Optimization of Fe/Ni, Fe/Cu bimetallic nanoparticle synthesis process utilizing concentrated Camellia sinensis extract solution and activity evaluation through methylene blue removal reaction

In this study, we introduce a synthesis process of bimetallic nanoparticles (BNPs) Fe/Ni and Fe/Cu utilizing concentrated Camellia sinenis extract that was optimized with a solvent ratio of ethanol/H2O 4/1 (v/v), a metal ratio of 5/1 (w/w), a total polyphenol content (TPC) in the solution of 12.5 g.l−1, pH = 3–4, 25 °C, and the reaction time ranging from 30 min to 50 min. The structural and morphological characteristics of the resulting materials were determined using several techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). The maximum removal efficiency of methylene blue (MB) by BNPs Fe/Ni and Fe/Cu materials was found to be 88.60% and 91.06%, respectively, at a concentration of MB = 25 mg.l−1 and 25 °C. According to the results of the kinetic modeling study, the adsorption process of MB on the two BNPs materials followed second-order kinetics, with the maximum adsorption capacities of MB on Fe/Ni and Fe/Cu BNPs being 26.94 mg.g−1and 28.00 mg.g−1, respectively.


Introduction
Currently, Fe°-based bimetallic nanoparticles (BNPs) with superior catalytic activity, high treatment efficiency, facile fabrication, and convenience in application are widely utilized for the remediation of contaminants in both surface and groundwater [1, 2].The treatment efficacy of Fe°-based BNPs towards contaminants is significantly influenced by the surface characteristics, redox potential, and inherent nature of the nanomaterial.Experimental investigations and small-scale applications of BNPs have been conducted for the treatment of water pollutants such as dyes [3], chlorinated organic compounds [1,4], nitrates [5], heavy metals [2, 6], and antibiotics [7].BNPs function both as catalysts to enhance the rate of pollutant degradation and as effective agents to hinder the surface oxidation of nanoparticles, surpassing the performance of monometallic zero-valent materials [8].Transition metals are often the subject of research and use as 2nd grade metals to enhance the activity of Fe-based BNPs [9,10].Platinum and other precious metals were studied very early [11] and could be used as cocatalysts but at high cost and in limited supply.Among transition metals, copper nanoparticles (CuNPs) are the most widely used because of their applicability [12].Besides, Ni-based nanomaterials are also a compelling choice for organic dye decomposition processes and some advanced applications involving magnetism [13].Therefore, the research and development of BNPs materials using familiar and inexpensive metals such as Fe, Cu, and Ni is essential.
In recent years, the synthesis of Fe-based BNPs using green chemistry approaches (employing plant extracts containing polyphenol derivatives) has emerged as a preferred strategy to substitute potent reducing agents [14,15].Among these, Camellia sinenis (green tea) has been extensively studied and widely recognized for its biological properties [16].Polyphenols present in green tea, notably catechins, constitute approximately 18% to 36% of the dry weight of tea [17], exhibit antioxidative capabilities, rendering plant extracts a subject of extensive investigation as modifiers and surface antioxidative agents of materials.Several techniques including ultrasound-assisted extraction, supercritical fluid extraction, microwave-assisted extraction, and high-pressure liquid extraction, have been employed for polyphenol extraction from plants [18].Solvent extraction techniques are widely adopted in industrial production because of their flexibility, reliability, and cost-effectiveness.Ethanol and water are favored solvents owing to their non-toxicity and environmental compatibility [19].Notably, water is a preferable solvent owing to its environmental friendliness and cost-effectiveness compared with organic solvents [20].
An inherent property of Fe°-based BNPs is their ability to adsorb and reduce pollutants via electron transfer mechanisms [21].Among the detrimental pollutants, MB is one of the most widely used cationic dyes.Endowed with a complex chemical structure, MB poses challenges in biodegradation and has adverse impacts on aquatic ecosystems by impeding sunlight penetration into water bodies [22].Various methodologies have been explored for MB removal from aqueous environments.Notably, Fe°-based BNPs have garnered attention because of their promising efficacy in MB remediation, representing a highly efficient treatment avenue.
In this study, we optimized the synthesis process of two types of Fe/Cu and Fe/Ni BNPs materials using concentrated Camellia sinenis extract as the extracting solution and ethanol, water as solvents.In addition, we evaluated the structure and activity of the synthesized materials by assessing their ability to treat MB in water.

Methods
2.2.1.Optimization of the synthesis process of Fe/Cu and Fe/Ni BNPs materials 2.2.1.1.Investigation of factors affecting the total polyphenol content (TPC) in concentrated green tea powder Green tea leaves, post-harvest, were preliminarily processed through cleaning, drying, and grinding into powder form, which was then sieved through a mesh screen of 1.0 mm size.The obtained powder (20g) was weighed and added to a stainless steel container containing 1 L of RO water.The container was covered, placed on an electric heater, and heated (the temperature was monitored using a thermometer).We proceeded to examine the influence of water temperature ranging from to 30 °C-95 °C, and the heating duration was investigated at intervals of 10, 30, 60, 90, 120, 150, and 180 min on the TPC present in the final product.After heating, the extracted solution was processed according to the steps referenced in the procedures outlined by Vuong (2010) [23] and Ramos-Escudero (2023) [24].

Determining the TPC in concentrated green tea powder
The TPC of the concentrated powder product was determined according to ISO 14502-1:2005 using the Folin-Ciocalteu method.Standard curve of gallic acid was established by preparing 1 ml of gallic acid solution with concentrations of 10, 20, 30, 40, and 50 μg.l−1 in test tubes, adding 5 ml of 10% Folin-Ciocalteu reagent to each tube, shaking well, then adding 4 ml of 7.5% Na 2 CO 3 , shaking well, and allowing the mixture to stand at 25 °C for 60 min, followed by measuring the optical absorption in a cuvette with a 10 mm path length compared to water on a UV-vis spectrophotometer (Dynamica, Switzerland) at a wavelength of 765 nm.Concentrated green tea powder (0.5 g) was dissolved in a mixture of 45 ml RO water and 5 ml acetonitrile.One milliliter of the resulting solution was diluted with 100 ml RO water.One milliliter of the diluted extract was pipetted into a test tube, 5 ml of 10% Folin-Ciocalteu reagent was added and shaken well, after 4 ml of 7.5% Na 2 CO 3 was added, shaken well, and the mixture was allowed to stand at 25 °C for 60 min, followed by measuring the optical absorption.After obtaining the linear value of the standard curve and the optical absorption value of the sample, the TPC of the sample was determined using the following formula: The green tea solution was then transferred into a separating funnel and slowly dropped into a round-bottom flask containing the prepared metal salt solution, with the reaction occurring under N 2 gas.After the complete addition, the mixture was stirred for 30 min at room temperature.Upon reaction completion, half of the solution volume was transferred to a clean, dry glass vial, and the sedimentation time of the BNPs particles was observed.The remaining volume was centrifuged, the supernatant was decanted, and the product was washed three times with ethanol before being placed in a desiccator under vacuum for 24 h at 25 °C to obtain a solid black product.Parameters such as the ethanol/H 2 O ratio, Fe/M weight ratio (w/w) (M: metal), TPC, reaction time, and reaction temperature were systematically varied to optimize the process.For pH influence, a small amount of precipitated product was placed in a test tube and the pH was adjusted using a 0.1M NaOH solution.In this study, the sedimentation time of BNPs particles in the solution and the overall synthesis efficiency of the particle product were chosen as the basis for determining the optimal conditions.

Characteristics of synthesized Fe/Cu, Fe/Ni BNPs materials
The surface morphology and particle size of the synthesized BNPs were determined using scanning electron microscopy (SEM, Hitachi S-4800, Japan) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100, Japan)).The distribution of metal elements was analyzed using energy dispersive x-ray (EDX, Shimadzu, Japan), while the x-ray diffraction (XRD, Panalytical, Netherlands) method identified the presence of crystalline phases in the materials.The chemical states of the elements on the surface of the BNPs were analyzed using x-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA).

The kinetics of MB adsorption
BNPs (Fe/Ni, Fe/Cu) (30 mg) were placed in a 50 ml glass beaker, followed by the addition of 10 ml of MB solution.The mixture was stirred for 10, 30, 60, 90, 120, 150, and 180 min at 25 °C.Subsequently, the solution was centrifuged at 5000 rpm for 5 min, the solid was discarded, and the optical density of the solution was measured using a UV-vis spectrophotometer at λ max = 668 nm.The experiment was conducted using MB concentrations of 25, 50, 75, and 100 mg.l −1 .The efficiency of MB adsorption by each material was evaluated using the following formula [25]: where A 0 is the optical density of the initial MB solution, A t is the optical density at time t, and H is the MB removal efficiency in the solution.
The amount of MB adsorbed per unit mass of adsorbent (BNPs) was calculated using the following formula: where C 0 is the initial concentration of MB, C is the MB concentration at time t, V is the volume of the solution (L), and m is the mass of BNPs (g).
An effective method for evaluating the efficiency of MB dye adsorption is to study the adsorption kinetics.We utilized experimental data to apply various kinetic models, including the pseudo-first-order, pseudosecond-order, and Elovich models as shown in table 1.
where q e , q t, and q max represent the amount of MB adsorbed at equilibrium, at time t, and the maximum adsorption capacity (mg.g −1 ) respectively; K 1 and K 2 are the equilibrium constants of the pseudo-first-order and pseudo-second-order kinetic models, respectively; a is the initial adsorption rate (mg.g −1 .min−1 ), b is the desorption constant (g.mg −1 ); K L is the Langmuir adsorption constant (L.mg −1 ); R L is the separation factor, and K F is the Freundlich adsorption constant.

Results and discussion
3.1.Optimization of the synthesis process of BNPs Fe/Ni, Fe/Cu 3.1.1.The conditions for synthesizing concentrated green tea extract Figure 1(a) shown the color changes corresponding to different concentrations of the mixture.We found that the maximum absorption wavelength of the standard gallic acid is λ = 747 nm on the UV-vis device with an optimized incubation time for the mixture (gallic acid + Folin + Na 2 CO 3 ) of 70 min.The results in figure 1(B) show the standard curve of gallic acid and the regression equation used to calculate the total polyphenol content.The correlation coefficient R 2 = 0.9996 demonstrated a strong linear relationship between the concentration and optical density of the gallic acid solution in the concentration range of 0-50 μg.mL −1 .Gallic acid was used as a standard to determine the TPC.
The results of the investigation on the influence of time and temperature of initial green tea extraction on TPC in concentrated green tea powder are depicted in figure 2. The response surface methodology (RSM) model was developed with a fixed initial tea powder amount of 20 g, based on formula (1).At a fixed extraction temperature of 95 °C, the TPC content in the final product increased as the extraction time increased, reaching a  Table 1.Kinetic and thermodynamic adsorption models were applied.

Model
Linear form Kinetic models Pseudo-firstorder ln(q e -q t ) = ln q e -K 1 t Pseudo-secondorder The optimized process for synthesizing concentrated green tea powder from green tea leaves is summarized in the generally diagram in figure 3.

The factors affecting the synthesis of BNPs
The results of the investigation into the factors affecting the sedimentation time and synthesis efficiency of BNPs are depicted in figure 4.

Effect of the ethanol/H 2 O ratio, metal ratio, TPC
The results of the influencing factors survey in figures 4(A), (B) reveal that, with an ethanol/H 2 O ratio of 4/1 (v/v), the sedimentation times for Fe/Cu and Fe/Ni were 174 h and 314 h, respectively, while the highest synthesis efficiency values were 29.25% and 31.76%.This aligns with the findings of Wang et al [27], where ethanol not only impedes oxidation, but also, because of its lower polarity compared to water, can stabilize BNPs during synthesis.Nanoparticles can disperse more easily in solvents with lower polarity, and an increase in ethanol concentration reduces polarity, resulting in a reduction in the surface tension of the ethanol/H 2 O solvent mixture to stabilize the formation of new nanoparticles.The Fe/Cu metal ratio of 5/1 (w/w) achieved the highest efficiency value of 29.4%, while the differences in this value for Fe/Ni 5/1 and 6/1 (w/w) are not significant (31.68% and 33.13%, respectively).The optimal TPC in the reaction solution was 12.5 g.l −1 , which maximized the reaction efficiency between the metal salts and polyphenols in the extract.Metal ions react with polyphenols in green tea to produce metal-polyphenol chelate complexes.In these complexes, functional groups of polyphenols act as ligands, creating tight bonds with metal ions and forming ring or chelate circuit structures [28].The metal ions are then 'agglomerated' to create nanoparticles, while polyphenols can act as protectants, keeping the nanoparticles from over-agglomerating.The process of metal cation reduction by polyphenols or other reducing agents present in plant extracts is demonstrated by the following reaction [29]: Therefore, we chose an ethanol/H 2 O ratio of 4/1 (v/v), a metal ratio of 5/1 (w/w), and a TPC of 12.5 g.l −1 in the solution to carry out the next optimization step.

Effect of reaction time and reaction temperature
The use of high temperature during synthesis affects the structure of the iron particles, causing more dissolution of the iron oxide shell.He and Zhao (2008) reported that higher reaction temperatures led to an increased rate of oxidation-reduction reactions, thereby increasing the number of nZVI nuclei formed, which accelerated the nucleation and growth processes.Additionally, iron compounds dissolve more at higher temperatures because, at elevated temperatures, the total energy of the atoms increases, resulting in an increased distance between the two iron particles [30].Consequently, water molecules can easily occupy larger spaces and react with iron, resulting in increased dissolution.The survey results showed that the sedimentation time and the synthesis efficiency of BNPs tended to slightly increase with higher reaction temperatures; however, this variation was insignificant, and higher temperature points were not investigated due to safety considerations.Temperature conditions typically ranging from to 25 °C-30 °C were chosen to facilitate large-scale BNPs synthesis processes.The optimal reaction time ranges from to 30-50 min for the optimal dispersion of BNPs in the solution.

Effect of pH
BNPs deposition occurred most slowly under pH conditions of 2-3 and gradually decreased within the pH range of 3-4 (figure 4(C)).Sedimentation time decreased rapidly within the pH range of 4-5 and shows no significant change within the pH range of 5-7 (after 0.5 h).The increase in pH leads to an enhanced formation of oxides and hydroxides on the surface of the BNPs nanoparticles owing to the interaction of OH-radicals with metal ions on the surface.The formation of oxides and hydroxides on the surface increases the density of the particles, thereby enhancing their coagulation and bonding through molecular forces into clusters, resulting in rapid sedimentation.These research findings are consistent with the previously published results by Rezaei et al (2018) [12], Yoshino et al (2018) [13].Therefore, the optimal conditions determined by the research team for synthesizing BNPs are an ethanol/H 2 O ratio of 4/1 (v/v), Fe/M ratio of 5/1 (w/w) (M: Cu, Ni), TPC concentration of 12.5 g.l −1 , reaction temperature of 25 °C-30 °C, and reaction time of 30-50 min.

The structure and morphology of BNPs
The results of the surface morphology analysis and particle size analysis of the synthesized BNPs are depicted in figures 5(A), (B).
The results in figures 5(A) and (B) indicate well-dispersed nanoparticles with Fe/Cu particles ranging from to 37-45 nm, and Fe/Ni particles averaging approximately 11-22 nm.The material surface appeared relatively porous with voids.TEM images revealed a multi-layered structure of BNPs with interparticle contact and a coating layer with an average thickness of 10-30 nm, which could be attributed to the formation of secondary metal and metal oxide.
The EDS mapping analysis in figure 6(A) reveals the distribution of Fe and Cu on the material surface, where the presence of Fe is prominently indicated by dense appearances, whereas Cu is less pronounced because of the controlled input ratio of Fe/Cu = 5/1 (m/m).Similar trends were observed in Fe/Ni, with a denser Fe presence.
The XRD patterns of Fe/Cu samples in figure 6(B) show the appearance of Fe(0) crystallites at a corresponding peak of 44.9°, along with peaks indicative of Cu(0) crystals at 43.1°and 50.3°, and Cu(I) at 36.2°.The XRD spectra of Fe/Ni NPs demonstrated distinct peaks of Fe(0) at 44.9°and Ni(0) crystallite formation at 51.8°, with potential overlapping peaks between Fe and Ni crystallites around 44-45°due to the dominance of Fe crystal intensity.These results are consistent with previous studies of BNPs, such as those conducted by Xiong et al (2018) [31] and Zhang et al (2018) [32].
X-ray photoelectron spectroscopy (XPS) was employed in this study to examine the composition and chemical states of the elements on the surfaces of Fe/Cu and Fe/Ni BNPs materials, as illustrated in figure 7. The distinct peaks of C, O, Fe, Cu, and Ni were clearly discernible in the XPS wide spectra (figure 7(C)).The presence of the C 1s and O 1s peaks may originate from the adsorption of bio-molecules from the green tea extract onto the nanoparticles surfaces [33].As depicted in figure 7(A), two peaks with binding energies at 711.08 eV and 724.28 eV are assigned to Fe 2p3/2 and Fe 2p1/2, confirming the presence of Fe(II) and Fe(III) in the iron oxidation products, while a broad peak at 705.58 eV indicates the ambiguous presence of Fe°on the surface of Fe/Cu BNP.The Cu 2p XPS spectrum reveals the main peaks of Cu 2p3/2 at 932.08 eV and Cu 2p1/2 at 952.08 eV, confirming the existence of metallic Cu° [34,35].The broad peaks at 935.08 eV and 954.78 eV, 943.18 eV for Cu 2p3/2 and Cu 2p1/2, respectively, are characteristic of CuO [34], indicating the coexistence of Cu°and CuO on the BNPs surface.The results in figure 7    with a broad peak in the 2p3/2 region at 851.88 eV corresponding to Ni°in the bimetallic NPs system [36].The presence of both metals on the surface suggests a higher tendency towards mixed-structure BNPs than coreshell structures.

Study of MB removal 3.3.1. Adsorption kinetics studies
The investigation of the influence of MB concentration on the adsorption process was conducted by the research group at various concentrations (25-100 mg.l −1 ) with a fixed amount of BNPs (30 mg) at 25 °C, and a solution volume of 10 ml.
The results shown in figures 8(a) and (b) indicated a decrease in the efficiency of MB removal as the initial concentration of MB increased.The MB removal efficiencies in water by Fe/Ni and Fe/Cu BNPs materials decreased from 88.6% to 80.72% and from 91.06% to 84.02%, respectively, as the MB concentration increased from 25 mg.l −1 to 100 mg.l −1 .The MB adsorption capacity of the Fe/Ni BNPs increased from 7.43 mg.g −1 to 26.94 mg.g −1 , and Fe/Cu BNPs increased from 7.59 mg.g −1 to 28.00 mg.g −1 .Thus, the higher the solution concentration, the better the adsorption capacity.The MB concentrations before and after adsorption on the two BNP samples are shown in figure 8(c), showing a high tendency to remove MB when MB concentration increases to 100 mg.l −1 .
The results obtained from table 2 and figure 9 reveal that the adsorption of MB onto Fe/Ni BNPs follows a pseudo-second-order kinetic model with a higher R 2 value (> 0.9993) than the R 2 value in the pseudo-firstorder kinetic model.In the pseudo-second-order kinetic model, it can be observed that the calculated q e,cal value is in close agreement with the experimental q e,exp value, indicating the suitability of this model.Furthermore, the K 2 value decreased gradually with increasing initial concentration of the MB dye, indicating that the adsorption of MB onto Fe/Ni BNPs is a physical adsorption reaction.The chi-square (c 2 ) test statistic was used to assess the disparity between the experimental data and model data.A small c 2 value indicates a good fit between the model and experimental data, whereas a high c 2 value suggests a poor fit of the model to the experimental data.According to the results obtained from the model, the c 2 values for the pseudo-first-order and pseudo-second-order models are 0.102 and 35.471, respectively.Thus, the lower c 2 value for the pseudo-second-order model indicates that the adsorption of the MB dye onto Fe/Ni follows this model.
The results obtained from the MB adsorption process on Fe/Cu BNPs materials also suggest strong agreement with the pseudo-second-order kinetic model with c 2 = 1.427 and R 2 > 0.9969.However, the difference between the c 2 values for MB adsorption on the Fe/Ni and Fe/Cu BNPs was not significant.The Elovich chemisorption model describes the interaction between the solid and liquid surfaces [37].As observed in table 2, the initial adsorption rate constant α was significantly higher than the desorption constant β, indicating a   10): Furthermore, this chemical adsorption process is also facilitated by the precipitation reaction of MB with Fe 2+ and Fe 3+ ions on the material surface, as well as the reaction between free hydroxyl radicals (OH•) and MB to produce CO 2 , H 2 O, and short-chain acids, as demonstrated by Abdelfatah et al (2021) [29].The presence of functional groups in the polyphenol coating layer on the surface of BNPs, mainly hydroxyl groups in phenolic, is also a necessary condition for MB adsorption through chemical processes [39].

Adsorption Isotherm Models
The isothermal parameters calculated for the adsorption of MB onto the Fe/Ni and Fe/Cu BNPs materials are presented in table 3.
Isothermal adsorption models are essential for describing the interactions between the adsorbent and adsorbate at equilibrium.The equilibrium data for MB adsorption onto Fe/Ni and Fe/Cu BNPs in this study were analyzed using two commonly used two-parameter isothermal models, Langmuir and Freundlich.The results presented in figure 11 and table 3 indicate that the adsorption of MB dye onto Fe/Ni BNPs fits well with the Freundlich model, with a correlation coefficient R 2 of 0.991, whereas the MB adsorption onto Fe/Cu BNPs materials showed a closer fit to the Langmuir model with an R 2 value of 0.995.MB molecules were adsorbed onto the surface of Fe/Ni BNPs mateirals through a multilayer adsorption mechanism, whereas monolayer adsorption occured on the mesoporous surface of the Fe/Cu BNPs.An experimental parameter n>1 (1.459 and 1.403) signifies a moderate adsorption intensity of MB onto the material.Furthermore, the calculated maximum adsorption capacities (q max ) from Langmuir are 38.46 mg•g −1 and 57.14 mg•g −1 , respectively.The dimensionless separation factor R L at different initial concentrations of the MB dye falls within the range of 0 to 1, indicating a relatively favorable adsorption capability of MB onto both BNPs materials.

Comparison with the reported Fe°-based BNPs
The MB adsorption capacities of the two materials in this study were compared with those of the previously studied BNPs materials.The results in table 4 demonstrate that the Fe/Cu and Fe/Ni BNPs exhibit excellent MB adsorption capabilities, suggesting their potential application as green adsorbents for treating dye wastewater.Compared to some BNP materials chemically synthesized with NaBH 4 and green chemistry using other plant extracts, the two BNPs materials in this study resulted in relatively high adsorption and removal of colored organic matter (except compared to modified mPEC-Fe/Pd).Typically, nanomaterials synthesized by green mechanisms are not as highly efficient as synthetic and chemically modified materials.However, through the condensation process, elevated polyphenol content may be responsible for increasing the adsorption capacity of BNPs.This is evident when compared to BNPs using Ficus leaves, Rooibos tea, and Eucalyptus leaf extract.

Conclusions
In conclusion, the synthesis of Fe/Cu and Fe/Ni BNPs mateirals using concentrated green tea extract as a solvent with optimized parameters: ethanol/H 2 O ratio of 4/1 (v/v), metal ratio of 5/1 (m/m), TPC concentration in the solution of 12.5 g.l −1 , pH = 3-4, reaction temperature of 25 °C, and reaction time ranging from 30 min to 50 min, resulted in the synthesis of BNPs materials with an average particle size of 11-45 nm and a multi-layered structure.The activity was demonstrated through the maximum removal efficiency of MB, which was 88.6% for Fe/Ni and 91.06% for Fe/Cu BNPs materials after 180 min at an MB concentration of 25 mg.l −1 and 25 °C.The MB adsorption on both BNPs materials followed a pseudo-second-order kinetic model.The results of the isothermal adsorption study indicated multilayer adsorption on Fe/Ni BNPs, whereas Fe/Cu BNPs materials exhibited monolayer adsorption on the mesoporous surface.

Figure 2 .
Figure 2. RSM model on the influence of extraction time and temperature on TPC in concentrated green tea powder.
value of 887.27 mg.g −1 at 120 min.Temperature is a factor influencing extraction efficiency because the thermal effect activates the release of phenolic compounds from the concentrated green tea powder into the solvent [26].The equation representing the dependence of TPC on extraction time (X 1 ) and extraction temperature (X 2 ) is encoded and expressed as follows: Y pp = 158.69

Figure 3 .
Figure 3.The process of synthesizing concentrated green tea powder from green tea leaves.

Figure 4 .
Figure 4.The factors influencing sedimentation time and the synthesis efficiency of BNPs are as follows: (A) Fe/Cu, (B) Fe/Ni, (C) pH.
(B) demonstrate the similarity in Fe 2p between the Fe/Cu and Fe/Ni materials, while the Ni 2p region spectrum shows the presence of Ni in various states.The Ni 2p3/2 peak (855.78 eV) demonstrated the existence of Ni 2+ as NiO and Ni(OH) 2 formed by surface oxidation,

Figure 6 .
Figure 6.The results of EDS mapping of (A) Fe/Cu, (B) Fe/Ni and (C) XRD analysis of two BNPs.

Figure 9 .
Figure 9.The kinetic models of MB adsorption on A) Fe/Ni BNPs and B) Fe/Cu BNPs.

Figure 10 .
Figure 10.Reduction reaction of MB to LMB.
Where X represents the total polyphenol content-TPC, expressed as a percentage of the dry weight, and Y represents the dry matter content of the sample.Where A sample is the optical density of the sample solution; a is the slope of the standard curve, and b is the y-intercept value.2.2.1.3.Investigation of factors influencing the synthesis process of BNPs materials Preparation of FeCl 3 •6H 2 O, CuCl 2 •2H 2 O, and NiCl 2 •6H 2 O, followed by dissolution in 40 ml of an ethanol/H 2 O solvent mixture according to the investigated ratios.Dissolution of concentrated green tea powder in 160 ml of ethanol/H 2 O solvent mixture.

Table 2 .
[38]kinetic parameters of MB dye adsorption onto Fe/Ni and Fe/Cu BNPs.and a higher adsorption rate, suggesting the effective adsorption tendency of MB on both BNPs materials.The relatively high R 2 value of the model implies that the MB adsorption mechanism by the BNPs may involve both physical and chemical adsorption.Physical adsorption can be attributed to the specific surface area and porous structure of BNPs.Additionally, chemical adsorption on BNPs is facilitated by the reduction of MB to leuco-MB (LMB), a colorless form[38]due to the release of electrons from Fe°, Ni o , and Cu o metal species present in BNPs materials, as follows (figure

Table 3 .
Isothermal parameters for the adsorption of MB dye onto Fe/Ni and Fe/Cu BNPs.