Synthesis and Application of Poly(N-isopropylacrylamide-co-methacrylic Acid) Hydrogels as Sorbent Materials for Wastewater Treatment

Abstract

N-isopropylacrylamide and methacrylic acid were copolymerized by a free radical polymerized mechanism. The obtained hydrogel poly(N-isopropylacrylamide-co-methacrylic acid) hydrogels, poly(NIPAM-co-MAA), were utilized as sorbent material for removal Cr(VI), Mn(II), and Pb(II) ions from simulated aqueous solutions. Hydrogel structures before and after heavy metal sorption are characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The swelling results indicate that poly(NIPAM-co-MAA) hydrogels are pH- and temperature-sensitive and have high swelling reversibility through three swelling/contraction cycles. The studied parameters of heavy metal sorption include the effect of pH, the initial concentration of heavy metal, the effect of temperature, and the desorption of metal ions. The maximum sorption capacities of poly(NIPAM-co-MAA) hydrogels were determined at pH 4.5 and 25 °C, and they are, for Cr(VI), Mn(II), and Pb(II) ions, 289.35 mg/g, 190.59 mg/g, and 349.71 mg/g, respectively. The pseudo-second-order model and the Langmuir adsorption isotherm best describe the sorption of heavy metal ions onto hydrogels. The removal of heavy metals is an exothermic reaction, and the interaction mechanism between the metal and the hydrogel is primarily physical in nature. Results of three sorption/desorption cycles show a good desorption ratio and sorption capacity of poly(NIPAM-co-MAA) hydrogels.

1. Introduction

Heavy metal pollution of surface- and wastewater is a critical topic today because these compounds negatively affect the environment in small quantities. Heavy metals are very toxic elements for biological systems and have an adverse effect on aquatic systems. Their negative properties, such as being non-degradable and bioaccumulable, allow them a prolonged environmental presence [1,2,3,4]. Industrial wastewater contains heavy metals such as mercury, chromium, copper, manganese, cadmium, and nickel. Chromium (Cr) in the hexavalent form exhibits carcinogenic, mutagenic, and teratogenic effects on humans and animals [5,6,7,8]. Hexavalent chromium has a toxic effect on plants [9], aquatic animals [10], and microorganisms [11]. Depending on the entry route, chromium in humans causes mucous membrane irritation, allergic dermatitis, nasal ulcers, asthma, hepatitis, and lung and digestive tract tumors [5,12,13,14]. Chronic exposure to moderate concentrations of manganese (Mn) through inhalation affects the central nervous system (CNS) of humans and may lead to neurological disorders similar to Parkinson’s disease. At high concentrations, manganese (Mn) damages deoxyribonucleic acid (DNA) and leads to mutations in mammalian cells [15,16]. Lead (Pb) has a high potential for absorption and bioaccumulation in plants, which can cause chlorosis, darkening of the root system, and impaired plant development [17]. In humans, lead is toxic to the CNS and urinary and hematopoietic systems [18,19].

Industrial heavy metal wastewater is a source of groundwater and surface water pollution, so it is necessary to develop an efficient and sustainable process for the removal of pollutants [20,21]. Various conventional treatment technologies, such as chemical precipitation, filtration, ultrafiltration, oxidation, solvent extraction, electrolysis, reverse osmosis, and ion exchange, are used to remove potentially toxic elements from water media. Most technologies have limitations and disadvantages: difficult applicability, lower efficiency, high process costs, production of secondary pollutants, and additional sludge treatment in later stages [21,22,23,24,25]. Among the alternative techniques for purifying water polluted by metals, sorption stands out as a remediation technique thanks to its shorter process time, high removal efficiency, and possible sorbent regeneration and metal recovery [21,24,26].

Increasing interest in the removal of heavy metals from industrial wastewater by researchers has increased the development and testing of new sorption materials. Hydrogels with chelating ligands, which represent synthetic sorbents, have been the focus of interest in recent years [3]. Hydrogels are cross-linked hydrophilic polymeric materials that can absorb and retain large amounts of water within a three-dimensional network without dissolving [27,28]. The ability to absorb a large amount of water, high porosity, selectivity towards certain metals, and the ability to regenerate, enable the application of these materials in heavy metal removal processes [29,30,31,32]. Hydrogels and heavy metal ions can establish different interaction mechanisms during the sorption process: electrostatic interactions, hydrogen bonding, π-π interactions, ion exchange, surface complexation, and chelation [33,34,35]. The hydrogel’s regenerative ability allows repeated sorbent application, reducing economic sorption costs. Hydrogels sensitive to external stimuli are ideal for the regeneration of heavy metals and other pollutants. These polymeric materials show significant physicochemical changes (e.g., swelling in an aqueous medium) with changes in environmental conditions (pH, temperature, light, electric, and magnetic fields) [36,37]. For pH-sensitive hydrogel sorbents, metal recovery is performed in an acidic (anionic hydrogel) or basic environment (cationic hydrogel) using an acid or base solution [22,38,39]. Recovery of Fe(III) and Au(III) ions by acrylic acid-based hydrogels, such as poly(acrylamide-co-acrylic acid), from wastewater/water can be achieved in an acidic environment at room temperature [40,41]. In the paper of Chowdhury et al. [40], 90% recovery of heavy metal, like Fe(III) ions, was achieved in 3 h at pH = 1 at room temperature. For dual-responsive hydrogels, like those that are pH- and temperature-sensitive, partial regeneration of the sorbent is possible by raising the temperature above the volume phase transition temperature (VPPT). Compared to single responsive hydrogels, in addition to more efficient regeneration, the incorporation of amino or amide group of a temperature-sensitive component, such as NIPAM, contributes to a more significant binding of heavy metal ions or other pollutants via a free electron pair on nitrogen, while a pH-sensitive component like acrylic or methacrylic acid enables the binding of pollutants via an ionized carboxyl group.

The available literature shows data on applying hydrogels as sorbent materials for heavy metals, dye, radioactive, and rare earth elements [22,42,43].

A sodium alginate hydrogel is used to produce membranes for the filtration of heavy metals and dyes [44,45,46]. A nanofiltration membrane obtained from sodium alginate, large molecular weight polyethylene glycol (pore-making agent), cellulose nanofibers, and carboxylated multiwalled carbon nanotubes (additive) shows a high degree of rejection for dyes: Crystal violet, Congo red, Tartrazine, and Methylene blue [46].

In this study, a series of dual-responsive hydrogels, poly(N-isopropylacrylamide-co-methacrylic acid) (poly(NIPAM-co-MAA)), were synthesized, and the hydrogel sorption capacities for Cr(VI), Mn(II), and Pb(II) ions were investigated in a batch system. The presence of the NIPAM structural unit allows hydrogels to respond to temperature changes, while the existence of ionizable groups provides pH sensitivity [47]. Changes in the pH value lead to changes in the degree of ionization of electrolyte side groups (MAA), leading to a change in swelling capacity [48,49]. In the literature, only the application of micro- and nanoparticles of poly(NIPAM-co-MAA) hydrogels to remove Cu(II) ions exists [35]. Acrylic acid-based hydrogels were used to sorb or remove Au(III) ions from simulated wastewater or Fe(III), Cr(III), and Hg(II) ions from aqueous solutions [40,41].

2. Materials and Methods

2.1. Materials

Monomer N-isopropylacrylamide (NIPAM, purity 99%) and initiator 2,2′-azobis(2-methylpropionitrile) (AZDN, purity 99%) were purchased from Acros organics (Fair Lawn, NJ, USA). The other reagents used for the synthesis of hydrogels, such as methacrylic acid (MAA; purity 98%) and cross-linker ethylene glycol dimethacrylate (EGDM; purity 97%), were purchased from Fluka (Fluka Chemical Corp., Buchs SG, Switzerland). Simulated single-component wastewaters were prepared from the following heavy metal salts: potassium dichromate, K₂Cr₂O₇ (Merck KGaA, Darmstadt, Germany), manganese(II)chloride tetrahydrate, MnCl₂·4H₂O (Acros Organics N.V., Fair Lawn, NJ, USA), and lead(II)acetate trihydrate, Pb(CH₃COO)₂·3H₂O (Merck KGaA, Darmstadt, Germany). All reagents and chemicals used for the synthesis of hydrogels and for removing contaminants from water were not previously purified.

2.2. Synthesis of Hydrogels

The serias copolymeric hydrogels poly(N-isopropylacrylamide-co-methacrylic acid), poly(NIPAM-co-MAA), were synthesized by radical polymerization from monomers of NIPAM and MAA at a molar ratio of 95/5, respectively. In the copolymer synthesis reactions, the content of the EGDM crosslinker has varied (1.5, 2, and 3 mol%), while the content of the AZDN initiator remained constant at 2.7 mol%. The molar percentages of crosslinkers and initiators were calculated in relation to the amount of the monomer mixture.

After the homogenization and dissolution of monomers, crosslinkers, and initiators in acetone (Centrochem, Belgrade, Serbia), the reaction mixtures were injected into glass tubes (diameter 5 mm). The glass tubes were then sealed by flame. The reaction mixture was thermally initiated in the following way: 0.5 h at 75 °C, 2 h at 80 °C, and 0.5 h at 85 °C. The synthesized hydrogels were cut into discs (5 mm diameter × 2 mm thickness). The hydrogel discs were immersed in 30 cm³ of methanol (Centrochem, Belgrade, Serbia) for 72 h to remove residual reactants. The hydrogel disks were then rinsed with methanol/distilled water solutions in the ratios 75/25%, 50/50%, 25/75%, and 0/100%, v/v, to remove methanol from copolymers. The hydrogels were dried in a drying oven for 3 h at 40 °C to a constant mass and stored in a desicator until they were used.

2.3. Characterization of Hydrogels

2.3.1. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectrum of the hydrogel sample with 1.5 mol % EGDM was recorded in a thin transparent pastille form on the Bomem Hartmann & Braun MB-series FTIR spectrophotometer (Hartmann & Braun, Baptiste, CA, USA). A total of 1 mg of the hydrogel sample and 150 mg of KBr (purity 99%, Merck KGaA, Darmstadt, Germany) were previously ground into powder on an amalgamator (WIG-L-Bug, Dentsply RINN, a Division of Dentsply International Inc., Smile Way, York, PA, USA) and homogenized. The pastille was prepared by vacuuming and pressing under a pressure of about 200 MPa. The sample was recorded in the range of wave numbers from 4000 to 400 cm⁻¹, and the obtained FTIR spectrum was processed with the Win-Bomem Easy software version 3.01C.

2.3.2. Scanning Electron Microscopy (SEM)

The morphology and microstructure of the synthesized poly(NIPAM-co-MAA) hydrogel were determined using the SEM method. Before recording, the sample was swollen to equilibrium in distilled water and lyophilized in a lyophilizer (Freeze Dryers Rotational-Vaccum-Concentrator GAMMA 1-16 LSC, Osterode, Germany). The lyophilized sample was immersed in liquid nitrogen, and a small fractured piece of hydrogel was sprayed by an alloy of gold and palladium (85/15) in a JEOL Fine Coat JFC-1100E Ion sputter (JEOL Co., Tokyo, Japan). The prepared sample was scanned on a JEOL Scanning Electron Microscope JSM-5300 (JEOL Ltd., Tokyo, Japan).

2.3.3. Swelling

The swelling process of poly(NIPAM-co-MAA) hydrogels was monitored gravimetrically. Dry hydrogel samples (xerogels) were immersed in solutions of specific pH values and temperatures for a certain period, after which the samples were taken out of the solution and a surplus of the solution was removed from their surface. After that, the mass of the hydrogel samples was measured in specific time periods until reaching equilibrium. The swelling degree in time interval t (α_t) was calculated according to Equation (1).

$${\alpha }_{\mathrm{t}}={\displaystyle \frac{m_{\mathrm{t}}-m_0}{m_0}}={\displaystyle \frac{w_{\mathrm{t}}}{m_0}}$$

(1)

where m₀ and m_t are the dry gel’s mass and the swollen hydrogel’s mass at time t, respectively; w_t represents the mass of liquid absorbed by the gel at time t.

The hydrogel’s pH sensitivity was determined in pH 4.5 and 6.8 solutions at room temperature, while its temperature sensitivity was investigated in the range from 20 to 80 °C at a pH 6 solution. Hydrogels were subjected to three cycles of alternating swelling/contraction under the influence of temperature (from 25 to 70 °C) to determine the swelling reversibility. Specific pH values of solutions were prepared by adding 0.1 M HCl solution (Zorka Pharma, Šabac, Serbia) or 0.1 M NaOH solution (Centrochem, Belgrade, Serbia). The acidity or alkalinity of the solution was adjusted by a digital pH meter (HI9318-HI9219, HANNA Instruments, Woonsocket, RI, USA) and the solution temperature in a water bath (Sutjeska, Belgrade, Serbia).

2.3.4. Determination of the Point of Zero Charge

The point of zero charge of the synthesized poly(NIPAM-co-MAA) hydrogel was determined by immersing 0.01 g of the copolymer in 40 cm³ of a 0.1 M KNO₃ solution. The initial pH (pHi) of the KNO₃ solution between 1 and 10 was adjusted by adding 0.1 M HCl solution or 0.1 M NaOH solution on a digital pH meter. The hydrogel in the solution was left for 24 h, after which the final pH value (pHf) was determined. From the plotted graphs of pHf–pHi versus pHi, the pHpzc of the adsorbent was obtained.

2.4. Removal of Heavy Metals from Simulated Wastewater by Hydrogels

Hydrogels weighing approximately 0.01 g were immersed in 25 cm³ of simulated wastewater containing Cr(VI)/Mn(II)/Pb(II) ions. Single-component wastewater with specific concentrations of heavy metals was produced by dissolving K₂Cr₂O₇, MnCl₂·4H₂O, and Pb(CH₃COO)₂·3H₂O salts in redistilled water. The removal of heavy metal ions from water by hydrogels was monitored for up to 72 h. Residual concentrations of heavy metals in water were determined using the method of quantitative analysis on an Optical Emission Spectrometer with Induced Coupled Plasma-ICP-OES Arcos FHX (Spectro Analytical Instruments GMBH & CO KG, Kleve, Germany), utilizing Argon 5.0 (purity 99.999%) as the carrier gas. For the ICP-OES analysis, a heavy metal water sample was diluted with water, which has a conductivity of 0.055 µS/cm at 1:99 (v/v). The obtained samples were filtered on a cellulose membrane filter with a pore diameter of 0.45 μm (Econofilters, Agilent Technologies, Waldborn, Germany).

The quantity of heavy metal removed from water by the hydrogel over the period t (q_t) was calculated as follows:

$$q_{\mathrm{t}}={\displaystyle \frac{c_0{V}_0-c_{\mathrm{t}}{V}_{\mathrm{t}}}{W}}$$

(2)

where q_t is the sorption capacity of the hydrogel towards heavy metal (mg/g), c₀ and c_t are the initial concentration of the metal ion solution and the concentration of the metal ion solution at the time t (mg/dm³), respectively, V₀ and V_t are the initial volume of the metal ion solution and the volume of the metal ion solution at the time t (dm³), respectively, and W is the mass of the xerogel (g).

2.5. The Effect of Process Parameters on the Sorption Capacity of Hydrogels

2.5.1. Solution pH

The sorption of heavy metal ions onto hydrogels from simulated solutions was monitored across a broad range of pH values (2.2, 3.5, 4.5, 5.5, 6.8, and 8). The experiments were conducted at 25 °C with an initial sorbate concentration of 500 mg/dm³.

2.5.2. Contact Time

The effect of contact time on the removal process was investigated at an initial concentration of heavy metals of 500 mg/dm³, a temperature of 25 °C, and a pH of 4.5. To identify a suitable model for describing the kinetics of heavy metal ion sorption onto hydrogels, pseudo-first-order, pseudo-second-order, and intra-particle diffusion models were applied.

2.5.3. Sorbate Initial Concentration

The removal capacity of hydrogels for metal ions was monitored at 25 °C in heavy metal solutions with a pH of 4.5. In the experiments, heavy metal concentrations varied from 50 to 500 mg/dm³. Equilibrium data were analyzed using the Langmuir and Freundlich sorption isotherm models.

2.5.4. Temperature (Thermodynamics)

Thermodynamic parameters for the sorption of metal ions were determined under the following experimental conditions: a metal concentration of 500 mg/dm³, a pH of 4.5, and temperatures of 25, 35, and 45 °C.

2.6. Desorption and Regeneration Studies

The desorption of heavy metal ions from hydrogel was carried out in a batch system at 25 °C for 4 h. The eluent used for regeneration is 0.5 M HCl. Metal sorption was conducted using 0.01 g of hydrogel at an initial concentration of 500 mg/dm³. Thereafter, the heavy metal ions were desorbed from hydrogels by 50 cm³ HCL solutions. Three consecutive sorption/desorption cycles were performed. Before each sorption, the sorbent hydrogel was washed several times with distilled water to remove the excess HCL and dried to a constant mass in a drying oven at 105 °C. The percentage of desorbed heavy metals (DP) is calculated by Equation (3).

$$\mathrm{DP}(\%)={\displaystyle \frac{q_{\mathrm{d}}}{q_{\mathrm{ad}}}}100$$

(3)

where q_d is the amount of desorbed heavy metal (mg/g), while q_ad is the amount of sorbed heavy metal onto hydrogel (mg/g).

2.7. Characterization of Hydrogels with Sorbed Heavy Metals

The poly(NIPAM-co-MAA) hydrogels with sorbed Cr(VI), Mn(II), and Pb(II) ions were characterized using FTIR and SEM techniques. The hydrogel samples with heavy metals were prepared in the same manner as the pure sample’s hydrogel, as shown in Section 2.3.1 and Section 2.3.2.

Energy-dispersive X-ray (EDX) spectroscopy determined the content of heavy metals in the sorbent hydrogels. To obtain EDX spectra, a microprobe installed on a scanning electron microscope (JSM-5300, JEOL Ltd., Tokyo, Japan) was used. The Link-Analytical QX-2000 microprobe operates at 20 kV.

3. Results and Discussion

3.1. Characterization Hydrogels

3.1.1. FTIR Analysis

The FTIR spectrum of the synthesized poly(NIPAM-co-MAA) hydrogel, sample 95/5/1.5, is shown in Figure 1a. In the sample designation, e.g., 95/5/1.5, the first number represents mol% of NIPAM, the second one mol% of MAA, and the third one mol% of crosslinker EGDM.

Functional groups from both monomers, NIPAM and MAA, are present in the FTIR spectrum of the poly(NIPAM-co-MAA) copolymer (Figure 1a). In the range of wave numbers 3000–3700 cm⁻¹, the spectrum shows a complex broad band with two maxima, the first one at 3433 cm⁻¹ from the valence OH vibrations of MAA, ν(OH), and the second one at 3312 cm⁻¹ from the valence vibrations of the N-H group of NIPAM, ν(NH). Asymmetric and symmetric valence vibrations of the C-H bond of methyl groups of monomers give absorption bands with maxima at 2974 and 2877 cm⁻¹, respectively. On the copolymer’s FTIR spectrum (Figure 1a), there is also a band of asymmetric deformational C-H vibrations in the plane of the CH₃-C group, δ_as(C-H), at 1461 cm⁻¹. The intense absorption band at 1722 cm⁻¹ corresponds to the valence vibrations of the C=O group of MAA or EGDM. The vibration of the NIPAM amide group in the hydrogel structure results in strong absorption bands at 1649 cm⁻¹ (amide band I) and 1547 cm⁻¹ (amide band II), which is in accordance with literature data by other authors [50]. Amide band I correspond to the valence vibrations of C=O, while amide band II is due to the coupling of in-plane N-H deformation vibrations, δ(N-H), and C-N bond valence vibrations, ν(C-N). By the absorption of IR radiation, the isopropyl groups, CH(CH₃)₂, of NIPAM in the hydrogel structure in FTIR spectrum copolymer give absorption bands from the deformation C-H vibrations at 1368, 1173, and 1130 cm⁻¹.

The binding of monomers during the polymerization reaction occurs through the breaking of double bonds (C=C). This is evidenced by the absence of valence and deformation vibrations of the vinyl groups in both monomers [51,52,53]. The absorption band with maxima at 3077 cm⁻¹ is probably the result of asymmetric valence vibrations of the vinyl group, ν_as(=C-H), of the dangling chains of the EGDM crosslinker. By decomposing the initiator, the unpaired electron of the AZDN radical interacts with the opposing spin electron of the monomer’s double bond, effectively breaking the weaker π bond [54]. The hydrogel chain grows by breaking the unsaturated bonds in the vinyl groups of the monomers, while the crosslinking of the linear chains occurs through the vinyl groups of the EGDM crosslinker.

Figure 1b–d shows the FTIR spectra of sorbent hydrogel after the sorption of Cr(VI), Mn(II), and Pb(II) ions. The peak absorption band values in the hydrogel’s FTIR spectrum before and after the sorption of heavy metal ions are shown in

Table 1. Maximum absorption band values in the FTIR spectrum of poly(NIPAM-co-MAA) before and after the sorption of heavy metal ions.

Vibration Assignment	Pure Hydrogel	Hydrogel After Sorption of Heavy Metal Ions
		Cr(VI)	Mn(II)	Pb(II)
	Wavenumber (cm−1)
ν(OH)	3433	3437 (+4)	3440 (+7)	3437 (+4)
ν(N-H)	3312	3343 (+31)	3313 (+1)	3321 (+9)
νas(=C-H)	3077	3079 (+2)	3079 (+2)	3078 (+1)
νas(C-H)	2974	2975 (+1)	2974 (0)	2974 (0)
νs(C-H)	2877	2878 (+1)	2877 (0)	2877 (0)
ν(C=O)	1722	1720 (−2)	1721 (−1)	1723 (+1)
Amide band I, ν(C=O)	1649	1649 (0)	1651 (+2)	1652 (+3)
Amide band II, coupling of δ(N-H) and ν(C-N)	1547	1550 (+3)	1545 (−2)	1549 (+2)
δas(C-H)	1461	1461 (0)	1461 (0)	1461 (0)

(+) Shift to higher wave numbers; (−) shift to lower wave numbers; (0) no shift.

.

By comparing the FTIR spectrum of the pure poly(NIPAM-co-MAA) copolymer (Figure 1a) with the spectra of the copolymer with sorbed metal ions (Figure 1b–d), the shifts of absorption band from the amide and carboxyl groups of the copolymer were observed. In the FTIR spectra of copolymers with heavy metal ions, there are shifts in the centroids of the absorption bands, which are the result of the OH group vibration, up to 7 units towards higher wave numbers compared to the same band on the FTIR spectrum of the pure copolymer. If the maximum shift of the absorption bands towards higher or lower wave numbers is greater, the bond between the hydrogel groups and the heavy metal ion is stronger. Shifts in the absorption bands assigned to C=O vibrations (Figure 1b–d) indicate that the carboxyl group of the copolymer poly(NIPAM-co-MAA) copolymer may play a role in binding Cr(VI), Mn(II), and Pb(II) ions. Significant shifts of the maximum absorption bands from the N-H bond (from 1 to 31 units towards higher wave numbers) occur in the FTIR spectra of copolymers with metal ions. The amide group can bind heavy metal ions, as confirmed by the shift in the absorption bands with maxima at 3312, 1649, and 1547 cm⁻¹ in the FTIR spectrum of pure poly(NIPAM-co-MAA) copolymer.

The FTIR spectrum of the poly(NIPAM-co-MAA) copolymer with sorbed Cr(VI) ions (Figure 1b) exhibits two absorption bands in the region of wave numbers below 1000 cm⁻¹ that were not observed in the FTIR spectrum of the pure copolymer (Figure 1a). The absorption band of medium intensity with a maximum at 951 cm⁻¹ corresponds to asymmetric valence CrO₃ vibrations of the sorbed chemical species of hexavalent chromium anion, ν_as(CrO₃) [55,56,57,58,59,60]. The chemical species of hexavalent chromium anions (Cr₂O₇²⁻, CrO₄²⁻, and HCrO₄⁻) depend on the pH value during heavy metal removal. Electrostatic interactions can be established between the protonated secondary amino groups (NH₂⁺) of the hydrogel and chromium anions. There is a possibility of binding cations of lead and manganese, Pb(II) and Mn(II), through electrostatic interaction with carboxylate anions (COO⁻) of poly(NIPAM-co-MAA) hydrogel. The mechanisms involved in the interaction of heavy metal cations with hydrogels during the sorption process may include electrostatic attraction, chelation formation, ion exchange, and physical sorption [34,35,61]. In this study, the removal mechanism of both anions and cations of heavy metals from simulated water will be determined based on thermodynamic results.

3.1.2. SEM Analysis

The SEM micrographs in Figure 2 show the microstructure of the lyophilized copolymer hydrogel sample, 95/5/1.5, before and after the sorption of heavy metal ions. The three-dimensional structure of the poly(NIPAM-co-MAA) hydrogel was observed at magnification of ×1000.

The lyophilized swollen poly(NIPAM-co-MAA) hydrogel has a porous structure with different pore sizes and distribution (5–30 μm). A uniform cross-linking process was not achieved within the copolymer structure (Figure 2a). In the paper of Qi et al., the synthesized poly(NIPAM-co-MAA) hydrogel in the swollen state exhibits a regular porous structure with a more uniform distribution of pores with a size of 28.3 ± 7.5 μm [62]. The surface and internal morphology of swollen hydrogels depend on the copolymer’s composition and swelling characteristics. The pore size of poly(NIPAM-co-AAD) swollen hydrogels, where AAD represents acrylic acid (AA) and its derivatives (MAA, β-ethyl acrylic acid, β-propyl acrylic acid), decreases with increasing hydrophobicity of the acrylic unit because the swelling ratio decreases [48].

Poly(NIPAM-co-MAA) hydrogels were classified as macroporous polymeric materials based on the swollen state’s pore sizes [63,64].

The hydrogel’s porous structure is ideal for transporting the simulated metal solution and binding the metal to functional groups (sorption centers) inside the hydrogel (absorption). SEM micrographs show that lyophilized samples of poly(NIPAM-co-MAA) hydrogels that swelled in solutions of chromium and lead salts (Figure 2b,d) show significant changes in their microstructure compared to the same hydrogel samples that swelled in water (Figure 2a). The heavy metal accumulations were bound to the functional groups on the hydrogel surface (adsorption). After removing manganese ions from the solution, significantly lower accumulations were observed on the hydrogel surface (Figure 2c), indicating that more lead ions may have penetrated into the hydrogel and bound to the sorption centers (absorption). After the sorption of Cd(II) ions onto poly(acrylamide-co-sodium methacrylate) and poly(2-hydroxyethyl acrylate-co-itaconic acid) hydrogels, the pore size of the copolymer decreases. The authors explain that this reduction occurs due to a decrease in the repulsive electrostatic interactions between the carboxylate (COO⁻) groups of the acid unit [65,66].

3.1.3. EDX Analysis

The results of elemental analysis of lyophilized hydrogels after the removal of metal ions from the solution are shown by EDX spectra in Figure 3. EDX spectroscopic elemental analysis confirmed the presence of all heavy metals within the structure of the hydrogels: Cr (Kα 5.411, Lα 0.573 keV; Figure 3a), Mn (Kα 5.894, Lα 0.637 keV; Figure 3b), and Pb (Lα 10.550, M 2.342 keV; Figure 3c). Additionally, the EDX spectra of the hydrogels, after the removal of heavy metal ions from water, also showed the presence of O at Kα 0.525 keV, N at Kα 0.392 keV, and C at Kα 0.277 keV.

3.1.4. Swelling Behavior

Swelling is a property of hydrogels that determines the application of these polymeric materials in systems for the delivery of drugs, removal of pollutants, or as superabsorbent materials. During the swelling process, first, the solvent molecules (water) contact the xerogel surface. Then, the solvent molecules penetrate the polymer network, whereby the distance between the network nodes expands. The expansion of the polymer network enables the penetration of other solvent molecules and the increase in the volume of the hydrogel until the equilibrium state is reached [63]. The equilibrium swelling degree (α_e) is a significant parameter for the characterization of hydrogels and describes the amount of water inside hydrogels in a state of equilibrium. For ionic hydrogels, α_e depends on the network structure, crosslinking density, hydrophilic groups, and ionization degree of functional groups [28,67]. A combination of intermolecular interactions, such as van der Waals forces, hydrophobic interactions, hydrogen bonding, and electrostatic interactions, determines the equilibrium swelling of hydrogels. The swelling degree of dual-responsive ionic hydrogels, such as the poly(NIPAM-co-MAA) hydrogels synthesized in this paper, depends highly on the properties of the swelling medium—pH value, temperaturę, and ionic strength of the solution [48,68,69].

The time dependence of the swelling degree of a series of poly(NIPAM-co-MAA) hydrogels at different pH values (2.2, 4.5, 6.8, and 9.1) at a temperature of 25 °C is shown in Figure 4.

Figure 4 shows an intense increase in the swelling degree of poly(NIPAM-co-MAA) hydrogels up to 300 min. Equilibrium swelling of hydrogel in a solution with a pH value of 6.8 is achieved for 48 h (Figure 4c). In pH 2.2 and 4.5 solutions (Figure 4a,b), the swelling degree of a hydrogel is inversely proportional to the concentration of the EGDM crosslinker. Increasing the crosslinker content during synthesis results in a denser polymer network, which subsequently has a lower absorption capacity for aqueous solutions of varying pH values. An exception is the behavior of the synthesized poly(NIPAM-co-MAA) hydrogels at pH values of 6.8 and 9.1, where the hydrogel with 2 mol% EGDM has a higher equilibrium swelling capacity than the hydrogel with 1.5 mol% crosslinker (Figure 4b,d).

Increasing the pH value of the aqueous medium increases the equilibrium swelling degree of poly(NIPAM-co-MAA). Thus, hydrogels show significantly high swelling degrees in solutions with pH values of 6.8 and 9.1 and a temperature of 25 °C (α_e = 87.96–170.09). At these medium pH values, the carboxyl groups (COO⁻) of the MAA unit of co-polymer are ionized, which, in turn, causes an increase in the degree of hydrogel swelling due to electrostatic repulsion between the ionized carboxyl groups of the polymer. Ionization and expansion of the polymer network are significantly more pronounced in a solution with a pH value of 9.1. The maximum absorption capacity of the synthesized poly(NIPAM-co-MAA) was obtained for sample 95/5/2 in a solution with a pH value of 9.1, α_e = 170.09. The poly(NIPAM-co-MAA) hydrogel synthesized in the paper of Brazel and Peppas with an 88 mole% of NIPAM and EGDM as a crosslinker achieved the highest volume swelling degree of about 20 in deionized water [70].

In a highly acidic environment (pH = 2.2), the lower swelling degree of hydrogel (α_e = 3.93–6.86) is probably a consequence of forming hydrogen bonds between the polymer chains, resulting in a more compact polymer structure. The swelling of poly(NIPAM-co-MAA) hydrogels at pH 2.2 is caused by the electrostatic repulsion of protonated amino groups (NH₂⁺) of the NIPAM unit. Higher degrees of hydrogel swelling were obtained in a solution with a pH value of 4.5, resulting from the partial ionization of carboxyl groups. According to some authors, the pKa values of the acid group of MAA are 4.46 [71]. Breaking hydrogen bonds can also contribute to a higher degree of swelling. The point of zero charges for poly(NIPAM-co-MAA) hydrogel, determined by the pH-drift method, is 4.5, at which the surface polymer structure is neutrally charged. At a pH below 4.5, the hydrogel surface is positively charged, while, at values higher than 4.5, it is negatively charged.

Based on the presented results, it can be concluded that the synthesized poly(NIPAM-co-MAA) hydrogels react to the change in the medium’s pH, and, in a basic environment, hydrogels show the properties of superabsorbent materials (α_t > 100 g/xerogel).

The temperature sensitivity of a series of poly(NIPAM-co-MAA) hydrogels in the range from 20 to 80 °C at pH 6 is shown in Figure 5.

Copolymers based on NIPAM and anionic comonomers (AA, MAA) show temperature sensitivity in water by building or dissociating hydrogen bonds with the solvent [72,73]. Increasing the temperature from 20 to 80 °C reduces the degree of swelling of hydrogels. They are classified in the group of negatively temperature-sensitive hydrogels because, above the VPTT (Volume Phase Temperature Transition), they contract. The decrease in the volume of hydrogels is caused by the breaking of hydrogen bonds with water molecules, so hydrophobic interactions between the side groups of the polymer network are more dominant [69]. At temperatures below the VPTT, in hydrogels, hydration of the side groups occurs, i.e., intermolecular hydrogen interactions with water molecules.

The poly(NIPAM-co-MAA) hydrogel, sample 95/5/1.5, was subjected to three cycles of alternating swelling and contraction in a pH 6 solution, varying the temperature from 25 to 70 °C. The results of the reversibility of the swelling hydrogel are shown in Figure 6.

Poly(NIPAM-co-MAA) hydrogels at 25 °C have a soft, transparent structure, while, above the phase transition, they acquire a white solid consistency. In the II cycle, the swelling degree of the poly(NIPAM-co-MAA) hydrogel, sample 95/5/1.5, at 25 °C decreases by 3.934% compared to the I cycle. This trend of reducing the absorption capacity continues in the III cycle, with the swelling degree lower than the I cycle by 7.707%. During the repeated swelling cycle, poly(NIPAM-co-MAA) hydrogels reach high absorption capacities of solutions, which can be significant for their application and regeneration for removing pollutants from the water.

3.2. Heavy Metal Ions Adsorption Studies

3.2.1. The Effect of pH

The pH value of the environment determines the ionization degree of the hydrogel functional groups and the chemical species of metal in sorption research. It also defines the medium’s concentration of H⁺ and OH⁻ ions. At low pH values, protons can compete with positive metal ions. Unlike protons, hydroxyl ions form insoluble hydroxides with metals at higher pH values [74,75,76,77,78].

Figure 7 shows the sorption capacities of hydrogel poly(NIPAM-co-MAA), sample 95/5/1.5, in the range of pH values from 2.2 to 8.1 for 72 h of removal at 25 °C. In removing Pb(II) ions, the sorption process was investigated up to a pH value of 5.5 because ions are precipitated in the form of Pb(OH)₂ at higher pH values.

The increase in the hydrogel poly(NIPAM-co-MAA) sorption capacity towards all heavy ions exists up to the solution’s pH value of 4.5 (Figure 7). Heavy metal ions electrostatically repel with protonated amino groups, explaining the low hydrogel sorption capacity for Pb(II) and Mn(II) ions in a pH solution of 2.2. The carboxyl group of hydrogels at this pH value has a low degree of ionization, so it does not represent a free sorption site for metal [77,79]. Increasing the pH value from 4.5 to 8.1 decreases the degree to which metal ions from water are bound to the hydrogel structure.

In a weakly acidic environment (pH = 4.5), a significant number of ionized carboxyl groups are available to establish electrostatic interactions with the Pb(II) and Mn(II) ions. The removal of the hexavalent chromium anion in the investigated pH range is the highest at a solution pH of 4.5 (Figure 7). This result is probably due to a higher degree of hydrogel swelling and the availability of protonated amino groups in the poly(NIPAM-co-MAA) structure. The results show that the pH value is considered one of the process parameters that affect the capacity to remove heavy metals. Further sorption studies were conducted at the optimal pH value (pH = 4.5).

3.2.2. The Effect of Contact Time and Sorption Kinetics

The change in the sorption degree of heavy metal ions from solution (pH 4.5 and 25 °C) onto poly(NIPAM-co-MAA) hydrogels is given in Figure 8.

Poly(NIPAM-co-MAA) hydrogels with a higher cross-linking degree show lower sorption capacities, which confirms that the mole content of EGDM is an important parameter that determines the removal of Cr(VI), Mn(II), and Pb(II) ions from water (Figure 8). A higher degree of cross-linking allows lower penetration and binding of heavy metal ions inside the hydrogel, which, in turn, reduces the sorption capacity.

The process contact time affects the removal of heavy metal ions by synthesized hydrogels poly(NIPAM-co-MAA) from solutions (Figure 8). Depending on the type of metal ions and the applied sorbent, a significant increase in the sorption capacity of hydrogels occurs during the first 5 or 24 h. Poly(NIPAM-co-MAA) hydrogels reach sorption capacities close to equilibrium for metal ions in 24 or 48 h (Figure 8).

The poly(NIPAM-co-MAA) hydrogel, sample 95/5/1.5, removed the highest amount of metal ions for 72 h of the process: q_m = 289.35 mg/g for Cr(VI) ions, q_m = 190.59 mg/g for Mn(II) ions, and q_m = 349.71 mg/g for Pb(II) ions. In the available literature, only Wu and Tian investigated the poly(NIPAM-co-MAA) polymer as a sorbent material for heavy metal ions. The authors sorbed metal ions from a CuSO₄ solution onto poly(NIPAM-co-MAA) micro/nanoparticles. They achieved a maximum sorption capacity of 0.765 mmol/g at an MAA/NIPAM ratio of 0.700 (particle size of 505.8 nm) in a solution of 50 °C and pH 6 [35]. Ju et al., in their investigation, achieved a removal capacity for Pb(II) ions of 120 mg/g for the poly(NIPAM) hydrogel [39].

The highest removal capacities of heavy metal ions from the aqueous medium by poly(NIPAM-co-MAA) hydrogels synthesized with 10 mol% MAA are shown in

Table 2. Maximum sorption capacities of poly(NIPAM-co-MAA) hydrogels with 10 mol% MAA for Cr(VI), Mn(II), and Pb(II) ions at the optimal pH value of the solution at an initial sorbate concentration of 500 mg/dm³ at 25 °C.

Heavy Metal Ion	Hydrogel Sample	qm (mg/g)
Cr(VI)	90/10/1.5	225.12
	90/10/2	165.54
	90/10/3	62.04
Mn(II)	90/10/1.5	131.92
	90/10/2	34.90
	90/10/3	29.58
Pb(II)	90/10/1.5	449.90
	90/10/2	77.32
	90/10/3	33.03

. Investigations were performed under the same optimal conditions as hydrogels with 5 mol% MAA: pH = 4.5, temperature 25 °C, and sorbate concentration of 500 mg/dm³. By comparing the sorption capacities of hydrogels with 5 and 10 mol% MAA, it can be observed that the general introduction of a higher content of MAA comonomers into the structure of hydrogels has a negative effect on the hydrogel’s ability to sorb heavy metal ions (Figure 8 and Table 2). Lower metal ion sorption capacities were obtained by the hydrogel with 10 mol% MAA at all crosslinker contents. The exception is the hydrogel sample 90/10/1.5, which showed a higher removal degree of Pb(II) ions (q_m = 449.90 mg/g) compared to the 95/5/1.5 sample (q_m = 349.71 mg/g). The obtained result may indicate that the temperature-sensitive component, NIPAM, is partly responsible for the sorption of metal ions from aqueous solutions onto hydrogels.

Natural polymers, such as chitosan, chitin, cellulose, alginate, pectin, and lignin, are suitable sorbent materials for removing heavy metals and dyes from the aquatic environment [80].

Table 3. Removal capacities of natural and synthetic polymers towards dyes and heavy metal ions.

Sorbent	Sorbat	Sorption Capacity (mg/g)	Reference
poly(acrylamide-co-acrylic acid)	Au(III)	up to 124	[41]
poly(NIPAM)	Pb(II)	120	[39]
poly(acrylamide-co-acrylic acid)	Fe(III), Cr(III)	276 for Fe(III), and 139 for Cr(III)	[40]
poly(NIPAM-co-MAA)	Cu(II)	0.765 mmol/g	[35]
N-methyl pyrrolidinium-based Polymeric Ionic Liquid	Orange G, Orange II, Sunset Yellow	198.4 for Orange G, 279.3 for Orange II, and 316.5 for Sunset Yellow	[81]
Chitosan/salicylaldehyde composite	Pb(II)	123.67	[82]
Cellulose/pectin composite	Fe(II)	98	[83]
Alginate/rice husk composite	Direct Blue 67	0.63	[84]
Alginate/pectin	Crystal Violet	619.22	[85]

shows the sorption capacities of natural and synthetic polymers towards dyes and heavy metal ions. The maximum sorption capacities of poly(NIPAM-co-MAA) hydrogels for heavy metal ions in this paper agree with the capacities obtained by other authors of natural and synthetic polymers for dyes and heavy metals (Table 3) and are even higher.

Sorption of pollutants on a porous sorbent takes place in four consecutive steps: (1) diffusion in the mass—transport of sorbate from the liquid phase to the boundary layer around the particle; (2) diffusion through the film or external diffusion—transport of sorbate through the boundary layer to the outer surface of the sorbent; (3) intra-particle diffusion—transport of sorbate inside the sorbent particles by molecular diffusion through the pores; (4) interaction of sorbate molecules with the centers of the sorbent. Slower processes, such as diffusion through the boundary layer or intra-particle diffusion, determine the sorption rate [86].

The kinetic models used to describe the process of removing heavy metals from simulated solutions with poly(NIPAM-co-MAA) hydrogels were pseudo-first-order, pseudo-second-order, and intraparticle diffusion models [87,88,89,90]. Linear forms of kinetic models are shown by Equations (4)–(6).

$$\log \left(q_{\mathrm{e}}-q_{\mathrm{t}}\right)=\log q_{\mathrm{e}}-{\displaystyle \frac{k_1}{2303}}t$$

(4)

$${\displaystyle \frac{t}{q_{\mathrm{t}}}}={\displaystyle \frac{1}{k_2{q}_{\mathrm{e}}^2}}+{\displaystyle \frac{1}{q_{\mathrm{e}}}}t$$

(5)

$$q_{\mathrm{t}}=k_{\mathrm{id}}{t}^{1/2}+C$$

(6)

where q_e and q_t represent the sorption capacities in the equilibrium state and at the moment of time t, respectively, k₁ is the pseudo-first-order rate constant (1/min), k₂ is the pseudo-second-order rate constant (g/mg min), k_id is the intra-particle diffusion rate constant (mg/g min^1/2), and C reflects the boundary layer effect. From the slope and intercept of the dependence, log(q_e − q_t) of t, t/q_t of t, and q_t of t^1/2, the kinetic parameters were calculated and are given in

Table 4. Kinetic parameters of sorption of heavy metal ions onto poly(NIPAM-co-MAA) hydrogels.

Hydrogel	qe, exp 1 (mg/g)	Pseudo-First-Order			Pseudo-Second-Order			Intra-Particle Diffusion
		k1 × 104 (1/min)	qe, cal 2 (mg/g)	R2	k2 × 106 (g/mg min)	qe, cal (mg/g)	R2	kid (mg/g min1/2)	C (mg/g)	R2
Cr(VI) ion
95/5/1.5	289.35	8.19	170.47	0.965	12.73	301.20	0.996	3.14	101.04	0.877
95/5/2	217.34	9.72	97.67	0.771	24.36	225.73	0.999	2.38	85.22	0.616
95/5/3	120.12	9.41	63.01	0.756	26.58	128.20	0.996	1.52	35.24	0.659
Mn(II) ion
95/5/1.5	190.59	8.33	147.43	0.989	8.37	213.22	0.999	2.63	31.52	0.922
95/5/2	148.87	13.46	97.99	0.904	20.27	160.77	0.996	1.79	47.08	0.793
95/5/3	115.28	8.37	74.67	0.917	19.94	124.67	0.995	1.42	30.71	0.818
Pb(II) ion
95/5/1.5	349.71	11.42	302.82	0.983	5.17	390.62	0.990	5.03	55.23	0.884
95/5/2	156.86	20.08	96.22	0.945	39.95	163.40	0.999	1.62	67.91	0.758
95/5/3	90.30	9.16	63.62	0.976	29.00	96.71	0.997	1.17	21.35	0.879

¹ Experimental sorption capacity of hydrogel; ² calculated sorption capacity of hydrogel.

.

The pseudo-second-order model has the highest correlation coefficients obtained by the linear regression method (R² = 0.990–0.999). According to this model, the calculated sorption capacity (q_e,cal) is the closest to the experimentally determined sorption capacity (q_e,exp) (Table 4). The removal of Cr(VI), Mn(II), and Pb(II) ions from solution by poly(NIPAM-co-MAA) hydrogels follows pseudo-second-order kinetics. The linear straight line of dependence of q_t on t^1/2, constructed based on experimental data, does not pass through the coordinate origin, indicating that intra-particle diffusion is not the only limiting step that determines the sorption rate during the sorption of ions onto hydrogels [22]. A better fit of the experimental data for the dependence of q_t on t^1/2 was obtained for two linear lines, as shown in Figure 9 for the 95/5/3 hydrogel sample. The multilinearity indicates that the removal of metal ions from water by poly(NIPAM-co-MAA) hydrogels occurs in two steps.

In the case of heavy metal removal by hydrogels, the sorption process occurs in two or three steps. The first linear line on the graph of q_t versus t^1/2 represents the external surface adsorption. In contrast, the second line represents the sorption phase in which intra-particle diffusion controls the process rate. The third linear line refers to the equilibrium phase, where diffusion of the heavy metal within the particles slowly decreases due to the extremely low concentration of the solute in the medium [91]. There is no equilibrium phase (third linear line) for removing metal ions from water by poly(NIPAM-co-MAA) hydrogels, and only surface sorption and intra-particle diffusion affect the sorption process, which is in accordance with studies by other authors [92].

3.2.3. The Effect of Heavy Metal Initial Concentration and Sorption Isotherms

The initial concentration of the sorbate solution is a parameter that provides a driving force that overcomes the resistance during the transfer of the pollutant mass between the liquid and solid phases [93] At higher initial concentrations, the amount of pollutant in the solution is higher; therefore, the percentage of pollutant binding to the sorbent’s sorption centers is higher [94]. The effect of initial concentration on the removal ability of poly(NIPAM-co-MAA) hydrogels for heavy ions from water was investigated in the concentration range of 50–500 mg/dm³ at 25 °C, and the results are shown in Figure 10.

In Figure 10, it is noticed that the degree of removal of metal ions by hydrogel poly(NIPAM-co-MAA), sample 95/5/2, depends on the initial concentration of heavy metal. The maximum sorption capacity depends on the type of sorbent, and it is in the following order: Cr(VI) > Pb(II) > Mn(II) ions. With the rise in heavy metal concentration up to 200 or 300 mg/dm³, the absorption capacity of hydrogel poly(NIPAM-co-MAA) increased intensively. At concentrations higher than 200 or 300 mg/dm³, the sorption sites in the copolymer are likely to be saturated, and there is less possibility of sorbent and sorbate interaction. Therefore, there is a small change in the degree of metal ion removal.

Sorption isotherms provide information on the relationship between the amount of bound sorbate and the sorbate concentration in solution and explain the interaction between the sorbent and the pollutant. Applying isotherms can characterize the sorption process and predict the pollutant removal capacity [22,86,95].

The obtained equilibrium data of metal ion sorption on poly(NIPAM-co-MAA) were analyzed using the linear regression method according to the Langmuir and Freundlich isotherm models.

The Langmuir model implies a monolayer binding of sorbates at specific localized sites (sorption centers) of an energetically homogeneous surface. According to this model, sorbate particles have no interactions [86,96]. Freundlich’s model is not limited to single-layer sorption but implies multi-layer sorption on an energetically heterogeneous sorbent surface. Unlike the Langmuir model, interactions exist between the sorbate particles [97]. Equations (7) and (8) mathematically present the linear form of the Langmuir and Freundlich models, respectively.

$${\displaystyle \frac{c_{\mathrm{e}}}{q_{\mathrm{e}}}}={\displaystyle \frac{1}{q_m{K}_{\mathrm{L}}}}+{\displaystyle \frac{c_{\mathrm{e}}}{q_m}}$$

(7)

$$\log q_{\mathrm{e}}=\log K_{\mathrm{F}}+{\displaystyle \frac{1}{n}}\log c_{\mathrm{e}}$$

(8)

where q_m represents the maximum adsorption capacity for a given sorbate (mg/g), c_e is the concentration of sorbate in the equilibrium state (mg/dm³), the Langmuir constant is K_L (dm³/mg), the Freundlich constant, K_F, indicates the strength of adsorption, i.e., refers to the sorption capacity, n refers to the energetic heterogeneity of the sorbent surface. The parameters of the isotherms are determined from the slope and intercept graph of the linear dependence of c_e/q_e on c_e and logq_e on logc_e (

Table 5. Langmuir and Freundlich isotherm parameters for sorption of heavy metal ions onto poly(NIPAM-co-MAA) hydrogel sample 95/5/2.

Heavy Metal Ion	Langmuir			Freundlich
	qm (mg/g)	KL × 103 (dm3/mg)	R2	KF	n	R2
Cr(VI)	309.60	6.14	0.988	6.58	1.66	0.954
Mn(II)	184.16	9.63	0.982	8.13	2.01	0.866
Pb(II)	202.43	8.38	0.970	7.84	1.95	0.842

).

The Langmir constant K_L is used to determine the separation factor (R_L), which indicates the nature of the sorption process: unfavored (R_L > 1), linear (R_L = 1), favored (0 < R_L < 1), and irreversible sorption (R_L = 0) [98]. The separation factor is calculated by the following equation:

$$R_{\mathrm{L}}={\displaystyle \frac{1}{1+K_{\mathrm{L}}{c}_0}},$$

(9)

where c₀ is the initial sorbate concentration (mg/dm³).

The Langmuir model (R² = 0.970–0.988) better fits the experimental equilibrium sorption data than the Freundlich model (R² = 0.842–0.954). Therefore, it is an adequate model for simulating the sorption process of Cr(VI), Mn(II), and Pb(II) ions using the poly(NIPAM-co-MAA) hydrogel. Heavy metal ions from water are probably monolayer bound to energetically homogeneous centers inside and on the surface of the hydrogel. The obtained values for R_L range from 0.272 to 0.701; thus, removing metal ions at 25 °C by the poly(NIPAM-co-MAA) hydrogel is the favored process.

3.2.4. Thermodinamic Studies

Basic thermodynamic quantities, such as Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°), are used to characterize the sorption process in terms of spontaneity, the nature of the reaction (endothermic and exothermic reaction), the sorption mechanism, and the randomness at the sorbate/solution interface. The Gibbs free energy change (J/mol) is given by Equation (10).

$$\Delta G^{\circ }=-RT\ln K_{\mathrm{c}}$$

(10)

where R is the universal gas constant (8.314 J/mol K), T is the absolute temperature (K), and K_c represents the equilibrium constant. K_c is calculated using the following expression:

$$K_{\mathrm{c}}={\displaystyle \frac{c_{\mathrm{ad}}}{c_{\mathrm{e}}}}$$

(11)

where c_ad and c_e are the equilibrium concentrations of pollutants in the sorbent and in the solution (mg/dm³), respectively.

Equation (12) shows the relationship between the thermodynamic quantities, ΔG°, ΔH°, and ΔS°.

$$\Delta G°=\Delta H°-T\Delta S°$$

(12)

Substituting Equation (11) into Equation (12) yields the following expression:

$$\ln K_{\mathrm{c}}={\displaystyle \frac{\Delta S^{\circ }}{R}}-{\displaystyle \frac{\Delta H^{\circ }}{RT}}$$

(13)

From the slope of the linear graph of the dependence of lnK_c on 1/T, ΔH° (J/mol) is calculated, and from the intercept ΔS° (J/mol K) [91,99,100].

The obtained sorption capacity of the poly(NIPAM-co-MAA) hydrogel, sample 95/5/1.5, for Cr(VI), Mn(II), and Pb(II) ions at temperatures of 25, 35, and 45 °C was used to calculate the process’s thermodynamic parameters (

Table 6. Thermodynamic parameters of heavy metal ion sorption onto the poly(NIPAM-co-MAA) hydrogel sample 95/5/1.5.

Heavy Metal Ion	Temperature (C°)	ΔG° (kJ/mol)	ΔH° (kJ/mol)	ΔS° (J/mol K)	R2
Cr(VI)	25	2.978	−9.116	40.607	0.984
	35	3.429
	45	3.788
Mn(II)	25	4.254	−5.947	34.183	0.991
	35	4.574
	45	4.939
Pb(II)	25	2.345	−8.936	37.788	0.984
	35	2.679
	45	3.103

).

From the obtained sorption data for investigating the thermodynamic parameters of the process, it is concluded that, with an increase in temperature from 25 to 45 °C, there is a decrease in the sorption capacity of poly(NIPAM-co-MAA) towards heavy metal ions. Positive values of ΔG° (Table 6) at all tested temperatures indicate that ion sorption does not represent a spontaneous reaction. The removal of heavy metal ions by hydrogels is an exothermic process (negative value of ΔH°). Based on the sign of ΔS° (Table 6), it can be concluded that, during the sorption of Cr(VI), Mn(II), and Pb(II) ions, there is an increase in the randomness at the solid–solution interface. Various interactions between the pollutant and the sorbent, such as van der Waals, hydrogen, dipole–dipole, ion exchange, and chemical bonds, contribute to the enthalpy change. The ΔH° value determines the interaction mechanism between the sorbent and sorbate, so the ΔH° value up to 20 kJ/mol indicates physical sorption, while the ΔH° value greater than 80 kJ/mol indicates chemical sorption. The binding of sorbate to the sorbent is based on ion exchange if the ΔH° value is between 20 and 80 kJ/mol [101,102]. According to these authors, the mechanism of sorbate binding to the sorbent is of a physical nature—hydrogen, Van der Waals, or non-specific electrostatic interactions (ΔH° = from −5.947 to −9.116 kJ/mol).

3.2.5. Regeneration and Reuse of Poly(NIPAM-co-MAA) Hydrogel

The regeneration and stability of the sorbent material are crucial in the sorption process as they determine the potential for reusing the sorbent. The use of reusable sorbents lowers sorption costs and enables the recovery of heavy metal ions from wastewater [22,74,77]. In this study, the sorption/desorption cycles of heavy metal ions were conducted three times consecutively. The percentage of desorbed metal ions (DPs) during these cycles is illustrated in Figure 11. A slightly decreased desorption degree in the 0.5 M HCl solution occurs after the second cycle of sorption/desorption (92.55–97.63%). After the third sorption/desorption cycle, the desorption percentages for Cr(IV), Mn(II), and Pb(II) ions are 85.37%, 82.01%, and 84.34%, respectively (Figure 11). Some authors suggest that the desorption of bound metal ions in hydrogels containing an acid component, such as acrylic and methacrylic acid, is the result of the protonation of the hydrogel’s carboxyl groups. This, in turn, leads to a decrease in electrostatic interactions between metal ions and carboxylate (COO⁻) [74,77]. The elution of Mn(II) and Pb(II) ions can be explained by this mechanism if electrostatic interactions of a physical nature exist between the carboxyl groups of the poly(NIPAM-co-MAA) hydrogel and metal ions. The results in Figure 11 show that lead and manganese cations and chromium anions, sorbed onto the hydrogel through weak interactions, can be desorbed using an HCl solution. The pH-sensitive hydrogel poly(NIPAM-co-MAA) contracts by heavy metal ions elution by acid. Another good characteristic of the synthesized hydrogels is the high percentage of the initial sorption capacities for metal ions after the third desorption cycle (75.68–78.75%). The poly(NIPAM-co-MAA) hydrogel proved to be a stable and good reusable sorbent material for removing heavy metals from water/wastewater.

4. Conclusions

Poly(NIPAM-co-MAA) hydrogels were synthesized by the radical polymerization process from the starting monomers NIPAM and MAA in the presence of the crosslinker EGDM. FTIR analysis indicated that the polymer synthesis occurred by breaking the double bonds of the monomers and the crosslinker. The synthesized pH- and temperature-sensitive hydrogels show a good absorption capacity of solutions and swelling reversibility. In the third swelling–contraction cycle, the swelling degree of poly(NIPAM-co-MAA) reaches about 92% of the swelling degree in the first cycle. SEM and EDX techniques provided evidence for the presence of Cr(VI), Mn(II), and Pb(II) ions within the structure of poly(NIPAM-co-MAA) hydrogels. The efficiency of heavy metal removal from simulated aqueous solutions onto hydrogels was found to be in the order of Pb(II) > Cr(VI) > Mn(II) ions. The pseudo-second-order model best fits kinetic sorption experimental data, while equilibrium data are best fitted by the Langmuir sorption isotherm. The sorption of heavy metals onto hydrogels is an exothermic process in which physical interactions play a crucial role in binding metals to the hydrogels. The examined poly(NIPAM-co-MAA) hydrogel exhibits high sorption capacity towards heavy metals through three consecutive sorption/desorption processes. Hydrogels can be regenerated and reused by 0.5 M HCL solution at least three times. The high heavy metal ion removal capacity and good desorption efficiency implied that poly(NIPAM-co-MAA) hydrogels could be used as sorbent material for the alternative treatment of industrial heavy metal effluents.

5. Patents

Granted patent RS58996B: Zdravković, A.; Nikolić, Lj.; Ilić-Stojanović, S.; Nikolić, V.; Savić, S.; Petrović, S. Procedure for application of temperature and pH-sensitive hydrogels for the adsorption of heavy metals, The Intellectual Property Office of the Republic of Serbia, https://e-reg.zis.gov.rs/patreg/?t=p (accessed on: 8 March 2025).