentages of 41.2%, 28.9%, and 30.0%, respectively. This composition confirms the purely organic nature of the heterocyclic framework and the absence of inorganic impurities.

Upon alumina loading, the EDX spectrum of Al₂O₃@Pyrano PY (Fig. 4b) displays a distinct aluminum signal with an atomic percentage of 15.4%, unequivocally confirming the successful deposition of Al₂O₃ nanoparticles onto the organic matrix. The relative decrease in the carbon nitrogen and oxygen signals further indicates partial surface coverage of the Pyrano PY framework by alumina species rather than simple physical mixing. This observation is consistent with the SEM results and supports the formation of a genuine hybrid composite.

After Cd²⁺ adsorption, the EDX spectrum (Fig. 4c) exhibits a clear cadmium signal with an atomic percentage of approximately 2.1%, providing direct evidence for the binding of cadmium ions onto the Al₂O₃@Pyrano PY surface. The appearance of cadmium is attributed to synergistic adsorption mechanisms involving coordination with nitrogen and oxygen donor atoms within the organic framework, as well as interactions with surface hydroxyl groups and Al–O functionalities of the alumina phase. The coexistence of Al and Cd signals further supports the cooperative role of the inorganic and organic components in metal-ion capture.

It is important to emphasize that EDX is a surface-sensitive and semi-quantitative technique, and the reported values represent relative atomic and weight percentages within the near-surface detection volume rather than absolute elemental contents. Accordingly, the apparent decrease in the relative oxygen percentage after Cd²⁺ adsorption does not indicate depletion of oxygen-containing functionalities. Instead, it primarily reflects surface coverage and signal redistribution effects associated with cadmium uptake. Because cadmium is a heavier element, the emergence of a Cd signal together with partial masking of oxygen-rich regions reduces the relative contribution of oxygen in the normalized EDX output. In the Al₂O₃@Pyrano PY composite, Cd²⁺ adsorption proceeds through coordination with oxygen- and nitrogen-donor sites as well as Al–O bonds and surface hydroxyl groups, leading to localized surface enrichment in cadmium and an apparent reduction in the measured oxygen percentage. Therefore, the observed EDX trend is fully consistent with Cd²⁺ binding through oxygen-containing sites rather than contradicting it and should be interpreted as a relative compositional shift inherent to surface EDX analysis.

Fig. 4

EDX spectra of (a) Pyrano PY, (b) Al₂O₃@Pyrano PY, and (c) Al₂O₃@Pyrano PY after Cd²⁺ adsorption, confirming alumina loading and effective cadmium binding.

Collectively, the SEM and EDX analyses demonstrate the successful fabrication of the Al₂O₃@Pyrano PY hybrid composite and reveal a clear correlation between surface morphology, elemental composition, and adsorption behavior. The preservation of porosity combined with the introduction of alumina-derived active sites provides a structurally and chemically optimized interface for efficient heavy-metal adsorption.

Adsorption performance

Effect of pH

Solution pH is a critical parameter governing adsorption processes, as it directly influences both the surface charge of the adsorbent and the speciation of adsorbate molecules in solutions. The effect of pH on the adsorption performance of Pyrano PY and the Al₂O₃@Pyrano PY composite toward methylene blue and Cd²⁺ ions was therefore systematically investigated to elucidate the underlying interaction mechanisms.

For methylene blue adsorption, experiments were conducted over a wide pH range from 2 to 12 at an initial dye concentration of 25 ppm, as shown in Fig. 5a. At acidic pH values, both adsorbents exhibited relatively low removal efficiencies, which can be attributed to protonation of surface functional groups and competition between excess H⁺ ions and the cationic dye molecules for available adsorption sites. As the pH increased, a gradual enhancement in methylene blue uptake was observed for both materials. Maximum removal efficiencies were achieved at pH 8, reaching 89.8% for Pyrano PY and 92.0% for the Al₂O₃@Pyrano PY composite, corresponding to equilibrium adsorption capacities (qₑ) of 33.7 mg g⁻¹ and 34.6 mg g⁻¹, respectively. Beyond this pH value, no significant improvement in adsorption was observed, indicating that electrostatic interactions dominate under mildly alkaline conditions.

The adsorption behavior of Cd²⁺ ions exhibits a similar but more constrained pH dependence, as shown in Fig. 5b. Experiments were performed at pH values of 4, 6, 7, and 8 using an initial cadmium concentration of 200 ppm. At lower pH values, cadmium removal was limited due to strong competition with protons for coordination sites on the adsorbent surface. As the pH increased, deprotonation of surface functional groups enhanced the availability of negatively charged binding sites, leading to a marked increase in Cd²⁺ uptake. The optimum pH for cadmium adsorption was identified as pH 7, at which Pyrano PY achieved a removal efficiency of 87.0% with a qₑ value of 255 mg g⁻¹, while the Al₂O₃@Pyrano PY composite exhibited superior performance with a removal efficiency of 90.1% and a qₑ value of 268 mg g⁻¹. At pH 8, partial precipitation of cadmium as Cd(OH)₂ was observed, rendering adsorption data unreliable and therefore unsuitable for mechanistic interpretation.

The observed pH-dependent trends correlate well with the measured points of zero charge (pH_ZPC) of the adsorbents, which were determined to be 5.9 for Pyrano PY and 5.6 for the Al₂O₃@Pyrano PY composite (Figure S4). At pH values above pH_ZPC, the surface of both materials becomes negatively charged, promoting electrostatic attraction toward cationic species such as methylene blue and Cd²⁺ ions. This electrostatic contribution is further complemented by coordination interactions involving nitrogen- and oxygen-containing functional groups within the pyranopyrazole framework, as well as Al–O and surface hydroxyl sites introduced by alumina loading. The slightly lower pH_ZPC value of the composite reflects the influence of nano-alumina and explains its consistently higher adsorption efficiency across the investigated pH range.

Overall, the pH-dependent adsorption behavior demonstrates that both Pyrano PY and Al₂O₃@Pyrano PY operate most effectively under near-neutral to mildly alkaline conditions. The enhanced performance of the composite highlights the synergistic effect of organic heterocyclic donor sites and inorganic alumina-derived functionalities, which collectively optimize surface charge characteristics and binding affinity toward cationic pollutants.

Fig. 5

Effect of solution pH on removal percentage and equilibrium adsorption capacity (qₑ) of (a) methylene blue and (b) Cd²⁺ by Pyrano PY and Al₂O₃@Pyrano PY composite.

Effect of contact time and kinetic studies

The effect of contact time on the adsorption behavior of methylene blue and Cd²⁺ ions was investigated to elucidate adsorption rates and equilibrium characteristics for both Pyrano PY and the Al₂O₃@Pyrano PY composite. As illustrated in Fig. 6, adsorption experiments were carried out over a contact time range of 15–180 min using initial concentrations of 25 ppm for methylene blue and 200 ppm for cadmium ions.

For both adsorbents and pollutants, a rapid increase in adsorption capacity was observed during the initial stage of contact. This behavior can be attributed to the high availability of vacant and energetically favorable adsorption sites on the external surface of the adsorbents at the beginning of the process. The open porous structure of Pyrano PY and the increased surface heterogeneity introduced by nano-alumina loading further facilitate fast mass transfer of adsorbate species from the bulk solution to the adsorbent surface.

As the contact time progressed, the adsorption rate gradually decreased, particularly after approximately 120 min. This deceleration is associated with progressive occupation of active sites and the development of repulsive forces between adsorbed species and those remaining in solution, which reduces the probability of further adsorption events⁴⁰. Equilibrium was effectively reached at around 120 min for both methylene blue and Cd²⁺ adsorption, indicating that this contact time is sufficient to achieve maximum uptake under the investigated conditions.

Fig. 6

Effect of contact time on the adsorption capacity of (a) methylene blue and (b) Cd²⁺ by Pyrano PY and Al₂O₃@Pyrano PY composite.

To gain deeper insight into the adsorption mechanism and rate-controlling steps, kinetic data were analyzed using pseudo-first-order (PFORE) and pseudo-second-order (PSORE) kinetic models. These models are widely applied to distinguish between diffusion-controlled processes and surface reaction–controlled adsorption mechanisms⁴¹.

The pseudo-first-order model assumes that adsorption rate is governed by diffusion and mass transfer processes and is sensitive to particle size and boundary-layer effects⁴². Linear plots of log(qₑ−qₜ) versus contact time are shown in Fig. 7, and the corresponding kinetic parameters are summarized in Table 1. The results reveal a reasonably good fit for the early stages of adsorption, with correlation coefficients (R²) exceeding 0.91 for both methylene blue and Cd²⁺. The Al₂O₃@Pyrano PY composite exhibited slightly higher rate constants and calculated adsorption capacities than the pristine Pyrano PY, reflecting the positive influence of alumina loading on initial adsorption kinetics. However, the qₑ values derived from the pseudo-first-order model deviate from the experimentally observed equilibrium values, indicating limitations in accurately describing the entire adsorption process.

Fig. 7

Pseudo-first-order kinetic model (PFORE) plots for methylene blue and Cd²⁺ adsorption by Pyrano PY and Al₂O₃@Pyrano PY composite.

The pseudo-second-order kinetic model was subsequently applied to further evaluate the adsorption behavior. This model assumes that the rate-limiting step involves surface interactions such as electron sharing or exchange between the adsorbent and adsorbate⁴³. Linear plots of t/qₜ versus contact time are presented in Fig. 8, and the corresponding kinetic parameters are listed in Table 1. The pseudo-second-order model provides an excellent fit for the experimental data, with correlation coefficients exceeding 0.99 for all systems investigated.

Notably, the equilibrium adsorption capacities calculated from the pseudo-second-order model closely match the experimentally observed values. For methylene blue, the calculated qₑ increased from 38.8 mg g⁻¹ for Pyrano PY to 51.5 mg g⁻¹ for the Al₂O₃@Pyrano PY composite. Similarly, for Cd²⁺ adsorption, qₑ increased significantly from 270.3 mg g⁻¹ to 384.6 mg g⁻¹ following alumina modification. These results clearly demonstrate the enhancement in adsorption performance resulting from the incorporation of nano-Al₂O₃.

Fig. 8

Kinetic model PSORE for MB and Cd(II) by Pyrano PY and Al₂O₃@ Pyrano PY composite.

A direct comparison between the two kinetic models highlights that the pseudo-second-order model consistently exhibits higher correlation coefficients and more accurate prediction of equilibrium adsorption capacities than the pseudo-first-order model. This observation indicates that the adsorption of both methylene blue and Cd²⁺ onto Pyrano PY and Al₂O₃@Pyrano PY is predominantly governed by chemisorption processes rather than simple diffusion. The involvement of nitrogen- and oxygen-containing functional groups within the pyranopyrazole framework, together with Al–O and surface hydroxyl sites introduced by nano-alumina, supports a mechanism dominated by strong surface interactions⁴⁴.

Overall, the kinetic analysis confirms that Al₂O₃ loading not only enhances adsorption capacity but also accelerates adsorption kinetics by increasing the number and accessibility of active sites. The excellent agreement with the pseudo-second-order model underscores the importance of surface chemistry and specific interactions in controlling the adsorption process for both organic dyes and heavy-metal ions.

Table 1 Kinetic parameters and correlation coefficients for methylene blue and Cd(II) adsorption on Pyrano PY and Al₂O₃@Pyrano PY.

Effect of initial concentration and adsorption isotherm studies

The influence of initial pollutant concentration on adsorption behavior provides critical insight into the capacity, affinity, and saturation characteristics of the adsorbent surface. Accordingly, the effect of initial methylene blue concentration on adsorption performance was investigated over a concentration range of 10–150 mg L⁻¹ at room temperature using a fixed adsorbent dose of 0.05 g, as illustrated in Fig. 9a.

At low initial concentrations, particularly at 10 mg L⁻¹, both Pyrano PY and Al₂O₃@Pyrano PY exhibited high removal efficiencies of 95.72% and 97.7%, respectively. These values correspond to equilibrium adsorption capacities (qₑ) of 14.3 mg g⁻¹ for Pyrano PY and 14.7 mg g⁻¹ for the composite. The high removal percentages observed at low concentrations can be attributed to the abundance of available active sites relative to the number of dye molecules present in solution. Under these conditions, adsorption proceeds efficiently without significant competition among adsorbate molecules.

As the initial methylene blue concentration increased beyond 15 mg L⁻¹, a gradual decrease in removal percentage was observed for both adsorbents. This behavior reflects progressive saturation of accessible adsorption sites, which limits the fraction of dye molecules that can be removed from solution. In contrast, the equilibrium adsorption capacity (qₑ) increased steadily with increasing concentration, driven by the higher concentration gradient between the solution and the adsorbent surface. At the highest investigated concentration of 150 mg L⁻¹, maximum adsorption capacities of approximately 180.3 mg g⁻¹ for Pyrano PY and 190 mg g⁻¹ for Al₂O₃@Pyrano PY were achieved. These results demonstrate the strong affinity of methylene blue toward the adsorbent surface and clearly highlight the enhanced uptake capacity imparted by alumina incorporation.

A similar trend was observed for Cd²⁺ adsorption, as shown in Fig. 9b. The effect of initial cadmium concentration was examined over the range of 15–200 mg L⁻¹. At the lowest concentration of 15 mg L⁻¹, removal efficiencies reached 95.67% for Pyrano PY and 98.5% for the Al₂O₃@Pyrano PY composite, corresponding to qₑ values of 21.5 and 29.56 mg g⁻¹, respectively. These results again reflect the favorable ratio between available adsorption sites and Cd²⁺ ions at low concentrations.

As the initial Cd²⁺ concentration increased, a pronounced increase in adsorption capacity was observed for both materials. At 200 mg L⁻¹, the maximum qₑ values reached approximately 250 mg g⁻¹ for Pyrano PY and 343 mg g⁻¹ for the Al₂O₃@Pyrano PY composite. The significantly higher capacity of the composite confirms that alumina loading increases both the number and accessibility of active binding sites, particularly those capable of strong coordination with metal ions. The results collectively demonstrate that the composite maintains high adsorption efficiency across a wide concentration range, which is essential for practical wastewater treatment applications.

Fig. 9

Effect of initial concentration on the adsorption performance of Pyrano PY and Al₂O₃@Pyrano PY toward (a) methylene blue and (b) Cd²⁺ ions.

To further elucidate the adsorption mechanism and surface characteristics of the prepared materials, equilibrium adsorption data were analyzed using the Langmuir, Freundlich, and Temkin isotherm models. These models provide complementary information regarding surface homogeneity, adsorption intensity, and energetic interactions between adsorbate and adsorbent.

The Langmuir isotherm model was applied to estimate the maximum monolayer adsorption capacity (q_max) and the affinity constant (K_L), assuming uniform adsorption sites and monolayer coverage. The linearized Langmuir equation is expressed as:

$$\:\frac{{C}_{eq}}{{q}_{eq}}=\frac{{C}_{eq}}{{q}_{max}}+\frac{1}{{K}_{L}{q}_{max}}$$

(9)

As summarized in Table 2 and illustrated in Figure S5, the Langmuir model provided a good description of the experimental data, with correlation coefficients exceeding 0.93 for both methylene blue and Cd²⁺ adsorption. The calculated q_max values were slightly higher than the experimentally observed maximum capacities. For example, Pyrano PY exhibited a calculated q_max of 227.27 mg g⁻¹ for methylene blue compared to an experimental uptake of 180 mg g⁻¹. This overestimation is commonly associated with the ideal assumptions of the Langmuir model, which neglect surface heterogeneity and possible multilayer adsorption. Nevertheless, the relatively high K_L values indicate strong affinity between the adsorbates and the sorbent surfaces.

The Freundlich isotherm model was employed to evaluate adsorption intensity and surface heterogeneity. The linearized Freundlich equation is given by:

$$Ln{\rm{ }}\left( {{q_{eq}}} \right){\rm{ }} = {\rm{ }}Ln{\rm{ }}\left( {{K_f}} \right){\rm{ }} + \frac{1}{n}~Ln~\left( {Ce} \right)$$

(10)

In this model, K_F represents adsorption capacity, while the Freundlich exponent n reflects adsorption favorability. The n values ranged from 1.36 to 1.53 for methylene blue and from 1.38 to 1.49 for Cd²⁺, indicating favorable adsorption on moderately heterogeneous surfaces. The 1/n values further confirm the presence of surface heterogeneity rather than ideal uniformity. Notably, the Freundlich constant K_F for Cd²⁺ adsorption increased from 19.9 mg g⁻¹ for Pyrano PY to 37.5 mg g⁻¹ for Al₂O₃@Pyrano PY, highlighting the enhanced binding strength provided by alumina modification. The excellent correlation coefficients (R² > 0.98) indicate that the Freundlich model accurately captures the adsorption behavior, particularly for heterogeneous surface interactions.

To further probe adsorbate–adsorbent interaction energies, the Temkin isotherm model was applied, as shown in Figure S5. The Temkin model accounts for indirect adsorbate–adsorbent interactions and assumes a linear decrease in adsorption heat with increasing surface coverage. The Temkin parameters A_T and B_T listed in Table 2 suggest moderate interaction energies for both methylene blue and Cd²⁺ adsorption. The higher A_T and B_T values observed for the Al₂O₃@Pyrano PY composite indicate stronger binding interactions and higher adsorption energy compared to the pristine sorbent. Correlation coefficients around 0.90 support the applicability of the Temkin model in describing the adsorption process⁴⁵.

Overall, the combined analysis of Langmuir, Freundlich, and Temkin isotherms provides a comprehensive understanding of the adsorption behavior of Pyrano PY and its alumina-modified counterpart. The results demonstrate that adsorption occurs on a partially heterogeneous surface with favorable interaction energies, and that Al₂O₃ incorporation significantly enhances both adsorption capacity and affinity toward cationic dye molecules and heavy-metal ions. This synergistic improvement underscores the effectiveness of the hybrid composite for high-performance wastewater remediation.

Table 2 Isotherm parameters and correlation coefficients for methylene blue and Cd(II) adsorption on Pyrano PY and Al₂O₃@Pyrano PY sorbents.

Effect of temperature and thermodynamic studies

The influence of temperature on the adsorption behavior of methylene blue and Cd²⁺ ions onto Pyrano PY and Al₂O₃@Pyrano PY was investigated to elucidate the thermodynamic nature of the adsorption process and assess the thermal stability of the prepared adsorbents. Batch adsorption experiments were conducted at temperatures ranging from 298 to 328 K under optimized conditions of pH 8 and a contact time of 120 min. A fixed adsorbent dose of 0.05 g was employed, with initial concentrations of 10 mg L⁻¹ for methylene blue and 15 mg L⁻¹ for Cd²⁺ ions. These conditions were selected to ensure sufficient adsorbate–adsorbent interaction while enabling reliable evaluation of temperature effects.

As illustrated in Fig. 10, an overall decrease in removal efficiency with increasing temperature was observed for both adsorbents and both pollutants. For methylene blue adsorption onto Pyrano PY, the removal efficiency decreased from 95.72% at 298 K to 79.1% at 328 K. In contrast, the Al₂O₃@Pyrano PY composite consistently exhibited higher removal efficiencies over the entire temperature range, decreasing from 97.7% to 84.3%. A similar trend was observed for Cd²⁺ adsorption. The removal efficiency of Pyrano PY decreased from 95.67% to 74.4% as temperature increased, whereas the composite maintained higher performance, with removal efficiencies decreasing from 98.53% to 82.2%.

The observed decline in adsorption efficiency with increasing temperature suggests that both methylene blue and Cd²⁺ adsorption processes are exothermic in nature. Elevated temperatures likely weaken the interactions between adsorbate molecules and surface functional groups, leading to partial desorption and reduced uptake. Notably, the Al₂O₃@Pyrano PY composite exhibits a less pronounced decline in adsorption efficiency compared to the pristine Pyrano PY, indicating enhanced thermal stability. This behavior can be attributed to the presence of nano-alumina, which introduces additional adsorption sites and strengthens surface interactions, thereby mitigating the adverse effect of temperature increase.

Fig. 10

Effect of temperature on the adsorption efficiency of (a) methylene blue and (b) Cd²⁺ by Pyrano PY and Al₂O₃@Pyrano PY composite.

Thermodynamic parameters were calculated to quantitatively assess the feasibility and energetic characteristics of the adsorption process. For methylene blue adsorption (Table 3; Fig. 11), Pyrano PY exhibits a negative enthalpy change (ΔH° = −5.44 kJ mol⁻¹), confirming the exothermic nature of the adsorption interaction. The negative entropy change (ΔS° = −12.90 J mol⁻¹ K⁻¹) indicates a decrease in randomness at the solid–solution interface, suggesting that methylene blue molecules adopt a more ordered arrangement upon adsorption. The Gibbs free energy values (ΔG° ranging from − 15.95 to − 12.08 kJ mol⁻¹) remain negative across the investigated temperature range, confirming that the adsorption process is spontaneous and thermodynamically favorable⁴⁶.

For the Al₂O₃@Pyrano PY composite, a smaller negative enthalpy change (ΔH° = −2.43 kJ mol⁻¹) and a less negative entropy change (ΔS° = −2.67 J mol⁻¹ K⁻¹) were observed. Despite the lower magnitude of ΔH°, the composite exhibits more negative Gibbs free energy values (ΔG° between − 16.40 and − 15.60 kJ mol⁻¹) compared to the pristine adsorbent. This behavior indicates enhanced spontaneity and suggests that adsorption on the composite surface benefits stronger overall driving forces, likely arising from synergistic interactions between organic functional groups and alumina-derived active sites. The lower exothermicity also explains the reduced sensitivity of the composite to temperature increases, contributing to its improved thermal stability.

Table 3 Thermodynamic parameters for methylene blue adsorption on Pyrano PY and Al₂O₃@Pyrano PY.

For Cd²⁺ adsorption (Table 4; Fig. 11), Pyrano PY again demonstrates a spontaneous and exothermic adsorption process, characterized by ΔH° = −6.86 kJ mol⁻¹ and ΔS° = −17.63 J mol⁻¹ K⁻¹. The negative Gibbs free energy values (− 16.08 to − 10.79 kJ mol⁻¹) confirm favorable adsorption across the studied temperature range. Upon incorporation of nano-Al₂O₃, the enthalpy change becomes slightly less negative (ΔH° = −4.74 kJ mol⁻¹), while the Gibbs free energy values become more negative (− 16.95 to − 13.88 kJ mol⁻¹), indicating improved spontaneity and adsorption efficiency.

The enhanced thermodynamic performance of the Al₂O₃@Pyrano PY composite can be attributed to increased surface heterogeneity, a higher density of accessible adsorption sites, and the presence of Al–O and surface hydroxyl groups that strengthen metal–surface interactions. These factors collectively improve adsorption stability and reduce sensitivity to thermal fluctuations.

Table 4 Thermodynamic parameters for Cd(II) adsorption on Pyrano PY and Al₂O₃@Pyrano PY.

Fig. 11

Van’t Hoff plots for (a) methylene blue and (b) Cd²⁺ adsorption on Pyrano PY and Al₂O₃@Pyrano PY.

Overall, the thermodynamic analysis confirms that the adsorption of both methylene blue and Cd²⁺ ions onto Pyrano PY and Al₂O₃@Pyrano PY proceeds via spontaneous and exothermic mechanisms. The alumina-modified composite exhibits superior thermodynamic favorability and enhanced thermal stability compared to the pristine sorbent, further validating its suitability for practical wastewater treatment under variable temperature conditions. The optimal conditions for both pollutants are summarized in Table S1.

Methylene blue and Cd(II) desorption and sorbent recycling

The reusability and regeneration capability of an adsorbent are critical parameters for assessing its practical applicability and economic feasibility in large-scale wastewater treatment. Accordingly, the regeneration performance of Pyrano PY and Al₂O₃@Pyrano PY was systematically evaluated through multiple adsorption–desorption cycles to examine their structural stability and long-term adsorption efficiency.

Desorption experiments were carried out using appropriate eluents selected based on the nature of the adsorbed pollutants. For methylene blue, desorption was achieved using 0.2 M NaOH solution, which promotes dye release through disruption of electrostatic interactions and hydrogen bonding. In contrast, Cd²⁺ desorption was conducted using 0.1 M HCl, where protonation of surface functional groups weakens metal–ligand coordination and facilitates ion release. After each regeneration step, the desorbing solutions were collected and neutralized to approximately pH 7 prior to disposal. The neutralized effluents were subsequently diluted with deionized water, ensuring minimal environmental impact and compliance with wastewater discharge guidelines.

The desorption efficiencies of both Pyrano PY and Al₂O₃@Pyrano PY for methylene blue and Cd²⁺ over five consecutive cycles are summarized in Table 5. Both sorbents exhibited high desorption efficiencies throughout the regeneration process, with values remaining above 90% even after the fifth cycle. Only a gradual and limited decline in desorption efficiency was observed, with an overall decrease (ΔDe) of less than 7–8%, indicating minimal loss of active adsorption sites and strong resistance to structural degradation.

Notably, the Al₂O₃@Pyrano PY composite consistently demonstrated slightly higher desorption efficiencies compared to the pristine Pyrano PY. This behavior suggests that nano-alumina incorporation contributes to improved surface stability and preserves adsorption sites during repeated adsorption–desorption cycles. The enhanced regeneration performance may also be attributed to the synergistic interaction between organic functional groups and Al–O surface sites, which enables efficient pollutant release without irreversible binding.

Overall, the regeneration results confirm that both Pyrano PY and Al₂O₃@Pyrano PY possess excellent recyclability and operational stability. Their ability to maintain high desorption efficiency over multiple cycles highlights their suitability for repeated use in wastewater treatment applications, significantly reducing operational costs and material consumption. The superior recycling performance of the Al₂O₃@Pyrano PY composite further reinforces its potential as a sustainable and durable adsorbent for the removal of cationic dyes and heavy-metal ions from contaminated water.

Table 5 Desorption efficiency of Cd(II) and methylene blue over five consecutive adsorption–desorption cycles.

Conclusion

In this work, a heteroatom-enriched pyranopyrazole adsorbent (Pyrano PY) and its waste-derived nano-alumina hybrid (Al₂O₃@Pyrano PY) were rationally developed as efficient and sustainable platforms for the simultaneous removal of methylene blue and Cd(II) ions from aqueous media. The study integrates green material synthesis, comprehensive physicochemical characterization, systematic adsorption evaluation, and density functional theory analysis to establish a clear and consistent structure–interaction–performance relationship.

The one-pot multicomponent construction of Pyrano PY yields a chemically versatile organic framework densely populated with nitrogen- and oxygen-containing functional groups, enabling strong coordination, electrostatic, hydrogen-bonding, and π-interaction pathways. Subsequent incorporation of nano-Al₂O₃ derived from aluminum waste introduces an inorganic phase that markedly amplifies surface reactivity by increasing accessible surface area, active-site density, and interfacial heterogeneity without disrupting the integrity of the organic scaffold. Structural, spectroscopic, and microscopic analyses confirm homogeneous alumina decoration and the formation of a genuine organic–inorganic hybrid interface.

Adsorption studies demonstrate that both materials exhibit rapid uptake kinetics, pronounced pH-dependent behavior, and high adsorption capacities toward both target pollutants. The Al₂O₃@Pyrano PY composite consistently outperforms the pristine adsorbent, achieving maximum experimental capacities of approximately 190 mg g⁻¹ for methylene blue and 343 mg g⁻¹ for Cd(II). Kinetic data show excellent agreement with the pseudo-second-order model, confirming that adsorption is governed by surface-controlled chemisorption processes. Equilibrium behavior is best described by the Freundlich isotherm, reflecting heterogeneous and energetically diverse adsorption sites, while thermodynamic analysis verifies that both adsorption processes are spontaneous and exothermic across the investigated temperature range.

The practical robustness of the developed adsorbents is further demonstrated by their high regeneration efficiency and structural stability over multiple adsorption–desorption cycles, with minimal loss of performance. The alumina-loaded composite exhibits enhanced thermal tolerance and operational durability, highlighting its suitability for repeated use under realistic water treatment conditions.

Importantly, density functional theory calculations provide molecular-level validation of the experimental observations. Frontier orbital analysis and global reactivity descriptors reveal preferential localization of electron density over nitrogen and oxygen centers, rationalizing strong Cd(II) coordination and stabilizing interactions with methylene blue through charge transfer and π–π stacking. Mulliken population analysis further identifies specific heteroatoms as dominant adsorption centers, establishing a direct electronic rationale for the observed kinetics, selectivity, and thermodynamic favorability. The strong agreement between theoretical predictions and experimental behavior confirms the robustness of the proposed adsorption mechanism.

Overall, this study demonstrates that the rational integration of heterocyclic organic frameworks with waste-derived inorganic nanophases offers a powerful strategy for designing high-performance, reusable, and environmentally responsible adsorbents. The Al₂O₃@Pyrano PY composite uniquely combines sustainable material sourcing, enhanced interfacial energetics, and superior adsorption performance within a single platform. Beyond the present system, the design principles established herein provide a transferable blueprint for the development of next-generation hybrid adsorbents guided by both experimental performance metrics and electronic-structure insights, with clear potential for scalable wastewater remediation applications.

Comparative evaluation of cadmium and methylene blue adsorption by pyrano PY and Al₂O₃@Pyrano PY composite relative to literature reports

To benchmark the practical significance of Pyrano PY and Al₂O₃@Pyrano PY, their adsorption capacities for methylene blue and Cd(II) were systematically compared with representative adsorbents reported in the literature, with reported capacities selected under comparable experimental conditions where available. The comparison encompasses conventional porous carbons, engineered biochars, and high-affinity porous frameworks, as summarized in Table 6. Overall, the present system demonstrates a balanced and application-oriented performance profile. The methylene blue uptake achieved by Pyrano PY and its alumina-loaded composite is competitive with many scalable carbonaceous adsorbents, while Cd(II) capture by Al₂O₃@Pyrano PY reaches a high-capacity regime that rivals or exceeds numerous hybrid sorbents. Importantly, this performance is achieved under relatively mild operating conditions and is accompanied by demonstrated regeneration capability and cycling stability. Taken together, the comparative analysis positions the developed composite among practically attractive adsorption platforms in which capacity, durability, and sustainable materials design co-exist within a single system.

Table 6 Comparison of methylene blue and Cd(II) adsorption capacities of Pyrano PY and Al₂O₃@Pyrano PY with selected literature adsorbents.

Proposed mechanism of methylene blue and Cd(II) adsorption onto Pyrano PY and Al₂O₃@Pyrano PY

The adsorption behavior of methylene blue and Cd²⁺ ions onto Pyrano PY and its alumina-modified composite is governed by an integrated interplay between surface functionality, electronic structure, and organic–inorganic interfacial synergy. Experimental observations supported by spectroscopic analysis, kinetic and thermodynamic modeling, elemental mapping, and density functional theory collectively converge toward a unified adsorption mechanism, schematically illustrated in Scheme 2.

For the pristine Pyrano PY adsorbent, adsorption originates from the heteroatom-rich pyranopyrazole framework, which provides multiple chemically accessible binding domains. FTIR spectral shifts observed after adsorption confirm the direct involvement of amino, hydroxyl, imine, and ether functionalities, indicating that these groups actively participate in pollutant binding rather than serving as passive structural motifs. This experimental evidence is fully consistent with DFT calculations, which reveal pronounced localization of HOMO electron density over nitrogen and oxygen centers within the pyranopyrazole scaffold. Such localization confers strong electron-donating character to these sites, rendering them particularly favorable for coordination with electrophilic metal ions.

Accordingly, Cd²⁺ adsorption on Pyrano PY proceeds predominantly through inner-sphere surface complexation involving amino and imine nitrogen atoms, with auxiliary stabilization provided by oxygen donor sites. The dominance of pseudo-second-order kinetics, together with negative Gibbs free energy values, confirms that Cd²⁺ uptake is controlled by chemisorption rather than diffusion-limited physical adsorption. The moderate HOMO–LUMO energy gap and chemical hardness derived from DFT further indicate that the electronic structure of Pyrano PY is sufficiently flexible to accommodate metal–ligand coordination without compromising molecular stability.

In contrast, methylene blue adsorption on Pyrano PY is governed by a cooperative combination of electrostatic attraction, hydrogen bonding, and π–π stacking interactions. At pH values exceeding the point of zero charge, surface deprotonation generates negatively charged domains that electrostatically attract the cationic dye. Simultaneously, the extended conjugated aromatic system of the pyranopyrazole framework enables effective π–π overlap with the aromatic rings of methylene blue, while hydrogen bonding interactions further stabilize the adsorbed species. DFT frontier orbital analysis supports this mechanism by demonstrating delocalized π-electron density across the aromatic regions, facilitating dye stacking and multilayer adsorption on the heterogeneous surface.

The incorporation of nano-Al₂O₃ fundamentally amplifies the adsorption mechanism by introducing an inorganic phase that modifies both surface chemistry and textural properties. SEM observations confirm uniform nano-alumina decoration over the organic matrix without pore blockage, effectively increasing surface roughness and accessible interfacial area. This nano-scale dispersion generates a higher density of exposed active sites while preserving mass-transfer pathways. FTIR signatures corresponding to Al–O and Al–O–Al vibrations verify the structural integration of alumina, while EDX analysis confirms its homogeneous distribution.

From a mechanistic perspective, alumina introduces Lewis acidic Al–O centers capable of forming strong inner-sphere Cd–O interactions through coordination and ion-exchange processes. These inorganic sites operate synergistically with organic nitrogen and oxygen donors, creating multidentate binding environments that significantly enhance metal ion affinity. DFT-supported thermodynamic trends, particularly the more negative Gibbs free energy values observed for the composite, reflect the stabilization imparted by this cooperative organic–inorganic coordination network.

In addition to metal binding, surface hydroxyl groups associated with nano-Al₂O₃ play a critical role in reinforcing adsorption of both methylene blue and Cd²⁺ through hydrogen bonding and electrostatic mediation. These hydroxyl-rich domains increase adsorption capacity while maintaining relatively low enthalpy changes, accounting for the improved thermal stability and reduced temperature sensitivity of the composite compared to pristine Pyrano PY. Moreover, the alumina phase contributes to surface charge modulation, enhancing electrostatic attraction toward cationic species under near-neutral conditions and broadening the effective operational pH window.

Overall, adsorption of methylene blue and Cd²⁺ onto Pyrano PY and Al₂O₃@Pyrano PY proceeds through a synergistic multi-pathway mechanism involving heteroatom coordination, surface complexation, hydrogen bonding, electrostatic attraction, and π–π interactions, as depicted in Scheme 2. The integration of experimental validation with electronic-structure insights provides a coherent mechanistic framework that explains the superior adsorption capacity, stability, and recyclability of the alumina-loaded composite. This mechanistic understanding underscores the critical role of nano-Al₂O₃ in simultaneously enhancing surface area, active site density, and interfacial energetics, thereby establishing Al₂O₃@Pyrano PY as a robust and multifunctional adsorbent platform for advanced wastewater remediation.

Scheme 2

Schematic representation of the adsorption mechanisms of Cd²⁺ ions and methylene blue molecules onto the Al₂O₃@Pyrano PY composite.

Density functional theory (DFT) analysis and electronic Structure–Adsorption correlation

Frontier molecular orbital and reactivity descriptor analysis

Density functional theory calculations were employed to provide a molecular-level interpretation of the experimentally observed adsorption behavior and to identify the electronic features governing interaction with heavy-metal ions. Frontier molecular orbital analysis offers a direct visualization of reactive regions within the molecule and serves as a powerful tool for correlating electronic structure with adsorption performance. As depicted in Fig. 12a and b, the highest occupied molecular orbital (HOMO) is predominantly localized over nitrogen- and oxygen-containing functional groups within the pyranopyrazole framework. This spatial distribution clearly identifies these heteroatoms as the primary electron-donating centers responsible for coordination with electrophilic metal cations. The localization of electron density on amino, imine, and oxygenated moieties facilitates efficient lone-pair donation to metal ions, which is consistent with the experimentally observed chemisorption-dominated kinetics and the negative enthalpy values obtained from thermodynamic analysis.

Fig. 12

Spatial distributions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of Pyrano PY, illustrating the localization of electron density over heteroatom-rich regions and the conjugated framework relevant to adsorption interactions.

The lowest unoccupied molecular orbital (LUMO), by contrast, is distributed over the conjugated backbone of the molecule, indicating that charge redistribution upon adsorption can be effectively delocalized across the π-system. This delocalization contributes to stabilizing the adsorbate–adsorbent complex and supports the formation of relatively strong surface complexes rather than weak physical interactions. The resulting HOMO–LUMO energy gap (ΔE_g = 8.556 eV), as summarized in Table 7, reflects a chemically stable yet sufficiently reactive system. Such a balance is particularly advantageous for adsorption applications, as it ensures structural robustness under aqueous conditions while preserving the ability to engage in strong metal-binding interactions.

Global reactivity descriptors further reinforce these observations. The relatively high ionization potential (I = 9.598 eV) indicates resistance toward oxidative degradation, an essential feature for adsorbents subjected to repeated adsorption–desorption cycles. At the same time, the moderate electron affinity (A = 1.042 eV) suggests that the molecule can accommodate electron density changes during metal binding without excessive energetic penalty. The calculated chemical hardness (η = 4.278 eV) and corresponding softness (σ = 0.117 eV⁻¹) classify the molecule as moderately soft.

According to Pearson’s Hard and Soft Acids and Bases (HSAB) theory, soft and borderline metal ions such as Cd²⁺ preferentially interact with polarizable, electron-rich donor sites⁵⁵. The moderate hardness of Pyrano PY, combined with the pronounced HOMO localization on nitrogen and oxygen atoms, confirms that the molecule behaves as a borderline base. This electronic character explains the strong affinity toward Cd²⁺ ions observed experimentally and aligns with the excellent agreement between kinetic data and the pseudo-second-order model, which implies surface-controlled chemisorption rather than diffusion-limited uptake.

The negative chemical potential (µ = −5.320 eV) further indicates a strong thermodynamic tendency toward electron donation, consistent with the spontaneous nature of adsorption reflected by negative Gibbs free energy values. In addition, the electrophilicity index (ω = 3.31 eV) suggests a pronounced ability of the molecule to stabilize charge redistribution during adsorption. Together, these descriptors confirm that adsorption proceeds through a synergistic mechanism involving chelation, charge transfer, and orbital overlap, rather than simple electrostatic attraction or physical adsorption⁵⁶.

Table 7 Global quantum chemical descriptors of Pyrano PY obtained from DFT calculations.

To further clarify the nature of active adsorption sites, Mulliken population analysis was performed to examine charge distribution across the molecular framework. As summarized in Table 8, a pronounced heterogeneity in electron density is evident, particularly around heteroatoms. Several nitrogen atoms exhibit strongly negative Mulliken charges, with N(15) showing the most negative value (− 0.6901), followed by N(18) (− 0.4286). These results unequivocally identify amine and imine nitrogen centers as the primary coordination sites for Cd²⁺ ions. In contrast, N(19) carries a positive charge (+ 0.6513), indicating its involvement in conjugative or electron-withdrawing effects rather than direct metal binding⁵⁷.

Oxygen atoms associated with oxygenated functional groups also display significant negative charges, particularly O(28) (− 0.2706), O(26) (− 0.2137), and O(27) (− 0.2004). These oxygen atoms are well positioned to participate in metal coordination through lone-pair donation, either independently or cooperatively with adjacent nitrogen atoms, enabling chelation-type binding modes that enhance adsorption stability.

The carbon framework exhibits moderate charge polarization, especially for carbons directly bonded to heteroatoms. Carbons such as C(12) (− 0.3581), C(7) (− 0.2422), and C(8) (− 0.2308) show enhanced negative charge density due to inductive and resonance effects. This polarization facilitates electron delocalization across the conjugated system, which in turn stabilizes the adsorbent–metal complex by distributing charge density away from localized coordination sites.

Importantly, the Mulliken charge distribution closely correlates with the HOMO localization pattern discussed earlier. The same nitrogen and oxygen atoms identified as highly negative in Mulliken analysis correspond to regions of high HOMO electron density, confirming that these sites are both energetically and electronically favored for electron donation⁵⁸. This strong agreement between orbital analysis and population analysis provides compelling electronic validation for the proposed adsorption mechanism.

Overall, the DFT results demonstrate that heavy-metal adsorption by Pyrano PY is fundamentally governed by heteroatom-centered coordination supported by conjugation-assisted charge delocalization. These electronic characteristics rationalize the high adsorption affinity, favorable thermodynamics, and excellent kinetic performance observed experimentally, and they provide a robust theoretical foundation for the superior adsorption behavior of Pyrano PY and its alumina-modified composite in aqueous environments.

Table 8 Mulliken atomic charges of Pyrano PY derived from DFT calculations.

Economic feasibility and practical implications

Economic feasibility is a decisive criterion for translating laboratory-scale adsorbents into deployable wastewater treatment technologies. In this context, the cost-effectiveness of Pyrano PY and Al₂O₃@Pyrano PY can be rationalized by considering feedstock accessibility, synthesis intensity, operational demands, and lifetime performance under repeated use.

Pyrano PY is produced through a one-pot multicomponent synthesis that relies on commercially available precursors and proceeds under mild conditions without inert atmospheres, high pressures, or specialized catalysts. The operational simplicity of the synthetic route, together with the high isolated yield and limited downstream purification, implies reduced solvent consumption, shortened processing time, and lower energy input per unit mass of adsorbent. These attributes are economically favorable when moving beyond batch laboratory preparation toward kilogram-scale production.

A central economic advantage of the composite arises from the use of waste aluminum cans as the precursor for nano-Al₂O₃. Converting an abundant post-consumer waste stream into a functional oxide phase eliminates the need to procure commercial alumina nanoparticles and simultaneously creates value from a low-cost feedstock. The alumina synthesis is based on straightforward acid–base processing and thermal conversion, avoiding expensive structure-directing agents, multi-step functionalization, or toxic organic solvents. This waste-to-resource pathway supports circular economy goals and reduces the raw-material cost component of the final composite.

From an operational perspective, the adsorption process is compatible with typical water treatment conditions. Both materials operate effectively at ambient pressure and moderate temperatures, reach equilibrium within a practical timeframe, and do not require continuous energy-intensive inputs. These features align well with scalable process configurations such as stirred tank contactors, packed-bed columns, or hybrid adsorption–filtration units, where the key economic drivers are hydraulic residence time, adsorbent dose, and the frequency of replacement or regeneration.

Reusability substantially strengthens the cost profile of the developed adsorbents. The high desorption efficiencies retained over repeated cycles indicate that the effective service life can extend across multiple treatment runs, lowering the adsorbent replacement rate and reducing the overall cost per treated volume. Moreover, the regenerants employed are dilute NaOH for methylene blue and dilute HCl for Cd(II), both of which are low-cost commodities widely available at industrial scale. Their use also suggests that regeneration can be implemented with standard chemical handling and neutralization infrastructure already present in most wastewater facilities, keeping operational complexity limited.

Compared with adsorption systems that require energy-intensive thermal reactivation, costly precursors such as MOF linkers, or multi-step nanomaterial fabrication, the present platform offers a favorable cost-to-performance balance. Importantly, the higher adsorption capacities of Al₂O₃@Pyrano PY toward both methylene blue and Cd(II) reduce the required adsorbent dose for a given treatment target, directly lowering material consumption and indirectly reducing sludge generation and handling costs associated with spent sorbent management.

From a scale-up standpoint, several practical assumptions support feasibility. First, the synthesis and modification steps rely on conventional unit operations compatible with scale-up, including mixing, filtration, drying, and thermal treatment. Second, the adsorption–desorption protocol is based on aqueous handling and dilute reagents, which are readily integrated into continuous or semi-continuous workflows. Third, the demonstrated stability over multiple regeneration cycles suggests that performance can be maintained under repeated process conditions, a key requirement for fixed-bed operation where mechanical integrity and sustained adsorption capacity govern long-term economics. Taking together, these factors indicate that the transition from laboratory batches to pilot-scale treatment is technically straightforward and economically defensible.

Overall, the combination of low-cost synthesis, waste-derived nano-alumina integration, mild operating conditions, and robust recyclability establishes Pyrano PY and particularly Al₂O₃@Pyrano PY as economically viable candidates for large-scale remediation of dye- and heavy-metal-contaminated wastewater. The platform is especially attractive for industrial settings where treatment costs must be minimized while maintaining high removal efficiency, regeneration capability, and sustainability-driven materials sourcing.