Description:1
FORM 2
THE PATENTS ACT 1970 (39 of 1970)
&
THE PATENTS RULES, 2003 COMPLETE SPECIFICATION
(See section10 and rule 13)
1. TITLE OF THE INVENTION
Process for Green Synthesis of Zinc Oxide Nanoparticles Using Allium Cepa Peel Extract for Removal of Cr(VI) from Contaminated Aqueous Solutions
2. APPLICANT
Sr. No.
NAME
NATIONALITY
ADDRESS
1.
Renuka Gupta
Indian
Department of Environmental Sciences, J.C. Bose University of Science & Technology, YMCA, Faridabad, Haryana, India
(i) PREAMBLE TO THE DESCRIPTION
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.
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FIELD OF INVENTION
The present invention relates to the field of environmental nanotechnology and wastewater treatment. More particularly, the invention pertains to a green and sustainable process for synthesizing zinc oxide nanoparticles (ZnO-O NPs) using Allium cepa (onion) peel extract via the co-precipitation method. The invention integrates principles of green chemistry and resource valorization to develop eco-friendly nanomaterials for industrial effluent remediation and pollution control.
BACKGROUND
Water is undoubtedly a necessity for life, and due to the growing world population, water demand has significantly increased for various uses. Therefore, scientists and experts are concerned about the sanctity of water sources consistently being poisoned with bio-resilient and toxic contaminants from numerous sectors.
Water contamination by heavy metals is a significant global concern due to the toxicity, persistence, and bioaccumulative nature of these pollutants. Among various heavy metals, hexavalent chromium [Cr(VI)] is particularly hazardous, known for its carcinogenic, mutagenic, and teratogenic effects on human health and its detrimental impact on aquatic ecosystems. Cr(VI) is commonly found in industrial wastewater originating from electroplating, leather tanning, textile dyeing, metal processing, and pigment manufacturing industries.
Various conventional techniques have been employed for the removal of Cr(VI) from contaminated water, including chemical precipitation, ion exchange, reverse osmosis, membrane filtration, electrochemical reduction, and adsorption. However, many of these methods suffer from limitations, such as high operational cost, complexity, limited selectivity, secondary pollution, and difficulty in the regeneration or disposal of spent materials.
In recent years, nanotechnology has emerged as a promising alternative for water treatment, offering high surface-to-volume ratio, tunable surface properties, and enhanced reactivity. Among the various nanomaterials studied, zinc oxide nanoparticles (ZnO-O NPs) usingAllium Cepapeel extract have gained significant attention due to their favorable properties, such as chemical stability, non-toxicity, photocatalytic activity, and antimicrobial effects.
While chemically synthesized ZnO-O nanoparticlesusingAllium Cepapeel extract have shown potential in the adsorption and degradation of pollutants, their production typically involves the use of toxic reagents and energy-intensive processes that are not environmentally benign. To address this, researchers have shifted focus toward green synthesis approaches, where plant-based extracts are used
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as reducing and stabilizing agents in nanoparticle formation. These methods align with the principles of green chemistry, offering a sustainable and safer route for nanoparticle fabrication.
Onion (Allium cepa) peels, a widely available kitchen biowaste, are rich in natural phytochemicals, such as flavonoids, phenolics, and sulfur compounds, which can serve as effective reducing and capping agents during nanoparticle synthesis. Despite their abundance, onion peels are typically discarded without utilization, contributing to organic waste buildup.
Accordingly, there exists a need for a cost-effective, eco-friendly, and scalable process for synthesizing ZnO nanoparticles using bio-waste materials like Onion (Allium cepa) peels that not only reduce environmental burden but also provide a high-performance adsorbent for Cr(VI) remediation.
The present invention addresses this need by providing a green co-precipitation method for synthesizing ZnO nanoparticles using Allium cepa peel extract and by evaluating their performance as nano-adsorbents for efficient and reusable Cr(VI) removal from aqueous systems.
OBJECTS OF THE INVENTION
The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available techniques and processes.Therefore, the current invention successfully overcomes all of the above-discussed shortcomings present in the art.
1. It is an object of the invention to develop an eco-friendly and sustainable process for synthesizing zinc oxide nanoparticles (ZnO NPs) using Allium cepa (onion) peel extract via a green co-precipitation method.
2. It is an object of the invention to utilize onion peel waste as a bio-reducing and stabilizing agent in nanoparticle synthesis, thereby promoting value addition to agricultural and kitchen biowaste in line with circular economy principles.
3. It is an object of the invention to produce ZnO nanoparticles with enhanced physicochemical properties, such as reduced particle size (~18.65 nm), increased surface area (5.98 m²/g), and improved morphology for efficient Cr(VI) adsorption.
4. It is an object of the invention to provide a nanoparticle-based adsorbent system capable of removing Cr(VI) ions from aqueous media under optimized conditions of pH, contact time, adsorbent dosage, and initial metal concentration.
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5. It is an object of the invention to investigate and establish adsorption isotherms and kinetics, especially demonstrating pseudo-second-order adsorption kinetics and fitting to both Langmuir and Freundlich isotherm models.
6. It is an object of the invention to examine the reusability and regeneration potential of the synthesized ZnO-O nanoparticles for Cr(VI) adsorption over multiple cycles, ensuring practical feasibility in industrial settings.
7. It is an object of the invention to characterize the synthesized nanoparticles using advanced analytical techniques, including XRD, TEM, BET, FTIR, DLS, and PZC to confirm structural, surface, and functional properties.
8. It is an object of the invention to offer a cost-effective and scalable solution for industrial wastewater treatment, particularly targeting hexavalent chromium contamination in effluents from tanneries, electroplating, and dyeing industries.
How the foregoing objects are achieved will be clear from the following brief description. In this context, it is clarified that the description provided is non-limiting and is only by way of explanation. Other objects and advantages of the invention will become apparent as the foregoing description proceeds, taken together with the accompanying drawings and the appended claims.
SUMMARY
This summary is provided to introduce a selection of concepts in a simplified format that is further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention nor is it intended for determining the scope of the invention.
The present invention relates to a green, sustainable, and cost-effective process for synthesizing ZnO NPs using Allium cepa (onion) peel extract via a co-precipitation method. The synthesized nanoparticles, referred to as ZnO-O NPs, exhibit enhanced physicochemical properties and show high efficiency in the removal of hexavalent chromium [Cr(VI)] from aqueous solutions.
Cr(VI) is a highly toxic and carcinogenic pollutant commonly released into the environment through industrial effluents from electroplating, tanning, and textile processing. Existing Cr(VI) removal methods, such as chemical precipitation, ion exchange, and membrane filtration, are
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often expensive, energy-intensive, and generate secondary waste. Nanotechnology-based adsorbents, particularly ZnO NPs, offer a promising alternative due to their high surface area and reactivity. However, conventional ZnO synthesis methods rely on toxic chemicals and harsh conditions.
This invention addresses these issues by utilizing Allium cepa peel waste—an abundant biowaste rich in phytochemicals—as a natural reducing and stabilizing agent for ZnO NP synthesis. This green route minimizes chemical usage, supports waste valorization, and aligns with circular economy principles.
The process involves preparing an onion-peel extract by boiling 5 g of powdered peels in 100 mL of distilled water at 90°C for 2 hours. Simultaneously, 6.4 g of zinc nitrate hexahydrate is dissolved in 225 mL water, and the pH is adjusted to 10 using 1.8 g NaOH dissolved in 450 mL water. Under continuous stirring at 550 rpm, 10 mL of the peel extract was added to the zinc nitrate solution. The mixture is allowed to precipitate, centrifuged, dried at 60°C, and calcined at 200°C to yield ZnO-O nanoparticles.
Characterization studies confirmed an average particle size of 18.65 nm (XRD), a surface area of 5.98 m²/g (BET), and the presence of functional groups like OH, CO, and amide (FTIR). TEM revealed mixed spherical and rod-like morphologies, and the pHpzc was determined to be 5.1, which is favorable for Cr(VI) adsorption at acidic pH.
Batch adsorption experiments demonstrated maximum Cr(VI) removal of 95.7% under optimal conditions: pH 3, 0.5 g adsorbent dose, 90 minutes contact time, and 30 mg/L Cr(VI) concentration. The adsorption behavior follows the pseudo-second-order kinetic model (R² = 0.997) and fits both Langmuir (qmax = 32.67 mg/g) and Freundlich isotherms (1/n = 0.618).
Reusability tests confirmed that ZnO-O NPs retained 49.6% removal efficiency after five cycles using ethanol as a desorbing agent, indicating good regeneration potential.
In conclusion, the invention offers a scalable and environmentally friendly solution for Cr(VI) remediation using bio-assisted ZnO nanoparticles. It provides high removal efficiency, reusability, and waste valorization—making it suitable for industrial wastewater treatment and adaptable to other heavy metal contaminants.
To further clarify the advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which
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are illustrated in the appended figures. It is appreciated that this figure depicts only typical embodiments of the invention and are therefore not to be considered limiting to its scope. The invention will be described and explained with additional specificity and detail with the accompanying figure.
BRIEF DESCRIPTION OF FIGURES
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying figures in which like characters represent like parts throughout the figures, wherein:
Figure 1, illustrates a view of a schematic illustration of ZnO and ZnO-O NPs synthesis for the present invention.
Figure 2, illustrates a view of FESEM and EDX of (a) ZnO and (b) ZnO-O NPs for the present invention.
Figure 3, illustrates a view of TEM (a-b) and average particle size (c-d) of ZnO and ZnO-O NPs for the present invention.
Figure 4, illustrates a view of FTIR (a-b) spectra and XRD (c-d) of ZnO (a, c) and ZnO-O NPs (b, d) for the present invention.
Figure 5, illustrates a view of N2 adsorption-desorption isotherm (a-b) and pore size distribution (c-d) plot of ZnO (a & c) and ZnO-O (b &d) NPs for the present invention.
Figure 6, illustrates a view of DLS of (a) ZnO and (b) ZnO-O NPs for the present invention.
Figure 7, illustrates a view of pHpzc of (a) ZnO and (b) ZnO-O NPs for the present invention.
Figure 8, illustrates a view of Effect of (a) pH, (b) metal concentration & time, (c) dose, and (d) temperature on removal of Cr(VI) by ZnO and ZnO-O NPs for the present invention.
Figure 9, illustrates a view of Various isotherms, such as (a-b) Langmuir, (c-d) Freundlich, and (e-f) Temkin isotherms performed for ZnO & ZnO-O NPs for the present invention.
Figure 10, illustrates a view of Pseudo-first-order (a-b) and Pseudo-second-order (c-d) kinetics of ZnO (a, c) & ZnO-O (b, d) NPs for the present invention.
Figure 11, illustrates a view of Reusability of ZnO & ZnO-O NPs up to five consecutive cycles for the present invention.
Figure 12,illustrates a view of Interaction mechanisms for Cr(VI) removal and ZnO/ZnO-O NPs for the present invention.
Further, skilled artisans will appreciate that elements in the figures are illustrated for simplicity and may not necessarily have been drawn to scale. For example, the flowcharts illustrate the method in terms of the most prominent steps involved to help to improve understanding of aspects of the present invention.
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DETAILED DESCRIPTION:
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the figures, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.
Reference throughout this specification to “an aspect”, “another aspect,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase “in an embodiment”, “in another embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or systems or elements or structures or components proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other systems or other elements or other structures or other components or additional devices or additional systems or additional elements or additional structures or additional components.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.
The terms “a” and “an” herein do not denote a limitation of quantity but rather denote the presence of at least one of the referenced items.
The terms “having”, “comprising”, “including”, and variations thereof signify the presence of a component.
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Now, the present invention will be described below in detail with reference to the following embodiment.
Example 1:Synthesis of zinc oxide nanoparticles
Onion (Allium cepa) peels are collected from the kitchen. After many rinses with distilled water, the onion peels are subjected to overnight drying at a temperature of 100ºC. The dried peels are powdered with a motor pestle and sieved through a 2 mm molecular sieve. 5 g peels powder is dissolved in 100 mL distilled water at 90ºC for 2 h, and the prepared extract is stored for further use. The solutions are prepared by dissolving 6.4 g of Zn(NO3)2.6H2O in 225 mL distilled water and 1.8 g NaOH in 450 mL distilled water. The pH of the Zn(NO3)2.6H2O solution is made up to 10 by employing NaOH dropwise. Following that, it is stirred for 30 min at 550 rpm; meanwhile, 10 mL of onion peel extract is added to the solution. The solution is kept for 45 min for precipitation and then centrifuged at 4000 rpm. The prepared precipitates are primarily heated at 60ºC for 24 h; afterwards, they are heated at high temperatures, i.e., 200ºC for 2 h (Figure 1).
Example2: Metal adsorption experiments
The batch mode experiments are performed for the removal of Cr(VI) from aqueous solution. A stock solution of 1000 mg/L (ppm) is prepared by dissolving 2.82 g K2Cr2O7 in 1 L distilled water, and the subsequent standards/solutions are prepared. The present invention examined the effects of multiple parameters, such as pH, contact time, temperature, adsorbent dosage, and initial metal ion concentration on the adsorption process. The pH of the solution is altered using 0.1 M HCl and 0.1 M NaOH. For a predetermined duration, the adsorption process is conducted within a 250 mL Erlenmeyer flask, with continuous agitation provided by an orbital shaker. The metal ion concentrations before and after adsorption are quantified by a Microwave Plasma Atomic Emission Spectrophotometer (MPAES). The metal removal percentage and adsorption capacity are calculated using Eq (1) & Eq (2):
Metal Removal percentage (%) = ????????-????????????????????????????? (Eq 1)
Adsorption capacity (qe) = ??????????-?????????????????? (Eq2)
Where Co (mg/L) represents the initial concentration of metal ions, Cf (mg/L) represents the final concentration, V (L) represents the volume of the solution, and m (g) indicates the mass of the adsorbent.
The adsorption experiments are subjected to Langmuir, Freundlich, and Temkin isotherms and kinetics modelling studies to inventZnO and ZnO-ONPs behavior and pathways.
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Example3: Surface morphology and characteristics of ZnO and ZnO-O NPs
The FESEM and EDX microphotographs of ZnO and ZnO-O NPs are used at various magnifications to analyze the texture, surface morphology, and elemental distribution. A coating of gold before scanning is conducted to mitigate surface charging and facilitate secondary electron emission so that the NPs behave evenly and provide a homogenous surface for imaging and analysis. The FESEM images revealed that the ZnO and ZnO-O NPs have rod-like structures, as portrayed in Figure 2.The average size for NPs is <30 nm and <20 nm for ZnO and ZnO-O, respectively. However, upon the modification of ZnO NPs with onion extract, a spherical-shaped structure with a size range of 40–120 nm is observed.
Transmission electron microscopy (TEM) is employed to examine the morphology of NPs at various dimensions. The invention revealed that the ZnO NPs exhibited a size range of 25-30 nm, while the ZnO-O NPs displayed a size range of 15-19.5 nm. The mean size for ZnO and ZnO-O NPs is 25.78 nm and 16.15 nm, respectively (Figure 3). The NPs are somewhat spherical and aggregated with rod-like structures. The similar particle morphology is also confirmed through the FESEM image.
The FTIR spectrum helps detect NP’s composition by determining the functional groups involved in Cr(VI) binding on ZnO and ZnO-O NPs. Figure 4a-b presents the FTIR spectra of ZnO and ZnO-O NPs. The presence of ZnO stretching can be inferred from the absorption bands at 452.45 cm-1 in the FTIR spectra of ZnO NPs. The stretching at 1454 cm-1 is due to C-H or –C=C stretching. A bending vibration of hydroxyl groups (-OH) can be inferred from the adsorption band observed within the 1320-1350 cm-1 spectral range. A sharp peak in the 540-420 cm-1 range corresponds to the stretching of ZnO. The presence of an amide group in the onion-peel-derived NPs is responsible for the band at 1346 cm-1, but the presence of C = O stretching causes a deep and sharp band at 1496 cm-1. A wide band detected at 3426 cm-1 suggests that the produced ZnO-O NPs include hydroxyl molecules. Thus, it is evident from FTIR graphs that ZnO and ZnO-O NPs are properly synthesized.
X-ray diffraction analysis (XRD) is a method that facilitates the acquisition of data about the crystal structure, physical characteristics, and chemical composition of NPs. The diffraction patterns of X-rays for both ZnO and ZnO-O NPs at specific angles, namely 2? (theta), including 31º, 34º, 36º, 47º, 56º, 62º, and 68º, corresponding to crystal planes 100, 002, 101, 102, 110, 103, and 112, are illustrated in Figure 4c-d. It is also evident that the onion extract did not affect the synthesis of ZnO and the crystalline phase. The average NP size for ZnO and ZnO-O are 28 nm and 18.65 nm, respectively, and their computation is based on the Debye-Scherer equation, as shown below:
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????=????????????.?????? ???????? (???????? ????)
Where "D" represents the average crystalline structure size, "?" stands for CuKa radiation (0.154 nm), "K" denotes the Debye-Scherer constant, and the symbol "ß" is used to represent the full width at half maximum (FWHM). In contrast, "?" is employed to indicate Bragg's angle.The degree of crystallinity shown by the NPs is 69.37% for ZnO NPs and 78.05% for ZnO-O NPs, calculated by the formula: ????????????????????????????????????????????????????=?????? ?????? ?????????? ?????? ?????? ?????????? ?????????????? ?????????? ????????????? (???????? ????)
Bragg's law is used to calculate d-spacing. The d-spacing for ZnO NPs is 0.189 nm, and for ZnO-O NPs, it is 0.206 nm, as per Bragg's Law:
??
=?????? ?????? (Eq 5)
Within this framework, the variable "n" denotes the order of diffraction, typically taking on the value of 1. The symbol "?" represents the wavelength of the X-ray, while "d" symbolizes the separation distance between two atomic planes. Lastly, the variable "?" denotes the peak location, measured in radians.
Example4: BET analysis and DLS
The BET analysis is a physical characterization of NPs that provides quantitative data on porosity distribution and specific surface area. The size distribution of pores of an adsorbent greatly influences its adsorption efficiency. Figure 5 illustrates the nitrogen (N2) adsorption-desorption isotherms to determine the pore diameter, specific surface area, and pore volume of ZnO and ZnO-O NPs. Among the six adsorption isotherm curve types, the ZnO and ZnO-O NPs graph represent Type-II isotherm followed in the P/Po range of 0.2-0.99. The observed forces result from the adsorbate and adsorbent combination-interaction activities and form a mesoporous structure. The measured specific surface area of ZnO NPs is found to be 3.05 m2/g, whereas it is 5.98 m2/g for ZnO-O NPs. On the other hand, the average pore diameter of ZnO and ZnO-O NPs is measured to be 38 nm and 19.84 nm, respectively. It is worth noting that the addition of onion extract led to the expansion of the surface area, which aids the adsorption capacity.
The Dynamic light scattering (DLS) characterization measures formulations particle size, stability and detects aggregation. The particle size is measured by scattering light from the laser and passing it through the colloidal solution. The hydrodynamic diameter for ZnO NPs is 204.6 nm, and for ZnO-O NPs, it is 340.3 nm (Figure 6).
pH at point zero charge
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The concept point of zero charge (PZC) signifies the state at which the surface of the adsorbent exhibits no overall charge and maintains a neutral condition. This occurs when the total number of positive charges (contribution of cations) and negative charges (anions) on the surface become equal, i.e., the net charge at that particular pH on the adsorbent surface is zero. It is predicted that the surface of adsorbent exhibits a higher positive charge when pHpHpzc. The technique employed to ascertain the pHpzc is the salt addition method. This experimental procedure introduced 0.5 g of ZnO & ZnO-O NPs into separate flasks containing 100 mL of 0.1 M NaNO3 solution each. The pH of each flask varied in the range of 2-12 while using 0.1 M NaOH and 0.1 M nitric acid (HNO3). Following that, the flasks are positioned in an orbital shaker and subjected to agitation for 48 h at 180 rpm. After 48 h, the change in pH is noted. A graph is plotted between the initial pH and ?pH (final-initial pH) to determine the pHpzc, as shown in Figure 7. The pHpzc for ZnO &ZnO-O NPs are found to be 6.4 and 5.1, corresponding to the near neutral and slightly acidic surface of synthesized NPs, respectively.
Effect of pH
The solution's pH levels significantly influence the surface charge and morphology of adsorbents, including metal binding, adsorption capacity, and selectivity. In the present invention, the effect of pH on Cr(VI) adsorption via ZnO and ZnO-O NPs is studied in the range of 3 -13. The other parameters are adsorbent dose 0.5g, metal concentration 30 mg/L, and temperature 35±2ºC at 180 rpm. The removal of Cr(VI) by ZnO NPs dropped from 89.5% to 54.4% as pH increased from 3 to 13, while in the case of ZnO-O NPs, reduction is from 95.7% to 56.2% (Figure 8a). The Cr(VI) adsorption is maximum at pH 3 for both the adsorbents. The removal of Cr(VI) is higher at low pH as it is reduced to Cr(III). The exception is shown at extremely acidic pH values (~2), where the difference in Cr(VI) present form and the abundance of H+ ions may obstruct ZnO and ZnO-O NPs active sites, resulting in a reduced removal efficiency. At acidic pH levels, Cr exists in two forms: as chromic acid (H2CrO4-) within the pH range of 1–2 and as hydrogen chromate ions (HCrO4-) within the pH range of 3–7. Specifically, the surface of ZnO NPs becomes protonated by hydrogen ions under strong acidic conditions, resulting in a positively charged surface that facilitates electrostatic attraction with Cr(VI) in the form of HCrO4-. The removal efficiency of Cr(VI) by ZnO NPs decreases under alkaline conditions due to the presence of Cr(VI) in the form of chromate ions (CrO42-), which competes with the hydroxide ions on the sorbent surface. It is worth mentioning that Cr(VI) adsorption capacity is higher at acidic pH 3, and its subsequent decrease with increasing pH values is due to the gradual conversion of CrO42- and Cr2O72- ions.
Effect of metal concentration and time
The effect of initial metal ion concentration on adsorption process and the equilibrium time attained is investigated by varying the initial Cr(VI) concentration of 10, 30 and 50 mg/L at time intervals (30, 60, 90, 120 and 150 min) and other constant parameters, i.e., pH 3, adsorbent dose 0.5g,
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temperature 35±2 ºC and rpm 180. It is found that ZnO and ZnO-O NPs achieved a maximum Cr(VI) removal percentage of 96.4% and 97.4%, respectively, for 10 mg/L at 90 min (Figure 8b). The results showed that as the concentration of Cr(VI) increased from 10 to 50 mg/L, the removal percentage decreased from 96.4% to 75.5% for ZnO and 97.4% to 81.5% for ZnO-O NPs at 120 min. The results showed that at a metal concentration of 10 mg/L, the equilibrium is achieved in the least time.
Effect of adsorbent dose
The effect of adsorbents dose on Cr(VI) removal is investigated at varying doses of ZnO and ZnO-O NPs (0.2, 0.4, 0.5, 0.6, 0.8 and 1.0 g). The findings revealed that Cr(VI) removal by ZnO and ZnO-O NPs increased initially with the increase in adsorbent dose from 0.2 g to 0.5 g with removal efficiency from 9.87% - 89.5% and 13.4% - 97.6%, respectively as shown in Figure 8c, afterwards, there is no further change in adsorption capacity in both cases with increment dose from 0.5 g - 1.0 g.
Effect of temperature
The effect of temperature on adsorbent performance for Cr(VI) removal is studied at a temperature range of 25º-45ºC. The increase in temperature from 25ºC to 45ºC resulted in the enhancement of Cr(VI) removal from 79% to 89.7% by ZnO NPs and 84.03% to 96.07% by ZnO-O NPs. Figure 8d exhibited that the adsorption of Cr(VI) increased with an increase in temperature with both the adsorbents used in the invention.
Example5: Isotherm adsorption analysis
The analysis of adsorption isotherms is crucial for advancing adsorption pathways and mechanisms. Linear regression interpretation is the most used tool for determining a fitted model because it evaluates adsorption systems and verifies theoretical assumptions of isothermal models. Langmuir, Freundlich, and Temkin isotherm models are applied to invent the adsorption behavior and interactions of Cr(VI) and synthesized adsorbents. The following expression represents the linear representation of the Langmuir isotherm:
????????????????=1????????????????????????+???????????????????????? (?????? 8)
The equation presented above involves the Ce (mg/L) denoting equilibrium concentration of the adsorbate. The qe, qmax, and KL (L/mg) expressed the adsorption capacity, maximum monolayer adsorption capacity (mg/g), and Langmuir constant, respectively. The 1/qe vs. 1/Ce plots for Cr(VI) adsorption onto ZnO and ZnO-O NPs have been graphically presented for the Langmuir isotherm model (Figure 9a-b). The correlation coefficient and Langmuir parameters for both NPs are presented in Table 1. The maximum adsorption capacity of ZnO-O NPs is found to be 32.67 mg/g,
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which is higher than the 14.58 mg/g observed for ZnO NPs. Using the linear calibration curve with the equation absorbance, the correlation coefficient (R2) is 0.973 and 0.982 in the case of ZnO and ZnO-O NPs, respectively.
The degree of adsorption on the adsorbent can be elucidated through the use of the dimensionless parameter denoted as the separation factor (RL) as per the following mathematical equation:
????????=????????+?????????????? (????????????)
where Ci (mg/L) is the initial concentration of the solution. The RL value for Cr(VI) adsorption by ZnO and ZnO-O NPs is 0.09 and 0.063, respectively. As obvious from the results, RL values ranged between 0 and 1, indicating that the process of Cr(VI) adsorption is favorable.
The adsorption processes on surfaces with diverse characteristics are well described by the empirical Freundlich adsorption isotherm model. The isotherm characterizes the presence of heterogeneity on the adsorbent surface, along with the exponential distribution of active binding sites. The representative equation is as below:
??
?????? ?? =?? ????????????+?????? ?? ?????????? (????????????????)
The Freundlich constant, associated with the adsorption capacity, is represented in the equation by Kf(L/mg), and the adsorption intensity, which indicates a measure of heterogeneity, is denoted by 1/n. For an adsorption process to be deemed successful, the slope (1/n) must fall between 0 and 1. If the value of the heterogeneity factor is close to 0, it shows that the surface is more heterogeneous. The parameters of the Freundlich adsorption isotherm are determined by plotting the log qe vs log Ce (Figure 9c-d). Table 1 illustrates the calculated values of 1/n and the correlation coefficient. As shown in Table 1, the value of 1/n is between 0 and 1, indicating that the adsorption process is favorable. A higher 1/n value as a result of Cr(VI) adsorption on ZnO-O is observed as compared to ZnO NPs. The relative adsorption capacity, KF, for Cr(VI) adsorption onto ZnO-ZnO-O NPs is higher 0.786, indicating a higher adsorption intensity compared to metal adsorption onto ZnO NPs (0.693). Further, a value of 1/n less than 1 represents the chemisorption of Cr(VI) for both NPs in the present invention.
The Temkin adsorption isotherm explains the effect of indirect interaction between adsorbate and adsorbent on the adsorption process. This isotherm is valid for only an intermediate range of ion concentration. The following equation 11 represents the linear form of the Temkin adsorption isotherm model:
????????=?????????????????????????? +???????????????????????????? (?????? 11)
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Here, the parameter "b" (Jmol-1) is associated with the heat of sorption, "R" is used to designate the universal gas constant, and "T" is employed to represent the absolute temperature (K). The equilibrium energy binding constant is denoted as "KT" (L/g). The parameters and correlation coefficient are given in Table 1, while the plot ln Ce vs. qe is shown in Figure 9e-f.The results as per the Temkin isotherm model the physical adsorption of Cr(VI) removal by ZnO and ZnO-O NPs. The low binding energy in the case of Cr(VI) adsorption onto ZnO indicated a weak interaction between the adsorbate and the biosorbent as compared to ZnO-O NPs. The low R2 values for both NPs suggested that Cr(VI) adsorption does not closely follow the Temkin isotherm (Table 1).
Table 1: Adsorption Isotherm constants for Cr(VI) adsorption on ZnO and Zn-O NPs
Parameters Values
Isotherms
Parameters
ZnO
ZnO-O
Langmuir
qmax
14.58
32.67
KL
0.303
0.493
RL
0.09
0.063
R2
0.973
0.982
Freundlich
1/n
0.378
0.618
Kf
0.693
0.786
R2
0.903
0.98
Temkin
B
3.018
3.449
KT
1.810
4.450
R2
0.875
0.925
Adsorption kinetics analysis
The pseudo-first-order and pseudo-second-order models are the most commonly used kinetic models. Lagergren's pseudo-first-order model describes the kinetic rate of the liquid and solid phase during the adsorption process in terms of adsorption capacity. This linear form of the model, which is frequently essential during the first stage of the adsorption process, is as follows: ?? ????(?? ?? -?? ?? )=?? ?????? ?? -??????????????.?????? ?????? (???????? ?????? )
In the above equation, "qe" and "qt" denote the adsorption capacity at equilibrium time and contact time t, and k1 (min-1) represents the equilibrium rate constant of the pseudo-first-order model. The value of k1 can be determined by calculating the slope of the linear graph of log (qe- qt) as a function
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of time (Figure 10a-b). Various parameters for a pseudo-first-order model for ZnO and ZnO-O NPs are illustrated in Table 2. The values of pseudo-first-order model's regression coefficients (R2) are low for both the adsorbents as shown in Table 2, indicating that the pseudo-first-order model is insufficient to describe the mechanism of uptake of Cr(VI) onto the adsorbents in the present invention.
Table 2: Kinetic model parameters for adsorption of Cr(VI) onto ZnO and ZnO-O NPs
Kinetic Order
Parameters
ZnO
ZnO-O
Pseudo-first-order
qe(mg/g)
0.803
0.830
K1
-3.27
-3.35
R2
0.913
0.910
Pseudo-second-order
qe (mg/g)
5.924
6.397
K2
0.010
0.008
R2
0.995
0.997
The pseudo-second-order model assumes that the adsorption rate depends on the adsorption capacity of the adsorbent, not on the adsorbate concentration, and that chemisorption is the limiting step. The equation (13) describes the linear version of the pseudo-second-order model as follows: ?? ?? ?? =?????????????? ??????+?? ?? ?? (???????? ?????? )
Where qt and qe are adsorption capacity at time (t) and equilibrium time, and k2 (min-1) is the equilibrium rate constant for the pseudo-second-order kinetic model (Figure 10c-d). The values of qe, qt, and rate constant k2 for both NPs. The results showed that the adsorption of Cr(VI) is best followed by pseudo-second-order kinetics, as indicated by high R2 values 0.995 for ZnO and 0.997 for ZnO-O NPs, as illustrated in Table 2.
Reusability analysis
The reusability/regeneration experiment investigated the removal efficiencies of both ZnO and ZnO-O NPs for Cr(VI) ions. Ethanol is used as a desorbing agent in the invention. The immediate coloration of ethanol showed the desorption of Cr(VI) from the surface of adsorbents ZnO and ZnO-O NPs. There are five repetitions for the complete cycles. The removal efficiency gradually decreased from 89.5% in the case of ZnO NPs to 42.3%, while in ZnO-O NPs, the percentage declined from 95.7% to 49.6% (Figure 11).
16
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching.
The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the spirit or scope of the present invention.
17
CLAIMS
WE CLAIM,
1. A process for green synthesis of zinc oxide nanoparticles (ZnO-O NPs) using Allium cepa peel extract for removal of hexavalent chromium [Cr(VI)] from aqueous solutions, the said process comprising the steps of:
i. Collecting onion (Allium cepa) peels from domestic or institutional kitchen waste;
ii. Washing the peels thoroughly with distilled water to remove surface impurities and drying them at 100°C for 24 hours;
iii. Powdering the dried peels and sieving through a 2 mm mesh to obtain uniform peel powder;
iv. Preparing the onion peel extract by mixing 5 grams of the peel powder in 100 mL distilled water and heating at 90°C for 2 hours, followed by filtration to obtain a clear extract containing bioactive compounds;
v. Preparing a zinc precursor solution by dissolving 6.4 grams of zinc nitrate hexahydrate [Zn(NO3)2·6H2O] in 225 mL distilled water;
vi. Preparing a sodium hydroxide (NaOH) solution by dissolving 1.8 grams of NaOH in 450 mL distilled water;
vii. Adjusting the pH of the zinc nitrate solution to 10 by adding NaOH solution dropwise under continuous stirring at 550 rpm;
viii. Adding 10 mL of onion peel extract to the zinc nitrate solution while stirring and maintaining the reaction at room temperature for 30 minutes;
ix. Allowing the mixture to undergo precipitation for 45 minutes and centrifuging the resulting solution at 4000 rpm to collect the nanoparticle precipitate;
x. Drying the obtained precipitate at 60°C for 24 hours followed by calcination at 200°C for 2 hours to yield the bio-fabricated ZnO-O nanoparticles;
xi. Characterizing the synthesized ZnO-O nanoparticles using techniques selected from: X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) analysis, and Dynamic Light Scattering (DLS);
xii. Utilizing the synthesized ZnO-O nanoparticles as nano-adsorbents in batch mode experiments for the removal of Cr(VI) ions from aqueous solutions,
18
wherein the final product is ZnO-ONPs having an average size of approximately 18.65 nm, surface area of 5.98 m²/g, and Cr(VI) removal efficiency of up to 95.7% under optimized conditions comprising pH 3, 0.5 g adsorbent dosage, 90 minutes contact time, and 30 mg/L Cr(VI) concentration.
2. The process as claimed in claim 1, wherein the zinc oxide nanoparticles synthesized using Allium cepa peel extract exhibit a rod-like and spherical morphology with particle sizes ranging from 15 nm to 19.5 nm, as determined by Transmission Electron Microscopy (TEM).
3. The process as claimed in claim 1, wherein the synthesized ZnO-O nanoparticles possess a specific surface area of 5.98 m²/g and average pore diameter of 19.84 nm, as determined by Brunauer–Emmett–Teller (BET) surface area analysis.
4. The process as claimed in claim 1, wherein the point of zero charge (pHpzc) of ZnO-O nanoparticles is 5.1, making them optimally active for Cr(VI) adsorption at a solution pH less than 5.1 due to favorable electrostatic interactions.
5. The process as claimed in claim 1, wherein the adsorption kinetics follow a pseudo-second-order model with a correlation coefficient (R²) of 0.997, indicating that the adsorption is governed by chemisorption involving valence forces through sharing or exchange of electrons.
6. The process as claimed in claim 1, wherein the adsorption equilibrium data for Cr(VI) removal fit Langmuir isotherm model, with maximum monolayer adsorption capacity (qmax) of 32.67 mg/g.
7. The process as claimed in claim 1, wherein the synthesized ZnO-O NPs demonstrate reusability for up to five adsorption-desorption cycles, retaining Cr(VI) removal efficiency from 95.7% in the first cycle to 49.6% in the fifth cycle, using ethanol as a desorbing agent.
8. The process as claimed in claim 1, wherein the optimal operational parameters for maximum Cr(VI) removal include a pH of 3.0, a contact time of 90 minutes, a nanoparticle dose of 0.5 g per 100 mL of solution, and an initial Cr(VI) concentration of 30 mg/L.
Dated this 14th day of April, 2025
SHIVANI (IN/PA-4661)
AGENT FOR THE APPLICANT(s)
19
ABSTRACT
Process for Green Synthesis of Zinc Oxide Nanoparticles Using Allium Cepa Peel Extract for Removal of Cr(VI) from Contaminated Aqueous Solutions
The present invention relates to a green and sustainable process for synthesizing zinc oxide nanoparticles (ZnO NPs) utilizing Allium cepa (onion) peel extract via the co-precipitation method, and their application in the removal of hexavalent chromium [Cr(VI)] from aqueous solutions. The process involves the preparation of onion peel extract as a reducing and stabilizing agent, followed by the reaction with zinc nitrate and sodium hydroxide under controlled pH and temperature conditions to yield ZnO nanoparticles. The synthesized bio-assisted ZnO-O nanoparticles exhibit a particle size of approximately 18.65 nm, an enhanced surface area of 5.98 m²/g, and favorable adsorption properties. Characterization is performed using techniques including XRD, TEM, BET, FTIR, and DLS. Batch adsorption experiments show a Cr(VI) removal efficiency of up to 95.7% under optimal conditions: pH 3, 0.5 g dosage, 90 minutes contact time, and 30 mg/L initial Cr(VI) concentration. Adsorption follows the Langmuir and Freundlich isotherms and pseudo-second-order kinetics. The invention offers an eco-friendly, cost-effective, and reusable nano-adsorbent solution for industrial wastewater treatment. , Claims:WE CLAIM,
1. A process for green synthesis of zinc oxide nanoparticles (ZnO-O NPs) using Allium cepa peel extract for removal of hexavalent chromium [Cr(VI)] from aqueous solutions, the said process comprising the steps of:
i. Collecting onion (Allium cepa) peels from domestic or institutional kitchen waste;
ii. Washing the peels thoroughly with distilled water to remove surface impurities and drying them at 100°C for 24 hours;
iii. Powdering the dried peels and sieving through a 2 mm mesh to obtain uniform peel powder;
iv. Preparing the onion peel extract by mixing 5 grams of the peel powder in 100 mL distilled water and heating at 90°C for 2 hours, followed by filtration to obtain a clear extract containing bioactive compounds;
v. Preparing a zinc precursor solution by dissolving 6.4 grams of zinc nitrate hexahydrate [Zn(NO3)2·6H2O] in 225 mL distilled water;
vi. Preparing a sodium hydroxide (NaOH) solution by dissolving 1.8 grams of NaOH in 450 mL distilled water;
vii. Adjusting the pH of the zinc nitrate solution to 10 by adding NaOH solution dropwise under continuous stirring at 550 rpm;
viii. Adding 10 mL of onion peel extract to the zinc nitrate solution while stirring and maintaining the reaction at room temperature for 30 minutes;
ix. Allowing the mixture to undergo precipitation for 45 minutes and centrifuging the resulting solution at 4000 rpm to collect the nanoparticle precipitate;
x. Drying the obtained precipitate at 60°C for 24 hours followed by calcination at 200°C for 2 hours to yield the bio-fabricated ZnO-O nanoparticles;
xi. Characterizing the synthesized ZnO-O nanoparticles using techniques selected from: X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) analysis, and Dynamic Light Scattering (DLS);
xii. Utilizing the synthesized ZnO-O nanoparticles as nano-adsorbents in batch mode experiments for the removal of Cr(VI) ions from aqueous solutions,
18
wherein the final product is ZnO-ONPs having an average size of approximately 18.65 nm, surface area of 5.98 m²/g, and Cr(VI) removal efficiency of up to 95.7% under optimized conditions comprising pH 3, 0.5 g adsorbent dosage, 90 minutes contact time, and 30 mg/L Cr(VI) concentration.
2. The process as claimed in claim 1, wherein the zinc oxide nanoparticles synthesized using Allium cepa peel extract exhibit a rod-like and spherical morphology with particle sizes ranging from 15 nm to 19.5 nm, as determined by Transmission Electron Microscopy (TEM).
3. The process as claimed in claim 1, wherein the synthesized ZnO-O nanoparticles possess a specific surface area of 5.98 m²/g and average pore diameter of 19.84 nm, as determined by Brunauer–Emmett–Teller (BET) surface area analysis.
4. The process as claimed in claim 1, wherein the point of zero charge (pHpzc) of ZnO-O nanoparticles is 5.1, making them optimally active for Cr(VI) adsorption at a solution pH less than 5.1 due to favorable electrostatic interactions.
5. The process as claimed in claim 1, wherein the adsorption kinetics follow a pseudo-second-order model with a correlation coefficient (R²) of 0.997, indicating that the adsorption is governed by chemisorption involving valence forces through sharing or exchange of electrons.
6. The process as claimed in claim 1, wherein the adsorption equilibrium data for Cr(VI) removal fit Langmuir isotherm model, with maximum monolayer adsorption capacity (qmax) of 32.67 mg/g.
7. The process as claimed in claim 1, wherein the synthesized ZnO-O NPs demonstrate reusability for up to five adsorption-desorption cycles, retaining Cr(VI) removal efficiency from 95.7% in the first cycle to 49.6% in the fifth cycle, using ethanol as a desorbing agent.
8. The process as claimed in claim 1, wherein the optimal operational parameters for maximum Cr(VI) removal include a pH of 3.0, a contact time of 90 minutes, a nanoparticle dose of 0.5 g per 100 mL of solution, and an initial Cr(VI) concentration of 30 mg/L.