Facile preparation and adsorption behavior studies of poly(acrylic acid)-based hydrogels reinforced by hydrogen bonds for methylene blue dye

doi:10.21203/rs.3.rs-1889584/v1

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 (q_m) 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.

Hydrogel

Adsorption

Methylene blue

Non-covalent crosslink

Adsorption isotherm

Kinetic study

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 (Katheresan et al. 2018). For instance, methylene blue (MB), a cationic dye, has harmful effects on humans, such as retching, vomiting, cyanosis, jaundice, and tissue necrosis (Thakur et al. 2016). Therefore, it is essential to explore and develop efficient treatment methods and technologies for dye-containing wastewater.

To eliminate organic dye contaminants, various methods and techniques have been adopted, including adsorption (Ai et al. 2020; Fallah et al. 2020; Fang et al. 2015; Ulu et al. 2022; Xu et al. 2022), biodegradation (Martín et al. 2011; Rodrigues et al. 2014), chemical oxidation (He et al. 2018; Navarro et al. 2017; Sun et al. 2020; Zhou et al. 2015), reverse osmosis (Nataraj et al. 2009), membrane separation (Liu et al. 2007; Liu et al. 2017), photo-catalysis (Hu et al. 2020; Wang et al. 2021b; Zhang et al. 2019; Zhao et al. 2020), and electrochemical methods (Abou-Shady 2017; Körbahti et al. 2011). Among them, adsorption is considered as a promising and effective method due to its significant advantages like simplicity, high efficiency, availability, environmental friendliness, low energy consumption, and economic feasibility. Different types of adsorbent materials have been employed to remove dyes from wastewater, such as carbon-based materials (Chenab et al. 2020), clay-based nanomaterials (Hong et al. 2009), metal-organic frameworks (MOFs) (Ahmadijokani et al. 2022), MXenes (Ibrahim et al. 2022), layered sodium titanate (Xu et al. 2021), and hydrogels (Sarmah and Karak 2020; Wang et al. 2021c; Zhou et al. 2014).

Compared with inorganic adsorbents, 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.(Sarmah and Karak 2020) synthesized a starch-based hydrogel with positive and negative charges, which could remove both cationic dye and anionic dye. Ren et al.(Ren et al. 2022) 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.(Melo et al. 2018) utilized cellulose nanowhiskers to modify chitosan-g-poly(acrylic acid) hydrogel as an MB adsorbent, which could be reused for at least 5 consecutive adsorption/desorption cycles. İsmail et al.(İsmail and Gökçe Kocabay 2021) 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.(Fan et al. 2022) 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.(Wang et al. 2020) 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.(Viana et al. 2020) 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.

2.1 Chemicals and Reagents

Acrylic acid (AA), acrylamide (AM), N,N′-methylene diacrylamide (MBA), polyvinylpyrrolidone (PVP-K30) urea, NaOH, and ammonium persulfate (APS) were chemical pure and purchased from Sinopharm Chemical Reagent Co., Ltd.. Allyloxypolyethyleneglycol(APEG-1200)was purchased from Shandong Zhuo Xing Chemical Co., Ltd. (Shandong, China). All of them were used as received without further purification.

2.2 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 10 mL H₂O 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 ^oC 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.

Table 1

Parameters and MB removal efficiency of hydrogels.
Sample	AA (g)	AM (g)	APEG (g)	PVP (g)	NaOH (g)	MBA (mg)	APS (mg)	Urea (g)	R_e(%)*
HGA1	2	0	0	0	0.8	4	10	0	90.5
HGA2	1.6	0.4	0	0	0.64	4	10	0	93.2
HGA3	1.2	0.8	0	0	0.48	4	10	0	88.8
HGA4	0.8	1.2	0	0	0.32	4	10	0	67.2
HGA5	0.4	1.6	0	0	0.16	4	10	0	59.8
HGA6	0	2	0	0	0	4	10	0	23.6
HGB1	1.6	0.4	0.04	0	0.64	4	10	0	93.6
HGB2	1.6	0.4	0.08	0	0.64	4	10	0	94.8
HGB3	1.6	0.4	0.12	0	0.64	4	10	0	92.4
HGB4	1.6	0.4	0.16	0	0.64	4	10	0	88.2
HGB5	1.6	0.4	0.24	0	0.64	4	10	0	82.8
HGB6	1.6	0.4	0.32	0	0.64	4	10	0	78.5
HGC1	2	0	0	0.1	0.8	2	10	0	86.4
HGC2	2	0	0	0.1	0.8	2	10	0.3	89.8
HGC3	2	0	0	0.1	0.8	4	10	0.3	94.2
HGC4	2	0	0	0.2	0.8	4	10	0.3	93.8
HGC5	2	0	0	0.3	0.8	4	10	0.3	88.4
HGC6	2	0	0	0.5	0.8	4	10	0.3	88.2
*Note: 10 mg of sample in 100 mL of 10 mg L^− 1 MB solution (pH = 9) at room temperature for 24 h.

2.3 Characterization

FTIR spectroscopy of samples was recorded on a VERTEX70 spectrometer. SEM observations of samples were carried out on scanning electron microscopy (SEM, JEOL JSM-6510) after they were sputtered a layer of platinum.

2.4 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 the formula (1).

where W₂ and W₁ 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.

2.5 Adsorption experiments

The effect of pH on MB removal was investigated in the pH range of 3–11. The pH value was adjusted using H₂SO₄ 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 formula (2) and (3), respectively.

$${q_t}(mg \cdot {g^{ - 1}})=\frac{{{C_0} \times {V_0} - {C_t} \times {V_t}}}{m}$$

2

$${R_e}(\% )=\frac{{{C_0} \times {V_0} - {C_t} \times {V_t}}}{{{C_0} \times {V_0}}} \times 100$$

3

where C₀ and C_t are the MB concentrations (mg L^− 1) before and after adsorption, V₀ and V_t are the volume values (L) of MB solution before and after adsorption, and m is the mass of hydrogel (g).

2.6 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.

3.1 Hydrogels characterization

The preparation process of three series of hydrogels was illustrated in Figure. 1. APEG is an important raw material of polycarboxylic acid-type water-reducing agent for cement paste (Li and Wang 2021; Sun et al. 2011). 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 (Han et al. 2021; Wang et al. 2013). According to R_e values in Table 1, 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 (Tang et al. 2021; Wang et al. 2020). The bands between 2856 and 2971 cm^-1 were assigned to the C-H symmetric and asymmetric stretching vibrations (Sarmah and Karak 2020; Zhang et al. 2020). The absorption band at1722 cm^-1 was attributed to the C = O asymmetric in the carboxylate anion (Wang et al. 2020). The absorption band at1663 cm^-1 was responsible for the stretching vibration of C = O in amide groups or pyrrolidone (Wang et al. 2021a; Zhang et al. 2020). The absorption bands at 1454, and 1404 cm^-1 were related to -COO^- stretching vibrations of carboxyl groups (Thakur et al. 2016; Wang et al. 2013). The absorption band at1283 cm^-1 corresponded to the C-N vibration of pyrrolidone (Wang et al. 2021a). The presence of bands at 1163 cm^-1 was associated with the C-N stretching vibration of MBA (Melo et al. 2018; Sarmah and Karak 2020), 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.

3.2 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.

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 (Sarmah and Karak 2020; Wang et al. 2020). 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 (Melo et al. 2018). 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^-) (Chen et al. 2021). When the pH further exceeded 9, the charge screening effect of the excess Na⁺ led to the reduction of their R_e values (Sarmah and Karak 2020).

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, 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's 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.

3.3 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 (Zhou et al. 2014), pseudo-second-order model (Omer et al. 2021; Ren et al. 2022), Elovich model (Melo et al. 2018), liquid film diffusion model (He et al. 2018), and intra-particle diffusion model (Tang et al. 2021) were adopted to analyze these adsorption data (Fig. 4b-f). The linear forms of these kinetic models were provided in equations (4)-(8) 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 K₁ (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 hydrogels at time t (min), and K₂ (g mg^-1 min^-1) is the rate constant of the model.

Elovich model:

$${q_t}=\frac{1}{\beta }\ln (\alpha \beta )+\frac{1}{\beta }\ln t$$

6

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 (Liu et al. 2022).

Intra-particle diffusion model:

$${q_t}={K_i} \times {t^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0pt} 2}}}+C$$

7

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 diffusion rate constant, and C (mg g^-1) is a constant related to the thickness of the boundary layer.

Liquid film diffusion model:

$$\ln (1 - \frac{{{q_t}}}{{{q_e}}})= - {K_d} \times t$$

8

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 (R²) values were listed in Table 2.

Table 2

Kinetic parameters for the MB adsorption of HGA2, HGB2, and HGC3 hydrogels.
Models	Parameters	HGA2	HGB2	HGC3
Pseudo-first-order	R²	0.869	0.972	0.837
	K₁ (min^-1)	0.00196	0.00243	0.00136
	q_e (mg g^-1)	4.457	8.607	3.867
Pseudo-second-order	R²	0.999	0.998	0.999
	K₂ (g mg^-1 min^-1)	0.00284	0.00100	0.00390
	q_e (mg g^-1)	18.741	19.316	18.592
	h₀ (mg g^-1 min^-1)	0.997	0.373	1.348
Elovich	R²	0.868	0.986	0.996
	α (mg g^− 1 min^− 1)	80.786	4.546	272.336
	β (g mg^− 1)	0.584	0.414	1.055
Intra-particle diffusion
Step I	R²	0.957	0.986	0.969
	K_i (mg g^-1 min^-1/2)	0.696	0.545	0.296
	C (mg g^-1)	7.446	7.020	13.287
Step II	R²	0.776	0.992	0.953
	K_i (mg g^-1 min^-1/2)	0.0471	0.247	0.0673
	C (mg g^-1)	16.394	11.237	16.254
Liquid film diffusion	R²	0.776	0.948	0.954
Liquid film diffusion	K_d (min^-1)	0.00252	0.00305	0.0022

As revealed in the fitting diagrams of the pseudo-first-order and second-order kinetics (Fig. 4b-c) and the R² values of them in Table 2, compared to the pseudo-first-order model, the pseudo-second- order kinetic model demonstrated R² values 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, MB adsorption of the three hydrogels could be well described by the pseudo-second-order kinetic model, which indicated that the adsorption process of MB in these hydrogels 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 (Chen et al. 2021; Sarmah and Karak 2020; Thakur et al. 2016; Ulu et al. 2022; Zhou et al. 2014). Based on the kinetic parameters of the pseudo-second- order kinetic model in Table 2, the initial adsorption rate (h₀) of the three samples could be obtained from the following Eq. (9) (Viana et al. 2020):

$${h_0}={K_2} \times {q_e}^{2}$$

9

It could be found that HGC3 had the highest h₀ value and HGB2 had the lowest h₀ value among the three hydrogels. The lowest h₀ 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 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 (Sarmah and Karak 2020). 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 (Ren et al. 2022; Zhou et al. 2014). On the contrary, the fitting plots of the liquid film diffusion model displayed poor R² 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.

3.4 Adsorption isotherms

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 (Zhou et al. 2014), Freundlich (Maijan et al. 2020; Omer et al. 2021), and Dubinin-Radushkevich (D-R) (Ren et al. 2022) isotherm models. The plotting curves were displayed in Fig. 5b-d. The linear forms of these isotherm models were expressed in equations (10)-(12), respectively. All the calculated isotherm model parameters and their R² values of HGA2, HGB2, and HGC3 hydrogels for the MB adsorption were recorded in Table 3.

Table 3

Isotherm model parameters for the MB adsorption of HGA2, HGB2, and HGC3 hydrogels.
Models	Parameters	HGA2	HGB2	HGC3
Langmuir	R²	0.994	0.999	0.997
	K_L (L mg^-1)	0.0133	0.0092	0.0053
	q_m (mg g^-1)	1838.2	1207.3	2483.2
	R_L	0.07 ~ 0.883	0.098 ~ 0.916	0.159 ~ 0.95
Freundlich	R²	0.97772	0.98286	0.99447
	K_f [(mg g^-1) (L mg^-1)^1/n]	28.03629	22.2564	19.60334
	1/n	0.80359	0.6702	0.79926
Dubinin-Radushkevich	R²	0.676	0.479	0.488
	K_DR (mol² kJ^-2)	0.766	0.522	0.916
	q_m (mg g^-1)	430.8	253.9	376.2
	E (kJ mol^-1)	0.808	0.979	0.739

Langmuir model:

$$\frac{{{C_e}}}{{{q_e}}}=\frac{1}{{{q_m} \times {K_L}}}+\frac{{{C_e}}}{{{q_m}}}$$

10

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:

$$\ln {q_e}=\ln {K_f}+\frac{1}{n}\ln {C_e}$$

11

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:

$$\ln {q_e}=\ln {q_m} - {K_{DR}} \times {\varepsilon ^2}$$

12

$$\varepsilon =RT\ln (1+\frac{1}{{{C_e}}})$$

13

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 (kJ² 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. (14) (Ren et al. 2022).

$$E=\frac{1}{{\sqrt {2{K_{DR}}} }}$$

14

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 R² values. As illustrated in Table 3, the Langmuir model demonstrated R² 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 appropriate to describe the MB adsorption process on the three hydrogels, which suggested that the adsorption of MB molecules on these hydrogels occurred in a monolayer coverage (Malatji et al. 2020). The separation constant (R_L) of the Langmuir model could be calculated via Eq. (15) (Melo et al. 2018).

$${R_L}=\frac{1}{{1+{K_L}{C_0}}}$$

15

As noted in 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.

Table 4

Comparative assessment of adsorption capacities with other hydrogel-based adsorbents.
Adsorbents	q_m (mg g^-1)	Reference
HGA2	1838.2	This work
HGB2	1207.3	This work
HGC3	2483.2	This work
TiO₂/sodium alginate crosslinked polyacrylic acid composite hydrogel	2257.36	(Thakur et al. 2016)
Chitosan/κ-carrageenan/acid-activated bentonite composite	18.8	(Ulu et al. 2022)
Partially hydrolyzed polyacrylamide/cellulose nanocrystal nanocomposite hydrogel	358.42	(Zhou et al. 2014)
Hydrophobic starch based amphoteric hydrogel	133.65	(Sarmah and Karak 2020)
Cellulose nanowhiskers/ chitosan-g-poly(acrylic acid) hydrogel	2074	(Melo et al. 2018)
Polyacrylamide/sodium alginate hydrogel	90.9	(İsmail and Gökçe Kocabay 2021)
Functionalized cotton charcoal/chitosan biomass-based hydrogel	590.72	(Fan et al. 2022)
Villi-like poly(acrylic acid) based hydrogel	2249	(Tang et al. 2021)
Polyacrylamide/chitosan/Fe₃O₄ composite hydrogel	1603	(Zhang et al. 2020)
Semi‑interpenetrated PVA/PAMPS hydrogel	14.7	(Omer et al. 2021)
CMC-cl-pAA hydrogel	1109.55	(Malatji et al. 2020)
P(NIPAM-co-AAc)/MoS₂ hydrogel	1258	(Yang et al. 2021)
CMC/ polyacrylamide/MMT hydrogel	410	(Ma et al. 2021)

3.5 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. 6. The R_e values of them remained higher than 80% even 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.

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 them 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.

Declarations of interest

The authors declared that they have no conflicts of interest in this work.

Jiale Yao: Investigation, Methodology, Writing - original draft.

He Yang: Experiment, Data Curation, Writing - review &editing.

Danying Zuo: Resources, Formal analysis, Review & Editing.

Jing Xu: Review & Editing.

Hongwei Zhang: Conceptualization, Writing - Review &Editing, Supervision.

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No competing interests reported.

Facile preparation and adsorption behavior studies of poly(acrylic acid)-based hydrogels reinforced by hydrogen bonds for methylene blue dye

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods