Magnetic Ti3C2 MXene Nanomaterials for Doxorubicin Adsorption from Aqueous Solutions: Kinetic, Isotherms, and Thermodynamic Studies.

In this work, the magnetic Ti3C2 MXene functionalized with β-cyclodextrin was prepared and characterized using scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectra, X-ray diffraction, X-ray photoelectron spectroscopy, vibrating sample magnetometry, and thermogravimetric analysis. The synthesized nanomaterial was used as an adsorbent to adsorb doxorubicin from aqueous solutions, and the experimental parameters that affected the adsorption efficiency were investigated. In addition, the adsorption characteristics including adsorption kinetics, adsorption isotherm, and thermodynamics were researched comprehensively. The adsorption kinetics of doxorubicin followed a pseudo-second-order kinetic model, which indicated that adsorption was the rate-limiting step, and the maximum adsorption capacity was 7.35 μg mg-1 by shaking for 60 min at pH 7.0. The adsorption isotherm was well described using the Freundlich model, which implied that multilayer adsorption took place over the prepared nanomaterial for doxorubicin adsorption. The negative values of Gibbs free energy change (ΔG 0 < 0) demonstrated that doxorubicin adsorption was a spontaneous process. The positive values of entropy change (ΔS 0 > 0) implied that doxorubicin adsorption was an increasing random process. Enthalpy change values were positive (ΔH 0 > 0) and indicated that the adsorption of doxorubicin was endothermic. The adsorption percentage of doxorubicin remained in the range of 41.05-44.09%, and the relative standard deviation (RSD) based on the adsorption percentage through five replicate adsorption and desorption processes was 2.8%. These results indicated that the magnetic Ti3C2 MXene nanomaterials can be an effective adsorbent to adsorb DOX from aqueous solutions.

Introduction

In recent years, two-dimensional (2D) structure materials have shown unique physical and chemical properties compared with other materials. MXene (e.g., Ti3C2T), a family of 2D materials, has attracted wide interest since its discovery in 2011[1] because of its relatively large specific surface area, high electrical conductivity, and hydrophilicity.[2] The general formula of MXene is often described as MXT (M, X, and T represent a transition metal, carbon/nitrogen, and surface functional groups such as −OH, −O, and −F),[3] and they are produced by selective etching of IIIA or IVA elements from the MAX phase (e.g., Ti3AlC2).[4,5] MXene has shown huge potential in multiple fields, such as sensors, energy storage, cancer therapy, and EM wave absorption and shielding. Because of the large specific surface area, hydrophilic performance, tunable surface chemistry, high negative zeta potential, and environment-friendly characteristics, MXene has attracted wide attention in adsorption and remediation of wastewater.[6] For example, Ti3C2 MXene exhibited a high adsorption efficiency with the removal rate enhanced up to ∼99.7% for ciprofloxacin from wastewater,[7] which revealed that MXenes can be used as a novel adsorption nanomaterial with great potential in environmental applications.[8]

Doxorubicin (DOX) is an effective anticancer drug, which can significantly inhibit the growth of tumor cells. However, it will lead to a variety of adverse side effects and multidrug resistance effects in cancer treatments because it cannot distinguish between normal cells and tumor cells.[9] Eventually, after administration to patients, it may be released into the environment through excretions as mixtures of its original form and their metabolites.[10] The detection of DOX in oncologic hospital wastewater in some areas was in the range of 0.26–1.35 μg L–1,[11] and 0.02–0.042 μg L–1 was detected in wastewater treatment plants.[12] Eco-genotoxicity studies revealed that DOX causes DNA damage to Ceriodaphnia dubia cells at a concentration of 0.05 μg L–1. Thus, DOX has been one of the contaminants in the list of emerging substances because of the potential risks.[13] The solubility of DOX is extremely low in neutral water and could pose harmful effects on growing eukaryotic organisms because of its toxicity, genotoxicity, mutagenicity, and teratogenicity.[14] Various treatment methods such as membrane technology, activated carbon, exchange resins method, and advanced oxidation technology have been tried for the elimination of pharmaceutical residues from water.[13,15,16] Unfortunately, secondary pollutants may be formed in aqueous solutions because of the complex structures and variable physicochemical properties of DOX.[17,18] Accordingly, it is necessary to find a more effective method for removing DOX.

Adsorption technology is considered to be a promising method for removing pollutants from effluents.[14] More kinds of absorbents such as carbon nitride, carbon nanotubes, and graphene nanoribbons have been used for adsorption of DOX from aqueous solutions,[19−21] but some shortcomings such as time consumption and difficult separation exist.[18] Recently, iron oxide nanoparticles (Fe3O4 NPs) have attracted extensive attention in the field of environmental remediation.[22,23] However, unmodified Fe3O4 NPs are difficult to recycle in practical application because they are easy to agglomerate and form clusters.[24] Up to now, various magnetic adsorbents that were modified or coated different groups on the surface of Fe3O4 NPs to provide functionality and stability have been prepared for inorganic, organic, and biological analysis.[25,26] For example, multilayer cucurbit[6]uril-based magnetic nanoparticles were prepared based on host-guest interactions by Jia’s group, and the compound was successfully used as a blood purification material to remove lipoprotein from plasma.[27]

β-cyclodextrin (β-CD) is a kind of nontoxic cyclic oligosaccharides, and the most significant structure is hydrophilic exterior and hydrophobic interior cavity.[28,29] It is well known that β-CD can form stable host-guest inclusion complex with a variety of aromatic molecules into its hydrophobic cavity though host-guest interactions.[30] However, the inherent solubility of β-CD hinders its practical application in a variety of areas.[31] β-CD has many reactive hydroxyl groups with high chemical reactivity, which offer abundant active centers for the reaction with organic functional groups, making it possible for new functionalization.[29] Furthermore, chemical modification can be conducted at the upper and lower edges to improve solubility and inclusion efficiency and further to construct various functional materials.[32]

In this study, magnetic Ti3C2 MXene functionalized with β-CD was prepared (Ti3C2@Fe3O4@β-CD) and used as an adsorbent for adsorption of DOX from aqueous solutions. The experimental parameters that affected the adsorption efficiency were investigated, and the experimental data were analyzed based on adsorption kinetics, adsorption isotherm models, and thermodynamics. After that, the performance of interference and regeneration was also investigated.

Experimental Section

Materials

Ti3AlC2, β-cyclodextrin (β-CD), hydrofluoric acid (HF), doxorubicin (DOX), allyl bromide, p-toluenesulfonyl chloride (TsCl), ethylene glycol, dimethylsulfoxide (DMSO), ferric chloride hexahydrate (FeCl3·6H2O), ammonium hydroxide (NH3·H2O, 25%, w/w), (3-mercaptopropyl) triethoxysilane (MPTMS), and polyethylene glycol (PEG-2000, Mn = 2000) were obtained from Macklin Reagent (Shanghai, China). Acetic acid, sodium chloride (NaCl), hydrochloric acid (HCl), sodium acetate, sodium hydroxide (NaOH), phosphoric acid (H3PO4), nitric acid (HNO3), sodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), and ethanol were purchased from Jiangsu Qiangsheng Chemical Co., Ltd. (Changshu, China). The DOX stock solution (100 μg mL–1) was prepared by deionized water. The grade of NdFeB strong magnet is N35, and the residual magnetic flux density is 11,700–12,200 Gs.

Methodology

The synthesized magnetic nanoparticles were characterized using scanning electron microscopy (SEM, S-4800, JEOL Company, Japan; QUANTA-250, Thermo, USA), transmission electron microscopy (TEM, H-7800, Hitachi, Japan), Fourier transform infrared (FT-IR, IS10, Thermo, USA) spectrometry, X-ray diffraction (XRD, X’PERT POWDER, PANalytical B.V., Netherland), X-ray photoelectron spectroscopy (XPS, D8 Advance, Bruker), vibrating sample magnetometry (VSM, MPMS SQUID XL-7, Quantum Design, USA), and thermogravimetric analysis (TGA, STA2500, Netzsch, Germany). In addition, the concentration of DOX was determined by measuring the absorbance using a UV–visible spectrophotometer (CARY100, Agilent, USA) at a wavelength of 483 nm.

Preparation of Ti3C2 MXene Nanosheets

Ti3AlC2 (1.0 g) was slowly added in HF (10 mL) to stir for 48 h at 50 °C for complete removal of the Al layers. Then, the solution was centrifuged and rinsed several times with deionized water to remove the unreacted HF until pH > 6.0. Then, the obtained Ti3C2 powder was vacuum-dried at 40 °C.

Preparation of Fe3O4 NPs

Fe3O4 NPs were prepared according to our previous work.[33] FeCl3·6H2O (2.7 g) was added to a flask and dissolved with ethylene glycol (50 mL). After stirring violently for 30 min, the color of the solution changed from colorless to orange. Then PEG-2000 (2.0 g) and sodium acetate (7.2 g) were added to the solution and stirred for 1 h. The brown sticky solution was transferred into a Teflon-lined stainless-steel autoclave and sealed, and the autoclave was heated to 200 °C for 9 h. After that, the obtained black precipitate was washed several times with ethanol and deionized water until neutral and was dried at an ambient temperature in vacuum.

Preparation of Ti3C2@Fe3O4@β-CD

Ti3C2 MXene (100 mg) and Fe3O4 NPs (50 mg) were dispersed in 80 and 20 mL of deionized water, respectively, and allowed to stir for 30 min. Then, the above solution was mixed and ultrasound-treated for 120 min under N2. After that, the resulting black precipitate was collected using an NdFeB strong magnet and washed several times with ethanol and deionized water. Finally, Ti3C2@Fe3O4 was obtained and dried at 50 °C overnight.

Ti3C2@Fe3O4@MPTMS was obtained with the surface of Ti3C2@Fe3O4 modified via the grafting of MPTMS as an organosilane. Ti3C2@Fe3O4 (0.1 g) was redispersed into the solution of DMSO/MPTMS (v/v, 80:2), and the mixture was stirred for 5 h at 80 °C to obtain Ti3C2@Fe3O4@MPTMS.

Allyl-β-CD was synthesized according to Zheng’s work.[34] Then, Ti3C2@Fe3O4@β-CD magnetic nanomaterials were prepared by thiol-ene click reaction between −SH of MPTMS and C=C of allyl-β-CD. allyl-β-CD, Ti3C2@Fe3O4@MPTMS, and AIBN (1 wt %) was added to the DMSO, and the mixture was allowed to stir for 4 h at 75 °C. The obtained Ti3C2@Fe3O4@β-CD magnetic nanomaterials were separated using an NdFeB strong magnet and then washed with ethanol and deionized water several times. The obtained products were dried under vacuum at 60 °C overnight. The synthesis process of Ti3C2@Fe3O4@β-CD was illustrated in Figure .

Figure 1

Synthesis of Ti3C2@Fe3O4@β-CD.

Adsorption Experiments

The DOX adsorption process onto the Ti3C2@Fe3O4@β-CD was conducted as follows: Ti3C2@Fe3O4@β-CD (5 mg) was mixed with DOX solution (15 mL, 5 μg mL–1), and then, the mixture was mechanically shaken for 60 min at 25 °C. The DOX-adsorbed Ti3C2@Fe3O4@β-CD was separated using an NdFeB strong magnet. To evaluate the amount of adsorbed DOX, the adsorption percentage was measured using a UV–visible spectrophotometer at 483 nm.

The adsorption percentage of DOX was calculated according to the eq .where Ce and C0 are the equilibrium concentrations and initial concentrations (μg mL–1), respectively.

In order to research and construct the adsorption isotherm of DOX onto the Ti3C2@Fe3O4@β-CD, a series of concentrations of 2, 4, 6, 8, 10, 20, 30, 50, 80, and 100 μg mL–1 DOX standard solutions (pH 7.0) were prepared and individually mixed with Ti3C2@Fe3O4@β-CD (5 mg). The mixture was mechanically shaken for 60 min at 25 °C, and the adsorption capacity (Qe) at equilibrium for DOX onto the Ti3C2@Fe3O4@β-CD was calculated as eq :[35]where Qe is the adsorption capacity at equilibrium (μg mg–1), V is the volume of solution in (mL), and m is the mass of the adsorbent (mg).

The kinetic study based on the DOX adsorption onto the Ti3C2@Fe3O4@β-CD was studied at different periods of time: 5, 10, 20, 30, 60, 90, 120, and 180 min, under the same batch conditions using 5 μg mL–1 DOX (pH 7.0) and individually mixed with Ti3C2@Fe3O4@β-CD (5 mg).

Desorption Experiments

Desorption experiments of DOX from Ti3C2@Fe3O4@β-CD was monitored that DOX-adsorbed Ti3C2@Fe3O4@β-CD was dispersed in ∼5 mL of phosphate buffer solution (PBS) with pH 9.0. The experiment was carried out in an orbital shaker and maintained at 160 rpm for 30 min. Then, the supernatant was completely separated from the mixture solution using an NdFeB strong magnet for data analysis by UV–visible spectrophotometer at 483 nm.

Characterization

The morphologies of Ti3C2 and Ti3C2@Fe3O4@β-CD were obtained by SEM and TEM and are presented in Figure . Figure a, b illustrates the typical Ti3C2 MXene with the layered structure. After being etched, the tightly packed layers are obviously separated from each other and transformed into a loose accordion-like structure with a significant layer spacing because of the loss of Al atoms.[36] After the incorporation of Fe3O4, a large number of regular spherical particles were immobilized uniformly into the structure of MXene nanosheets. Because of the thiol-ene click reaction between −SH of MPTMS and C=C of allyl-β-CD, the surface roughness of Ti3C2@Fe3O4@β-CD was increased (Figure c, d).

Figure 2

SEM images and TEM images of (a and b) Ti3C2 and (c and d) Ti3C2@Fe3O4@β-CD.

The groups and structure of Ti3C2, Ti3C2@Fe3O4, and Ti3C2@Fe3O4@β-CD were characterized by FT-IR analysis. As shown in Figure , the strong peaks of 3440, 1100, and 598 cm–1 were measured in Ti3C2@Fe3O4 and Ti3C2@Fe3O4@β-CD, which should be from the stretching vibration of O–H, C–F, and Fe–O–Fe, respectively.[37] The O–H stretching vibration peak at 3440 cm–1 exhibited a typical large red-shift relative to that of free O–H mode. Accordingly, it is reasonable to suppose that β-CD coexisted in the magnetic nanomaterial. In particular, the vibration of the Ti–O bond may account for the peak at 663 cm–1.[38] Thus, the Ti3C2, Ti3C2@Fe3O4, and Ti3C2@Fe3O4@β-CD have been successfully synthesized.

Figure 3

FT-IR spectra of Ti3C2, Ti3C2@Fe3O4, and Ti3C2@Fe3O4@β-CD.

The crystal structures of Ti3C2, Ti3C2@Fe3O4, Ti3C2@Fe3O4@β-CD, Fe3O4, and Ti3AlC2 were investigated by XRD. As depicted in Figure , Fe3O4, Ti3C2@Fe3O4, and Ti3C2@Fe3O4@β-CD show similar diffraction patterns. Six characteristic peaks (2θ = 30.09, 35.47, 43.11, 53.61, 56.95, and 62.63°) were indexed to (220), (311), (400), (422), (511), and (440), respectively,[39] which match the face center cubic phase of Fe3O4 (JCPDS Card No. 19-629). There are a set of characteristic peaks at 2θ = 19.11, 33.96, 38.74, 41.76, and 60.18°, which corresponded to the (004), (101), (104), (105), and (109) planes of the Ti3AlC2 MAX phase.[40] Three peaks in the Ti3C2 MXene XRD pattern at 2θ = 18.27, 33.96, and 60.18° were observed. However, the peak at 38.74° (104) disappeared completely in the XRD pattern of the obtained Ti3C2 MXene nanosheets, which revealed that the aluminum phase in Ti3AlC2 MAX was exfoliated during the etching process.[41] Thus, the nanosheet structure of Ti3C2 MXene was successfully formed. Moreover, the (004) peak of the Ti3C2 MXene shifted toward the low-angle direction, which suggested the expansion of the MXene interlayer distance.[42] The characteristic diffraction peaks at 18.27, 33.96, and 60.18° in Ti3C2@Fe3O4 and Ti3C2@Fe3O4@β-CD were attributed to the crystal planes (004), (101), and (109) of Ti3C2 MXene, respectively. The above results further demonstrated that Ti3C2@Fe3O4@β-CD maintain the primary structure of Ti3C2 MXenes.

Figure 4

XRD patterns of Ti3C2, Ti3C2@Fe3O4, Ti3C2@Fe3O4@β-CD, Fe3O4, and Ti3AlC2.

Figure a demonstrated the survey XPS spectrum of Ti3C2 and Ti3C2@Fe3O4@β-CD, which mainly contained five elements corresponding to C, Ti, O, F, and Fe elements. The high-resolution XPS spectra of C 1s (Figure b) could be decomposed into four components. The peaks at 288.5, 286.3, 284.8, and 281.5 eV might be assigned as C=O, C–O, C–C, and C–Ti, respectively. The XPS spectra of O 1s are presented in Figure c. The peaks at 532.7, 531.0, and 530.0 eV were ascribed to the Ti–O, C=O, and C–O group, respectively. The Ti 2p spectra of Ti3C2@Fe3O4@β-CD (Figure d) were also studied. Four functional groups appeared at 464.6, 459.5, 458.3, and 454.9 eV, which were associated with Ti–O, Ti–O 2p1/2, Ti–O 2p3/2, and Ti–C, respectively.[43,44] In addition, the relative elemental compositions of Ti3C2 and Ti3C2@Fe3O4@β-CD are summarized in Table . The relative content of Fe 2p and O 1s increased obviously compared with Ti3C2 but Ti 2p decreased. The results further indicated that Ti3C2@Fe3O4@β-CD was successfully synthesized.

Figure 5

Survey spectra of (a) Ti3C2 and Ti3C2@Fe3O4@β-CD; (b) C 1s XPS spectra of Ti3C2@Fe3O4@β-CD; (c) O 1s XPS spectra of Ti3C2@Fe3O4@β-CD; (d) Ti 2p XPS spectra of Ti3C2@Fe3O4@β-CD.

Table 1

Elemental Compositions and Relative Contents of Ti3C2 and Ti3C2@Fe3O4@β-CD

	elemental compositions (%)
materials	Fe 2p	O 1s	Ti 2p	C 1s	N 1s
Ti3C2	0.06	43.78	35.71	19.88	0.56
Ti3C2@Fe3O4@β-CD	11.80	47.18	20.92	19.46	0.64

Figure illustrates the magnetization curves of Fe3O4, Ti3C2@Fe3O4, and Ti3C2@Fe3O4@β-CD, and the saturation magnetization values were calculated to be about 82.61, 35.57, and 29.48 emu g–1, respectively. The results demonstrated that three samples have magnetic properties and they can be readily separated from the reaction media. The saturation magnetization of the Ti3C2@Fe3O4@β-CD was lower than that of two others, mainly due to the introduction of Ti3C2 and β-CD.

Figure 6

Magnetization curve of Fe3O4, Ti3C2@Fe3O4, and Ti3C2@Fe3O4@β-CD.

Figure shows the weight losses of Ti3C2@Fe3O4@β-CD upon heating under air atmosphere from 10 to 1000 °C. Under the low temperature, it may be caused by the evaporation of free water adsorbed on the surface of Ti3C2@Fe3O4@β-CD nanomaterials. Under the high temperature, it may be caused by the functional groups of Ti3C2@Fe3O4@β-CD. The less weight loss indicated that Ti3C2@Fe3O4@β-CD showed remarked thermogravimetric stability.

Figure 7

TGA thermogram of Ti3C2@Fe3O4@β-CD.

Results and Discussion

Optimization of Adsorption Conditions

Figure a depicts that the adsorption percentage of DOX onto the Ti3C2@Fe3O4@β-CD varies with the sample pH. The adsorption percentage increased from 10.79 to 34.87% when the pH enhanced from 2.0 to 7.0. The reason may be that the Ti3C2@Fe3O4@β-CD surface was positively charged when sample pH was low (pHPZC is about 6.5); meanwhile, the electrostatic repulsion existed between DOX and Ti3C2@Fe3O4@β-CD surface.[9] Besides, a competition between H+ ions and positively charged DOX molecules existing in the solution was presented on the surface of Ti3C2@Fe3O4@β-CD14. With increased pH, the electrostatic repulsion will decrease. The molecular structure of DOX contains −OH and −NH2 groups, which can form hydrogen bonds with C=O and −OH groups on the surface of Ti3C2@Fe3O4@β-CD. The hydrogen-bonding interaction formed under natural conditions is stronger than that formed under acidic conditions. Regarding the structure and color alteration of DOX at the pH above 8.5, the adsorption percentage decreased obviously.[17] Thus, pH 7.0 was chosen in further experiments.

Figure 8

Effect of (a) pH; (b) adsorption time; and (c) initial concentration.

The influence of adsorption time was evaluated in the range of 5–180 min, as shown in Figure b. The adsorption percentage can be obtained as 41% within 60 min and then increased by only 7% from 60 to 180 min, which indicated that longer adsorption time is of little significance for increasing the adsorption percentage. Thus, 60 min was adopted as the adsorption time for subsequent analysis.

Figure c describes the effect of different initial concentrations of DOX on the adsorption percentage. The results indicated that the adsorption percentage of DOX onto the Ti3C2@Fe3O4@β-CD increased observably when the initial concentration was increased from 2 to 15 μg mL–1, but it increased slowly later.

Adsorption Kinetics

Adsorption kinetics experiments help understand the dynamics and mechanism of the adsorption process. The kinetic study of DOX is also essential for designing and modeling the adsorption process by selecting optimal parameter conditions. In order to study the adsorption kinetics of DOX on adsorbents, the experimental data were analyzed by pseudo-first-order and pseudo-second-order kinetics models.[45−47] The linear relationships are formed according to eqs and 4,where qe and qt are the amounts of DOX adsorbed at equilibrium and at time t (μg mg–1), respectively. k1 and k2 are the rate constant of the pseudo-first-order models (min–1) and the pseudo-second-order models (mg μg–1 min–1), respectively.[28,30]

The kinetic parameters for DOX adsorption onto the Ti3C2@Fe3O4@β-CD are illustrated in Table . The correlation factors for the pseudo-first-order kinetic plot was less than 0.99, whereas the pseudo-second-order kinetic plot exhibited correlation factors of 0.9960 for DOX adsorption. Pseudo-second-order rate constants qe and k2 were calculated to be 7.35 μg mg–1 and 0.0144 mg μg–1 min–1 under experimental conditions, which could be determined from the slope and intercept of the line. The fitted value of qe was very close to the experimental value of qe (7.12 μg mg–1), which further verified that DOX adsorption onto the Ti3C2@Fe3O4@β-CD are of the pseudo-second-order model. This implied that DOX adsorption could be dominated by chemisorption rather than physical adsorption. Chemisorption could be a result of host–guest interactions between DOX and β-CD or hydrogen-bonding interactions between Ti3C2@Fe3O4@β-CD and DOX.

Table 2

Kinetic Parameters for the Adsorption of DOX onto the Ti3C2@Fe3O4@β-CD

	pseudo-first-order model			pseudo-second-order model
qe,exp (μg mg–1)	k1 (min–1)	qe,cal (μg mg–1)	R2	k2 (mg μg–1 min–1)	qe,cal (μg mg–1)	R2
7.12	0.0018	15.95	0.9010	0.0144	7.35	0.9960

Adsorption Isotherms

Adsorption isotherms provide valuable information about adsorption behavior, surface properties, and affinity of DOX toward the adsorbent. The amount of DOX-adsorbed on the Ti3C2@Fe3O4@β-CD surface and the amount of DOX remaining in the solution at a fixed temperature and pH at equilibrium can be evaluated by isotherms. The equilibrium adsorption of the Ti3C2@Fe3O4@β-CD was analyzed using Langmuir and Freundlich adsorption isotherms.[35]

The Langmuir isotherm assumes monolayer adsorption on the homogeneous surface, where the adsorbed each adsorbate species has equal adsorption activation energy. The expression for the Langmuir isotherm is as follows:where Ce represents the equilibrium concentration of DOX in the supernatant (μg mL–1); qe is DOX uptake at the adsorption equilibrium per weight of the adsorbent (μg mg–1); qmax is the maximum DOX adsorption capacity per weight of the adsorbent (μg mg–1); and b is the Langmuir adsorption constant (mL μg–1). The slope and intercept of the linear plot of Ce/qe against Ce yield the values of qmax and b.[48]

Also, the Langmuir isotherm can be employed to predict whether or not an adsorption process is favorable by a dimensionless constant separation factor, RL, which is represented as follows:where C0 is the initial concentration of DOX (μg mL–1). The value of RL indicates that the shape of the isotherm and is defined as follows: RL = 0 (irreversible), 0 < RL < 1 (favorable), RL > 1 (unfavorable), and RL = 1 (linear).

The Freundlich isotherm is considered as model multilayer adsorption, which occurs on a heterogeneous surface, and its linear form can be expressed as follows:where k is the Freundlich constant indicative of adsorption capacity; 1/n is an empirical parameter that represents the adsorption capacity of the adsorbent and the energy of adsorption.[30,49]k and n can be obtained from the intercept and the slope of the linear plot of log(qe) versus log(Ce), respectively. For the favorable adsorption process, the value of n should lie in the range of 1–10.

In this study, Langmuir and Freundlich isotherm models were fitted with experimental data, and then, the constants and correlation factors were calculated and listed in Table . Based on the correlation factor (R2) values, it can be deduced that the adsorption process of DOX onto the Ti3C2@Fe3O4@β-CD followed the Freundlich isotherm model owing to a high correlation value of 0.9916. This implied that multilayer adsorption took place over the Ti3C2@Fe3O4@β-CD. The assumption for this isotherm is that the interaction between the adsorbent surface and adsorbed molecules occurs on a heterogeneous surface, and the energy is distributed nonuniformly. The Langmuir isotherm model did not correlate with the experimental data. The plot of Ce/qe versus Ce does not follow a linear relationship, presenting a bad correlation coefficient (R2 = 0.4934). It is apparent that the Langmuir model was unsuitable to describe the adsorption behaviors of DOX onto the Ti3C2@Fe3O4@β-CD.

Table 3

Parameters of the Langmuir and Freundlich Isotherms for DOX Adsorption

Langmuir model				Freundlich model
qmax (μg mg–1)	b (mL μg–1)	R2	RL	1/n	K	R2
–90.09	–0.035	0.4934	–0.400–1.075	1.1424	2.194	0.9916

Thermodynamic Studies

The adsorption behaviors of Ti3C2@Fe3O4@β-CD for DOX were critically investigated at 298, 308, and 318 K. Thermodynamic parameters are calculated from eqs , 9, and 10:[48]where Kd is the distribution coefficient; R is the ideal gas constant (8.314 J mol–1 K–1); and T is the temperature (K). Gibbs free energy change (ΔG0) was calculated using ln Kd values for different temperatures. Enthalpy change (ΔH0) and entropy change (ΔS0) are calculated from the slopes and intercepts of eq .

According to eq , the thermodynamic parameters were summarized in Table . The value of ΔH0 confirmed that the DOX adsorption progress onto the Ti3C2@Fe3O4@β-CD was endothermic and the adsorption percentage of DOX increased with temperature. The value of ΔS0 indicated a greater order of reaction during DOX adsorption and the negative values of ΔG0 reflected the DOX adsorption progress onto the Ti3C2@Fe3O4@β-CD was a spontaneous and feasible process.

Table 4

Thermodynamic Parameters for Adsorption of DOX onto the Ti3C2@Fe3O4@β-CD

T/K	ΔG0/kJ mol–1	ΔH0/kJ mol–1	ΔS0/J mol–1 K–1
298	–2.68	11.63	48.05
308	–3.19
318	–3.63

Desorption and Regeneration

To investigate the possibility of Ti3C2@Fe3O4@β-CD regeneration, the desorption experiments were performed. The results showed that PBS solutions (pH 9.0) could be used for Ti3C2@Fe3O4@β-CD regeneration, and the desorption percentage was above 70%. The molecular structure of DOX contains −OH and −NH2 groups, which can form hydrogen-bonding interactions with C=O and −OH groups on the surface of Ti3C2@Fe3O4@β-CD.[18,24] With pH increased, the hydrogen-bonding interaction was broken, which may result in the desorption of DOX from the surface of Ti3C2@Fe3O4@β-CD. The adsorption percentage remained in the range of 41.05–44.09%, and RSD based on the adsorption percentage through five replicate adsorption and desorption process was 2.8%. The desorption and regeneration results indicated that Ti3C2@Fe3O4@β-CD can be employed as a kind of recyclable adsorbents for DOX adsorption.

Interference Experiments

The interference on the adsorption of DOX was investigated when Na+, Cl–, NO3–, SO42–, Co2+, and Cu2+ ions as the interfering ions coexisted with DOX in solution. The tolerance of interfering ions is defined as a 5% reduction in signal intensity. Results in Table showed that the interfering ions cause no significant interference with the determination of DOX of 5 μg mL–1 at pH 7.0. This is probably because the chemisorption based on host–guest interactions and hydrogen-bonding interactions are the primary adsorption.

Table 5

Effect of Interfering Ions

coexisting ions	tolerance limit/μg mL–1
Na+, Cl–	1000
SO42–, NO3+, Cu2+, Co2+	100

Comparison of Ti3C2@Fe3O4@β-CD Performance with the Other Adsorbents

The comparisons of Ti3C2@Fe3O4@β-CD with other adsorbents for adsorption of DOX are summarized in Table . Higher adsorption capacities of some other’s work are related to the higher initial concentration and more adsorbent dosage used in these studies than the presented work. However, the proposed method can offer several advantages, including that it does not require any expensive instruments, complex preparation steps, and the operation is much easier.

Table 6

Comparison of Ti3C2@Fe3O4@β-CD with Other Adsorbents Applied for Adsorption of DOX

adsorbents	pH	initial concentration (mg mL–1)	temperature (°C)	adsorbent dosage	adsorption capacity	adsorption time	ref
GO@Fe3O4@ZnO@CS	7.0	10	25	0.01 g	390 mg g–1	45 min	(14)
green magnetic/graphene oxide/chitosan/alliumsativum/quercus nanocomposite	6.3	2		1.4 g L–1		10 min	(9)
Fe3O4 NPs	6.0	20	30	0.5 g L–1	32 mg g–1	48 h	(18)
Fe3O4@SiO2-Glu	7.4	20	30	10 mg	64.51 mg g–1	24 h	(17)
Ti3C2@Fe3O4@β-CD	7.0	5	25	5 mg	7.35 μg mg–1	60 min	this work

Conclusions

The synthesis procedure and structural characterization of Ti3C2@Fe3O4@β-CD and then Ti3C2@Fe3O4@β-CD as an efficient adsorbent to adsorb DOX from aqueous solutions are reported in the present work. SEM analysis confirmed that the surface of Ti3C2 MXene was obviously a loose accordion-like structure. After preparing Ti3C2@Fe3O4@β-CD, a large number of regular spherical particles were uniformly immobilized in the structure of MXene nanosheets. XRD analysis confirmed that Ti3C2@Fe3O4@β-CD maintains the primary structure of Ti3C2 MXenes and Fe3O4. VSM analysis demonstrated that the prepared materials had magnetic properties and they could be readily separated from the reaction media. TGA analysis indicated that Ti3C2@Fe3O4@β-CD possessed remarked thermogravimetric stability. The batch study confirmed that the adsorption of DOX onto the Ti3C2@Fe3O4@β-CD depended on the parameters such as pH, adsorption time, and initial concentration. Kinetic models and adsorption isotherms demonstrated that DOX adsorption fitted well to the pseudo-second-order kinetics models and Freundlich isotherm models. The thermodynamic study indicated that DOX adsorption onto the Ti3C2@Fe3O4@β-CD was endothermic, and the negative ΔG0 values implied spontaneous adsorption. The positive value of ΔS0 showed that randomness increased at the solid/solution interface. Moreover, Ti3C2@Fe3O4@β-CD could be effectively regenerated using a solution of PBS (pH 9.0). The adsorption percentage remained in the range of 41.05–44.09%, and RSD based on the adsorption percentage through five replicate adsorption and desorption process was 2.8%. These remarkable results indicated that Ti3C2@Fe3O4@β-CD possessed potential applications to adsorb DOX from aqueous solutions.