Orange peel-derived Cu₂O/RGO nanocomposite: Mesoporous binary system for degradation of doxycycline in water

Abstract

In recent times, there is a mammoth challenge for the world and mankind to deal with the frequent use and misuse of antibiotics and its casual discard to the water bodies. The scavenging degradation of antibiotics which are no longer in use from the environment is a growing concern and compulsively needs to be addressed. Herein, we have devised a novel and green protocol for the synthesis of Cu₂O decorated on reduced graphene oxide (Cu₂O/RGO) nanocomposite (NCs) using agro-waste, i.e., orange pomace extract (OPE) as a reducing and stabilizing agent for the degradation of antibiotic. The biogenically synthesized Cu₂O/RGO NCs proved to emerge as an excellent degradation catalyst exhibiting efficiency of 98.68% within 15 min and 86.38% within 30 min for 10 mg/L DC concentration assisted by ultrasound waves and solar light respectively in separate reactions. The complete degradation process followed a pseudo-first-order kinetics with a rate constant of 0.29 min^− 1 and 0.0542 min^− 1 for sonocatalytic and photocatalytic degradation process, respectively. Surface area analysis showed that with the increase in the GO amount, the doxycycline degradation increases. An in-depth mechanistic account of sonocatalytic and photocatalytic process has been discussed followed by a radical scavenging test which validated the major role of the synthesized NCs in the degradation of DC. The extraordinary catalytic indulgence of biogenically synthesized graphene-based nanocatalyst opens newer avenues for future research in green chemistry and catalytic field.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s10668-022-02895-2.

Introduction

The continued advancement in the medical sector has led to the development of different antibiotics targeting various disorders that has been proved to be a boon for the livings beings as well as improved the living rates (Fair & Tor, 2014). There has been a tremendous increment in the usage of pharma products in human disease treatment, livestock, and poultry farms. In India and China alone, approximately, 200,000 tons of antibiotics are consumed annually (Prabagar et al., 2022). However, this increased consumption of antibiotics has resulted in the increased discharge of pharma residues in the environment and has led to their bioaccumulation which are rendered harmful for all the living organisms. Antibiotics are highly active molecules which have the ability to affect the physiological changes in humans and animals even at low concentration. The long stay of antibiotics in the environment in various water bodies leads to the sprouting of drug-resistant bacteria causing additional damage to the environment and living species (Polianciuc et al., 2020). The presence of pharmaceutical residuals in environment may be due to domestic waste like human excretion which contains the non-metabolized residues of the consumed antibiotics, improper disposal of unused and expired medicines, untreated release of effluents from pharmaceutical industries and hospitals etc.(Zur et al., 2020) The presence of such antibiotic residues in the water sources results in the generation of drug resistant bacteria and genes which eventually catalyzes the growth of antibiotic resistance which will have devastating effect on the health of the living beings (Abdurahman et al., 2021). It also leads to ecological imbalance thus affecting the life of all the living organisms.

Doxycycline (DC), a member of the tetracycline family, is a semisynthetic antibiotic derived from oxytetracycline and is one of the most consumed antibiotics due to its high activity against a wide range of infections caused due to bacteria or parasites (Kadayifci et al., 2004). During the COVID-19 pandemic its usage escalated rapidly due to its promising results in the treatment process (Comber et al., 2020). However, their extensive usage has led to the development of drug resistance in bacteria which can prove crucial to the human immunity system. The presence of DC in water bodies causes an imbalance in the aquatic life and ultimately enters the food chain thereby increasing the risk factor (Pan & Chu, 2017). Several conventional water treatment methods like osmosis (Samsami et al., 2022), coagulation (Zaidi et al., 2019), adsorption (Abdulsahib et al., 2020), etc., are being utilized extensively for the removal of DC from the contaminated water; however, these methods are not capable to completely remove such polar, water soluble, and non-biodegradable pharmaceutical pollutants from the wastewater (Martinez, 2009) as well as often leads to secondary pollutant which will require additional treatment thus rendering the purification process non-economical. These drawbacks of the conventional purification processes have made the development of new advanced purification processes more crucial. Advanced oxidation processes (AOP) have gained enormous attention in these recent years due to their ability to degrade such harmful pollutants directly into CO₂ and H₂O without the release of any secondary pollutants (Tan et al., 2011). The usage of ultrasound waves and electromagnetic radiations specifically ultraviolet and solar light for the water treatment processes are considered some of the most novel protocols due to several merits like high efficiency, low power consumption, and economical (Khurana et al., 2019). The usage of metal oxide (MO)  nanoparticles (NPs) as effective catalyst for such AOPs has been elevated in the recent years due to their large surface area and amazing affinity toward varying pollutants and fast kinetics (Dou et al., 2021; Joshi et al., 2022). Among the various MO NPs synthesized, cuprous (I) oxide NPs have gained special attention due to their simple synthesis procedure, non-toxic nature, surface modifiability, and great biocompatibility (Kerour et al., 2018; Yadav et al., 2021). Cu₂O NPs is a p-type of semiconductor possessing a bandgap of around 1.9–2.1 eV (Hadiyan et al., 2021) which makes them an amazing catalyst which facilitates the generation of e⁻(electrons)-h⁺(holes) pairs at a lower energy and allows the production of various reactive oxidation species. However, Cu₂O NPs have the tendency to agglomerate that leads to a reduction in the effective surface area thus limiting its efficiency. Apart from the agglomeration of NPs, due to the low band gap in Cu₂O, the generated e⁻-h⁺ pairs have a high recombining capacity which further hinders the generation of ROS thus proving detrimental to the removal process. Therefore, Cu₂O NPs are often fabricated onto various supporting materials like graphene, carbon nanotubes, molecular organic framework, etc., which prevents the agglomeration of the MO NPs as well as acts as an amazing electron transport system thus preventing the e⁻-h⁺ pairs recombination. The fabrication of Cu₂O NPs onto the RGO surface not only prevents the agglomeration and e⁻-h⁺ pairs recombination, but also shows an additional affinity toward the antibiotic molecules due to the π–π interaction between the π bonds of the RGO surface and aromatic rings present in DC which further improves the contact between the catalyst and the pollutant (Zhang & Shao, 2014).

Furthermore, the usage of plant derivatives as a reducing and stabilizing agent for the nanomaterial synthesis over the conventional protocols provides an added advantage and encourages us to shift toward the development of a sustainable environment. The agro-waste are often dumped into the landfills or are burned leading to environmental pollution. Fruit pomace, the residue left after the extraction of juice from fruits, is rich in various flavonoids, sugars, and polyphenols that contains various hydroxyl groups which can act as a great reductant and capping agent during the nanomaterial synthesis (Kumar et al., 2020).

Hence, we used agro-waste OPE to fabricate Cu₂O/RGO NCs and utilized these in degradation of drugs from wastewater. To the best knowledge of author’s, this is first report where OPE is utilized for the fabrication of Cu₂O NPs and Cu₂O/RGO NCs. Further, these synthesized NCs have been used for the degradation of DC drug. The benefits of this invention are utilizing agro-waste and making it prominent in environment sustainability along with the minimal cost efforts. The twin benefit of the current research lies in its target of one waste material becoming scavenging agent for another waste product thereby endorsing the trash to treasure scheme.

Experimental

Materials

Orange pomace (OP) was sourced from the local juice vendors. All the chemicals were used as received without any additional purification. The chemicals used were Graphite Powder (Loba Chemie Pvt. Ltd., 98%), H₃PO₄ (Molychem, 85%,), H₂SO₄ (Thomas Baker, 98%), KMnO₄ (Thomas Baker, 99%), H₂O₂ (Chemigens, 50%), CuSO₄.5H₂O (Thomas Baker,99%), NaOH (Merck, 97%), potassium sodium tartrate tetrahydrate (Thomas Baker, 99%), HCl (Thomas Baker, 35%), ammonium oxalate (Merck, 99%), sodium azide (s.d. fine-chem ltd, 99%), and KI (Thomas Baker, 99%). All the solutions were prepared using distilled water.

Characterization techniques

The physical, chemical, and optical properties of the biogenically synthesized Cu₂O NPs and Cu₂O/RGO NCs were investigated using various characterization techniques. The crystalline structure of the synthesized Cu₂O NPs and Cu₂O/RGO NCs were analyzed using the Powder X-ray diffractometer (P-XRD) at USIC, University of Delhi (Bruker High Resolution X-ray diffractometer) operated at 40 kV and 40mA, Cu Kα radiation and 2θ in the scan range of 20–80°. The optical properties of the synthesized NCs and NPs were analyzed using a double beam UV-Visible spectrophotometer (Motras Scientific Instrument) operated within a wavelength range of 200–800 nm at 1-nm resolution. Fourier transform infrared (FTIR) spectrometer (Thermo Scientific NICOLET iS50, ABX-FTIR) in the ATR mode was used for the identification of various functional groups. Raman spectra of the synthesized nanomaterials were analyzed using the RENISHAW inVia Raman Microscope at USIC, Delhi University. Nitrogen adsorption–desorption isotherms of the biogenically synthesized NCs were noted using a porosity and surface area analyzer (micromeritics Gemini-V) at 77 K. The morphologies of the biosynthesized NPs and NCs were studied using the scanning electron microscope, SEM (JEOL-JEM 2100). The elemental analysis of the synthesized NCs was analyzed using the energy-dispersive X-ray study (EDX). All the degradation study were done by using ultrasonicator (Motras Scientific Instruments) and UV–visible spectrophotometer.

Preparation of Fehling solution

Fehling’s reagent test is commonly used for distinguishing between reducing and non-reducing sugars. Fehling's solution is prepared by mixing Fehling A and Fehling B solutions in equal parts (1:1 ratio). Fehling A solution was prepared by adding (6.9 g, 0.02 mol) copper (II) sulfate pentahydrate (CuSO₄.5H₂O) in 100 ml distilled water. For the preparation of Fehling B solution, 34.6 g sodium potassium tartrate tetrahydrate and 12 g sodium hydroxide (NaOH) were dissolved in 100 ml distilled water.

Preparation of orange pomace extract (OPE)

20 g OP was added to 100 ml distilled water in a 250 ml Erlenmeyer flask, and the contents were heated at 60 °C for 20 min. Subsequently, the OP solution was filtered through ordinary filter to collect the OPE. The prepared OPE was refrigerated for further use.

Synthesis of GO

GO was synthesized from the natural graphite using the Modified Hummer’s method (Zaaba et al., 2017). Briefly, H₂SO₄ and H₃PO₄ were taken in a round-bottom flask in a ratio of 9:1 and stirred well for several minutes. 0.375 g of graphite powder was added to the above acid mixture followed by slow addition of 2.250 g potassium permanganate with constant stirring at 50 °C for 12 h. After 12 h, the reaction mixture was allowed to cool till room temperature, and 100 ml ice cold distilled water was added into it followed by the dropwise addition of 1 ml hydrogen peroxide in order to quench the reaction (pictorial representation is given in Scheme S1). The synthesized GO solution was then centrifuged and washed with distilled water and ethanol and dried at 60 °C overnight. The brown color powdered GO was obtained.

Biogenic synthesis Cu₂O NPs using OPE

Cu₂O NPs were prepared by the reduction in a mixture of Fehling solution using OPE as the reducing and stabilizing agent. A mixture of Fehling solution and distilled water in the ratio of 1:50 was taken in a beaker and stirred well for 15 min at room temperature followed by the dropwise addition of 20 ml OPE. A color change from blue to yellow within 0.5 h indicated the synthesis of Cu₂O NPs (pictorial representation is shown in Scheme S2). Thereafter, the synthesized Cu₂O NPs were centrifuged and washed with distilled water and ethanol and then dried in oven at 60 °C overnight.

Biogenic synthesis of Cu₂O/RGO NCs using OPE

OPE was used as the reducing agent and stabilizing agent for the Cu₂O/RGO NCs synthesis. 1:50 ratios of Fehling solution and distilled water and GO in different amounts (20 mg, 10 mg, and 5 mg) were taken in a beaker and was allowed to sonicate for 30 min for the fine dispersion of GO. Afterward, 10 ml OPE was added dropwise to it. A color change from blue to reddish brown was observed (as shown in Scheme S3) which confirmed the synthesis of Cu₂O/RGO NCs. The brown colored solution was then refluxed at 95 °C for 1 h for the complete conversion of GO to RGO. Thereafter, the synthesized NCs were centrifuged and washed with water and ethanol and dried in oven at 60 °C overnight. Cu₂O/RGO NCs synthesized from different amounts (20 mg, 10 mg, and 5 mg) of GO were named as CRO-20, CRO-10, and CRO-5, respectively.

Photocatalytic and sonocatalytic degradation of DC using Cu₂O/RGO NCs

The photocatalytic and sonocatalytic activity of the synthesized Cu₂O NPs and Cu₂O/RGO NCs was evaluated by analyzing the degradation of DC, one of the most extensively utilized antibiotic as a target pollutant. The photodegradation studies were conducted under solar light, while the sonocatalytic degradation studies were carried out using an ultrasonic bath of power 60 kHz. In brief, for the photocatalytic studies, 10 mg Cu₂O NPs and Cu₂O/RGO NCs were dispersed into 100 ml of DC solution (10 mg/L), and the solution was then exposed to solar light accompanied by constant stirring and this photodegradation was continued for 60 min. Similarly, reaction mixture of the same concentration was kept in an ultrasonic bath for analyzing the sonocatalytic degradation, and the reaction was carried out for 40 min. The pH of the solution was maintained between 6 and 7 using NaOH and HCl. 3 ml of aliquots were drawn at a regular interval of 5 min, were centrifuged and then analyzed under a UV–visible spectrophotometer. The absorption spectra were recorded in the range of 200−400 nm, and utilizing the absorbance data, the degradation percentage of DC was determined using the following equation:

$$\text{Degradation}(\%)=\left(1-\frac{\mathit{Ct}}{\mathit{Co}}\right)\times 100=\left(1-\frac{\mathit{At}}{\mathit{Ao}}\right)\times 100$$

where $\mathit{Ct}$ and $\mathit{Co}$ are DC concentration at t min and 0 min, respectively, while $\mathit{At}$ and $\mathit{Ao}$ corresponds to the maximum absorbance values at λ_max.

For the determination of various radicals involved in the degradation process, ammonium oxalate (0.5 mM), sodium azide (0.5 mM), and potassium iodide (0.5 mM) were added into the reaction mixture as radical scavengers for h⁺, O₂^– and OH radicals, respectively.

Results and discussion

The growth in industries has fueled the overall development of the country as well as have a tremendous impact on the living rates. However, this progress is accompanied by various negative impacts with degradation of environment being one of the most crucial. The untreated disposal of waste materials especially wastewater containing various effluents from various industries directly into the water resources poses as a major factor responsible for the deterioration of environment. Therefore, the development of new sustainable technologies for both the synthesis process as well as the treatment of such contaminated water resources has become extremely necessary. Plant extracts are a storehouse of various biomolecules that can be utilized as an effective reducing and stabilizing agent for the NPs synthesis.

In this current study, we designed a novel green protocol for the synthesis of Cu₂O NPs and Cu₂O/RGO NCs using orange waste without the consumption of any other harmful chemicals. OPE consists of various flavanones containing various hydroxyl groups and reduces the copper metal ion and GO. The utilization of orange pomace which is an agro-waste for the nanomaterial synthesis provides a non-toxic and green alternative to the conventional method and further facilitates the waste management. Further, we utilized the synthesized Cu₂O/RGO NCs for the antibiotic degradation which helps in the environmental purification.

OPE is rich with various phytochemicals that have the potential to act as a reducing agent for the NCs synthesis. To determine these biomolecules, present in them, OPE was subjected to various functional group test which included the test for Saponins (Froth Test; Fig. S1a), Carbohydrates (Molish test; Fig. S1b), Steroids (Liebermann Test, Fig. S1c), Flavonoids (Alkaline Reagent Test; Fig. S1d), Proteins (Biuret Test, Fig. S1e), Tannins (FeCl₃ test, Fig. S1f), and Alkaloids (Mayer Test, Fig. S1g). The presence of carbohydrates, flavonoids, and saponins was confirmed by the positive results observed in the case of Molish test, Alkaline Reagent Test, and Froth Test. The hydroxyl groups present in flavanones which are generally found in OPE like hesperidin and naringenin acts as the reducing and stabilizing group in the synthesis of Cu₂O NPs and Cu₂O/RGO NCs. A simple probable mechanism for the synthesis of Cu₂O/RGO NCs using OPE is provided in Scheme 1.

Scheme 1.

Mechanism for synthesis of Cu₂O/RGO NCs using OPE

The presence of flavonoids in the OPE and formation of Cu₂O NPs and Cu₂O/RGO NCs were confirmed with the help of UV–visible spectroscopy as shown in Fig. 1. Three absorption peaks were observed in case of OPE (Fig. 1b) due to the presence of flavonoids; peaks at 213 and 280 nm correspond to the π–π^* transitions of aromatic chromophores, whereas the peak at 313 nm corresponds to the n to π^* excitation of carbonyl chromophores, respectively (Sisa et al., 2010). Maximum absorbance at 225 nm was observed in case of GO corresponding to the π–π^* transitions of the C-C bonds (Fig. 1a) (Pham, 2015). For the biogenically synthesized Cu₂O NPs (Fig. 1c), maximum absorbance was observed at 379 nm which is in accordance to the reported data (Chen et al., 2015). In case of Cu₂O/RGO NCs of different concentrations, one peak at 251 nm, 275 nm, and 243 nm attributed to the π–π^* transitions of RGO, and second peak at 394 nm, 391 nm, and 363 nm corresponding to the Cu₂O NPs was observed for CRO-20 (Fig. 1d), CRO-10 (Fig. 1e), and CRO-5 (Fig. 1f), respectively. This indicated the successful fabrication of Cu₂O NPs onto the RGO surface.

Fig. 1.

UV–visible absorption spectra of a GO, b OPE, c Cu₂O NPs, d CRO-20, e CRO-10, and f CRO-5

The analysis of the optical behavior and evaluation of band gap of the biogenically synthesized Cu₂O NPs and Cu₂O/RGO NCs were observed with the help of Tauc’s plots. The coefficient of absorption for the biogenically synthesized nanomaterials were evaluated using the following equation:

$$\alpha h\nu =A{(h\nu -Eg)}^n$$

where E_g corresponds to the gap in the optical energy, A corresponds to the incident photon constant, hv corresponds to the energy of the incident photon, and n corresponds to the nature of transitions. When n = ½, this implies that the transitions are indirect and allowed.

$$\alpha \left(\nu \right)=\left(\text{Absorbance}\times 2\text{.303}\right)/t$$

where α(v) is the absorption coefficient defined by the Beer–Lambert’s law and calculated from absorption spectra and t is thickness of the film. To evaluate the band gap of the biogenically synthesized Cu₂O NPs and Cu₂O/RGO NCs, Tauc plots ((αhν)^1/n vs. hν) for n = 2 were plotted as shown in Fig. 2. The intersection of the tangent of the linear segment of the absorption curve on X-axis is taken as the band gap, i.e., E_g. The E_g value for Cu₂O NPs, CRO-20, CRO-10, and CRO-5 was found to be 1.91 eV, 1.35 eV, 1.40 eV, and 1.43 eV, respectively, for indirect transitions (Fig. 2). It was observed that with the increase in the concentration of GO in the NCs, the band gap also reduced.

Fig. 2.

Tauc’s plots for a Cu₂O, b CRO-20, c CRO-10, and d CRO-5

The synthesis of the Cu₂O NPs and Cu₂O/RGO NCs were further validated using the FTIR spectrum (Fig. 3). The FTIR spectra of GO (Fig. 3a) showed spectral peaks mainly at 3311 cm^− 1 corresponding to the O–H stretching vibration, 1719 cm^− 1 attributed to the C = O carbonyl stretching, 1613 cm^− 1 corresponding to the C = C stretching of aromatic ring, and 1218 cm^− 1 corresponding to the C-O alkoxy stretching. The FTIR spectrum of the biogenically synthesized Cu₂O NPs (Fig. 3b) shows spectral peaks mainly at 3321 cm^− 1, 1605 cm^− 1, 1317 cm^− 1, 1179 cm^− 1, and 610 cm^− 1. The formation of the Cu₂O NPs was confirmed by the presence of characteristic peak of Cu-O at 610 cm^− 1. The broad peak at 3321 cm^− 1 corresponds to the hydroxyl (OH) groups. The spectral peak 1605 and 1317 cm^− 1 corresponds to the asymmetric and symmetric C = O carbonyl group stretching, respectively. The spectral peak at 1179 cm^− 1 represents the C-O stretching vibrations (Amrani et al., 2016; Bonev & Alexandrov, 1993). A complete disappearance of the peak at 1719 cm^− 1 corresponding to the C = O carbonyl stretching frequency of GO was observed in the case of Cu₂O/RGO NCs which confirms the successful reduction of GO using OPE (Fig. 3c, d, e). The presence of the characteristic Cu-O vibration peak at 610 cm^− 1 in Cu₂O/RGO NCs confirms the successful fabrication of Cu₂O NPs on to the RGO surface (Kumar et al., 2016).

Fig. 3.

FTIR spectrum of a GO, b Cu₂O, c CRO-5, d CRO-10, and e CRO-20

Further, the purity and crystalline nature of the synthesized Cu₂O NPs and Cu₂O/RGO NCs were analyzed using the P-XRD pattern as shown in Fig. 4. The biogenically synthesized Cu₂O NPs showed 2θ values of diffraction peaks at 29.33, 36.48, 42.64, 61.82, 74.13, and 78.12° corresponding to the main facets (110), (111), (200), (220), (222), and (311), respectively (Chen et al., 2015). The presence of all the main diffraction peaks of Cu₂O in the XRD pattern of Cu₂O/RGO NCs confirmed the presence of Cu₂O in the NCs (Kumar et al., 2016). The biogenically synthesized Cu₂O/RGO NCs was further verified using the Raman spectra (Fig. 5). The Raman spectrum of the synthesized NCs clearly showed the presence of two characteristic peak of RGO: the D band at 1355 cm^− 1 and G band at 1597 cm^− 1. The D and G bands are attributed to the breathing modes of ring of A_1g symmetry and E_2g, respectively, and in plane stretching vibration of sp² hybridized carbon atoms (C = C bonds). The presence of a sharp D band at 1355 cm^− 1 and a small 2D band at 2709 cm^− 1 signifies the graphitic character of synthesized Cu₂O/RGO NCs. Furthermore, the presence of three peaks at 413 cm^− 1, 527 cm^− 1, and 627 cm^− 1 confirmed the presence of Cu₂O in the NCs (Thi Thanh Nhi et al., 2020). The characteristic surface area of CRO NCs were analyzed using the isothermal N₂ adsorption–desorption analysis (Fig. 6). The isothermal curves showed an improvement in the surface area of the NCs with the increase in the GO amount. This improved surface area with the increase in the GO amount plays a vital role in the doxycycline degradation. The outcomes visibly confirm that the biogenically synthesized Cu₂O/RGO NCs show mesoporous nature of composites.

Fig. 4.

P-XRD spectra of biogenically synthesized a Cu₂O NPs, b CRO-5, c CRO-10, and d CRO-20

Fig. 5.

Raman spectrum of a GO, b Cu₂O, c CRO-5, d CRO-10, and e CRO-20

Fig. 6.

N₂ adsorption–desorption isotherms and BJH pore size distribution plot of a CRO-20, b CRO-10, and c CRO-5

The morphology of the synthesized Cu₂O NPs and Cu₂O/RGO NCs was analyzed using the SEM techniques as shown in Fig. 7. The biogenically synthesized Cu₂O NPs showed spherical morphology with the particles agglomerated together; however, in case of Cu₂O/RGO NCs, the particles were evenly distributed over the RGO sheets. The elemental analysis of the synthesized nanomaterials was studied using the EDX spectrum. The EDX spectrum of Cu₂O NPs confirmed the presence of Copper and Oxygen as the constituent elements however in case of Cu₂O/RGO NCs carbon along with copper and oxygen were present as the constituent element due to the presence of RGO.

Fig. 7.

SEM images of a, b GO, c Cu₂O, e CRO-20, g CRO-10, and i CRO-5 and EDX image of d Cu₂O, f CRO-20, h CRO-10, and j CRO-5

DC degradation studies

The sonocatalytic and photocatalytic efficiency of the synthesized Cu₂O NPs and Cu₂O/RGO NCs were investigated in the presence of ultrasound waves and solar light by using the antibiotic, DC as a model pollutant and the degradation efficiency was monitored using a UV–visible spectrophotometer. The UV–visible spectrum of DC showed two absorption peaks at 273 and 347 nm; however, a red shift in the band at 347 to 368 nm was observed in the presence of the Cu₂O/RGO NCs and Cu₂O NPs. A comparison of the degradation efficiency of the synthesized Cu₂O/RGO NCs with previously synthesized nanomaterial is provided in Table 1.

Table 1.

Comparison of DC degradation of Cu₂O/RGO NCs with other nanomaterials

Sr. No.	Catalyst	Synthesis	Degradation methodology	Degradation efficiency (%)	Time (minutes)	Reference
1	ZnO/NiCo2S4 QDs	Green synthesis	Photocatalytic	99	160	(Swedha et al., 2022)
2	MLG/ZnO	Green synthesis	Photocatalytic	95	300	(Sebuso et al., 2022)
3	Ag/AgCl-CdMoO4	Conventional method	Photocatalytic	82.37	60	(Wen et al., 2019)
4	BiOBr/FeWO4	Conventional method	Photocatalytic	90	60	(Gao et al., 2018)
5	Magnetic polymer-ZnO	Conventional method	Photocatalytic	76.5	3600	(Mohammadi & Pourmoslemi, 2018)
6	MWCNTs/α-Bi2O3	Conventional method	Photocatalytic	91	120	(Liu et al., 2019)
7	Fe, Cu-co-doped TiO2-SiO2	Conventional method	Photocatalytic	98.60	20	(Rani et al., 2021)
8	La2CuO4	Conventional method	Photocatalytic	85	120	(Prabagar et al., 2022)
9	Cu2O/RGO NCs	Green synthesis	Sonocatalytic and photocatalytic	98.68 and 86.38	15 and 30	Present work

Initially without the addition of any catalyst, no noticeable change was observed in case DC solution indicating the negligible degradation of DC in the absence of catalyst. Figures 8 and 9 illustrate the DC degradation results under ultrasound waves and solar light by CRO-20, CRO-10, CRO-5, and Cu₂O NPs, respectively.

Fig. 8.

DC degradation with time in the presence of ultrasound waves; a CRO-20, b CRO-10, c CRO-5, and d Cu₂O; DC concentration = 10 mg/L; Catalyst concentration = 0.1 g/L; pH = 7; Temp = RT

Fig. 9.

DC degradation with time in the presence of solar light; a CRO-20, b CRO-10, c CRO-5, and d Cu₂O; DC concentration = 10 mg/L; Catalyst concentration = 0.1 g/L; pH = 7; Temp = RT

The DC degradation efficiency increased to 84.08% in 35 min upon the utilization of bare Cu₂O in the presence of ultrasound waves (Fig. 8d). The DC removal efficiency was further enhanced upon the addition of Cu₂O/RGO NCs. The degradation efficiency was found to be 93.52, 96.36, and 98.68% within 15 min only when CRO-5 (Fig. 8c), CRO-10 (Fig. 8b), and CRO-20 (Fig. 8a) were utilized, respectively, indicating that CRO-20 showed the highest degradation efficiency. The bar graph for the comparison of sonocatalytic degradation efficiency is shown in Fig. S2a.

Similarly in the presence of solar light, the removal efficiency of DC using pure Cu₂O NPs was found to be 76.38% in 55 min (Fig. 9d) which was further enhanced upon the addition of Cu₂O/RGO NCs. The photocatalytic degradation efficiency for DC in the presence of CRO-5 (Fig. 9c), CRO-10 (Fig. 9b), and CRO-20 (Fig. 9a) was found to be 81.18%, 83.04%, and 86.38% in just 30 min, respectively, again revealing that CRO-20 showed the highest degradation efficiency. The bar graph of photocatalytic degradation efficiency is shown in Fig. S2b. This observation revealed that with the increase in the concentration of RGO in the catalyst, its catalytic properties were also enhanced. Furthermore, it was concluded that sonocatalytic degradation showed the best degradation efficiency in a small amount of time as compared to the photocatalytic degradation thus making it a much more efficient method for the DC degradation.

Since CRO-20 showed the best performance among all the three Cu₂O/RGO NCs and Cu₂O NPs (Fig. S2), it was utilized for the further studies. To discover the optimum reaction conditions for the DC removal, degradation studies were performed under various conditions including the catalyst concentration, DC concentration and pH. Table 2 illustrates the effect of various reaction conditions on the degradation efficiency.

Table 2.

Effect of various reaction conditions on the degradation efficiency

Sr. No.	Catalyst	Catalyst dosage (g/L)	DC dosage(mg/L)	pH	Degradation method	Degradation efficiency (%)	Time
1	CRO-20	0.1	10	7	Sonocatalytic	98.68	15
2	CRO-10	0.1	10	7	Sonocatalytic	96.36	15
3	CRO-5	0.1	10	7	Sonocatalytic	93.52	15
4	Cu2O	0.1	10	7	Sonocatalytic	84.08	35
5	CRO-20	0.1	10	7	Photocatalytic	86.38	30
6	CRO-10	0.1	10	7	Photocatalytic	83.04	30
7	CRO-5	0.1	10	7	Photocatalytic	81.18	35
8	Cu2O	0.1	10	7	Photocatalytic	76.38	55
9	CRO-20	0.05	10	7	Sonocatalytic	89.78	15
10	CRO-20	0.2	10	7	Sonocatalytic	87.48	15
11	CRO-20	0.05	10	7	Photocatalytic	75.37	30
12	CRO-20	0.2	10	7	Photocatalytic	72.11	30
13	CRO-20	0.1	20	7	Sonocatalytic	94	15
14	CRO-20	0.1	30	7	Sonocatalytic	80.72	15
15	CRO-20	0.1	20	7	Photocatalytic	79.62	30
16	CRO-20	0.1	30	7	Photocatalytic	71.76	30
17	CRO-20	0.1	10	2	Sonocatalytic	65.22	15
18	CRO-20	0.1	10	11	Sonocatalytic	56.59	15
19	CRO-20	0.1	10	2	Photocatalytic	57.44	30
20	CRO-20	0.1	10	11	Photocatalytic	46.79	30

Effect of catalyst dosage

Catalyst dosage is an important factor that influences the degradation efficiency. The experiment was observed for different CRO-20 dosages of 0.05 g/L (Table 2 entry 9 and 11), 0.1 g/L (Table 2 entry 1 and 5) and 0.2 g/L (Table 2 entry 10 and 12) to analyze the degradation of 10 mg/L of DC solution under both ultrasound waves and solar light. Figure S3 presents the effect of catalyst dosage on DC degradation both sonocatalytically and photocatalytically.

As shown in the Fig. 10, it is evident that upon increasing the catalyst dosage from 0.05 g/L (Table 2 entry 9) to 0.1 g/L (Table 2 entry 1), a huge improvement in the degradation efficiency from 89.78 to 98.68% was observed in case of sonocatalytic DC degradation. Thereafter, when the catalyst concentration was increased to 0.2 g/L (Table 2 entry 10), a relative decrease in the removal efficiency was observed. This improvement in removal efficiency can be attributed to the fact that the increase in catalyst dosage leads to an enhancement in the surface area and releases more active species that are responsible for the sonocatalytic degradation, thus enhancing the removal efficiency. However, upon increasing the catalyst dosage beyond the optimum condition, a partial aggregation of the catalyst occurs that leads to a decrease in the effective surface area thus limiting the catalytic properties of the catalyst, and a decrease in degradation efficiency was observed.

Fig. 10.

Sonocatalytic degradation of DC; a removal efficiency of DC under different catalyst dosage as a function of time b plot of C/C_ο versus time, c plot of -ln(C/C_ο) versus time, and d Rate constant (k) (min^− 1) for DC degradation; DC concentration = 10 mg/L; pH = 7; Temp = RT

The kinetic modeling for the DC degradation is shown in Fig. 10c and d, and the pseudo-first-order model was employed using the equation;

$$-ln\frac{\mathit{Ct}}{C0}=kt$$

The kinetic study revealed that the degradation follows pseudo-first-order kinetics with the highest rate constant of 0.29 min^− 1 for 0.1 g/L catalyst dosage (R² = 0.970). Therefore, 0.1 g/L was considered as the optimum catalyst dosage for the sonocatalytic DC degradation.

In case of photocatalytic degradation, it is clear from Fig. 11 that an increase in catalyst concentration from 0.05 g/L (Table 2 entry 11) to 0.1 g/L (Table 2 entry 5) enhanced the degradation efficiency largely from 75.37 to 86.38% but any further increase in the catalyst dosage lowers the removal efficiency. The enhancement in degradation efficiency as a result of increased catalyst dosage may be due to the presence of more active sites that improves the adsorption capability of the catalysts and the improve electron-hole pair generation which further enhances the degradation efficiency. However, the presence of a very high concentration of catalyst in the DC solution hinders the light penetration into the reaction solution which would be deleterious to the removal efficiency. The kinetic study (Fig. 11c and d) revealed that the degradation follows pseudo-first-order kinetics with 0.1 g/L of CRO-20 showing the highest degradation rate constant of 0.05042 min^− 1 (R² = 0.921). Thus, 0.1 g/L catalyst dosage was considered as the optimum dosage for photocatalytic degradation.

Fig. 11.

Photocatalytic degradation of DC; a Removal efficiency of DC under different catalyst dosage as a function of time, b plot of C/C_ο versus time, c plot of -ln(C/C_ο) versus time, and d Rate constant (k) (min^− 1) for DC degradation; DC concentration = 10 mg/L; pH = 7; Temp = RT

Effect of pollutant concentration

The presence of pollutant in water also has an enormous effect upon the efficiency of the catalyst; therefore, the sonocatalytic and photocatalytic properties of CRO-20 were investigated in the presence of different DC concentration; 10 mg/L (Table 2 entry 1 and 5), 20 mg/L (Table 2 entry 13 and 15) and 30 mg/L (Table 2 entry 14 and 16) in the presence of 0.1 g/L catalyst dosage (Fig. S4).

With the increase in DC concentration from 10 mg/L (Table 2 entry 1) to 20 mg/L (Table 2 entry 13) during the sonocatalytic degradation, a slight decrease in efficiency from 98.68 to 94% (Fig. 12a) was observed which upon further increment of the DC concentration to 30 mg/L (Table 2 entry 14) depleted to 80.72%. Thus, upon increasing the DC concentration from 10 mg/L to 30 mg/L in the presence of ultrasound waves, a decline in the degradation efficiency was observed. This decrement in degradation efficiency is because of the fact that as the concentration of DC increases, more antibiotic molecules get adsorbed onto the catalyst surface and these excessive DC molecules compete with generated active radicals necessary for degradation. However, the number of reactive species being generated and the catalyst surface remains the same which results in an increased degradation burden onto the catalyst and further is considered detrimental to the removal efficiency. The kinetic study (Fig. 12b and c) suggested that the reaction follow a pseudo-first-order reaction with DC concentration of 10 mg/L showing the highest rate constant of 0.29 min^− 1. Therefore, optimum DC concentration of 10 mg/L was considered for the further sonocatalytic degradation studies.

Fig. 12.

Sonocatalytic degradation of DC under varying concentration of DC a removal efficiency of DC as a function of time, b plot of C/C_ο versus time, c plot of -ln(C/C_ο) versus time, d Rate constant (k) (min^− 1) for DC degradation; Catalyst dosage = 0.1 g/L; pH = 7; Temp = RT

From Fig. 13a, it was observed that on increasing the DC concentration from 10 mg/L (Table 2 entry 5) to 20 mg/L (Table 2 entry 15) in the presence of solar light, the degradation efficiency decreased from 86.39 to 79.62% and finally reduced to 71.76% when the antibiotic concentration reached 30 mg/L (Table 2 entry 16). This reduction in degradation efficiency is due to the increased adsorption of DC molecules on the catalyst surface that hinders the photons from reaching to the catalyst surface thus lowering the generation of electron–hole pairs and other active radical species that are vital for DC degradation and thus results in the lowering of the degradation efficiency.

Fig. 13.

Photocatalytic degradation under varying DC concentration a Removal efficiency of DC as a function of time, b plot of C/C_ο versus time, c plot of -ln(C/C_ο) versus time, and d Rate constant (k) (min^− 1) for DC degradation; Catalyst concentration = 0.1 g/L; pH = 7; Temp = RT

The kinetic modelling (Fig. 13c and d) revealed that the reaction follows a pseudo-first-order kinetics with DC concentration of 10 mg/L showing the highest rate constant of 0.050 min^− 1. Hence, 10 mg/L is considered as the optimum DC concentration for photocatalytic degradation studies.

Effect of pH

The effect of pH on the sonocatalytic and photocatalytic activity of CRO-20 was evaluated at three different pH (2,7 and 11). At pH 7, the degradation efficiency was found to be the highest in the presence of both ultrasound waves and solar light. However, the DC degradation was greatly subdued when the pH was changed to 2 and 11. Such detrimental change in the removal efficiency with the change in pH reveals that pH is an important factor influencing the catalytic performance of the synthesized CRO-20 NCs.

This change in removal efficiency upon pH variation may be due to the fact that the variation in pH effect the charge on the catalyst surface as well as have a significant influence on the structure of the pollutant molecules due to the presence of constituents having different pKa values in the doxycycline structure (Ibarra et al., 2011). At acidic pH, the catalyst surface is positively charged and the DC molecules also exists in their cationic form. The presence of same charges on the catalyst surface and DC results in repulsion, and a decline in degradation efficiency is observed. Similarly at higher pH, the presence of repulsion between the negatively charged catalyst surface and anionic form of DC results in the lowering of the degradation efficiency. However, at neutral pH, DC exists in the form of a zwitter ion, and the catalyst surface is also neutral which favors the adsorption of DC onto the catalyst surface and have a positive effect onto the DC degradation (Fig. 14).

Fig. 14.

Doxycycline structure variation under different pH

Since the CRO-20 showed the highest degradation efficiency in neutral conditions (Figs. 15c and 16c), pH 7 was decided as the optimum pH, and the further degradation studies were carried out in neutral pH for both the degradation methods.

Fig. 15.

Sonocatalytic degradation of DC in different pH conditions; a Removal efficiency of DC as a function of time, b plot of C/C_ο versus time, and c bar graph of DC removal efficiency; DC concentration = 10 mg/L; Catalyst dosage = 0.1 g/L; Temp = RT

Fig. 16.

Photocatalytic degradation of DC in different pH conditions; a Removal efficiency of DC as a function of time, b plot of C/C_ο versus time, and c bar graph of DC removal efficiency; DC concentration = 10 mg/L; Catalyst dosage = 0.1 g/L; Temp = RT.

Stability and reusability of the catalyst

The stability and reusability are also important factors influencing the practical application of a catalyst. Therefore, the stability and reusability of Cu₂O/RGO NCs were investigated by running five cycles of the DC degradation reaction in the presence of ultrasound waves and solar light. After the first cycle of DC degradation, the catalyst was recovered via centrifugation followed by proper washing with water and ethanol and subsequent drying at 60 °C. Cu₂O/RGO NCs showed amazing catalytic activity even after five cycles as revealed in Fig. S5. However, after the fifth cycle, a reduction in the degradation efficiencies to 92.14% and 80.74% were observed in the presence of ultrasound waves and solar light, respectively. This slight reduction in efficiency may be due to the unavoidable catalyst loss during the recycle process. The FTIR spectrum of the reused catalysts (Fig. S6) showed no significant variation in comparison with the freshly synthesized catalysts even after 5 cycles. These results indicated that the synthesized Cu₂O/RGO nano-catalyst possess good stability and have great reusability properties which makes their utilization in everyday life more feasible and economic.

Contribution of various different radicals: radical scavenging studies

To identify the various radicals involved in the degradation process, ammonium oxalate (AO), sodium azide (SA), and potassium iodide (KI) were used as radical scavengers for h⁺, O₂⁻, and OH radicals, respectively. As shown in Fig. 17a, decline in the degradation efficiency was observed to various degrees upon the addition of the scavengers. However, the presence of AO suppressed the DC degradation the most thus confirming that holes had the biggest contribution toward the DC degradation process.

Fig. 17.

Degradation efficiency of DC in the absence and presence of various scavengers; a sonocatalytic and b photocatalytic

Doxycycline degradation in real water sample

To analyze the efficiency of the catalyst in day-to-day usage, degradation of doxycycline using CRO-20 NCs was also studied in real water sample. For the real water analysis, tap water was spiked with 10 mg/L of doxycycline solution and 0.1 g/L of CRO-20 catalyst, and the solution was subjected to ultrasound waves and solar light for 15 and 30 min, respectively. A degradation efficiency of 88.42% (Fig. S7a) in case of ultrasound waves and 77.84% (Fig. S7b) in case of solar light were observed, respectively. This observation proved that the synthesized CRO NCs can effectively be utilized in the purification of water for daily purposes as well.

Probable mechanism for sonocatalytic DC degradation

AOPs using ultrasound waves are utilized extensively for toxic pollutant removal. The bubbles produced during the cavitation phenomenon helps in the degradation process. The collapse of the bubble during the cavitation phenomenon leads to the production of various hot spots as well as leads to the phenomenon of sonoluminescence. The generation of sonoluminescence may lead to the production of light emission that carry photons having some average energy. These photons excite the Cu₂O/RGO NCs catalyst, thus leading to the excitation of electrons from the valence band to the conduction band and in turn creating holes in the valence band. This leads to the generation of e⁻–h⁺ pairs. These generated electrons and holes further aids in the production of various reactive radical species like O₂⁻ and OH which helps in the degradation process. Further, the presence of RGO in the Cu₂O/RGO NCs leads to an enhanced adsorption of the DC molecules onto the catalyst surface owing to the π–π interactions between the π bonds of the RGO surface and aromatic rings of the DC. This improves the contact between the pollutant molecules and the catalyst surface. Furthermore, the presence of catalyst provides more active sites for the cavitation phenomenon to occur thus enhancing the electron-hole pair and reactive radicals production. The generated reactive radicals act as a very strong oxidizing agent and oxidizes the DC molecules into non-toxic components like CO₂ and H₂O. A diagrammatic representation of the degradation mechanism is provided in Fig. 18.

Fig. 18.

Probable mechanism for sonocatalytic DC degradation using Cu₂O/RGO NCs

Probable mechanism for photocatalytic DC degradation

The photocatalytic degradation of the pollutant begins upon the irradiation of light source that can provide photons having energy sufficient to overcome the bandgap of the catalyst. The lower band of the synthesized Cu₂O/RGO NCs, the energy provided by the solar light, is sufficient to overcome the bandgap. Irradiation of the Cu₂O/RGO NCs surface with solar radiation leads to the generation of electron–hole pairs. The presence of RGO in the catalyst facilitates the transfer of electrons from the conduction band onto the RGO surface which prevents the recombination of the generated electron–hole pairs. The electrons transferred onto the RGO surface can reduce the O₂ molecules present on the surface and generates O₂⁻. The generated holes oxidize the H₂O molecules into the hydroxyl radicals or can also directly degrade the DC molecules present on the catalyst surface. The generated O₂⁻ and OH are highly reactive species and effectively degrades the DC molecules into biodegradable and non-toxic components like CO₂ and H₂O. Furthermore, RGO present in the NCs also enhances the adsorption of the DC molecules onto the Cu₂O/RGO NCs surface thus improving the contact between the pollutant molecules and catalyst surface that further improves the pollutant degradation. A diagrammatic representation for the proposed mechanism for photocatalytic DC degradation is provided in Fig. 19.

Fig. 19.

Probable mechanism for photocatalytic DC degradation using biogenic Cu₂O/RGO NCs

Conclusion

In this current work, Cu₂O NPs and Cu₂O/RGO NCs were successfully synthesized using orange pomace as a reducing and capping agent and further utilized as an effective catalyst for the sonocatalytic and photocatalytic degradation of DC present in water sample. The various flavanones present in the OPE reduces the Cu⁺ ion and GO allowing the synthesis and stabilization of the NCs. The utilization of synthesized nanomaterials for DC degradation showed amazing degradation efficiency particularly in case of sonocatalytic degradation which showed a much higher removal efficiency as compared to the photocatalytic process. It was observed that at an optimum catalyst dosage of 0.1 g/L, CRO-20 showed an excellent degradation efficiency of 98.68% within 15 min and 86.38% within 30 min for 10 mg/L DC concentration in the presence of ultrasound waves and solar light, respectively. The degradation efficiency followed a pseudo-first-order kinetics with a rate constant of 0.29 min^− 1 and 0.0542 min^− 1 for sonocatalytic and photocatalytic degradation process, respectively. Radical scavenging test confirmed that holes were the major contributor toward the DC degradation. The synthesized NCs showed amazing stability and removal efficiency even after fifth cycles thus confirming the high recyclability of the CRO-20 NCs. The development of CRO-NCs as an effective catalyst for the degradation of DC provides a very simple, economical, and time-efficient route for water purification without the generation of any secondary pollutant thus rendering this process capable for its application in the daily use. The dual achievement in successfully conducting a biogenic synthesis and then degrading a drug which is harming the ecosystem envisages a novel perspective for nanocatalysis and environmental remediation.

Supplementary Information

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Data availability

All data generated or analyzed during this study are included in this published article (and its supplementary information files).

Conflict of interest

On the behalf of all coauthor’s, there is no conflict of interest to declare.