Kinetic Modelling and pH-Dependent Adsorption of Malachite Green onto Pectin/Poly(NIPAm-co-Acrylic Acid) Nano-hydrogel: Mechanistic Insights

INTRODUCTION
Water contamination by synthetic dyes from textile, paper, and leather industries has become a pressing environmental concern worldwide. Among these pollutants, malachite green (MG), a triphenylmethane cationic dye, is extensively used in dyeing processes, aquaculture, and as an antifungal agent. Despite its wide application, MG poses serious health risks including carcinogenicity, mutagenicity, and respiratory toxicity, even at low concentrations [1,2]. Its molecular stability and resistance to natural degradation necessitate the development of effective remediation strategies. Several techniques have been explored for dye removal from wastewater, including photocatalytic degradation, coagulation, membrane filtration, and biological treatment [3,4]. Nevertheless, adsorption has gained considerable attention due to its operational simplicity, cost-effectiveness, and high removal efficiency across a broad spectrum of pollutant concentrations [5,6].
Hydrogel-based adsorbents have emerged as promising candidates owing to their three-dimensional cross-linked polymer networks, tuneable swelling capacity, and abundant functional groups that facilitate pollutant binding [7,8]. Stimuli-responsive hydrogels, in particular those incorporating poly(N-isopropylacrylamide) (PNIPAm) and pH-sensitive monomers such as acrylic acid, exhibit dual responsiveness to temperature and pH changes, which can be exploited to enhance adsorption performance and enable regeneration under controlled conditions [9,10]. The incorporation of natural polysaccharides like pectin into the hydrogel matrix further improves biocompatibility, hydrophilicity, and the density of carboxyl functional groups available for electrostatic interaction with cationic pollutants [11,12].
Despite the considerable progress made in hydrogel-based adsorption, detailed kinetic and pH-dependent mechanistic studies on pectin-based multi-responsive hydrogels for MG removal remain scarce. Most existing studies either focus solely on equilibrium data or employ single-component hydrogels lacking the synergistic benefits of combined natural and synthetic polymers [13,14]. This study therefore aims to synthesize a pectin/poly(NIPAm-co-acrylic acid) nano-hydrogel through free-radical copolymerization and to systematically investigate the kinetic behaviour and pH-dependent adsorption mechanism for MG dye. Particular attention is given to determining the rate-controlling step via pseudo-first-order, pseudo-second-order, and Weber–Morris diffusion models, alongside elucidating the role of surface charge through point of zero charge analysis. The findings presented herein contribute to a deeper understanding of structure–performance relationships in multi-responsive hydrogels and offer practical insights for designing efficient adsorbents for cationic dye removal.

MATERIALS AND METHODS
Materials
Pectin (from citrus peel), N-isopropylacrylamide (NIPAm, 97%), acrylic acid (AA, 99%), N,N′-methylenebisacrylamide (MBA, cross-linking agent), and potassium persulphate (KPS, initiator) were purchased from Sigma-Aldrich and used without further purification. Malachite green oxalate (C₂₃H₂₅ClN₂, molecular weight 364.91 g/mol, λmax = 621.6 nm) was obtained from Merck. Hydrochloric acid (HCl, 1 M) and sodium hydroxide (NaOH, 1 M) were employed for pH adjustment. Sodium chloride (NaCl, 0.1 M) was used in point of zero charge determination. All solutions were prepared using deionized water.

Synthesis of Pectin/Poly(NIPAm-co-Acrylic Acid) Nano-hydrogel
The nano-hydrogel was synthesized through free-radical copolymerization in aqueous solution. Briefly, 0.5 g of pectin was dissolved in 30 mL of deionized water under continuous stirring at 40°C. After 30 min, 0.4 g of NIPAm was added to the solution with stirring, followed by the addition of 4 mL of acrylic acid after 15 min. Subsequently, 0.04 g of MBA (cross-linker) dissolved in 2 mL of deionized water was introduced into the mixture under nitrogen atmosphere. In a separate vessel, 0.04 g of KPS (initiator) was dissolved in 2 mL of water and then added to the reaction mixture while maintaining nitrogen purging and continuous stirring at 40°C. The resulting pre-gel solution was transferred into glass tubes and placed in a water bath at 70°C for 6 h to complete the polymerization. The obtained nano-hydrogel was cut into small pieces, washed repeatedly with deionized water (replacing water every 60 min) to remove unreacted monomers and oligomers, and subsequently dried in an oven at 70°C for 48 h until a constant weight was achieved. The dried nano-hydrogel was finely ground and sieved through a 100 μm mesh for subsequent adsorption experiments. The gel content was determined gravimetrically and found to be 93.23%, confirming a high degree of cross-linking.

Characterization Techniques
The functional groups of the synthesized nano-hydrogel were identified by Fourier-transform infrared spectroscopy (FTIR, Shimadzu 8400S) in the range 400–4000 cm⁻¹ using KBr pellets. The crystalline structure was analysed by X-ray diffraction (XRD, Shimadzu XRD-6000) with Cu-Kα radiation (λ = 1.5406 Å) over 2θ range of 5–80°. Surface morphology was examined using field-emission scanning electron microscopy (FE-SEM, TESCAN MIRA3) at 20 kV accelerating voltage. Thermal stability was assessed by thermogravimetric analysis (TGA, Perkin Elmer TGA4000) from 30 to 750°C at a heating rate of 10°C/min under nitrogen atmosphere. BET surface area and pore size distribution were determined by nitrogen adsorption–desorption isotherm at 77 K (Quantachrome NOVA 2200e).

Point of Zero Charge Determination
The point of zero charge (pHPZC) was determined using the pH drift method. A mass of 0.02 g of the nano-hydrogel was added to 25 mL of 0.1 M NaCl solution at various initial pH values (pHi = 2–12), adjusted using 1 M HCl or NaOH. The suspensions were agitated at 120 rpm for 24 h at 25°C. The final pH (pHf) was measured, and ΔpH (= pHf – pHi) was plotted against pHi. The pHPZC corresponds to the pHi at which ΔpH = 0 [15].

Batch Adsorption Experiments
All adsorption experiments were performed in batch mode. A stock solution of MG (1000 mg/L) was prepared by dissolving 1 g of MG in 1000 mL of deionized water. Working solutions were obtained by serial dilution. The maximum absorption wavelength was confirmed at 621.6 nm using a UV–Visible spectrophotometer (Shimadzu UV-1800).

Effect of adsorbent dosage
Various masses of nano-hydrogel (0.006–0.06 g) were added to 10 mL of MG solution (500 mg/L) at pH 7 and 25°C. After shaking for 3 h, the residual MG concentration was measured spectrophotometrically.

Effect of contact time
The optimum dosage (0.008 g) was contacted with 10 mL of MG solution (500 mg/L) at pH 7 and 25°C for varying time intervals (1–150 min). Samples were withdrawn at predetermined times and analysed.

Effect of pH
The solution pH was varied from 2 to 10 using 1 M HCl or NaOH. The adsorbent (0.008 g) was added to 10 mL of MG solution (500 mg/L) and agitated at 120 rpm until equilibrium was reached.
The equilibrium adsorption capacity (qe, mg/g) and removal efficiency (R%) were calculated using Eqs. 1 and 2:

qₑ = (C₀ – Cₑ) × V / m                                            (1)

R% = [(C₀ – Cₑ) / C₀] × 100                                 (2)

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

Regeneration and Reusability Experiments
The reusability of the nano-hydrogel was evaluated over five consecutive adsorption–desorption cycles (n = 3 per cycle). In each cycle, 0.008 g of the nano-hydrogel was equilibrated with 10 mL of MG solution (500 mg/L, pH 7) at 25°C for 90 min. After adsorption, the supernatant was analysed spectrophotometrically to determine the residual MG concentration. The spent nano-hydrogel was then desorbed by immersion in 20 mL of 0.1 M HCl in 50% (v/v) ethanol and shaken at 150 rpm for 60 min. The desorbed nano-hydrogel was washed with deionized water until neutral pH, dried at 60°C for 12 h, and re-used in the next adsorption cycle. Desorption efficiency was calculated as the ratio of MG released during desorption to MG adsorbed during the preceding adsorption step × 100. All measurements were performed in triplicate, and the results are reported as mean ± standard deviation (SD). Statistical significance across cycles was assessed by one-way ANOVA at the 95% confidence level (p < 0.05).

Applicability in Real Water Matrices
To evaluate the practical applicability of the nano-hydrogel, adsorption experiments were conducted using four different water matrices: (i) deionized water (control), (ii) tap water (Al-Diwaniyah municipal supply; total dissolved solids, TDS ≈ 480 mg/L, pH 7.3), (iii) river water (collected from the Diwaniyah River; TDS ≈ 890 mg/L, pH 7.8, turbidity 18 NTU), and (iv) simulated industrial wastewater (prepared by supplementing deionized water with 100 mg/L Na2SO4, 50 mg/L NaCl, 20 mg/L CaCl2, 10 mg/L humic acid, and 5 mg/L phenol, pH adjusted to 7.0). Each matrix was spiked with 500 mg/L MG, and adsorption was conducted under optimum conditions (0.008 g adsorbent, 90 min, 25°C). All experiments were performed in triplicate (n = 3). One-way ANOVA followed by Tukey’s post-hoc test was used to compare qe values across matrices (p < 0.05).

RESULTS AND DISCUSSION
Characterization of the Nano-hydrogel
The FTIR spectrum of the synthesized nano-hydrogel confirmed the successful copolymerization and cross-linking. A broad absorption band centred at 3794 cm⁻¹ was attributed to overlapping O–H and N–H stretching vibrations from pectin and NIPAm, respectively. Bands at 2925 and 2857 cm⁻¹ corresponded to asymmetric and symmetric C–H stretching of aliphatic chains. The carbonyl stretching band at 1730 cm⁻¹ confirmed the presence of ester groups from acrylic acid and pectin, while the amide I band at 1600 cm⁻¹ and amide II band at 1547 cm⁻¹ verified the incorporation of NIPAm into the polymer network. Additional bands at 1458 cm⁻¹ (CH₂ bending), 1258 cm⁻¹ (C–O–C stretching), and 1024 cm⁻¹ (C–O stretching of pectin sugar rings) further supported the composite structure [16,17].
The XRD pattern displayed a broad hump between 15° and 30° (2θ) with a diffuse peak at approximately 20.7°, characteristic of a semi-crystalline to amorphous polymeric material. This pattern is consistent with the disordered cross-linked network formed between pectin, PNIPAm, and polyacrylic acid chains [18]. The absence of sharp crystalline peaks confirmed that the nano-hydrogel matrix was predominantly amorphous.
FE-SEM micrographs revealed a folded sheet-like morphology with wrinkles and surface undulations typical of dried cross-linked hydrogels. Statistical analysis of 142 particulate features yielded a mean diameter of 84.1 nm (σ = 30.4 nm), with visible surface cavities and pores that serve as potential adsorption sites [19]. BET analysis showed a specific surface area of 6.62 m²/g, a total pore volume of 0.027 cm³/g, and an average pore diameter of 16.56 nm, classifying the material as mesoporous according to IUPAC nomenclature. The nitrogen adsorption–desorption isotherm followed a Type IV pattern with an H3 hysteresis loop, indicative of slit-shaped mesopores arising from stacked polymeric sheets [20]. TGA revealed three-stage weight loss: dehydration (30–150°C, 6.1%), organic decomposition (150–400°C, 58.8%), and advanced carbonization (400–750°C, 9.5%), with a total mass loss of 74.4% [21].

Effect of Adsorbent Dosage
The influence of adsorbent dosage on MG removal was investigated by varying the nano-hydrogel mass from 0.006 to 0.06 g while keeping other parameters constant (C₀ = 500 mg/L, V = 10 mL, pH = 7, T = 25°C). As shown in Fig. 3a, the removal efficiency increased steadily from 86.90% at 0.006 g to essentially 100% at 0.06 g, which is logically attributed to the greater number of active binding sites available at higher dosages [22]. Conversely, the equilibrium adsorption capacity (qₑ) declined from 724.13 to 83.33 mg/g over the same range, reflecting the underutilization of available sites when the adsorbent is present in excess relative to the quantity of solute molecules. Such behaviour is commonly reported in the literature and arises because, at elevated adsorbent loadings, a large fraction of binding sites remain unsaturated [23]. A dosage of 0.008 g was selected as the optimum working mass, as it provided a favourable balance between adsorption capacity (596.04 mg/g) and removal percentage (95.37%). This dosage was therefore employed in all subsequent experiments. All batch experiments throughout this study were performed in triplicate (n = 3), and results are expressed as mean ± standard deviation (SD) unless stated otherwise.

Effect of Contact Time and Equilibrium
The kinetics of MG adsorption onto the nano-hydrogel were studied at the optimum dosage (0.008 g) with a contact time ranging from 1 to 150 min (Fig. 3b). Three distinct phases were observed in the time-dependent adsorption profile. During the initial rapid uptake phase (0–10 min), the adsorption capacity rose sharply from 515.29 to 577.29 mg/g, corresponding to the immediate occupation of readily accessible surface sites by MG molecules through electrostatic attraction and hydrogen bonding. In the second phase (10–60 min), the rate of uptake decelerated progressively as the surface sites became increasingly saturated and diffusion into the interior pore network began to govern the process. Finally, an equilibrium plateau was attained between 60 and 90 min, with no statistically significant change in qₑ beyond this interval (qₑ = 604.89 mg/g, R% = 96.78%). The rapid initial uptake may be attributed to the strong electrostatic interaction between the cationic MG molecules and the deprotonated carboxyl groups (–COO⁻) on the nano-hydrogel surface, which are abundant due to the acrylic acid and pectin components of the polymer network [24]. The existence of a clear equilibrium plateau confirms monolayer-to-multilayer transition saturation of the available binding sites [25].

Adsorption Kinetic Modelling
To gain mechanistic insights into the adsorption process, three kinetic models were applied in both linear and non-linear forms: the pseudo-first-order (PFO), pseudo-second-order (PSO), and Weber–Morris intraparticle diffusion models.

Pseudo-First-Order Model
The Lagergren pseudo-first-order model assumes that the rate of change in solute uptake is directly proportional to the difference between the equilibrium capacity and the amount adsorbed at any given time. Its linearized form is expressed as:

log(qₑ – qₜ) = log qₑ – (k₁/2.303)t                     (3)

Although the PFO model yielded a reasonable regression coefficient (R² = 0.8596 for the linear form), the calculated equilibrium capacity (qₑ,calc) deviated considerably from the experimental value (qₑ,exp = 604.89 mg/g). This discrepancy is typical of systems in which chemisorption, rather than simple physisorption, governs the rate of uptake. The PFO model is generally applicable only to the initial stages of adsorption and tends to underperform when the process involves chemical bonding between the adsorbate and adsorbent surface [26].

Pseudo-Second-Order Model
The Ho and McKay pseudo-second-order model presumes that the overall rate of adsorption is controlled by chemisorption involving electron sharing or transfer between the adsorbate and active surface sites:

t/qₜ = 1/(k₂qₑ²) + t/qₑ                                          (4)

The PSO model provided an excellent fit to the experimental data, with R² ≈ 1.000 (RMSE = 0.89 mg/g, SSE = 10.61) for the linear regression. The calculated equilibrium capacity (qₑ,calc = 605.13 mg/g) was in close agreement with the experimental value (604.89 mg/g), and the rate constant k₂ was determined as 3.654 × 10⁻³ g/mg·min (i.e., 0.003654 g/mg·min). The initial adsorption rate (h = k₂qₑ²× 10⁻⁶ g/mg·min. The initial adsorption rate (h = k₂qₑ²) was calculated as 1338.14 mg/g·min, reflecting the strong affinity between MG molecules and the nano-hydrogel surface functional groups [27]. The dominance of the PSO model confirms that the adsorption of MG onto the nano-hydrogel is predominantly governed by chemisorption, involving electron-pair sharing between the nitrogen atoms in the MG structure and the carboxyl and amide groups on the polymer network, alongside electrostatic interactions between the cationic dye and the anionic surface [28].

Weber–Morris Intraparticle Diffusion Model
The Weber–Morris model was employed to assess the contribution of intraparticle diffusion to the overall rate:

qₜ = kᵢᵈ t¹˲² + C                                                        (5)

The plot of qₜ versus t¹˲² exhibited two distinct linear segments, suggesting a multi-step diffusion process. The first segment (kᵢᵈ,₁ = 26.49 mg/g·min¹˲²) represented rapid external surface adsorption and boundary layer diffusion, while the second segment (kᵢᵈ,₂ = 1.67 mg/g·min¹˲²) corresponded to the slower intraparticle diffusion within the mesoporous network of the nano-hydrogel. Importantly, the linear segments did not pass through the origin (C ≠ 0), which indicates that intraparticle diffusion was involved but was not the sole rate-limiting step. The non-zero intercept values suggest that boundary layer effects and chemical interactions at the surface also contributed significantly to the overall rate control [29]. This multi-step mechanism is consistent with the mesoporous structure revealed by BET analysis (average pore diameter = 16.56 nm), which provides pathways for gradual MG diffusion into the interior of the nano-hydrogel particles after the external surface sites are saturated [30].

Point of Zero Charge (pHₚ₁ₜ)
The point of zero charge is a fundamental parameter that dictates the surface charge of an adsorbent as a function of solution pH and consequently governs the electrostatic interactions between the adsorbent and ionic adsorbates. The pH drift results for the synthesized nano-hydrogel are presented in Fig. 5. The plot of ΔpH versus pHi revealed that the pHPZC was located at pH 4.2, which is the pHi value at which ΔpH equals zero. At pH < 4.2, the ΔpH values were positive, indicating that the surface consumed protons from solution (i.e., the surface was positively charged). At pH > 4.2, the ΔpH values became negative, signifying proton release and a net negative surface charge [15].
The acidic pHPZC value (4.2) is attributed to the abundance of carboxylic acid groups from two sources: acrylic acid (pKa ≈ 4.25) and galacturonic acid residues in pectin (pKa ≈ 3.5). At pH values above the pHPZC, deprotonation of these carboxyl groups (–COOH → –COO⁻ + H⁺) generates a negatively charged surface that readily attracts cationic species such as MG through electrostatic attraction [31]. This finding is central to understanding the pH-dependent adsorption behaviour discussed in the following section.

Effect of Solution pH on MG Adsorption
The solution pH exerts a profound influence on adsorption performance by modulating both the surface charge of the adsorbent and the speciation of the dye in solution. Fig. 5 illustrates the variation of qₑ with pH in the range 2–10. At pH 2, the lowest adsorption capacity was recorded (qₑ = 515.21 mg/g, R% = 82.43%). This can be explained by the protonation of carboxyl and amide groups on the nano-hydrogel surface at pH values well below the pHPZC, which renders the surface positively charged and creates electrostatic repulsion with the cationic MG molecules. Moreover, the high concentration of H⁺ ions in strongly acidic media competes with MG cations for the limited negatively charged binding sites [32]. As the pH increased from 2 to 4, a noticeable improvement in qₑ was observed, coinciding with the approach to the pHPZC (4.2) where the surface transitions from a net positive to a net negative charge. Beyond pH 4, the progressive ionization of –COOH groups to –COO⁻ significantly enhances the anionic character of the surface, thereby strengthening electrostatic attraction towards MG cations. Notably, qₑ values remained high and relatively stable across the pH range 4–10 (592–609 mg/g), with the maximum observed at pH 10 (qₑ = 608.96 mg/g). This broad operational pH window is of considerable practical importance, as it indicates that the nano-hydrogel can function effectively without stringent pH control in real wastewater applications [33]. The pH dependence of adsorption also provides mechanistic information. The fact that a substantial removal (82.43%) was achieved even at pH 2—where the surface is positively charged—implies that non-electrostatic interactions such as hydrogen bonding between MG and the amide (–CONH–) and hydroxyl (–OH) groups of the nano-hydrogel, as well as π–π interactions involving the aromatic rings of MG and the polymer backbone, contribute meaningfully to the overall adsorption mechanism. At alkaline pH, the combination of electrostatic attraction, hydrogen bonding, and π–π stacking results in maximum uptake. These findings are corroborated by the FTIR analysis of the nano-hydrogel after MG adsorption, which revealed shifts in the O–H, C=O, and amide I bands, alongside the appearance of new peaks attributable to MG structural vibrations [34,35].

Regeneration and Reusability
The practical viability of any adsorbent is critically dependent on its regeneration potential and reusability over multiple cycles. Fig. 6 presents the adsorption capacity, removal efficiency, and desorption efficiency of the pectin/poly(NIPAm-co-AA) nano-hydrogel over five successive adsorption–desorption cycles using 0.1 M HCl in 50% ethanol as the eluent (n = 3 per cycle). The adsorption capacity decreased gradually from 604.89 ± 8.12 mg/g in the first cycle to 531.27 ± 12.64 mg/g in the fifth cycle, corresponding to a cumulative capacity retention of 87.8% (Fig. 6a). Similarly, the removal efficiency declined from 96.78 ± 1.30% to 85.00 ± 2.02%. One-way ANOVA confirmed that the decrease in qe across cycles was statistically significant (F = 38.80, p < 0.001), reflecting progressive loss of active sites due to irreversible chemisorption and structural relaxation of the polymer network [39]. The desorption efficiency also decreased from 94.32 ± 2.14% (cycle 1) to 82.67 ± 3.45% (cycle 5) (Fig. 6b), suggesting that a fraction of MG molecules are retained by strong covalent-like interactions (consistent with the PSO mechanism) that resist elution. Nonetheless, the retention of >85% of the initial adsorption capacity after five cycles compares favourably with other hydrogel-based adsorbents reported in the literature [40,41] and confirms the suitability of the nano-hydrogel for repeated use in practical water treatment scenarios.

Applicability in Real Water Matrices
The performance of the nano-hydrogel was further examined in four water matrices of increasing complexity to assess its tolerance to competing ions, natural organic matter (NOM), and other co-contaminants (Fig. 7). All experiments were conducted in triplicate (n = 3) under optimum conditions (C₀ = 500 mg/L, 0.008 g, pH 7, 25°C, 90 min). In deionized water, the nano-hydrogel achieved qe = 604.89 ± 8.12 mg/g (R = 96.78 ± 1.30%). A modest reduction was observed in tap water (qe = 578.34 ± 10.67 mg/g, R = 92.53 ± 1.71%), attributable to the competitive adsorption of Ca2+ and Mg2+ ions that partially screen the anionic surface sites. In river water, the adsorption capacity decreased further to 552.17 ± 13.24 mg/g (R = 88.35 ± 2.12%), which can be ascribed to the combined effects of elevated TDS, NOM fouling of surface sites, and colloidal competition [42]. The most pronounced reduction occurred in simulated industrial wastewater (qe = 498.63 ± 15.89 mg/g, R = 79.78 ± 2.54%), where humic acid and phenol compete for hydrogen-bonding and π–π stacking sites, and the high ionic strength compresses the electrical double layer, diminishing electrostatic attraction [43]. One-way ANOVA revealed a statistically significant difference in qe across the four matrices (F = 53.80, p < 0.001). Tukey’s post-hoc analysis indicated that the DI–river and DI–industrial pairs were significantly different (p < 0.01), whereas the DI–tap pair was not (p = 0.078). Despite the reductions observed in complex matrices, the nano-hydrogel retained >79% removal efficiency even in the most challenging medium, demonstrating its robustness for practical water treatment applications.

Mechanism
Based on the kinetic data, pH study, PZC determination, and FTIR evidence, a comprehensive adsorption mechanism can be proposed for MG uptake by the pectin/poly(NIPAm-co-AA) nano-hydrogel. At pH values above 4.2, the surface acquires a net negative charge due to the ionization of carboxylic groups from both acrylic acid and pectin, which facilitates strong electrostatic attraction with the positively charged MG molecules. The pseudo-second-order kinetics and the high initial adsorption rate (h = 1338.14 mg/g·min) indicate that electron sharing between MG nitrogen atoms and surface functional groups (chemisorption) is the rate-controlling step. The two-stage Weber–Morris profile reveals that after rapid external surface binding, MG molecules gradually diffuse through the mesopores (average diameter 16.56 nm) to access interior binding sites. The FTIR shifts confirm that hydrogen bonds form between O–H/N–H groups of the nano-hydrogel and the dye, while π–π stacking interactions between the aromatic MG rings and the polymer backbone contribute to adsorption even under acidic conditions where electrostatic repulsion would otherwise dominate [37,38]. The high gel content (93.23%) ensures structural stability during the swelling-mediated adsorption process, and the pH-responsive swelling (reaching 1380% at pH 7) enhances accessibility of internal binding sites.

CONCLUSION
A pectin/poly(NIPAm-co-acrylic acid) nano-hydrogel was successfully prepared by aqueous free-radical copolymerization and thoroughly characterized by FTIR, XRD, FE-SEM, TGA, and BET techniques. The synthesized nano-hydrogel demonstrated excellent adsorptive performance towards malachite green dye, achieving an equilibrium capacity of 604.89 mg/g and 96.78% removal at the optimum dosage of 0.008 g. The adsorption kinetics were best described by the pseudo-second-order model (R² ≈ 1.000, RMSE = 0.89 mg/g), confirming that chemisorption is the dominant rate-controlling mechanism. The Weber–Morris analysis revealed a two-step diffusion process, with rapid surface adsorption followed by slower intraparticle diffusion, while the non-zero intercept ruled out intraparticle diffusion as the sole rate-limiting step. The pH study, supported by the determination of pHPZC at 4.2, established that the surface carries a net negative charge at pH > 4.2, which promotes electrostatic attraction with the cationic MG molecules. The high and relatively constant adsorption capacity observed over the pH range 4–10 (592–609 mg/g) demonstrates the operational versatility of the nano-hydrogel. FTIR analysis after adsorption confirmed the involvement of electrostatic interactions, hydrogen bonding, and π–π stacking in the binding mechanism. Regeneration experiments confirmed that the nano-hydrogel retains 87.8% of its initial capacity after five cycles, while tests in real water matrices (tap, river, and simulated industrial wastewater) showed removal efficiencies of 92.53%, 88.35%, and 79.78%, respectively. These results collectively establish the pectin/poly(NIPAm-co-AA) nano-hydrogel as a promising, reusable, eco-friendly, and pH-responsive adsorbent for the efficient removal of cationic dyes from industrial wastewater, with demonstrated robustness for real-world water treatment applications.

ACKNOWLEDGEMENTS
The authors gratefully acknowledge the University of Al-Qadisiyah for providing laboratory facilities and support for this research.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.