Synergistic DTPA-functionalized alginate/ZIF-90-derived oxide composite for ultraefficient removal of lead and methylene blue from aquaculture wastewater

Abstract

Industrial wastewater often contains various pollutants such as heavy metals and organic dyes, but most of the current adsorbents only show high performance under single-substance conditions and are still limited in adsorption capacity, kinetic rate, and reusability. To address this research gap, the work proposes a ZZO@ALG–DTPA composite material integrating ZIF-90/zeolite hybrid oxide (ZZO) and Diethylenetriaminepentaacetic acid-functionalized alginate (ALG–DTPA), creating a multifunctional adsorption platform for the separate treatment of Pb²⁺ and methylene blue (MB). The material shows outstanding adsorption efficiency and capacity, reaching a maximum q_max = 1114.7 mg g⁻¹ (Pb²⁺) and 787.41 mg g⁻¹ (MB) followed by the Langmuir equation (R² = 0.988–0.975). The kinetics followed pseudo-second-order (R² = 0.999), confirming the chemical adsorption mechanism dominated by the coordination interaction –COO⁻/–NH₂ with Pb²⁺ and charge interaction with MB. The optimum conditions were achieved at pH = 7, dosage 20 mg, equilibrium time 20 min, with the removal efficiencies of 96.98% (Pb²⁺) and 98.85% (MB), respectively. Ca²⁺ reduced Pb²⁺ removal by 50%, while monovalent ions had minimal impact, and the efficiency remained ≥85% in multi-ion systems. The material remained stable after 5 regeneration cycles, 85.43% efficiency for Pb²⁺ and 89.94% for MB, while demonstrating good ion resistance and high efficiency in complex real-water matrices (75.47–87.35% for Pb²⁺ and 80.55–88.96% for MB). These results prove that ZZO@ALG–DTPA is a promising, highly efficient, and renewable-based sorbent material for the treatment of multi-polluted wastewater.

Industrial wastewater carries heavy metals and organic dyes, yet most adsorbents excel only in single-substance conditions and lack high capacity, fast kinetics, and reusability.

1. Introduction

Wastewater containing heavy metals, synthetic dyes, together with diverse microbial and biological impurities has emerged as a major worldwide environmental concern driven by accelerated industrial development, expanding populations, and intensifying freshwater shortages.^1–3 Among these harmful contaminants, methylene blue (MB)⁴ and Pb²⁺ (ref. 5) are particularly problematic because of their persistence, toxicity, and tendency to bioaccumulate. MB is a cationic dye with an intense blue color, widely employed in textile dyeing, printing, paper coatings, leather processing, paints, and cosmetics.^6,7 Although MB is not considered highly toxic, prolonged exposure has been associated with adverse effects such as nausea, vomiting, increased heart rate, and neuronal apoptosis.⁸ Its release into aquatic environments can result in significant ecological problems, including water coloration, reduced light penetration, and toxicity to aquatic organisms.⁹ Human exposure to MB may cause profuse sweating, nausea, vomiting, neuronal apoptosis, methemoglobinemia, and mental confusion.¹⁰ Textile effluents frequently contain traces of heavy metals such as Pb²⁺, which exacerbate environmental hazards.¹¹ Pb²⁺ is among the most common toxic metals released through pigment production, fuel additives, battery manufacturing, aquaculture, and leaded glass industries.¹² Even at low concentrations, Pb²⁺ can accumulate in the human body via the food chain due to its water solubility and non-biodegradable nature.¹³ The World Health Organization (WHO) recommends a maximum Pb²⁺ concentration of 0.01 mg L⁻¹ in drinking water¹⁴ and a discharge limit of 0.1 mg L⁻¹ for wastewater.¹⁵ Exceeding these levels can lead to severe health consequences, including liver and kidney damage, insomnia, central nervous system disorders, hallucinations, anemia, and irritability.^16,17

To remove these pollutants, a variety of traditional approaches have been used, including chemical precipitation, ion exchange, membrane filtration, electrochemical treatment, biological processes, and especially adsorption.^18–22 Among them, adsorption on porous frameworks has drawn particular attention because it delivers high removal efficiency, operational simplicity, low cost, reusability, and minimal secondary pollution. A highly promising class of such adsorbents is zeolitic imidazolate frameworks (ZIFs). ZIFs combine the structural robustness of zeolites with the versatile chemistry of coordination frameworks, offering high porosity, tunable surfaces, and exposed metal sites. Studies have shown that different ZIFs (e.g., ZIF-8, ZIF-67) can strongly adsorb cationic dyes like MB.²³ For instance, Kang et al. demonstrated comparable MB uptake on both ZIF-8 and ZIF-67, attributing the adsorption to electrostatic interactions between the imidazolate linkers and MB.²³ In another work, hierarchical ZIFs engineered via selective ligand removal exhibited dramatically enhanced MB adsorption capacity over 40 times higher than conventional ZIF-8, thanks to added mesopores that facilitate dye diffusion.²⁴ Further, functionalized ZIFs have been explored: for example, ZIF-67 incorporated with phosphotungstic acid (HPW) showed very high MB adsorption (446 mg g⁻¹), driven by electrostatic attraction and π–π interactions.²⁵ There are also composite systems: ZIF-67/PAN nanofibers achieved MB adsorption >1100 mg g⁻¹ plus catalytic degradation via peroxymonosulfate activation.²⁶ Studies have also demonstrated that various ZIFs can efficiently adsorb Pb²⁺. For example, ZIF-8-based composites such as SA@ZIF-8/biochar showed a maximum Pb²⁺ uptake of 300 mg g⁻¹, with adsorption fitting a Langmuir isotherm and pseudo-second-order kinetics, indicating strong chemisorption via –OH and –COOH sites.²⁷ In another work, a flexible membrane made by growing ZIF-8 on fungal hyphae exhibited an extremely high dynamic Pb²⁺ adsorption capacity (1443 mg g⁻¹) under flow conditions.²⁸ Moreover, ZIF-8 and ZIF-67 have both been shown to remove Pb²⁺ from water with very high capacity up to 1978.6 mg g⁻¹ for ZIF-8 in one study.²⁹ Composite materials also perform well: a magnetic ZIF-8/graphene oxide (GO) material achieved a Pb²⁺ adsorption capacity of 625 mg g⁻¹ within minutes.³⁰

Beyond their pristine form, ZIFs can be thermally converted into metal-oxide or carbon-based composites.³¹ Through controlled calcination, the metal nodes and organic linkers rearrange into stable oxide/carbon phases while retaining parts of the original morphology. These derived materials often present improved stability, more accessible active sites, and enhanced adsorption performance toward both heavy metal ions like Pb²⁺ and organic dyes like MB. Among ZIFs, ZIF-90 has gained considerable research interest in recent years owing to its excellent thermal resistance, well-defined crystalline architecture, and the presence of abundant aldehyde (–CHO) functionalities.³² These surface functionalities impart intrinsic polarity and enable post-synthetic modification or transformation into porous carbon and metal-oxide-derived materials through thermal treatment.³³ Unmodified ZIF-90 has been investigated for eliminating both metal ions and organic dye molecules from aqueous media. For example, SA@CA-ZIF-90 material efficiently adsorbed Pb²⁺via coordinative bonding with carboxyl groups,³⁴ while amino-functionalized ZIF-90 achieved up to 93.30% adsorption of Au³⁺, demonstrating strong binding toward multivalent cations.³² In addition, recent studies have confirmed that ZIF-90 can adsorb cationic dyes such as methylene blue, with removal mechanisms involving electrostatic attraction and π–π interactions with the –CHO and imidazole groups.²⁴ However, pristine ZIF-90 tends to experience structural instability in ion-rich aqueous solutions, limiting its long-term application. To overcome this, approaches such as incorporating thermally resistant zeolitic phases followed by air calcination have been proposed. Such thermal conversion can generate metal-oxide-containing porous structures while preserving, to some extent, the morphological features of the parent ZIF and introducing additional oxygen-containing surface sites.³⁵ These structural and chemical characteristics can improve surface hydrophilicity and provide additional sites for metal-ion binding through surface complexation and electrostatic interactions. Such modifications enable ZIF-90-derived oxide composites to combine the advantages of the original ZIF framework with improved chemical stability and practical applicability in wastewater treatment.

Sodium alginate (ALG) is a natural biopolymer consisting of alternating α-l-guluronate and β-d-mannuronate units.³⁶ Its chain contains abundant hydroxyl and carboxyl functionalities that strongly interact with metal ions through coordination and chemical binding, making ALG a promising biosorbent.¹⁹ Nevertheless, practical use is constrained by pronounced swelling, weak structural durability in aqueous media, and limited alkali tolerance. To overcome these limitations, grafting chelating agents has been employed to strengthen the polymer matrix and improve metal uptake. Among them, diethylenetriaminepentaacetic acid (DTPA) is a multidentate chelator with 3 tertiary amines and 5 carboxyl groups, is particularly effective due to its strong complexation ability with a wide range of metal cations.³⁷ Grafting DTPA onto ALG chains can significantly increase both selectivity and adsorption efficiency toward heavy metals.³⁸

Inspired by this strategy, a composite system combining ZIF-90/zeolite-derived oxides as well as DTPA-functionalized alginate (ZZO@ALG–DTPA) was constructed to achieve simultaneous structural stabilization and improved contaminant capture. In this material, the oxide phase serves as a rigid porous scaffold, whereas the modified alginate matrix introduces additional chelating sites that promote cooperative binding interactions. This integrated architecture is anticipated to enhance Pb²⁺ and MB uptake, preserve mechanical stability, and ensure reusability under aqueous conditions. The composite was fabricated and examined to ascertain its morphology and surface characteristics. Batch uptake studies were implemented to ascertain the effects of pH, dosage, concentration, and contact duration. Kinetic and equilibrium analyses were performed to clarify the uptake behavior and interaction pathways. Reusability and stability tests, including multiple adsorption–desorption cycles and experiments in real wastewater, were implemented to verify practical applicability. Overall, this work provides a basis for developing efficient and durable oxide-polymer sorbents capable of concurrently eliminating metal ions and organic dyes from contaminated water.

2. Experimental section

2.1. Materials

The reagents employed in this study were of analytical grade and utilized without additional purification. Imidazole-2-carboxaldehyde (C₄H₄N₂O, 99%, Merck) and zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, ≥99.5%, Merck) were selected as precursors for framework construction. Absolute ethanol (C₂H₅OH, ≥99.7%, Merck) and N,N-dimethylformamide (DMF, ≥99.5%, Xilong) were used as organic solvents, diethylenetriaminepentaacetic acid (DTPA, ≥99.5%, Xilong) and sodium alginate (C₆H₇NaO₆, ≥99.5%, Merck) were employed for polymer functionalization. Hydrochloric acid (HCl, 37%, Xilong) and sodium hydroxide (NaOH, ≥98%, Sigma-Aldrich) were employed for pH adjustment. Methylene blue (MB, C₁₆H₁₈ClN₃S·2H₂O, Xilong) served as the model adsorbate. Methanol (CH₃OH, 99.7%, Merck) was applied as the reaction medium and zeolite particles (0.5–2.5 µm, Sigma-Aldrich) acted as the inorganic support. All aqueous solutions were prepared using deionized water (DI water, 18.2 MΩ cm).

2.2. Production of ZZO

ZIF-90 crystals were obtained via a solvothermal method carried out in a Teflon-lined stainless-steel reactor. Specifically, Zn(NO₃)₂·6H₂O (2.96 g, 10.00 mmol), imidazole-2-carboxaldehyde (3.84 g, 40.0 mmol), and sodium formate (1.34 g, 20.00 mmol) were introduced into 350 mL of CH₃OH and stirred at 650 rpm for 4 h until a homogeneous and clear solution was formed. The resulting mixture was subsequently sealed in the reactor and maintained at 90 °C for 24 h to promote crystallization of the framework. After allowing the system to cool naturally to ambient conditions, a yellow precipitate adhering to the inner surface of the vessel was collected. The solid product was washed 3–5 times employing methanol until the supernatant became transparent, followed by vacuum drying at 80 °C for 10 h to produce water-free ZIF-90 crystals.

To generate the ZIF-90/zeolite-derived oxide, a two-step procedure involving liquid-phase dispersion followed by thermal treatment was adopted. First, 0.5 g of as-prepared ZIF-90 was dispersed along with 0.3 g of zeolite (Al/Si ≈ 1/25) in 30 mL of CH₃OH. The resulting suspension was placed in an ultrasonic bath for 20 min to achieve a homogeneous mixture and to facilitate the adhesion of zeolite particles onto the ZIF-90 crystals. Stirring was then continued for 2.5 h under ambient conditions using a mechanical stirrer, ensuring that the solids remained well suspended. The mixture was subsequently centrifuged to separate the solid product, which was then rinsed with methanol 3 times and dried overnight at 70 °C. After drying, the material was subjected to calcination at 550 °C in air for 2 h, with the temperature raised gradually at 5 °C per minute. This step served to eliminate the organic components from the ZIF-90 framework, resulting in the formation of a porous oxide composite. Following natural cooling to room temperature, the sample was gently ground into a fine, light gray powder. This final product is denoted as ZZO.

2.3. Production of ZZO@ALG–DTPA

The ZZO@ALG–DTPA composite was obtained through a sequential incorporation and gelation process. The formulation consisted of sodium alginate and DTPA in a 2 : 3 ratio were hydrated in 30 mL of deionized water, the pH being carefully regulated to 6.0–6.5 to favor polymer activation. The mixture was allowed to stir for 1 h until a uniform viscous phase was achieved. Separately, ZZO (0.1 g) was redispersed in 10 mL of a 1 : 1 water/C₂H₅OH medium and exposed to ultrasonic agitation for 15 min to promote full deagglomeration of oxide particles. The oxide dispersion was then introduced gradually into the polymeric solution, maintaining continuous stirring for a further hour to ensure intimate integration. Gelation was initiated by gradually introducing a 2% (m V⁻¹) CaCl₂ solution over 4 h, leading to crosslinked hydrogel bead formation with the oxide phase immobilized within the network. The formed beads were separated by vacuum filtration, repeatedly rinsed with deionized water to remove residual ions, and oven-dried overnight at 80 °C, ultimately yielding the ZZO@ALG–DTPA material (Fig. S1).

2.4. Characterization

Surface charge characteristics and colloidal stability of the samples were first assessed by zeta potential and dynamic light scattering analyses using a Malvern Zetasizer Nano ZS (UK), which also provided information on hydrodynamic particle size in aqueous dispersion. The functional groups present on the surface of the materials were subsequently examined by Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet iS50, USA), recorded between 4000 and 400 cm⁻¹ with a spectral resolution of 4 cm⁻¹. Textural features including specific surface area and mesoporous characteristics were determined via nitrogen adsorption–desorption isotherms collected at −196 °C on a Micromeritics ASAP 2020 analyzer (USA), with BET and BJH models applied for calculation of surface area and pore-size distribution, respectively. Morphological attributes and microstructural arrangement were visualized using a JEOL JSM-7600F field-emission scanning electron microscope (Japan, 15 kV), complemented by energy-dispersive X-ray spectroscopy for elemental mapping and compositional confirmation. Additional nanoscale structural information, including particle geometry and dispersion state, was gained from transmission electron microscopy (TEM, JEOL JEM-2100Plus, Japan) operating at 180 kV. Finally, the crystalline phases of ZZO and ZZO@ALG–DTPA were resolved through X-ray diffraction employing a PANalytical Empyrean diffractometer (Malvern Panalytical, Netherlands) with Cu Kα radiation (λ = 1.5406 Å). Measurements were conducted at 45 kV and 35 mA. The diffraction profiles were collected over a 2θ range of 5–80° with a scanning rate of 3° min⁻¹.

2.5. Uptake investigations

To examine contaminant uptake, a fixed amount of ZZO@ALG–DTPA (20 mg) was dispersed in 100 mL aqueous media containing either Pb²⁺ (40 mg L⁻¹) or MB (20 mg L⁻¹). The system was allowed to interact under ambient conditions, and liquid samples were withdrawn at selected durations between 5 and 60 min to track concentration changes. After eliminating the solid fraction, the remaining solute level in the liquid phase was determined spectroscopically (UV-Vis spectra). For Pb²⁺ determination, 1 mL of the filtered solution was transferred to a 25 mL volumetric flask and mixed with 3 mL CH₃COONa, 2 mL NH₃, and 2 mL PAR to form a colored Pb–PAR complex. The mixture was allowed to equilibrate for 20 min, and the absorbance was subsequently measured at 530 nm using an Agilent 8453 UV-Vis spectrophotometer. The Pb²⁺ concentration was calculated from a calibration curve established using standard Pb²⁺ solutions. Independent repetitions (n = 3) were carried out for each condition to minimize experimental deviation. The uptake amount (q_t, mg g⁻¹) together with the removal percentage (R, %) was derived from concentration differences before and after contact:^39,40

1

2

The symbols C₀ and C_t (mg L⁻¹) describe the pollutant concentration at the beginning of the experiment and after a contact time t. The parameter V (L) denotes the solution volume, and m (g) represents the quantity of adsorbent introduced into the system.

2.6. Uptake kinetics

To investigate the adsorption kinetics of Pb²⁺ and MB on ZZO@ALG–DTPA, batch experiments were conducted under optimized conditions (pH 7, 20 mg adsorbent in 100 mL solution, initial concentrations of 40 mg L⁻¹ for Pb²⁺ and 20 mg L⁻¹ for MB, 30 °C) with contact times ranging from 0 to 60 min. The kinetic behavior was interpreted using 4 commonly applied kinetic models:^41–43

3

Intraparticle diffusion: q_t = C + k_difft^1/2 4

5

First-order: ln(q_e − q_t) + k₁·t = ln q_e 6

In the kinetic evaluation, each model introduces its own characteristic parameters to describe how Pb²⁺ and MB migrate toward and interact with the ZZO@ALG–DTPA surface. The pseudo-first-order and pseudo-second-order equations employ the constants k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹), respectively, which quantify the rate at which adsorption progresses under their assumed mechanisms. The intra-particle diffusion analysis provides 2 additional quantities: the intercept C (mg g⁻¹), representing the external mass-transfer contribution, and the diffusion coefficient k_diff (mg g⁻¹ min⁻¹), which characterizes the movement of adsorbate species into the inner framework of the composite. The capacities q_t and q_e (mg g⁻¹) represent the amount taken up at a given time and the final equilibrium value. In the Elovich model, the β (g mg⁻¹) and α (mg g⁻¹ min⁻¹) serve to characterize the first chemisorption rate and the extent to which surface heterogeneity influences the activation process.

2.7. Uptake isotherm

Adsorption isotherms were established by contacting 20 mg of the ZZO@ALG–DTPA composite with 100 mL of Pb²⁺ or MB solutions spanning a wide concentration range (10–300 mg L⁻¹) at pH 7 and 30 °C. The suspensions were stirred at 250 rpm for 60 min, a duration previously confirmed to be sufficient for attaining equilibrium. The equilibrium uptake values obtained from these tests were subsequently interpreted using 4 complementary isotherm models, like Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich. Each model was fitted using its respective linearized form, allowing extraction of characteristic parameters that collectively describe site homogeneity, adsorption energetics, and the nature of surface–adsorbate interactions:^44,45

7

C_e·q_max⁻¹ + (K_L·q_max)⁻¹ = C_e·q_e⁻¹ 8

9

q_e = B ln K_t + B ln C_e 10

ln q_max = K_D–Rε² + ln q_e 11

12

13

In the Freundlich framework, K_n serves as an indicator of how strongly the material can accommodate a given contaminant, while n captures how uneven or varied the surface energy landscape is. When n falls between 1 and 10, the adsorption process is generally considered favorable, meaning the surface offers enough energetically accessible sites for effective uptake.⁴⁶ The Temkin model yields an equilibrium binding constant K_T (L mg⁻¹) and a term, B = R_T/b_T, which relates directly to the heat of adsorption. Conversely, the D–R model is grounded in Polanyi potential theory, where ε = RT ln(1 + 1/C_e). Its constant, K_D–R (mol² J⁻²), correlates to the adsorption energy, allowing the mean free energy E_D–R = 1/√(2K_D–R) to be calculated, which is crucial for characterizing the nature of the adsorption interaction.⁴⁷ Generally, E_D–R < 8 kJ mol⁻¹ are associated with physical removal, whereas those between 8 and 16 kJ mol⁻¹ refers to the chemical removal.⁴⁸ In the Langmuir framework, the parameter q_max (mg g⁻¹) defines the maximum monolayer adsorption capacity, while K_L (L mg⁻¹) reflects the affinity between the adsorbent and adsorbate.⁴⁹ The corresponding separation factor, R_L = 1/(K_LC_o + 1), is commonly applied to evaluate the favorability of the adsorption system, where values within the range 0 < R_L < 1 indicate favorable uptake behavior.⁵⁰ In this model, q_e (mg g⁻¹) represents the equilibrium adsorption capacity, and C_e (mg L⁻¹) denotes the solute concentration at equilibrium, whereas C₀ (mg L⁻¹) corresponds to the initial concentration of the solution.

3. Results and discussions

3.1. Composite characteristics

The XRD profiles of ZZO, ZZO@ALG–DTPA, and ZZO@ALG–DTPA after adsorption (Fig. 1a) display clear structural variations due to the incorporation of alginate and DTPA, together with pollutant loading. In the unmodified ZZO, well-defined diffraction reflections are observed at 2θ = 11.8°, 15.5°, 18.5°, 20.1°, 22.3°, 23.2°, 26.6°, 28.9°, and 30.3°, indicating the characteristic crystalline features of ZZO. The diffraction pattern is consistent with a well-crystallized zeolitic structure, although the observed reflections are not sufficient to unambiguously identify a specific zeolite framework. No distinct peaks corresponding to wurtzite-type ZnO (JCPDS 00-036-1451) were detected, implying that zinc is either present in an amorphous form or the oxide crystallites are too small to generate noticeable diffraction signals.⁵¹ Following functionalization with DTPA and ALG, the intensity of the zeolite peaks slightly diminishes, and the background signal within the 10–30° region becomes more pronounced, suggesting partial amorphization. This effect originates from the anchoring of –COOH and –OH functionalities derived from alginate together with the amine/carboxyl moieties of DTPA onto the oxide surface, which locally disturbs the crystalline framework of the zeolite. Importantly, these modifications generate additional active sites with the ability to bind metal ions, demonstrating that the organic modification of the ZZO surface has been successfully accomplished.

Fig. 1. (a) XRD, (b) FT-IR and (c) TGA of ZZO, ZZO@ALG–DTPA, ZZO@ALG–DTPA-Pb²⁺, and ZZO@ALG–DTPA-MB.

Following the Pb²⁺ decontamination, the characteristic peaks of ZZO@ALG–DTPA decreased markedly in intensity, particularly within the 2θ = 15–30° region, accompanied by a noticeable broadening of the diffraction background. These changes indicate an increase in amorphization as the coordination centers of DTPA, together with the carboxyl and hydroxyl groups of alginate, interact strongly with Pb²⁺. The reduced sharpness of the faujasite peaks suggests that Pb²⁺ occupied ion-exchange sites within the zeolite framework and formed surface complexes with negatively charged functional groups, leading to localized distortions of the crystal lattice. Importantly, no new diffraction peaks characteristic of lead precipitates were observed, indicating that Pb²⁺ was retained predominantly through coordination and ion-exchange rather than by forming separate crystalline phases. For the sample after MB adsorption, structural alterations were even more pronounced. The background signal in the 2θ = 10–30° region increased sharply, and several framework peaks became obscured, reflecting strong interactions between MB and the material surface. The attachment of MB through its amine functionalities and positively charged heterocyclic rings likely resulted in the formation of a semi-amorphous organic layer that significantly reduced the intensity and definition of the crystalline peaks. As expected, no new peaks attributable to MB appeared due to its inherently non-crystalline nature. These observations confirm that adsorption induces the formation of strong surface interaction complexes between MB and the –COO⁻, –OH groups and the N, O donor sites of DTPA, leading to a noticeable reduction in the structural order of the composite material.

The FT-IR spectra of ZZO and ZZO@ALG–DTPA (Fig. 1b) clearly demonstrate the chemical modifications introduced by alginate and DTPA. Pristine ZZO exhibits a broad absorption band at 3300–3450 cm⁻¹ associated with O–H stretching of surface hydroxyl groups and physically sorbed water, together with the H–O–H bending mode near 1644 cm⁻¹.⁵² The strong vibrations in the region of 967–433 cm⁻¹ correspond to T–O–T (T = Zn, Al, Si) stretching of the zeolite structure.⁵³ Upon functionalization, the broad O–H/N–H stretching band shifts slightly from 3390 to 3375 cm⁻¹, reflecting changes in the hydrogen-bonding and chemical environment of hydroxyl- and amine-containing groups introduced by ALG–DTPA.⁵⁴ The intensified absorptions at 1594 and 1416 cm⁻¹ can be assigned to the asymmetric and symmetric stretching vibrations of carboxylate (COO⁻) groups, respectively, indicating the increased contribution of carboxyl-containing functionalities from alginate and DTPA.⁵⁵ The pronounced COO⁻ bands are particularly relevant to the chelating functionality of ZZO@ALG–DTPA, as these groups can provide coordination sites for metal-ion binding. Meanwhile, the contribution of N-containing groups from DTPA further enriches the potential chelating sites on the functionalized surface. The shift and enhancement of these characteristic bands, together with the retention of the T–O–T vibrations of ZZO, support the successful incorporation of the ALG–DTPA functional layer onto the metal oxide framework.

Following Pb²⁺ adsorption, ZZO@ALG–DTPA-Pb²⁺ exhibits clear spectral changes indicative of metal coordination. The O–H/N–H vibration shifts to 3370 cm⁻¹, while the bands at 1580 and 1408 cm⁻¹ assigned to C–N and C–O groups undergo noticeable shifts and intensity alterations. New shoulders at 977 and 421 cm⁻¹ appear, consistent with the perturbation of C–O and T–O–T environments due to Pb²⁺ binding. These changes collectively demonstrate the interaction of Pb²⁺ with carboxyl, hydroxyl, and nitrogen-containing groups from the alginate-DTPA matrix. For the dye-loaded sample ZZO@ALG–DTPA-MB, the O–H/N–H vibration is observed at 3399 cm⁻¹, higher than that of ZZO@ALG–DTPA, suggesting enhanced hydrogen bonding after MB adsorption. The characteristic C–N band at 1568 cm⁻¹ becomes more intense, and the band corresponding to C–O stretching shifts to 986 cm⁻¹. In addition, a new absorption band emerging at around 1193 cm⁻¹ is observed, which can be attributed to the aromatic C–N⁺ stretching of MB molecules.⁵⁶ These modifications are consistent with the attachment of MB molecules onto the functionalized surface through electrostatic attraction, hydrogen bonding, and interactions involving amine and aromatic groups.

The TGA curves of ZZO and ZZO@ALG–DTPA (Fig. 1c) display markedly distinct thermal decomposition behaviors. For ungrafted ZZO, a slight reduction in mass is observed below 100 °C, associated with the elimination of physically retained water and loosely attached hydroxyl species. Afterward, the weight remains nearly constant, preserving approximately 83 wt% at 700 °C, demonstrating the excellent thermal resistance of the oxide-zeolite framework originating from the calcination of ZIF-90. Conversely, the ZZO@ALG–DTPA sorbent presents successive mass-loss steps. The initial stage occurring below 115 °C is attributed to the evaporation of absorbed moisture. The second major stage between 300–350 °C, accompanied by a sharp weight decrease and 77.93 wt% mass loss at 325 °C, is related to the breakdown of ALG chains and the organic moieties of DTPA. The final stage from 325–520 °C is assigned to additional decomposition and carbonization of the residual organic components, leading to 58.16 wt% remaining mass at 520 °C, which stays nearly unchanged up to 700 °C. Although ZZO@ALG–DTPA shows reduced thermal stability compared with ZZO due to its higher organic fraction, the persistence of a significant residue confirms the preservation of a robust inorganic skeleton. These findings validate the organic–inorganic hybrid structure of ZZO@ALG–DTPA and indicate that the ALG–DTPA polymer coating provides abundant functional sites for adsorption while retaining sufficient structural stability for practical wastewater treatment operations.

SEM observations clearly illustrate the morphological evolution of the material through 3 stages ZZO, ZZO@ALG–DTPA, and ZZO@ALG–DTPA after adsorption, while the accompanying EDX spectra validate compositional changes associated with surface modification and pollutant capture. For the pristine ZZO (Fig. 2(a and b)), the particles exhibit a coarse and compact agglomerated structure with irregular particle surfaces and distinct plate-like or crystallite-like features. The observed morphology reflects the heterogeneous surface structure of the starting inorganic material, although the specific phases responsible for individual morphological features cannot be determined solely from the SEM images. The EDX spectrum confirms that the material consists primarily of O, Si, Al, and Zn, with minor amounts of Na, Mg, and Ca, consistent with zeolitic phases. A very low carbon signal indicates the absence of an organic coating on the surface. Following decoration with DTPA and ALG (Fig. 2c and d), the morphology transforms notably. The surface appears smoother and the particles are more extensively aggregated, consistent with the incorporation of the organic components. After adsorption of Pb²⁺ (Fig. 2e and f), the SEM images show a denser surface morphology and increased particle aggregation compared with the ZZO@ALG–DTPA composite before adsorption. These changes in surface morphology are accompanied by the appearance of Pb in the EDX spectrum, supporting the incorporation of Pb²⁺ onto the composite surface. EDX spectra display clear Pb peaks near 2.3–2.5 keV and ∼10–12 keV, and Pb content increases substantially (7.42%) was detected in the adsorbed sample. The presence of Pb in the EDX spectrum provides direct evidence of Pb-containing species on the composite surface. Minor shifts in Na, Ca, and Mg contents imply ion-exchange processes during adsorption. These findings indicate that Pb removal proceeds via a combination of complexation and ion exchange. In contrast, after adsorption of MB, the SEM images reveal changes in surface morphology, with a more compact appearance and increased surface irregularity compared with the pre-adsorption sample. No distinct particulate deposits or continuous surface film can be clearly distinguished from the SEM images. EDX confirms the presence of S (6.55%), characteristic of the sulfur-containing thiazine ring of MB, alongside slight increases in C and N relative to the pre-adsorption sample. The Si, Al, and Zn signals remain clearly detectable after MB adsorption, while the appearance of S and the changes in the relative elemental composition indicate the presence of MB-associated species on the composite surface. In summary, SEM and EDX analyses demonstrate that ALG–DTPA modification fundamentally alters the surface characteristics of ZZO, enhances active functional sites, and is accompanied by distinct morphological and compositional changes following adsorption of Pb²⁺ and MB. For MB, the changes are reflected in the surface morphology and the incorporation of S-containing species, whereas the characteristic inorganic elements of the ZZO@ALG–DTPA composite remain detectable. These observations, together with the elemental composition obtained from EDX, support the successful incorporation of MB onto the ALG–DTPA-modified surface.

Fig. 2. (a) SEM and EDX of (a, b and i) ZZO, (c and d) ZZO@ALG–DTPA, (e, f and j) ZZO@ALG–DTPA-Pb²⁺, and (g, h and k) ZZO@ALG–DTPA-MB.

As illustrated in the TEM micrographs at various magnifications (Fig. 3a–d), the ZZO@ALG–DTPA composite exhibits a distinctive nanoscale architecture following surface modification. At lower magnification, the sample displays loosely aggregated domains with blurred boundaries, coated by a uniform polymeric layer, suggesting efficient incorporation and dispersion of the oxide phase within the alginate–DTPA framework. When the structure is examined at intermediate magnifications, elongated oxide nanostructures (nanorods/nanofibers) become evident, distributed throughout the polymer network without noticeable agglomeration, indicative of a strong anchoring effect that restricts particle growth. At the highest magnification, the oxide species appear as densely packed nano-sized entities (tens of nanometers) with random spatial orientation, confirming that ALG–DTPA modification effectively prevents the collapse and enlargement of oxide crystallites during formation. This open and highly porous nanostructure provides numerous exposed active sites and enhances surface accessibility of –COOH and –NH₂ functional groups, which is advantageous for adsorption processes. Collectively, the TEM observations demonstrate that the ALG–DTPA polymer matrix creates a stable dispersion environment, suppresses particle aggregation, and promotes the development of a highly accessible, reactive surface, making the material particularly suitable for simultaneous remediation of organic and metal species contaminants.

Fig. 3. TEM images of the (a–d) ZZO@ALG–DTPA.

The N₂ adsorption–desorption profiles of ZZO and ZZO@ALG–DTPA (Fig. 4a) reveal type-IV isothermal accompanied by H3 hysteresis loops, characteristic of mesoporous frameworks with slit-shaped pores. For the pristine ZZO sample, the sharp uptake at high relative pressures (0.8 < P/P₀) suggests the presence of an interconnected open-pore framework and substantial large-pore volume. Following surface modification with alginate–DTPA, the hysteresis loop becomes narrower and steeper, implying that portions of the mesopores are partially blocked or constricted by the deposited polymer layer. This structural evolution is also verified by the BJH pore size distribution curves (Fig. 4b), in which both ZZO and ZZO@ALG–DTPA exhibit broad mesopore distributions with the main contribution centered around approximately 20 nm. The distribution after ALG–DTPA modification shows only a subtle change compared with pristine ZZO, with no clear evidence of a transition toward a localized smaller-pore range. Quantitative textural parameters (Table S1) reinforce these observations. The original ZZO displays a high BET surface area of 362.97 m² g⁻¹, with 145.79 m² g⁻¹ contributed by micropores, indicating a well-developed micro–mesoporous capillary network. Its total pore volume reaches 2.20 cm³ g⁻¹, including 0.24 cm³ g⁻¹ of micropores, and an average pore width of 10.29 nm, representing the calculated average pore width rather than the position of the maximum in the BJH distribution. After ALG–DTPA deposition, the material shows a significant reduction in surface area to 195.39 m² g⁻¹ (≈46% decrease) and a sharp decline in microporosity (micropore area 40.35 m² g⁻¹, micropore volume 0.07 cm³ g⁻¹). Interestingly, the total pore volume remains almost unchanged (2.25 cm³ g⁻¹), while the average pore size increases to 11.49 nm, indicating a modest change in the calculated average pore width rather than a substantial enlargement of the mesopores. Despite the decrease in specific surface area and microporosity, ZZO@ALG–DTPA exhibits markedly enhanced adsorption for both Pb²⁺ and MB. This improvement indicates that the uptake is governed primarily by chemical interactions introduced by the –COOH and –NH₂ functional groups, including complexation and ion-exchange mechanisms with Pb²⁺ and electrostatic interactions with MB, rather than relying on purely physical surface area as in the unmodified ZZO. Therefore, ALG–DTPA modification enriches the surface with additional functional groups and alters the surface chemical environment, providing additional sites and stronger surface interactions that contribute to the enhanced adsorption performance of ZZO@ALG–DTPA.

Fig. 4. N₂ adsorption/desorption and pore size distributions of the (a) ZZO and (b) ZZO@ALG–DTPA.

3.2. Removal performance

3.2.1. Optimization of modifying conditions

The removal efficiencies of Pb²⁺ and MB for ALG, DTPA, ZZO, ZZO–DTPA, and the ZZO@ALG–DTPA composite are presented in Fig. S2a. For each material, 20 mg was contacted with 100 mL of solution containing either Pb²⁺ (40 mg L⁻¹) or MB (20 mg L⁻¹). After the contact period, ALG, ZZO, ZZO–DTPA, and ZZO@ALG–DTPA were separated by centrifugation, whereas DTPA, as a water-soluble chelating agent, remained in the aqueous phase and interacted with Pb²⁺ through complex formation. Thus, the Pb²⁺ removal observed for DTPA represents its contribution through aqueous-phase Pb²⁺ complexation and provides a reference for evaluating the role of DTPA in the composite. A significant performance disparity can be observed between the composite material and the individual components. For Pb²⁺, ZZO exhibited a removal efficiency of 57.65%, reflecting the contribution of Si–O–Al active sites and surface metal oxides participating in ion exchange. DTPA displayed a slightly higher removal efficiency (60.65%) owing to its abundant –COOH and –NH₂ coordination groups, whereas the ZZO–DTPA composite achieved a markedly higher Pb²⁺ removal efficiency of 85.44%, demonstrating the enhanced contribution of DTPA-functionalized ZZO toward Pb²⁺ binding. Meanwhile alginate showed only 35.65%, attributed to the limited number of accessible sites and its swelling behavior that restricts effective complexation in aqueous media. In stark contrast, the ZZO@ALG–DTPA composite achieved a remarkably high Pb²⁺ removal efficiency of 96.28%, significantly surpassing the individual components. This result confirms a strong synergistic effect arising from the simultaneous contributions of Pb-carboxyl/amine chelation, ion exchange via guluronate units of alginate, and the structural anchoring role of the oxide network, enabling multiple adsorption mechanisms to operate cooperatively. A similar trend was observed for MB adsorption: ALG, ZZO, DTPA, and ZZO–DTPA exhibited removal efficiencies of 56.43, 79.44, 77.55, and 90.87%, respectively, whereas ZZO@ALG–DTPA reached 98.65%. This dramatic improvement is attributed to the polymer layer forming a flexible binding network that provides abundant sites for electrostatic interaction and hydrogen bonding, while the oxide matrix facilitates diffusion of the bulky MB molecules. Additionally, interactions between MB molecules and the functional groups within the coating enhance adsorption, resulting in dense surface coverage as supported by SEM-EDX observations.

3.2.2. Role of the ZZO relative to ALG content

The ZZO/ALG ratio exhibited a pronounced influence on the remediation capability of the sorbent toward both Pb²⁺ and MB (Fig. S2b). As the ratio increased from 1 : 1 to 1 : 2.5, the removal efficiency for Pb²⁺ rose markedly from 77.42 to 96.43%, while that for MB improved from 80.54 to 99.51%. This distinct enhancement suggests that a higher alginate content enhances the dispersion of ZZO particles, effectively suppresses agglomeration, and enables greater exposure of active sites to the solution. In addition, increasing the ALG content increases the relative contribution of alginate-derived carboxylate and hydroxyl groups, which can provide additional metal-binding and hydrogen-bonding sites. However, although the 1 : 2.5 ratio produced the highest removal performance, the improvement relative to the 1 : 2 ratio was marginal. At the 1 : 2 composition, the removal efficiencies already reached 95.54% for Pb²⁺ and 98.78% for MB, indicating that further increasing the alginate content provided little additional benefit. This indicates that excessive alginate may form an overly thick polymer coating that partially shields active centers, hindering direct interaction with metal ions and limiting access to electrostatically active groups. Therefore, the 1 : 2 ratio was identified as the optimal composition, achieving high simultaneous removal of both contaminants while avoiding unnecessary polymer consumption and preserving an effective adsorption architecture. Based on these considerations, the 1 : 2 ZZO/ALG ratio was selected for subsequent experiments, balancing treatment efficiency and material economy.

3.2.3. Role of the ZZO relative to DTPA content

The investigation of the ZZO/DTPA mass ratio revealed a characteristic non-linear adsorption behavior for both Pb²⁺ and MB (Fig. S2c). At the low ratio of 1 : 1, the removal efficiencies were modest, with values of 60.55% for Pb²⁺ and 75.55% for MB, indicating that the limited amount of DTPA provided an insufficient density of coordination and electrostatic interaction sites. As the ratio increased to 1 : 2 and reached an optimum at 1 : 3, the adsorption efficiencies rose sharply, with maximum values of 95.54% for Pb²⁺ and 98.63% for MB. This enhancement reflects the creation of a more stable hybrid framework in which the increased density of DTPA-derived carboxyl and amine groups provides abundant chelating and coordination sites, thereby promoting efficient complexation with Pb²⁺ and strengthening electrostatic and hydrogen-bonding interactions with MB molecules. When the DTPA content increased further to 1 : 4, a slight decline in elimination performance was discovered, which can be ascribed to excessive organic coverage shielding the vacant sites on the ZZO interface and reducing pollutant accessibility. Overall, the mass ratio of 1 : 3 represents the optimal balance between functional group density and surface permeability, demonstrating the synergistic contribution of the oxide framework and the complexing ligand in enhancing the treatment efficiency for both metal ions and organic dyes. Taken together, the optimization of the ZZO/ALG and ZZO/DTPA ratios indicates that the relative contributions of ALG and DTPA were effectively incorporated into the composite design, with their functional sites complementing rather than simply increasing the total number of adsorption sites.

3.2.4. Effect of duration time

The adsorption kinetics profiles showed that both Pb²⁺ and MB were removed extremely rapidly at the initial stage and achieved near-equilibrium after approximately 20 min (Fig. S3). Within the first 5 min, the removal efficiencies already reached 85.43% for Pb²⁺ and 88.66% for MB, indicating a very fast adsorption rate driven by the high density of accessible empty sites on the ZZO@ALG–DTPA framework and the strong affinity of its carboxyl and amine groups toward both metal ions and dye molecules. The efficiencies continued to increase and reached 95.56% for Pb²⁺ and 98.34% for MB at 20 min, suggesting that after the rapid initial uptake, adsorption progressed mainly through diffusion into remaining sites of lower affinity. After 20 min, the kinetic curves gradually leveled off, reaching stable maximum values of 96.98% for Pb²⁺ and 98.85% for MB at 60 min, revealing the rapid attainment of uptake equilibrium. The fast saturation of active sites reflects not only the high uptake capability but also the predominance of chemisorption, which aligns with the second-order kinetic model and demonstrates the material's suitability for practical applications requiring short treatment times and high removal efficiency.⁵⁷

The kinetic evaluation provided clear evidence of how Pb²⁺ and MB were adsorbed onto the ZZO@ALG–DTPA composite and the rate-determining processes involved (Fig. 5a–d and Table 1). The second-order equation offered an excellent fit for both contaminants, with R² values reaching 0.999. Moreover, the calculated equilibrium capacities (199.64 mg g⁻¹ for Pb²⁺ and 99.61 mg g⁻¹ for MB) matched closely with the experimental q_e values, verifying that chemisorption mediated by ligand exchange or strong electrostatic attraction between –COOH/–NH₂ functionalities and the adsorbates governed the overall kinetics.⁵⁸ In contrast, the pseudo-first-order model predicted unrealistically low q_e values (3.41 and 1.89 mg g⁻¹) and produced much weaker correlations, confirming that physisorption alone cannot account for the adsorption behavior. Insights from the intraparticle diffusion model discovered that the uptake proceeded through 2 kinetic regions. The plots did not intersect the origin, and the initial stage exhibited large diffusion coefficients (k_diff = 60.79 mg (g⁻¹ min⁻¹) for Pb²⁺ and 31.93 mg (g⁻¹ min⁻¹) for MB), corresponding to rapid mass transfer from the bulk solution to the exterior surface of the material. This was followed by a second, markedly slower stage, where k_diff decreased sharply (0.883 and 0.138 mg (g⁻¹ min⁻¹)), representing restricted diffusion within inner pore structures until equilibrium was attained. The high C values (187.24 and 97.53) in this region further indicated a pronounced boundary-layer effect and highlighted the role of surface diffusion.⁵⁹ The Elovich model also supported a chemisorption-dominated pathway, as reflected by its α coefficients, higher for Pb²⁺ (9.33 mg g⁻¹ min⁻¹) than for MB (3.69 mg g⁻¹ min⁻¹) and by β values related to activation energies on a heterogeneous surface.⁶⁰ Taken together, these analyses consistently demonstrate that adsorption on ZZO@ALG–DTPA is primarily controlled by chemical interactions on an energetically diverse surface, with pore diffusion acting as an auxiliary process.

Fig. 5. Uptake kinetics (a) intraparticle diffusion (b) first-, (c) second-order, and (d) Elovich models.

Table 1. Kinetic sorption parameters.

	Parameters	Value
		Pb2+	MB
Elovich	α (mg g−1 min−1)	9.33	3.69
	R 2	0.873	0.702
	β (g mg−1)	158.57	85.01
Second-order	q e,exp (mg g−1)	193.94	98.85
	R 2	0.999	0.999
	k 2 (g mg−1 min−1)	0.01	0.005
	q e,cal (mg g−1)	199.64	99.61
Intra-particle diffusion	First-stage
	R 1 2	0.888	0.900
	C	7.91	3.91
	k diff (mg g−1 min−1)	60.79	31.93
	Two-stage
	R 2 2	0.954	0.999
	C	187.24	97.53
	k diff (mg g−1 min−1)	0.883	0.138
First order	R 2	0.928	0.314
	q e,cal (mg g−1)	3.41	1.89
	k 1 (min−1)	0.09	0.06

3.2.5. Effect of pH

The pH-dependent uptake behavior of ZZO@ALG–DTPA reveals a highly sensitive interplay between surface-charge regulation, ligand protonation equilibria, and the speciation of both pollutants in solution (Fig. 6a).⁶¹ The pH speciation diagram of Pb²⁺ (Fig. S3b) further shows the transition from predominantly free Pb²⁺ species at near-neutral pH to hydrolyzed Pb species under alkaline conditions, supporting the observed pH-dependent adsorption behavior. Because the material exhibits a pH_pzc of 6.3 (Fig. 6b), even small shifts in pH induce substantial reorganization of the electrostatic microenvironment and the accessibility of chelating groups, thereby dictating distinct adsorption pathways for Pb²⁺ and MB. For Pb²⁺, the adsorption efficiency reached its maximum (96.48%) at pH 7, where the surface of ZZO@ALG–DTPA becomes predominantly negatively charged, and the DTPA-based coordination sites are present in their optimal deprotonated state.⁶² Under this condition, 2 mechanisms operate synergistically: (i) long-range coulombic attraction that accelerates the migration and accumulation of Pb²⁺ near the surface and (ii) short-range inner-sphere complexation between Pb²⁺ and multidentate –COO⁻/–NH groups, which stabilizes strongly bound surface complexes. This cooperative effect explains the sharp rise in adsorption efficiency immediately after crossing the pH_pzc. In contrast, at acidic pH (3–5), the adsorption capacity decreases markedly (70.4 3–85.29%) due to a combination of factors intrinsic to the proton-rich environment. First, protonation of carboxylate and amine groups suppresses their electron-donating ability, diminishing the formation of stable chelating Pb-ligand complexes.⁶³ Second, the surface attains a net positive charge, reversing the direction of electrostatic forces and generating substantial repulsion against Pb²⁺. Third, competitive adsorption of H⁺ at active sites lowers the density of available complexation centers. The simultaneous suppression of both long-range (electrostatic) and short-range (coordination) interactions results in a steep decline in metal uptake.⁶⁰ A different limitation emerges in the alkaline region (pH 9–11), where the surface remains strongly negative, yet the adsorption decreases again (down to 74.81%). This decline is governed not by surface chemistry but by solution speciation: Pb²⁺ undergoes progressive hydrolysis to Pb(OH)⁺, Pb(OH)_2, and other hydroxo complexes with significantly lower affinity toward DTPA and carboxylate groups.⁶⁴ The progressive decrease of dissolved Pb²⁺ concentration diminishes the thermodynamic driving force for surface complexation, thereby resulting in incomplete occupation of active sites even though the material maintains a favorable charge state. This phenomenon highlights that the adsorption limit at high pH is controlled by equilibrium thermodynamics of metal hydrolysis rather than the intrinsic capacity of the composite.

Fig. 6. Effect of (a) pH, (b) pH_pzc, (c and d) ZZO@ALG–DTPA mass, and (e and f) pollutants content. The error bars demonstrate the reproducibility of 3 repetitions.

For MB, a cationic dye with a planar aromatic structure, the adsorption trend contrasts sharply with the behavior of Pb²⁺. As the pH increases, MB removal follows a monotonic rise, exceeding 98% from pH ≥ 7. Above the pH_pzc, the negatively charged surface significantly enhances electrostatic attraction toward MB, enabling rapid diffusion and accumulation at the interface.⁴⁵ Beyond electrostatic forces, the deprotonated functional groups in the ZZO@ALG–DTPA coating provide additional favorable interactions with MB, thereby contributing to its enhanced uptake performance. These combined electrostatic interactions, π–π, and non-polar, reinforce each other at neutral to alkaline pH, leading to near-quantitative removal. Conversely, at pH 3, extensive protonation of functional groups renders the surface positively charged and disrupts π-electron density, thereby generating strong repulsion toward MB and reducing the adsorption efficiency to its minimum (80.44%).⁶⁵ Collectively, these results demonstrate that pH = 7 provides a mechanistically optimal operating point for the co-removal of Pb²⁺ and MB. At this pH, the material achieves (i) a sufficiently negative surface potential to maximize mass-transfer-driven accumulation of both pollutants, (ii) full accessibility of chelating groups for Pb²⁺ complexation, and (iii) enhanced π–π and electrostatic interactions for efficient MB adsorption. The convergence of these conditions underscores the crucial role of pH-induced interfacial chemistry in governing the dual-pollutant removal capability of composite sorbent.

3.2.6. Effect of ZZO@ALG–DTPA dosage

The variation of ZZO@ALG–DTPA adsorbent dosage from 10 to 30 mg exposed a core tension inherent to adsorption systems (Fig. 6c and d), where a direct and unavoidable competition between the metrics of removal efficiency (R_e) and adsorption capacity (q_e) for both Pb²⁺ and MB.⁶⁶ Experimentally, escalating the dosage delivered near-quantitative removal, driving efficiencies from 78.67 to 99.57% for Pb²⁺ and from 80.29 to 99.99% for MB, a predictable consequence of increasing the population of active sites. Yet, this gain came at a severe cost to the material's intrinsic capacity, which plummeted from 313.72 to 132.76 mg g⁻¹ for Pb²⁺ and from 160.86 to 65.65 mg g⁻¹ for MB. The underlying mechanism is one of diminishing returns: at lower dosages, the system operates in a site-limited regime where every active center is saturated, which leads to maximizing capacity.⁶⁷ In contrast, higher dosages force the system into a particle-interaction regime, where overcrowding and aggregation occlude accessible surface area and create an unsaturated, inefficient state per unit mass of adsorbent.⁶⁸ An amount of 20 mg was thus identified as the operational optimum, representing a point of equilibrium that reconciles high decontamination performance, with remediation efficiencies of 96.54% for Pb²⁺ and 98.39% for MB, while still maintaining meaningful elimination capabilities of 193.08 mg g⁻¹ for Pb²⁺ and 98.39 mg g⁻¹ for MB. The above balance establishes a viable and cost-effective paradigm for practical implementation.

3.2.7. Effect of Pb²⁺ content

When challenged with higher initial concentrations, the adsorption kinetics of ZZO@ALG–DTPA devolved into a complex competition, a direct tug-of-war between the density of pollutants and the finite capacity of the material (Fig. 6e and f). In the low concentration range from 10 to 40 mg L⁻¹, the material showed excellent capture ability with removal efficiency (R_e) reaching 99.46–98.91% for Pb²⁺ and 99.29–90.55% for MB. During this period, the adsorption capacity (q_e) increased almost linearly from 49.25 to 193.1 mg g⁻¹ for Pb²⁺ and from 49.72 to 181.12 mg g⁻¹ for MB, demonstrating a significant surplus of active sites relative to the pollutant concentration, allowing the adsorption process to occur almost entirely through both chemical complexation and physical interaction mechanisms.⁶⁹ As the concentration increased to the intermediate range (100 mg L⁻¹), the removal efficiency began to decline markedly, reaching 90.39% for Pb²⁺ and 80.57% for MB, while the adsorption capacity continued to increase to 452.15 mg g⁻¹ and 402.65 mg g⁻¹, respectively. This behavior indicates that, with increasing initial adsorbate concentration, a progressively smaller fraction of the adsorbate could be removed, while the amount adsorbed per unit mass of adsorbent continued to increase.¹⁹ At high concentrations of 200 and 300 mg L⁻¹, the removal efficiency declined markedly, dropping to 79.87 and 69.99% for Pb²⁺ and to 72.32% and 61.42% for MB, respectively. This behavior reflects the increasing adsorbate loading relative to the available adsorption capacity of the material, resulting in progressively lower removal efficiencies at higher initial concentrations.⁷⁰ Meanwhile, the adsorption capacity approached its saturation threshold, reaching 1049.85 mg g⁻¹ for Pb²⁺ and 921.35 mg g⁻¹ for MB. Overall, these results indicate that the material performs optimally up to about 100 mg L⁻¹ for Pb²⁺ and 40 mg L⁻¹ for MB while further increases in initial concentration progressively limit the removal efficiency despite the continued increase in adsorption capacity.

3.2.8. Effect of various ions

Interference experiments involving mono- and multivalent cations as well as common anions further elucidate how competitive adsorption, ionic strength, hydration behavior, and charge-screening effects collectively influence the adsorption pathways of ZZO@ALG–DTPA. A concentration of 0.1 M was used for each cation and anion in the individual interference experiments, and the concentration of each ion was maintained at 0.1 M in the “All” experiment. Although the presence of background ions introduced measurable variations in performance, the composite retained a consistently high removal capacity, underscoring its strong intrinsic affinity for both Pb²⁺ and MB (Fig. S4a). For Pb²⁺, the inhibition sequence correlated closely with the charge density of competing cations. Monovalent species (Na⁺, K⁺, NH₄⁺) caused only marginal suppression (88.55–92.43%), indicating that singly charged ions exert insufficient electrostatic competition to disrupt the polydentate Pb-DTPA coordination.⁷¹ Their relatively large hydrated radii and weak surface-binding tendencies minimize their ability to occupy carboxylate and amine sites on the composite. In contrast, divalent ions, particularly Ca²⁺, produced a much more pronounced decrease, with Ca²⁺ lowering Pb²⁺ removal by 50.43%. This strong inhibitory effect reflects the high charge density and smaller hydration sphere of Ca²⁺, facilitating its interaction with deprotonated –COO⁻ groups and enabling competitive binding at the same chelation sites targeted by Pb²⁺. This finding is particularly relevant to real wastewater, where Ca²⁺ is often present at concentrations considerably higher than those of Pb²⁺, potentially resulting in a high Ca²⁺/Pb²⁺ concentration ratio and consequently stronger competition for the available binding sites. Thus, the substantial suppression by Ca²⁺ suggests that Pb²⁺ removal in high-hardness wastewater may be lower than that observed in single-solute systems, and Ca²⁺ interference should be considered when assessing the practical selectivity of the composite. When all cations coexisted, the removal efficiency (85.62%) exceeded that of Ca²⁺ alone, implying that competitive interplay among coexisting ions limits the dominant suppression imposed by any individual species.⁷² Maintaining Pb²⁺ removal above 85% even in this multicomponent system demonstrates the robustness of the chelation-driven mechanism, a critical feature for real wastewater matrices. MB adsorption exhibited similar but consistently weaker cation-related effects. Monovalent ions again produced negligible suppression (89–94%), consistent with an adsorption pathway governed primarily by electrostatic attraction and π–π interactions rather than metal-like coordination. Even with Mg²⁺ or Ca²⁺ present, MB removal exceeded 81%, confirming that its adsorption relies largely on surface-driven interactions that are not easily displaced by competing cations. The mixed-cation condition maintained an efficiency of 88.32%, further demonstrating the resilience of MB adsorption to ionic competition.

Anion interference followed a distinct mechanistic pattern (Fig. S4b). For Pb²⁺, SO₄²⁻ induced the weakest inhibition (90.23%), although the low solubility of PbSO₄ suggests that precipitation may also influence the observed Pb²⁺ removal under these conditions. NO₃⁻ produced more substantial suppression (70.42%), which is more reasonably attributed to increased ionic strength and electrostatic screening of the Pb²⁺ charge near the negatively charged adsorbent surface rather than to precipitation. CO₃²⁻ caused a moderate reduction in performance, lowering the Pb²⁺ removal efficiency to 74.43%. Given the very low solubility of PbCO₃, the presence of CO₃²⁻ may promote the formation of Pb-carbonate species in solution, thereby reducing the concentration of dissolved Pb²⁺ available for adsorption.⁷³ Under mixed-anion conditions, Pb²⁺ removal remained high and reached 81.32%, indicating that the overall Pb²⁺ removal remained substantial despite the presence of competing anions and possible changes in Pb speciation. For MB, the influence of anions was uniformly mild. Even in the presence of CO₃²⁻ or SO₄²⁻, MB adsorption consistently exceeded 85%, and the mixed-anion condition achieved 88.44%. These results indicate that the presence of the tested anions had a limited effect on MB adsorption under the investigated conditions. Collectively, these findings show that ZZO@ALG–DTPA possesses strong resistance to ionic interference. For Pb²⁺, the observed effects of the anions likely reflect a combination of changes in Pb speciation, possible precipitation of sparingly soluble Pb-containing species, and electrostatic screening.

3.2.9. Elimination of multiple contaminants

The pollutant removal capability of ZZO@ALG–DTPA strongly depends on the intrinsic chemical properties and molecular configurations of the target contaminants (Fig. S4c). This study was designed to probe how the material responds when confronted with pollutants that differ fundamentally in molecular structure and interaction behavior, namely heavy metals, cationic dyes, and antibiotics, a combination frequently encountered in real wastewater and known for generating competitive and often unpredictable adsorption pathways. To obtain an unbiased assessment of the material's selectivity and its practical treatment capability, the evaluation was carried out under a unified experimental framework: heavy metals were tested at 40 mg L⁻¹, dyes and antibiotics at 20 mg L⁻¹, with a fixed material dosage of 0.2 g L⁻¹, and all uptake conducted at the native pH of the environment. This systematic approach allows the adsorption behavior across pollutant classes to be compared on equal footing, thereby revealing the intrinsic affinity and interference resistance of the material under realistic operating conditions. Among the examined species, cationic dyes exhibited the highest adsorption performance, with efficiencies of 99.65% for MB and 98.76% for JGB, followed by Pb²⁺ at 96.76%. The superior affinity toward MB and JGB is attributable to the combined effects of electrostatic attraction, hydrogen bonding, surface complexation, and other favorable interactions between the dye molecules and the functional groups and active adsorption sites of ZZO@ALG–DTPA. Pb²⁺ adsorption is predominantly governed by strong polydentate chelation with the DTPA moieties, forming highly stable coordination complexes. Conversely, transition metal ions such as Ni²⁺ (70.45%) and Cu²⁺ (80.33%) displayed noticeably lower uptake, which can be correlated with their higher hydration energies and ionic radii, reducing the likelihood of effective coordination with surface functional groups. Similarly, larger and more structurally complex pharmaceuticals, including TCH (60.33%), TA (65.45%), and CIP (75.45%), exhibited reduced removal efficiency. Their bulky conformations and multifunctional amphoteric groups likely induce steric hindrance and competitive interactions, restricting accessibility to empty sites on the material's framework. Overall, the findings demonstrate that ZZO@ALG–DTPA preferentially adsorbs simple cationic dye molecules and metal ions capable of forming strong coordination bonds, verifying its high potential for practical wastewater treatment involving diverse coexisting pollutants, particularly mixtures of toxic heavy metals and cationic organic dyes.

3.3. Uptake isothermal

Analysis of the adsorption isotherms using 4 classical models provided deeper insight into how ZZO@ALG–DTPA interacts with each contaminant (Fig. 7a, d and Table 2). Although nonlinear regression is widely adopted because it minimizes potential errors arising from mathematical linearization,^74–76 the linearized forms were retained in this study to provide a consistent basis for directly comparing the fitting performance and characteristic parameters of the four isotherm models. The Langmuir equation gave the best overall description for both Pb²⁺ and MB, with strong correlation coefficients (R² = 0.988 and 0.975, respectively), indicating that the Langmuir model provides the most suitable empirical description of the adsorption data among the models evaluated. This result is consistent with predominant monolayer adsorption, although the isotherm fitting alone does not establish a specific adsorption mechanism or uniformity of all adsorption site.³⁹ Nonetheless, the distinct chemical behaviors of the 2 pollutants became evident through their Langmuir parameters. Pb²⁺ exhibited a markedly higher maximum capacity (q_max = 1114.70 mg g⁻¹) compared with MB (787.41 mg g⁻¹), consistent with the strong complexation affinity of –COO⁻ and –NH₂ groups toward divalent metal ions, a highly selective and directional interaction.⁶¹ Although MB also followed Langmuir-type adsorption, its binding is more likely associated with the electrostatic attraction between the cationic MB molecules and the negatively charged surface, with the enhanced surface charge density at higher DTPA content favoring MB uptake. The Freundlich model simultaneously produced high-quality fits (R² = 0.980 for Pb²⁺ and 0.971 for MB), indicating that this model also provides a reasonable description of the adsorption data and that the surface cannot be characterized solely on the basis of the Langmuir fit. Importantly, the 1/n values (0.48 for Pb²⁺ and 0.38 for MB) reveal meaningful differences. The value of ∼0.5 for Pb²⁺ reflects moderately favorable adsorption within the Freundlich framework.⁷⁷ The smaller 1/n for MB indicates a higher adsorption favorability according to the Freundlich parameter, but does not by itself demonstrate greater surface heterogeneity or adsorbate–adsorbate interactions. The Temkin model yielded comparatively lower correlations (R² = 0.7–0.8), implying that the heat of adsorption does not decline linearly and that adsorbate–adsorbate interactions are more complex than the model assumes.⁷⁸ Importantly, the Dubinin–Radushkevich analysis produced exceptionally high adsorption energy values (E_D–R = 2224.97 kJ mol⁻¹ for Pb²⁺ and 3429.97 kJ mol⁻¹ for MB), far surpassing the common 8 kJ mol⁻¹ boundary for chemisorption.⁷⁹ However, these unusually high values should be interpreted with caution, as the D–R model parameter alone is insufficient to establish the formation of specific chemical bonds. Altogether, the results show that the Langmuir model provides the best overall fit, while the favorable Freundlich parameters indicate that alternative descriptions of the adsorption behavior are also possible. The differences between Pb²⁺ and MB adsorption are likely related to their distinct physicochemical characteristics, including charge, interaction with the functional groups, and molecular size.

Fig. 7. Uptake isotherms (a) Langmuir (b) Dubinin Radushkevich, (c) Temkin, and (d) Freundlich models.

Table 2. Isotherm sorption parameters.

Equations	Properties
Langmuir		RMSE	K L (L mg−1)	p	R 2	q max (mg g−1)
	Pb2+	0.002	0.0009	0.021	0.988	1114.70
	MB	0.003	0.0013	0.025	0.975	787.41
Temkin		RMSE	K T (L mg−1)	p	R 2	B T (J mol−1)
	Pb2+	191.98	157.16	0.154	0.723	223.02
	MB	131.18	107.92	0.147	0.781	233.26
Dubunin–Radushkevich		RMSE	E L (kJ mol−1)	p	R 2	K D–R (mol2 kJ−2)
	Pb2+	0.513	2224.97	0.050	0.626	1.01 × 10−7
	MB	0.661	3429.97	0.055	0.544	4.25 × 10−8
Freundlich		RMSE	1/n	p	R 2	K F (L mg−1)
	Pb2+	0.35	0.48	0.056	0.980	4.92
	MB	0.31	0.38	0.055	0.971	4.94

3.4. Uptake thermodynamic

Spontaneous adsorption does not occur arbitrarily, it results from the combined effects of energetic stabilization and increased molecular disorder. In environmental systems, assessing both of these contributions through enthalpy and entropy changes is essential for understanding whether a remediation process is thermodynamically favorable and how it proceeds at the molecular level. In the present work, the thermodynamic characteristics of Pb²⁺ and MB uptake on the ZZO@ALG–DTPA was quantified by evaluating the standard Gibbs free energy (ΔG°), entropy (ΔS°), and enthalpy (ΔH°) changes. These quantities were derived from equilibrium constants obtained at multiple temperatures and calculated using the conventional thermodynamic expressions shown below:^80–82

ΔG° = −RT ln K_c 14

15

16

In this context, C_e denotes the equilibrium content in solution (mg L⁻¹), from which the equilibrium constant K_c is obtained. The thermodynamic calculations were implemented utilizing the absolute temperature T (K) and the universal gas constant R = 8.314 J mol⁻¹ K⁻¹. The corresponding equilibrium uptake, q_e (mg g⁻¹), reflects the mass of pollutant retained by the material at equilibrium.

Thermodynamic parameters offer valuable insights into the uptake mechanisms of Pb²⁺ and MB on the ZZO@ALG–DTPA composite (Fig. 8a, b and Table S2). The negative values of ΔG° across the studied temperature range (−13.98 to −11.23 kJ mol⁻¹ for Pb²⁺ and −16.61 to −10.64 kJ mol⁻¹ for MB) demonstrate that both adsorption processes occur spontaneously.^83,84 The gradual increase of ΔG° with temperature indicates a reduction in spontaneity at elevated temperatures, implying that the process is more favorable under lower thermal conditions. This tendency is more pronounced for MB, reflecting its sensitivity to temperature-related weakening of weak physical forces. The markedly negative ΔH° values (−38.69 kJ mol⁻¹ for Pb²⁺ and −89.66 kJ mol⁻¹ for MB) confirm that the uptake is exothermic and further reveal the substantial strength of the adsorption interactions. According to Deng et al.,⁸⁵ enthalpy values in the range of 20–420 kJ mol⁻¹ typically correspond to chemisorption, whereas values below 20 kJ mol⁻¹ are characteristic of multilayer or weak physical adsorption. Hence, the significantly larger ΔH° for MB suggests the involvement of multiple concurrent mechanisms (electrostatic attraction, hydrogen bond formation, and van der Waals interactions), whereas Pb²⁺ adsorption predominantly proceeds through surface complexation at functional groups including –COOH and –NH₂. The highly negative ΔS° values (−81.81 J mol⁻¹ K⁻¹ for Pb²⁺ and −242.02 J mol⁻¹ K⁻¹ for MB) indicate decreased randomness at the solid-solution interface during adsorption. As highlighted by Nguyen et al., negative ΔS° is associated with the formation of highly ordered adsorbate structures on the adsorbent surface.⁸⁶ The much larger entropy loss for MB suggests ordered molecular assembly and tight packing on the polymer-oxide surface, while Pb²⁺ ions are individually coordinated to discrete active sites. Overall, the thermodynamic parameters of Pb²⁺ are consistent with findings reported by Pham et al. on metal-biochar adsorption,⁸⁷ whereas the values obtained for MB closely match those of Alqadami et al. for dye adsorption on MOF-based adsorbents.⁸⁸ This strong agreement with the literature validates the accuracy of the experimental results and confirms the practical relevance of the ZZO@ALG–DTPA framework for real wastewater treatment applications.

Fig. 8. Effect (a and b) temperature on the Pb²⁺ and MB sorption.

3.5. Mechanism of contaminants elimination

The mechanism of Pb²⁺ and MB capture on ZZO@ALG–DTPA is a multistep process, in which directional chemical interactions (chemisorption) determine the ultimate stability of the adsorbent, while physical interactions and structural effects (diffusion, confinement) control the adsorption rate and selectivity. This material integrates 3 main functional components: (i) a ZZO (Zn/Si/Al oxide) hybrid oxide phase providing Lewis acid (M–O) centers and a mesoporous framework, (ii) an alginate network containing guluronate/mannuronate units with ion-exchangeable –COOH/–OH groups, (iii) DTPA bound or grafted onto the polymer, acting as a multidentate ligand, supplying concentrated O and N donors for complexation.⁸⁹ The combination not only increases the number of active sites but also diversifies the site types (Lewis acid, Brønsted base, chelating donor), forming the basis for the “multi-mode” adsorption mechanism of the composite.

At the molecular scale, the adsorption mechanism may be categorized into multiple stages: diffusion across the boundary layer (film diffusion), transport into the capillary pores (pore diffusion), initial surface interaction (electrostatic attraction/outer-sphere complexation), and final chemical rearrangement at the active sites (inner-sphere complexation/chemisorption) (Fig. 9). In the first step, under conditions where a significant fraction of –COOH groups are deprotonated to –COO⁻, the composite surface becomes negatively charged, and electrostatic attraction thus pulls cations (Pb²⁺ or cationic dyes) toward the surface, enhancing local concentration and the likelihood of interaction with active sites.⁹⁰ This explains the initial rapid adsorption phase observed in kinetic curves. In the next step, the spatial distribution of sites between alginate and DTPA enables 2 main complex forms: outer-sphere complexes dominated by ion–ion and hydration interactions, and inner-sphere complexes where the metal ion replaces one or more hydration water molecules to coordinate directly with O/N donors of DTPA or –COO⁻ groups. Pb²⁺, with its relatively large ionic radius and moderate hydration energy, readily loses its hydration shell and forms stable inner-sphere complexes with O/N donors, generating polynuclear Pb–O and Pb–N bonds.⁹¹ The coordination reactions can be generally expressed as:

2R-COO⁻ + Pb²⁺ ⇌ (R-COO)₂Pb (inner-sphere), 17

and

DTPAⁿ⁻ + Pb²⁺ ⇌ [Pb-DTPA]²⁻ⁿ 18

Fig. 9. Mechanism adsorption of Pb²⁺ and MB.

These chemical bonds exhibit strong covalent character and dominate the adsorption, consistent with the pseudo-second-order kinetic model.

Ion exchange also plays a significant complementary role in the alginate regions: guluronate units associated with Na⁺/Ca²⁺/Mg²⁺ can be stoichiometrically replaced by Pb²⁺, as evidenced by the reduction of intrinsic cations in EDX and the concomitant increase of Pb. The simple equilibrium can be described as:

2R-Na⁺/Ca²⁺/Mg²⁺ + Pb²⁺ → (R)₂-Pb +2Na⁺/Ca²⁺/Mg²⁺ 19

Ion exchange generates additional immobilization sites and can lead to local precipitation under suitable pH and ionic conditions, forming Pb-rich clusters observed in SEM. Consequently, Pb²⁺ is typically immobilized as a combination of inner-sphere complexes and minor precipitation/clustering, explaining the relative decrease in C, O, and N signals in EDX due to shielding by the Pb-rich layer.

For aromatic organic molecules such as MB, the adsorption mechanism differs from that of metal ions. The initial access of MB to the surface is strongly influenced by its cationic charge, aromatic structure, and hydrogen bonding capability.⁹² Electrostatic attraction draws MB toward –COO⁻ groups, while hydrogen bonding between –NH groups of MB and –OH/–COO⁻ groups of ALG–DTPA, together with interactions with the surface active sites of ZZO, contributes to the retention of MB.⁹³ These interactions act complementarily: electrostatic forces bring the molecule into proximity, while hydrogen bonding and surface interactions help retain it without forming strong coordination bonds as observed with Pb²⁺. This behavior explains the uniform thin-film morphology observed in SEM/EDX, with slightly increased S content and elevated C/N, in contrast to the particle agglomeration seen for Pb. The ZZO phase (ZnO–SiO₂–Al₂O₃) contributes in 2 important ways. It provides Lewis acid (M–O) sites that facilitate labile ligand exchange on cations and mediate local electron interactions. It also maintains the mechanical integrity of the mesoporous capillary framework. The Lewis centers enhance the adsorption of electron-rich molecules and stabilize the complex structure. Importantly, the majority of Pb²⁺ and MB adsorption on the composite is not a redox reaction, confirming that inner-sphere complexation and ion exchange dominate. Thermodynamic and kinetic observations support the predominance of chemisorption. Adsorption follows pseudo-second-order kinetics, which indicates that the rate depends on the number of chemical sites.⁹⁴ The isotherms fit the Langmuir model suggesting equivalent, finite sites, and monolayer adsorption. Negative ΔG° values indicate that the uptake is spontaneous and reversible. The isotherms fit the Langmuir model suggesting equivalent, finite sites, and monolayer adsorption. Negative ΔG° values indicate that the uptake is spontaneous and reversible. The magnitude and sign of ΔH° reflect whether the process is endothermic or exothermic, with chemisorption generally associated with larger enthalpy changes due to bond formation and cleavage.

The capillary-mesoporous structure and polymer coating have 2 key effects. The polymer reduces the physical BET surface area while increasing the effective chemical area, meaning the number of coordination-active sites is higher. The coating also modulates the pore size, creating a confinement effect that favors medium-sized molecules, slows diffusion rates, prolongs retention time, and increases the likelihood of strong interactions. As a result, the transport mechanism can be described as a two-phase process. The first phase is fast and dominated by film diffusion and easily accessible surface sites. The second phase is slower, controlled by intraparticle diffusion into mesopores where full coordination occurs. Selectivity can be explained by the Hard–Soft Acid–Base (HSAB) principle.⁹⁵ Pb²⁺, being a soft-to-intermediate acid, interacts strongly with the soft O/N donors of DTPA and alginate, forming stable complexes. In contrast, cations with high hydration energy or harder character, such as Cu²⁺ and Ni²⁺, have difficulty shedding their hydration shells, which limits inner-sphere coordination and reduces adsorption efficiency.⁹⁶ The presence of –NH₂ groups from DTPA and oxy-carboxylate groups in the polymer further allows pH-dependent selectivity, because protonation or deprotonation alters the coordination ability. In summary, adsorption on ZZO@ALG–DTPA results from a synergistic combination of chemical and physical interactions. Coordination sites such as DTPA, –COO⁻, and T–O–T provide stable inner-sphere complexation and ion exchange. Electrostatic, π–π, and hydrogen bonding interactions promote rapid surface capture. Meanwhile, the mesoporous capillary system, together with the polymer coating, regulates transport, increases retention time, and enhances the probability of effective interactions. This comprehensive mechanism explains the high adsorption performance, selectivity, and applicability of the composite for treating multicomponent wastewater.

3.6. Characterization after Pb²⁺/MB adsorption

To further support the proposed adsorption mechanisms, the morphological evolution observed in the TEM images clearly reflects how Pb²⁺ and MB interact differently with the composite surface. Compared with the pre-uptake morphology, where ZZO@ALG–DTPA exhibited a uniformly coated polymer network and well-dispersed oxide nanorods/nanofibers, the post-adsorption samples revealed distinct microstructural changes that closely align with the different interaction mechanisms of Pb²⁺ and MB. After Pb²⁺ adsorption (Fig S5(a and b)), the dark contrast regions became noticeably thicker and denser, the interface between the polymer layer and oxide domains appeared blurred, and irregular nanoscale clusters emerged. These features are characteristic of Pb-carboxyl/amine coordination, combined with ion-exchange processes and localized aggregation, supporting the proposed inner-sphere complexation mechanism. In contrast, the MB-adsorbed sample (Fig. S5(c and d)) displayed a softer, more uniform morphology without the dense aggregates observed for Pb²⁺. Instead, a thin, diffuse organic film covered the entire surface, lowering the image contrast and partially masking the underlying oxide structure. This coating behavior is consistent with MB adsorption dominated by electrostatic attraction, hydrogen bonding, and π–π interactions between the dye's aromatic ring and the electron-rich functional groups within the polymer matrix. Collectively, the TEM results provide direct visual confirmation that Pb²⁺ induces localized complexation and clustering, whereas MB forms a continuous organic layer, thereby validating the distinct adsorption mechanisms proposed for these two structurally and chemically different pollutants.

The zeta potential analysis provided important evidence of the change in surface charge during the material modification and Pb²⁺/MB adsorption (Fig. 10a). The zeta potential value of the initial ZZO sample was −11.43 mV, indicating a weak negative surface charge, typical for zeolite-ZnO materials with a limited number of ionizable functional groups. After decoration with DTPA and ALG functionalities, this value dropped to −22.76 mV, demonstrating a significant enhance in negative charge density due to the dense presence of –COO⁻ and –NH⁻ groups after deprotonation. This increase in negative charge confirmed that the organic coating was successfully attached to ZZO and created a favorable platform for the capture of positively charged pollutants through electrostatic and coordination interactions. After adsorption of Pb²⁺, the zeta potential shifted to a slightly positive value of 0.54 mV, reflecting the neutralization and charge reversal due to the direct attachment of Pb²⁺ ions to the –COO⁻ and –NH₂ coordination centers. This marked change suggests that Pb²⁺ not only adheres to the surface by electrostatic attraction but also forms inner-sphere complexation coordination bonds, leading to the surface charge shifting to a near-neutral or slightly positive state. For MB, the zeta potential value after adsorption was −2.56 mV, still negative but approaching zero, indicating that MB is adsorbed mainly through directional electrostatic interactions, hydrogen bonding, and π–π interactions, instead of a complete ion exchange mechanism as with Pb²⁺. The decrease in negative intensity but no reversal demonstrated that MB formed a thin film-like surface-distributed coating, consistent with SEM/EDX characterization showing a continuous organic coating but no local precipitate formation.

Fig. 10. (a) Zeta measurement of ZZO, ZZO@ALG–DTPA, ZZO@ALG–DTPA-MB, ZZO@ALG–DTPA-Pb²⁺, (b) reusability of ZZO@ALG–DTPA. The error bars demonstrate the reproducibility of 3 repetitions.

3.7. Comparison of elimination performance among various sorbents

The performance assessment of ZZO@ALG–DTPA in comparison with previously reported adsorbents clearly highlights its superior adsorption capacity as well as its strong potential for practical deployment. As summarized in the comparison Table 3, existing materials display a wide disparity in their ability to simultaneously remove Pb²⁺ and MB. Conventional adsorbents such as walnut-based activated carbon or polyvinyl alcohol/xanthan gum hydrogels exhibit limited performance, typically below 100 mg g⁻¹ for at least one pollutant, reflecting their non-specialized surface chemistry and restricted interaction mechanisms. More advanced materials, including carboxyl-functionalized walnut shells and aluminum-modified magadite, demonstrate notable improvement by introducing targeted functional groups or lamellar structures, achieving capacities exceeding 100 mg g⁻¹ and even approaching 220 mg g⁻¹ for MB in the case of magadite. A further performance leap is observed in composite systems such as zeolite-A/reduced graphene oxide or carbon@chitosan@montmorillonite, where synergistic interfacial effects enable adsorption capacities of 416.7–666.7 mg g⁻¹. However, these materials generally operate under demanding conditions, most notably, very high initial pollutant concentrations (up to 1500 mg L⁻¹ for Pb²⁺), which limit their feasibility for conventional wastewater treatment scenarios. Similarly, acrylic-based biochar@chitosan@EDT presents exceptionally high performance (Pb²⁺: 1105.78 mg g⁻¹, MB: 590.72 mg g⁻¹) owing to the strong chelating ability of EDT ligands, yet its synthesis complexity and chemical selectivity can restrict broad applicability. Within this contextual landscape, the ZZO@ALG–DTPA material developed in this study not only exhibits the highest adsorption capacities among the compared systems (Pb²⁺: 1114.7 mg g; MB: 787.41 mg g⁻¹) but also offers clear practical relevance through its straightforward preparation and ease of operation. Its superiority is reflected in 3 fundamental aspects: (1) dual-pollutant excellence ZZO@ALG–DTPA is among the very few materials capable of achieving outstanding adsorption for both heavy metals and organic dyes simultaneously, overcoming the common trade-off observed in many adsorbents that are optimized for only one pollutant class, (2) exceptional material utilization efficiency. The optimal performance is realized at a remarkably low dosage (0.2 g L⁻¹), highlighting the high density and accessibility of active sites, and underscoring significant economic benefits in large-scale applications, (3) real-condition compatibility. The material operates effectively under near-neutral pH and moderate pollutant concentrations, aligning well with the characteristics of real industrial wastewater streams and avoiding the drawbacks of harsh pH adjustment or artificially elevated contaminant loads. The synergistic structural design, where alginate provides a stable three-dimensional framework, the DTPA ligand introduces highly selective chelation sites for Pb²⁺, and the hierarchical porous network enhances physical interactions with MB, creates a robust and versatile hybrid adsorbent. Taken together, these attributes position ZZO@ALG–DTPA as a highly promising material capable of addressing the longstanding challenge of multi-pollutant removal in a practical, efficient, and sustainable manner.

Table 3. Sorption capability between various sorbents.

Composites	Fixed parameters	q max (mg g−1)	Ref.
Walnut wood activated carbon	[Composite] = 0.5–1.0 g L−1, pH = 5.2–5.7, [MB] = 15 mg L−1, [Pb2+] = 50 mg L−1	Pb2+: 58.82	101
		MB: 18.51
Porous polyvinyl alcohol/xanthan gum hydrogel	[Composite] = 0.25 g L−1, pH = 7, [MB] = 50 mg L−1, [Pb2+] = 50 mg L−1, T = 25 °C	Pb2+: 58.50	102
		MB: 94.47
Carboxylic acid functionalized walnut shell	[Composite] = 0.73 g L−1, pH = 4.33, [MB] = 55 mg L−1, [Pb2+] = 115.5 mg L−1	Pb2+: 128.4	103
		MB: 98.89
Al-incorporated magadiite	[Composite] = 0.5 g L−1, pH = 3, [MB] = 100 mg L−1, [Pb2+] = 25–100 mg L−1	Pb2+: 113.5	104
		MB: 220.2
Cross-linked lignosulfonate bio-adsorbent	[Composite] = 1.0 g L−1, pH = 6, [MB] = 100 mg L−1, [Pb2+] = 50 mg L−1	Pb2+: 64.9	105
		MB: 132.6
Zeolite-A/reduced graphene oxide	[Composite] = 1.0 g L−1, pH = 7, [MB] = 200 mg L−1, [Pb2+] = 200 mg L−1, T = 25 °C	Pb2+: 416.7	106
		MB: 666.7
Carbon@chitosan@montmorillonite	[Composite] = 0.5 g L−1, pH = 6, [MB] = 400 mg L−1, [Pb2+] = 1500 mg L−1, T = 40 °C	Pb2+: 884.19	107
		MB: 326.21
Biochar acrylic-based@Chitosan@EDT	[Composite] = 0.1 g L−1, pH = 4, [MB] = 25–250 mg L−1, [Pb2+] = 100–1000 mg L−1	Pb2+: 1105.78	108
		MB: 590.72
ZZO@ALG–DTPA	[Composite] = 0.2 g L−1, pH = 6, [MB] = 40 mg L−1, [Pb2+] = 100 mg L−1	Pb2+: 1114.7	This study
		MB: 787.41

3.8. Performance validation in complex aquatic matrices

The transition from ideal laboratory conditions to chemically complex aquatic environments represents a critical benchmark for evaluating the practical applicability of any adsorbent. In order to evaluate the practical efficiency of ZZO@ALG–DTPA, adsorption tests were carried out with environmental water samples obtained from 6 representative sites corresponding to 2 distinct pollution scenarios: (i) Pb²⁺-contaminated saltwater associated with marine aquaculture systems, and (ii) dye-containing wastewater originating from industrial activities. For Pb²⁺ evaluation, water samples were collected from 3 coastal and nearshore locations associated with marine aquaculture activities, including the Tien Hai coastal area (Thai Binh province), the Dam Ha coastal aquaculture area, and the Ha Long Bay aquaculture area (Quang Ninh province, Vietnam). These brackish and saline environments are representative of offshore and nearshore aquaculture systems, where trace metal contamination may arise from antifouling coatings, feed residues, and anthropogenic coastal discharges, thereby posing potential risks to aquatic organisms and promoting metal bioaccumulation. The elevated ionic strength and complex inorganic composition of these waters provide a stringent testing matrix for Pb²⁺ adsorption under marine-related conditions. Simultaneously, the adsorption performance toward MB was evaluated using real wastewater samples collected from 3 dye-affected sources, namely the La Khe textile dyeing wastewater channel (Duong Noi Ward), an industrial wastewater discharge point in Tuong Giang (Bac Ninh province), and a mixed wastewater stream in the Nhue River basin. These sampling sites represent typical industrial wastewater matrices generated by textile dyeing, printing, and finishing operations, in which synthetic dyes are prevalent contaminants, and therefore serve as appropriate systems for evaluating dye removal efficiency. Across all tested environmental water matrices, ZZO@ALG–DTPA exhibited consistently high removal efficiencies (Table S3). For Pb²⁺, removal efficiencies ranged from 75.47 to 87.35%, while MB removal efficiencies varied between 80.55 and 88.96%, despite the chemical complexity of the sampled waters. Notably, with increasing initial pollutant concentrations, the removal efficiencies for both Pb²⁺ and MB remained stable or even exhibited a slight increase, rather than declining as commonly 6 observed for conventional adsorbents. This behavior indicates a high adsorption capacity and strong binding affinity, effectively delaying surface saturation, which is a key operational advantage for practical water treatment systems exposed to fluctuating contaminant loads.⁹⁷ Variations in removal efficiency among different sampling locations further highlight matrix-dependent effects on adsorption behavior. In the case of Pb²⁺, superior and more stable performance was observed for samples from Ha Long Bay aquaculture area, suggesting a favorable chemical environment, potentially associated with relatively stable pH conditions or specific ionic compositions that enhance coordination interactions between Pb²⁺ ions and the functionalities of the sorbent, even in saline media. In contrast, slightly reduced MB removal efficiencies observed in certain industrial wastewater samples can be attributed to competitive adsorption from coexisting organic compounds and dissolved organic matter, which may partially occupy active sites or hinder access to the adsorbent surface. Overall, these results extend beyond laboratory-scale validation and demonstrate the robustness, sustainability, and functional versatility of ZZO@ALG–DTPA under realistic environmental conditions. The material shows effective performance in 2 well-defined application contexts: (i) Pb²⁺ mitigation in saltwater environments associated with marine aquaculture, and (ii) treatment of dye-contaminated industrial wastewater. This dual validation highlights the potential of ZZO@ALG–DTPA as a high-performance, application-oriented adsorbent for source-specific water quality management in complex aquatic systems.

3.9. Reuse of adsorbent

Reusability is a critical determinant for the practical application of adsorbent materials in water decontamination, where regeneration cost and performance retention are major challenges in scaling from laboratory to industrial use. The reusability of ZZO@ALG–DTPA was evaluated over 5 consecutive adsorption–desorption cycles for both Pb²⁺ and MB. In Fig. 10b, the material demonstrated exceptional stability, with Pb²⁺ removal efficiency declining only from 95.43% in the initial cycle to 85.43% in the fifth cycle, while MB removal decreased slightly from 98.43 to 89.94%. This minimal performance loss confirms the structural integrity of the composite and the resilience of its key functional groups (–COO⁻/–NH₂) against chemical regeneration. Notably, the reusability of ZZO@ALG–DTPA stands out when benchmarked against previously reported dual-adsorbent systems for Pb²⁺ and MB. EC–AB beads (EDTA–chitosan/alginate) retained only 76.64% Pb²⁺ and 80.29% MB after 5 cycles.⁹⁸ The β-CD@MBCP composite also showed a marked decline, dropping from 98.1 to 82.5% for Pb²⁺ and from 97.4 to 84.2% for MB within 4 cycles.⁹⁹ Likewise, the MCB/ALG biochar–alginate hydrogel maintained merely 85% MB after 5 uses.¹⁰⁰ The high regeneration efficiency underscores the material's economic viability and operational durability, highlighting its strong potential for long-term, large-scale wastewater decontamination applications.

4. Conclusions

This study developed a new multifunctional adsorbent, ZZO@ALG–DTPA, by integrating ZIF-90-derived oxide/zeolitic frameworks with DTPA-modified alginate, thereby creating an inorganic–organic hybrid material rich in O/N-donor chelation sites while retaining the mechanical robustness and hydrophilicity of the oxide-mesopore network. The most distinctive feature of this system is its ability to combine, within a single architecture, three typically independent adsorption routes multi-dentate coordination, ion-exchange through the alginate backbone, and electrostatic/π–π interactions, enabling the simultaneous remediation of both heavy metals and cationic dyes with high efficiency. Under fixed conditions (pH 7, 0.2 g L⁻¹, 20 min), the material reached uptake efficiencies of 96.98% for Pb²⁺ and 98.85% for MB. The Langmuir model displayed an excellent description of the uptake behavior, yielding exceptionally high maximum capabilities of 1114.7 mg g⁻¹ (Pb²⁺) and 787.41 mg g⁻¹ (MB), while the adsorption kinetics followed a pseudo-second-order model (R² = 0.999), confirming the dominance of chemisorption. Mechanistic analysis revealed that Pb²⁺ was captured primarily via inner-sphere coordination with –COO⁻/–NH₂ groups and ion exchange sites, whereas MB uptake involved a combination of hydrogen bond formation, and electrostatic attraction with the organic domains of the composite. The material retained strong performance in the occurrence of species competing (>85%) and showed notable durability, preserving more than 85% of its initial efficiency after 5 regeneration cycles. Importantly, ZZO@ALG–DTPA also performed effectively in complex real-water matrices, achieving Pb²⁺ removal of 75.47–87.35% and MB removal of 80.55–88.96%, highlighting its practical applicability in treating complex, multi-contaminant wastewater. Overall, ZZO@ALG–DTPA represents a robust, sustainable, and high-performance adsorbent with clear potential for real-world water decontamination applications.

Conflicts of interest

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

Data availability

Data will be provided to the extent necessary upon request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra07492h.