Nanoparticles Composed of β-Cyclodextrin and Sodium p-Styrenesulfonate for the Reversible Symmetric Adsorption of Rhodamine B

Abstract

Nanomaterials have been extensively employed for the efficient removal of the cationic dye rhodamine B (RhB) from aqueous solutions. However, challenges such as complex synthesis processes, limited adsorption capacity, and poor cycling stability remain to be addressed. In this research, a novel nanoparticle-based β-cyclodextrin and sodium p-styrenesulfonate composite (M-β-SCDP) was synthesized via a two-step method to enhance its adsorption capabilities for RhB from water. The modification resulted in a material enriched with active sites (−OH, −SO³⁻) and a mesoporous structure, greatly enhancing its adsorption capacity to 2392.34 mg·g⁻¹ for RhB removal from water solutions. The M-β-SCDP exhibited excellent reversible symmetric adsorption, which remained stable after 10 regeneration cycles with no loss in adsorption capacity. The simple manufacturing process, along with its effective adsorption capabilities and outstanding reusability, indicates that M-β-SCDP has great potential for real-world use in efficiently treating RhB in water.

1. Introduction

Organic dyes are widely used in sectors like textiles, printing, and cosmetics because of their vivid colors and remarkable longevity. Nonetheless, they are major contaminants in industrial wastewater, presenting severe risks to the environment and human health. Rhodamine B (RhB), a cationic xanthene dye that is hydrophilic, is one of these dyes and has been identified as a potential carcinogen, being associated with eye, gastrointestinal, skin, and respiratory irritations and infections [1,2]. Given the hazards associated with RhB, it is imperative to treat wastewater effectively to support industrial production, enhance product quality, protect the human environment, and maintain ecological balance.

At present, various methods have been created to effectively eliminate dyes from wastewater, including adsorption, biodegradation, ion exchange, advanced oxidation processes (AOPs), and membrane filtration [3,4,5,6,7,8]. Among these methods, adsorption stands out for its cost-effectiveness, efficiency, and environmentally safe properties [9,10,11]. Nevertheless, the application of traditional adsorption materials, including hematite, montmorillonite, and activated carbon, is constrained by several limitations such as narrow pore size distribution, low porosity, limited adsorption capacity, poor selectivity, and high energy demands for recycling and regeneration [12,13,14]. Therefore, it is crucial to create new adsorbents with multi-scale pore structures, high porosity, and numerous binding sites to improve adsorption capacity.

β-Cyclodextrin (β-CD) is a ring-shaped oligosaccharide that traps pollutants within its cavity, creating specific host–guest complexes [15,16,17,18]. However, its limited solubility in water constrains its application in wastewater treatment. Therefore, creating insoluble β-CD derivatives via polymerization, cross-linking, or immobilization methods is crucial for improving efficient and selective separation and purification processes [19,20,21,22,23,24]. Rohith et al. have recently advanced the field by synthesizing a green citric acid cross-linked β-CD polymeric sorbent aimed at efficiently removing RhB from wastewater, and the maximum degree of sorption of RhB was 96.43%. However, this adsorbent suffered severe regeneration efficiency decay, with the RhB removal rate dropping from 96.43% to 58.83% after three cycles [25]. Li et al. also designed a β-CD porous polymer that features a large pore volume, ideal for efficiently eliminating aromatic pollutants from wastewater. The distribution coefficient of the adsorbent was in the range of 10³~10⁶ mL·g⁻¹, much higher than those of other β-cyclodextrin-based adsorbents. However, the reagents employed in the adsorbent preparation process were highly toxic, and the reaction conditions were exceptionally stringent, which resulted in significant challenges for scaling up to industrial production [26]. Ultimately, the primary aim of formulating β-CD derivatives as adsorbents is to obtain the porous structure and the abundant active sites to improve the adsorption efficiency of RhB. In recent years, novel adsorbents functionalized with sulfonate groups (-SO₃⁻) have been fabricated for the removal of cationic dyes. Hong et al. prepared an adsorbent with sulfonate-functionalized graphene, which demonstrated outstanding adsorption capacity (675 mg·g⁻¹) towards RhB. Nevertheless, the reaction conditions were very demanding, and it was difficult to achieve large-scale production [27]. Therefore, it is significant to prepare the novel adsorbents functionalized with sulfonate groups and β-CD through a straightforward synthesis route for the removal of RhB. Furthermore, the synergistic effect of sulfonate groups and β-CD can construct abundant active sites and porous structures to enhance the adsorption capacity and the reversible symmetric adsorption of the adsorbent during the removal of RhB from water.

In this research, we created a novel mesoporous adsorbent (M-β-SCDP) through the addition polymerization of sodium p-styrenesulfonate (SS) and β-cyclodextrin derivatives modified with methacrylic anhydride (MA). The adsorption efficiency of M-β-SCDP was thoroughly analyzed, considering the influence of RhB pH, concentration, contact time and temperature. Additionally, the adsorption behavior of RhB on M-β-SCDP was explained using isothermal models, adsorption kinetics, and thermodynamic analysis. This research provides both practical and theoretical insights for creating effective adsorption materials, showing great promise for cleaning water polluted with organic dyes.

2. Materials and Methods

2.1. Materials

Methacrylic anhydride (MA, 94%, RHAWN, Shanghai, China), β-Cyclodextrin (β-CD, 98%, RHAWN, Shanghai, China), triethylamine (TEA, 99%, RHAWN, Shanghai, China), sodium p-styrenesulfonate (SS, 90%, RHAWN, Shanghai, China), ammonium persulfate (APS, 98.5%, RHAWN, Shanghai, China), N,N-dimethylformamide (DMF, AR, 97%, RHAWN, Shanghai, China), and methanol (MeOH, AR, 99.5%, RHAWN, Shanghai, China) were prepared. Aqueous solutions of rhodamine B (RhB) were prepared using deionized water at varying initial concentrations. All chemical solvents underwent purification and drying through a distillation purification system.

2.2. Preparation of M-β-CD

A 500 mL flask was used to mix 11.35 g (0.01 mol) of β-CD and 2 g (0.02 mol) of TEA in 200 mL of DMF. Subsequently, 1.54 g (0.01 mol) of MA was added to the mixed liquid. The flask containing the aforementioned mixture was allowed to react for 2 days at 0 °C. The resulting mixture was then dialyzed in water for 2 days and subsequently dried at 50 °C for 6 h to produce M-β-CD [28].

2.3. Preparation of M-β-SCDP

The M-β-CD (5 g), SS (5 g), and APS (0.2 g) were added in 100 mL DMF, and then the mixture was stirred mechanically while being heated to 80 °C. After reacting for 1 h, the mixture was filtered, and excess DMF was employed to eliminate any remaining M-β-CD, SS, and APS. The recovered solids underwent freeze-drying for 48 h to obtain M-β-SCDP. The prepared samples were named M-β-SCDPx according to different stirring rates (x being 500 rpm, 1000 rpm, and 2000 rpm, respectively). And the synthetic strategy of M-β-SCDP was shown in Scheme 1.

2.4. Adsorption Experiments

Aqueous solutions of RhB with varying concentrations (50~1000 mg·L⁻¹) and varying dosages (20~100 mg) were prepared at a certain volume (V = 100 mL) to explore the adsorption properties of the adsorbents. The pH (2~12) of RhB solutions was altered with HCl and NaOH, each having a concentration of 0.1 mol·L⁻¹. Furthermore, the thermodynamics for RhB by M-β-SCDP were determined by batch experiments. The experiments for adsorption were conducted in reagent bottles with M-β-SCDP agitated at 150 rpm by a shaker equipped with a thermostatic water bath. The adsorbent’s equilibrium adsorption capacity was calculated using the following equation [29,30]:

$$q_e=(C_0-C_e)\times V/m$$

(1)

In this context, q_e (mg·g⁻¹) denotes the equilibrium adsorption capacity, and C₀ and C_e (mg·L⁻¹) indicate the initial and equilibrium concentrations of pollutant, respectively; V (L) refers to the water volume; m (g) signifies the adsorbent dose.

The regeneration of the adsorbent after pollutant adsorption was studied by using a certain volume of MeOH for desorption. A UV/VIS/NIR spectrometer (Lambda35, High Wycombe, UK) was selected to collect the UV-vis absorption spectra of the solutions. The adsorbents were separated through a centrifugal machine (SC-3616, Hefei, China) after the adsorption of RhB with the loss rate of blank experiments less than 1%. The experiments were repeated multiple times, and the average outcomes were recorded.

2.5. Characterization

The chemical structures of the samples were identified by Fourier Transform Infrared Spectroscopy (FTIR Spectrum two, High Wycombe, UK) with the range of 4000–400 cm⁻¹. To verify the findings from FTIR spectroscopies, ¹³C NMR (AVANCE400, Ettlingen, Germany) and XPS spectrometers (ESCALAB250, Waltham, MA, USA) were employed. The Brunauer–Emmett–Teller (Micromeritics ASAP 2020 V4, Norcross, GA, USA) method was used to estimate the pore width and volume of the materials. Field emission scanning electron microscopy (FESEM) (SU8010, Tokyo, Japan) was used to examine the morphologies of the specimens that were prepared.

3. Results and Discussion

3.1. Structural Characterization

M-β-CD was created through an esterification reaction involving MA and β-CD, and its chemical structure was identified using FTIR, as depicted in Figure 1a. A broad and characteristic peak around 3410 cm⁻¹ in the FTIR spectrum of M-β-CD corresponds to the presence of hydroxyl groups (–OH) [31,32]. In contrast to β-CD, the 1725 cm⁻¹ is linked to the O-C=O stretching vibration of the methacrylate group in M-β-CD [28,33]. The FTIR of M-β-SCDP, as shown in Figure 1b, revealed new peaks at 1186 and 1047 cm⁻¹, associated with the S=O vibration from -SO³⁻ groups [34,35]. These characteristic peaks represent the distinctive absorption of SS, confirming its successful incorporation into the M-β-SCDP.

As shown in Figure 1c, the ¹³C MAS-NMR spectra of M-β-SCDP exhibited resonances at δ = 98.9 ppm (C1), 45.8 ppm (C2), 41.5 ppm (C3), 46.5 ppm (C4), 74.4 ppm (C5), and 66.6 ppm (C6), which are associated with β-CD spectra. Resonance at δ = 131.5 ppm (C-7), 121.62 ppm (C-8), 120.13 ppm (C-9), and 165.36 ppm (C-10) corresponded to the spectra of SS. A broad signal was observed at δ = 178.9 ppm (C11) and 25.8 ppm (C12), indicating that the carbocation addition reaction occurred between SS and M-β-CD [36].

Furthermore, to ascertain the elemental composition of M-β-SCDP, XPS analysis was undertaken, with the results shown in Figure 2. The survey spectrum (Figure 2a) confirmed the presence of oxygen (O), carbon (C) and sulfur (S). As depicted in Figure 2b, C-C (284.53 eV), C-O (285.63 eV), and C=O (287.51 eV) could be detected in C 1s spectrum. The peaks at 169.80 eV (S 2p₁/₂) and 168.47 eV (S 2p₃/₂) are specifically associated with sulfates derived from the SS, as illustrated in Figure 2c and Table S1 [37]. This could further assist in the successful integration of SS monomers onto M-β-SCDP. The morphologies of the as-prepared specimens were investigated through FESEM, as shown in Figure 2d–f. It was found that different morphologies were clearly presented for the specimens, which were dependent on the stirring rate of the reaction mixtures. The porous structure of the specimens could be observed, which are beneficial to enhance their adsorption capacities.

The porous characteristics of the materials were investigated using nitrogen sorption measurements. As shown in Figure 3a, there was a steady increase in uptake beginning from a relative pressure (P/P₀) of 0.045 and reaching 0.85 for M-β-SCDP500, M-β-SCDP1000, and M-β-SCDP2000. This behavior suggests the presence of mesopores within these materials [38]. This inference is further corroborated by the pore size distribution results, which indicated the presence of mesopores ranging from 2 to 50 nm, as illustrated in Figure 3b and Table S2. As previously discussed, the synthesized M-β-SCDP materials possessed mesoporous structures, which facilitate the utilization of active adsorption sites [39]. This finding prompted further investigation into their adsorption properties concerning pollutants.

3.2. Evaluation of Adsorption Performance of M-β-SCDP for RhB

3.2.1. Solution pH

To investigate whether the adsorption performance is influenced by environmental conditions, specifically pH, various pH levels were selected for the RhB solutions. The effectiveness of M-β-SCDPs (M-β-SCDP500, M-β-SCDP1000, and M-β-SCDP2000) in adsorbing RhB was analyzed with pH levels from 2 to 12, with adjustments made using 0.1 mol·L⁻¹ HCl and NaOH solutions. The RhB removal efficiency of M-β-SCDPs at different pH levels is presented in Figure 4a, indicating that RhB removal was pH-dependent. The maximum removal efficiency for RhB was observed at pH 4, with a removal rate exceeding 95%. In theory, the pH level of RhB solution can affect the dissociation of functional groups at the active sites of the M-β-SCDPs, surface charges, and the molecular form of RhB in the solution.

As shown in Figure 4b, the zeta potential of M-β-SCDPs was negative, which contributed to the electrostatic interaction towards RhB at lower pH values. The protonation of hydroxy and sulfonic acid groups in M-β-SCDPs led to repulsive interactions with RhB, restricting the adsorption capacity for RhB. As the pH increased, the degree of electrostatic repulsion diminished, while electrostatic attractions between RhB and the adsorbents were enhanced due to reduced protonation of the hydroxy groups, resulting in an increased removal efficiency for RhB. At pH levels above 4, the hydroxy groups in M-β-SCDPs and the dimethylamine group in RhB were likely to lose protons, limiting the adsorption of RhB. Moreover, the adsorption capacity of M-β-SCDPs stayed mostly consistent due to the plentiful binding sites present on the adsorbent surfaces. Consequently, M-β-SCDP500 was selected for further investigation into kinetics, adsorption isotherms, thermodynamics, removal mechanisms, and recyclability.

3.2.2. Adsorbent Dosage

Research into the variation of adsorbent quantities is essential in adsorption studies, as it influences the effectiveness of pollutant removal and the economic feasibility of the treatment method. This research analyzed how the adsorption of Rhodamine B (RhB) was influenced by different adsorbent loadings, using varying amounts of adsorbent material (20, 30, 40, 50, 60, 70, 80, 100 mg). Figure 4c displays how the adsorption efficiency of RhB varied with different dosages of M-β-SCDP500. When the amount of M-β-SCDP500 was raised from 0.02 g to 0.1 g, the efficiency of RhB removal increased from 74.76% to 99.32%, but the capacity of RhB for adsorption dropped from 149.52 mg·g⁻¹ to 39.73 mg·g⁻¹. The decrease in adsorption capacity is due to the higher M-β-SCDP500 dosage, which offers more active sites, improving RhB removal efficiency, but eventually causing adsorbent saturation. Consequently, an adsorbent dosage of 40 mg was identified as the optimal amount for batch experiments.

3.2.3. Kinetic Studies

The adsorption kinetics of M-β-SCDP for pollutants were investigated across varying contact times. As illustrated in Figure 5, the adsorbents demonstrated rapid adsorption capabilities, achieving a removal efficiency exceeding 70% within 1 min and reaching equilibrium uptake within 30 min. This indicates that pollutants can be effectively adsorbed by M-β-SCDP500 in a relatively short period. The outstanding adsorption capability of M-β-SCDP500 is due to the plentiful functional groups on nanoparticle surfaces and their mesoporous architecture. The kinetic parameters for adsorption were obtained using pseudo-first-order, pseudo-second-order, and particle diffusion models, shown in

Table 1. Adsorption of RhB by M-β-SCDP500 at different contact times.

Adsorbent	Pollutant	Pseudo-First Order			Pseudo-Second Order			Particle Diffusion Model
		q1e (mg·g−1)	K1 (min−1)	R2	q2e (mg·g−1)	K2 (g·mg−1·min−1)	R2	Kp	C	R2
M-β-SCDP500	RhB	479.16	1.31	0.974	497.51	$4.15\times {10}^{-3}$	0.999	11.38	399.60	0.581

and explained by Equations (2)–(4) [40,41].

$$\mathrm{Pseudo}\text{-}\mathrm{first} \mathrm{order}: \ln \left(q_e-q_t\right)=ln{q}_e-K_{1}t$$

(2)

$$\mathrm{Pseudo}\text{-}\sec \mathrm{ond} \mathrm{order}: t/q_t=1/K_2{q}_e^2+t/q_e$$

(3)

$$\mathrm{Particle} \mathrm{diffusion} \mathrm{model}: q_t=K_p{t}^{1/2}+C$$

(4)

q_e and q_t represent the quantities of pollutants adsorbed at equilibrium and at a specific contact time t (min), respectively; K₁ (min⁻¹) represents the pseudo-first-order rate constant, while K₂ (g·mg⁻¹·min⁻¹) represents the pseudo-second-order kinetic rate constant; K_p represents the diffusion rate constant (mg·(g·min^1/2)⁻¹) within particles, and C represents the adsorption constant (mg·g⁻¹) in the particle diffusion model. Figure 5 and Table 1 display the relative curves and parameters based on the fitting results.

The adsorption kinetics of M-β-SCDP500 for RhB are illustrated in Figure 6. The pseudo-second-order model (R² of 0.999) provided a better fit for the adsorption kinetics of RhB on M-β-SCDP500 than the pseudo-first-order model, which had an R² of 0.974. This points to the adsorption process being in agreement with chemical adsorption.

3.2.4. Adsorption Isotherm

Typically, to evaluate an adsorbent’s adsorption capacity, the adsorption equilibrium is analyzed at different pollutant concentrations under a constant temperature in 30 min. To determine isotherm parameters, the Langmuir and Freundlich isotherm models are frequently used, as shown in Equations (5) and (6) [42,43].

$$\mathrm{Langmuir} \mathrm{model}: C_e/q_e=1/b{q}_m+C_e/q_m$$

(5)

$$\mathrm{Freundlich} \mathrm{model}: ln{q}_e=ln{K}_F+{\displaystyle \frac{1}{n}}ln{C}_e$$

(6)

C_e (mg·L⁻¹) represents the concentration of the RhB solution at equilibrium, while q_e (mg·g⁻¹) denotes the adsorption capacity at this concentration, and q_m (mg·g⁻¹) is the highest adsorption capacity; b (L·mg⁻¹) and K_F (mg^1−1/n·L^1/n·g⁻¹) are the constants of the Langmuir and Freundlich models, respectively; 1/n represents the intensity of adsorption.

The Langmuir model describes the adsorbent surface as having multiple identical adsorption sites, all with equal affinity for the adsorbate. The occupation of one active site does not influence adjacent sites. In contrast, the Freundlich model portrays the M-β-SCDP500 surface as varied, enabling multilayer adsorption interactions with adsorbate molecules. In this model, stronger active sites are preferentially occupied, leading to a decrease in adsorbent affinity for adsorbate molecules as surface coverage increases [44].

Compared to the Freundlich model, which had a correlation coefficient (R²) of 0.792, the Langmuir isotherm model, characterized by an R² of 0.998, showed a superior fit for the adsorption of RhB by M-β-SCDP500. This discovery suggests that the Langmuir model better represents the RhB adsorption process, implying that pollutants were adsorbed on a uniform monolayer surface of the adsorbents. As detailed in

Table 2. The adsorption capacity of M-β-SCDP500 for RhB compared to other adsorbents.

Adsorbents	Pollutants	qm (mg·g−1)	Reference
Quercetin-based novel porous organic polymer (QPOP)	RhB	499.52	[9]
Bamboo-based columnar activated carbons (BAC-10)	RhB	300.3	[11]
Groundnut husk biochar (GHB)	RhB	182.24	[45]
Mesoporous manganese silicate (MH)	RhB	420.63	[46]
Carbon nanotube composites (SCTNs/AC)	RhB	3242.04	[47]
M-β-SCDP500	RhB	2392.34	This work

, M-β-SCDP500 has an adsorption capacity for RhB of 2392.34 mg·g⁻¹, which is higher than that of many other adsorbents reported in the literature.

3.2.5. Thermodynamic Studies and Recyclability

In this investigation, different temperatures were chosen to study the impact of temperature on RhB nanoparticle adsorption, allowing for the determination of thermodynamic parameters and the assessment of temperature effects. The adsorption capacity of M-β-SCDP500 for RhB exhibited a significant increase with rising solution temperatures.

Table 3. Thermodynamic characteristics of RhB adsorption on M-β-SCDP500 across different temperatures.

Adsorbents	Pollutant	ΔH° (kJ·mol−1)	ΔS° (J·mol−1·K−1)	ΔG° (kJ·mol−1)
				298.15 K	308.15 K	313.15 K
M-β-SCDP500	RhB	12.17	78.46	−11.21	−24.18	−24.96

displays the thermodynamic parameters for RhB adsorption at different temperatures, calculated using Equations (7)–(9) [48].

$$K_d=q_e/C_e$$

(7)

$$ln{K}_d={∆S}^o/R-∆H^o/RT$$

(8)

$${∆G}^o=-RTln{K}_d$$

(9)

C_e represents the concentration at equilibrium (mg·L⁻¹); q_e denotes the quantity of pollutants adsorbed per gram of adsorbent at equilibrium (mg·g⁻¹); K_d is the constant for adsorption equilibrium; T represents the absolute temperature measured in Kelvin; and R stands for the gas constant, which is 8.314 J·mol⁻¹·K⁻¹. ΔH° and ΔS° were derived from the slope and intercept of the linear regression of lnK_d against the inverse of T (Figure 7d). Equation (9) was used to calculate the values of ΔG° at various temperatures.

Table 3 displays positive ΔH° values for RhB adsorption, indicating that these processes were endothermic. Conversely, negative ΔG° values for the adsorbate suggests that the process was thermodynamically spontaneous. Similarly, the ΔS° value for the adsorption of RhB was positive, demonstrating heightened randomness at the M-β-SCDP500–solution interface throughout the adsorption process, which compensated for the absorbed heat and drove the adsorption process at increased temperatures, ultimately achieving the spontaneous and efficient adsorption of Rhodamine B [49].

The adsorbents demonstrated outstanding adsorption performance, as illustrated in Figure 8. However, the reversible symmetric adsorption of adsorbents is crucial for practical applications. To evaluate the reusability of the adsorbents, M-β-SCDP500 with adsorbed pollutants was soaked in methanol (MeOH) for 30 min at room temperature to release RhB. Afterward, the adsorbents were extracted from the solutions through filtration and reused to eliminate RhB from water. Figure 8a indicates that the R_t value remained largely unchanged after 10 recycling cycles without any loss of adsorption capacity. Thus, considering the energy-demanding and degradative regeneration methods for activated carbons (ACs) [50], which also exhibit excellent adsorption performance, the M-β-SCDP500 adsorbents show great potential for effectively removing RhB from water and being reused efficiently.

3.2.6. Adsorption Mechanism

As illustrated in Figure 8b, FTIR was selected to display the chemical bonds of M-β-SCDP in both the presence and absence of RhB. A new stretching vibration of C=N with relatively weak intensity was detected at 1710 cm⁻¹ in M-β-SCDP@RhB, indicating that the RhB molecule has been adsorbed onto the M-β-SCDP [51,52]. The emergence of these peaks implies that RhB were physically encapsulated within the pores of M-β-SCDP, not merely adsorbed onto the surface. Additionally, the hydroxyl group (-OH) bands in M-β-SCDP@RhB exhibited a slight upward shift and increased broadness compared to those in pure M-β-SCDP [53], indicating that hydrogen bonds were formed between M-β-SCDP and RhB. Furthermore, considering the results of the influence of RhB solution pH values on the adsorption process and the zeta potential of M-β-SCDP in Figure 4b, the adsorption process of M-β-SCDP for RhB also includes electrostatic interaction. Consequently, the adsorption mechanism of M-β-SCDP for the removal of RhB mainly included hydrogen bond interaction, and electrostatic interaction.

4. Conclusions

A novel adsorbent with rapid adsorption capabilities and exceptional performance was synthesized based on β-cyclodextrin, methacrylic anhydride, and p-styrenesulfonate. The fabricated specimens demonstrated a highly porous structure, contributing to their rapid and remarkable adsorption capacity for RhB, measured at 2392.34 mg·g⁻¹, along with outstanding recyclability. Adsorption kinetics and isotherm experiments revealed that the adsorption process mainly relies on electrostatic adsorption, complexation, and hydrogen-bonding interaction. Additionally, M-β-SCDP exhibited excellent cyclic adsorption and desorption performance, with adsorption efficiency remaining virtually unchanged with no loss of adsorption capacity. In results, M-β-SCDP shows promise for the effective treatment of organic and inorganic pollutants in water because of its simple fabrication, excellent adsorption performance, and reversible adsorption symmetry.