Fully chitosan-based thermoresponsive hydrogel for efficient Cu²⁺ adsorption and regeneration

Abstract

To efficiently remove Cu²⁺ from wastewater, a fully chitosan thermoresponsive hydrogel (IPS_gel) was prepared using chitosan as the raw material. IPS_gel exhibited pronounced thermoresponsive behavior as well as a high adsorption capacity toward Cu²⁺. When the temperature exceeded its volume phase transition temperature (VPTT = 35.4 °C), enhanced hydrophobic interactions among with the alkyl groups along the polymer chains induced rapid dehydration, shrinkage, and a reversible volume phase transition. The effects of initial Cu²⁺ concentration and pH on adsorption performance were systematically evaluated. The adsorption process followed the Langmuir isotherm and pseudo-second-order kinetic models, and a maximum adsorption capacity of 24.5 g/g was achieved. During regeneration, only 0.23 L of 0.1 M hydrochloric acid per gram of IPS_gel was required for rapid Cu²⁺ desorption, representing a reduction of approximately 88 to 95% compared to the conventional acid consumption (2 to 5 L/g) of traditional hydrogels. These findings indicate that by integrating thermoresponsiveness, high adsorption capacity, and efficient regeneration, the chitosan-based IPS_gel shows strong potential for wastewater treatment applications and provides a new strategy for the green design of purification materials.

Full Article

Fully Chitosan-Based Thermoresponsive Hydrogel for Efficient Cu²⁺ Adsorption and Regeneration

Xiaohan Zha  ,^a,b,# Zhuoying Meng  ,^a,# Chen Wang,^a,b Yiwen Ye,^a,b Jiale Zhou,^a,b

Ruilin Zhang,^a,b Jingyuan Li,^a,b Shifeng Ma,^a,b Chengyu Tan,^a,b,* and Ye Tian  ,^a,b,c,*

To efficiently remove Cu²⁺ from wastewater, a fully chitosan thermoresponsive hydrogel (IPS_gel) was prepared using chitosan as the raw material. IPS_gel exhibited pronounced thermoresponsive behavior as well as a high adsorption capacity toward Cu²⁺. When the temperature exceeded its volume phase transition temperature (VPTT = 35.4 °C), enhanced hydrophobic interactions among with the alkyl groups along the polymer chains induced rapid dehydration, shrinkage, and a reversible volume phase transition. The effects of initial Cu²⁺ concentration and pH on adsorption performance were systematically evaluated. The adsorption process followed the Langmuir isotherm and pseudo-second-order kinetic models, and a maximum adsorption capacity of 24.5 g/g was achieved. During regeneration, only 0.23 L of 0.1 M hydrochloric acid per gram of IPS_gel was required for rapid Cu²⁺ desorption, representing a reduction of approximately 88 to 95% compared to the conventional acid consumption (2 to 5 L/g) of traditional hydrogels. These findings indicate that by integrating thermoresponsiveness, high adsorption capacity, and efficient regeneration, the chitosan-based IPS_gel shows strong potential for wastewater treatment applications and provides a new strategy for the green design of purification materials.

DOI: 10.15376/biores.21.3.8001-8014

Keywords: Chitosan; Thermoresponsive; Hydrogel; Cu²⁺ adsorption; Reusability

Contact information: a: Key Laboratory of Environment Controlled Aquaculture, Dalian Ocean University, Ministry of Education, Dalian 116023, China; b: College of Marine Science and Environment, Dalian Ocean University, Dalian 116023, China; c: College of Biosystems Engineering and Food Science (BEFS), Zhejiang University, Hangzhou 310058, China; #: Xiaohan Zha and Zhuoying Meng contributed equally to the work, they are jointly listed as first authors;

* Corresponding author: [email protected]; [email protected]

INTRODUCTION

Chitosan, a natural polysaccharide derived from chitin, is the only basic polysaccharide found in nature. It has high biological activity and economic value, so it can be used as a biomedical material, an aquatic feed additive, or an industrial wastewater treatment material (Arafa et al. 2024; Zha et al. 2026). However, one notable thing about chitosan is that it dissolves in acidic solutions but not in alkaline ones. This limits how it can be processed and makes it more difficult to prepare chitosan-based materials (Crini and Lichtfouse 2019; Nankawa et al. 2025). Therefore, exploring and optimizing chitosan processing strategies is both theoretically important and practically useful.

Industrial wastewater containing copper ions is commonly discharged into the environment, and the potential hazards of this pollutant cannot be overlooked. Excessive Cu²⁺ can be readily adsorbed by soils and sediments, subsequently accumulating through the food chain and ultimately entering the human body (Zhen et al. 2022). The long-term intake of Cu²⁺-contaminated food may lead to chronic toxicity, causing irreversible damage to multiple organs, including the respiratory and digestive systems (Jeremias et al. 2023; Sahu et al. 2025). Moreover, exposure to Cu²⁺ may increase carcinogenic risks. Common wastewater treatment methods for the removal of Cu²⁺ include adsorption, ion exchange, chemical precipitation, and membrane filtration (Cai et al. 2024). Among these, adsorption is considered an efficient and promising approach due to its operational simplicity, low cost, and ease of regeneration (Chen et al. 2025). In recent years, hydrogel-based adsorbents have attracted significant attention for heavy metal removal because of their three-dimensional network structure, hydrophilicity, and tunable functionality (Chen et al. 2022). In particular, natural polysaccharide-based hydrogels, such as those derived from chitosan, exhibit excellent biocompatibility and biodegradability. The abundant amino and hydroxyl groups of these hydrogels serve as effective binding sites, enabling coordination interactions with Cu²⁺ that provide enhanced adsorption capacity and selectivity (Badsha et al. 2021; Khan et al. 2021).

However, challenges, such as difficult recovery, potential secondary pollution, and insufficient cycling stability, have limited the practical application of hydrogel-based adsorbents. Moreover, conventional desorption methods typically rely on large volumes of concentrated acidic eluents, leading to high operational costs and severe secondary pollution. There is a need to develop environmentally friendly alternatives (De Luna et al. 2015; Park and Lee 2022; Zhi et al. 2025). To address the above challenges, stimulus-responsive hydrogels, particularly thermoresponsive systems, have emerged as promising alternatives for adsorbent recovery and reuse. These materials undergo reversible swelling–shrinkage transitions when the environmental temperature crosses the volume phase transition temperature (VPTT) (Sun et al. 2024; Wang et al. 2025), enabling mild regeneration and the efficient release of adsorbed pollutants (Suljovrujic et al. 2019; Zhi et al. 2025).

Based on these considerations, a fully chitosan thermoresponsive hydrogel was developed in this study for the efficient removal of Cu²⁺. Chitosan was first treated via an alkaline dissolution method and subsequently modified to obtain thermoresponsive, 2-hydroxy-3-isopropoxypropyl chitosan (hereafter denoted as IPS).. This approach endowed the chitosan with temperature sensitivity while also improving its solubility, thereby enhancing its processability.

Subsequently, IPS was employed as the sol-gel framework to construct a molecularly functionalized thermoresponsive hydrogel (denoted as IPS_gel) through a covalent crosslinking strategy, enabling improved Cu²⁺ adsorption performance. The structure and morphology of the hydrogel were characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Then, the thermoresponsive swelling–shrinkage behavior of IPS_gel was systematically investigated. Considering the prospective application of this material in saline wastewater, NaCl was used as an example to observe how the ionic environment affects the thermosensitive behavior of the hydrogel. The introduction of salt ions competes with hydrophilic groups for water molecules, weakens hydrogen bonding, and allows hydrophobic interactions to gradually dominate, thereby lowering the VPTT of the hydrogel. In addition, pH significantly alters the existing forms and adsorption properties of Cu²⁺ in solution. Therefore, this work considered the effect of pH on the adsorption capacity and identified the optimal pH condition, under which all subsequent adsorption isotherm and kinetic experiments were carried out. These results provide a reference for understanding how ions and pH synergistically regulate the adsorption process. Effects of pH (1.5 to 6.0) and Cu²⁺ concentration (25 to 200 mg/L) were studied relative to the Cu²⁺ adsorption capacity. The adsorption process was analyzed using isotherm models (Langmuir and Freundlich) and kinetic models (pseudo-first-order and pseudo-second-order). In addition, the reusability and regeneration performance of IPS_gel were comprehensively assessed.

EXPERIMENTAL

Materials

Isopropyl glycidyl ether (IPGE), ethylene glycol diglycidyl ether (EDGE), and lithium hydroxide (LiOH, analytical grade) were obtained from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Anhydrous copper sulfate (CuSO₄, analytical grade) was purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). Potassium hydroxide (KOH, analytical grade) and chitosan (degree of deacetylation ≥ 95%, viscosity 100 to 200 mPa·s) were obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH, analytical grade) was supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

Methods

Synthesis of thermoresponsive IPS_gel

A mixed aqueous solution containing 4.5 wt% LiOH, 4.5 wt% KOH, and 3.5 wt% urea was first prepared. Chitosan (CS) was then added to achieve a concentration of 2.8 wt%, followed by continuous stirring. This mixture was frozen at −40 °C for 3 h to obtain a transparent CS solution. Subsequently, the pH of the solution was adjusted to approximately 12 by adding 1M NaOH, and the mixture was heated in a 60 °C water bath with stirring for 1 h. The IPGE was then introduced, and the reaction was maintained for 5 h. The product was purified by dialysis for 72 h, followed by rotary evaporation and freeze-drying to obtain purified IPS. The obtained IPS powder was redissolved, and NaOH was added until complete dissolution was achieved. The EDGE was slowly added in a 45 °C water bath to form the hydrogel via covalent crosslinking. The swelling–shrinkage behavior of this hydrogel was repeatedly evaluated using deionized water at 22 °C and 40 °C. Finally, the hydrogel was freeze-dried to obtain dried IPS_gel (Dai et al. 2022; Zha et al. 2026).

Morphological and structural characterization of IPS_gel

Fourier transform infrared spectra of CS, IPS, and IPS_gel were recorded in the range of 800 to 4000 cm⁻¹ using an IRTracer-100 spectrometer (Shimadzu, Japan). For morphological analysis, the surfaces of dried shrunken and swollen IPS_gel samples were sputter-coated with gold prior to observation. The SEM images were obtained using a JSM-7800F field emission scanning electron microscope (JEOL, Japan).

Swelling properties of IPS_gel

The swelling behavior of IPS_gel was investigated using a gravimetric method. Dried IPS_gel samples (200 mg) were immersed in deionized water at a temperature of 22 °C for 24 h to reach swelling equilibrium. The IPS_gel samples were then transferred to a 40 °C water bath for 15 min to induce shrinkage. After removing excess surface water, the samples were weighed. This process was repeated several times to ensure reproducibility.

To evaluate the effect of ionic strength on the VPTT, NaCl solutions with concentrations of 0, 4, 8, 12, and 20 g/L were used. The swelling ratio (SR) (Zha et al. 2026) of IPS_gel was calculated according to Eq. 1,

 (1)

where W_t is the weight (g) of the IPS_gel sample (including all water in the system after immersion) and W_d is the weight (g) of the dried IPS_gel.

Cu²⁺ adsorption capacity of IPS_gel

Adsorption experiments were conducted by adding IPS_gel (15 mg) to Cu²⁺ aqueous solutions. The solution pH was adjusted to 6.0 using 0.1 M HCl or NaOH, and an incubator shaker (300 rpm, 22 °C) was employed for 4 h to achieve adsorption equilibrium. The residual concentration of Cu²⁺ in each solution was determined by atomic absorption spectroscopy (USA -Perkin Elmer-AAnalyst 400).

To investigate the effect of pH on adsorption performance, Cu²⁺ solutions (100 mg/L) with pH values of 1.5 to 6.0 were prepared using 0.1 M HCl and NaOH. The adsorption capacity of IPS_gel for Cu²⁺ was calculated using Eq. 2 (Dai et al. 2022),

 (2)

where C₀ and C_e are the initial and equilibrium concentrations (mg/L) of Cu²⁺, respectively; V is the volume (L) of the Cu²⁺ solution; and m is the mass (g) of the dried IPS_gel.

Adsorption isotherms

To better understand the interactions between the polymer network of IPS_gel and Cu²⁺, adsorption experiments were conducted at pH 6 and 22 °C with initial Cu²⁺ concentrations of 25, 50, 75, 100, 125, 150, and 200 mg/L. The experimental data were then fitted with the Langmuir and Freundlich isotherm models. The Langmuir isotherm model (Langmuir 1933) describes monolayer adsorption on a homogeneous surface. In contrast, the Freundlich isotherm model (Azizian 2004) is used to evaluate adsorption on heterogeneous surfaces and is not restricted to monolayer adsorption. The linear forms of the Langmuir and Freundlich models are respectively presented in Eqs. 3 and 4.

Langmuir model:

 (3)

Freundlich model:

 (4)

In the isotherm equations, C_e is the equilibrium concentration of Cu²⁺ in solution (mg/L) and q_e is the adsorption capacity of IPS_gel at equilibrium (mg/g). The parameter q_max represents the maximum adsorption capacity. The parameter n is an empirical constant related to adsorption intensity in the Freundlich model. K_L is the Langmuir adsorption constant (L/mg), and K_F is the Freundlich capacity of the adsorbent (mg/g).

Adsorption kinetics

The adsorption kinetics of Cu²⁺ on IPS_gel were evaluated using the widely used pseudo-first-order and pseudo-second-order kinetic models. The kinetic experiments were conducted at pH 6.0 and 22 °C with an initial Cu²⁺ concentration of 100 mg/L. The pseudo-first-order and pseudo-second-order models (Ho and McKay 1999) are described by Eqs. 5 and 6:

Pseudo-first-order model:

 (5)

Pseudo-second-order model:

 (6)

where q_e and q_t are the adsorption capacities at equilibrium (mg/g) and at time t (min), respectively. k₁ and k₂ are the rate constants of the pseudo-first-order (min⁻¹) and pseudo-second-order models (g/(mg·min)), respectively.

Desorption and regeneration studies

To evaluate the desorption efficiency of IPS_gel, samples were first immersed in a 100 mg/L Cu²⁺ solution until adsorption equilibrium was achieved. Then, these samples were subjected to desorption in 0.1 M HCl at 22 °C and 40 °C for 60 min. The desorption efficiency was calculated using Eq. 7,

 (7)

where C_d is the concentration (mg/L) of Cu²⁺ in the HCl solution after desorption, V_d is the volume (L) of the desorption solution, m is the mass (g) of the adsorbent, and q_e is the equilibrium (mg/g) adsorption capacity for Cu²⁺ at an initial concentration of 100 mg/L.

To evaluate the reusability of IPS_gel, desorption was performed via immersion in 0.1 M HCl for 30 min, followed by regeneration using 0.1 M NaOH. The regenerated samples were washed with deionized water to a neutral pH and then dried before being reused in the subsequent adsorption cycle. The adsorption–desorption process was repeated for 20 cycles.

RESULTS AND DISCUSSION

Structural Characterization of IPS_gel

The synthesis route of IPS_gel is illustrated in Fig. 1(a). First, chitosan was treated via an alkaline dissolution method, followed by modification with hydrophobic isopropyl groups to obtain water-soluble IPS. This IPS was then employed as the sole gel framework to construct IPS_gel through covalent crosslinking.

The FTIR spectra of chitosan (CS) with a 95% degree of deacetylation, IPS, and IPS_gel are shown in Fig. 1(b). The broad peak at approximately 3390 cm⁻¹ (Yan et al. 2017) was attributed to the stretching vibrations of the O–H and N–H groups in CS, IPS, and IPS_gel. In the FTIR spectra of IPS and IPS_gel, this peak was less intense than that of CS. The peaks at about 2889 and 2780 cm⁻¹ (Liu et al. 2023) were ascribed to the stretching vibrations of alkyl C–H groups. The peak near 1590 cm⁻¹ (Zhang et al. 2018) was associated with the bending vibration of the amide I band related to residual acetyl groups in chitosan. The peak at approximately 1368 cm⁻¹ (Li et al. 2019) was assigned to the CH₂ wagging vibration at the C₆ position of the anhydroglucose unit. The peak at 1020 cm⁻¹ (Wang et al. 2018) was attributed to the asymmetric stretching vibration of the C–O–C group in the epoxy ring, confirming the successful formation of IPS_gel.

To investigate the thermoresponsive properties of IPS_gel, its volume changes at different temperatures were examined by observing its microstructural morphology under varying temperature conditions. Figure 1(c) shows SEM images of IPS_gel in the swollen state at 22 °C and the shrunken state at 40 °C. At 22 °C, IPS_gel exhibited an irregular and dense porous network structure with relatively numerous large pores. In contrast, raising the temperature to 40 °C induced noticeable volume shrinkage, leading to a more compact internal structure and a marked reduction in the number of large pores. These observations indicate that IPS_gel showed pronounced thermoresponsive swelling–shrinkage behavior.

Fig. 1. (a) Synthesis schematic of IPS_gel; (b) FTIR spectra of CS, IPS, and IPS_gel; (c) SEM images of IPS_gel in the swollen state at 22 °C and in the shrunken state at 40 °C

Thermoresponsive Behavior of IPS_gel

The swelling ratio of IPS_gel over the temperature range of 21 to 48 °C was obtained to evaluate its VPTT and thermoresponsive behavior, as shown in Fig. 2(a). When the temperature was below the VPTT, the hydrophobic interactions of IPS were markedly weakened and hydrophilic interactions became dominant (Zha et al. 2026), resulting in increased water uptake and enhanced swelling. A sharp decrease in the swelling ratio was observed at approximately 35 °C, indicating the volume phase transition. Above 38 °C, the swelling ratio stabilized. When the temperature exceeded the VPTT, the hydrogen bonding interactions between water molecules and the IPS polymer chains were weakened, while hydrophobic interactions became dominant. This led to the shrinkage of the IPS_gel network and a reduction in pore size.

The effect of NaCl concentration (0, 4, 8, 12, and 20 g/L) on the VPTT of IPS_gel was investigated, as shown in Fig. 2(b). As the NaCl concentration increased, the equilibrium swelling ratio of IPS_gel markedly decreased. Despite the hydrophobic modification of IPS_gel, residual hydroxyl and amino groups remained on the polymer chains. The introduced Na⁺ and Cl⁻ ions screened the electrostatic repulsion between these charged groups, leading to polymer network contractions (Mussel et al. 2019). In addition, due to their strong hydration ability and ability to compete with polymer chains for water molecules, the salt ions disrupted the hydration shell of the hydrogel network (He et al. 2022). The presence of salt further weakens the interactions between water molecules and hydrophobic groups (such as isopropyl moieties) in thermoresponsive hydrogels, enhancing hydrophobic interactions. As a result, hydrogels undergo more pronounced shrinkage at lower temperatures or under the same temperature conditions (Sun et al. 2024; Zhou et al. 2025).

Fig. 2. (a) Equilibrium swelling ratio of IPS_gel as a function of temperature; (b) Effect of NaCl concentration (0, 4, 8, 12, 20 g/L) on the VPTT of IPS_gel

Cu²⁺ Adsorption Performance of IPS_gel

The adsorption performance of IPS_gel toward Cu²⁺ was investigated using initial Cu²⁺ concentrations of 25 to 200 mg/L. As shown in Fig. 3(a), the adsorption capacity rapidly increased at lower concentrations (C₀ ≤ 100 mg/L) and then gradually approached a plateau as the initial concentration was further increased. At low Cu²⁺ concentrations, the rapid increase in adsorption capacity with increasing concentration can be attributed to the availability of abundant active binding sites on IPS_gel. However, at higher concentrations, the number of available binding sites becomes limited, and the complexation capacity of the functional groups is gradually saturated, resulting in a slower increase in adsorption capacity until equilibrium is reached (Yan et al. 2017).

The pH value, which influences both the speciation of copper ions in solution and the physicochemical properties of the hydrogel adsorbent (Jung et al. 2022), is a key factor affecting the adsorption of Cu²⁺ by IPS_gel. The effect of pH on the adsorption capacity of IPS_gel is shown in Fig. 3(b). At pH values below 3.5, IPS_gel showed a relatively low adsorption capacity for Cu²⁺. This behavior is likely due to the protonation of amino groups on the IPS_gel surface, particularly those not involved in crosslinking. Protonation results in electrostatic repulsion between positively charged Cu²⁺ and the hydrogel surface as well as the competitive occupation of active sites by H⁺, inhibiting adsorption (Moralez et al. 2025). However, when the pH was raised above 3.5, the concentration of H⁺ decreased, reducing this competitive effect and enhancing Cu²⁺ adsorption. As a result, the adsorption capacity rapidly increased. At pH 6.0, IPS_gel exhibited a maximum adsorption capacity of 24.5 mg/g.

Fig. 3. (a) Effect of initial Cu²⁺ concentration on adsorption capacity of IPS_gel (C₀ = 25 to 200 mg/L, T = 22 °C); (b) Effect of solution pH on Cu²⁺ adsorption efficiency of IPS_gel (C₀ = 100 mg/L, T = 22 °C)

Adsorption Isotherms and Kinetics

To better understand the interaction between the IPS_gel polymer network and Cu²⁺, the adsorption data were fitted using the Langmuir and Freundlich models to describe the variation of adsorption capacity with concentration (Xu et al. 2025). The Langmuir isotherm which assumes monolayer adsorption on a uniform surface, provided a better fit (R² = 0.989) than the Freundlich model (R² = 0.974), as shown in Fig. 4(a). The excellent fits to both models suggests that the Cu²⁺ adsorption on IPS_gel was predominantly monolayer in nature.

The adsorption kinetics were further evaluated to understand the Cu²⁺ adsorption rate. As shown in Fig. 4(b), the experimental data were fitted with both the pseudo-first-order (R² = 0.987) and pseudo-second-order (R² = 0.990) kinetic models, both of which provided satisfactory mathematical descriptions of the adsorption process. The adsorption of Cu²⁺ proceeded rapidly and approached a plateau within 60 min, reaching equilibrium at approximately 120 min with an equilibrium adsorption capacity of 24.5 mg/g (Ofomaja et al. 2010).

Fig. 4. (a) Adsorption isotherm of Cu²⁺ on IPS_gel (C₀ = 100 mg⋅L⁻¹, T = 22 °C, pH = 6); (b) Adsorption kinetics of Cu²⁺ on IPS_gel (C₀ = 100 mg⋅L⁻¹, T = 22 °C, pH = 6)

Table 1. Isotherm Parameters for Cu²⁺ Adsorption onto IPS_gel

Table 2. Kinetic Parameters for Cu²⁺ Adsorption onto IPS_gel

Regeneration and Reusability of IPS_gel

In conventional adsorbent systems, the desorption and regeneration of the adsorbent material typically require large amounts of acid, alkali, or other eluents, which increases operational costs and may also cause secondary environmental pollution. As shown in Fig. 5(a), IPS_gel in the shrunken and swollen states was immersed in a volume of 0.1 M HCl solution for desorption. At 40 °C, the IPS_gel contracted, expelled a large amount of water, and exhibited a noticeable reduction in volume. Consequently, compared with the swollen state, the shrunken state required substantially less HCl for effective desorption. As illustrated in Fig. 5(b), the desorption efficiency of IPS_gel in the shrunken state reached 83% within 50 min, while a desorption efficiency of only 66% was achieved in the swollen state over the same period, indicating markedly enhanced desorption performance under the shrunken condition (Cai et al. 2021; Wen et al. 2022).

Reusability is also a key aspect of practical adsorption applications. Therefore, adsorption–desorption cycling experiments were conducted to evaluate the regeneration performance of IPS_gel. As shown in Fig. 5(c), after 20 cycles, the adsorption capacity of IPS_gel slightly decreased from 25.6 mg/g to 22.3 mg/g, demonstrating a capacity retention of 86.9%. These results demonstrate that IPS_gel possesses excellent regeneration properties and shows strong potential for heavy metal removal in wastewater treatment applications.

Fig. 5. (a) Elution efficiency comparison of Cu²⁺-loaded IPS_gel under swollen (22 °C) and shrunken (40 °C) states; (b) Desorption efficiency of IPS_gel under swollen and shrunken states within 70 min; (c) Adsorption capacity of IPS_gel after 20 adsorption cycles

CONCLUSIONS

1. IPS_gel exhibited the adsorption capacity of 24.5 mg/g for Cu²⁺, and its adsorption performance was influenced by solution pH and initial Cu²⁺ concentration; the adsorption isotherm followed the Langmuir model, and the kinetics followed the pseudo‑second‑order model.
2. The Cu²⁺‑loaded IPS_gel could be effectively regenerated using a small amount of HCl solution, representing an 88 to 95% reduction in acid consumption compared to conventional hydrogels; even after 20 adsorption–desorption cycles, IPS_gel maintained good adsorption performance.
3. IPS_gel is a thermoresponsive, high-capacity chitosan-based adsorbent that enables efficient Cu²⁺ removal and low-acid regeneration, unlike conventional adsorbents requiring high acid consumption, thereby providing a green strategy and showing promise for Cu²⁺ removal from wastewater.

ACKNOWLEDGMENTS

The authors are grateful for the support of the National Natural Science Foundation of China (Nos. 22278062, 31901775); Dalian, Science and Technology Innovation Fund (2024JJ13GX047); Liaoning Provincial (2025-MSLH-126).

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Article submitted: April 16, 2026; Peer review completed: May 9, 2026; Revisions accepted: June 19, 2026; Published: July 10, 2026.

DOI: 10.15376/biores.21.3.8001-8014