Green Synthesis of Zinc Oxide Nanoparticles from Allium cepa Skin: Photocatalytic Degradation and Antibacterial Properties

The present study uses a bio-waste i.e., skin of Allium cepa, for green synthesis of zinc oxide nanoparticles. The nanoparticles A.Cepa-ZnONP were tested for their photo catalytic degradation e�cacy towards harmful dyes. Additionally, the anti-bacterial properties of A.Cepa-ZnONP were evaluated against four organisms, namely Escherichia coli (EC), Pseudomonas aeruginosa (PA), Bacillus cereus (BC), and Staphylococcus aureus (SA). The synthesized A.Cepa-ZnONP was characterized with scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) methods. The A.Cepa-ZnONP demonstrated e�cient degradation of Crystal Violet (CV), Rhodamine B (RB), and Methylene blue (MB) dyes, achieving maximum degradation percentages of 99.21%, 95.63%, and 92.34%, respectively, while operating under optimal process conditions. The rate constant values for CV, RB, and MB dyes at a temperature of 328K were determined to be 0.1063, 0.0758, and 0.0447 min -1 , respectively. The activation energy values for CV, RB, and MB dyes were determined to be 12.28, 18.437, and 50.623 kJ/mol, respectively. The successful regeneration of photo catalytic material A.Cepa-ZnONP is a crucial milestone in guaranteeing their long-term effectiveness and practical usability.


Introduction
The textile industry, a vital component of global economic activity, faces a signi cant environmental challenge: the generation of wastewater containing hazardous compounds [1,2].The problem occurs as a result of the substantial water usage linked to several wet processing techniques.The procedure yields e uent waste comprising a diverse range of chemicals, such as acids, alkalis, dyes, hydrogen peroxide, starch, surfactants, dispersion agents, and metallic soaps.The repercussions of discharging wastewater in this manner further enhance the textile industry's standing as a notable global water consumer and a signi cant contributor to pollution.The enduring nature and ability to dissolve of intricate arti cial dyes in water pose substantial risks to the environment and human well-being [3,4].
Traditional methods employed for wastewater management have demonstrated restricted e cacy in achieving full decomposition of dyes.Semiconductor metal oxides have arisen as a potential solution for the degradation of dyes in wastewater treatment, in response to this challenge [5].This study examines the use of zinc oxide nanoparticles (ZnONP) obtained from onion skin as a sustainable and long-lasting method for treating industrial wastewater contaminated with dyes.Zinc oxide nanoparticles (ZnONPs) may be used with light to e ciently remove dyes from wastewater, showing great potential as a method for dye removal.During the process, exposure to light sources, such as UV light or solar irradiation, leads to the formation of positively charged holes (h+) in the valence band (VB) and negatively charged electrons (e) in the conduction band (CB).Photons possessing energy exceeding the band gap of ZnO induce the migration of electrons from the valence band to the conduction band.This procedure facilitates the oxidation and reduction of pollutants, such as dyes, by utilizing photo-induced charge carriers [6,7].Zinc oxide (ZnO) is highly esteemed in the eld of semiconductor-based photo catalysis due to its lack of toxicity, cost-effectiveness, broad adsorption range, abundant availability, exceptional electron mobility, and impressive thermal, chemical, and physical stability.These characteristics make ZnO an excellent option for addressing the challenges posed by dye-contaminated wastewater in the textile industry [8,9].Traditional methods of nanoparticle manufacturing can include hazardous chemicals and necessitate substantial energy use.This approach aligns with current scienti c endeavours to develop green synthesis techniques, emphasizing the utilization of environmentally sustainable and renewable substances [10].
The use of bio-waste, speci cally the outer layer of Allium cepa (onion), supports a sustainable and environmentally conscious method in the eco-friendly production of zinc oxide nanoparticles (A.cepa-ZnONP).This approach adheres to the tenets of green chemistry, highlighting the signi cance of utilizing easily accessible and harmless substances.The work centers on two primary factors: the effectiveness of photo catalytic degradation and its antimicrobial attributes.The decision to utilize the skin of Allium cepa is based on its high concentration of bioactive chemicals, which function as both reducing and capping agents in the creation of nanoparticles.Phytochemicals present in plant extracts have a vital function in controlling the features of nanoparticles, improving their stability, compatibility with biological entities, and overall safety [11,12].This green synthesis strategy is in line with the increasing focus on environmentally friendly and sustainable approaches in the production of nanomaterials.It supports the ideas of a circular economy by utilizing agricultural waste.The onion peel, abundant in antioxidants and bioactive constituents, provides a cost-e cient and easily obtainable substitute for chemical reagents, thereby advancing the concepts of green chemistry and possible health advantages.The primary emphasis is on the ability of A.Cepa-ZnONP to degrade harmful dyes, such as Crystal Violet (CV), Rhodamine B (RB), and Methylene blue (MB) from industrial e uent.We evaluate the effectiveness of A.Cepa-ZnONP under optimal conditions by analyzing degradation percentages and rate constants.
The application of scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) in the analysis of A.Cepa-ZnONP enhances our comprehension of this environmentally friendly nanomaterial.

Materials:
Dyeing wastewater was collected from dyeing companies located near Mangalagiri, Andhra Pradesh, India.The wastewater used for dyeing comprises cationic dyes crystal violet (CV), rhodamine b (RB), and methylene blue (MB).The outer layer of Allium Cepa was obtained from a nearby vegetable market in Visakhapatnam, Andhra Pradesh, India.The chemicals ZnSO 4 7H 2 O and NaOH were acquired from Lotus Enterprises, Visakhapatnam.The nutrient agar medium was obtained from the National Collection of Industrial Microorganisms (NCIM) in India.Bacterial strains of Escherichia coli(EC), Pseudomonas aeruginosa (PA), Bacillus cereus (BC) and Staphylococcus aureus (SA) were acquired from this collection.The bacteria indicated earlier were each sub cultured separately, and a small amount of each culture was placed into a new nutritional broth.

Preparation of stock solution:
The wastewater collected is stored in chemically inert glass containers.A minute quantity of acid, such as hydrochloric acid, is included to aid in the preservation of the samples.The samples are stored in a location with low temperatures and the absence of light to reduce the occurrence of photochemical reactions and microbiological activity.The properties of the wastewater, including pH, color, chemical oxygen demand (COD), total dissolved solids (TDS), and dye content, were measured.Further, the dilution samples were prepared by carefully choosing the dilution factors and using distilled water as the dilutant.Preparation of Allium cepa extract: The outer layer of Allium cepa (A.cepa) was washed with distilled water, dried using sunshine, and then crushed into a ne powder.40 g. of A. cepa powder was added with 500 ml of distilled water and agitated at a temperature of 90 0 C for 40 min.The mixture was subjected to a cooling process and thereafter underwent ltration.The solution obtained was stored for further use.

Synthesis and Characterization of Allium Cepa-Zinc oxide nanoparticles ( A.Cepa -ZnONP):
To prepare the mixture, 100 ml of A. cepa extract was combined with 500 ml of a 0.1M zinc sulphate solution.The mixture was then agitated for 3 hrs at room temperature, while ensuring that the pH remained at 8. The solution is subjected to centrifugation at 3600 rpm for 20 min.After centrifugation, the liquid portion (supernatant) is discarded and the solid portion (sediment) is collected.The collected sample was subsequently dehydrated in an oven at a temperature of 100°C, pulverized into a ne powder using a mortar and pestle, and preserved in an air-tight sealed container.
The characterization of the prepared material entails a thorough examination to comprehend its morphological attributes and structural composition.Multiple sophisticated methods are utilized to evaluate different facets of the material's characteristics.The primary characterization techniques used are Scanning Electron Microscopy with Energy Dispersive X-ray Analysis (SEM-EDAX), X-ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR).
Antimicrobial action against A.Cepa-ZnONP: The antibacterial e cacy of A. Cepa-ZnONPs was evaluated using nutrient agar plates and the well diffusion technique.Four aseptic conical asks, each holding an accurately measured 2.8 g of nutrientrich agar, were lled with 100 ml of distilled water.The asks were subsequently autoclaved at a temperature of 121˚C and a pressure of 15 lb for 20 min.After the chilling procedure to achieve room temperature, the test organisms EC, PA, BC, and SA were placed into separate asks and vigorously agitated until the sterile agar reached a temperature of around 40˚C.After introducing the test organism onto sterile Petri plates, the clear agar medium was then put in a laminar air ow chamber to facilitate sedimentation.After 20 min, little indentations or hollows were created in Petri dishes using a glass borer.
A volume of 100 µl of A.cepa-ZnONP was used to ll the cavities.The plates were subsequently incubated for 24 hrs at a temperature range from 32 to 34˚C.Throughout this period, a meticulous examination was carried out to monitor the development and advancement of the microorganisms found within the plates [7].
Batch experimental studies for CV, RB, MB dyes using A.Cepa-ZnONP: The photo catalytic effectiveness of A.Cepa-ZnONP for degrading CV, RB, and MB dyes in dyeing wastewater was analyzed.100 ml of dye solution of speci ed concentration was taken in a conical ask and necessary amount of photo catalyst was added.The solution was vigorously stirred using a magnetic stirrer at a speed of 400 rpm, while being subjected to ultraviolet (UV) light for different periods of time, pH levels, and temperatures.The experiment was carried out in controlled conditions in a sealed chamber, where the sole source of light was focused on the dye solution.Figure 2 depicts the experimental setup used for the process of photo catalytic degradation.After conducting the experimental trials, the solutions were subjected to centrifugation at a rotational speed of 3000 rpm for 20 min.UV-Spectrophotometer was used to measure the optical density of the supernatant solution produced after centrifugation.The measurements were taken at the maximum wavelengths of each dye, as speci ed in reference [13].
The % degradation of dye was determined by employing Eq. 1.
% degradation = Where, C o -the initial concentration of the dye and C t -the dye concentration at time t.

Dye degradation kinetics:
The pseudo rst order kinetic reaction was used to study the dye degradation kinetics (Eq.2) [8].Activation energy calculations: The assessment of activation energy in the photo catalytic degradation of dyes using zinc oxide nanoparticles produced from Allium Cepa skin offers useful insights into the kinetics of the degradation process.The activation energy (Ea) is the minimum amount of energy required to initiate a chemical reaction, serving as a barrier that must be surpassed.Calculating this value can provide insights into the fundamental mechanisms involved in the process of photo catalytic degradation.Eq. 3 is used to calculate activation energy [14][15][16]. 3 Where k = reaction rate constant, min − 1 , E A = activation energy, kJ/mol, R = universal gas constant, T = temperature, K, A = Arrhenius pre-exponential factor.
Results & Discussion

Characterization of catalyst:
The scanning electron microscope (SEM) analysis of A.cepa-ZnONP reveals irregularly shaped particles with a size lower than 100 nm (Fig. 3a).The particles are uniformly scattered, forming a stack of samples.Agglomeration is seen, leading to the formation of a microcrystalline rough structure that is dispersed over A.Cepa-ZnONP [2].The EDX analysis of A.Cepa-ZnONP (Fig. 3b) indicates the existence of oxygen and zinc.The weight percentages of Zn and O were determined to be 28% and 70%, respectively.The remaining components have weight fractions below 2%.The X-ray diffraction (XRD) uses the dual wave characteristic of X-rays to see the arrangement of crystalline materials.The degree of sharpness shown by the detected peaks provides information on the crystalline characteristics of the A.Cepa-ZnONP [2].The peaks seen in Fig. 3c  The Fourier transform infrared (FT-IR) approach is a spectroscopic technique used to identify alterations in the functional groups of the adsorbent formed during the adsorption process.FTIR was performed to identify the bio molecules and functional groups contained in A.Cepa-ZnONP. Figure 4 illustrate the FTIR images of A.Cepa-ZnONP before and after being exposed to dyeing wastewater, respectively.The prominent points identi ed in the FTIR spectrum of A.Cepa-ZnONP at wave numbers 3642.22,3600.43,3519.70, and 3498.40 cm − 1 are no longer discernible in the FTIR spectrum of A.Cepa-ZnONP following exposure to dyeing wastewater.In addition, it was noted that the peaks at wave numbers 1635.31cm − 1 and 1114.86 cm − 1 underwent a change, leading to new peaks of 1656.48 cm − 1 and 1118.45cm − 1 , respectively [2,12].The changes observed in the functional groups and biomolecules inside A.Cepa-ZnONP can be ascribed to the interaction with dyes present in the dyeing wastewater.The e cacy of A.Cepa-ZnONP skin as a degradation agent was demonstrated by analyzing the FTIR images, which revealed the breakdown of colors in dyeing wastewater.
Anti-bacterial activity of skin of A.Cepa-ZnONP: The antibacterial e cacy of A.Cepa-ZnONP was assessed by quantifying the zone of inhibition against the test microorganisms.The measurements of the areas of growth inhibition are displayed in Table 2 and depicted in Fig. 5.The ndings demonstrated that A.Cepa-ZnONP had signi cant antibacterial e cacy against all strains examined.The A.Cepa-ZnONP solution undergoes dissolution and then diffuses across the surrounding media.Any substance or compound that exhibits antibacterial properties hinders or halts the proliferation of germs within its vicinity [7].The growth of microorganisms in the media is shown by the turbidity observed in the medium 24 hrs after inoculation, whereas the control medium remains clear and transparent.Therefore, the test liquid exhibits a distinct or see-through area surrounding the pit, indicating its high level of diffusion effectiveness.Effect of pH: The pH of different organic pollutants has a crucial role in determining the process of photo catalytic destruction.The pH of a photo catalyst affects certain properties, such as surface charge and potential.
Additionally, the photo degradation rate is in uenced by the electrostatic attraction or repulsion between the catalyst's surface and the organic molecule, which depends on the ionic form of the organic compound (anionic or cationic).An investigation was conducted to examine the impact of pH levels ranging from 2 to 9 on the photo catalytic degradation of CV, RB, and MB dyes.The highest percentage of dye degradation achieved was 79.65% at pH 6 for the CV dye.For the RB and MB dyes, the greatest percentages of dye degradation recorded were 75.88% and 72.56% respectively, at pH 8 (Fig. 6).The degradation of the dye was reduced as the pH was further increased.This should be feasible since the optimal pH for dye degradation often occurs at a neutral or slightly alkaline pH.However, the effectiveness of degradation reduces signi cantly at highly acidic or basic pH levels [14].
Kinetic studies: The rate constant was determined by graphing the natural logarithm of the initial concentration divided by the concentration at a given time against time.This was done for CV, RB, and MB dyes, and the results are published in a table and represented in Fig. 7.The rate constants at a temperature of 328K for CV, RB, and MB dyes were 0.1063 min − 1 , 0.0758 min − 1 , and 0.0447 min − 1 , respectively.The corresponding R 2 values were 0.838, 0.8937, and 0.9761, indicating that the rst-order reaction was well-suited for all three dyes [8].
Activation energy studies: The determination of activation energy offers valuable insights into the kinetics of dye photo catalytic degradation.This contributes to a better comprehension of the underlying mechanisms and aids in optimizing photo catalytic processes for wastewater treatment.The analysis of activation energy indicated that the degradation of CV dye had the lowest activation energy, implying that the degradation process for CV is characterized by a lower energy barrier in comparison to RB and MB dyes.This observation can be bene cial for comprehending the comparative responsiveness of various dyes and enhancing the process of photo catalytic degradation.The reusability study involved subjecting the A.Cepa-ZnONP photo catalyst to many deterioration cycles in a controlled laboratory environment.After each deterioration cycle, the photo catalyst was carefully retrieved, subjected to comprehensive examination, and then used for following degradation tests.Upon reusability testing, the A.Cepa-ZnONP photo catalyst demonstrated exceptional stability, maintaining its elevated catalytic activity after numerous degradation cycles.In order to replicate real-life conditions, the reusability of the photo catalyst was assessed by employing the identical dyeing wastewater for every cycle.
Following each run, the A.Cepa-ZnONP underwent a meticulous recovery procedure, which included numerous rinses with distilled water.Afterward, the A.Cepa-ZnONP was subjected to a desiccation process in a high-temperature oven at 80°C for 12 hrs before to its use in further procedures.Signi cantly, three successive trials were carried out using the previously utilized photo catalyst under the most favorable circumstances.Despite a slight decrease in e ciency during the second and third cycles, the overall performance demonstrated the reusability of the photo catalyst, indicating its potential for longterm environmental remediation.
Comparison study: The catalytic activity of A.Cepa-ZnONP for the degradation of CV, RB and MB dyes in dyeing wastewater was compared with some other photo catalysts in the literature.

Conclusion
Zinc oxide nanoparticles were synthesized using the outer layer of Allium Cepa.The SEM-EDX analysis has con rmed the formation of ZnONP, which has an estimated size of approximately 100 nm.The X-ray diffraction (XRD) analysis demonstrates that A.Cepa-ZnONP possesses a hexagonal wurtzite crystal structure.The recorded measurements for the zone of inhibition, speci cally 25 mm, 22 mm, 20 mm, and 20 mm for the test organisms EC, PA, BC, and SA respectively, demonstrate the antibacterial effectiveness of A.Cepa-ZnONP.The A.Cepa-ZnONP shown signi cant e cacy in the degradation of CV, RB, and MB dyes present in dyeing e uent.The optimized process settings resulted in degradation percentages of 99.21%, 95.63%, and 92.34% for CV, RB, and MB dyes, respectively.The rate constants for the CV, RB, and MB dyes at a temperature of 328K were found to be 0.1063, 0.0758, and 0.0447 min-1, respectively.The activation energy values for CV, RB, and MB dyes were determined to be 12.28, 18.437, and 50.623 kJ/mol, respectively.The stability and reusability of A.Cepa-ZnONP were examined throughout the course of three experimental runs.

Declarations
Figures reaction rate constant, C o = initial dye concentration, C t = dye concentration at the reaction time t.

Table 1
Concentrations of CV, RB, and MB present in dyeing wastewater.

Table 4
Comparison of catalytic activity of skin of A.Cepa-ZnONP for the degradation of CV, RB and MB dyes