Photocatalytic Removal of Ciprofloxacin in Water by Novel Sandwich-like CuFe2O4 on rGO/Halloysite Material: Insights into Kinetics and Intermediate Reactive Radicals

Abstract

In this study, the CuFe₂O₄ on rGO/halloysite material was made in an uncomplicated manner. The catalyst has a sandwich-like shape with a uniform coating of the active phase on the rGO sheets and halloysite tubes. The catalyst’s large specific surface area (130 m²/g) and small band gap energy (1.9 eV) allow it to adsorb photons and photocatalyze organic contaminants effectively. In approximately 1 h of light, the catalyst showed high performance in achieving almost complete conversion in photodegrading CIP for an initial CIP concentration of 20 ppm. A pseudo-first-order rate law was followed by the process, as revealed by the experimental results. In addition, the pH effect and the contribution of intermediate reactive radicals that emerged during the photochemical process were explored. The results indicated that hydroxyl radicals and holes had a major impact on CIP decomposition, suggesting that the addition of these radicals could enhance CIP degradation efficiency at a larger scale. This study also confirmed the superiority of catalysis and photochemical processes in environmental treatments by the neutral pH values.

1. Introduction

Antibiotics are undeniably crucial in today’s world. They have helped people overcome many major diseases, and they are also used to promote growth in animals. Numerous investigations have discovered the presence of antibiotics in aquatic organisms, animal feces, soil, sediments, and drinking water as well as in the aquatic environment (wastewater, surface water, groundwater) [1,2]. Concentrations of a few 100 mg/L or mg/kg dry weight concentrations have been recorded in several locations [3]. Antibiotic concentrations in wastewater from hospitals, industrial areas, and waste dumps have all been reported to be excessive [4,5,6]. This will have a detrimental effect on human health, increasing the risk of pollution and disease spreading in the community.

Ciprofloxacin (CIP) is a fluoroquinolone drug prescribed for the treatment of infections involving bacteria. CIP is now widely utilized not only in people but also in animals and plants. CIP concentrations ranging from 0.0018 nmol/L to 19,617 nmol/L were measured in the surface water [7].

Publications have pointed out that CIP is susceptible to direct photochemical alterations generated by direct ultraviolet irradiation as well as the inclusion of photocatalysts and hydrogen peroxide (H₂O₂) [8,9,10]. Photocatalysis is a highly effective, low-cost, and ecologically benign approach for treating CIP-containing water. Among the challenges of photocatalysis is to improve the efficiency and selectivity of semiconductor photocatalysts under visible-light irradiation. Conventional TiO₂, which is widely used as a photocatalyst, has some limitations such as a large band gap (3.0–3.2 eV) that restricts its absorption to the ultraviolet region of the solar spectrum, and a high rate of recombination of photogenerated charge carriers that reduces its photocatalytic activity. Therefore, various strategies have been proposed to overcome these drawbacks, such as doping with metals or non-metals, coupling with other semiconductors, modifying with noble metal nanoparticles, and designing novel nanostructures [11,12]. Several photocatalysts for CIP treatment have been researched and produced, including ZnO-based photocatalysts [13], Ag-TiO₂ photocatalysts [14], and Cu₂O photocatalysts [15,16]. Since the photo-Fenton effect and the existence of a metal combination are present, CuFe₂O₄ is a new material that has a small band gap energy and exhibits high and consistent activity during reactions [17,18]. Using a support with the catalyst spread across the surface has been suggested as one way to boost the catalyst’s effectiveness. Materials that have large specific surfaces, great strength, and stability are considered supports. Typical supports include Al₂O₃, SiO₂, SBA-15, and MIL 141. Among the disadvantages of using catalyst support in a photocatalyst is that it may reduce the photocatalytic activity of the catalyst by blocking the active sites or increasing the charge recombination rate. Another disadvantage is that it may affect the stability and recyclability of the photocatalyst by leaching or corrosion. Therefore, choosing a suitable catalyst support material is crucial for enhancing the photocatalytic performance and durability of the catalyst. Some of the factors that should be considered when selecting a catalyst support are the surface area, porosity, chemical stability, optical properties, and compatibility with the photocatalyst [19,20].

Halloysite is a natural kaolin-group mineral that comes in a variety of tubular forms ranging in size from 100 to 200 nm. Halloysite also has a specific area ranging from 50 to 150 m²/g. Furthermore, halloysite nanotubes (HNTs) are highly stable, resistant to organic solvents, and simple to reuse after the reaction. A significant number of recent research efforts have targeted on utilizing halloysite as a support material, where nanoscale metal particles are placed on the exterior of halloysite acting as the catalytic component [21,22,23].

Further, graphene’s superior adsorption capability in environmental treatment processes has been investigated and verified [24]. Several methods for making graphene-based photocatalysts have been presented in recent studies, and their effectiveness has also been evaluated. In this case, graphene acts as a center for receiving and transferring electrons, enabling the catalyst to participate in photo-excited reactions with reduced absorbed energies [25,26]. However, released studies show some difficulties with the graphene synthesis process, specifically with the re-adhesion of carbon monolayers in graphene.

Few studies have focused on the process of combining halloysite and graphene to produce nanocomposites because of the advantages of these two materials as independent entities as well as their limitations. This composite material has only been tested and orientated for use in direct adsorption or as a supercapacitor [27], building a membrane for the separation of water and oil [28], combining materials with epoxy resin to increase mechanical qualities and heat resistance [29], or utilizing materials in conjunction with metallic silver as a biocide [30]. This material has not been targeted as a catalyst since there is little information available about it, thus it is frequently used in its natural state, especially for photocatalysis. As far as we are concerned, there are no publications on the use of the nanocomposite in organic pollutant degradation.

This study describes the process of extracting halloysite from its natural ore form. Reduced graphene will next be mixed with the nanoscale halloysite tube to generate the composite material. This material will be employed as a support for copper ferrite active-phase distribution. Photocatalytic activity, kinetics, pH effect as well as the presence of reactive radicals during the reaction were also investigated and examined.

2. Experimental

2.1. Chemicals

To produce the rGO/HNT composite, graphite and raw halloysite ore were used as the starting materials. After being dried for a day at 80 °C to eliminate the water content, they were crushed and sifted The reagents used for the oxidation of graphite and the functionalization of rGO were KMnO₄ (98%), H₂O₂ (30%), HCl, H₂SO₄ (98%), Na₂NO₃, hydrazine, isopropyl alcohol (99.7%), EDTA (99%), p-benzoquinone (99%), and DMSO (99%), all purchased from Xilong Scientific Co., Ltd., Guanzhou, China. The metal salts used for the preparation of CuFe₂O₄ nanoparticles were Cu(NO₃)₂ (>99.9%) and Fe(NO₃)₃ (>99.95%), obtained from Sigma Aldrich, Darmstadt, Germany. The model pollutant used for the photocatalytic degradation experiments was ciprofloxacin (99.99%), supplied by Merck, Darmstadt, Germany. All the chemicals used were of analytical grade and were used as they were, without any additional purification.

2.2. Graphene Oxide (GO) Elaboration

Graphite powder was converted into graphene oxide by the authors using the Hummers method [19]. Graphite was mixed with 98% H₂SO₄ at less than 5 °C for 30 min until the color changed to black. KMnO₄ was introduced to the mixture and it was maintained at a temperature of 20 °C for a duration of 4 h. The mixture was gradually combined with distilled water and heated to a temperature below 50 °C for a period of three hours. H₂O₂ was introduced to the mixture and it was agitated for a duration of 20 min, resulting in a light brown color. Then, it was filtered and rinsed with a 0.1 M HCl solution until it reached a pH of 7, then it was dried at a temperature of 70 °C. GO was obtained after ultrasonic treatment for 30 min, following the method of Ngo et al. (2023) from a previous publication [31].

2.3. Extraction of Halloysite Nanotubes

Raw halloysite, obtained from a kaolin deposit in Vietnam, was ground and sifted to eliminate impurities. The unprocessed halloysite was dried at a temperature of 100 °C for a duration of three hours, after which 20 g of it was combined with 27.5 mL of purified water. A 98% H₂SO₄ solution of 1 mL was slowly added, and the mixture was agitated for two hours at 90 °C. The resulting mixture was strained and repeatedly washed with distilled water to eliminate any excess H₂SO₄. The dried sample was added to 750 mL of distilled water, which was then agitated for 24 h before being left to settle for 96 h. The solution was filtered, and the top was separated from the bottom residue. The obtained solid was repeatedly rinsed with water, then filtered, centrifuged, and dried to yield halloysite. The purification process was carried out according to the procedure outlined by Ngo et al. (2023) [31].

2.4. Elaboration of rGO/HNT

To make rGO/HNT composite nanomaterials, the following procedure was followed: a mixture was created by combining 100 mg of HNT with 50 mL of purified water, and it was then ultrasonically stirred for 10 min (solution 1). In addition, 200 mg of GO was dispersed in 200 mL of water, and the dispersion was then ultrasonically treated for 10 min (solution 2). The two solutions were combined, agitated for a duration of 10 min, and ultrasonically processed for 30 min at 90 °C. The next phase included adding 80% hydrazine of 2 mL gradually while stirring constantly for two hours. The residual material was filtered and dried to produce rGO/HNT material. The method of Ngo et al. (2023) was followed for the fabrication of rGO/HNT [31].

2.5. Elaboration of the CuFe₂O₄/Nanocomposite

The synthesis of the CuFe₂O₄/rGO/HNT composite was carried out as follows: first, 0.5 g of the rGO/HNT composite was mixed into 50 mL of purified water and subjected to ultrasonic processing for 15 min. After that, a solution of Fe(NO₃)₃ and Cu(NO₃)₂ at a molar ratio of 2:1 was prepared by dissolving the salts in 20 mL of 30% H₂O₂. This solution was gradually added to the rGO/HNT mixture while it was being subjected to ultrasonic processing for an additional 15 min. The pH level of the mixture was raised to 10 by including 25% NH₄OH solution. The solid material that formed in the mixture was separated by spinning it at a high speed (5000 revolutions per minute) using a centrifuge. The solid material was then repeatedly rinsed with ethanol and water. The CuFe₂O₄/rGO/HNT composite was obtained by drying the solid material at 80 °C overnight and then calcining it in a N₂ atmosphere at 650 °C for 1 h. The effect of the active-phase content on the catalytic activity of the composite was investigated by varying CuFe₂O₄ loading from 30% to 70% by weight.

2.6. Material Characterizations

A JSM-6701F (JEOL) scanning electron microscope (SEM) was used to examine the morphological characteristics of the prepared material. Prior to conducting SEM analysis, a small quantity of the material suspension was deposited onto a silicon wafer and left to dry. High-resolution transmission electron microscopy (HRTEM) images were captured by JEM2100 device (JEOL, Tokyo, Japan). The crystal composition of the material was ascertained using a D8-Bruker X-ray diffraction (XRD) instrument with Cu Kα radiation (λ = 1.5403 Å) from 5 to 80° of 2θ. Fourier-transform infrared (FTIR) spectroscopy with a Jasco FT/IR-4600 instrument enabled the investigation of the material’s functional groups and bonding features. An ESEM-XL30 apparatus was used to verify the elemental composition of the material by energy dispersive X-ray (EDX) spectroscopy. UV–vis spectroscopy with a Jasco V-750 instrument in the wavelength between 200 and 800 nm enabled the measurement of the material’s optical properties and bandgap energy. N₂ adsorption-desorption isotherm technique (BET) at 77.34 K with a CHEMBET-3030 instrument allowed the determination of the material’s specific surface area and pore size distribution. Finally, The drift method was used to measure the point of zero charge (pzc) of the nanocomposite [32].

2.7. Catalytic Activity Evaluation

The following conditions were used for the photocatalytic degradation of CIP: 50 mL of 20 ppm CIP in aqueous solution with 10 mg of the nanocomposite and 0.25 mL of 30% H₂O₂ as an oxidant. The mixture was agitated at 400 rpm for 30 min in the dark to achieve the adsorption equilibrium and to prevent the photolysis effect. After that, the photocatalytic reaction was initiated by switching the UV light source on (4 UV lamps, 60 W each). The samples were taken 30 min after the dark adsorption and every 1 h under the illumination. The samples were filtered with filter paper to eliminate residual catalyst and UV–vis spectroscopy was used to determine the CIP concentration.

2.8. Experimental Results Calculation

The photocatalytic degradation efficiency of CIP using the CuFe₂O₄/rGO/HNT catalyst was evaluated by measuring the conversion of CIP under different experimental conditions, such as the addition of scavengers and the variation of pH value. The following equation enabled the calculation of the conversion of CIP:

$\mathrm{Conversion}\left( \%\right)=\left(\frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_{\mathrm{o}}}\right)\times 1$

where C_o is the initial concentration of CIP in the solution and C_t is the final concentration. The concentrations were determined by UV–vis spectroscopy using the calibration curve, which was built based on the intensity of the absorbance peak at 273 nm.

3. Results and Discussion

3.1. Material Characterizations

Various physicochemical techniques were used to characterize the structural features of the prepared catalysts.

Figure 1 shows the X-ray diffraction patterns of the catalysts between 2θ of 10 and 80°. The characteristic peaks at 18.5°, 30.1°, 35.6°, 38.4°, 42.9°, 53.2°, 56.8°, and 62.2° correspond to the spinel phase of CuFe₂O₄ (JCPDS No. 34-0425) [33,34]. The presence of rGO in the catalysts is indicated by the peak at 26°, which is also attributed to HNT. The peak at 49° reveals the impurity of copper oxide (JCPDS 80–1268). The existence of each catalytic phase was further confirmed by Raman, FT-IR, and EDX analyses.

Figure 2 shows the results of material characterization by Raman spectroscopy. At wavelengths of 1345.9 cm⁻¹ and 1593.7 cm⁻¹, respectively, the measurement data demonstrate the typical peaks of HNT and rGO [35]. The signals measured at 278 and 613 cm⁻¹ indicate the presence of CuFe₂O₄ [36].

FT-IR spectroscopy was used to acquire more accurate information on the distinctive bonds in the materials. The material characterization results using FT-IR are shown in Figure 3. The distinctive vibrations with wave numbers at 1083 cm⁻¹ (corresponds to Si-O), 912 cm⁻¹ (corresponds to Al-OH), and 752 cm⁻¹ (corresponds to Si-O-Si) demonstrate the presence of HNTs in rGO/HNT. No vibration of the carboxyl group (1714 cm⁻¹) was observed, showing that elaboration of rGO/HNT involved the reduction of GO to rGO. The C=C and C=O groups of rGO have stretching vibrations that cause peaks at 1633 cm⁻¹ and 1086 cm⁻¹ [37,38]. A weak vibration at the wavenumber of 2360 cm⁻¹ is characteristic of the C=O bond, indicating that a small portion of graphene oxide has not been completely reduced. In addition, vibrations at wavenumbers of 1383 cm⁻¹ and 3445 cm⁻¹ indicate the presence of OH groups in the halloysite nanotube composition, in rGO and adsorbed water in the synthesized catalyst. The existence of CuFe₂O₄ bonding with rGO/HNT is shown by the 530 cm⁻¹ characteristic vibrations [39,40].

The surface morphology of the materials was investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Additionally, the photocatalyst’s specific surface area, pore volume, and pore diameter were measured by N₂ adsorption isotherm technique at 77.3 K using BET method. Figure 4A,B presents the SEM results while Figure 4C,D indicate the results obtained by HRTEM. The findings revealed by BET are illustrated in Figure 5a,b.

Accordingly, rGO sheets are prevented from adhering to each other by the insertion of nanoscale halloysite tubes between them. Additionally, SEM images reveal the CuFe₂O₄ particles deposited on rGO sheets and halloysite tubes. The catalyst particle size is approximately 100 nm according to SEM scans. HRTEM images reveal that halloysite nanotube tubes are inserted between layers of rGO, with the active phase being distributed on both the tube walls and the surface of rGO. Furthermore, the crystal structure of the catalyst material that has been synthesized is also shown in HRTEM images.

With the use of the BET data, it was determined that the catalyst material’s specific surface area is approximately 130 m²/g with the average pore diameter is approximately 5 nm. Additionally, the N₂ adsorption-desorption hysteresis of the material has a type IV shape according to IUPAC, which confirms the mesoporous material. The synthesized catalyst has a mesoporous system and a large specific surface area, which are ideal for the removal of organic pollutants using the adsorption–catalysis process.

Then, the presence and proportion of elements in the photocatalyst are verified by using the EDX technique. The findings are introduced in Figure 6.

All the elements that make up the composite are present, according to the results. Active-phase of CuFe₂O₄ has a percentage composition of 64%, which is comparable to the initial calculation made while synthesizing (the sample contains 70% active phase by mass).

The bandgap energy of the catalyst material was identified by UV–vis diffuse reflectance spectroscopy. The relation between [F(R)hv]² and hν can be used to determine the band gap energies of the materials.

The Wood–Tauc equation is used to determine the band gap energy of the materials:

(α·hν)^1/n = A(hν − Eg)

In this equation: α is the optical absorbance; hν is the photon energy; A and n are constant (n = 2); Eg is the band gap energy; n is a constant and n = 2. Figure 7 reveals the results.

The E_g value of the CuFe₂O₄/rGO/HNT composite is 1.9 eV, as can be observed. A smaller bandgap demonstrates that the material could have better photocatalytic activity and makes it able to operate in the visible light spectrum.

3.2. Impact of Active-Phase Loading

The material’s photocatalytic activity was assessed in the CIP photodecomposition process. A comparison of the catalyst sample activities with various active-phase contents is summarized in Figure 8.

The CIP conversion with the involvement of CuFe₂O₄-70%/rGO/HNT after 2 h of UV is 99%, which is higher than the value reported by Tamaddon et al. [18], who used the similar active phase with an initial CIP concentration of only 3 ppm. The results also showed better efficiency compared to values in previous publications [41,42]. This value is greater than that of samples with 30% and 50% wt. of active phase, which are at 85% and 86% correspondingly. In addition, an efficiency over 90% can be observed in all cases after 5 h of UV irradiation. This demonstrates that CuFe₂O₄-70%/rGO/HNT catalytic activity is superior to that of the catalysts with 30% wt. and 50% wt. of active phase. Due to the higher content of copper and iron, catalysts could react with H₂O₂ and water to produce more •OH radicals. The stability of the ferrite system is maintained during the decomposition process and •OH agents and then radicals are continually created with continuous conversion from Fe³⁺ to Fe²⁺ and Cu²⁺ to Cu⁺. This •OH radical could interact with CIP to create CO₂ and water, following the reactions:

Cu²⁺ + H₂O + hv (h⁺, e⁻) → •OH + Cu⁺ + H⁺

H₂O₂ + Cu⁺ + hv (h⁺, e⁻) → HO⁻ + •OH + Cu²⁺

Fe³⁺ + H₂O + hv (h⁺, e⁻) → •OH + Fe²⁺ + H⁺

H₂O₂ + Fe²⁺ + hv (h⁺, e⁻) → HO⁻ + •OH + Fe³⁺

•OH + H⁺ + CIP → CO₂ + H₂O + other simple products

To ensure accuracy and clearly evaluate the influence of scavengers, the material with 30% wt. of active phase was selected to perform the related experiments.

3.3. Photodegradation Kinetics of CIP

Experiments for investigating the kinetics of the reaction were conducted under the conditions: 50 mL of 20 ppm CIP solution, 0.25 mL of H₂O₂ and catalyst loading of 2.5 mg, 5 mg and 7.5 mg, respectively. The system was irradiated under UV light, and the lamps emitted light continuously within the range of 340–410 nm. The conversion of CIP is presented in Figure 9.

The effect of increasing the catalyst mass on the conversion rate of the reaction is shown in Figure 9. As the catalyst mass grows, more adsorption surface is available; and as the number of ions (Fe²⁺, Cu²⁺) rises, more free radicals (•OH, h⁺, •O₂⁻, e⁻) are produced. As a result, CIP photochemically degrades more quickly.

The following graphs in Figure 10a,b are plotted using experimental data from the equations of the pseudo-first- and second-order kinetic models. The photodegradation of CIP utilizing the catalyst CuFe₂O₄/rGO/HNT followed the pseudo-first-order rate law, which has a remarkably high R² coefficient, as shown in Figure 10a. The kinetic data of the reaction are illustrated more clearly in Table 1.

Table 1 reveals that as the catalyst volume rises, the reaction rate constant increases as well, resulting in a greater conversion. However, in some cases, using too much catalyst might result in a reduction in the effectiveness of the reaction. This behavior can be attributed to less effective light penetration into the reaction mixture that contains more catalyst. In other words, the amount of light scattering induced by catalyst particles increases with the amount of catalyst loading. This is in accordance with previous studies in the literature [43,44,45].

3.4. Impact of Solution pH

3.4.1. CuFe₂O₄/rGO/HNT Point of Zero Charge (pH_pzc) Determination

The pH_pzc value of CuFe₂O₄/rGO/HNT can be obtained using the drift method. The results are shown in Table 2 and Figure 11.

The pH_pzc value of CuFe₂O₄/rGO/HNT is approximately 6 (the intersection of the graphs y = x and y = 0.0929x² − 0.7621x + 7.2402). The adsorbent’s surface has a neutral charge at pH = pH_pzc. Depending on whether the pH is below or above the pH_pzc value, the surface of the adsorbent has a positive or negative charge. This improves its ability to adsorb anions or cations, respectively.

3.4.2. Influence of pH

Depending on the pH of the solution, CIP can exist in three forms because it is an ampholytic molecule with a carboxylic group and a nitrogen on the piperazinyl ring that has pKa values of 5.9 and 8.89, respectively. The three forms are: the cationic form at pH values under 5.9, the zwitterionic form at pH values between 6.9 and 8.89, and the anionic form at pH values over 8.89 [7].

Thus, the removal of CIP by CuFe₂O₄/rGO/HNT was tested at three pH values of 4, 7, and 10 to determine how pH influences the efficiency. The experiments used a fixed condition of 50 mL of CIP 20 ppm solution, 2.5 mL of H₂O₂ and 7.5 mg CuFe₂O₄/rGO/HNT. The results are introduced in Figure 12.

According to the data, removal of CIP increases as pH is adjusted from acidic to neutral, and it starts to fall at pH 10. The conversion of CIP at pH = 4, 7, and 10 after 5 h of UV irradiation was 86%, 98.00%, and 71%, respectively. Some inferences might be drawn from the findings found above:

The catalyst has a point of zero charge (pH_pzc) of 6.02. When the pH is lower than pH_pzc, the adsorbent’s surface area will have a positive charge and attract anions, and the opposite will happen when the pH is higher than pH_pzc.

At pH = 4, the degradation rate decreases and electrostatic repulsion occurs because the photocatalyst surface and the CIP molecule are both positively charged. In addition, least conversion was observed at pH = 10; the hydroxyl anions were more attracted to it, which lowers the creation of hydroxyl radicals. The system has less free hydroxyl ions, which limits the formation of hydroxyl radicals [46].

At pH = 7, the zwitterionic form or isoelectric point of ciprofloxacin is at pH 7.4. Photocatalytic degradation of ciprofloxacin was most effective at a pH close to this value. Because the CIP zwitterionic form and the material surface had a strong electrostatic interaction due to its slightly negative charge, the conversion was the highest at pH = 7 [16].

At pH = 10, electrostatic repulsion occurs and CIP degradation decreases because the photocatalyst and CIP molecule are both negatively charged. Additionally, the photocatalyst blocks UV light and lowers removal efficiency because of the considerable amount of hydroxide ions per unit volume. In a solution with a high pH, CO₂ transforms into HCO₃⁻ and CO₃²⁻. These substances can eliminate hydroxyl radicals. The rate of oxidation decreases when bicarbonates convert into carbonate ions and reduce OH radical [47].

In conclusion, pH = 7 conditions are optimal for the photochemical breakdown of CIP utilizing CuFe₂O₄/rGO/HNT (specifically, pH between 6 and 9). According to certain studies, hospital wastewater has a pH value of 6 to 9 [48], making the use of CuFe₂O₄/rGO/HNT to remove antibiotic residues at normal temperature and pressure totally appropriate and superior.

3.5. The Scavengers Test

To pinpoint the active species causing the photodegradation of ciprofloxacin, experiments with scavengers were conducted. Different scavengers are employed to identify the respective active species (•O₂⁻ radical), holes (h⁺), hydroxyl radicals (•OH), and electrons (e⁻), including p-benzoquinone (p-BQ), disodium ethylenediaminetetraacetate (2Na-EDTA), isopropyl alcohol (IPA), and dimethyl sulfoxide (DMSO), respectively. In these experiments, CuFe₂O₄-30%/rGO/HNT was used, and the samples were taken after 5 h of irradiation. The CuFe₂O₄-30%/rGO/HNT catalyst was chosen for this experiment to enable tracking of CIP conversion changes over time. Catalysts with a higher amount of active phase can rapidly achieve full conversion, making it imprecise to assess the influence of scavengers on active intermediate radicals. The other conditions were kept intact as those applied in photocatalytic activity tests.

The involvement of different reactive radicals in the photodecompostion of CIP antibiotic is depicted in Figure 13.

As can be observed, the reaction system’s CIP degradation efficiency drastically decreased once traps were included. In particular, the efficiency of the process with the addition of 2Na-EDTA (h⁺ trap), IPA (•OH trap), and DMSO (e⁻ trap) reduced from 84% to 51%, 67%, and 71%, respectively. This demonstrates how active species such as holes (denoted as h⁺), hydroxyl radicals (denoted as •OH), and electrons (denoted as e⁻) are crucial to the CIP degradation process. The reaction efficiency rises from 84% to 98% using the p-BQ trap (also known as the “•O₂⁻”), and this phenomenon is caused by the trap’s mechanism. The response is as follows [49]:

$\mathrm{BQ}\underset{{•{\mathrm{O}}_2}^{-},{\mathrm{H}}^{+}}{\to }\mathrm{QH}• \underset{{•{\mathrm{O}}_2}^{-},{\mathrm{H}}^{+}}{\to }\mathrm{H}2\mathrm{Q}$

p-BQ not only interacts with •O₂⁻, but also with H⁺, causing a reduction in the quantity of H⁺, as can be observed from the reactions.

Along with that, the catalyst’s photodegradation process results in:

Cu²⁺ + H₂O + hv → •OH + Cu⁺ + H⁺ (1),

H₂O₂ + Cu⁺ + hv → HO⁻ + •OH + Cu²⁺ (2),

Fe³⁺ + H₂O + hv → •OH + Fe²⁺ + H⁺ (3), and

H₂O₂ + Fe²⁺ + hv → HO⁻ + •OH + Fe³⁺ (4).

Reactions (1) and (3) will change in a way that produces more H⁺ ions while also producing a lot of •OH as the amount of h+ drops. The sample with the 1,4-BQ trap has a greater conversion than the sample without the trap because of the abundant •OH generated during the reaction process. •O₂⁻ hence plays a small role in the conversion of CIP. This implies that the active radicals participated in the decomposition of CIP as follows: h⁺ > •OH > e⁻ > •O₂⁻.

3.6. The CIP Photodegradation Mechanism with CuFe₂O₄/rGO/HNT

3.6.1. Calculation of the Conduction Band and Valence Band Energy (E_CB, E_VB)

The following empirical equations are used to determine the components’ conduction band (CB) and valence band (VB) energy [50].

(1)${\mathrm{E}}_{\mathrm{C}\mathrm{B}}=\mathsf{\chi }-{\mathrm{E}}^{\mathrm{e}}-\frac{1}{2}{\mathrm{E}\mathrm{g}}^{\begin{array}{c}\\ \end{array}}$

(2)${\mathrm{E}}_{\mathrm{VB}}=\mathrm{Eg}+{\mathrm{E}}_{\mathrm{CB}}$

where

E_VB: valence band energy.

E_CB: conduction band energy.

E^e represents the free electrons’ energy in comparison to hydrogen, which is 4.5 eV.

Eg denotes the semiconductor’s bandgap.

χ is the semiconductor electronegativity. Equation (3) is often used to identify this value:

(3)$\mathsf{\chi }={\left[\mathrm{x}{\left(\mathrm{A}\right)}^{\mathrm{a}}\mathrm{x}{\left(\mathrm{B}\right)}^{\mathrm{b}}\mathrm{x}{\left(\mathrm{C}\right)}^{\mathrm{c}}\right]}^{\frac{1}{\mathrm{a}+\mathrm{b}+\mathrm{c}}}$

Additionally, each component’s electronegativity value used in Equation (3) is shown in Table 3 below.

From the UV–vis DRS results presented previously, the bandgap energy E_g value of CuFe₂O₄ is 1.95 eV. Additionally, from the data in Table 3, the value of χ is 5.853. The values of E_CB = 0.378 eV and E_VB = 2.328 eV are calculated by exchanging the values of χ and Eg in Equations (1) and (2). Oxidizing species including •OH, h⁺, and •O₂⁻ have significant contributions to the destruction of organic contaminants. However, from the scavengers test results, it can be seen that •O₂⁻ plays a very minor role in the CIP degradation process.

Theoretically, the electrons in the CuFe₂O₄ conduction band might readily produce •O₂⁻ from O_2, since the material’s conduction band energy (E_CB = 0.378 eV) is less positive than the energy required for the formation of •O₂⁻ radicals (O₂/•O₂⁻ = 0.64 eV) [51]. However, •O₂⁻ only plays a very minor role in the oxidation of CIP. This implies that the reaction between the generated •O₂⁻ and the proton creates H₂O₂ and leads to the subsequent formation of •OH.

In comparison to the hole’s potential on the CuFe₂O₄ valence band (E_VB = 2.328 eV), the OH/•OH oxidation–reduction reaction is less positive (1.99 eV). Therefore, direct production of •OH is conceivable. In addition, •O₂⁻ can be used to form •OH. On the contrary, the surface holes of CuFe₂O₄ also participate in photocatalytic activity. The degradation of CIP over the CuFe₂O₄/rGO/HNT catalyst is most likely a localized surface reaction because h⁺ can interact with organic pollutants when they are deposited on the catalytic surface.

3.6.2. The CIP Photochemical Degradation Mechanism with CuFe₂O₄/rGO/HNT

From the presented investigations, the CIP photochemical degradation mechanism using CuFe₂O₄/rGO/HNT could be suggested as follows:

Step 1: When the catalyst surface absorbs the energy from UV light, the electrons are stimulated and generate e⁻_cb, h⁺_υb.

CuFe₂O₄ + hυ → CuFe₂O₄ + (e⁻_cb +h⁺_υb)

Step 2: •OH radicals are formed. Holes would be left in the valence band of the semiconductor. These holes can oxidize donor molecules and react with water to produce hydroxyl radicals that have a high oxidizing ability and are essential for contaminant destruction.

Cu²⁺ + H₂O + hν (h⁺, e⁻) → •OH + Cu⁺ + H⁺,

H₂O₂ + Cu⁺ + hν (h⁺, e⁻) → HO⁻ + •OH + Cu²⁺,

Fe³⁺ + H₂O + hν (h⁺, e⁻) → •OH + Fe²⁺ + H⁺, and

H₂O₂ + Fe²⁺ + hν (h⁺, e⁻) → HO⁻ + •OH + Fe³⁺.

Step 3: Transformation of oxygen adsorbed to the surface into superoxide radical. Superoxide ions are formed when electrons in the conduction band combine with dissolved oxygen species.

e⁻_cb + O₂ → •O₂⁻

Step 4: From scavenger tests and considering the E_CB, E_VB, ˙O₂⁻ formed could react with proton produces H₂O₂ and subsequently ˙OH is formed according to the following reactions:

•O₂⁻ + H⁺ → •HO₂, and

2•HO₂ → H₂O₂ + O₂.

Along with the participation in a direct reaction with ions on the catalyst surface, hydrogen peroxide can function as an electron sink, resulting in the enhancement the charge separation. Moreover, it can generate more hydroxyl radicals through the following reactions:

H₂O₂ + e⁻ → OH⁻ + •OH

H₂O₂ + O₂ → OH⁻ + •OH + O₂

Step 5: The following scheme illustrates the degradation of CIP by radicals.

•OH, h⁺, e⁻ + ciprofloxacin → CO₂ + H₂O + simpler molecules

3.6.3. Total Organic Carbon Measurement

Total organic carbon (TOC) is an important parameter for assessing the quality of drinking water, wastewater, and natural water sources. High levels of TOC can indicate contamination, disinfection by-product formation, biological growth, and fouling potential. In our study, TOC was measured to determine the presence of intermediate organic compounds that were generated but not fully mineralized. Through this, we will be able to evaluate the final effectiveness of the fabricated photocatalytic material. The results are shown in Figure 14.

Analysis of Figure 14 reveals a reduction in TOC levels from 25 mg/L to approximately 4.5 mg/L following a 5 h reaction period with the CuFe₂O₄-70%/rGO/HNT catalyst, corresponding to a mineralization rate exceeding 80%. The data indicate that the catalyst exhibits a high conversion rate for the target antibiotic CIP. Despite the test’s antibiotic concentration being significantly higher than real-world measurements [52,53], the catalyst demonstrated remarkable efficacy. It can be inferred that at lower CIP concentrations, the catalyst possesses the ability to fully mineralize this waterborne pollutant.

3.7. Reusability of the Photocatalyst

Among the important factors for evaluating the performance of a photocatalyst is its reusability, which reflects its stability and durability under repeated cycles of illumination conditions. A photocatalyst that can maintain its activity and structure after multiple uses is desirable for practical applications. Figure 15 reveals the results after five cycles of catalyst regeneration.

Following five cycles of reaction and regeneration of the catalyst, a minor reduction in CIP conversion was noted. The initial use of the catalyst yielded a CIP conversion rate of 99%, while after five cycles of regeneration, the rate decreased to approximately 91%. The CuFe₂O₄ material demonstrated stable catalytic activity and chemical durability, with minimal loss or contamination of the active center. XRD spectra (Figure 16) and SEM images (Figure 17) taken after four regeneration cycles further confirmed this stability, with no significant changes observed in the crystal structure or surface morphology of the material.

4. Conclusions

Important properties of CuFe₂O₄/rGO/HNT catalytic materials were explored in this work. The XRD, Raman, FTIR, BET, HRTEM and SEM results show that the synthesized CuFe₂O₄/rGO/HNT catalyst has exceptional qualities such as a large specific surface area (130 m²/g) and mesopore size which of approximately 5 nm. In addition, its interstitial structure may generate greater space efficiency than the original rGO. These characterizations with low bandgap energy, which is approximately 1.9 eV, make the material extremely efficient in photocatalytic applications. Indeed, when the catalyst was used, the antibiotic CIP at a concentration of 20 ppm was entirely transformed after 1 h of photocatalytic activity. Additionally, investigation on the reaction kinetics revealed photodegradation of CIP by CuFe₂O₄/rGO/HNT followed pseudo-first-order rate law.

Further, the experimental results with scavengers indicate that the radicals involved in CIP photodegradation with the CuFe₂O₄/rGO/HNT catalyst have the following order of importance: h⁺ > ^●OH > e⁻ > ^●O₂⁻. The point of zero charge (pH_pzc) of CuFe₂O₄/rGO/HNT was determined experimentally to be 6.02. The findings of pH effects at three values of 4, 7, and 10 lead to the conclusion that the most efficient photodegradation of CIP with CuFe₂O₄/rGO/HNT is reached at pH values ranging from 6 to 9, under ambient temperature and atmospheric pressure. The catalyst material has shown a high ability to mineralize CIP, with a mineralization rate exceeding 80% after a 5 h reaction. The material can be regenerated and reused, maintaining its activity and structural stability even after five regeneration cycles. Notably, when the catalyst material is regenerated five times, the conversion rate of CIP remains above 90%. This shows the wide-ranging potential of the material for the advanced removal of antibiotic residues from wastewater.

Author Contributions

N.-T.T., data curation and investigation. H.-C.L., data curation and formal analysis. T.-L.N.: supervision, resources, methodology and writing—original draft. H.-S.N.: formal analysis, supervision, writing—review and editing and validation. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Figures, tables, and references in this article contain the data that are presented in this study.

Acknowledgments

Our gratitude goes to Hanoi University of Mining and Geology for providing us with the laboratory facilities used in this work and to the Vietnam Ministry of Education and Training for supporting this study with the grant (B2021-MDA-02).

Conflicts of Interest

The authors state that none of their known personal or financial conflicts could have been expected to have affected the research reported in this publication.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. XRD patterns of CuFe2O4/rGO/HNT series at diffracted planes.

Figure 2. Raman spectra of CuFe2O4-70%/rGO/HNT.

Figure 3. FT-IR spectra of CuFe2O4-70%/rGO/HNT.

Figure 4. SEM (A,B) and HRTEM images (C,D) of CuFe2O4-70%/rGO/HNT.

Figure 5. (a) N2 adsorption–desorption isotherm of CuFe2O4-70%/rGO/HNT. (b) Pore size distribution of CuFe2O4-70%/rGO/HNT.

Figure 5. (a) N2 adsorption–desorption isotherm of CuFe2O4-70%/rGO/HNT. (b) Pore size distribution of CuFe2O4-70%/rGO/HNT.

Figure 6. EDX results of CuFe2O4-70%/rGO/HNT.

Figure 7. Energy bandgap of the CuFe2O4-70%/rGO/HNT catalyst.

Figure 8. CIP conversion with CuFe2O4/rGO/HNT series.

Figure 9. CIP conversion with different loading of CuFe2O4-70%/rGO/HNT.

Figure 10. (a) Pseudo-first-order kinetic model. (b) Pseudo-second-order kinetic model.

Figure 11. CuFe2O4/rGO/HNT point of zero charge (pHPZC).

Figure 12. Impact of pH on CIP conversion rate.

Figure 13. CIP conversion with the involvement of different scavengers.

Figure 14. TOC values before the reaction and after 5 h of illumination with the involvement of CuFe2O4-70%/rGO/HNT.

Figure 15. CIP conversion after five cycles of regeneration (catalyst: CuFe2O4-70%/rGO/HNT; reaction time: 5 h).

Figure 16. XRD spectra of CuFe2O4-70%/rGO/HNT before and after five cycles of regeneration.

Figure 17. SEM images of CuFe2O4-70%/rGO/HNT before (A) and after (B) five cycles of regeneration.

Table 1

CIP degradation kinetic model with the involvement of CuFe₂O₄/rGO/HNT.

	Pseudo-First-Order Rate Law			Pseudo-Second-Order Rate Law
Catalyst Weight	Linear Equation	k	R2	Linear Equation	k	R2
2.5 mg	y = 0.1226x + 1.0001	0.1226	0.999	y = 0.0366x + 0.0394	0.0366	0.9664
5 mg	y = 0.2115x + 0.9088	0.2115	0.9997	y = 0.1058x − 0.1227	0.1058	0.9322
7.5 mg	y = 0.3793x + 0.7277	0.3793	0.9984	y = 0.3679x − 0.7391	0.3679	0.8292

Table 2

Drift method experimental results.

Initial pH	11	8.94	7.02	5	3
Final PH	10.1	7.94	6.16	6.02	5.7

Table 3

Value of electronegativity of each component.

	Cu (eV)	Fe (eV)	O (eV)
Ionization energy	7.726	7.902	13.618
Electron affinity	1.2271	0.1627	1.4613
χ	4.47655	4.0324	7.53965

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