A novel cobalt-based MOF for simultaneous removal of Pb(ii) and Congo red from real textile wastewater: synergistic adsorption behavior and mechanistic insights

Abstract

Currently, the co-removal of heavy metals and anionic dyes from industrial wastewater is quite difficult due to the stability of both pollutants and their combined co-existence. Most previous studies have focused on single-pollutant adsorption, and few reports have investigated the performance of multifunctional MOFs with an effective removal efficiency towards co-existing inorganic and organic pollutants. Herein, a newly developed azo-ligand cobalt-based metal–organic framework (Co-MOF) is developed using a solvothermal method, and its adsorption performance in the simultaneous removal of Pb(ii) and Congo Red (CR) from aqueous solutions and real textile wastewater is examined. Characterization using FTIR, FESEM, EDX, XRD, TGA, and zeta-potential demonstrates the successful creation of a highly stable crystalline framework with abundant active sites that are accessible. Under the optimum conditions of a pH of 5–6, an adsorbent dosage of 0.03 g and room temperature (298 K), very fast adsorption rates were observed (50 min for Pb(ii) and 20 min for CR). The calculated maximum adsorption capacities based on the Langmuir model were 344.8 mg g⁻¹ for Pb(ii) and 476.2 mg g⁻¹ for CR, respectively, with removal of more than 93% for both pollutants. The kinetics and isotherms were fitted to the pseudo-second-order and Langmuir models, and the adsorption process was spontaneous and endothermic. In addition to high reuse capacity (six recovery cycles), the Co-MOF showed remarkable adsorption efficiency toward mixed pollutants in a real textile wastewater sample. The presented reusable Co-MOF is therefore a promising adsorbent for simultaneous remediation of mixed inorganic/organic pollutants.

Currently, the co-removal of heavy metals and anionic dyes from industrial wastewater is quite difficult due to the stability of both pollutants and their combined co-existence.

1. Introduction

Rapid industrialization and urbanization have significantly increased the discharge of hazardous pollutants into aquatic environments, causing serious ecological and human health problems.¹ Among these pollutants, heavy metals and synthetic dyes are particularly dangerous because of their toxicity, persistence, and bioaccumulation or recalcitrance in the environment.² Textile industries are major contributors to wastewater contamination, as their effluents commonly contain both toxic metal ions and stable organic dyes.³ Therefore, the development of efficient materials for treating multi-component wastewater systems has become an important environmental challenge.⁴

Lead(ii) (Pb(ii)) is one of the most hazardous heavy metals frequently detected in industrial wastewater generated from the textile dyeing, electroplating, mining, and battery industries.⁵ Pb(ii) exposure can cause neurological disorders, kidney damage, reproductive toxicity, and developmental abnormalities even at low concentrations.⁶ In addition, Congo Red (CR), a widely used anionic azo dye, exhibits carcinogenicity, mutagenicity, and strong resistance to biodegradation due to its complex aromatic structure and azo functional groups.⁷ The coexistence of Pb(ii) and Congo red in textile wastewater significantly increases treatment complexity and environmental risk.⁸

Various physicochemical and biological technologies, including membrane filtration, coagulation, photocatalysis, ion exchange,⁹ solvent extraction, electrode-deposition, flotation, oxidation–reduction, reverse osmosis, biosorption, and biological degradation have been widely applied for wastewater remediation.¹⁰ Despite their effectiveness, many of these methods are limited by high operational cost, sludge generation, membrane fouling, energy demand, or incomplete removal efficiency.¹¹ In contrast, adsorption is widely regarded as an efficient and economical treatment approach due to its simplicity, versatility, high removal performance, and low operating requirements.¹²

Solvent extraction, electrode-deposition, flotation, oxidation–reduction, reverse osmosis, biosorption. Recently, metal-organic frameworks (MOFs) have attracted considerable attention as advanced porous adsorbents for environmental remediation applications.¹³ MOFs possess exceptionally high surface areas, tunable pore structures, abundant active sites, and excellent adsorption capacities.¹⁴ In particular, cobalt-based MOFs have demonstrated strong potential for removing both heavy metals and dyes because their frameworks provide accessible active sites, good structural stability, and multiple interaction pathways with pollutants.¹⁵ These interactions include electrostatic attraction, coordination interaction, hydrogen bonding, and π–π interactions, which together help explain their high removal efficiency in aqueous systems.¹⁶

The main objective of this research work is to design, synthesize, and characterize a novel azo-ligand cobalt-based Co-MOF (metal–organic framework) as a multifunctional adsorbent for the simultaneous elimination of Pb(ii) and CR from aqueous solutions and real textile wastewater. The novelty of this work lies in the molecular design of an azo-functionalized ligand that introduces multiple adsorption-active sites into a cobalt-based MOF framework. Unlike conventional Co-MOF adsorbents that primarily depend on limited coordination or surface interactions, the introduced azo, amino, and carboxyl functionalities provide complementary binding mechanisms, enabling efficient interaction with both Pb(ii) ions and CR molecules. This functionalized architecture creates a recyclable multifunctional adsorbent capable of addressing chemically distinct pollutants through synergistic adsorption pathways, including coordination, electrostatic attraction, hydrogen bonding, and π–π interactions. The effects of various parameters on adsorption were optimized, followed by kinetic, isotherm, and thermodynamic studies to understand the mode of adsorption and thermodynamic nature. Mechanistic studies were carried out to determine the dominant interactions during Pb(ii) and CR adsorption. Reusability, regeneration, and real textile wastewater treatment tests were performed to confirm the applicability of the designed material.

2. Experimental

2.1. Apparatus

The following instruments were used in adsorption experiments and characterization studies: a pH meter (Hanna), centrifuge (PLC Series), shaking water bath (model YCW-012S, GEMMYCO), sensitive balance (Accuris Instruments), and FAAS (Pye Unicam Ltd, Cambridge, England). A Shimadzu UV-visible double-beam spectrophotometer (model UV-1800, Japan) with a fixed 1 nm bandwidth and a 1 cm quartz cell was utilized for spectrophotometric measurements. An FT-IR spectrophotometer (Shimadzu) was also used. The shapes of particles were analyzed via FESEM in conjunction with EDX (Tescan MIRA3 FEG-SEM, Czech Republic) at 15 kV after being coated with a thin layer of gold. The EDX detector was set to SE mode. Thermogravimetric analysis (TGA) was conducted using an STA-200 synchronous thermal analyzer (Guangdong Newguai Instrument Co., Ltd, China); XRD patterns were recorded with an X-ray diffractometer using a Cu Kα spectral line at 45 kV and 40 mA, and a 2θ range between 5 and 80.

2.2. Materials

2-Amino benzoic acid (C₆H₄(NH₂)(CO₂H), CAS-Nr: 118-92-3, M.W.: 137.14 g mol⁻¹, Sigma-Aldrich), 3-chloro-4-methyl aniline (ClC₆H₃(CH₃)NH₂, CAS-Nr: 95-74-9, M.W.: 141.60 g mol⁻¹, Sigma-Aldrich), sodium nitrite (NaNO₂, CAS-Nr: 7632-00-0, M.W.: 69.0 g mol⁻¹, Sigma-Aldrich), methanol (CH₃OH, CAS-Nr: 67-56-1, M.W.: 32.04 g mol⁻¹, Sigma-Aldrich), N,N-dimethylformamide (DMF; HCON(CH₃)₂, CAS-Nr: 68-12-2, M.W.: 73.09 g mol⁻¹, Sigma-Aldrich), ethanol (C₂H₅OH, CAS-Nr: 64-17-5, M.W.: 46.07 g mol⁻¹, Sigma-Aldrich), sodium hydroxide (NaOH, CAS-Nr: 1310-73-2, M.W.: 39.9971 g mol⁻¹, Scharlau), hydrochloric acid (HCl, CAS-Nr: 7647-01-0, M.W.: 36.46 g mol⁻¹, BDH), cobalt(ii) nitrate (Co(NO₃)₂·6H₂O, CAS-Nr: 10026-22-9, M.W.: 291.03 g mol⁻¹, Fluka), ethylene diamine tetra acetic acid (EDTA; C₁₀H₁₆N₂O₈, CAS-Nr: 6381-92-6, M.W.: 292.244 g mol⁻¹, Merck), nitric acid (HNO₃, CAS-Nr: 7697-37-2, M.W.: 63.01 g mol⁻¹, Sigma-Aldrich), Congo red (C₃₂H₂₂N₆Na₂O₆S₂, CAS-Nr: 573-58-0, M.W.: 696.66 g mol⁻¹, Sigma-Aldrich), lead nitrate (Pb(NO₃)₂, CAS-Nr: 10099-74-8, M.W.: 331.21 g mol⁻¹).

2.3. Preparation of Co-MOF

The Co-MOF was synthesized following a modified literature procedure, as shown in Scheme 1.¹⁷ In a typical synthesis, 0.2810 g of compound AA (HOOC–C₆H₃(NH₂)–N N–C₆H₃(Cl)(CH₃), 0.97 mmol) previously synthesized according to the reported method¹⁸ and 1.010 g of Co(NO₃)₂·6H₂O (3.47 mmol) were dissolved in 25 mL of DMF. The mixture was ultrasonically treated until a clear and homogeneous solution was obtained. Subsequently, the solution was transferred into glass-lined Teflon autoclaves and subjected to solvothermal treatment at 100 °C for 24 h. Upon completion of the reaction, the autoclaves were allowed to cool naturally to room temperature. The resulting microcrystalline product was collected by filtration, thoroughly washed with methanol five times to remove unreacted precursors and residual solvent, and then immersed in fresh methanol for 6 days with daily solvent exchange to ensure effective solvent activation. Finally, the activated material was dried under vacuum, affording the Co-MOF product.

Scheme 1. Schematic diagram depicting the stepwise synthesis of the novel Co-MOF.

2.4. Adsorption studies

Batch adsorption experiments were conducted to investigate the adsorption process. In a standard experimental setup, 100 mL of a 100 mg L⁻¹ solution of either Congo Red (CR) or Pb(ii) was prepared. A precisely weighed quantity of the adsorbent (0.01–0.06 g) was added to the solution, which was then agitated at controlled temperature (15, 20, 25, 30, 35, 40, and 50 °C) in a thermostatic water bath shaker for a predetermined time interval (5, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180, and 240 minutes for Pb(ii); 1, 3, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 90 and 120 minutes for CR). The effect of pH was examined over the range of 2–8, with pH adjustments being made using 0.1 N HCl and NaOH solutions. After agitation, the supernatant was centrifuged to evaluate the possible sorption of Pb(ii) and CR. The absorbance of the filtrate was measured at the respective λ_max (498 nm) using a UV-vis spectrophotometer for CR, and FAAS for Pb(ii). The percent adsorption and the adsorption capacity were calculated using the following equations:

1

2

where C₀ is the initial adsorbate concentration (mg L⁻¹), C_e is the final adsorbate concentration (mg L⁻¹), q_e is the adsorption capacity at equilibrium (mg g⁻¹), w is the adsorbent weight (g), and v is the adsorbate solution volume (mL).^19,20

3. Results and discussion

3.1. Characterization of the new Co-MOF

3.1.1. FTIR analysis

Fig. 1 shows the FTIR spectra of Co-MOF, Co-MOF-CR (loaded with Congo red), and Co-MOF-Pb(ii) (loaded with lead). The spectrum of the pure Co-MOF shows a broad absorption band at about 3415 cm⁻¹, which is the stretching vibration of the amine and hydroxyl groups that are coordinated to water molecules and surface hydroxyl groups. The presence of the absorption bands at about 1605 cm⁻¹ and 1390 cm⁻¹ is attributed to the asymmetric and symmetric vibrations of the coordinated carboxylate (COO–) groups, respectively, confirming the coordination of the organic linker with the Co(ii) ions. Moreover, the presence of bands in the 1100–1250 cm⁻¹ region can be ascribed to the C–O stretching vibrations of the organic ligand, whereas the typical band at lower wavenumbers (below 700 cm⁻¹) represents the Co–O bond vibrations, and thus indicates the formation of the Co-MOF structure. After the adsorption of Congo red, the FTIR spectrum was greatly altered. The wide OH stretching band moved and widened, revealing the existence of more hydrogen bonds between the dye molecules and the MOF surface. Moreover, the asymmetric carboxylate stretching vibration at 1605 cm⁻¹ moved to about 1629 cm⁻¹, whereas the symmetric stretching vibration was slightly displaced to about 1380 cm⁻¹, indicating the participation of carboxylate oxygen atoms in the dye adsorption process. Several other absorption bands were present in the 1500–1200 cm⁻¹ region, including bands at around 1490, 1450, 1310 and 1235 cm⁻¹, which are typical of the aromatic C C vibrations, azo (–N N–) stretching, and C–N vibrations of Congo red. Additional spectral changes were observed after the adsorption of Pb(ii). The strength of the hydroxyl stretching band was reduced, and it moved slightly to around 3405 cm⁻¹, which suggests that the hydroxyl groups are involved in the coordination of Pb(ii). Similarly, the asymmetric carboxylate stretching vibration moved to around 1585 cm⁻¹ rather than 1592 cm⁻¹, whereas the symmetric stretching band was reduced at 1390 cm⁻¹, which confirms the coordination of the Pb(ii) ions with the oxygen functional groups of the MOF. The weakening of the Co–O vibration and its shift from around 487 cm⁻¹ to 480 cm⁻¹ also indicates modification of the local coordination environment of the cobalt centers following the adsorption of Pb(ii). However, none of the characteristic framework bands disappeared, which indicated that the crystalline structure of the Co-MOF did not change after the Congo red and Pb(ii) adsorption processes. Overall, the changes in the hydroxyl and carboxylate stretching vibrations, as well as the emergence of typical Congo red bands and the minor shift of the Co–O vibration, provide very convincing evidence that the mechanism of adsorption relies on hydrogen bonding, electrostatic attraction, π–π interactions, and coordination between the Pb(ii) and the oxygen donor atoms in the framework. The high stability of the adsorbent, indicated by the maintenance of the characteristic Co-MOF vibrations following adsorption, demonstrates that the adsorbent is structurally very stable, thereby enabling the simultaneous separation of organic dyes and heavy metal ions from aqueous media.

Fig. 1. FTIR spectra of pristine Co-MOF and Co-MOF after Pb(ii) and Congo red adsorption.

3.1.2. FESEM analysis

The pure Co-MOF shows a very rough, blocky, and extremely porous crystalline framework structure that is made up of micro-scale blocks and scattered granular aggregates, as shown in Fig. 2a. Under higher magnification (500 nm scale), individual nanoparticles with measured dimensions in the range of 48 nm to 67 nm forming an open network of inter-particle micro/mesopores can clearly be seen. The fact that this coarse and uninhibited architecture offers a high specific area with a wealth of active coordinating sites that are accessible is significant.²¹

Fig. 2. FE-SEM micrographs of the synthesized Co-MOF (a) before adsorption, (b) after Congo Red (CR) adsorption, and (c) after Pb(ii) adsorption at different magnifications.

Fig. 2b shows that the adsorption of Congo red dye on Co-MOF transforms it dramatically to produce a highly uniform, densely packed nanoball morphology with the particle size being constrained to a narrow range between 54 nm and 65 nm. The angular crystalline blocks that were visible in the pristine sample are completely concealed, and the texture is much smoother and continuous. This transition in structure indicates that the large, aromatic CR dye molecules were able to fit inside the pores of the framework and bind to its exterior surface via strong interactions such as electrostatic attraction, hydrogen bonding, and π–π stacking, effectively filling in the rough edges of the starting material^22,23

The surface morphology changes to larger, irregularly shaped, and intensely fused agglomerates with individual features growing significantly up to 59 nm to 106 nm upon heavy metal Pb(ii) ion uptake, as demonstrated in Fig. 2c. At the macro-scale, the material is very dense, with very few fine details, and is chemically compacted in appearance. The pronounced agglomeration and particle enlargement can be directly correlated with the strong chemical coordination and surface complexation between the lead ions and the active donor atom of the MOF linkers, which usually causes the surface charges to disappear and leads to the fusion of structures or co-precipitation onto the framework.²¹

3.1.3. EDX analysis

As presented in Fig. 3a, the EDX spectrum and elemental mapping of the pristine Co-MOF confirmed the presence of C, N, O, Co, and Cl, supporting the successful formation of a chemically homogeneous cobalt-based framework with well-distributed constituent elements. The appearance of a distinct S signal after Congo red adsorption (Fig. 3b) was assigned to the sulfonate groups of the dye, and the uniform distribution of sulfur over the mapped surface indicated effective anchoring of CR molecules across accessible adsorption sites. This observation agrees with earlier reports showing that Co-based and other MOF adsorbents remove anionic azo dyes through electrostatic attraction, hydrogen bonding, and π–π interactions.²⁴ Following Pb(ii) adsorption, the emergence of Pb peaks in the EDX spectrum and their homogeneous distribution in the elemental maps confirmed the successful uptake of lead ions by the Co-MOF surface, as can be clearly observed in Fig. 3c. The efficient Pb(ii) binding is consistent with literature showing that MOFs provide abundant N- and O-containing coordination sites, porous channels, and surface-active centers that promote chemisorption, electrostatic interaction, chelation, and surface complexation of metal ions.²⁵ Collectively, these findings indicate that the synthesized Co-MOF possesses a well-dispersed elemental framework, accessible active sites, and favorable porous characteristics for the effective removal of both Congo red and Pb(ii) from aqueous media, which is in line with recent studies reporting the high adsorption capacity, good stability, and reusable performance of Co-based and MOF-derived adsorbents in wastewater treatment.²³

Fig. 3. EDX spectra and elemental mapping of Co-MOF: (a) before adsorption, (b) after Congo red adsorption, and (c) after Pb(ii) adsorption onto the Co-MOF surface.

3.1.4. TGA analysis

TGA/DSC analysis of the synthesized Co-MOF revealed a multistep thermal decomposition pattern, confirming the formation of a thermally stable porous framework, as shown in Fig. 4. An initial weight loss of approximately 10 wt% below 160 °C was assigned to the removal of physically adsorbed water and residual solvent molecules trapped within the pores, which is consistent with the commonly observed early-stage desorption of guest molecules in porous cobalt-based MOFs.²⁶ The second decomposition step was centered at 224.5 °C, with a mass loss of nearly 20 wt%, and was attributed to the release of coordinated solvent species and the onset of weakening of the Co–ligand coordination environment.²⁷ The principal degradation stage occurred at about 311.9 °C and involved an approximately 30 wt% mass loss, indicating collapse of the Co-MOF framework and decomposition of the organic linker, as also reflected by the sharp DSC event. Beyond 350 °C, the TGA trace became relatively stable, leaving a residual mass of approximately 45–50 wt%, which can be ascribed to the formation of cobalt oxide residues, most likely CoO and/or Co₃O₄, in agreement with previous reports on the thermal conversion of cobalt-containing precursors and Co-based MOFs at elevated temperatures.²⁶ Overall, the obtained results indicate that the synthesized Co-MOF maintains good thermal stability up to nearly 300 °C, a property that is advantageous for adsorption-based wastewater treatment, as MOF performance in environmental remediation depends strongly on structural robustness, porosity, and resistance to degradation during operation.^28,29

Fig. 4. TGA/DSC curves of Co-MOF showing thermal decomposition and cobalt oxide residue formation.

3.1.5. Zeta potential analysis

The zeta potential and electrophoretic mobility results indicate that the synthesized Co-MOF possesses a positively charged surface in aqueous medium, with a zeta potential of approximately +25 to +30 mV (Fig. 5), confirming the cationic nature of the adsorbent surface.³⁰ This value is consistent with a moderately stable dispersion, although zeta potential should not be used as a standalone criterion for colloidal stability, as its interpretation also depends on aggregation behavior and the electrokinetic model applied. The positive surface charge is favorable for the adsorption of anionic pollutants such as Congo red, since electrostatic attraction between the sulfonate groups of the dye and the Co-MOF surface can enhance uptake; this mechanism has been widely reported in Co-based and other MOF adsorbents, together with hydrogen bonding and π–π interactions.^31,32 In contrast, the role of zeta potential in Pb(ii) adsorption is less direct, as Pb(ii) is cationic and therefore not inherently favored by a positively charged surface. Instead, Pb(ii) removal in MOF-based systems is commonly governed by coordination or complexation with O-, N-, or S-containing functional groups.³³ Overall, the zeta potential data mainly support the suitability of the prepared Co-MOF for anionic dye adsorption, whereas Pb(ii) uptake should be interpreted primarily in terms of specific metal-binding interactions rather than electrostatic attraction alone.

Fig. 5. Zeta potential and electrophoretic mobility distributions of the synthesized Co-MOF adsorbent.

3.1.6. PXRD analysis

The powder X-ray diffraction (PXRD) pattern of the synthesized Co-MOF shown in Fig. 6 confirms the successful formation of a crystalline cobalt-based metal–organic framework with good structural purity. The presence of distinct diffraction peaks in the 2θ range of approximately 18–45° indicates an ordered arrangement of Co(ii) centers and organic linkers within the coordination framework, consistent with the crystalline 2D and 3D architectures reported for Co-based MOFs characterized using single-crystal and powder X-ray diffraction.²³ The sharp and intense reflections further suggest a high degree of crystallinity and a well-defined framework, in line with the general behavior of MOFs, for which PXRD is the primary method used to verify crystal structure and phase identity.³⁴ The absence of additional diffraction peaks supports the formation of a largely phase-pure material without detectable crystalline impurity phases, which agrees with prior reports in which PXRD was used to confirm phase purity in cobalt-containing porous frameworks.³⁵ The broad background feature observed at higher diffraction angles may reflect a limited amorphous contribution associated with linker disorder or structural defects, although this assignment should be made cautiously, because the supplied studies mainly establish PXRD as a tool for confirming crystallinity and purity rather than directly quantifying amorphous content.³⁶

Fig. 6. PXRD pattern of the synthesized Co-MOF adsorbent showing the characteristic crystalline structure and phase purity of the cobalt-based metal–organic framework.

3.2. Adsorption studies

3.2.1. Effect of time

The adsorption efficiency plots of Co-MOF for Pb(ii) ions and Congo red (Fig. 7) show rapid initial adsorption that approaches equilibrium in the long term, a typical feature of porous adsorbents in which abundant active sites are available at the initial stage of adsorption and subsequently fill up over time.³⁷

Fig. 7. Influence of contact time on Pb(ii) ion and CR adsorption onto Co-MOF (initial concentration: 100 mg L⁻¹; natural pH; adsorbent dosage: 0.03 g; temperature: 35 °C).

The Congo red removal efficiency rose very quickly from about 76% at 2 min to nearly 99% at 20 min and was almost constant thereafter, showing fast adsorption kinetics and equilibrium. This behavior is attributed to strong electrostatic attraction between CR molecules and the Co-MOF framework, π–π interactions, and hydrogen bonding.^23,38 However, the removal of Pb(ii) rose slowly from ∼71% at 5 min to ∼93% after 50–60 min, until equilibrium was reached. The lower rate of adsorption is attributed to the movement of hydrated Pb(ii) ions inside the porous structure and their coordination with oxygen- and nitrogen-containing functional groups on the surface of Co-MOF.^39,40

The overall results showed that Co-MOF has good adsorption capacity for both pollutants, with Congo red and Pb(ii) reaching equilibrium at 20 and 50 minutes, respectively. The adsorption sites of Congo red molecules tend to be more abundant and, therefore, their surface interactions are more rapid than those of Pb(ii), which, in turn, may be associated with the diffusion of Pb(ii) ions and their subsequent coordination with active sites on the adsorbent.⁴¹

3.2.2. Effect of pH

Solution pH (Fig. 8) is very important in determining the adsorption effectiveness of Co-MOF for the removal of Pb(ii) and Congo red due to the effect of pH on the surface charge of the adsorbent and the speciation of the adsorbates.⁴²

Fig. 8. Influence of pH on Pb(ii) ion and CR adsorption onto Co-MOF (initial concentration: 100 mg L⁻¹; contact time: 20 min for CR and 50 min for Pb(ii); adsorbent dosage: 0.03 g; temperature: 35 °C).

For Pb(ii), the removal efficiency was higher at pH 5–6 (95%) than at pH 2 (66%) due to less competition with the H⁺ ions and more coordination between the oxygen- and nitrogen-containing active sites within Co-MOF and Pb(ii) ions. The removal efficiency decreased at pH > 6, probably due to the hydrolysis of the lead ions and a decrease in adsorption interactions.⁴³

The Congo red removal was almost complete (98–100%) at pH values ranging from 2 to 5, in which range strong electrostatic attraction, π–π interactions, and hydrogen bonding interactions between the Congo red and Co-MOF were found. The adsorption efficiency gradually decreased to ∼59% at pH 8 due to increased electrostatic repulsion from surface deprotonation of the adsorbent.^22,44

In general, Co-MOF was most effective when the pH of the solution was in the range of 5–6, with the highest capacities of around 95% for the adsorption of Pb(ii) and 99% for Congo red. The results show that the simultaneous removal of the heavy metal and anionic dye over Co-MOF is mainly controlled by surface complexation for the adsorption of Pb(ii) and electrostatic attraction and π–π interactions for the removal of CR, respectively, indicating that Co-MOF has an excellent ability for the simultaneous removal of a heavy metal and anionic dye. Similar results have been reported by Wang and Zhang for Pb(ii),^39,43 and by Dalvi with Chen for Congo red.^23,45

3.2.3. Effect of dose

Fig. 9 shows that the removal efficiency of both pollutants increased with increasing Co-MOF dosage, owing to the greater number of available active adsorption sites and larger surface area. As the Co-MOF dosage increased from 0.01 to 0.03 g, the removal efficiency of Pb(ii) increased from about 69% to 94%, while that of CR increased from about 84% to almost 99%. The better adsorption at higher dosages is believed to result from the larger number of coordination sites for the adsorption of Pb(ii) ions and the larger number of adsorption sites for the CR molecules.⁴⁶ When the amount of adsorbent was increased above 0.03 g, the removal efficiencies remained almost constant for both pollutants, suggesting that adsorption equilibrium had been reached and that further increase in adsorbent dose did not make a significant difference. This plateau occurs because enough adsorption sites are present to make the amount of Pb(ii) ions and CR molecules the limiting factor. In addition, particle aggregation at high doses can also affect the proportion of the total surface area that is actually available and can decrease the number of new adsorption sites that are utilized.⁴⁷

Fig. 9. Influence of adsorbent dose on Pb(ii) ion and CR adsorption onto Co-MOF (initial concentration: 100 mg L⁻¹; contact time: 20 min for CR and 50 min for Pb(ii); natural pH; temperature: 35 °C).

Overall, the optimum dose was determined to be 0.03 g, which showed the highest removal efficiency for both Pb(ii) and Congo red (94% and 99%, respectively). These findings indicate that Co-MOF has a high density of accessible active sites to effectively remove pollutants from water at relatively low dosages of adsorbent, which is beneficial for practical wastewater treatment applications. Similar results were clearly observed by Naseem.⁴⁸

3.2.4. Effect of concentration

As shown in Fig. 10, at lower concentrations (20–100 mg L⁻¹), the removal efficiency of both Pb and CR was high (>95%), which is an indication that the adsorbent has enough vacant active sites to capture the adsorbate species.⁴⁹ When the initial concentration was increased beyond 120 mg L⁻¹, the removal efficiency of the compounds decreased gradually with the progressive filling of the available binding sites and increased competition among the adsorbate molecules for the finite surface functional groups.⁵⁰ The removal efficiency for Pb decreased to ∼55% at 180 mg L⁻¹, while that for CR remained relatively high at ∼70% at 200 mg L⁻¹, indicating that multiple interaction mechanisms, such as electrostatic attraction, hydrogen bonding, and π–π interactions, were involved in CR removal .^51,52 The overall result depicted by this diagram indicates that adsorption is controlled by the availability of active sites. Increasing the adsorbate concentration enhances the driving force for mass transfer, but decreases the removal efficiency because of the decrease in the number of available adsorption sites relative to the amount of adsorbate.^53,54

Fig. 10. Influence of the concentration of Pb(ii) ion and CR on their adsorption onto Co-MOF (contact time: 20 min for CR and 50 min for Pb(ii); natural pH; adsorbent dosage: 0.03 g; temperature: 35 °C).

3.2.5. Effect of temperature

The removal efficiency of both Pb and CR also improved significantly with increasing temperature, ranging from about 58% to 94% for Pb and 75% to 98% for CR up to a temperature of 35 °C; further increasing the temperature to 50 °C produced negligible improvement in the removal efficiency (Fig. 11). The improvement of adsorption at higher temperatures suggests an endothermic character for the adsorption process, with higher temperature leading to higher mobility of the Pb ions and CR molecules, lowering the diffusion resistance and enhancing their transfer to the adsorbent surface.⁵⁵ Furthermore, higher temperatures make surface binding sites more accessible and more active due to increased flexibility of the surface functional groups, as well as increased interactions between the adsorbate and active surface sites.⁵⁶ However, once the easily accessible binding sites are saturated (∼35 °C), further increasing the temperature offers little or no advantage, leading to a nearly constant removal efficiency. This means that ultimately, it is not the available thermal energy that determines the adsorptive capacity, but instead the number of accessible binding sites.⁵⁷

Fig. 11. Influence of temperature on Pb(ii) ion and CR adsorption onto Co-MOF (initial concentration: 100 mg L⁻¹; contact time: 20 min for CR and 50 min for Pb(ii); adsorbent dosage: 0.03 g; natural pH).

3.3. Kinetic studies

The adsorption of lead ions (Pb(ii)) and Congo Red (CR) onto Co-MOF was studied using pseudo-first-order, pseudo-second-order, intra-particle diffusion, and Elovich kinetic models (Table 1). The pseudo-first-order model showed comparatively weaker fitting for both Pb(ii) and CR adsorption (Fig. 12a), and the calculated equilibrium capacities were substantially lower than the experimental values, as presented in Table 2, indicating that physisorption alone was not the dominant adsorption pathway.⁵⁸

Table 1. Kinetic and isotherm model equations.

Models	Equation form	Parameters
Pseudo-1st-order model		q e (mg g−1) adsorption capacity at equilibrium, and qt (mg g−1) at a specific time
Pseudo-2nd-order model		k 1 (min−1) is speed of pseudo-1st-order, and k2 is a rate of adsorption under 2nd-order conditions (g mg−1 min−1)
Elovich		α is the initial adsorption rate (mg g−1 min−1), and β is the desorption constant (g mg−1)
Intraparticle diffusion	q t = Kidt1/2 + C	K id is the intraparticle rate constant (mg g−1 min−0.5), t1/2 is the square root of time, and C is the boundary layer thickness
Langmuir		C e (mg g−1) is the adsorbate concentration at equilibrium. KL is the Langmuir equilibrium constant (L mg−1), qmax is the maximum adsorption capacity (mg g−1) and RL is a separation factor
Freundlich		K f and 1/n are Freundlich constants
D–R		ε is the Polanyi potential, β is the Dubinin–Radushkevich constant, R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature, and E (kJ mol−1) is the mean adsorption energy
	ln qe = ln qmax + βε2
Sips		K s is the Sips equilibrium constant (L mg−1), n is the heterogeneity factor

Fig. 12. Kinetic analysis of Pb(ii) ion and CR adsorption onto Co-MOF using (a) pseudo-first-order, (b) pseudo-second-order, (c) intra-particle diffusion, and (d) Elovich models.

Table 2. Kinetic model parameters for the adsorption of Pb(ii) and CR on Co-MOF.

Kinetic model	Parameters	Pb(ii)	CR
	q max (exp.) (mg g−1)	309.7	329.5
Pseudo-1st-order	q max (cal.) (mg g−1)	97.90	75.08
	k 1 (min−1)	0.0485	0.1142
Pseudo-2nd-order	q max (cal.) (mg g−1)	312.5	322.6
	k 2 (g mg−1 min−1)	0.0023	0.0107
Intra-particle diffusion	C (mg g−1)	204.3	245.6
	K id (mg g−1 min−1/2)	14.93	18.31
Elovich	α (mg g−1 min−1)	14 256	2 152 770
	β (g mg−1)	0.0329	0.0445

In contrast, the pseudo-second-order model gave an excellent fit for both pollutants, as demonstrated in Fig. 12b. The calculated adsorption capacities were very close to the experimental values, indicating that adsorption on Co-MOF was governed predominantly by chemisorption at active sites.⁵⁹ Similar pseudo-second-order behavior has been widely reported for MOF-based adsorbents used for Pb(ii) and Congo red removal, including Pb(ii) uptake on Zn-MOF-derived carbon, Ti-based MOFs, and magnetic Ti-MOF/chitosan composites, as well as CR uptake on ZnCuCr-TpIm MOF, La-BDC@CS, and magnetic La-MOF systems.^39,60,61 The higher k₂ value for CR than for Pb(ii) further suggests that CR adsorption proceeded more rapidly under the tested conditions, consistent with reports that some MOF adsorbents show very fast dye uptake and can reach high removal within minutes.⁶²

Fig. 12c depicts the intra-particle diffusion model and also shows high correlation coefficients, indicating that pore diffusion contributed to mass transfer during adsorption. However, the nonzero intercept implies that intra-particle diffusion was not the only rate-limiting step; rather, adsorption likely proceeded through multiple coupled steps involving surface sorption, film diffusion, and diffusion within the porous structure. A larger K_id value for CR than for Pb(ii) is therefore consistent with faster diffusion of dye molecules through accessible Co-MOF pores under the studied conditions.^46,63

The Elovich model (Fig. 12d) also described the kinetic data well, supporting the view that the Co-MOF surface is energetically heterogeneous and that adsorption involves chemisorptive interactions on sites with different activation energies. A high α value reflects a rapid initial adsorption stage caused by abundant vacant active sites, whereas β is associated with progressive surface coverage and the activation energy of chemisorption.⁶⁴ Comparable Elovich behavior has also been reported in dye and ion adsorption systems in which pseudo-second-order fitting dominates but heterogeneous-surface effects remain important.⁶⁵

Overall, the adsorption of Pb(ii) ions and CR onto Co-MOF showed better agreement with the pseudo-second-order model, indicating that the adsorption rate is influenced by interactions between adsorbates and active sites. However, this kinetic model alone cannot confirm chemisorption as the governing mechanism. The adsorption process may also involve intra-particle diffusion, boundary-layer effects, and interactions with heterogeneous adsorption sites.

3.4. Isotherm studies

Adsorption equilibrium behavior can be evaluated using various theoretical and empirical models. Classical density functional theory (cDFT) offers a molecular-level description of adsorption systems by considering the spatial distribution of adsorbate density and the interactions occurring within the adsorption environment. Despite its theoretical advantages, the application of cDFT often requires complex mathematical treatments and considerable computational resources.⁶⁶ Therefore, the adsorption equilibrium of Pb(ii) ions and CR on Co-MOF was evaluated using the Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Sips isotherm models (Table 1). These models were selected because they provide practical approaches for interpreting adsorption characteristics, evaluating surface heterogeneity, and understanding the interaction between Pb(ii)/CR molecules and the active sites of the Co-MOF adsorbent.

The equilibrium data for Pb(ii) and Congo red adsorption on Co-MOF were best described by the Langmuir isotherm model (Fig. 13a and Table 3), indicating predominantly monolayer uptake on relatively homogeneous adsorption sites. This interpretation is consistent with multiple recent adsorbent studies in which better Langmuir model fits were taken as evidence for uniform surface sites and monolayer coverage, including CR adsorption on Co-based and mixed-metal MOFs and Pb(ii) adsorption on metal-oxide and carbonaceous sorbents.⁶⁷ The close agreement between the calculated Langmuir capacities and the experimentally determined capacities further supports the robustness of the model fit for both contaminants, with q_max values of 344.8 and 476.2 mg g⁻¹ for Pb(ii) and CR, respectively. Similar agreement between calculated and experimental capacities has been reported for MOF-derived and MOF-based adsorbents, with the Langmuir model successfully capturing Pb(ii) and CR equilibrium behavior with high correlation coefficients and realistic monolayer capacities.⁶⁸ The low R_L values (0 < R_L < 1) confirm that adsorption was highly favorable, while the Freundlich 1/n values below unity also indicate strong adsorbate affinity toward the Co-MOF surface.³⁸

Fig. 13. Equilibrium isotherms of the adsorption of Pb(ii) ions and CR onto Co-MOF using the (a) Langmuir, (b) Freundlich, (c) Dubinin–Radushkevich (D–R), and (d) Sips models.

Table 3. Isotherm model parameters for the adsorption of Pb(ii) and CR on Co-MOF.

Model	Parameters	Pb(ii)	CR
Langmuir	q max (exp.) (mg g−1)	347.2	475.1
	q max (cal.) (mg g−1)	344.8	476.2
	K L	1.206	0.3809
	R L	(0.0390–0.0045)	(0.1160–0.0120)
Freundlich	1/n	0.2833	0.2415
	K f	144.8	220.0
Dubinin–Radushkevich	q max (mg g−1)	339.3	475.3
	E (kJ mol−1)	11.18	15.81
Sips	N	1.0053	1.0011
	K s	0.3722	0.6434

The Freundlich parameters (Fig. 13b and Table 3) also indicated favorable adsorption, with the 1/n values of 0.2833 for Pb(ii) and 0.2415 for CR suggesting strong affinity between the adsorbates and Co-MOF. However, the superior fit of the Langmuir model relative to the Freundlich one suggests that monolayer adsorption predominates over multilayer adsorption in this system.

The Dubinin–Radushkevich (Fig. 13c and Table 3) adsorption energies of 11.18 kJ mol⁻¹ for Pb(ii) and 15.81 kJ mol⁻¹ for CR are consistent with a mechanism dominated by chemisorptive interactions rather than weak physical sorption. This interpretation aligns with studies that have used pseudo-second-order behavior and strong adsorbent–adsorbate interactions as evidence of chemical adsorption for both Pb(ii) and CR on MOF-based materials.⁵⁹

The Sips exponent values close to unity (Fig. 13d) further indicate that the adsorption approaches ideal Langmuir behavior, while still allowing for minor surface heterogeneity. This conclusion is also compatible with reports in which Langmuir-like fits dominated, but some heterogeneity remained detectable through Freundlich or mixed-model behavior, particularly in composite and bimetallic MOF systems.³³

Overall, Pb(ii) and CR adsorption onto Co-MOF is best described by the Langmuir model, indicating favorable monolayer adsorption on largely homogeneous sites. At the same time, the D–R and Sips parameters support a predominantly chemisorptive mechanism with only slight heterogeneity.

3.5. Thermodynamic studies

The thermodynamic behavior of the adsorption of Pb(ii) ions and CR onto Co-MOF was evaluated using eqn (3)–(5) to calculate thermodynamic parameters, including (ΔH), (ΔS), and (ΔG), which refer to enthalpy change, entropy change, and a change in Gibbs free energy, respectively.

3

4

ΔG = −RT ln K_d 5

where T is the absolute temperature in kelvin (K), R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), and plotting ln K_dvs. 1/T can be used to obtain the Van't Hoff slope and intercept to estimate the enthalpy change (ΔH) and entropy change (ΔS), as shown in Fig. 14. The adsorption distribution coefficient (K_d) was determined from the ratio of adsorption capacity to equilibrium concentration (q_e/C_e).⁶⁹

Fig. 14. Thermodynamic analysis of Pb(ii) and Congo Red (CR) adsorption onto Co-MOF based on the Van't Hoff relationship.

The thermodynamic analysis indicates that adsorption of Pb(ii) ions and CR onto Co-MOF was spontaneous over the investigated temperature range, as evidenced by the negative ΔG values for both adsorbates, as presented in Table 4. The progressive decrease in ΔG with increasing temperature further shows that adsorption became more favorable at elevated temperatures, reflecting a stronger thermodynamic driving force and enhanced affinity of Pb(ii) and CR toward the Co-MOF surface.⁷⁰

Table 4. Thermodynamic parameters concerning the adsorption of Pb(ii) and CR on Co-MOF.

Adsorbate	T (K)	ΔG (kJ mol−1)	ΔH (kJ mol−1)	ΔS (J mol−1 K−1)
Pb(ii)	288	−3.64	+79.43	+288.9
	293	−5.23
	298	−6.49
	303	−9.57
	308	−10.06
	313	−10.17
	323	−10.53
CR	288	−5.51
	293	−7.14	+112.26	+406.9
	298	−8.42
	303	−11.03
	308	−13.77
	313	13.32
	323	−13.53

The positive ΔH values suggest that adsorption was thermally favored under the investigated conditions. This behavior may be associated with enhanced diffusion, dehydration effects, and increased accessibility of adsorption sites at elevated temperatures. Similar temperature-enhanced and endothermic adsorption behavior has been reported for Pb(ii) uptake on UiO-66-EDA and for CR adsorption on Fe₃O₄@MIL-53(Al) and ZIF-67, for which increasing temperature improved adsorption performance.^71–73

The large positive ΔS values indicate increased randomness at the solid–solution interface during adsorption. In related adsorption systems, positive entropy changes have been interpreted as evidence of greater interfacial disorder caused by solvent displacement and adsorbate reorganization during uptake.⁷⁰

The marked enthalpy values in the present system suggest that adsorption was not governed solely by weak physical interactions, but likely involved stronger adsorbate–surface interactions. This interpretation is consistent with MOF studies of Pb(ii), in which adsorption was linked to electron exchange, electron sharing, electrostatic attraction, and covalent interaction with surface functional groups, and with CR systems in which adsorption involved electrostatic, π–π, and coordination interactions.⁷⁴

3.6. Regeneration and reusability

The desorption and regeneration results demonstrate that the synthesized Co-MOF is a recyclable adsorbent with strong potential for practical wastewater treatment, as shown in Fig. 15. After each adsorption cycle, the spent Co-MOF was regenerated using an appropriate desorbing agent, either dilute HCl (0.1 M) for Pb(ii) ions or absolute ethanol solution for CR, for 24 h, followed by filtration, repeated washing with deionized water, and drying before reuse. The recovered adsorbent was subjected to subsequent adsorption cycles to evaluate its recyclability. This strategy is consistent with established regeneration procedures for MOF-based adsorbents.⁷⁵

Fig. 15. Reusability performance of Co-MOF during successive adsorption–desorption cycles for the removal of Pb(ii) ions and Congo red.

The Co-MOF retained a high adsorption capacity over six consecutive cycles, indicating good structural stability and effective recovery of active sites. The small decrease in removal efficiency after repeated use likely arose from incomplete desorption of strongly bound species, partial blockage of adsorption sites, and minor framework deterioration during repeated adsorption–desorption operation.⁷⁶ To evaluate the structural stability of the Co-MOF during repeated adsorption–desorption cycles, cobalt leaching was monitored via flame atomic absorption spectroscopy. No detectable Co ions were observed in the regenerated solutions after the adsorption cycles, indicating negligible cobalt release and confirming the high stability of the Co-MOF framework under the applied regeneration conditions.

This behavior is consistent with recent reports on MOF-based adsorbents for Pb(ii) and CR, including Sn-BDC-MOF/GO, TMIP-206, HKUST-1@Urea, ZnCuCr-TpIm MOF, and PEI@MDC, all of which retained substantial adsorption performance after multiple regeneration cycles.^59,75,77 Collectively, these results indicate that Co-MOF combines high reusability, structural robustness, and economic promise, supporting its application as a sustainable adsorbent for repeated removal of Pb(ii) ions and Congo red from contaminated water.

3.7. Adsorption mechanism

Fig. 16 shows that the adsorption of the Pb(ii) metal ions and the dye Congo red onto Co-MOF can be described as a predominantly chemisorption-driven process that occurs in a Langmuir-type fashion. The adsorption of Pb(ii) ions is mainly driven by coordination and surface complexation with O- and N-containing functional groups, with electrostatic attraction, and in some systems, ion exchange or donor–acceptor interactions between electron-rich surface atoms and the Pb(ii) metal ion centers playing supporting roles.^44,75

Fig. 16. Schematic illustration of the proposed adsorption mechanism of Pb(ii) ions and Congo red onto Co-MOF.

In addition to electrostatic attraction between the anionic sulfonate groups of CR and the positively charged metal sites, π–π stacking, hydrogen bonding, coordination to unsaturated nodes, and pore filling are important in stabilizing the molecules in the framework, and hierarchical porosity and intraparticle diffusion are crucial for achieving improved mass transfer of both Pb(ii) and CR to the internal active sites, particularly in defective or hollow MOF-based architectures that contain numerous accessible adsorption regions.²¹ The excellent agreement with pseudo-second-order kinetics and Langmuir-type equilibrium models across related Pb(ii) and CR removal systems therefore supports a mechanism dominated by monolayer adsorption on relatively homogeneous active sites, with chemisorption as the principal pathway and electrostatic and pore-transport effects acting as cooperative contributions rather than independent controlling steps.^21,44

3.8. Comparison with previously reported adsorbents

To evaluate the performance of the synthesized Co-MOF, its adsorption behavior was compared with that of recently reported MOF-based adsorbents for Pb(ii) and CR removal (Table 5). Most reported MOFs exhibit adsorption capacities of approximately 268–520 mg g⁻¹ under optimized conditions. The synthesized Co-MOF demonstrated competitive adsorption performance, excellent regeneration ability (>90% efficiency after six cycles), and high structural stability. Its superior performance is attributed to the synergistic effects of its abundant Co active sites, porous framework, and multiple adsorption mechanisms, including coordination complexation, electrostatic attraction, hydrogen bonding, and π–π interactions. Moreover, unlike many previous studies performed in single-component synthetic solutions, the present work demonstrates the simultaneous removal of Pb(ii) and CR from real textile wastewater, highlighting the practical potential of Co-MOF for sustainable wastewater treatment.

Table 5. Comparison of the adsorption performance of the synthesized Co-MOF with that of reported MOF-based adsorbents for Pb(ii) and Congo red.

Pollutant	Adsorbents	Adsorption capacity (mg g−1)	Optimum conditions: time (min), dose (g L−1), temp. (K)	Regeneration cycles	References
CR	Zn(ii)-based metal–organic framework	262.2	12 h, pH 5.2, 0.166, 308	5	78
	MOF-5/Cu	357.42	2, pH 5.8, 10 mg, 298	5	79
	Al-AZB	456.6	1440, pH 4.0, 0.3, 300	3	80
	Zr-AZB	128.9
	UiO-66	83.01	120, pH 6.5, —, 303.15	5	81
	BHP-MOFs	176.06	180, pH 6.0, 0.10, 298	7	82
	ZT-MOF@Ag@C	416.6	60, pH 5.0, 0.10, 308	6	83
	Hierarchical Zr-MOF/PVA/melamine sponge composite	303.95	120, pH 6.0, 0.50, 298	5	84
	MOF-5	351.8	60, pH 5.8, 0.20, 298	5	85
	ZnCuCr-TpIm MOF	325	60, pH 6–7, 0.40, 328	5	86
	Cd-MOF	500	30 h, pH 7.0, 0.20,298	5	87
	Co-MOF	475.1	20, pH 5.0–6.0,0.30, 308	6	This study
Pb(ii)	MIL-100(Fe)	23.46	180, pH 5.5, 3.0, 298	3	88
	Cr-MOF/AC composite	127	40, pH 5.0, 0.25, 298	5	89
	UiO-66-NH2	320.73	90, pH 6.0, —, 328	4	90
	UiO-66	291.39
	Hierarchical Zr-MOF/PVA/melamine sponge composite	326.8	120, pH 5.5, 0.50, 298	5	84
	Co-MOF	347.2	50, pH 5.0–6.0, 0.3, 308	6	This study

3.9. Application

To evaluate the practical applicability of the synthesized Co-MOF, its removal efficiency toward Congo Red (CR) dye and Pb(ii) ions was assessed using three real water matrices, namely textile fiber wastewater, textile washing wastewater, and municipal sewage effluent, as shown in Fig. 17. In textile fiber wastewater with initial Pb(ii) and CR concentrations of 80.9 and 140.03 mg L⁻¹, respectively, the Co-MOF achieved simultaneous removal efficiencies of 72.38% and 82%. A marked enhancement in performance was observed for textile washing wastewater (Pb(ii): 34.3 mg L⁻¹; CR: 63.7 mg L⁻¹), with the removal efficiencies increasing to 86.20% and 93.09%, respectively. The highest removal efficiencies, 93% for Pb(ii) and 99.9% for CR, were recorded in the sewage wastewater sample, which exhibited the lowest initial pollutant concentrations (11.9 and 35.0 mg L⁻¹, respectively). These simultaneous experiments were conducted under the optimized conditions: a natural pH of 5–6, an adsorbent dose of 0.05 g, a contact time of 60 min, and a temperature of 35 °C. This inverse relationship between initial pollutant concentration and removal efficiency can be attributed to the finite number of active adsorption sites on the Co-MOF surface, which become progressively saturated at higher pollutant loadings.

Fig. 17. Schematic illustration of Co-MOF adsorption and removal of Pb(ii) and Congo red from real wastewater samples.

The optimized adsorption conditions obtained in the individual Pb(ii) and CR systems were successfully maintained during the simultaneous removal process. Comparable adsorption capacities were achieved in the presence of both pollutants, as presented in Table 6. These results collectively demonstrate the robustness and versatility of the Co-MOF adsorbent in complex, real-world aqueous environments containing competing ions and organic matter, underscoring its strong potential for practical application in industrial wastewater and dye-contaminated water treatment.

Table 6. Comparison of Pb(ii) and CR adsorption capacities in single-component and simultaneous systems in real wastewater samples.

Pollutant	Textile fiber wastewater qe, mg g−1		Textile washing wastewater qe, mg g−1		Municipal sewage effluent qe, mg g−1
	Individual	Simultaneous	Individual	Simultaneous	Individual	Simultaneous
Pb	152.09	117.12	67.10	58.45	23.8	22.13
CR	256.26	229.66	126.80	118.6	69.99	69.93

4. Conclusion

In this study, a successful synthesis of an azo-functionalized cobalt-based metal–organic framework (Co-MOF) was reported, and its use as an adsorbent for the simultaneous adsorption of Pb(ii) and CR from aqueous solution was investigated. Physicochemical characterization confirmed the formation of a crystalline cobalt-containing framework with surface functional groups capable of interacting with both inorganic and organic pollutants. Its adsorption performance was found to be considerably affected by solution pH, adsorbent dose, the concentrations of the pollutants, contact time, and temperature. Under the optimum conditions, the Co-MOF displayed efficient adsorption performance with maximum Langmuir adsorption capacities of 344.8 mg g⁻¹ and 476.2 mg g⁻¹, respectively, for Pb(ii) and CR. Adsorption was, in general, fast, with the adsorption of Pb(ii) requiring about 50 min and that of CR about 20 min to reach equilibrium. Adsorption kinetic and equilibrium data were best fit by pseudo-second-order kinetics and the Langmuir adsorption isotherm, suggesting a strong interaction between the pollutant molecules and the surface of the Co-MOF as well as favorable adsorption behavior. Furthermore, thermodynamic analyses revealed that the adsorption processes of the pollutants were spontaneous and favorable at elevated temperatures under the conditions of the current investigation. Changes in the FTIR spectra, zeta potential, and morphology after adsorption indicate that electrostatic interactions, hydrogen bonding, coordination of the pollutants, and active sites of the Co-MOF surface may play an important role in the capture of the target compounds; however, the comparative contributions of the various adsorption mechanisms remain to be checked and verified. The synthesized material also showed good reusability for adsorption–desorption, and good removal percentages of the targeted pollutants can be obtained up to the sixth cycle. Moreover, adsorption experiments using real wastewater displayed the significant Pb(ii) and CR uptake ability of the Co-MOF, demonstrating its applicability in environmental remediation. Overall, the results indicated that an azo-functionalized Co-MOF can serve as an efficient adsorbent for the simultaneous removal of Pb(ii) and Congo red from aqueous solutions. The research presented herein provides a potential avenue for more extensive investigations, including surface characterization, area/pore characterization, competition studies, metal leaching studies, and multiple-cycle durability tests.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.