Photocatalytic degradation of tetracycline antibiotic and organic dyes using biogenic synthesized CuO/Fe2O3 nanocomposite: pathways and mechanism insights

Tetracycline (TC) is a frequently administered antibiotic in many countries, due to its low price and excellent potency. However, certain antibiotics can be hazardous to living creatures due to their accumulation by complexation with metal ions which can contribute to teratogenicity and carcinogenicity. In this investigation, copper oxide-ferric oxide nanocomposite (CuO/Fe2O3 nanocomposite) was synthesized employing Psidium guajava (P. guajava) leaf extract as a reductant as well as a capping agent in an environment friendly and economical green synthesis method. The as-synthesized CuO/Fe2O3 nanocomposite was comprehensively characterized using various sophisticated techniques and its efficiency as a photocatalyst for degradation of tetracycline (TC) antibiotic and toxic dyes, i.e., rhodamine B (RhB) and methylene blue (MB) were investigated. The CuO/Fe2O3 nanocomposite exhibited exceptional efficiency for degradation of TC antibiotic (88% removal in 80 min), RhB (96% removal in 40 min), and MB (93% elimination in 40 min) with apparent rate constant of 0.048, 0.068, and 0.032 min–1, respectively. In the degradation experiments, photocatalytic activity of CuO/Fe2O3 nanocomposite was studied by varying different factors such as time of contact, catalyst dose, and solution pH. The role of reactive species in antibiotics and dye degradation was validated by radical scavenging studies which indicated that.OH radical played a critical role in photocatalytic decomposition. Furthermore, liquid chromatography-mass spectrometry (LC–MS) investigations were employed to anticipate a plausible mechanism for TC degradation.


Introduction
The effluents from industries and households have resulted in significant perturb and hazard to human and aquatic organisms. The wastewater discharged from different industries comprises organic pollutants of harmful and carcinogenic nature (Sha et al. 2016). The colored dye pollutants distress the aquatic bodies as they screen the sunlight, resulting in diminished dissolved oxygen concentrations (Farré et al. 2008). More than 10,000 organic dyes are executed in diverse practices like pharmaceutical, leather tanning, paper, textiles, plastic, and cosmetics industries (Singh et al. 2019). Out of the total annual production of dyes, approximately 15% is assessed to be unproductive in industrial operations and is alleged to adulterate natural water bodies (Lavanya 2014;Mamba and Mishra 2016). Also, because of complicated structure and resistance to degradation, elimination of organic dyes is considered as a major hindrance in treatment of effluents (Buthiyappan et al. 2016).
Antibiotics are widely utilized to cure bacterial infections . Huge quantities of antibiotics are regularly discharged in effluents as a result of widespread usage of antibiotics (Daghrir and Drogui 2013;Mamba et al. 2018). Tetracycline (TC) is an extensively used antibiotic due to its low price and versatility (Li et al. 2011). TC is partially digested by organisms, and difficult to breakdown. Hence, TC is widely distributed in aquatic environment (Watkinson et al. 2007). After entering the natural environment, antibiotics like TC can cause severe toxicity in aquatic or terrestrial species and promote the evolution of antibioticresistant genes and bacteria, thus, posing an ecological danger and a hazard to human health .
Effective strategies for elimination of these toxic substances from contaminated water bodies are urgently needed as they have harmful influence on animals, plants, and human health. For this purpose, various water treatment strategies like electrolytic, electron beam, photocatalysis, biodegradation, adsorption, and photo electrochemical techniques have been utilized for treating industrial effluents (Mehta et al. 2016;Wen et al. 2010;Ji et al. 2009;Oturan et al. 2013;Kaushal et al. 2018). Unfortunately, these procedures frequently transmit contaminants from one phase to the other or produce secondary toxins like cancer causing aromatic amines which can lead to other major health issues.
Scientists and researchers across the globe focused on fabrication of monometallic nanoparticles initially. However, in the recent times, bimetallic nanocomposites have gained popularity because of their synergistic effects in a variety of applications, especially in organic pollutants degradation (Mittal et al. 2013). CuO/Fe 2 O 3 nanocomposite has been synthesized by a few research groups and explored for various applications like oxidation of n-hexane, oxidation of carbon monoxide, degradation of organic pollutants (Liang et al. 2008;Todorova et al. 2010;Luo et al. 2022;Panthawan et al. 2022).
The annual production of Cu and Fe NPs reaches around millions of tons and is expected to expand every year owing to their unique usage in various industries (Tang and Wang 2020;Hu et al. 2010). The chemical processes produce a variety of toxins as precursors which are extremely harmful to environment (Duan et al. 2015;Mamba et al. 2017). As a result, green methods must be adopted to fulfil the requirement of these nanoparticles in a healthy and safe manner. In green chemistry, there are several methods for the synthesis of NPs which utilize biological waste and plant extracts. The plant-mediated technique was encouraged for the preparation of CuO/Fe 2 O 3 nanocomposite because of the extensive and effortless accessibility of plants (Hussain et al. 2016).
In order to fulfil our goal for the synthesis of CuO/Fe 2 O 3 nanocomposite in an eco-friendly manner, Psidium guajava (P. guajava) often known as guava (commonly grown in northern part of India) was used as a plant source. The hazardous organic pollutants including TC, RhB, and MB were degraded to determine the effectiveness of as-synthesized photocatalyst under UV/visible light exposure. Consequently, this research adds to existing corpus of knowledge in the synthesis of green and economical photocatalysts for removing toxic substances from polluted water with high degradation performance while posing no threat to the environment.

Plant material and reagents
Fresh green leaves of P. guajava were taken from an orchid in Fatehgarh Sahib (Punjab) India. Tetracycline (Sigma Aldrich), ferrous sulfate, copper sulfate, methylene blue, rhodamine B were purchased from LOBA Chemie Pvt Ltd. All the reagents were of analytical grade. Deionized (DI) water was employed for preparing different solutions.

Preparation of guava leaf extract
The P. guajava plant (guava) was chosen for this study due to its easy accessibility and cost efficacy. Fresh leaves (10-20) from a healthy guava tree were taken and washed thoroughly with DI water. These leaves were cut into small pieces and put into a beaker containing 300 mL DI water. The beaker containing the leaves was heated on a magnetic stirrer for 4-6 h at 60 °C until the color changes to reddish brown. After cooling, the extract was filtered through Whatmann filter paper and the filtrate was kept at 4 °C, for using in the synthesis of nanoparticles.

Preparation of CuO and Fe 2 O 3 monometallic nanoparticles
Took 0.5 M each of the ferrous sulfate and copper sulfate solutions in different beakers and added 50 mL distilled water in each beaker. Both the solutions were stirred for half an hour and then guava leaf extract (20-25 mL) was added while maintaining the temperature at 60 °C. The solution was stirred again till the precipitates were formed. The precipitates were allowed to settle down for 2-3 days.

Preparation of CuO/Fe 2 O 3 nanocomposite
Ferrous sulfate and copper sulfate solutions were added to the leaf extract taken in a 250-mL round bottom flask and stirred overnight at 60 °C on a reflux condenser. The round bottom flask was removed from the condenser and the precipitates were allowed to settle down for 3 days. The solution in the flask was filtered and the impurities were removed by double washing of precipitates with ethanol, and then with DI water. The precipitates obtained were dried at 60 °C in an air oven. The process was used to synthesize the nanocomposites with varied molar ratios of Fe 2 O 3 : CuO, i.e., 0.25:1.00, 0.50:1.00, 0.75:1.00, and 1.00:1.00. The nanocomposite sample with Fe 2 O 3 :CuO of 0.50:1.00 was observed to be the most active catalyst among various CuO/Fe 2 O 3 nanocomposite samples synthesized, and as a consequence, it was selected as a typical catalyst for further exploration.

Characterization of CuO/Fe 2 O 3 nanocomposite
Perkin Elmer RXI FTIR spectrometer was employed to record Fourier transform infrared (FTIR) spectra. The morphology of CuO/Fe 2 O 3 nanocomposite was studied by field emission scanning electron microscopy (FESEM) (Carl Zeiss Supra 55) and high-resolution transmission electron microscope (HRTEM) (Jeol Jem 2100 Plus). The structure of the nanocomposite was investigated by X-ray diffractometer (XRD, X'Pert PRO) with Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific Escalab Xi + ) with 300 W Al Kα radiation was employed to determine the elemental constitution of nanocomposite. The adsorption characteristics of nanocomposite were investigated by Brunner-Emmett-Teller (BET) method. The composition of nanocomposite was estimated by energy-dispersive X-ray spectroscopy (EDS) (Oxford). UV-Vis spectrophotometer (UV-Vis) was employed for studying the degradation of dyes as well as antibiotics.

Photocatalytic studies
The performance of CuO/Fe 2 O 3 nanocomposite for degradation of TC, RhB and MB was investigated under diverse conditions like catalyst dosage, pH of solution etc. DI water was used to prepare the solution of each dye (20 mg L -1 ). The CuO/Fe 2 O 3 nanocomposite photocatalyst (10-50 mg) was added to 50 mL of dye solution at ambient temperature and the resultant mixture was placed in dark for 30 min. Then, the above mixture was irradiated with UV (for TC)/ visible (for MB and RhB) radiations. Magnetic stirrer was used for controlled stirring of the mixture. About 4 mL of the solution was withdrawn at regular intervals and centrifuged for 10 min at 3500-4500 rpm to remove the suspended CuO/ Fe 2 O 3 nanocomposite. The UV-vis absorption spectrum was used to determine the degree of dye degradation employing the following equation: where C o and C t denote the concentration of pollutant at equilibrium and time t, respectively.
According to the Langmuir-Hinshelwood kinetics model, first-order kinetic equation given below was employed to investigate the kinetics of photodegradation reaction of RhB, MB and TC.
where k and t represent the rate constant and irradiation time, respectively. The photocatalytic activity was determined using the linear plot with slope as the k value. The formation of radicals ( . OH and . O 2 − ), electrons (e − ), and holes (h + ) was tracked by introducing various radical oxygen species (ROS) scavengers such as benzoquinone (BQ), isopropyl alcohol (IPA), triethylamine (TEA) and potassium dichromate (K 2 Cr 2 O 7 ) to investigate the primary photodegradation mechanism. Each aqueous scavenger solution (10 -3 M) was added to the sample under investigation (20 mg L -1 ). Three sets of measurements were carried out to verify the repeatability of the outcomes, and the average findings were presented.

Results and discussion
CuO, Fe 2 O 3 NPs, and CuO/Fe 2 O 3 nanocomposite were synthesized by a green biogenic method using P. guajava leaf extract (Fig. S1). Diverse techniques such as UV-Vis, FTIR, BET, XPS, FESEM, EDX, HRTEM, and XRD were employed to characterize these nanoparticles. Metallic nanoparticles exhibited absorbance in the UV-Vis region and hence, UV-vis spectrophotometric investigation was used as a rapid preliminary test to validate nanoparticle formation. The UV-vis spectra of biogenic synthesized Fe 2 O 3 , CuO NPs and CuO/Fe 2 O 3 nanocomposite with P. guajava leaf extract is given in Fig. S1a. The formation of CuO NPs was validated by a prominent absorption band at 282 nm in the colloidal solution of final product (Abdel et al. 2019). The UV-vis spectra of Fe 2 O 3 NPs displayed a strong absorption at 305 nm which decreased with increase in wavelength. The spectra obtained conforms to that reported in literature (Degen et al. 2008) The as-synthesized CuO/Fe 2 O 3 nanocomposite has a broad and "shallow" band centered around 289 nm with a tail extending up to 800 nm. The band gap energies of CuO, Fe 2 O 3 NPs and CuO/Fe 2 O 3 nanocomposite were found to be 1.8, 2.8, and 2.4 eV, respectively, signifying that the incorporation of CuO diminishes the band gap of Fe 2 O 3 NPs (Fig. S1b). In the light of these findings, it was revealed that slight valence band tailing was responsible for the significant band gap narrowing of Fe 2 O 3 NPs. These findings imply that the simultaneous shifting of the valence band maxima and conduction band minima that occurs in the CuO/Fe 2 O 3 nanocomposite reduces the band gap of    nanocomposite has certain unexplained peaks which could be attributed to the crystallization of biomolecules present in P. guajava leaf extract. BET analysis was utilized to discover more about the surface area and distribution of pores in CuO/Fe 2 O 3 nanocomposite (Fig. 1c). The surface area, total volume of pores and mean pore diameter of the biogenic synthesized nanocomposite was estimated to be 36.8 m 2 g −1 , 0.31 cm 3 g −1 , and 9.10 nm, respectively. As a result, CuO/Fe 2 O 3 nanocomposite has an extremely precise exterior zone and appropriate pore structure, thus, making it ideal for surface interaction activities. The composition of CuO/Fe 2 O 3 nanocomposite was explored by energy-dispersive X-ray spectroscopy (EDS) (Fig. 1d) that demonstrated the existence of three elements Fe, Cu, and O.
The FESEM images of as-synthesized CuO/Fe 2 O 3 nanocomposite (Fig. 2a, b) revealed that the nanocomposite comprised monodisperse sphere-like particles. The CuO/Fe 2 O 3 nanocomposite has uniform size, as shown in the typical highmagnification FESEM image (Fig. 2b). The nanoparticles have a diameter of 25-30 nm and a sphere-like shape which is consistent with the XRD results. Further, it was revealed from HR-TEM images that the nanocomposite contained two distinct, i.e., dark and light phases which might be CuO and Fe 2 O 3 NPs (Fig. 2c-e). The crystalline character of the as synthesized CuO/Fe 2 O 3 nanocomposite was confirmed by the selected area electron diffraction (SAED) pattern (Fig. 2f). The existence of a quasi-ring-like diffraction pattern revealed polycrystalline structure of the nanocomposite.
XPS was employed to identify the elemental composition and valence states of CuO/Fe 2 O 3 nanocomposite. The C 1 s spectrum (Fig. 3a) is deconvoluted into two peaks at 284.02 eV and 285.35 eV, ascribed to C-C/C = C and C-O configurations, respectively. Two prominent peaks of Cu 2p attributed to Cu 2p 3/2 and Cu 2p 1/2 were observed at 952.3 eV and 932.8 eV, respectively (Fig. 3b) (Bai et al. 2016). Two distinct peaks at binding energy 710.6 and 724.2 eV in Fe 2p spectrum (Fig. 3c) correspond to Fe 2p 3/2 and Fe 2p 1/2 , respectively (Qi et al. 2017). Another peak at 713.6 eV appears to originate primarily from Fe 2p 3/2 peak for FeOOH (Momose et al. 2020). The high resolution scan of O 1s (Fig. 3d)

Photocatalytic degradation of tetracycline antibiotic over CuO/Fe 2 O 3 nanocomposite
The photocatalytic performance of CuO/Fe 2 O 3 nanocomposite was studied towards TC (374 nm) degradation under Fig. 2 a-

b) FESEM and c-e)
HRTEM images and f) SAED pattern of CuO/Fe 2 O 3 nanocomposite UV irradiation. UV-vis spectrum of TC revealed two strong absorption bands at 275 and 374 nm. The generated chromophores as well as aromatic rings B-D were responsible for absorption by TC at 360 nm (Wang et al. 2011). The said absorption band gradually faded with increase in irradiation period, indicating that the phenolic groups linked to the aromatic ring B were fragmenting (Zhu et al. 2013). The generation of acylamino and hydroxyl groups was responsible for decline of absorbance at 270 nm. The decrease in absorbance at 360 nm is attributed to the formation of 4a,12a-anhydro-4-oxo-4-dedimethylaminotetracycline (Addamo et al. 2005).
It was observed that maximum degradation of TC by CuO/Fe 2 O 3 nanocomposite reached 88% after 50 min of illumination (Fig. 4a). It was observed that the degradation of TC is insignificant in the dark and without CuO/Fe 2 O 3 nanocomposite photocatalyst. After adding CuO/Fe 2 O 3 nanocomposite, the concentration of TC diminished steadily with time, confirming continuing photocatalytic degradation of TC (Fig. 4b). The reduction in TC concentration can be attributed to oxidation-reduction reactions taking place on the surface of photocatalyst and efficient electron-hole splitting (Huang et al. 2019).
As the amount of CuO/Fe 2 O 3 nanocomposite is increased, the photocatalytic performance is also enhanced. It was observed (Fig. 4b) that 18%, 39%, 65%, 86%, and 88% of TC was degraded during light exposure for 80 min with 10 mg, 20 mg, 30 mg, 40 mg, and 50 mg of the nanocomposite, respectively. When the loading of photocatalyst reaches 40 mg, the degradation effect is almost best, and the degradation rate can reach 86%. This is due to the fact that with higher dose of photocatalyst, exposure to radiation produces more charge carriers capable of participating in redox processes. The degradation efficacy remains almost the same at a photocatalyst dose of 50 mg. In fact, the addition of photocatalyst makes the solution turbid and high doses can cause light screening, thus, reducing the catalyst's ability to absorb light (Che et al. 2018;Xu et al. 2018).
The graphs of ln(C t /C 0 ) vs. time for different photocatalyst samples are linear (Fig. 4c), suggesting that TC degradation is governed by pseudo-first-order reaction kinetics. The rate constant (k) of the photocatalytic degradation reaction is calculated from the slope of the graph. The rate constant (k) values for PC-I, PC-II, PC-III, PC-IV, and PC-V were determined to be 0.005, 0.015, 0.034, 0.047, and 0.048 min −1 , respectively. It has been observed from the data obtained that the rate constant (k) increased with increase in photocatalyst dosage.
For photocatalytic reactions, pH of the solution influences the rate of decomposition of organic substances. The photocatalyst's surface charge and creation of hydroxyl radicals ( . OH) is affected by the pH of solution (Zhang et al. 2020). The impact of pH on the deterioration of TC utilizing Fe 2 O 3 -CuO nanocomposite was explored in the pH range of  Fig. 4d (Hunge et al. 2022). Tetracycline has different dissociation constant values at different pH values as it exists as a cationic (pH < 6.0), molecular (pH 6.0-7.5) or an anionic species (pH > 7.5) (Ahmadi et al. 2017). The pH range of 6.0-7.5, where TC is present in molecular form, is believed to be beneficial for TC sorption and hence, boosts photocatalytic efficiency. In the pH range of 3.0-6.0 (acidic conditions), the maximum photocatalytic degradation efficiency has been noticed to be 70% after 80 min whereas in pH range of 7.5-10.0 (basic conditions), the maximum photocatalytic degradation was found to be 57% in the same interval of time. These results demonstrated that both acidic and basic conditions are not favourable for photocatalytic decomposition of TC (Hunge et al. 2022), and highest decomposition of 88% has been observed at pH 7 (neutral pH).

Photocatalytic mechanism
The radical scavenging investigations were performed to confirm the involvement of reactive species in TC degradation. Benzoquinone (BQ), isopropyl alcohol (IPA), triethylamine (TEA), and potassium dichromate (K 2 Cr 2 O 7 ) were employed as scavengers for superoxide ( . O 2 − ) radicals, hydroxide ( . OH) radicals, holes (h + ) and electrons (e − ), respectively (Hunge et al. 2022;Xie et al. 2018). The scavenging investigations were done in similar conditions as those for the photocatalytic experiments. Fig. S2 shows that the addition of K 2 Cr 2 O 7 and TEA had only a minor influence (< 20%) on TC breakdown whereas BQ and IPA inhibit TC degradation to a large extent, with inhibition rates of 38.2% and 52.3%, respectively. Thus, it is clear that . OH radicals have a crucial role in photocatalytic decomposition of TC.
In contemplation to go deep into the photocatalytic decomposition process of TC over CuO/Fe 2 O 3 nanocomposite, LC-MS technique was utilized to accurately perceive the intermediates generated during photocatalytic degradation reaction (Fig. S3). As displayed in Fig. S3, only primary peak with m/z = 445.16, credited to TC molecule was noticed, showing that there was no degradation of TC prior to irradiation. The peak intensity of TC steadily reduced as the irradiation time increased, and some series of new peaks emerged (Fig. S3b-f). The charge-to-mass ratio of intermediate products was investigated, and possible TC degradation routes are depicted in Fig. 5. To begin with, CuO/Fe 2 O 3 nanocomposite with high surface area adsorbed TC molecules (m/z = 445.16). When the TC solution containing CuO/Fe 2 O 3 nanocomposite was irradiated with UV-light after attaining adsorption-desorption equilibrium, abundant . O 2 − , . OH, h + , and e − were produced quickly. The active species attacked double bonds, aromatic ring, and amino group, signalling that ring-opening reactions or Fig. 4 a) UV-vis absorbance of TC vs. irradiation time; b) concentration changes of TC; c) graph of ln(C 0 /Ct) vs. time; and d) influence of pH on the decomposition of TC over CuO/ Fe 2 O 3 nanocomposite photocatalyst rupture of the primary carbon bonds formed the majority of intermediates (He et al. 2014).
On the addition of a hydroxyl group, P1 with m/z = 461.2 was produced. According to early literature on the theoretical degradation products of TC in solution, double bond at C11a-C12 location of TC is the highly sensitive site for attack of oxidants, and a reorganization of hydroxyl group at C12 position might result in P1 (Hou et al. 2019). P2 (m/z = 427) is formed by the dehydration of P1 at C-6 and C-11a (Zhang et al. 2021). The compound P3 (m/z = 410) was formed as a result of removal of hydroxyl group at C-3 and oxidation at C-12. P4 (m/z = 398) is produced via N-dealkylation of C4 tertiary amine position owing to lesser bond dissociation energy of N-C bond . The excessive oxidation of product P4 could lead to the breakage of ring and the formation of products P5 (m/z = 255), P6 (m/z = 241), P7 (m/z = 233), P8 (m/z = 183), P9 (m/z = 139) and P10 (m/z = 103) which are well matched with those reported in literature (Li et al. 2021;Kaushal et al. 2022). These products of ring-opening reactions were
The existence of self-sensitization process was studied without the insertion of photocatalyst, and it was observed that negligible amount of both the dyes degraded. Contrarily, with the addition of photocatalyst in the dye solutions, significant quantity of dyes was degraded that clearly showed that the decomposition of dyes is primarily due to the photocatalytic process.
The kinetics of photodecomposition of RhB and MB dyes was analyzed by the following equation: ln(C 0 /C) = kt, where k is 1 st order rate constant, t is reaction time, C 0 and C are the dye concentrations before and after irradiation, respectively (Fig. 8a, b). For all the samples of Fe 2 O 3 -CuO nanocomposite, the R 2 value is greater than 0.95 for RhB and MB degradation. The values of rate constant (k) for the Organic pollutants are degraded in the heterogeneous photocatalytic reaction by reactive intermediates like . O 2 − , . OH, e − , and h + which are created under optimum light exposure. A number of tests on quenching of active species were undertaken by introducing different scavengers to the photocatalytic reaction system to discover which active species played a key role in dye photodegradation with CuO/Fe 2 O 3 nanocomposite. The different trapping agents employed in this investigation were BQ ( . O 2 − quencher), IPA ( . OH quencher), K 2 Cr 2 O 7 (e − quencher), TEA (h + quencher) for the decomposition of RhB and MB dyes. The photocatalytic reaction was somewhat hindered as a result of quenching, resulting in limited decomposition of dyes. The amount of reduction in decomposition produced by each scavenger reflected the relevance of the related reactive species.
By comparing the degradation extents of RhB and MB dyes during light irradiation, the impact of the series of scavengers was investigated. It was demonstrated (Fig. S4) that when IPA was added as an . OH scavenger, photodegradation of RhB and MB employing CuO/Fe 2 O 3 nanocomposite was considerably reduced, implying that . OH was a key intermediate in photocatalytic decomposition. However, when BQ, K 2 Cr 2 O 7 , and TEA were added, the photocatalytic performance of CuO/Fe 2 O 3 nanocomposite marginally decreased, suggesting that . O 2 − , h + and e − contribute a little but have a synergistic role.
The position of band gap is crucial for the formation of active species as well as photocatalysis performance. The conduction band (CB) and valence band (VB) position for CuO and Fe 2 O 3 was determined by using the following empirical formula: where E CB and E VB denote the energy of conduction and valence band, respectively. χ denotes the absolute electronegativity of semiconductor and its values for CuO and Fe 2 O 3 are 5.81 and 5.86 eV, respectively. On the hydrogen scale, E e denotes the energy of free electron which is 4.5 eV. The band gap energy of semiconductors is denoted by E g . The band gap energies of CuO and Fe 2 O 3 are 3.5 eV and 3.0 eV, respectively, according to the DRS study. The conduction and valence band locations of CuO and Fe 2 O 3 have been determined to be -0.44/2.86 eV and -0.14/3.06 eV, respectively. On the basis of results obtained, the tentative Photocatalytic degradation mechanism of TC, MB and RhB over CuO/Fe 2 O 3 nanocomposite mechanism for the exceptional photocatalytic action of CuO/ Fe 2 O 3 nanocomposite has been proposed in Fig. 9.
To further explore the photocatalytic efficiency of CuO/ Fe 2 O 3 nanocomposite, the COD values of the dye solutions were estimated before and after the light exposure in the presence of photocatalyst. The estimated COD for MB and RhB dye solutions diminished from 200 to 40 mg L −1 and 220 to 146 mg L −1 , respectively. After exposure to radiations, the mineralization yield of nanocomposite was estimated to be 80% and 67%, separately, for the MB and RhB dyes.

Reusability and stability
While exploring the photocatalytic applications, reusability and stability of photocatalyst are the critical considerations. Four successive cycles of TC, RhB, and MB degradation experiments were performed under identical reaction circumstances, to assess the reusability of CuO/Fe 2 O 3 nanocomposite (PC-IV). Fig. S5 represents the removal efficiency of TC, RhB and MB in each cycle, and the variation among each cycle was easily discernible. TC, RhB and MB were found to have a removal effectiveness of 88%, 96%, and 93%, respectively in the 1 st cycle, with CuO/Fe 2 O 3 nanocomposite. However, after 4 th cycle, the decomposition efficiency of TC, RhB and MB was diminished to 84%, 92%, and 88%, respectively, which is slightly less than that in the 1 st cycle. Thus, the removal efficiency of the nanocomposite remained excellent after the 4 th cycle. It demonstrated that the material's stability and recyclability were good. The slight decline in the activity of the nanocomposite after 4 th cycle was induced by the unavoidable percolation of the catalyst throughout the reclamation process, as well as the obstruction of surface active sites by tetracycline and its decomposition by-products Kumari et al. 2022).

Conclusion
To summarize, CuO/Fe 2 O 3 nanocomposite was successfully obtained using P. guajava leaf extract in a convenient, lowcost, and environment friendly green approach. The different characterization techniques like FTIR, XRD, XPS, BET, FE-SEM, HR-TEM and UV-vis spectroscopy were employed to explore the structure, bonding, and numerous other characteristics of the CuO/Fe 2 O 3 nanocomposite. The optical band gap of biogenic CuO/Fe 2 O 3 nanocomposite synthesized was 2.5 eV, indicating that it has a high photocatalytic potential when exposed to sunlight. The potency of the as-synthesized materials was explored against simultaneous degradation of commonly used TC antibiotic and RhB and MB dyes. The as-prepared CuO/Fe 2 O 3 nanocomposite photocatalyst exhibited remarkably high photocatalytic efficiency than that of bare Fe 2 O 3 and CuO NPs for the decomposition of TC antibiotic and RhB & MB dyes under UV/visible light irradiation. The 40 mg of CuO/Fe 2 O 3 nanocomposite sample had the best efficiency, with 88%, 96%, and 93% degradation of TC, RhB and MB in 80, 40 and 40 min, respectively under UV/visible light irradiation. Among different types of scavengers introduced in the decomposition reaction, it was noticed that . OH was the key intermediate engaged in the decomposition of antibiotic and dyes over CuO/Fe 2 O 3 nanocomposite. Thus, the current work alludes to the biogenic/green synthesis of photocatalysts and their successful utilization in waste water remediation to mitigate the harmful organic pollutants due to their great stability and phenomenal photocatalytic capacity.