Synergistic photocatalytic removal of moxifloxacin from aqueous solutions using ZnO-Fe3O4-chitosan composites

The escalating contamination of water bodies with antibiotic residues is an urgent environmental and public health issue. This study aimed to fabricate an innovative photocatalytic composite (CMZ) by combining chitosan, magnetic iron oxide (Fe3O4), and zinc oxide (ZnO) for efficiently removing antibiotic moxifloxacin (MFX) water. Comprehensive characterization of the fabricated CMZ was performed using various techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), photoluminescence spectroscopy (PL) and nitrogen adsorption/desorption isotherm analysis. The synergistic incorporation of ZnO, Fe3O4, and chitosan in the CMZ composite altered the structural properties of ZnO and chitosan The band gap energy of CMZ was 2.58 eV, significantly boosting its photocatalytic effectiveness under visible light exposure. The CMZ composites exhibited a high efficiency in catalyzing MFX degradation in aqueous environments. The optimal conditions for MFX degradation were established, including a neutral pH level of 7, a 90 min exposure to irradiation, and employing 0.1 g of the CMZ catalyst. The degradation process obeyed closely to the first-order kinetic model. The CMZ material showed consistently high performance in degrading MFX across four consecutive reuse cycles, emphasizing its practical applicability for mitigating antibiotic pollution.


Introduction
Antibiotics are a great invention of mankind.The appearance of antibiotics has cured people of many dangerous diseases.However, because antibiotics are considered highly effective drugs, people sometimes overuse and abuse antibiotics, leading to an excess of antibiotics in the water environment.This has caused antibiotic pollution in the water environment.According to recent studies, antibiotics not only exist in wastewater treatment areas but also in surface water and groundwater [1][2][3].Although found in small concentrations, antibiotics are compounds that are difficult to decompose.Hence, in the long term, they will have negative effects on the natural environment and humans [2].In particular, the presence of antibiotics in water is one of the main causes of antibiotic resistance [4].Antibiotic resistance is occurring in many countries around the world [3,5,6].Therefore, removing antibiotic residues from water is a very urgent issue.Moxifloxacin (MFX) is an antibiotic of the fluoroquinolone group that is widely used in the pharmaceutical industry because it is bactericidal against both gram-positive and gram-negative bacteria [7].Consequently, the pollution of MFX in the aquatic environment is a great concern and the removal of MFX from wastewater before discharge to the environment is essential [1,2,4].MFX can be removed from aqueous solution by various techniques such as adsorption [8], biodegradation [9], and photodegradation on heterogeneous catalysis [10][11][12].Among these approaches, photodegradation stands out due to its cost-effectiveness, operational simplicity, and environmentally benign nature, making it a viable and sustainable alternative [13][14][15][16].A variety of materials, such as TiO 2 [17], Ag-TiO 2 , and ZnO [18], have been synthesized and employed for the photodegradation of antibiotics in aquatic environments.ZnO stands out for its unique advantages, notably its capacity to generate reactive oxygen species (ROS), such as hydroxyl radicals.This ability is remarkably efficient in decomposing antibiotic molecular structures into simpler, less harmful entities.This mechanism substantially reduces both the concentration and the toxic impact of antibiotics in water, thereby establishing ZnO as a crucial element in water purification techniques [19,20].The synthesis of ZnO is both economically feasible and environmentally sustainable, attributed to its low production cost and abundant availability [21].ZnO exhibits exceptional chemical stability under diverse environmental conditions, ensuring sustained effectiveness and durability of this photocatalyst within water treatment systems.Additionally, the flexibility in ZnO synthesis permits customized modifications of its size, shape, and crystalline structure, which significantly influence its photocatalytic efficiency [20,22].However, the application of ZnO in antibiotic photodegradation presents certain challenges.The recovery of ZnO nanoparticles after treatment remains a considerable challenge, complicating recycling efforts [23,24].Furthermore, the photocatalytic activity of ZnO predominantly relies on UV light absorption, a consequence of its wide bandgap energy (approximately 3.37 eV).This dependency limits its operational efficiency primarily to UV light sources, reducing its effectiveness under solar irradiation, predominantly composed of visible light.Consequently, advancing the recyclability of ZnO and lowering its bandgap energy is imperative for enhancing its applicability in wastewater treatment, thus addressing both economic and environmental imperatives [23,24].
The combination of ZnO with Fe 3 O 4 represents a notable advancement in overcoming the challenges associated with photocatalyst recovery during antibiotic photodegradation, introducing a synergistic approach that capitalizes on the strengths of both constituents [23], [24,25].Fe 3 O 4 , renowned for its magnetic properties, has magnetic responsiveness to the ZnO-Fe 3 O 4 composite, enabling efficient magnetic separation of the photocatalyst from treated water.Such magnetic recoverability substantially streamlines the recovery process, enhancing both efficiency and cost-effectiveness and thereby mitigating a major limitation inherent in employing nanoscale photocatalysts like ZnO independently [26].The incorporation of Fe 3 O 4 into ZnO potentially augments photocatalytic performance owing to the establishment of a heterojunction interface between the two materials.This heterojunction may enhance charge separation and diminish the recombination rate of photoinduced electron-hole pairs, consequently amplifying the production of reactive oxygen species, which are crucial for antibiotic compound degradation [23][24][25][26].While ZnO predominantly absorbs UV light, the integration of Fe 3 O 4 could modify the electronic structure of the composite, possibly extending its absorption spectrum into the visible range.Such an expansion in the absorption profile might elevate the photocatalytic efficacy under solar irradiation, rendering the photodegradation process more viable in practical scenarios characterized by limited UV light availability.
Chitosan, a biopolymer derived from natural sources, is distinguished by its biocompatibility and biodegradability [27].Incorporating chitosan into the nanocomposite serves dual purposes: it bolsters the environmental safety of the material and introduces functional groups capable of interacting with pollutant molecules, thereby potentially augmenting the adsorption capacity of the composite [28,29].The chitosan fraction plays a pivotal role in the overall adsorption dynamics of the nanocomposite, enabling the preconcentration of pollutant molecules at the catalyst interface [30].This pre-concentration phenomenon potentially amplifies the photodegradation efficiency by fostering more robust interactions between the pollutants and the photocatalytic sites.Furthermore, chitosan may enhance the mechanical and thermal stability of the nanocomposite.It functions as a cohesive agent, coalescing the Fe 3 O 4 and ZnO particles [31,32], which prevent the agglomeration of nanoparticles and preserve the structural integrity of the composite across diverse environmental conditions.
The integration of chitosan into the Fe 3 O 4 and ZnO nanocomposite thus not only elevates the environmental suitability and photocatalytic prowess of the material but also confers practical benefits in terms of recyclability and reusability.The unique composition of the chitosan-Fe 3 O 4 -ZnO composite (CMZ) harnesses the synergistic effects of robust photocatalytic properties of ZnO, magnetic separability of Fe 3 O 4 , and high adsorption capacity of chitosan, offering a multifaceted approach to water treatment [30,33].Unlike conventional photocatalytic systems, which primarily rely on UV activation and often suffer from low adsorption capacities and challenging recovery processes [17], CMZ material shows enhanced photocatalytic degradation of MFX under both UV and visible light, facilitated by the improved separation of electron-hole pairs and an increased reactive surface area.This innovative approach can give a compelling alternative to current conventional systems for the removal of antibiotics like MFX from water, contributing significantly to the advancement of wastewater treatment technologies.
This study focuses on the preparation of CMZ comprising chitosan, magnetic nanoparticles (Fe 3 O 4 ), and ZnO, utilizing a one-pot synthesis method, and assesses the efficacy of CMZ in removing MFX from aqueous environments.The synthesized materials are characterized by employing a range of techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), photoluminescence spectroscopy (PL), vibrating-sample magnetometry (VSM), scanning electron microscopy (SEM), and nitrogen adsorption/desorption isotherms.Key parameters influencing the degradation of moxifloxacin are thoroughly investigated, including pH, reaction time, the initial concentration of moxifloxacin, and the mass of the material.Additionally, this research proposes detailed analyses of the kinetics, recyclability of the material, and a photocatalytic degradation mechanism model for MFX.

Synthesis of CMZ material
In a 40 ml aqueous solution of 1 M HCl, nitrogen gas was bubbled for 10 min to create an inert atmosphere.Subsequently, a mixture of 0.625 g of FeCl 2 •6H 2 O and 1.694 g of FeCl 3 •6H 2 O was dissolved to prepare the iron solution.This was followed by the addition of 20 ml of a 20% chitosan solution, which had been previously prepared by dissolving chitosan in a 2% acetic acid solution.The solution was stirred at 200 rpm and maintained at 80 °C for 15 min.Then, 40 ml of a 2 M NaOH solution was rapidly introduced to the system, leading to the immediate formation of a black precipitate.The reaction was further progressed by the addition of 20 ml of 2 M Zn(CH 3 COO) 2 and 10 ml of 2 M Na 2 CO 3 solutions, under continuous stirring for an additional 30 min at the temperature of 80 °C.After that, the mixture was transferred to an autoclave and subjected to a temperature of 220 °C for 10 min.The synthesized material (CMZ) was separated using a magnetic, washed with double distilled water until a neutral pH of 7 was achieved, and dried at 100 °C for 4 h.For comparison, Fe 3 O 4 and ZnO nanoparticles were separately synthesized from the same precursors and under the same conditions but without the presence of chitosan.

Characterization
The crystalline structure of the synthesized material was characterized using a D2 Phase X-ray diffraction (XRD) instrument (Bruker).The elemental composition was analyzed using energy-dispersive X-ray spectroscopy (EDX) on a JEOL 5410 system.The composition of CMZ was determined using FT-IR spectroscopy (SpectrumTwo) and Raman spectroscopy (XploRa Plus).PL spectra of the samples were recorded on an FLS 1000 instrument.To examine the surface morphology, scanning electron microscopy (SEM) analyses were conducted using an FE-SEM S-4800 (Hitachi).The specific surface area, along with the pore volume and capillary diameter characteristics, were evaluated through nitrogen adsorption-desorption isotherms on a TriStar II system.Magnetic property measurements of the materials were carried out using a vibrating sample magnetometer (VSM) system, model DMS 880.The optical property was assessed via UV-visible diffuse reflectance spectroscopy (UV-vis DRS) using a Shimadzu UV-2600 spectrophotometer.The point of zero charge (pH PZC ) of the material was determined by mixing CMZ with 0.1 M NaCl solutions in 24 h at different initial pH values (from 2 to 12).The pH pzc was estimated from the plot of pH final -pH initial versus pH initial .

MFX removal study
To investigate the effect of CMZ and light on the MFX removal efficiency, three separate experiments were conducted.The first experiment involved exposing the system to UVA light (Philips, 12 W).The second experiment was conducted in the absence of light but with the presence of CMZ, and the third experiment was carried out with both CMZ and UVA light.Each experiment was conducted with an initial MFX concentration of 5 mg l −1 , using 0.05 g of CMZ, in a 40 ml solution volume, at a pH of 7. The reaction systems were stirred at 150 rpm.After specified intervals, 1 ml samples were collected for MFX concentration analysis using a photospectrometer (V-770, Jasco) at a wavelength of 295 nm.The formula to calculate the MFX removal efficiency (RE) is as follows: where C o and C t are the MFX concentrations at the beginning and after t minutes.
To study the effects of pH, reaction time, and CMZ mass on the reaction efficiency (RE), the reaction system was initially kept in the dark for 30 min before exposure to UVA light.The impact of pH was explored by adjusting the initial pH value of the solution from 1 to 11.The influence of reaction time was examined across a range from 5 to 150 min, while the effect of CMZ mass was investigated by altering the mass from 0.025 g to 0.15 g.To ensure reliability, each data point was replicated, with an acceptable error margin maintained below 5%.
The lattice constants (a and c) were estimated using the following equation: where h, k, l are the Miller indices.
As presented in table 1, the interplanar distances for (100) and (002) planes slightly decreased from 2.83 Å and 2.61 Å in pure ZnO to 2.81 Å and 2.80 Å in ZnO within CMZ, respectively.The calculation of lattice constants also shows a decrease in a and c about 0.02 Å.
This decrease indicates a slight reduction in the lattice constant of ZnO when it is part of the CMZ composite, compared to its pure form.The presence of Fe 3 O 4 within the CMZ composite is confirmed by the distinct diffraction peaks at approximately 30.32°and 43.21°, which are attributed to the reflections from the (400) and (220) planes in Fe 3 O 4 crystals (ICDD 01-071-6339), respectively [35].The identification of Fe 3 O 4 peaks is somewhat complicated due to their overlap with ZnO peaks, making the Fe 3 O 4 characteristic peaks at around 35.54°, 57.55°, and 63.87°less detectable.Additionally, the integration of chitosan into the CMZ composite leads to notable alterations in its structural profile.In the standalone chitosan sample, peaks at approximately 10.18°and 20.11°are prominent [36].However, within the CMZ composite, the first peak shifts to 11.39°, and the second peak becomes undetectable.The XRD results indicate that the successful formation of CMZ and the synthesis process affected the structure of its individual components.
The composition of CMZ was further investigated using FT-IR and Raman spectra and the results are illustrated in figure 2. The FT-IR spectrum of CMZ (figure 2(a)) shows some characteristic bands of chitosan, ZnO, and Fe 3 O 4 .The broad absorption at 3409 cm −1 is assigned to the O-H stretching vibrations and a peak at 2927 cm −1 corresponds to C-H stretching in chitosan.The band at about 1631 cm −1 is attributed to N-H bending in amide linkages, with additional bands at 1502 and 1538 cm −1 suggesting asymmetric stretching of N-H vibrations.Peaks around 1380 cm −1 , and within the 1000 to 1100 cm −1 range are C-O stretching vibrations.Additionally, the lower frequency peaks at 580, 519, and 442 cm −1 confirm the presence of metaloxygen bonds from Zn-O and Fe-O vibrations.The Raman spectrum of CMZ (figure 2(b)) shows the peaks at 102 and 442 cm −1 , correlating with the E 2 (low) and E 2 (high) vibrational modes of the hexagonal wurtzite structure of ZnO.The peaks at about 664 and 332 cm −1 can be indexed to the characteristic peaks of Fe 3 O 4 .FT-IR and Raman data further confirm the formation of the chitosan-Fe 3 O 4 -ZnO composite.
The SEM technique was used to investigate the morphologies of chitosan, Fe3O4, ZnO, and the CMZ composite; the results are depicted in figure 3.As presented in figure 3(D), both Fe 3 O 4 and ZnO nanoparticles within the CMZ sample can be observed and their distribution is uniform across the CMZ surface.The diameters of the ZnO nanoparticles are observed to range between 20 and 60 nm, whereas those of the Fe 3 O 4 nanoparticles fall within a narrower span of 5-10 nm.A significant observation from the SEM images is the absence of the original morphology of chitosan (as seen in figure 3(A)) in the CMZ composite.This alteration likely results from the covering of chitosan by ZnO and Fe 3 O 4 nanoparticles, combined with structural changes in chitosan during the CMZ formation process.Chitosan plays an essential role in altering both the morphology and properties of Fe 3 O 4 and ZnO nanoparticles within composites.Research indicates that the integration of chitosan with ZnO nanoparticles facilitates the formation of more uniform and evenly distributed nanocomposites, thereby modifying their inherent characteristics.Functioning as a templating matrix, chitosan introduces porous structures on the surfaces of ZnO nanoparticles, which significantly enhances their efficacy as catalysts in the degradation of organic pollutants [37].Chitosan significantly influences the characteristics of Fe 3 O 4 nanoparticles.It enhances their properties by reducing particle size, increasing surface area, and improving both catalytic and antibacterial efficacy [38].Moreover, the modification of Fe 3 O 4 nanoparticles with chitosan not only augments their crystallite size but also enhances their surface plasmon resonance properties, thereby expanding their potential in various applications [39].The morphological transformation of the components in CMZ underscores the complex interactions and modifications occurring among them, indicative of a synergistic integration within the CMZ structure.Figure 4(a) presents the absorption spectrum and deconvolution of the CMZ sample.In addition to a peak in the ultraviolet region, the spectrum reveals a distinct peak at approximately 503 nm within the visible spectrum.This suggests that CMZ may serve as an effective catalyst for photodegradation processes under visible light conditions.Bandgap energy (E g ) is an important parameter of a catalytic material in a photodegradation process.In this work, the E g of CMZ was calculated using from using a combination of the Kubelka-Munk theory [40] and Tauc plot [41].The equations of Kubelka-Munk theory are  where F (R ∞ ) is the Kubelka-Munk function, R s is the reflectance measured for the sample, and R r is the reflectance measured for a reference.E g can be extrapolated using the Tauc plot of (F(R∞)hυ) 1/n again hυ (eV).The exponent n is 1/2 for dipole-allowed transitions occurring at a direct band gap and 2 for dipole-allowed transitions near an indirect gap.In this work, the experimental data were fitted to the directly allowed transition using n = 1/2 and the results are presented figure 4(b).The estimated E g is 2.58 eV which is lower than that of pure ZnO (3.28 eV), indicating that the absorption wavelength of CMZ is longer than that of ZnO.The red shift of the adsorption light of CMZ when compared to ZnO can be explained by the interaction between ZnO and Fe 3 O 4 .Similar results were also reported in different materials containing ZnO and Fe 3 O 4 such as ZnO/Fe 3 O 4 nanocomposite (E g = 2.93 eV) [23] and ZnO/Fe 3 O 4 heteronanostructures (E g = 2.85 eV) [24].When ZnO is combined with Fe 3 O 4 , heterojunctions are formed at the interface of the two materials.These heterojunctions can alter the electronic structures of both components.In ZnO, the presence of Fe 3 O 4 can lead to a bending of energy bands at the interface, which effectively narrows the bandgap of ZnO.The band alignment between ZnO and Fe 3 O 4 is such that the conduction band of Fe 3 O 4 is lower in energy than that of ZnO.This alignment can facilitate the transfer of electrons from Fe 3 O 4 to ZnO, leading to a modification in the electronic structure of ZnO and a consequent reduction in its bandgap.The reduction of E g can broaden the application of the synthesized material in the photodegradation under sunlight conditions.
Figure 5 depicts the photoluminescence (PL) spectra of ZnO and CMZ samples under an excitation wavelength of 325 nm.The PL spectrum of ZnO nanoparticles features a narrow near-ultraviolet emission band centered at 397 nm, accompanied by a visible emission band peaking at 633 nm.The violet emission peak at 390 nm is attributed to the band-band recombination of electron and hole pairs in the ZnO material.Additionally, the emission band in the visible spectrum is associated with oxygen vacancies (V O ) and pseudozinc structure (V Zn ).The green emission peak at 520 nm arises primarily from the oxygen vacancies created during the synthesis process, whereas the red emission peak at 633 nm is linked to the zinc pseudo-structure of the ZnO material.In contrast, the PL spectrum of CMZ exhibits a blue shift in the ultraviolet region and a narrowing of the emission band's half-width.These changes are attributed to the interaction between ZnO and Fe 3 O 4 nanoparticles, resulting in decreased electron-hole pair recombination between the conduction band and the valence band.Consequently, this adjustment enhances the fluorescence lifetime and photocatalytic properties of the CMZ material relative to ZnO nanoparticles.
Magnetic properties play a critical role in the post-treatment recovery of materials.In this research, the magnetic characteristics of CMZ and Fe 3 O 4 were assessed using a vibrating sample magnetometer (VSM).The hysteresis loop curves for both CMZ and Fe 3 O 4 , observed under a magnetic field of 10 kOe, are depicted in figure 6.The magnetization saturation (MS) for the Fe 3 O 4 sample was found to be approximately 67.24 emu g −1 , aligning closely with the MS values reported in previous studies [26].Conversely, the MS value for CMZ was recorded at 15.04 emu g −1 .This decrease in MS value is attributed to the presence of non-magnetic components, namely ZnO and chitosan, within the CMZ composite.Despite the reduced MS value, the CMZ material demonstrates sufficient magnetic properties to enable its magnetic recovery from water treatment systems, as illustrated in figure 6 (inset).
Porous structure and surface area are important features of the materials for water treatment because they directly affect the adsorption and catalysis capability of the materials.Figure 7 shows the nitrogen adsorption/ desorption isotherms of CMZ.The isotherm exhibits a typical Type IV behavior with a hysteresis loop, indicative of mesoporous materials according to the IUPAC classification [42].This suggests that the material has pores with diameters in the range of 2 to 50 nm.The hysteresis loop, apparent in the desorption branch, is characteristic of capillary condensation occurring within mesopores and is often associated with complex pore structures or networks.The steep initial rise of the adsorption isotherm at low relative pressures (<0.1) indicates a high affinity for nitrogen, suggesting the presence of microporosity or highly active sites on the material's surface [42].This is a typical feature of materials with a significant surface area, which could be beneficial for applications requiring high adsorption capacities, such as gas storage, separation, and catalysis.The surface area calculated using the Brunauer-Emmett-Teller (BET) method is 66.3819 m 2 /g.This is a moderate surface area indicative of a material that is suitable for moderate adsorption applications, catalysis, and environmental pollutant removal.The estimated average pore widths using the Barrett-Joyner-Halenda (BJH) method are 9.0635 nm (for adsorption) and 9.4538 nm (desorption), indicating a predominance of mesopores.Moreover, the close alignment between adsorption and desorption average pore sizes suggests a relatively uniform pore size distribution, which is beneficial for consistent performance in adsorption processes and catalytic reactions.The chemical composition of CMZ was determined using the EDX method and the result is presented in figure 8(a).The peaks at about 1.01, 8.63, and 9.71 keV are the characteristic of Zn and the determined mass percentage of this element is 47.56%, showing that Zn is the most major element in the composition of CMZ.The presence of Fe is evidenced by the appearance of the peaks at about 0.71, 6.40, and 7.10 keV with a mass percentage of 22.05%.The peaks at about 0.28 and 0.53 keV correspond to the typical peaks of C and O, accounting for 26.88 and 3.63% of CMZ, respectively.The elemental mapping analysis presented in figure 8(b) shows that Fe, O, C, and Zn were uniformly distributed across the samples, serving as the principal elements.This confirms the successful formation of the composites.

Effect of irradiation and material on the MFX removal efficiency
To evaluate the effects of the materials and irradiation on the degradation of MFX, experiments were conducted under various conditions: light irradiation alone, with CMZ, and with both CMZ and light irradiation.Figure 9 illustrates that light irradiation alone resulted in a negligible change in MFX concentration after 150 min.In contrast, the presence of CMZ led to a 27% reduction in MFX concentration within the initial 30 min, with little further decrease thereafter, indicating that an equilibrium was reached.This reduction is primarily attributed to the adsorption of MFX onto CMZ.Notably, the combined effect of light irradiation and CMZ facilitated a substantial decrease in MFX concentration by approximately 83%, underscoring CMZ's dual functionality as both an adsorbent and a photocatalyst in the removal of MFX from aqueous solutions.In previous research [26], it was demonstrated that light used in conjunction with ZnO/Fe 3 O 4 materials could degrade 80% of methyl orange within 120 min, highlighting the effectiveness of ZnO/Fe 3 O 4 in the degradation of organic compounds.However, the adsorption capacity of these materials was considered negligible.In contrast, our study reveals that CMZ exhibits both adsorptive and photodegradation capabilities for MFX removal, potentially enhancing the elimination of by-products generated during the photodegradation process.

Effect of pH on the MFX removal efficiency
The pH value plays a critical role in both photodegradation and adsorption processes, affecting the generation of free radicals and the interaction dynamics between the adsorbate and adsorbent.This study investigated the impact of pH on the efficiency of MFX removal by adjusting the pH of MFX solutions across a range from 1 to 11.The results are depicted in figure 10.
The results demonstrated a marked increase in MFX removal efficiency as the pH rose from 1 to 7, followed by a decrease at pH levels of 9 and 11.This phenomenon can be elucidated by considering the adsorption of MFX onto CMZ and the MFX photodegradation capability of CMZ.For the adsorption, the form of MFX in the solution and the charge of the surface of CMZ are important factors.In the acidic solution (pH = 1-5), MFX, having pK a1 of 6. 3 [43] is protonated to form positively charged species.As presented in figure 11, the determined point of zero charge (pH PZC ) of CMZ is 6.8, meaning that the surface of CMZ is positive at pH below 6.8 and negative at pH above pH PZC .Hence, there is an electrostatic repulsion between MFX positively charged species and CMZ surface, resulting in a decrease in the removal efficiency.As pH is above 9, MFX with pK a2 of 9.3 [44] becomes negatively charged species that are repulsed by the negatively charged surface of CMZ, leading to the reduction in adsorption capability.The types and amounts of reactive oxygen species (ROS) such as hydroxyl radicals • OH and superoxide radicals • O 2 -generated during the photocatalysis process can be influenced by pH.At pH above 7, the generation of the • OH radicals may be enhanced due to the increased availability of hydroxide ions (OH -) that can react with photogenerated holes.

Effect of irradiation time and initial concentration on the MFX removal efficiency
The influence of irradiation duration and initial MFX concentration on removal efficiency was systematically examined by varying the MFX concentrations from 5 to 20 mg l −1 and extending the irradiation period up to 150 min.Results shown in figure 12 clearly demonstrate a significant reduction in MFX concentration with prolonged irradiation, underscoring the effectiveness of the photocatalytic degradation process.Specifically, at an initial concentration of 20 mg l −1 , the degradation efficiency approached nearly 90% after 150 min of irradiation.This enhancement in degradation efficiency with increased irradiation time can be attributed to the augmented generation of electron-hole pairs, which in turn, facilitates the photocatalytic adsorption of MFX.However, it was observed that beyond 90 min, the rate of MFX degradation on the photocatalytic material began to diminish.This attenuation in degradation rate is likely due to the accumulation of intermediate compounds formed during photocatalytic activity, which compete for adsorption sites with MFX molecules, thereby impeding further decomposition of MFX on the catalyst surface [45].The experimental data revealed a correlation between the MFX initial concentration and the removal efficiency.Notably, at the end of a 150minute treatment period, the removal efficiencies were observed to be 99.61%,96.15%, 93.01%, and 89.81% for initial concentrations of 5, 10, 15, and 20 mg l −1 , respectively.This trend can be attributed to the fact that, as the concentration of MFX increases while the amount of photocatalyst remains constant, the availability of adsorption sites on CMZ does not change.Consequently, this leads to competitive adsorption between MFX molecules and the existing adsorption centers.Additionally, at higher MFX concentrations, the proportion of MFX molecules relative to photocatalytic sites increases, contributing to a decrease in overall removal efficiency.Similar trends have been observed in various studies examining the effects of time and initial concentration of pollutants on removal efficiency, such as the decomposition of colorants and antibiotics on GO-ZnO [13], and decomposition of RhB on GO-ZnO.AC-TiO 2 material [46], and decomposition of sulfamethazole on TiO 2 [47].

Effect of material mass on the MFX removal efficiency
The impact of CMZ mass on MFX removal efficiency was systematically assessed by varying the CMZ mass from 0.025 to 0.150 grams.As shown in figure 13, a clear trend is evident where MFX removal efficiency increases with an increase in CMZ mass from 0.025 g grams to 0.100 g.However, a marginal reduction in efficiency is observed  when the CMZ mass is increased further to 0.125 and 0.150 g.This phenomenon can be explained by the fact that a higher CMZ mass provides more active sites, enhancing the adsorption of MFX molecules and facilitating the generation of electron-hole pairs under light irradiation.This, in turn, promotes the photocatalytic reaction, leading to increased MFX degradation efficiency.Nonetheless, beyond a certain CMZ mass, efficiency slightly declines, likely due to the saturation of adsorption sites or the hindrance of light penetration through the reaction medium, which affects photocatalytic activity.This observation is consistent with findings in the literature documenting the influence of material mass on similar photocatalytic processes [23,24].

Kinetics of the MFX removal process
The dye degradation process via heterogeneous photocatalysis involves a series of intricate steps.These steps include the physical adsorption of pollutants onto the catalyst's surface, the initiation of chemical reactions under light exposure, and the subsequent degradation of the dye, often accompanied by the formation of secondary by-products.The complexity of this degradation pathway presents challenges in accurately determining the influence of various factors on the rate of photocatalytic degradation.Despite the complex nature of these processes and the emergence of intermediate products, the overall reaction can be effectively represented using the first-order kinetic model.This model is mathematically represented by the following equations: where k ap is the first-order reaction rate constant (min −1 ), C o and C t are the MFX initial concentration, and after t (min).The data presented in figure 14 and table 2 demonstrate that the removal process follows first-order kinetics, as evidenced by high correlation coefficients (R 2 > 0.98), approaching unity.This indicates a strong fit to the first-order kinetic model.Notably, there is an observable inverse relationship between the initial concentration of MFX and the reaction rate constant.Specifically, as the initial concentrations of MFX are increased to 5.18, 10.15, 15.02, and 20.12 mg l −1 , the corresponding decomposition rate constants show a decrease to 0.032, 0.021, 0.016, and 0.014 min −1 , respectively.This observed trend aligns with findings from several previous studies.For instance, similar patterns have been reported in the decomposition of rhodamine B using AC/TiO 2 photocatalytic materials [46], as well as in the degradation of antibiotics with TiO 2 /T 25 material [16].

Reusability of CMZ for removal of MFX
The evaluation of the reusability of CMZ is critical for ascertaining its practical applicability in real-world scenarios.In this work, a series of experiments were conducted in which CMZ was repeatedly employed for the removal of MFX through continuous batch operations.After each cycle, magnetic separation was utilized to separate CMZ from the reaction mixture, followed by thorough washing with hot water and subsequent drying at 100 °C for 8 h.Photocatalytic degradation experiments proceeded under optimized conditions, comprising 0.1 g of CMZ, 5 mg l −1 of MFX, a neutral pH (7), and an irradiation period of 90 min.As illustrated in figure 15, CMZ underwent four successive reuse cycles.The degradation efficiency percentages for MFX across these cycles were 94.23%, 90.47%, 88.93%, and 85.62%, respectively.This trend suggests that CMZ maintains considerable photocatalytic efficacy even after multiple utilizations.The slight decline in efficiency observed over the four cycles can predominantly be ascribed to (i) inadvertent loss of catalyst particles during the recovery and drying phases, and (ii) potential occlusion of the active sites of CMZ, likely resulting from interactions with MFX [13,48,49].The findings of this study conclusively demonstrate the stability and high reusability of the CMZ nanocomposite for the photocatalytic degradation of moxifloxacin (MFX).These outcomes underscore the potential of CMZ as an efficient and sustainable catalyst for MFX removal, thereby affirming its applicability in wastewater treatment processes.

This study
Various materials used for the removal of MFX from water are summarized in table 3. It is evident that the photodegradation of MFX can be catalyzed using diverse materials.The kinetics of the degradation process generally follow a first-order kinetic model.In terms of removal efficiency, CMZ exhibits comparable performance to N-doped ZnO N-doped ZnO [50], and CoFe 2 O 4 /PMS [51].Furthermore, the removal efficiency of CMZ is higher than that of other materials such as Ni/Mo.S2/MOF-5/GO [52], TiO 2 /UVC [16], BiOCl/Cu 2 O [49], and Ti 3 C 2 /Btex [53], indicating that CMZ is a promising material for effectively removing MFX from water.

Proposed mechanism for photocatalytic degradation of MFX
The removal mechanism of moxifloxacin (MFX) by the CMZ composite is a comprehensive process that synergistically integrates adsorption and photocatalysis.Initially, MFX molecules interact with the chitosan component of CMZ.Derived from chitin, chitosan is a biopolymer rich in amino (-NH 2 ) and hydroxyl (-OH) functional groups.These groups enable hydrogen bonding and electrostatic interactions with MFX, thereby enhancing its adsorption.The adsorption process is further enhanced by the porous structure of chitosan, which increases the surface area available for interactions with MFX molecules.Upon irradiation, the ZnO component within the composite becomes photoactivated, leading to electron excitation from the valence to the conduction band and generating electron-hole pairs.These pairs are crucial to the photocatalytic process, although their recombination can typically reduce photocatalytic efficiency.The inclusion of Fe 3 O 4 nanoparticles within the composite addresses this limitation by acting as an electron sink.This reduces the recombination rate of electron-hole pairs, significantly enhancing the photocatalytic activity of the composite through a synergistic interaction between ZnO and Fe 3 O 4 [23,25,26].The holes generated in the valence band interact of ZnO with water molecules or hydroxide ions to produce hydroxyl radicals ( • OH), while the trapped electrons can reduce oxygen molecules to form superoxide anions ( • O 2− ).These reactive oxygen species (ROS) are potent oxidizers capable of degrading organic pollutants, including MFX.The degradation of MFX involves the breakdown of its aromatic rings, dealkylation, and other oxidative transformations, culminating in its mineralization to innocuous end products such as water and carbon dioxide.The reactions can be summarized as follows:

Conclusions
The innovative adsorbent CMZ (chitosan, Fe 3 O 4 , and ZnO integrated composite) was successfully synthesized, demonstrating a bandgap energy of 2.58 eV, a porous structure with a specific surface area of 59.18 m 2 /g, and a magnetic saturation value of 15.04 emu g −1 .This study extensively investigated the efficacy of CMZ in the photocatalytic degradation of the antibiotic Moxifloxacin (MFX) in aqueous solutions, revealing optimal degradation efficiency at a neutral pH (pH = 7) and with a CMZ dosage of 0.1 g.It was observed that the photocatalytic activity of CMZ in decomposing MFX increased with prolonged irradiation time and decreased as the initial MFX concentration rose.The removal process followed a first-order kinetic model.Durability and reusability tests confirmed the stable photocatalytic performance of CMZ through four consecutive cycles.The findings in this study indicate that CMZ is a promising material used in water purification technologies.Its high synergistic photocatalytic efficiency in degrading moxifloxacin from aqueous solutions positions it as a viable solution for addressing pharmaceutical pollutants.The integration of these composites into existing water treatment infrastructures could significantly enhance contaminant removal capabilities, offering a sustainable and eco-friendly alternative to traditional methods.Further research needs to be carried out on scaling production, optimizing performance in diverse water qualities, and evaluating long-term stability and reusability, which are critical for commercial success and environmental impact.

Figure 4 .
Figure 4. (a) Absorption spectrum of CMZ and the deconvolution of the spectrum, and (b) Tauc plot for estimating E g of CMZ.

Figure 6 .
Figure 6.Hysteresis loops curves of Fe 3 O 4 and CMZ with the inset showing the recovery of CMZ using a magnet.

Figure 8 .
Figure 8. EDX analysis (a) and elemental mapping analysis of CMZ.

Figure 9 .
Figure 9.The degradation of MFX with different experiment conditions.

Figure 10 .
Figure 10.Effect of pH on the MFX removal efficiency of CMZ.

Figure 11 .
Figure 11.Experimental data for determining pH pzc of CMZ.

Figure 12 .
Figure 12.Effects of MFX initial concentration and irradiation time on the removal efficiency.

Figure 13 .
Figure 13.MFX degradation efficiency at different masses of CMZ.

Figure 14 .
Figure 14.The experimental data and the data predicted by the first-order kinetic model.

Table 1 .
The interplanar distances and lattice constants calculated for ZnO lattice cells in ZnO and CMZ.

Table 2 .
Kinetic parameters for MFX removal process.

Table 3 .
Comparison of various materials used for removal of MFX.