Facile Preparation and Adsorption Behavior Studies of Poly(acrylic acid)-Based Hydrogels Reinforced by Hydrogen Bonds for Methylene Blue Dye

To suppress the extreme swelling of poly(acrylic acid) (PAA) hydrogels, two series of PAA-based hydrogels with additional non-covalent crosslinks were successfully synthesized by incorporating allyloxypolyethyleneglycol (APEG) or polyvinylpyrrolidone (PVP) to form hydrogen bonds. The developed hydrogels were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR). The swelling and methylene blue (MB) adsorption experiments of them were performed. Both series of hydrogels exhibited moderate swelling ratios of 213–235 g g−1 and excellent removal efficiency of MB (> 92%). Based on kinetic assessments, the MB adsorption process could be well-described by the pseudo-second-order kinetic and Langmuir isotherm models, suggesting a monolayer chemisorption mechanism. The maximum theoretical adsorption capacity (qm) of the PAA-based hydrogel with PVP could achieve 2483.2 mg g−1. Regeneration experiments showed that both series of hydrogels could endure adsorption/desorption for at least 5 consecutive cycles with MB removal efficiency of higher than 80%. Therefore, it was believed that this study could provide a facile method to develop hydrogel adsorbents and a promising adsorbent material with high adsorption capacity for the removal of MB from dye-containing wastewater.


Introduction
With the rapid development of the printing and dyeing industry, huge quantities of dyeing wastewater have been discharged into the nearby water body, which results in great harm to the ecological environment and humans [16]. For instance, methylene blue (MB), a cationic dye, has harmful effects on humans, such as retching, vomiting, cyanosis, jaundice, and tissue necrosis [36]. Therefore, it is essential to explore and develop efficient treatment methods and technologies for dye-containing wastewater.
Hydrogels have cross-linked three-dimensional (3D) polymer networks with large quantities of hydrophilic functional groups, which provide sufficient binding sites for dyes. Besides, they are easily separated from wastewater after adsorption, which is favorable for avoiding secondary water pollution. Sarmah et al. [32] synthesized a starchbased hydrogel with positive and negative charges, which could remove both cationic dye and anionic dye. Ren [30] reported a double network gelatin/chitosan hydrogel as a dye adsorbent, which demonstrated a maximum adsorption capacity of 209.2 mg g −1 for MB. Melo et al. [26] utilized cellulose nanowhiskers to modify chitosang-poly(acrylic acid) hydrogel as an MB adsorbent, which could be reused for at least 5 consecutive adsorption/desorption cycles. İsmail et al. [15] prepared a polyacrylamide/ sodium alginate hydrogel for the removal of MB, which displayed a maximum adsorption capacity of 90.90 mg g −1 .
Fan et al. [8] combined modified biochar and chitosan with the acrylic acid monomer to obtain a composite hydrogel by radical polymerization, which exhibited improved stability and a maximum adsorption capacity of 590.72 mg g −1 for MB. Wang et al. [40] doped the montmorillonite into a poly(acrylamide-co-acrylic acid) hydrogel to fabricate a hybrid hydrogel, which indicated a superior adsorbability for MB and a maximum adsorption capacity of 717.54 mg g −1 . Viana et al. [38] adopted the aminated graphene oxide as a reinforce nanofiller and cross-linker of polyacrylamide to form a hybrid hydrogel, which showed enhanced thermal properties and a maximum adsorption capacity of 205.4 mg g −1 for MB. However, hydrogels often swell in water and absorb a large amount of water, which consequently induces low mechanical strength and unsatisfactory regeneration performance. It is necessary to suppress the swelling of hydrogels and maintain their adsorption capacity for dye at the same time.
In this work, we introduce additional non-covalent crosslinks (hydrogen-bonding interaction) into poly(acrylic acid)-based hydrogels to reduce their swelling. The as-prepared hydrogels were firstly characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Subsequently, the effects of pH, initial MB concentration, and contact time on MB adsorption were examined. Thereafter, the adsorption kinetics and adsorption isotherms were investigated by different models. At last, the MB desorption was performed to study the adsorbent regeneration. The performance studies suggested that the obtained hydrogels might be promising adsorbents for MB removal from dye-containing wastewater.

Preparation of Hydrogels
All hydrogels were prepared by radical polymerization in aqueous solutions. Predetermined amounts (Table 1) of raw materials, crosslinking agent, and initiator were dissolved in Table 1 Parameters and MB removal efficiency of hydrogels a 10 mg of sample in 100 mL of 10 mg L −1 MB solution (pH = 9) at room temperature for 24 h Sample AA (g) AM (g) APEG (g) PVP (g) NaOH (g) MBA (mg) APS (mg) Urea (g) R e (%) a 10 mL H 2 O at room temperature. Then the as-obtained solution was kept at 60 °C for 24 h to complete the polymerization. Finally, the as-prepared hydrogel was washed with distilled water and dried at 60 °C for 24 h. The P(AA-co-AM) hydrogels, P(AA-co-APEG-co-AM) hydrogels, and PAA/PVP hydrogels were labeled as HGA, HGB, and HGC.

Characterization
FTIR spectroscopy of samples was recorded on a VERTEX70 spectrometer in the ranging from 4000 to 400 cm −1 at a resolution of 2 cm −1 . The content of hydrogel samples in KBr pellets was about 1.5 wt%. The hydrogels were freeze-dried at − 50 °C and the surface were sputter coated with a thin layer of platinum. SEM observations of samples were carried out on scanning electron microscopy (SEM, JEOL JSM-6510).

Swelling Behaviors
The dried hydrogel sample was immersed in distilled water at room temperature for 48 h. After swelling equilibrium was achieved, the swollen hydrogel was weighed. The equilibrium swelling ratio (SR) of the sample was obtained by Eq. (1).
where W 2 and W 1 are the weights of the swollen and dry samples. All the tests were performed using three samples in parallel and the average value of them was calculated.

Adsorption Experiments
The effect of pH on MB removal was investigated in the pH range of 3-11. The pH value was adjusted using H 2 SO 4 or NaOH solution. To examine the MB adsorption capacity of hydrogels, a certain amount of hydrogel was soaked in 100 mL MB aqueous solution with different concentrations. The adsorption process was carried out for 48 h at room temperature. The residual concentration of MB was monitored at 664 nm using an ultraviolet-visible spectrophotometer. The adsorption capacity (q t or q e , mg g −1 ) and the removal efficiency (R e , %) of MB were determined using the Eqs. (2) and (3), respectively.
where C 0 and C t are the MB concentrations (mg L −1 ) before and after adsorption, V 0 and V t are the volume values (L) of MB solution before and after adsorption, and m is the mass of hydrogel (g).

Regeneration Experiments
The MB-adsorbed samples were collected and immersed in 0.05 mol L −1 NaCl aqueous solution for desorption. Regenerated samples were washed with distilled water and tested in a new adsorption/desorption cycle. The regeneration efficiency (RE) was calculated with Eq. (4) as shown in the literature [2].
where q reg and q 0 are the adsorption capacity of regenerated adsorbent and the adsorption capacity of fresh adsorbent, respectively.

Hydrogels Characterization
The preparation process of three series of hydrogels was illustrated in Fig. 1. APEG is an important raw material of polycarboxylic acid-type water-reducing agent for cement paste [18,33]. When it is introduced into P(AA-co-AM) hydrogels, its ether and hydroxyl groups are prone to form hydrogen bonds with carboxylic acid groups and carbonyl groups. PVP is a water-soluble linear polymer, whose carbonyl groups can also form hydrogen bonds with carboxylic acid groups of PAA [10,39]. The R e values of all hydrogels were presented in Table 1. For the series of HGA hydrogels, both HGA1 and HGA2 had R e values over 90%, but HGA1 became very brittle after the MB adsorption due to the excess swelling, which was unfavorable to regeneration. On the contrary, HGA2 had a moderate swelling owing to the interaction between carboxyl and amidogen.
With the increase of AM contents, HGA3, HGA4, HGA5, and HGA6 showed gradually reduced R e values lower than HGA2 because of the obvious decrease of reactive sites for MB adsorption. For the series of HGB hydrogels, HGB2 had a higher R e value than HGB1 because a small number of hydrogen bonds could decrease the swelling and promote the MB adsorption by shortening the diffusion distance of MB molecules. However, excessive hydrogen bonds would result in compacter structures with the increasing of APEG contents and hinder the MB adsorption process by suppressing the diffusion of MB molecules. As for the series of HGC hydrogels, the heterocyclic groups of PVP could provide additional adsorption sites for MB molecules. But the simultaneous increase of PVP and the crosslinking agent could lead to reduced R e values similarly owing to compacter structures. Therefore HGC3 exhibited the highest R e value. As a consequence, HGA2, HGB2, and HGC3 hydrogels were selected for further study. Figure 2a showed the FTIR spectra of HGA2, HGB2, and HGC3 hydrogels. The wide absorption band detected at 3446 cm −1 was due to the stretching vibration of O-H and N-H [35,40]. The bands between 2856 and 2971 cm −1 were assigned to the C-H symmetric and asymmetric stretching vibrations [32,48]. The absorption band at1722 cm −1 was attributed to the C=O asymmetric in the carboxylate anion [40]. The absorption band at 1663 cm −1 was responsible for the stretching vibration of C=O in amide groups or pyrrolidone [42,48]. The absorption bands at 1454, and 1404 cm −1 were related to -COO − stretching vibrations of carboxyl groups [36,39]. The absorption band at1283 cm −1 corresponded to the C-N vibration of pyrrolidone [42]. The presence of bands at 1163 cm −1 was associated with the C-N stretching vibration of MBA [26,32], supporting the presence of the cross-linker.
The cross-section morphology images of HGA2, HGB2, and HGC3 hydrogels before adsorption were presented in Fig. 2b-d. It could be clearly observed that three samples had well-connected 3D porous structures with a pore diameter in the range of 10-30 μm, which provided a large surface area and enough channels for the adsorption and diffusion of water and MB molecules. Moreover, the HGB2 and HGC3 hydrogels indicated thicker pore-wall than the HGA2 hydrogel, which might be caused by the additional non-covalent crosslinks.

Swelling and Adsorption Behaviors
The moderate swelling capacity of hydrogels is necessary and beneficial to the transportation and adsorption of dye molecules. Nevertheless, excessive amounts of water in hydrogels are also disadvantaged for desorption and re-usage. Figure 3a showed the swelling ratio of HGA2, HGB2, and HGC3 hydrogels. Their SR values of them were 582.6, 213.2, and 234.5 g g −1 , respectively. It was reasonable to deduce that the additional non-covalent crosslinks in HGB2 and HGC3 hydrogels increased their crosslinking densities and consequently reduced their swelling capacities. Although the SR values in MB solution of them had a similar trend as in water, these values were was accordingly reduced to 265.8, 108.4, and 125.6 owing to the decreased osmotic pressure difference between the hydrogel network and the external MB solution, which resulted from the anion-anion electrostatic repulsion of MB and carboxyl [49].
The pH of MB solution had an important influence on the adsorption of hydrogels. As shown in Fig. 3b, the removal efficiency of three samples exhibited the highest values in the pH environment of 7-9, which was consistent with other reports [32,40]. Close to neutrality or under weak alkaline conditions, R e values could reach higher than 93%, which meant the blue MB solution could be changed to a transparent solution with slight cyan after adsorption (Fig. 3c). In the acidic environment, anionic adsorption sites in hydrogels were mostly occupied by protons (H + ), which suppressed the adsorption of MB molecules [26]. While in a weak alkaline environment, the most of anionic adsorption sites in hydrogels were exposed to MB molecules due to deprotonation of the carboxylic groups, which facilitated the electrostatic attractions between MB and carboxylate (-COO − ) [5]. When the pH further exceeded 9, the charge screening effect of the excess Na + led to the reduction of their R e values [32].
The effect of initial MB concentration on the adsorption of HGA2, HGB2, and HGC3 hydrogels was also explored. As illustrated in Fig. 3d-f, the R e values of them decreased with the increasing MB concentration from 10 to 1000 mg L −1 . Such a finding might be attributed to the availability of active adsorption sites. At a lower initial MB concentration, there were enough active adsorption sites for MB molecules, which resulted in higher removal efficiency of MB. In contrast, at higher initial MB concentration, the available adsorption sites for MB molecules became insufficient due to the competition between more and more MB molecules, Fig. 2 a FTIR spectra of HGA2, HGB2, and HGC3 samples. SEM images of b HGA2 hydrogel, c HGB2 hydrogel, d HGC3 hydrogel which led to lower removal efficiency of MB. On the contrary, the adsorption capacities of three hydrogels increased with the increasing MB concentration from 10 to 800 mg L −1 . Then the adsorption capacities of HGA2 and HGB2 hydrogels decreased with the further increase of MB concentration. It is noteworthy to mention that the adsorption capacity of HGC3 hydrogel continued to ascend even in the initial MB concentration of 2000 mg L −1 , indicating an outstanding adsorption ability for MB. The highest adsorption amounts of three hydrogels were 1260, 913, and 1996.8 mg g −1 , respectively.

Influence of Contact time and Kinetics Parameters
The influence of contact time on the MB adsorption capacity of three hydrogels was shown in Fig. 4a. It could be seen that the adsorption rates were very fast in the initial 100 min and then slowed down in the following adsorption stage. To better understand the adsorption kinetics, the pseudo-first-order model [51], pseudo-second-order model [29,30], Elovich model [26], liquid film diffusion model [11], and intraparticle diffusion model [35] were adopted to analyze these adsorption data (Fig. 4b-f). The linear forms of these kinetic models were provided in Eqs. (5)-(9) respectively.
The pseudo-first-order model: where q e (mg g −1 ) is the equilibrium adsorption capacity of hydrogels, q t (mg g −1 ) is the adsorption capacity of hydrogels at time t (min), and k 1 (min −1 ) is the rate constant of the model. The pseudo-second-order model: where q e (mg g −1 ) is the equilibrium adsorption capacity of hydrogels, q t (mg g −1 ) is the adsorption capacity of where α (mg g −1 min −1 ) is the initial adsorption rate, q t (mg g −1 ) is the adsorption capacity of hydrogels at time t (min), and β (g mg −1 ) is a constant related to the desorption process [21]. Intra-particle diffusion model: where q t (mg g −1 ) is the adsorption capacity of hydrogels at time t (min), K i (mg g −1 min −1/2 ) is the intra-particle (7) q t = 1 ln( ) + 1 ln t (8) q t = K i × t 1∕2 + C diffusion rate constant, and C (mg g −1 ) is a constant related to the thickness of the boundary layer. Liquid film diffusion model: where q e (mg g −1 ) is the equilibrium adsorption capacity of hydrogels, q t (mg g −1 ) is the adsorption capacity of hydrogels at time t (min), and K d (min −1 ) is the liquid film diffusion rate constant. All the above-involved kinetic parameters and the correlation coefficient (R 2 ) values were listed in Table 2.
As revealed in the fitting diagrams of the pseudo-firstorder and second-order kinetics (Fig. 4b, c) and the R 2 values of them in Table 2, compared to the pseudo-first-order . 4 a The effect of contact time on the MB adsorption of HGA2, HGB2, and HGC3 hydrogels (MB concentration 10 mg L −1 , 100 mL; temperature 25 °C, pH: 7). MB adsorption kinetic models of three samples: b pseudo-first-order, c pseudo-second-order, d Elovich, e intra-particle diffusion, and f liquid film diffusion models model, the pseudo-second-order kinetic model demonstrated R 2 values were close to 1 and a remarkable consistency between the theoretical q e values in Table 2 and the experimental equilibrium amounts in Fig. 4a. Therefore, the pseudo-second-order kinetic model could best describe the BM adsorption process of the three hydrogels, which indicated that the adsorption process of MB was dominantly controlled by the chemisorption via the exchange or sharing of electrons between MB molecules and polymer chains of hydrogels, such as electrostatic attraction, hydrogen-bonding interaction, and n-π interaction [5,32,36,37,51]. Based on the kinetic parameters of the pseudosecond-order kinetic model in Table 2, the initial adsorption rate (h 0 ) of the three samples could be obtained from Eq. (10) [38]: It could be found that HGC3 had the highest h 0 value and HGB2 had the lowest h 0 value among the three hydrogels. The lowest h 0 value of HGB2 was probably related to its lowest SR value. In addition, according to the Elovich model, which describes the chemical adsorption mechanism for heterogeneous adsorption processes, the initial adsorption rates (α) of the three hydrogels in Table 2 also had a similar order. It could be inferred that the introduction of (10) h 0 = k 2 × q e 2 PVP might be more feasible to enhance the initial adsorption rate of HGA hydrogels than the introduction of APEG.
To further understand the adsorption mechanism of MB in these hydrogels, the intra-particle diffusion model and the liquid film diffusion model were also used to evaluate these kinetic results from the perspective of MB molecule diffusion. It could be viewed that the fitting curves of the intra-particle diffusion model in Fig. 4e exhibited two linear regions with different slopes for three samples. Furthermore, these fitting plots did not pass through zero. These results indicated the overall adsorption process might be controlled by multiple mechanisms [32]. The step I straight lines with large slopes originated from the rapid transfer of MB molecules on the external surface of hydrogels through boundary layer diffusion, while the step II straight lines with small slopes associated with the intra-particle or pore diffusion of MB molecules in hydrogels [30,51]. On the contrary, the fitting plots of the liquid film diffusion model displayed poor R 2 values lower than those of the intra-particle diffusion model (Table 2) and did not pass through the origin (Fig. 4f), which meant that the liquid film diffusion model was not applicable for the MB adsorption in the three samples. Figure 5a showed the adsorption isotherms of HGA2, HGB2, and HGC3 hydrogels for MB. It could be found that the adsorption capacities of hydrogels increased with the increase of the MB equilibrium concentration. To better understand the interaction between these hydrogels and MB molecules, the equilibrium data were evaluated by Langmuir [51], Freundlich [23,29], and Dubinin-Radushkevich (D-R) [30] isotherm models. The plotting curves were displayed in Fig. 5b-d. The linear forms of these isotherm models were expressed in Eqs. (11)-(13), respectively. All the calculated isotherm model parameters and their R 2 values of HGA2, HGB2, and HGC3 hydrogels for the MB adsorption were recorded in Table 3.

Adsorption Isotherms
Langmuir model: where q e (mg g −1 ) is the equilibrium adsorption capacity of hydrogels, q m (mg g −1 ) is the maximum adsorption capacity of hydrogels, C e (mg L −1 ) is the MB equilibrium concentration, and K L (L mg −1 ) is the Langmuir constant, which associates with the free energy and affinity of adsorption. Freundlich model: 12) ln q e = ln K F + ln C e n where q e (mg g −1 ) is the equilibrium adsorption capacity of hydrogels, C e (mg L −1 ) is the MB equilibrium concentration, and K F ((mg g −1 ) (L mg −1 ) 1/n ) is the Freundlich constant, and 1/n is the heterogeneity factor. D-R model: where q e (mg g −1 ) is the equilibrium adsorption capacity of hydrogels, q m (mg g −1 ) is the maximum theoretical adsorption capacity of hydrogels, C e (mg L −1 ) is the MB equilibrium concentration, K DR (kJ 2 mol -2 ) is the adsorption constant related to the adsorption energy, R (8.314 J K −1 mol −1 ) is the universal gas constant, and T (K) is the testing temperature in Kelvin. Additionally, the mean adsorption free energy (E, kJ mol −1 ) of MB can be obtained from Eq. (15) [30].  Based on the E value, the nature of the isotherm can be judged as physisorption (E < 8 kJ mol −1 ) and chemisorption (E = 8-16 kJ mol −1 ).
The applicability of these isotherm models to the adsorption of HGA2, HGB2, and HGC3 hydrogels for MB could be determined through R 2 values. As illustrated in Table 3, the Langmuir model demonstrated R 2 values between 0.994 and 0.999, which were larger than those of the Freundlich and D-R models. This revealed that the Langmuir model was more suitable to describe the adsorption behaviour of the three hydrogels, which suggested that the adsorption of MB molecules on these hydrogels occurred in a monolayer coverage [24]. The separation constant (R L ) of the Langmuir model could be calculated via Eq. (16) [26]. Table 3, all the R L values were in the range of 0 -1, which meant that the MB adsorption on the three hydrogels was a favorable process. In the case of the Langmuir model, the maximum theoretical adsorption capacities of them respectively were 1838.2, 1207.3, and 2483.2 mg g −1 , which were higher than or comparable to that of other hydrogel-based adsorbents in literature (Table 4). This comparison demonstrated that the HGC3 hydrogel might be an effective adsorbent for MB removal. Figure 6 showed the FTIR spectra of three hydrogels after the adsorption of MB. Compared with the FTIR Fig. 2a, there appeared the characteristic peaks of MB at 1599, 1397, and 667 cm −1 , which were attributed to the typical vibration of the aromatic ring, the symmetric bending vibration of -CH 3 , and the stretching vibration of C-S-C, respectively [5,40]. The result revealed that MB had been successfully adsorbed on these hydrogels. It was noteworthy that the bands at 3446 (O-H stretching vibration) and 1454 cm −1 (-COO − stretching vibration) in Fig. 2a shifted to 3442 and 1457 cm −1 , which indicated the strong interactions between MB and the functional groups of hydrogels [37].

Regeneration
From a practical point of view, the regeneration and reusability of adsorbents are very desirable issues. Regeneration experiments of three hydrogels were performed and the results were exhibited in Fig. 7. The R e values of them remained higher than 80% even after 5 consecutive adsorption/desorption cycles. The regeneration efficiency of the HGC3 hydrogel still remained 88.21% after 5 consecutive adsorption/desorption cycles. As a consequence, the HGC3 hydrogel could be employed as a promising recyclable adsorbent for MB removal from wastewater due to its attractive properties, such as low SR value, fast initial adsorption rate, large adsorption capacity, and excellent reusability.

Conclusion
In summary, two series of PAA-based hydrogels with additional non-covalent crosslinks were successfully synthesized by introducing APEG or PVP. Their morphologies of well-connected 3D porous structures were revealed by SEM images. They exhibited reduced SR ratios of 213-235 g g −1 and effective adsorption ability for MB. The analysis for adsorption data indicated that the kinetics of the MB adsorption process followed well the pseudo-second-order kinetic model and intra-particle diffusion model. Additionally, the equilibrium data also obeyed the Langmuir isotherm model, suggesting a monolayer chemisorption mechanism. Based on the Langmuir model, the maximum theoretical adsorption capacity of the hydrogel containing PVP achieved 2483.2 mg g −1 . After five consecutive adsorption/desorption cycles, the R e values of these prepared polymeric adsorbent samples still maintained more than 80%. As a result, the hydrogel containing PVP could be served as an effective adsorbent material for MB removal from wastewaters.