Enhanced photocatalytic degradation of tetracycline using cobalt-substituted ZnFe₂O₄ porous microspheres under visible-LED irradiation

Abstract

We developed an advanced method for separation of organic materials from water in adsorption process. Advanced functional inorganic materials were designed and synthesized using hydrothermal method. Photocatalytic processes are among the most effective and sustainable methods for eliminating synthetic toxic organic pollutants and pharmaceuticals. The removal of toxic contaminants from water necessitates the implementation of advanced oxidation processes (AOPs) that utilize highly efficient photocatalysts. Furthermore, the design of visible-light-responsive photocatalysts for AOPs requires a balance of cost-effectiveness and efficiency. This study focuses on developing a sustainable synthetic route to obtain a photocatalyst with enhanced activity while minimizing environmental impact. Specifically, pure (ZFC0) and 10wt% cobalt- substituted ZnFe₂O₄ (ZFC10) microspheres were synthesized using a hydrothermal method, and their morphology and crystallinity were studied. These microspheres were employed as catalysts for the degradation of three commercial pharmaceuticals: tetracycline, paracetamol, and antihistamines. Notably, the combination of ZF and cobalt facilitates a surface contact that effectively inhibits the recombination of electron-hole pairs, thereby enhancing charge transfer between the two materials and resulting in improved photocatalytic oxidation. The photocatalytic activity was assessed for varying concentrations of tetracycline and ZFC10. Under irradiation from a mercury vapor lamp, ZFC10 achieved a tetracycline degradation efficiency of up to 83% within 180 min (60 min in darkness and 120 min under light), nearly double the degradation observed under LED light. Additionally, ZFC10 exhibited excellent reusability and chemical stability after three cycles. The study also discusses the underlying photocatalytic mechanisms.

Introduction

The widespread use of pharmaceuticals has increased globally owing to advancements in healthcare systems aimed at controlling and preventing diseases^1–3. However, the excessive use of these substances and their resistance to wastewater treatment can lead to adverse effects on aquatic environments and human health^4,5. Consequently, it is essential to effectively separate the pharmaceutical pollutants from effluents before they are discharged. Various methods have been developed for water treatment, including biological methods^6,7, ion exchange⁸, adsorption technology⁹, and membrane technology¹⁰. Generally, these methods have the capability to transfer pollutants from one environment to another, potentially resulting in secondary environmental contamination and incurring high operational costs. Specifically, the removal of toxic and resistant pollutants, such as pharmaceuticals, presents particular challenges¹¹. In this context, the ring-opening reaction is a crucial and fundamental step in the mineralization of aromatic pollutants to carbon dioxide and water¹². To address these issues, advanced oxidation processes (AOPs) have emerged as an innovative and forward-looking approach¹³. Among AOPs, photocatalytic processes utilizing semiconductors have gained significant attention, as they can operate under sunlight and do not produce any secondary toxic byproducts^14–17. However, the use of high doses of catalysts can pose limitations for deployment in industrial-scale systems. Therefore, it is necessary to both enhance photocatalytic activity and reduce the dosage of catalysts. Semiconductors with various morphologies demonstrate improved optical and antibacterial properties compared to bulk semiconductors due to their increased surface-to-volume ratio and quantum confinement effects^18,19. Moreover, modifying semiconductors to utilize visible light is essential, and this can be achieved by doping the crystalline lattice with different elements to inhibit electron–hole pair recombination and maintain efficient charge separation²⁰. In this regard, metal ferrite nanocrystals have garnered considerable interest, particularly due to their significance in understanding the principles of nanomagnetic and their diverse technological relevance, including uses in data-recording media, smart ferrofluids, magnetic refrigeration units, magnetic resonance imaging, and heterogeneous catalysis. Notably, zinc ferrite, with a direct bandgap of 1.9 eV, has attracted considerable attention as an advanced magnetic material, photocatalyst, and gas sensor^21–26.

In this study, we investigate the catalytic properties of cobalt -substituted zinc ferrite with a 10 wt% of cobalt for the removal of tetracycline. This research aims to analyze the impact of cobalt doping on the photocatalytic activity of zinc ferrite and evaluate its potential for reducing pharmaceutical contaminants in aquatic environments.

Experimental

Materials and methods

The research involved a variety of materials, specifically Fe (NO₃)₂·9H₂O, Zn (NO₃)₂·6H₂O, Co (NO₃)₂·6H₂O (98%), Urea, and Ascorbic acid (> 99%), all procured from Merck Co., Germany. The phase composition and lattice structure of the obtained specimens were characterized by XRD employing Cu-Kα radiation (λ = 1.54 Å, operated at 40 kV). UV–Vis spectroscopic analysis was carried out with a Varian Cary 100 instrument to evaluate the optical characteristics of the materials. To evaluate the presence of functional groups within the materials, infrared characterization (FT-IR) was performed on an AVATAR 360 spectrophotometer after forming the materials into KBr-based pellet discs. Spectral data were collected within the range of 400 to 4000 cm⁻¹. The morphological characteristics and elemental composition of the samples were assessed utilizing a Mira 3-XMU scanning electron microscope. Additionally, Nitrogen adsorption–desorption data obtained on a Belsorp II instrument were used to calculate the BET-specific surface area of the materials⁶¹.

Synthesis of ZF and Cobalt-substituted ZF

A straightforward one-step hydrothermal technique was employed to fabricate porous microspheres of zinc ferrite. The method is taken from our previous study⁶¹. In a typical process, urea 3.3 g and ascorbic acid 3.17 g were dissolved in deionized water to form a homogenous solution. This solution was then gradually introduced into a mixture containing Zn (NO₃)₂·6H₂O) 1.3 g and Fe (NO₃)₂·9H₂O 3.24 g. The resulting mixture was stirred for 30 min, after which it was transferred to a 50 ml Teflon-lined stainless-steel autoclave. The autoclave was maintained at a temperature of 160 °C for a duration of 6 h. Upon completion, cooling occurred naturally to room temperature, and the synthesized products were separated by centrifugation, followed by thorough washing with deionized water and ethanol. The collected materials were then dried at 60 °C for 12 h, followed by calcination at 500 °C for 2 h⁶¹.

For the preparation of cobalt-substituted zinc ferrite, zinc nitrate was combined with cobalt nitrate at a concentration of 10 wt%, following the same synthesis procedure as described for the pure zinc ferrite. The final products were labeled as ZFC0 for the pure zinc ferrite and ZFC10 for the zinc ferrite with 10 wt% substitutional Co doping⁶¹.

Photocatalysis-based testing for pharmaceutical compound degradation

The photocatalytic degradation activities of the ZFC0 and ZFC10 samples were evaluated at room temperature and PH 6 for the photodegradation of tetracycline in an aqueous solution under a 125 W Hg vapor lamp. Initially, a specific amount of the photocatalyst (0.03, 0.1, and 0.3 g/L) was dispersed in 60 mL of a drug solution with concentration of 10, 30, and 50 mg/L. The suspension was stirred in darkness for 60 min to achieve adsorption-desorption equilibrium. Sampling of the suspension was conducted at 20-min intervals, followed by centrifugation to separate solid particles and obtain a clear liquid phase. The absorbance of TC at 357 nm was obtained using a Yoke UV-1100 spectrophotometer, and its photocatalytic degradation percentage was computed following Eq. (1):

1

where R denotes the photocatalytic degradation efficiency, and C₀ corresponds to the absorbance of the drug solution after 60 min of dark equilibration, and C_t is the absorbance of the drug solution at time t. Figure 1 presents a schematic representation of the photocatalysis setup.

Fig. 1.

Schematic of the photocatalytic setup.

Characterization

FESEM and elemental analysis via EDX

To evaluate the surface morphology and dimensions of the porous microspheres of pure and Co-substituted ZF, comprehensive analyses were performed using FESEM. The FESEM images (Fig. 2) illustrate that the porous flower-like microspheres of ZFC0 have diameter of ≈ 1 μm, characterized by intricately twisted nanosheets that interlace to form the unique flower-like architecture. Following cobalt doping, as demonstrated in Fig. 2 for ZFC10, the fundamental morphology of the microspheres remains largely unaltered, preserving their distinctive porous flower-like appearance with minimal variation in sphere diameter. Notably, the higher magnification images indicate a pronounced decrease within the thickness variation of the curved nanosheet petals with increasing cobalt weight%. The measured thicknesses are ≈ 20 nm for ZFC0 and 16 nm for ZFC10 (measured from SEM images using ImageJ). The interplay between particle size and porosity is a critical factor, as it profoundly impacts the specific surface area. This relationship will be further examined in the subsequent sections through the analysis of N₂ adsorption and desorption isotherms. Also, Substitutional cobalt centers within the spinel act as redox-active sites that facilitate photoinduced charge transfer and improve visible-light utilization⁶¹.

Fig. 2.

FESEM images and EDS analysis of synthesized samples.

The elemental composition of samples was evaluated through EDX, with the results illustrated in Fig. 2. The detailed elemental tables in Fig. 3 provide insight into the purity and composition of the samples. The EDX spectrum for the ZFC0 reveals that it is composed solely of Zn and Fe, confirming the successful synthesis of ZF. This finding underscores the effectiveness of the synthesis method in producing high-purity materials. In comparison, the EDX spectrum of ZFC10 clearly reveals cobalt signals, demonstrating that the doping procedure was successfully achieved. The distinct presence of cobalt in the spectra not only signifies the successful introduction of this cation but also suggests that cobalt has effectively substituted into the lattice of the zinc ferrite matrix. The elemental characterization confirms that the samples preserve their target compositions and architectures, supporting further exploration of their structural and performance-related features⁶¹.

Fig. 3.

(a–c) XRD pattern and (d,e) FTIR spectra.

Structural and spectroscopic analysis

XRD results indicate that the ZF sample crystallizes in a cubic spinel phase with a lattice constant of a = 0.835 nm. The broadened diffraction peaks appearing at different 2θ positions are attributed to the [220], [311], [400], [422], [511], [440], and [533] crystal planes (Fig. 3a,b²⁷. Notably, the sharp diffraction peak at 2θ = 35.63 is indicative of the distinct ZF structure. The primary diffraction peaks in the XRD pattern of pure ZF align well with its spinel-type crystal structure, as confirmed by the JCPDS Card No. 82-1049, and no impurity peaks are detected, demonstrating the high purity of the synthesized material. In contrast, the XRD pattern of ZFC10 does not show a distinct cobalt peak, which can be attributed to the successful incorporation of cobalt ions within the ZF lattice, whereby cobalt effectively substitutes into the zinc sites, resulting in the dissolution of cobalt in the crystalline matrix (Fig. 3c)^28–30. The limited concentration of cobalt may also be insufficient to produce detectable diffraction peaks within the XRD patterns, as corroborated by previous studies²⁹. The Scherrer equation was employed to evaluate the average crystallite size (D)³¹:

2

in which θ denotes the diffraction angle, β refers to the FWHM of the peak, and λ indicates the wavelength of the employed X-ray source. Based on the XRD analysis, the average nanocrystal sizes were determined to be 6.54 nm, and 7.57 nm for the ZFC0, and ZFC10 samples, respectively. Significantly, while the peak positions exhibited slight shifts with cobalt -substituted, the intensities of these peaks showed considerable changes. The introduction of Co²⁺ ions, having a smaller ionic radius than Zn²⁺ ions, allows cobalt to occupy the tetrahedral sites within the ZF lattice, creating vacancies and modifying the original structure. Additionally, variations in the FWHM serve as a crucial indicator of structural strain and nanocrystal size, confirming that the incorporation of cobalt influences both the microstructural characteristics and the crystallinity of the resulting materials^32,33. These findings collectively underline the successful synthesis of cobalt-substituted zinc ferrites with noteworthy structural properties, laying the groundwork for their potential applications in various fields⁶¹.

FTIR analysis was conducted to confirm the spinel structure in the cobalt-substituted ZFs, as depicted in Fig. 3d,e. FTIR spectra are instrumental in identifying the characteristic vibrational modes of spinel ferrites, particularly those occurring below 600 cm⁻¹. In the FTIR spectra, two significant vibrational bands indicative of zinc ferrite synthesis were observed. The absorption band at ~ 560 cm⁻¹ is related to Zn–O stretching at tetrahedral sites, and the band at ~ 430–470 cm⁻¹ corresponds to Fe–O stretching at octahedral sites³⁴. Notably, doping the zinc ferrite matrix with cobalt, the vibrational band corresponding to M–O stretching at the tetrahedral position exhibited a clear shift. This spectral variation is attributed to the redistribution of Zn²⁺, Co²⁺, and Fe²⁺ ions between tetrahedral and octahedral positions, indicating notable cation–lattice interactions within the spinel framework. A band at 1095 cm⁻¹ was also detected, corresponding to M–O–H vibrations and likely arising from overlapping modes associated with carbon-based groups originating from acetate residues³⁵. The spectra exhibit asymmetric stretching features of nitrate groups around 1400 cm⁻¹, supporting the chemical identity of the synthesized compound. Additionally, the appearance of a peak near 2300 cm⁻¹ signifies the incorporation of CO₂ molecules within the material’s structure³⁶. The band at ~ 3400 cm⁻¹ corresponds to O–H stretching, while the peak at ~ 1630 cm⁻¹ is attributed to H–O–H bending of adsorbed water³⁷. These findings underscore the effectiveness of FTIR analysis in elucidating the structural and chemical characteristics of cobalt-substituted zinc ferrites, confirming their formation and highlighting the intricate interactions among metal ions in the lattice⁶¹.

BET analysis

The specific surface area and pore size are critical parameters that significantly influence the performance of sensors. To investigate these properties, BET analysis was conducted on the samples. The gas adsorption-desorption hysteresis presented in Fig. 4 reveals that these microspheres exhibit high porosity, characterized by an H₃ type hysteresis loop. Notably, for P/P₀ values greater than 0.4, a marked increase in adsorption on the sample surface is observed, indicating enhanced surface interactions. The BET characterization results, summarized in Table 1, demonstrate that the cobalt–substituted sample (ZFC10) possesses the highest specific surface area of 68.13 m²/g, while the ZFC0 shows a lower effective surface area of 51.557 m²/g. This significant difference in surface area suggests that the introduction of cobalt improves both the structural framework and the surface-related attributes. Furthermore, while ZFC10 has the largest pore volume, it also exhibits a larger average pore diameter, which is almost twice that of ZFC0. This unique combination of high surface area and favorable pore characteristics in the ZFC10 sample demonstrates its strong suitability for integration into advanced catalytic systems⁶¹.

Fig. 4.

BET-N2 adsorption–desorption curves.

Table 1.

Surface area, pore volume and pore diameter of the ZFC0, and ZFC10.

Sample		Pore volume	Average pore diameter ()
ZFC0	51.557	0.153	13.21
ZFC10	68.13	0.221	18.87

Optical results

This section presents a detailed assessment of the optical responses of the ZFC0 and ZFC10 powders, as these features fundamentally govern their photocatalytic activity. The electronic band-gap values were extracted from the diffuse-reflectance data by applying the Kubelka–Munk transformation. F(R) is derived from reflectance (R) using Eq. (3)³⁸:

3

The plot of F(R) versus wavelength for the two samples is shown in Fig. 5a,b, where the absorption edge is observed around 330–350 nm. The modest displacement of the absorption edge observed upon cobalt incorporation reflects a modification in the electronic configuration of the semiconductor. The optical band gap energy (E_g) was further calculated through Tauc’s equation, expressed as follows²⁸:

4

where A denotes a proportionality constant, while hν corresponds to the energy carried by the incoming photons. By extrapolating the plots of F(R) against photon energy, the band gap values were determined for the samples ZFC0, and ZFC10, yielding values of 1.73 eV, and 1.67 eV respectively, as indicated in Fig. 5d,e. The observed decrease in band gap energy from 1.73 eV for ZFC0 to 1.67 eV for ZFC10 suggests that substitutional cobalt doping effectively tunes the electronic properties of the material. The reduction in band gap energy can be attributed to various factors, such as alterations in crystal size, lattice parameters, carrier concentration, and the introduction of defects in the material⁶¹. Specifically, the introduction of cobalt ions contracts the lattice parameters and generates supplementary electronic states within the band-gap region. These newly formed states promote additional electronic transitions between the valence and conduction bands, thereby driving a further reduction in the band-gap energy. The narrowed band gap in the Co-substituted samples enhances the production of electron–hole pairs (e⁻/h⁺), which in turn strengthens their photocatalytic response. By lowering the band-gap energy, the material becomes capable of harvesting a wider portion of the visible spectrum and more effectively utilizing lower-energy photons—an essential factor in achieving improved photocatalytic efficiency⁶¹. Consequently, modifying the optical properties through cobalt doping positions ZF as a promising candidate for advanced photocatalytic applications^39,40. In addition to the band gap narrowing, the incorporation of Co²⁺/Co²⁺ introduces localized states that can act as charge trapping and transfer centers. These sites facilitate electron migration and reduce the probability of direct e⁻/h⁺ recombination, thereby prolonging carrier lifetimes⁶¹. This indirect evidence from optical analysis supports our conclusion that cobalt -substituted suppresses charge recombination and enhances charge separation, consistent with prior reports on transition-metal-doped ferrites^41,42. As shown in the UV–Vis DRS spectra (Fig. 5c), cobalt doping causes a noticeable red-shift of the absorption edge compared to pristine ZF, which corroborates the observed decrease in band gap energy.

Fig. 5.

(a,b) Variation of Kubelka–Munk function F(R), (c) UV–Vis absorption spectrum, and (d,e) Optical band gap spectra.

Photocatalytic performance

Photocatalytic response under LED and mercury-vapor illumination

To compare the photocatalytic efficiency toward tetracycline degradation for ZFC0 and ZFC10, as well as to investigate the effect of lamp type on the photocatalytic process, two light sources were utilized: a 100-watt LED lamp and a 126-watt mercury vapor lamp. As illustrated in Fig. 6a, it was observed that under LED light, both samples exhibited lower degradation rates. The degradation percentage of tetracycline using the mercury vapor lamp for samples ZFC0 and ZFC10 increased from 33 to 83%. Furthermore, in both cases, the ZFC10 sample demonstrated a significantly higher degradation percentage compared to the ZFC0 sample. Control experiments also showed that under visible light irradiation without catalyst, only negligible TC degradation occurred, and in the dark with the presence of catalyst, only partial absorption was observed; therefore, TC removal is primarily attributed to photocatalysis on the catalyst.

Fig. 6.

(a) Comparison of two LED and Hg lamps. (b) Influence of initial pollutant concentration. (c) Influence of catalyst concentration. Error bars indicate standard deviation from three independent experiments (n = 3). (d) Recycled degradation.

Elimination of different organic pollutants

To assess the selective degradation capability of the samples toward tetracycline, two other drugs, acetaminophen (AC) and antihistamine (AH), were also investigated. ZFC10 could effectively degrade tetracycline with a removal rate of more than 83% in three hours. While the removal rates of AC and AH only reached 42% and 34% in 3 h, respectively, which may be related to the chemical properties of the sample surface and the chemical structure of TC. As a result, ZFC10 demonstrates multifunctional photocatalytic capability, enabling effective removal of a broad range of organic contaminants from aqueous systems.

Effect of contaminant concentration

One of the effective factors for optimizing photocatalytic systems is the concentration of contaminants during the photocatalytic process of drugs. Increasing the contaminant concentration can increase their adsorption in the photocatalytic process because due to the availability of contaminants, the adsorption reaction on the photocatalyst surface increases, more hydroxyl radicals are produced, and the decomposition process becomes easier⁴³. However, the efficiency of contaminant concentration can be affected by various factors such as the amount of photocatalyst in the system, the properties of the photocatalyst, including surface area, composition, and stability, so that different photocatalysts can have different capabilities in degrading pharmaceutical compounds at different contaminant concentrations^44,45. In Fig. 6b, the plot is plotted for the degradation of tetracycline at different concentrations of 10, 30, and 50 mg/L using ZFC10 against a Hg vapor lamp. As can be seen, the highest degradation (83%) was obtained for a concentration of 50 mg/L TC after 60 min of stirring in the dark and 120 min under light.

Influence of catalyst concentration

The efficiency of the photocatalytic degradation process is significantly affected by the concentration of the catalyst particles present. Increasing the catalyst dosage improves the degradation efficiency by promoting the generation of hydroxyl radicals (·OH), as a larger amount of active surface sites becomes available for photocatalytic reactions^46,47. However, excessively high catalyst levels can lead to a decrease in degradation efficiency, primarily because they increase the turbidity of the solution, which in turn reduces light penetration. Additionally, high catalyst concentrations may promote agglomeration of particles, thereby reducing the accessible surface required for effective catalytic interactions⁴⁸. The influence of catalyst dosage is further governed by several additional operational and material-related factors. The choice of catalyst and its specific properties, such as surface area, composition, and stability, can affect the degradation process’s efficiency at different concentrations. In this study, we investigated the impact of ZFC10 catalyst concentrations at three different weights: 0.03 g, 0.1 g, and 0.3 g. Although ZFC10 can be considered an effective catalyst for tetracycline degradation, the maximum degradation efficiency was achieved at a catalyst loading of 0.3 g, yielding degradation efficiencies of 20%, 35%, and 83% for the three concentrations over 3 h (Fig. 6c). All photocatalytic degradation and characterization experiments were conducted in triplicate under identical conditions. The data are reported as average values, with error bars representing the standard deviation obtained from three separate experimental runs⁶¹. For instance, at tetracycline concentrations of 10, 30, and 50 mg/L, the standard deviations were 4.2, 3.8, and 2.9, respectively. Similarly, for catalyst dosages of 0.03, 0.1, and 0.3 g/L, the standard deviations were 3.8, 3.3, and 2.9, respectively. Finally, the stability and repeatability of the ZFC10 photocatalyst were also tested, and the findings indicated that the catalyst preserved its performance over three successive photocatalytic cycles, demonstrating satisfactory operational stability (Fig. 6d)⁶¹. The catalyst retained its photocatalytic performance over repeated cycles under identical conditions, suggesting that the active sites remain intact and that potential Co loss, if any, is minimal within our testing window. The degradation kinetics were analyzed using the pseudo-first-order model (− ln (C_t/C₀) = k_app·t) during the visible-light irradiation period (t = 60–180 min). As shown in Table 2, the apparent rate constants (k_app) for tetracycline degradation vary with both the initial contaminant concentration and catalyst dosage. For initial concentrations of 10, 30, and 50 mg/L, the k_app values were 0.0040 ± 0.0003, 0.0027 ± 0.0002, and 0.0135 ± 0.0006 min⁻¹, respectively (R² ≥ 0.98). Similarly, for catalyst loadings of 0.03, 0.10, and 0.30 g/L/, the k_app values were 0.0039 ± 0.0004, 0.0070 ± 0.0003, and 0.0159 ± 0.0015 min⁻¹, respectively (R² ≥ 0.95). The higher kinetic constants at elevated concentration and loading conditions indicate enhanced collision frequency between TC molecules and reactive species, consistent with a Langmuir–Hinshelwood regime. These results confirm that photocatalysis is the dominant degradation pathway, as control tests (light without catalyst, and catalyst in dark) showed only negligible TC loss. The kinetic constants obtained here compare favorably with those reported for similar spinel ferrite photocatalysts in recent literature^49,50, underscoring the efficiency of Co-substituted ZnFe₂O₄ under visible-light irradiation⁶¹.

Table 2.

Apparent pseudo-first-order rate constants (k_app) for tetracycline degradation under visible light (t = 60–180 min). The error bars reflect the standard deviation calculated from three independent trials (n = 3).

Variable	Level	kapp (min− 1)	±SE (min− 1)	R 2
Initial concentration of TC	10 mg/L	0/004	0/0002	0/983
Initial concentration of TC	30 mg/L	0/0027	0/0001	0/985
Initial concentration of TC	50 mg/L	0/0135	0/0006	0/991
Catalyst loading	0.03 g/L	0/0039	0/0004	0/948
Catalyst loading	0.10 g/L	0/007	0/0003	0/992
Catalyst loading	0.30 g/L	0/0159	0/0015	0/958

The conduction and valence band edge positions were determined using the electronegativity method, yielding E_CB ≈ − 1.33 eV and E_VB ≈ + 0.34 eV for pristine ZnFe₂O₄. These values are in close agreement with those obtained from experimental Mott–Schottky analyses reported in the literature for ZnFe₂O₄-based photocatalysts^51,52. Such consistency demonstrates that our optical/electronegativity-derived band structure provides a reliable description of the charge transfer processes in cobalt-substituted ZnFe₂O₄ and is fully consistent with its enhanced photocatalytic activity.

Proposed mechanism of cobalt-substituted ZnFe₂O₄ in photocatalytic degradation of TC

The band-edge positions of ZnFe₂O₄ were estimated by the absolute electronegativity method^53,54:

5a

5b

where X is the absolute electronegativity and Ee is the free-electron energy on the hydrogen scale (≈ 4.5 eV). Using these relations, E_CB ≈ − 1.335 eV and E_VB ≈ + 0.335 eV were obtained for ZF. Under visible-light irradiation, electrons (e⁻) are promoted from the valence band (VB) to the conduction band (CB), leaving holes (h⁺) in the VB^55,56. In pristine ZF, fast e⁻/h⁺ recombination limits activity. Upon cobalt substitution, Co²⁺/Co²⁺ redox-active centers and localized states are introduced within the spinel lattice, which (i) act as efficient electron/hole traps suppressing direct recombination and (ii) induce band-gap narrowing (from 1.73 eV for ZFC0 to 1.67 eV for ZFC10, Fig. 5), thereby enhancing visible-light absorption and prolonging carrier lifetimes. Consistently, the decay of the 357-nm TC band during irradiation corroborates chromophore disruption prior to ring opening. Photogenerated electrons reduce dissolved O₂ to superoxide (·O₂⁻). Sequential proton-coupled electron-transfer steps generate HO₂· and H₂O₂, ultimately yielding hydroxyl radicals (·OH). Both ·O₂⁻ and ·OH are potent oxidants that attack TC; VB holes also contribute directly or via ·OH formation from surface –OH/H₂O^57,58. The overall sequence is:

6

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9

10

11

12

13

14

15

Drawing on LC–MS and scavenger/EPR studies of cobalt -modified ferrites and ZnFe₂O₄ photocatalysts (including the reviewer-suggested reference), TC typically undergoes hydroxylation and N-demethylation at the C4-dimethylamino moiety, followed by C–N/C–C bond cleavage and aromatic ring opening, yielding lower-mass oxygenates that further oxidize toward mineralization. While LC–MS was not conducted here, our UV–Vis/kinetic evidence and these established literature reports jointly support the assignment of ·O₂⁻ and ·OH as the dominant ROS driving TC degradation in the present system^{59]– [60}. Figure 7 shows the photocatalytic degradation pathway of tetracycline with cobalt-substituted zinc ferrite⁶¹.

Fig. 7.

Proposed mechanism for the photocatalytic degradation process.

Conclusion

In summary, the photocatalytic oxidation process presents a promising and environmentally sustainable approach for the removal of pharmaceutical compounds from wastewater. This study highlights the effectiveness of photocatalytic processes in eliminating synthetic toxic organic pollutants, particularly through the use of advanced oxidation processes that employ high-performance photocatalysts. The successful synthesis of pure and 10% by weight cobalt-substituted ZnFe₂O₄ (ZF) microspheres via hydrothermal methods demonstrated enhanced morphology and crystallinity, contributing to their efficacy as catalysts for the degradation of tetracycline, paracetamol, and antihistamines. The combination of ZF and 10wt% cobalt (ZFC10) not only suppresses the recombination of photogenerated charge carriers but also significantly improves charge transfer, thereby increasing photocatalytic oxidation performance. Under the irradiation of a Hg vapor lamp, ZFC10 achieved a remarkable tetracycline degradation efficiency of 83% within 180 min, nearly doubling the degradation rate observed under LED light. Furthermore, ZFC10 exhibited exceptional reusability and chemical stability over three recycling cycles. Overall, this study provides valuable insights into the mechanisms of photocatalytic degradation and underscores the potential of using visible-light-responsive photocatalysts in advanced oxidation processes for effective environmental remediation.

Author contributions

A.S.: Writing, model, simulation, supervision, software, methodology. S.B.E.: Writing, model, simulation, validation, software, resources. N.A.: Writing, model, simulation, validation, analysis, methodology. All authors reviewed the manuscript.

Data availability

The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.