Swelling behavior observation of gel resin in heavy metal adsorption process: gel resin containing sodium carboxylate

Article information

Environmental Engineering Research. 2026;31(1)

Publication date (electronic) : 2025 June 12

doi : https://doi.org/10.4491/eer.2024.577

Bo Yu ¹^,², Ming Zhang ¹^,²^,

, Lei Wang ¹^,², Huan Zhang ³, Xiangming Liu ¹^,²^,⁴, Pengfei Shen ¹, Di Wang ², Longgang Ren ¹^,²

¹China Tiegong Investment & Construction Group CO., LTD., Beijing 100000, China

²China Railway Water Group CO., LTD., Xi’an 710000, China

³College of Chemistry and Material, Weinan Normal University, Weinan 714099, China

⁴Ningxia Shuirun Testing Technology CO., Ltd., Yinchuan, 750000, China

^†Corresponding author: E-mail: mingzhang1993@sina.com, Tel: +86 13540265732 Fax: +86 13540265732, ORCID: 0000-0002-5395-3900

Received 2024 September 29; Revised 2025 June 4; Accepted 2025 June 9.

Abstract

In previous researches, the adsorption process of gel resin has been described as ion exchange process. The strong hydrophilic and quick ionization property of −COONa group in solution have not been considered. Thus, this article aimed to elucidate the swelling behavior of gel resin in heavy metal solution, in which the active adsorption site was −COONa group. The experimental conditions included initial copper concentration, adding dose, reaction time, continuous adsorption, and regeneration experiment. Three indexes, including adsorption capacity, released sodium ion amounts, and water absorbency, were measured simultaneously. The results exhibited that the dried resin particles swelled quickly once immersing in heavy metal solution. When the adsorption saturation condition was not reached, gel resin absorbed water molecules and existed as the swelled particles. Significant negative correlation was found between adsorption capacity and water absorbency. The heavy metal adsorption process occurred in tandem with the initial water absorption and subsequent water loss processes. Up until the saturation point of heavy metal adsorption, the slow release of sodium ions and water molecules from the inflated gel was seen. The study findings offer a thorough grasp of gel resin adsorbing heavy metals and absorbing water molecules tendencies.

Abstract

Graphical Abstract

Keywords: Gel resin; Heavy metal adsorption; Water absorbency; Swell

1. Introduction

Ion exchange process has been recognized to explain heavy metal adsorption mechanism for gel resin. This process consists of four parts: (1) diffusion of heavy metal ions from the aqueous solution into the surface layer of adsorbents; (2) diffusion of heavy metal ions in the inner layer of the adsorbents; (3) replacement of heavy metals with the substituted ions in the adsorbents; (4) release of the substituted ions from the particles into the outer solution [1–4]. These descriptions highlight the entire mass transferring and migrating process of heavy metal ions from diffusing in aqueous solution to be adsorbed by solid adsorbents. In cation exchange resins, the exchangeable sites are initially occupied by hydrogen or sodium ions, which are exchanged with M²⁺ (such as cadmium, copper, lead, and zinc ions). In anion exchange resins, sites bear counter balanced by chloride ions, which is displaced by anionic metals (such as dichromate ion). The essence of this reaction is the combination of active functional groups to heavy metal ions [5]. Specifically, the active functional groups usually are −COO⁻ and −SO₃⁻ in cation exchange resin, −N(CH₃)₃⁺ in anion exchange resin. Herein, considering the strong ionization property of sodium acetate (CH₃COONa), −COONa groups in polymer chains exhibit the ionization reaction when immersing the resin into aqueous solutions. As hydrophilic group, −COONa groups in the polymer network chains could also absorb water molecules. However, most researches focus on detecting heavy metal ions concentration in residual solution. Resin volumetric change, especially for expanding characteristic, has not been observed synchronously. The water absorption property of the −COO⁻ groups has not been considered in this theory.

The −COONa functional groups in adsorbents have been applied to combine heavy metals, such as surface modification of citric acid to peach stone, grafting of functional groups in biomass, and synthesis of acrylic acid (AA) into polymer or inorganic metal compounds [6–9]. These behaviors parallel those of super-absorbent resins (SAR), which likewise swell via −COO⁻ hydration. SAR can absorb water and retain water molecules in network structures [10–12]. Hence, the same property of −COONa groups to absorb water molecules might also be applicable for ion exchange resin. Manaila et al. synthesized Poly(AM-co-AA) resin through electron beam irradiation, and this resin exhibited 85 g/g water absorbency in distilled water [13]. Zhao et al. prepared Poly(AM/AA/cellulose) resin to adsorb heavy metals, and the maximum adsorption capacities for cationic copper, lead, and cadmium ions were 157.51, 393.28, and 289.97 mg/g [14]. Marjub et al. prepared Poly(chitosan/AA) resin to adsorb copper and lead ions, and the water absorbency of Poly(chitosan/AA) was 256.04 g/g in distilled water [15]. Early studies have focused on obtaining the adsorption capacities of resin or the removal rates to heavy metals [16]. However, the swelling characteristics of resin containing −COONa group in heavy metal solution have not been investigated. The responsive behavior of hydrophilicity for −COONa should be comprehensively investigated for resin in heavy metal solution.

In this work, extensive experiments have been conducted to examine the heavy metal adsorption and swelling behaviors for gel resin containing −COONa groups. The radical polymerization was used to create gel resin with a −COONa group. Heavy metal adsorption capacity and water absorbency in CuSO₄ solution were investigated simultaneously. In a variety of adsorption condition settings, including initial copper concentration, adding dose, reaction time, continuous adsorption, and regeneration experiment, the associations between gel resin’s ability to adsorb heavy metals and absorb water molecules were established. The morphologies of gel resin were also noted. The primary aim of this study is to give an additional analysis of gel resin’s water absorption behavior in heavy metal solution.

2. Materials and Methods

2.1. Materials

Acrylic acid (AA), acrylamide (AM), ammonium persulfate (APS), N,N-methylene bisacrylamide (MBA) were provided by Kelong Chemical Reagent Plant (Chengdu, China). Hydrochloric acid (HCl), sodium hydroxide (NaOH), copper sulfate (CuSO₄), were. purchased from Sinopharm Chemical Reagent CO., Ltd. And all reagents that used were analytical grades.

2.2 Preparation of Product

Firstly, 5 mL AA was neutralized with 28.3 mL of 10 wt.% NaOH solution to reach 100% neutralization degree. Then, 2 g of AM and 0.3 g of MBA were added under the magnetic stirring at 70°C. Thereafter, the dissolved APS solution (0.2 g of APS to 5 mL distilled water) was added to the mixed solution along with heating at 70°C. Once the mixed solution was turned into a solid gel, the magnetic stirrer was turned off, and the reaction was continued for 3 h at 70°C. Finally, the gel was cut into granular particles, washed with absolute alcohol for three times to remove unreacted monomers, and dried at 70°C for 24 h. Through smashing and sieving, P(AANa/AM) gel resin had a particle size within 0.45 mm to 1 mm.

2.3. Heavy Metal Adsorption Experiments

Three indexes, including adsorption capacity, released sodium ion amounts by resin, and water absorbency, were measured together [17]. Firstly, CuSO₄ was dissolved in distilled water to prepare 1000 mg/L reserve solution and diluted to different concentrations according to experimental requirements. Adsorption experiments were carried out by adding different samples to 100 mL CuSO₄ solution with the initial concentration of 250 mg/L. Samples were mechanically agitated at 180 rpm at 25°C. After reaching the equilibrium state, P(AANa/AM) were separated by stainless steel mesh. The concentrations of copper and sodium ions in residual solution were detected by a flame adsorption atomic spectrophotometer (AAS, AA-7050, Beijing East & West Analytical Instruments CO., Ltd., China). The filtered adsorbents were weighted by a analytical balance. The adsorption capacity, removal rate to copper, released sodium ion amounts by resin, and water absorbency of P(AANa/AM) were calculated by the following Eq. (1), Eq. (2), Eq. (3), and Eq. (4). To check the reproducibility, each test specimen was prepared in triplicate.

(1) Qad=(C0-Ce)×VC0

(2) η=C0-CeC0×100%

(3) QRNa=C1×Vm0

(4) Qab=m1-m0m0

where Q_ad (mg/g) and η mean the adsorption capacity and removal rate to Cu²⁺, respectively. Q_RNa (mg/g) means the released sodium ion amounts by resin. Q_ab (mg/g) means the water absorbency for resin in aqueous solution. C₀ (mg/L) and C_e (mg/L) are the initial and equilibrium concentrations of Cu²⁺, respectively. V (L) is the volume of CuSO₄ solution. m₀ (g) and m₁ (g) are the mass of the dried adsorbent and the filtered adsorbent [18, 19].

The relationship between adsorption capacity, released sodium ion amounts by resin, and water absorbency was evaluated through continuous adsorption. P(AANa/AM) of 0.1 g was added into 100 mL of 50 mg/L CuSO₄ solution, and stirred for 1 h. The adsorbents were filtered and reused for a fresh adsorption in another 50 mg/L CuSO₄ solution. This process was repeated until the concentration of Cu²⁺ in residual solution did not decrease. The adsorption capacity, released sodium ion amounts by resin, and water absorbency values in each process were calculated and evaluated of correlation analysis by SPSS.

2.4. Desorption and Regeneration

The swelling characteristic of P(AANa/AM) in regeneration experiments was observed. A fixed amount of 0.1 g of P(AANa/AM) was immersed sequentially in 100 mL of 250 mg/L CuSO₄ solution, 0.1 mol/L HCl solution, and 0.1 mol/L NaOH solution. The bottle was placed in a shaker for 1 h at 180 rpm. And the end of the experiment, the solid adsorbent was filtered and then immersed in distilled water. Water absorbency of adsorbent in each steps were measured.

2.5. Characterization

The surface metal ion contents were scanned by Bruker EDX detector and coated with Au vapor. The accelerating voltage was set to 15 kV. The working distance was set to 12.5 mm. The macro appearance of adsorbents in each batch experiments was observed by an industrial camera with light source and 30 times magnification (GP-300C, Kunshan Gaopin Precision Instrument CO., LTD., China).

3. Results and Discussion

3.1. Effect of Initial Concentration on Adsorption

Fig. 1(a) illustrated the influence of initial copper ion concentration on heavy metal adsorption capacity, released amount of sodium ions, and water absorbency. Heavy metal adsorption capacity increased dramatically with increasing the initial copper ion concentration. The maximum adsorption capacity was 201.63 mg/g (converting into 3.17 mmol Cu²⁺/g), in which the saturation threshold of initial copper ion concentration was 250 mg/L (converting into 7.41 mmol Na⁺/g). The adsorption saturation state for P(AANa/AM) gel resin was reached when initial copper ion concentration was higher than that of 250 mg/L. Otherwise, the functional adsorption sites in adsorbent were not all occupied by heavy metals. The curve trend of the released sodium amounts was same as adsorption capacity. And the maximum value of the released sodium amounts was 170.48 mg/g, which was derived from the diffusion of −COONa in adsorbent. Significantly, water absorbency of gel adsorbent was synchronously measured in adsorption experiment. When initial Cu²⁺ concentrations were 10, 250, and 500 mg/L, water absorbency values of adsorbent were 39.04, 3.56, and 3.67 g/g, respectively. Fig. 1(b) shows the relativity of adsorption capacity, released sodium ion amounts, and water absorbency for P(AANa/AM) gel adsorbent under different initial Cu²⁺ concentrations. Strong positive correlation between adsorption capacity and the released sodium ion amounts indicated that copper ion in solution was exchanged with sodium ion in adsorbent. Furthermore, strong negative correlation between adsorption capacity and water absorbency was found, which indicated water absorbency could be served as an indicator to determine the endpoint of adsorption saturation for gel adsorbent [20–22].

Fig. 1

(a) Effect of copper ion initial concentration and (b) correlation plot (**p<0.01) on adsorption capacity, released sodium ions amounts, and absorbency water (m=0.01g, V=100mL, t=1h).

Fig. S1 showed the morphologies of P(AANa/AM) adsorbent after immersing in CuSO₄ solutions with different initial concentrations. The original morphology of P(AANa/AM) gel resin was dried particle with the size of 0.45 to 1 mm. The swelling behavior was observed upon immersion in a 10 mg/L CuSO₄ solution. According to our earlier research, the −COONa functional group was classified as a negative group that exhibited the capacity to adsorb cationic heavy metal ions [23]. Herein, the hydrophilicity of −COONa for gel adsorbent should be taken into account simultaneously in heavy metal adsorption experiments. The swollen particle was the P(AANa/AM) adsorbent’s current state when adsorption saturation was not attained. Water molecules were now dispersed in the inner area of resin. Once the initial copper ion concentration exceeded 250 mg/L, the massive shrinking phenomenon was noticed. These findings supported the theory that gel resin would likewise exhibit swelling behavior during the heavy metal adsorption process. [24–26].

3.2. Effect of Adding Dose on Adsorption

Fig. 2 illustrated the influence of P(AANa/AM) gel resin adding dose on removal rate, adsorption capacity, released sodium ion amounts, and water absorbency. Removal rates to copper ion was increased dramatically with increasing the adding dose of adsorbent, and the maximum removal rate was 94.08%. This observation can be illustrated by that the increase of adding dose of adsorbents may multiply the amount of the adsorption active sites, hence leading to the increase of removal rates. When the adding dose of adsorbent was lower than 0.10 g, adsorption capacity, released sodium ion amounts, and water absorbency values remained roughly constant of 200.83 mg/g, 177.93 mg/g, and 3.35 g/g, respectively. In these moments, adsorption saturation states were all reached for P(AANa/AM). With further rising in adding dose, water absorbency was increased dramatically, which was mainly related to the binding degree of −COONa to heavy metals. The remaining vacant sites in adsorbent were not all occupied by copper ions, but to combine with water molecules [27, 28]. It was related that the shrank phenomena in Fig. S2 were observed when the adding dose of P(AANa/AM) was lower than 0.12 g. The gel adsorbent swelled with further increasing adding dose. Thereafter, the hydrophilicity property of −COONa should be attached to study the heavy metal adsorption process.

Fig. 2

Effect of adsorbent adding dose on removal rate, adsorption capacity, released sodium ion amounts, and water absorbency (C₀ = 250 mg/L, V = 100 mL, t = 1 h).

3.3. Effect of Reaction Time on Adsorption

Fig. 3 illustrated the influence of reaction time on adsorption capacity, released sodium amounts, and water absorbency in CuSO₄ solutions under different initial concentrations of 20, 100, 250, and 500 mg/L. It can be seen that adsorption capacity and released Na⁺ amounts increased rapidly during the first 15 min, then plateaued by 40 min. The number of the adsorbed copper ion by adsorbent was increased, which was accompanied with the releasing process of Na⁺ from adsorbent into the aqueous solution. To understand water molecules releasing mechanisms during heavy metal adsorption process, water absorbency was measured simultaneously. Swelling behavior of P(AANa/AM) gel resin was also occurred in heavy metal solution. P(AANa/AM) swelled quickly after immersing the dried adsorbent particles into CuSO₄ solution. At 7.5 min, water absorbency values were 35.95, 32.66, 27.29, 18.51, 12.42 g/g in distilled water, CuSO₄ solutions with initial concentration of 20, 100, 250, and 500 mg/L, respectively. There was a gradual decrease of water absorbency during the subsequent soaking process. At 60 min, water absorbency values were 39.82, 38.67, 23.48, 3.71, and 3.69 g/g. Such behavior is due to a synergistic effect of −COONa to combine heavy metal and water molecules. As a polyelectrolyte, water molecules could easily release into the inner area of gel resin and combine with −COONa groups through hydrogen bond. The stretching network polymer chains provide a sufficient space to accommodate water molecules. Further immersing adsorbents in CuSO₄ solution, water absorbency was decreased with the increase of adsorption capacity. This could be attributed to the fact that the copper ions and water molecules competed for −COO⁻ sites for resin in heavy metal adsorption process.

Fig. 3

Kinetic adsorption capacity, released sodium ion amounts, and water absorbency values of P(AANa/AM) gel resin in distilled water and CuSO₄ solutions with initial concentration of 20 mg/L, 100 mg/L, 250 mg/L, and 500 mg/L (m= 0.10 g, V = 100 mL).

3.4. Continuous Adsorption Experiment

In order to further confirm the relationship of adsorption capacity, released Na⁺ amounts, and water absorbency in heavy metal adsorption process, the continuous adsorption experiment in initial 50 mg/L CuSO₄ solution was conducted, and the results were showed in Fig. 4 and Fig. S3. In first immersion in 50 mg/L CuSO₄ solution, values of adsorption capacity, released Na⁺ amounts, and water absorbency were 33.85 mg/g, 36.64 mg/g, and 34.85 g/g, respectively. After immersing in fresh 50 mg/L CuSO₄ solutions for five times, the saturation state was reached. And the values of adsorption capacity, released sodium ion amounts, and water absorbency were 199.01 mg/g, 170.80 mg/g, and 3.72 g/g. The reduction in water absorbency accompanied with the increase of adsorption capacity and released sodium ion amounts. A significant positive correlation was found between adsorption capacity and released sodium ion amounts. This result revealed that Na⁺ was released from adsorbent particles into the outer solution. Meanwhile, adsorption capacity had a significant negative correlation with water absorbency, which indicated that water molecules were also lost during the heavy metal adsorption process [29, 30]. Such phenomena are due to the hydrophilic property of −COONa groups in the crosslinked net polymer chains. The quickly swelling behavior would be occurred once soaking the dried gel adsorbent into aqueous solution. Synchronous releases of sodium ion and water molecule were accompanied with the adsorption of copper ions. Hence, on the basis of the exchanging process between heavy metals and sodium ions, water molecule acts as a co-transported species in the ion exchange process [31–33]. The dynamic swelling and shrinking changes should be added to study adsorption mechanism of gel adsorbent in heavy metal solution.

Fig. 4

(a) values and (b) correlation analysis (**p<0.01) of adsorption capacity, released sodium ion amounts, and water absorbency under five times continuous adsorption (C₀ = 50 mg/L, m = 0.10 g, V = 100 mL).

3.5. Regeneration Experiment

Prior research on the regeneration experiment mostly assessed the sustainability of adsorption capacity or removal rate for adsorbent [34–36]. It has not been possible to observe the morphological changes that occurs over the whole soaking cycle of an adsorbent in acid or alkali solutions. Fig. 5 and Fig. 6 illustrate water absorbency and morphologies of P(AANa/AM) gel resin during the whole regeneration process. Soaking the dried adsorbent into 250 mg/L CuSO₄ solution, water absorbency of adsorbent was 3.68 g/g. Later immersing in 0.1 mol/L HCl solution and distilled water, water absorbency were 0.97 and 1.26 g/g. All of the adsorbed Cu²⁺ in adsorbent was rinsed out in this phase. The resin with −COOH groups exhibited the extremely low water absorption capacity. Finally, recovery of −COONa functional groups was achieved following immersion in 0.1 mol/L NaOH solution and distilled water, which can be confirmed by the listed EDS analysis results in Fig. 5. The final state of P(AANa/AM) gel resin was the swelled particle with 39.51 g/g water absorbency. Immersing in 250 mg/L CuSO₄ solution once again, water loss phenomenon was observed for resin. Morphology observations in Fig. 6 were compatible with the water absorbency results. The swelling and shrinking behavior for gel adsorbent should be recommended in column filter application. The loading height or quality of gel adsorbent must be correlated by water absorbency.

Fig. 5

Water absorbency and EDS analysis of P(AANa/AM) gel adsorbent in the heavy metal adsorption and desorption experiments.

Fig. 6

Morphologies of P(AANa/AM) gel adsorbent in the heavy metal adsorption and desorption experiments.

3.6. Mechanism Analysis

Fig. 7 illustrates the heavy metal adsorption process of gel resin containing −COONa groups. Crosslinked polymer backbones provide the insoluble but swelling property for gel resin. The −COONa groups are linked in the polymer chains, in which the mobility is limited. Attributing to the hydrophilic property of −COO⁻, gel resin can absorb water molecules through hydrogen bond. Thus, Immersing dried gel resin into heavy metal solution, the preferential behavior of absorbing water is firstly observed. The polymer chains in the swollen particles generate the elastic stretch synchronously, reflecting in the increase of the particles size. Meanwhile, the ionization of −COONa is accompanied with the quick expanding behavior. Part of Na⁺ was released into the outer solution.

Fig. 7

Schematic illustration of gel resin with −COONa groups adsorbing heavy metals and swelling behavior.

The −COONa group in the polymer network chains could also adsorb heavy metals. In case −COO⁻ group is not combined with heavy metal, −COO⁻ group in the polymer chains combines with water molecule through hydrogen bond. The maximum water absorbency would be obtained in the early stage. As the adsorption reaction proceeding, the adsorption active sites in −COO⁻ groups would gradually be occupied by copper ions. sodium ion in resin was exchanged and released into the outer solution. In fact, the electrostatic attraction theory explains the adsorption force between anionic −COO⁻ group and cationic copper ion. Ion exchange theory emphasizes the mass migration process to maintain zero charge in adsorbent and aqueous solution. The orbital hybridization theory perfectly interprets the combining form for unequal charge number between −COO⁻ and copper. To achieve a stable conformation, divalent copper ion needs to bind with four numbers of −COO⁻ groups to develop a tetrahedral structure through dsp² orbital hybridization. Owing to this special ionic crosslinkage, the space area for accommodating water molecules is decreased. Thus, water absorbency is decreased along with the adsorption process. When adsorption saturation is reached, all water molecules and sodium ions are released. The final state of the adsorbent is the shrinkage particle. Thus, the absorption and release of water by gel resin represent the participation of the water molecules during heavy metal adsorption process.

4. Conclusion

The adsorption of heavy metal and the swelling characteristics of gel resin with −COONa groups were noted. Immersing in heavy metal solution, water absorption also took place due to the −COONa group’s exceptional hydrophilic properties. When initial Cu²⁺ concentration were 10, 250, and 500 mg/L, water absorbency values of gel resin were 39.04, 3.56, and 3.67 g/g, respectively. Kinetic experiments exhibited that the heavy metal adsorption process was accompanied by the initial expansion and subsequent contraction for gel resin. Water molecules and sodium ions were partially retained in the enlarged particles under unsaturated adsorption condition. Conversely, after adsorption saturation was achieved, no water molecules or sodium ions were present in the adsorbents. Furthermore, in the regeneration experiment, the gel resin’s end state was a swelled particles with 39.51 g/g water absorbency. According to this study, gel resin exhibited the swelling behavior during the heavy metal adsorption process. The hydrophilic influence of −COONa group for gel resin is another important parameter that required in practice heavy metal adsorption application.

Supplementary Information

eer-2024-577-supplementary.pdf

Notes

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of Interest

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

Author Contributions

B.Y. (Senior Engineer) contributed to the project administration. M.Z. (PhD) contributed to the manuscript conceptualization, validation, methodology, investigation, and writing original draft. L.W. (Professor) contributed the manuscript methodology, investigation, and resources. H.Z. (Assistant Professor) contributed to the manuscript conceptualization. X.M.L. (Engineer) contributed to the validation and data analysis. P.F.S. (PhD) revised and corrected the manuscript. L.G.R. (Senior Engineer) reviewed and edited the manuscript. All the authors read and approved the final manuscript.

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