High Performance of Ciprofloxacin Removal Using Heterostructure Material Based on the Combination of CeO₂ and Palygorskite Fibrous Clay

Abstract

Ciprofloxacin, a second-generation fluoroquinolone, is widely used in human and veterinary medicine. However, it is known for its environmental persistence and ability to promote bacterial resistance, causing genotoxic impacts and chronic toxicity in various aquatic life forms. Adsorption is an effective technique for water treatment, removing multiple organic molecules, even in minimal concentrations. Hybrid materials based on fibrous clay minerals, such as palygorskite, are promising for environmental remediation, significantly when modified with oxides to improve their adsorption properties. This work prepared and characterized a CeO₂/palygorskite hybrid material using various physicochemical techniques (XRD, FTIR, BET, SEM), which indicated the formation of the heterostructure material with interesting textural properties. This CeO₂/palygorskite was evaluated as an adsorbent of the antibiotic drug ciprofloxacin. The influence of pH (3, 7, and 9) and ciprofloxacin concentration (6, 8, 10, and 14 ppm) on adsorption were studied, using pseudo-first- and pseudo-second-order kinetic models. The pseudo-second-order model showed the best fit (R² > 0.99) and the lowest squared error (SSE), indicating chemisorption. The Langmuir, Freundlich, and Temkin isotherms were applied to the experimental data, where the Langmuir model had the best fit, indicating monolayer adsorption with a maximum capacity of 15 mg·g⁻¹. Post-adsorption characterization by FTIR confirmed the structural stability of the material, highlighting its promising application in environmental remediation due to its high concentration of adsorbents.

1. Introduction

Pharmaceutical compounds are a global concern due to their widespread presence in aquatic environments, with concentrations ranging from ng L⁻¹ to µg L⁻¹. Antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), synthetic hormones, and psychiatric medications are frequently found in surface, wastewater, and treated water. These compositions represent a challenge due to their potential biological effects on aquatic organisms, whose acute and chronic ramifications are not yet fully understood. Ciprofloxacin (CIP), a second-generation fluoroquinolone widely used in human and veterinary medicine, stands out for its environmental persistence and ability to promote bacterial resistance [1]. CIP can cause genotoxic impacts and chronic toxicity in various forms of aquatic life. Given this reality, adsorption emerges as an effective technique for water treatment, efficiently removing a variety of organic molecules, even in minimal concentrations. This method uses porous solid adsorbents that capture contaminants by surface adhesion. Adsorption efficiency depends on factors such as adsorbent properties, pollutant particle size, contact time, and pH of the medium [2].

In this sense, studies have shown that the natural or modified palygorskite clay mineral can be used effectively to remove pharmaceuticals such as antibiotics [3], anti-inflammatories [4], and other pharmaceutical contaminants from wastewater [5]. Palygorskite, also known as attapulgite, is a fibrous clay mineral of microfibrous nature, belonging to the class of phyllosilicates that has an empirical structural formula (Mg, Al)₅(Si, Al)₈O₂₀(OH)₂·8H₂O. [6,7]. This silicate has a relatively little isomorphic substitution of Al for Si in the tetrahedral structure. Here, Mg tends to occupy edge positions, whereas the smaller Al ion primarily occupies interior places. This fibrous clay mineral shows a discontinuity in its octahedral sheets due to regular inversions of the silicon tetrahedron orientation. This discontinuity generates an alternation of tunnels and blocks that grow towards the fiber (c-axis), and due to this, the palygorskite presents a high surface area (around 150 m²/g), which is defined by the tiny particle size, fibrous shape, and the presence of micropores [7]. In addition, the formation of tunnels and channels in the structure determines the presence of silanol groups (≡Si-OH) at the silicate’s fibers exterior surface, which can be functionalized to introduce new properties. Therefore, numerous organic and inorganic species, including polymers [8] and oxides [9], can easily access these free Si-OH silanol groups, enabling the development of hybrid compounds, which have implications for a wide range of industrial applications [10]. Given this, the surface modification of clays with metal oxides, such as FeO, Al₂O₃ [11], MnOx [12], or ZnO [10], can significantly improve their performance in a specific application. These modifications can alter the physical and chemical properties of silicate, resulting in an interesting interface for a variety of surface chemistry reactions, including adsorption, since it is reported that the deposition of metal oxides can increase the specific surface area and porosity of palygorskite, providing more adsorption sites for interactions with contaminants [13]. Among the studied oxides, cerium oxide (CeO₂) becomes very attractive due to its unique physicochemical properties, redox capacity, thermal stability, and high surface area, making it suitable for various applications, including catalysis, pollutant adsorption, and biomedical applications. The presence of cerium oxide can create new adsorption sites on the surface, improving physicochemical adsorption by introducing new functional groups that can interact strongly with target pollutants and increase adsorption capacity and selectivity [14]. The support of CeO₂ on palygorskite has been explored in the scientific literature due to its unique catalytic and adsorption properties. This composite material combines the structural characteristics of palygorskite, a fibrous clay mineral with a high surface area, with the catalytic properties of cerium oxide. Recent advances in research have focused on controlled synthesis to improve the dispersion of CeO₂ in palygorskite, optimizing its adsorption and catalytic capabilities. These efforts are aimed at applying the material in the treatment of contaminated water, gas purification, and as catalysts for industrial reactions. In conclusion, CeO₂ supported on palygorskite represents a promising line of research with the potential for advanced environmental and industrial solutions, prompting further studies to expand its practical applications and its impact on sustainability [15,16]. With the basis of these premises, this study proposes the synthesis of hybrid materials based on the wet impregnation of CeO₂ nanoparticles on palygorskite clay mineral. To elucidate the structure and possible interactions between both inorganic phases, these hybrid materials were characterized by diverse physicochemical techniques. The CeO₂/palygorskite heterostructure material was applied as an adsorbent in the adsorption of the ciprofloxacin antibiotic drug, and kinetic models were used to understand the main mechanisms involved in the process. This strategy represents a significant advancement in the search for effective and sustainable solutions to address the challenges of organic contamination in water resources.

2. Materials and Methods

2.1. Materials

Cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99%) and sodium hydroxide pearls (NaOH, 99%) were obtained by Êxodo (São Paulo, Brazil). Ethyl alcohol (C₂H₅OH, 99.5%) was obtained from Isofar (Rio de Janeiro, Brazil), and the ciprofloxacin drug (≥98%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Brazilian palygorskite was obtained from Piauí State—Brazil, and detailed characterization of this clay mineral can be found in previous works [17,18]. Deionized water (18.0 µS·cm⁻¹) was obtained with a Millipore System (Merck Millipore, Burlington, MA, USA).

2.2. Synthesis of Cerium Oxide—CeO₂

The CeO₂ was obtained via a hydrothermal method. Initially, 35 mL of deionized water was added to an 80 mL Teflon container. Subsequently, 0.868 g of Ce(NO₃)9.6H₂O and 9.6 g of NaOH were added and stirred at room temperature for 1 h. Then, the solution was transferred to an 80 mL stainless steel-lined Teflon autoclave, sealed, and maintained at 180 °C for 24 h. Afterward, the autoclave was allowed to cool to room temperature, and the resulting light-yellow paste was collected. The washing process commenced using 15 mL of deionized water and 15 mL of ethyl alcohol in a Falcon tube, with each wash centrifuged at 4000 rpm for 5 min and dried in an oven at 60 °C for 12 h.

2.3. Synthesis of Hybrid Material

The synthesis of the CeO₂ supported on palygorskite hybrid material (CeO₂/Paly) was carried out using the wet impregnation method. Initially, a 1:1 ratio between CeO₂ and the palygorskite support (Paly) was employed. For this, 0.1302 g of Paly was dispersed in 50 mL of deionized water for 1 h. This homogenous dispersion was slowly dripped into 50 mL of an aqueous solution containing 0.1302 g of CeO₂, which was kept under magnetic stirring at room temperature for 24 h. The resulting product was collected through centrifugation at 4000 rpm and then dried in an oven at 60 °C for 12 h.

2.4. Characterization of Heterostructure Material

The obtained samples were characterized using X-ray diffraction (XRD) analysis performed on a BRUKER D8 Advance instrument (Karlsruhe, Germany), equipped with the LynxEye linear detector. The scan range was set from 2° to 70°, with X-rays generated by a Cu anode (Kα), λ = 1.54056 Å, operating at 35 mA current and 40 kV potential. Fourier Transform Infrared Spectroscopy (FTIR) was recorded on the materials using a KBr pellet in transmission mode over the range of 4000 to 500 cm⁻¹, and a resolution of 2 cm⁻¹ (SHIMADZU IR-Prestige-21, Tokyo, Japan). To ascertain the specific surface area (Brunauer, Emmett, and Teller—BET method), pore size distribution (Barrett, Joyner, and Halenda—BJH method), and total pore volume of the synthesized materials, the nitrogen adsorption/desorption analysis was carried out in Quantachrome equipment (Boynton Beach, FL, USA) at −196 °C. Before this, the samples were degassed under vacuum for four hours at 100 °C. The morphological characterization of the samples was carried out by Scanning Electron Microscopy (SEM) on Philips XL-30 FEG (Field Emission Gun) equipment (Eindhoven, The Netherlands) operating with a 5 kV electron beam and I probe of 150 pA. The sample was prepared by putting the as-grown products in water and then dropping a few drops of the resulting suspension containing the synthesized materials onto the silicon support. After drying, the materials were directly observed without any conductive coating on the surface. X-ray photoemission spectroscopy (XPS) measurements were conducted using a Scienta Omicron ESCA + spectrometer system, which includes an EA 125 hemispherical analyzer and an XM 1000 monochromated X-ray source (Scientia Omicron, Uppsala, Sweden), employing Al Kα (1486.7 eV) as the X-ray source. The data were analyzed with CasaXPS processing software version 2.3.15 (Casa Software Ltd., Teignmouth, UK).

2.5. Point of Zero Charge

The zero point of charge (pH_PZC) was determined by adding 50 mg of the pure adsorbent (CeO₂), Paly, and the hybrid CeO₂/Paly to separate 25 mL Erlenmeyer flasks containing 0.1 mol·L⁻¹ KCl solution with fixed initial pH values ranging from 2 to 11. The solutions were subjected to magnetic stirring at a constant temperature for 24 h. Subsequently, the final pH was measured using a Hanna Instruments (Woonsocket, RI, USA), model HI-2002 pH meter. Using a graphical method, ΔpH (pHi − pHf) vs. pHi, the pH at the point of zero charge for each solid was estimated under ΔpH = 0.

2.6. Adsorption Study

The adsorption studies evaluated the effectiveness of CeO₂, Paly, and CeO₂/Paly as adsorbents by considering various parameters. The experiments were carried out in batch mode at a constant temperature of 25 °C and an agitation rate of 300 rpm. At predetermined intervals, aliquots of 2 mL were withdrawn and centrifuged at 4000 rpm for 5 min to separate the adsorbent from the supernatant for further analysis.

2.6.1. Influence of pH

The influence of pH on ciprofloxacin adsorption was investigated at room temperature using three different pH values: 3, 7, and 9. These were adjusted using 0.1 mol L⁻¹ NaOH and 0.1 mol L⁻¹ HCl solutions. Briefly, 50 mL of the 10 ppm ciprofloxacin solution was shaken with 50 mg of each adsorbent individually for 120 min. Aliquots were then withdrawn at predetermined time intervals, centrifuged, and analyzed using a pre-established calibration curve on a UV-Vis spectrophotometer (IL-593, HexaSystems Inc., West Palm Beach, FL, USA) at a wavelength λ = 270 nm to determine the equilibrium concentration of ciprofloxacin at each pH. Equation (1) calculated the synthesized material’s equilibrium adsorption efficiency (R%) for ciprofloxacin removal.

$$\mathrm{R}\left(\mathrm{\%}\right)=100\times {\displaystyle \frac{\mathrm{C}\mathrm{i}-\mathrm{C}\mathrm{f}}{\mathrm{C}\mathrm{i}}}$$

(1)

2.6.2. Influence of Concentration

The influence of the initial ciprofloxacin concentration (C_i) on the adsorption process was investigated using solutions with concentrations of 6, 8, 10, and 14 ppm at a constant pH of 7. The experiments were carried out in batch mode at a constant temperature. Each experiment employed 50 mg of adsorbent and 50 mL of the adsorbate solution. The residual concentration of ciprofloxacin (C_t) was determined by UV-Vis spectrophotometry (λ = 270 nm), employing the Lambert–Beer Law. This allowed for the quantification of ciprofloxacin adsorbed at various concentrations over predetermined time intervals at room temperature. The amount of ciprofloxacin adsorbed (q_t, in mg·g⁻¹) for each synthesized material was calculated using Equation (2):

$${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{({\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{t}})}{\mathrm{m}}}\times \mathrm{V}$$

(2)

2.6.3. Influence of Contact Time

The influence of contact time on ciprofloxacin adsorption by CeO₂, Paly, and CeO₂/Paly adsorbents was investigated. Flasks containing 50 mL of 10 ppm ciprofloxacin solution at pH 7 were spiked with 50 mg of each adsorbent. The mixtures were shaken at constant agitation (25 °C and 300 rpm). Aliquots of 2 mL were withdrawn at predetermined intervals (5, 15, 30, 60, 90, and 120 min) and centrifuged at 4000 rpm for 5 min to separate the adsorbent. The amount of ciprofloxacin adsorbed (q_t) and the adsorption efficiency (%) were calculated using Equations (1) and (2), respectively, to evaluate the effect of contact time.

2.6.4. Kinetic Study Adsorption

The kinetic studies of ciprofloxacin adsorption onto the CeO₂/Paly material were performed in triplicate at room temperature and pH 7. Each experiment involved contacting 50 mg of the adsorbent with 50 mL of the ciprofloxacin solution under constant agitation (300 rpm) for 120 min. After the designated contact time, aliquots were withdrawn, centrifuged, and analyzed using a pre-established calibration curve on a UV-Vis spectrophotometer. Pseudo-first-order and pseudo-second-order kinetic models were applied to evaluate the adsorption process. The linearized forms of Equations (3) and (4) were used, which represent these models, respectively. In these equations, q_e and q_t represent the amounts of ciprofloxacin adsorbed per gram of adsorbent (mg·g⁻¹) at equilibrium and time t, respectively. k_l is the adsorption rate constant for the pseudo-first-order model (min⁻¹), and k₂ is the rate constant for the pseudo-second-order model (g mg⁻¹ min⁻¹).

$$\ln \left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\ln {\mathrm{q}}_{\mathrm{e}}-{\mathrm{k}}_1.\mathrm{t}$$

(3)

$${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{\mathrm{k}}_2({\mathrm{q}}_{\mathrm{e}}{)}^2}}+{\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}}\mathrm{t}$$

(4)

2.6.5. Isothermal Adsorption Models

Isothermal adsorption models were employed to determine the maximum adsorption capacity of the adsorbents for ciprofloxacin. Experiments were conducted using different initial ciprofloxacin concentrations (6, 8, 10, and 14 mg/L) at room temperature and a contact time of 120 min. The equilibrium adsorption data were fitted to the Langmuir [19] (Equation (5)), Freundlich [14] (Equation (6)), and Temkin [20] (Equation (7)) models. The following linearized forms of these equations were used for the fitting process:

$${\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}}=\left({\displaystyle \frac{1}{{\mathrm{q}}_{\max }}}\right){\mathrm{C}}_{\mathrm{e}}+{\displaystyle \frac{1}{{\mathrm{K}}_{\mathrm{L}}\times {\mathrm{q}}_{\max }}}$$

(5)

$$\ln {\mathrm{q}}_{\mathrm{e}}=\ln {\mathrm{K}}_{\mathrm{F}}+{\displaystyle \frac{1}{\mathrm{n}}}\ln {\mathrm{C}}_{\mathrm{e}}$$

(6)

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{\mathrm{R}\mathrm{T}}{{\mathrm{b}}_{\mathrm{T}}}}\mathrm{l}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}-{\displaystyle \frac{\mathrm{R}\mathrm{T}}{{\mathrm{b}}_{\mathrm{T}}}}\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{T}}$$

(7)

where q_max (mg·g⁻¹) is the maximum adsorption capacity; K_L (L mg⁻¹) is the Langmuir constant related to the affinity between adsorbent and adsorbate; K_F [(mg·g⁻¹)(L·mg⁻¹)^1/n)] is the Freundlich constant; 1/n (dimensionless) is the Freundlich exponent; R is the gas constant (8.314 J·mol⁻¹·K⁻¹); T is the absolute temperature (K); K_T (L·g⁻¹) is the Temkin equilibrium binding constant (L·g⁻¹); and b_T is the Temkin constant related to the heat of adsorption (kJ·mol⁻¹).

2.6.6. Regeneration and Recyclability Process

The adsorbent regeneration and recycling process was carried out using ciprofloxacin at 10 ppm. For this, the heterostructure sample loaded with ciprofloxacin, i.e., after adsorption assays at 10 ppm drug solution, was washed with 30 mL of a 70% acetone solution as extractant. The system was shaken for 1 h, the solid was isolated by centrifugation (4000 rpm for 10 min), and the removed supernatant was analyzed in the UV-Vis spectrophotometer. The isolated sample was dried in an oven at 60 °C and reused in a new cycle of adsorption of ciprofloxacin using the same conditions described previously in the adsorption studies section.

3. Results and Discussion

The synthesized hybrid material CeO₂/Paly demonstrated stability in the adsorption process at different concentrations. The adsorptive activity of the hybrid material was previously investigated through kinetic and isothermal studies. The following section presents detailed results of the characterization of the synthesized materials.

3.1. PXRD Measurements

The structural characteristics of the powdered hybrid materials were investigated using the PXRD technique (Figure 1). CeO₂ exhibits characteristic diffraction peaks at the planes (111), (200), (311), (222), (400), and (420), with 2θ values of 28.6°, 33.1°, 47.6°, 56.4°, 59.1°, 69.4°, 76.8°, and 79.1°, respectively. These diffraction peaks correspond to a cubic structure with space group Fm3m, as described in the JCPDS card number 34-0394 [21], and the absence of secondary impurity peaks suggests the high purity of the CeO₂ sample. The palygorskite mineral shows diffraction peaks corresponding to an orthorhombic crystalline structure with space group Pncn by the JCPDS 31-0783 card [22]. The Paly clay mineral exhibits peaks of lower intensity, corresponding to quartz impurity (JCPDS 85-0794) [23]. Paly shows the highest intensity diffraction peak at 2θ of 8.3°, corresponding to the primary reflection planes (110), and also presents other planes (200), (130), (040), (121), (310), (400), and (102) [19,24]. On the other hand, in the hybrid material, it is possible to observe the maximum diffraction intensity associated with the clay mineral occurring at 8.3°, as well as the presence of intense characteristic reflections of CeO₂. This way, the association of Paly to CeO₂ nanoparticles in the hybrid material is confirmed, where both phases are preserved throughout the synthesis process, highlighting the significant contribution of the support.

3.2. FTIR Analysis

Figure 2 depicts the FTIR spectra for the CeO₂, Paly, and the CeO₂/Paly heterostructure samples. The CeO₂ semiconductor exhibits a band at 3498 cm⁻¹, attributed to the O-H stretching vibration in OH⁻ groups. Additionally, a band at 1592 cm⁻¹ related to residual water bending vibration was observed, along with bands at 1374, 841, and 538 cm⁻¹ assigned to Ce-O-Ce and Ce-O vibrations, respectively [25]. The Paly clay mineral shows bands at 3617 and 3543 cm⁻¹ associated with hydroxyls’ (OH) stretching vibration bound to Mg and Al [26]. Other important bands were identified at 3405 and 1654 cm⁻¹, related to zeolitic water, and at 1080 and 950 cm⁻¹, attributed to Si-O stretching, with an additional band at 489 cm⁻¹ corresponding to Si-O deformation [27]. For the CeO₂/Paly hybrid material, bands at 3604, 3532, 3410, 1653, 1386, 1180, 1023, 956, and 489 cm⁻¹ were observed. These bands indicate the presence of functional groups from CeO₂ and Paly, confirming the formation of the hybrid material and its vibrational interaction. Unfortunately, analogously to sepiolite fiber clay mineral, the band associated with the Si-OH located on the surface of the clay mineral at approximately 3710 cm⁻¹ [28] cannot be appreciated in the spectra due to the use of KBr employed in the pellet formation for the measurement, causing a shift in this band toward lower frequencies, being overlapped by the wide range associated with OH groups [29].

3.3. Nitrogen Desorption and Adsorption Analysis

The textural properties of the synthesized materials were analyzed by nitrogen adsorption and desorption measurements, as presented in Figure 3. The Paly and CeO₂/Paly materials (Figure 3a) exhibit characteristics of Type IV isotherms, with desorption of the H3 hysteresis loop indicating a distinct trend with a sharp and rapid increase in adsorption at lower relative pressures [19,30]. In contrast, CeO₂ (Figure 3a) semiconductor material demonstrates a Type IV isotherm behavior, typical of mesoporous materials [31,32]. Figure 3b shows the pore size distribution for all samples, and it is observed that the heterostructure material contributes a wide range of pore sizes, likely due to pore size contribution from both CeO₂ and Paly moieties.

Table 1. Textural characteristics of the materials.

Samples	SBET (m2/g)	Pore Diameter (nm)	Pore Volume (cm3/g)
CeO2	91.50	11.9	0.21
Paly	306.0	1.91	0.58
CeO2/Paly	443.3	1.91	0.59

exhibits the data obtained from the isotherms, which can determine the specific surface area (S_BET), pore volume, and diameter of the synthesized materials. The combination of the semiconductor with the silicate in the CeO₂/Paly sample shows an increase in surface area in comparison to CeO₂ (91.50 m²/g) and Paly (306 m²/g) alone, exhibiting an S_BET of 443.3 m²/g, indicating the formation of a new porous structure resulting from the interaction between the components. Furthermore, it demonstrated a relatively larger pore volume of 0.59 cm³/g of the heterostructure material in comparison to stating moieties. These results indicate that CeO₂/Paly heterostructure material exhibits improved textural properties, such as adsorption properties, which are very interesting for surface chemistry reaction purposes.

3.4. SEM

The morphology of the CeO₂/Paly was investigated using SEM, and the images are revealed in Figure 4. The Paly (Figure 4a) exhibited a fibrous morphology, resembling needles, with agglomerated bundles of several elongated fibers. When the CeO₂ nanoparticles (Figure 4b) were combined on Paly in the heterostructure materials (Figure 4c), it was possible to evidence a new texture where the particles of the semiconductor seem to act as agglutinate of the palygorskite fibers, which is a characteristic typical of composites containing CeO₂ and clay minerals [33,34,35].

3.5. XPS

Figure 5 shows the XPS spectrum of the CeO₂ used in our studies, displaying multiple peaks corresponding to the Ce 3d core level. Such an analysis is crucial for understanding cerium’s chemical state and surface composition. A U2 Tougaard-type background function has been applied for background, and an LF line shape function has been used to fit the peaks in this spectrum. The 3d_5/2 (designated as v) and 3d_3/2 (designated as u) components were identified, comprising 10 peaks. These peaks were curve-fitted into four doublets, representing Ce⁴⁺ (v, v″, v″′, u, u″, and u″′) and Ce³⁺ (v⁰, v′, u⁰, u′) species. Such results allowed us to calculate the quantities of Ce⁴⁺ (60.1%) and Ce³⁺ (39.8%).

3.6. Point of Zero Charge

The point of zero charge (pH_PZC) offers valuable insights into the electrical properties of an adsorbent, playing a crucial role in understanding the interaction between the adsorbent and adsorbate during the adsorption process. As shown in Figure 6, the pH_PZC values for CeO₂, Paly, and CeO₂/Paly were 6.11 and 6.13, respectively. These results indicate that when the solution pH is higher than the pH_PZC, the surfaces of CeO₂ and CeO₂/Paly become negatively charged. Conversely, the surfaces become positively charged at a pH lower than the pH_PZC. This variation in surface charge across different pH ranges significantly impacts electrostatic interactions and, consequently, the effectiveness of the adsorption process. Notably, the clay mineral Paly did not exhibit a pH_PZCwithin the investigated pH range, suggesting a permanently negative surface charge, which could be related to its basic character given the high content of basic salts associated with its geological formation [18].

3.7. Adsorption Study

The adsorption experiments were conducted following a calibration curve to ensure that the linear regression coefficient was close to one, ensuring the results’ accuracy. Additionally, the pH variation’s influence on the efficiency of the adsorption process was examined.

3.7.1. Influence of pH

The effect of pH on the adsorption efficiency of ciprofloxacin was evaluated using solutions with varying pH values (3, 7, and 9) at a constant ciprofloxacin concentration of 10 ppm (Figure 7). The results revealed significant differences in ciprofloxacin removal rates among the synthesized adsorbents. The pH of the solution significantly affects the ciprofloxacin molecule due to its two pKa values (5.90 and 8.89) [1].

At pH 7, all three materials (CeO₂, Paly, and CeO₂/Paly) exhibited high removal efficiencies due to the interaction between the negatively charged surfaces of the adsorbents and the zwitterionic form of ciprofloxacin (the predominant form at pH 7) [2]. The removal rates at pH 7 were 25% for CeO₂, 75% for Paly, and 94% for CeO₂/Paly. Notably, at pH 9, there is a reduction in the drug removal efficiency to 54% for Paly, and 90% for CeO₂/Paly (pH higher than the materials’ pH_PZC values), and the surface charge of these adsorbents becomes more negative. Since ciprofloxacin predominantly exists in its deprotonated (anionic) form at this pH, electrostatic repulsion between the negatively charged species decreases removal efficiency. However, some adsorption still occurs, likely due to interactions between the deprotonated ciprofloxacin and positively charged sites on the adsorbents. A significant increase in the removal rate for CeO₂ at this pH (73%) suggests a greater participation of Ce(IV) in interactions with the adsorbate, likely through complexation or coordination, which can overcome the electrostatic repulsion despite the overall negative surface charge. This enhanced interaction contributes to the higher adsorption efficiency observed. In the CeO₂/Paly composite, the presence of Ce(IV) also mitigates the reduction in removal efficiency compared to Paly alone, indicating that Ce(IV) plays a crucial role in maintaining adsorption performance even at higher pH levels.

At pH 3, the removal rates were approximately 22% for CeO₂, 91% for Paly, and 81% for CeO₂/Paly. In this acidic medium, CeO₂ and CeO₂/Paly have positively charged surfaces, while Paly remains negatively charged. Ciprofloxacin is primarily in its cationic form due to the protonation of the amine group. Consequently, a strong electrostatic attraction exists between the positively charged adsorbents (CeO₂ and CeO₂/Paly) and the cationic ciprofloxacin, leading to the high removal efficiencies observed for Paly and CeO₂/Paly at this pH. Overall, the CeO₂/Paly heterostructure material exhibited higher drug removal efficiency (pH 7 and 9), possibly due to the combined effect of the semiconductor and clay mineral, increasing the available active sites for drug adsorption, as observed in the BET analysis. Therefore, pH directly influences adsorption by influencing the adsorbents’ surface charge and ciprofloxacin’s speciation, ultimately affecting the removal efficiency. The varying ionic forms of ciprofloxacin due to its pKa values result in different interactions with the adsorbent materials, leading to the observed removal efficiencies. Based on these results, subsequent studies were conducted at pH 7, where the CeO₂/Paly material demonstrated the highest adsorption capacity.

3.7.2. Influence of Concentration

The influence of ciprofloxacin concentration on the adsorption behavior of CeO₂, Paly, and CeO₂/Paly materials was investigated at pH 7 to understand their performance under varying conditions and applicability in removing pharmaceuticals from aqueous solutions (Figure 8). Similar removal efficiencies were observed for ciprofloxacin concentrations of 6, 8, 10, and 14 ppm. The CeO₂/Paly hybrid material exhibited stable adsorption capacities across this range, with values of 5.74 mg·g⁻¹, 7.43 mg·g⁻¹, 9.43 mg·g⁻¹, and 12.40 mg·g⁻¹, respectively. This indicates a consistent adsorption capacity within the investigated concentration range, even with the increasing drug concentration. Moreover, the system demonstrated rapid and favorable adsorption kinetics, reaching equilibrium within a short time interval of 15 min. Throughout all evaluated concentrations, the hybrid material consistently displayed superior efficacy in ciprofloxacin removal, achieving approximately 98% removal compared to CeO₂ and Paly alone. This suggests that the incorporation of CeO₂ nanoparticles improved the surface properties of the Paly clay mineral, since this heterostructure material showed efficient drug removal with high adsorption rates at different concentrations. This study highlights the potential of this material for developing efficient strategies for contaminant removal under varying concentration scenarios. Due to this, the CeO₂/Paly sample was selected for further experiments to understand the adsorption process.

3.7.3. Influence of Contact Time

Considering that the adsorption efficiency of ciprofloxacin (%R) for CeO₂/Paly material was 95.0, 93.0, 94.0, 87.0, and 88.0% at the concentrations of 6, 8, 10, and 14 ppm, respectively, the concentration of 10 ppm was chosen for contact time and subsequent kinetic studies. Considering this, the results revealed rapid initial ciprofloxacin adsorption within 15 min, followed by a gradual decrease in the adsorption rate until equilibrium was reached. Figure 9 illustrates the increase in adsorption rate over time, with a higher removal efficiency of 94.3% in the initial minutes due to the abundant binding sites available. As these sites became occupied, the adsorption rate stabilized, leading to equilibrium. The CeO₂/Paly material exhibited rapid adsorption within 5 min, followed by a gradual increase up to 120 min, reaching an adsorption capacity of 9.43 mg·g⁻¹ at equilibrium. This is superior to the capacities of CeO₂ (2.54 mg·g⁻¹) and Paly (7.107 mg·g⁻¹). The ciprofloxacin removal efficiency mirrored this trend, with 98% removal for the heterostructure material, 25% for CeO₂, and 71% for Paly. These findings highlight the effectiveness of the CeO₂/Paly heterostructure material in removing ciprofloxacin from aqueous solutions, indicating its potential for applications in wastewater treatments.

3.7.4. Kinetic Study of Adsorption

The pseudo−first−order and pseudo-second-order kinetic models were applied to the experimental data obtained for ciprofloxacin removal onto CeO₂/Paly (Figure 8c) to identify the model that best describes the adsorption process, as in Figure 10. The corresponding kinetic parameters are summarized in

Table 2. Parameters of the first-order and second-order kinetic models for the adsorption of ciprofloxacin onto CeO₂/Paly heterostructure material.

Parameter/Model	Pseudo-First-Order	Pseudo-Second-Order
qe experimental (mg·g−1)	9.43
qe calculated (mg·g−1)	0.17	9.41
Constant k1 (min−1)	1.66 × 10−3	-
Constant k2 (g mg−1 min−1)	-	5.83 × 10−1
Coefficient of determination R2	0.080	0.999
Sum of squares errors (SSE)	3.24 × 10−3	6.38 × 10−3

R² = measure of how well the data fits the model; SSE: sum of the squared differences between experimental and predicted values, indicating the precision of the model fit.

, and the fitting results are shown in Figure 10.

Figure 10b illustrates that the pseudo−second−order model exhibits a better fit to experimental data, with linearity coefficients R² exceeding 0.99. This model suggests that the rate-limiting step involves physicochemical interactions between the adsorbate (ciprofloxacin) and the functional groups present on the adsorbent surface, implying a chemisorption process [36]. The analysis is further supported by comparing the adsorbed drug (q_e) amount in the pseudo−first−order and pseudo−second−order models (Table 2). This conclusion is further supported by comparing the q_e (amount of adsorbed ciprofloxacin) and k (rate constant) values obtained from both models (Table 2). The pseudo-second-order model provides a more accurate representation of the adsorption kinetics based on these parameters. These findings align with several studies reported in the literature, which suggest that pseudo-second-order models provide a more accurate description of adsorption kinetics when dealing with heterostructure adsorbents based on clay minerals. For example, ref. [37] demonstrated that the adsorption of methylene blue onto palygorskite- cerium oxide composites followed pseudo-second-order kinetics with excellent correlation coefficients (R² > 0.99). Similarly, ref. [38] reported that the adsorption of heavy metal ions onto clay–metal oxide hybrid materials predominantly followed the pseudo-second-order model. These studies, along with others, highlight the prevalence of the pseudo-second-order model in describing the adsorption kinetics of systems involving heterostructure materials, emphasizing the importance of specific chemical interactions between the adsorbate and adsorbent.

To better assess the goodness of fit, the sum of squared errors (SSE) for each model was also calculated. The SSE provides a quantitative measure of the discrepancy between the experimental data and the model predictions. A lower SSE indicates a better fit, and as shown in Table 2, the pseudo-second-order model yielded a lower SSE compared to the pseudo-first-order model, suggesting that the pseudo-second-order model more accurately describes the adsorption kinetics of ciprofloxacin onto the CeO₂/Paly heterostructure material.

3.7.5. Isothermal Adsorption Models

Figure 11 shows the adsorption isotherms of the ciprofloxacin drug over CeO₂/Paly, which relate to the amount adsorbed and the equilibrium concentration of the adsorbate. The heterostructure exhibited a higher adsorption capacity at different drug concentrations, reaching values very close to the initial ciprofloxacin concentrations, suggesting a high affinity between the adsorbate and CeO₂/Paly adsorbent. To further analyze this behavior, the Langmuir, Freundlich, and Temkin models were applied to the adsorption data of the CeO₂/Paly heterostructure, aiming to describe the mathematical relationship between the adsorbed amount and the equilibrium concentration.

To deepen the understanding of the interaction between the adsorbate and the adsorbent, the experimental adsorption data were analyzed using Langmuir, Freundlich, and Temkin isotherm models in Figure 12, with the calculated parameters presented in

Table 3. Isothermal parameters for adsorption removal of ciprofloxacin onto CeO₂/Paly using Langmuir, Freundlich, and Temkin Models.

Model	Parameter	Values
Langmuir	qm (mg·g−1)	15.0
	KL (L·mg−1)	2.07
	R2	0.958
	SSE	2.7 × 10−4
Freundlich	KF [(mg·g−1)(L·mg−1)1/n)]	9.75
	n	2.54
	R2	0.946
	SSE	1.9 × 10−2
Temkim	KT (L g−1)	19.5
	b (J mol−1)	74.103
	R2	0.923
	SSE	2.08

. The Freundlich model indicates the presence of heterogeneous adsorption sites on the surface of the adsorbent and that the adsorbed species mutually influence each other [39]. Additionally, adsorption in this model is not limited to a single layer. In contrast, the Langmuir model predicts that the adsorbed particles are independent of each other and form a single monolayer on the surface of the adsorbent [40]. The Temkin model suggests that the interaction between the adsorbent and adsorbate, and consequently the heat of adsorption, decreases linearly with coverage for all molecules in the layer [20]. Among the evaluated models, the Langmuir model provided the best fit for the experimental adsorption data of ciprofloxacin on CeO₂/Paly, with a correlation coefficient (R² = 0.958) superior to those of the Freundlich and Temkin models (R² = 0.946 and 0.923, respectively). This indicates a monolayer adsorption process, where only a single layer of ciprofloxacin molecules can be adsorbed onto the CeO₂/Paly adsorbent surface. The maximum adsorption capacity (q_m) was calculated from the Langmuir isotherm to be 15.0 mg·g⁻¹, a value very close to that obtained experimentally (9.43 mg·g⁻¹), providing additional confirmation that the Langmuir model is appropriate for the adsorption data.

3.7.6. Regeneration and Recyclability Processes

The regeneration and reuse of adsorbents have significant potential for water decontamination applications. The adsorption capacities of the CeO₂/Paly hybrid material for ciprofloxacin over three cycles are shown in Figure 13. From this figure, it can be observed that CeO₂/Paly is promising after consecutive cycles, where it remained active with a ciprofloxacin removal percentage of 94% and 61% for the first and second reuse cycles. However, the data also indicate a gradual decline in the adsorption capacity of the heterostructure material, mainly after the third reuse cycle, showing an adsorption efficiency of 34%. This significant reduction in the adsorption capacity after the third reuse cycle can be attributed to the incomplete regeneration of the adsorption sites during the material regeneration process. As the reuse cycles progressed, the adsorption sites of the material gradually became saturated, leading to an equilibrium state for drug adsorption.

3.7.7. Characterization after the Adsorption Process

The heterostructure material was characterized by FTIR spectroscopy after the adsorption process to observe potential structural change (Figure 14). The results indicated that the material’s overall structure remained almost the same. However, a slight decrease in the intensity of the band at 3586 cm⁻¹, corresponding to lattice water, was observed. Additionally, the analysis demonstrated that the material remained stable after adsorption, as the characteristic vibration bands of the semiconductor and the clay mineral were still present. Consequently, the heterostructure material appears promising for environmental remediation applications, as it exhibited a high adsorption rate of the contaminant at different concentrations and maintained structural stability.

4. Conclusions

This study showed the synthesis and characterization of hybrid material based on the wet impregnation of CeO₂ nanoparticles in palygorskite clay, demonstrating a significant advance in the search for effective and sustainable solutions for the removal of organic contaminants in water resources. PXRD analysis confirmed that both phases were preserved during the association of CeO₂ nanoparticles with palygorskite, at the same time the combination of the semiconductor with the silicate in the CeO₂/Paly sample provides an increase in the surface area and pore volume, indicating the formation of a new porous structure. SEM images confirm the formation of a new texture, where CeO₂ nanoparticles act as binders of the palygorskite fibers. The adsorption efficiency of ciprofloxacin by the CeO₂/Paly sample at different pH values showed a higher removal efficiency at pH 7 due to the interaction between the negatively charged surfaces of the adsorbents and the zwitterionic form of ciprofloxacin. At varying concentrations of ciprofloxacin, the CeO₂/Paly heterostructure material maintained stable and rapid adsorption capacities, where the Langmuir model provided the best fit to the experimental data, suggesting a monolayer adsorption process. Kinetic analysis indicated that the pseudo-second-order model best fitted the experimental data, suggesting that the physicochemical interactions between the adsorbate and the functional groups of the adsorbent are the rate-limiting step. This material underwent regeneration and recyclability processes, showing promise after consecutive cycles of adsorption. FTIR characterization after the adsorption process revealed that the structure of the material remained practically unchanged, indicating its stability and potential for applications in environmental remediation.