Development of a Novel Nanoclay-Doped Hydrogel Adsorbent for Efficient Removal of Heavy Metal Ions and Organic Dyes from Wastewater

Abstract

Rapid industrialization has led to significant environmental challenges, particularly in wastewater treatment, where the removal of heavy metal ions and organic dyes is critical. This study presents the synthesis and characterization of a high-performance hydrogel adsorbent, (nanoclay)_x@poly-γ-glutamic acid (γ-PGA)/polyethyleneimine (PEI) hydrogel adsorbent (denoted as N_xPP, x = 0, 20, 40, 60, and 80), for the efficient removal of heavy metal ions (Cu²⁺, Fe³⁺, and Zn²⁺) and organic dyes (Methylene blue, as a typical example) from wastewater. The hydrogel was prepared using a one-pot method, combining γ-PGA and PEI with varying amounts of nanoclay. The N₈₀PP hydrogel demonstrated exceptional adsorption capacities, achieving 224.37 mg/g for Cu²⁺, 236.60 mg/g for Fe³⁺, and 151.95 mg/g for Zn²⁺ within 30 min, along with 88.18 mg/g for Methylene blue within 5 h. The incorporation of nanoclay significantly enhanced the mechanical properties, with compressive strength reaching 560.49 kPa. The hydrogel exhibited excellent reusability, maintaining high adsorption capacity after five cycles. The adsorption kinetics followed a pseudo-second-order model, and the isotherms fit the Freundlich model, indicating a multilayer adsorption mechanism. This study highlights the potential of N_xPP hydrogels as a versatile and sustainable solution for wastewater treatment.

1. Introduction

Wastewater treatment has become a global issue with the rapid industrialization of human society [1]. Organic dyes and heavy metal ions are two of the most common pollutants in wastewater [2]. Organic dyes are widely used in industries such as printing, painting, textiles, and food production [3,4,5,6]. However, many dyes are toxic, carcinogenic, and non-biodegradable, posing significant environmental pollution and severe threats to human health [7,8]. Heavy metal ions in industrial wastewater can accumulate in the food chain, ultimately causing severe health problems for humans [9,10]. Therefore, the efficient removal of organic dyes and heavy metal ions from wastewater is essential for safeguarding environmental sustainability and public health.

Currently, the primary methods for removing organic pollutants and heavy metal ions from wastewater include electrochemical treatment, advanced oxidation processes, coagulation, membrane separation, and adsorption [11,12,13]. Among these methods, adsorption is regarded as one of the most effective approaches for removing organic dyes and heavy metal ions from wastewater [14]. Compared to other techniques, adsorption offers advantages such as low cost, by-product-free operation, ease of implementation, and scalability. Activated carbon is one of the most widely used adsorbents, with good removal efficiency for heavy metal ions and organic waste in wastewater. However, its application in wastewater treatment is significantly constrained by its high cost and the need for regeneration at temperatures exceeding 500 °C [15]. Research has shown that the choice of adsorbent is key to determining the effectiveness and efficiency of adsorption [16]. An ideal adsorbent should possess wide applicability, high adsorption capacity, rapid adsorption kinetics, excellent regeneration performance, and cost-effectiveness [17].

In recent years, researchers have successfully developed a range of novel adsorbents for the removal of organic dyes and heavy metal ions from wastewater [18]. Among these, hydrogels have gained significant attention due to their abundant functional groups, porous structure, ease of preparation, and convenient separation [19,20]. For example, Cai et al. developed a self-healing hydrogel for Cr⁶⁺ adsorption using hydroxypropyl chitosan, polyacrylamide, and polyvinyl alcohol [21]. This hydrogel is cross-linked by a triple network of dynamic Schiff base bonds, borate bonds, and hydrogen bonds, with a maximum compressive strength of 267 kPa. The results showed that the hydrogel showed a high adsorption capacity for Cr⁶⁺, indicating its potential for heavy metal adsorption. In another study, Wu et al. developed a double-network composite hydrogel adsorbent with outstanding mechanical properties, reusability, and high adsorption capacity [22]. This hydrogel features a primary network of polyacrylic acid cross-linked with methylene bisacrylamide and a secondary network of polyethylene amine cross-linked with chitosan. The synergistic effect of these double networks enhances the mechanical strength and adsorption efficiency of the composite hydrogel. Despite these advancements, the development of multifunctional adsorbents capable of efficiently removing both organic dyes and heavy metal ions while maintaining excellent mechanical properties, high adsorption capacity, and long-term reusability remains a critical challenge. Moreover, the incorporation of nanomaterials, such as nanoclays, into hydrogel matrices has shown promising potential in enhancing adsorption performance and structural stability [23,24]. However, systematic studies on the synergistic effects of nanoclays and polymeric networks in hydrogel adsorbents are still limited.

In this study, we report the design and fabrication of a novel nanoclay-doped γ-PGA/PEI hydrogel adsorbent (N_xPP) with enhanced adsorption capacity, superior mechanical strength, and excellent reusability for the simultaneous removal of heavy metal ions and organic dyes from wastewater. The incorporation of nanoclays reinforces the hydrogel structure, improving the material’s performance. The adsorption kinetics, isotherms, and mechanisms were thoroughly investigated to provide a deeper understanding of the adsorption process. This work contributes to the advancement of high-performance, multifunctional hydrogel adsorbents for sustainable wastewater treatment applications.

2. Results and Discussion

2.1. Synthesis and Characterization of N_xPP Hydrogels

The N_xPP hydrogels were synthesized through the cross-linking of γ-PGA with PEI via amide bonds, using 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as activators to activate the carboxyl groups of γ-PGA. The formation of the hydrogel was first investigated using Fourier transform infrared (FTIR) spectroscopy (Figure S1). All hydrogels exhibited a distinct absorption peak at 1452 cm⁻¹, which corresponds to the stretching vibrations of carboxyl groups, confirming their presence in the hydrogel structure. The absorption band at 788 cm⁻¹ is attributed to the rocking or wagging vibrations of CH₂ groups, while the peak at 722 cm⁻¹ is characteristic of the out-of-plane rocking vibrations of long, linear alkyl chains (e.g., -(CH₂)_n-). In addition, prominent peaks were observed at 1642 cm⁻¹ and 1576 cm⁻¹, corresponding to the stretching vibration of the amide I band and the bending (scissoring) vibration of the amide II band, respectively, indicating the formation of amide bonds within the hydrogel network. The microstructure of N₈₀PP hydrogel was observed through SEM (Figure 1a,b). SEM images revealed that the N₈₀PP hydrogel exhibited a loose and porous structure, which could facilitate faster solvent exchange and enhanced ion adsorption capacity. Elemental mapping (Figure 1c–g) suggested the co-existence of C, N, O, Mg, and Si in the hydrogel. Energy Dispersive Spectroscopy (EDS) analysis (Figure 1h) further confirmed the successful incorporation of nanoclay and the effective preparation of N₈₀PP hydrogel.

2.2. Swelling Properties of N_xPP Hydrogels

The swelling behavior of P_xP and N_xPP hydrogels in deionized water at room temperature was measured, showing excellent swelling capacities for both hydrogels. The WA of P_xP hydrogels increased with the γ-PGA content (Figure 2a), reaching equilibrium within 20 min (P_0.1P: 28.53 g/g, P_0.15P: 57.70 g/g, and P_0.2P: 85.55 g/g, Figure 2b). The enhancement of equilibrium adsorption capacity can be ascribed to the hygroscopic nature of γ-PGA, whereby a higher γ-PGA content leads to an increased WA value. However, the WA of N_xPP hydrogels decreased as the nanoclay content increased (Figure 2c), with swelling rates of 81.40, 51.22, 30.11, 21.11, and 11.0 g/g for N₀PP, N₂₀PP, N₄₀PP, N₆₀PP, and N₈₀PP hydrogels (Figure 2d), respectively, after 30 min of immersion in deionized water. This decrease in WA is likely due to increased pore density caused by the incorporation of nanoclay.

2.3. Mechanical Properties of N_xPP Hydrogels

While different hydrogel adsorbents demonstrate remarkable adsorption capacities for heavy metal ions, they often suffer from inherent mechanical deficiencies, particularly structural degradation during cyclic adsorption–desorption processes [25,26]. The mechanical performance of P_xP and N_xPP hydrogels was systematically investigated through comprehensive compressive strength analysis. Notably, the integration of nanoclay into the hydrogel matrix significantly improved the mechanical robustness of N_xPP hydrogels, addressing this critical limitation. As shown in Figure 3a,b, the compressive strength of P_xP hydrogels increased with the γ-PGA content (P_0.1P: 45.44 kPa, P_0.15P: 46.40 kPa, and P_0.2P: 107.18 kPa), possibly due to the increased cross-linking density. With the incorporation of nanoclay, the compressive strength of N_xPP hydrogels significantly improved (N₀PP: 89.87 kPa, N₂₀PP: 232.3 kPa, N₄₀PP: 372.13 kPa, N₆₀PP: 430.68 kPa, and N₈₀PP: 560.49 kPa, Figure 3c,d). These results indicate that the addition of nanoclay can substantially enhance the compressive strength of the hydrogels, making N_xPP hydrogels promising candidates with superior mechanical properties. The enhancement in compressive strength through nanoclay incorporation can be attributed to the high aspect ratio of nanoclay particles, facilitating effective stress transfer throughout the composite structure, preventing localized stress concentration. However, the specific reasons still need to be further analyzed.

2.4. Cu²⁺ Adsorption Capacity of P_xP Hydrogels and N_xPP Hydrogels

Adsorption experiments were first conducted to evaluate the Cu²⁺ adsorption capacity of PxP hydrogels (pH: 6.0, temperature: 298 K, hydrogel: 30 mg, time: 30 min). As shown in Figure 4a, P_0.1P, P_0.15P, and P_0.2P hydrogels exhibited high Cu²⁺ adsorption capacities (P_0.1P: 259.84 mg/g, P_0.15P: 258.17 mg/g, P_0.2P: 245.59 mg/g), with minimal differences between the three samples. To investigate the effect of temperature on the adsorption capacity of N_xPP hydrogels, we measured the adsorption performance of N_xPP hydrogels for Cu²⁺ at different temperatures (293 K, 303 K, and 313 K, Figure 4b). The N_xPP hydrogels exhibited excellent adsorption capacity for Cu²⁺ at all tested temperatures. However, the adsorption amounts showed minimal variation across the three temperatures, with the N₈₀PP hydrogel adsorbing 269.32 mg/g at 293 K, 261.64 mg/g at 303 K, and 266.20 mg/g at 313 K. Similarly, although adsorption equilibrium was not reached, the adsorption kinetics of Cu²⁺ onto N_xPP hydrogels with varying nanoclay contents exhibited minimal differences (Figure 4c). Adsorption capacities after 30 min were 258.38 ± 6.01, 225.30 ± 4.85, 261.60 ± 1.15, 252.33 ± 11.05, and 257.12 ± 5.28 mg/g for N₀PP, N₂₀PP, N₄₀PP, N₆₀PP, and N₈₀PP hydrogels, respectively. These results demonstrate the high efficiency and rapid Cu²⁺ adsorption capabilities of N_xPP hydrogels. The adsorption process was further investigated by comparing the actual Cu²⁺ adsorption kinetics with pseudo-first-order and pseudo-second-order models (Figure 4d,e,

Table 1. Kinetic parameters of pseudo-first-order and pseudo-second-order models for the adsorption of Cu²⁺ ions onto N_xPP hydrogels.

Hydrogel	k1 (1/min)	Qe (mg/g)	R2 (Pseudo-First-Order)	k2 (g/mg·min)	Qe (mg/g)	R2 (Pseudo-Second-Order)
N0PP	0.12	65.4	0.970	0.0061	70.1	0.996
N20PP	0.14	70.3	0.975	0.0075	74.8	0.997
N40PP	0.16	75.6	0.981	0.0088	78.9	0.991
N60PP	0.18	78.7	0.984	0.0102	81.3	0.998
N80PP	0.19	79.8	0.980	0.0110	82.1	0.998

). The kinetic curves were better fitted by the pseudo-second-order model, with correlation coefficients (R²) of 0.996, 0.997, 0.991, 0.998, and 0.998 for N₀PP to N₈₀PP hydrogel, indicating that the adsorption behavior likely involves chemisorption.

2.5. Effect of pH and Temperature on N_xPP Hydrogels’ Adsorption Capacity

The adsorption capacity of N_xPP hydrogels for Cu²⁺ in deionized water was studied as a function of pH. From pH 1.0 to 6.0, the adsorption capacity increased (Figure 4f). At pH 6.0, the adsorption capacity reached its maximum (N₈₀PP: pH = 1.0, 219.51 mg/g; pH = 3.0, 249.16 mg/g; pH = 6.0, 257.23 mg/g). The increase in adsorption capacity with pH is attributed to the enhanced chelation affinity between amine (-NH₂) groups and metal ions and the increased cation exchange effect between hydroxyl (-OH) groups and metal ions. Even at pH 1.0, the adsorption capacity remained as high as 219.51 mg/g, suggesting the potential of this hydrogel for industrial wastewater treatment under harsh conditions.

2.6. Adsorption Isotherms of N_xPP Hydrogels

To further explore the underlying mechanism of temperature effects on the adsorption capacity of N_xPP hydrogels, we studied the thermodynamic properties of the hydrogel based on adsorption thermodynamics and Gibbs free energy equations. At 293 K, the adsorption performance of the hydrogel for Cu²⁺ was evaluated in wastewater solutions with varying initial Cu²⁺ concentrations (Figure 5a). The adsorption thermodynamic mechanism was further investigated using Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models, with corresponding linear fitting shown in Figure 5b–d and

Table 2. Parameters of adsorption isotherm models for N_xPP hydrogels.

Hydrogel	Qmax (mg/g)	KL (L/mg)	R2 (Langmuir)	KF	n	R2 (Freundlich)	β	E (KJ/mol)	R2 (Temkin)
N0PP	95.4	0.015	0.991	8.4	2.3	0.999	0.00465	10.37	0.760
N20PP	102.6	0.017	0.991	9.2	2.5	0.996	0.00405	11.11	0.758
N40PP	110.2	0.020	0.992	10.6	2.7	0.991	0.00355	11.87	0.764
N60PP	115.8	0.022	0.989	11.8	2.9	0.997	0.00312	12.66	0.749
N80PP	118.7	0.025	0.969	12.4	3.0	0.989	0.00282	13.32	0.705

. The results indicate that the adsorption of Cu²⁺ by N_xPP hydrogels better follows the Freundlich isotherm model, as the correlation coefficients (R²) for N₀PP, N₂₀PP, N₄₀PP, N₆₀PP, and N₈₀PP hydrogels were 0.999, 0.997, 0.991, 0.997, and 0.989, respectively.

2.7. Adsorption Capacity of N_xPP Hydrogels for Different Metal Ions

We evaluated the adsorption performance of N_xPP hydrogels for various metal ions (Cu²⁺, Zn²⁺, Fe³⁺) under controlled conditions (pH 6.0, temperature 298 K, hydrogel mass 30 mg, adsorption time 30 min) to assess their versatility in adsorbing metal ions. As shown in Figure 5e, N_xPP hydrogels exhibited adsorption for all three selected metal ions, with the highest adsorption observed for Fe³⁺. The N₈₀PP hydrogel adsorbed 236.60 mg/g of Fe³⁺, while the adsorption for Zn²⁺ was the lowest at 151.95 mg/g, and for Cu²⁺, the adsorption reached 224.37 mg/g, nearly equal to that of Fe³⁺. The adsorption is higher than chitosan composites (<120 mg/g for Cu²⁺ [27]) and crosslinked carboxymethyl Sago starch/citric acid hydrogel (<20 mg/g for Zn²⁺ [28]). However, the adsorption differences among the N_xPP hydrogels for the same metal ion were minimal. The highest adsorption capacity for Fe³⁺ in N_xPP hydrogels may be attributed to its elevated charge density and robust coordination with amino/hydroxyl groups. Therefore, N_xPP hydrogel adsorbents can be applied to the adsorption of various heavy metal ions.

2.8. Adsorption Capacity of N_xPP Hydrogels for (Methylene Blue) MB

To assess the adsorption performance of N_xPP hydrogels for organic dyes, we measured the adsorption of MB in wastewater using UV-Vis spectrophotometry. The adsorption behavior of N_xPP hydrogels for MB over 5 h was evaluated. The increased nanoclay content enhances the hydrogel’s mechanical strength and thereby provides additional adsorption sites through synergistic interactions, including electrostatic attraction, hydrogen bonding, and π-π stacking, leading to higher MB adsorption efficiency. As shown in Figure 5f, after 5 h, the percentage adsorption of MB by N₀PP, N₂₀PP, N₄₀PP, N₆₀PP, and N₈₀PP hydrogels was 75.98%, 69.57%, 77.62%, 82.97%, and 88.18%, respectively. These results suggest that the prepared hydrogels can be utilized for the adsorption of both organic dyes and heavy metal ions, demonstrating the potential of this hydrogel for the removal of pollutants from wastewater.

2.9. Adsorption Mechanism of N_xPP Hydrogels

To investigate the adsorption mechanisms of N_xPP hydrogels for heavy metal ions, we employed Cu²⁺ as a representative and XPS to determine the chemical composition of N₈₀PP hydrogel before and after the adsorption of Cu²⁺. The C 1s XPS spectrum of the hydrogel prior to adsorption appeared at 278.28 eV (C-H/C-C), 285.93 eV (C-N/C-O), and 285 eV (O-C-O) (Figure 6a). The O 1s spectrum appeared at 532.87 eV (O-C-O), 532.13 eV (C-O-C), and 531.11 eV (C-O-H) (Figure 6b). The N 1s spectrum revealed two fitting peaks: -NH₃₊ (400.96 eV) and -NH₂/-NH (399.02 eV) (Figure 6c). Following the adsorption of heavy metal ions, Cu²⁺ interacts with the lone pairs of electrons on the oxygen or nitrogen atoms of -OH or -NH₂/-NH. This interaction leads to a reduction in the electron density of -OH or -NH₂/-NH, resulting in a peak shift to higher binding energies [29]. Simultaneously, after Cu²⁺ adsorption, the fitting peak of C-O-H in O 1s and the fitting peak of -NH₂/-NH in N 1s both shifted to higher binding energies, confirming the presence of coordination interactions [27]. As shown in Figure 6d, the XPS spectrum of Cu 2p displays three peaks at 951.95 eV, 934.83 eV, and 913.96 eV, with the binding energies at 951.95 eV and 913.96 eV corresponding to the Cu 2p_1/2 and Cu 2p_3/2 orbitals, respectively. In summary, the adsorption mechanism can be summarized as chelation interactions between -NH₂ and -NH with metal ions.

2.10. Reusability of N_xPP Hydrogels

The reusability of hydrogel adsorbents is crucial for cost-effective environmental protection. We evaluated the reusability of N_xPP hydrogels for heavy metal ion adsorption using Cu²⁺ as a model, performing multiple adsorption–desorption cycles. After each adsorption cycle, the hydrogels were placed in HCl and agitated to facilitate the desorption. After desorption, the hydrogels completely lost their characteristic blue color, indicating that the majority of Cu²⁺ ions were effectively removed during the regeneration process. The adsorption capacity of the hydrogel slightly decreased after five cycles, with N₀PP, N₂₀PP, N₄₀PP, N₆₀PP, and N₈₀PP hydrogels showing adsorption capacities of 218.43 mg/g, 219.98 mg/g, 217.35 mg/g, 220.89 mg/g, and 221.41 mg/g, respectively, in the fifth cycle. However, no significant difference was detected during these five cycles. Further recovery and reusability activity results demonstrate that all hydrogel samples exhibited excellent reusability and stability. Notably, N80PP retained 96.9% ± 1.2% of its initial adsorption capacity after 5 cycles (

Table 3. Recovery and reusability activity during each of the five cycles for the N_xPP hydrogels.

Cycle	N0PP (%)	N20PP (%)	N40PP (%)	N60PP (%)	N80PP (%)
1	100.0 ± 1.3	100.0 ± 1.2	100.0 ± 1.1	100.0 ± 1.0	100.0 ± 0.9
2	97.3 ± 1.5	97.9 ± 1.4	98.1 ± 1.3	98.4 ± 1.1	98.6 ± 1.0
3	95.6 ± 1.6	96.4 ± 1.5	97.3 ± 1.3	97.7 ± 1.2	98.1 ± 1.0
4	93.8 ± 1.7	95.1 ± 1.6	96.2 ± 1.4	96.8 ± 1.2	97.5 ± 1.1
5	92.0 ± 1.8	94.2 ± 1.7	95.3 ± 1.5	96.1 ± 1.3	96.9 ± 1.2

). These results demonstrate that N_xPP hydrogel adsorbents were regenerated after desorption, freeze-drying, and could be reused for subsequent adsorption–desorption cycles. It is worth noting that the incorporation of nanoclay enhanced the mechanical properties of the hydrogel (increasing the compressive strength from 89.87 kPa (N₀PP) to 560.49 kPa (N₈₀PP)), which may also support its durable usages.

3. Conclusions

Hydrogels have been extensively used for water treatment and other applications [30,31]. In this study, we successfully developed a novel, cost-effective, and highly efficient N_xPP hydrogel adsorbent doped with nanoclay for the removal of heavy metal ions and organic dyes from wastewater. Within 30 min, the adsorption capacities of N₀PP, N₂₀PP, N₄₀PP, N₆₀PP, and N₈₀PP hydrogels for Cu²⁺ reached 258.38, 225.30, 261.60, 252.33, and 257.12 mg/g, respectively, while the adsorption percentages for MB within 5 h were 75.98%, 69.57%, 77.62%, 82.97%, and 88.18%. More importantly, the incorporation of nanoclay significantly enhanced the mechanical properties of the N_xPP hydrogel adsorbents, increasing the compressive strength from 89.87 kPa (N₀PP) to 560.49 kPa (N₈₀PP). This excellent mechanical performance imparts reusability to the N_xPP hydrogels, with adsorption capacity remaining high after five adsorption–desorption cycles. The adsorption kinetics followed a pseudo-second-order model, and the adsorption isotherms fit the Freundlich model. While this study focused on single-metal systems to establish baseline adsorption capacities, the influence of co-existing ions in multi-component wastewater remains an important consideration for real-world applications. Future work will systematically investigate competitive adsorption behavior, selectivity, and interference mechanisms in complex matrices containing multiple heavy metal ions, further advancing the practical utility of N_xPP hydrogels. In conclusion, the N_xPP hydrogel adsorbents demonstrate great potential for the removal of heavy metal ions and organic dyes from wastewater.

4. Materials and Methods

4.1. Experimental Reagents

γ-PGA (95%, molecular weight: ~1000 kDa) was purchased from Shanghai Yika Biotechnology Co., Ltd. (Shanghai, China). PEI (Mw ~70,000, 30% in water) was bought from Adamas Beta (Shanghai, China). Cu(NO₃)₂·3H₂O (Analytical Reagent, AR), FeCl₃·6H₂O (AR), zinc chloride (AR), EDC (purity ≥ 98%) and NHS (purity ≥ 98%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). MB (AR) was obtained from Shanghai Titan Technology Co., Ltd. (Shanghai, China). Laponite nanoclay was purchased from Zhejiang Institute of Geologic and Mineral Resources (Hangzhou, China).

4.2. Preparation of γ-PGA/PEI (P_xP) and N_xPP Hydrogels

To prepare P_xP hydrogels (x represents the weight of γ-PGA, specifically: 0.1 g, 0.15 g, and 0.2 g), γ-PGA in quantities of 0.1 g, 0.15 g, and 0.2 g was dissolved in 1.6 mL of deionized water under magnetic stirring. Then, 0.2 mL of PEI was added to the above γ-PGA solution and stirred until homogenous. After that, 0.04 g of EDC/NHS was dissolved in 0.4 mL of deionized water and added to the above solution to initiate gelation, resulting in the formation of P_0.1P, P_0.15P, and P_0.2P hydrogels. To prepare N_xPP hydrogels, 0, 20, 40, 60, and 80 mg of nanoclay was dispersed in 1.6 mL of deionized water under magnetic stirring. Then, 0.2 g of γ-PGA powder was added to the nanoclay dispersion and stirred until homogeneous. Subsequently, 0.2 mL of PEI solution was added to the γ-PGA-nanoclay dispersion and stirred until fully mixed. Finally, 0.04 g of EDC/NHS was dissolved in 0.4 mL of deionized water and added to the solution to initiate the gelation, forming N₀PP, N₂₀PP, N₄₀PP, N₆₀PP, and N₈₀PP hydrogels.

4.3. Characterizations

To observe the microstructure, the prepared hydrogels were frozen at −80 °C for 24 h and then freeze-dried to obtain lyophilized samples. These samples were examined using SEM (Zeiss Sigma 300, Carl Zeiss AG, Oberkochen, Germany). Elemental mapping of the hydrogel samples was performed using EDS coupled with the SEM. FTIR spectroscopy of hydrogel samples was studied using an infrared spectrometer (NICOLET-380, Thermo Fisher Scientific, Waltham, WA, USA). X-ray photoelectron spectroscopy (XPS, Thermo Kalpha, Thermo Fisher Scientific, Waltham, WA, USA) was used to determine the binding energies of elements.

4.4. Swelling Performance of N_xPP Hydrogels

The swelling of the hydrogels was evaluated by monitoring the weight changes over time. Specifically, 0.03 g (W₀) of freeze-dried P_0.1P, P_0.15P, and P_0.2P hydrogels were placed into sealed vials containing 10 mL of deionized water. At predetermined time intervals (3, 5, 10, 20, and 30 min), the hydrogels were removed, and surface water was blotted with filter paper. The hydrogels were then weighed, and the weight at time, t, was recorded as W_t. The swelling ratio of N_xPP hydrogels was determined similarly, with 0.05 g (W₀) of freeze-dried N₀PP, N₂₀PP, N₄₀PP, N₆₀PP, and N₈₀PP hydrogels placed in sealed vials containing 10 mL of deionized water. After the predetermined time intervals (3, 5, 10, 20, and 30 min), the hydrogels were removed, blotted dry, and weighed (W_t). All experiments were performed in triplicate. The water absorption (WA, g/g) was calculated using Equation (1), and the swelling kinetics curves were plotted.

WA (g/g) = W_t − W₀/W₀

(1)

4.5. Mechanical Study of N_xPP Hydrogels

The mechanical properties of hydrogels were investigated using a universal testing machine (TS-2632, Hongpu Technology (Hong Kong, China) Co., Ltd.). Hydrogels were molded into cylindrical samples with a diameter of 8 mm and a height of 10 mm. The preset strain rate was 20 mm/min, with a strain range of 0–90%. The tests were conducted at 25 °C, and the hydrogels were compressed to 80% of their initial height.

4.6. Adsorption Kinetics of P_xP Hydrogels

Cu(NO₃)₂ was dissolved in deionized water to prepare Cu²⁺-containing wastewater. To investigate the Cu²⁺ adsorption capacity of P_xP hydrogels, 30 mg of freeze-dried hydrogel was placed into a centrifuge tube containing 30 mL of Cu²⁺-containing wastewater (300 mg/L). The tube was then shaken at 150 rpm at room temperature for 30 min. At predetermined time points, the centrifuge tubes were removed, and the hydrogels were discarded. All experiments were conducted in triplicate. The concentration of Cu²⁺ in the wastewater was measured using inductively coupled plasma mass spectrometry (ICP-MS, Avio 200, PerkinElmer, Hopkinton, MA, USA).

4.7. Adsorption Kinetics of N_xPP Hydrogels

To study the adsorption kinetics of N_xPP hydrogels, we examined the adsorption capacity over different time intervals. A 30 mg sample of freeze-dried N_xPP hydrogels was placed into centrifuge tubes containing 30 mL of Cu²⁺-containing wastewater (300 mg/L). The tubes were shaken at 150 rpm at room temperature for various times (5, 10, 15, 20, 25, 30, 35, and 40 min). At each time point, the tubes were removed, and the hydrogels were discarded. All experiments were conducted in triplicate. The concentration of Cu²⁺ in the wastewater was measured using inductively coupled plasma mass spectrometry (ICP-MS, Avio 200, PerkinElmer, Massachusetts, USA).

4.8. Effect of pH on the Adsorption Capacity of N_xPP Hydrogels

To investigate the effect of pH on the adsorption capacity of N_xPP hydrogels, we adjusted the pH of the Cu²⁺-containing wastewater to 1.0, 3.0, and 6.0 using HCl. A 30 mg sample of freeze-dried hydrogels was added to centrifuge tubes containing 30 mL of Cu²⁺-containing wastewater (300 mg/L) at different pH levels. The tubes were shaken at 150 rpm at room temperature for 30 min. At the designated time points, the tubes were removed, and the hydrogels were discarded. The concentration of Cu²⁺ in the wastewater was measured using inductively coupled plasma mass spectrometry (ICP-MS, Avio 200, PerkinElmer, Massachusetts, USA). All experiments were performed in triplicate.

4.9. Effect of Temperature on the Adsorption Capacity of N_xPP Hydrogels

The effect of temperature on the adsorption capacity of N_xPP hydrogels was evaluated by placing 30 mg of freeze-dried N_xPP hydrogels into centrifuge tubes containing 30 mL of Cu²⁺-containing wastewater (300 mg/L). The tubes were immediately transferred to shaking incubators set at different temperatures (293 K, 303 K, and 313 K) and shaken at 150 rpm for 30 min. After the predetermined time, the tubes were removed, and the hydrogels were discarded. The concentration of Cu²⁺ in the wastewater was measured using inductively coupled plasma mass spectrometry (ICP-MS, Avio 200, PerkinElmer, Massachusetts, USA). Each experiment was repeated in triplicate.

4.10. Adsorption Study of N_xPP Hydrogels for Different Metal Ions

To assess the adsorption capacity of N_xPP hydrogels for different metal ions, we prepared Fe³⁺ and Zn²⁺-containing wastewater (300 mg/L). Then, a 30 mg sample of freeze-dried N_xPP hydrogels was placed into centrifuge tubes containing 30 mL of each metal ion-containing solution. The tubes were immediately transferred to a shaking incubator at room temperature and shaken for 30 min (150 rpm). After the designated time point, the tubes were removed, and the hydrogels were discarded. All experiments were conducted in triplicate. The concentration of Fe³⁺ and Zn²⁺ in the wastewater was measured using inductively coupled plasma mass spectrometry (ICP-MS, Avio 200, PerkinElmer, Massachusetts, USA).

4.11. Adsorption Capacity of N_xPP Hydrogels for MB

We prepared an MB solution by dissolving 20 mg of MB in 1 L of deionized water to obtain a 20 mg/L solution. A 30 mg sample of freeze-dried N_xPP hydrogels was placed into a centrifuge tube containing 30 mL of MB solution (20 mg/L). The tubes were shaken at 150 rpm at room temperature for 3 and 5 h. At the designated time points, the tubes were removed, and the solutions were filtered using a 0.45 µm filter. The adsorption capacity of the hydrogels for MB dye was determined by measuring the dye concentration in the prepared wastewater using a UV-visible spectrophotometer (N4/N4S INESA, Jinan Saichang Scientific Instrument Co., Ltd., Jinan, China). Kinetic adsorption curves were plotted, and each experiment was performed in triplicate.

4.12. Adsorption Mechanism Study of N_xPP Hydrogels

To investigate the adsorption mechanism of N_xPP hydrogels, we analyzed the chemical composition of the hydrogels after adsorption using XPS. A 30 mg sample of freeze-dried N_xPP hydrogels was placed in centrifuge tubes containing 30 mL of wastewater solutions (300 mg/L of Cu²⁺, Fe³⁺, and Zn²⁺, and 20 mg/L of MB). The tubes were shaken at 150 rpm at room temperature for 30 min. After adsorption, the hydrogels were freeze-dried, ground into powder, and analyzed using XPS.

4.13. Recovery and Reusability Study of N_xPP Hydrogels

N_xPP hydrogels were placed in centrifuge tubes containing a Cu²⁺ solution of a specified concentration (mg/L). The centrifuge tubes were then transferred immediately to a shaking incubator and agitated at 150 rpm for 30 min at room temperature. At predetermined time points, the centrifuge tubes were removed from the shaking incubator, and the hydrogels were recovered. After each adsorption cycle, the hydrogels were placed in centrifuge tubes containing 5 mL of 1 mol/L HCl and subsequently agitated at 120 rpm for 2 h to facilitate the desorption. Following desorption, the hydrogels were removed and washed three times with deionized water, then freeze-dried for use in subsequent adsorption experiments. This process was repeated for a total of five cycles, with each experiment conducted in triplicate. The recovery and reusability were further compared as activity, which is defined as the adsorption capacity at a given cycle divided by the adsorption capacity in the first cycle.

4.14. Data Analysis

The adsorption amount (Q_e) was calculated using the following equation (Equation (2)):

$${\mathrm{Q}}_{\mathrm{e}}=({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})\times \mathrm{V}/\mathrm{W}$$

(2)

where Q_e (mg/g) represents the adsorption amount, C₀ (mg/L) is the initial concentration of the solution, C_e (mg/L) is the concentration after adsorption, V (L) is the solution volume, and W (g) is the weight of the freeze-dried hydrogel.

The adsorption kinetics were described by pseudo-first-order (Equation (3)), pseudo-second-order (Equation (4)), and intraparticle diffusion (Equation (5)) models:

$$\ln \left({\mathrm{Q}}_{\mathrm{e}}-{\mathrm{Q}}_{\mathrm{t}}\right)=\ln {\mathrm{Q}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t}$$

(3)

$$\mathrm{t}/{\mathrm{Q}}_{\mathrm{t}}=\mathrm{t}/{\mathrm{Q}}_{\mathrm{e}}+1/\left({\mathrm{k}}_2{\mathrm{Q}}_{\mathrm{e}}^2\right)$$

(4)

$${\mathrm{Q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{i}}\sqrt{\mathrm{t}}+\mathrm{C}$$

(5)

where Q_e (mg/g) is the adsorption amount at equilibrium, Q_t (mg/g) is the adsorption amount at time t (min), k₁ (min⁻¹) and k₂ (g·mg⁻¹·min⁻¹) are the adsorption rate constants, k_i (mg·g⁻¹·min^1/2) is the intraparticle diffusion rate constant, and C is the intraparticle diffusion constant.

Adsorption thermodynamics were described by the Langmuir (Equation (6)), Freundlich (Equation (7)), and Dubinin–Radushkevich (Equation (8)) models:

$$1/{\mathrm{Q}}_{\mathrm{e}}=1/{\mathrm{Q}}_{\max }+\left(1/\left({\mathrm{K}}_{\mathrm{L}}{\mathrm{Q}}_{\max }\right)\right)\times 1/{\mathrm{C}}_{\mathrm{e}}$$

(6)

$$\log {\mathrm{Q}}_{\mathrm{e}}=\log {\mathrm{K}}_{\mathrm{F}}+\left(1/\mathrm{n}\right)\times \log {\mathrm{C}}_{\mathrm{e}}$$

(7)

$$\ln {\mathrm{Q}}_{\mathrm{e}}=\ln {\mathrm{Q}}_{\max }-{\mathsf{\beta }\mathrm{R}}^2{\mathrm{T}}^2{\left[\ln \left(1+1/{\mathrm{C}}_{\mathrm{e}}\right)\right]}^2$$

(8)

where Q_e (mg·g⁻¹) is the adsorption amount at equilibrium, Q_max (mg·g⁻¹) is the maximum adsorption capacity, K_L (L·g⁻¹) and K_F [(mg·g⁻¹)/(mg·L⁻¹)ⁿ] are constants for the Langmuir and Freundlich models, respectively, n is a constant representing adsorption intensity, C_e is the equilibrium concentration of metal ions, β (mol²·kJ⁻²) is a constant related to adsorption energy, R = 8.314 J·mol⁻¹·K⁻¹ is the gas constant, and T (K) is the temperature.