Design, synthesis, and characterization of glyoxime-functionalized hydroxypropyl methylcellulose/acrylic acid hydrogel for efficient removal of lead (II) ions from aqueous media

Abstract

The present work describes the fabrication of a functionalized hydroxypropyl methylcellulose/acrylic acid hydrogel with glyoxime (OXIM-HPMC/AAC) through the free radical approach for the effective removal of Pb (II). Hydrogel characterization has been performed using FTIR, SEM, and EDX techniques. SEM showed the presence of porous three-dimensional structures that improve diffusion of metal ions in the hydrogel, but pores were partially blocked following adsorption because of the neutralization of charges. EDX results validated the Pb (II) adsorption by the presence of characteristic peaks. The gel fraction varied depending on the composition and ranged from 86.3% at OXIM-HPMC/AAC (70/30) to 77.6% at (30/70). The swelling capacity improved with increasing HPMC content to reach its maximum value of 219.6% after 5 h. Pb (II) removal efficiency was investigated under different experimental conditions, and the optimum values were determined to be pH 3.0, 45 min contact time, and a V/m ratio of 0.1 L/g. Adsorption kinetics followed the pseudo-second-order rate model with adsorption capacity (q_e = 9.42 mg/g) and R² = 0.94, whereas equilibrium data followed the Langmuir isotherm model with monolayer adsorption of 20.4 mg/g and correlation coefficient of 0.95. Thermodynamic analysis showed the process to be spontaneous and exothermic.

Introduction

Heavy metals, particularly lead Pb (II), when released into wastewater, provide a considerable threat to human health and the environment^1–3. Pb (II) pollution is a significant waste of industrial activities in various sectors, including battery production and metal plating^4,5. Pb (II) contamination in water results in its accumulation within the human body, especially in bones, muscles, and the brain, leading to severe damage that may be lethal^6,7. Despite the wide application of conventional techniques such as precipitation⁸, membrane filtration, reverse osmosis⁹, ion exchange¹⁰, and electrochemical methods for heavy metal removal¹¹, their practical implementation remains constrained by several drawbacks, including high cost, secondary pollution, limited efficiency at low concentrations, and operational complexity¹². Moreover, membrane-based processes are often hindered by fouling, high energy consumption, and maintenance requirements, which restrict their long-term sustainability^13–15. In this context, adsorption has emerged as one of the most promising and versatile approaches due to its simplicity, high efficiency, and adaptability to different water matrices. The effectiveness of adsorption largely depends on the nature of the adsorbent, particularly its surface chemistry, porosity, and the presence of functional groups capable of interacting with metal ions^16–18. In recent years, polymer-based adsorbents, particularly hydrogels, have gained considerable attention due to their tunable network structure¹⁹, high swelling capacity, and ability to incorporate diverse functional groups that enhance their interaction with metal ions^20,21. Hydrogels are a type of material synthesized using several techniques, including free polymerization²². A multitude of hydrogels can be synthesized from either natural or synthetic substances²³. Cellulose-derived hydrogels have emerged as sustainable materials owing to their biodegradability and environmental compatibility²⁴. Among them, hydroxypropyl methylcellulose (HPMC) is widely recognized for its outstanding hydrophilicity, biocompatibility, film-forming properties, and ability to form stable hydrogel networks with high swelling capacity, making it highly suitable for advanced adsorption and environmental applications^25,26. However, pristine HPMC lacks sufficient functional groups required for strong interaction with heavy metal ions, which limits its adsorption performance²⁷. To overcome this limitation, chemical modification or grafting with functional monomers is often employed to introduce active binding sites. Glyoxime-based functional groups are particularly attractive for metal ion adsorption due to their strong chelating ability through nitrogen and oxygen donor atoms²⁸. These groups can form stable complexes with metal ions, enhancing adsorption efficiency even at relatively low concentrations²⁹. The integration of glyoxime functionalities into polymeric hydrogels has been shown to significantly improve metal ion uptake through combined electrostatic and coordination interactions^30,31. In this context, the development of glyoxime-functionalized HPMC/AAC hydrogel represents a promising strategy for enhancing Pb (II) removal performance. The synergistic combination of HPMC as a biodegradable backbone, AAC as a functional monomer, and glyoxime groups as chelating sites is expected to provide a highly efficient adsorption system with improved affinity toward Pb (II) ions. Additionally, the porous structure and swelling behavior of the hydrogel facilitate rapid diffusion and interaction with metal ions, leading to enhanced removal efficiency. Therefore, this study focuses on the synthesis, characterization, and application of OXIM-HPMC/AAC hydrogel for Pb (II) removal from aqueous media. The adsorption behavior is systematically investigated under different operational conditions, and the mechanism is interpreted through kinetic, isotherm, and thermodynamic analyses to provide a comprehensive understanding of the adsorption process.

Experimental

Materials

Hydroxypropyl methylcellulose (HPMC) and acrylic acid (AAc) were purchased from Merck, Darmstadt, Germany (99%) and were used as received without further purification. N,N′-Methylenebisacrylamide (MBA, ≥ 99%, Sigma-Aldrich, USA) and ammonium persulfate (APS, ≥ 98%, Sigma-Aldrich, USA) were supplied by Sigma-Aldrich. Lead nitrate (Pb(NO₃)₂, 99%) and dimethylglyoxime (DMG, 99%) were also supplied by Sigma-Aldrich, USA and used as received without additional treatment.

Preparation of glyoxime-functionalized HPMC/AAC (OXIM-HPMC/AAC) hydrogel

The OXIM-HPMC/AAC hydrogel was synthesized using free radical polymerization. Initially, 1 g of HPMC was dissolved in 50 mL of warm distilled water to create solution A. Next, 0.2 g of dimethylglyoxime was dissolved in 10 mL of acrylic acid to produce solution B. Solution B was then sequentially introduced to solution A with different ratio (A/B; 30/70, 50/50, and 70/30) while being strongly stirred with a magnetic stirrer. Afterward, 5 mL of APS (20%) and 5 mL of MBA (30%) were included into the solution under continuous stirring. The solution was thereafter sonicated for 5 min to eliminate oxygen. The solution was subsequently transferred into sealed glass tubes, purged with nitrogen to also eliminate oxygen, and immersed in a water bath at 70 °C for 24 h. The samples were cooled, and the hydrogel was sectioned into approximately equal fragments and rinsed with a water/ethanol solution to eliminate any unreacted particles. The samples were dried and then stored in sealed plastic bags for further characterization and application.

Swelling properties

To evaluate the swelling behavior of the OXIM-HPMC/AAC hydrogel, a known weight of the dried sample was immersed in 100 mL of deionized water at ambient temperature. At predetermined time intervals, the sample was removed, gently blotted to eliminate excess surface water, and then weighed. This procedure was repeated until a constant weight was attained, indicating that equilibrium swelling had been achieved. The swelling ratio was subsequently determined using the following equation:

1

Ws and Wd refer to the measured masses of the hydrated and dried hydrogel samples, respectively³². All experiments were performed in triplicate, and the average values are reported.

Batch adsorption

A Pb (II) stock solution (1000 mg/L) was prepared by dissolving an accurately weighed amount of lead nitrate (Sigma-Aldrich, Germany, 99%) in double-distilled water. Batch adsorption experiments were carried out to systematically investigate the influence of key operational parameters, including shaking time, solution pH, V/m ratio, initial metal ion concentration, and temperature. The optimum conditions were determined based on the maximum removal efficiency. The concentration of Pb (II) was measured using an atomic absorption spectrometer (AAS, Thermo Fisher Scientific, USA) at a wavelength of 283.30 nm. For equilibrium studies, 0.025 g of the synthesized OXIM–HPMC/AAC hydrogel was added to 5.0 mL of Pb (II) solution and agitated in a thermostatically controlled shaker at 25 °C until equilibrium was reached.

The removal efficiency (R, %) of Pb (II) was calculated according to the following expression:

2

where C_o and C_e denote the initial and equilibrium concentrations of Pb (II), respectively³³.

Characterization

OXIM-HPMC/AAC hydrogel before and after extraction of Pb (II) were analyzed by FTIR. FTIR spectra were recorded using a Shimadzu infrared spectrometer (Model 400, Shimadzu Corporation, Kyoto, Japan) using the KBr pellet technique over the spectral range of 4000 – 400 cm⁻¹. The surface morphology and elemental composition of the prepared hydrogel were examined using a JEOL JXA-840A scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDXS) analyzer (JEOL Ltd., Tokyo, Japan). SEM images and EDS spectra were acquired at an accelerating voltage of 20 kV under high-vacuum conditions.

Results and discussion

Characterization

FTIR investigations

Prior to adsorption of Pb (II) There are lots of distinct peaks throughout the spectrum start with peak at around 1705 cm⁻¹ is assigned to the C=O stretching vibration of the carboxylic acid moiety in acrylic acid. The signal about 1630 cm⁻¹ was assigned to the stretching of C=N in the glyoxime moiety, which confirms the successful attachment of glyoxime to HPMC. The absorption band at ca. 1450 cm⁻¹ is associated with the C–H bending in the polymer chain. The band centered at ∼1160 cm⁻¹ is attributed to the C–O stretching of ether functionalities in HPMC and the C–O stretching of –COOH.Post-Pb (II) adsorption:

After adsorption of Pb (II) Significant changes were observed. The interaction between the functional groups and the Pb (II) ions was indicated by the great decrease of the size of all the peaks. The shift of the C=O peak at ∼1705 cm⁻¹ to a higher wavenumber with a decrease in intensity suggested the involvement of carboxylate groups in chelation of Pb (II) by ion exchange. The C=N stretch at about 1630 cm⁻¹ was shifted and broadened, indicating that the nitrogen atom of the glyoxime moiety participated in complexation with Pb (II). The C–O signal at about 1160 cm⁻¹ shifted and this shows that hydroxyl and ether oxygen atoms are also involved in binding. The reduction of transmittance and the peaks shift show that –COOH, C=N and –OH groups are the active sites for Pb (II) adsorption. The interaction is via coordination and chelation of Pb (II) ions by the oxygen and nitrogen donor atoms of the glyoxime-modified hydrogel (Fig. 1)[34-37].

Fig. 1.

FTIR spectra of OXIM-HPMC/AAC hydrogels before and after Pb (II) removal.

SEM and EDX analysis of the OXIM-HPMC/AAC hydrogels before and after Pb (II) removal

Scanning electron microscopy (SEM) images clearly show that hydrogels possess an interior porosity network. A significant quantity of carboxyl anions (COO−) exists within the hydrogel network. Electrostatic repulsion among the carboxyl anions (COO−) induces the hydrogel network to expand, hence increasing the quantity and dimensions of the pores, as shown in Fig. 2A. The hydrogel network’s numerous tiny pores facilitate the facile diffusion of metal ions in and out of the network. Consequently, hydrogel exhibits a substantial adsorption capability. The SEM picture of the hydrogel, following the adsorption of metal ions, is presented in Fig. 2B. A distinct morphological variation is clear between the hydrogels before and after removal of Pb (II). The pore size of the loaded hydrogel decreased after removal of Pb (II) when the electrostatic repulsion inside the hydrogel network decreases following the adsorption of positive metal ions³⁸. An elemental composition of OXIM-HPMC/AAC hydrogels was obtained using EDX both before and after the adsorption of metal ions. The distinctive peaks of heavy metal ions were not present in the EDX spectrum of the OXIM-HPMC/AAC hydrogel before it was immersed in the metal ions solutions, as can be seen in Fig. 2C, D. On the other hand, the spectrum revealed the appearance of carbon, oxygen, and nitrogen, which demonstrated that the elemental composition of the OXIM-HPMC/AAC hydrogels had been confirmed before their removal. The presence of Pb (II) in the OXIM-HPMC/AAC hydrogel was clearly demonstrated by the EDX spectra after the hydrogel had been loaded with Pb (II). This suggests that the OXIM-HPMC/AAC hydrogel was effectively coated with the heavy metal ions³². As well as Scheme 1, represent the adsorption mechanism of lead (II) in the OXIM-HPMC/AAC hydrogel from aqueous solution at pH = 3.

Fig. 2.

The SEM of the OXIM-HPMC/AAC hydrogel before (A), and after (B) adsorption of Pb (II). The EDX analysis and elemental weight % table of the OXIM-HPMC/AAC hydrogel before (C), and after (D) adsorption of Pb (II).

Scheme 1.

Adsorption mechanism of Pb (II) in the OXIM-HPMC/AAC hydrogel from aqueous solution.

Hence, the removal of Pb (II) by the OXIM-HPMC/AAC hydrogel is mainly due to chelation between Pb (II) and O/N donor atoms of glyoxime and carboxyl groups, and ion exchange and electrostatic forces. High gel percentage (86.3%) and swelling (219.6%) provide structural integrity and allow Pb (II) to reach the interior binding sites. Also, the first adsorption is attributed to the electrostatic attraction between the negatively charged –COO– groups and the positively charged Pb²⁺ ions. The hydrogel exhibited little color and morphology change after binding, indicating considerable chemical interaction rather than physical adsorption. As well as Scheme 1, represent the adsorption mechanism of lead (II) in the OXIM-HPMC/AAC hydrogel from aqueous solution at pH = 3.

Gel fraction and swelling properties of OXIM-HPMC/AAC hydrogel

The gel fraction is largely dependent on the composition of acrylic acid monomer and HPMC together. Figure 3 shows the gel percentage at different HPMC/AAC ratios. It was observed that with increasing AAC concentration in the prepared solution, the gel percentage increased to reach 86.3% in the OXIM-HPMC/AAC (70/30) hydrogel. Meanwhile, the gel fraction decreased to 77.6% for the OXIM-HPMC/AAC (30/70) hydrogel.

Fig. 3.

The Gel fraction and swelling% of the OXIM-HPMC/AAC hydrogel at different ratio of HPMC/AAC 30/70, 50/50, and 70/30.

Multiple factors, including chemical and structural characters as well as the swelling characteristics of hydrogels, affect the removal of metal ions. The swelling characteristics of the synthesized hydrogels at varying concentrations were examined. Figure 3 illustrates the swelling behavior of the hydrogels in distilled water for a duration of 8 h. The data indicated a gradual increase in the percentage of swelling over time, reaching its peak after 5 h. The swelling percentages for the OXIM-HPMC/AAC (30/70, 50/50, and 70/30) hydrogels were 142.3, 200.4, and 219.6, respectively. The OXIM-HPMC/AAC (50/50) hydrogel was used for the metal ion removal method due to its satisfactory fractional gel and swelling percentage for this study.

Adsorption experiments

Effect of solution pH

The solution pH is one of the most influential parameters governing the removal of Pb (II) using the OXIM-HPMC/AAC hydrogel, as it directly controls both the surface chemistry of the hydrogel and the behavior of Pb (II) in solution. The experimental results clearly demonstrate a strong dependence of removal efficiency on the initial pH, Fig. 4. At highly acidic conditions pH 1.0, the removal efficiency was very low 19.10%, which can be explained by the excessive presence of hydrogen ions that compete effectively with Pb (II) ions for the available active sites. Under these conditions, the functional groups present in the hydrogel structure, including carboxyl, hydroxyl, and oxime groups, remain in their protonated form, which significantly limits their ability to interact with metal ions³⁹. As the pH increased to 2.0, the removal efficiency improved slightly to 29.55%, indicating a partial reduction in proton competition. However, the most notable change occurred at pH 3.0, where a sharp increase in removal efficiency was observed, reaching 88.21%. This behavior reflects a critical transition in the system, where a considerable fraction of the functional groups becomes deprotonated, allowing stronger electrostatic attraction and coordination interactions with Pb (II) ions⁴⁰. Further increases in pH to 4.0 and 5.0 resulted in only a slight improvement in removal efficiency, reaching 92.10% and 93.61%, respectively. This indicates that most of the active binding sites are already effectively utilized at pH 3.0, and that the system approaches a near-saturation condition at higher pH values. From a practical and experimental perspective, pH 3.0 can be considered the most suitable operating condition for subsequent studies. At this pH, a high removal efficiency is achieved while maintaining stable chemical conditions and minimizing potential complications associated with higher pH values, such as changes in solution chemistry or the onset of precipitation⁴¹.

Fig. 4.

Effect of solution pH on the removal efficiency of Pb (II) using OXIM-HPMC/AAC hydrogel. Experimental conditions: initial Pb (II) concentration (100.0 mg/L) , V/m (0.1 L/g), temperature = (25 ± 1 °C), Shaking time = (60.0 min).

Effect of shaking time

The shaking time has a significant influence on the removal of Pb (II) using the OXIM-HPMC/AAC hydrogel, as it determines the extent of contact between Pb (II) ions and the active functional sites, and the results are presented in Fig. 5a. The obtained results clearly show a continuous increase in removal efficiency with increasing contact time until equilibrium is reached. At the initial stage, within 1.0 min, the removal efficiency was relatively low at 33.57%, which is mainly due to the limited interaction time between Pb (II) ions and the hydrogel surface. When the shaking time was increased to 5 min and 15 min, the removal efficiency rose significantly to 53.43% and 77.37%, respectively. This sharp increase indicates that a large number of active sites are readily available at the beginning, allowing rapid surface binding and fast diffusion of Pb (II) ions from the bulk solution. As the shaking time extended to 30 min, the removal efficiency reached 83.29%, followed by 89.21% at 45 min. At this stage, most of the accessible functional groups on the hydrogel surface become occupied, and the rate of removal starts to slow down due to partial saturation and increased diffusion resistance inside the hydrogel network. After 60 min, only a slight improvement was observed, reaching 90.65%, suggesting that the system is approaching equilibrium conditions. A final removal efficiency of 95.77% at 90 min indicates that the adsorption sites are almost fully utilized and no significant further uptake occurs beyond this point. Based on the experimental trend, 45.0 min can be considered the equilibrium contact time, as the increase beyond this point becomes marginal, confirming that most of the removal process has already been completed within this period. This behavior is consistent with recent studies on hydrogel-based adsorbents, which reported that rapid uptake occurs in the first stage followed by equilibrium due to site saturation and diffusion limitations⁴². Similar systems based on acrylic acid and cellulose derivatives have also shown that equilibrium is often achieved within 30–60 min depending on surface functionality and porosity⁴³.

Fig. 5.

Influence of (a) shaking time on the removal efficiency of Pb (II) using OXIM-HPMC/AAC hydrogel, (b) Non-linear PFO and PSO models were studied at pH 3.0 under controlled conditions, including an initial Pb (II) concentration of 100.0 mg/L, a V/m ratio of 0.1 L/g, and a temperature of 25 ± 1 °C.

Adsorption kinetics The adsorption kinetics of Pb (II) removal using OXIM-HPMC/AAC hydrogel were studied to clarify the rate-controlling mechanism and the nature of interaction between Pb (II) ions and the active functional groups. The kinetic behavior was evaluated using both the pseudo-first-order and pseudo-second-order models, as shown in Fig. 5b, and the obtained results are summarized in Table 1. The pseudo-first-order model assumes that adsorption occurs through physical interaction where the rate depends mainly on the number of available sites. However, in this study, the calculated adsorption capacity (q_e = 8.79 mg/g) was far lower than the experimental value (q_exp = 11.01 mg/g), and the correlation coefficient was moderate (R² = 0.834). This noticeable deviation indicates that the pseudo-first-order model does not adequately describe the behavior of Pb (II) removal in this system⁴⁴. On the other hand, the pseudo-second-order model provided a much better description of the experimental data, as clearly shown in Fig. 5b. The calculated adsorption capacity (q_e = 9.42 mg/g) was much closer to the experimental value, and the correlation coefficient was very high (R² = 0.94), confirming an excellent fit. The relatively high rate constant (K₂ = 0.35 g/mg· min) also reflects a strong affinity between Pb (II) ions and the functional groups of the hydrogel. The applicability of the pseudo-second-order kinetic model indicates that the adsorption process is predominantly controlled by chemisorption. In this mechanism, Pb (II) ions interact with the active functional groups present on the hydrogel surface such as hydroxyl, carboxyl, and oxime groups—through coordination bonding and electron-sharing interactions. These processes promote the formation of stable surface complexes, which accounts for the high removal efficiency observed experimentally.

Table 1.

Non-linear Kinetic model parameters for the removal of Pb (II) ions from aqueous solution using OXIM-HPMC/AAC hydrogel.

Kinetic	Non linear equations	Parameters	Metal ion
			Pb (II)
Pseudo-first order		qe (mg/g)	8.79
		K1 (g/mg min)	0.22
		R2	0.834
		χ2	3.48
Pseudo-second order		qe (mg/g)	9.42
		K2 (g/mg min)	0.35
		R2	0.94
		χ2	1.26
qexp, mg/g			11.01

Effect of initial metal ion concentration

The influence of initial metal ion concentration on the removal efficiency of Pb (II) using OXIM-HPMC/AAC hydrogel was investigated under optimized conditions (pH 3.0, shaking time 45 min, and V/M = 0.1 L/g), as illustrated in Fig. 6a. The results clearly demonstrated that the removal efficiency is strongly dependent on the initial concentration of Pb (II), exhibiting a clear decreasing trend with increasing metal ion concentration. At a low initial concentration of 100 mg/L, the hydrogel achieved a high removal efficiency of 89.5%, which can be attributed to the abundant availability of active sites relative to the number of Pb (II) ions in solution. Under these conditions, most metal ions can easily access and interact with the external surface of the hydrogel, resulting in efficient uptake. When the concentration increased from 200 to 500 mg/L, the removal efficiency gradually decreased from 77.82 to 40.84%. This reduction indicates that the number of available adsorption sites begins to become relatively limited compared to the increasing number of Pb (II) ions. As a result, competition for active sites becomes more pronounced, leading to partial saturation of the hydrogel surface⁴⁶.

Fig. 6.

Influence of (a) metal ion concentration on the removal efficiency of Pb (II) using OXIM-HPMC/AAC hydrogel, (b) Non-linear fitting of the isotherm models investigated for the removal efficiency of Pb (II) at pH 3.0 under controlled conditions, including an shaking time of 45.0 min, a V/m ratio of 0.1 L/g, and a temperature of 25 ± 1 °C.

Adsorption isotherm study The equilibrium adsorption behavior of Pb (II) onto OXIM-HPMC/AAC hydrogel was investigated using different non-linear isotherm models to describe the interaction mechanism and surface characteristics of the adsorbent. The experimental data, together with the model fittings presented in Fig. 6b, and the parameters summarized in Table 2, were analyzed using Freundlich, Langmuir, and Temkin isotherm models. Among all evaluated models, the Langmuir isotherm (Fig. 6b) showed the best agreement with the experimental data, as confirmed by the highest correlation coefficient (R² = 0.95). The calculated maximum adsorption capacity (Q_o = 20.4 mg/g) is very close to the experimental value (q_exp = 21.88 mg/g), indicating a strong consistency between theoretical prediction and practical results. In addition, the separation factor (R_L = 0.123) confirms that the adsorption process is highly favorable under the studied conditions. This indicates that Pb (II) ions are predominantly adsorbed as a monolayer onto a uniform distribution of active sites on the hydrogel surface. The Freundlich model (Fig. 6b) exhibited a lower correlation (R² = 0.92), indicating a limited contribution of surface heterogeneity and multilayer adsorption. The Temkin model (Fig. 6b) showed improved fitting (R² = 0.958), suggesting a gradual decrease in adsorption energy with surface coverage due to weak repulsive interactions between adsorbed species².

Table 2.

Non-linear adsorption isotherm constants and R² values for the removal of Pb (II) ions from aqueous solution using OXIM-HPMC/AAC hydrogel.

Isotherm	Non-linear form equation	Parameters	Metal ion
			Pb2+
Langmuir		Qo (mg/g)	20.4
		b (ml/mg)	0.071
		RL	0.123
		R2	0.95
		χ2	3.69
Freundlich		Kf (mg/g)	6.15
		n	0.214
		R2	0.92
		χ2	6.26
Temkin		KT	1.75
		B	3.27
		R2	0.958
		χ2	3.39
qexp, mg/g			21.88

Effect of (V/m) ratio

The effect of the solution volume-to-adsorbent mass ratio (V/m) on the removal efficiency of Pb (II) using OXIM-HPMC/AAC hydrogel was investigated, and the results are presented in Fig. 7a. The experiments were conducted at the optimum conditions of pH 3.0, shaking time 45.0 min, and an initial Pb (II) concentration of 100 mg/L. This parameter plays a key role in controlling the number of available active sites relative to the amount of metal ions in solution. As shown in Fig. 7a, the removal efficiency increased with decreasing V/m ratio. At a relatively high V/m value of 0.25 L/g, the removal efficiency was 66.64%, indicating that the amount of adsorbent was insufficient compared to the volume of solution, resulting in limited availability of active sites. When the V/m ratio decreased to 0.2 and 0.166 L/g, the removal efficiency improved to 72.7% and 78.85%, respectively, reflecting a gradual increase in accessible binding sites. A more pronounced enhancement was observed at lower V/m values. At 0.1 L/g, the removal efficiency reached 89.42%, while further decreasing the ratio to 0.066 and 0.05 L/g led to significant increases to 93.67% and 96.99%, respectively. This trend indicates that increasing the adsorbent dose relative to the solution volume provides a greater number of active sites, allowing more Pb (II) ions to be effectively removed from solution²⁵.

Fig. 7.

Effect of (a) V/m ratio on the removal efficiency of Pb (II) using OXIM-HPMC/AAC hydrogel, (b) temperature on the removal percent and (c) thermodynamic parameters was studied at pH 3.0 under controlled conditions, including an shaking time of 45.0 min, and V/m ratio of 0.1 L/g.

Effect of solution temperature

Temperature is a critical factor that influences both the mobility of metal ions in solutions and the interaction strength between Pb (II) ions and the hydrogel surface. The effect of temperature on the removal efficiency of Pb (II) using OXIM-HPMC/AAC hydrogel was investigated, and the results are presented in Fig. 7b. As shown in Fig. 7b, the removal efficiency decreases steadily with increasing temperature, confirming the temperature sensitivity of the adsorption process. The hydrogel achieved its highest efficiency at 25 °C (89.97%), which slightly declined to 85.68% at 35 °C, indicating the onset of weaker interactions between Pb (II) ions and the active sites. A more pronounced reduction was observed at higher temperatures, where the efficiency dropped to 63.26% at 45 °C, 44.06% at 55 °C, and reached 33.66% at 65 °C. This continuous decline demonstrates that elevated temperatures negatively influence the adsorption performance. This behavior can be explained by the weakening of the interaction between Pb (II) ions and the hydrogel surface at higher temperatures. As temperature increases, the ions become more mobile and tend to leave the surface more easily, which reduces the removal efficiency. In addition, slight changes in the hydrogel structure may decrease the availability of active sites. Therefore, the results indicate that the adsorption process is exothermic and mainly controlled by weak physical interactions, which is consistent with the kinetic and isotherm findings⁴⁷.

Thermodynamic parameters Thermodynamic parameters for the removal of Pb (II) onto OXIM-HPMC/AAC hydrogel were calculated to assess the feasibility and characterize the nature of the process. The evaluated thermodynamic data, including ΔG°, ΔH°, and ΔS°, are listed in Table 3 and graphically represented in Fig. 7c, providing a clear understanding of the adsorption characteristics. As shown in Fig. 7c, the ΔG° values are negative over the temperature range studied (-22.89 to − 19.0 kJ mol⁻¹), confirming that the sorption process is spontaneous. However, the gradual increase in ΔG° with temperature indicates a reduction in the driving force of adsorption, indicating reduced favorability at higher temperatures. The negative enthalpy change (ΔH° =  − 51.93 kJ mol⁻¹) confirms the exothermic nature of the adsorption process, which is consistent with the observed decrease in removal efficiency at elevated temperatures. The obtained ΔH° value also implies that the binding of Pb (II) ions to the hydrogel surface is predominantly associated with weak intermolecular forces rather than strong chemisorptive interactions. The negative entropy change (ΔS° =  − 97.43 J mol⁻¹ K⁻¹) indicates a decrease in randomness at the solid–solution interface during adsorption, implying that Pb (II) ions become more ordered upon attachment to the hydrogel active sites⁴⁸.

Table 3.

Thermodynamic parameters for the removal of Pb (II) ions from aqueous solution using OXIM-HPMC/AAC hydrogel.

Metal ions	T, K	ΔG°, k J mole−1	ΔH°, k J mole−1	ΔS°, J mole−1 K−1
Pb (II)	298	− 22.89	− 51.93	− 97.43
	308	− 21.92
	318	− 20.95
	328	− 19.97
	338	− 19.0

Desorption and reusability investigations

For optimal separation of the metal ions in question, the desorption process is crucial, and repeatability is a key consideration when selecting an adsorbent. To accomplish this task, the loaded OXIM-HPMC/AAC hydrogel was subjected to several stripping agents in a controlled laboratory environment ([Pb (II)] = 100 ppm, contact time: 60 min, v/m: 0.1 L/g, temperature: 25 °C). Table 4 shows that after one desorption cycle, the maximum percentage of Pb (II) desorption with 0.5 HNO₃ was 75.50%. Desorption of Pb (II) was completely achieved using 0.5 M HNO₃ after two desorption cycle. The reconstituted hydrogel was rinsed with distilled water until a neutral pH was achieved and then dried for reuse in the subsequent cycle of adsorption under the same conditions. This procedure was repeated for five consecutive cycles of adsorption and desorption was given in Table 5.

Table 4.

Desorption values of Pb (II) loaded OXIM-HPMC/AAC using various reagents.

Desorption reagent, M	Desorption%
Citric acid 0.5 M	34.5%
HCl, 0.1 M	32%
HCl, 0.50 M	36.7%
HNO3, 0.1 M	50%
HNO3, 0.50 M	75.50

Table 5.

Adsorption capacity and desorption efficiency of desorbed-OXIM-HPMC/AAC hydrogel over 5 cycles using 0.5 HNO₃.

Cycle	Adsorption capacity (mg/g)	Desorption efficacy %
1	21.88	94.20
2	21.10	92.8
3	20.35	91.50
4	19.62	90.10
5	18.90	88.70

Sorption of Pb (II) from multicomponent system

Researching the adsorption properties of OXIM-HPMC/AAC in a multi-components metal system is practically significant since real wastewater often contains multiple metal ions. In the current study multicomponent system contains (Ca²⁺, Mg²⁺, and Na⁺) with concentration of 100 ppm for each metal ion. The experimental condition was (pH 3, shaking time of 45.0 min, and V/m ratio of 0.1 L/g. The finding results of this study established that in a multi-metal system, take the order of (Pb²⁺  > Mg²⁺ > Ca²⁺ > Na⁺) with removal efficiencies, 80%, 30%, 21.5%, 11.5%. The results show that Pb (II) is more advantageous in the multicomponent system, and that OXIM-HPMC/AAC can offer more adsorption sites for Pb (II). In particular, a stronger attraction is associated with an electronegativity that is higher. Pb (II) has an electronegativity value of 2.3, Mg (II) of 1.31, Ca (II) of 1.0, and Na (I) of 0.93 . These values are in agreement with what has been found experimentally in terms of the selective adsorption properties.

Comparison with other related finding results

The adsorption performance of the OXIM–HPMC/AAC hydrogel was assessed by comparison with previously reported adsorbents, as summarized in Table 6. The obtained adsorption capacity (21.88 mg/g) is comparable to or higher than most reported bio-based and composite materials, including chitosan/MAA (13.72 mg/g), chitosan/CMC (16.35 mg/g), magnetite–chitosan hydrogel beads (18.26 mg/g), and nano-bentonite crosslinked chitosan (7.93 mg/g). Only cellulose nanofiber–sodium alginate hydrogel (22.67 mg/g) and polyacrylic acid/nano-clay hydrogel (33.33 mg/g) exhibit higher values. Consequently, the OXIM–HPMC/AAC hydrogel can be regarded as a promising adsorbent for the effective removal of Pb (II) ions from aqueous systems.

Table 6.

Adsorption capacities of different adsorbents of Pb²⁺.

Adsorbent	Qo, mg/g	References
OXIM-HPMC/AAC hydrogel	21.88	This work
Cellulose nanofiber-sodium alginate hydrogel	22.67	49
Chitosan/methacrylic acid (MAA)	13.72	50
Chitosan/CMC bio-based composite	16.35	51
Magnetite hydrogel beads (CMC–Chitosan–Fe3O4)	18.26	52
Polyacrylic acid/nano clay embedded hydrogel composite	33.33	53
NanoBentonitecrosslinked Chitosan	7.93	54

Conclusion

The hydrogel based on HPMC/AAC and functionalized by oxime were prepared by free radical polymerization in the presence of APS and MBA and were characterized by FTIR, EDX, and SEM. The presence of absorption bands at 1705, 1450, 1160, and 1630  cm⁻¹ confirms that the bands of carboxylic and oxime groups of the OXIM-HPMC/AAC hydrogel. The FTIR spectra after adsorption show a clear decrease in the intensity of all characteristic peaks, confirming that both carboxyl and oxime groups are directly involved in the Pb (II) removal process. This is further confirmed by SEM images, which show a decrease in the pore size after Pb (II) adsorption. Also, the appearance of characteristic Pb (II) peaks in the EDX spectra after adsorption, along with the initial presence of carbon, oxygen, and nitrogen, confirms the successful and efficient removal of heavy metal ions by the OXIM-HPMC/AAC hydrogel. The OXIM-HPMC/AAC (50/50) hydrogel was identified as the optimal formulation for removing metal ions, as it gave an acceptable gel fraction in the preparation process with a swelling% of up to 200.4% after 5 h. The Pb (II) removal using OXIM-HPMC/AAC hydrogel was investigated under various operating conditions. The results indicated that the system achieves its highest efficiency under mild acidic conditions, with a contact time of 45 min and a V/m ratio of 0.1 L/g. Equilibrium modeling showed that adsorption followed the Langmuir isotherm, confirming monolayer adsorption on uniform surface and a maximum adsorption capacity of 21.88 mg/g. Thermodynamic analysis showed that (ΔH° =  − 51.93 kJ mol⁻¹) which is confirmed that sorption of Pb (II) on The OXIM-HPMC/AAC is exothermic process. The best desorption reagent for Pb (II) was found to be 0.5 M HNO₃. Furthermore, the limitation of this study is to working for leading removal from low pH values so we will develop a new hydrogel type which works in the long PH range.

Author contributions

El-Sayed Khafagy, Nourah Alshahrani, Amr Selim Abu Lila, Ahmed Al Saqr, Mohammed F. Aldawsari, Bjad K. Almutairy, M. A. Mostafa, Ashraf M. Ashmawy: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Project administration; Resources; Software; Validation; Visualization; Roles/Writing—original draft; Writing—review and editing.

Data availability

The datasets used during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.