Eco-Friendly Synthesis of Copper Nanoparticles from Fiber of Trapa natans L. Shells and Their Impregnation Onto Polyamide-12 for Environmental Applications

ABSTRACT In this study, copper nanoparticles were synthesized from Trapa natans L. shells and loaded within polyamide-12 for use as an adsorbent. The synthesized nanocomposite was used for the first time to remove toxic methyl orange and brilliant green dye molecules from synthetic aqueous solutions. In order to characterize the plant-based nanocomposite, its functional groups, surface texture, elemental composition, structure, surface area, and crystallinity were determined. Adsorption efficiency was found to be affected by factors like pH (2–10), time (5–240 min.), and temperature (288–308 K). It was at pH 6 that the adsorbent reached the point of zero charge. During the adsorption process, electrostatic attraction played an influential role. Adsorption isotherms and kinetics data were well-fitted by Langmuir isotherms and pseudo-second-order models, respectively. A maximum adsorption capacity of 166.60 mg/gm was calculated for 93.90% methyl orange removal and 55.24 mg/gm for 97.30% brilliant green removal. The adsorption process was endothermic, spontaneous, and physical. The rapid desorption of dye molecules confirms the potential, and practicality of the nanocomposite as a promising adsorbent for decolorizing colored wastewater on an industrial scale.


Introduction
There is a continuous increase in dye pollution of water.As reported by the world health organization and the environmental pollution agency, their toxic effects on the environment are concerning.Since dye toxicity is expensive and time-consuming, most industries don't pay attention to it.Methyl orange (C 14 H 14 N 3 NaO 3 S) is a vibrant orange-yellow azo dye used primarily as a pH indicator to titrate inorganic acids and alkalis.As a result of its industrial applications, methyl orange (MO) is released into the water environment through effluent stream contributing to the depletion of the water environment.A study has indicated that about 20% of azo dyes disposed of by textile factories are discharged directly into the environment as a waste product (Robinson et al. 2001).Brilliant green (C 27 H 34 N 2 O 4 S) dye is a cationic, triphenylmethane-based organic dye with no odor.BG is illegally used not only in aquaculture to treat fish diseases but also in pulp and paper production to color paper towels used in hospitals, factories, and other locations to dry hands after washing (Oplatowska et al. 2011).In humans, its fatal dose is 50-500 mg/kg, which makes it a highly toxic dye (Pohanish 2012).In order to treat dye-containing wastewater, many physical-chemical techniques have been implemented such as photodegradation (Siong et al. 2019), photo-Fenton (Muhammad et al. 2020), photocatalysis (Ikram et al. 2021), etc.In addition to their advantages, these technologies have some drawbacks as well.Among them, adsorption has long been the most popular and effective method for removing aqueous pollutants (Uddin 2017;Uddin and Rahaman 2017) and treating dyes containing wastewater for many years (Husein et al. 2021;Uddin et al. 2021).For this technique, an adsorbent material that is porous and efficient is needed.Many water treatment processes already use biomass (Rao, Ikram, and Kashif Uddin 2015;Khatoon, Uddin, and Rao 2018).Scientists who work on the protection of the environment and water treatment are interested in the search for new adsorbents.In order to reduce the toxic effects of water, nanotechnology and green chemistry can be combined with safe biological materials, such as plants, to fabricate nontoxic nanoparticles (Jadoun et al. 2021).A green synthesis process uses plant extracts that are cheap, widely available, and safe for humans.The biosynthesis of nanoparticles (NPs) by using a green approach has gained considerable attention due to the fact that plants and their wastes are readily available, are a cheaper alternative, show high removal rates, and can be reused (Giri et al. 2020).The synthesis of NPs is therefore based on natural resources, such as plant extracts, fungi, and bacteria.In nanomaterials, carbon is sourced primarily from plant extracts (Saddik et al. 2020).In recent reviews conducted by Qamar and Ahmad (Qamar and Nazeer Ahmad 2021) and Yang et al (Yang et al. 2022).concluded that two dominant approaches are used to synthesize nanoparticles (the bottom-up approach and top-down approach).ZnFe 2 O 4 nanoparticles synthesized by using plants and their properties have also been recently reviewed for their use as catalysts, biomedical materials, and biomedical agents, among others (Nguyen et al. 2023).The preparation methods also have some limitations, such as top-down method requires wide size distribution and varies particle size, whereas bottom-up method requires chemical purification and cannot be produced at large scales.Among metal oxide nanoparticles, copper oxide nanoparticles have gained considerable attention for their many characteristics and applications.A unique antimicrobial activity of copper nanoparticles have been demonstrated (Thandapani et al. 2023).According to a recent review, there have been many attempts to synthesize copper oxide nanoparticles using different plant extracts (Chakraborty et al. 2022).The biosynthesis of Cu and Cu 2 O has been done by several plants (Al-Hakkani et al. 2021).Studies have demonstrated that metallic nanoparticles can effectively target cancer cells and manage them (Saddik et al. 2021(Saddik et al. , 2022)).
It is relatively easy to mix copper oxide nanoparticles with polymers, and their chemical and physical properties are relatively stable (Allaker 2012).Developing polymer nanocomposite by incorporating organic or inorganic nanoparticles into polymeric matrices is a rapidly emerging research field (Wani et al. 2022).Adding nanoparticles to polymers allows modification of physical properties and implementation of new features (Hanemann and Vinga Szabó 2010).The use of polymer nanocomposites in many applications has been widely accepted (Alarifi, Prasad, and Kashif Uddin 2020).Polyamide-12 (PA-12) is a semi-crystalline thermoplastic polymer and is considered an efficacious material due to its ideal properties and multiple applications (Aldahash et al. 2022).A major characteristic of PA-12 is its inhomogeneous particle size, high specific surface area, high lamellar crystal size, and low crystallinity (Simha et al. 2021).Polyamide has attracted scientists to the development of new materials with an intriguing perspective and a large potential in many industrial fields, including wastewater treatment (Basaleh, Al-Malack, and Saleh 2019;Saleh, Tuzen, and Sarı 2018).In a recent critical review, polyamide-based desalination membranes were highlighted as being capable of separating toxic organic compounds from water (Shin et al. 2022).Currently, PA-12 is a relatively new material in the field of environmental science.A PA-12/cement nanocomposite has been synthesized and used to remove four different dye molecules in our latest article (Aldahash et al. 2022).Biosorbents, which are derived from plants, are among the most widely used low-cost materials for removing pollutants from water (Rao, Rehman, and Kashifuddin 2012;Uddin and Nasar 2020).The water chestnut (Trapa natans L.) is an aquatic plant that is also known as the horn chestnut, the Jesuit nut, and the water caltrop (Borojevic 2009).Various species of Trapa natans L. are grown in confined areas of water in the countries of China, India, and Thailand for human consumption (Borojevic 2009).Table S1 shows the composition of Trapa natans L. shells (Sverguzova et al. 2020).Recently, Qaiyum et al. have used alkali-modified shells of Trapa natans L. to remove malachite green from water (Qaiyum et al. 2022).Trapa natans L. peels treated with citric acid were used to remove brilliant green dye using a biosorption procedure (Hussain, Hussain et al. 2021).In Khan et al.'s study, it was used to remove rhodamine B dye from an aqueous solution through biosorption (Khan, Nazir, and Khan 2013).Trapa natans L. husk activated carbon was tested for its ability to remove norfloxacin drug from aqueous solutions at a low cost (Xie et al. 2011).This study aims to produce eco-friendly copper nanoparticles by green synthesis from Trapa natans L. shells, and encapsulate them within PA-12 material for use as a novel adsorbent.A feature of this method is that nanoparticles can be synthesized without much equipment and at a low cost from plant extracts.The use of plant extracts for nanoparticle synthesis has advantages over other biological synthesis methods.The prepared material is tested for the removal of methyl orange and brilliant green dye molecules from aqueous solutions for industrial wastewater treatment.

Materials and methods
Description of methods and methods are available as supplementary material.

Fourier Transform Infrared (FTIR)
The FTIR spectrums of PA-12/CuONPs, MO and BG adsorbed PA-12/CuONPs are depicted in Figure 1.As observed in PA-12/CuONPs, the broad peak at 3411 cm −1 was allocated to -OH stretching vibration.The peaks at 2,922.65 and 2852.12cm −1 indicate asymmetric and symmetric stretching vibration of C -H (Rao and Kashifuddin 2014).The peaks at 2374.31 and 2343.86 cm −1 represent the adsorbed atmospheric carbon dioxide gas molecule.The peak at 1712.75 cm −1 is due to the stretching vibration of carbonyl (C=O) and may be the result of carboxylic acids or their esters (Feng Ting et al. 2007).Observation of the peak at 1639 cm −1 indicates stretching vibration of the Cu -O bond of CuONPs (Raul et al. 2014).Trapa natans L. exhibits characteristic lignin components that exhibit nonsymmetric stretching vibrations of C=O and COO near 1500 cm −1 (Ceylan and Kilic Pekgozulu 2019).Narrow peaks at 1561, 1553.01 and 1461.58cm −1 were ascribed to amine (amide II band) stretching vibrations of N -H and C -N, respectively (Yin, Zhu, and Deng 2016).The peaks at 1434.55, 1418.27 and 1383.73 cm −1 are caused by the interaction of carboxylic acid functional groups with the O−H bonds.The peaks at 1221 and 1118.34 cm −1 are attributed to the stretching of the C -O bond due to the amorphous/crystalline structure of Cu 2+ (Uddin, Ali Khan Rao, and Chandra Mouli 2018).The peak at 873 cm −1 can be correlated to stretching of and Cu-O-H while the various peaks appeared in the range from 779 to 425 cm −1 indicating the formation of CuO-NPs (Bialy et al. 2020).A small shift, weakening, and movement of the peak intensities indicated the interface between dye molecules and PA-12/CuONPs.As a result of MO adsorption, new absorption bands of wavenumbers ~ 3744, 2339, 2064, 1630, 1419, 1365 and 1270 cm −1 appeared, while after BG adsorption, new absorption bands of wavenumbers ~ 3778, 3402, 2064, 1465, 1547, 1187, 1088 and 600 cm −1 appeared.Figure S1 shows individual FTIR spectra of Trapa natans L., PA-12, CuONPs, MO, and BG.Table S2 specify the vibrational bands that appeared after the adsorption in each case.

Brunauer-Emmett-Teller (BET) analysis
As shown in the Figure 2, the N 2 adsorption-desorption isotherm of PA-12/CuONPs belongs to type III according to IUPAC having H3 type hysteresis loop, indicating a typical mesoporous structure in the adsorbent.The H3 loop is attributed to wedge-shaped (tapering a thin edge at one end) pores formed by the loose collection of flaky particles composed of nonrigid aggregates of plate-like particles (Longfei et al. 2020).As type III indicates narrower mesopores, wider micropores, and a wider pore size distribution, which promotes sharp adsorption capacity (Mukhtar et al. 2020).Using BET, 34.309 m 2 /gm of surface area is calculated.As a result of the BJH method adsorption, the surface area is 26.3936 m 2 /gm, the pore volume is 0.299852 cc/gm, and the pore radius is 1.6919 nm, whereas as a result of the BJH method desorption, the surface area is 39.3279 m 2 /gm, the pore volume is 0.308586 cc/gm, and the pore radius is 8.63211 nm.As PA-12/CuONPs have pore widths not exceeding about 2 nm, they can be characterized as microporous solids.A recent study by Liu et al. showed that polymeric adsorbents with smaller pores could adsorb phenol and anionic surfactants more effectively (Liu et al. 2021).

High Resolution Transmission Electron Microscopy (HR-TEM)
HR-TEM analyses revealed a branched, stable structure comprised of segments interconnected by crosslinks and forming network aggregates.HR-TEM images at different magnifications (Figure 3a-c) show nanoparticles with small, homogeneous and uniform particles, round surfaces, chains, and network structures.Also, PA-12 is encrusted with copper nanoparticles that have a darker color.Based on Image J software, a 23.21 nm average size was calculated.

Scanning Electron Microscopy (SEM) and energy dispersive X-ray (EDS) analysis
As shown in the SEM images (Figure 4a, b), the PA-12/CuONPs nanocomposite exhibits a rice-grainshaped morphology with a spontaneous, narrow particle size distribution.There are agglomerated semi-spherical nano-sized particles, which are relatively homogenous.In the presence of MO and BG, the surface of PA/12-CuNPs underwent considerable changes (Figure 4c, d).MO and BG ions surrounded and accumulated on the surface after adsorption.As shown by the bright patches on the dye-loaded adsorbent's surface, this facilitates the binding of dye molecules to PA-12/CuONPs.Figures 5a-c and 6a-c shows the elemental content and their abundances of PA-12/CuONPs, before and after adsorption by elemental mapping and EDS, respectively.According to the analysis, PA-12/ CuONPs contains 80.71% copper, 7.82% carbon, 6.47% oxygen, 2.81% sulfur, and 0.75% sodium.In the element analysis of dye adsorbed PA-12/CuONPs, the rise in the C element indicates successful adsorption of MO and BG onto PA-12/CuONPs.In addition, the change in their respective weight confirms dye adsorption, which is also evident in the elemental mapping of PA-12/CuONPs where the detected elements are distributed on the surface.

X-Ray diffraction analysis (XRD)
According to the XRD pattern for PA-12/CuONPs, there are intense diffraction peaks (Figure 7a) indicating that PA-12 is loaded due to the occurrence of intense diffraction bands at 2θ values and crystal planes at ~ 35.55° and ~ 38.74° in accordance with the JCPDS (01-089-5895).The XRD also revealed a monoclinic crystal system of the adsorbent when the polyamide was intercalated.On the other hand, CuO peaks have been observed at ~ 32.43°, ~35.46°, ~38.59°, ~48.53°, ~57.97°, ~61.42° and ~ 66.02° (Figure 7a, b).Data from the JCPDS (01-089-5898) suggested the volume of the cell to be 81.16pm.In PA-12, there are two distinct peaks at the 2θ values of ~ 20.9° and ~ 22.0°Corresponding to the unstable and intermediate phases of α and γ, respectively (Sommereyns et al. 2021) (Figure 7c).With the Miller indices, XRD peaks are identified (Figure 7a-c).In addition, the crystallite sizes of CuONPs before and after synthesis were calculated using the Scherrer equation and found to be 53.9696nm and 13.32696 nm, respectively.

Thermo Gravimetric Analysis (TGA)
The TGA thermogram (Figure 8) of PA-12/CuONPs shows that weight percentage decreases with temperature, indicating that mass continuously changes due to thermal treatment.TGA curve showed that water was removed from steps 1-2 resulting in a 20% weight loss.Prior to melting temperature for step-1, no weight loss was observed, indicating that it is anhydrous.Starting at 420°C in steps-2-3, the melting peak is followed by a degradation peak indicating significant weight loss.Approximately 98% of it is degraded up to the end of a reaction.When the temperature is high, volatiles are lost, causing weight loss.

UV-Visible (UV-Vis) spectroscopy
Fig. S2 shows UV-vis of CuONPs and PA-12/CuONPs.In this study, copper sulfate solute (CuSO 4 .5H 2 O) was reduced into copper oxide nanoparticles using UV-VIS spectroscopy (Perkin Elmer, USA).Upon synthesis of copper oxide nanoparticles and subsequent addition of Polyamide 12, the color of the solution gradually changed and was observed through Surface Plasmon Resonance (Fig. S2).A 3 ml sample of copper oxide nanoparticles and PA-12/CuONPs were injected into the cuvette and absorbance was measured between 400 and 600 nm, following the method of Sarwar et al (Sarwar et al. 2021).It was observed that the maximum absorption of copper nanoparticles before addition of PA-12 occurred at 500 nm, indicating the change in color from blue to green, whereas after addition of PA-12 it occurred at 590 nm, forming a dark brown paste.

Effect of pH and point of zero charge
The pH plays a significant role in the adsorption study because it affects the surface binding sites of the adsorbent due to its effect on the ionizing power of dye molecules in the aqueous mixture.Figure 9 shows the effects of different pH values (2-10) on the adsorption of MO and BG on PA-12/CuONPs.For both dyes, the trend was the same and at pH 6, MO and BG adsorb best, but after that there is a gradual decline in qe values.In previous studies, pH 6 was also identified as the optimum pH for MO (Baig, Kashif Uddin, and Gondal 2020) and BG (Aldahash et al. 2022) adsorption.To understand how  pH affects sorption, the point of zero surface (pHpzc) is an important parameter.pHpzc refers to the point where (pH i -pH f ) becomes zero.Above pHpzc the adsorbent has a negatively charged surface, while below this point, the adsorbent has a positively charged surface (Uddin and Nasar 2020).A pH6 PZC value was obtained for PA-12/CuONPs (Figure 10), thus, the surface of PA-12/CuONPs is positively and negatively charged below and above this pH, respectively.When pH is low, MO molecules generate a monovalent ion on nitrogen, which causes electrostatic repulsion with PA-12/ CuONPs active sites (at < pH 6), thus decreasing MO adsorption.BG is a cationic dye and dissociate H + ion which causes electrostatic repulsion at PA-12/CuONPs active sites (at < pH 6).At pH > 6, OH¯ ions associated with the PA-12/CuONPs surface caused precipitation and competition between MO and BG with negative adsorption sites, which hindered and decreased MO and BG adsorption at PA-12/CuONPs surfaces.It can therefore be assumed that MO and BG were adsorbing on PA-12/CuONPs through electrostatic interaction mechanism.

Adsorption isotherm
By using the isotherm equations, the experimental data of MO and BG adsorption onto PA-12/ CuONPs were fitted into non-linear plots based on Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherm models (Figure 11a-c).Langmuir and Freundlich models both perform  equally well in describing the adsorption process of MO and BG onto PA-12/CuONPs based on their generated correlation coefficients (R 2 ) (Table 1).These results indicate that both the dye molecules are adsorbed initially as monolayers, and later as multilayers onto PA-12/CuONPs surfaces.RL is less than 1 indicating a favorable adsorption process.A value of n of 1.04 is obtained MO, and a value of 1.397 is acquired for BG.Based on the outcome that b, K F , RL and n are low and positive values, it is clear that dye molecules interact favorably with PA-12/CuONPs with high affinity.In comparison, the PA-12/CuONPs obtained maximum adsorption capacity (qm) values are higher than previously reported qm values for various MO and BG adsorbents.It was observed that MO exhibited an excellent qm of 166.60 mg/gm, which was greater than CO 3 O 4 nanoparticles (46.08 mg/gm) (Uddin and Baig 2019), and ZnO supported red mud (102.64 mg/gm) (Qiao et al. 2022).A reasonable qm of 55.24 mg/gm was obtained for BG, which is superior to Peganum harmala-L-seeds (35.97 mg/gm) (Agarwal et al. 2017), and modified areca nut husk (18.21 mg/gm) (Sukla Baidya and Kumar 2021).This result demonstrates the feasibility and practicality of using PA-12/CuONPs to remove MO and BG from industrial wastewater.In order to confirm the nature of the adsorption process, D -R isotherm parameters were calculated.Adsorption is considered physical if adsorption energy (E) is less than 8 kJ/mol, and chemical if it is equal to or greater than 8 kJ/mol.For both MO and BG, the calculated E values were less than 8 kJ/mol, which supports the physical adsorption of MO and BG on PA-12/CuONPs via ion exchange.In addition, the Temkin isotherm model also BG adsorption with a reasonable R 2 .When the heat of sorption (B) value is less than 20-40 kJ/mol, physical adsorption occurs, and when it is 80-240 kJ/mol, chemical adsorption occurs.Adsorption involves both physical and chemical adsorption if the B value is between 20-40 kJ/mol (Uddin and Baig 2019).In terms of B, the value comes out to be 0.277 kJ/mol for MO and 0.296 kJ/mol for BG, again suggesting a physical adsorption process, also supported by the D-R isotherm model.

Adsorption thermodynamics
Using vant's Hoff plots, the thermodynamic parameters for MO and BG adsorption onto PA-12/ CuONPs were determined at various temperatures and shown in Table 2 and Figure   the results, the values of -ΔG values increased with temperature, which indicates that MO and BG were adsorbed more with increasing temperature due to the endothermic and spontaneous nature of the process.The adsorption process of MO and BG was further attributed to physical adsorption since -ΔG values were below 20 kJ/mol (Ahmadi, Hazrati Niari, and Kakavandi 2017).For both the dyes, the positive values of ΔH presented the same indication of endothermic and spontaneous processes while the positive values of ΔS indicate increased disorder at the solid/solution boundary during the favorable adsorption of MO and BG onto PA-12/CuONPs.

Adsorption kinetic
As a result of a higher R 2 and closer qe (calculated) to qe (experimental) (Table 3), pseudo -secondorder (PSO) was found to be superior to pseudo -first-order (PFO) in fitting the MO and BG adsorption data (Figure 13a, b).In general, PSO works well for the adsorption data since its equation does not take the effect of experimental errors into account (Plazinski, Rudzinski, and Plazinska 2009).As the values of PFO and PSO rate constants are low, K 1 and K 2 , respectively, equilibrium was achieved at higher concentrations.Elovich's model parameters of MO and BG adsorption as obtained by linear plots (Figure 13c) show reasonable R 2 values and higher values of A (adsorption constant) than B (desorption constant) (Table 3), confirming that adsorption is more feasible than desorption.
The negative B values indicate endothermic processes, which confirms the thermodynamic results.To assess molecular diffusion, intraparticle diffusion model (IPD) plots were used (Figure 13d).It is evident from Figure 13d that MO adsorption is influenced by particle diffusion, as the initial phase of surface adsorption occurs quickly, followed by a gradual internal adsorption of MO onto PA-12/ CuONPs, until the diffusion is constant.In terms of BG adsorption, IPD plot has a plateau profile that does not pass through the origin and thus not controlled by film diffusion.

Desorption
In Figure 14, the percentage desorption efficiency of MO and BG adsorbed PA-12/CuONPs nanocomposite after five regeneration cycles is shown.According to this result, there is a constant decline in the percentage removal of MO and BG, which is suitable for the recycling of PA-12/CuONPs as well as its practicality and durability.Furthermore, it indicates that MO and BG were readily desorbed from PA-12/CuONPs, indicating a physical adsorption process.After five adsorption-desorption cycles, PA-12/CuONPs has stability of 52.57and 48.63% for BG and MO, respectively.

Conclusion
This study presents a simple and convenient method for synthesizing copper oxide nanoparticles with polyamide using Trapa natans L. shells.In SEM images, the nanocomposites appear ricegrain-shaped with narrow, spontaneous particle sizes.The BET analysis of the adsorbent revealed an H3 type hysteresis loop, indicating a typical mesoporous structure.For MO and BG adsorption, pH 6 was found to be optimal.As a result of its excellent adsorption capacity toward MO and BG, the material is both feasible and practical.A maximum removal of 93.9% was achieved for MO as well as 97.3% for BG when using the adsorbent.Adsorption mechanisms involved electrostatic attraction and physical adsorption.The most fitted adsorption isotherm models for MO and BG are Langmuir and Temkin, respectively.According to the thermodynamic process, both dye molecules adsorb endothermically and spontaneously.Despite five adsorption-desorption cycles, the adsorbent showed good stability, which made it useful for industrial use.In addition to use of polyamide in buildings, polyamide nanocomposites can also be used to make products that treat water in houses.
• In order to impregnate the nanoparticles, PA-12 was used.
• The rapid desorption of dye molecules confirms the adsorptive potential of the synthesized nanocomposite.
• As a result of its excellent adsorption capacity toward MO and BG, the material is both feasible and practical.
• PA-12 can also be used to make nanocomposites that can treat water in houses.
• The synthesized nanocomposite has stability of 52.57and 48.63% for BG and MO.

Figure
Figure Effect of pH on MO and BG adsorption onto PA-12/CuONPs.
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