A comparative study of CuO nanoparticle and CuO/PVA-PVP nanocomposite on the basis of dye removal performance and antibacterial activity in wastewater treatment

ABSTRACT Contaminated waterways, particularly effluent from the dye industry, are one of the major issues today. Among various methods adsorption is the most straightforward, effective, and successful to treat such effluents. For this purpose Copper oxide nanoparticles (CuO) and Copper oxide nanoparticles embedded within Polyvinyl alcohol (PVA) and Polyvinyl pyrrolidone (PVP) mixed polymer (CuO/PVA-PVP) were synthesized using precipitation methods and different analytical techniques such as dynamic light scattering (DLS), UV-Vis, band gap measurement, powdered X-ray diffraction(XRD), Fourier-transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) were used to characterize morphology and size. Synthesized nanomaterials were used for the adsorption of malachite green (MG) dye in aqueous solution by the batch adsorption method. Various experimental conditions e.g. contact time, concentrations, pH, were optimized to obtain the best adsorption outcome. Adsorption kinetics and isotherm suggested that it follows the pseudo-second-order kinetic and Freundlich isotherm that have maximum adsorption capacity of 40 and 76.92 mg/g for CuO and CuO/PVA-PVP, respectively. The in vitro antibacterial activity was investigated using disk diffusion and minimum inhibitory concentration (MIC) test against Staphylococcus aureus (S. aureus), a gram-positive and Escherichia coli (E. coli), a gram-negative bacteria, where both the synthesized nanoparticles showed a decent antibacterial activity against both the bacterial strain.


Introduction
Nanotechnology is one of the most important and useful innovations of contemporary science and has gained an enormous attention during the last three decades because of vast range of applications [1].Nanotechnology is a branch of study that deals with extremely small particles.In the realm of material science, where tiny size plays a key role in defining fundamental characteristics, amazing evolutions have occurred during recent times.As the subject of study expands, commercial exploration evolves day by day [2].The nanoparticle having smaller surface area and greater activity in interacting with metallic species outperforms the larger particles in the adsorption phenomenon.
Pollution of potable water resources has become one of the main headaches for the scientific society.Researchers are trying to develop applications to reduce pollution, which is a tragic problem in the recent situation and especially in the case of water.Water pollution has become one of the most severe issues nowadays due to being polluted by various toxic materials, such as agricultural practices, household waste, especially industrial pollution.Among them, textile dye is one of the most polluting aspects of the global fashion industry.Though the progress of the textile and dye industries is a profitable venture, the drawback is the creation of dangerous organic wastes, which eventually pollute the water and disturb the ecosystem.Dyes are mainly contained with a complex mixture of dye bases, salts, acid, compounds, organ chlorinated, auxiliary chemicals, and heavy metals which are water soluble so it is very difficult to remove them with the conventional methods.Coloring dye is the first contaminant detected in water, and it must be removed from wastewater before it may be discharged into bodies of water [3].Which can obstruct light penetration, slows photosynthetic activity and biota growth, and has a proclivity for chelating metal ions, resulting in micro-toxicity in fish and other species and reduction in dissolved oxygen level [4][5][6][7][8].Among many harmful dyes, malachite green (MG) is one of them, which is cationic in nature and used as a coloring agent in silk, leather, paper industries also used as a tropical anti-protozoal agent, fungicide, and antiseptic in the aquaculture industry to control disease and fish parasites [9].However, MG is highly cytotoxic and also can be a liver tumor-enhancing agent.The toxicity of the MG dye increases with exposure time, temperature and concentration.There are many conventional procedures to remove dyes from water, such as electrocoagulation [10], potentiometric [11], degradation using visible light [10,12], adsorption [13] and bio-degradation [14][15][16][17][18].The most promising technologies are adsorption and photocatalytic degradation, which has a high removal efficiency [19][20][21][22][23][24][25], as well as low energy consumption and environmental friendliness.Following the photocatalytic degradation process, the treated water will include degraded compounds, some of which may be harmful to human health.However, in the adsorption process no contaminants are left behind, as all of the adsorbents are absorbed by the adsorbate, which can be beneficial for human health.Additionally, UV light source is an essential material required for the most photocatalytic degradation process, which is another disadvantage of the process as it enhances the cost, whereas adsorption requires no external light source or particular conditions.The adsorption process is an extensively used process with various advantages.As adsorption offers a very simple and successful treatment procedure for huge volumes of wastewater effluents, it is considered the most effective and cost-effective alternative technology for removing hazardous dyes from wastewater [26][27][28][29][30][31].Due to its great efficiency in removing harmful contaminants, ease of operation, and low cost, liquid phase adsorption offers a significant advantage over other approaches [28,30,[32][33][34][35][36].And the nanoscale metal oxides having small size and high adsorption effectiveness, possibly provide a more cost-effective water treatment and remediation technique [37].Nanoparticles have attracted a lot of attention in the field of water treatment, particularly for removing hazardous pigments.The high specific surface area, extraordinary catalytic activity, and chemical stability of metal oxide nanoparticles could explain their noteworthy adsorption behaviour [38,39].As nanoparticles have bigger surface-to-volume ratios and a higher number of particles per given mass, they have better antibacterial activity than microscaled particles (bulk particles).When metal ions on the surface of nanoparticles are in touch with biomolecules, they become coordinated unsaturated, or 'active centres', which are very reactive and easily released into the surrounding environment [40][41][42].Many metal oxides (Zn, Sn, Fe, Co, Ni, Cu, etc.) are used to remove toxic dyes from waste water [43,44].Among them, Copper oxide (CuO) nanoparticles have been discovered to be a good adsorbent for heavy metals and dyes [45][46][47].CuO nanoparticles also act as a photocatalyst under sunlight irradiation.It has gained a tremendous role in science and technology due to its unique features such as high specific surface area, chemical stability, electrochemical activity, high electron communication features, fungicide, optical switch, metallurgy reagent, gas sensor, pigment, field emission (FE) emitter, magnetic storage media and so on.Furthermore, it is also one of the valuable antimicrobial agents because it is inexpensive and easily mixed with water and polymers [48,49].The antibacterial activity was evaluated using Escherichia coli (E.coli) and Staphylococcus aureus (S. aureus) two of the most commonly used and well-understood model organisms in microbial research for gram-negative and gram-positive strains, respectively.Several studies [50][51][52][53][54][55][56][57][58][59][60] used these model organisms to investigate the antibacterial activity of nanomaterials.As a result, we used the plate-counting technique to test the antibacterial activity of CuO NPs and CuO/PVA-PVP NCs in E. coli and S. aureus.Nanoparticles that have been freshly synthesized are highly unstable and tend to agglomerate into larger formations.The aqueous phase stability of nanoparticles is critical in particle processing and applications, that is why CuO nanoparticles must be shielded from aggregation in order to maintain their characteristics.To preserve the nanoparticles, different surfactants, polymers, and biological templates are utilized [61,62].The goal of this study is to make CuO nanoparticles that are coated with a PVA and PVP blend matrix in a simple and straightforward way.Both amorphous polymers are highly water soluble, synthetic, and biocompatible.When two polymers are mixed, their aqueous blending form inter-chain hydrogen bonds within PVA and PVP develop potential applications that are more similar to those of the homopolymer.The stability and performance of nanoparticle in their application field become better with coated with these blend matrices [63][64][65][66].The purpose of our present work is to determine the effect of the morphology of the prepared nanoparticles and to compare the MG dye adsorption efficiency and antibacterial activity between CuO nanoparticles (NPs) and CuO/PVA-PVP nanocomposites (NCs).

Synthesis of copper oxide (CuO) nanoparticles
CuO nanoparticles were synthesized by the aqueous precipitation method [67].200 mL of a 7% CH 3 COOH solution was prepared and 4 gm of copper (II) acetate mixed with it, then the mixture was stirred for a few minutes until the solution turned green.A 0.9 (M) sodium hydroxide solution was mixed with it in a dropwise manner, maintaining its pH~12, which is followed by heating at 100 o C for 1 h.The mixture was cooled at room temperature and a black precipitate was obtained and separated by centrifugal force (4000 rpm) and washed several times with distilled water and ethanol.The resulting product was dried at 80 o C in a hot air oven for 8 h to obtain the dry powder of CuO nanoparticles.

Synthesis of CuO/PVA-PVP blend nanocomposite
An equal amount (1 gm of each polymer mixed with double distilled water) of PVA and PVP solution was dissolved in 120 mL water under magnetic stirring at 100 o C for 1 h.[63].After the dissolution of polymer composites, CuO nanoparticles (pre-synthesized in section 2.2) were mixed with the polymer solution and sonicated for 1 h until a homogenous solution was obtained.The mixture was stirred for the next 1 h, and further ultrasonication was performed for complete dispersal of CuO in PVA-PVP solution.It was then spread on a clean, level glass plate and dried in a hot air oven for 24 h at 60 o C. The dried films were taken from the glass plate and cut into little pieces for later usage.

Characterizations
The hydro dynamical size distribution of the particles were measured by Litesizer 500 (Anton Paar, Austria), at a scattering angle of 90 o at 28 o C .For the optical characteristics of the samples UV/Vis spectroscopy had been applied by using a UV/Vis spectrophotometer (Thermo Scientific Evolution 201, U.S) within a wavelength range of 200-800 nm, using a 1 cm path length quartz cuvette.Fourier-transform infrared spectroscopy (FTIR, alphaT, Bruker, Germany) was adopted to analysis the functional groups on the surface, the scanning was executed in the range of 500−3500 cm −1 .The crystal structures were characterized using X-ray diffraction (XRD, Explorer, GNR Italy) with Cu Kα radiation.For size and morphology transmission electron microscopy (TEM) was performed on a JEM-2100 (JEOL, Japan) operating at an accelerating voltage of 200 KV, equipped with energy dispersive X-ray spectroscopy (EDS) for the element composition and selected area electron diffraction (SAED) for the crystalline phase identification.For the centrifugation force, a centrifuge (Remi R-4C) was used.And the pH of the solutions was adjusted using a pH meter (Fisher Scientific, Accumet)

Adsorption studies
The adsorption experiments of CuO NPs and CuO/PVA-PVP NCs were performed by batch method at room temperature.A known amount of malachite green and nanoparticles were taken in a conical flask, and then the mixture was shaken thoroughly for 150 min.The samples were collected at regular intervals and the absorbances were measured at the maximum absorption wavelength, i.e. 618 nm for MG using a 1 cm path length cuvette in UV-vis spectrophotometer Various parameters such as contact time (30-150 min), dye concentration in solution (2-26 mg/L) and nanomaterial concentration (2-10 mg), pH (2-10) of the solution were varied during the experiments, pH was adjusted using dilute NaOH and dilute HCl.
The acquired results were verified using adsorption isotherm models such as Langmuir and Freundlich, as well as pseudo-first-order and pseudo-second-order equations to simulate the kinetics of the experiments.
The amount of adsorption percentage (%) and adsorption capacity was evaluated by two equations ( 1) and ( 2), respectively.
Co and Ce designate the colorant<apos;>s initial and final concentrations (mg/L), q e the amount of colorant removed by the adsorbents (CuO and CuO/PVA-PVP) at the final phase of the reaction (mg/g), V is the volume (L), and M is the weight of adsorbents (CuO and CuO/PVA-PVP) (gm).

Bacteria culture preparation
Gram-negative bacteria E. coli (ATCC 25922) and Gram-positive bacteria S. aureus (ATCC 29213) were cultured in Luria Broth (LB) for 24 h at 37 o C and standardized to the 0.5 McFarland standards.

Disk diffusion test
The antibacterial effect of CuO NPs and CuO/PVA-PVP NCs against E. coli and S. aureus was performed by disk diffusion method.100 µL of each bacteria cell suspension (0.5 McFarland standard) was spread over the LB-Agar plate and allowed to dry for 5 minutes.Then a sterile paper disk (5mm) was placed on the plate and 10 µL of solution (concentration of 1mg/mL) of respective compounds were poured on each disk.The plates were then sealed with parafilm and incubated at 37 o C for 24 h, after which the diameter of each inhibitory zone was measured.Water was taken as anegative control and Chloramphenicol as a positive control [68].

Minimum inhibitory concentration (MIC) determination
The minimum inhibitory concentration of the CuO NPs and CuO/PVA-PVP NCs were determined by broth dilution method.50 µL LB was dispensed in each well of column 1-10.50 µL of then 50 µL serially diluted solution of the tested compound was poured into column 1-10, concentration sequence from 4 mg/mL to 0.0078 mg/mL followed by addition of standardized bacteria inoculums (1 ×10 5 cfu/mL).Column 11 was for positive control (only bacteria) and column 12 contain 100 µL LB (for sterility).And the micro plates were incubated at 37 o C for 24 h.And the lowest concentration of compound with no visible growth of bacteria was determined as MIC [69,70].

DLS experiment
CuO NPs and CuO/PVA-PVP NCs were characterized by dynamic light scattering (DLS) measurements, from which the hydrodynamical size distribution was investigated.Both CuO NPs and CuO/PVA-PVP NCs showed single peak having hydrodynamic diameters of 421 nm and 512 nm and polydispersity index 28.5% and 11.3% respectively.These results reveal that both samples were monodispersed i.e. all particles were in the similar size range (Fig. S1).

UV-Vis analysis
The UV-Vis spectra of both nanoparticles CuO NPs and CuO/PVA-PVP NCs are shown in the Figure 1 which range was in between 300-400 nm confirm their good formation and appeared due to surface plasmon resonance (SPR) of metal oxide [71].This UV-Vis characterization is mainly observes for the confirmation of nanoparticles by knowing their absorption peak.

Study of band gap (E g )
CuO nanoparticles are thermally and chemically durable medium bandgap semiconductor with a bandgap energy ranging from 1.2 to 2.8 eV depending on the preparation condition [72][73][74][75].The direct bandgap of CuO nanoparticles and CuO/PVA-PVP nanocomposite is 2.83 and 3.30 eV, respectively (Figure 2).When the CuO crystalline is smaller, a greater band gap is expected, which can be linked to the growth of CuO grain and the improvement of CuO nanocrystal.While the implementation of CuO nanoparticles with PVA-PVP polymers, changed the energy of the bandgap to right, from 2.83 to 3.30 eV, indicating reduction in the size of CuO nanoparticles.As a result, the CuO/PVA-PVP nanocomposite outperforms CuO nanoparticles.

FTIR analysis
The Fourier Transform Infra-Red (FTIR) spectrum of CuO NPs and CuO/PVA-PVP NCs are shown in the Figure 3(a) and Figure 3(b) respectively.In Figure 3(a) the band at 597.87 cm −1 is attributed to the Cu-O stretching vibration [76].The appearance of the peak 1408.01 cm −1 is due to the adsorption of CO 2 molecules and the peak 1548.stretching vibration.The band 2923.94cm −1 and 3221.69 cm −1 belong to the symmetric and asymmetric stretching vibration of the O-H bond, respectively [77], while for CuO/ PVA-PVP NCs in Figure 3(b) the peak at 586.23 cm −1 and 837.85 cm −1 indicated the Cu-O stretching vibration [31] and the C-C stretching vibration 1090.17cm −1 is responsible for stretching C = O and bending of -OH, 1245.67 cm −1 is for the C-N stretching vibration [78].1368.65 cm −1 , 1425.90 cm −1 are appear due to -CH 2 bending and C-N stretching.
Another strong band at 1647.83 cm −1 and 1713.56 cm −1 is responsible for the C-N stretching and stretching vibration of the C = O bond.A weak band 2943.38 cm −1 is assigned to asymmetric stretching and a wide and strong band at 3293.31 cm −1 is due to -OH stretching vibration of the sample [79].

Powder XRD analysis
The CuO nanoparticle XRD pattern in Figure 4

TEM and EDS analysis
Transmission electron microscopy (TEM) reveals the size and morphology of CuO NPs and CuO/PVA-PVP NCs shown in Figure 5 with a regular and rod shaped narrow size distribution (Fig. S3).The average diameters of CuO NPs and CuO/PVA-PVP NCs are 16.04 and 14.55 nm respectively.The bigger size of the CuO NPs compared to the CuO/PVA-PVP NCs may be due to the fact that the uncoated nanoparticles might aggregate due to dipole-dipole interaction but in the case of CuO/PVA-PVP NCs the interparticle distance is enhanced in the absence of interparticle aggregates, as the range of dipolar coupling connected to the space between particles and most of the particles discrete.Resulting, a decrease in the size of CuO/PVA-PVP NCs [81].The structure of generated nanoparticles and nanocomposites was investigated using selected area electron diffraction (SAED).The SAED contrast images reveal a series of diffraction rings with bright spots, which could be due to the polycrystalline and amorphous nature of the nanomaterial.In addition, TEM-EDS (Fig. S3) was performed to determine the composition of the nanoparticles, which indicates the presence of Cu and O molecules, but the presence of carbon(C) in CuO NPs is due to the copper grid and incase of CuO/PVA-PVP NCs, the presence of carbon (C) confirmed the presence of PVA-PVP polymer.

Thermogravimetric analysis (TGA)
TGA is an appropriate technique for examining the thermal characteristics of materials across a wide temperature range to study the weight loss of the CuO NPs and CuO/PVA-PVP NCs also to verify that PVA-PVP participated in the formation of the CuO.The weight loss of CuO NPs before and after generated using a blend polymer of PVA-PVP was investigated using thermogravimetric analysis (TGA).The results are displayed in Figure 6, which reveals that the mass loss of CuO and CuO/PVA-PVP before 223 o C is primarily due to water evaporation.The curve(Figure 6) of CuO indicated that mass of CuO changes slightly after 223 o C, and the mass loss is caused by the binding water between the molecules (Cu(OH) 2 = CuO + H 2 O) [82].Apart from that, in the case of CuO/PVA-PVP NCs it can be observed that the first step of thermal degradation appears around 44-107 o C and this is due to the water absorbed in the blend matrix.The second thermal decomposition occurs from 216 to 350 o C, which can be assigned to the melting of the PVA/PVP chain.
The third stage of decomposition from 387 to 435 o C is the degradation of the remaining blend matrix.The PVA-PVP blend matrix creates a chemical link with CuO, which is then dissolved by heating, resulting in a fast mass loss.The sole weight loss elicited by the decomposition of organic contaminants is what we<apos;>re interested in here [83][84][85].

Effect of adsorbent dose
Adsorptions experiments were carried out by testing different CuO NPs and CuO/PVA-PVP NCs dosages varying from 2-10 mg in 10 mL of MG dye solution containing 2 mg/L as initial dye concentration at room temperature and natural pH.The results are presented in Figure 7 for CuO NPs and CuO/PVA-PVP NCs, respectively.When the dosages were raised from 2 to 10 mg, the MG removal rose from 25.71 to 64.33% for CuO NPs and from 53.60 to 92.40% for CuO/PVA-PVP NCs was observed.More adsorbent sites and surface area for the adsorbate were clearly provided by a greater adsorbent dosage.As the adsorbent dosage was increased, more surface area became accessible for adsorption, leading to an increase in the number of active sites on the adsorbent, making adsorbate ion penetration to the sorption sites easier.From the graph, it can be observed that as the sorbent dosage increases, the available site of adsorption also increases, which causes the highest percent of removal efficiency [86].

Dye adsorption isotherm study for MG dye
Adsorption isotherms are the most important parameters for exploring the interaction between adsorbate and adsorbent, and represent the result accurately.For this experiment using the batch method, 10 mL dye solution with different initial concentrations (2-26 mg/L) and a known amount of adsorbent i.e.CuO NPs and CuO/PVA-PVP NCs (10 mg) was taken in a borosilicate Erlenmeyer flask in room temperature (28 o C) and natural pH and was shaken at constant rpm for 150 minutes To determine the adsorption isotherm, which basically established the equilibrium relationship of adsorption between dye and nanoparticles, two equilibrium models, the Langmuir and Freundlich, were interpreted and correlation coefficients were calculated to identify the best fit.Adsorption isotherms represent the adsorbate's interaction with the adsorbent and provide information on the material's adsorption capability.The correlation coefficients (R 2 ) and statistical characteristics were examined using the linear form of the Langmuir and Freundlich models.
The linear form of the Langmuir isotherm equation is given below in equation 3 [68] .
Where Q e is the amount of adsorbed dye on the adsorbent at equilibrium (mg/g), C e is the equilibrium concentration of dye solution (mg/L), K L is the Langmuir constant (L/mg) and the maximum adsorption capacity is Q m .A slope of 1/ Q m is obtained from the plot C e Q e vs. C e from where the maximum adsorption capacity was determined [Figures 8(a) and 8(c)].The values of Q m ; K L which were calculated from the isotherm are listed in Table 1.
Another dimensionless constant which called the equilibrium parameter is R L to describe the essential characteristics of Langmuir isotherm using equation 4 below: Where C 0 (mg/g) is the initial adsorbate concentration, K L is the Langmuir constant.The value of R L is classified as 0<R L <1 the adsorption process is satisfactory and for R L ≥1 for unsatisfactory.
The Freundlich isotherm describes the hetero generous surface adsorption.The linear form of equation is given in equation ( 5).
Where K F (L/g) and 1  n are the adsorption capacity and adsorption intensity or surface heterogeneity respectively, which are obtain by the plot of logQ e vs. logC e from equation 5.The 1 n is lower than 1 means that the adsorption is favorable.Langmuir [Figure 8(a) and (c)] and Freundlich plots [Figure 8(b) and (d)] of CuO NPs and CuO/PVA-PVP NCs are shown in Figure 8, respectively.From Table 1, the R 2 value of both CuO/PVA-PVP NCs and CuO NPs reveals that the Freundlich isotherm model having R 2 close to 1 (R 2 -0.994 and 0.998) is well fitted compared to that Langmuir isotherm (R 2 -0.928 and 0.952) for both cases.The Freundlich model predicts a heterogeneous surface with a non-uniform distribution of adsorbate and multilayer adsorption of adsorbate on the adsorbent surface.The lower value of 1  n and R L i.e. within 0 to 1, indicates that the adsorption is favorable for both cases [84,85].

Dye adsorption kinetic study for MG dye
One of the key factors in determining adsorption efficiency is kinetic adsorption.Kinetic adsorption experiences were carried out taking a fixed amount of CuO NPs and CuO/PVA-PVP NCs (10 mg) in 10 mL MG dye (2 mg/L) solution at room temperature and natural pH for 150 minutes and samples were collected at every 25 minutes interval and absorbance was measured.The adsorption process, which included mass transfer and chemical reaction, was investigated using kinetics models and kinetic study was investigated using two models: Pseudo-first-order and pseudo-second-order models.The linear form equations of these models are given below: Where Q e and Q t are the adsorption capacity of dye at equilibrium time and time t (mg/g).K 1 (1/min) and K 2 (g/mg min) are the equilibrium rate constant of Pseudofirst-order and Pseudo-second-order respectively All kinetic data for both models are represented in Table 2.
In Table 2 all kinetic parameters (K 1 and K 2 ) as well as the correlation coefficients (R 2 ) are listed.The correlation factor (R 2 ) value is used to determine the best-fitted kinetic model (Figure 9).According to Table 2, the Pseudo-second-order-kinetic model with higher correlation coefficient value (R 2 ) is better defined the adsorption of MG dye than the Pseudo-first- order-kinetic model for both nanoparticles, these results suggest that the Pseudo-secondorder kinetics will be used in the adsorption of MG dye onto CuO NPs and CuO/PVA-PVP NCs.This shows chemical adsorption and a lack of internal diffusion resistance.The adsorbate and the adsorbent exchange or share electrons, resulting in valency forces.

Effect of pH
The most important variable that affects the adsorption onto adsorbent has been discovered as the pH of the solution.The effect of pH on the removal efficiency of the MG dye was investigated by changing the pH from 2 to 10 while keeping at room temperature, dye concentration (2 mg/L, 10 mL) and adsorbent amount (10 mg) fixed.Solution pH is one of important parameter during the adsorption process.In Figure 10 it was found that the nanoparticle removal efficiency increases with increasing the pH value; the highest removal efficiency of the dye was at pH 10 for both nanoparticles.The percentage of removal efficiency is comparatively less in the acidic range of pH [87].Increasing pH leads to the formation of negative charge on the adsorbent surface whereas at low pH the presence of excess H + ions, the surface of these nanoparticles was positively charged.The negative charge enables high removal efficiency due to the electrostatic attraction between the adsorbent and the positively charged adsorbate (MG dye), but at low pH due to the strong repulsive force of the two positive charged dye and the adsorbents, the decolorization process decreases [88].This behavior can be observed because the point of zero charge (pHzpc) was 7.6 and 6.74 for CuO NPs. and CuO/PVA-PVP NCs. and the surface of both adsorbents were negatively charged at pH>pHzpc, which indicated the sorption of MG cationic ions onto the adsorbents.The electrical neutrality of the adsorbents and surface at a specific pH value is indicated by pHzpc, which is an important feature.When the pH of the solution exceeds pHzpc (pH>pHzpc), the adsorbent<apos;>s surface becomes negatively charged, boosting electrostatic contact between the MG dye<apos;>s cations and the adsorbents [36].

Comparison with other adsorbents
Table 3 compares the maximum monolayer adsorption capacity of several adsorbents for the adsorption of MG dyes in the literature.Due to its increased surface area, porous structure, and more active adsorbent sites, the CuO/PVA-PVP nanocomposite is more efficient than other adsorbents.These findings point to a new technique for synthesizing CuO nanoparticles embedded with PVA-PVP polymers for the simultaneous and effective removal of MG dye from aqueous solutions in the future.

Antibacterial activity of CuO NPs and CuO/PVA-PVP NCs
The antibacterial activity of CuO and CuO/PVA-PVP against both E. coli and S. aureus in terms of Zone of inhibition (ZOI) is shown in Fig. S4 and tabulated in Table 4.The CuO/PVA-PVP NCs showed a slightly better ZOI value than the CuO NPs against both bacteria (Fig. S4).Negative control did not show any zone of inhibition.The MIC value against E. coli and S. aureus was found to be 0.5 mg/mL for CuO, whereas CuO/PVA-PVP exhibited a value of 0.25 mg/mL and 0.5 mg/mL (Table 4).It is confirmed that both synthesized nanoparticles have reasonable and indistinguishable antibacterial properties against E. coli and S. aureus.

Conclusions
Copper oxide nanoparticle (CuO) and copper oxide nanoparticle embedded within PVA and PVP mixed polymer (CuO/PVA-PVP) were successfully prepared successfully by the chemical precipitation method.These two nanomaterials were examined for their ability to remove malachite green (MG) color.The results indicated that both nanoparticles fit well with the Freundlich isotherm and follow the pseudo-secondorder kinetic model.The maximum adsorption capacity of CuO NPs and CuO/PVA-PVP NCs were found 40 and 76.92 mg/g respectively which indicating that CuO/ PVA-PVP NCs had a higher adsorption capacity than CuO NPs.The π-π interaction and electrostatic interaction between CuO/PVA-PVP and dye molecules resulted in a high adsorption capacity.Besides, in case of antibacterial test against E. coli and S. aureus CuO/PVA-PVP NCs showed a better result than CuO NPs.This research found that CuO/PVA-PVP NCs might be worth more than CuO NPs and it would be an environmentally acceptable and efficient option for treating organic dyes and decent antibacterial agent in the sectors of water environment and resource management.

Figure 7 .
Figure 7. Effect of the dose on dye removal through (a) CuO nanoparticles and (b) CuO/PVA-PVP nanocomposites.

Table 1 .
Adsorption isotherm parameters for MG dye on CuO/PVA-PVP and CuO adsorbents.

Table 2 .
Adsorption kinetic parameters for MG dye onto CuO/PVA-PVP and CuO adsorbents.

Table 3 .
Comparison of adsorption capacity of CuO/PVA-PVP with other adsorbents.

Table 4 .
Results of antibacterial activity of CuO NPs and CuO/PVA-PVP NCs against both E. coli and S. aureus.