Synthesis and Characterization of Parthenium hysterophorus - Mediated ZnO Nanoparticles for Methylene Blue Dye Degradation

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Introduction
Aquatic pollution by industrial efuents and emissions from wastewater treatment plants is a serious threat facing humanity and the ecosystem [1,2].Among the industrial wastewater contaminating our water systems are organic dyes from textile, cosmetic, paper, plastic, and pharmaceutical factories [3][4][5].Tese organic dyes endanger water quality, and some, such as methylene blue, are nonbiodegradable and toxic because of their mutagenic and carcinogenic properties, threatening human health [6].
Chemical, physical, and biological methods are conventional methods employed in wastewater treatment to remove contaminants, including dyes [5].However, these methods have drawbacks in removing the contaminants at low concentrations.Recently, researchers have embraced green nanotechnology to develop materials in nanoscale size that can efectively remediate the pollutants before being discharged into the water system [7][8][9][10][11].Chemical, biological, and physical methods are the primary methods employed in the synthesis of nanoparticles [11,12].Chemical and physical processes of synthesizing nanoparticles include coprecipitation, pyrolysis, thermal decomposition, sol-gel, solvothermal, laser ablation, and ball milling [13,14].However, both chemical and physical processes of synthesizing nanomaterials have drawbacks of using toxic chemicals and being time-consuming, energy inefcient, and cost-inefective [14][15][16].Researchers fnd biological methods for synthesizing nanomaterials more attractive because they are environmentally friendly and cost-efective [16,17].Biological methods involve the use of microorganisms and plants.However, the use of microorganisms is characterized to be labor-intensive and involves delicate procedures in microbe isolation, growth, maintenance, longer synthesis time, and the possibility of forming toxic byproducts [18,19].To promote environmentally friendly chemistry, using aqueous plant extracts in synthesizing metallic nanoparticles is preferred because of their cost-efectiveness, simplicity, little or no toxicity in products, and ease in large-scale production of nanoparticles [15,[18][19][20][21].
Zinc oxide nanoparticles (ZnO NPs) fnd wide application in cosmetic, photocatalysis, plasmonic, sensors, pure water technologies, and optoelectronics, among others [14,22].Te ZnO NPs possess improved chemical and physical properties which make them suitable for removing pollutants from the environmental compartments [14,22].Te use of plant extracts for synthesizing ZnO NPs makes the process cost-efective and nontoxic to carry out [23].
Terefore, this study reports a simple, cost-efective, environmentally friendly, and easy approach to decontaminating a highly toxic MB dye from our wastewater system using readily available plant materials, Parthenium hysterophorus, as raw materials, which are renewable and biodegradable, for preparing the ZnO NPs.Parthenium hysterophorus plant is an invasive weed that abundantly grows in cultivated and agricultural lands and along the roadsides [24].Previous studies report that Parthenium hysterophorus possesses medicinal and bioherbicidal properties [25,26].Te phytochemical profling of the Parthenium hysterophorus shows the presence of tannins, favonoids, phenols, saponins, and terpenoids, essential for reducing, capping, and stabilizing metal ions from high oxidation state to zero-valent species [25].Te synthesized ZnO NPs were characterized via UV-Vis spectroscopy, FTIR, SEM, TEM, XRD, and DLS techniques and investigated on their ability to degrade methylene blue (MB) dye.Te catalytic properties of ZnO NPs against MB dye were also explored at varying conditions of pH, ZnO NP dosage, MB dye concentration, interaction time, and temperature.Te study places ZnO NPs obtained in Parthenium hysterophorus-mediated synthesis as powerful plant-based nanoparticles that aim to reduce increasing aquatic pollution from dyes, a signifcant issue currently pressing humanity.In this light, a comparative overview of the efciency of Parthenium hysterophorus-mediated ZnO NPs in the removal of MB dye with other nanomaterials that have previously been studied for the removal of MB dye in aquatic systems is provided in this study.

Sample Collection and Preparation.
Te Parthenium hysterophorus plant sample was collected from Kalimoni, Juja, Kiambu County, Kenya.Mr. John Kamau authenticated the plant sample, and a voucher specimen was deposited at Jomo Kenyatta University of Agriculture and Technology (JKUAT) Botany Herbarium and assigned accession number DMN-JKUATBH/001/2023A-C as described in our previous study [24].Te sample was washed with running tap water, rinsed with distilled water, and shed-dried for two weeks at room temperature [26].We followed the methods of Datta et al. in which the dry sample was ground to a fne powder by using a milling machine, after which the phytochemicals were extracted by dissolving 20 g of the plant sample in 200 mL of distilled water [26].Te extraction was carried out at 45 °C for 45 minutes and stored at 4 °C in a freeze drier before further being used to synthesize zinc oxide nanoparticles [26].[27,28].Te formation of ZnO NPs was monitored by color change and UV-Vis spectrophotometric measurement [28][29][30].

UV-Vis Analysis.
Te UV-Vis absorption analysis of ZnO NPs was performed by scanning a double beam UV 1800 UV-Vis spectrophotometer (Shimadzu, Japan) to confrm their surface plasmonic resonance at 200-800 nm wavelength range [24].

FTIR Analysis.
Te functional groups present on the surface of the ZnO NPs were acquired by Bruker Tensor II FT-IR spectrophotometer (Bruker, Ettlingen, Germany).Te samples were directly placed on the sample holder, pressed, and scanned in the frequency range of 4000−400 cm −1 [31,32].

XRD Analysis.
Te crystallinity nature of the nanoparticles was determined by using X-ray difraction, and the difractograms were obtained by using a Bruker D8 Advance Difractometer (Bruker, Ettlingen, Germany) with a copper tube operating under a voltage and current of 40 kV and 40 mA.Te samples were irradiated with a monochromatic CuKα radiation of 0.1542 nm, and the difractograms were acquired between 2θ values of 5 °-90 °at 0.05 °intervals with a measurement time of 1 second per 2θ intervals.Te nanoparticles' crystalline size was calculated by using the Debye Scherrer's equation as follows: where D is the average particle size (nm), K is a constant equal to 0.94, λ is the wavelength of X-ray radiation, β is 2 Journal of Chemistry full-width at half maximum of the peak in radians, and theta is the difraction angle (degree) [24,29,33].

SEM Analysis.
Te surface morphology analysis of the nanoparticles was visualized by using the Tescan Mira3 LM FE scanning electron microscope (Tescan, Brno-Kohoutovice, Czech Republic), operating under an accelerating voltage of 3 kV.Before analysis, the samples were sputter coated with 4 nm gold to avoid charging efects by using a AGB7340 Agar Sputter Coater (Agar Scientifc, Essex, United Kingdom) [32,34].
2.4.5.TEM Analysis.Surface morphology and shape of the nanoparticles were observed using TEM and was performed on a Tecnai G2 Spirit (Termo Fischer Scientifc, Oregon USA) under an operating voltage of 120 kV equipped with Veleta 2048 × 2048 wide angle and Eagle 4096 × 4096 bottom mount detectors.Te dry samples were suspended in ultrapure water (Barnstead GenPure, Termo Scientifc, Germany) and ultrasonicated to obtain a solution that was drop casted on 300 mesh carbon flms before analysis [32,34].
2.4.6.DLS Analysis.To determine the particle size distribution and polydispersity index of the suspended nanoparticles, the metallic nanoparticles were resuspended in ultrapure water (18 MΩ•cm Barnstead GenPure UV-TOC, Termo Scientifc, Germany) and ultrasonicated to obtain a solution of suspended nanoparticles.Te solutions were fltered through 0.25 μM PTFE syringes into glass vials, and 45 μL of each solution was transferred onto quartz cuvettes before analysis.Te particle size distribution and polydispersity index were then measured by using a Beckman Coulter DelsaMax PRO dynamic light scattering analyzer (Beckman Coulter, Indianapolis, United States) [35,36].

Degradation Studies.
Methylene blue (MB) dye was used as a model organic dye in the degradation studies using ZnO NPs.Te degradation of MB dye was monitored by using a UV-Vis spectrophotometer by measuring the change in absorbance of the dye alone; the dye spiked with hydrogen peroxide and the dye spiked with hydrogen peroxide and ZnO NPs at a constant time of 150 minutes.Te degradation studies of MB dye by ZnO NPs were also conducted at varying reaction conditions, including the concentration of the MB dye solution, the dosage of ZnO NPs, interaction time, pH, and temperature [37][38][39].Te recyclability capability of ZnO NPs in the degradation of MB dye was also investigated in this study [17,[39][40][41][42].For this study, 5 mg/L of the MB dye solution was prepared as the standard MB concentration except where the variation of MB concentration was studied.Te degradation efciency was calculated by using the following equation, where A o is the absorbance of MB at time � 0, and A t is the absorbance after a particular time, t [24]: Te kinetics of degradation of MB dye was determined by assuming a pseudo-frst-order kinetic model (equation ( 3)) and a pseudo-second-order kinetic model (equation ( 4)) [38,40,41].
where Ao � is the absorbance of MB at time � 0, At � is the absorbance after a particular time, and t � is the time in minutes.
Te thermodynamics of the degradation reaction was determined by using the linear form of Van't Hof 's equation to obtain the change in heat and enthalpy of the reaction as follows: where ∆H and ∆S are the change in heat and entropy of the degradation reaction, respectively.Te T is the temperature in Kelvin, and R is the universal gas constant [38,43].

Results and Discussion
3.1.Biosynthesis of ZnO NPs.Parthenium hysterophorus aqueous extract contains phytochemicals such as saponins, favonoids, terpenoids, and phenols [25].Tese phytochemicals contain functional groups, which, upon reaction with zinc metal solution, reduce Zn 2+ to Zn 0 , as previously described in the biosynthesis mechanism of metallic nanoparticles [44][45][46].Figure 1 illustrates a generalized mechanism for the synthesis of Parthenium hysterophorusmediated ZnO NPs.As can be seen in Figure 2, the color changed from brown to dark brown after the zinc ion solution was mixed with Parthenium hysterophorus aqueous extract.Tis color change can be attributed to secondary metabolites in the aqueous extract that reduces zinc ions (Zn 2+ ) into Zn 0 species [47].Furthermore, the formation of ZnO NPs was confrmed by its intrinsic optical absorbance band by UV-Vis spectrophotometer centered at 337 nm.Our synthesis of ZnO NPs agreed with UV-Vis results from previous studies [27,28,47,48].As can be seen in Figure 3, the plant extract has peaks at 3299 cm −1 , 1632 cm −1 , 1362 cm −1 , and 1222 cm −1 .Tese peaks are attributable to broad O-H, C�O, and C-N stretching bands characterized by phenols, favonoids, and amine metabolites in the plant extract [26].A slight shift in the functional groups present in the extract was evident in the IR spectra of the ZnO NPs, indicating that some metabolites bonded or capped ZnO during its formation [49].Observable vibration bands in ZnO NPs spectra were at 3173 cm −1 , 2923 cm −1 , 1591 cm −1 , 1375 cm −1 , 1259 cm −1 , 1001 cm −1 , 809 cm −1 , and 543 cm −1 .Tree new peaks appeared in the IR spectra of ZnO NPs at 1001 cm −1 , 809 cm −1 , and 543 cm −1 .Te peak at 1001 cm −1 showed that ZnO NPs underwent C-H bending.Te peak at 543 cm −1 was characteristic of the Zn-O stretching vibration band.Te FTIR analysis showed the role of functional groups in the plant metabolites as reducing and capping agents in forming ZnO NPs, which were consistent with previous studies using plant extracts as sources of reducing and capping agents in the formation of ZnO NPs [28,40,47,49].Te surface morphology from the SEM micrograph revealed that the ZnO NPs were nearly spherical.Tese fndings agreed with previous studies, which showed that ZnO NPs synthesized using the cofee leaf extract and Elaeagnus angustifolia leaf extracts were spherical [27,48].Te micrograph revealed less evidence of agglomeration of the nanoparticles, which indicates that the synthesis method was efective in obtaining highly homogenous nanoparticles with desired catalytic properties [26,49,50].Te particle diameter size ranged from 10 to 70 nm, with the average particle size diameter of the ZnO NPs calculated using ImageJ software being 38.47 nm.Te particle diameter size was in the range of previously prepared ZnO NPs [12,51].As shown in Figure 5, the TEM images were spherical with little agglomeration evidence, which agrees with SEM image results.Te average particle size of the nanoparticles was calculated from ImageJ data to be 7.54 nm.Tese results agree with previous studies that synthesized ZnO NPs using cofee leaf extract [48].Tese crystallographic planes reveal that the ZnO NPs had a hexagonal wurtzite structure [12,48,49].Te crystallite size was calculated by using the Debye Scherrer equation and was found to be of 42.6 nm.In a previous study, the particle size of ZnO NPs was 66.43 nm [52].In another study, the crystalline particle size of ZnO NPs was calculated to be 52.23 nm [47].Terefore, we confrm the formation of ZnO NPs from the XRD pattern.

XRD Analysis
3.7.DLS Analysis.Figure 7 depicts the size distribution of ZnO NPs as observed by using the dynamic light scattering (DLS) technique.Te particle size distribution revealed that the particles were polydisperse, with most of the particles below 500 nm.Te polydispersity index (PDI) obtained in the DLS determination was ≤0.3, which proved that the particles were polydisperse.In contrast, individual groups of particles were monodisperse, supporting possible particle agglomeration [49,53].Te monodispersity of the particles shows that the particles were homogenous, which is desired in the catalytic activity of the nanoparticles.As shown in Figure 8(a), there was no observable change in absorption intensity for the MB dye after 150 minutes.However, spiking the same amount of MB dye with 1 mL hydrogen peroxide, there was a slight decrease in the absorbance intensity of the dye as shown in Figure 8(b).A signifcant change in the absorbance intensity of the MB dye was obtained when the same amount of the MB dye was reacted with 1 mL of H 2 O 2 and 10 mg of ZnO NPs as shown in Figure 8(c).Tis indicated that the ZnO NPs have the catalytic properties to degrade the MB dye [39,54].Te degradation studies were then investigated by varying MB dye concentration, pH, ZnO NP dosage, temperature, and interaction time [37][38][39].

Efect of MB Dye Concentration on Degradation Efciency of ZnO NPs.
To study the infuence of dye concentration on the degradation efciency of ZnO NPs, the study was conducted using 2.5 mg/L, 5 mg/L, 7.5 mg/L, 10 mg/L, and 12.5 mg/L concentrations, and their efect on degradation efciency is depicted in Figure 9.  Te concentration of dyes afects the activity of the nanoparticles because of the saturation of active sites at higher dye concentrations.It was observed that the percentage of degradation of MB dye decreased as the dye concentration increased.At 2.5 mg/L of MB dye, the degradation percentage was 35.03%, while at 12.5 mg/L (the highest concentration of MB dye used for this study), the degradation percentage was 14.68%.Te decrease in the efciency of ZnO NPs in MB degradation studies can be attributed to the saturation of the nanoparticles' active sites by more MB dye molecules [40,55].Simultaneously, more molecules of MB dye were available with insufcient active sites on the surface of the nanoparticles because the ZnO NPs were saturated [55].Te MB solution's absorbance increased as the MB dye concentration increased.Tis means that the nanoparticles become highly saturated at higher concentrations of MB dye, and little dye breakdown occurs.

Efect of Nanoparticle Dosage on Degradation Efciency of
ZnO NPs.In this study, the dosage of ZnO NPs was conducted at 10 mg, 20 mg, 30 mg, 40 mg, and 50 mg, and their degradation efciency was determined within 2.5 hours, and the results are illustrated in Figure 10.
It was observed that the percentage of degradation of MB dye increased proportionally to the dosage of the ZnO NPs.Te degradation percentage increased from 22.32% at 10 mg to 44.52% at 50 mg within 2.5 hours.Te increase in degradation percentage of MB upon increasing the amount of ZnO NPs was probable because, at higher ZnO NPs dosage, the total surface area of the nanoparticles was higher, making more active sites available to bind the MB dye molecules [3,56].Te decrease in absorbance at a higher nanoparticle dose (50 mg) revealed that the MB dye molecules were degrading towards completion because more active sites were present on the surface of the nanoparticles to bind, interact, and result in the degradation of MB dye [40].Our results agreed with previous studies which reported that the degradation effciency increased as the amount of the nanoparticles was increased against the same amount of the pollutant [3,24,40,50,55,56].It was observed that as the interaction time of MB dye with ZnO NPs was increased, the degradation percentage increased from 18.09% at 30 min to 56.87% within 360 minutes.Te corresponding absorbance of MB decreased as the reaction time increased, implying that molecules were allowed enough time to interact with the surface of ZnO NPs and break down.Te increase in the degradation percentage of MB dye was also supported in previous studies employing ZnO NPs [12,50].

Efect of pH on Degradation Efciency of ZnO NPs.
To monitor the optimal pH that results in the higher degradation of MB using ZnO NPs, the solution pH was adjusted to pH 2, 4, 8, and 12, and their efect on the degradation efciency is depicted in Figure 12.
Te acidic conditions of the MB dye solution were adjusted using 0.1 M of HCl, while the basic conditions of the MB dye were adjusted using 0.1 M of NaOH.At pH 2 and 4, the degradation efciency was 26.00% and 29.91%, respectively.At pH 8 and 12, the degradation efciency was 45.34% and −114.72%,respectively.Te interaction of the acid and the base resulted in the generation of free radicals from the hydrogen peroxide used as a catalyst and radical generator for this reaction.In acidic pH 2 and 4, the degradation efciency could be attributed to the increase in hydrogen radicals generated from H 2 O 2 and the positive  charge imparted on the surface of nanoparticles, which activates the active sites to improve reaction with the dye molecules [55].In basic pH 8 and 12, degradation efciency was attributed to the generation of hydroxyl radicals, which increased the degradation ability of the nanoparticles towards the MB molecules [3].Uniquely, at pH 12, a negative percentage of degradation was observed within the 2.5 hours used for this study.At pH 12, it implies that MB dye molecules were destroyed to yield probable degradation products with higher absorbance than those obtained in other pH values.

Efect of Temperature on Degradation Efciency of ZnO
NPs. Te temperature of the MB dyes was varied at 25 °C, 35 °C, 45 °C, 55 °C, and 65 °C, and the highest degradation efciency was observed at higher temperatures, as illustrated in Figure 13.
Te temperature signifcantly infuences the activation of the surface of nanoparticles and, therefore, improves the degradation efciency of ZnO NPs on the degradation of MB dye.Te increase in temperature was observed to increase the percentage of degradation of MB dye to 53.08% at 65 °C, which is attributed to the Brownian motion, which increases the kinetic energy of MB dye molecules [38].Another probable reason for the increase in degradation efciency at higher temperatures is that the MB dye molecules have enough energy to react and undergo degradation [53].
Te change in heat and entropy of the reaction was obtained by Van't Hof's plot of lnK v(1/T), as shown in Figure 14.
Figure 14 shows that the degradation of MB dye using ZnO NPs was an endothermic reaction, demonstrating that the MB degradation rate increases as the temperature increases.Te change in the heat of the degradation reaction was determined experimentally and calculated from the slope of Van't Hof's plot to be 14.199 kJ•mol −1 , and the change in entropy was calculated from the intercept value to be 1.37264 J•K −1 .Te entropy indicated enhanced system disorder due to the increase in temperature resulting from the breakdown of the MB dye molecules [57].

Degradation of MB Dye Using ZnO NPs at Optimal
Conditions.Te degradation of MB dye by ZnO NPs was also investigated by combining all the conditions that resulted in the highest degradation efciency of MB dye, as described in the previous section.Figure 15 depicts the change in absorption intensity of MB dye obtained using 5 mg/L concentration of MB dye, temperature maintained at 65 °C, 50 mg of ZnO NPs, and solution at pH 12.
As shown in Figure 15, a signifcant change in the absorption intensity of MB dye was observed to occur within 32 minutes in optimal conditions described in this study.Degradation efciency of 55.69% was obtained under the combined optimal conditions.

Kinetics of Degradation of Methylene Blue Dye
Te kinetics of degradation of MB dye using ZnO NPs was determined by ftting the pseudo-frst-order and pseudosecond-order kinetics at 298, 308, 318 328, and 338 K, and the results are depicted in Table 1 and Figures 16 and 17.
Te kinetic data were analyzed to ft the pseudo-frstorder other than the pseudo-second-order because most R 2 (correlation coefcient) values were higher compared to the R 2 of the pseudo-second-order kinetic model.From this determination, it can be concluded that the degradation of MB was highly dependent on temperature.Te increase in temperature increases the reacting molecules' Brownian motion, resulting in the highest degradation efciency [58,59].Figure 16 shows the frst-order kinetic plots of ln(At/Ao)v T, and Figure 17 shows the second-order kinetic plots of (1/At) v T.

Functional Group Analysis of ZnO NPs after Degradation Studies
Te analysis of changes in the frequencies of functional groups present in ZnO NPs after degradation studies with MB dye was investigated.Te IR spectra in Figure 18 show changes in the peaks before and after degradation studies.Te frequencies of functional groups in the IR spectrum of ZnO NPs before degradation were observed at 3173 cm −1 , 2923 cm −1 , 1591 cm −1 , 1375 cm −1 , 1259 cm −1 , 1001 cm −1 , 809 cm −1 , and 543 cm −1 .Te peak at 3173 cm −1 was attributed to the O-H group of phenols and 2923 cm −1 was due to the C-H band, while the bands at 1375 cm −1 and 1259 cm −1 were characteristic of C-N.Te peak at 1001 cm −1 and 809 cm −1 was attributed to C�C bending vibration.Te peak at 543 cm −1 was characteristic of Zn-O stretching vibration [3,49].Te shifts in the IR spectrum of ZnO NPs after degradation studies with MB dye were observed at 3023 cm −1 , 2958 cm −1 , 1734 cm −1 , 1447 cm −1 , 1364 cm −1 , 1215 cm −1 , 1095 cm −1 , 891 cm −1 , and 521 cm −1 .Te peaks at 3023 cm −1 were due to O-H of phenols, the band at 2958 cm −1 was due to C-H band, 1734 cm −1 was attributable to C�O, 1445 cm −1 band was attributable to C-H bending, and the bands at 1364 cm −1 and 1215 cm −1 were attributable to C-N stretching band.Te 1095 cm −1 and 891 cm −1 peaks were due to C-O and C�C bending bands, respectively.Te characteristic peak at 521 cm −1 was associated with Zn-O vibration bands.Te slight changes in the frequencies of the functional groups after reaction with MB dye indicated that a molecule of MB was adsorbed onto the surface of the ZnO NPs, which resulted in a slight shift in the functional group frequencies and their intensities after degradation [60].

Recyclability of ZnO NPs
Te recyclability ability of ZnO NPs was investigated experimentally in four cycles for MB dye degradation.Te recyclability studies were conducted under the same conditions (20 mg of ZnO NPs and 5 mg/L of MB concentration).Te degradation cycle was determined at constant time (2.5 hours), after which the solution was decanted and washed with water, and the nanoparticles were dried at 80 °C in an oven for 6 hours [48].Te degradation of MB was observed by the change in intensity of the absorbance monitored using a UV-Vis spectrophotometer, and the results are plotted as shown in Figure 19.After each cycle, fresh 5 mg/L of MB solution was spiked with 1 mL of H 2 O 2 .As observed in Figure 19, the percentage of degradation of MB decreased after each cycle of using the ZnO NPs.Te decrease in the degradation efciency of ZnO NPs with multiple uses in the recyclability process indicates a loss in the catalytic potential of the nanoparticles [17].Te reduction in degradation efciency of ZnO NPs can be attributed to the loss of nanoparticle catalytic ability during the separation, washing, and drying procedures after each application cycle [45,61].Tese procedures result in a decrease in the number of active sites available on the surface of ZnO NPs to interact with the fresh MB dye molecules.We also performed FTIR analysis to get insights into the stability of the nanoparticles by observing structural changes of ZnO NPs after their use in the frst cycle and after their fourth cycle, and the results are depicted in Figure 20.
Te structure of ZnO NPs from the IR spectrum revealed that no remarkable changes in the functional groups were

Journal of Chemistry
observed for a single cycle and when four recyclability processes were used during the degradation of MB dye.Tis can support that the ZnO NPs exhibited strong stability and were not altered after several uses, a valuable characteristic of nanoparticles [13,62].

Mechanism of Degradation of Methylene Blue Dye Using ZnO NPs
Te mechanism of degradation of MB dye by ZnO NPs has been explained in previous studies [11,50,[63][64][65][66]. Te incident light leads to the excitation of an electron from the valence band (VB) to the conduction band (CB) on the ZnO NPs surface, leading to the creation of a positive hole in the VB (h + VB ) [50,63,64].Te electron on the CB (e − CB ) is taken up by oxygen adsorbed onto the surface of ZnO NPs, generating a superoxide anion radical 2 ) anion radical is then involved in the degradation of MB dye [64,67].Te positive holes in the valence band react with H 2 O 2 used as a radical generator on the surface of ZnO NPs to produce OH radical which reacts with the dye, leading to the generation of degradation products.Te positive hole in the VB then moves onto the surface of ZnO NPs, releasing more oxygen that produces (O

Journal of Chemistry
Figure 21 further provides an illustration of the probable mechanism of degradation of MB by ZnO NPs.
Te ZnO NPs prepared using plant extracts have been reported to be less toxic than chemically prepared ZnO NPs [68,69].Te reducing agents such as hydroxyl radicals and superoxide radical anions reduce the methylene blue dye to less toxic degradation products such as carbon dioxide and water which are less harmful and environmentally friendly [33,50,[68][69][70][71].

Comparative Overview of Degradation Efficiency of MB Dye Using Other Nanomaterials
Table 2 provides an overview of previously utilized nanomaterials in removing MB dye from aquatic systems and their performance efciency.From

Conclusion
ZnO NPs were successfully synthesized using an aqueous extract of the Parthenium hysterophorus plant as a reducing, capping, and stabilizing agent.Te ZnO NPs were analyzed via UV-Vis spectroscopy, which revealed an intrinsic optical absorbance band associated with ZnO NPs that occurred at 337 nm.Te functional group analysis by FTIR confrmed the presence of secondary metabolites in the extract, which were responsible for reducing Zn 2+ to Zn 0 , with a characteristic Zn-O vibration band being observed at 543 cm −1 .SEM and TEM analyses revealed that the ZnO NPs were spherical with an average particle size of 38 nm.Te XRD analysis confrmed the hexagonal wurtzite structure of ZnO NPs, and the crystallite size calculated using the Debye Scherrer equation was 42 nm.Te pH, temperature, dosage of ZnO NPs, concentration of MB solution, and interaction time infuenced the degradation ability of ZnO NPs against MB dye.Under optimal conditions set at 65 °C, pH 12, 50 mg of ZnO NPs, and 5 mg/L of MB dye concentration, a degradation efciency of 55.69% was obtained within 32 minutes.Te stability of ZnO NPs after multiple uses was confrmed by running an FTIR analysis in which there were no observable changes in the position of the functional groups.Terefore, Parthenium hysterophorus-mediated ZnO NPs demonstrated to be fruitful in degrading MB dye making the nanoparticles suitable for addressing aquatic pollution by dyes.However, future prospects should consider understanding the toxicity profle of the degradation products on the environment and encapsulation of the ZnO NPs with polymeric adsorbents to enhance their degradation efciency, recyclability, and large-scale application.Te green synthesis of ZnO NPs is still in its early stages and should be exploited by using other plant materials and studied in the degradation of other organic dyes before they are deployed in large-scale waste water treatment.

Figure 2
depicts the formation of ZnO NPs by color change and observation of the intrinsic optical absorbance band by UV-Vis spectrum.
Figure 3 compares the IR spectra of Parthenium hysterophorus aqueous extract and ZnO NPs identifed by infrared spectrometry.
Figure 4  shows the SEM micrograph of ZnO NPs and the histogram used to determine particle size distribution using ImageJ software.

Figure 1 :Figure 2 :Figure 3 :
Figure 1: Schematic representation of the plausible mechanism of biosynthesis of ZnO NPs using aqueous extracts of Parthenium hysterophorus plant.
Figure 5 depicts the TEM images and particle diameter size distribution histogram of the synthesized ZnO NPs.

Figure 8 (
Figure 8(a) shows the UV-Vis spectrum of MB alone observed after 150 min, Figure 8(b) shows the changes in absorbance intensity of MB after it is reacted with 1 mL of H 2 O 2 , and Figure 8(c) shows the changes in absorbance intensity of MB after it is reacted with 1 mL of H 2 O 2 and 10 mg of ZnO NPs.As shown in Figure8(a), there was no observable change in absorption intensity for the MB dye after 150 minutes.However, spiking the same amount of MB dye with 1 mL hydrogen peroxide, there was a slight decrease in the absorbance intensity of the dye as shown in Figure8(b).A signifcant change in the absorbance intensity of the MB dye was obtained when the same amount of the MB dye was reacted with 1 mL of H 2 O 2 and 10 mg of ZnO NPs as shown in Figure8(c).Tis indicated that the ZnO NPs have the catalytic properties to degrade the MB dye[39,54].Te degradation studies were then investigated by varying MB dye concentration, pH, ZnO NP dosage, temperature, and interaction time[37][38][39].

Figure 4 :
Figure 4: SEM micrograph and particle size distribution histogram of ZnO NPs.

Figure 5 :
Figure 5: TEM micrographs and particle size distribution histogram of ZnO NPs.

Figure 11
illustrates the efect of interaction time on the percentage of degradation of MB dye by ZnO NPs.

Figure 10 :
Figure 10: Efect of nanoparticle dosage on the degradation efciency of ZnO NPs against MB dye.

Figure 9 :
Figure 9: Efect of concentration of MB dye on degradation effciency of ZnO NPs.

Figure 11 :
Figure 11: Efect of interaction time on degradation efciency of ZnO NPs against MB dye.

Figure 12 :
Figure 12: Efect of pH on degradation efciency of ZnO NPs against MB dye.

Figure 13 :
Figure 13: Efect of temperature on degradation efciency ZnO NPs against MB dye.

Figure 14 :Figure 15 :
Figure 14: Van't Hof's plot of the degradation of MB dye used to obtain the change in heat and entropy of the degradation process.

Figure 16 :
Figure 16: Pseudo-frst-order kinetics for degradation of MB dye and ZnO NPs at varying temperatures.

Figure 18 :Figure 19 :
Figure 18: IR spectra showing ZnO NPs before and after the degradation process.

Figure 20 :
Figure 20: IR spectra of ZnO NPs after a single and four recycling cycles.

Figure 21 :
Figure 21: Probable mechanism of degradation of MB dye by using ZnO NPs.
zinc oxide nanoparticles MB dye Solar irradiation, pH 9, 10 ppm of MB dye, 100 mg/L of nanoparticle dose 2 irradiation, ZnO dosage of 250 mg/L, pH 10, 20 mg/L of MB, 2 hours 85 [41] CeO 2 NPs/graphene oxide/polyacrylamide composite MB dye UV light, 2.5 mg of CeO 2 , pH 12, 5 mg/L of MB, 90 min 90 [74] ZnO-NPs MB dye UV irradiation, 10 mg/L of MB, pH 2, 1.8 g/L of ZnO NPs 140 min 86 [59] 2% Fe-ZnO MB dye UV irradiation, 10 mg/L of MB dye, 1.8 g/L of 2% Fe-ZnO, irradiation time 140 light, 0.1 g of CuO, 10 ppm of MB, pH 9mg/L of MB, 10 mg of ZnO NPs, 1 mL of H L of MB, 10 mg of ZnO NPs, pH 4, 1 mL of H L of MB, 10 mg of ZnO NPs, pH 8, 1 mL of H L of MB, 10 mg of ZnO NPs, 65 °C, 150 min 53.08 50 mg of ZnO NPs, 5 mg/L of MB, pH 12, 65 °C, 1 mL of H MB dye and thus are promising in future exploitation of biosynthesized nanoparticles in environmental remediation of Our study confrms that aqueous extract from the Parthenium hysterophorus plant can act as reducing, capping, and stabilizing agents in the formation of ZnO nanoparticles.Te presence of secondary metabolites makes Parthenium hysterophorus a potential universal candidate in the formation of other nanoparticles in an eco-friendly and cost-efective manner.
Solution and Synthesis of ZnO NPs.0.01 M zinc solution of zinc nitrate hexahydrate (Zn (NO 3 ) 2 .6H 2 O) was prepared by using distilled water.Te nanoparticles were synthesized by modifcation of a previously established protocol of Iqbal et al. and Naseer et al. in which the zinc ions solution was mixed with plant extract in a ratio of 1 : 4 and heated in a hot plate at 60 °C for 1 hour with constant stirring

Table 1 :
Experimental data ftted assuming pseudo-frst-order kinetics and pseudo-second-order kinetic models at diferent temperatures.

Table 2 ,
it can be concluded that ZnO NPs prepared from Parthenium hysterophorus aqueous extract are attractive in the degradation of

Table 2 :
Comparative overview of nanomaterials previously utilized in the removal of MB dye.