Egg White Protein–Soybean Protein Isolate Hierarchical Network Hydrogel for Enhanced Adsorption of Methylene Blue

Abstract

In terms of environmental protection and the sustainable development of society, the constraint of dye concentrations in industrial wastewater is vitally important for the development of every country. In this study, egg white protein (EWP)–soybean protein isolate (SPI) hierarchical-network hydrogel beads reinforced with calcium alginate are devised using a one-step chemical crosslinking. The prepared EWP/SPI beads, with a specific surface area of 26.55 m²∙g⁻¹, possess a self-floating ability that enhances their solid–liquid separation of methylene blue (MB) from industrial sewage and achieves adsorption equilibrium within 60 min. The investigation of adsorption behavior indicates that the results fitted well with the Langmuir isotherm mode and pseudo-first-order kinetic model. Based on the pseudo-first-order kinetic model and the Langmuir model, the equilibrium adsorption capacity and maximum adsorption capacity of the EWP/SPI hydrogel beads towards MB are 187.495 and 336.265 mg∙g⁻¹, respectively. Furthermore, the favorable regeneration of the EWP/SPI hydrogel is demonstrated, with a removal efficiency towards MB decreasing from 94% to 82% (10 mg dose, 100 mg∙L⁻¹ MB, pH 7, 25 °C) after five adsorption–desorption cycles. The resulting EWP/SPI hydrogel beads with hydrophilicity exhibited good self-floating stability (above 80%) in wastewater for 7 days, suggesting their potential for recycling in diverse complex environments. Therefore, the inexpensive and sustainable floating EWP/SPI hydrogel beads provide a new insight for organic pollutant treatment in wastewater.

1. Introduction

With the rapid development of the textile industry, the utilization of dyes has dramatically increased [1]. Without proper treatment, these toxic and carcinogenic dyes will remain in soils and water bodies and accumulate in the body at various levels of the food chain for a long time in a manner that seriously threatens human health [2,3]. Therefore, the constraint of dye concentrations in industrial wastewater in the environment is extremely important [4,5]. In recent decades, a great many treatments have already been employed to remove various organic dye molecules, such as photodegradation [6,7,8], chemical oxidation [9], filtration [10,11], or adsorption [12,13,14]. Among all these treatments, adsorption is now considered an attractive approach due to its high adsorption efficiency, low cost, easy operation, and reusability. As a result, adsorption is widely used to alleviate the harm caused by organic dyes. However, the application of the adsorption method faces limits in terms of the low adsorption capability and inferior separation efficiency caused by the precipitation of traditional adsorbents to the bottom of the container, which reduces the contact between the adsorbent and the dye contaminants [15,16]. Consequently, devising an adsorbent that can remove dyes efficiently, coupled with environmental friendliness and an acceptable mechanical property of reusability, is still a challenging task.

Hydrogels with a three-dimensional (3D) network structure formed by the crosslinking of polymers exhibit an excellent dye removal ability due to their high specific surface area. In particular, a new generation of biomass-based hydrogels have attracted a lot of attention as they are plentiful, inexpensive, sustainable, and biodegradable [17]. Numerous kinds of biomass-based hydrogels, including alginate, chitosan, starch, kaolinite, and cellulose, have been employed to remove pollutants containing organic dyes. Kalidhasan et al. synthesized soy lecithin (SL)–Au (LG) and SL-Au-Cu (LGC) catalysts efficiently and degraded rhodamine B (RB, 100%) in the presence of H₂O₂ under light irradiation [18]. Zhang et al. designed alginate/organo-selenium hydrogel beads for both dye adsorption and bacterial deactivation [19]. Sayyar et al. prepared a low-cost chitosan modified with a montmorillonite/ZnO hydrogel nanocomposite for the adsorption of ciprofloxacin from water [20]. In our country, the biomass-derived raw materials are based on abundant agricultural resources [21,22,23]. Among these natural resources, protein-based adsorbents are especially well received in different sectors due to their prominent properties, such as massive functional groups, biodegradability, biocompatibility, and lower toxicity compared with artificial synthetic polymers [24,25,26,27].

Egg white (EW) is a common material with multifunctional properties, such as gelling, heat setting, foaming, bonding, and emulsifying. As a result, EW is widely used as an ideal ingredient in the food industry. In view of the other advantages of EW, such as its low cost, multiple functions, sustainability, and, eco-friendliness, it has excellent prospects for applications in the preparation of new functional materials. Yan et al. constructed an egg white protein (EWP)–gallic acid–xanthan gum-based emulsion and oleogel [28]. Soy protein isolate (SPI) from soybean consists of about 20 different amino acids, such as lysine, aspartic, leucine, glutamic acid, and tyrosine. SPI with desirable functionalities has been widely applied in biodegradable material preparation. Moreover, SPI with positive chelating properties has been reported as an adsorbent for wastewater treatment. Revadekar et al. developed an SPI–sodium alginate-based aerogel for the efficient uptake of organic dye from effluents [29]. Lu et al. designed a double-crosslinked chitosan sponge encapsulated with ZrO₂–SPI amyloid fibril nanoparticles for the removal of fluoride ions from water [30].

In this study, we propose a novel approach to preparing an EWP/SPI hierarchical-network hydrogel reinforced with calcium alginate using a simultaneous one-step chemical crosslinking. In this hydrogel, the crosslinking of EWP forms the first-order network structure, and the crosslinking of SPI generates the second-order network structure. Alginates are added into hydrogels to build a third-order network and enhance the mechanical properties of hydrogels through crosslinking with Ca²⁺. The synthesized hydrogel with a three-level network can be used as an adsorbent for the removal of organic dyes. The obtained experimental results indicate that the EWP/SPI hydrogel has a favorable adsorption capacity and adsorption efficiency for methylene blue.

2. Materials and Methods

2.1. Materials

Egg white protein (EWP), soybean protein isolate (SPI), sodium alginate (SA), calcium chloride anhydrous (CaCl₂), methylene blue (MB), glutaraldehyde, and hydrochloric acid (HCl) were provided by Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. All experimental reagents were of analytical grade and could be used directly without further purification.

2.2. Preparation of EWP/SPI Hydrogel Beads

To prepare the EWP/SPI hydrogel beads, we firstly prepared an EWP/SPI/sodium alginate mixture. Briefly, 1.0 g SPI and 1.0 g EWP were added into 50.0 mL deionized water to form an EWP/SPI mixture [24,31,32]. Then, the mixture was strongly stirred for 30 min, followed by 20 min ultrasonication using an ultrasonic device with a power of 300 W to uniformly disperse the mixture. After that, the EWP/SPI mixture was added into an SA solution (1 wt%) and stirred for 1 h. The mixture was dropped into a solution with CaCl₂ (3 wt%) and glutaraldehyde (1 wt%) at 95 °C using a syringe pump to obtain EWP/SPI hydrogel beads [33,34,35]. To remove superfluous reagents, the beads were washed several times using deionized water. Finally, EWP/SPI hydrogel beads were desiccated using freeze-drying technology. The synthetic process of EWP/SPI hydrogel beads is exhibited in Figure 1.

2.3. Characterisations

The morphology and chemical compositions of the as-fabricated beads were characterized by scanning electron microscope (SEM) with energy-dispersive spectrometry (EDS). The functional groups were analyzed by Fourier transform infrared spectroscopy (FTIR). The specific surface area within the relative pressure range (P/P₀) was calculated by surface area using Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. The concentrations of MB in the solution were measured by UV–Vis spectrometer (UV–Vis). The thermal stability of the beads was measured with a thermogravimetric analyzer (TGA) from 30 to 600 °C under a nitrogen atmosphere. The static contact angle was measured using a contact angle meter at the room temperature.

2.4. Adsorption Experiments

2.4.1. Adsorption of MB

In total, 10 mg EWP and EWP/SPI hydrogel beads were added into a 50 mL conical flask including 20 mL dye aqueous solution (50 mg∙L⁻¹) with a pH value of 7. The flask was stirred at 25 °C for 60 min. Then, 1 mL solution was taken out and diluted before UV–Vis spectrometry measurement.

2.4.2. Optimization of the Performance of EWP/SPI Hydrogel Beads

To optimize the adsorption performance of EWP/SPI hydrogel beads, influence of adsorption dose, pH, and adsorption time were investigated at 25 °C. Briefly, 5–25 mg beads were added to 20 mL MB solution with a concentration from 50 to 300 mg∙L⁻¹ (pH from 4 to 10) for 480 min in a shaking incubator at 200 rpm. After adsorption completion, the adsorption performance of the beads was characterized by the comparison of the residual MB concentration with the initial concentration using a UV–Vis spectrophotometer. The equilibrium adsorption capacity (q_e, mg∙g⁻¹), adsorption capacity at time t (q_t, mg∙g⁻¹), and removal rate (%) were calculated using the following formulas [31]:

$$q_e={\displaystyle \frac{C_0-C_e}{m}}\times V$$

(1)

$$q_t={\displaystyle \frac{C_0-C_t}{m}}\times V$$

(2)

$$\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}={\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(3)

where C₀ (mg∙L⁻¹) is the initial contaminant concentration, C_e (mg∙L⁻¹) is the MB concentration at the equilibrium, C_t (mg∙L⁻¹) is the MB concentration at time t, V (L) is the volume of the MB solution, and m is the weight of the EWP/SPI hydrogel beads (g).

2.5. Stability of EWP/SPI Hydrogel Beads

2.5.1. Wettability of EWP/SPI Hydrogel Beads

To check the floatability of EWP/SPI hydrogel beads, we firstly evaluated the wettability of beads via measuring the water contact angle (WCA). Briefly, 10 μL deionized water was dropped onto the surface of the EWP/SPI hydrogel and then the WCA was measured using a contact angle meter. Note that at least five locations on the hydrogel were gauged to estimate the average WCA.

2.5.2. Floatability of EWP/SPI Hydrogel Beads

A total of 25 pieces of EWP/SPI hydrogel beads were immersed in 50 mL water with MB for 7 days, and we recorded the number of floating EWP/SPI beads. Floating rate (%) was determined by Equation (4).

$$\mathrm{F}\mathrm{l}\mathrm{o}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}={\displaystyle \frac{F_t}{25}}\times 100$$

(4)

where F_t and 25 are the number of hydrogel beads floating at time t and the initial number of hydrogel beads.

2.6. Recyclability Experiments

The regeneration performance of the beads was determined by five adsorption–desorption cycles. In each cycle, 10 mg EWP/SPI beads were added into a 50 mL conical flask including MB solution (100 mg∙L⁻¹, 20 mL, pH 7) under 120 rpm in a shaker. After an equilibration for 12 h, EWP/SPI beads absorbed with MB were immersed in 1 mol/L HCl for desorbing. After a desorption for 24 h, the beads were filtrated, washed, dried, and reused in the next adsorption cycle.

3. Results and Discussion

3.1. Characterization of EWP/SPI Hydrogel Beads

To analyze adsorption performance of EWP/SPI hydrogel beads, the morphological structures were precisely investigated. As shown in Figure 2a, the surface of pure EWP hydrogel composing of massive lamella exhibited some crosslinking points. The diameters of the EWP hydrogel ranged from 13 to 18 μm (Figure 2c). As a comparison, the surface of EWP/SPI hydrogel beads exhibited an apparent 3D network structure with massive crosslinking points (Figure 2b). The range of pore diameter was from 0.32 to 3.02 μm, which is much smaller than those of EWP, as shown in Figure 2d, indicating that the addition of SPI into the EWP hydrogel decreases the pore diameter and increases the porosity the hydrogel. Based on the above results, the porous network structure may be due to the synergistic effect of the gelling of EWP and SPI, hydrogen bond interaction between EWP and SPI, and the complexation reaction with Ca²⁺ facilitating the crosslink of EWP, SPI with SA, leading to the formation of more porous structures.

The N₂ adsorption/desorption isotherms of EWP and EWP/SPI hydrogel beads are presented in Figure 2e. The specific surface area of EWP/SPI (26.55 m²∙g⁻¹) increased by ~3 times more than EWP (6.50 m²∙g⁻¹). Such increases are consistent with a decrease in mean pore diameter from about 10.89 of EWP (Figure 2g) to 4.89 nm of EWP/SPI (Figure 2h), indicating the primarily mesopore structure in EWP and EWP/SPT hydrogel. The obtained isotherm conforms to typical Langmuir IV type, indicating a mesoporous structure in the beads as well. The N₂ adsorption/desorption amount of EWP/SPI is obviously higher than that of EWP at both low and high pressure. When p/p₀ > 0.5, the adsorption capacity increases sharply, and the desorption curve separates from the adsorption curve, exhibiting an H1 hysteresis loop [36]. This confirms that the hydrogel beads are chiefly composed of mesoporous structures, with capillary condensation of N₂ to fill the mesopores occurred in the adsorption process. Hence, mesopores are favorable for the adsorption of MB molecule via mesopore-filling mechanism.

To investigate the adsorption performance of the EWP/SPI hydrogel beads, EDS was employed to verify the elemental composition of the beads. As shown in Figure 2f, the elements C, O, N, S, Ca, and Cl can be identified in the spectrometry (red curve). C, O, and N are the most common elements of EWP and SPI. The calcium element originates from calcium alginate due to the reaction between sodium alginate and CaCl₂. As a comparison, pure EWP is just composed of C, O, N, and S (blue curve).

To further investigate the adsorption performance of the EWP/SPI hydrogel beads, the functional groups of the beads were characterized using FTIR spectroscopy, as shown in Figure 3a. A broadband peak between 3100 and 3700 cm⁻¹ corresponds to −OH stretching vibration of EWP (in blue curve). A peak at 2960 cm⁻¹ is attributable to stretching vibration of −CH. The peaks at 1650 and 1530 cm⁻¹ are derived from the vibration of C=O and −NH, respectively. The peaks at 1363 and 1070 cm⁻¹ belong to the vibration of C−O and C−O−C. As a comparison, the peaks in the red curve at 1644, 1538, 1420, and 1030 cm⁻¹ belong to C=O stretching (amide I), N−H bending (amide II), C−N stretching (amide III), and C−C, confirming the successful formation of EWP/SPI hydrogel beads.

TGA was used to characterize the thermal stability of the EWP/SPI hydrogel beads. As shown in Figure 3b, the TGA curves of EWP/SPI show a significant amount of weight loss, of about 3.5%, due to dehydration before 85 °C. Between 85 and 321 °C, EWP/SPI hydrogel beads exhibit an obvious mass loss of 33.4% due to the initial decomposition and dehydroxylation of organic components as well as the destruction of hydrogen bonds [29]. In the last stage (321−600 °C), a mass loss of 31.1% from EWP/SPI hydrogel beads is caused by the further decomposition and carbonization of organic components in EWP/SPI hydrogel beads. Comparison with the residue of EWP/SPI (32%), the residue content of EWP hydrogel is lower (28.9%). These obtained results declare a higher thermal stability of EWP/SPI hydrogel beads, probably attributed to the formation of hydrogen bonds between EWP, SPI, and SA.

3.2. Adsorption Performance of EWP/SPI Hydrogel Beads

Figure 4 shows the UV–Vis spectra of MB in contact with EWP, EWP/SPI hydrogel beads. The UV–Vis absorption peak at 665 nm was used to monitor the changes in concentration before and after the treatment of various hydrogels. The UV–Vis absorption spectra of MB treatment with EWP and EWP/SPI hydrogel beads for 60 min are shown in Figure 4. The absorbance of MB treated with EWP/SPI is lower than that treated with EWP, suggesting that EWP/SPI hydrogel beads adsorb much more MB compared with EWP hydrogel. This result indicates that the EWP/SPI hydrogel beads can enhance the adsorption capacity of the hydrogel compared with pure EWP hydrogel through the synergistic effect between EWP, SPI, and SA.

To maximize the adsorption capacity of the prepared beads, we investigated the influence of adsorption parameters (adsorbent dosage, pH, and time) in the adsorption process. The effects of absorbent dosage on adsorption capacity and percentage contaminant removal were analyzed by adding 5–25 mg EWP/SPI beads into 100 mg∙L⁻¹ MB solution with a volume of 20 mL. As shown in Figure 5a, with the increase in the amount of the adsorbent, the removal rate increased from 75 to 99% due to the increase in the adsorption sites and surface area. However, with the increase in the dose of the adsorbent, q_e decreased from 300 to 79.2 mg∙g⁻¹. The decrease in q_e arose mainly due to the lower utilization of active sites employed to adsorb MB molecules on the adsorbent surface.

To evaluate the influence of pH in the solution, the adsorption capacity of the EWP/SPI beads was investigated across a pH range of 4 to 10, shown in Figure 5b. When the pH increased from 4 to 10, the MB adsorption capacity increased from 60 to 194 mg∙g⁻¹. The low adsorption capacity in acidic condition is attributed to positively charged H⁺ occupying the adsorption sites and competing with cationic dyes in the adsorption process. With an increase in solution pH, the carboxyl group and hydroxyl group of EWP/SPI beads were deprotonated and enhanced the interactions with the MB cation. Therefore, weakly basic solutions were chosen for further measurements.

To evaluate the adsorption efficiency of EWP/SPI hydrogel beads in the adsorption process, we investigated the relation of adsorption capacity to contact time. As shown in Figure 6, the adsorption capacity of EWP/SPI hydrogel beads MB sharply increased to 180.0 mg∙g⁻¹ within 60 min and eventually reached an equilibrium concentration of 188.7 mg∙g⁻¹ at approximately 480 min at 25 °C with a pH value of 7. At the beginning of the adsorption, the fast adsorption rate was owing to abundant adsorption sites on EWP/SPI hydrogel beads, resulting from the porous structure and the massive functional groups on the surface of the hydrogel. This boosted intraparticle diffusion and efficient adsorption of the MB molecule. Subsequently, after 60 min, the adsorption rate lowered due to a decrease in available active sites on the EWP/SPI hydrogel beads, leading to a dynamic adsorption–desorption process in the beads.

3.3. Adsorption Kinetics of EWP/SPI Hydrogel Beads

To better understand the adsorption process, adsorption kinetics was used to analyze the adsorption reaction of EWP/SPI hydrogel beads. The adsorption process was studied using a pseudo-first-order kinetic model, a pseudo-second-order kinetic model, the Elovich kinetic model, and the intraparticle diffusion model. The pseudo-first-order model (Equation (5)), pseudo-second-order model (Equation (6)), Elovich kinetic model (Equation (7)), and intraparticle diffusion model (Equation (8)) are formulated as follows [31]:

$$q_t=q_e(1-e^{-k_{1}t})$$

(5)

$$q_t={\displaystyle \frac{q_e^2{k}_{2}t}{1+{k_{2}q}_{e}t}}$$

(6)

$$q_t={\displaystyle \frac{\mathrm{l}\mathrm{n}\left(\alpha \beta \right)}{\beta }}+{\displaystyle \frac{\mathrm{l}\mathrm{n}\left(t\right)}{\beta }}$$

(7)

$$q_t=k_i{t}^{1/2}+c_i$$

(8)

where q_t and q_e in the equilibrium are the adsorption capacity at time t and equilibrium adsorption capacity. k₁ (min⁻¹) and k₂ (g·mg⁻¹·min⁻¹) are the rate constant of the pseudo-first-order model and the pseudo-second-order model. α (mg·g⁻¹·min⁻¹) denotes the initial adsorption rates, β (g·mg⁻¹) refers to the parameters related to chemisorption activation energy and surface coverage extent. k_i (mg·g⁻¹·min^−1/2) is the rate constant of the intraparticle diffusion model and c_i (mg mg⁻¹·g) is a constant associated with the thickness of the boundary layer.

As seen in Figure 7a, the pseudo-first-order model, pseudo-second-order model, and Elovich kinetic model were simulated by nonlinear regression analysis. The kinetic parameters and correlation coefficients are summarized in

Table 1. Kinetics parameters of MB adsorption on EWP/SPI hydrogel beads.

Kinetic Model	Parameter	Values
Pseudo-first-order kinetic model	k1 (min−1)	0.0546
	qe (mg·g−1)	187.495
	R2	0.985
Pseudo-second-order kinetic model	k2 × 104 (g·mg−1·g−1)	4.212
	qe (mg·g−1)	198.417
	R2	0.95293
Elovich kinetic model	α (mg·g−1·min−1)	190.590
	β (g·mg−1)	0.04051
	R2	0.875
Intraparticle diffusion model	ki,1 (mg·g−1·min−1/2)	19.561
	ci,1 (mg·g−1)	1.0883
	R2	0.908
	ki,2 (mg·g−1·min−1/2)	0.337
	ci,2 (mg·g−1)	180.952
	R2	0.904

. Compared with the pseudo-second-order model (R² = 0.953) and Elovich kinetic model (R² = 0.875), the pseudo-first-order model (R² = 0.985) is more consistent, with the kinetic adsorption data describing the adsorption process. To further explore the interaction of the EWP/SPI beads and MB, all data were fitted by the intraparticle diffusion model. As shown in Figure 7b, the fitting line is nonlinear and can be divided into two parts with different slopes. The first stage (R² = 0.908), with a slope of 19.561, is a rapid adsorption stage due to transportation of the MB from the solution to the surface of the EWP/SPI beads in this process. In the second stage, the slope of the fitting line (R² = 0.904) is reduced to 0.337, which implies that the adsorption process is much slower than the first stage. This stage is dominated by the reaction between the EWP/SPI beads and the MB.

3.4. Adsorption Isotherm of EWP/SPI Hydrogel Beads

To further explain the interaction between solid EWP/SPI beads and MB molecule in solution, the adsorption isotherm models (Langmuir (Equation (9)), Freundlich (Equation (10)), and Temkin (Equation (11)) were used to perform nonlinear fitting to these experimental data and explore the adsorption equilibrium in the adsorption process. The Langmuir model assumes monolayer formation at specific homogeneous sites within the adsorbent, while the Freundlich model refers to the heterogeneity of adsorption sites and multilayer adsorption [37]. Finally, the Temkin model considers indirect interactions between adsorbent and adsorbate molecules.

The expressions of the adsorption isotherm models are presented below [31,36].

$$q_e={\displaystyle \frac{q_m{K}_L{C}_e}{1+K_L{c}_e}}$$

(9)

$$q_e=K_f{C}_e^{1/n}$$

(10)

$$q_e=B_T\mathrm{l}\mathrm{n}\left(A_T{C}_e\right)$$

(11)

Here, K_L (L·g⁻¹) and K_f (mg^1−1/n·L^1/n·g⁻¹) represent the absorption constants of the Langmuir model, the Freundlich model, and the Temkin model, respectively. B_T (J·mol⁻¹) is the energy constant in the Temkin isotherm model. C_e is the concentration of MB at equilibrium. A_T describes (L·mg⁻¹) the equilibrium binding constant. n represents the degree of irregularity of the adsorbent surface.

The equilibrium adsorption relationships between MB molecule and EWP/SPI hydrogel beads are explained using the adsorption isotherm. The adsorption isotherm models are fitted using the nonlinear regression technique to avoid some errors in linearization. The parameters of the adsorption isotherms are listed in

Table 2. Isotherm parameters of MB adsorption on EWP/SPI hydrogel beads.

Isotherm Models	Parameter	Values
Langmuir	qm (mg·g−1)	336.265
	KL (L·mg−1)	0.183
	R2	0.989
Freundlich	Kf (mg1−1/n·L1/n·g1)	107.353
	n	3.909
	R2	0.910
Temkin	BT (J·mol−1)	61.083
	AT (L·mg−1)	2.677
	R2	0.949

. As shown in Figure 8, all the isotherm models had a good correlation with the experimental data. The correlation coefficient (R²) of the isotherm models ranged from 0.910 to 0.989. Among the three isotherm models, the adsorption process expressed a greater consistency with the Langmuir model (R² = 0.989) compared with the other two models, indicating monolayer adsorption on EWP/SPI hydrogel beads with effective adsorption ability at each adsorption point. According to the fitting results, the adsorption of MB on the EWP/SPI beads was conducted through reaction with oxygen-containing functional groups. The value of the separation factor (R_L) was calculated to be 0.052, indicating spontaneous and favorable adsorption. The maximum adsorption capacity of the EWP/SPI beads calculated by the Langmuir model was 336.265 mg·g⁻¹.

3.5. Thermodynamic Studies of EWP/SPI Hydrogel Beads

To further analyze the adsorption mechanisms of MB on the EWP/SPI beads, the change in thermodynamic adsorption parameters (Gibbs free energy (G), entropy (S), and enthalpy (H)) were calculated the following mathematical expressions [31,36]:

$$∆G=∆H-T∆S$$

(12)

$$\ln \left({\displaystyle \frac{q_e}{C_e}}\right)=-{\displaystyle \frac{∆H}{RT}}+{\displaystyle \frac{∆S}{R}}$$

(13)

In the expression, T (K) is absolute temperature in Kelvin, and R (8.314 J·mol⁻¹·K⁻¹) is the universal gas constant. The values of ΔH and ΔS can be calculated from the linear fit of expressions (14) using ln(q_e/c_e) versus 1/T. The thermodynamic parameters of MB adsorption are estimated using the Van’t Hoff equation (Equation (13)). The calculated results of ΔH, ΔS, and ΔG values are −40.085 kJ·mol⁻¹, −105.955 J·mol⁻¹·K⁻¹, and −8.511 kJ·mol⁻¹. The value of ΔG is negative (ΔG < 0) and between −20 to 0 kJ/mol at experimental temperatures, which indicates that this adsorption process occurred spontaneously and physisorption predominated the adsorption of MB. Moreover, the absolute value of increases with decrease in temperature suggests that the physisorption predominated adsorption and lower temperatures are conducive to the adsorption of MB [38]. The value of ΔH is also negative, which implies that the adsorption process is exothermic. The negative value of entropy changes shows that the adsorption reduces disorder at the solid–solute interface.

To further understand adsorption mechanism, the Dubinin–Radushkevich (D–R) isotherm model was employed to describe adsorption on both heterogeneous and homogenous surfaces. The D–R model is expressed as follows:

$$\mathrm{l}\mathrm{n}{q}_e=ln{q}_m-B{\epsilon }^2$$

(14)

Here, B represents a constant related to the mean free energy of adsorption (mol²∙kJ⁻²), q_m refers the theoretical saturation capacity, and ε represents the Polanyi potential, which is calculated as follows:

$$\epsilon =RT\mathrm{l}\mathrm{n}(1+{\displaystyle \frac{1}{C}})$$

(15)

T (K) refers to the absolute temperature, and R (J mol⁻¹∙K⁻¹) indicates the gas constant. The mean energy of adsorption (E) can be estimated using the following expression:

$$E={\displaystyle \frac{1}{\sqrt{-2B}}}$$

(16)

According to Equations (14)–(16), E is calculated as 2.017 kJ∙mol⁻¹, which is below 5 kJ∙mol⁻¹, providing important information regarding adsorption mechanism. Note that the value of E ranges from 1 to 8 kJ∙mol⁻¹, 8 to 16 kJ∙mol⁻¹, and 20 to 40 kJ∙mol⁻¹, where the adsorption process corresponds to physical adsorption, ion exchange, and chemisorption, respectively. E is calculated as 2.017 kJ∙mol⁻¹, which is below 5 kJ∙mol⁻¹, validating that the physical adsorption dominates the adsorption process of EWP/SPI towards MB.

3.6. Floatability Performance of EWP/SPI Hydrogel Beads

EWP/SPI hydrogel beads are completely hydrophilic as the WCA is ~0°and absorbed water droplets immediately when the water droplet is placed on the surface of the hydrogel (inset of Figure 9). The floatability of hydrogel beads is favorable for MB absorption and separation after the adsorption process [33]. Figure 9 shows the floating performance of EWP/SPI hydrogel beads in water over 7 days. The floatation rates of beads are 100% for the first two days, followed by a decrease with the increase in immersion time. Even after soaking for 7 days, the floating rates of all beads are still higher than 80%. These results imply that the EWP/SPI hydrogel beads provide great potential for reuse during practical applications.

3.7. Regeneration Performance of EWP/SPI Hydrogel Beads

To evaluate the regeneration performance of EWP/SPI hydrogel beads, a repetitious adsorption–desorption of MB was performed for five cycles, where 10 mg EWP/SPI beads were added to the MB solution with a concentration of 100 mg·mL⁻¹. After an adsorption of 12 h, EWP/SPI beads with MB were filtered and immersed in 1 mol·L⁻¹ HCl to desorb the absorbed MB. Under strongly acidic condition, the interaction between MB and EWP/SPI hydrogel beads was electrostatic repulsion. The H⁺ derived from HCl interacted with the adsorptive site EWP/SPI hydrogel beads more strongly, leading to the desorption of MB. After a careful collection and cleaning, the beads were employed for repeated absorption. Then the adsorption capacities were calculated and compared with the initial value. The adsorption capacity and adsorption efficiency of EWP/SPI hydrogel beads is shown in Figure 10. After five adsorption–desorption cycles, the adsorption efficiency of EWP/SPI beads decreased to 82%. Thus, the EWP/SPI beads are adsorbent with preferable reusability.

FTIR spectra of EWP/SPI adsorbing MB (the first cycle) and EWP/SPI (after desorption of MB) were performed to gain more adsorption mechanisms in Figure 11. Compared with EWP/SPI hydrogel beads, a new absorption band at 3420 cm⁻¹ in the blue curve of MB adsorbed EWP/SPI, indicating an overlapping peak from −OH stretching vibration and -NH in the MB molecule. A decrease in peak intensity at 3420 cm⁻¹ of −OH and −NH indicates a hydrogen bonding interaction between −NH and −OH. Furthermore, a decrease in peak intensity at 1650 (C=O), 1030 cm⁻¹ (C−O), and a shift from 1650 to 1640 cm⁻¹ indicates the successful adsorption of MB molecules on EWP/SPI hydrogel beads. After a desorption of MB from EWP/SPI using HCl, the disappearance of characteristic absorption peak at 3420 cm⁻¹ (-NH), and increase in peak intensity at 1650 (C=O) and 1030 cm⁻¹ (C−O), confirm the successful desorption of MB from EWP/SPI hydrogel beads.

3.8. Comparison of Adsorption Performance of EWP/SPI Hydrogel Beads with Other Adsorbents

EWP/SPI beads showed a relatively high adsorption capacity for MB. Based on the Langmuir model, the maximum adsorption capacity of EWP/SPI hydrogel beads for MB was 336.265 mg g⁻¹. A comparison of adsorption capacity with other reported adsorbents is listed in

Table 3. Comparison of maximum adsorption of EWP/SPI hydrogel beads with various adsorbents.

Type of Adsorbent	qm (mg·g−1)	References
EWP/graphene oxide	91.7	[24]
Polyvinyl alcohol-alginate/bentonite	51.34	[39]
Modified soybean straw activated carbon	41.96	[40]
Superabsorbent polymer-activated charcoal	213.2	[41]
EWP/SPI hydrogel	336.265	This work

. The adsorption capacity of EWP/SPI hydrogel beads was larger than that of EWP/graphene oxide (91.7 mg∙g⁻¹) and superabsorbent polymer-activated charcoal (213.2 mg∙g⁻¹), and much higher than that of polyvinyl alcohol-alginate/bentonite (51.34 mg∙g⁻¹) and modified soybean straw activated carbon (41.96 mg∙g⁻¹). These data declare that the synthesized EWP/SPI hydrogel beads can be employed as an adsorbent for MB.

4. Conclusions

In this paper, low-cost and efficient EWP/SPI hydrogel beads reinforced with calcium alginate were successfully synthesized by a one-step simple chemical crosslinking. Here, the crosslinking of SPI formed a first-order network, and then the crosslinking of EWP component formed a second-order network. The addition of alginate into hydrogels acted as a framework and mechanical property reinforcement agent to form a third-order network and enhance the mechanical property of hydrogels through crosslinking with Ca²⁺. The as-prepared EWP/SPI hydrogel beads exhibited a distinctive 3D porous structure with excellent structural stability and floatability. The maximum adsorption capacity of EWP/SPI hydrogel beads towards MB was 336.265 mg∙g⁻¹. Moreover, the prepared hydrogel beads exhibited obvious reusability after five cycles of adsorption–desorption. These results indicate that the EWP/SPI hydrogel beads show a favorable adsorption performance in water treatment. We envision that as-prepared beads offer a possibility for generating multifunctional beads applied in water disposal.