Introduction

Every aspect of water resources has been contaminated as a result of the rapid growth of the population, development, the Industrial Revolution, and climate change1,2,3,4,5,6,7. The excessive and indiscriminate use of antibiotic poses a significant threat to Earth’s aquatic ecosystems8,9,10,11,12. Various water sources are increasingly contaminated due to factors such as heightened antibiotic use during global pandemics like COVID-19 to manage secondary bacterial infections and the general rise in bacterial infections13,14. Antibiotic concentrations vary across environmental compartments, with acceptable levels ranging from 0.01 to 1 µg/L in surface waters, below 0.1 µg/L (100 ng/L) in drinking water, 0.1–1 mg/kg (dry weight) in soils and sediments to prevent bioaccumulation, and 1–50 µg/L in wastewater, reflecting higher contamination in treated and untreated effluents15,16. Extensive use of antibiotics in veterinary practices, particularly for livestock, contributes substantially to agricultural runoff17,18,19. The production and distribution of unlicensed antibiotics and the widespread use of antibiotics without proper medical prescriptions further exacerbate the problem20,21. This misuse fosters antibiotic resistance and leads to the accumulation of these drugs in water bodies, where they decompose into potentially toxic products22. Addressing this challenge is crucial, as removing complex antibiotics from our environment and protecting natural water sources require urgent attention.

Tetracycline (TC) has been widely used for a long time to prevent infectious diseases in both animals and humans, leading to numerous environmental problems. They rank as the second most consumed antibiotics globally due to their affordability, simple production process, and effective antimicrobial properties, which work by inhibiting protein synthesis23. The detection of antibiotics like TC, in surface and groundwater highlights the growing issue of pharmaceutical pollution in aquatic ecosystems24.TC accumulates in wastewater systems due to inputs from hospitals, pharmaceutical industries, and livestock, with domestic wastewater showing low TC levels (1 µg L−1) in contrast to the higher levels observed in hospital wastewater (100 µg L−1)7,25. Although numerous methods have been employed in environmental studies to eliminate these pollutants, they frequently prove to be both inefficient and costly26. In contrast, photocatalysis stands out as an eco-friendly approach, demonstrating exceptional effectiveness in the degradation of organic contaminants in water26,27,28. Photocatalysis stands out among advanced oxidation processes (AOPs) due to its unique advantages, including high efficiency and the use of renewable solar energy29. Photocatalysis uses solar or artificial light, providing an eco-friendly and sustainable method to degrade a wide range of persistent pollutants, including antibiotics, without harmful by-products or sludge. It operates under mild conditions, requiring no high pressures or temperatures26,30.

In recent decades, there has been a concerted effort to develop visible-light-driven (VLD) photocatalysts. A study found that Al-substituted ZnO nanoparticles resulted in 99.7% degradation of methylene blue (MB) and 99.6% degradation of reactive red 120 (RR120), with corresponding rate constants of 0.01701 min⁻¹ and 0.0680 min⁻¹, respectively31. Another study reported the synthesis of a FeCoWO₄/g-C₃N₄ heterojunction composite, which efficiently degraded 91.5% of MB under visible light within 120 min32. Numerous VLD semiconductor materials have been explored, including metal oxides, metal ferrites, and various carbon-based materials such as reduced graphene oxide (rGO)33,34, multi-walled carbon nanotubes (MWCNTs)35,36,37,38, activated carbon (AC)39, and carbon quantum dots (CQDs)40. Metal oxides like copper oxide (CuO)41,42,43, tin oxide (SnO2)44 and metal ferrites like cobalt ferrite.

(CoFe2O4)8,33,45 are widely utilized in catalysis, supercapacitor electrodes, sensors, solar cells, and water splitting, owing to their exceptional properties. CuO often struggles with the rapid recombination of photogenerated electron-hole pairs, limiting its photocatalytic efficiency. Significant efforts have been dedicated to addressing these limitations and enhancing its catalytic performance. Among the various strategies explored, the fabrication of heterojunction structures has garnered substantial interest from researchers. For example, creating p-n junctions like TiO2/CuO4647,, CuO/BiVO450, and ZnO/CuO43can significantly boost the catalytic efficiency of CuO. Additionally, a wide range of semiconductors, such as WO343, NiO48, and Ag2O49, have been coupled with CuO to form heterojunction structures. While electron-hole pairs may be successfully separated by conventional heterojunctions, the transfer of these charges frequently decreases the composites’ oxidation and reduction potentials, which makes photocatalytic performance for treating refractory contaminants less effective50.

In photocatalysis, the incorporation of MWCNTs into metal oxide and ferrite nanoparticles effectively reduces electron-hole pair recombination. MWCNTs are particularly advantageous for adsorption due to their large specific surface area, high electrical conductivity, layered hollow structure, and electron-rich surface51. In this synergistic system, photogenerated electrons from a photocatalyst with a higher conduction band (CB) can transfer to another photocatalyst with a lower CB, and subsequently migrate to the surface of MWCNTs. Here, they react with O2 to generate reactive superoxide radicals (O2• −), while holes (h+) in the higher valence band (VB) react with H2O to form highly reactive hydroxide radicals (OH•)5253,54,. These radicals are pivotal in decomposing large, complex molecules into smaller, less harmful substances. Thus, this integrated approach enhances the efficient separation of electrons and holes and maintains robust redox capabilities. Consequently, fabricating a CuO photocatalyst integrated with CoFe2O4 and MWCNTs heterostructures holds substantial promise for achieving outstanding photocatalytic performance in the remediation of pollutants from surface and groundwater.

Spinel ferrites have been utilized for the photocatalytic breakdown of TC under visible light, capitalizing on their narrow band gap of around 2 eV. Notable examples like CoFe2O437, NiFe2O438,39,56, and ZnFe2O457 have proven effective in removing pharmaceutical residues from water. Additionally, synergistic effects have been observed with MnFe2O4and molybdenum disulfide (MoS) for TC degradation55, while magnesium ferrite (MgFe2O4) and manganese ferrite (MnxFe3−xO4) show promise against TC and sulfamethoxazole (SMX), respectively20. The use of ferrites for photocatalytic antibiotic degradation presents a promising solution for treating antibiotic-contaminated water due to their gentle magnetic properties, strong catalytic capabilities, and ease of recovery via magnetic separation. Recent studies have demonstrated notable progress, such as CoFe2O4@TiO2@rGO achieving 84.7% TC degradation under visible light in just 60 min8. Furthermore, a CoFe2O4/MWCNTs nanocomposite achieved 90% TC degradation efficiency within 120 min36. Ongoing research aims to enhance CoFe2O4 performance through modifications with CuO and MWCNTs, focusing on stabilizing nanoparticles, preventing aggregation, and facilitating CoFe2O4 recovery from aqueous environments.

In this study, a novel CuO/CoFe2O4/MWCNTs (CCT) heterostructure photocatalyst was synthesized through binary CoFe2O4/MWCNTs (CT) composite using a straightforward in situ hydrothermal method followed by a photoreduction process. Incorporating CoFe₂O₄ and MWCNTs into the CuO matrix aims to enhance photocatalytic efficiency via synergistic effects, contributing to innovative methods for antibiotic pollutant removal from water and providing a sustainable solution to mitigate environmental risks. The photocatalytic activity was assessed by the photodegradation of TC molecules under visible light. Structural and morphological properties were characterized using XRD, HR-SEM, and HR-TEM, while.

UV-Vis analysis measured the materials’ band gap critical for photocatalysis. BET analysis provided details on specific surface area, pore size, and volume, aiding in understanding adsorption reactions. EDX confirmed the elemental composition of pure and composite materials. PL analysis showed effective electron-hole recombination suppression, highlighting the nanocomposite’s photocatalytic suitability. Stability and active species trapping tests evaluated catalyst efficiency and identified active species, leading to a proposed mechanism and degradation pathway based on energy band analysis and trapping results. The chemical structure of TC antibiotics is shown in Fig. 1.

Fig. 1
figure 1

Acidic dissociation constants (pKa) of TC structure.

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Materials and methods

Materials

Cobalt chloride (CoCl2·6H2O, 98% purity), copper chloride (CuCl2·2H2O, 99% purity), iron chloride (FeCl3·6H2O, 98% purity), multi-walled carbon nanotubes (MWCNTs; diameter 10–30 nm, length 5–40 μm, purity > 95%), sodium hydroxide (NaOH), hydrochloric acid (HCl), ethanol, and deionized water were utilized for synthesizing the hybrid nanocomposite. TC with a purity exceeding 98%, alongside other reagents, was procured from Che-O-Chem Scientific Company (India). Additionally, isopropyl alcohol (IPA), chloroform (CH), and ammonium oxalate (AO), all with a purity of 99%, were sourced from Merck Company (Germany).

Synthesis of CoFe2O4 nanoparticles

A 0.4 M FeCl3·6H2O solution (25 mL) and a 0.2 M CoCl2·6H2O solution (25 mL) were mixed in double-filtered, deionized water to prevent contamination. Subsequently, a 25 mL solution of 3 M NaOH was gradually added to this mixture while magnetic stirring was used to continuously monitor and maintain the pH between 11 and 12 59. Oleic acid was utilized as a surfactant and coating agent with care. The mixture was then heated to 80 °C and stirred for a further hour. The precipitate was rinsed twice with distilled water and once with ethanol when it cooled to room temperature to eliminate surplus surfactant and create particles devoid of salt and chlorine. The mixture was separated from the supernatant by centrifuging it for 15 min at 3000 rpm. After being dried at 100 °C for the whole night, the thick, black precipitate was crushed into a fine powder. The CoFe2O4 powder nanocatalyst was further calcined for 10 h at 600 °C to remove any remaining water molecules. X-ray diffraction and other methods were used to characterize the finished product to verify its purity and structure.

Synthesis of CuO nanoparticles

The hydrothermal method was employed to synthesize CuO nanoparticles. First, 50 mL of a precursor solution containing copper chloride was combined with double-distilled water57. 50 mL of an aqueous NaOH solution was progressively added to the mixture while it was being constantly agitated. After that, the resultant solution was sealed in an autoclave and heated to 120 °C for a whole day. After this procedure, the precipitates were cleaned using deionized water and ethanol57,58. After that, the crystalline CuO nanoparticles were dried in a furnace at 90 °C and annealed for 3 h at 400 °C.

Synthesis of CT nanocomposite

An ultrasonic bath dissolved 0.25 g of MWCNTs in 20 mL of ethanol for 30 min. The resultant suspension was then combined with 0.25 g of CoFe2O4 nanoparticles. A magnetic stirrer was utilized to ensure the continuous stirring of the CT nanocomposite. The nanocomposite was then vacuum-dried at 50 °C for a whole night. After that, the CT mixture was placed into a Teflon-lined autoclave and heated to 180 °C for 12 h. Before vacuum drying, the combinations went through several ethanol washes and centrifugation. After that, the sample was annealed at 400 °C for 4 h.

Synthesis of CCT nanocomposite

The synthesis of CCT nanocomposite starts by mixing 0.2 g of CT with a solution of 40 mL ethanol and an equal amount of CuO, followed by an hour of ultrasonic bath treatment. Subsequently, the entire mixture was stirred for 4 h to ensure uniformity. The resulting precipitate was rinsed twice with distilled water and once with ethanol to remove impurities. The mixture was then dried in a furnace at 180 °C for 12 h, followed by annealing at 400 °C for 2 h, resulting in the formation of the CCT nanocomposite. Figure 2 illustrates the complete synthesis process of the nanocomposite.

Fig. 2
figure 2

Illustrate detailed synthesis procedures for CCT nanocomposite.

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Photocatalytic degradation of TC using CCT nanocomposite

A stock solution of TC at 1000 mg/L concentration was prepared from pure TC powder and stored in the dark at 4 °C to prevent any premature degradation. Experiments were conducted in a batch system, using 50 mL aliquots at an ambient temperature of 24 ± 2 °C in a 500 mL reactor. This reactor was submerged in a cooling water chamber and continuously rotated to maintain thermal stability. Magnetic stirring at 350 rpm ensured thorough mixing of the samples. Solution pH was adjusted as necessary using 0.1 N HCl or NaOH. The photodegradation studies were conducted with a visible light source supplied by a low-cost solar simulator (LSO1O6, 150 W Xe light source) fitted with a filter holder and a 90° beam turner. In a 100 mL beaker, 50 mL of TC solution with an initial concentration of 25 mg L−1 was exposed to 0.1 g of CCT composite particles for degradation. The solutions were gently swirled at 350 rpm for 120 min under simulated visible-light irradiation, with 20 cm gap between the UV lamp and the test solution. Samples were collected at various intervals to investigate degradation kinetics. The concentrations of TC (λmax = 360 nm), were measured using a UV-Vis spectrophotometer (Perkin Elmer, Lambda 750). To determine the effect of CCT loading on degrading efficiency, the composite particle dose was changed from 0.1 to 0.5 g. Similarly, the effect of starting concentrations of TC on degrading capacity was investigated by varying their concentrations from 5 to 25 mg L−1. The influence of pH on the degradation process was examined by changing the pH level from 2 to 10. In addition, control tests were carried out in the absence of light to determine adsorption-desorption equilibrium. The deterioration efficiency was calculated using Eq. 1 as follows:

$$Efficiency\:(\%)=((C_0-C_t)/C_0)\times100$$
(1)

where Co is the initial concentration of TC and Ct is the concentration of TC at time t.

Results and discussions

Characterization of synthesized samples

Figure 3 displays the XRD patterns of the synthesized pure MWCNTs, CoFe₂O₄, CuO, as well as the CT and CCT composite materials. The XRD results depicted in Fig. 3 (a) reveal that MWCNTs exhibit two distinct peaks at 2θ = 25.9° and 43.3°, corresponding to the (002) and (100) planes, respectively. Figure 3 (b) shows diffraction peaks at 2θ = 30.2°, 35.6°, 43.2°, 53.6°, 57.2°, and 62.8°, which are assigned to the (220), (311), (400), (422), (511), and (400) crystallographic planes of CoFe2O4 nanoparticles, indicating a cubic crystal structure (JCPDS card no. 022–1086). The XRD pattern of pure CuO in Fig. 3 (c) displays peaks at 32.2°, 35.7°, 38.7°, 46.1°, 48.5°, 52.6°, 57.8°, 61.5°, 66.2°, and 72.0°, corresponding to the (110), (002),

(−111), (111), (−112), (−202), (020), (202), (−113), (022), and (−311) crystal planes of the monoclinic system (JCPDS card no. 041–0254). The crystallite sizes of pure CoFe₂O₄ and CuO, as calculated using the Scherrer formula (Eq. 2), are 46.8 nm and 37.5 nm, respectively. The XRD patterns in Fig. 3 (d) and (e) confirm CT nanocomposite and the presence of both CuO and CT phases in the CCT photocatalyst. Using the Scherrer equation and the most prominent peak of CCT, the average crystallite size is calculated to be 53 nm. The (002) peak associated with MWCNTs is not visible, likely due to overlapping with the diffraction peaks of CuO or a low loading percentage of MWCNTs. A well-defined crystal structure is essential for boosting photocatalytic efficiency, as it facilitates the effective separation of charge carriers. The presence of crystalline phases like CoFe₂O₄ and CuO is frequently linked to enhanced photocatalytic activity.

Scherrer equation can be written as

$$\:D=\frac{K\lambda\:}{\beta\:cos\theta\:}$$
(2)

where K = dimensionless shape factor close to unity.

λ = X-ray wavelength.

β = FWHM ((full width half maximum) value in radians.

θ = Bragg angle.

Fig. 3
figure 3

XRD spectra of (a) MWCNTs (b) CoFe2O4, (c) CuO, (d) CT, and (e) CCT.

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Figure 4(a) to (d) present HR-SEM images of the synthesized nanocomposite materials, highlighting the uniform distribution of CoFe₂O₄ and CuO nanoparticles on the surface of MWCNTs. The images depict a well-defined, three-dimensional network structure where MWCNTs serve as a supportive matrix for the nanoparticles. The tight adherence of the nanoparticles to the MWCNT surface suggests strong interactions within the composite, with a relatively narrow particle size distribution indicating a controlled synthesis process. The homogeneous dispersion of nanoparticles along the MWCNTs is crucial for maintaining a high surface area, which is essential for effective interaction with external agents in potential applications. The typical tubular structure of MWCNTs is observed, with slight nanoparticle accumulation in some areas, although this does not significantly impact the overall uniformity of the nanocomposite. The CuO nanoparticles are primarily spherical or quasi-spherical, whereas the CoFe₂O₄ nanoparticles exhibit a more irregular morphology, consistent with their distinct crystalline structures. The strong nanoparticle adherence suggests a synergistic effect between the composite components, potentially enhancing properties such as electrical conductivity, magnetic behaviour, and catalytic activity.

Fig. 4
figure 4

HR-SEM images of (a) CoFe2O4, (b) MWCNTs, (c) CT, and (d) CCT nanocomposites.

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The elemental composition of the CoFe₂O₄, MWCNTs, and CuO in the CCT composite was determined via EDX analysis. The results confirmed the presence of iron (Fe), cobalt (Co), oxygen (O), copper (Cu), and carbon (C), aligning with the expected molar ratios. The EDX analysis, performed across multiple sample areas, is detailed in Fig. 5. The EDX spectra identified CoFe₂O₄, CuO, and MWCNTs in the synthesized composite nanostructure. The CoFe₂O₄ nanoparticles contained 14.62% Co, 37.59% Fe, and 47.79% O, as shown in.

Figure 5 (a). The MWCNTs exhibited 100% C, as depicted in Fig. 5 (b). Additionally, the atomic weight percentages of Co, Fe, O, and C in the CT composite, shown in Fig. 5 (c), were 3.06%, 5.71%, 22.56%, and 68.67%, respectively. Figure 5 (d) presents the atomic weight percentages of the ternary CCT nanocomposite, with Cu, Co, Fe, O, and C recorded at 21.59%, 4.43%, 8.15%, 26.44%, and 39.40%, respectively. Doping with specific metals or heteroatoms can modify the electronic structure of a photocatalyst, improving electron-hole separation and boosting photocatalytic efficiency.

Fig. 5
figure 5

EDX analysis of (a) CoFe2O4, (b) MWCNTs, (c) CT, and (d) CCT nanocomposites.

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To examine the morphology and structure of the synthesized materials, HR-TEM was employed, utilizing high-energy electron beams. Figure 6 (a) and (b) depict the HR-TEM images of pristine CoFe₂O₄ and MWCNTs, respectively. The CoFe₂O₄ nanoparticles exhibit a nearly spherical morphology with an average particle size of approximately 28.2 nm, while the MWCNTs display a tubular structure with a diameter of 15.65 nm, aligning well with the findings from XRD analysis. Figure 6 (c) illustrates the HR-TEM image of the CT nanocomposite, revealing CoFe₂O₄ particles with an average size of around 26.8 nm. Notably, the image indicates that magnetic interactions among CoFe₂O₄ nanoparticles result in agglomeration, leading to the formation of larger clusters or assemblies. Figure 6 (d) presents the HR-TEM image of the CCT nanocomposite, which shows an increased average particle size of 49.2 nm. The CoFe₂O₄ and CuO nanoparticles in the CCT nanocomposite exhibit excellent crystallinity while maintaining the structural integrity of the MWCNTs. Surface texture and particle size greatly influence light absorption and photocatalytic efficiency. Smaller particles, with more surface defects and a higher surface area-to-volume ratio, enhance light absorption and accelerate reaction rates.

Fig. 6
figure 6

HR-TEM images of (a) CoFe2O4, (b) MWCNTs, (c) CT, and (d) CCT nanocomposites.

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The optical properties of the synthesized materials were characterized using UV-Vis spectroscopy, as shown in Fig. 7(a) to (d). This method is pivotal in assessing the photocatalytic potential of the catalysts59. The results reveal that all synthesized samples exhibit strong absorption within the visible light spectrum. Interestingly, when CoFe2O4 and CuO nanostructures were integrated into MWCNTs, the absorption spectrum of the CCT nanocomposite displayed an absorption edge that closely mirrors that of the CT nanocomposite. The integration of pristine CoFe2O4 and CuO nanostructures resulted in a small decrease in light absorption, which was most likely owing to the MWCNTs’ entire encapsulation. However, the combination of CT with CuO resulted in higher photonic absorption than pure CoFe2O4. These findings indicate that the CCT nanocomposite is better at absorbing visible light. Typically, the photon absorption capability of photocatalysts is intrinsically related to their band gap energy (Eg). The band gap energies of the synthesized nanostructures were ascertained using Tauc plots, with (αhυ)² vs. hυ curves depicted in the inset of Fig. 7. The band gap energies of pristine CoFe2O4, CuO, CT, and CCT were calculated to be 2.27, 1.8, 2.0, and 1.6 eV, respectively. The reduced Eg values are due to enhanced electronic interactions between the semiconductor and MWCNTs, which improve charge separation. The combination of materials with varying energy levels shifts the conduction and valence bands, narrowing the band gap. Furthermore, MWCNTs act as electron scavengers, facilitating electron transfer from the conduction band of CuO or CoFe₂O₄ and reducing electron-hole recombination, thereby boosting photocatalytic performance, and lowering the band gap. Materials with narrower band gaps, such as CT (2.0 eV) and the CCT nanocomposite (1.6 eV), exhibit enhanced absorption capabilities within the visible region of the solar spectrum. In the CCT nanocomposite, the integration of CoFe₂O₄ and MWCNTs into the CuO matrix effectively reduces the band gap to 1.6 eV, thus extending light absorption into the visible range. This reduction in band gap significantly enhances photocatalytic performance by increasing light-harvesting efficiency. These findings imply that while the CT and CCT photocatalysts share comparable photon absorption abilities, a decrease in the recombination rate of electron-hole pairs (e− - h+) could substantially enhance their photocatalytic performance, a topic that will be further elaborated in the following section.

Fig. 7
figure 7

UV-Vis spectra of (a) CoFe2O4, (b) CuO, (c) CT, and (d) CCT nanocomposites.

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The specific surface area and pore size distribution of the synthesized materials were determined using nitrogen (N2) adsorption-desorption measurements60. The BET technique was employed to accurately determine the surface area, while the Barrett-Joyner-Halenda (BJH) analysis was used to measure pore size distribution61. The BET analysis results for CoFe2O4, MWCNTs, CT, and CCT nanocomposites are presented in Fig. 8 (a) to (d). The specific surface areas for CoFe2O4, MWCNTs, CT, and CCT nanocomposites were found to be 49.4, 289.63, 68.52, and 82.34 m²/g, respectively. The CoFe2O4, CT, and CCT nanocomposites exhibited type IV isotherms with an accompanying H3 hysteresis loop, indicating the mesoporous nature of these samples. Additionally, the pore volumes derived from the BJH analysis for CoFe2O4, CT, and CCT nanocomposites were 0.23, 0.29, and 0.37 cm³/g, respectively. The high specific surface area and pore volume of these materials suggest that they provide numerous active sites, making them highly suitable for photocatalytic degradation applications.

Fig. 8
figure 8

BET analysis of (a) CoFe2O4, (b) MWCNTs, (c) CT, and (d) CCT nanocomposites.

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The PL spectra of synthesized materials provide information about photon-induced electron-hole (e−-h+) pair separation efficiency59. The recombination rate of e−-h+ pairs can be directly assessed by analyzing the intensity of the PL spectrum of the corresponding materials. A more intense PL spectrum in the nanostructure suggests a higher recombination rate of e−-h+pairs. Conversely, a lower PL intensity indicates that more photoexcited electrons are effectively trapped and transferred across the catalyst interface62. The PL spectra of the synthesized materials, as illustrated in Fig. 9, exhibit emission peaks within the 350 to 450 nm range. The PL intensity of CuO and CoFe2O4 nanoparticles surpasses that of the composite materials, reflecting a higher e−-h+recombination rate in the pure nanoparticles. The PL intensity decreases further upon incorporating MWCNTs into these materials, implying a reduced recombination rate in the composites, which may contribute to enhanced photocatalytic efficiency63. The decrement in PL intensity is due to the rapid delocalization of photo-excited electrons to the MWCNTs and reducing the e−-h+ pair recombination probability. The incorporation of MWCNTs with CuO and CoFe2O4 caused a slight red shift in the PL spectra, which is probably due to molecular interactions among these components. These findings indicate that the synthesized CCT nanocomposite enhances light absorption and effectively prevents the recombination of photoinduced electron-hole pairs, significantly improving its photocatalytic efficiency.

Fig. 9
figure 9

PL spectra of synthesized materials at room temperature.

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Effect of operational parameters

Effect due to catalyst dosage

The research aims to investigate the impact of different catalyst quantities on the degradation of the antibiotics TC. The experiment involves varying the photocatalyst dose from 0.1 to 0.5 g/L to determine its influence on the degradation process. Figure 10(a) shows the degradation rate of antibiotics with different quantities of CCT nanocomposite at pH = 7, starting concentration of 25 mg/L, duration of 120 min, and visible light exposure. The degradation rate of the target antibiotics increased as the loading of CCT was raised from 0.1 to 0.3 g/L, but a decrease in the degradation rate was observed when the loading was further increased from 0.4 to 0.5 g/L. This behaviour may be related to an increase in catalyst amount, which improves the surface-to-volume ratio and encourages the development of more active sites64. Increasing the number of photocatalyst particles allows more photons to contact the catalyst surface, thereby initiating the photocatalytic process36. Because of the lack of active sites on the catalyst surface, a restricted number of reactive sites are formed per unit of time at 0.1 g/L, resulting in a lower degradation rate. However, exceeding 0.3 g/L of CCT resulted in catalyst particle aggregation and reduced active site availability. This aggregation increased solution turbidity, reducing light penetration and increasing light scattering65. Furthermore, the nanocomposite’s large surface area and high energy facilitated aggregation, reducing light’s effectiveness in interacting with the photocatalyst at high concentrations. As shown in the bar diagram Fig. 10 (b), when the concentration exceeded 0.3 g/L, the removal efficiency subsequently decreased. Figure 10 (c) demonstrates the impact of different nanocomposite quantities on the concentration ratios for TC degradation. Figure 10 (d) illustrates the UV-Vis spectra showing the degradation of TC antibiotics using 0.3 g/L of the nanocomposite, demonstrating the highest degradation rate.

Fig. 10
figure 10

(a) Impact on the percentage of TC degradation by various catalyst dosages, (b) Bar diagram of TC removal at various CCT dosages, (c) Effect of concentration ratios on TC degradation using CCT nanocomposite dosages, and d) UV-Vis spectra of TC antibiotic degradation by 0.3 g/L of CCT nanocomposite.

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Effect of initial TC concentration

The effect of TC antibiotic concentration on degradation was assessed using a CCT nanocomposite at a dosage of 0.3 g/L, pH = 7, and under 120 min of visible light exposure. As illustrated in Fig. 11 (a), the degradation percentage varied with different TC concentrations. Specifically, the bar diagram in Fig. 11 (b) shows that increasing the TC concentration from.

5 mg/L to 25 mg/L decreased the degradation rate from 91 to 80.1%. This indicates a higher degradation efficiency at lower TC concentrations. Several factors contribute to this observed effect. Firstly, under the given conditions, the catalyst dosage, time, pH level, and density of free radicals (OH•) remain constant. Hence, the interaction between TC and OH•radicals is more efficient at lower TC concentrations66. Secondly, at higher TC concentrations, pollutant molecules tend to cover the active sites of the catalyst, hindering the production of radicals. Lastly, with increased TC concentration, the drug molecules absorb more radiation and cover the catalyst particles, thereby obstructing the radiation from reaching the catalyst67. Consequently, the degradation rate decreases as the catalyst particles are less activated by light. Furthermore, increased initial TC concentrations resulted in the formation of a greater number of intermediates. These intermediates compete with TC molecules for active sites on the catalyst surface, thereby impeding the degradation process of TC52. Figure 11 (c) demonstrates the dark adsorption and the impact of varying concentration ratios in the reaction using 0.3 g/L catalyst with different TC concentrations. Additionally, Fig. 11 (d) presents the UV-Vis spectra for the reaction using a 0.3 g/L dosage with a TC concentration of 5 mg/L showing maximum degradation results.

Fig. 11
figure 11

(a) Concentration effects of antibiotics on the degradation process, (b) Bar diagram of TC removal at different concentrations by 0.3 g/L of CCT composite, (c) Effect of concentration ratios with various TC concentrations, and (d) UV-Vis spectra of TC antibiotics of 5 mg/L.

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Effect of pH

The pH of the solution significantly influences the degradation process. As illustrated in Fig. 12 (a), the degradation rate of TC increases as the pH rises from 2 to 10, indicating that optimal degradation occurs under alkaline conditions. Furthermore, Fig. 12 (b) presents a bar diagram depicting the degradation rates, validating the enhanced degradation efficiency in alkaline environments. This behaviour can be attributed to the point of zero charge (pHpzc) of the synthesized nanocomposite, which is determined to be 6.2. At pH values below the pHpzc, the photocatalyst surface is predominantly positively charged, whereas at pH values above the pHpzc, it becomes negatively charged. Similarly, TC molecules are generally positively charged in acidic conditions and negatively charged in basic conditions. Consequently, in an acidic medium, both the photocatalyst and TC molecules carry the same positive charge, leading to electrostatic repulsion68. This repulsion reduces the adsorption of TC onto the nanocomposite surface, resulting in a lower degradation rate under acidic conditions. In addition, acidic conditions promote nanoparticle aggregation and accumulation, which substantially decrease the catalyst’s surface area available for TC adsorption, thereby reducing the degradation efficiency. Furthermore, the elevated concentration of H+ ions relative to OH−ions in acidic environments results in diminished hydroxyl radical production, further lowering the removal efficiency16.

In an alkaline medium, both the catalyst and TC molecules acquire negative charges. Nevertheless, the hydroxyl radicals (OH•), generated by the reaction between hydroxyl ions (OH−) and holes (h+), exhibit high activity in semiconductor photocatalytic reactions. As a result, TC degradation efficiency is enhanced under alkaline conditions68,69. Experimental data reveal that the highest degradation rate occurs at a pH of 10. This can be explained by the increased electron density in an alkaline medium, which promotes more effective radical attacks on TC molecules. Figure 12 (c) shows dark adsorption and the influence of concentration ratios at different pH levels, while Fig. 12 (d) displays the UV-Vis spectra of the TC degradation reaction, indicating that a pH of 10 yields the maximum degradation rate.

Fig. 12
figure 12

(a) The effect of pH on the degradation of TC using CCT nanocomposite, (b) Bar diagram of TC removal at different pH values, (c) The effect of concentration ratios in different pH of the solution, and d) UV-Vis spectra of TC antibiotics of the solution at pH = 10.

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Effect of TC degradation on pure and composite materials

A set of experiments was meticulously carried out under ideal conditions, employing pure materials, to comprehensively examine the photocatalytic efficiency of CuO/CoFe2O4/MWCNTs (CCT) in the degradation of TC under visible light irradiation.

Figure 13 (a) illustrates the photocatalytic efficiency under optimal conditions: a photocatalyst dosage of 0.3 g/L, TC concentration of 5 mg/L, pH = 10, and 120 min of visible light exposure. The CCT nanocomposite demonstrates significantly enhanced degradation performance compared to the pure materials alone. Figure 13 (b) offers a comparative bar chart of the degradation efficiencies of both pure and composite materials. In the absence of light, the CCT nanocomposite achieved an adsorption capacity of less than 15% for TC degradation, highlighting its limited effectiveness under dark conditions, as shown in Fig. 13(c). Under dark conditions, introducing the CCT photocatalyst into a TC solution initiates an adsorption process where TC molecules adhere to the catalyst’s surface. In the absence of light, no photocatalytic degradation occurs, so the observed reduction in TC concentration arises solely from adsorption onto the catalyst surface70. The composite’s high surface area, attributed primarily to MWCNTs, offers numerous active sites for TC attachment. Additionally, the CuO and CoFe₂O₄ components in the composite contribute functional groups that promote strong interactions with TC molecules through electrostatic forces, hydrogen bonding, and π-π stacking interactions. The porous architecture of the CCT nanocomposite supports TC adsorption both on the surface and within the pores, with MWCNTs significantly enhancing the mesoporous structure. This mesoporosity increases the adsorption capacity by effectively trapping TC molecules within the composite matrix. Under optimal conditions with visible light exposure, the CCT nanocomposite was able to achieve an impressive 98.1% degradation of TC within 120 min. This is a remarkable result compared to the performance of the individual pure materials. For instance, pure CoFe₂O₄ achieved a 74% degradation rate, pure CuO achieved 86%, and the CT nanocomposite (CoFe2O4/MWCNTs) achieved 92%. These comparisons indicate that the full CCT nanocomposite, which includes all three components (CuO, CoFe₂O₄, and MWCNTs), is far more effective at degrading TC under visible light than any of its individual components or partial combinations.

Fig. 13
figure 13

(a) Impact on degradation efficiency of pure and composite (b) Bar chart of TC degradation efficiency (c) Effect of catalysts on dark adsorption.

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Kinetic study of TC antibiotic degradation

The kinetic reaction of TC antibiotic degradation was analyzed using a CCT nanocomposite of 0.3 g/L under pH 7 and 120 min of visible light irradiation. Initial TC concentrations varied between 5 mg/L and 25 mg/L—the degradation of TC in the aqueous solution adhered to pseudo-first-order kinetics, as represented by Eq. 3.

$$\:\varvec{l}\varvec{n}{\varvec{C}}_{0}/{\varvec{C}}_{\varvec{t}\:}=\varvec{k}\varvec{t}$$
(3)

where k = rate constant.

C0 = initial concentration (mg/L).

Ct= concentration of TC at time t (mg/L).

t = time (min).

Figure 14 (a) and (b) depict the pseudo-first-order kinetic curves and a bar diagram of the k values, respectively, with the associated results outlined in Table 1. As the initial concentration increases, the rate constant decreases. Specifically, the rate constants for initial concentrations of 5 mg/L and 25 mg/L were 0.0181 m⁻¹ and 0.0119 m⁻¹, respectively. These findings indicate that the pseudo-first-order kinetic model appropriately describes the reaction rate, fitting the data well across all tested concentrations. The coefficient of determination (R2) values, which are close to 1, further validate the model’s adequacy. At higher TC concentrations, the reaction rate decreases, likely due to a higher concentration of intermediate products that limit the availability of active hydroxyl radicals, thereby reducing the degradation rate constant66.

Table 2 shows the most recent reported studies of TC degradation using different nanocomposites.

Fig. 14
figure 14

(a) Pseudo-first-order kinetic plots for different concentrations of antibiotics. (b) Bar graph showing rate constants and associated error values for different concentrations.

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Table 1 Pseudo-first-order kinetic parameters for TC decomposition at varying concentrations.
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Table 2 Summary of research outcomes on TC degradation using various nanocomposites.
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Reusability and stability of the CCT nanocomposite

Assessing a catalyst’s durability and stability is crucial to determine its potential for industrial applications. To evaluate the recyclability of the CCT photocatalyst, we conducted five sequential cycles of TC photodegradation under optimized conditions: 0.3 g/L CCT nanocomposite, 5 mg/L TC concentration, pH 10, and 120 min of visible light irradiation. This setup tested the reusability of the CCT catalyst. After each cycle, the CCT was collected, washed with various solvents (deionized water, 0.1 M HCl, ethanol, or a 4:3 mixture of ethanol and 0.1 M HCl), and dried at 80 °C for 5 h to remove any adsorbed organic compounds before being reused in the next cycle. This procedure was repeated over five cycles. Remarkably, the CCT catalyst maintained stable catalytic performance throughout, with only a slight decline in TC degradation efficiency—a 2.1% reduction in the fifth cycle, as illustrated in Fig. 15(a). This reduction is mainly attributed to the formation of intermediate compounds that compete with TC molecules for free radical reactions, as well as the blockage of pores and active sites by TC and reaction byproducts56. The bar chart in Fig. 15 (b) shows a 2.1% decrease in efficiency from the first to the fifth cycle.

XRD analysis of the CCT photocatalyst, as shown in Fig. 15 (c), revealed no significant changes in the spectra before and after the photocatalytic cycles, indicating strong structural stability. Additionally, HR-TEM images (Fig. 15 (d)) taken after five consecutive cycles showed that the CuO and CoFe₂O₄ nanoparticles remained uniformly distributed on the MWCNTs, preserving the microsphere morphology of the CuO and CoFe₂O₄ components. These XRD and HR-TEM findings demonstrate that the CCT photocatalyst maintains excellent stability for visible-light-driven TC photodegradation, with consistent chemical and structural properties even after five cycles of reuse.

Fig. 15
figure 15

(a) Impact on the relative concentration of TC degradation in each cycle (b) Bar diagram of TC degradation efficiency in each cycle (c) XRD spectra and d) HR-TEM of CCT nanocomposite before and after degradation.

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Role of reactive species in the reaction

The study reveals that the CCT photocatalyst significantly boosts the production of hydroxyl radicals (OH•). In typical photocatalytic processes, photons with adequate energy excite electrons within the photocatalyst, moving them from the valence band (VB) to the conduction band (CB) and generating electron-hole pairs76. Effective separation of these charge carriers enables oxidation and reduction reactions of pharmaceutical compounds, largely determined by the material’s band edge positions77. Within the CCT nanocomposite, photoexcited electrons from the VB to the CB of CuO are effectively trapped by CoFe₂O₄ nanoparticles and MWCNTs, which serve as electron sinks, thereby reducing electron-hole recombination and enhancing redox activity. The remaining holes in the VB participate in the oxidation of hydroxide ions (OH-), producing OH• radicals. Increased electron capture by CoFe₂O₄ and MWCNTs leads to greater availability of holes, encouraging further OH•radical generation. These radicals subsequently interact with pharmaceutical molecules, aiding in their breakdown. In addition, the holes may also directly participate in degrading pharmaceutical contaminants78. The improvement in photocatalytic efficiency is primarily attributed to better charge carrier separation, where CB electrons are effectively prevented from recombining with VB holes79. This separation allows these electrons and holes to engage in redox reactions with the surrounding environment, thus accelerating the degradation of pharmaceutical compounds.

Mechanism of photocatalytic reaction

To better understand the operation of CCT, the role of reactive oxygen species (ROS) in the photodegradation of TC is investigated using isopropyl alcohol (IPA) as a hydroxyl radical (OH•) scavenger, ammonium oxalate (AO) as a hole (h+) scavenger, and chloroform (CH) as a superoxide ion radical (O2• −) scavenger. These scavengers were introduced at a concentration of 1 mM each, while the weight of CCT, concentration of TC, photodegradation duration, and pH remained as optimum values. After 120 min of visible light exposure, TC degradation fell considerably to 20%, 22%, and 72% in the presence of hydroxyl radical, hole, and superoxide radical scavengers (Fig. 16). These results indicate that photogenerated hydroxyl radicals (OH•) and holes (h+) are the principal factors impacting TC degradation when CCT hybrids are used as photocatalysts. Furthermore, when isopropanol (IPA) and ammonium oxalate (AO) were added at the same concentration and volume, the degradation efficiency decreased by around 20%. As a result, it may be concluded that OH• has a bigger influence than h+ and both h+ and OH• are required for efficient photocatalysis.

Fig. 16
figure 16

The effect of scavengers (IPA, AO, and CH) on TC degradation.

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To understand the reaction mechanism, it is essential to determine the valence band (VB) and conduction band (CB) potential edges of the catalysts. These potential edges can be derived using Eqs. 4 and 5.

$$E_{CB}=\chi-E^c-0.5\:E_g$$
(4)
$$E_{VB}=E_{CB}+E_g$$
(5)

Here \(\:{E}_{CB\:},\:{E}_{VB\:},\:\)and χ signifies the energy potentials linked to the CB, VB, and Mulliken absolute electronegativity of an atom, respectively. \(\:{E}^{c}\:\)represents the energy level of free electrons on the hydrogen scale valued at 4.5 eV80. The electronegativity is calculated using Eqs. 6 and 7.

$$\:{\varvec{\chi\:}}_{\varvec{a}\varvec{t}\varvec{o}\varvec{m}}=\frac{1}{2}\:\left({\varvec{E}}_{\varvec{E}\varvec{A}}+\:{\varvec{E}}_{\varvec{I}\varvec{E}}\right)$$
(6)
$$\:\varvec{\chi\:}=[\:{{\varvec{\chi\:}\left(\varvec{A}\right)}^{\varvec{a}\:}.\:{\varvec{\chi\:}\left(\varvec{B}\right)}^{\varvec{b}}\:.\:{\varvec{\chi\:}\left(\varvec{C}\right)}^{\varvec{c}\:}]}^{\raisebox{1ex}{$1$}\!\left/\:\!\raisebox{-1ex}{$\left(\varvec{a}+\varvec{b}+\varvec{c}\right)$}\right.}$$
(7)

where \(\:{E}_{EA}\) and \(\:{E}_{IE}\) represent electron affinity and ionization energy of atoms a, b, and c represent atomic numbers of the compounds. Therefore, Mulliken absolute electronegativity of CoFe2O4and CuO can be written as 5.80 and 5.79 eV respectively23,56. So, VB and CB edge potential for CoFe2O4 is 2.44 and 0.17 eV at the same time for CuO is 2.19 and 0.39 eV respectively. The estimated band edge positions for CoFe2O4 and CuO are elucidated in Table 3.

Table 3 Mullikan absolute electronegativity, band gap energies, and calculated CB and VB edges for CoFe2O4 and CuO.
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The degradation mechanism of TC using CCT nanocomposite can be clearly explained through Eq. 8 to 15. The electrons (e−) in the CB of the CCT catalyst can be represented as e− CB as well as holes (h+) in the VB represented by h+ VB

$$\:Co{Fe}_{2}{O}_{4}\:\:\:+\:h\upsilon\:\:\to\:\:Co{Fe}_{2}{O}_{4}\:(\:{e}_{CB}^{-}+\:{h}_{VB}^{+})$$
(8)
$$\:CuO\:\:+\:h\upsilon\:\:\to\:\:CuO\:(\:{e}_{CB}^{-}+\:{h}_{VB}^{+})$$
(9)
$$\:{O}_{2}\:+\:MWCNTs\:\left(\:{e}_{CB}^{-}\:\right)\to\:\:{O}_{2}^{{\bullet\:}\:-}$$
(10)
$$\:{O}_{2}^{{\bullet\:}\:-}+\:\:MWCNTs\:\left(\:{e}_{CB}^{-}\right)+\:2{H}^{+}\:\to\:\:{OH}^{-}\:+\:{OH}^{{\bullet\:}}$$
(11)
$$\:{O}_{2}+\:3{e}_{CB}^{-}\:+\:2{H}^{+}\:\to\:\:2{OH}^{-}$$
(12)
$$\:{H}_{2}O\:+\:{h}_{VB}^{+}\:\:\:\to\:\:{OH}^{{\bullet\:}}+\:{H}^{+}$$
(13)
$$\:{OH}^{-}\:+\:{h}_{VB}^{+}\:\to\:\:{OH}^{{\bullet\:}}$$
(14)
$$\:{OH}^{{\bullet\:}}/\:{h}_{VB}^{+}\:\:+\:TC\:\to\:\:products$$
(15)

The photocatalytic degradation mechanism is described in Fig. 17. Exposure to visible light generates electrons (e⁻) and holes (h⁺) in the CB and VB of catalysts CoFe₂O₄ and CuO (Eqs. 8 and 9)81. Photogenerated electrons are transmitted to MWCNTs, which act as electron acceptors and transport mediums in the photocatalytic system82. This prevents the recombination of photogenerated e--h+ pairs. Because of their enormous specific surface area and active sites, the reaction occurs on the photocatalyst CCT surface and the MWCNTs. The electrons transported to the surface of MWCNTs react with surface-adsorbed oxygen to create superoxide radicals (O₂•-) (Eq. 10). These radicals next combine with hydrogen ions (H⁺) to make hydroxyl radicals (OH•) (Eq. 11). The CB potential edge of CoFe₂O₄ (0.17 eV) is more negative than the conventional redox potential of O₂/OH⁻ (0.40 eV), allowing photogenerated electrons to convert O₂ to OH⁻ (Eq. 12)59. CuO has a higher VB edge potential (2.19 eV) than OH⁻/OH• (1.99 eV) and H₂O/OH• (1.23 eV), enabling h⁺ to oxidise OH⁻ and H2O to generate OH•(Eqs. 13 and 14)33. The production of OH• through oxidation and reduction reactions is critical in breaking TC antibiotics. Additionally, the h⁺ in CuO’s VB can directly oxidise TC molecules. These highly reactive species interact with TC molecules, causing an effective photocatalytic process (Eq. 15).

Fig. 17
figure 17

Mechanism of photocatalytic degradation using CCT nanocomposite.

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Conclusion

This study presents the successful synthesis of a novel CuO/CoFe2O4/MWCNTs (CCT) nanocomposite using a hydrothermal approach, designed to enhance the photocatalytic degradation of tetracycline (TC) antibiotics under visible light irradiation. Comprehensive characterization of the CCT nanocomposite through XRD, HR-SEM, EDX, HR-TEM, UV-Vis, BET, and PL analyses confirmed its unique structural, morphological, and optical properties. The XRD analysis revealed that the crystallite sizes of the CoFe2O4, CuO, and CCT nanocomposite were 46.8 nm, 37.5 nm, and 53 nm, respectively. HR-SEM and HR-TEM images further corroborated these findings, showing the cubic and tubular structures, with an average particle size of 49.2 nm for the nanocomposite. The UV-Vis analysis and Tauc plots demonstrated that the CCT nanocomposite possesses narrow band gap energies, making it highly responsive to visible light, which is critical for efficient photocatalysis. EDX analysis validated the elemental composition of the composite materials, underscoring the successful integration of CoFe2O4, CuO, and MWCNTs. PL and BET analyses highlighted the nanocomposite’s superior photocatalytic efficiency and high surface area, respectively, making it a promising candidate for catalytic applications. Photocatalytic experiments revealed that the CCT nanocomposite achieved a remarkable 98.1% degradation efficiency of TC under optimal conditions, significantly outperforming the individual pure materials. The degradation process was consistent with first-order reaction kinetics, as indicated by a high regression coefficient (R² = 0.9833) and a rate constant (k = 0.014 min⁻¹). Moreover, the stability and reusability tests demonstrated that the CCT nanocomposite retained its photocatalytic efficiency over five consecutive cycles, indicating its potential for long-term applications in water treatment. In summary, this study not only advances the development of high-performance photocatalysts for antibiotic degradation but also provides a solid foundation for the practical application of the CCT nanocomposite in sustainable water treatment technologies. The results underscore the CCT nanocomposite’s potential to address the growing challenge of antibiotic pollution in aquatic environments, contributing to the broader goal of environmental protection and public health.