Photocatalytic degradation of tetracycline antibiotic and organic dyes using biogenic synthesized CuO/Fe 2 O 3 nanocomposite: Pathways and mechanism insights

doi:10.21203/rs.3.rs-1743967/v1

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Photocatalytic degradation of tetracycline antibiotic and organic dyes using biogenic synthesized CuO/Fe 2 O 3 nanocomposite: Pathways and mechanism insights

https://doi.org/10.21203/rs.3.rs-1743967/v1

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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, ferrous oxide-copper oxide nanocomposite (CuO/Fe₂O₃ nanocomposite) were synthesised employing Psidium guajava (P. guajava) leaf extract as a reductant and a capping agent in an environment friendly and economical green synthesis method. The as-synthesized CuO/Fe₂O₃ nanocomposite were comprehensively characterized using various sophisticated techniques and their efficiency as photocatalyst for degradation of TC antibiotic and toxic dyes i.e. rhodamine B (RhB) and methylene blue (MB) was 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. In the degradation experiments, photocatalytic activities of CuO/Fe₂O₃ nanocomposite were 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 has a critical role in photocatalytic decomposition. Furthermore, liquid chromatography-mass spectrometry (LC-MS) investigations were employed to anticipate a plausible mechanism for TC degradation.

Green synthesis

CuO/Fe2O3 nanocomposite

degradation

tetracycline antibiotic

dyes

The effluents from industries and households have resulted in significant perturb and hazard to human and aquatic organisms. The waste water discharged from different industries comprises of organic pollutants of harmful and carcinogenic nature [1]. The coloured dye pollutants distress the aquatic bodies as they screen the sunlight, resulting in diminished dissolved oxygen concentrations [2]. More than ten thousand organic dyes are executed in diverse practices like pharmaceutical, leather tanning, paper, textiles, plastic and cosmetics industries [3]. 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 [4]. Also, because of complicated structure and resistance to degradation, elimination of organic dyes is considered as a major hindrance in treatment of effluents [5].

Antibiotics are widely utilised to cure bacterial infections [6]. Huge quantities of antibiotics are regularly discharged in effluents as a result of widespread usage of antibiotics [7]. Tetracycline (TC) is an extensively used antibiotic due to its low price and versatility [8]. TC is partially digested in organisms, and difficult to breakdown. Hence, TC is widely distributed in aquatic environment [9]. After entering the natural environment, antibiotics like TC can cause severe toxicity in aquatic or terrestrial species and promote the evolution of antibiotic-resistant genes and bacteria, thus, posing an ecological danger and a hazard to human health [10].

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 [11–15]. 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 [16]. CuO/Fe₂O₃ nanocomposite have 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 [17–20].

The annual production of Cu and Fe NPs reaches around millions of tonnes, and is expected to expand every year owing to their unique usage in various industries [21–22]. The chemical processes produce a variety of toxins as precursors which are extremely harmful to environment [23]. As a result, green methods must be adopted to fulfil the needs for these nanoparticles in a healthier and safer 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 these CuO/Fe₂O₃ nanocomposite because of the extensive and effortless accessibility of plants [24].

In order to fulfil our goal for the synthesis of CuO/Fe₂O₃ 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.

2.1 Plant material and reagents

Fresh green leaves of P. guajava were taken from an orchid in Fatehgarh Sahib (Punjab) India. Tetracycline (Sigma Aldrich), ferrous sulphate, copper sulphate, 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.

2.2 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 hours at 60ºC till the colour changes. After cooling, the extract was filtered through Whatmann filter paper and the filtrate was kept at 4°C for use in synthesis of nanoparticles [3].

2.3 Preparation of Fe & Cu monometallic nanoparticles

Took 0.5 M each of the ferrous sulphate and copper sulphate 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 ºC. The solution was stirred again till the precipitates were formed. The precipitates were allowed to settle down for 2–3 days.

2.4 Preparation of CuO/Fe₂O₃ nanocomposite

Ferrous sulphate and copper sulphate solutions were added to the leaf extract taken in a 250 mL round bottom flask and stirred overnight at 60 ºC 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 ºC in an air oven.

2.5 Characterization of CuO/Fe₂O₃ nanocomposite

Perkin Elmer RXI FTIR spectrometer was employed to record fourier transform infrared spectroscopy (FTIR) spectra. The morphology of CuO/Fe₂O₃ 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 nanocomposite was investigated by X-ray diffractometer (XRD, X’Pert PRO) with Cu Ka irradiation (k = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific Escalab Xi⁺) with 300W 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.

2.6 Photocatalytic studies

The performance of CuO/Fe₂O₃ 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/Fe₂O₃ nanocomposite photocatalyst (10–50 mg) was added to with 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 solution was withdrawn at regular intervals and centrifuged for 10 min at 3500–4500 rpm to remove the suspended CuO/Fe₂O₃ nanocomposite. The UV-vis absorption spectrum was used to determine the degree of dye degradation employing the following equation:

Degradation efficiency (%) = $\frac{{C}_{0}-{C}_{t}}{{C}_{0}}$ ×100

where C_e 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.

$$ln\frac{{C}_{0}}{{C}_{t}}=kt$$

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 .O₂⁻), 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 (K₂Cr₂O₇) 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.

CuO, Fe₂O₃ NPs and CuO/Fe₂O₃ 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 Fe₂O₃, CuO NPs and CuO/Fe₂O₃ 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 [25]. The UV-Vis spectra of Fe₂O₃ NPs displayed a strong absorption at 305 nm which decreased with increase in wavelength. The spectra obtained conforms to that reported in literature [26] The as-synthesized CuO/Fe₂O₃ nanocomposite have a broad and “shallow” band centered around 289 nm with a tail extending up to 800 nm. The band gap energies of CuO, Fe₂O₃ NPs and CuO/Fe₂O₃ nanocomposite were found to be 3.5, 3.0 and 2.5 eV, respectively, signifying that the incorporation of Fe₂O₃ diminishes the band gap of CuO NPs [Fig. S1b]. Furthermore, this reduction confirmed electronic coupling between Fe₂O₃ and CuO NPs.

FTIR spectrum of as synthesized Fe₂O₃ NPs, Cu NPs and their corresponding CuO/Fe₂O₃ nanocomposite has been shown in Fig. 1a. The IR bands near 3700, 3650, 2300, 1700, 1520, 1030, 800 cm^− 1 were observed. IR peaks at 3750 − 3200 cm^− 1 may be attributed to stretching vibrations of hydroxyl group (O-H). The band at 2370 − 2330 cm^− 1 was attributed to the asymmetric stretching vibrations of C = O = N bond. The bands at 1700 cm^− 1 and 1520 cm^− 1 correspond to the stretching vibrations of C = O bond. A sharp peak at 1030 cm^− 1 has been assigned to stretching vibrations of C-O bond [27]. IR bands in 800 − 600 cm^− 1 region represent bending vibrations of C-H bond [28]. In addition to this, variation in intensity of old peaks and formation of new peaks confirmed interactions between CuO and Fe₂O₃ nanocomposite.

XRD patterns of Fe₂O_3, CuO and CuO/Fe₂O₃ nanocomposite are given in Fig. 1b. The observed planes of pure Fe₂O₃ NPs correspond to (314), (999), (712), (192), (341), (411), (086), (264), (256), (088) and (055) at 2θ values of 24.14, 33.17, 35.71, 40.91, 49.51, 54.12, 57.62, 62.41, 63.95, 72.09 and 75.51, respectively, with JCPDS card number 89-8103. These planes correlate to d-spacing of 3.71, 2.72, 2.53, 2.27, 2.10, 1.65, 1.59, 1.49, 1.46, 1.31 and 1.25Å, respectively, which can be easily assigned to the rhombohedral structure [29]. The diffraction peaks for CuO NPs are observed at 2θ values of 25.98, 35.51, 38.21 and 43.31 with (012), (022), (004) and (220) planes, respectively. The said planes correspond to d-spacing of 3.36, 2.52, 2.34 and 2.07 Å (JCPDS card number 48-1548), and indicate monoclinic phase of CuO NPs. Fe₂O₃ in CuO/Fe₂O₃ nanocomposite exhibits a sharp peak at 2θ value of 33.9° which resembles (999) reflection. As per Scherer equation, the average particle size of Fe₂O₃, CuO and CuO/Fe₂O₃ nanocomposite was observed to be 15.91, 14.16 and 22.7 nm, respectively. The XRD pattern of CuO/Fe₂O₃ nanocomposite has certain unexplained peaks which could be attributed to the crystallisation 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/Fe₂O₃ 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 m²g^− 1, 0.31cm³g^− 1 and 9.10 nm, respectively. As a result, CuO/Fe₂O₃ nanocomposite have an extremely precise exterior zone and appropriate pore structure, thus, making it ideal for surface interaction activities. The composition of CuO/Fe₂O₃ 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-synthesised CuO/Fe₂O₃ nanocomposite (Fig. 2a, b) revealed that the nanocomposite comprised monodisperse sphere-like particles. The CuO/Fe₂O₃ nanocomposite has uniform size, as shown in the typical high-magnification FESEM image (Fig. 2b). The nanoparticles had a diameter of 25–30 nm and a sphere-like shape, which was consistent with the XRD results. Further, it was revealed from HR-TEM images that BNPs contained two distinct i.e. dark and light phases which might be CuO and Fe₂O₃ NPs (Fig. 2c-e). The crystalline character of the as synthesized CuO/Fe₂O₃ 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/Fe₂O₃ nanocomposite. The C 1s 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) [30]. Two distinct peaks at binding energy 710.6 and 724.2eV in Fe 2p spectrum (Fig. 3c) correspond to Fe 2p_3/2 and Fe 2p_1/2, respectively [31], and the another peak at 713.6eV appears to originate primarily from Fe 2p_3/2 peak for FeOOH [32]. The high resolution scan of O 1s (Fig. 3d) represented three main peaks at 530.5 (Fe–O), 531.3 eV (C–O–Fe) and 532.2 eV (Fe–OH), demonstrating the linkage of Fe₂O₃ with CuO through the C–O–Fe bond in CuO/Fe₂O₃ nanocomposite.

3.1 Photocatalytic Degradation of Tetracycline Antibiotics over CuO/Fe₂O₃ nanocomposite

The photocatalytic performance of CuO/Fe₂O₃ nanocomposite was studied towards TC (374 nm) degradation under 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 [33]. The said absorption band gradually faded with increase in irradiation period, indicating that the phenolic groups linked to the aromatic ring B were fragmenting [34]. 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 [35].

It was observed that maximum degradation of TC by CuO/Fe₂O₃ 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/Fe₂O₃ nanocomposite photocatalyst. After adding CuO/Fe₂O₃ 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 [36].

As the amount of CuO/Fe₂O₃ 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 bimetallic photocatalyst, respectively. When the loading of photocatalyst reaches 40 mg, the degradation effect is almost best, and the degradation rate can reach more than 88%. 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 decreased somewhat 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 [37–38].

The graphs of ln(C_t/C₀) 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 slopes of the graphs. 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 the 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 [39]. The impact of pH on the deterioration of TC utilizing Fe-Cu BNP was explored in pH range of 3.0–10.0 and shown in Fig. 4d [40]. 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) [41]. The pH range of 6.0–7.5, where the 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 [40], and highest decomposition of 88% has been observed at pH 7 (neutral pH).

Photocatalytic mechanisms 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 (K₂Cr₂O₇) were employed as scavengers for superoxide (.O₂^-) radicals, hydroxide (.OH) radicals, holes (h⁺) and electrons (e^-), respectively [40, 42]. The scavenging investigations were done in similar conditions as those for the photocatalytic experiments. Fig. S2 shows that the addition of K₂Cr₂O₇ 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/Fe₂O₃ nanocomposite, LC-MS technique was utilized to accurately perceive the intermediates generated during photocatalytic degradation reaction (Fig. S3). As displayed in Fig. S2, 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. 2S b-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/Fe₂O₃ nanocomposite with high surface area adsorbed TC molecules (m/z = 445.16). When the TC solution containing CuO/Fe₂O₃ nanocomposite was irradiated with UV-light after attaining adsorption-desorption equilibrium, abundant .O₂⁻, .OH, h⁺ and e⁻ were produced quickly. The active species attacked double bonds, aromatic ring, and amino group, signalling that ring-opening reactions or rupture of the primary carbon bonds formed the majority of intermediates [43].

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 [44]. P2 (m/z = 427) is formed by the dehydration of P1 at C-6 and C-11a [45]. 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 [46]. 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/z183), P9 (m/z = 139) and P10 (m/z = 103) which are well matched with the reported literature [46–47]. These products of ring-opening reactions were ultimately oxidized to CO₂ and H₂O due to consistent attack of reactive intermediates.

Photocatalytic degradation of organic dyes

The activity of the as-synthesized CuO/Fe₂O₃ nanocomposite towards photodegradation of RhB and MO dyes was investigated in visible radiations (Fig. 7a, b), by measuring the absorbance of RhB & MB at 554 and 664 nm, respectively. It was revealed from the initial adsorption examinations of both RhB and MB dyes on CuO/Fe₂O₃ nanocomposite for 30 minutes in the absence of UV radiation that insignificant amounts of dyes were removed by the nanocomposite. The photocatalytic activity of CuO/Fe₂O₃ nanocomposite was optimized w.r.t. variation in pH of dye solution, dose of photocatalyst and time, to achieve better degradation results. The degradation of dyes over CuO/Fe₂O₃ nanocomposite was performed in the pH range of 3–10. Both the RhB and MB dyes deteriorated to the greatest extent in neutral solution, in contrast to acidic or basic solutions. This could be attributed to the creation of Fenton's reagent in acidic solutions, and leaching of CuO/Fe₂O₃ nanocomposite photocatalyst into solution, forming chemical sludge at alkaline pH values [48–49]. Thus, the as-synthesized photocatalyst was more efficacious for degradation of neutral dye solution (pH 7).

It was noticed that 22.1%, 45%, 77%, 92% and 96% of RhB dye (Fig. 8a), and 18.2%, 53.8%, 74.3%, 89% and 93% of MB dye was decomposed with 10 mg, 20 mg, 30 mg, 40 mg and 50 mg of BNPs catalyst, respectively, after 40 min of exposure to visible light (Fig. 8b).

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 analysed by the following equation: ln(C₀/C) = kt, where k is 1st order rate constant, t is reaction time, C₀ and C are the dye concentrations before and after irradiation, respectively (Fig. 9a, b). For all the samples of Fe-Cu BNPs, the R² value is greater than 0.95 for RhB and MB degradation. The values of rate constant (k) for the decomposition of RhB over PC-I, PC-II, PC-III, PC-IV and PC-V photocatalysts were determined to be 0.019, 0.024, 0.032, 0.062, 0.068 min^− 1, respectively (Fig. 7b). Thus, it was inferred that the decomposition efficiency was improved by enhanced loading of CuO/Fe₂O₃ nanocomposite photocatalyst. Similarly, for the decomposition of MB dye, the k values were estimated to be 0.016, 0.020, 0.024, 0.031, 0.032 min^− 1 for PC-I, PC-II, PC-III, PC-IV and PC-V photocatalysts, respectively. The k values for both RhB and MB dyes have shown drastic increase from PC-III to PC-IV and negligible increase from PC-IV to PC-V, indicating that PC-IV exhibited better photodegradation performance.

Organic pollutants are degraded in the heterogeneous photocatalytic reaction by reactive intermediates like .O₂⁻,.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/Fe₂O₃ nanocomposite. The different trapping agents employed in this investigation were BQ (.O₂⁻ quencher), IPA (.OH quencher), K₂Cr₂O₇ (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 a .OH scavenger, photodegradation of RhB and MB employing CuO/Fe₂O₃ nanocomposite was considerably reduced, implying that .OH was a key intermediate in photocatalytic decomposition. However, when BQ, K₂Cr₂O₇ and TEA were added, the photocatalytic performance of CuO/Fe₂O₃ nanocomposite marginally decreased, suggesting that .O₂⁻, 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 Fe₂O₃ was determined by using the following empirical formula:

E_CB = χ- E^e-0.5E_g

E_VB = E_CB+ E_g

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 Fe₂O₃ 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 Eg. The band gap energies of CuO and Fe₂O₃ are 3.5 eV and 3.0 eV, respectively, according to the DRS study. The conduction and valence band locations of CuO and Fe₂O₃ 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 mechanism for the exceptional photocatalytic action of CuO/Fe₂O₃ nanocomposite has been proposed in Fig. 10.

To further explore the photocatalytic efficiency of CuO/Fe₂O₃ 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/Fe2O3 nanocomposite (PC-IV). Fig. S5, represent 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 1st cycle, with CuO/Fe₂O₃ nanocomposite. However, after 4th 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 1st cycle. Thus, the removal efficiency of the BNPs remained excellent after the 4th cycle. It demonstrated that the material's stability and recyclability were good. The slight decline in the activity of BNPs after 4th 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 [50].

To summarise, CuO/Fe₂O₃ nanocomposite were successfully obtained using P. guajava leaf extract in a convenient, low-cost, and environmentally 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/Fe₂O₃ nanocomposite. The optical band gap of biogenic CuO/Fe2O3 nanocomposite synthesised was 2.5 eV, indicating that they have 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/Fe₂O₃ nanocomposite photocatalyst exhibited remarkably high photocatalytic efficiency than that of bare Fe₂O₃ and CuO NPs for the decomposition of TC antibiotic and RhB & MB dyes under UV/visible light irradiation. The 40 mg of CuO/Fe₂O₃ nanocomposite sample had the best efficiency, with 88%, 96% and 93% degradation of TC, RhB and MB in 80 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/Fe₂O₃ 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.

Funding

There is no funding received to declare for this manuscript.

Author Contribution

SK - Proposed the work, write up and all the work done under his supervision.

AK - All the calculations and figures are made by him.

HB -She has done the synthesis and experimental work in the research work.

PS - Proposed the work and all the work done under his supervision.

Conflict of the interest

There are no conflicts to declare.

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No competing interests reported.

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Photocatalytic degradation of tetracycline antibiotic and organic dyes using biogenic synthesized CuO/Fe 2 O 3 nanocomposite: Pathways and mechanism insights

Archived Versions:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methodology

2.1 Plant material and reagents

2.2 Preparation of guava leaf extract

2.3 Preparation of Fe & Cu monometallic nanoparticles

2.4 Preparation of CuO/Fe₂O₃ nanocomposite

2.5 Characterization of CuO/Fe₂O₃ nanocomposite

2.6 Photocatalytic studies

3. Results And Discussion

3.1 Photocatalytic Degradation of Tetracycline Antibiotics over CuO/Fe₂O₃ nanocomposite

Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Archived Versions:

Version 1

Photocatalytic degradation of tetracycline antibiotic and organic dyes using biogenic synthesized CuO/Fe 2 O 3 nanocomposite: Pathways and mechanism insights

Archived Versions:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methodology

2.1 Plant material and reagents

2.2 Preparation of guava leaf extract

2.3 Preparation of Fe & Cu monometallic nanoparticles

2.4 Preparation of CuO/Fe2O3 nanocomposite

2.5 Characterization of CuO/Fe2O3 nanocomposite

2.6 Photocatalytic studies

3. Results And Discussion

3.1 Photocatalytic Degradation of Tetracycline Antibiotics over CuO/Fe2O3 nanocomposite

Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Archived Versions:

Version 1

2.4 Preparation of CuO/Fe₂O₃ nanocomposite

2.5 Characterization of CuO/Fe₂O₃ nanocomposite

3.1 Photocatalytic Degradation of Tetracycline Antibiotics over CuO/Fe₂O₃ nanocomposite