Hydroxypropyl-β-Cyclodextrin Improves Removal of Polycyclic Aromatic Hydrocarbons by Fe₃O₄ Nanocomposites

Abstract

The contamination of water bodies by polycyclic aromatic hydrocarbons (PAHs) poses a significant concern for the ecological systems, along with public health. Magnetic adsorption stands out as a green and practical solution for treating polluted water. To make the process more efficient and economical, it is important to create materials that not only absorb contaminants effectively but also allow for easy recovery and reuse. This study proposes a simple yet effective method for coating Fe₃O₄ nanoparticles with hydroxypropyl-β-cyclodextrin polymer (HP-β-CDCP). The physicochemical properties of the synthesized sorbent were characterized using a transmission electron microscope (TEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and Vibrating Sample Magnetometer (VSM) analysis. The adsorption performance of HP-β-CDCP/Fe₃O₄ nanoparticles was well-described by the pseudo-second-order kinetic model, thermodynamic analysis, and the Freundlich isotherm model, indicating multiple interaction mechanisms with PAHs, such as π–π interactions, hydrogen bonding, and van der Waals forces. Using HP-β-CDCP/Fe₃O₄ nanoparticles as the adsorbent, the purification rates for the fifteen representative PAHs were achieved within the range of 33.9–93.1%, compared to 15.3–64.8% of the unmodified Fe₃O₄ nanoparticles. The adsorption of all studied PAHs onto HP-β-CDCP/Fe₃O₄ nanocomposites was governed by pH, time, and temperature. Equilibrium in the uptake mechanism was obtained within 15 min, with the largest adsorption capacities for PAHs in competitive adsorption mode being 6.46–19.0 mg·g⁻¹ at 20 °C, pH 7.0. This study points to the practical value of incorporating cyclodextrins into tailored polymer frameworks for improving the removal of PAHs from polluted water.

1. Introduction

Pollution of water bodies with polycyclic aromatic hydrocarbons (PAHs) has raised widespread concern, largely because of the serious risks they pose to both the biosphere and human populations [1,2]. These compounds are commonly released into the environment through a range of industrial processes, such as oil refineries, gas plants, coal tar plants, etc. [3,4]. These compounds are resistant to degradation/biodegradation and may remain in the environment for a long time [5]. Benzopyrene was officially classified as a Group 1 carcinogen (indicating conclusive evidence of carcinogenicity in humans) by the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) in 2017. Currently, 16 PAHs were designated as priority control pollutants in the United States, with eight of these compounds included in China’s list of environmental priority control pollutants, underscoring their recognized risk to human health and the environment [6,7,8]. Accordingly, the implementation of reliable remediation approaches for their extraction from aquatic matrices is essential.

Their limited solubility and resistance to breakdown make PAHs difficult to eliminate using common methods like oxidation and ultrasound-based treatments [9,10]. Among various treatment strategies, adsorption stands out for its efficiency and low cost. Activated carbon and resins are commonly employed to remove PAHs from contaminated water [11,12]. However, the prolonged processing time and elevated expenses associated with these materials restrict their use for large-scale wastewater treatment [13]. Magnetic adsorption is recognized as a simplified and low-cost approach to removing pollutants from wastewater [14,15]. For example, Liu et al. reported the use of ionic liquids-functionalized magnetic nanocomposites as sorbents to isolate five PAHs [16]. Hashemi et al. evaluated magnetic activated carbon aimed at removing aniline from contaminated water [17]. Badruddoza et al. used cyclodextrin polymer-functionalized magnetic nanocomposites as sorbents to selectively extract heavy metals from industrial effluents [18]. Even with the advances achieved so far, scholars are still seeking magnetic adsorbents that offer both high efficiency and low production cost.

Cyclodextrins are cyclic oligosaccharides consisting of 6(a), 7(β), 8(γ) glucopyranose units linked via (1–4) linkages. They adopt a torus-shaped ring structure featuring a nonpolar internal cavity, with primary hydroxyl groups positioned externally and secondary hydroxyl groups located within [19]. Hydroxypropyl-β-cyclodextrin is the derivative of cyclodextrin [20]. It demonstrates the ability to encapsulate a diverse spectrum of organic molecules via inclusion complex formation [21,22,23]. Magnetic nanocomposites have shown promising results for removing heavy metals from wastewater. However, their potential for adsorbing PAHs in environmental contexts remains largely overlooked [24,25].

For the purpose of enhancing the adsorbability of Fe₃O₄ nanoparticles, hydroxypropyl-β-cyclodextrin polymer (HP-β-CDCP) was prepared and applied to graft onto Fe₃O₄ nanoparticles. By grafting the polymer onto magnetic nanoparticles, a recyclable and easily retrievable adsorbent was developed, which effectively removed a range of fifteen PAHs (including naphthalene (NaP), fluorene (FLu), acenaphthene (Ana), anthracene (Ant), benzo[a]anthracene (BaA), benzo[b]fluoranthene (BbF), benzo[k] fluoranthene (BkF), benzo[ghi]perylene (BpE), benzo[a]pyrene (BaP), chrysene (ChR), dibenz[a,h]anthracene (DbA), fluoranthene (Flt), indeno[1,2,3-cd]pyrene (Ipy), phenanthrene (Phe), and pyrene (Pyr)) from contaminated water. Figure 1 illustrates the chemical structures of fifteen PAHs. Attempts were intended to clarify the effects of adsorption conditions and to explore the reusability of these nanocomposites.

2. Materials and Methods

2.1. Chemicals and Reagents

PAHs were purchased from O2Si Smart Solutions (Charleston, SC, USA) with a concentration of 100 mg·L⁻¹ and a purity was 99.5%. A standard stock solution of 200 μg·mL⁻¹ was prepared in acetonitrile, and working solutions were freshly obtained by diluting the stock solution with the mobile phase prior to use.

Hydroxypropyl-β-cyclodextrin, carbinol, ethanol, epoxy chloropropane, acetone, sodium chloroacetate, ferrisulphas, and ferric trichloride were sourced from Shanghai Chemical Reagent Corporation (Shanghai, China). Acetonitrile was obtained from Merck Reagents (Darmstadt, Germany). Acetonitrile utilized in this study was of high-performance liquid chromatography (HPLC) grade, whereas the remaining solvents and chemicals conformed to analytical-grade standards. Ultrapure water was obtained via the Milli-Q™ Advantage A10 water purification system (Madrid, Spain).

2.2. Synthesis of HP-β-CDCP/Fe₃O₄ Nanocomposites

The nanocomposites were synthesized by the protocol shown in Figure 2A. Carboxymethyl-HP-β-CDCP was produced according to Badruddoza et al. [18]. HP-β-CDCP/Fe₃O₄ nanocomposites were prepared through a single-step co-precipitation technique. A total of 1.21 g of FeSO₄.7H₂O, 1.42 g FeCl₃, and 3.0 g carboxymethyl-HP-β-CDCP were dispersed in 40 mL of ultrapure water and stirred at 1200 rpm. After heating the solution to 90 °C, 5 mL of NH₄OH (25%) was introduced. Resulting nanoparticles were prepared after stirring for 1 h.

2.3. Analytical Technique

Magnetic solid-phase extraction procedure is shown in Figure 2B. First, HP-β-CDCP/Fe₃O₄ nanocomposites were introduced to a 100 mL water sample with fifteen kinds of PAHs (pH 7.0), and the product was transferred to a platform shaker at low speed and equilibrated for 15 min at 30 °C. A neodymium magnet was positioned under the conical flask, extracting the nanocomposites from the suspension. The liquid supernatant was analyzed with an HPLC system equipped with a fluorescence detector (FLD), model Agilent 1260, purchased from Agilent Technologies, Inc. (Santa Clara, CA, USA). The Agilent 1260 detector thermal condition was remained maintained at 25 °C and 2.2 mL min⁻¹. A mixture of acetonitrile and water was adopted as the mobile phase.

Table 1. The gradient elution program.

	Time/Min	0.00	0.66	20.0	25.0	27.0	30.0
Mobile Phase
Acetonitrile (%)		40	40	100	100	40	40
Water (%)		60	60	0	0	60	60

outlines the gradient elution program, while

Table 2. The time program of fluorescence detection.

	Time/Min	0.0	11.5	12.5	13.5	14.5	19.0	25.2
Wavelength
λex (nm)		280	254	254	290	270	290	305
λem (nm)		340	350	400	460	390	410	500

summarizes the time program of fluorescence detection.

2.4. Adsorption Kinetics

In the procedure, 60 mg (m) of HP-β-CDCP/Fe₃O₄ nanocomposites was added into a centrifuge tube with each PAH (BaP and Ant) of 100 mL (V) of working solution. To promote efficient adsorbate–adsorbent interaction, the solution was oscillated at 20 °C across a series of contact times (1, 2, 4, 6, 8, 12, 15, 20, 25, and 30 min). After extraction, the adsorbent was magnetically extracted, and the residual content of PAHs was analyzed using HPLC. The removal rate of PAHs was quantified as follows:

$$R\% = {\displaystyle \frac{{C_{0 }- C}_{\mathrm{e}}}{C_0}}\times 100\%$$

(1)

where R represents the removal rate of PAHs, and C_e and C₀ represent the ultimate concentration and initial concentration of PAHs (μg·L⁻¹), respectively:

$$Q_t = {\displaystyle \frac{c_0 - c_t}{m}}V$$

(2)

Q_t (mg·g⁻¹) represents the adsorption capacity of the adsorbent for each PAH as a function of time, and C_t (μg·L⁻¹) represents the concentration of ions remaining in the solution at time t (h):

$$Q_{\mathrm{e}}={\displaystyle \frac{c_0 - c_{\mathrm{e}}}{m}}V$$

(3)

Q_e (mg·g⁻¹) represents the equilibrium constant.

The kinetic models of pseudo-first and pseudo-second order were evaluated. The parameters of these models are expressed as follows:

$$\mathrm{Pseudo-First} \mathrm{Order} \mathrm{kinetic} \mathrm{model}: Q_t = Q_{\mathrm{e}}(1- e^{-k_{1}t})$$

(4)

k₁ represents the rate constant (min⁻¹).

$$\mathrm{Pseudo-Second} \mathrm{Order} \mathrm{kinetic} \mathrm{model}: Q_t={\displaystyle \frac{t}{(\frac{1}{{k_{2}Q}_{\mathrm{e}}^2})+\frac{t}{Q_{\mathrm{e}}}}}$$

(5)

k₂ represents the constant rate (g·mg⁻¹·min⁻¹).

2.5. Adsorption Isotherms

The isothermal adsorption model could be used to analyze the mechanism of equilibrium adsorption between solid and liquid. At 298 K, by changing the initial concentration of PAHs, the maximum adsorption capacity of HP-β-CDCP/Fe₃O₄ nanocomposites was investigated, and the Langmuir (6) and the Freundlich (7) isotherm models were utilized for fitting:

$${\displaystyle \frac{C_{\mathrm{e}}}{Q_{\mathrm{e}}}}={\displaystyle \frac{1}{K_{\mathrm{L}}{Q}_{m\mathrm{a}\mathrm{x}}}}+{\displaystyle \frac{C_{\mathrm{e}}}{Q_{m\mathrm{a}\mathrm{x}}}}$$

(6)

$$\ln Q_{\mathrm{e}}=\ln K_{\mathrm{F}}+({\displaystyle \frac{1}{n_{\mathrm{F}}}})\ln C_{\mathrm{e}}$$

(7)

In these equations, C_e represents the equilibrium concentration (mg·L⁻¹), and Q_e and Q_max represent the equilibrium adsorption amount and the maximum adsorption amount (mg·g⁻¹), respectively. K_L was the Langmuir constant (L·mg⁻¹), n_F and K_F were the Freundlich constants (mg⁽¹⁻ⁿ⁾·g⁻¹·Lⁿ).

2.6. Adsorption Thermodynamics

In order to investigate whether the adsorption process on HP-β-CDCP/Fe₃O₄ nanocomposites was endothermic or exothermic and whether it was a spontaneous process, the thermodynamic parameters of adsorption were calculated. The calculation of thermodynamic parameters under standard conditions was as follows:

$$k_{\mathrm{d}}={\displaystyle \frac{(C_0-C_{\mathrm{e}})\times V}{m\times C_{\mathrm{e}}}}$$

(8)

$$\ln k_{\mathrm{d}}={\displaystyle \frac{\Delta S^{\theta }}{R}}-{\displaystyle \frac{\Delta H^{\theta }}{RT}}$$

(9)

$$\Delta {\mathrm{G}}^{\theta }=\Delta H^{\theta }-T\Delta S^{\theta }$$

(10)

Among them, ΔS^θ was the entropy change (J·(mol·K)⁻¹), ΔH^θ was the enthalpy change (J·(mol·K)⁻¹), R was the gas constant (8.3145 J·mol⁻¹·K⁻¹), k_d was the distribution coefficient, and ΔG^θ was the Gibbs free energy (kJ·mol⁻¹).

3. Results

3.1. Characterization of HP-β-CDCP/Fe₃O₄ Nanocomposites

Figure 3A illustrates the transmission electron microscopy (TEM) images of HP-β-CDCP/Fe₃O₄. As exhibited in the images, the nanoparticles possessed a well-defined spherical core–shell structure, with an average diameter of 20 nm (statistically calculated from more than 50 randomly selected particles in the TEM field of view), which was consistent with the typical size range of cyclodextrin-functionalized magnetic nanomaterials reported in the literature [26].

Figure 3B presents FTIR spectra of HP-β-CDCP/Fe₃O₄ nanocomposites (curve a), Fe₃O₄ nanocomposites (curve b), carboxymethyl-HP-β-CDCP (curve c), HP-β-CDCP (curve d), and hydroxypropyl-β-cyclodextrin (curve e). Characterization data are provided below.

Carboxymethyl-hydroxypropyl-β-cyclodextrin polymer (curve c): A stretching vibration peak of C=O (ν(C=O)) appeared at 1710 cm⁻¹, and a stretching vibration peak of O–H (ν(O–H)) was observed at 3200 cm⁻¹. These peaks were consistent with the characteristic infrared responses of carboxyl groups (–COOH), confirming that –COOH has been successfully grafted onto the backbone of the hydroxypropyl-β-cyclodextrin polymer.

Hydroxypropyl-β-cyclodextrin polymer/Fe₃O₄ nanocomposites (curve a): The absorption peak at 586 cm⁻¹ corresponded to the characteristic vibration of Fe–O bonds in the tetrahedral sites of Fe₃O₄ (a typical infrared marker for magnetic nanoparticles). Meanwhile, C–H stretching vibration peaks (ν(C–H)) were detected at 2400 cm⁻¹ and 2850 cm⁻¹, along with a C=O vibration peak at 1628 cm⁻¹ (which underwent a redshift compared to curve c, attributed to the coordination interaction between C=O and the Fe₃O₄ surface). The coexistence of these characteristic peaks, combined with the observation that only the 586 cm⁻¹ peak exists in curve b (pure Fe₃O₄), confirmed that the carboxymethyl-hydroxypropyl-β-cyclodextrin polymer has been successfully coated on the surface of Fe₃O₄ nanoparticles.

Figure 3C shows the carbon mass fraction (C%) in hydroxypropyl-β-cyclodextrin polymer-coated Fe₃O₄ nanocomposites as a function of the molar feed ratio (n) of hydroxypropyl-β-cyclodextrin polymer to FeSO₄·7H₂O. As n increased from 0 to 1, C% rose sharply from 0.2 to 0.8: this rapid increase was attributed to the progressive coating of the Fe₃O₄ surface by the carbon-rich cyclodextrin polymer. When n was further increased to 2, C% reached approximately 1.0 and then stabilized. The observed plateau suggested that the Fe₃O₄ nanoparticles were fully encapsulated by the polymer, as the finite surface area of Fe₃O₄ restricts further polymer adsorption—consistent with the limited number of coordination sites available on Fe₃O₄ for polymer binding.

As shown in Figure 3D, the XRD pattern of the HP-β-CDCP/Fe₃O₄ nanocomposites exhibits six distinct diffraction peaks at 2θ values of 30.29°, 35.33°, 44.23°, 54.10°, 57.42°, and 63.92°, which correspond well with the reference pattern of Fe₃O₄ (JCPDS card No. 19-629) from the International Centre for Diffraction Data.

Figure 3E shows the hysteresis curves of Fe₃O₄ particles and HP-β-CDCP/Fe₃O₄ nanocomposites, respectively. The measured saturation magnetization values for the two materials were 74.6 emu·g⁻¹ and 36.8 emu·g⁻¹, respectively. The coercivity values of both adsorbents were found to be close to zero, indicating excellent superparamagnetic behavior. After modification of Fe₃O₄ nanoparticles with non-magnetic HP-β-CDCP, a reduction in saturation magnetization was observed. However, the HP-β-CDCP/Fe₃O₄ nanocomposites still maintained strong magnetic properties, enabling effective magnetic separation under an external magnetic field.

To analyze the specific surface area and porosity of the material, N₂ adsorption–desorption isotherms were performed, with the corresponding results presented in Figure 3F. As shown in Figure 3F, the N₂ adsorption–desorption isotherm of the material exhibited a Type IV isotherm with H3 hysteresis loop observed in the relative pressure (P/P₀) range of 0.4–1.0, indicating a mesoporous structure (pore size range: 2–50 nm). Based on the isotherm data, the Brunauer–Emmett–Teller (BET) specific surface area was equal to 91.5 m²·g⁻¹, and the total pore volume was calculated as 0.230 cm³·g⁻¹.

3.2. Effects of Operation Parameters on PAH Removal

3.2.1. Effect of HP-β-CDCP

In this study, Fe₃O₄ nanocomposites and HP-β-CDCP/Fe₃O₄ nanocomposites were synthesized. The adsorption performance of PAHs with HP-β-CDCP/Fe₃O₄ nanocomposites as sorbent was significantly elevated compared to Fe₃O₄ nanocomposites in Figure 4A, due to the inclusion interaction between PAHs and HP-β-CDCP of HP-β-CDCP/Fe₃O₄ nanocomposites.

3.2.2. Effect of pH

The solution’s pH impacts the adsorption process, especially the efficiency of the adsorbent–analyte interaction. The influence of initial solution pH for the adsorption of fifteen PAHs onto HP-β-CDCP/Fe₃O₄ nanocomposites was investigated at pH 4.0–9.0. Based on the removal performance for all analytes (Figure 4B), we could come to the following conclusions. The adsorption efficiency of PAHs enhanced with the pH surged from 4.0 to 7.0. The extraction efficiency of NaP, Flu, Ana, and Phe increased further with a further surge in pH and reached the maximum at pH 8.0; meanwhile, the extraction efficiency of other PAHs decreased gradually. The removal performance of all analytes onto HP-β-CDCP/Fe₃O₄ nanocomposites reached almost the optimal value (33.9–93.1%) in pH 7.0. The experimental results demonstrated that the cavity size and hydrophobicity of the HP-β-CDCP/Fe₃O₄ nanocomposites were closely correlated with the protonation state of the surface hydroxyl groups. Under acidic conditions, protonation of the hydroxyl moieties elevated the cavity polarity, which attenuated the hydrophobic interactions between the cavity and PAHs and, consequently, impaired the inclusion efficiency. At neutral pH, the HP-β-CD cavities exhibited an optimal hydrophobic microenvironment, which facilitated the embedding of PAH molecules and thus maximized the host–guest inclusion effect. In alkaline media, deprotonation of the hydroxyl groups induced subtle alterations in the cavity structure. This conformational change favored the encapsulation of specific PAHs with matching molecular sizes (e.g., NaP and Flu), while diminishing the encapsulation affinity for PAHs with mismatched dimensions. So, pH 7.0 was selected for this work.

3.2.3. Effect of Extraction Time

To investigate the influence of extraction time on the removal efficiency of PAHs, various durations ranging from 1 to 30 min were tested. The extraction efficiency increased with time and plateaued after 15 min, indicating equilibrium. Therefore, 15 min was selected as the optimal extraction time.

3.2.4. Effect of Nano-Adsorbent Dosage

Amount of HP-β-CDCP/Fe₃O₄ nanocomposite impact on the adsorption efficiency of PAHs. Different amounts of nanocomposites were analyzed with 20~80 mg (Figure 4C). An increase in HP-β-CDCP/Fe₃O₄ nanocomposites led to a corresponding rise in adsorption efficiency, and this reached the maximum when the amount of nanocomposites was 60 mg. A further increase in adsorbent dosage (exceeding 60 mg) did not improve the adsorption efficiency, which was attributed to the aggregation of nanocomposite particles. Excessive adsorbents tended to form aggregates in the solution, which reduced the specific surface area accessible to PAHs and hindered the diffusion of PAH molecules to the internal active sites, resulting in the plateau of adsorption efficiency. Therefore, 60 mg was identified as the most effective.

3.2.5. Effect of Sample Volume

For improved removal performance in reduced operational time, the sample volume was studied from 30 to 200 mL with a fixed amount of PAHs and nanocomposites. As the sample volume > 100 mL, the extraction performance decreased in Figure 4D. This study used a sample volume of 100 mL.

3.3. Adsorption Kinetics of HP-β-CDCP/Fe₃O₄ Nanocomposites

To evaluate the adsorption kinetics of the HP-β-CDCP/Fe₃O₄ nanocomposites, benzo[a]pyrene (BaP) and anthracene (Ant) were selected as representative PAHs. The kinetic behavior was simulated via both pseudo-first-order and pseudo-second-order equations (Figure 5A,B). The kinetic parameters obtained from the fitting results are detailed in

Table 3. Fitting analysis for adsorption kinetics constant of HP-β-CDCP/Fe₃O₄ nanocomposites.

Model	Parameter	Value
		BaP	Ant
Pseudo-first order kinetic model	R2	0.9877	0.9801
	k1	0.237	0.195
	Qe	18.3	15.6
Pseudo-second order kinetic model	R2	0.9965	0.9929
	k2	0.015	0.070
	Qe	18.3	15.6

. Among the two models, the pseudo-second-order model demonstrated a better fit, with higher correlation coefficients (R²), suggesting it more accurately describes the extraction process. This outcome indicates that the adsorption mechanism of HP-β-CDCP/Fe₃O₄ involves a combination of physisorption and chemisorption processes [27]. As in Figure 5C, the Elovich model demonstrated enhanced fitting performance, indicating that the adsorption process was controlled by chemical adsorption mechanisms. These mechanisms involve specific interactions between heterogeneous active sites on the composite surface and PAH molecules. The hydrophobic cavity of HP-β-CD can encapsulate the benzene rings of PAHs through hydrophobic effects and van der Waals forces, resulting in the formation of host–guest complexes. This interaction exhibits a certain degree of specificity and can be regarded as a type of weak chemical adsorption.

3.4. Adsorption Isotherms of HP-β-CDCP/Fe₃O₄ Nanocomposites

The isothermal adsorption model was employed to investigate the equilibrium adsorption mechanism between solid and liquid phases. At 298.15 K, the maximum adsorption capacity of HP-β-CDCP/Fe₃O₄ nanocomposites was evaluated by varying the initial concentrations of Ant and Bap. The experimental data were fitted using the Langmuir Equation (6) and the Freundlich Equation (7), and the data results are presented in

Table 4. Fitting results of adsorption isotherm constants of Ant and Bap on HP-β-CDCP/Fe₃O₄ nanocomposites.

Analyst	Langmuir Model			Freundlich Model			Elovich Model
	Qmax	KL	R2L	nF	KF	R2F	a	b	R2
	(mg·g−1)
Ant	16.8	0.55	0.9747	1.17	12.75	0.9838	6.87	0.227	0.9564
Bap	19.2	0.21	0.9840	1.45	15.63	0.9903	10.9	0.208	0.9534

and Figure 5D,E. Comparative analysis of the linear correlation coefficients revealed that the R² values for PAHs in the Freundlich isothermal model (Ant: 0.9838; Bap: 0.9903) were higher than those in the Langmuir model (Ant: 0.9747; Bap: 0.9838), suggesting a heterogeneous adsorption surface and the occurrence of multilayer adsorption. Furthermore, all 1/n_F values were less than 1, indicating a nonlinear relationship between analyte concentration and adsorption capacity. This implies that while analyte concentration influences adsorption, other factors—such as chemical interactions—also contribute significantly, thereby enhancing the overall adsorption potential.

3.5. Adsorption Thermodynamics of HP-β-CDCP/Fe₃O₄ Nanocomposites

The adsorptions of Ant and Bap onto HP-β-CDCP/Fe₃O₄ nanocomposites were examined at 298.15, 308.15, and 318.15 K. Thermodynamic parameters, calculated using Equations (8)–(10), were summarized in

Table 5. Parameters of thermodynamics.

Analyst	ΔHθ/ (kJ·mol−1)	ΔSθ/(J (mol·K)−1)	ΔGθ/(kJ·mol−1)
			298.15 K	308.15 K	318.15 K
Ant	−5.76	1.83	−6.29	−6.37	−6.32
Bap	−5.35	4.42	−6.67	−6.70	−6.76

. The negative ΔG values indicated a spontaneous adsorption process. Given that ΔG lies between –20 kJ mol⁻¹ and 0 kJ mol⁻¹, the adsorption of PAHs was interpreted as predominantly physical, such as π–π interactions, hydrogen bonding, and Van der Waals forces. The negative ΔH further confirmed the exothermic nature of adsorption. In addition, the positive ΔS suggested that adsorptions of both Ant and Bap were spontaneous across the studied temperature range.

3.6. Analyses

3.6.1. Regeneration and Reusability Study

To study the regeneration and reusability potential of these nanocomposites as an adsorbent, desorption experiments were conducted. Acetonitrile was, consequently, chosen as the optimal eluent, and the nanocomposites were rinsed with 3 mL acetonitrile twice following each PAH’s adsorption. PAHs’ adsorptions on the nanocomposites are dominated by hydrophobic interactions and host–guest inclusion complex formation between PAHs’ molecules and the hydrophobic cavities of HP-β-CD. Acetonitrile’s solubility parameter (24.5 MPa^1/2) was highly compatible with that of PAHs (18.0–22.0 MPa^1/2), facilitating strong solvation of PAHs and promoting their transfer from HP-β-CD cavities to the eluent phase. According to Figure 6, the highest removal efficiencies (%) for PAHs on the surface of the HP-β-CDCP/Fe₃O₄ nanocomposites were achieved in the 1st cycle. After the 6th cycle, the removal efficiencies have exhibited a decline of 3% to 6%. This decrease in the removal efficiency may be attributed to the damage of some adsorption sites and the blockage of the others due to the failure to desorb the PAHs as a result of the strong interaction between PAHs and HP-β-CDCP/Fe₃O₄ nanocomposites. Recyclability tests demonstrated that the prepared nanoadsorbents can be used many successive times for the removal of PAHs from wastewater.

3.6.2. Adsorption Capacity

The adsorption capacity (i.e., the equilibrium dosage of PAHs adsorbed per gram of the nanocomposites in a competitive system) is an important factor for evaluation of the HP-β-CDCP/Fe₃O₄ nanocomposites. In this study, the maximum uptakes for NaP, FLu, Ana, Phe, Ant, Flt, Pyr, BaA, ChR, BbF, BkF, BaP, DbA, BpE, and Ipy in competitive adsorption mode were 6.46 mg·g⁻¹, 10.2 mg·g⁻¹, 11.8 mg·g⁻¹, 13.6 mg·g⁻¹, 15.4 mg·g⁻¹, 16.6 mg·g⁻¹, 17.6 mg·g⁻¹, 18.1 mg·g⁻¹, 17.9 mg·g⁻¹, 18.2 mg·g⁻¹, 18.5 mg·g⁻¹, 18.4 mg·g⁻¹, 18.0 mg·g⁻¹, 18.6 mg·g⁻¹, and 19.0 mg·g⁻¹, respectively.

3.6.3. Analysis of Simulated Wastewater

To validate its practical applicability, the adopted approach was employed to quantify PAHs in simulated wastewater. Figure 7 illustrates HPLC-FLD chromatograms of wastewater samples before and after extraction. By comparing the peak area of curve a and curve b, the absorption efficiency is enhanced with the increasing ring number of PAHs. This indicated that the clathration is a key factor influencing this adsorption process. Correction curve, correlation coefficient, and removal efficiency of PAHs are listed in

Table 6. The correction curve, correlation coefficient, and removal efficiency of fifteen kinds of PAHS.

Compound	Molecular Formula	Molecular Weight	Correction Curve	R2	Removal Efficiency (%)
NaP	C10H8	128.17	y = 135.11x + 0.68	0.9999	33.9
FLu	C13H10	166.22	y = 231.56x + 0.97	0.9998	51.5
Ana	C12H10	154.21	y = 419.60x + 3.94	0.9997	58.5
Phe	C14H10	178.23	y = 411.59x + 3.36	0.9998	68.0
Ant	C14H10	178.23	y = 1555.9x + 12.11	1	77.3
Flt	C16H10	202.25	y = 115.4x + 0.40	1	83.2
Pyr	C16H10	202.25	y = 597.09x − 0.12	0.9998	86.3
BaA	C18H12	228.29	y = 737.14x − 0.10	0.9998	89.7
ChR	C18H12	228.29	y = 688.39x + 0.044	0.9998	89.6
BbF	C22H12	252.31	y = 139.27x + 0.84	1	90.1
BkF	C22H12	252.31	y = 1108x + 5.21	0.9999	91.8
BaP	C20H12	252.3	y = 1022.4x − 11.88	0.9998	91.9
DbA	C22H14	278.35	y = 602.19x − 1.48	0.9997	90.3
BpE	C22H12	276.33	y = 294.04x − 2.33	0.9991	92.1
Ipy	C22H12	276.33	y = 55.38x − 0.31	1	93.1

.

3.7. Comparative Study

To evaluate the adsorption performance of the HP-β-CDCP/Fe₃O₄ nanocomposite, a comparative analysis was conducted against various adsorbents documented in the existing literature. The findings are presented in

Table 7. Comparison of PAHs’ removal performance of different adsorbents.

Material	Pollutant	Adsorption Quantity (mg·g−1)	Removal Efficiency (%)	Ref.
Granular activated carbon (GAC)	Ana, Phe	2.63, 7.36	-	[28]
G-mCS/GO	NaP, Ant, Flt	11.9, 15.8, 15.4	93.6, 89.2, 62.3	[29]
DNA magnetic nanoparticles	Phe	0.220	88.0	[30]
Polyester microfibers	3- and 4-ring PAHs	0.0250–1.42	62.0–88.0	[31]
β-CD-MSNs	Pyr, Ant, Phe, Flu, Flt	0.3–1.65	-	[32]
HP-β-CDCP/Fe3O4 nanocomposites	Fifteen representative PAHs	6.46–19.0	33.9–93.1	This work

. Conventional adsorbents, such as GAC, exhibited relatively weak adsorption capacities toward PAHs. Although the G-mCS/GO composite demonstrated superior adsorption performance for Nap compared with HP-β-CDCP/Fe₃O₄ nanocomposites, the latter displayed a stronger affinity for PAHs with polycyclic skeletons (e.g., Flt). Furthermore, the adsorptive performance of the material developed in this work outperforms that of β-CD-MSNs, which could be attributed to the fact that β-CD polymers provided more binding sites for PAHs than β-CD monomers, while the incorporation of Fe₃O₄ endowed the composite with magnetically separable capability. When benchmarked against emerging adsorbents reported in recent years (e.g., DNA magnetic nanoparticles and polyester microfibers), HP-β-CDCP/Fe₃O₄ nanocomposites also show advantages in both adsorption capacity and applicable PAH range. The results demonstrated that the HP-β-CDCP/Fe₃O₄ nanocomposite developed herein exhibits distinct advantages in the adsorption capacity towards PAHs, coupled with superior overall adsorption performance. These characteristics indicate that the HP-β-CDCP/Fe₃O₄ nanocomposite represents a promising high-performance adsorbent for the remediation of trace persistent organic pollutants in wastewater.

4. Conclusions

In this study, Fe₃O₄ nanocomposites modified with HP-β-CDCP were prepared and demonstrated excellent adsorption capacity for PAHs in wastewater. The pH, temperature, and extraction time that altered the interaction of PAHs with the nanocomposites were evaluated by single-factor tests. The adsorption kinetics were evaluated using pseudo-first-order and pseudo-second-order models, with the results showing that the pseudo-second-order model more accurately described the adsorption behavior of PAHs. Isotherm data were fitted to both Langmuir and Freundlich models, and the Freundlich model provided a better fit to the experimental data, indicating heterogeneous surface adsorption with multilayer characteristics. Thermodynamic analysis revealed that the adsorption of PAHs was spontaneous and exothermic, involving multiple interaction mechanisms such as π–π interactions, hydrogen bonding, and van der Waals forces. This nano-adsorbent showed exceptional adsorption performance and enabled efficient extraction of PAHs from large sample volumes within a short time. Therefore, we can expect extensive application of HP-β-CDCP/Fe₃O₄ nanocomposites in wastewater treatment.